INFLUENCES OF LIGAND ENVIRONMENT ON THE SPECTROSCOPIC PROPERTIES AND DISPROPORTIONATION REACTIVITY OF COPPER-NITROSYL COMPLEXES

In studies of the chemistry of new copper-nitrosyl complexes supported by -(trifluoromethyl)5-methylpyrazol-1-yl)hydroborate (Tp(CF3,CH3)) a nd tris(3-mesitylpyrazol-1-yl)hydroborate (Tp(Ms,H)), Significant effe cts of the scorpionate ligand substituents on the properties of the {C uNO}(11) unit were found that have implications for environmental infl uences on similar species in biological and catalytic systems. The cop per(I) complexes Tp(Ms,H)Cu(THF) and Tp(CF3,CH3)Cu(CH3CN) were structu rally characterized by X-ray crystallography, and their respective CO and NO adducts were studied by FTIR, EPR, NMR, and/or UV-vis spectrosc opies in solution. Both nitrosyl complexes disproportionate in the pre sence of excess NO to N2O and Tp(R,R)'Cu(NO2); an X-ray structure of t he latter product supported by Tp(CF3,CH3) was determined. Unlike prev iously studied paramagnetic [CuNO](11) compounds that exhibit EPR sign als with g < 2.0 and large A(NO) values at temperatures below similar to 40 K (Ruggiero, C. E.; Carrier, S. M.; Antholine, W. E.; Whittaker, J. W.; Cramer, C. J.; Tolman, W. B. J. Am. Chem. Sec. 1993, 115, 1128 5-11298), Tp(Ms,H)Cu(NO) is EPR silent at 4.2 K and exhibits an NMR sp ectrum (238 K, toluene-d(8)) with sharp signals. Peak assignments for the NMR spectrum were deduced from integrated intensities, temperature -dependent isotropic shifts, and the nuclear relaxation rates. The uni que NMR spectral behavior for the Tp(Ms,H) complex, which only differs from those of analogues with simple phenyl substituents by virtue of the shape of the substrate binding pocket enforced by the mesityl meth yl groups, suggests that caution should be exercised in characterizing such adducts in proteins and heterogeneous systems; subtle environmen tal effects may determine the applicability of EPR versus NMR methods. The electron-withdrawing effects of the trifluoromethyl substituents in Tp(CF3,CH3)Cu-(NO) perturb nu(TO) and the Cu(I) --> NO MLCT energy in the respective FTIR and UV-vis spectra and induce a significant slo wing of its disproportionation rate. These results, in conjunction wit h those obtained from kinetic and spectroscopic studies on the Tp(Ms,H ) system, support a mechanism for the disproportionation involving gen eration of the CuNO adduct from NO and the Cu(I) precursor in a preequ ilibrium step, followed by electrophilic attack of a second NO molecul e on the adduct that is rate-controlling.