Remote Oxidative Activation of a [Cp*Rh] Monohydride

Abstract

Half-sandwich rhodium monohydrides are often proposed as intermediates in catalysis, but little is known regarding the redox-induced reactivity accessible to these species. Herein, the bis(diphenylphosphino)ferrocene (dppf) ligand has been used to explore the reactivity that can be induced when a [Cp*Rh] monohydride undergoes remote (dppf-centered) oxidation by 1e⁻. Chemical and electrochemical studies show that one-electron redox chemistry is accessible to Cp*Rh(dppf), including a unique quasi-reversible Rh^(II/I) process at −0.96 V vs. ferrocenium/ferrocene (Fc^(+/0)). This redox manifold was confirmed by isolation of an uncommon Rh^(II) species, [Cp*Rh(dppf)]⁺, that was characterized by electron paramagnetic resonance (EPR) spectroscopy. Protonation of Cp*Rh(dppf) with anilinium triflate yielded an isolable and inert monohydride, [Cp*Rh(dppf)H]+, and this species was found to undergo a quasireversible electrochemical oxidation at +0.41 V vs. Fc^(+/0) that corresponds to iron-centered oxidation in the dppf backbone. Thermochemical analysis predicts that this dppf-centered oxidation drives a dramatic increase in acidity of the Rh−H moiety by 23 pK_a units, a reactivity pattern confirmed by in situ ¹H NMR studies. Taken together, these results show that remote oxidation can effectively induce M−H activation and suggest that ligand-centered redox activity could be an attractive feature for the design of new systems relying on hydride intermediates.

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