Stoichiometry, products and kinetics of oxidation of neutral 2,6-diacetylpyridine-bis( N4-azacyclic thiosemicarbazone)cobalt(II) complexes by dioxygen in aprotic solvents

Abstract

Low-spin, square planar cobalt(II) complexes are obtained by reaction of equimolar cobalt(II) acetate with potentially five-coordinate 2,6-diacetylpyridine-bis( N 4-azacyclic thiosemicarbazone) ligands. When the N 4-azacyclic moiety is piperidyl, morpholyl and N-methylpiperazyl, the cobalt(II) complexes are abbreviated as [Co(DAPP)], [Co(DAPM)] and [Co(DAPN)], respectively. The complexes are freely soluble in methylene chloride and nitrobenzene. They rapidly react with dioxygen in these solvents to form the corresponding diamagnetic μ-peroxodicobalt(III) complexes with stoichiometry 2Co(II)+O2→(Co(III)) 2O 2 as indicated by manometric oxygen uptake, cryoscopic measurements in nitrobenzene and the appearance of a v(OO) band at 860 cm −1. Comparison of the infrared spectra of the cobalt(II) and cobalt(III) complexes and the 1H NMR spectra of cobalt(III) complexes with corresponding data for the structurally known nickel(II) complex [Ni(DAPM)] indicates that the bis- thiosemicarbazone ligands act as dibasic tetradentate molecules. However, cobalt is unsymmetrically bonded to the thiohydrazone portions of the ligand. Reduction of dioxygen to bound peroxide by large excesses of these cobalt(II) complexes in nitrobenzene and methylene chloride is governed by the third-order rate law d[(Co(III)) 2O 2]/dt= k 2[co(II)] 2[O 2]. Very negative Δ S 2 ≠, small δH 2 ≠ and minimal solvent effects suggest that assembly of the activated complex is rate-determining. Rate constants k 2 at 25 °C decrease in order [Co(DAPN)]>[Co(DAPM]>[Co(DAPP)].