The preparation and catalytic properties of copper(II) complexes derived from a pyrazole containing ligand. X-ray crystal structure of [Cu(pzmhp)(BF 4)](BF 4)

Abstract

The synthesis of the new multidentate ligand N, N′-bis(3,5-dimethylpyrazol-l-ylmethyl)-1,3-diamino-2-hydroxypropane (pzmhp) is described along with the preparation of a series of copper(II) complexes derived from this ligand. These complexes have the form [Cu(pzmhp)X]X where X = BF 4 or ClO 4 and [Cu(pzmhp)X 2] where X = Cl or Br. The complexes have been characterized on the basis of elemental analysis, IR spectroscopy, magnetic susceptibility, solution conductivity, and fast atom bombardment mass spectrometry. These methods show the ligand to coordinate to copper(II) in a mononuclear fashion with the ligand acting as an N 4 donor set with an uncomplexed alcohol oxygen. The single-crystal X-ray structure of [Cu(pzmhp)(BF 4)](BF 4) ( 1) shows it to be triclinic with space group P1, with cell dimensions α = 7.4518(5), b = 11.367(1), c = 15.037(1) A ̊ , α = 104.124(5)°, β = 95.815(7)°, γ = 98.755(8)° and with Z = 2 . The asymmetric unit comprises twice the formula unit. Each copper ion is in a distorted square planar environment of four nitrogens from the ligand ( CuN = 2.0 A ̊ ) ; additionally, two fluorine atoms are in (Semi-)coordinate axial positions ( CuF = 2.7 A ̊ ) , completing the distorted elongated octahedron around copper(II). Cyclic voltammetry shows that each complex is reduced reversibly in a one-electron process; reduction potentials range from a high of 0.00 V for [Cu(pzmhp)(BF 4)](BF 4) ( 1) to −0.205 V for [Cu(pzmhp)(Cl) 2] versus Ag/AgCl. The ability of the complexes to catalyze the oxidation of 3,5-di-tert-butylcatechol to 3,5-di-tert-butylquinone was studied showing all of the complexes to be catalytically active. The complexes with weakly corrdinating counterions, ([Cu(pzmhp)(BF 4)](BF 4) ( 1) and [Cu(pzmhp)(ClO 4)][ClO 4) ( 2)), have activities of 0.031 and 0.028 mmol substrate per mg catalyst per min respectively, while the complexes with stronger coordinating anions ([Cu(pzmhp)(Br) 2] ( 3) and [Cu 2(pzmhp)(Cl) 2]∗dH 2O ( 4)), have lower catalytic capabilities with activities of 0.015 and 0.007 mmol substrate per mg catalyst per min respectively.