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.