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

The reaction of 1,1'-(4-methylpyrazole-3,5-diyl)diacetaldehyde dioxime (H3L) with CuCl2.2H2O produces two different isomeric dinuclear co-or dination compounds according to the temperature. Analytical results in dicated the same composition [Cu2(H2L)2Cl2].2H2O. One isomer (1) cryst allized at 15-degrees-C in the monoclinic space group P2(1)/a with a = 11.490(5), b = 16.287(14), c = 6.253(4) angstrom, beta = 97.78(4)degr ees and Z = 2. Refinement of 1983 reflections with I > 3sigma(I) gave a final R = 0.025 (R' = 0.027). The second isomer (2) crystallized at 25-degrees-C in the triclinic system, space group P1BAR and a = 8.520( 15), b = 8.607(18), c = 8.671(14), alpha =75.27(15), beta = 69.96(17), gamma = 76.66(15)degrees and Z = 1; R = 0.027 (R' = 0.031) for 2964 s ignificant reflections. The molecular structures of the two isomers ar e closely related; the centrosymmetric dinuclear [Cu2(H2L)2Cl2] unit i s almost planar. Each copper atom is five-co-ordinated in a distorted square-based pyramid involving an N3O in-plane donor set, with the oxy gen atom of the zwitterionic form of the oxime group, the nitrogen ato m of the classical form of the oxime group and two nitrogen atoms of t he pyrazole heterocycle; the axial ligand is a chloride ion. The two w ater 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 nu(N-H) vibration is observed only for compound 1. Magnetic susceptibility measurements pe rformed from 5 to 300 K provides evidence of the existence of strong a ntiferromagnetic 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.