Interplay between Terminal and Bridging Diiron Hydrides in Neutral and Oxidized States.

This study describes the structural, spectroscopic, and electrochemical properties of electronically unsymmetrical diiron hydrides. The terminal hydride Cp*Fe(pdt)Fe(dppe)(CO)H ([1(t-H)]0, Cp*- = Me5C5-, pdt2- = CH2(CH2S-)2, dppe = Ph2PC2H4PPh2) was prepared by hydride reduction of [Cp*Fe(pdt)Fe(dppe)(CO)(NCMe)]+. As established by X-ray crystallography, [1(t-H)]0 features a terminal hydride ligand. Unlike previous examples of terminal diiron hydrides, [1(t-H)]0 does not isomerize to the bridging hydride [1(μ-H)]0. Oxidation of [1(t-H)]0 gives [1(t-H)]+, which was also characterized crystallographically as its BF4- salt. Density functional theory (DFT) calculations indicate that [1(t-H)]+ is best described as containing an Cp*FeIII center. In solution, [1(t-H)]+ isomerizes to [1(μ-H)]+, as anticipated by DFT. Reduction of [1(μ-H)]+ by Cp2Co afforded the diferrous bridging hydride [1(μ-H)]0. Electrochemical measurements and DFT calculations indicate that the couples [1(t-H)]+/0 and [1(μ-H)]+/0 differ by 210 mV. Qualitative measurements indicate that [1(t-H)]0 and [1(μ-H)]0 are close in free energy. Protonation of [1(t-H)]0 in MeCN solution affords H2 even with weak acids via hydride transfer. In contrast, protonation of [1(μ-H)]0 yields 0.5 equiv of H2 by a proposed protonation-induced electron transfer process. Isotopic labeling indicates that μ-H/D ligands are inert.