Nickel(II)-phenoxyl radical complexes: Structure-radical stability relationship

Nickel(II) complexes of N ₃O-donor tripodal ligands, 2,4-di-tert-butyl-6-{([bis(2-pyridyl)methyl]amino)methyl}phenol (HtbuL), 2,4-di-tert-butyl-6-{([(6-methyl-2-pyridyl)methyl](2-pyridylmethyl)amino)methyl} phenol (HtbuLMepy), and 2,4-di-tert-butyl-6-{([bis(6-methyl-2-pyridyl)methyl] amino)methyl}phenol (HtbuL(Mepy) ₂), were prepared, and [Ni(tbuL)-Cl(H ₂O)] (1), [Ni(tbuLMepy)Cl] (2), and [Ni(tbuL(Mepy) ₂)Cl] (3) were structurally characterized by the X-ray diffraction method. Complexes 1 and 3 have a mononuclear structure with a coordinated phenolate moiety, while 2 has a dinuclear structure bridged by two chloride ions. The geometry of the Ni(II) center was found to be octahedral for 1 and 2 and 5-coordinate trigonal bipyramidal for 3. Complexes 1-3 exhibited similar absorption spectra in CH ₃CN, indicating that they all have a mononuclear structure in solution. They were converted to the phenoxyl radicals upon oxidation with Ce(IV), giving a phenoxyl radical π-π* transition band at 394-407 nm. ESR spectra at low temperature and resonance Raman spectra established that the radical species has a Ni(II)-phenoxyl radical bond. The cyclic voltammograms showed a quasi-reversible redox wave at E _1/2 = 0.46-0.56 V (vs Ag/AgCl) corresponding to the formation of the phenoxyl radical, which displayed a first-order decay with a half-life of 45 min at room temperature for 1 and 26 and 5.9 min at -20°C for 2 and 3, respectively. The radical stability increased with the donor ability of the N ligands.