From Divalent to Pentavalent Iron Imido Complexes and an Fe(V) Nitride via N-C Bond Cleavage.

As key intermediates in metal-catalyzed nitrogen-transfer chemistry, terminal imido complexes of iron have attracted significant attention for a long time. In search of versatile model compounds, the recently developed second-generation N -anchored tris -NHC chelating ligand t ris -[2-(3-mesityl- im idazole-2-ylidene)- m ethyl]ami n e (TIMMN Mes ) was utilized to synthesize and compare two series of mid- to high-valent iron alkyl imido complexes, including a reactive Fe(V) adamantyl imido intermediate en route to an isolable Fe(V) nitrido complex. The chemistry toward the iron adamantyl imides was achieved by reacting the Fe(I) precursor [(TIMMN Mes )Fe I (N 2 )] + ( 1 ) with 1-adamantyl azide to yield the corresponding trivalent iron imide. Stepwise chemical reduction and oxidation lead to the isostructural series of low-spin [(TIMMN Mes )Fe(NAd)] 0,1+,2+,3+ ( 2 Ad - 5 Ad ) in oxidation states II to V. The Fe(V) imide [(TIMMN Mes )Fe(NAd)] 3+ ( 5 Ad ) is unstable under ambient conditions and converts to the air-stable nitride [(TIMMN Mes )Fe V (N)] 2+ ( 6 ) via N-C bond cleavage. The stability of the pentavalent imide can be increased by derivatizing the nitride [(TIMMN Mes )Fe IV (N)] + ( 7 ) with an ethyl group using the triethyloxonium salt Et 3 OPF 6 . This gives access to the analogous series of ethyl imides [(TIMMN Mes )Fe(NEt)] 0,1+,2+,3+ ( 2 Et - 5 Et ), including the stable Fe(V) ethyl imide. Iron imido complexes exist in a manifold of different electronic structures, ultimately controlling their diverse reactivities. Accordingly, these complexes were characterized by single-crystal X-ray diffraction analyses, SQUID magnetization, and electrochemical methods, as well as 57 Fe Mössbauer, IR vibrational, UV/vis electronic absorption, multinuclear NMR, X-band EPR, and X-ray absorption spectroscopy. Our studies are complemented with quantum chemical calculations, thus providing further insight into the electronic structures of all complexes.