Ligand Contributions to the Electronic Structures of the Oxidized Cobalt(II) salen Complexes

Square planar cobalt­(II) complexes of salen ligands N,N′-bis­(3-tert-butyl-5R-salicylidene)-1,2-cyclohexanediamine),
where R = OMe (1) and tert-butyl (2), were
prepared. 1 and 2 were electrochemically
reversibly oxidized into cations [1-H2O]+ and [2-H2O]+ in CH2Cl2. The chemically generated [1-H2O]­(SbF6)·0.68 H2O·0.82CH2Cl2 and [2-H2O]­(SbF6)·0.3H2O·0.85CH2Cl2 were characterized
by X-ray diffraction and NIR spectroscopy. Both complexes are paramagnetic
species containing a square pyramidal cobalt ion coordinated at the
apical position by an exogenous water molecule. They exhibit remarkable
NIR bands at 1220 (7370 M–1 cm–1) and 1060 nm (5560 M–1 cm–1),
respectively, assigned to a CT transition. DFT calculations and magnetic
measurements confirm the paramagnetic (S = 1) ground
spin state of the cations. They show that more than 70% of the total
spin density in [1-H2O]+ and [2-H2O]+ is localized on the
metal, the remaining spin density being distributed over the aromatic
rings (30% phenoxyl character). In the presence of N-methylimidazole 1 and 2 are irreversibly
oxidized by air into the genuine octahedral cobalt­(III) bis­(phenolate)
complexes [1-im2]+ and [2-im2]+, the former being structurally
characterized. Neither [1-im2]+ nor [2-im2]+ exhibits
a NIR feature in its electronic spectrum. 1 and 2 were electrochemically two-electron oxidized into [1]2+ and [2]2+. The cations were identified as Co­(III)–phenoxyl
species by their characteristic absorption band at ca. 400 nm in the
UV–vis spectrum. Coordination of the phenoxyl radical to the
cobalt­(III) metal ion is evidenced by the EPR signal centered at g = 2.00.