Described are syntheses, characterizations, and photochemical reactions of the nitrosyl complexes Ru(salen)-(ONO)(NO) (I, salen ) N,N′-ethylenebis(salicylideneiminato) dianion), Ru(salen)(Cl)(NO) (II), Ru( t Bu 4 salen)(Cl)-(NO) (III, t Bu 4 salen ) N, N′-ethylenebis(3,5-di-tert-butylsalicylideneiminatoUpon photolysis, these Ru(L)(X)(NO) compounds undergo NO dissociation to give the ruthenium(III) solvento products Ru(L)(X)(Sol). Quantum yields for 365 nm irradiation in acetonitrile solution fall in a fairly narrow range (0.055− 0.13) but decreased at longer λ irr . The quantum yield (λ irr ) 365 nm) for NO release from the water soluble complex [Ru(salen)(H 2 O)(NO)]Cl (VII) was 0.005 in water. Kinetics of thermal back-reactions to re-form the nitrosyl complexes demonstrated strong solvent dependence with second-order rate constants k NO varying from 5 × 10 -4 M -1 s -1 for the re-formation of II in acetonitrile to 5 × 10 8 M -1 s -1 for re-formation of III in cyclohexane. Pressure and temperature effects on the back-reaction rates were also examined. These results are relevant to possible applications of photochemistry for nitric oxide delivery to biological targets, to the mechanisms by which NO reacts with metal centers to form metal−nitrosyl bonds, and to the role of photochemistry in activating similar compounds as catalysts for several organic transformations. Also described are the X-ray crystal structures of I and V.
Several beneficial biological effects of carbon monoxide (CO) have been identified. CO contributes to vascular tone, is anti-inflammatory and antiapoptotic, and protects tissue against hypoxia. In response, a number of novel COreleasing systems, mostly transition-metal carbonyl compounds, have been reported. Recent research in the area of photochemical CO-releasing molecules (PhotoCORMs) is reviewed. The photophysical and chemical properties of these PhotoCORMs are described, along with studies of biological activity in some cases. Spectral data and quantum yields, when available, are compared. In addition, various methods for the detection and quantification of CO are discussed.
Described are studies directed toward elucidating the controversial chemistry relating to the solution phase reactions of nitric oxide with the iron(II) porphyrin complex Fe(TPP)(NO) (1, TPP = meso-tetraphenylporphinato2-). The only reaction observable with clean NO is the formation of the diamagnetic dinitrosyl species Fe(TPP)(NO)2 (2), and this is seen only at low temperatures (K(1) < 3 M(-1) at ambient temperature). However, 1 does readily react reversibly with N2O3 in the presence of excess NO to give the nitro nitrosyl complex Fe(TPP)(NO2)(NO) (3), suggesting that previous claims that 1 promotes NO disproportionation to give 3 may have been compromised by traces of air in the nitric oxide sources. It is also noted that 3 undergoes reversible loss of NO to give the elusive nitro species Fe(TPP)(NO2) (4), which has been implicated as a powerful oxygen atom transfer agent in reactions with various substrates. Furthermore, in the presence of excess NO2, the latter undergoes oxidation to the stable nitrato analogue Fe(TPP)(NO3) (5). Owing to such reactivity of Fe(TPP)(NO2), flash photolysis and stopped-flow kinetics rather than static techniques were necessary for the accurate measurement of dissociation equilibria characteristic of Fe(TPP)(NO2)(NO) in 298 K toluene solution. Flash photolysis of 3 resulted in competitive NO2 and NO dissociation to give Fe(TPP)(NO) and Fe(TPP)(NO2), respectively. The rate constant for the reaction of 1 with N2O3 to generate Fe(TPP)(NO2)(NO) was determined to be 1.8 x 10(6) M(-1) s(-1), and that for the NO reaction with 4 was similarly determined to be 4.2 x 10(5) M(-1) s(-1). Stopped-flow rapid dilution techniques were used to determine the rate constant for NO dissociation from 3 as 2.6 s(-1). The rapid dilution experiments also demonstrated that Fe(TPP)(NO2) readily undergoes further oxidation to give Fe(TPP)(NO3). The mechanistic implications of these observations are discussed, and it is suggested that NO2 liberated spontaneously from Fe(P)(NO2) may play a role in an important oxidative process involving this elusive species.
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