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, <i>S</i> = 0, which is accessible through multiple methods and stabilized by a tripodal <i>tris</i>-N-heterocyclic carbene (NHC) ligand. Reaction of the Fe^I precursor [(^Me2TIMMN^Mes)Fe^I(η¹-N₂)](PF₆) (<b>1</b>) (^Me2TIMMN^Mes = <i>tris</i>-[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) (<b>3</b>). One-electron oxidation of the <i>in situ</i> generated <b>3</b> with [Cp₂Fe][PF₆] provides access to a rare, nonmagnetic (<i>d</i>⁴, <i>S</i> = 0) Fe^IV oxo complex, namely [(^Me2TIMMN^Mes)Fe^IV(O)](PF₆)₂ (<b>4</b>). Alternatively, tetravalent <b>4</b> could also be obtained by treatment of the divalent iron complex [(^Me2TIMMN^Mes)Fe^II(THF)](PF₆)₂ (<b>2</b>-<b>THF</b>) with 1 equiv of trimethylamine <i>N</i>-oxide (TMAO) or by the photolysis reaction of the ferrous sulfoxide isotopomers [(^Me2TIMMN^Mes)Fe^II(MeS(^16/18O)Ph)](PF₆)₂ (<b>2</b>-^{<b>16</b>/<b>18</b>}<b>OSR</b>_<b>2</b>). Although the fleeting nature of trivalent oxide <b>3</b> precludes extensive characterization and isolation in the solid state, the one-electron oxidation product [(^Me2TIMMN^Mes)Fe^IV(O)](PF₆)₂ (<b>4</b>) 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 <b>4</b> 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, <i>E</i>_1/2, of 1.25 V <i>vs</i> Fe(Cp)₂/Fe(Cp)₂⁺ in the cyclic voltammogram of <b>4</b> suggests the existence of an Fe(V) oxo species.