Cobalt complexes with redox-active anthraquinone-type ligands

Abstract

Three anthraquinone-type multidentate ligands, HL^1–3 (HL = 2-R-1H-anthra[1,2-d]imidazole-6,11-dione; HL¹: R = (2-pyridyl), HL²; R = (4,6-dimethyl-2-pyridyl), HL³; R = (6-methoxy-2-pyridyl)), were prepared, and their complexation behaviour was investigated. Three bis-chelate cobalt complexes with the formula [Co^II(L^1–3)₂]·n(solv.) (1, 2, and 3 for HL¹, HL², and HL³, respectively), in which the ligands adopted tridentate binding modes, were synthesized and structurally characterized by single-crystal X-ray analyses. Electrochemical studies of 1–3 in CH₂Cl₂ reveal three reversible redox waves, assigned to ligand and cobalt-centred processes. Additional complexes were obtained in which HL¹ adopted a bidentate binding mode, stabilising the mono-chelate [Co^II(HL¹)(NO₃)₂(DMF)₂] (4) species and tris-chelate [Co^III(L¹)₃] (5) complex in which the cobalt ion was in its 3+ state. The electrochemical properties of complex 5 were investigated in DMF, and the Co(II)/Co(III) redox couple was found to be negatively shifted compared to that of complex 1, while the ligand-based processes became irreversible. Tridentate chelation is found to stabilise the anthraquinone ligands and unlocks their redox multi-stability.