A series of alkali metal capped cerium(IV) imido complexes, [M(solv)][Ce═N(3,5-(CF)CH)(TriNOx)] (M = Li, K, Rb, Cs; solv = TMEDA, THF, EtO, 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-(CF)CH)(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)][Ce═N(3,5-(CF)CH)(TriNOx)] complex cleaves the Si-O bond in (MeSi)O, yielding the [(MeSiO)Ce(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.
Structurally authenticated, terminal lanthanide-ligand multiple bonds are rare and expected to be highly reactive. Even capped with an alkali metal cation, poor orbital energy matching and overlap of metal and ligand valence orbitals should result in strong charge polarization within such bonds. We expand on a new strategy for isolating terminal lanthanide-ligand multiple bonds using cerium(IV) complexes. In the current case, our tailored tris(hydroxylaminato) ligand framework, TriNOx(3-), provides steric protection against ligand scrambling and metal complex oligomerization and electronic protection against reduction. This strategy culminates in isolation of the first formal Ce═N bonded moiety in the complex [K(DME)2][Ce═N(3,5-(CF3)2C6H3)(TriNOx)], whose Ce═N bond is the shortest known at 2.119(3) Å.
The formation and stabilization of multiple bonds between a lanthanide cation and an anionic p-block element fragment are challenging, and only a few examples of such terminal complexes have been reported to date. Notably, all reported synthetic routes to lanthanide-imido moieties employed deprotonation as the key formation step. In the present report, we describe the generation of a Ce(IV) imido complex by a trimethylsilyl group transfer pathway. Experimental and computational evidence supports an equilibrium proceeding by a bimolecular nucleophilic substitution mechanism.
Chemical oxidation of cerium complexes can be unpredictable because of labile metal-ligand bonds leading to ligand redistribution. The use of tripodal frameworks such as silyl-substituted tren ligands (NN' = [N(CHCHN(SiMeBu))]) and a tris(hydroxylaminato) ligand, [((2- BuNO)CHCH)N] (TriNOx), has been shown to mitigate ligand redistribution effects to allow access to tetravalent cerium complexes with different apical ligands. In the current work, the coordination chemistry of Ce with the related tripodal atrane (Hatrane = [N(CHC(CH)OH)]) ligand framework was examined. A series of Ce(atrane) complexes with supporting chloride, silylamide, and aryloxide ligands were synthesized and characterized by X-ray crystallography. The solution-state behaviors of these complexes were studied using H and diffusion-ordered (DOSY) NMR spectroscopies. The electrochemical stabilization of the Ce cation within the atrane framework was examined. Our results showed that a combination of suitable apical ligands and the atrane framework provided excellent stabilization for the Ce cation.
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