Redox-active ligand-mediated Co-Cl bond-forming reactions at reducing square planar cobalt(III) centers

Synthetic routes to new square planar cobalt complexes with redox-active amidophenolate chelates are presented. Contrary to previous reports, steric bulk on the ligands is not a prerequisite to formation of the low-coordinate materials. X-ray crystal structure metrical data of the neutral S = 1/2 complexes are most consistent with cobalt(III) bound to one iminobenzoseminonate(1-) radical and one amidophenolate(2-) ligand. Addition of 1e(-) affords reduced congeners that are also square planar cobalt(III) because the redox-active ligand accepts an electron to generate bis(amidophenolate) species. The redox-activity of the ligands facilitates reactions with chlorine electrophiles to generate square pyramidal products containing new Co-Cl bonds. The bond-forming reactions all formally require oxidation of the metal fragment but there is no change in formal cobalt oxidation state. Instead, the reaction proceeds with oxidation of the amidophenolate ligands. Control of ligand oxidation state provides a mechanism for 1e(-) versus 2e(-) selectivity in the bond-forming redox reactions. (C) 2009 Elsevier Ltd. All rights reserved.