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

Abstract

The mixed-valence diiron hydrido complex (μ-H)­Fe₂(pdt)­(CO)₂(dppv)₂ ([H<b>1</b>]⁰, where pdt =1,3-propanedithiolate and dppv = <i>cis</i>-1,2-C₂H₂(PPh₂)₂), was generated by reduction of the differous hydride [H<b>1</b>]⁺ using decamethylcobaltocene. Crystallographic analysis shows that [H<b>1</b>]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 [H<b>1</b>]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 (<i>sym</i>) isomer wherein both dppv ligands occupy apical–basal sites. The unsymmetrical (<i>unsym</i>) arrangement of the dppv ligands is reflected in the values of <i>A</i>(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 [H<b>1</b>]⁰ and to facilitate spectral simulation and assignment of EPR parameters including ¹H and ³¹P hyperfine couplings. The EPR spectra of [H<b>1</b>]⁰ and [D<b>1</b>]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···Fe^I. The coupling to the hydride is <i>A</i>(¹H) = 55 and 74 MHz for <i>unsym</i>- amd <i>sym</i>-[H<b>1</b>]⁰, respectively. Treatment of [H<b>1</b>]⁰ with H⁺ gives 0.5 equiv of H₂ and [H<b>1</b>]⁺. 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