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

Abstract

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)Ce^IV(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.