Influences of Ligand Environment on the Spectroscopic Properties and Disproportionation Reactivity of Copper−Nitrosyl Complexes

Abstract

In studies of the chemistry of new copper−nitrosyl complexes supported by tris(3-(trifluoromethyl)-5-methylpyrazol-1-yl)hydroborate (TpCF3,CH3) and tris(3-mesitylpyrazol-1-yl)hydroborate (TpMs,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 TpMs,HCu(THF) and TpCF3,CH3Cu(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 TpR,R‘Cu(NO2); an X-ray structure of the latter product supported by TpCF3,CH3 was determined. Unlike previously studied paramagnetic [CuNO]11 compounds that exhibit EPR signals with g < 2.0 and large ANO 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), TpMs,HCu(NO) is EPR silent at 4.2 K and exhibits an NMR spectrum (238 K, toluene-d8) with sharp signals. 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 TpMs,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 heterogeneous systems; subtle environmental effects may determine the applicability of EPR versus NMR methods. The electron-withdrawing effects of the trifluoromethyl substituents in TpCF3,CH3Cu(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 TpMs,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.