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Published on: August 12, 2019
A Structurally Authenticated Closed-Shell Iron(IV) Oxo Ferryl Complex: Synthesis, Properties, and Reactivity
Weiqing Mao1, Zihan Zhang1, Martin Keilwerth1
1Department of Chemistry and Pharmacy, Inorganic Chemistry, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Egerlandstraße 1, 91058 Erlangen, Germany.
Abstract:
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 FeI precursor [(Me2TIMMNMes)FeI(η1-N2)](PF6) (1) (Me2TIMMNMes = 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 FeIII oxo complex [(Me2TIMMNMes)FeIII(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) FeIV oxo complex, namely [(Me2TIMMNMes)FeIV(O)](PF6)2 (4). Alternatively, tetravalent 4 could also be obtained by treatment of the divalent iron complex [(Me2TIMMNMes)FeII(THF)](PF6)2 (2-THF) with 1 equiv of trimethylamine N-oxide (TMAO) or by the photolysis reaction of the ferrous sulfoxide isotopomers [(Me2TIMMNMes)FeII(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 [(Me2TIMMNMes)FeIV(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, FeIV≡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, E1/2, 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.
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