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)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).