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Divalent Rare Earth Chemistry

SmR2carbene-render3We have been synthesizing rare earth and main group alkyl compounds as catalyst precursors and as catalysts in their own right. Unlike many transition metal complexes, main group and rare earth alkyl compounds containing β-H atoms (i.e., M–C–C–H) often resist β-H elimination that leads to metal-carbon bond cleavage.

Making use of this fact, we synthesized divalent and trivalent organometallics containing the β-H rich ligand C(SiHMe2)3. For example, the series of compounds M{C(SiHMe2)3THF2 (M = Ca, Yb, Sm) have been synthesized and persist without β-H elimination, even at elevated temperature. These compounds react with Lewis acids such as B(C6F5)3 through β-H abstraction rather than alkyl group abstraction to give metal hydridoborate compounds such as (Me2HSi)3CYb(μ-H)B(C6F5)3. Effectively, the Lewis acid acts to facilitate β-H elimination to give a metal hydride coordinated by tris(perfluorophenyl)borane. Accordingly, the byproduct is a head-to-tail [2+2] dimer of the silene Me2Si=C(SiHMe2)2. See our J. Am. Chem. Soc. paper or full Organometallics paper.

Elimination TOC GraphicRecently, we have found than N-heterocyclic carbenes coordinate to give divalent Ln{C(SiHMe2)3}2ImtBu (Ln = Yb, Sm). These compounds are structurally interesting, in that the Yb compound contains two Yb↼Si-H from the same alkyl ligand, while the two Sm↼Si-H form from different ligands (see the figure above). Moreover, discounting the nonclassica Ln↼Si-H, these compounds contain only carbon coordinated to the rare earth center. Unlike the THF-coordinated analogues, Ln{C(SiHMe2)3}2ImtBu catalyze dehydrocoupling of amines and silanes. Remarkably, kinetic studies show the catalytic reaction is inhibited by excess silane. See Organometallics.