Non-Innocent Ligand Behavior of a Bimetallic Ni Schiff-Base Complex Containing a Bridging Catecholate

The geometric and electronic structure of a bimetallic Ni Schiff-base complex and its one-electron oxidized form have been investigated in the solid state and in solution. The two salen units in the neutral complex 1 are linked via a bridging catecholate function. The one-electron oxidized form [1]⁺ was determined to exist as a ligand radical species in solution, with the electron hole potentially localized on the redox-active dioxolene, the phenolate ligands, or delocalized over the entire ligand system. Electrochemical experiments and UV–vis–NIR spectroscopy, in combination with density functional theory (DFT) calculations, provide insight into the locus of oxidation and the degree of delocalization in this system. The one-electron hole for [1]⁺ was determined experimentally to be localized on the dioxolene bridge with a small amount of spin density on the outer phenolate moieties predicted by the calculations. The resonance Raman spectrum of [1]⁺ (λ_ex = 413 nm) in CH₂Cl₂ solution clearly exhibited a new band at 1315 cm^–1 in comparison to 1, which is predicted to be a combination of dioxolene ring and C–O bond stretching modes, consistent with oxidation of the bridging moiety in [1]⁺. Analysis of the NIR bands for [1]⁺, in association with time-dependent DFT calculations, suggests that the low energy bands are ligand to ligand charge transfer transitions from the terminal phenolates to the central dioxolene unit. In combination, this data is consistent with a description of the overall electronic structure of [1]⁺ as a bridge-localized semiquinone ligand radical species. This is in contrast to the mixed-valence ground state description for many one-electron oxidized Ni salen monomer systems, and analysis in terms of intervalence charge transfer (IVCT) theory.