Insight into the reactivity and electronic structure of dinuclear dinitrosyl iron complexes

A combination of N/S/Fe K-edge X-ray absorption spectroscopy (XAS), X-ray diffraction data, and density functional theory (DFT) calculations provides an efficient way to unambiguously delineate the electronic structures and bonding characters of Fe-S, N-O, and Fe-N bonds among the direduced-form Roussin's red ester (RRE) [Fe2(μ-SPh)2(NO)4]2-(1) with {Fe(NO)2}10-{Fe(NO)2}10 core, the reduced-form RRE [Fe2(μ-SPh)2(NO)4]-(3) with {Fe(NO)2}9-{Fe(NO)2}10 core, and RRE [Fe2(μ-SPh)2(NO)4] (4) with {Fe(NO)2}9-{Fe(NO)2}9 core. The major contributions of highest occupied molecular orbital (HOMO) 113α/β in complex 1 is related to the antibonding character between Fe(d) and Fe(d), Fe(d), and S atoms, and bonding character between Fe(d) and NO(π∗). The effective nuclear charge (Zeff) of Fe site can be increased by removing electrons from HOMO to shorten the distances of Fe⋯Fe and Fe-S from 1 to 3 to 4 or, in contrast, to increase the Fe-N bond lengths from 1 to 3 to 4. The higher IR νNO stretching frequencies (1761, 1720 cm-1 (4), 1680, 1665 cm-1 (3), and 1646, 1611, 1603 cm-1 (1)) associated with the higher transition energy of N1s →σ∗(NO) (412.6 eV (4), 412.3 eV (3), and 412.2 eV (1)) and the higher Zeff of Fe derived from the transition energy of Fe1s → Fe3d (7113.8 eV (4), 7113.5 eV (3), and 7113.3 eV (1)) indicate that the N-O bond distances of these complexes are in the order of 1 > 3 > 4. The N/S/Fe K-edge XAS spectra as well as DFT computations reveal the reduction of complex 4 yielding complex 3 occurs at Fe, S, and NO; in contrast, reduction mainly occurs at Fe site from complex 3 to complex 1. (Graph Presented).