Cobalt Half-Sandwich, Sandwich, and Mixed Sandwich Complexes with Soft Tripodal Ligands

Reaction of sodium hydrotris(methimazolyl)borate (NaTmMe) with cobalt halides leads to the formation of paramagnetic pseudotetrahedral [Co(TmMe)X] (X = Cl, Br, I), of which the bromide has been crystallographically characterized. Mass spectrometry reveals the presence of higher molecular weight fragments [Co(TmMe)2]+ and [Co2(TmMe)2X]+ in solution. Aerial oxidation in donor solvents (e.g. MeCN) leads to formation of the [Co(TmMe)2]+ cation, which has been crystallographically characterized as the BF4-, ClO4-, Br-, and I-, salts. Attempts to prepare the mixed sandwich complex, [Co(Cp)(TmMe)]+, resulted in ligand decomposition to yield [Co(mtH)3I]I (mtH = 1-methylimidazole-2-thione), but with the more electron donating methylcyclopentadienyl (CpMe) ligand, [Co(CpMe)(TmMe)]I was isolated and characterized. Electrochemical measurements reveal that the cobalt(III) TmMe complexes are consistently more difficult to reduce than their Tp and Cp congeners.