Understanding Ligand Effects on Bielectronic Transitions: Chemo- and Electroreduction of Rhodium Bis(Diphosphine) Complexes to Low Oxidation States.

Abstract

Rhodium complexes in the -I and 0 oxidation states are of great potential interest in catalytic applications. In contrast to their rhodium +I congeners, however, the structural and electronic parameters governing their access and stability are far less understood. Herein, we investigate the two-electron reduction of a parameterized series of bis(diphosphine) Rh complexes [Rh(dxpy)2]NTf2 (x=P-substituent, y=alkanediyl bridging P atoms). Through (electro)reductions from the RhI parents, Rh-I d10-complexes were obtained and characterized spectroscopically, including 103Rh NMR data. The reductive steps convolute with structural rearrangements from square planar to tetrahedral coordination. We found that the extent of these reorganisations defines whether the first E0(RhI/0) and second E0(Rh0/-I) reduction potentials are normally ordered, leading to monoelectronic stepwise transitions, or inverted, giving bielectronic events. Reductionist approaches based on Hammett parameters or the P-Rh-P bite angles provide only partial correlations with the redox potentials. However, we identified the C-O stretch of analogue diphosphine complexes as an expedient computational parameter that enables these correlations through both electronic and geometric features, even in a predictive manner. Gaining control over two-electron reduction behaviors through rationalized ligand effects has potential impact beyond Rh complexes, for molecular and enzymatic metal sites commonly exhibiting bielectronic transitions.