Preparations, Structures, and Properties of Cu(II) Complexes with Tripodal Tetradentate Ligand, Tris(6-pivaloylamino-2-pyridylmethyl)amine (Htppa), and Reaction of Its Cu(I) Complex with Dioxygen

Abstract

Abstract Mononuclear copper complexes with a tripodal tetradentate ligand, tris(6-pivaloylamino-2-pyridylmethyl)amine (Htppa), and several anions have been prepared as a model metal centre of copper enzymes, whose structures have been examined by electronic absorption, ESR and NMR spectral, cyclic voltammetry, and X-ray diffraction methods. Those with an anion such as OH−, Cl−, Br−, and I− were shown to form a trigonal-bipyramidal geometry. The X-ray structures of [Cu(Htppa)](ClO4)2·2H2O·CH3OH, [CuCl(Htppa)]ClO4, [Cu(OH)(Htppa)]ClO4, [Cu(OH)(tppa)], and [Cu(tppa)](BPh4), obtained as a single crystal, revealed all trigonal-bipyramidal geometry with four nitrogen atoms of Htppa in trigonal plane and axial position and with the anion in another apical position. The reaction with NaN3 gave [Cu(N3)2(Htppa)], whose crystal structure was a square-pyramid with three nitrogen atoms of Htppa and N3− in square-planar positions and with another N3− in apical position. The redox potentials of [CuCl(Htppa)]ClO4 in MeCN or CH2Cl2 showed almost reversible CuI/CuII couples at 0.225 and 0.300 V vs. Ag/AgCl, respectively, which are characteristically high for the usual copper complexes. The addition of dioxygen to the Cu(I)–Htppa complex prepared from [Cu(MeCN)4]ClO4 and Htppa (1 : 1) in methanol solution at −78 °C resulted in an immediate absorption spectral change, with two well-separated absorptions at 657 (ε = 110 M−1 cm−1) and 803 nm (ε = 120 M−1 cm−1) and an intense band at 315 nm (ε = ca. 4000 M−1 cm−1) as a shoulder. Simultaneous ESR experiments of the same complex solution exhibited a silent spectrum, and the NMR spectrum at −80 °C was diamagnetic. These facts indicate the formation of CuII–O2−species. Interestingly, the CO gas bubbling into a CH3OH/THF/EtCN (4 : 3 : 3) solution of the superoxo complex species at −78 °C led to a significant color change from yellow-green to pale yellow, and alternate bubblings of O2 and CO to the solution exhibited a reversible spectral change with an isosbestic point at 521 nm due to the reversible formation of the CuII–O2− species.