Photochemistry of the Dinitrosyl Iron Complex [S5Fe(NO)2]- Leading to Reversible Formation of [S5Fe(μ-S)2FeS5]2-: Spectroscopic Characterization of Species Relevant to the Nitric Oxide Modification and Repair of [2Fe−2S] Ferredoxins

The reaction of [PPN][Fe(CO)(3)(NO)] and S(8) in a 1:1 molar ratio in THF proceeded to give the dinitrosyl iron complex [PPN][S(5)Fe(NO)(2)] (1) and the known [PPN](2)[S(5)Fe(mu-S)(2)FeS(5)] (2). EPR signals of g values g(z) = 2.0148, g(x) = 2.0270, and g(y) = 2.0485 at 77 K confirmed the existence of the unpaired electron in compound 1. The temperature-dependent magnetic moment of complex 1 indicates that the ground state is one unpaired electron with (S(t), S(L)) = ((1)/(2), 1) at very low temperature (S(t) is the total spin quantum number of the system; S(L) is the sum of the spin quantum numbers of two NO ligands). The O K-edge absorptions of complex 1 and [(NO)Fe(S(2)CNEt(2))(2)] at 532.1 and 532.5 eV are assigned to the transition of 1s --> pi(NO) and 1s --> pi(NO(+)), respectively. For the electronic structure of the [Fe(NO)(2)] core, DFT calculations, magnetic susceptibility measurement, EPR, and Fe K-/L-edge XAS spectroscopy of complex 1 lead to a description of [Fe(1+)(.NO)(2)](9). [2Fe-2S] cluster 2 treated with nitric oxide in THF shows that cluster 2 is transformed into the dinitrosyl iron complex 1 identified by IR, UV-vis, and X-ray diffraction analysis. The reaction may be reversed by the photolysis of the THF solution of 1 in the presence of the NO-accepting reagent [(C(4)H(8)O)Fe(S,S-C(6)H(4))(2)](-) to reform 2. This result demonstrates a successful biomimetic reaction cycle of the degradation and reassembly of [2Fe-2S] cluster [S(5)Fe(mu-S)(2)FeS(5)](2)(-) relevant to the repair of nitric oxide-modified [2Fe-2S] ferredoxin by cysteine desulfurase and l-cysteine in vitro.