Steric and hydrogen-bonding effects on the stability of copper complexes with small molecules

A series of the copper(II) complexes with tripodal tetradentate tris(pyridyl 2-methyl)amine-based ligands possessing the hydrogen-bonding 6-aminopyridine units (tapa, three amino groups; bapa, two amino groups; mapa, one amino group) have been synthesized, and their copper(II) complexes with a small molecule such as dioxygen and azide have been studied spectroscopically and structurally. The reaction of their Cu(II) complexes with NaN3 have given the mononuclear copper complexes with azide in an end-on mode, [Cu(tapa)(N-3)]ClO4 (1a), [Cu(bapa)(N-3)]ClO4 (2a), [Cu(mapa)(N-3)]ClO4 (3a), and [Cu(tpa)(N-3)]ClO4 (4a) (tpa, no amino group). The crystal structures have revealed that the coordination geometries around the metal centers are almost a trigonal-bipyramidal rather than a square-planar except for la with an intermediate between them. The UV-vis and ESR spectral data indicate that the increase of NH2 groups of ligands causes the structural change from trigonal-bipyramidal to square-pyramidal geometry, which is regulated by a combination of steric repulsion and hydrogen bond. The steric repulsion of amino groups with the azide nitrogen gives rise to elongation of the Cu-N-py bonds, which leads to the positive shift of the redox potentials of the complexes. The hydrogen bonds between the coordinated azide and amino nitrogens (2.84-3.05 Angstrom) contribute clearly to the fixation of azide. The Cu(l) complexes with bapa and mapa ligands have been obtained as a precipitate, although that with tapa was not isolated. The reactions of the Cu(I) complexes with dioxygen in MeOH at -75 degreesC have given the trans-mu-1,2 peroxo dinuclear Cu(II) complexes formulated as [{(tapa)Cu}(2)(O-2)](2+) (1c), [{(bapa)Cu}(2)(O-2)](2+) (2c), and [{(mapa)Cu}(2)(O-2)](2+) (3c), whose characterizations were confirmed by UV-vis, ESR, and resonance Raman spectroscopies. UV-vis spectra of 1c, 2c, and 3c exhibited intense bands assignable to pi*(O-2(2-))-to-d(Cu) charge transfer (CT) transitions at lambda(max)/nm (epsilon/M(-1)cm(-1)) = 449 (4620), 474 (6860), and 500 (9680), respectively. The series of the peroxo, adducts generated was ESR silent. The resonance Raman spectra exhibited the enhanced features assignable to two stretching vibrations v(O-16-O-16/O-18-O-18)/cm(-1) and v(Cu-O-16/Cu-O-18)/cm(-1) at 853/807 (1c), 858/812 (2c), 847/800 (3c), and at 547/522 (2c), 544/518 (3c), respectively. The thermal stability of the peroxo-copper species has increased with increase in the number of the hydrogen-bonding interactions between the peroxide and amino groups.