SYNTHESIS AND CHARACTERIZATION OF COPPER(I)-PYRAZOLE COMPLEXES - X-RAY CRYSTAL-STRUCTURE OF [CU(PZH)2CL]2 AND NMR INVESTIGATION OF THE FLUXIONAL BEHAVIOR OF [CU(PZH)2(CO)CL]

The product of the reduction of [Cu(pzH)4Cl2] (pzH = pyrazole, C3H4N2) with Cu powder has been shown by single crystal X-ray analysis to the dimeric species [Cu(pzH)2Cl]2. The crystals are monoclinic, space group P21/n, with a 6.327(1), b 9.610(1), c 15.066(1) \uc5, \u3b2 96.91(1), and Z = 2. The refinements performed by full-matrix least-squares methods, for 1417 independent significant reflexions, gave a final R value of 0.030. Each dimeric unit, located on a crystallographic inversion centre, contains two Cu atoms (Cu...Cu\u2032 3.401(1) \uc5), each surrounded by two bridging C1 atoms and two pzH ligands, in a pseudo-tetrahedral arrangement. The reaction of carbon monoxide with [Cu(pzH)2Cl]2 gave the new species [Cu(pzH)2(CO)Cl], which was characterized by IR and NMR spectroscopy. Fluxional behaviour of the pyrazole ligands, equalizing their 3 and 5 positions, was revealed by both 1H and 13C NMR spectra. Variable temperature 13C NMR experiments in different conditions (solvent or concentrations) lead to different Ea values, indicating that the exchange process is not intramolecular, and is favoured in donor solvents, at temperatures above 250 K. In concentrated solutions in CD2Cl2 a double-slope Arrhenius plot was obtained, suggesting the occurrence of two different processes, whose natures are discussed