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

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 CO. The one electron reduction of [Co(salophen)] 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(salophen)K] complexes or the [Co(bis-salophen)M] (M = Li, Na) dimers that are present in solution in equilibrium with the respective [Co(salophen)M] complexes. The two electron reduction of [Co(salophen)] results in both ligand centered and metal centered reduction affording the Co(I)-Co(II)-Co(I) [Co(tris-salophen)Na(THF)], 6 complex supported by a bridging deca-anionic tris-salophen ligand where three salophen 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(salophen))Na][Na(cryptand)], 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(bis-salophen)NaPy][Na(cryptand)], 8. Complex 6 reacts with ambient CO leading to multiple CO reduction products. The product of CO addition to the salophen ligand, [Co(salophen-CO)Na][Na(cryptand)], 9, was isolated but CO 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.