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 S8 in a 1:1 molar ratio in THF proceeded to give the dinitrosyl iron complex [PPN][S5Fe(NO)2] (1) and the known [PPN]2[S5Fe(μ-S)2FeS5] (2). EPR signals of g values gz = 2.0148, gx = 2.0270, and gy = 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 (St, SL) = (1/2, 1) at very low temperature (St is the total spin quantum number of the system; SL is the sum of the spin quantum numbers of two NO ligands). The O K-edge absorptions of complex 1 and [(NO)Fe(S2CNEt2)2] at 532.1 and 532.5 eV are assigned to the transition of 1s → π*(NO) and 1s → π*(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 {Fe1+(•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 [(C4H8O)Fe(S,S-C6H4)2]- to reform 2. This result demonstrates a successful biomimetic reaction cycle of the degradation and reassembly of [2Fe−2S] cluster [S5Fe(μ-S)2FeS5]2- relevant to the repair of nitric oxide-modified [2Fe−2S] ferredoxin by cysteine desulfurase and l-cysteine in vitro.