Iron acyl thiolato carbonyls: structural models for the active site of the [Fe]-hydrogenase (Hmd)

Phosphine-modified thioester derivatives are shown to serve as efficient precursors to phosphine-stabilized ferrous acyl thiolato carbonyls, which replicate key structural features of the active site of the hydrogenase Hmd. The reaction of Ph(2)PC(6)H(4)C(O)SPh and sources of Fe(0) generates both Fe(SPh)(Ph(2)PC(6)H(4)CO)(CO)(3) (1) and the diferrous diacyl Fe(2)(SPh)(2)(CO)(3)(Ph(2)PC(6)H(4)CO)(2), which carbonylates to give 1. For the extremely bulky arylthioester Ph(2)PC(6)H(4)C(O)SC(6)H(3)-2,6-(2,4,6-trimethylphenyl)(2), oxidative addition is arrested and the Fe(0) adduct of the phosphine is obtained. Complex 1 reacts with cyanide to give Et(4)N[Fe(SPh)(Ph(2)PC(6)H(4)CO)(CN)(CO)(2)] (Et(4)N[2]). (13)C and (31)P NMR spectra indicate that substitution is stereospecific and cis to P. The IR spectrum of [2](-) in ν(CN) and ν(CO) regions very closely matches that for Hmd(CN). XANES and EXAFS measurements also indicate close structural and electronic similarity of Et(4)N[2] to the active site of wild-type Hmd. Complex 1 also stereospecifically forms a derivative with TsCH(2)NC, but the adduct is more labile than Et(4)N[2]. Tricarbonyl 1 was found to reversibly protonate to give a thermally labile derivative, IR measurements of which indicate that the acyl and thiolate ligands are probably not protonated in Hmd.