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

Abstract

Synthesis and magnetic characterization of a family of cobalt-dioxolene complexes [(Me₂TPA)Co(36-DBCat)] (1), [(Me₂TPA)Co(36-DBCat)](PF₆) (2) and [(Me₂TPA)Co(diox-(OMe)₃)](BPh₄) (3) (Me₂TPA = bis(6-methyl-2-pyridyl)methyl-(2-pyridylmethyl)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 1a 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 ⇄ 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 oxocyclohexadienolate 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.