the direct conversion 
 of solar energy to a chemical fuel is an 
 essential part of future sustainable energy systems that are independent 
 of fossil reserves .
hydrogen is an environmentally 
 benign energy carrier of high - energy density and can be produced by 
 photocatalytic water reduction .
platinum 
 and other noble metals can serve as heterogeneous hydrogen evolution 
 catalysts ; however , their limited earth abundance and cost precludes 
 further development and/or large - scale applications . on the other hand ,
organometallic compounds are attractive catalysts 
 for this transformation considering the variety of complexes that 
 can be prepared , the synthetic ease with which electronic properties 
 ( and hence catalyst activity ) can be modulated , and the ability to 
 study their mechanisms in detail . in nature , hydrogenase enzymes , 
 especially those containing fe2 active sites , efficiently 
 catalyze proton reduction to molecular dihydrogen .
models 
 of the [ fefe]-hydrogenases can act as biomimetic hydrogen evolution 
 catalysts , although they often suffer 
 from limited stability , especially when catalysis is driven photochemically 
 in conjunction with photosensitizers .
the 
 necessity of an external matrix to stabilize the catalyst is thus 
 evident .
metal  organic frameworks ( mofs ) , also referred 
 to as porous 
 coordination polymers , have emerged as an intriguing class of microporous 
 crystalline materials due to their intrinsic topology and porosity and have been studied for a range of applications 
 in gas storage / separation , chemical sensing , drug delivery , and 
 catalysis . unlike other porous materials 
 such as zeolites , the organic ligand component of mofs allows for 
 functionalization of internal channels and/or cavities either through 
 direct solvothermal synthesis or by postsynthetic 
 modification reactions that include the metathesis of metal ions and 
 organic linkers under relatively mild conditions .
hupp , cohen , and others have reported on this postsynthetic exchange 
 ( pse ) phenomenon ( also termed solvent - assisted linker exchange , sale ) , 
 including in highly robust mofs , such as the materials of the institute 
 lavoisier , zeolitic imidaozolate framework , and university of oslo 
 ( uio ) materials .
all of these materials are considered to be  inert  
 with exceptionally high thermal and chemical stability , and can provide 
 a robust platform for the incorporation of potentially labile molecular 
 catalysts .
incorporation of catalytic sites into mofs has resulted 
 in heterogeneous 
 catalysts that promote a wide range of organic reactions .
the heterogeneous 
 nature of mof catalysts allows for their easy separation , reusability , 
 and enhanced stability . in the context of light - to - fuel 
 conversion schemes , homogeneous photosensitizers such as ir polypyridine 
 complexes and porphyrins
have been incorporated in mofs and were shown to drive 
 photochemical hydrogen production catalyzed by pt nanoparticles .
organometallic ir and re catalysts have been 
 incorporated as the ligand linker part of the mof and were shown to 
 catalyze ce - promoted water oxidation and photochemical co2 reduction , respectively .
although excellent proof - of - concept studies , 
 in both cases , resource - limited 
 precious metal catalytic sites were used . moreover ,
the scope of these 
 reports is somewhat limited , as the solvothermal procedures that were 
 used for the synthesis of the mofs require organometallic units that 
 are thermally robust .
herein , we describe the incorporation of an 
 organometallic fe2 complex that bears structural resemblance 
 to the active site of [ fefe ] h2ases into a mof .
[ fefe](bdt)(co)6 ( 2 ) ( bdt = benzenedithiolate ) has previously 
 been shown to be an effective proton reduction catalyst in electro- 
 and photochemical schemes .
decoration of complex 2 at the bdt ligand with two carboxylates results in complex 1 which can be introduced into mofs by pse of 1,4-benzenedicarboxylate 
 ( bdc ) ligands , which is a common ligand linker in many mofs ( figure 1 ) .
pse thus allowed for the introduction of a thermally 
 unstable [ fefe](bdt)(co)6 moiety into the thermally stable 
 zr(iv)-based uio-66 mof ( figure 2 ) .
x - ray absorption 
 spectroscopy ( xas ) was used to confirm the coordination environment 
 of the fe2 site in the mof .
importantly , uio-66-[fefe](dcbdt)(co)6 was found to be a highly active hydrogen production catalyst 
 in photochemical arrays with [ ru(bpy)3 ] as 
 a photosensitizer and ascorbate as an electron donor .
the catalytic 
 performance of the mof exceeds that of an analogous homogeneous reference 
 system .
[ fefe ] hydrogenase active site model complexes [ fefe](dcbdt)(co)61 and [ fefe](bdt)(co)62 and the bdc ligand
2,3-dimercaptoterephthalic 
 acid was prepared from benzene-1,2-dithiole 
 via lithiation and carboxylation and directly 
 converted to complex 1 ( [ fefe](dcbdt)(co)6 , dcbdt = 1,4-dicarboxylbenzene-2,3-ditiolate ) by combining the ligand 
 with fe3(co)12 in thf .
single crystal x - ray 
 analysis of complex 1 shows the usual distorted octahedral 
 coordination sphere around the fe ions , with the dcbdt ligand perpendicular 
 to the fe  fe bond vector ( see also esi ) .
the uio-66 framework , 
 consisting of zr(iv)-based secondary building 
 units ( sbus ) and bdc ligand ( zr6o4(oh)4(bdc)6 ) , was chosen for the incorporation of complex 1 because of its exceptionally high structural stability with 
 respect to water and weak acids .
highly crystalline uio-66 was synthesized 
 under solvothermal conditions using zrcl4 , bdc , and benzoic 
 acid ( as a crystal growth modulator ) in dmf for 24 h , followed by 
 washing with meoh and activation under vacuum .
field - emission scanning 
 electron microscopy ( fe - sem ) shows an octahedral morphology of the 
 resultant uio-66 crystals with a particle size ranging from 200 
 to 500 nm ( figure 3 ) .
fe - sem image of ( a ) uio-66 
 and ( b ) uio-66-[fefe](dcbdt)(co)6 .
scale bar is 1 m . attempts to directly include 1 during solvothermal 
 synthesis ( > 50 c ) resulted in decomposition of the cluster , 
 presumably due to the labile bonds between the fe centers and the 
 highly electron - deficient dcbdt ligand .
taking advantage of the structural 
 homology of the bdc and dcbdt ligands in complex 1 ( figure 1 ) , we employed pse as a mild functionalization approach 
 to introduce complex 1 into uio-66 ( figure 2 ) .
optimization of the pse conditions revealed that the use 
 of deoxygenated , ultrapure water ( room temperature for 24
organic solvents , including meoh , dmf , 
 and chcl3 , gave lower incorporation , consistent with previous 
 observations on the solvent dependence of pse .
as expected from the attempted solvothermal syntheses , 
 pse at elevated temperatures ( > 50 c ) gave results that also 
 suggested partial decomposition of 1 .
interestingly , 
 it was found that pse was facilitated by using a highly microcrystalline 
 form of uio-66 that was synthesized in the presence of a benzoic acid 
 modulator .
in contrast , conventionally synthesized uio-66 ( without 
 modulator ) resulted in a less crystalline material and a less efficient 
 exchange process .
the linker - exchanged material , uio-66-[fefe](dcbdt)(co)6 , was isolated as an orange microcrystalline powder after 
 washing 
 thoroughly with fresh meoh and activation under vacuum .
g / cm , measured 
 with nitrogen at 77 k. this value is close to the bet surface area 
 of pristine uio-66 ( 1475  89
g / cm ) , suggesting 
 a true pse process between 1 and the framework , and not 
 simple trapping of the iron complex in the mof pores ( which would 
 be expected to produce a much lower surface area ) .
n2 absorption / desorption 
 isotherms of uio-66 and uio-66-[fefe]dcbdt(co)6 do indicate 
 a modest decrease in pore size distribution ( figure 
 s1 ) .
uio-66 is known to possess two pore types , tetrahedral 
 and octahedral cages , with pore widths of 8 and 11 
  , respectively ( figure s2 ) .
pristine uio-66 gave a median pore width of 11.8  , 
 while after incorporation of 1 , a reduction in the median 
 pore width to 10.9  was observed , consistent with incorporation 
 of the [ fe2s2 ] functionality in uio-66-[fefe]dcbdt(co)6 .
powder x - ray diffraction ( pxrd ) patterns before and after 
 pse confirmed the retention of the crystalline uio-66 framework ( figure 4a ) .
fe - sem showed that uio-66-[fefe](dcbdt)(co)6 possesses a nearly identical particle size and octahedral 
 particle morphology compared to uio-66 , again indicative of a pse 
 mechanism ( figure 3 ) .
asterisks 
 indicate remaining benzoic acid ( modulator ) and the black square indicates 
 dcbdt .
the degree of pse was characterized 
 by energy - dispersed x - ray spectroscopy 
 ( edx ) , elemental analysis ( ea ) , thermogravimetric analysis ( tga ) , 
 and proton nuclear magnetic resonance spectroscopy ( h 
 nmr ) .
the ratio of heavy elements in uio-66-[fefe](dcbdt)(co)6 was determined to be 3.52:1:1.01 ( zr : fe : s , normalized to 
 fe ) via edx , which suggests that 14% of bdc in uio-66 was 
 exchanged for [ fefe](dcbdt)(co)6 ( figure 
 s3 ) .
the expected 1:1 fe / s ratio determined by edx also further 
 supports that the cluster is intact within the mof .
treatment of uio-66-[fefe](dcbdt)(co)6 with dilute hf / d - dmso was used to digest the mof but also decomposed [ fefe](dcbdt)(co)6 to dcbdt .
integration of the proton resonances for bdc and 
 dcbdt in the h nmr confirmed the degree of pse at 14% 
 ( figure 4 ) , giving an overall formula for uio-66-[fefe](dcbdt)(co)6 as zr6o4(oh)4(bdc)5.1([fefe](dcbdt)(co)6)0.92ch3oh .
based on this given formula , c / h / n / s elemental analysis also 
 confirmed the degree of functionalization [ c(% ) : 32.75 ( obs ) , 32.80 
 ( calcd ) ; h(% ) : 1.75 ( obs ) , 1.68 ( calcd ) ; n(% ) : 0.00 ( obs ) , 0.00 ( calcd ) ; 
 s(% ) : 2.97 ( obs ) , 2.84 ( calcd ) ] . unlike pristine uio-66 , which displays 
 only one major decomposition step at 400 c , the tga 
 trace of uio-66-[fefe](dcbdt)(co)6 exhibits two decomposition 
 steps at 80200 and 350400 c , 
 respectively ( figure 4 ) .
the first mass loss 
 is likely due to partial thermal liberation of the carbonyl ligands 
 attached to fe centers ( obs : 7% ; calcd : 7.2% ) .
both bdc and [ fefe](dcbdt)(co)6 start to decompose at 350 c , leading to a combination 
 of zro2 and fe2o phases ( obs : 46.3% ; calcd : 
 43.6% , percent weight residual mass ) . to confirm that compound 1 was being incorporated 
 into the framework lattice to give uio-66-[fefe]dcbdt(co)6 ,
additional experiments were performed to exclude the possibility 
 that compound 1 was merely trapped in the pores of the 
 mof . in one experiment ,
compound 2 ( figure 1 ) contains the same cluster core but lacks the coordinating 
 carboxylate ligands required for mof formation and hence pse .
incubation 
 of uio-66 with compound 2 showed no evidence of substantial 
 incorporation into the mof as shown by a lack of color change ( figure s4 ) and a low iron content in the edx 
 analysis ( figure s5 ) . in a second experiment , 

pse between uio-66 and 1 was performed in d2o , and the presence of bdc was observed in the reaction solution , 
 as determined by h nmr ( figure s6 ) , indicative of displacement of bdc by 1 .
importantly , 
 uio-66 in d2o in the absence of 1 does not 
 show release of free bdc into solution .
these nmr observations are 
 also indicative of a ligand metathesis process and argue against simple 
 inclusion of 1 into the pores of the mof . finally , as 
 stated above , performing pse between uio-66 and 1 in 
 other solvents ( meoh , dmf , and chcl3 ) was not efficient , 
 achieving negligible incorporation ( < 2% ) , consistent with the known 
 solvent dependence of pse processes .
if 
 complex 1 was only being included into uio-66 via sorption 
 into the pores , then inclusion would not be expected to be strongly 
 solvent dependent . taken together , these experiments provide strong 
 evidence , consistent with reported pse studies , that the iron cluster 
 is becoming part of the uio-66 framework via a ligand pse process 
 and that the data do not support simple inclusion of the cluster into 
 the pores of the mof . in order to further demonstrate the incorporation 
 of the intact 
 fe2s2 dinuclear cluster into the mof , we employed 
 fourier - transformed infrared spectroscopy ( ftir ) and diffuse reflectance 
 uv  vis spectroscopy .
ftir of uio-66-[fefe](dcbdt)(co)6 exhibited three prominent co stretching vibration bands at 2078 , 
 2038 , and 2001 cm , while no such absorption bands 
 were observed for pristine uio-66 material between 2100 and 2000 cm ( figure 4 ) .
moreover , the 
 relative intensity of these three characteristic bands was identical 
 to that of free 1 , suggesting the dinuclear cluster is 
 intact in the mof .
solid - state uv  vis spectroscopy of uio-66-[fefe](dcbdt)(co)6 also showed a characteristic absorption at 350 nm , 
 which is consistent with the spectral features of 1 ( figure s7 ) . due to the potentially labile 
 nature of [ fefe](dcbdt)(co)6 , we sought to provide data 
 to confirm the coordination environment 
 of fe2s2 core in the mof .
fe k - edge extended 
 x - ray absorption fine structure spectroscopy ( exafs ) was performed 
 on both 1 and uio-66-[fefe](dcbdt)(co)6 .
as 
 shown in figure 5 , fourier - transformed exafs 
 in r space revealed nearly identical coordination environments of 
 the fe centers in [ fefe](dcbdt)(co)6 before and after pse 
 into the uio-66 framework .
both sets of data were best fit using the 
 first and second neighboring atoms of fe from the single - crystal x - ray 
 structure obtained for [ fefe](dcbdt)(co)6 , where fe centers 
 occupy a distorted octahedral geometry ( see esi ) .
exafs of uio-66-[fefe](dcbdt)(co)6 suggests three carbon atoms from carbonyl groups and two 
 sulfur atoms bridging the dinuclear fe2 center at distances 
 of 1.7961.814 and 2.2832.285  , 
 respectively ( table 1 ) .
importantly , these 
 bond lengths are in good agreement with the crystallographic data 
 of 1 ( see esi ) , showing 1.7971.811  
 ( fe  c ) and 2.2552.257  ( fe  s ) 
 .
in addition , x - ray absorption near - edge structure indicates a common 
 fe(i ) oxidation state of the cluster within the framework and in 1 ( figure s8 ) . fe k - edge exafs fourier 
 transforms and exafs spectra ( inset ) for 
 ( a ) 1 and ( b ) uio-66-[fefe](dcbdt)(co)6 .
solid 
 black lines show the experimental data , dashed red lines show the 
 fits based on crystallographic data of 1 , and dotted 
 gray lines show the fitting 
 window . fitting 
 data
having observed that pse could be used to incorporate 
 complex 1 into a robust mof , we explored the suitability 
 of this material 
 as a catalyst in photochemical hydrogen production schemes .
thus , 
 uio-66-[fefe](dcbdt)(co)6 was suspended in a 1.0 m acetate 
 buffer solution of [ ru(bpy)3 ] photosensitizer 
 ( 0.5 mm ) , and ascorbate electron donor ( 100 mm ) at ph 5 . as depicted 
 in figure 6 , the projected sequence for photocatalytic 
 proton reduction by uio-66-[fefe](dcbdt)(co)6 commences 
 with the reductive quenching of photoexcited [ ru(bpy)3 ] by the electron donor ascorbate with a rate constant of 
 1  10 m s. following the analysis of schmehl 
 et al .
, 14% of * [ ru(bpy)3 ] excited states can be expected to form the [ ru(bpy)2(bpy ) ] reductant 
 ( [ ascorbate ] = 0.1 m ; (*[ru(bpy)3 ] ) 
 = 500 ns ) .
charge recombination between photogenerated [ ru(bpy)3 ] and oxidized ascorbate can be expected to occur 
 close to the diffusion limit and will 
 compete with the productive heterogeneous electron transfer ( et ) between 
 [ ru(bpy)3 ] and uio-66-[fefe](dcbdt)(co)6 .
the driving force for the et can be estimated at 300 
 mv from the electrochemically obtained reduction potentials , assuming 
 that the reduction potential of the fe2 site in uio-66-[fefe](dcbdt)(co)6 is similar to that obtained for complex 1 in 
 solution . from solution studies
, it is well established that the electrochemical 
 reduction of 1 , and its bdt derivative 2 , is a two - electron process due to inverted electrochemical potentials . 

assuming that the fe2 complex in uio-66-[fefe](dcbdt)(co)6 shows similar reductive chemistry as complex 2 in solution , the driving force for electron transfer from a second 
 photogenerated [ ru(bpy)3 ] to the previously 
 produced monoreduced fe2 site in the mof will be even > 300 
 mv . the dianionic fe2 site 1
a second plausible 
 pathway to 1 is through disproportionation 
 of two singly reduced 1 in the mof . 

this disproportionation is thermodynamically feasible as evidenced 
 by the inverted electrochemical potentials of the 1/1 and 1/1 couples .
reaction scheme for the 
 photocatalytic reduction of protons . as demonstrated in figure 7 , uio-66-[fefe](dcbdt)(co)6 is indeed a proton reduction catalyst . under the reaction 
 conditions described above , hydrogen production is observed and can 
 be quantified with a hydrogen specific solid - state sensor ( see esi 
 for details ) .
it is thus clear that heterogeneous electron transfer 
 between photogenerated [ ru(bpy)3 ] and uio-66-[fefe](dcbdt)(co)6 can compete with homogeneous charge recombination with oxidized 
 ascorbate .
the fe2 sites within the mof can be reduced 
 in a light - driven reaction and are themselves catalysts for the reduction 
 of protons to molecular hydrogen . as the rate of electron transfer 
 from photogenerated [ ru(bpy)3 ] decreases exponentially 
 with distance , it can be assumed that only fe2 sites that 
 reside within a few nm from the surface of the mof particles will 
 be viable acceptor sites .
the de facto concentration of operating 
 catalysts in the mof may thus be substantially smaller than the total 
 concentration of 1 in the mof . comparing the activity 
 of uio-66-[fefe](dcbdt)(co)6 with that of a homogeneous 
 reference system that contains complex 1 at concentrations 
 similar to the total amount of fe2
complex in uio-66-[fefe](dcbdt)(co)6 shows that the activity of the former is not only preserved 
 but actually exceeds that of the latter , both in terms of initial 
 rate as well as overall amount of produced hydrogen .
control experiments without uio-66-[fefe](dcbdt)(co)6 or with unmodified uio-66 ( which does not contain [ fefe](dcbdt)(co)6 ) do not show meaningful amounts of hydrogen generation ( figure 7 ) .
photocatalytic hydrogen production in the presence of 
 uio-66-[fefe](dcbdt)(co)6 ( blue trace , 5 mg mof , 0.59 
 mol catalyst ) , 
 complex 1 ( red , 0.59 mol ) , uio-66 ( black , 
 5 mg mof ) , and background ( gray ) .
conditions : 1 m acetate buffer ph 
 5 , 100 mm ascorbic acid , 0.5 mm [ ru(bpy)3 ] .
quantitative comparison between the homogeneous and heterogeneous 
 hydrogen production systems must be done with great care , as hydrogen 
 production in both systems is not limited by an intrinsic step of 
 the catalytic cycle but by insufficient photoproduction of the [ ru(bpy)2(bpy ) ] reductant .
nevertheless , figure 7 clearly shows that the heterogeneous system outperforms 
 the homogeneous one both in overall hydrogen production as well as 
 initial rate .
as shown in a recent study , the photoproduction of [ ru(bpy)2(bpy ) ] is not strongly influenced by homogeneous complex 2(39 ) and probably also not by uio-66-[fefe](dcbdt)(co)6 .
the amount of available reductant can thus considered to 
 be comparable in both systems . also , the heterogeneous et rate constant ket , het in figure 6 is 
 presumably not higher than the corresponding ket , hom in the homogeneous reference system .
therefore , the 
 reasons for the superior catalytic performance of uio-66-[fefe](dcbdt)(co)6 compared to that of the homogeneous reference system must 
 be due to differences in the catalyst .
a trivial but nevertheless 
 important rationale for the increased hydrogen production yield in 
 the mof is the stabilization of the catalyst when inside the framework . 
 supporting this notion ,
it was found that uio-66-[fefe](dcbdt)(co)6 recovered after 1 h of photocatalysis still shows the characteristic 
 co bands in the ir spectrum ( figure s10 ) .
in contrast , and in accordance with published work , complex 1 decomposes under identical 
 photocatalysis conditions , as evidenced by the lack of any ir signals 
 in the typical co region .
as for all photochemical reduction 
 schemes based on [ ru(bpy)3 ] , one - electron 
 photochemistry needs to be coupled 
 to a two - electron catalytic process . with the limited availability 
 of reductant ,
most productive et events will produce singly reduced 
 fe2 sites ( 1 ) , while dianionic 
 fe2 species ( 1 ) are unlikely 
 to be formed by an encounter with a second equivalent of [ ru(bpy)2(bpy ) ] due 
 to the low concentrations of both species . as discussed above , it 
 is thermodynamically feasible that two 1 sites disproportionate to form the catalytically active 1 site that reacts with two protons to form hydrogen . 

disproportionation as well as protonation needs to occur before the 
 reduced species recombine with oxidized ascorbate .
here , the fe2 site in uio-66-[fefe](dcbdt)(co)6 has an undisputable 
 advantage over the homogeneous system , as its incorporation into the 
 mof spatially protects from unproductive charge recombination .
moreover , 
 the presence of many fe2 sites within the mof may promote 
 disproportionation as soon as two monoreduced sites are present .
it 
 is these kinetic advantages that explain the higher initial hydrogen 
 production rates in uio-66-[fefe](dcbdt)(co)6 .
we employed pse as an efficient and mild approach to obtain the 
 first mof that contains a multinuclear , organometallic , nonprecious - metal - based 
 proton reduction catalyst .
the resulting uio-66-[fefe](dcbdt)(co)6 is a hybrid material that combines the advantages of molecular 
 catalysts with a highly ordered and stable inorganic support .
[ fefe](dcbdt)(co)6 ( 1 ) is among the most complex structure that 
 has ever been introduced into a mof , and its presence and molecular 
 integrity within the uio-66 framework could be confirmed by exafs , 
 ftir , and other methods .
uio-66-[fefe](dcbdt)(co)6 exhibits 
 high efficiency for photochemical hydrogen evolution , exceeding that 
 of the homogeneous reference system in terms of rate and total hydrogen 
 production yield .
incorporation of the fe2 complex in uio-66-[fefe](dcbdt)(co)6 results in a higher stability under the photocatalysis conditions , 
 protects reduced species from nonproductive charge recombination , 
 and may promote disproportionation reactions to produce catalytically 
 active dianion 1 .
 a molecular 
 proton reduction catalyst [ fefe](dcbdt)(co)6 ( 1 , dcbdt = 1,4-dicarboxylbenzene-2,3-dithiolate ) with 
 structural similarities to [ fefe]-hydrogenase active sites has been 
 incorporated into a highly robust zr(iv)-based metal  organic 
 framework ( mof ) by postsynthetic exchange ( pse ) . 
 the pse protocol 
 is crucial as direct solvothermal synthesis fails to produce the functionalized 
 mof . 
 the molecular integrity of the organometallic site within the 
 mof is demonstrated by a variety of techniques , including x - ray absorption 
 spectroscopy . in conjunction with [ ru(bpy)3]2 + as a photosensitizer and ascorbate as an electron donor 
 , mof-[fefe](dcbdt)(co)6 catalyzes photochemical hydrogen evolution in water at ph 
 5 . 
 the immobilized catalyst shows substantially improved initial rates 
 and overall hydrogen production when compared to a reference system 
 of complex 1 in solution . improved catalytic performance 
 
 is ascribed to structural stabilization of the complex when incorporated 
 in the mof as well as the protection of reduced catalysts 1 and 12 from undesirable 
 charge recombination with oxidized ascorbate . 
Introduction
Results and Discussion
Conclusion