From d 8 to d 1 : 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)] + ( 1 ) (TIMMN Mes = t ris-[(3-mesityl im idazol-2-ylidene) m ethyl]ami n e) to the low-valent iron(0) complex [(TIMMN Mes )Fe(CO) 3 ] ( 2 ) 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, η 1 -N 2 , CO, py) ( 3 ) are synthesized and fully characterized. Complexes 1 - 3 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 3-N 2 in a reaction with benzophenone to yield the corresponding, charge-separated iron(II) radical complex [(TIMMN Mes )Fe(OCPh 2 )] + ( 4 ). 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 3-N 2 , where H atom abstraction with TEMPO triggers the formation of the metallacycle [(TIMMN Mes *)Fe(py)] + ( 5 ), 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+ ( 6 ) 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.