Carbon Radical Generation by d⁰ Tantalum Complexes with α-Diimine Ligands through Ligand-Centered Redox Processes

High-valent tantalum complexes having redox-active α-diimine ligands, (α-diimine)TaCln (n = 3, 4), are prepared by the reaction of TaCl5, α-diimine ligands, and an organosilicon-based reductant, 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene. Reductive cleavage of the C–Cl bond of polyhaloalkanes is accomplished by trichlorotantalum complexes having dianionic α-diimine ligands via electron transfer from the dianionic ligands, whereas oxidative decomposition of tetraphenylborate is observed using tetrachlorotantalum complexes with monoanionic α-diimine ligands through electron transfer to the monoanionic ligands. Chemically oxidized or reduced complexes of (α-diimine)TaCl4 are isolated as ligand-centered redox products, [Cp2Co][(α-diimine)TaCl4] and [(α-diimine)TaCl4][WCl6], where the α-diimine ligand coordinates to the metal center as a dianionic or neutral ligand, respectively. On the basis of EPR measurements of (α-diimine)TaCl4 complexes (which are key intermediates for reductive cleavage of C–Cl bond and oxidative decomposition of tetraphenylborate), two redox isomers―a tantalum-centered radical and ligand-localized radical―are present in solution.