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

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