Ate-complexes of tris-dioxolene tin anion with nickel (or cobalt) bis-(2,2ʹ-dipyridine)-dioxolene cation. EPR study of spin migration dynamics. Solvent and counterion effects

Abstract

Reaction of bis-o-semiquinonato nickel complex (2,2ʹ-dipyridine)Ni(dioxolene)2 (1) (dioxolene here – mono- or double-reduced forms of 3,6-di-tert-butyl-o-benzoquinone) or its cobalt analogue (2,2ʹ-dipyridine)Co(dioxolene)2) with (2,2ʹ-dipyridine)Sn(dioxolene)2 occur through ligands exchange and results in ate-complex [(2,2ʹ-dipyridine)2M(dioxolene)]+ [Sn(dioxolene)3]- (M = Ni (2), Co(3)). According to single crystal x-ray diffraction study both products are isostructural. Geometry of tin anion is intermediate between octahedral and trigonal prism. Analysis of bonds lengths in chelate cycles in tin anion one of dioxolene ligands is semiquinone anion-radical whereas the other two are catecholato-dianions. Geometry of inner coordination core of both cations is close to octahedral. Cobalt being in 3+ oxidation state is connected to catecholato-dianion. Nickel being in 2+ oxidation state is connected to semiquinonato anion-radical. Variable temperature magnetic susceptibility measurements of 2 indicate high spin nickel ferromagnetically coupled with semiquinonato anion-radical connected to it and independent semiquinone coordinated to tin. Cobalt cation is diamagnetic. EPR studies of ate-complex 3 in solution indicate two independent dynamic processes: solvation and migration of unpaired electron between three structurally identical ligands. Rate of this migration depends on solvent and temperature: from slow (CCl4, toluene) to fast (MeOH, CH3CN) in EPR time scale. Solvent role is solvation of the tin anion on one hand and the extent of isolation of ions: from “close pair” in CCl4 to “solvate separated pair” in methanol on the other hand. It is determined by solvent permittivity and dipole moment. Temperature dependence of hyperfine coupling constant with tin magnetic nuclei allowed estimating enthalpy and entropy of solvation. Temperature dependence of EPR spectrum shape together with it simulation allowed to evaluate the rates of spin migration and it activation parameters. In general, the regularities of ate-complex 2 behavior are the same but EPR lines are broader because of presence of high spin nickel and cation polarity effect is less than that in 3 because of smaller charge separation.