Biplab Mondal

Nitric Oxide Reduction of Copper(II) Complexes: Spectroscopic Evidence of Copper(II)−Nitrosyl Intermediate

Two copper(II) complexes, 1 and 2 with L(1) and L(2) [L(1) = 2- aminomethyl pyridine; L(2) = bis-(2-aminoethyl)amine], respectively, in degassed acetonitrile solvent, on exposure to NO gas, were found to form a thermally unstable [Cu(II)-NO] intermediate which then resulted in the reduction of the copper(II) centers. The formation of the [Cu(II)-NO] intermediate was evidenced by UV-visible, FT-IR, and EPR spectroscopic studies. The reduction of the copper(II) centers by nitric oxide afforded ligand transformation through diazotization at the primary amine coordination site, in both cases. The modified ligands, in each case, were isolated and characterized.

Fluorescence-based detection of nitric oxide in aqueous and methanol media using a copper(II) complex

The quenched fluorescent intensity of a copper(II) complex, 1, of a fluorescent ligand, in degassed methanol or aqueous (buffered at pH 7.2) solution, was found to reappear on exposure to nitric oxide. Thus, it can function as a fluorescence based nitric oxide sensor. It has been found that the present complex can be used to sense nanomolar quantities of nitric oxide in both methanol and pH 7.2 buffered-water medium.

Heme-Copper-Dioxygen Complexes: Toward Understanding Ligand-Environmental Effects on the Coordination Geometry, Electronic Structure, and Reactivity

The nature of the ligand is an important aspect of controlling the structure and reactivity in coordination chemistry. In connection with our study of heme-copper-oxygen reactivity relevant to cytochrome c oxidase dioxygen-reduction chemistry, we compare the molecular and electronic structures of two high-spin heme-peroxo-copper [Fe(III)O(2)(2-)Cu(II)](+) complexes containing N(4) tetradentate (1) or N(3) tridentate (2) copper ligands. Combining previously reported and new resonance Raman and EXAFS data coupled to density functional theory calculations, we report a geometric structure and more complete electronic description of the high-spin heme-peroxo-copper complexes 1 and 2, which establish mu-(O(2)(2-)) side-on to the Fe(III) and end-on to Cu(II) (mu-eta(2):eta(1)) binding for the complex 1 but side-on/side-on (mu-eta(2):eta(2)) mu-peroxo coordination for the complex 2. We also compare and summarize the differences and similarities of these two complexes in their reactivity toward CO, PPh(3), acid, and phenols. The comparison of a new X-ray structure of mu-oxo complex 2a with the previously reported 1a X-ray structure, two thermal decomposition products respectively of 2 and 1, reveals a considerable difference in the Fe-O-Cu angle between the two mu-oxo complexes ( angleFe-O-Cu = 178.2 degrees in 1a and angleFe-O-Cu = 149.5 degrees in 2a). The reaction of 2 with 1 equiv of an exogenous nitrogen-donor axial base leads to the formation of a distinctive low-temperature-stable, low-spin heme-dioxygen-copper complex (2b), but under the same conditions, the addition of an axial base to 1 leads to the dissociation of the heme-peroxo-copper assembly and the release of O(2). 2b reacts with phenols performing H-atom (e(-) + H(+)) abstraction resulting in O-O bond cleavage and the formation of high-valent ferryl [Fe(IV)=O] complex (2c). The nature of 2c was confirmed by a comparison of its spectroscopic features and reactivity with those of an independently prepared ferryl complex. The phenoxyl radical generated by the H-atom abstraction was either (1) directly detected by electron paramagnetic resonance spectroscopy using phenols that produce stable radicals or (2) indirectly detected by the coupling product of two phenoxyl radicals.

Reduction of copper(II) complexes of tripodal ligands by nitric oxide and trinitrosation of the ligands.

The copper(II) centers in two copper(II) complexes of tripodal amine ligands, in acetonitrile solvent, upon exposure to nitric oxide have been found to produce a thermally unstable [Cu(II)-NO] intermediate followed by the reduction of copper(II) to copper(I). This reduction resulted in the concomitant trinitrosation of the ligands at their terminal amine centers.

A peroxynitrite complex of copper: formation from a copper–nitrosyl complex, transformation to nitrite and exogenous phenol oxidative coupling or nitration

Reaction of nitrogen monoxide with a copper(I) complex possessing a tridentate alkylamine ligand gives a Cu(I)–(·NO) adduct, which when exposed to dioxygen generates a peroxynitrite (O=NOO−)–Cu(II) species. This undergoes thermal transformation to produce a copper(II) nitrito (NO2–) complex and 0.5 mol equiv O2. In the presence of a substituted phenol, the peroxynitrite complex effects oxidative coupling, whereas addition of chloride ion to dissociate the peroxynitrite moiety instead leads to phenol ortho nitration. Discussions include the structures (including electronic description) of the copper–nitrosyl and copper–peroxynitrite complexes and the formation of the latter, based on density functional theory calculations and accompanying spectroscopic data.

Heme/O2/•NO Nitric Oxide Dioxygenase (NOD) Reactivity: Phenolic Nitration via a Putative Heme-Peroxynitrite Intermediate

An oxy-heme complex, the heme-superoxo species (tetrahydrofuran)(F(8))Fe(III)-(O(2)(*-)) (2) (F(8) = an ortho-difluoro substituted tetraarylporphyrinate), reacts with nitrogen monoxide (*NO; nitric oxide) to produce a nitrato-iron(III) compound (F(8))Fe(III)-(NO(3)(-)) (3) (X-ray). The chemistry mimics the action of *NO Dioxygenases (NODs), microbial and mammalian heme proteins which facilitate *NO detoxification/homeostasis. A peroxynitrite intermediate complex is implicated; if 2,4-di-tert-butylphenol is added prior to *NO reaction with 2, o-nitration occurs giving 2,4-di-tert-butyl-6-nitrophenol. The iron product is (F(8))Fe(III)-(OH) (4). The results suggest that heme/O(2)/*NO chemistry may lead to peroxynitrite leakage and/or exogenous substrate oxidative/nitrative reactivity.

Ruthenium monoterpyridine complexes incorporating α,α′-diimine based ancillary functions. Synthesis, crystal structure, spectroelectrochemical properties and catalytic aspect

Abstract Ruthenium monoterpyridine complexes of the type [RuII(trpy)(L′)(X)](ClO4)m·2H2O (1–2) [trpy=2,2′:6′,2″-terpyridine; L′=NC5H4C(H)N(C6H4)nNH2 (n=1 and 2); X=Cl−, m=1 (1); X=H2O, m=2 (2)] have been synthesized via the selective hydrolysis of one of the imine functions present in the preformed stable dinucleating bridging functions, NC5H4C(H)N(C6H4)nNC(H)H4C5N (L) (n=1, 2). The single crystal X-ray structures of the dinucleating bridging function (L1) and the chloro complex (1a) (in both cases n=1) have been determined. The complexes stabilize preferentially in one particular isomeric form where the Cl− or H2O molecule is in the trans configuration with respect to the N(imine) center. The chloro complexes (1) exhibit strong MLCT bands near 500 nm whereas in the case of the aqua complexes (2) the MLCT bands are blue shifted near 470 nm. The chloro complexes (1) exhibit weak emissions in EtOH–MeOH (4:1 v/v) glass at 77 K near 600 nm (quantum yield, Φem=0.015–0.03). In acetonitrile solvent 1 display a ruthenium(III)–ruthenium(II) couple near 0.8 V and terpyridine based reduction near 1 V versus SCE. The aqua complexes (2) exhibit a concerted 2e−/2H+ oxidation process in the acidic region and in the alkaline region, the complexes display a 2e−/H+ oxidation process. The potentials are observed to decrease linearly with the increase in pH. The proton coupled redox processes in the acidic and basic regions correspond to [RuII(trpy)(L′)(H2O)]2+–[RuIV(trpy)(L′)(O)]2+ and [RuII(trpy)(L′)(OH)]+–[RuIV(trpy)(L′)(O)]2+ couples, respectively. The chemical oxidation of 2 by excess CeIV solution in 1 (N) H2SO4 also leads to the formation of the corresponding [RuIV(trpy)(L′)(O)]2+ (3). The oxo complexes (3) are stable only in the presence of excess CeIV ion, otherwise they slowly catalyze the oxidation of water to dioxygen and return back to the parent aqua species. The electrochemically generated oxo-species are found to catalyze the benzyl alcohol oxidation process.

Role of ligand to control the mechanism of nitric oxide reduction of copper(II) complexes and ligand nitrosation.

The nitric oxide reactivity of two copper(II) complexes, 1 and 2 with ligands L(1) and L(2), respectively, [L(1) = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, L(2) = 5,5,7-trimethyl-[1,4]-diazepane] have been studied. The copper(II) center in complex 1 was found to be unreactive toward nitric oxide in pure acetonitrile; however, it displayed reduction in methanol solvent in presence of base. The copper(II) center in 2, in acetonitrile solvent, on exposure to nitric oxide has been found to be reduced to copper(I). The same reduction was observed in methanol, also, in case of complex 2. In case of complex 1, presumably, the attack of nitric oxide on the deprotonated amine is the first step, followed by electron transfer to the copper(II) center to afford the reduction. Alternatively, first NO coordination to the Cu(II) followed by NO(+) migration to the secondary amine is the most probable in case of complex 2. The observation of the transient intermediate in UV-visible and FT-IR spectroscopy prior to reduction in case of complex 2 also supports this possibility. In both cases, the reduction resulted into N-nitrosation; in 1, only mononitrosation was observed whereas complex 2 afforded dinitrosation as major product along with a minor amount of mononitrosation. Thus, it is evident from the present study that the macrocyclic ligands prefer the deprotonation pathway leading to mononitrosation; whereas nonmacrocyclic ones prefer the [Cu(II)-NO] intermediate pathway resulting into nitrosation at all the available sites of the ligand as major product.

Reaction of a copper(II)–nitrosyl complex with hydrogen peroxide: putative formation of a copper(I)–peroxynitrite intermediate

The reaction of a Cu(II)-nitrosyl complex (1) with hydrogen peroxide at -20 °C in acetonitrile results in the formation of the corresponding Cu(I)-peroxynitrite intermediate. The reduction of the Cu(II) center was monitored by UV-visible spectroscopic studies. Formation of the peroxynitrite intermediate has been confirmed by its characteristic phenol ring nitration reaction as well as isolation of corresponding Cu(I)-nitrate (2). On air oxidation, 2 resulted in the corresponding Cu(II)-nitrate (3). Thus, these results demonstrate a possible decomposition pathway for H(2)O(2) and NO through the formation of a peroxynitrite intermediate in biological systems.

Reduction of copper(II) complexes of tridentate ligands by nitric oxide and fluorescent detection of NO in methanol and water media.

Two copper complexes, 1 and 2, with tridentate N-donor ligands, L(1) and L(2) [L(1)= (1-methyl-1H-imidazol-2-ylmethyl)-(2-pyridin-2-yl-ethyl)amine, L(2) = (2-pyridin-2-yl-ethyl)-pyridin-2 yl-methylamine] respectively, have been synthesized and characterized. On exposure to nitric oxide, the copper(II) centers in complexes 1 and 2 were found to undergo reduction in various solvents. In acetonitrile solvent the reduction was accompanied by a simultaneous N-nitrosation on the secondary amine center on the ligand frameworks. Complexes 3 and 4 were prepared with ligands L(3) and L(4), respectively. L(3) and L(4) [L(3) = 5-dimethylamino-naphthalene-1-sulfonic acid (1-methyl-1H-imidazol-2-ylmethyl)-(2-pyridin-2-yl-ethyl)-amide; L(4) = 5-dimethylamino-naphthalene-1-sulfonic acid(2-pyridin-2-yl-ethyl)-pyridin-2-ylmethyl-amide] are the dansyl derivatives of L(1) and L(2), respectively. Complex 4, due to paramagnetic quenching, does not display any fluorescence; however, on addition of nitric oxide to a methanol or water solution of complex 4, the fluorescence intensity of the fluorophore has been found to be restored. This is attributed to the reduction of the Cu(II) center by nitric oxide to diamagnetic Cu(I). The turn-on of quenched fluorescence intensity has been observed both in methanol and water media.