Flash and continuous photolysis studies of Roussin's red salt dianion Fe2S2(NO)(4)(2-) in solution

Flash photolysis studies of Roussin's red salt dianion, Fe2S2(NO)(4)(2-) (R RS) demonstrated high quantum yield formation of an intermediate believed t o be Fe2S2(NO)(3)(2-) formed by photodissociation of NO. This species react s competitively with nitric oxide via second-order kinetics (k(NO) = 9.1 x 10(8) M-1 s(-1)) to reform RRS and with dioxygen (k(Ox) = 5.6 x 10(7) M-1 s (-1)) to give a secondary intermediate. The latter species is the likely pr ecursor of the eventual photoproduct, Roussin's black salt anion, Fe4S3(NO) (7)(-) (RBS). In deaerated aqueous solution, photolysis of the red salt als o leads to formation of RBS but with a quantum yield (0.0039) much smaller than that in aerated solution (0.14). Notably, under these conditions N2O i s also found to be a product. In deoxygenated aprotic media, a different ph otoproduct, the black salt dianion Fe4S3(NO)(7)(2-) is formed in low yield. The independently prepared dianion Fe4S3(NO)(7)(2-) was shown to reduce NO to N2O in the presence of a proton source. A scheme for the observed photo chemistry of RRS based on these observations is proposed. (C) 2000 Elsevier Science S.A. All rights reserved.