Magnetically bistable cobalt-dioxolene complexes with a tetradentate N-donor base

Synthesis and magnetic characterization of a family of cobalt-dioxolene complexes [(Me(2)TPA)Co(36-DBCat)] (1), [(Me(2)TPA)Co(36-DBCat)](PF6) (2) and [(Me(2)TPA)Co(diox-(OMe)(3))](BPh4) (3) (Me(2)TPA = bis(6-methyl-2-pyridyl)methyl-(2-pyridyl)methyl)amine; 36-DBCat = dianion of 3,6-di-tert-butylcatechol; diox(OMe)(3) - 2,5-di-tert-butyl-3,3,4-trimethoxy-6-oxocyclohexa-1,4-dienolate) is reported. The neutral complex 1 is found to form hexa-(CoO2N4, 1a) and pentacoordinated (CoO2N3, 1b) isomers. Variable temperature single crystal X-ray diffraction analysis of la and 1b clearly indicates the presence of the high-spin divalent metal ion and the dianionic catecholate form of the dioxolene ligand. Oxidation of 1 by ferrocenium hexafluorophosphate results in the formation of the ionic octahedral complex 2, demonstrating thermally induced valence-tautomeric transition (ls-Co-III-36-DBCat reversible arrow hs-Co-II-36-DBSQ) in the solid state with T-1/2 = 175 K (36-DBSQ = radical-anionic semiquinonate form of the redox-ligand). In contrast, aerial oxidation of 1 is accompanied by changes in the structure of dioxolene resulting in oxocyclo-hexadienolate ligand and the formation of an ionic complex of high-spin divalent cobalt (3). Compounds 1a, 1b, and 3 are found to demonstrate a field-induced single-ion magnet behavior. The analysis of the electronic structures of 1, 2 and 3 with the aid of DFT and SA-CASSCF/NEVPT2 calculations is also given.