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

The mixed-valence diiron hydrido complex (μ-H)Fe2(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 A, but the Fe–P bonds only change subtly. Although the Fe–H distances are indistinguishable in the precursor, they differ by 0.2 A 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(31P), which range from 31 MHz for the basal phosphines to 284 MHz f...