Nickel(II) Complexes of tpa Ligands with 6-Phenyl Substituents (Phntpa). Structure and H2O2-Reactivity

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Nickel(II) complexes supported by a series of tpa {tris[(pyridin-2-yl)methyl]amine} ligand derivatives containing different numbers of phenyl substituent on the 6-position of pyridine donor group (Phntpa, n = 0–3) have been prepared and structurally characterized. Ligand tpa and its derivatives with one or two 6-Ph substituent(s) (Ph0tpa, Ph1tpa, and Ph2tpa) afforded nickel(II) complexes 1[0], 1[1], and 1[2] with a distorted octahedral geometry, whereas the ligand having three 6-Ph groups (Ph3tpa) gave nickel(II) complex 1[3] with a five-coordinate trigonal bipyramidal structure. All the complexes exhibited a high spin ground state (S = 1). Reactivity of the nickel(II) complexes 1[0] and 1[1] toward H2O2 was very poor, whereas 1[2] and 1[3] readily gave bis(μ-oxo)dinickel(III) complexes 2[2] and 2[3], respectively, in the reaction with H2O2 at a low temperature. The bis(μ-oxo)dinickel(III) complexes 2[2] and 2[3] gradually decomposed to cause an aromatic hydroxylation reaction of one of the 6-Ph groups of the supporting ligands. The ligand substituent effects on the formation and decomposition processes of the bis(μ-oxo)dinickel(III) complexes are discussed based on the structures and the redox potentials of the starting nickel(II) complexes.