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

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 [CoIIIL2]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 CoII, formed the complexes [CoIIIL(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 [CoIIIL22]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 [CoIIIL12]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 CoI was observed. The different reactivity is attributed to the more strained co-ordination in [CoIIIL12]ClO4 with respect to that in [CoIIIL22]ClO4.