Nitric oxide-release study of a bio-inspired copper(i)-nitrito complex under chemical and biological conditions.

The selective and efficient nitrite reduction process is ubiquitous in biological systems. To understand copper-mediated nitrite reduction, we developed a bio-inspired model system to investigate the mechanism of copper-containing nitrite reductase. A well-characterized copper(i)-nitrate complex with amino functionalized 2-(diphenylphosphino)aniline ligands, [(Ph2PC6H4(o-NH2))2Cu(ONO)], demonstrated the aniline protonation will cause NO release in an acidic environment. To further understand NO releasing ability, we also performed pH-dependency experiments and confocal imaging to release NO under physiological buffer conditions. According to titration and spectroscopic studies on the protonation reaction of complex [(Ph2PC6H4(o-NH2))2Cu(ONO)], we proposed a mechanistic pathway for proton transfer and NO release. Furthermore, DFT calculations predicted that the release of NO takes place via aniline in both organic and aqueous media. These results highlight the importance of the proton-rich microenvironment around the copper(i)-nitrite core to induce nitrate reduction in a chemical and biological environment.

Structure and nitrite reduction reactivity study of bio-inspired copper(i)-nitro complexes in steric and electronic considerations of tridentate nitrogen ligands.

Two copper(i)-nitro complexes [Tpm3-tBuCu(NO2)] (1) and [(Ph3P)2N][Tp3-tBuCu(NO2)] (2), containing steric bulky neutral tris(3-tert-butylpyrazolyl)methane and anionic hydrotris(3-tert-butylpyrazolyl)borate ligands, have been synthesized and characterized. Complex 2 adopts a unique κ2-binding mode of Tp3-tBu around the copper(i)-nitro environment in the solid state and shows a four-coordinated tetrahedral geometry surrounded by a nitro and three pz3-tBu groups in solution. Both complexes 1 and 2 allow for the stoichiometric reduction of NO2- to NO with H+ addition. The results of this effort show that increasing steric bulk and electron donation properties on the nitrogen ancillary ligand will improve the nitrite reduction ability of the copper(i)-nitro model complexes.

Reactivity Study of Unsymmetrical ß-Diketiminato Copper(I) Complexes: Effect of the Chelating Ring.

ß-Diketiminato copper(I) complexes play important roles in bioinspired catalytic chemistry and in applications to the materials industry. However, it has been observed that these complexes are very susceptible to disproportionation. Coordinating solvents or Lewis bases are typically used to prevent disproportionation and to block the coordination sites of the copper(I) center from further decomposition. Here, we incorporate this coordination protection directly into the molecule in order to increase the stability and reactivity of these complexes and to discover new copper(I) binding motifs. Here we describe the synthesis, structural characterization, and reactivity of a series of unsymmetrical N-aryl-N'-alkylpyridyl ß-diketiminato copper(I) complexes and discuss the structures and reactivity of these complexes with respect to the length of the pyridyl arm. All of the aforementioned unsymmetrical ß-diketiminato copper(I) complexes bind CO reversibly and are stable to disproportionation. The binding ability of CO and the rate of pyridyl ligand decoordination of these copper(I) complexes are directly related to the competition between the degree of puckering of the chelate system and the steric demands of the N-aryl substituent.

Synthesis of Sodium Complexes Supported with NNO-Tridentate Schiff Base Ligands and Their Applications in the Ring-Opening Polymerization of L-Lactide.

A series of sodium complexes bearing NNO-tridentate Schiff base ligands with an N-pendant arm were synthesized and used as catalysts for the ring-opening polymerization of L-lactide (L-LA). Electronic effects of ancillary ligands coordinated by sodium complexes substantially influence the catalysis, and ligands with electron-donating groups increase the catalytic activity of the sodium complexes for catalyzing L-LA polymerization. In particular, a sodium complex bearing a 4-methoxy group has the highest activity with conversion up to 95% within 30 s at 0 °C and a low polydispersity index of 1.13, whereas the 4-bromo group showed the poorest performance with regard to the catalytic rate of L-LA polymerization in the presence of benzyl alcohol (BnOH). (1)H NMR pulsed-gradient spin-echo diffusion experiments and single-crystal X-ray analyses showed that sodium complexes [L(H)Na(THF)]2 and [L(4-Cl)Na(THF)]2 were dinuclear species in both solution and the solid state. The kinetic results indicated a first-order dependence on each of [[L(4-Cl)Na]2], [l-LA], and [BnOH].

Anion reduction dominated cathodic limit of metal-free ionic liquid: experimental and theoretical proofs.

The cathodic limit of the electrochemical window in the second-generation ionic liquids (composed of air- and water-stable metal-free cations and anions) is traditionally believed to be determined by the reduction of the cation. More and more exceptions, however, were found in various ionic liquids. In this study, the cathodic limit of the electrochemical window in 1-butyl-1-methylpyrrolidinium salicylate ionic liquid (BMP-SAL IL) was studied. It has been found that the cathodic limit of BMP-SAL is determined by the reduction of SAL(-) anion rather than the reduction of BMP(+) cation. The cyclic voltammetric behavior, NMR spectra, and MALDI-TOF MS spectra of BMP-SAL recorded before and after the IL was electrolyzed at the cathodic limit provide sufficient evidence that the major reaction at the cathodic limit of BMP-SAL is the reduction of SAL(-) anion. The theoretical calculations support the experimental data, and the results indicate that anion reduction dominated cathodic limit should be a common phenomenon in ionic liquids.

Copper(I) nitro complex with an anionic [HB(3,5-Me2Pz)3]− ligand: a synthetic model for the copper nitrite reductase active site.

The new copper(I) nitro complex [(Ph(3)P)(2)N][Cu(HB(3,5-Me(2)Pz)(3))(NO(2))] (2), containing the anionic hydrotris(3,5-dimethylpyrazolyl)borate ligand, was synthesized, and its structural features were probed using X-ray crystallography. Complex 2 was found to cocrystallize with a water molecule, and X-ray crystallographic analysis showed that the resulting molecule had the structure [(Ph(3)P)(2)N][Cu(HB(3,5-Me(2)Pz)(3))(NO(2))]·H(2)O (3), containing a water hydrogen bonded to an oxygen of the nitrite moiety. This complex represents the first example in the solid state of an analogue of the nitrous acid intermediate (CuNO(2)H). A comparison of the nitrite reduction reactivity of the electron-rich ligand containing the CuNO(2) complex 2 with that of the known neutral ligand containing the CuNO(2) complex [Cu(HC(3,5-Me(2)Pz)(3))(NO(2))] (1) shows that reactivity is significantly influenced by the electron density around the copper and nitrite centers. The detailed mechanisms of nitrite reduction reactions of 1 and 2 with acetic acid were explored by using density functional theory calculations. Overall, the results of this effort show that synthetic models, based on neutral HC(3,5-Me(2)Pz)(3) and anionic [HB(3,5-Me(2)Pz)(3)](-) ligands, mimic the electronic influence of (His)(3) ligands in the environment of the type II copper center of copper nitrite reductases (Cu-NIRs).

Characterization of a new copper(I)-nitrito complex that evolves nitric oxide.

The complexes [Cu(kappa(2)-Ph(2)PC(6)H(4)(o-OMe))(2)(CH(3)CN)](BF(4)) (1) and [CuCl(Ph(2)PC(6)H(4)(o-OMe))(2)] (2) have been prepared by treating [Cu(CH(3)CN)(4)](BF(4)) or CuCl with two equivalents o-(diphenylphosphino)anisole (Ph(2)PC(6)H(4)(o-OMe)) at room temperature, respectively. The reaction of 1 and (PPN)(NO(2)) in acetonitrile solution affords a neutral compound [Cu(Ph(2)PC(6)H(4)(o-OMe))(2)(ONO)] (3). In contrast to the synthesis of 3, mixing NaNO(2) and 1 in MeOH yielded a unique dicopper(I) cationic species, [((Ph(2)PC(6)H(4)(o-OMe))(2)Cu)(2)(mu-NO(2))](+) (4) after ether/CH(2)Cl(2) crystallization. The molecular structures of 1-4 have been determined by an X-ray diffraction study. The copper(I)-nitrito adduct 3 containing phosphine-ether ligands forms nitric oxide gas from the reaction with acetic acid, suggesting the first example and model compound in the asymmetric O-bound copper(I) nitrite intermediate microenvironment of copper nitrite reductases (Cu-NIRs).