From d to d: Iron(0) and Iron(I) Complexes Complete the Series of Eight Fe Oxidation States within the TIMMN Ligand Framework.

Reduction of the ferrous precursor [(TIMMN)Fe(Cl)] () (TIMMN = ris-[(3-mesitylidazol-2-ylidene)ethyl]amie) to the low-valent iron(0) complex [(TIMMN)Fe(CO)] () 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)Fe(L)] (L = free site, η-N, CO, py) () are synthesized and fully characterized. Complexes - demonstrate the notable steric and electronic flexibility of the TIMMN 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 in a reaction with benzophenone to yield the corresponding, charge-separated iron(II) radical complex [(TIMMN)Fe(OCPh)] (). 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 , where H atom abstraction with TEMPO triggers the formation of the metallacycle [(TIMMN*)Fe(py)] (), and the reactivity of the highly unstable Fe(VII) nitride complex [(TIMMN)Fe(N)(F)] to give the metallacyclic Fe(V) imido complex [(TIMMN)Fe(N)(MeCN)] () 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.