research into the synthesis 
 of noble metal 
 nanoparticles is motivated by the opportunity to harness advantages 
 not present in their bulk equivalents , such as high surface atom to 
 volume ratios , semiconductor properties , and , in some cases , magnetic properties . however , nanosized ir particles have received 
 limited attention from the scientific community compared to that of 
 other noble metal counterparts .
synthetic methods involving highly 
 specified equipment , extreme reaction conditions , and/or specialty chemicals have been the 
 primary culprits of this conundrum . in our recent studies ,
we 
 reported the successful synthesis of dodecanethiolate - capped pd nanoparticles 
 generated from sodium s - dodecylthiosulfate after 
 the cleavage of the sulfite group .
the 
 overall lower reactivity of this ligand precursor compared to alkanethiol 
 allowed the successful synthesis of pd nanoparticles without the formation 
 of oxidized pd(ii ) species during and after the reaction .
further studies from our group using the thiosulfate synthetic method 
 led to the discovery of a way to control ligand surface coverage that 
 has a direct correlation with catalytic activity and selectivity of 
 alkanethiolate - capped pd nanoparticles .
many 
 studies have previously revealed that the indistinguishable metal  sulfur 
 bonds result from the adsorption of thiosulfate precursors and from 
 the direct adsorption of thiols on metal nanoparticles and flat metal 
 surfaces . for ir nanoparticles ,
however , such stable alkanethiolate - capped 
 nanoparticles were unattainable by employing alkanethiols directly 
 as a ligand and nabh4 as a reducing agent thus far . in this article
, we relate a facile synthesis 
 of 1.2  0.3 nm ir nanoparticles by employing the modified 
 brust  schiffrin system along with the aforementioned thiosulfate 
 ligand precursor strategy .
new insights into the mechanistic disparities 
 between sodium s - alkanethiosulfate and alkanethiol 
 are obtained by investigating the formation chemistry of alkanethiolate - stabilized 
 ir nanoparticles using these two organic ligand precursors .
the following reagents were obtained from 
 the indicated suppliers and used as received : tetra - n - octylammonium bromide ( 98% ) , 1-dodecanethiol ( 98% ) , and sodium borohydride 
 ( 98% ) were obtained from acros .
sodium thiosulfate pentahydrate ( na2s2o35h2o ) along with 
 solvents such as toluene , acetone , ethanol , and methanol was purchased 
 from fisher scientific .
water was purified by using a barnstead nanopure diamond ion exchange 
 resins purification unit .
tetra - n - octylammonium bromide ( 2.0 mmol ) was dissolved in 25 ml 
 of toluene .
precisely measured k2ircl6 ( 0.40 
 mmol ) dissolved in 25 ml of water was added to the organic solution . 

the mixture was continuously stirred until the aqueous layer was cleared 
 signifying the completion of the phase transfer .
the organic layer 
 was separated and placed in a 250 ml round - bottomed flask .
sodium s - dodecylthiosulfate ( 0.80 mmol ) along with another portion 
 of tetra - n - octylammonium bromide ( 2.0 mmol ) was added 
 to the round - bottomed flask with the aid of 10 ml of aq methanol ( 40% 
 v / v ) .
the reaction mixture was placed in 60 c water bath and 
 stirred for 15 min .
subsequently , 8.0 ml of a freshly prepared solution 
 of sodium borohydride ( 1 m ) was rapidly poured into the reaction mixture . 
 within 4 min , the color of the reaction changed from red to yellow 
 to brown to finally black indicating the formation of nanoparticles . 

after stirring for 1 h , the nanoparticle mixture was removed from 
 the water bath . upon cooling to room temperature
, the organic layer 
 was washed with several aliquots of nanopure water and isolated using 
 a separatory funnel .
lastly , the nanoparticle crude was extensively washed with ethanol , 
 methanol , and acetone on a course frit funnel ( f ) .
iridium nanoparticles 
 were ultimately recollected by reconstitution with chloroform , in 
 which the solution was vacuum - dried to obtain solid powders .
h nmr spectra were recorded using bruker ac400 ft - nmr 
 operating at 400 mhz .
infrared spectra were acquired 
 by attenuated total reflectance ( atr ) using a perkinelmer spectrum 
 100 ft - ir spectrometer .
transmission electron microscope ( tem ) images 
 were obtained with a jeol 1200 ex ii electron microscope operating 
 at 90 kev .
samples were prepared by placing 25 l of an ir nanoparticle 
 toluene solution ( 1 mg / ml ) on a 200 mesh copper grid with 
 formvar film .
size distribution analysis of ir nanoparticle core microscope 
 images was executed with scion image beta release 2tm .
thermogravimetric analysis 
 ( tga ) was conducted using ta instruments sdt q600 with a flow rate 
 of 100 ml / min of n2 with heating from room temperature 
 to 600 c .
xps measurements were performed using a mg k 
 source ( xr50 , specs gmbh ) and a hemispherical electron energy analyzer 
 ( phoibos 100 , specs gmbh ) .
spectra were acquired with 10 ev pass energy 
 and normalized to the o1s binding energy at 528.5 ev .
similar to the studies by lennox 
 et al . , the mechanistic aspects of the 
 nanoparticle synthesis were evaluated by comparing h nmr 
 and uv  vis spectra of the reaction precursors , intermediates , 
 and final products formed by the direct application of either 1-dodecanethiol 
 or sodium s - dodecylthiosulfate as a ligand precursor 
 ( scheme 1 ) . since the size , composition , and 
 physical properties of the nanoparticles synthesized by the modified 
 brust 
schiffrin 
 reactions are highly dependent upon the specific reaction conditions , 
 additional mechanistic insights into this procedure regarding different 
 types of capping ligands and atypical metals may allow further expansion of available nanomaterials toward nanoparticles 
 with exotic characteristics . at the onset of this synthesis , 

aqueous ircl6 was transferred to toluene - d8 by employing a large excess of tetra - n - octylammonium bromide as a phase transfer agent .
this 
 transfer took place just as observed for gold nanoparticles accompanying 
 the colorimetric change of both aqueous and organic phases ( figure 1 : from a to b ) .
the 
 [ r4n]2[irx6 ] complex in organic phase , however , failed to form a homogeneous 
 solution at room temperature and could only be fully mixed after continuous 
 stirring at a higher temperature of 60 c .
vis spectra 
 of irx6 in the organic phase showed 
 two strong absorption bands at 440 and 500 nm , which correspond to 
 the presence of ir - x ( either cl or br ) bonds , with a 
 weak absorption band at 595 nm ( figure s1 , supporting 
 information ) .
these absorption 
 bands for ir - x appeared at a slightly higher wavelength , 
 compared to that of the same bands observed for ircl6 in water before the phase transfer , but were without 
 any significant change in their intensity .
the h nmr spectra 
 of [ r4n]2[irx6 ] in toluene - d8 revealed the ch2-n peaks at  2.65 ppm , which have 
 been dramatically shifted to more upfield from their original chemical 
 shift of  3.40 ppm ( figure 2a ) .
have reported similar dramatic shifts of ch2-n peaks of the tetra - n - octylammonium complex during the brust 
schiffrin reaction 
 using haucl4 and proposed that this phenomena was due to 
 the formation of inverse micelle structures with aucl4 complexation in their pore .
have shown that the same shifts resulted 
 from the ion - pair complex formation without the presence of an inverse 
 micelle of tetraoctylammonium .
on the basis of the 
 latter hypothesis , our nmr data support the formation of an ion - pair 
 complex of tetra - n - octylammonium with irx6 .
the interaction between the positively charged 
 quaternary ammonium nitrogen and irx6 was the primary reason for the upfield shifts of both ch2-n peaks and ch2-ch2-n peaks of tetra - n - octylammonium 
 salts .
picture of solutions containing ( a ) k2ircl6(aq ) ( 0.4 mmol ) , ( b ) [ r4n]2[ircl6](toluene ) , ( c ) 1-dodecanthiol 
 ( 0.8 mmol ) added to b , ( d ) nabh4 ( 4.0 mmol ) added to c , 
 and ( e ) s - dodecylthiosulfate ( 0.8 mmol ) and nabh4 ( 4.0 mmol ) added to b. h nmr spectra of ( a ) [ r4n]2[irx6 ] in 
 toluene - d8 obtained after mixing the organic 
 ( toluene - d8 ) layer of toab ( 2.0 mmol ) 
 and the aqueous ( d2o ) layer of k2ircl6 ( 0.4 mmol ) , ( b ) 1-dodecanthiol ( 0.8 mmol ) added to a , ( c ) 1-dodecanthiol 
 ( 1.6 mmol ) added to a , ( d ) nabh4 ( 4.0 mmol ) added to c , 
 ( e ) s - dodecylthiosulfate ( 0.8 mmol ) added to a , and 
 ( f ) ir nanoparticle in cdcl3 after purification .
the multiplet 
 resonance at  2.09 ppm is due to the presence of toluene solvent 
 impurity .
the addition of 2 equiv of 1-dodecanthiol 
 under the reaction temperature of 60 c resulted in color change 
 from dark brown to bright yellow as demonstrated in figure 1c and the reaction progression in figure s2 ( supporting information ) .
the color change indicates 
 the probable reduction of ir to ir , which 
 has also been observed from the reduction of ircl6 ( dark brown ) to ircl6 ( brownish yellow ) .
previously , zhu 
 et al . were able to confirm the complete reduction of au to au by the addition of 2 equiv of alkanethiols , which 
 were oxidized to disulfides , during the brust 
schiffrin 
 synthesis of ir nanoparticles using 1-dodecanethiol , which subsequently 
 showed small peaks representative of dodecyldisulfide (  2.54 
 ppm ) and 1-dodecanethiolate (  2.18 ppm ) , along with the ch2-n peaks of tetra - n - octylammonium 
 downshifted back to their original chemical shift of  3.40 
 ppm .
this change in nmr spectra implies that the reduced ir complex 
 is no longer directly associated with tetra - n - octylammonium 
 salts , which now exclusively form the ion - pair complex with halide 
 ions ( figure 2b ) .
clear evidence of the 
 reaction between irx6 with 1-dodecanethiol 
 was shown in figure s3 ( supporting information ) , which illustrates the uv  vis spectra of the reaction mixture 
 after the addition of 4 equivs of 1-dodecanethiol .
the spectra showed 
 complete disappearances of the strong absorption bands at 440 and 
 500 nm , indicating the cleavage of ir - x ( either cl or 
 br ) bonds .
the appearance of absorption 
 bands at 310 nm is most likely due to the formation of ir - sr bonds .
the h 
 nmr spectra of the reaction mixture after the addition of 4 equivs 
 of dodecanethiol shown in figure 2c also support 
 the formation of dodecyldisulfide from 1-dodecanethiol after the reaction 
 with irx6 .
the relative intensity of 
 these peaks at  2.54 ppm and  2.18 ppm was 1 to 3 in 
 favor of dodecanethiolate , confirming the formation of the ir(sr)3 complex .
one equiv of 1-dodecanthiol was , therefore , used 
 for the reduction of ir to ir , and the other 
 3 equivs of thiol participated in the complexation with ir .
the direct evidence of ir - sr bond formation could 
 be obtained from a closer analysis of h nmr spectra .
the 
 peaks at  2.18 ppm exhibited a multiplicity ( triplet of triplet ) 
 rather than a triplet typically observed from ch2ch2-s or a quartet observed 
 from ch2ch2-sh .
spin coupling 
 of ir  h ( j ) and h  h ( j ) for 
 the ir(sr)3 complex . to our knowledge , however , such an 
 interaction of three bond ( ir  s - c - h ) ir  h coupling has never been reported .
interestingly , 
 the h nmr studies of ir  h complexes have only shown 
 the tendency for ir to self - decouple for j coupling .
considering the spin number ( i = 3/2 ) of ir , the coupling of ir  h ( j ) should 
 produce a quartet splitting instead of a triplet splitting as shown 
 in figure 2c , inset . despite this inconclusive 
 nmr splitting
, the presence of this abnormal multiplicity strongly 
 supports the presence of an ir 
h ( j ) interaction 
 and thus the formation of the ir  s bond .
the addition 
 of nabh4 , a highly basic reducing agent , was enacted in 
 order to produce ir nanoparticles from an ir(sr)3 complex . 
 upon the addition of nabh4 , the color of the solution turned 
 into strong orange as shown in figure 1d .
this 
 color change was followed by the formation of insoluble gel - like materials 
 during further stirring of the reaction mixture ( figure s4 , supporting information ) .
moreover , uv  vis 
 spectra of the reaction mixture did not resemble that of a typical 
 nanoparticle solution but instead showed strong absorption bands centered 
 at 330 nm .
this result indicated the probable formation of 
 the hydrolysis product rather than the desired reduction of ir to ir after the addition of basic nabh4 solution .
insoluble ir(oh)6(na)y would precipitate out 
 in toluene as shown in figure s4 ( supporting information ) , and more soluble ir(oh)6(r4n)y would result in 
 the appearance 
 of strong absorption bands at 330 nm . the uv data also suggested 
 the reaction did not involve the reformation of irx6 or the formation of iroxnh2o , which have strong absorption 
 bands at 440/500 nm and 580 nm , respectively .
the nmr spectra of the reaction mixture after 
 the addition of nabh4 shown in figure 2d indicated the full conversion of the dodecanethiolate ligand 
 of the ir(sr)3 complex to its disulfide form in the presence 
 of basic sodium borohydride solution .
in contrast to the alkanethiol , 
 the addition of s - dodecylthiosulfate to the reaction 
 mixture containing the [ r4n]2[irx6 ] adduct did not trigger any change in 
 the solution color and absorption bands of ir - x at 440 
 and 500 nm in uv  vis spectra .
the h nmr spectra 
 of [ r4n]2[irx6 ] in toluene - d8 were also nearly unaffected 
 by the addition of s - dodecylthiosulfate , which showed 
 ch2-s peaks at  3.28 
 ppm ( figure 2e ) .
a slight upfield shift from 
  2.65 ppm to  2.48 ppm was observed for the ch2-n peaks .
since the structure of s - dodecylthiosulfate is highly reminiscent of hydrophobic 
 ionic salts , the formation of an ion - pair complex of [ r4n][rs2o3 ] and the rapid equilibrium with [ r4n]2[irx6 ] would cause such a change 
 in the chemical shifts .
the overall spectroscopic results clearly 
 demonstrated the low reactivity of the thiosulfate ligand precursors 
 compared to thiols against the iridium complex .
the addition 
 of nabh4 to the reaction mixture containing [ r4n]2[irx6 ] and s - dodecylthiosulfate initially turned the solution color 
 from dark brown to faint yellow implying the cleavage of the ir - x 
 bond , but the solution turned into dark black / blown within 5 min of 
 nabh4 addition ( figure s5 , supporting 
 information ) .
this result suggested that the formation of ir 
 nanoparticles is slower than that of other nanoparticles including 
 au , ag , and pd , which undergo color changes into black / blown immediately 
 upon the addition of nabh4 .
vis spectra of the 
 isolated ir nanoparticles showed an exponential decay in absorbance 
 with a decrease in energy , which is a typical characteristic of small 
 nanosized colloids lacking surface plasmon resonance as is the case 
 with ir ( figure s6 , supporting information ) .
the h nmr of the ir nanoparticles 
 isolated and redissolved in cdcl3 revealed three broad 
 resonances at  1.801.60 ,  1.501.20 , 
 and  0.900.80 ppm for the -ch2-ch2-s , -ch2- , and -ch3 , respectively ( figure 2f ) .
these peaks resulted from the attached dodecanethiolate 
 stabilizers on the nanoparticle surface , as peak broadening has been 
 indicative of ligand immobilization for other ligand - capped metal 
 nanoparticles .
interestingly enough , the small 
 core size of these nanoparticles allow the -protons of attached 
 organic ligand stabilizers to be visible ; a rare event only observed 
 for the small and monodisperse metal nanoparticles .
moreover , the high purity of the isolated ir nanoparticles 
 was proven to be attributed to the absence of resonances at  
 3.40 ( tetra - n - octylammonium : ch2-n ) ,  3.28 ppm ( s - dodecylthiosulfate : 
 ch2-n ) ,  2.54 ( didodecyl 
 disulfide : ch2-s ) , and  
 2.18 ( dodecanethiolate : ch2-s ) .
ft - ir spectra of ir nanoparticles were also identical 
 to those of metal nanoparticles previously generated from both dodecanethiol 
 and s - dodecylthiosulfate ( figure s7 , supporting information ) .
only the ch2 and ch3 stretches 
 ( 30002800 cm ) and bendings ( 1450 
 cm ) of dodecanethiolate monolayers were observed 
 in the spectra .
any other significant peaks including sulfonate ( so3 ) stretches at 1350 ( ass = o ; strong ) and 1175 cm ( ss = o ; strong ) were not observed implying 
 the cleavage of the thiosulfate s  s bond after the adsorption 
 of thiosulfate on the ir nanoparticle surface . the tem image and the 
 core size histogram of the final purified product are presented in 
 figure 3 .
the average core size of the ir nanoparticles 
 was determined to be 1.2  0.3 nm by counting a total of 2300 
 particles from multiple images .
the small size and high monodispersity 
 of these nanoparticles are quite novel for ir nanostructures because 
 such cluster - like , stable , and isolable ir nanoparticles have never 
 been reported previously .
the overall organic and metallic weight 
 fractions could be determined by thermogravimetric analysis ( figure 
 s8 , supporting information ) .
the initial 
 major volatilization of these iridium nanoparticles took place close 
 to 150 c , which is quite similar to that in our prior 
 studies involving palladium nanoparticles .
after the final temperature of 600 c was reached , 61.5% of 
 the residual sample was still present signifying an organic fraction 
 of 38.5% for these ir nanoparticles .
xps spectra of the ir4f7 ( 60.8 ev ) and 
 ir4f5 ( 63.8 ev ) region for ir nanoparticles are shown in 
 figure 4a .
the 
 broad xps s2p signal at 162.2 ev ( figure 4b ) revealed the presence of an ir 
small traces of 
 oxidized s , which are clearly differentiated from the high intense 
 peaks observed for oxidized s species of the thiosulfate ligand precursor 
 ( figure 4c ) , were observed at 1668 
 ev . since the absence of unbound thiosulfate 
 was confirmed by nmr and ir , the small amount of oxidized s species 
 observed in xps spectra was most likely due to the small presence 
 of chemisorbed oxidized sulfur species such as sulfur trioxide or 
 sulfite that were cleaved off from thiosulfate precursors .
this result demonstrated that the removal of surface adsorbed oxidized 
 sulfur species is somewhat incomplete for ir nanoparticles .
( a ) ir4f xps spectra and ( b ) s2p xps spectra of dodecanethiolate - capped 
 iridium nanoparticles ( c = 2 mg ml ) .
ultimately , one of the main reasons this reaction produces 
 stable and isolable ir nanoparticles was that all necessary reagents 
 ( the ir complex , ligands , and reducing agent ) for nucleation  growth 
passivation 
 of ir nanoparticles form ionic pair complexes with tetraoctylammonium 
 salts and , therefore , are present at close proximity to each other . 

the activation energy of the reaction is likely decreased thereby 
 aiding the completion of the nanoparticle stabilization stage . in 
 comparison ,
the thiol protocol involves the nonionic ir(sr)3 complex as a key intermediate , and the hydrophobic alkyl tail parts 
 of tetraoctylammonium might act as a kinetic barrier for the further 
 reduction of ir to ir by ion - paired bh4 .
small ir clusters have been predicted 
 to have magnetic moments larger than 0.5 b / atom . however , based on our knowledge , there has never 
 been any published report on experimental work related to stable and 
 unsupported ir nanoparticles with a detectible magnetic moment .
the 
 successful synthesis of small ir nanoparticles capped with dodecanethiolate 
 ligands allowed us , for the first time , to examine the magnetic properties 
 of the cluster - like ir particles .
macroscopic magnetization measurements 
 have been performed at temperatures ranging from 5 to 300 k using 
 quantum design physical property measurement system .
the ir nanoparticles 
 were found to have a relatively large magnetic moment which increases 
 with stronger external fields as shown in figure 5a .
the saturation magnetization , determined using tga analysis 
 of the nanoparticle sample , was found to be 12.2 emu / g of ir at 300 
 k and 13.0 emu / g of ir at 5 k. this value is higher than the saturation 
 magnetization of dendrimer - encapsulated ni nanoparticles and other alkaenthiolate - capped metal nanoparticles . the preservation of the magnetic signal from 5 to 300 k ( only a 
 decrease of 6% ) is observed .
the hysteresis loop at 300 and 
 5 k shows the nanoparticles to have a coercitivity of 54 and 105 oe , 
 respectively .
in addition , both the zero - field - cooled ( zfc ) and field - cooled 
 ( fc ) temperature dependence of the magnetization were measured in 
 a 100 oe applied field ( figure 5b ) . the maximum 
 in the zfc curve and divergence in the zfc and
fc curves indicate 
 a blocking temperature , tb , of more than 
 300 k. the absence of any significant amount of magnetic impurity 
 such as fe was confirmed from xps results .
the overall characteristics 
 of ir nanoparticles suggest that they are  soft  magnetic 
 materials with ferromagnetic properties .
more in - depth studies on 
 the magnetic behavior of ir nanoparticles with different core sizes 
 and ir / thiolate compositions will be done and reported in the near 
 future .
( a ) magnetization curves of dodecanethiolate - capped iridium nanoparticles 
 obtained at 300 and 5 k ; the inset shows the similar coercive fields 
 at 300 and 5 k. ( b ) temperature dependence of the magnetization of 
 iridium nanoparticles measured in a 100 oe applied magnetic field .
the synthesis of stable 
 and isolable iridium nanoparticles with an average core size of 1.2 
 nm was achieved by employing sodium s - dodecylthiosulfate 
 as a ligand precursor .
it was also demonstrated that employing dodecanethiol 
 directly under identical conditions fails to yield iridium nanoparticles . 
 to our knowledge
, our article stands as the lone method for producing 
 stable alkanethiolate - capped ir nanoparticles using sodium borohydride 
 reduction .
schiffrin 
 mechanism and the reason thiosulfate ligands are preferred over thiol 
 ligands for the synthesis of ir nanoparticles .
finally , the produced 
 ir nanoparticles exhibited strong magnetic moments demonstrating the 
 potential of these ir nanoparticles for various technological applications .
 the 
 synthesis of stable and isolable iridium nanoparticles with an average 
 core size of 1.2  0.3 nm was achieved by employing sodium s - dodecylthiosulfate as a ligand precursor during the modified 
 brust  
 schiffrin reaction . 
 transmission electron microscopy 
 ( tem ) of the isolated ir nanoparticles revealed a high degree of monodispersity . 
 
 further characterizations with 1h nmr , ft - ir , uv  vis 
 spectroscopy , thermogravimetric analysis ( tga ) , and x - ray photoelectron 
 spectroscopy ( xps ) confirmed that the synthesized ir nanoparticles 
 are stabilized by dodecanethiolate ligands produced upon the adsorption / cleavage 
 of s - dodecylthiosulfate on the growing ir nanoparticle 
 surface . by comparison , synthetic attempts employing dodecanethiol 
 as a stabilizing ligand led to the formation of ir - thiolate species 
 ( ir(sr)3 ) as an intermediate and ir - hydroxide species at 
 the completion of reaction . 
 mechanistic investigations of these two 
 reactions using s - dodecylthiosulfate and dodecanethiol 
 provided deeper understandings on the novelty of thiosulfate ligands , 
 which allow the successful formation of stable thiolate - capped ir 
 nanoparticles . 
 moreover , these ir nanoparticles were shown to have 
 strong magnetic properties . 
Introduction
Experimental
Methods
Results and Discussion
Conclusions