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
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.