Helical-Chain Copper(II) Complexes and a Cyclic Tetranuclear Copper(II) Complex with Single Syn−Anti Carboxylate Bridges and Ferromagnetic Exchange Interactions

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Abstract
Tridentate Schiff-base carboxylate-containing ligands, derived from the condensation of 2-imidazolecarboxaldehyde with the amino acids β-alanine (H2L1) and 2-aminobenzoic acid (H2L5) and the condensation of 2-pyridinecarboxaldehyde with β-alanine (HL2), d,l-3-aminobutyric acid (HL3), and 4-aminobutyric acid (HL4), react with copper(II) perchlorate to give rise to the helical-chain complexes {[Cu(μ-HL1)(H2O)](ClO4)}n (1), {[Cu(μ-L2)(H2O)](ClO4)·2H2O}n (2), and {[Cu(μ-L3)(H2O)](ClO4)·2H2O}n (3), the tetranuclear complex {[Cu(μ-L4)(H2O)](ClO4)}4 (4), and the mononuclear complex [Cu(HL5)(H2O)](ClO4)·1/2H2O (5). The reaction of copper(II) chloride with H2L1 leads not to a syn−anti carboxylate-bridged compound but to the chloride-bridged dinuclear complex [Cu(HL1)(μ-Cl)]2 (6). The structures of these complexes have been solved by X-ray crystallography. In complexes 1−4, roughly square-pyramidal copper(II) ions are sequentially bridged by syn−anti carboxylate groups. Copper(II) ions exhibit CuN2O3 coordination environments with the three donor atoms of the ligand and one oxygen atom belonging to the carboxylate group of an adjacent molecule occupying the basal positions and an oxygen atom (from a water molecule in the case of compounds 1−3 and from a perchlorate anion in 4) coordinated in the apical position. Therefore, carboxylate groups are mutually cis oriented and each syn−anti carboxylate group bridges two copper(II) ions in basal−basal positions with Cu···Cu distances ranging from 4.541 Å for 4 to 5.186 Å for 2. In complex 5, the water molecule occupies an equatorial position in the distorted octahedral environment of the copper(II) ion and the Cu−O carboxylate distances in axial positions are very large (>2.78 Å). Therefore, this complex can be considered as mononuclear. Complex 6 exhibits a dinuclear parallel planar structure with Ci symmetry. Copper(II) ions display a square-pyramidal coordination geometry (τ = 0.06) for the N2OCl2 donor set, where the basal coordination sites are occupied by one of the bridging chlorine atoms and the three donor atoms of the tridentate ligand and the apical site is occupied by the remaining bridging chlorine atom. Magnetic susceptibility measurements indicate that complexes 1−4 exhibit weak ferromagnetic interactions whereas a weak antiferromagnetic coupling has been established for 6. The magnetic behavior can be satisfactorily explained on the basis of the structural data for these and related complexes.

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