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

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.