From d⁸ to d¹: Iron(0) and Iron(I) Complexes Complete the Series of Eight Fe Oxidation States within the TIMMN^Mes Ligand Framework

Reduction of the ferrous precursor [(TIMMN^Mes)Fe(Cl)]⁺ (<b>1</b>) (TIMMN^Mes = <b>t</b>ris-[(3-mesityl<b>im</b>idazol-2-ylidene)<b>m</b>ethyl]ami<b>n</b>e) to the low-valent iron(0) complex [(TIMMN^Mes)Fe(CO)₃] (<b>2</b>) is presented, where the tris(N-heterocyclic carbene) (NHC) ligand framework remains intact, yet the coordination mode changed from 3-fold to 2-fold coordination of the carbene arms. Further, the corresponding iron(I) complexes [(TIMMN^Mes)Fe(L)]⁺ (L = free site, η¹-N₂, CO, py) (<b>3</b>) are synthesized and fully characterized. Complexes <b>1</b>-<b>3</b> demonstrate the notable steric and electronic flexibility of the TIMMN^Mes ligand framework by variation of the Fe-N anchor and Fe-carbene distances and the variable size of the axial cavity occupation. This is further underpinned by the oxidation of <b>3-N</b>_<b>2</b> in a reaction with benzophenone to yield the corresponding, charge-separated iron(II) radical complex [(TIMMN^Mes)Fe(OCPh₂)]⁺ (<b>4</b>). We found rather surprising similarities in the reactivity behavior when going to low- or high-valent oxidation states of the central iron ion. This is demonstrated by the closely related reactivity of <b>3-N</b>_<b>2</b>, where H atom abstraction with TEMPO triggers the formation of the metallacycle [(TIMMN^Mes*)Fe(py)]⁺ (<b>5</b>), and the reactivity of the highly unstable Fe(VII) nitride complex [(TIMMN^Mes)Fe(N)(F)]³⁺ to give the metallacyclic Fe(V) imido complex [(TIMMN^MesN)Fe(N^Mes)(MeCN)]³⁺ (<b>6</b>) upon warming. Thus, the employed tris(carbene) chelate is not only capable of stabilizing the superoxidized Fe(VI) and Fe(VII) nitrides but equally supports the iron center in its low oxidation states 0 and +1. Isolation and characterization of these zero- and monovalent iron complexes demonstrate the extraordinary capability of the tris(carbene) chelate TIMMN to support iron in eight different oxidation states within the very same ligand platform.