Keywords: Iron / Hydrogenases / Substitution / Electrocatalysis / Ligand effects / Enzyme catalysis / SeleniumThe displacement of terminal CO ligands in Fe 2 (µ-Se 2 C 3 H 5 CH 3 )(CO) 6 (1) by triphenylphosphane, trimethyl phosphite, and bis(diphenylphosphanyl)ethane (dppe) ligands is investigated. Treatment of 1 with 1 equiv. of triphenylphosphane afforded Fe 2 (µ-Se 2 C 3 H 5 CH 3 )(CO) 5 (PPh 3 ) (2). The mono-and disubstituted phosphite complexes Fe 2 (µ-Se 2 C 3 H 5 CH 3 )(CO) 5 P(OMe) 3 (3)andFe 2 (µ-Se 2 C 3 H 5 CH 3 )(CO) 4 -[P(OMe) 3 ] 2 (4) were obtained from the reaction of 1 with excess P(OMe) 3 at reflux in toluene. In contrast, the reaction of 1 with 1 equiv. of dppe in the presence of Me 3 NO·2H 2 O gave
In order to elucidate the role of the "on-off " coordination mode of the thioether group in the [2Fe3S] complex 1, which is related to the active site of [FeFe] hydrogenases, substitution studies of CO ligands by phosphite and phosphine ligands at compound Fe 2 (μ-S 2 (C 3 H 6 ) 2 S-μ)(CO) 5 (1) have been investigated. The reaction of 1 with 1 equiv of trimethylphosphite gave the kinetically controlled product Fe 2 (μ-S 2 (C 3 H 6 ) 2 S)(CO) 5 P(OMe) 3 (2) or the thermodynamically controlled product Fe 2 (μ-S 2 (C 3 H 6 ) 2 S-μ)(CO) 4 P(OMe) 3 (3) depending on the reaction conditions. Moreover, Fe 2 (μ-S 2 (C 3 -H 6 ) 2 S)(CO) 4 [P(OMe) 3 ] 2 (4) and Fe 2 (μ-S 2 (C 3 H 6 ) 2 S)(CO) 4 (PMe 3 ) 2 (5) were obtained from the reactions of 1 with excess P(OMe) 3 and excess PMe 3 , respectively. These novel complexes have been characterized by IR, 1 H, 13 C{ 1 H}, and 31 P{ 1 H} NMR spectroscopy, mass spectrometry, elemental analysis, and X-ray single-crystal structure analysis.
Dodecacarbonyltriiron reacts with 3,3,5,5‐tetraphenyl‐1,2,4‐trithiolanes (1e) to give the ortho‐metalated complex Fe2(CO)6[κ,μ‐S,η2‐(C13H10S)] (9a), complexes of the type (Ph2C)S2Fe2(CO)6 and the well known trinuclear complex Fe3S2 (CO)9 as by‐products. Complex 9a can also be obtained from the reaction of Fe3(CO)12 with thiobenzophenone (2a). In a similar way, 4,4′‐bis(dimethylamino)thiobenzophenone (2b) reacts with Fe3(CO)12 to give Fe2(CO)6[κ,μ‐S,η2‐(C17H20N2S)] (9b). The cyclic aromatic thioketones such as dibenzosuberenethione (2c) and xanthione (2d) react with Fe3(CO)12 to give the cyclometalated products Fe2(CO)6[κ,μ‐S,η2‐(C15H12S)] (9c) and Fe2(CO)6[κ,μ‐S,η2‐(C13H8OS)] (9d), respectively, and a small amount of Fe3S2(CO)9. Complexes 9a–d have been characterized by IR and NMR spectroscopies, elemental analyses, and X‐ray single crystal structure analyses.
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.
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