A Structurally Authenticated Closed-Shell Iron(IV) Oxo Ferryl Complex: Synthesis, Properties, and Reactivity

We report the synthesis and comprehensive characterization of a closed-shell iron(IV) oxo (ferryl) complex, S = 0, which is accessible through multiple methods and stabilized by a tripodal tris-N-heterocyclic carbene (NHC) ligand. Reaction of the Fe^I precursor [(^Me2TIMMN^Mes)Fe^I(η¹-N₂)](PF₆) (1) (^Me2TIMMN^Mes = tris-[2-(3-mesityl-4,5-dimethyl-imidazolin-2-ylidene)methyl]amine) with an excess of N₂O at -78 °C in THF yields the reactive Fe^III oxo complex [(^Me2TIMMN^Mes)Fe^III(O)](PF₆) (3). One-electron oxidation of the in situ generated 3 with [Cp₂Fe][PF₆] provides access to a rare, nonmagnetic (d ⁴, S = 0) Fe^IV oxo complex, namely [(^Me2TIMMN^Mes)Fe^IV(O)](PF₆)₂ (4). Alternatively, tetravalent 4 could also be obtained by treatment of the divalent iron complex [(^Me2TIMMN^Mes)Fe^II(THF)](PF₆)₂ (2-THF) with 1 equiv of trimethylamine N-oxide (TMAO) or by the photolysis reaction of the ferrous sulfoxide isotopomers [(^Me2TIMMN^Mes)Fe^II(MeS(^16/18O)Ph)](PF₆)₂ (2-^16/18OSR₂). Although the fleeting nature of trivalent oxide 3 precludes extensive characterization and isolation in the solid state, the one-electron oxidation product [(^Me2TIMMN^Mes)Fe^IV(O)](PF₆)₂ (4) is isolable and was reproducibly synthesized as well as fully characterized, including CHN elemental analysis, multinuclear NMR, IR vibrational, UV/vis electronic absorption, zero- and applied-field ⁵⁷Fe Mössbauer spectroscopy, and single-crystal X-ray crystallography studies. Diamagnetic 4 features a remarkably short Fe-O bond (d(Fe-O) = 1.576(1) Å) and a notably negative isomer shift (δ = -0.38 mm s^-1) in the ⁵⁷Fe Mössbauer spectrum. Computational analyses corroborate the + IV oxidation state and advocate an iron-oxygen triple bond, Fe^IV≡O. Preliminary reactivity studies show that the closed-shell iron(IV) oxo complex can mediate intermolecular oxygen-atom transfer chemistry. A reversible redox event at a half-wave potential, E_1/2, of 1.25 V vs Fe(Cp)₂/Fe(Cp)₂⁺ in the cyclic voltammogram of 4 suggests the existence of an Fe(V) oxo species.