BOND BREAKING IN THE CHEMICAL-VAPOR-DEPOSITION PRECURSOR (1,1,1,5,5,5-HEXAFLUORO-2,4-PENTANEDIONATO)(ETA-2-1,5-CYCLOOCTADIENE)-COPPER(I) STUDIED BY VARIABLE-TEMPERATURE X-RAY CRYSTALLOGRAPHY AND SOLID-STATE NMR-SPECTROSCOPY

Bond breaking in chemical vapor deposition (CVD) and fluxional processes in the solid state depend on the same fundamental molecular properties, and cross-fertilization between these two separate areas is attempted herein. X-ray crystallograpic and solid-state NMR studies of (hfac)Cu(I)(COD) (hfac = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione; COD = 1,5-cyclooctadiene) reveal that the Cu atom is disordered between two sites. The two disordered molecules (represented by the disordered atoms Cu and Cu') are crystallographically inequivalent, but they have the same connectivity, with eta2 coordination of Cu to the COD ligand (in contrast to our previous proposal of eta2 and eta4 sites in equilbrium: Kumar, R.; et al. Chem. Mater. 1992, 4, 577). X-ray structural data recorded at three temperatures lead to estimates of the relative populations of Cu and Cu, which correspond to an energy difference of 3-5 kJ mol-1. (CuC13H13F6O2, monoclinic, space group P2(1)/c; Z = 4. At 195 K, a = 9.980(5), b = 9.690(2), c = 15.547(7) angstrom; beta = 107.31(2)degrees, R = 0.044 and R(w) = 0.056 for 4517 reflections and 256 parameters; populations Cu:Cu' = 0.944:0.056. At 110 K, a = 9.917(5), b = 9.583(2), c = 15.452(7) angstrom; beta = 106.69(4); R = 0.034 and R(w) = 0.039 for 3524 reflections and 255 parameters; populations 0.990: 0.010.) C-13 CP/MAS spectra show increasingly rapid interconversion between the two sites at higher temperatures, with two separate resonances at 127 and 105 ppm (free and bound -CH= sites in the same COD ligand) coalescing into a single resonance at 340 K. Line-shape analysis yields an interconversion barrier of ca. 60 kJ mol-1. This barrier is similar to previously reported experimental activation barriers for dissociation of olefins from Cu surfaces or from Cu(I) complexes adsorbed on Cu surfaces, suggesting that the Cu-olefin bond may be nearly broken in the transition state for the Cu <-> Cu' interconversion.