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

Abstract

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