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

Abstract

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}^<i>n</i> 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-<i>N</i>-heterocyclic carbene ligand tris[2-(3-mesitylimidazol-2-ylidene)ethyl]amine (TIMEN^Mes), which is known to support iron in high and low oxidation states. Reaction of NOBF₄ with [(TIMEN^Mes)Fe]²⁺ resulted in formation of the {Fe-NO}⁶ compound [(TIMEN^Mes)Fe(NO)(CH₃CN)](BF₄)₃ (<b>1</b>). 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 (<b>2</b>-<b>4</b>). Reduction of {Fe-NO}⁹ with Cs electride finally yields the highly reduced {Fe-NO}¹⁰ intermediate, key to formation of [Cs(crypt-222)][(TIMEN^Mes)Fe(NO)], (<b>5</b>) 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 [(TIMEN^Mes)Fe(N)](BF₄). Surprisingly, the resulting {Fe-NO}⁸ species is electronically and structural similar to the [(TIMEN^Mes)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 [(TIMEN^Mes)Fe(N)]⁺ as a {Fe-N}⁸ species.