Synthesis, characterization, and thermal stability of new mononuclear hydrogenperoxocopper(II) complexes with N3O-type tripodal ligands bearing hydrogen-bonding interaction sites

In order to understand the effect of an oxygen-containing Ligand on the physico-chemical properties and reactivities of hydrogenperoxocopper complexes, new copper(II) complexes with the N3O-type tripodal ligand bearing pivalamido groups, N,N-bis(6-pivalamido-2-pyridylmethyl)glycine (Hbpga), and N,N-bis(6-pivalamido-2-pyridylmethyl)-beta-alanine (Hbpaa), have been designed and synthesized. Copper(II) complexes without any external ligand and those with a monodentate ligand, such as azido and chloro, have been prepared and characterized with the aid of electronic absorption and ESR spectroscopic, cyclic voltammetric, and X-ray structure analytical methods. The redox potential values of the Cu(II) complexes, when they were compared with the Cu(II) complex of bis(6-pivalamido-2-pyridylmethyl)(2-pyridylmethyl)amine (bppa), reported previously, shifted toward the negative side upon the introduction of a carboxylate group in the place of one pyridine of bppa. Reactions of [Cu(bpga)]ClO4 (1a) and [Cu(bpaa)]PF6 (2a) with hydrogen peroxide in the presence of triethylamine in both MeCN and MeOH solutions gave mononuclear copper(II) complexes with hydrogenperoxide(1-), Cu-bpga-OOH (1d) and Cu-bpaa-OOH (2d) systems, respectively. The intense absorption bands, assignable to LMCT (HOO- --> Cu(II)) and d-d bands, and ESR and resonance Raman spectra have revealed that they form trigonal bipyramidal copper complexes with OOH- in an end-on fashion. The thermal stabilities of 1d and 2d have also been studied by following the reduction rate of the LMCT bands at 283 K. Those of copper(II) complexes with hydrogenperoxide(1-) have been reduced in the order 1d > 2d > [Cu(bppa)(OOH)](+) (3d), all of which are rather stable compared with that of Cu(II)-tpa-OOH (tpa = tris(2-pyridylmethyl)amine). These findings indicate that the hydrogenperoxocopper(II) complexes are activated by introducing carboxylate coordination, although they are stabilized by hydrogen-bonding interactions.