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 −NO 2 - 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(CH 3 CN)]PF 6 (L = L i -Pr 3, 1,4,7-triisopropyl-1,4,7-triazacyclononane, or L Bn 3, 1,4,7-tribenzyl-1,4,7-triazacyclononane) in MeOH with excess NaNO 2 yielded the novel dicopper(I,I) complexes [(LCu) 2 (μ-NO 2 )]PF 6 . The complex with L = L i -Pr 3 was cleaved by PPh 3 to afford [L i -Pr 3 Cu(PPh 3 )]PF 6 and L i -Pr 3 Cu(NO 2 ), a structural model for the substrate adduct of copper nitrite reductase. Oxidation of the dicopper(I,I) compound (L = L i -Pr 3 ) with (Cp 2 Fe)(PF 6 ) in CH 2 Cl 2 yielded the deep red, mixed valent, dicopper(I,II) species [(L i -Pr 3 Cu) 2 (μ-NO 2 )](PF 6 ) 2, which was structurally characterized as its [B(3,5-(CF 3 ) 2 C 6 H 3 ) 4 ] - salt (crystal data: triclinic space group P 1̄, 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 R w = 0.070). A diamagnetic heterodinuclear Cu I Zn II analog, [L i -Pr 3 Cu(μ-NO 2 )ZnL i -Pr 3 ](O 3 SCF 3 ) 2, was assembled by mixing L i -Pr 3 Cu(NO 2 ), Zn(O 3 SCF 3 ) 2, and L i -Pr 3 and was shown to adopt a structure similar to that of its Cu I Cu II relative (crystal data: monoclinic space group P 2 1 / c, a = 10.8752(1) Å, b = 15.6121(3) Å, c = 25.8020(5) Å, β = 90.094(1)°, Z = 4, R 1 = 0.0472, and w R 2 = 0.1082). Both compounds exhibit an intense electronic absorption feature that was assigned as a Cu I → NO 2 - MLCT transition on the basis of resonance Raman spectroscopic results. Functional modeling of copper nitrite reductase was accomplished by treating solutions of L i -Pr 3 Cu(NO 2 ) with protonic acids or Me 3 SiO 3 SCF 3 . Nitric oxide evolution was accompanied by the formation of L i -Pr 3 Cu(O 2 CCH 3 ) 2 and L i -Pr 3 Cu(O 3 SCF 3 ) 2 when acetic acid or Me 3 SiO 3 SCF 3 was used. The latter crystallized as a water adduct [L i -Pr 3 Cu(H 2 O)(O 3 SCF 3 )](O 3 SCF 3 ) (crystal data: monoclinic space group P 2 1 / 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 R w = 0.064). The involvement of the Cu I Cu II species as an intermediate in the reaction of L i -Pr 3 Cu(NO 2 ) with Me 3 SiO 3 SCF 3 at low temperature and a mechanism for NO generation involving both L i -Pr 3 Cu(NO 2 ) and [(L i -Pr 3 Cu) 2 (μ-NO 2 )] 2+ are discussed.