Dinuclear cobalt(II) and cobalt(III) complexes of bis-bidentate napthoquinone ligands
The combination of bridging bis-bidentate redox-active ligands derived from 3,3-bis-2-hydroxy-1,4-naphthoquinone (bhnqH2), ancillary ligands based on tris(2-pyridylmethyl)amine (tpa) and cobalt salts has afforded a new family of dinuclear cobalt complexes. Compounds of the complexes [Co2(bhnq)(tpa)2]2+ (1), [Co2(bhnq)(Metpa)2]2+ (2), [Co2(bhnq)(Me2tpa)2]2+ (3) [Co2(bhnq)(Me3tpa)2]2+ (4), [Co2(bhnq)(tpa)2]4+ (5), [Co2(bhMenaph)(tpa)2]2+ (6) and [Co2(bhPronaph)(tpa)2]2+ (7) (Mentpa involves n = 0, 1, 2 and 3 methyl groups at the 6-position of the tpa pyridine rings; bhMenaphH4 = bis-3,4-dihydroxy-4-methoxynaphthalene-1-one; bhPronaphH4 = bis-3,4-dihydroxy-4-(2-oxopropyl)naphthalen-1(4H)-one) have been characterised by single crystal X-ray diffraction. While complexes 1–4 possess divalent cobalt centres, trivalent cobalt is evident in complexes 5–7. The bis-bidentate redox-active bridging ligand remains in the diamagnetic quinone bhnq2− redox state in complexes 1–5. Metal-catalysed reaction with methoxide or acetone enolate ions gives rise to the derivatised bridging ligands present in 6 and 7. The electronic properties of compounds of 1–7 have been explored in the solid state by infrared spectroscopy and variable temperature magnetic measurements and in solution by electronic absorption spectroscopy and cyclic voltammetry.