Versatility of squaric acid as ligand. Crystal, molecular structures and magnetic studies of four poly- or dimetallic Cu(II) complexes

Abstract

The synthesis and magnetic data of four copper(II) complexes obtained from squaric acid and bipyridyl or imidazole as ligand are described. Cu(C4O4)(imidazole)2•2H2O (compound I) is made of chains of squarato-1,3-bridged Cu(II) ions. The compound crystallizes in the monoclinic system, space group P21/n, and lattice parameters a = 7.455(2) Å, b = 8.196(2) Å, c = 11.235(2) Å and β = 100.55(2)°. RF = 0.067 for 1076 observed reflections. [Cu2(bipyridyl)2(C4O4)(H2O)4](C4O4H)2-4H2O (compound II) is a C4O4H− salt of a squarato-1,3 bridged centrosymmetric Cu(II) dimeric cation. The compound crystallizes in the triclinic system, space group P-1and lattice parameters a = 9.021(2) Å, b = 9.017(2) Å, c = 12.581(4) Å, α = 98.58(2)°,β = 106.81(2)°, and γ = 106.03(2)°. RF = 0.038 for 2376 reflections. Each copper atom is pentacoordinated in a distorted square-based pyramidal and is linked by two nitrogen atoms of the bipyridyl, one oxygen atom of the squarato bridge, and one oxygen atom of a water molecule in the plane and one water molecule out of plane. [Cu2(bipyridyl)2(C4O4) (H2O)2Br2] (compound III) is a squarato-1,3 bridged copper(II) dimer. The compound crystallizes in the monoclinic system, space group P21/n, and lattice parameters a = 10.480(4) Å, b = 8.503(1) Å, c = 14.590(2) Å, and β = 99.75(2)°. RF = 0.048 for 1423 reflections. Each copper atom is pentacoordinated in a distorted square-based pyramidal and is linked by two nitrogen atoms of the bipyridyl, one oxygen atom of the squarato bridge and one oxygen atom of a water molecule in the plane, and one bromine atom out of the plane.In complexes I, II, and III, the magnetic susceptibility data could be best described by neglecting the exchange coupling. The magnetic data of [Cu(bipyridyl)(C4O4)•-4H2O]n (IV) are best explained in terms of an alternating Heisenberg ferrornagnet. The epr spectra ratify results obtained from magnetic and crystallographic measurements. Keywords: squaric acid, 3,4-dihydroxy-3-cyclobutene-1,2-dion, copper(II) dimers, copper(II) chains, magnetic properties.