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.