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

Abstract

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)n(PPhH)2 (n = 2 (edpH2) and n = 3 (pdpH2)) with Fe3(CO)12 gave excellent yields of Fe2(edp)(CO)6 (1) and Fe2(pdp)(CO)6 (2). Substitution of Fe2(edp)(CO)6 with PMe3 afforded Fe2(edp)(CO)2(PMe3)4 (3; νCO 1855 and 1836 cm–1). Crystallographic analysis showed that 3 adopts an idealized C2 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 3 with H(OEt2)2BArF4 (ArF = C6H3-3,5-(CF3)2) gave exclusively the C2-symmetric μ-hydride complex [HFe2(edp)(CO)2(PMe3)4]+. This result contrasts with the behavior of the analogous ethanedithiolate Fe2(edt)(CO)2(PMe3)4 (edt = 1,2-C2H4S2), protonation of which gives both the bridging and terminal 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) (4; dppv = cis-1,2-C2H2(PPh2)2) and Fe2(edp)(CO)4(dppbz) (5; 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, 4 was shown to protonate faster than Fe2(pdt)(CO)4(dppv).