Electrochemical Kinetic Study of [Cp*Rh] Complexes Supported by Bis(2-pyridyl)methane Ligands

Redox-induced reactions of organometallic complexes are ubiquitous in molecular electrochemistry and electrocatalysis research. However, a detailed knowledge of the kinetic parameters associated with individual elementary steps in these reactions is often challenging to obtain, limiting an understanding of the reactivity pathways that can be used to construct new catalytic cycles. Here, the kinetics of redox processes in model [Cp*Rh] complexes have been explored with substituted bis­(2-pyridyl)­methane (dipyridylmethane, dpma) ligands. Complementing prior work with [Cp*Rh] complexes bearing 2,2′-bipyridyl ligands, we find that the redox chemistry in these species is strongly affected by the disrupted inter-ring conjugation of dpma ligand frameworks. In particular, [Cp*Rh] complexes bearing κ2-dpma ligands with varying substitution at the bridging methylene position undergo a unique electrochemical–chemical (EC) process upon reduction from Rh­(II) to Rh­(I) as observed by cyclic voltammetry; transient electrogenerated Rh­(I) species undergo a ligand rearrangement that results in facial η2 coordination of one pyridine motif on the dpma platform. Studies of a family of [Cp*Rh] complexes bearing dimethyl (Me2dpma)-, dibenzyl (Bn2dpma)-, methyl,methylpyrenyl- (MePyrdpma)-, and bis­(methylpyrenyl) (Pyr2dpma)-substituted dpma ligands reveal a uniform trend in the first-order rate constants associated with this EC process involving ligand rearrangement, providing kinetic insight into a key process that enables the stabilization of low-valent rhodium by substituted dpma-type ligands.