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 Å, but the Fe–P bonds only change subtly.
Although the Fe–H distances are indistinguishable in the precursor,
they differ by 0.2 Å 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 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 1H and 31P hyperfine
couplings. The EPR spectra of [H1]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, FeII–H···FeI. The coupling to the hydride is A(1H)
= 55 and 74 MHz for unsym- amd sym-[H1]0, respectively. Treatment of [H1]0 with H+ gives 0.5 equiv of H2 and [H1]+. Reduction of D+ affords D2, leaving the hydride ligand intact. These
experiments demonstrate that the bridging hydride ligand in this complex
is a spectator in the hydrogen evolution reaction.