Homoleptic Divalent Dialkyl Lanthanide-Catalyzed Cross-Dehydrocoupling of Silanes and Amines

The rare-earth bis­(alkyl)
compound Sm­{C­(SiHMe2)3}2THF2 (1b) is prepared
by the reaction of samarium­(II) iodide and 2 equiv of KC­(SiHMe2)3. This synthesis is similar to that of previously
reported Yb­{C­(SiHMe2)3}2THF2 (1a), and compounds 1a,b are
isostructural. Reactions of 1b and 1 or 2 equiv of B­(C6F5)3 afford SmC­(SiHMe2)3HB­(C6F5)3THF2 (2b) or Sm­{HB­(C6F5)3}2THF2 (3b), respectively, and 1,3-disilacyclobutane
{Me2Si-C­(SiHMe2)2}2 as
a byproduct. Bands from 2300 to 2400 cm–1 assigned
to νBH in the IR spectra and highly paramagnetically
shifted signals in the 11B NMR spectra of 2b and 3b provided evidence for Sm-coordinated HB­(C6F5)3. Compounds 1a,b react with the bulky N-heterocyclic carbene (NHC) 1,3-di-tert-butylimidazol-2-ylidene (ImtBu) to
displace both THF ligands and give three-coordinate monoadducts Ln­{C­(SiHMe2)3}2ImtBu (Ln = Yb
(4a), Sm (4b)). Complexes 4a,b catalyze cross-dehydrocoupling of organosilanes with
primary and secondary amines at room temperature to give silazanes
and H2, whereas 1a,b are not
effective catalysts under these conditions. Second-order plots of
ln­{[Et2NH]/[Ph2SiH2]} vs time for 4a-catalyzed dehydrocoupling are linear and indicate first-order
dependences on silane and amine concentrations. However, changes in
the experimental rate law with increased silane concentration or decreased
amine concentration reveal inhibition by silane. In addition, excess
ImtBu or THF inhibit the reaction rate. These data,
along with the structures of 4a,b, suggest
that the bulky carbene favors low coordination numbers, which is important
for accessing the catalytically active species.