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H2O2-Reactivity of Copper(II) Complexes Supported by Tris[(pyridin-2-yl)methyl]amine Ligands with 6-Phenyl Substituents

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posted on 15.12.2008, 00:00 by Atsushi Kunishita, Minoru Kubo, Hirohito Ishimaru, Takashi Ogura, Hideki Sugimoto, Shinobu Itoh
The structure and H2O2-reactivity of a series of copper(II) complexes supported by tris[(pyridin-2-yl)methyl]amine (TPA) derivatives having a phenyl group at the 6-position of pyridine donor group(s) [(6-phenylpyridin-2-yl)methyl]bis[(pyridin-2-yl)methyl]amine (Ph1TPA), bis[(6-phenylpyridin-2-yl)methyl][(pyridin-2-yl)methyl]amine (Ph2TPA), and tris[(6-phenylpyridin-2-yl)methyl]amine (Ph3TPA) have systematically been examined to get insights into the aromatic substituent (6-Ph) effects on the coordination chemistry of TPA ligand system. The X-ray crystallographic analyses have revealed that [CuII(TPA)(CH3CN)](ClO4)2 (CuTPA) and [CuII(Ph3TPA)(CH3CN)](ClO4)2 (3) exhibit a trigonal bipyramidal structure, whereas [CuII(Ph1TPA)(CH3CN)](ClO4)2 (1) shows a slightly distorted square pyramidal structure and [CuII(Ph2TPA)(CH3CN)](ClO4)2 (2) has an intermediate structure between trigonal bipyramidal and square pyramidal. On the other hand, the UV−vis and ESR data have suggested that all the copper(II) complexes have a similar trigonal bipyramidal structure in solution. The redox potentials of CuTPA, 1, 2, and 3 have been determined as E1/2 = −0.34, −0.28, −0.16, and −0.04 mV vs Ag/AgNO3, respectively, demonstrating that introduction of each 6-Ph group causes positive shift of E1/2 about 0.1 V. Notable difference in H2O2-reactivity has been found among the copper(II) complexes. Namely, CuTPA and 1 afforded mononuclear copper(II)-hydroperoxo complexes CuTPA-OOH and 1-OOH, respectively, whereas complex 2 provided bis(μ-oxo)dicopper(III) complex 2-oxo. On the other hand, copper(II) complex 3 was reduced to the corresponding copper(I) complex 3red. On the basis of the H2O2-reactivity together with the X-ray structures and the redox potentials of the copper(II) complexes, the substituent effects of 6-Ph are discussed in detail.