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With the discovery of molecular complexes exhibiting slow relaxation of the magnetization and magnetic hysteresis at low temperature, research activity in the field of molecular magnetism based on coordination compounds has experienced spectacular growth. [1] These nanomagnets, called single-molecule magnets (SMMs), [1][2][3] straddle the quantum/ classical interface showing quantum effects, such as quantum tunneling of the magnetization and quantum phase interference, and have potential applications in molecular spintronics, ultra-high density magnetic information storage, and quantum computing at the molecular level. [3] The motivation of much of this research activity has been provided by the prospect of integrating SMMs into nanosized devices. The origin of the SMM behavior is the existence of an energy barrier that prevents reversal of the molecular magnetization, [1] although the currently observed energy barriers are (relatively) low and therefore SMMs act as magnets only at very low temperature. To increase the height of the energy barrier and therefore to improve the SMM properties, systems with large spin-ground states and/or with large magnetic anisotropy are required. The early examples of SMMs were clusters of transition metal ions, [2] but recently mixed 3d/4f metal aggregates, [4] low-nuclearity 4f metal complexes, [5] and even mononuclear complexes (called single-ion magnets, SIMs) of lanthanide, [6] actinide, [7] and transition-metal ions [8] have been reported to exhibit slow relaxation of the magnetization.It should be noted that for integer-spin systems with D < 0 fast quantum tunneling of the magnetization (QTM) through the mixing of AE Ms levels may suppress the observation of slow magnetic relaxation through a thermally activated mechanism. QTM is promoted by transverse zero-field splitting (E), hyperfine interactions, and/or dipolar interactions. [1] The application of a small direct current (dc) field, stabilizing the negative Ms levels with regard to the positive ones, may remove the degeneracy of the AE Ms levels on either side of the energy barrier, tilting the system out of resonance and, on occasion, enabling the thermally activated mechanism. For non-integer spin systems with D < 0, the mixing of the degenerate ground state AE Ms levels through transverse anisotropy (E) is forbidden, thus favoring observation of the thermally activated relaxation process. [9] This situation, together with the fact that mononuclear species can exhibit larger anisotropies than their multinuclear counterparts (the

Tridentate Schiff-base carboxylate-containing ligands, derived from the condensation of 2-imidazolecarboxaldehyde with the amino acids beta-alanine (H2L1) and 2-aminobenzoic acid (H2L5) and the condensation of 2-pyridinecarboxaldehyde with beta-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(mu-HL1)(H2O)](ClO4)]n (1), [[Cu(mu-L2)(H2O)](ClO4).2H2O]n (2), and [[Cu(mu-L3)(H2O)](ClO4).2H2O]n (3), the tetranuclear complex [[Cu(mu-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)(mu-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 A for 4 to 5.186 A 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 A). 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 (tau = 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.