Isolation of a Mixed Valence Diiron Hydride: Evidence for a Spectator Hydride in Hydrogen Evolution Catalysis

The mixed-valence diiron hydrido complex (mu H)Fe-2(pdt)(CO)(2)(dppv)(2) ([H1](0), where pdt =1,3-propanedithiolate and dppv = cis-1,2-C2H2(PPh2)(2)), was generated by reduction of the differous hydride [H1](+) using decamethylcobaltocene. Crystallographic analysis shows that [H1](0) retains the stereochemistry of its precursor, where one dppv ligand spans two basal sites and the other spans apical and basal positions. The Fe-Fe bond elongates to 2.80 from 2.66 angstrom, but the Fe-P bonds only change subtly. Although the Fe-H distances are indistinguishable in the precursor, they differ by 0.2 angstrom in [H1](0). The X-band electron paramagnetic resonance (EPR) spectrum reveals the presence of two stereoisomers, the one characterized crystallographically and a contribution of about 10% from a second symmetrical (sym) isomer wherein both dppv ligands occupy apical-basal sites. The unsymmetrical (unsym) arrangement of the dppv ligands is reflected in the values of A(P-31), which range from 31 MHz for the basal phosphines to 284 MHz for the apical phosphine. Density functional theory calculations were employed to rationalize the electronic structure of [H1](0) and to facilitate spectral simulation and assignment of EPR parameters including H-1 and P-31 hyperfine couplings. The EPR spectra of [HI](0) and [D1](0) demonstrate that the singly occupied molecular orbital is primarily localized on the Fe center with the longer bond to H, that is, Fe-II-H center dot center dot center dot Fe-I. The coupling to the hydride is A(H-1) = 55 and 74 MHz for unsym- amd sym[H1](0), respectively. Treatment of [H1](0) with H+ gives 0.5 equiv of H-2 and [H1](+). Reduction of D+ affords D-2, leaving the hydride ligand intact. These experiments demonstrate that the bridging hydride ligand in this complex is a spectator in the hydrogen evolution reaction.