A mixed-bridged Cu3 cluster with an isosceles-triangular array of copper(II) ions: synthesis, crystal structure and magnetic properties

Abstract

The reaction of H₂L [H₂L =N,N′-bis(1,3-dimethyl-5-nitrosopyrimidine-2,4(1H,3H)-dion-6-yl)propylenediamine] with an excess of Cu(ClO₄)₂·6H₂O in ethanol gave the dinuclear complex [Cu(µ-HL)-Cu(H₂O)₂(EtOH)][ClO₄]₃1, magnetic studies of which revealed that the antiferromagnetic exchange interaction mediated by the oximate groups is so strong that it causes a complete spin coupling at room temperature. This compound reacted with NaCl in ethanol–water (5:1) to afford the mixed-bridged trinuclear copper(II) complex [{Cu(µ-HL)Cu(H₂O)Cu(µ-L)}(µ-Cl)][ClO₄]₂·4.88H₂O 2 the crystal structure of which was solved by single-crystal X-ray diffraction. Compound 2 consists of discrete trinuclear cations [{Cu(µ-HL)Cu(H₂O)Cu(µ-L)}(µ-Cl)]²⁺, non-co-ordinated perchlorate anions and lattice water molecules. Within the trinuclear cation, which has C_2v symmetry the three copper ions are arranged at the corners of an isosceles triangle. Each terminal copper atom Cu(2) and the central copper atom Cu(1) are bibridged by syn,syn oximate groups with a Cu(1)⋯ Cu(2) distance of 3.373(4)Å, whereas a chloride anion bridges the two terminal copper atoms with a Cu(2)⋯ Cu(2) distance of 4.770(4)Å. The angles Cu(2)–Cu(1)–Cu(2) and Cu(2)–Cl(1)–Cu(2) are 90.0(1) and 144.4(4)°, respectively. The central Cu(1) exhibits a square pyramidal CuO₅ co-ordination environment with four oxygen atoms of oximate groups in the basal plane and the water oxygen at the apical site. The terminal Cu(2) atoms occupy the N₄ inner site of the ligand with the chloride bridging anion occupying the axial position of the square pyramidal CuN₄Cl co-ordination environment. From magnetic susceptibility measurements, complex 2 was found to exhibit a very strong antiferromagnetic exchange interaction through the oximate bridges (J=–189 cm^–1), leading to complete spin coupling at 120 K. Magnetic parameters are discussed on the basis of the structural results and compared with those of other related complexes.