A Series of Iron Nitrosyl Complexes {Fe–NO}^6–9 and a Fleeting {Fe–NO}¹⁰ Intermediate en Route to a Metalacyclic Iron Nitrosoalkane

Iron–nitrosyls have fascinated chemists for a long time due to the noninnocent nature of the NO ligand that can exist in up to five different oxidation and spin states. Coordination to an open-shell iron center leads to complex electronic structures, which is the reason Enemark−Feltham introduced the {Fe–NO}n notation. In this work, we succeeded in characterizing a series of {Fe–NO}6–9 complexes, including a reactive {Fe–NO}10 intermediate. All complexes were synthesized with the tris-N-heterocyclic carbene ligand tris[2-(3-mesitylimidazol-2-ylidene)ethyl]amine (TIMENMes), which is known to support iron in high and low oxidation states. Reaction of NOBF4 with [(TIMENMes)Fe]2+ resulted in formation of the {Fe–NO}6 compound [(TIMENMes)Fe(NO)(CH3CN)](BF4)3 (1). Stepwise chemical reduction with Zn, Mg, and Na/Hg leads to the isostructural series of high-spin iron nitrosyl complexes {Fe–NO}7,8,9 (2–4). Reduction of {Fe–NO}9 with Cs electride finally yields the highly reduced {Fe–NO}10 intermediate, key to formation of [Cs(crypt-222)][(TIMENMes)Fe(NO)], (5) featuring a metalacyclic [Fe−(NO−NHC)3−] nitrosoalkane unit. All complexes were characterized by single-crystal XRD analyses, temperature and field-dependent SQUID magnetization methods, as well as 57Fe Mössbauer, IR, UV/vis, multinuclear NMR, and dual-mode EPR spectroscopy. Spectroscopy-based DFT analyses provide insight into the electronic structures of all compounds and allowed assignments of oxidation states to iron and NO ligands. An alternative synthesis to the {Fe–NO}8 complex was found via oxygenation of the nitride complex [(TIMENMes)Fe(N)](BF4). Surprisingly, the resulting {Fe–NO}8 species is electronically and structural similar to the [(TIMENMes)Fe(N)]+ precursor. Based on the structural and electronic similarities between this nitrosyl/nitride complex couple, we adopted the strategy, developed by Wieghardt et al., of extending the Enemark−Feltham nomenclature to nitrido complexes, rendering [(TIMENMes)Fe(N)]+ as a {Fe–N}8 species