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 tris-[2-(3-mesityl-imidazole-2-ylidene)-methyl]amine (TIMMNMes) 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 [(TIMMNMes)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 [(TIMMNMes)Fe-(NAd)](0,1+,2+,3+) (2(Ad)-5(Ad)) in oxidation states II to V. The Fe(V) imide [(TIMMNMes)Fe(NAd)](3+) (5(Ad)) is unstable under ambient conditions and converts to the air-stable nitride [(TIMMNMes)Fe-V(N)](2+) (6) via N-C bond cleavage. The stability of the pentavalent imide can be increased by derivatizing the nitride [(TIMMNMes)Fe-IV(N)]+ (7) with an ethyl group using the triethyloxonium salt Et3OPF6. This gives access to the analogous series of ethyl imides [(TIMMNMes)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 Fe-57 Mossbauer, 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.

Automated summary

Terminal imido complexes of iron have been widely studied in metal-catalyzed nitrogen-transfer chemistry. In this study, a new tris-NHC chelating ligand (TIMMNMes) was used to synthesize and compare two series of iron alkyl imido complexes, including a reactive Fe(V) adamantyl imido intermediate. The stability and reactivity of these complexes were investigated using various characterization techniques and quantum chemical calculations.