Nickel(II)−Phenoxyl Radical Complexes: Structure−Radical Stability Relationship

Nickel(II) complexes of N3O-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(H2O)] (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 CH3CN,
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 E1/2 = 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.