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

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Abstract
The crystal and molecular structure of [{Cu(bpzt)}2]·3H2O [H2bpzt = 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 C3 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)Å 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|=≈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|.

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