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 tris(3-trifluoromethyl)-5-methylpyrazol-1-yl)hydroborate (Tp(CF₃,CH₃) and tris(3-mesitylpyrazol-1-yl)hydroborate (Tp(Ms,H)), significant effects of the scorpionate ligand substituents on the properties of the {CuNO}¹¹ unit were found that have implications for environmental influences on similar species in biological and catalytic systems. The copper(I) complexes Tp(Ms,H)Cu(THF) and Tp(CF₃,CH₃)Cu(CH₃CN) were structurally characterized by X-ray crystallography, and their respective CO and NO adducts were studied by FTIR, EPR, NMR, and/or UV-vis spectroscopies in solution. Both nitrosyl complexes disproportionate in the presence of excess NO to N₂O and Tp(R,R')Cu(NO₂); an X-ray structure of the latter products supported by Tp(CF₃,CH₃) was determined. Unlike previously studied paramagnetic [CuNO]¹¹ compounds that exhibit EPR signals with g < 2.0 and large A(NO) values at temperatures below ~40 K (Ruggiero, C. E.; Carrier, S. M.; Antholine, W. E.; Whittaker, J. W.; Cramer, C. J.; Tolman, W. B. J. Am. Chem. Soc. 1993, 115, 11285-11298), Tp(Ms,H)Cu(NO) is EPR silent at 4.2 K and exhibits an NMR spectrum (238 K, toluene-d₈) with sharp animals. Peak assignments for the NMR spectrum were deduced from integrated intensities, temperature-dependent isotropic shifts, and the nuclear relaxation rates. The unique 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 methyl groups, suggests that caution should be exercised in characterizing such adducts in proteins and heterogenous systems; subtle environmental 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 ν(NO) and the Cu(I) → NO MLCT energy in the respective FTIR and UV-vis spectra and induce a significant slowing of its disproportionation rate. These results, in conjunction with those obtained from kinetic and spectroscopic studies on the Tp(Ms,H) system, support a mechanism for the disproportionation involving generation of the CuNO adduct from NO and the Cu(I) precursor in a preequilibrium step, followed by electrophilic attack of a second NO molecule on the adduct that is rate-controlling.