Low-spin octahedral cobalt(II) complexes of CoN6 and CoN4P2 chromophores. Synthesis, spectroscopic characterisation and electron-transfer properties†

Abstract

The reaction of 2-(arylazo)pyridines (NC₅H₄)NNC₆H₄R L¹–L⁷ (R = H, o-Me/Cl, m-Me/Cl, p-Me/Cl) with cobalt(II) perchlorate hexahydrate in absolute ethanol under anaerobic conditions afforded low-spin [Co^IIL₃]²⁺ complexes, isolated as ClO₄^– salts. At room temperature the complexes are one-electron paramagnetic in nature, low-spin Co^II, t_2g⁶e_g¹, S = ½ and behave as 1∶2 electrolytes in acetonitrile solvent. In acetonitrile solvent they show a ligand-to-metal charge-transfer (LMCT) band near 400 nm, an intraligand transition near 300 nm and ligand-field d–d transitions in the range 860–600 nm. The complexes exhibit quasi-reversible Co^II–Co^III couples near 1 V and six sequential ligand reductions (NN groups) in the range 0.2 to –1.8 V versus saturated calomel electrode (SCE). At room temperature in the solid state they exhibit isotropic EPR spectra but at 77 K, both in the polycrystalline state and in the dichloromethane solution, display rhombic spectra. Reaction of [Co^IIL₃]²⁺ with 2,2′-bipyridine (bpy) and 1,10-phenanthroline (phen) resulted in complete ligand-exchanged products with concomitant metal oxidation, low-spin [Co^III(bpy)₃]³⁺ and low-spin [Co^III(phen)₃]³⁺ respectively. The reaction of PPh₃ with the [Co^II(L⁷)₃]²⁺ [L⁷ = 2-(p-chlorophenylazo)pyridine] yielded a partial ligand-exchanged product, low-spin [Co^II(L⁷)₂(PPh₃)₂]²⁺, isolated as its ClO₄^– salt. The complex is one-electron paramagnet and a 1∶2 electrolyte in acetonitrile solvent. It displays an LMCT band at 401 nm, an intraligand transition at 305 nm and four d–d transitions in the range 870–640 nm. It exhibits irreversible Co^II to Co^III oxidation at 1.33 V (E_pa) and four successive ligand reductions in the range –0.30 to –1.1 V versus SCE. At 77 K the complex displays an axial EPR spectrum.