Structural and electrochemical comparison of copper(II) complexes with tripodal ligands

A series of copper(II) complexes with tripodal polypyridylmethylamine ligands, such as tris(2-pyridylmethyl)amine (tpa), ((6-methyl-2-pyridyl)methyl)bis(2-pyridylmethyl)amine (Me(1)tpa), bis((6-methyl-2-pyridyl)methyl)(2-pyridylmethyl)amine (Me(2)tpa), and tris((6-methyl-2-pyridyl)methyl)amine (Me(3)tpa), have been synthesized and characterized by X-ray crystallography. [Cu(H2O)(tpa)](ClO4)(2) (1) crystallized in the monoclinic system, space group P2(1)/a, with a = 15.029(7) Angstrom, b = 9.268(2) Angstrom, c = 17.948(5) Angstrom, beta = 113.80(3)degrees and Z = 4 (R = 0.061, R(w) = 0.059). [CuCl(Me(1)tpa)]ClO4 (2) crystallized in the triclinic system, space group <P(1)over bar>, with a = 13.617(4) Angstrom, b = 14.532(4) Angstrom, c = 12.357(4) Angstrom, alpha = 106.01(3)degrees, beta = 111.96(2)degrees, gamma = 71.61(2)degrees, and Z = 4 (R = 0.054, R(w) = 0.037). [CuCl(Me(2)tpa)]ClO4 (3) crystallized in the monoclinic system, space group P2(1)/n, with a = 19.650(4) Angstrom, b = 13.528(4) Angstrom, c = 8.55(1) Angstrom, beta = 101.51(5)degrees and Z = 4 (R = 0.071, R(w) = 0.050). [CuCl(Me(3)tpa)][CuCl2(Me(3)tpa)]ClO4 (4) crystallized in the monoclinic system, space group P2(1)/a, with a = 15.698(6) Angstrom, b = 14.687(7) Angstrom, c = 19.475(4) Angstrom, beta = 97.13(2)degrees, and Z = 4 (R = 0.054, R(w) = 0.038). All the Cu atoms of 1-4 have pentacoordinate geometries with three pyridyl and one tertiary amino nitrogen atoms, and a chloride or aqua oxygen atom. Nitrite ion coordinated to the Cu(II) center of Me(1)tpa, Me(2)tpa, and Me(3)tpa complexes with only oxygen atom to form nitrito adducts. The cyclic voltammograms of [Cu(H2O)(Me(n)tpa)](2+) (n = 0, 1, 2, and 3) in the presence of NO2- in H2O (pH 7.0) revealed that the catalytic activity for the reduction of NO2- increases in the order Me(3)tpa << Me(2)tpa << Me(1)tpa < tpa complexes.