CRYSTAL-STRUCTURE, MAGNETIC-PROPERTIES AND SPECTROSCOPIC STUDIES OF BIS[1,3-BIS(PYRAZOL-3-YL)TRIAZENIDO]DICOPPER(II) TRIHYDRATE - A DINUCLEAR COMPLEX WITH AN ASYMMETRIC DOUBLE PYRAZOLATE BRIDGE

The crystal and molecular structure of [{Cu(bpzt)}(2)]. 3H(2)O [H(2)bp zt = 1,3-bis(pyrazol-3-yl)triazene] was determined by X-ray diffractio n methods. The complex has a quasi-planar dinuclear structure with an asymmetric double pyrazolate bridge. The asymmetry results from the pr esence, on the C-3 Of each bridging pyrazolate, of a pyrazolyltriazeni de chelating arm which completes the equatorial CuN4 co-ordination. Th e centrosymmetric dinuclear units have intramolecular Cu ... Cu distan ces of 3.852(5) and 3.839(4) Angstrom respectively. The units are stac ked in columns along the b axis. The cell also contains crossing chann els filled with water molecules. The complex was characterized by spec troscopic and magnetic measurements. Magnetic susceptibility data show antiferromagnetic behaviour with an exchange parameter \J\ = 151 cm(- 1) significantly decreased with-respect to symmetric doubly bridged co mplexes (\J\ = approximate to 200 cm(-1)). The X-band powder EPR. spec trum recorded at 77 K is typical of a triplet spin state lying above t he singlet ground state. The results obtained for this complex are com pared to recent literature data on other planar doubly bridged pyrazol ate dicopper(II) complexes. A correlation, valid for symmetric as well as asymmetric systems, is described between the exchange parameter J and the deviation of the Cu-N-N bridge angle from the optimum value co rresponding to the highest \J\.