Synthesis and characterization of copper(I)-pyrazole complexes. X-Ray crystal structure of [Cu(pzH) 2Cl] 2 and NMR investigation of the fluxional behaviour of [Cu(pzH) 2(CO)Cl

Abstract

The product of the reduction of [Cu(pzH) 4Cl 2] (pzH = pyrazole, C 3H 4N 2) 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 P2 1/ n, with a 6.327(1), b 9.610(1), c 15.066(1) Å, β 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′ 3.401(1) Å), 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 E a values, indicating that the exchange process is not intramolecular, and is favoured in donor solvents, at temperatures above 250 K. In concentrated solutions in CD 2Cl 2 a double-slope Arrhenius plot was obtained, suggesting the occurrence of two different processes, whose natures are discussed.