Flash Photolysis Studies of the Ruthenium(II) Porphyrins Ru(P)(NO)(ONO). Multiple Pathways Involving Reactions of Intermediates with Nitric Oxide1

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Abstract

Described are the spectra and kinetics of transients formed by laser flash photolysis of the ruthenium nitrosyl nitrito complexes Ru(P)(NO)(ONO), P = TPP (meso-tetraphenylporphyrin), OEP (octaethylporphyrin), TmTP (tetra(m-tolyl)porphyrin), and FTTP (tetra(m-trifluoromethylphenyl)porphyrin) in benzene solutions. Two transient decay processes are seen on the time frame (<1 ms) of the flash photolysis experiment, and a residual difference spectrum, which decays to baseline on a longer time frame, is noted as well. The accumulated evidence points to the formation of two primary photoproducts, Ru(P)(ONO) (A) formed by NO photolabilization and Ru(P)(NO) (B) formed by NO2 photolabilization. Both decay by NO dependent pathways, the reaction of A with NO to re-form Ru(P)(NO)(ONO) being substantially faster (2.4−5.5 × 108 M-1 s-1 in ambient temperature benzene) than the reaction of B with NO (2.4−10 × 107 M-1 s-1). The product of the latter reaction is apparently the dinitrosyl complex Ru(P)(NO)2, which undergoes a much slower thermal reaction with excess NO to give again Ru(P)(NO)(ONO). The possibility of B being the oxo complex ORu(P)(NO) formed by NO loss from coordinated nitrite was considered but concluded to be a minor pathway at best. Isotopic exchange reactions using either labeled complex or labeled NO in cyclohexane demonstrate photochemical exchange of NO into both the nitrosyl and nitrito complexes, and time-resolved infrared experiments are consistent with formation of a long-lived nitrosyl-containing intermediate. Flash photolysis studies of the respective nitrosyl chloro complexes Ru(TPP)(NO)Cl and Ru(OEP)(NO)Cl indicate that only a single transient species, presumably Ru(P)Cl, is formed in each case, and this decays by a single NO dependent pathway back to starting material.

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