Effects of Alkane Linker Length and Chalcogen Character in [FeFe]‐Hydrogenase Inspired Compounds

Abstract

Models of [FeFe]‐hydrogenases containing diselenolato ligands with different bridge linker length have been prepared: Fe2(μ‐Se(CH2)4Se‐μ)(CO)6 (4DS), and Fe2(μ‐Se(CH2)5Se‐μ)(CO)6 (5DS) as well as dithiolato Fe2(μ‐S(CH2)4S‐μ)(CO)6 (4DT) and compared with Fe2(μ‐S(CH2)3S‐μ)(CO)6 (PDT) and Fe2(μ‐Se(CH2)3Se‐μ)(CO)6 (PDS). Compounds 4DT, PDS, 4DS, and 5DS were characterized by spectroscopic techniques including NMR, IR, mass spectrometry, ultraviolet photoelectron spectroscopy (UPS), elemental analysis, and X‐ray crystal structure analysis. Combinations of electrochemical measurements, UPS, and density functional theory calculations indicate that oxidations of these five compounds are not significantly affected by chalcogen character but instead are governed by linker length. Cations for all compounds are calculated to adopt a bridged CO “rotated” structure with a vacant site on one of the Fe centers. In 4DT, 4DS, and 5DS, the alkane linker forms an agostic interaction with the vacant site on the rotated Fe. The reduction potentials for these compounds shift positively on average 0.16 V for each carbon added to the alkane linker with shifts being as large as 0.23 V between PDT and 4DT, and as small as 0.09 V between 4DS and 5DS. Catalytic reduction of protons from acetic acid in CH2Cl2 occurs at −1.79 and −1.86 V for PDT and 4DT and −2.02, −2.09, and −2.04 V for PDS, 4DS, and 5DS, indicating that chalcogen character is the primary factor that affects catalytic potential. On average the S‐containing compounds catalyze proton reduction at potentials, which are 0.23 V less negative than the Se‐containing compounds in this study.

Phenyl-functionalized diiron diselenolato complexes with intra- or inter-molecular bridging bidentate phosphine ligands

“…Me 3 NOÁ2H 2 O, dppm and dppf were commercially available and used as received. 2-Phenyl-1,3-bis(selenocyanato)propane and Fe 3 (CO) 12 were prepared according to literature protocols [25,26]. Preparative TLC was carried out on glass plates (25 cm 9 20 cm 9 0.25 cm) coated with silica gel G (10-40 lm).…”

“…By reaction of 2-phenyl-1,3-dibromopropane with potassium selenocyanate, 2-phenyl-1,3-bis(selenocyanato)propane was readily acquired [29]. As shown in Scheme 1, reaction of 2-phenyl-1,3-bis(selenocyanato)propane with Fe 3 (CO) 12 afforded complex 1. Reaction of complex 1 with dppm in refluxing xylene gave the intra-molecular bridging complex 2.…”

“…Owing to the ease of access to the [2Fe2S] subsite, and especially its responsibility for catalytic production of hydrogen, the classical diiron dithiolato complexes, (l-SCH 2 XCH 2 S-l)Fe 2 (CO) 6 (X = NH, CH 2 , S or O), and their derivatives have been the focus of much research concerning the biomimetic chemistry of [FeFe] hydrogenases [8][9][10]. Recently, diiron diselenolato analogs, [(lSeCH 2 XCH 2 Se-l)]Fe 2 (CO) 6 [X = CH 2 , CHOH, NH, NCH 2 CH 2 OH, O, S or Se], and their derivatives were prepared, and the ability to catalyze production of hydrogen from various acids was evaluated [11][12][13][14][15][16][17][18][19]. Detailed investigations based on photoelectron spectra and theoretical computations pertinent to the diiron diselenolato complexes revealed that replacement of sulfur in diiron dithiolato models by the more electropositive selenium leads to an enhancement of the catalytic activity of some models of [FeFe] hydrogenases, which may be ascribed to a higher electron density and a faster rate of electron transfer in [2Fe2Se] cores than in [2Fe2S] cores.…”

[FeFe]‐Hydrogenase Models Containing Long Diselenolato Linkers

“…It is found that the electron density at the Fe centres in these models is increased and hence the protonation of these centres became possible by strong or moderate acids (HBF 4 ⋅Et 2 O, CF 3 CO 2 H; p K a =0.1 and 12.7 in MeCN, respectively). Moreover, we reported different diselenolate mimics having the general formula [Fe 2 (CO) 6 { μ ‐(SeCH 2 ) 2 (CH 2 ) n }] (n=2, 3, 4) with different bridging linker length from the reaction of Fe 3 (CO) 12 with diselenocyanatoalkanes . In the case of n=3 and 4, the reaction produced another compound that contains four iron atoms of the formula [Fe 2 (CO) 6 { μ ‐(SeCH 2 ) 2 (CH 2 ) n }] 2 .…”

“…The alkyl linker in these complexes forms an agostic interaction with the vacant site on the rotated Fe core. In addition, the reduction potentials of these complexes shift positively by increasing the linker‘s length …”