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 CH ₂Cl₂. The chemically generated [1-H₂O](SbF ₆)·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-im ₂]⁺ 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.