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 (Me₁tpa), bis((6-methyl-2-pyridyl)methyl)(2-pyridylmethyl)amine (Me₂tpa), and tris((6-methyl-2-pyridyl)methyl)amine (Me₃tpa), have been synthesized and characterized by X-ray crystallography. [Cu(H₂O)(tpa)](ClO₄)₂ (1) crystallized in the monoclinic system, space group P2₁/a, with a = 15.029(7) Å, b = 9.268(2) Å, c = 17.948(5) Å, β = 113.80(3)°, and Z = 4 (R = 0.061, R_w = 0.059). [CuCl(Me₁tpa)]ClO₄ (2) crystallized in the triclinic system, space group P1̄, with a = 13.617(4) Å, b = 14.532(4) Å, c = 12.357(4) Å, α = 106.01(3)°, β = 111.96(2)°, γ = 71.61(2)°, and Z = 4 (R = 0.054, R_w = 0.037). [CuCl(Me₂tpa)]ClO₄ (3) crystallized in the monoclinic system, space group P2₁/n, with a = 19.650(4) Å, b = 13.528(4) Å, c = 8.55(1) Å, β = 101.51(5)°, and Z = 4 (R = 0.071, R_w = 0.050). [CuCl(Me₃tpa)][CuCl₂(Me₃tpa)]ClO₄ (4) crystallized in the monoclinic system, space group P2₁/a, with a = 15.698(6) Å, b = 14.687(7) Å, c = 19.475(4) Å, β = 97.13(2)°, 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 Me₁tpa, Me₂tpa, and Me₃tpa complexes with only oxygen atom to form nitrito adducts. The cyclic voltammograms of [Cu(H₂O)(Me_ntpa)]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 Me₃tpa ≪ Me₂tpa ≪ Me₁tpa < tpa complexes.