A Novel Series of Heterotrinuclear Complexes Involving Imidazolate Bridging Schiff's Base Ligands. Synthesis, Crystal Structure, and Magnetic Properties

Abstract

Mononuclear copper(II) complexes with an unsymmetrical tridentate Schiff's base involving two imidazole moieties, [Cu(HL)(H2O)](ClO4)2 (1) and [Cu(L)(hfac)] (3), have been prepared and characterized, where HL stands for the 1:1 condensation product of 2-imidazolecarboxaldehyde and histamine. Under basic conditions, 1 easily undergoes a self-assembly process affording an insoluble imidazolate-bridged polymeric species {[Cu(L)(H2O)](ClO4)}n (2). By using 1 and 3 as “ligand complexes” toward M(hfac)2 (hfac = hexafluoroacetylacetonate) a series of imidazolate-bridged heterometal trinuclear complexes with the formula [Cu(L)(hfac)M(hfac)2Cu(hfac)(L)] (M = ZnII, CuII, NiII, MnII) (4−7) can be prepared. The structures of these complexes have been solved by X-ray crystallographic methods. All the trinuclear complexes are isostructural and consist of trinuclear molecules with a V-shaped conformation, which are formed by two CuL(hfac) “ligands” bonded through imidazolate bridges to the central metal in cis-position. The ligand environment about the central metal atom can be described as a distorted octahedron, with the nitrogen atoms from the imidazolate groups and two oxygen atoms of the hfac ligands in the equatorial plane and the remaining two oxygen atoms from the hfac ligands filling the axial positions at longer distances. The coordination geometry of the copper(II) ion is very close to a square pyramid with one oxygen atom from the hfac in a more distant axial position. The magnetic susceptibility data (2−300 K) revealed antiferromagnetic interactions between copper(II) ions and the central metal. The magnetic susceptibility data were quantitatively analyzed using the theoretical expressions deduced from the spin Hamiltonian for a symmetrical three-spin system H = −JCuM(SCu1·SM + SM·SCu2) to give the coupling parameters JCuCu = −60.6 cm-1, JCuNi = −21.6 cm-1, and JCuMn = −3.2 cm-1. These magnetic behaviors are discussed on the basis of the localized-orbital model of exchange interactions.