Discrimination of Mononuclear and Dinuclear Dinitrosyl Iron Complexes (DNICs) by S K-Edge X-ray Absorption Spectroscopy: Insight into the Electronic Structure and Reactivity of DNICs

in addition to probing the formation of dinitrosyl iron complexes (DNICs) by the characteristic Fe K-edge pre-edge absorption energy ranging from 7113.4 to 7113.8 eV, the distinct S K-edge pre-edge absorption energy and pattern can serve as an efficient tool to unambiguously characterize and discriminate mononuclear DNICs and dinuclear DNICs containing bridged-thiolate and bridged-sulfide ligands. The higher Fe-S bond covalency modulated by the stronger electron-donating thiolates promotes the Fe-NO pi-electron back-donation to strengthen the Fe-NO bond and weaken the NO-release ability of the mononuclear DNICs, which is supported by the Raman v(Fe-NO) stretching frequency. The Fe-S bond covalency of DNICs further rationalizes the binding preference of the {Fe(NO)(2)}. motif toward thiolates following the trend of [SEt](-) > [SPh](-) > [SC7H4SN](-). The relative d-manifold energy derived from S K-edge XAS as well as the Fe K-edge pre-edge energy reveals that the electronic structure of the {Fe(NO)(2)}(9) core of the mononuclear DNICs [(NO)(2)Fe(SR)(2)](-) is best described as {Fe-III(NO-)(2)}(9) compared to [{Fe-III(NO-)(2)}(9)-{Fe-III(NO-)(2)}(9)] for the dinuclear DNICs [Fe-2(mu-SEt)(mu-S)(NO)(4)](-) and [Fe-2(mu-S)(2)(NO)(4)](2-).