First example of equatorial-equatorial disposition of end-to-end thiocyanate bridges in a polynuclear copper(II) complex and its relation to the very efficient transmission of the magnetic interaction

The preparation, X-ray structure, and spectroscopic and magnetic properties of [Cu(NCS)(mu-NCS) (Hmtpo)(H2O)](2) (where Hmtpo=4H,7H-5-methyl-7-oxo[1,2,4]triazolo[1,5-a]pyrimidine) are described. This compound crystallizes in triclinic space group P (1) over bar, with unit cell dimensions a = 8.408(2) Angstrom, b = 10.581(3) Angstrom, c = 7.927(2) Angstrom, alpha = 89.45(2)degrees, beta = 77.44(2)degrees, gamma = 69.75(2)degrees, and Z = 1 (binuclear unit). The structure was refined to final R = 0.033 for 2143 data with I greater than or equal to 3.0 sigma(I). The structure is comprised of polymeric 1-D copper(Il) complexes supported by two different kinds of thiocyanate bridges. The coordination geometry around each of the metal centers is a distorted octahedron (N3S + O + S*). The equatorial positions are occupied by a monodentate Hmtpo ligand, coordinated via N3, two bridging thiocyanate groups coordinated in a ''symmetric'' end-to-end fashion, and the N-donor atom of an asymmetric end-to-end thiocyanate bridge. Finally, a water molecule and a weakly bonded S-donor atom from the asymmetric end-to-end thiocyanate bridge, with a Cu ... S separation of 2.959(1) Angstrom, occupy the apical sites. The presence of two ''symmetric'' thiocyanate bridges with short Cu-SCN and Cu-NCS distances of 2.402(1) and 1.938(3) Angstrom, respectively, results in a Cu-Cu separation of 5.499(1) Angstrom. The magnetic susceptibility data measured in the 11-290 K temperature range shows a strong antiferromagnetic coupling between the copper ions through the ''symmetric'' thiocyanate bridges. Fitting the data to the Bleaney-Bowers equation gives the exchange parameter 2J = -148.2 cm(-1), which is many times higher than those previously found in other copper(II) thiocyanate bridged complexes, and g = 2.20. The unprecedented equatorial-equatorial disposition of the thiocyanate bridging entities appears to be responsible for the magnetic behavior of this compound.