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 <i>N</i>,<i>N</i>′-bis­(3,5-di-<i>tert</i>-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. <i>Inorg. Chem.</i> <b>2012</b>, <i>51</i>, 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. <i>J. Am. Chem. Soc.</i> <b>2009</b>, <i>131</i>, 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 <i>p</i>-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 <i>S</i> = 1 spin system. ¹H 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