Synthetic modeling of nitrite binding and activation by reduced copper proteins. Characterization of copper(I)-nitrite complexes that evolve nitric oxide

In an effort to provide precedence for postulated intermediates in copper-protein-mediated nitrite reduction, a series of novel complexes containing the Cu-I-NO2- unit, including monocopper(I), dicopper(I,I), and mixed valence dicopper(I,II) and copper(I)-zinc(II) species, were prepared, fully characterized, and subjected to reactivity studies designed to probe their ability to produce nitric oxide. Treatment of solutions of [LCu(CH3CN)]PF6 (L = L(i-Pr3), 1,4,7-triisopropyl-1,4,7-triazacyclononane, or L(Bn3), 1,4,7-tribenzyl-1,4,7-triazacyclononane) in MeOH with excess NaNO2 yielded the novel dicopper(I,I) complexes [(LCu)(2)(mu-NO2)]PF6. The complex with L = L(i-Pr3) was cleaved by PPh(3) to afford [L(i-Pr3)(Cu(PPh(3))]PF6 and L(i-Pr3)Cu(NO2), a structural model for the substrate adduct of copper nitrite reductase. Oxidation of the dicopper(I,I) compound (L = L(i-Pr3)) with (Cp(2)Fe)(PF6) in CH2Cl2 yielded the deep red, mixed valent, dicopper(I,II) species [(L(i-Pr3)Cu)(2)(mu-NO2)](PF6)(2), which was structurally characterized as its [B(3,5-(CF3)(2)C6H3)(4)](-) salt (crystal data: triclinic space group P (1) over bar, a = 13.439(8) Angstrom, b = 13.777(5) Angstrom, c = 14.471(8) Angstrom, alpha = 108.22(4)degrees, beta = 92.08(5)degrees, gamma = 90.08(4)degrees, Z = 1, T = 177 K, R = 0.074, and R(w) = 0.070). A diamagnetic heterodinuclear (CuZnII)-Zn-I analog, [L(i-Pr3)Cu(mu-NO2)ZnL(i-Pr3)](O3SCF3)(2), was assembled by mixing L(i-Pr3)Cu(NO2), Zn(O-3-SCF3)(2), and L(i-Pr3) and was shown to adopt a structure similar to that of its (CuCuII)-Cu-I relative (crystaldata: monoclinic space group P2(1)/c, a = 10.8752(1) Angstrom, b = 15.6121(3) Angstrom, c = 25.8020(5) Angstrom, beta = 90.094(1)degrees, Z = 4, R1 = 0.0472, and wR2 = 0.1082). Both compounds exhibit an intense electronic absorption feature that was assigned as a Cu-I --> NO2- MLCT transition on the basis of resonance Raman spectroscopic results. Functional modeling of copper nitrite reductase was accomplished by treating solutions of L(i-Pr3)Cu(NO2) with protonic acids or Me(3)SiO(3)SCF(3). Nitric oxide evolution was accompanied by the formation of L(i-Pr3)Cu(O2CCH3)(2) and L(i-Pr3)Cu(O3SCF3)(2) when acetic acid or Me(3)SiO(3)SCF(3) was used. The latter crystallized as a water adduct [L(i-Pr3)Cu(H2O)(O3SCF3)](O3SCF3) (crystal data: monoclinic space group P2(1)/c, a = 8.59(1) Angstrom, b = 26.04(1) Angstrom, c = 12.838(4) Angstrom, beta = 108.26(6)degrees, Z = 4, T = 173 K, R = 0.067, and R(w) = 0.064). The involvement of the (CuCuII)-Cu-I species as an intermediate in the reaction of L(i-Pr3)Cu(NO2) with Me(3)SiO(3)SCF(3) at low temperature and a mechanism for NO generation involving both L(i-Pr3)Cu(NO2) and [(L(i-Pr3)Cu)(2)(mu-NO2)](2+) are discussed.