Electronic Structure Description of a Doubly Oxidized Bimetallic Cobalt Complex with Proradical Ligands

The geometric and electronic structure of a doubly oxidized bimetallic Co complex containing two redox-active salen moieties connected via a 1,2-phenylene linker was investigated and compared to an oxidized monomeric analogue. Both complexes, namely, CoL(1) and Co2L(2), are oxidized to the mono- and dications, respectively, with AgSbF6 and characterized by X-ray crystallography for the monomer and by vis-NIR (NIR = near-infrared) spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, superconducting quantum interference device (SQUID) magnetometry, and density functional theory (DFT) calculations for both the monomer and dimer. Both complexes exhibit a water molecule coordinated in the apical position upon oxidation. [CoL(1)-H2O](+) displays a broad NIR band at 8500 cm(-1) (8400 M(-1) cm(-1)), which is consistent with recent reports on oxidized Co salen complexes (Kochem, A. et al., Inorg. Chem., 2012, 51, 10557-10571 and Kurahashi, T. et al., Inorg. Chem., 2013, 52, 3908-3919). DFT calculations predict a triplet ground state with significant ligand and metal contributions to the singularly occupied molecular orbitals. The majority (∼75%) of the total spin density is localized on the metal, highlighting both high-spin Co(III) and Co(II)L(•) character in the electronic ground state. Further oxidation of CoL(1) to the dication affords a low-spin Co(III) phenoxyl radical species. The NIR features for [Co2L(2)-2H2O](2+) at 8600 cm(-1) (17 800 M(-1) cm(-1)) are doubly intense in comparison to [CoL(1)-H2O](+) owing to the description of [Co2L(2)-2H2O](2+) as two non-interacting oxidized Co salen complexes bound via the central phenylene linker. Interestingly, TD-DFT calculations predict two electronic transitions that are 353 cm(-1) apart. The NIR spectrum of the analogous Ni complex, [Ni2L(2)](2+), exhibits two intense transitions (4890 cm(-1)/26 500 M(-1) cm(-1) and 4200 cm(-1)/21 200 M(-1) cm(-1)) due to exciton coupling in the excited state. Only one broad band is observed in the NIR spectrum for [Co2L(2)-2H2O](2+) as a result of the contracted donor and acceptor orbitals and overall CT character.