*This article was submitted to Inorganic Chemistry, a section of the journal Frontiers in Chemistry*

*Received: 30 January 2019 Accepted: 24 April 2019 Published: 21 May 2019*

### *Citation:*

*Talukdar K, Issa A and Jurss JW (2019) Synthesis of a Redox-Active NNP-Type Pincer Ligand and Its Application to Electrocatalytic CO2 Reduction With First-Row Transition Metal Complexes Front. Chem. 7:330. doi: 10.3389/fchem.2019.00330*

**38**

(Benito-Garagorri and Kirchner, 2008; van der Vlugt and Reek, 2009; van der Vlugt, 2012; Chakraborty et al., 2015; Bauer and Hu, 2016). Yet, only a limited number of pincer complexes have been reported for electrocatalytic CO2 reduction (Arana et al., 1992; Chiericato et al., 2000; Elgrishi et al., 2014, 2015; Sheng et al., 2015; Rao et al., 2016; Cope et al., 2017; Liu et al., 2018; Therrien et al., 2018; Myren et al., 2019). Early examples from Arana et al. (1992) and Chiericato et al. (2000) were reported with redox-active terpyridine and bis(imino)pyridinebased ligands (**Figure 1**). The metal ions decorated with these NNN-type ligands showed catalytic activity under reducing potentials in CO2-saturated solutions. Detailed product analysis was not reported, but formic acid was a notable reduced carbon product obtained with a cobalt bis(imino)pyridine complex. Recently, Liu et al. (2018) prepared a related pincer ligand, based in part on bis(imino)pyridine, in which steric bulk was added and one arm was replaced with pyridine to access a bipyridyl unit, producing a cobalt catalyst that is selective for CO2 reduction to formate. Rao et al. (2016) reported a PNPligated manganese-tricarbonyl complex which can reduce CO2 to CO without any added Brønsted acid. Mixed-donor Nheterocyclic carbene-based pincer ligands have also proven to be effective supporting frameworks for transition metal catalysts for CO2 conversion (Sheng et al., 2015; Cope et al., 2017; Therrien et al., 2018; Myren et al., 2019).

Given these results, we sought to design a new redoxactive mixed-donor ligand with extended conjugation for CO2 reduction employing first-row transition metals. The ligand design aims to exploit the well-established redox activity of bipyridine at a lower overpotential by extending its conjugation. In addition, the triphenylphosphine donor fragment serves as a good π-acceptor to facilitate metal-based reductions at lower potentials. We note that closely-related NNP ligands have been recently reported (Basu et al., 2018; Kamitani et al., 2018) and applied to the synthesis of new Fe and Co complexes for catalytic hydrosilylation and dehydrogenative silylation. Herein, firstrow metal complexes of a rigid NNP pincer ligand and their electrocatalytic activity toward CO2 reduction are reported.

### EXPERIMENTAL

### Materials and Methods

All synthetic manipulations were carried out using standard Schlenk techniques or in an MBraun glovebox under nitrogen atmosphere. Freshly distilled acetonitrile (CH3CN) and N,N-dimethylformamide (DMF) were used in synthesis and electrochemistry. Tetrahydrofuran, toluene, and diethyl ether were dried with a Pure Process Technology solvent purification system. 1,1′ -Bis(diisopropylphosphino)-ferrocene (DiPPF), diphenylphosphine, iron(II) trifluoromethanesulfonate, and copper(II) perchlorate hexahydrate were purchased from Strem Chemicals. Zinc(II) trifluoromethanesulfonate and sodium tert-butoxide were purchased from Acros Organics. Nickel perchlorate hexahydrate and palladium(II) acetate were purchased from Alfa Aesar and Chem-Impex International, respectively. All other chemicals were reagent or ACS grade, purchased from commercial vendors, and used without further

purification. 1H, 31P, and 13C NMR spectra were obtained using Bruker spectrometers operating at 500 MHz (1H), 167 MHz ( 31P), or 126 MHz (13C). Spectra were calibrated vs. observed solvent peaks. Chemical shifts are reported in parts per million (ppm). Solution magnetic susceptibilities were determined by NMR using the Evans method. High-resolution electrospray ionization mass spectra (HR-ESI-MS) were obtained with a Waters SYNAPT HDMS Q-TOF mass spectrometer and elemental analyses of carbon, hydrogen, and nitrogen were conducted by Atlantic Microlab, Inc., Norcross, Georgia. Gas samples were analyzed by a custom Agilent 7890B Gas Chromatograph (Agilent PorapakQ column, 6 ft, 1/8 in. OD) with a dual detector system (TCD and FID). Calibration curves for the observed gases were prepared from commercial standards of known concentration obtained from buycalgas.com.

### Electrochemical Measurements

Cyclic voltammetry was performed with a typical three-electrode setup using a CH Instruments 600E Series potentiostat. The electrochemical cell was equipped with a glassy carbon disk working electrode (3 mm diameter), a platinum wire counter electrode, and a silver wire quasi-reference electrode. Acetonitrile or DMF solutions containing 0.1 M Bu4NPF6 as the supporting electrolyte were used in all studies as specified. Ferrocene was added at the end of experiments and served as an internal standard to reference the potential. Electrolysis solutions and electrochemical cells were thoroughly degassed with nitrogen or carbon dioxide for 10–20 min prior to each experiment. Freshly made solutions were used for each experiment. Controlled potential electrolyses (CPE) were conducted in an airtight twocompartment cell with a glassy carbon rod working electrode (type 2, Alfa Aesar, 2 mm diameter), a silver wire quasi-reference electrode, and a high-surface area platinum mesh counter electrode. The platinum counter electrode was positioned inside a small isolation chamber with a fine frit. The isolation chamber contained the same solution, but without catalyst. The electrolysis solution in the working electrode compartment was continuously stirred during the experiments. Evolved gases were quantified by taking aliquots (0.30 mL) from the headspace using a sealable gastight syringe for injection into the gas chromatograph for analysis.

## X-Ray Crystallography

A single crystal coated with Paratone-N hydrocarbon oil was mounted on the tip of a MiTeGen micromount. The temperature was maintained at 200 K with an Oxford Cryostream 700 during data collection at the University of Mississippi, Department of Chemistry and Biochemistry, X-ray Crystallography Facility. Samples were irradiated with Mo-Kα radiation with λ = 0.71073 Å using a Bruker Smart APEX II diffractometer equipped with a fine-focus sealed tube source and APEX-II detector. The Bruker APEX2 v. 2009.1 software package was used to integrate raw data which were corrected for Lorentz and polarization effects (APEX2, 2009). A semi-empirical absorption correction (SADABS) was applied (Sheldrick, 2000). The space group was identified based on systematic absences, E-statistics, and successive refinement of the structure. The structure was solved using direct methods and refined by least-squares refinement on F2 and standard difference Fourier techniques using SHELXL (Sheldrick, 1990, 2008, 2014). Thermal parameters for all non-hydrogen atoms were refined anisotropically, and hydrogen atoms were included at ideal positions. Crystallographic data in CIF format was deposited into The Cambridge Crystallographic Data Center (CCDC); see deposition number CCDC 1909986 (**NiL2**+ **2** ) and **Table S1**.

## Synthesis of the Ligand

Ligand precursor 8-bromo-2-(pyridin-2′ -yl)quinoline (**2**) was synthesized as previously reported (Mao et al., 2005; Wickramasinghe et al., 2015). New ligand (8- (diphenylphosphaneyl)-2-(pyridin-2′ -yl)quinoline) was prepared using a modified procedure (Murata and Buchwald, 2004; Zhang et al., 2017).

### 8-(diphenylphosphaneyl)-2-(pyridin-2′ -yl)quinoline, L

Inside the glovebox, to an oven-dried pressure flask with a Teflon screw cap was added 8-bromo-2-(pyridin-2′ -yl)quinoline, **2** (0.80 g, 2.81 mmol), Pd(OAc)2 (0.032 g, 0.140 mmol), 1,1′ bis(diisopropylphosphino)ferrocene (0.071 g, 0.168 mmol), and NaOtBu (0.324 g, 3.37 mmol). Then 5 mL anhydrous toluene was added and the mixture was stirred for 1 h. Diphenylphosphine (0.41 mL, 2.34 mmol) was added to the flask via a micropipette and the flask was re-sealed and heated at 120◦C for 36 h. After cooling to room temperature, the reaction mixture was concentrated and purified by a deactivated silica column eluting with hexanes:ethyl acetate (5:1). The product was obtained as a light yellow solid, which is air stable in the solid phase (0.667 g, 73 %). 1H NMR (CD3CN, 500 MHz): δ 8.64 (dq, J = 0.7 Hz, J = 4.8 Hz, 1H), 8.54 (d, J = 8.6 Hz, 1H), 8.40 (d, J = 8.6 Hz, 1H), 8.04 (d, J = 8.0 Hz, 1H), 7.95 (d, J = 8.2 Hz, 1H), 7.75 (td, J = 1.7 Hz, J = 7.8 Hz, 1H), 7.50 (td, J = 0.8 Hz, J = 8.0 Hz, 1H), 7.43–7.34 (m, 11H), 7.13 (qd, J = 1.3 Hz, J = 3.8 Hz, 1H). 13C{1H} NMR (CDCl3, 126 MHz): δ 156.18 (s), 154.94 (s), 148.86 (s), 148.63 (d), 139.26 (d), 137.62 (d), 137.13 (s), 136.98 (s), 134.54 (s), 134.38 (s), 134.06 (s), 128.74 (s), 128.56 - 128.43 (m), 127.93 (s), 126.96 (s), 124.11 (s), 122.37 (s), 119.00 (s). 31P{1H} NMR (CDCl3, 167 MHz) δ−13.35. HR-ESI-MS (M+) m/z calc. for [L+Cs+] +, 523.0340, Found, 523.0327.

### Synthesis of the Metal Complexes Common Synthetic Procedure

An acetonitrile or methanol solution (5 mL) of the NNP ligand **L** (0.050 g, 0.128 mmol) was prepared in a two-neck round bottom flask equipped with a reflux condenser before 0.5 equivalents of the corresponding metal precursor was added to the solution. The reaction mixture was refluxed for 6 h under nitrogen. After cooling to room temperature, the solvent was evaporated under reduced pressure and the residue was washed with Et2O.

### FeL2+ 2 , [FeL2](OTf)2

The complex was prepared from Fe(OTf)2 (0.023 g, 0.064 mmol) and 2 equivalents of **L** (0.050 g, 0.128 mmol) in acetonitrile. A short size-exclusion column (Sephadex **R** LH-20) was run, eluting with methanol, and a single dark band was collected and dried under vacuum. The complex was further purified by crystallization from a concentrated solution of methanol by slow diffusion of diethyl ether.  HR-ESI-MS (M+) m/z calc. for [FeL2(OTf)]+, 985.1441, Found, 985.1454.

### [FeL2](Br)2

This complex was made in analogous fashion to [FeL2](OTf)2 as detailed above using FeBr2 (0.014 g, 0.064 mmol). Yield = 0.058 g (91%). 1H NMR (DMSO-d6, 500 MHz): δ 9.29 (d, J = 8.8 Hz, 2 H), 9.03 (d, J = 8.85 Hz, 2H), 8.95 (d, J = 8.1 Hz, 2H), 8.54 (d, J = 8.05 Hz, 2H), 8.46 (br t, 2H), 8.32 (t, J = 7.6 Hz, 2H), 7.79 (t, J = 7.8 Hz, 2H), 7.26 (d, J = 5.55 Hz, 2H), 7.13 (t, J = 7.45 Hz, 2H), 7.08 (t, J = 7.35 Hz, 2H), 7.02 (t, J = 6.25 Hz, 2H), 6.85 (t, J = 7.55 Hz, 4H), 6.68 (t, J = 7.45 Hz, 4H), 6.27 (br t, 4H), 6.03 (br t, 4H). 13C{1H} NMR (DMSO-d6, 126 MHz): δ 160.19 (s), 156.43 (t), 156.26 (s), 151.31 (s), 140.07 (s), 138.35 (s), 137.65 (s), 133.98 (s), 130.81 (s), 130.25 (t), 129.72–129.59 (m), 129.33 (s), 129.15 (t), 128.53 (t), 128.37 (t), 128.14 (s), 127.95 (s), 127.79 (s), 127.67– 127.59 (m), 126.77 (s), 126.61 (s), 126.46 (s), 125.49 (s), 121.16 (s). 31P{1H} NMR (DMSO-d6, 167 MHz) δ 59.07. HR-ESI-MS (M+) m/z calc. for [FeL2Br]+, 915.1104, Found, 915.1077.

### CoL2+ 2 , [CoL2](OTf)2]

The complex was prepared from Co(CH3CN)2(OTf)2 (0.028 g, 0.064 mmol) and 2 equivalents of **L** in acetonitrile. A short size-exclusion column (Sephadex R LH-20) was run, eluting with methanol, and a single dark band was collected and dried under vacuum. Yield = 0.067 g (92%). HR-ESI-MS (M+) m/z calc. for [CoL2(OTf)]+, 988.1424, Found, 988.1389. The complex was fully characterized as a Co(III) complex, prepared by stirring a methanol solution of the Co(II) species overnight under air. Crystals of the Co(III) complex were grown from methanol by slow diffusion of diethyl ether. Elem. Anal. calc. for C52H38CoN4P2(CF3SO3)3·(H2O): C, 50.62; H, 3.09; N, 4.29. Found: C, 50.19; H, 3.05; N, 4.12. HR-ESI-MS (M+) m/z calc. for [CoL2(OTf)2] +, 1137.0944. Found, 1137.0947.

### NiL2+ 2 , [NiL2](ClO4)2

The complex was synthesized by reacting Ni(ClO4)2·6H2O (0.023 g, 0.064 mmol) and **L** (0.050 g, 0.128 mmol) in methanol. The complex was purified by recrystallization from hot methanol. Yield = 0.057 g (86%). Elem. Anal. calc. for C52H38Cl2N4NiO8P2·(H2O)2: C, 58.13; H, 3.94; N, 5.21. Found: C, 58.33; H, 3.86; N, 5.20. HR-ESI-MS (M+) m/z calc. for [NiL2(ClO4)]+, 937.1410. Found, 937.1398.

### CuL2+ 2 , [CuL2](ClO4)2

The complex was prepared from Cu(ClO4)2·6H2O (0.024 g, 0.064 mmol) and two equivalents of **L** in acetonitrile. The complex was found to be unstable in solutions and could not be fully purified. Yield = 0.061 g (92%). ESI-MS (M+) m/z calc. for [CuL2(ClO4)]+, 942.14. Found, 942.13.

### ZnL2+ 2 , [ZnL2](OTf)2

Treatment of Zn(OTf)2 (0.023 g, 0.064 mmol) with 2 equivalents of **L** in methanol gave the product as a light green powder. Yield = 0.065 g (89%). 1H NMR (CD3CN, 500 MHz): δ 9.13 (d, J = 8.75 Hz, 2H), 8.80 (d, J = 8.75 Hz, 2H), 8.56 (t, J = 8.55 Hz, 4H), 8.01–7.94 (m, 6H), 7.44 (br d, 2H), 7.22 (t, J = 7.35 Hz, 2H), 7.13 (t, J = 7.15 Hz, 2H), 6.94–6.89 (m, 10H), 6.70 (br m, 4H), 6.38 (br d, 4H). 13C{1H} NMR (acetone-d6, 126 MHz): δ 152.88 (s), 148.71 (s), 148.56 (s), 148.07 (t), 144.93 (s), 142.76 (s), 142.35 (s), 141.95 (s), 134.96 (t), 134.49 (s), 132.37 (s), 132.18 (t), 131.02 (t), 130.67 (d), 130.11 (t), 129.92 (t), 128.92 (s), 128.30 (t), 127.41 (t), 125.63 (s), 122.07 (s). 31P{1H} NMR (CDCl3, 167 MHz) δ −34.69. HR-ESI-MS (M+) m/z calc. for [ZnL2(OTf)]+, 993.1383, Found, 993.1382.

# RESULTS AND DISCUSSION

# Synthesis of the Ligand and the Metal Complexes

Ligand precursor 2,8-dibromoquinoline **(1)** was prepared via a three-step synthetic route starting with 2-bromoaniline and cinnamoyl chloride (Wickramasinghe et al., 2015). A CombiFlash R Rf+ system was utilized to obtain high purity product from the crude reaction mixture. As shown in **Figure 2**, Stille coupling of 2-(tributylstannyl)pyridine and **1** afforded 8 bromo-2-(pyridin-2′ -yl)quinoline **(2)** (Wickramasinghe et al.,

2015). The final ligand, 8-(diphenylphosphaneyl)-2-(pyridin-2′ yl)quinoline **(L)**, was prepared from **2** and diphenylphosphine using a modified cross-coupling reaction (Murata and Buchwald, 2004; Zhang et al., 2017). A deactivated silica gel column allowed purification of the product, in which 2-(pyridin-2′ -yl)quinoline was found to be the only major side-product.

The metal complexes were synthesized by refluxing the appropriate metal(II) salts with 2 equivalents of the ligand **L** in acetonitrile or methanol. Metal complexes in powder form were obtained after evaporation of the solvent and washing with diethyl ether. The Fe and Co complexes were initially purified by a Sephadex R column and obtained in high yields of ∼90%. Further purification of all complexes was done by recrystallization from concentrated methanol or acetonitrile solutions, generally at −20◦C or by slow diffusion of diethyl ether into the solution. The complexes are stable in the solid state to ambient light, air, and moisture.

In all cases, formation of the bis-chelated complexes was favored in the reaction conditions employed here. For reactions involving Co(II), Ni(II), Cu(II), and Zn(II), treatment of the appropriate metal precursor with 1 equivalent of **L** in polar solvents initially led to the formation of a mixture of mono- and bis-chelated metal complexes. Stirring the reaction mixture for prolonged periods led to clean formation of the bis-chelated complexes. In the case of Fe(II), the reaction of **L** and Fe(OTf)2 in a 1:1 ratio immediately formed bischelated complex as the only product. A similar result was observed when FeBr2 was used instead of Fe(OTf)2. This observation with iron complexes is analogous to the [(Ph-PNN)2Fe]2+ complexes reported by Zell et al. (2013). In their studies, DFT calculations indicated that formation of the bischelated complexes is thermodynamically favored relative to the mono-chelated compounds in all conditions. They could tune this coordination behavior by altering the substituents of the phosphine donor where bulky tert-butyl groups allowed formation of the mono-chelated complex only and isopropyl substituents gave access to both the mono- and bis-chelated complexes depending on the initial ratio of metal precursor and ligand during metalation. Basu et al. (2018) followed the same principle to avoid bis-chelation in their report.

Synthesis of the mono-chelated Co(II) complex was attempted by treating a tetrahydrofuran solution of CoCl2 with 1 equivalent of **L**. A green precipitate appeared immediately which was identified by mass spectroscopy to be mono-chelated **L**-CoCl2 complex. However, due to limited solubility in traditional noncoordinating solvents, it could not be further characterized. Dissolving the complex in polar solvents such as acetonitrile, methanol, or DMF led to a gradual color change to a reddishbrown solution, indicating formation of the bis-chelated **CoL2+ 2** complex. This behavior is similar to the observation made by Harris et al. (1969) for Co(tpy) complexes.

Stirring 2:1 mixtures of the ligand and the appropriate metal precursors in methanol or acetonitrile for ∼12 h led to quantitative formation of the bis-chelated complexes, ML2+ 2 . Elevated temperatures significantly reduced the reaction time. Each of the ML2+ 2 complexes are stable in the solid state, and apart from the Cu(II) complex, they are also stable in solution. **CuL2+ 2** slowly decomposes in solution and could not be purified or fully characterized. We note that the cobalt(II) complex is easily oxidized to the cobalt(III) complex in solution when exposed to air.

### Characterization of Metal Complexes

Elemental analyses and mass spectrometry of each ML2+ 2 complex show the 2:1 ratio of ligand to metal. The Co(II), Ni(II), and Cu(II) complexes are paramagnetic. As expected of a d10 metal center, the Zn(II) complex is diamagnetic. The diamagnetic nature of the d6 Fe(II) complex indicates a low-spin octahedral electronic state. Two distinct sets of 1H signals were observed for the phenyl groups in these complexes, which suggests that no plane of symmetry passes through the metal center. Notably, only one peak is observed in the 31P NMR spectra of **FeL2**+ **2** and **ZnL2**+ **2** signifying that the phosphorus atoms are chemically equivalent, consistent with κ 3 coordination modes for both NNP pincer ligands and octahedral complexes.

Indeed, a crystal structure of the octahedral nickel complex was obtained as shown in **Figure 3**. Single crystals of **NiL2**+ **2** were grown from a concentrated solution of hot methanol that was allowed to cool to room temperature. The nickel-nitrogen bond distances range from 2.054(4) to 2.093(4) Å, with pyridine donors trans to the phosphine donor having an average Ni-N bond distance of 2.090 Å while those that are trans to another pyridine have an average Ni-N bond distance of 2.058 Å. Significantly longer bond distances are observed for the phosphine donors at 2.4323(13) for Ni-P(1) and 2.4183(13) for Ni-P(2) in the solid state.

For the three reported paramagnetic complexes, solution magnetic susceptibilities (µeff) were determined by the Evans method (Evans, 1959). At room temperature, the values are 2.6, 2.8, and 2.0 for **CoL2**+ **2** , **NiL2**+ **2** , and **CuL2**+ **2** , respectively. The experimental values are close to previously reported values of similar octahedral complexes and characteristic of 1, 2, and 1 unpaired electron(s) for the Co(II), Ni(II), and Cu(II) complexes,

respectively. The relatively high µeff value for cobalt (2.6) is typical of cobalt polypyridyl complexes, and indicates significant orbital contribution (Chen et al., 2018).

### Electrochemistry

Cyclic voltammetry was performed on the series of metal complexes to assess and compare the redox potentials in acetonitrile/0.1 M Bu4NPF6 solutions under inert atmosphere. The cyclic voltammograms are shown in **Figure 4** and E1/2 values (peak potentials for the irreversible redox features) are summarized in **Table S2**. All potentials are reported against the ferrocenium/ferrocene couple (V vs. Fc+/0 ).

The cyclic voltammogram (CV) of **ZnL2+ 2** shows four redox couples at −1.36, −1.54, −2.16, and −2.35 V. Since this complex has a redox-inactive metal center, these processes are assigned to ligand-based redox events. The zinc complex is helpful in identifying metal- and ligand-based redox processes in the remaining complexes. Notably, **ZnL2+ 2** is unstable under repeated scans. The redox couples at −2.16 and −2.35 V become irreversible and a visible change in peak intensities is observed presumably due to passivation of the working electrode.

The reduction waves at Ep,c = −2.22 and −2.36 V of **CuL**2+ 2 can be safely assigned to ligand-based reductions. A sharp irreversible oxidation peak at −0.76 V is also observed, indicative of adsorption on the electrode surface. The redox features of the copper complex also lose their reversibility and peak shape under repeated scans.

Reversible metal-based MIII/II redox couples occur at E1/2 = 0.80, −0.46, and 0.66 V for **FeL2+ 2** , **CoL2+ 2** , and **NiL2+ 2** , respectively. In addition, three reversible redox events are

observed at negative potentials with the iron and cobalt complexes in the potential range studied here. The nickel complex shows slightly different behavior with two reversible reductions at −1.28 and −1.53 V, and a quasireversible reduction at −2.25 V. The quasireversible reduction has two small return oxidations at −2.15 and −1.83 V. The most negative features of these complexes are assigned to ligand-based reductions by comparison to the zinc analog.

# Electrochemical Reduction of CO2

Next, the complexes were analyzed by cyclic voltammetry to probe their reactivity toward CO2 reduction. We focused on **FeL2**+ **2** , **CoL2**+ **2** , and **NiL2**+ **2** as the copper and zinc complexes were not stable under electrochemical reduction. DMF solutions containing 0.1 M Bu4NPF6 were employed for electrocatalytic studies for two reasons. First, using acetonitrile resulted in a significant amount of precipitation during controlled potential electrolysis (CPE) experiments. This is consistent with previously reported observations with 3d transition metal terpyridine complexes and ruthenium polypyridyl complexes. It was concluded that the build-up of HCO− 3 /CO2− 3 during bulk electrolyses resulted in precipitation of the complexes as HCO− 3 or CO2− 3 salts (Chen et al., 2011; Elgrishi et al., 2014). We confirmed the formation of carbonate in CPE experiments with **CoL2**+ **2** by barium triflate titration (Yang et al., 2018), consistent with the reductive disproportionation of CO2 to CO and CO2− 3 . The use of DMF solved this solubility issue as no precipitate was observed during electrolyses over extended periods of time. Second, using DMF gave us the opportunity to directly compare the performance of these complexes with the previously reported terpyridine systems (Elgrishi et al., 2014, 2015).

Under N2 atmosphere, CVs of **FeL2**+ **2** , **CoL2**+ **2** , **NiL2**+ **2** , and **ZnL2**+ **2** in DMF (shown in **Figure S1**) exhibit similar behavior to

*Glassy carbon working electrode, platinum wire counter electrode, and silver wire quasireference electrode.*

that in acetonitrile, but with significantly lower stability for the nickel and zinc compounds. Redox potentials of the complexes in DMF are summarized in **Table 1**. Comparing the CV of the cobalt complex to its zinc counterpart, we assign the redox couples (E1/2) at −1.06 and −1.74 V to a metal-based CoII/I process and to a ligand-based process, respectively. The open circuit potential of the **CoL2**+ **2** was measured at −0.61 V, which is consistent with these assignments. A reversible ligand-based wave was also observed at E1/2 = −2.07 V. Adding free ligand to the solution increases the intensity of this peak (**Figure S2**), which further supports the assignment of this feature as a ligandbased reduction. Scanning past this ligand-based reduction gives rise to an irreversible ligand-based reduction at −2.65 V, which was found to be unstable under repeated scans (**Figure S3**).

For **FeL2**+ **2** , reversible waves with E1/2 values at −1.40, −1.64, and −2.27 V are attributed to ligand-based reductions by comparison with **ZnL2**+ **2** . Different behavior was observed with **NiL2**+ **2** relative to the Fe and Co complexes with one reversible redox feature at E1/2 = −1.27 V, followed by one irreversible reduction at Ep,c = −1.51 V and two quasireversible reductions at Ep,c = −1.83 and −2.25 V. The open circuit potential was determined to be −0.68 V. Thus, the reversible redox process at −1.27 V is expected to be a metal-centered NiII/I couple, while the remaining waves are consistent with ligand-based redox events. Irreversible reductions are presumably due to dissociation of one of the tridentate ligands from the complex. This redox behavior is different from the related Co(tpy)2+ 2 and Ni(tpy)2+ 2 systems (Hamacher et al., 2009; Elgrishi et al., 2014). CVs of **FeL2**+ **2** , **CoL2**+ **2** , and **NiL2**+ **2** at scan rates ranging from 10 to 1,000 mVs−1 show that the peak currents change linearly with the square root of the scan rate (**Figures S5**–**S7**), consistent with diffusion-controlled homogeneous species. The diffusion coefficients were calculated from the slopes of the linear fits of the first reduction peaks using the Randles-Sevcik equation (Bard and Faulkner, 2001) and are presented in **Table 1**.

Water was added as a proton source to investigate the proton-coupled reduction of CO2 with these complexes. The concentration of added water was kept fixed at 5% in these studies. No significant current enhancement was observed in the CVs when water was introduced to the systems under N2 (**Figure 5**). Likewise, there was little change to the CV of **CoL2+ 2** with added H2O. The third (most negative) reduction peak of **FeL2**+ **2** and **NiL2**+ **2** becomes irreversible in the presence of H2O.

FIGURE 5 | Cyclic voltammograms of FeL2+ 2 , CoL2+ 2 , and NiL2+ 2 at 0.5 mM concentrations in DMF/0.1 M Bu4NPF6 under N2 (black), CO2 (red), N2 with 5% H2O (green), and CO2 with 5% H2O (blue). Scan rate = 100 mV/s; glassy carbon disk working electrode.

In CO2-saturated anhydrous solutions, the third reduction of all three complexes loses reversibility and shows current enhancement, suggesting catalytic reactivity toward CO2 (**Figure 5**). A 2.4-fold current increase was observed at the third reduction peak of **FeL2**+ **2** with the appearance of a new peak at Ep,c = −2.23 V. The new peak showed current enhancement of 1.9-fold. In **CoL2**+ **2** , the most negative reduction peak was catalytically enhanced with an icat/ip value of 2.9 (where icat is the limiting catalytic current obtained under CO2 and ip is the reductive peak current of the catalyst in the absence of substrate). CVs of **NiL2**+ **2** showed current enhancement of 3.5- and 1.7-fold in the third and fourth reduction waves, respectively.

Additional current enhancement at the terminal reduction peaks of the **FeL2**+ **2** and **CoL2**+ **2** systems was observed when 5% water was added to CO2-saturated solutions, indicating that the complexes are capable of catalyzing the proton–coupled reduction of CO2 (**Figure S10**). The CV of **NiL2**+ **2** under the same conditions showed a decrease in the initial catalytic waves, but an additional and larger catalytic wave appeared at Ep,c = −2.40 V (**Figure 5**).

To better understand the reactivity of the complexes and to quantify the products, a series of controlled potential electrolyses (CPEs) were performed at different applied potentials with and without added H2O. Gaseous products were analyzed by periodic sampling of the headspace of the airtight electrochemical cell. **Table 2** summarizes the product distribution and Faradaic efficiencies (FE) of each complex over the first 2 h. Representative charge vs. time plots are provided in **Figures S11**–**S13**, **S16**. Carbon monoxide and hydrogen were the major products detected in all cases. A trace amount of methane was also found in the electrolyses of **CoL2**+ **2** and **FeL2**+ **2** but corresponds to <1% of the total charge passed.

The maximum overall FE was found to be 58% for **CoL2**+ **2** at an applied potential of −2.11 V in the presence of 5% water (**Figure 6**). A maximum FE of 23% for evolved CO was recorded at Eappl = −2.06 V. The production of H2 varied with the applied potential from 7 to 42%. When methanol—a more acidic proton source—was used, the overall FE decreased, but the ratio for H2 production increased (**Table 2**). No evolved H2 was observed when electrolyses were performed in anhydrous solutions. However, Faradaic efficiencies for CO were lower. Rinse tests were performed on the glassy carbon rod after CPEs to investigate possible adsorption or deposition of a heterogeneous catalyst on the electrode surface. There was no visible deposition and the glassy carbon rod did not exhibit any catalytic activity in fresh CO2-saturated DMF/0.1 M Bu4NPF6 solution, which suggests catalysis is governed by a molecular cobalt species.

The turnover frequency (TOF) of **CoL2**+ **2** was calculated as previously described (Narayanan et al., 2016; Yang et al., 2018) from scan rate-dependent linear sweep voltammograms performed in N2- and CO2-saturated solutions containing 5% H2O. A plot of the icat/ip current ratio at different scan rates as a function of potential is shown in **Figure S8**. Although a scan rateindependent TOF was not completely reached at scan rates up to 1 V/s, an estimated TOF of 12.4 s−1 was determined (**Figure S9**).

CPEs of the **NiL2**+ **2** complex show that the complex is selective for CO2 reduction over proton reduction, albeit with very low FEs. The maximum FE observed with the nickel complex was

*For the CPE with ZnL2+ 2 , a 2 mM concentration of the complex was used. All values are the average of at least two runs.*

8%. The total charge passed was also very low compared to its iron and cobalt counterparts. The **FeL2**+ **2** complex primarily catalyzed proton reduction in the presence of H2O. Indeed, the FE for CO production was ∼1% in anhydrous DMF solutions or with 5% added H2O. Under anhydrous conditions, proton reduction was suppressed, but CO production remained low. Rinse tests were also performed following electrolyses of both the nickel and iron complexes. For **FeL2**+ **2** , the electrode did not show any sign of deposition. On the other hand, visible signs of deposition on the glassy carbon rod were observed with **NiL2**+ **2** and the electrode showed catalytic activity in fresh solutions that signals decomposition of the complex occurs to form an active heterogeneous material.

### DISCUSSION

In general, to be an effective catalyst for CO2 or H+ reduction, a metal complex should be able to activate the substrate molecule via an open coordination site at the metal center. Thus, molecular catalysts are typically designed with at least one open or labile coordination site (Chiericato et al., 2000; Benson et al., 2009; Thoi et al., 2013; Su et al., 2018). In contrast, the complexes investigated here form an active catalyst in situ by ligand loss or dissociation of a donor moiety during electrolysis. Given the rigidity of the ligand, we hypothesize that one of the ligands completely dissociates. Thus, the bis-chelated **ML2**+ **2** complex acts as a precatalyst, which under reducing conditions loses a ligand to generate a mono-chelated, catalytically active species (bearing a single pincer ligand) with available coordination sites for substrate activation and conversion. Our attempts to isolate the mono-chelated active species were unsuccessful. However, controlled potential electrolysis of **CoL2**+ **2** was performed under inert atmosphere at an applied voltage of −2.65 V, and the electrolysis solution was analyzed by mass spectrometry, which showed evidence of free ligand that was not present in preelectrolysis solutions (**Figure S4**). In addition, CVs taken before and after electrolysis of **CoL2**+ **2** show the formation of new redox features, indicating that a new electroactive species is formed under reducing conditions (**Figure S14**). Together, these experiments indicate that a mono-chelated complex is formed when the pre-catalyst is subjected to electrochemical reduction, and it is this species that is responsible for the catalytic activity observed in the presence of substrates (CO2 and H+).

Given the structural similarities, **CoL2**+ **2** likely operates by the mechanism proposed by Elgrishi et al. (2014) and Elgrishi et al. (2015) for Co(tpy)2+ 2 complexes. The low Faradaic efficiencies are also consistent with their findings, which showed that the reduced mono-chelated active species can dimerize to give a deactivated species that does not reduce CO2. Moreover, the redox-active ligand liberated during creation of the active catalyst can be reduced further, contributing to the low overall Faradaic efficiencies (**Table 2**). Proposed mechanisms for CO2 reduction as well as the competing hydrogen evolution reaction are shown in **Figure S18** for the cobalt(II) complex.

The **FeL2**+ **2** and **NiL2**+ **2** complexes behave differently than the Fe and Ni bis(terpyridine) complexes reported by Elgrishi et al. (2014). In their studies, Fe(tpy)2+ 2 did not show any current enhancement in CO2-saturated solutions containing a proton source. Conversely, the Ni(tpy)2+ 2 complex showed considerable catalytic activity under comparable conditions to those employed here and selectively produced CO in bulk electrolyses. The redox potentials of the bis(tpy) complexes and the present systems are also significantly different under N2 atmosphere. It was concluded from CVs of Fe(tpy)2+ 2 , Ni(tpy)2+ 2 , and Zn(tpy)2+ 2 that they undergo ligand-based reductions exclusively in the potential window studied. The current enhancement observed in CVs of the complexes studied here under anhydrous conditions may be a result of CO2 binding to the reduced active species to form a stable intermediate, which is not effective for catalysis and does not liberate a reduced carbon product. Related observations have been reported for molecular nickel complexes that have poor Faradaic efficiencies for CO2 reduction despite encouraging results from cyclic voltammetry (Narayanan et al., 2016; Lieske et al., 2018). The irreversible reductions and dissimilarity between the anodic and cathodic scans of **NiL2**+ **2** under N2

indicates formation of a new species. Consistent with the rinse test, ligand dissociation likely occurs from the nickel complex during electrolysis, which is followed by formation of a heterogeneous material on the electrode surface. Decomposition of the molecular species following reduction may account for the low Faradaic efficiencies.

**ZnL2**+ **2** was electrochemically and spectroscopically analyzed to better understand the underlying mechanism and stability of the catalysts. The zinc complex did not show any notable current increase in CO2-saturated solutions by cyclic voltammetry (**Figure S15**). It also performed poorly in CPEs with an overall FE of <1%. However, the diamagnetic nature of **ZnL2**+ **2** was used to obtain insight into the decomposition pathway of the reported complexes. We conducted an electrochemical experiment as described by Elgrishi et al. (2014) and Fuchs et al. (1981), in which **ZnL2**+ **2** was subjected to bulk electrolysis and the electrolyzed product was studied by 1H NMR. NMRs were taken of the solution before and after electrolysis, and after adding iodomethane as an alkylating agent (**Figure S17**). Free ligand peaks were visible in the 1H NMR spectrum of the electrolyzed solution after a 3-h CPE, consistent with ligand dissociation. Changes in the aromatic region of the NMR spectrum and appearance of new peaks in the aliphatic region after alkylation suggest possible dearomatization and carboxylation of the ligand (Fuchs et al., 1981; Elgrishi et al., 2014).

While our cobalt system suffers from low overall Faradaic efficiency and poor selectivity, the ligand structure is amenable to electronic and steric modifications. Elgrishi et al. (2015) were able to tune the selectivity of Co(tpy)2+ 2 complexes by introducing different substituents on the tpy ligand. They improved selectivity toward CO production by disfavoring the hydrogen evolution reaction (HER), but overall Faradaic efficiencies remained low. The best derivative featuring bulky tert-butyl substituents on the tpy ligand led to the highest overall FE at 41%, presumably by reducing catalyst deactivation via dimerization. However, these results indicate that ligand modifications are largely ineffective for improving catalyst stability and thus they were not pursued with the NNP ligand investigated here.

An early example of electrochemical CO2 reduction was reported by Arana et al. (1992) involving bis-chelated Fe, Co, and Ni complexes of redox-active NNN-type ligands. These complexes showed CO2 reduction activity to various extents based on their current response in cyclic voltammetry studies. Among them, Co(dapa)2+ 2 and Ni(dapa)2+ 2 complexes (dapa = 2,6-bis-[1-(phenylimino)ethyl]pyridine) showed the best activity in CVs. Under inert atmosphere, both complexes showed one metal-based and two ligand-based reductions, similar to the complexes reported here. Catalytic current was observed at the second reduction for these systems, whereas catalysis begins in earnest at the third reduction with our system. However, the stability and substrate selectivity of the catalysts were not reported. Only a limited product analysis was done for the cobalt system, which had a ∼60% FE for formic acid production. This clearly suggests that this system follows a different catalytic mechanism than ours and the Fontecave systems where CO was the sole carbon-containing product.

As discussed previously, introducing steric bulk in pincer ligands can enforce mono-chelation, thereby avoiding the ligand dissociation step needed to access available coordination sites for substrate binding to the metal core. In addition, they can obstruct dimerization of reduced intermediates. Recently, electrocatalytic CO2 reduction with cobalt and nickel complexes featuring modified dapa ligands were reported by Kang and Rochford, respectively. Mono-chelation was achieved by introducing diisopropyl groups in the ligand framework (Narayanan et al., 2016; Liu et al., 2018). The Co complex showed minimal reactivity toward CO2 and was not studied further, whereas the Ni complex showed a good current response in CVs under CO2. However, during bulk electrolysis, the Ni complex produced only a small amount of CO with nearly quantitative formation of H2. Liu et al. (2018) hypothesized that catalysis could be improved by replacing one imino group with a pyridyl moiety. Their new NNN ligand has a redox-active bipyridine unit with extended π-conjugation and a sterically bulky imino group on one side. With this ligand, the corresponding cobalt complex shows metaland ligand-based redox events under argon. Adding water to the system gave rise to new waves corresponding to a Co-hydride species, which was found to mediate formate production in CO2 saturated solution. Consistent with DFT calculations, the reduced species in the presence of substrate can access two different pathways to generate both CO and formate, where formate is produced via a CoII-H intermediate. Despite similarities between this ligand and the NNP ligand reported here, our **CoL2**+ **2** complex has different product selectivity and CO was found to be the only 2e− reduced C1 product. This is likely due to a difference in hydricity of the CoII-H intermediate formed in our case, in which CO2 insertion into the metal-hydride to make a formato intermediate is not competitive with protonation to form H2 (Thoi et al., 2013; Kang et al., 2015).

Examples of phosphine-based pincer-ligated 3d metal complexes are still scarce among CO2 reduction electrocatalysts. Recently, a manganese tri-carbonyl complex with a symmetric PNP-pincer ligand was reported by the Richeson group. This complex shows an unusual coordination environment with a mer-M(CO)3 motif rather than the usual fac-M(CO)3 configuration (Rao et al., 2016). Moreover, this complex also triggers CO2 reduction at its first reduction. Addition of H2O as a weak proton source increased the current and lowered the catalytic overpotential, but also reduced substrate selectivity as HER activity was enhanced.

Pincer ligands with N-heterocylic carbene (NHC) donors have also been employed in CO2 reduction. A nickel-based catalyst for CO2 reduction supported by a mixed donor CNCtype ligand was reported by Sheng et al. (2015). This distorted square planner complex has a labile acetonitrile ligand at the fourth position and shows three irreversible reductions under N2. Among them, the first reduction at −1.19 V (vs Fc+/0 ) was assigned to the NiII/I couple. Catalysis occurs at the most negative reduction in CO2-saturated CH3CN. Added water was found to enhance catalysis without compromising product selectivity. Myren et al. (2019) recently reported two manganese tricarbonyl complexes with similar CNC pincer ligands which can reduce CO2 to CO at the first reduction potential. Though the overpotential is large, these catalysts also exhibit high selectivity in the presence of a proton source.

Cope et al. (2017) replaced the pyridyl group of the Sun catalyst with an aryl donor to prepare a Ni complex ligated by an anionic CCC-pincer ligand. This more electron-rich ligand on nickel results in faster catalysis (30x TOF), at the expense of a higher overpotential, and produces a mixture of CO and formate. Electrocatalysis occurs at the first reduction with this catalyst and added water enhances CO2 reduction and the rate of catalysis. Bulk electrolysis experiments confirmed that the catalyst is selective, producing CO and formate with an optimal FE of 81% for reduced carbon products. However, catalysis slows down after ∼1 h of electrolysis for unspecified reasons.

## CONCLUSIONS

A rigid NNP-type pincer ligand and its mid-to-late firstrow transition metal complexes have been synthesized. The ML2+ 2 complexes reported here are the first examples of an asymmetric phosphine-substituted pincer ligand employed for electrocatalytic CO2 reduction. Despite the steric bulk of the phenyl substituents on this ligand, the metal ions prefer to coordinate with two ligands to form pseudo-octahedral bischelated complexes. Electrochemical studies confirm that the cobalt complex is capable of catalyzing the electrochemical reduction of CO2 to CO. However, catalytic activity is greatly limited by deleterious side reactions. We reason that the rigid NNP framework inhibits ligand dissociation from the reduced

### REFERENCES

complex and, thus, impedes formation of the catalytically active species.

# AUTHOR CONTRIBUTIONS

KT synthesized and characterized the catalysts, collected and analyzed data, and wrote the original draft. AI synthesized and characterized the catalysts and collected data. JJ conceptualized the experiments, analyzed data, supervised research activities, acquired funding, and revised the manuscript.

### FUNDING

We thank the National Science Foundation for financial support through seed grant funding (OIA-1539035) and a CAREER Award (CHE-1848478). We are also grateful for start-up support from the University of Mississippi (UM).

### ACKNOWLEDGMENTS

We thank Prof. Jared Delcamp (UM) for allowing us to use his gas chromatograph and automated flash chromatography system. We also thank Senthil Kumar Eswaramoorthy and Prof. Amala Dass (UM) for their support with high-resolution mass spectrometry.

### SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem. 2019.00330/full#supplementary-material

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2019 Talukdar, Issa and Jurss. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

**48**

# Improved Conditions for the Visible-Light Driven Hydrocarboxylation by Rh(I) and Photoredox Dual Catalysts Based on the Mechanistic Analyses

Kei Murata† , Nobutsugu Numasawa, Katsuya Shimomaki, Jun Takaya and Nobuharu Iwasawa\*

*Department of Chemistry, Tokyo Institute of Technology, Tokyo, Japan*

### Edited by:

*Soumyajit Roy, Indian Institute of Science Education and Research Kolkata, India*

### Reviewed by:

*Munetaka Akita, Tokyo Institute of Technology, Japan Yang Yang, University of Central Florida, United States*

### \*Correspondence:

*Nobuharu Iwasawa<EMAIL_ADDRESS>
### †Present Address:

*Kei Murata, Institute of Industrial Science, The University of Tokyo, Tokyo, Japan*

### Specialty section:

*This article was submitted to Inorganic Chemistry, a section of the journal Frontiers in Chemistry*

Received: *07 March 2019* Accepted: *06 May 2019* Published: *22 May 2019*

### Citation:

*Murata K, Numasawa N, Shimomaki K, Takaya J and Iwasawa N (2019) Improved Conditions for the Visible-Light Driven Hydrocarboxylation by Rh(I) and Photoredox Dual Catalysts Based on the Mechanistic Analyses. Front. Chem. 7:371. doi: 10.3389/fchem.2019.00371* The improved catalytic conditions and detailed reaction mechanism of the visible-light driven hydrocarboxylation of alkenes with CO2 by the Rh(I) and photoredox dual catalysts were investigated. The use of the benzimidazoline derivative, BI(OH)H, as a sacrificial electron donor was found to increase the yield of the hydrocarboxylated product by accelerating the reduction process. In addition, the incorporation of the cyclometalated Ir(III) complex as a second photosensitizer with [Ru(bpy)3] 2+ photosensitizer also resulted in the promotion of the reduction process, supporting that the catalytic cycle includes two photochemical elementary processes: photoinduced electron and energy transfers.

Keywords: carboxylation, CO2 fixation, photoredox catalyst, rhodium catalyst, visible light

# INTRODUCTION

Catalytic hydrocarboxylation of unsaturated hydrocarbons with CO2 is one of the promising methods for the CO2 fixation (Luan and Ye, 2018; Yan et al., 2018; for recent reviews, see: Tortajada et al., 2018). The most common strategy to accomplish the hydrocarboxylation is to utilize a metal hydride complex as an active species. However, in these reactions, more than a stoichiometric amount of highly active, metallic reductants such as ZnEt2, AlEt3, or hydrosilanes are usually required to promote the reduction process in the catalytic cycle (Takaya and Iwasawa, 2008; Williams et al., 2008; Fujihara et al., 2011; Li et al., 2011; Hayashi et al., 2015; Wang et al., 2015; Zhu et al., 2015; Kawashima et al., 2016). In order to realize a more efficient and environmentallyfriendly system, the reaction which necessitates just a catalytic amount of metallic reagents is highly desirable. Meanwhile, the photochemical reduction process has been widely employed in the field of artificial photosynthesis, such as photocatalytic hydrogen generations (for review, see Esswein and Nocera, 2007) and CO2 reductions (for reviews, see Morris et al., 2009; Doherty et al., 2010; Takeda and Ishitani, 2010) in homogeneous systems. In these reactions, transition-metal catalysts are combined with redox photosensitizers and sacrificial electron donors to drive the multielectron transfer processes under visible-light irradiation. When the electron transfer is accompanied by the

**Abbreviations:** bpy, 2,2′ -bipyridyl; dtbbpy, 4,4′ -di-tert-butyl-2,2′ -bipyridyl; bpz, 2,2′ -bipyrazine; ppy, 2-phenylpyridiine; dF(CF3)ppy, 3,5-difluoro-2-[5-(trifluoromethyl)-2-pyridinyl]phenyl; SCE, saturated calomel electrode; BIH, 1,3 dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole; BI(OH)H, 1,3-dimethyl-2-(o-hydroxyphenyl)-2,3-dihydro-1Hbenzo[d]imidazole; SED, sacrificial electron donor; PC, photoredox catalyst; SCE, saturated calomel electrode; vt, virtual coupling. For the abbreviations of the rhodium complexes, see **Supplementary Table 1**.

proton transfer, metal hydrides can act as an active species in the catalytic cycle (for reviews, see Stoll et al., 2015; Adams et al., 2018). Although transition-metal/photoredox dual catalysis has been actively studied in the field of organic synthesis (for recent reviews, see Fabry and Rueping, 2016; Skubi et al., 2016; Twilton et al., 2017), few examples have been reported for catalytic organic transformations driven by photochemically-generated metal hydrides (Ghosh et al., 2015).

We recently developed the visible-light driven hydrocarboxylation of alkenes with CO2 for the first time by means of the photochemical generation of Rh(I) hydride species (Murata et al., 2017). 4-Cyanostyrene was transformed to the branched hydrocarboxylated product by using a Rh(I) hydride or chloride complex as a carboxylation catalyst, [Ru(bpy)3] 2+ as a photoredox catalyst, iPr2NEt as a sacrificial electron donor, with visible-light irradiation under CO2 atmosphere at room temperature (**Figure 1**). The photoredox catalysis made it possible to take electrons from tertiary amines and drive the reduction process without using a metallic reductant. Since then, several photoredox-catalyzed hydrocarboxylation reactions of unsaturated hydrocarbons with CO2 have been reported by other groups (Seo et al., 2017b; Hou et al., 2018b; Meng et al., 2018). Concomitantly, difunctionalizations of alkenes such as thiocarboxylation (Ye et al., 2017), carbocarboxylation and silylcarboxylation (Yatham et al., 2017; Hou et al., 2018a) have also been developed by incorporating an appropriate radical precursor with CO2. Furthermore, in addition to unsaturated hydrocarbons, various substrates such as aryl and alkyl halides (Meng et al., 2017; Shimomaki et al., 2017), amines (Seo et al., 2017a), imines and enamides (Fan et al., 2018; Ju et al., 2018) have been carboxylated with CO2 by photoredox catalysis so far (for review, see Yeung, 2019). These examples demonstrated wide applicability of the photoinduced electron transfer to carboxylation reactions.

On the basis of our previous experiments, the reaction mechanism of the hydrocarboxylation by Rh(I) and photoredox dual catalysts was proposed as shown in **Figure 2**. Initially, the hydrometallation of a styrene derivative by Rh(I) hydride species **A** gave the Rh(I) benzyl species **B** (i), and the visible-light promoted nucleophilic addition to CO2 afforded the Rh(I) carboxylate species **C** (ii). Then, the reductive quenching cycle of [Ru(bpy)3] 2+ with iPr2NEt mediated 2-electron, 2 proton transfers afforded the Rh(III) dihydride carboxylate species **D** (iii), followed by the base-promoted liberation of the carboxylated product to regenerate the active species **A** (iv). Although this reaction demonstrated fundamental aspects of the application of photochemical reduction processes to catalytic carboxylation reactions, there still has been room for improvement from the viewpoint of the applicability in organic synthesis: (i) The efficiency of the reaction was not very high. Good yield was obtained with 4-cyanostyrene and moderate yields were obtained with several other substrates. (ii) A large excess amount of a tertiary amine and long reaction time (>24 h) were necessary for completion of the reaction even for the reactive substrates, (iii) A significant amount of the hydrogenated product was produced as a byproduct. In order to resolve these problems, further screenings of the catalytic conditions were desired.

Herein, we explored the improved conditions of the visible-light driven hydrocarboxylation, and the catalytic efficiency was analyzed based on the detailed mechanistic study with a series of stoichiometric reactions of the rhodium intermediates. Through the investigation, the hydrocarboxylation was successfully improved by the alteration of the sacrificial electron donor or the incorporation of the second photosensitizer. The mechanistic study suggested that the promotion of the photochemical reduction process was crucial for the enhancement of the catalytic reaction.

# RESULTS AND DISCUSSION

### Screening of Reaction Conditions

On the basis of our previous experiments in terms of the screening of catalytic conditions and the observation of the reaction intermediates under the catalytic conditions, the followings were demonstrated: (i) As a carboxylation catalyst, Rh(I) hydride or chloro complexes with triarylphosphines were applicable. In particular, the µ-chloro bridged Rh(I) dimer [Rh(P(4-CF3C6H4)3)2Cl]2 (**4**) was the most effective catalyst. (ii) When **4** was employed under the catalytic conditions, the resting state was the corresponding Rh(I) carboxylate complex, indicating that the rate-determining step was its transformation to the Rh(I) hydride species. This result suggested that the promotion of the reduction process was crucial for the improvement of the catalytic reaction. According to these considerations, the reaction conditions were screened in terms of the photoredox catalyst and sacrificial electron donor,

which would have taken an important part in the reductive quenching cycle.

### Photoredox Catalyst

Photoredox catalysts were initially screened by performing the reaction of 4-cyanostyrene (**1a**) in a mixture of 2.0 mol% of a photosensitizer, 3.5 mol% of **4** and 4.0 equiv. of iPr2NEt under a CO2 atmosphere at room temperature (**Table 1**). In the case of [Ru(bpy)3](PF6)2 (E II∗ /I 1/2 = +0.77 V, E II/I 1/2 = −1.33 V vs. SCE) (Kalyanasundaram, 1982) as a photoredox catalyst, the hydrocarboxylated (**2a**) and hydrogenated (**3a**) products were obtained in 54 and 25% yields, respectively, after visiblelight irradiation for 24 h (**Table 1**, entry 1). A small amount of polymerized product of **1a** was also produced as byproduct. Though no other photosensitizers overcame this activity, the yield of **2a** was found to be strongly dependent on the photoredox catalyst. For instance, when [Ru(bpz)3](PF6)2 (E II∗ /I 1/2 = +1.45 V, E II/I 1/2 = −0.80 V vs. SCE) (Crutchley and Lever, 1980) or fac-Ir(ppy)3 (E III∗ /II 1/2 = +0.31 V, E III/II 1/2 = −2.19 V vs. SCE) (Flamigni et al., 2007) was employed, the yields substantially decreased compared to [Ru(bpy)3](PF6)2 (**Table 1**, entries 3, 6). On the other hand, when using [Ir(dF(CF3)ppy)2(dtbbpy)](PF6) (E III∗ /II 1/2 = +1.21 V, E III/II 1/2 = −1.37 V vs. SCE) or [Ir(ppy)2(dtbbpy)](PF6) (E III∗ /II 1/2 = +0.66 V, E III/II 1/2 = −1.51 V vs. SCE) (Lowry et al., 2005), moderate yields were obtained (**Table 1**, entry 4, 5). These results indicated that both sufficient oxidizing ability of the excited state and reducing ability of the one-electron reduced species are at least necessary for the photosensitizer. However, the detailed dependency was not simple, as other factors such as absorption properties, excited-state energies and photochemical stability of the photosensitizer could also affect the catalytic performances. Meanwhile, the screenings of additives with [Ru(bpy)3] 2+ photosensitizer demonstrated that the addition of Cs2CO3 as an inorganic base significantly improved the yield of the hydrocarboxylated product by suppressing the formation of the hydrogenated byproduct: the yield of **2a** increased to 67% while the yield of **3a** decreased to 1% (**Table 1**, entry 11).

### Sacrificial Electron Donor and Additives

Sacrificial electron donors were then screened in the presence of an excess amount of Cs2CO3. In order to highlight the reactivity, a less reactive alkene, 3,5-bis(trifluoromethyl)styrene (**1b**), was used as a substrate. The reactions of **1b** were performed in a mixture of 2.0 mol% of [Ru(bpy)3](PF6)2, 3.5 mol% of **4**, 4.0 equiv. of sacrificial electron donor and 1.2 equiv. of Cs2CO3 under a CO2 atmosphere at room temperature (**Table 2**). When iPr2NEt was employed as a sacrificial electron donor, 32% yield of the hydrocarboxylated product (**2b**) and a trace amount of the hydrogenated product (**3b**) were obtained after visiblelight irradiation for 12 h (**Table 2**, entry 1). Although the use of TEOA (triethanolamine) slightly increased the yield of **2b**, the formation of **3b** became pronounced probably due to the increase of proton concentration (**Table 2**, entry 3). On the other hand, the use of BI(OH)H (1,3-dimethyl-2-(o-hydroxyphenyl)- 2,3-dihydro-1H-benzo[d]imidazole) successfully accelerated the hydrocarboxylation and increased the yield of **2b** considerably with maintaining the low yield of **3b** (**Table 2**, entry 5). Furthermore, the incorporation of BI(OH)H made it possible to reduce the amounts of the photoredox catalyst and the sacrificial electron donor: the use of only 1.0 mol% of [Ru(bpy)3](PF6)2 and 2.0 equiv. of BI(OH)H gave 70% yield of **2b** (**Table 2**, entry 7). When the reaction was performed under an Ar atmosphere in the presence of Cs2CO3, no hydrocarboxylated product **2b** was obtained. This result confirmed that the carbonate did not work as a source of CO2 in the present reaction (**Table 2**, entry 9). BI(OH)H has been known to work as a 2-electron, 2-proton

TABLE 1 | Optimization of the reaction conditions of the hydrocarboxylation of 4-cyanostyrene.

*aNMR yield.*

*c1 mol% of [Ru(bpy)3](PF6)2.*

*d5 mol% of [Ru(bpy)3](PF6)2.*

donor with high reducing ability in redox photosensitizing reactions (Hasegawa et al., 2005, 2006; Tamaki et al., 2015). Since a tertiary amine contributed to the reductive quenching cycle of [Ru(bpy)3] 2+, the increase in the yield of **2b** was attributed to the promotion of the reduction process of the Rh(I) carboxylate species, which was the rate-determining step in the hydrocarboxylation. These results indicated that the redox property of a sacrificial electron donor is one of the crucial factors for the efficient promotion of the reaction.

### Generality of the Hydrocarboxylation Under the Improved Conditions

Based on the improved conditions using **1b** as discussed above, the generality of the hydrocarboxylation was examined using various alkene substrates. 1.0 mol% of [Ru(bpy)3](PF6)2, 3.5 mol% of **4**, 2.0 equiv. of BI(OH)H and 1.2 equiv. of Cs2CO3 were employed for the hydrocarboxylation (**Table 3**). In the cases of using styrenes with an electron-withdrawing group such as **1a** and 4-methoxycarbonyl styrene (**1d**), the reaction was almost completed after irradiation for 12 h, and the yields of the corresponding hydrocarboxylated products were significantly improved compared with those obtained in the previous conditions where 2.0 mol% of [Ru(bpy)3](PF6)2 and 4.0 equiv. of iPr2NEt were employed. Moreover, 4-trifluoromethyl styrene (**1c**) and non-substituted styrene (**1e**), which exhibited quite low reactivities in the previous conditions, did react to afford significant amounts TABLE 2 | Optimization of the reaction conditions of the hydrocarboxylation of 3,5-bis(trifluoromethyl)styrene.

*aNMR yield.*

*bGC yield. c1 mol% of [Ru(bpy)3](PF6)2 was used. dWithout CO2, under an Ar atmosphere.*

*e Isolated yield was 63%.*

of the corresponding hydrocarboxylated products though the yields were still not sufficiently high. The yields of the hydrocarboxylated products were also improved in the case of alkyl acrylates (**1f** and **1g**). Consequently, the introduction of BI(OH)H electron donor with Cs2CO3 base successfully resulted in the increase in the yields of the present hydrocarboxylation reaction.

### Mechanistic Study

In order to reveal the reaction mechanism of the photocatalytic hydrocarboxylation, the stoichiometric reactions of the possible rhodium intermediates, which corresponded to each elementary step in the proposed catalytic cycle, were examined.

### Rh Hydride Formation

Initially, the Rh(I) hydride formation step was investigated using Rh(PPh3)2(OAc) (**5**) as a model complex of the Rh(I) carboxylate intermediate. The DMA solution of **5** was irradiated by visible-light in the presence of a catalytic amount of [Ru(bpy)3](PF6)2, an excess amount of iPr2NEt and 1.5 equivalent of PPh3. After visible-light irradiation for 6 h, the Rh(I) monohydride complex, Rh(PPh)3H (**6**) was successfully obtained in 57% yield with the recovery of ca. 30% of **5**

*bGC yield.*

*aNMR yield.*

*b Isolated yield after methyl esterification: 57% (*2a*), 59% (*2d*).*

*cWith 2 mol% of [Ru(bpy)*3*](PF*6*)*2.

(based on 1H NMR using an internal standard) (**Figure 3A**; **Supplementary Figure 1**). Control experiments demonstrated that [Ru(bpy)3](PF6)2, iPr2NEt, and visible-light were all essential for the transformation. Since hydrogen evolution was not evident during the reaction, the contribution of gaseous hydrogen was excluded. Thus, this transformation was considered to proceed via (i) stepwise 2-electron, 2-proton transfers from the tertiary amine by the photoredox catalysis to give the Rh(III) dihydride carboxylate (**7**), and (ii) the baseassisted elimination of the carboxylic acid to give **6**. In terms of step (i), the similar mechanisms have been proposed in photocatalytic hydrogen generation systems by a Rh(I) catalyst (Stoll et al., 2015). The initial single electron transfer to the protonated form of **5** would give the Rh(II) carboxylate monohydride, and the following electron and proton transfers or disproportionation of the two Rh(II) hydride species would give **7** (**Figure 4**). The presence of the Rh(III) dihydride intermediate was also supported by the fact that Rh(PCy3)2(OAc) (**5** ′ ) was transformed to Rh(PCy3)2(OAc)(H)2 (**7** ′ ) almost quantitatively under the similar conditions although the reaction was relatively slow (**Figure 3B**; **Supplementary Figure 2**). In this case, PCy3 ligands with strong σ-donation were considered to stabilize the Rh(III) dihydride intermediate to inhibit the following elimination reaction.

In order to confirm the carboxylic acid elimination step (ii), the reactivity of **7** was investigated in the presence of base. **7** was alternatively synthesized by the hydrogenation of **5** with H2, and was treated with an excess amount of iPr2NEt in the presence of PPh3 in C6D6. The reaction readily gave a mixture of **7** and **6** with liberation of [iPr2NHEt]+[CH3COO]−. Furthermore, addition of a small excess amount of [iPr2NHEt]+[CH3COO]− to the C6D6 solution of **6** resulted in the quantitative formation of **7**. These results demonstrated that **7** was in equilibrium with **6** in the presence of iPr2NEt and PPh3 (**Figure 5**). When the treatment of **7** with iPr2NEt was similarly conducted in DMA, **6** was detected as a sole rhodium species in the reaction mixture, indicating that the equilibrium was almost completely shifted to the product side owing to the solvent effect of DMA.

The possible mechanisms for generation of the hydrogenated product were (i) 2-electron, 2-proton transfers to the Rh(I) benzyl intermediate by the photoredox catalysis to give Rh(III) benzyl dihydride intermediate, which would undergo reductive elimination of the hydrogenated product, and (ii) the alkene insertion to the Rh(III) dihydride intermediate and the successive reductive elimination. Both pathways could be inhibited by lowering proton concentrations, as proton transfers would become inefficient in the former, and the competing carboxylic acid elimination from the dihydride complex would be promoted in the latter. Therefore, the inhibition of the hydrogenated product formation by the addition of Cs2CO3 was attributed to the decrease of the proton concentration in the catalytic system.

The photochemical formation of Rh(I) monohydride species was also feasible by using Rh(I) chloride complex as a Rh(I) source. It was demonstrated by the fact that Wilkinson's type complex Rh(PPh3)3Cl was converted to Rh(PPh)3H (**6**) by visible-light irradiation in the presence of a catalytic amount of [Ru(bpy)3](PF6)2 and an excess amount of iPr2NEt. Therefore, the Rh(I) chloride complex was confirmed to work as a precursor of the Rh(I) hydride active species.

### Hydrometalation and Carboxylation

Since the Rh(I) monohydride species was successfully generated from the Rh(I) carboxylate species by photoredox catalysis, the hydrometallation and subsequent carboxylation processes were then investigated to complete the catalytic cycle. Treatment of Rh(PPh)3H (**6**) with an excess amount of **1a** at room temperature readily formed the Rh(I) benzyl species, Rh(PPh3)2(η 3 -CHCH3(4-CNC6H4)) (**8**), almost quantitatively with the liberation of a PPh3 ligand (**Figure 6**i). The benzyl ligand in **8** was found to possess η 3 -coordination to the Rh(I) center based on NMR spectroscopic data (Werner et al., 1994). However, the attempt for isolation of **8** was not successful due to the presence of an equilibrium with **6**. Therefore, in situ generated **8** was directly used for the carboxylation step.

To investigate the carboxylation process with CO2, a DMA solution of a 1:1 mixture of in-situ generated **8** and PPh3 was exposed to the atmospheric pressure of CO2 under various conditions. The carboxylation did not proceed under dark even by heating, which was against our expectations based on the general reactivity of organorhodium(I) complexes with CO2 (Ukai et al., 2006; Mizuno et al., 2011; Suga et al., 2014; Kawashima et al., 2016). Quite interestingly, when the mixture was irradiated by visible-light for 30 min in the presence of 30 mol% of [Ru(bpy)3](PF6)2, **8** was successfully converted to

*dYield obtained by the previous conditions with [Ru(bpy)*3*](PF*6*)*2 *(2 mol%),* 4 *(3.5 mol%). iPr*2*NEt (4 equiv.) and irradiation for 24 h.*

the Rh(I) carboxylate complex, Rh(PPh3)3(η 1 -O2CCHCH3(4- CNC6H4)) (**9**), almost quantitatively (**Figure 6**ii). 31P{1H} NMR spectroscopy confirmed the clean formation of **9**: a pair of the doublet of doublet signals attributed to **8** completely disappeared with the PPh3 signal, and the doublet of doublet and doublet of triplet signals attributed to **9** appeared in 2: 1 ratio by visible-light irradiation (**Figure 7**). The control experiments demonstrated that CO2, [Ru(bpy)3](PF6)2 and visible-light were all essential for the carboxylation, suggesting that the nucleophilic addition of **8** to CO2 was facilitated by the photosensitization of [Ru(bpy)3] 2+. The luminescence quenching experiment demonstrated that the excited state of [Ru(bpy)3] 2+ was effectively quenched by **8** (**Figure 8**). The quenching constant was determined to be Kq = 2.07 × 103 , which was much larger than that by iPr2NEt (Kq =

1.56 × 102 , **Supplementary Figure 3A**). This result indicates that either photoinduced electron transfer or triplet-triplet energy transfer to **8** contributed to the quenching (Campagna et al., 2007; Arias-Rotondo and McCusker, 2016; Strieth-Kalthoff et al., 2018). However, the photoinduced electron transfer mechanism was unlikely in this case since (i) the carboxylation of **8** proceeded with a catalytic amount of [Ru(bpy)3](PF6)2 even in the absence of the sacrificial electron donor, and (ii) the cyclic voltammogram of **8** showed no significant redox peak within the window where the oxidative quenching of [Ru(bpy)3] 2+ was possible. Therefore, the photoinduced triplet-triplet energy transfer from the excited [Ru(bpy)3] 2+ to **8** was considered to be the most likely process in this carboxylation process. Although not very common, several examples on the photocatalytic organic transformations mediated by the triplet-triplet energy transfer were previously reported (Ikezawa et al., 1986; Osawa et al., 2001; Islangulov and Castellano, 2006; Lu and Yoon, 2012; Farney and Yoon, 2014).

In order to investigate the detailed effect of the photoinduced energy transfer, the electronic structure analyses were performed in terms of the ground (S0) and the lowest excited triplet (T1) states of **8** based on DFT/TD-DFT methods. The calculated energy level of the T1 state of **8** (1.07 eV, based on the comparison between the S0 and T1 optimized geometries) was much lower than that of [Ru(bpy)3] 2+ (2.17 eV), indicating that the triplet-triplet energy transfer from the excited [Ru(bpy)3] 2+ to **8** was feasible. In terms of the optimized structures, a notable difference was found on the coordination manner of the benzyl ligand between the S0 and T1 geometries. In the S0 optimized structure, the η 3 -coodination of the benzyl ligand was represented by the similar three Rh-C distances, which coincided with the results of the 1H NMR observation (**Figure 9A**; **Table 4**). On the other hand, in the T1 optimized structure, while the Rh-C1 (benzyl carbon) distance remained unchanged, the Rh-C2/C3 distances significantly elongated compared with those of the S0 structure. These results indicated that the benzyl ligand changed its coordination-mode from η 3 -type (αbenzyl) to η 1 -type (σ-benzyl) in the T1 state. According to the analysis on the electronic transition characters, the T1 state was mainly contributed by the transitions of HOMO→LUMO (88%) and HOMO→LUMO+16 (4%) (**Supplementary Table 2**). The molecular orbital distribution indicated that LUMO and LUMO+16 mainly localized on the Rh (dπ) and benzyl ligand (π ∗ ) while the HOMO localized on the Rh (dσ) center (**Figure 9B**). As these LUMOs partially possessed the antibonding character on the Rh-C2/C3 bonds, the photoexcitation induced the dissociation of these Rh-C bonds, which resulted in the isomerization to the σ-benzyl species.

Concerning the acceleration of the carboxylation step, one possibility is the generation of the coordination site by taking σ-benzyl structure in the T1 state, which would promote the following carboxylation by facilitating coordination of CO2 to Rh center. Indeed, a similar thermal process has been proposed as a plausible mechanism for the carboxylation of organorhodium(I)

TABLE 4 | Selected bond lengths and NBO natural charges in the optimized geometries of 8(S0) and 8(T1).

complexes (Darensbourg et al., 1987). Another possibility is the direct nucleophilic addition of the benzyl carbon to CO2 in the T1 state. The NBO analysis demonstrated that the natural charge on the C1 atom significantly shifted to the negative side while that on the Rh atom shifted to the positive side in the T1 state. The increase of the electron density on the C1 atom in the T1 state would result in the acceleration of the nucleophilic addition to CO2. Therefore, the structural and/or electronic factors associated with the transition to the T1 state are thought to contribute to the carboxylation of **8**.

The effect of the photoactivation of the Rh(I) π-benzyl complex was also supported by the reactivity of the Rh(I) σ-alkyl complex with CO2. When a mixture of **6** and an excess amount of methyl acrylate (**1f**) was subjected to a CO2 atmosphere for 3 h even under dark, the quantitative formation of Rh(PPh3)3(η 1 - O2CCHCH3(CO2CH3)) (**10**) was indicated by 31P{1H} NMR spectroscopy. The carboxylation of **1f** was confirmed by the fact that the corresponding hydrocarboxylated product (**2f**) was obtained from the reaction mixture. This result indicated that the photosensitization by [Ru(bpy)3] 2+ was not essential in this case. Thus, the major role of the excitation was thought to be the transformation from π-benzyl to σ-benzyl complexes to generate a coordination site and to make them more nucleophilic.

### Addition of the Second Photosensitizer

The above mechanistic study revealed that a photosensitizer played two key roles in the hydrocarboxylation cycle: one is a "photoredox catalyst" to reduce the Rh(I) carboxylate species, and the other is a "triplet photosensitizer" to promote carboxylation of the Rh(I) benzyl species. With a single photosensitizer, the excited state of the photosensitizer was quenched by either a tertiary amine for the electron transfer or a Rh(I) benzyl species for the energy transfer, and these two processes competed during the reaction. Since the former was related to the rate-determining step when using iPr2NEt as a sacrificial electron donor, the incorporation of the second photosensitizer possessing suitable redox properties for the reductive quenching cycle was expected to facilitate the catalytic reaction.

On the basis of the idea, 2.0 mol% of a cyclometalated Ir(III) complex was added as a second photosensitizer to a mixture of **1b**, 3.5 mol% of **4**, 2.0 mol% of [Ru(bpy)3](PF6)2 and 4.0 equiv. of iPr2NEt, and the solution was irradiated under CO2 atmosphere at room temperature (**Table 5**). To excite both photosensitizers, a wide range of UV-visible-light (380–750 nm) was applied to the reactions. As expected, the addition of the second photosensitizer was found to be effective. For instance, when [Ir(ppy)2(dtbbpy)](PF6) was added, the reaction was completed after irradiation for only 6 h, and the yield of **2b** was increased more than five-fold compared to that of the reaction without the second photosensitizer (**Table 5**, entry 2). According to the redox properties of [Ir(ppy)2(dtbbpy)](PF6), the acceleration of the reaction was thought to be attributed mainly to the

TABLE 5 | Screening of the reaction conditions of the hydrocarboxylation of 3,5-bis(trifluoromethyl)styrene with a second photosensitizer.

*aNMR yield.*

*bGC yield. cBI(OH)H (1.2 equiv.) was used instead of iPr2NEt (4.0 equiv.).*

high reducing ability of the one-electron reduced species to promote the reduction process. The yield of **2b** further increased when incorporating [Ir(dF(CF3)ppy)2(dtbbpy)](PF6) as a second photosensitizer (**Table 5**, entry 3), and its concentration could be reduced to 1.0 mol% without lowering the yield (**Table 5**, entry 4). This result was assumed to be due to the high oxidizing ability of the excited state in addition to the sufficient reducing ability of the one-electron reduced species. The excited state of [Ir(dF(CF3)ppy)2(dtbbpy)](PF6) was found to be able to work as an energy transfer agent of **8** based on the luminescence quenching experiment (Kq = 2.76 × 104 , **Supplementary Figure 3B**). However, it is considered to contribute to the reaction mainly as an electron transfer agent under the catalytic conditions owing to the efficient quenching by the sacrificial electron donor. On the other hand, the addition of fac-Ir(ppy)3 resulted in only a small acceleration, which was probably attributable to the inferior oxidizing ability in the excited state (**Table 5**, entry 6). These results demonstrate that photosensitizers possessing both high oxidizing ability of the excited state and high reducing ability of the one-electron reduced species are advantageous as a second photosensitizer. The positive result on the addition of the two appropriate photosensitizers reflected the fact that the catalytic cycle was composed of the multiple photochemical processes, and the acceleration of the reduction process led to the enhancement of the catalytic activity when using iPr2NEt as a sacrificial electron donor.

### Rate-Determining Step in the Hydrocarboxylation With BI(OH)H

The previous experiments demonstrated that the ratedetermining step of the catalytic cycle was the reduction process of the Rh(I) carboxylate species **9** when employing iPr2NEt as a sacrificial electron donor. In order to investigate the contribution of BI(OH)H to the catalytic cycle, the similar examination was carried out using BI(OH)H as a sacrificial electron donor instead of iPr2NEt. Interestingly, the resting-state was found to be Rh(I) π-benzyl intermediate **8** when a mixture of **1a**, catalytic amounts of **6** and [Ru(bpy)3](PF6)2, and 1.2 equiv. of BI(OH)H was irradiated by visible-light under CO2 atmosphere (**Supplementary Figure 4**). In that case, **9** was not detectable even after prolonged irradiation, indicating that the rate-determining step obviously altered from the reduction process to the carboxylation process by changing the sacrificial electron donor. This result was also supported by the fact that the acceleration of the reaction by the addition of a second photosensitizer was not observed in the case of the reaction using BI(OH)H as a sacrificial electron donor (**Table 5**, entry 8). These observations suggested that the use of BI(OH)H strongly accelerated the reductive quenching cycle of [Ru(bpy)3] 2+ to promote the reduction process.

### CONCLUSION

In this study, the improved catalytic conditions of the visiblelight driven hydrocarboxylation by Rh(I) and [Ru(bpy)3] 2+ catalysts were explored, and the detailed reaction mechanism was investigated. On the basis of the stoichiometric reactions of the possible rhodium intermediates, the proposed catalytic cycle was confirmed to be composed of (i) the hydrometallation of alkenes by Rh(I) monohydride species, (ii) the photochemical carboxylation of the Rh(I) benzyl species with CO2, (iii) the photoinduced 2-electron, 2-proton transfers to the Rh(I) carboxylate species, and (iv) the base-assisted carboxylic acid elimination. One strategy for the enhancement of the catalytic reaction was to employ BI(OH)H possessing superior reducing ability as a sacrificial electron donor instead of iPr2NEt. It successfully improved the efficiency of the reaction, which had been major challenges in the previous catalytic conditions. The alteration of the resting-state by changing the sacrificial electron donor indicated that the addition of BI(OH)H significantly promoted the reduction process of the Rh(I) carboxylate species through the enhancement of the reductive quenching efficiency of [Ru(bpy)3] 2+. Another strategy for the enhancement of the efficiency was to add the second photosensitizer in charge of the reductive quenching cycle. The acceleration of the catalytic reaction by the addition of the appropriate cyclometalated Ir(III) complex together with [Ru(bpy)3] 2+ supported this hypothesis. These two effective strategies suggested that the promotion of the reduction processes was a key to enhance the catalytic activity in the present system. In addition to expand the versatility of the present hydrocarboxylation, this study would provide fundamental insights into the catalytic organic transformations by transitionmetal/photoredox dual catalysis.

### EXPERIMENTAL

### General

All operations were carried out under an argon atmosphere unless otherwise noted. 1H, 13C, and 31P NMR spectra were recorded on Bruker DRX-500, JEOL ECZ-500, ECX-400, and ECS-400 spectrometers. 31P and 19F NMR chemical shifts were calibrated using external 85% H3PO4 (δ: 0.0 ppm) and neat C6F6 (δ: −164.9 ppm), respectively. IR spectra were recorded on an SC-100-VIR with an ATR PRO450-S accessory (JASCO Co., Ltd.). Emission spectra were recorded on an FP-6500 spectrofluorometer (JASCO Co., Ltd.). FAB-MS and FD-MS spectra were recorded on a JEOL JMS-700 spectrometer and a JMS-T100 spectrometer, respectively. Gas chromatography (GC-FID / TCD) was recorded on a Shimadzu GC-2010 spectrometer. Analytical thin-layer chromatography (TLC) was performed with a glass plate coated with silica gel (Wakogel B-5F). Visiblelight irradiation was performed with a Relyon Twin LED Light (3W × 2, λirr. = 425 ± 15 nm), and UV-visible-light irradiation was performed with an USHIO Optical Modulex OPM2-502XQ (500 W Xe lamp) with a super cold filter ZSC0750 (ASAHI Spectra Inc.).

THF, toluene, pentane and diethyl ether were purified by a solvent purification system by Glass Contour. Dehydrated dimethylacetamide (DMA) and dimethylformamide (DMF) were purchased from Kanto Chemical Co., Inc., degassed by argon bubbling and stored in a glovebox. Tertiary amines were distilled, degassed three times by freeze-pump-thaw method and stored under N2. Solvents for NMR measurements were dried over molecular sieves, degassed three times by freeze-pump-thaw method and stored under N2. All other solvents were distilled, degassed by argon bubbling and stored in a glovebox. CO2 and H2 gases were purchased from Taiyo Nippon Sanso Corporation. [Rh(coe)2Cl]2 (Van der Ent et al., 1990), P(4-CF3C6H4)3 (Suomalainen et al., 2001), Rh(PPh3)2(OAc) (**5**) (Grushin et al., 1995), [Rh(cod)(OAc)]2 (Chatt and Venanzi, 1957), Rh(PPh3)3H (**6**) (Annibale and Song, 2014), [Ru(bpy)3](PF6)2 (Damrauer et al., 1997), [Ru(dmbpy)3](PF6)2 (Damrauer et al., 1997), [Ru(bpz)3](PF6)2 (Schultz et al., 2015), Ir(ppy)2(dtbbpy)(PF6) (Tellis et al., 2014), [Ir(dF(CF3)ppy)2(dtbbpy)](PF6) (Slinker et al., 2004), fac-Ir(ppy)3 (Tamayo et al., 2003), and diisopropylethylammonium acetate (Anouti et al., 2008) were prepared according to the published methods. 4-Cyanostyrene (**1a**) was prepared by Wittig reaction of 4-cyanobenzaldehyde (Falk et al., 2013). Other chemicals were purchased and used as received.

### Photocatalytic Reactions

For screening conditions with alkenes (**1a**–**1g**), a DMA solution (0.6 mL) of an alkene (0.060 mmol), [Rh(P(4-CF3C6H4)3)2Cl]2 (**4**, 4.5 mg, 0.0021 mmol), photoredox catalyst(s), sacrificial electron donor and inorganic base (defined amounts) was prepared in a glass tube (ϕ 2.0 cm, 18 cm) under an argon atmosphere. Then the headspace gas was replaced by an atmospheric pressure of CO2, and the reaction vessel was put in a water bath placed at a distance of 10 mm from light sources. The mixture was irradiated with visible-light from blue LED lamp (λirr. = 425 nm, two sockets) or UV-visible-light from Xe lamp (λirr. = 380–800 nm) for defined time in the closed system. The product mixture was analyzed by 1H NMR and GC to determine the NMR yield of the hydrocarboxylated product (**2a**–**2g**) and the GC yield of the hydrogenated product (**3a**, **3b**), respectively (internal standard: 1,1,2,2-tetrachloroethane).

For isolation of the methyl esters of the hydrocarboxylated products (**2a**, **2b**, **2d**), a DMA solution (1.2 mL) of a styrene (0.12 mmol), **4** (9.0 mg, 0.0042 mmol), [Ru(bpy)3](PF6)2 (1.0 mg, 0.0012 mmol), BI(OH)H (58 mg, 0.24 mmol), and Cs2CO3 (47 mg, 0.14 mmol) was prepared in a glass tube (ϕ 2.0 cm, 18 cm), and irradiated with visible-light from blue LED lamp (λirr. = 425 nm, three sockets) for defined time after replacement of the headspace gas by an atmospheric pressure of CO2. After irradiation, the reaction mixture was diluted with diethyl ether and extracted with H2O three times. The combined aqueous layer was acidified by 1N HCl aq., and then extracted with diethyl ether three times. The combined organic layer was dried over MgSO4, filtered and evaporated under reduced pressure to give the hydrocarboxylated product. Then, the product was dissolved in Et2O-MeOH, and TMSCHN2 (excess) was added at 0 ◦C. The mixture was stirred at 0◦C for 30 min and the solvent was removed under reduced pressure. The crude product was purified by preparative TLC (AcOEt/n-hexane = 1/5) to give the corresponding methyl-esterified product.

# Preparations of Rhodium Complexes and Their Stoichiometric Reactions

### Preparation of [Rh(P(4-CF3C6H4)3)2Cl]2 (4)

A solution of P(4-CF3C6H4)3 (200 mg, 0.429 mmol) in toluene (2 mL) was added dropwise to a solution of [Rh(coe)2Cl]2 (77 mg, 0.107 mmol) in toluene (2 mL) and the mixture was stirred at room temperature overnight. After removal of solvent under reduced pressure, the crude product was dissolved in THF and then pentane was added to induce precipitation. The precipitates were collected to give the target product (218 mg, 0.102 mmol, 95% yield). 1H NMR (500 MHz, THFd8, r.t., δ/ppm): δ 7.73–7.67 (m, 24 H, PAr3), 7.39 (d, J = 8 Hz, 24 H, PAr3). 13C{1H} NMR (125 MHz, r.t., THF-d8, δ/ppm): δ 139.3 (vt, N = 22 Hz, PAr3), 135.7 (s, PAr3), 132.1 (q, JC−F = 33 Hz, PAr3), 125.0 (s, PAr3), 124.6 (q, JC−F = 272 Hz, -CF3). 31P{1H} NMR (202 MHz, THF-d8, r.t., δ/ppm): δ 53.4 (d, J = 194 Hz). 19F NMR (471 MHz, THF-d8, r.t., δ/ppm): δ −60.6 (s). ESI-MS: m/z = 1035 [M/2 − Cl]+. Anal. Found (calcd for C84H48Cl2F36P4Rh2): C, 46.94 (47.11); H, 2.28 (2.26).

### Preparation of Rh(PCy3)2(OAc) (5′ )

[Rh(cod)(OAc)]2 (60 mg, 0.111 mmol) and PCy3 (125 mg, 0.444 mmol) were suspended in DMA (4 mL), and the mixture was irradiated with UV-visible-light (500 W Xe lamp, λirr. = 380– 800 nm) at room temperature with vigorous stirring for 30 h. The precipitate was filtered, washed with DMA and cold pentane (−35◦C), and then recrystallized from a minimum volume of pentane at −35◦C to yield the target compound (98 mg, 0.135 mmol, 61%). IR (KBr): ν(OCOas) = 1,528, ν(OCOsym) = 1,445 cm−1 . 1H NMR (500 MHz, C6D6, r.t., δ/ppm): δ 2.35 – 1.24 (m, 69 H, PCy3 and O2CCH3). 13C{1H} NMR (125 MHz, r.t., C6D6, δ/ppm): δ 188.2 (s, O2CCH3), 35.9 (vt, N = 10 Hz, PCy3), 31.1 (s, PCy3), 28.4 (vt, N = 5 Hz, PCy3), 27.3 (s, PCy3), 24.9 (s, O2CCH3). 31P{1H} NMR (202 MHz, C6D6, r.t., δ/ppm): δ 59.1 (d, JP−Rh = 198 Hz). HR-MS (FAB): m/z = 722.3817 [M]+ (calcd for [C38H69O2P2Rh]+: 722.3828).

### Preparation of an Authentic Sample of Rh(PCy3)2(OAc)(H)2 (7′ )

[Rh(cod)(OAc)]2 (60 mg, 0.111 mmol) and PCy3 (125 mg, 0.444 mmol) were dissolved in THF (3 mL), and the mixture was stirred under H2 (1 atm) at room temperature overnight. After removal of solvent, the crude product was dissolved in toluene and filtered through Celite R . The resultant solid after evaporation was washed with cold diethyl ether (−35◦C) to give the target compound (113 mg, 0.156 mmol, 70%). IR (KBr): ν(RhH) = 2,143, ν(OCOas) = 1,551, ν(OCOsym) = 1,436 cm−1 . 1H NMR (500 MHz, C6D6, r.t., δ/ppm): δ 2.20 – 1.22 (m, 69 H, PCy3 and O2CCH3), −23.7 (dt, JH−Rh = 24, JH−P = 15 Hz, 2 H, Rh-H) 13C{1H} NMR (125 MHz, r.t., C6D6, δ/ppm): δ 180.2 (s, O2CCH3), 35.8 (vt, JC−P = 10 Hz, PCy3), 30.5 (s, PCy3), 28.3 (vt, N = 5 Hz, PCy3), 27.1 (s, PCy3), 24.8 (s, O2CCH3). 31P{1H} NMR (202 MHz, C6D6, r.t., δ/ppm): δ 51.1 (d, JP−Rh = 115 Hz). HR-MS (FD): m/z = 724.3975 (calcd for [C38H71O2P2Rh]+: 724.3984).

### Preparation of Rh(PPh3)2(η 3 -CHCH3(4-CNC6H4)) (8)

To a solution of Rh(PPh3)3H (**6**) (5.4 mg, 0.0060 mmol) in THFd8 (0.6 mL) in a J. Young NMR tube was added 4-cyanostyrene (**1a**) (1.6 µL, 0.012 mmol) at room temperature. Rh(PPh3)2(η 3 - CHCH3(4-CNC6H4)) (**8**) formed almost quantitatively. 1H NMR (500 MHz, THF-d8, −10◦C, δ/ppm): δ 7.47 – 7.00 (m, 30 H, PPh3), 6.81 (brd, J = 8 Hz, 1 H, Ar), 6.57 (brd, J = 7 Hz, 1 H, Ar), 6.10 (brd, J = 8 Hz, 1 H, Ar), 4.89 (brd, J = 7 Hz, 1 H, Ar), 2.47 – 2.40 (m, 1 H, -CHCH3), 0.92 – 0.87 (m, 3 H, - CHCH3). 31P{1H} NMR (202 MHz, THF-d8, −10◦C, δ/ppm): δ 46.2 (dd, JP−Rh = 263 Hz, JP−P = 31 Hz), 39.6 (dd, JP−Rh = 178 Hz, JP−P = 31 Hz). HR-MS (FAB): m/z = 757.1539 (calcd for [C45H38NP2Rh]+: 757.1535).

### Preparation of Rh(PPh3)3(η 1 -O2CCHCH3 (4-CNC6H4)) (9)

To a solution of Rh(PPh3)3H (**6**) (60 mg, 0.090 mmol) in toluene (3 mL) was added dropwise a solution 2-(4 cyanophenyl)propionic acid (16 mg, 0.090 mmol) in toluene (2 mL), and the mixture was stirred at room temperature for 4 h. After removal of solvent under reduced pressure, the crude product was dissolved in toluene and then pentane was added to induce precipitation. The precipitates were collected to give the target product as a 9: 1 mixture with Rh(PPh3)2(η 2 - O2CCHCH3(4-CNC6H4)) which was formed by dissociation of PPh3 from **9** (59 mg). **9** IR (KBr): ν(CN) = 2,224, ν(OCOas) = 1,604, ν(OCOsym) = 1,342 cm−1 . 1H NMR (500 MHz, THF-d8, −60◦C, δ/ppm): δ 7.56 – 6.79 (m, 47 H, PPh3, O2CCHCH3Ar), 6.30 (brd, J = 8 Hz, 2 H, O2CCHCH3Ar), 1.65 (brq, J = 7 Hz, 1 H, O2CCHCH3Ar), 0.25 (brd, J = 7 Hz, 3 H, O2CCHCH3Ar). 13C{1H} NMR (125 MHz, r.t., THF-d8, −30◦C, δ/ppm): δ 176.6 (s, O2CCHCH3Ar), 150.8 (s, O2CCHCH3Ar), 135.7 (vt, N = 6 Hz, PPh3), 131.1 (s, O2CCHCH3Ar), 129.6 (s, O2CCHCH3Ar), 129.2 (s, PPh3), 127.8 (s, PPh3), 127.5 (s, PPh3), 127.4 (s, PPh3), 119.8 (s, -CN), 108.9 (s, O2CCHCH3Ar), 47.8 (s, O2CCHCH3Ar), 18.1 (s, O2CCHCH3Ar). 31P{1H} NMR (202 MHz, THF-d8, −30◦C, δ/ppm): δ 51.5 (dt, JP−Rh = 174 Hz, JP−P = 41 Hz), 34.9 (dd, JP−Rh = 153 Hz, JP−P = 41 Hz). HR-MS (FAB): m/z = 801.1453 [M−(PPh3)]+ (calcd for [C46H38NO2P2Rh]+: 801.1433). Rh(PPh3)2(η 2 -O2CCHCH3(4- CNC6H4)) 31P{1H} NMR (202 MHz, THF-d8, −30◦C, δ / ppm): δ 57.6 (brd, JP−Rh = 210 Hz).

### Redox-Photosensitized Reaction of Rh(PPh3)2(OAc) (5)

Rh(PPh3)2(OAc) (**5**) (2.7 mg, 0.0040 mmol), [Ru(bpy)3](PF6)2 (1.0 mg, 0.0012 mmol), PPh3 (1.5 mg, 0.0060 mmol), iPr2NEt (41 µL, 0.24 mmol) and DMA (0.6 mL) were added in a glass tube with a magnetic stirrer. The mixture was irradiated with visible-light (λirr. = 425 nm) at room temperature for 6 h. The solvent was removed under reduced pressure, and the resulting solid was analyzed by 1H and 31P NMR spectroscopies (THF-d8, internal standard: mesitylene). The Rh(I) monohydride species corresponding to Rh(PPh3)3H (**6**) formed in 57% yield (based on the Rh-hydride signal) and ca. 30% of the starting material **5** remained in the product mixture. **6** was highly fluxional in the reaction solution at room temperature in the presence of triphenylphosphine. 1H NMR (500 MHz, THF-d8, δ/ppm, r.t.): −8.45 (brd, JH−Rh = 13 Hz, Rh-H). 31P NMR (202 MHz, THF-d8, δ/ppm, r.t.): 41 (br). The intensity of the Rh-hydride signal was significantly increased at room temperature when Rh(PPh3)3H (**6**) synthesized alternatively was added to the reaction mixture, also supporting the formation of **6**. In addition, when the solution was cooled to −90◦C, the signals attributed to Rh(PPh3)4H were observed instead, indicating **6** was converted to Rh(PPh3)4H at low temperature. 1H NMR (500 MHz, THF-d8, δ/ppm, −90◦C): −13.5 (dq JH−Rh = 118, JH−P = 15 Hz, Rh-H). 31P NMR (202 MHz, THF-d8, δ/ppm, −90◦C): 33.4 (dm, JP−Rh = 112), 30.2 (dd, JP−Rh = 162, JP−P = 32 Hz). The spectrosopic data was analogous to the values reported previously (Dewhirst et al., 1968; Strauss and Shriver, 1978).

### Redox-Photosensitized Reaction of Rh(PCy3)2(OAc) (5′ )

Rh(PCy3)2(OAc) (**5** ′ ) (2.9 mg, 0.0040 mmol), [Ru(bpy)3](PF6)2 (1.0 mg, 0.0012 mmol), iPr2NEt (41 µL, 0.24 mmol) and DMA (0.6 mL) were added in a glass tube with a magnetic stirrer. The mixture was irradiated with visible-light (λirr. = 425 nm) at room temperature for 12 h. The solvent was removed under reduced pressure, and the resulting solid was analyzed by 1H and 31P NMR spectroscopies (C6D6, internal standard: mesitylene). Rh(PCy3)(OAc)(H)2 (**7** ′ ) was formed in 94% yield (based on the rhodium hydride signal). The spectroscopic feature well agreed with the complex **7** ′ synthesized alternatively (vide supra). No other Rh hydride signal was observed even by addition of 1 equiv. of PCy3 to the reaction mixture.

### Reaction of Rh(PPh3)2(OAc)(H)2 (7) and iPr2NEt

A solution of Rh(PPh3)2(OAc) (**5**) (2.7 mg, 0.0040 mmol) in C6D6 (0.6 mL) was put in a J. Young NMR tube under hydrogen atmosphere at room temperature. After 2 h, Rh(PPh3)2(OAc)(H)2 (**7**) formed in situ. 1H NMR (500 MHz, C6D6, δ/ppm, r.t.): 7.86 – 7.00 (m, PPh3), −20.8 (dt, JH−Rh = 22 Hz, JH−P = 17 Hz, Rh-H). 31P{1H} NMR (202 MHz, C6D6, δ/ppm, r.t.): 41.4 (d, JP−Rh = 121 Hz). Then, the reaction solution was degassed by freeze-pump-thaw method 3 times to remove the hydrogen gas. Addition of 1 equiv. of PPh3 (1.5 mg, 0.0060 mmol) and 80 equiv. of iPr2NEt (41 µL, 0.24 mmol) gave Rh(PPh3)3H (**6**) in 58% yield (based on the rhodium hydride signal). Ca. 20% of **7** remained in the reaction mixture. The spectroscopic data of **6** was identical with that of the sample prepared by the photosensitizing reaction of Rh(PPh3)2(OAc) (**5**). The same reaction was also performed in DMA. In this case, **7** converted fully after addition of PPh3 and iPr2NEt, and **6** was obtained as a major product.

### Reaction of Rh(PPh3)3H (6) and [iPr2NHEt]+ [CH3COO]−

To a solution of Rh(PPh3)3H (**6**) (3.6 mg, 0.0040 mmol) in C6D6 (0.6 mL) was added diisopropylethylammonium acetate (1.1 mg, 0.0060 mmol) and the mixture was stirred at room temperature for 10 min. Rh(PPh3)2(OAc)(H)2 (**7**) formed quantitatively. The spectroscopic data of the product in C6D6 was identical with those of the sample prepared by the hydrogenation of Rh(PPh3)2(OAc) (**5**).

### Redox-Photosensitized Reaction of Rh(PPh3)3Cl

Rh(PPh3)3Cl (3.7 mg, 0.0040 mmol), [Ru(bpy)3](PF6)2 (1.0 mg, 0.0012 mmol), iPr2NEt (41 µL, 0.24 mmol) and DMF-d7 (0.6 mL) were added in a J. Young NMR tube. The mixture was irradiated with visible-light (λirr. = 425 nm) at room temperature for 6 h. Rh(PPh3)3H (**6**) formed in 52% yield (based on the Rh-hydride signal) and ca. 40% of the starting material remained in the reaction mixture. 1H NMR (500 MHz, δ/ppm, −50◦C): −8.48 (ddt, JH−Rh = 101 Hz, JH−P = 18 Hz, JH−P = 15 Hz, Rh-H). 31P NMR (202 MHz, δ/ppm, −50◦C): 43.7 (dd, JP−Rh = 170 Hz, JP−P = 25 Hz), 38.9 (dm, JP−Rh = 149 Hz). The spectrosopic data was analogous to the values reported previously (Dewhirst et al., 1968; Strauss and Shriver, 1978).

### Reaction of Rh(PPh3)3H (6) With 4-Cyanostyrene (1a) and CO2

To a solution of Rh(PPh3)3H (**6**) (3.6 mg, 0.0040 mmol) in DMA (0.6 mL) was added 4-cyanostyrene (**1a**) (2.6 µL, 0.020 mmol) in a J. Young NMR tube at room temperature. Rh(PPh3)2(η 3 -CHCH3(4-CNC6H4)) (**8**) was generated in situ almost quantitatively. Then, [Ru(bpy)3](PF6)2 (1.0 mg, 0.0012 mmol) was added, and the mixture was irradiated with visible-light (λirr. = 425 nm) under a CO2 atmosphere (1 atm) at room temperature for 30 min. Rh(PPh3)3(η 1 -O2CCHCH3(4- CNC6H4)) (**9**) was formed almost quantitatively, as the spectroscopic data well agreed with the complex synthesized alternatively.

### Reaction of Rh(PPh3)3H (6) With Methyl Acrylate (1f) and CO2

To a solution of Rh(PPh3)3H (**6**) (3.6 mg, 0.0040 mmol) and [Ru(bpy)3](PF6)2 (1.0 mg, 0.0012 mmol) in DMA (0.6 mL) was added methyl acrylate (**1f**) (1.8 µL, 0.020 mmol) in a J. Young NMR tube at room temperature. The mixture was exposed to a CO2 atmosphere (1 atm) under dark at room temperature for 3 h. The observed 31P{1H} NMR spectral feature was analogous to that of **9**, indicating the formation of Rh(PPh3)3(η 1 -O2CCHCH3(CO2CH3)) (**10**). 31P{1H} NMR (162 MHz, DMA, −15◦C, δ/ppm): δ 50.6 (dt, JP−Rh = 174 Hz, JP−P = 44 Hz), 33.5 (dd, JP−Rh = 153 Hz, JP−P = 44 Hz). The reaction solution was treated with NaHCO3 aq., and then acidified with 1N HCl aq. The organic layer was extracted with diethyl ether three times, dried over MgSO4, filtered and evaporated under reduced pressure to give methyl methylmalonate in 54% yield (based on **6**).

### Observation of the Reaction Intermediates Under Catalytic Conditions

Rh(PPh3)3H (**6**) (3.6 mg, 0.0040 mmol), [Ru(bpy)3](PF6)2 (1.0 mg, 0.0012 mmol), BI(OH)H (17.3 mg, 0.072 mmol), 4-cyanostyrene (**1a**) (7.7 µL, 0.060 mmol) and DMA (0.6 mL) were added in a J. Young NMR tube. The mixture was irradiated with visible-light (λirr. = 425 nm) under CO2 atmosphere (1 atm) at room temperature for 30 min. The 31P{1H} NMR spectra were observed at −15◦C before and after irradiation, and only Rh(PPh3)2(η 3 - CHCH3(4-CNC6H4)) (**8**) was observed as a resting-state in both spectra.

### Theoretical Study

Theoretical calculations were performed at the DFT level with the Gaussian 09 package. The geometry optimizations were performed using the mPW1PW91 functional (Adamo and Barone, 1998). The LanL2DZ basis set was used for all atoms and extended by a polarization function (except for H) (Dunning and Hay, 1976; Wadt and Hay, 1985a,b). To address solvation effects, the conductor-like polarizable continuum model (CPCM, N,N-Dimethylacetamide) (Tomasi et al., 2005) was used for the ground and excited states. For validation, vibrational frequencies were calculated for the ground and excited states. The orbital plots as well as the graphical representations were performed using Molekel (Varetto, 2009). Natural bond orbital (NBO) analysis was used to predict and interpret the computational results (Glendening et al., 2001). Total ZPE energies and cartesian coordinates of computed structures are given in **Supplementary Table 3**.

### DATA AVAILABILITY

All datasets generated for this study are included in the manuscript and/or the **Supplementary Files**.

# AUTHOR CONTRIBUTIONS

NN and KM performed the experiments. KS instructed the experiments of NN. KM performed the theoretical calculations. NI and JT supervised the project. KM and NI wrote the paper.

# FUNDING

The authors are grateful for the financial supports by an ACT-C program JPMJCR12Y3 from JST and JSPS KAKENHI Grant no. 15H05800, 17H06143, and 24245019.

# ACKNOWLEDGMENTS

The authors also thank Prof. Osamu Ishitani for valuable discussions on the use of photosensitizers and sacrificial electron donors.

### REFERENCES

# SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem. 2019.00371/full#supplementary-material

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The reviewer MA declared a shared affiliation, with no collaboration, with the authors NN, KS, JT, NI, to the handling editor at the time of review.

Copyright © 2019 Murata, Numasawa, Shimomaki, Takaya and Iwasawa. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Electronic Effects of Substituents on fac-M(bpy-R)(CO)3 (M = Mn, Re) Complexes for Homogeneous CO2 Electroreduction

Laura Rotundo1,2,3, Emanuele Azzi 1 , Annamaria Deagostino1 , Claudio Garino1,2,3 , Luca Nencini 1 , Emanuele Priola1,2, Pierluigi Quagliotto1,2, Riccardo Rocca1,2,3 , Roberto Gobetto1,2,3 \* and Carlo Nervi 1,2,3 \*

### Edited by:

Hitoshi Ishida, Kitasato University, Japan

### Reviewed by:

Baojian Shen, China University of Petroleum, Beijing, China Yusuke Tamaki, Tokyo Institute of Technology, Japan

### \*Correspondence:

Roberto Gobetto<EMAIL_ADDRESS>Carlo Nervi<EMAIL_ADDRESS>
### Specialty section:

This article was submitted to Inorganic Chemistry, a section of the journal Frontiers in Chemistry

Received: 29 January 2019 Accepted: 21 May 2019 Published: 05 June 2019

### Citation:

Rotundo L, Azzi E, Deagostino A, Garino C, Nencini L, Priola E, Quagliotto P, Rocca R, Gobetto R and Nervi C (2019) Electronic Effects of Substituents on fac-M(bpy-R)(CO)3 (M = Mn, Re) Complexes for Homogeneous CO2 Electroreduction. Front. Chem. 7:417. doi: 10.3389/fchem.2019.00417 1 Department of Chemistry, Università degli Studi di Torino, Turin, Italy, 2 NIS Interdepartmental Centre, Università degli Studi di Torino, Turin, Italy, 3 Consorzio Interuniversitario Reattività Chimica e Catalisi (CIRCC), Bari, Italy

Synthesis and characterization of 14 new 2,2′ -bipyridine metal complexes fac-M(bpy-R)(CO)3X (where M = Mn, X = Br or M = Re, X = Cl and R = -CF3, -CN, -Ph, -PhOH, -NMe2) are reported. The complexes have been characterized by NMR, IR spectroscopy and elemental analysis. Single crystal X-Ray diffraction structures have been solved for Re(dpbpy)(CO)3Cl (dpbpy = 4,6-diphenyl-2,2′ -bipyridine) and Re(hpbpy)(CO)3Cl (hpbpy = 4-(2-hydroxy-phenyl)-6-phenyl-2,2′ -bipyridine). Electrochemical behaviors of the complexes in acetonitrile under Ar and their catalytic performances for CO2 reduction with added water and MeOH have been investigated by cyclic voltammetry and controlled potential electrolysis. The role of the substituents on the electrochemical properties and the related over potentials required for CO2 transformation have been analyzed. The complexes carrying only electron withdrawing groups like -CF3, -CN totally lose their catalytic activities toward CO2 reduction, whereas the symmetric -NMe2 substituted and push-pull systems (containing both -NMe2 and -CF3) still display electrocatalytic current enhancement under CO2 atmosphere. The complexes carrying a phenyl or a phenol group in position 4 show catalytic behaviors similar to those of simple M-bpy systems. The only detected reduction product by GC analysis is CO: for example, fac-Re (bpy-4,4′ -NMe2)(CO)3Cl gives CO with high faradic efficiency and a TON of 18 and 31, in absence of external proton source and with 5% MeOH, respectively. DFT calculations were carried out to highlight the electronic properties of the complexes; results are in agreement with experimental electrochemical data.

Keywords: CO2 electroreduction, Mn complexes, Re complexes, DFT calculations, bipy ligands, homogeneous catalysis, electron-withdrawing, electron-donating

# INTRODUCTION

Nowadays the CO2 concentration in the atmosphere is continuously increasing alongside with the overall world energy demand. Converting carbon dioxide via electrochemical reduction into useful chemicals and fuels for energy storage is an attractive and promising approach. CO2 reduction is a competition between its thermodynamic and kinetic: the one electron reduction in water occurs at a very negative potential (−1.90 V vs. SCE at pH 7) (Hammouche et al., 1988; Saveant, 2008) because it requires a drastic change in the geometry, from the linear CO2 molecule to the bent CO−• 2 radical anion. The reason of such high negative overpotential is due to the slow kinetics of the electron transfer, which is associated to the different geometries of the neutral and reduced species, respectively. However, the reduction reactions involving multiple electron transfers coupled with proton transfers provide a significant lowering of the thermodynamic barrier. To avoid the CO−• 2 as intermediate and to lower the energy cost of the reduction process, key catalytic strategies have been developed with the aim of obtaining the various products selectively. The best electrocatalysts currently studied work at a potential 100 mV negative with respect to E0 CO2/P (where P generically indicates the reduction products, CO, HCOOH, HCHO, CH3OH, CH4) (Francke et al., 2018; Franco et al., 2018). Despite the numerous advantages of heterogeneous electrocatalysis (Sun et al., 2016; Rotundo et al., 2019a), clever integration with the homogeneous counterpart allows a rational design of the catalysts, by tuning both the metal center and/or the ligand. One of the greatest challenge of the homogeneous approach lies in the search of stability, durability and improved turnover number (TON) efficiencies (Grice, 2017; Takeda et al., 2017; Wang et al., 2019). Bipyridine transition metal complexes represent one of the most studied classes of molecular electrocatalysts since the 1980s (Stanbury et al., 2018). Bipyridine has been extensively studied as ligand in the field of electro and photo-catalysis because of the capability to store electrons and subsequently delocalize electronic density on its π orbitals (Vlcek, 2002; Elgrishi et al., ˇ 2017). Both fac-Re(bpy)(CO)3Cl and fac-Mn(bpy)(CO)3Br are capable of reducing CO2 to CO with high faradaic efficiency (Hawecker et al., 1984; Bourrez et al., 2011). Comparing these two complexes, Mn exhibits a catalytic peak that is shifted more anodically (around 300 mV, Ep = −1.51 V vs. Ag/AgCl in MeCN) in comparison with the second reduction of Re analogous (Ep = −1.8 V vs. Ag/AgCl in MeCN). Another important difference is that usually the Mn-bpy complexes show their catalytic activities only in the presence of external proton sources. To better shed light on this uniqueness, our research group synthesized Mn(pdbpy)(CO)3Br (Franco et al., 2014, 2017), in which two pendant phenolic groups act as local proton source (Costentin et al., 2012), capable of reducing CO2 even in anhydrous acetonitrile. In this case a considerable amount of HCOOH was also detected. Conversely the analogous complex in which the OH groups were replaced by methoxy groups did not show any catalytic activity without the addition of Brønsted acid.

The electrochemical behavior of the two well-known Re and Mn-bpy complexes could be reasonably altered by varying the bipyridine moiety, i.e., introducing electron-withdrawing and electron-donating groups (Machan et al., 2014; Walsh et al., 2015; Stanbury et al., 2018). Kubiak and his group investigated the effect of 4,4′ -di-tertbutyl-2,2′ -bipyridine (tBu2-bpy) firstly on rhenium carbonyl complexes, secondarily on manganese (Smieja and Kubiak, 2010; Smieja et al., 2013). In other works they studied the role of the modification in the 6,6′ position of the bipyridine (Sampson et al., 2014; Sampson and Kubiak, 2016). A similar approach has already been applied to Mo and W-bpy complexes (Franco et al., 2017; Rotundo et al., 2018), to both Mn and Rebpy complexes by some of us (Franco et al., 2017; Rotundo et al., 2019b) and in the current work.

The target of the modification is the reduction potential of the catalyst, namely E 0 cat: usually a less negative E 0 cat corresponds to a decreased rate of CO2 conversion (Francke et al., 2018). Electronic properties of organic groups are commonly described by the inductive (±I) and mesomeric (±M) effects. Substituents with electron withdrawing groups like -CF3 (strong –I effect) and -CN (weaker –I effect) and with electron donating groups, like - N(Me)2 (strong +M effect), -Ph and -PhOH (weaker +M effect), were placed in the 4,4′ , 4,6, and 5,5′ positions of the bipyridine ligand coordinated to the metals. Combining both push and pull effects in the so called "push-pull" system, an electronic gradient is forced through the bipyridine. In this paper we explore the electrocatalytic properties of novel Mn and Re bpy-type complexes, bearing 7 differently substituted ligands (**Scheme 1**). More generally electron-donating groups are expected to convey greater nucleophilicity to the metal-center, although catalysis should require higher overpotentials. DFT calculations have been used as complementary tool to better correlate experimental electrochemical data, whereas Controlled Potential Electrolysis (CPE) experiments are useful to elucidate the stability and durability of the catalysts in acetonitrile solutions.

### RESULTS

# Synthesis and Structure

Fourteen new 2,2′ -bipyridine metal complexes fac-M(bpy-R)(CO)3X where M = Mn, X = Br or M = Re, X = Cl are reported, namely Mn(bpy-4,4′ -CF3)(CO)3Br (**1a**), Mn(bpy-5,5′ -CF3)(CO)3Br (**1b**), Mn(bpy-4,4′ -CN)(CO)3Br (**1c**), Mn(bpy-4,4′ -CF3-NMe2)(CO)3Br (**1d**), Mn(dpbpy)(CO)3Br (**1e**), Mn(hpbpy)CO)3Br (**1f**), Mn(bpy-4,4′ -NMe2)(CO)3Br (**1g**), Re(bpy-4,4′ -CF3)(CO)3Cl (**2a**), Re(bpy-5,5′ -CF3)(CO)3Cl (**2b**), Re(bpy-4,4′ -CN)(CO)3Cl (**2c**), Re(bpy-4,4′ -CF3-NMe2)(CO)3Cl (**2d**), Re(dpbpy)(CO)3Cl (**2e**), Re(hpbpy)(CO)3Cl (**2f**), Re(bpy-4,4′ -NMe2)(CO)3Cl (**2g**).

The ligands and the corresponding Mn and Re complexes have been synthesized according to the procedure reported in the experimental section. The complexes have been characterized by NMR, IR spectroscopy and elemental analysis. Single crystal X-Ray diffraction structures have been solved for **2e** and **2f** (for XRD data see **Tables S1**–**S6**).

The complex Re(dpbpy)(CO)3Cl (**2e**) crystallized from both acetonitrile and benzene solutions by slow evaporation, forming prismatic orange platelets of a phase of the pure molecular product (structure A, **Figure S1a**) and a solvate with two benzene

molecules (structure B, **Figure S1b**). The first presents the monoclinic centrosymmetric P21/a space group (**Figure S1c**) while the second the monoclinic centrosymmetric P2/n space group (**Figure S1d**). The complex Re(hpbpy)(CO)3Cl (**2f**) crystallized in dark by slow evaporation of solutions of toluene and benzene as yellow platelets and from ethyl acetate as orange prisms, both stable to air (**Figure S2a**). The crystal structure has been obtained from a platelet obtained from toluene solution and has monoclinic P21/n space group type (**Figure S2b**). The structures of both **2e** and **2f** present a quite distorted octahedral geometry around the rhenium center, as can be seen by the values in **Table 1**. The coordination bond of N1 is 0.1 Å longer than that of N2 (see **Figure 1** for numeration) and a similar asymmetry can be observed in the coordination of CO in trans position to the nitrogens. This asymmetric coordination, also present in other terpyridine derivatives (Anderson et al., 1990; Civitello et al., 1993; Wang et al., 2013; Klemens et al., 2016) is completely different from the very symmetrically bonded bpy derivatives (with N-Re average distances equal in the two coordinating pyridyl rings and long 2.17 Å). At the same time, while most of the 2,2′ -bipyridine derivatives of fac-Re(CO)3Cl complex unit are almost planar respect to the basal OC–Re–CO plane (Kurz et al., 2006; Smieja and Kubiak, 2010; Bullock et al., 2012; Machan et al., 2014; Manbeck et al., 2015), in the case of **2e** and **2f**, the ligand is distorted outside this plane (see **Table 1**). This behavior can be detected in all the terpyridines, in which the third pyridine ring is not coordinated to the metal center and is equivalent to a phenyl ring, and in 6-phenyl substituted 2,2′ -bipyridine derivatives. The reason of such distortion is clarified by considering the steric hindrance of the vicinal phenyl ring (the distance between the nearest CO group and the centroid of the phenyl ring in ortho to N1 is about 3.2 Å) that pushes up all the framework of the organic ligand, modifying the Re environment. This phenyl group is rotated to follow the shape of carbonyls with a torsion angle respect to the central pyridine ring of about 130◦ .

The distortion effects observed in the solid state are predicted also by molecular DFT calculations. Indeed, the optimized geometries of **2e** and **2f** perfectly overlap with the experimentally determined structures (**Figure 1D** and **Figure S3**), confirming that these anomalies originate from the coordination of 6-phenyl substituted 2,2′ -bipyridine ligands to carbonyl complexes and are not induced by crystal packing contributions.

The crystal packing of **2e** (both structure A and structure B) and **2f** is dominated by weak C–H···Cl, C–H···O and π···π stacking interactions. In the case of **2f**, the presence of - OH group on the organic ligand interacting with the chloride induces the formation of hydrogen-bonded molecular chains (see **Figure 1C** and **Figure S2c**). For a more detailed analysis on the crystal packing consults the Supplementary Material (see **Figures S4–S9**).

### Cyclic Voltammetry Under Ar

Cyclic Voltammetries (CVs) of all manganese complexes are reported in **Figure 2**. The CV of the Mn(bpy)(CO)3Br under our experimental conditions (in black) is included for comparison. This complex undergoes two successive irreversible reduction reactions and two reoxidation peaks (Bourrez et al., 2011). The first (Ep1 = −1.29 V vs. Ag/AgCl) and the second (Ep2 = −1.51 V vs. Ag/AgCl) reduction processes lead to the formation of the dimer and the mononuclear pentacoordinated anion species, respectively. Reoxidations of the pentacoordinated anion and of the dimer are located at −1.09 and −0.21 V vs. Ag/AgCl, respectively. The new synthesized complexes are supposed to display similar electrochemical behavior. **Table 2** reports the peak potentials of the first and second reductions. As expected, the presence of –CF3 (**1a** and **1b**) and –CN (**1c**) shifts first and second potentials toward more positive values, when compared to Mn(bpy)(CO)3Br. In a recent paper (Rawat et al., 2019) DFT calculations suggested that electron-withdrawing substituents like -CF3 stabilize the radical anion. Furthermore,

the formation of all Mn-Mn dimers was indicated as unfavorable. However, the electrochemical mechanism outlined above is commonly accepted, and the Mn dimer is strongly favored. We experimentally found that all complexes undergo a first and second chemically irreversible reductions. For example, **1a** shows two chemical irreversible processes followed by the reoxidations of the pentacoordinated radical anion and that of the dimer (**Figure S10**), even at high scan rates (1 V/s, **Figure S11**), thus confirming the general mechanism. CV of **1g** confirms that the strong electron-donating properties of dimethyl amino group result in more negative reduction potentials with respect to Mn(bpy)(CO)3Br. The push-pull system **1d**, **1e** and **1f** display similar potential values to the unsubstituted bpy-complex. In some complexes decomposition processes occurring after reduction generate small peaks (i.e., for **1e** and **1f** Ep = −1.65 and −1.63 V vs. Ag/AgCl are observed, respectively). The very negative peak around −3V vs. Ag/AgCl, is commonly assigned to a ligand-centered reduction.

CVs of all rhenium complexes are reported in **Figure 3**. CV of the reference Re(bpy)(CO)3Cl under our experimental conditions is included for comparison. This complex exhibits a first reversible reduction, due to the formation of the radical anion, which is more stable than the analogous with manganese, thus resulting in electrochemical reversibility and no presence of the reoxidation peak of the dimer (Hawecker et al., 1984). The second reduction leads to the pentacoordinated anion. Intrinsically, rhenium complexes require slightly higher overpotentials with respect to the corresponding manganese ones. The first reversible reduction of Re(bpy)(CO)3Cl is located at E1/2 = −1.35 V vs. Ag/AgCl, whereas the second chemically irreversible reduction is at Ep2 = −1.80 V vs. Ag/AgCl. Similarly to manganese, the new synthesized rhenium complexes show no significant difference in the electrochemical pathway under Ar when compared to the reference Re(bpy)(CO)3Cl (**Table 2**). Electron withdrawing groups (**2a**, **2b,** and **2c**) shift the reduction processes toward less negative values; electron donating groups like in **2g** essentially merge the first and second reductions into a single peak. The push-pull system **2d**, in analogy with the case of **1d**, do not significantly alter the positioning of the reduction potentials. The third peak, around −2.5V, also for these complexes, is generally attributed to a bpy-centered reduction.

The reduction potentials estimated by DFT calculation are in excellent agreement with the experimental values obtained for the first reduction potential of all rhenium complexes (**Figure 4**). This confirms the reversible nature of the first reduction process, leading to stable radical anions characterized by a very small increase (about 0.05 Å) in the Re–Cl bond length (**Figure S12**). Conversely, in the case of manganese complexes, DFT calculations, which compute thermodynamic reduction potentials, are not suitable to estimate the irreversible electrochemical behavior of the compounds (**Figure S13**). Indeed, the high instability of the radical anion leads to the weakening of the Mn–Br bond, as clearly evidenced by the significant increase of the Mn–Br bond length in the anion structures (**Figure S14**). Our DFT calculations that include weak interactions, are in agreement with the chemically irreversibility of the first reduction and dimer production, even in the case of electron-withdrawing substituents. For example, the formation of the dimer [Mn(bpy)(CO)3]2 from its radical anion is favored by 143.5 kJ/mol, and in the case of **1a** is favored by 102.6 kJ/mol.

### Cyclic Voltammetry Under CO2

The electrochemical behavior of manganese complexes under CO2 and with H2O (5%v) is reported in **Figure 5** for **1d** to

**1g** and in **Figure S15** for **1a**, **1b,** and **1c**, (these complexes are catalytically inactive toward CO2 reduction). CVs of **1a-1g** under CO2 with 5%v MeOH are included for comparison in **Figure S15** too. All complexes do not show significant current increases switching from Ar to CO2 atmosphere. TABLE 2 | Reduction peak potentials [V] from the CV of Mn and Re complexes.

While this is expected for manganese complexes, **1g** shows a current increase, though limited, even in absence of Brønsted acid addition.

The electrochemical behavior of rhenium complexes under CO2 and with 5%v MeOH is reported in **Figure 6** for **2d** to **2g** and in **Figure S16** for **2a**, **2b,** and **2c**. CVs of **2a-2g** under CO2 with 5%v H2O are included for comparison in **Figure S16** too. All complexes exhibit catalytic current switching from Ar to CO2 atmosphere, even in absence of Brønsted acid, as expected for rhenium complexes.

### Controlled Potential Electrolysis Experiments of the Complexes

Bulk electrolysis experiments of all manganese and rhenium complexes series under CO2 were performed upon setting the potential at values slightly negative to the second reductions, with and without external added Brønsted acids (water and methanol, 5%). A CO2 flow of 50 mL min−1 was kept constant during the experiments, gaseous products were determined by gas chromatography, and formate, if present, was assessed by NMR spectroscopy at the end of the experiments. **Table 3** summarizes the results obtained during these CPE experiments. A general trend can be outlined: addition of water results in increased TONs for Mn complexes (**Table 3** and **Figure S17**), differently from methanol, which drops them to lower values (**Table S7**). On the other hand, for the case of Re complexes, addition of methanol (**Table 3** and **Figure S18**) seems to enhance the catalytic activity with respect to water, except for complex **2d**, which is catalytically inactive, despite the promising current increase in presence of MeOH under CO2.

# CONCLUSIONS

In summary, a systematic study of the effect of the electronic properties of the substituents on 2,2′ -bipyridine Mn and Re complexes was conducted. Electron-withdrawing substituents shift the reduction potentials to more positive values, and

eventually inhibit the catalytic activities of the corresponding Mn and Re complexes toward CO2 reduction. In the case of electron-donating substituents the opposite trend is observed. These observations are in agreement with the induced electron density localized on the metal, strongly influencing the change in the reactivity with the weak electrophile CO2. Increasing or decreasing the electron density on the metal should facilitate or prevent the formation of intermediate in which the CO2 is coordinated to the metal. Another interesting effect in varying the electron properties of the substituents is the merging of the first and second reduction processes, observed in some Re and Mn complexes. A judicious selection of bpy substituents provides an alternative way to the use of bulky substituents to prevent dimer formation (Sampson et al., 2014), with the aim of transforming two 1e reduction into a single **2e** reduction process (CO2 to CO reduction requires **2e**).

It is interesting to note that in spite of the presence of a - OH group in **1f**, no formic acid is detected, probably because in contrast with Mn carrying local proton sources (Franco et al., 2014, 2017) the hydroxyl group is located far from the metal center, thus the generation of the metal-hydride, commonly considered catalyst for formate production, is no longer entropically favored. It is interesting to note how CVs under CO2 of the push-pull systems **1d** and **2d** show enhanced catalytic currents; during CPE the Mn derivative **1d** displays the higher TON value, while the corresponding Re derivative **2d**, albeit from CV appears to be a potentially highly active catalyst, undergoes decomposition. While Mn catalysts suffer from the presence of MeOH, they appear to work better in water, which seems to react promptly with reduced Mn. In fact, even if the reduction potential of **1g** is rather negative (**Table 2**), no hydrogen is produced. This is in line with the high TON values observed for the CO2 electrochemical reduction in pure water by Mn electrocatalysts supported on electrode surface (Walsh et al., 2015; Reuillard et al., 2017; Rotundo et al., 2019b). DFT calculations performed with dispersion correction agree with

the experimental data for both Mn and Re complexes. The Mn–Br bonds computed for the Mn radical anions undergo a significant elongation, around 0.2 Å, which indicate nonnegligible weakening of the Mn–Br bonds. The release of halogen is also very probably favored by the polar and coordinating solvent MeCN; indeed Br substitution by MeCN has been experimentally observed not only in the radical anion, but also in neutral Mn species (Franco et al., 2017). We demonstrated here how the appropriate choice of the electron properties of the ligands is of critical importance in the design of more effective bipyridine Mn and Re electrocatalysts for CO2 reduction.

# MATERIALS AND METHODS

### General Considerations