Unique Ligand-Radical Character of an Activated Cobalt Salen Catalyst That Is Generated by Aerobic Oxidation of a Cobalt(II) Salen Complex

Abstract

The Co(salen)(X) complex, where salen is chiral N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine and X is an external axial ligand, has been widely utilized as a versatile catalyst. The Co(salen)(X) complex is a stable solid that has been conventionally described as a Co(III)(salen)(X) complex. Recent theoretical calculations raised a new proposal that the Co(salen)(H2O)(SbF6) complex contains appreciable contribution from a Co(II)(salen(•+)) electronic structure (Kochem, A.; Kanso, H.; Baptiste, B.; Arora, H.; Philouze, C.; Jarjayes, O.; Vezin, H.; Luneau, D.; Orio, M.; Thomas, F. Inorg. Chem. 2012, 51, 10557-10571), while other theoretical calculations for Co(salen)(Cl) indicated a triplet Co(III)(salen) electronic structure (Kemper, S.; Hrobárik, P.; Kaupp, M.; Schlörer, N. E. J. Am. Chem. Soc. 2009, 131, 4172-4173). However, there have been no experimental data to evaluate these theoretical proposals. We herein report key experimental data on the electronic structure of the Co(salen)(X) complex (X = CF3SO3(-), SbF6(-), and p-MeC6H4SO3(-)). The X-ray crystallography shows that Co(salen)(OTf) has a square-planar N2O2 equatorial coordination sphere with OTf as an elongated external axial ligand. Magnetic susceptibility data indicate that Co(salen)(OTf) complexes belong to the S = 1 spin system. (1)H NMR measurements provide convincing evidence for the Co(II)(salen(•+))(X) character, which is estimated to be about 40% in addition to 60% Co(III)(salen)(X) character. The CH2Cl2 solution of Co(salen)(X) shows an intense near-IR absorption, which is assigned as overlapped transitions from a ligand-to-metal charge transfer in Co(III)(salen)(X) and a ligand-to-ligand charge transfer in Co(II)(salen(•+))(X). The present experimental study establishes that the electronic structure of Co(salen)(X) contains both Co(II)(salen(•+))(X) and Co(III)(salen)(X) character.