Dipicolylamine complexes of copper(II): two different coordination geometries in the same unit cell of Cu(Dipica)₂(BF₄)₂

Abstract

Bright blue Cu(Dipica)₂(BF₄)₂ crystallizes in the monoclinic space group P2₁/n, with unit cell parameters a = 23.406(6) Å, b = 9.338(2) Å, c = 25.573(7) Å, β = 95.39(2)° , and Z = 8. The structure was solved by conventional Patterson and Fourier methods. The R and R_w values for 5282 observed reflections were 7.06% and 9.10% respectively. Two structurally different complex cations are present in the same unit cell, one hexacoordinate and the other pentacoordinate. In the hexacoordinate cation, the two tridentate bis(2'-picolyl)amine ligands are trans-facially coordinated with two pyridine nitrogens and the two secondary amine nitrogens situated on four positions in a plane [Cu-_Npyr = 2.189(6), 2.146(6) Å; Cu-N_sat = 2.207(6), 2.201(5) Å]. The remaining two pyridine nitrogens constitute the axis [Cu-N_pyr = 2.035(5), 2.038(5) Å] in an equatorially expanded pseudooctahedral geometry. The pentacoordinate cation possesses a square-pyramidal configuration, the two secondary nitrogens being mutually cis, with one Dipica equatorially tridentate [Cu-N_pyr = 2.044(5), 2.027(5) Å, Cu-N_sat = 1.995(5) Å]. The other Dipica functions as a bidentate ligand, with one of the pyridine nitrogens occupying the equator [Cu-N_pyr = 1.986(5) Å] and the aliphatic nitrogen defining the axial copper position [Cu-N_sat = 2.344(5) Å]. Its second pyridine is uncoordinated but hydrogen-bonded to the coordinated NH of the other ligand. Solution properties offer no clear distinction between the two cation stereochemistries. The ternary chelates [Cu(Dipica)(Acac)]ClO₄ and [Cu(Dipica)(Bipy)](ClO₄)₂ are also described.