Insight into chalcogenolate-bound {Fe(NO)₂}⁹ dinitrosyl iron complexes (DNICs): covalent character versus ionic character

The synthesis, characterization and transformation of the thermally unstable {Fe(NO)₂}⁹ dinitrosyl iron complex (DNIC) [(OMe)₂Fe(NO)₂]⁻ (2) were investigated. The {Fe(NO)₂}⁹ DNIC 2 characterized by single-crystal X-ray diffraction is exclusively stabilized by the weak intermolecular [Fe(OMe)₂⋯(K⁺)] interactions (O(3)⋯K(1) and O(4)⋯K(1) distances of 2.818(3) and 2.810(3) Å, respectively). The binding affinity of chalcogenolate-containing ligands toward the {Fe(NO)₂}⁹ motif follows the series [SEt]⁻ > [SPh]⁻ > [OPh]⁻ > [OMe]⁻, which is dictated by the synergistic cooperation of the electron-donating order ([SEt]⁻ > [SPh]⁻ > [OPh]⁻) and the soft–hard order (from soft to hard, [SEt]⁻ ∼ [SPh]⁻ > [OPh]⁻ > [OMe]⁻). In comparison with the XAS Fe K-edge pre-edge energy of {Fe(NO)₂}⁹ [(RS)₂Fe(NO)₂]⁻ (R = Ph (4), Et (5)) and [(PhO)₂Fe(NO)₂]⁻ (6) DNICs falling within the reported range of 7113.4–7113.9 eV, the distinctive pre-edge energy of 7114.2 eV exhibited by complex 2 suggests that the electronic structure of {Fe(NO)₂}⁹ DNIC 2 may be qualitatively described as a {Fe^III(NO⁻)₂}⁹ electronic structure induced by the dominant ionic character of Fe–OMe bonds, instead of the resonance hybrids of {Fe^II(NO⁻)(˙NO)}⁹ and {Fe^III(NO⁻)₂}⁹ electronic structures induced by the dominant metal–ligand covalency of {Fe(NO)₂}⁹ DNICs 4–6. As shown in TD-DFT computation, the increased population of NO ligands in MO 125β (45.1% NO) attenuating the OMe-induced polarization imposed on the Fe center through the delocalized covalent nature of Fe–NO bonds supports the lower/synergistic NO/OMe → Fe charge transfer energy (1216 nm) observed in the solid-state UV-vis spectrum of complex 2 compared to those (1140 nm) of complexes 4–6.