Azonaphtholate ligands for the nickel(III) cation: design, synthesis and electrochemical properties

Abstract

Nickel(II) complexes of tridentate meridionally spanning azonaphtholate ligands, 1-(2-pyridylazo)-2-naphthol (HL¹) and 1-(2-methylsulfanylphenylazo)-2-naphthol (HL²), have been synthesised. The paramagnetic nickel(II) centres are six-co-ordinated, including the donor atoms of the pendant arms. In dichloromethane solution the nickel(III)–nickel(II) redox couple was observed by cyclic voltammetry for both the complexes [NiL¹₂] and [NiL²₂], E_½ being +0.93 and +0.65 V respectively versus saturated calomel electrode. Constant-potential electrolysis generated the nickel(III) complexes in solution. In frozen (77 K) dichloromethane solution the nickel(III) complexes display axial EPR spectra (g_∥≈ 2.20, g_⊥≈ 2.07) indicating an axial compression and (d_x²–y²)¹ ground state. The results demonstrate the significant role of the chelating nature of the ligands and thioether co-ordination in stabilising the trivalent nickel centre.