Molecular and electronic structure of four- and five-coordinate cobalt complexes containing two o-phenylenediamine- or two o-aminophenol-type ligands at various oxidation levels: an experimental, density functional, and correlated ab initio study

The bidentate ligands N-phenyl-o-phenylenediamine, H(2)((2)L(N)IP), or its analogue 2-(2-trifluoromethyl)anilino-4,6-di-tert-butylphenol, ((4)L(N)IP), react with Co(II)(CH(3)CO(2))(2)]4H(2)O and triethylamine in acetonitrile in the presence of air yielding the square-planar, four-coordinate species Co((2)L(N))(2)] (1) and Co((4)L(O))(2)] (4) with an S=1/2 ground state. The corresponding nickel complexes Ni((4)L(O))(2)] (8) and its cobaltocene reduced form Co(III)(Cp)(2)]Ni((4)L(O))(2)] (9) have also been synthesized. The five-coordinate species Co((2)L(N))(2)(tBu-py)] (2) (S=1/2) and its one-electron oxidized forms Co((2)L(N))(2)(tBu-py)](O(2)CCH(3)) (2 a) or Co((2)L(N))(2)I] (3) with diamagnetic ground states (S=0) have been prepared, as has the species Co((4)L(O))(2)(CH(2)CN)] (7). The one-electron reduced form of 4, namely Co(Cp)(2)]Co((4)L(O))(2)] (5) has been generated through the reduction of 4 with Co(Cp)(2)]. Complexes 1, 2, 2 a, 3, 4, 5, 7, 8, and 9 have been characterized by X-ray crystallography (100 K). The ligands are non-innocent and may exist as catecholate-like dianions ((2)L(N)IP)(2-), ((4)L(N)IP)(2-) or pi-radical semiquinonate monoanions ((2)L(N)ISQ)(*) (-), ((4)L(N)ISQ)(*) (-) or as neutral benzoquinones ((2) L(N)IBQ)(0), ((4) L(N)IBQ)(0); the spectroscopic oxidation states of the central metal ions vary accordingly. Electronic absorption, magnetic circular dichroism, and EPR spectroscopy, as well as variable temperature magnetic susceptibility measurements have been used to experimentally determine the electronic structures of these complexes. Density functional theoretical (DFT) and correlated ab initio calculation have been performed on the neutral and monoanionic species Co((1)L(N))(2)](0,-) in order to understand the structural and spectroscopic properties of complexes. It is shown that the corresponding nickel complexes 8 and 9 contain a low-spin nickel(II) ion regardless of the oxidation level of the ligand, whereas for the corresponding cobalt complexes the situation is more complicated. Spectroscopic oxidation states describing a d(6) (Co(III)) or d(7) (Co(II)) electron configuration cannot be unambiguously assigned.