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)bpzt = 1,3-bis(pyrazol-3-yl)triazene] was determined by X-ray diffraction methods. The complex has a quasi-planar dinuclear structure with an asymmetric double pyrazolate bridge. The asymmetry results from the presence, on the C-3 Of each bridging pyrazolate, of a pyrazolyltriazenide chelating arm which completes the equatorial CuN4 co-ordination. The centrosymmetric dinuclear units have intramolecular Cu ... Cu distances of 3.852(5) and 3.839(4) Angstrom respectively. The units are stacked in columns along the b axis. The cell also contains crossing channels filled with water molecules. The complex was characterized by spectroscopic 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 complexes (\J\ = approximate to 200 cm(-1)). The X-band powder EPR. spectrum recorded at 77 K is typical of a triplet spin state lying above the singlet ground state. The results obtained for this complex are compared to recent literature data on other planar doubly bridged pyrazolate 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 corresponding to the highest \J\.