Helical-chain copper(II) complexes and a cyclic tetranuclear copper(II) complex with single syn-anti carboxylate bridges and ferromagnetic exchange interactions

Tridentate Schiff-base carboxylate-containing ligands, derived from the con densation of 2-imidazolecarboxaldehyde with the amino acids beta-alanine (H 2L1) and 2-aminobenzoic acid (H2L5) and the condensation of 2-pyridinecar-b oxaldehyde with beta-alanine (HL2), D,L-3-aminobutyric acid (HL3), and 4-am inobutyric acid (HL4), react with copper(II) perchlorate to give rise to th e helical-chain complexes {(Cu(mu-HL1)(H2O))(ClO4)}(n) (1), {[Cu(mu-L-2)(H2 O)](ClO4) .2H(2)O}, (2), and {[Cu(mu-L-3)(H2O)](ClO4).2H(2)O}(n) (3), the t etranuclear complex {[Cu(mu-L-4)(H2O)](ClO4)}(4) (4), and the mononuclear c omplex [Cu(HL5)(H2O)](ClO4).1/2H(2)O (5). The reaction of copDer(II) chlori de with H2L1 leads not to a syn-anti carboxylate-bridged compound but to th e chloride-bridged dinuclear complex [Cu(HL1)(mu-Cl)](2) (6). The structure s of these complexes have been solved by X-ray crystallography. In complexe s 1-4, roughly square-pyramidal copper(II) ions are sequentially bridged by syn-anti carboxylate groups. Copper(II) ions exhibit CuN2O3 coordination e nvironments with the three donor atoms of the ligand and one oxygen atom be longing to the carboxylate group of an adjacent molecule occupying the basa l positions and an oxygen atom (from a water molecule in the case of compou nds 1-3 and from a perchlorate anion in 4) coordinated in the apical positi on. Therefore, carboxylate groups are mutually cis oriented and each syn-an ti carboxylate group bridges two copper(II) ions in basal-basal positions w ith Cu Cu distances ranging from 4.541 Angstrom for 4 to 5.186 Angstrom for 2. In complex 5, the water molecule occupies an equatorial position in the distorted octahedral environment of the copper(n) ion and the Cu-O carboxy late distances in axial positions are very large (>2.78 Angstrom). Therefor e, this complex can be considered as mononuclear. Complex 6 exhibits a dinu clear parallel planar structure with Ci symmetry. Copper(n) ions display a square-pyramidal coordination geometry (tau = 0.06) for the N2OCl2 donor se t, where the basal coordination sites are occupied by one of the bridging c hlorine atoms and the three donor atoms of the tridentate ligand and the ap ical site is occupied by the remaining bridging chlorine atom. Magnetic sus ceptibility measurements indicate that complexes 1-4 exhibit weak ferromagn etic interactions whereas a weak antiferromagnetic coupling has been establ ished for 6. The magnetic behavior can be satisfactorily explained on the b asis of the structural data for these and related complexes.