Preparations, Structures, and Properties of Copper(II) Complexes with a New Tripodal Tetradentate Ligand, N-(2-Pyridylmethyl)bis(6-pivalamido-2-pyridylmethyl)amine, and Reactivities of the Cu(I) Complex with Dioxygen

Abstract

Abstract Copper complexes of a tripodal tetradentate ligand, N-(2-pyridylmethyl)bis(6-pivalamido-2-pyridylmethyl)amine (Hbppa), have been prepared as a model of metal centers of mononuclear copper enzymes; their structures and properties have been examined together with those having several secondary ligands by electronic absorption, ESR and FAB mass spectral, cyclic voltammetric, and X-ray diffraction methods. The complexes, [Cu(Hbppa)](ClO4)2 and [Cu(Hbppa)(N3)]ClO4·H2O, were obtained as single crystals, whose crystal structures revealed square-planar and trigonal-bipyramidal geometries, respectively. The electronic absorption and ESR spectra for the CuII(Hbppa)–X systems (X = no, Cl−, Br−, I−, N3−, and CH3COO−) allowed us to conclude that the complexes form several coordination geometries, such as square-planar, square-pyramidal, and trigonal-bipyramidal, depending upon the solvents (MeOH, MeCN). The redox potentials of [Cu(Hbppa)Cl]Cl in MeCN, THF, MeOH, and CH2Cl2 showed quasi-reversible CuI/CuII couples in the range of −0.056 — +0.085 V vs. Ag/AgCl at room temperature. The addition of dioxygen to the Cu(I)–Hbppa, which was prepared from [Cu(MeCN)4]PF6 and Hbppa (1 : 1) in EtCN at −78 °C, resulted in a graduate absorption spectral change with two well-separated absorption maxima at 665 (ε = 162 M−1 cm−1) and 837 nm (ε = 305 M−1 cm−1) and an intense band at 375 nm (ε = 641 M−1 cm−1) as a shoulder. Simultaneous ESR experiments of the same complex solution exhibited a silent spectrum, indicating that the complex is diamagnetic. A similar electronic absorption spectral change was observed in MeOH with absorption peaks at 387 nm (ε = 843 M−1 cm−1), 640 (ε = 183 M−1 cm−1) and 828 (ε = 289 M−1 cm−1), although the ESR spectrum did not continue to be completely silent. Increasing the temperature of the solution up to room temperature demonstrated the formation of [Cu(Hbppa)(OH)]− species, whose X-ray structure was [Cu(Hbppa)(OH)]PF6·H2O. The reaction of the [Cu(Hbppa)]ClO4–sodium benzoylformate system with dioxygen in DMF resulted in production of carbon dioxide and benzoic acid, as analyzed by GC and HPLC.