Cerium(IV) Imido Complexes: Structural, Computational, and Reactivity Studies

A series of alkali metal capped cerium­(IV) imido complexes, [M­(solv)x]­[CeN­(3,5-(CF3)2C6H3)­(TriNOx)] (M = Li, K, Rb, Cs; solv = TMEDA, THF, Et2O, or DME), was isolated and fully characterized. An X-ray structural investigation of the cerium imido complexes demonstrated the impact of the alkali metal counterions on the geometry of the [CeN­(3,5-(CF3)2C6H3)­(TriNOx)]− moiety. Substantial shortening of the CeN bond was observed with increasing size of the alkali metal cation. The first complex featuring an unsupported, terminal multiple bond between a Ce­(IV) ion and a ligand fragment was also isolated by encapsulation of a Cs+ counterion with 2.2.2-cryptand. This complex shows the shortest recorded CeN bond length of 2.077(3) Å. Computational investigation of the cerium imido complexes using DFT methods showed a relatively larger contribution of the cerium 5d orbital than the 4f orbital to the CeN bonds. The [K­(DME)2]­[CeN­(3,5-(CF3)2C6H3)­(TriNOx)] complex cleaves the SiO bond in (Me3Si)2O, yielding the [(Me3SiO)­CeIV(TriNOx)] adduct. The reaction of the rubidium capped imido complex with benzophenone resulted in the formation of a rare Ce­(IV)–oxo complex, that was stabilized by a supramolecular, tetrameric oligomerization of the CeO units with rubidium cations.