Ligand and Metal Based Multielectron Redox Chemistry of Cobalt Supported by Tetradentate Schiff Bases

Abstract

We have investigated the influence of bound cations on the reduction of cobalt complexes of redox active ligands and explored the reactivity of reduced species with CO₂. The one electron reduction of [Co^II(^Rsalophen)] with alkali metals (M = Li, Na, K) leads to either ligand-centered or metal-centered reduction depending on the alkali ion. It affords either the [Co^I(^Rsalophen)K] complexes or the [Co^II₂(bis-salophen)M₂] (M = Li, Na) dimers that are present in solution in equilibrium with the respective [Co^I(salophen)M] complexes. The two electron reduction of [Co^II(^OMesalophen)] results in both ligand centered and metal centered reduction affording the Co(I)-Co(II)-Co(I) [Co₃(tris-^OMesalophen)Na₆(THF)₆], 6 complex supported by a bridging deca-anionic tris-^OMesalophen^10- ligand where three ^OMesalophen units are connected by two C-C bonds. Removal of the Na ion from 6 leads to a redistribution of the electrons affording the complex [(Co(^OMesalophen))₂Na][Na(cryptand)]₃, 7. The EPR spectrum of 7 suggests the presence of a Co(I) bound to a radical anionic ligand. Dissolution of 7 in pyridine leads to the isolation of [Co^I₂(bis-^OMesalophen)Na₂Py₄][Na(cryptand)]₂, 8. Complex 6 reacts with ambient CO₂ leading to multiple CO₂ reduction products. The product of CO₂ addition to the ^OMesalophen ligand, [Co(^OMesalophen-CO₂)Na]₂[Na(cryptand)]₂, 9, was isolated but CO₃^2- formation in 53% yield was also detected. Thus, the electrons stored in the reversible C-C bonds may be used for the transformation of carbon dioxide.