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 Ph₂PC₆H₄C(O)SPh and sources of Fe(0) generates both Fe(SPh)(Ph₂PC₆H₄CO)(CO) ₃ (1) and the diferrous diacyl Fe₂(SPh) ₂(CO)₃(Ph₂PC₆H₄CO) ₂, which carbonylates to give 1. For the extremely bulky arylthioester Ph₂PC₆H₄C(O)SC₆H 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 Et₄N[Fe(SPh)(Ph₂PC₆H ₄CO)(CN)(CO)₂] (Et₄N[2]). ¹³C and ³¹P NMR spectra indicate that substitution is stereospecific and cis to P. The IR spectrum of [2]^- in v_CN and v_CO regions very closely matches that for Hmd^CN. XANES and EXAFS measurements also indicate close structural and electronic similarity of Et ₄N[2] to the active site of wild-type Hmd. Complex 1 also stereospecifically forms a derivative with TsCH₂NC, but the adduct is more labile than Et₄N[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.