Synthetic Modeling of Nitrite Binding and Activation by Reduced Copper Proteins. Characterization of Copper(I)−Nitrite Complexes That Evolve Nitric Oxide

Abstract

In an effort to provide precedence for postulated intermediates in copper-protein-mediated nitrite reduction, a series of novel complexes containing the CuI−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 = Li-Pr3, 1,4,7-triisopropyl-1,4,7-triazacyclononane, or LBn3, 1,4,7-tribenzyl-1,4,7-triazacyclononane) in MeOH with excess NaNO2 yielded the novel dicopper(I,I) complexes [(LCu)2(μ-NO2)]PF6. The complex with L = Li-Pr3 was cleaved by PPh3 to afford [Li-Pr3Cu(PPh3)]PF6 and Li-Pr3Cu(NO2), a structural model for the substrate adduct of copper nitrite reductase. Oxidation of the dicopper(I,I) compound (L = Li-Pr3) with (Cp2Fe)(PF6) in CH2Cl2 yielded the deep red, mixed valent, dicopper(I,II) species [(Li-Pr3Cu)2(μ-NO2)](PF6)2, which was structurally characterized as its [B(3,5-(CF3)2C6H3)4]- salt (crystal data: triclinic space group P1̄, a = 13.439(8) Å, b = 13.777(5) Å, c = 14.471(8) Å, α = 108.22(4)°, β = 92.08(5)°, γ = 90.08(4)°, Z = 1, T = 177 K, R = 0.074, and Rw = 0.070). A diamagnetic heterodinuclear CuIZnII analog, [Li-Pr3Cu(μ-NO2)ZnLi-Pr3](O3SCF3)2, was assembled by mixing Li-Pr3Cu(NO2), Zn(O3SCF3)2, and Li-Pr3 and was shown to adopt a structure similar to that of its CuICuII relative (crystal data: monoclinic space group P21/c, a = 10.8752(1) Å, b = 15.6121(3) Å, c = 25.8020(5) Å, β = 90.094(1)°, Z = 4, R1 = 0.0472, and wR2 = 0.1082). Both compounds exhibit an intense electronic absorption feature that was assigned as a CuI → NO2- MLCT transition on the basis of resonance Raman spectroscopic results. Functional modeling of copper nitrite reductase was accomplished by treating solutions of Li-Pr3Cu(NO2) with protonic acids or Me3SiO3SCF3. Nitric oxide evolution was accompanied by the formation of Li-Pr3Cu(O2CCH3)2 and Li-Pr3Cu(O3SCF3)2 when acetic acid or Me3SiO3SCF3 was used. The latter crystallized as a water adduct [Li-Pr3Cu(H2O)(O3SCF3)](O3SCF3) (crystal data: monoclinic space group P21/c, a = 8.59(1) Å, b = 26.04(1) Å, c = 12.838(4) Å, β = 108.26(6)°, Z = 4, T = 173 K, R = 0.067, and Rw = 0.064). The involvement of the CuICuII species as an intermediate in the reaction of Li-Pr3Cu(NO2) with Me3SiO3SCF3 at low temperature and a mechanism for NO generation involving both Li-Pr3Cu(NO2) and [(Li-Pr3Cu)2(μ-NO2)]2+ are discussed.