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.