Synthesis, structure and reactivity of cobalt(III) complexes with tridentate imino- and amino-oxime ligands

Abstract

The tridentate iminooximes HL² and HL¹ have been prepared by condensation of biacetyl monoxime with 2-(2-aminoethyl)pyridine and 2-(aminomethyl)pyridine, respectively. They reacted with cobalt(II) ion in the presence of oxygen to give the complexes [Co^IIIL₂]ClO₄(L = L¹ or L²). The iminooxime may be hydrogenated in the presence of NaBH₄ to yield the corresponding aminooximes HL³ and HL⁴, which, on reaction with Co^II, formed the complexes [Co^IIIL(HL)][ClO₄]₂(L = L³ or L⁴). All these complexes have been structurally characterized by single-crystal X-ray diffraction. The cobalt has an octahedral environment with the iminooxime ligands in mer configuration and the aminooxime ones in fac configuration. The reduction of [Co^IIIL²₂]ClO₄ with NaBH₄ produces a nucleophilic cobalt(I) species which, on reaction with MeI, gives a stable organocobalt dinuclear complex, previously reported. On the contrary, the reduction of [Co^IIIL¹₂]ClO₄ involves hydrogenation of the ligand from imino- to amino-oxime, with the formation of a stable cobalt(II) species. Under these conditions, no reduction to Co^I was observed. The different reactivity is attributed to the more strained co-ordination in [Co^IIIL¹₂]ClO₄ with respect to that in [Co^IIIL²₂]ClO₄.