Normal and inverted redox potentials and structural changes tuned by medium effects in [M2Mo(η5-C5Me5)2(S2C6H4)2(CO)2] (M: Co, Rh)

Abstract

Redox potential inversion was observed during the two-electron (2e⁻) reduction of two novel heterometal trinuclear complexes, [Co₂Mo(η⁵-C₅Me₅)₂(S₂C₆H₄)₂(CO)₂] (1) and [Rh₂Mo(η⁵-C₅Me₅)₂(S₂C₆H₄)₂(CO)₂] (2), the metalladithiolenes of which were found to communicate electronically via a Mo(CO)₂ moiety through two Co/Rh–Mo bonds. The redox properties and structural changes associated with each complex were investigated. 1 showed both two-step one-electron (1e⁻) reductions with a normal-order redox potential difference and a one-step 2e⁻reduction with a redox potential inversion that depended on the solvent and electrolyte. The size of the counter cation was found to significantly influence the reduction behaviour of 1. In contrast, 2 showed only a one-step 2e⁻reduction that didn't depend on the solvent–electrolyte conditions. The CO coordination mode in each reduction underwent a reversible structural reorganization upon reduction, from a semibridging configuration between the Co/Rh atoms and the Mo atom in the neutral form to a bridging configuration in the reduced form. The local coordination structure of the metal atoms in the dianion 2²⁻2− was experimentally determined for the first time. The interactions between Co and CO in 1 were weaker than the interactions between Rh and CO in 2, and differences in the interaction strengths, together with the influence of the solvent–electrolyte medium, were found to determine the redox processes available to the complexes.