From d<sup>8</sup> to d<sup>1</sup>: Iron(0) and Iron(I)\nComplexes Complete the Series of Eight Fe Oxidation States within\nthe TIMMN<sup>Mes</sup> Ligand Framework

Reduction of the ferrous precursor [(TIMMN<sup>Mes</sup>)Fe(Cl)]<sup>+</sup> (<b>1</b>) (TIMMN<sup>Mes</sup> = <b>t</b>ris-[(3-mesityl<b>im</b>idazol-2-ylidene)<b>m</b>ethyl]ami<b>n</b>e)\nto the low-valent iron(0) complex [(TIMMN<sup>Mes</sup>)Fe(CO)<sub>3</sub>] (<b>2</b>) is presented, where the tris(N-heterocyclic\ncarbene) (NHC) ligand framework remains intact, yet the coordination\nmode changed from 3-fold to 2-fold coordination of the carbene arms.\nFurther, the corresponding iron(I) complexes [(TIMMN<sup>Mes</sup>)Fe(L)]<sup>+</sup> (L = free site, η<sup>1</sup>-N<sub>2</sub>, CO, py) (<b>3</b>) are synthesized and fully characterized.\nComplexes <b>1</b>–<b>3</b> demonstrate the notable\nsteric and electronic flexibility of the TIMMN<sup>Mes</sup> ligand\nframework by variation of the Fe–N anchor and Fe–carbene\ndistances and the variable size of the axial cavity occupation. This\nis further underpinned by the oxidation of <b>3-N</b><sub><b>2</b></sub> in a reaction with benzophenone to yield the corresponding,\ncharge-separated iron(II) radical complex [(TIMMN<sup>Mes</sup>)Fe(OCPh<sub>2</sub>)]<sup>+</sup> (<b>4</b>). We found rather surprising\nsimilarities in the reactivity behavior when going to low- or high-valent\noxidation states of the central iron ion. This is demonstrated by\nthe closely related reactivity of <b>3-N</b><sub><b>2</b></sub>, where H atom abstraction with TEMPO triggers the formation\nof the metallacycle [(TIMMN<sup>Mes</sup>*)Fe(py)]<sup>+</sup> (<b>5</b>), and the reactivity of the highly unstable Fe(VII) nitride\ncomplex [(TIMMN<sup>Mes</sup>)Fe(N)(F)]<sup>3+</sup> to give the metallacyclic\nFe(V) imido complex [(TIMMN<sup>MesN</sup>)Fe(N<sup>Mes</sup>)(MeCN)]<sup>3+</sup> (<b>6</b>) upon warming. Thus, the employed tris(carbene)\nchelate is not only capable of stabilizing the superoxidized Fe(VI)\nand Fe(VII) nitrides but equally supports the iron center in its low\noxidation states 0 and +1. Isolation and characterization of these\nzero- and monovalent iron complexes demonstrate the extraordinary\ncapability of the tris(carbene) chelate TIMMN to support iron in eight\ndifferent oxidation states within the very same ligand platform.