Contrasting Protonation Behavior of Diphosphido vs Dithiolato Diiron(I) Carbonyl Complexes

This paper reports on the protonation of phosphine-substituted diiron diphosphido carbonyls, analogues of diiron dithiolato centers at the active sites of hydrogenase enzymes. Reaction of the diphosphines (CH₂)_<i>n</i>(PPhH)₂ (<i>n</i> = 2 (edpH₂) and <i>n</i> = 3 (pdpH₂)) with Fe₃(CO)₁₂ gave excellent yields of Fe₂(edp)­(CO)₆ (<b>1</b>) and Fe₂(pdp)­(CO)₆ (<b>2</b>). Substitution of Fe₂(edp)­(CO)₆ with PMe₃ afforded Fe₂(edp)­(CO)₂(PMe₃)₄ (<b>3</b>; ν_CO 1855 and 1836 cm^–1). Crystallographic analysis showed that <b>3</b> adopts an idealized <i>C</i>₂ symmetry, with pairs of phosphine ligands occupying apical–basal sites on each Fe center. Relative to that in the dithiolato complex, the Fe–Fe bond (2.7786(8) Å) is elongated by 0.15 Å. Treatment of <b>3</b> with H­(OEt₂)₂BAr^F₄ (Ar^F = C₆H₃-3,5-(CF₃)₂) gave exclusively the <i>C</i>₂-symmetric μ-hydride complex [HFe₂(edp)­(CO)₂(PMe₃)₄]⁺. This result contrasts with the behavior of the analogous ethanedi<i>thiolate</i> Fe₂(edt)­(CO)₂(PMe₃)₄ (edt = 1,2-C₂H₄S₂), protonation of which gives both the bridging <i>and terminal</i> hydride complexes. This difference points to the participation of the sulfur centers in the formation of terminal hydrides. The absence of terminal hydride intermediates was also revealed in the protonation of the diphosphine diphosphido complexes Fe₂(pdp)­(CO)₄(dppv) (<b>4</b>; dppv = <i>cis</i>-1,2-C₂H₂(PPh₂)₂) and Fe₂(edp)­(CO)₄(dppbz) (<b>5</b>; dppbz = 1,2-C₆H₄(PPh₂)₂). Protonation of these diphosphine complexes afforded μ-hydrido cations with apical–basal diphosphine ligands, which convert to the isomer where the diphosphine is dibasal. In contrast, protonation of the dithiolato complex Fe₂(pdt)­(CO)₄(dppv) gave terminal hydrides, which isomerize to μ-hydrides. In a competition experiment, <b>4</b> was shown to protonate faster than Fe₂(pdt)­(CO)₄(dppv)