Synthesis, crystal structure and magnetic properties of a dinuclear copper(II) complex with 1,1′-(4-methylpyrazole-3,5-diyl)diacetaldehyde dioxime. Evidence for both classical and zwitterion structures

Abstract

The reaction of 1,1′-(4-methylpyrazole-3,5-diyl)diacetaldehyde dioxime (H3L) with CuCl2·2H2O produces two different isomeric dinuclear co-ordination compounds according to the temperature. Analytical results indicated the same composition [Cu2(H2L)2Cl2]·2H2O. One isomer (1) crystallized at 15 °C in the monoclinic space group P21/a with a= 11.490(5), b= 16.287(14), c= 6.253(4)A, β= 97.78(4)° and Z= 2. Refinement of 1983 reflections with I > 3σ(I) gave a final R= 0.025 (R′= 0.027). The second isomer (2) crystallized at 25 °C in the triclinic system, space group P and a= 8.520(15), b= 8.607(18), c= 8.671(14), α= 75.27(15), β= 69.96(17), γ= 76.66(15)° and Z= 1; R= 0.027 (R′= 0.031) for 2964 significant reflections. The molecular structures of the two isomers are closely related; the centrosymmetric dinuclear [Cu2(H2L)2Cl2] unit is almost planar. Each copper atom is five-co-ordinated in a distorted square-based pyramid involving an N3O in-plane donor set, with the oxygen atom of the zwitterionic form of the oxime group, the nitrogen atom of the classical form of the oxime group and two nitrogen atoms of the pyrazole heterocycle; the axial ligand is a chloride ion. The two water molecules are linked by hydrogen bonds to the oxime function. In 1 the bonding is O–H ⋯ OH2(classical structure), in 2 it is N–H ⋯ OH2(zwitterionic structure). In the IR spectrum the ν(N–H) vibration is observed only for compound 1. Magnetic susceptibility measurements performed from 5 to 300 K provides evidence of the existence of strong antiferromagnetic exchange interactions between the copper(II) ions (J=–159 cm–1 for isomer 2). These results are discussed on the basis of the structural features.