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

Ryan M. Clarke^†, Khatera Hazin^†, John R. Thompson^†, Didier Savard^†, Kathleen E. Prosser^†, and Tim Storr^*†

^† Department of Chemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada

Inorg. Chem., 2016, 55 (2), pp 762–774

DOI: 10.1021/acs.inorgchem.5b02231

Publication Date (Web): December 31, 2015

Copyright © 2015 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acs.inorgchem.5b02231

Abstract

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, CoL¹ and Co₂L², 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. [CoL¹-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 CoL¹ to the dication affords a low-spin Co(III) phenoxyl radical species. The NIR features for [Co₂L²-2H₂O]²⁺ at 8600 cm^–1 (17 800 M^–1 cm^–1) are doubly intense in comparison to [CoL¹-H₂O]⁺ owing to the description of [Co₂L²-2H₂O]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, [Ni₂L²]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 [Co₂L²-2H₂O]²⁺ as a result of the contracted donor and acceptor orbitals and overall CT character.