A series of new charge transfer (CT) chromophores of “α-diimine-M^II-catecholate” type (where M is 3d-row transition metals—Cu, Ni, Co) were derived from 4,4′-di-tert-butyl-2,2′-bipyridyl and 3,6-di-tert-butyl-o-benzoquinone (3,6-DTBQ) in accordance with three modified synthetic approaches, which provide high yields of products. A square-planar molecular structure is inherent for monomeric [Cu^II(3,6-Cat)(bipy^tBu)]∙THF (1) and Ni^II(3,6-Cat)(bipy^tBu) (2) chromophores, while dimeric complex [Co^II(3,6-Cat)(bipy^tBu)]₂∙toluene (3) units two substantially distorted heteroleptic D-M^II-A (where D, M, A are donor, metal and acceptor, respectively) parts through a donation of oxygen atoms from catecholate dianions. Chromophores 1–3 undergo an effective photoinduced intramolecular charge transfer (λ = 500–715 nm, extinction coefficient up to 10⁴ M⁻¹·cm⁻¹) with a concomitant generation of a less polar excited species, the energy of which is a finely sensitive towards solvent polarity, ensuring a pronounced negative solvatochromic effect. Special attention was paid to energetic characteristics for CT and interacting HOMO/LUMO orbitals that were explored by a synergy of UV-vis-NIR spectroscopy, cyclic voltammetry, and DFT study. The current work sheds light on the dependence of CT peculiarities on the nature of metal centers from various groups of the periodic law. Moreover, the “α-diimine-M^II-catecholate” CT chromophores on the base of “late” transition elements with differences in d-level’s electronic structure were compared for the first time.

The new bis-o-iminobenzosemiquinonate nickel and cobalt complexes (imSQ^t-Bu)₂M (M = Ni (1), Co (2)), where imSQ is a radical anion of 4,6-di-tert-butyl-N-(tert-butyl)-o-iminobenzoquinone, were synthesized and characterized in detail. The molecular structures of 1 and 2 have been established by single-crystal X-ray analysis. The metal atoms in 1 and 2 have a distorted tetrahedral environment, and the dihedral angles between the planes of two radical imSQ ligands are approximately 80° in both complexes. According to the structural and spectroscopy data along with magnetic susceptibility measurements the electronic structure of the complexes should be interpreted definitely as a high spin metal center Ni^II (d⁸, S = 1) in 1 and Co^II (d⁷, S = 3/2) in 2 bonded with two o-iminobenzosemiquinonate radicals (S_rad = 1/2). The strong antiferromagnetic metal–ligand spin interactions in both complexes lead to the observed S_t = 0 and S_t = 1/2 ground states in 1 and 2, respectively. The computational DFT UB3LYP/6-311++G(d,p) studies performed on 1 and 2 are in good agreement with experimental data. Complexes 1 and 2 have similar electrochemical properties. The electrochemical reduction of the complexes includes two quasi-reversible one-electron-transfer waves in the cathodic region corresponding to the formation of the anions [M(AP)₂]²⁻ and [(imSQ)M(AP)]¹⁻ (AP – dianion of 4,6-di-tert-butyl-N-(tert-butyl)-o-iminobenzoquinone), while in the anodic region only one quasi-reversible redox process was registered. All redox processes are shown to be ligand-based.