Kinetic, Mechanistic, and DFT Study of the Electrophilic Reactions of Nitrosyl Complexes with Hydroxide

Abstract

We present a kinetic study of OH(-) additions to several nitrosyl complexes containing mainly ruthenium and different coligands (polypyridines, amines, pyridines, cyanides). According to a first-order rate law in each reactant, we propose a fast ion pair formation equilibrium, followed by addition of OH(-) to the [MX(5)NO](n) moieties, with formation of the [MX(5)NO(2)H](n(-1)) intermediates. Additional attack by a second OH(-) gives the final products, [MX(5)NO(2)]((n-2)). A linear plot was found for ln k(4) (the addition rate constant) against the redox potential for nitrosyl reduction, E(NO+/-NO), showing a free-energy relationship with a slope close to 20 V(-1), consistent with an associative mechanism. Theoretical DFT calculated descriptors, as the charge density in the [MNO] moieties and the LUMO energies, qualitatively correlate with the rate constants. A linear to bent transformation was calculated for the nitrosyl complexes, as they evolve to the angular MNO(2)H and MNO(2) complexes. The geometries were optimized for the different complexes and adduct-intermediates, showing significant changes in the relevant distances and angles upon OH(-) addition. IR vibrations and electronic transitions were also calculated. The complete reaction profile was studied for the nitroprusside ion, including the description of the transition state structure. Experimental activation parameters revealed that both the activation enthalpies and entropies increase when going from the negatively charged to the positively charged complexes. As the rate constants increase in the same direction, we conclude that the reactions are entropically driven, compensating, this function, the increase in the activation enthalpies. The latter trend can be explained by the energies involved in angular reorganization after OH(-) coordination, which are larger as the positive charge in the nitrosyl moiety becomes larger. The use of E(NO+/-NO) as a predictive tool for electrophilic reactivity could be extended to similar reactions implying other nucleophiles, such as amines and thiolates.

The nature of Ru–NO bonds in ruthenium tetraazamacrocycle nitrosyl complexes—a computational study

“…[56][57][58] It should be stressed that controlling the properties of the complex, such as electronic spectra, reduction potential, and specific rate constant of the release of NO, is crucial for biological applications. [15][16][17][18][19][20][21][22][23]37,38,[59][60][61][62][63] Tetraazamacrocycles such as cyclam and its derivatives are fairly flexible; they adopt five different configurations, depending on the spatial alignment of the NH protons (Fig. 1).…”

Addition and Redox Reactivity of Hydrogen Sulfides (H2S/HS−) with Nitroprusside: New Chemistry of Nitrososulfide Ligands

“…[45] Depending on the metal fragments, ill-defined intermediates have been proposed in these reactions, and N 2 O and free NH 2 OH have been observed. The latter species is known to be formed by the reaction of HNO with thiols, generating the corresponding disulfide.…”

Chromatographic Investigation of Ruthenium Nitrosyl Complex: No Interconversion and Reactions With Biological Reductants

“…29,34 It has been reported that when solutions of {MNO} 6 complexes are made alkaline, 36,40 a product with absorption band from 300 -400 nm is observed. 31,52,53 Additionally, this transformation is fully reversible upon addition of acid. Currently, there is no debate regarding the nature of this reaction corresponding to the interconversion of the nitrosyl complex into the nitrite.…”

“…Currently, there is no debate regarding the nature of this reaction corresponding to the interconversion of the nitrosyl complex into the nitrite. 31,52,53 This resulted by a nucleophilic attack of the hydroxyl into the NO + group, and a new band observed in 400 nm may be assigned to a metal to ligand charge transfer from Ru(II) to the NO 2 -ligand.…”

“…[27][28][29][30][31][32][33][34] These species exhibit quite distinct overall charge, which can strongly alter interaction with reverse phase column.…”