β‑SiH-Containing Tris(silazido) Rare-Earth Complexes as Homogeneous and Grafted Single-Site Catalyst Precursors for Hydroamination

A series of homoleptic
rare-earth silazido compounds and their
silica-grafted derivatives were prepared to compare spectroscopic
and catalytic features under homogeneous and interfacial conditions.
Trivalent tris­(silazido) compounds Ln­{N­(SiHMe2)tBu}3 (Ln = Sc (1), Y (2), Lu (3)) are prepared in high yield by salt metathesis
reactions. Solution-phase and solid-state characterization of 1–3 by NMR and IR spectroscopy and X-ray
diffraction reveals Ln↼H–Si interactions. These features
are retained in solvent-coordinated 2·Et2O, 2·THF, and 3·THF. The change in spectroscopic features characterizing
the secondary interactions (νSiH, 1JSiH) from the unactivated SiH in the silazane
HN­(SiHMe2)tBu follows the trend 3 > 2 > 1 ≈ 2·Et2O > 2·THF ≈ 3·THF. Ligand lability follows the same
pattern, with Et2O readily dissociating from 2·Et2O while THF is displaced only
during surface grafting reactions. 1 and 2·THF graft onto mesoporous silica nanoparticles (MSN) to give Ln­{N­(SiHMe2)tBu}n@MSN (Ln = Sc (1@MSN), Y (2@MSN)) along with THF and protonated silazido
as HN­(SiHMe2)tBu and H2NtBu. The surface species are characterized by multinuclear
and multidimensional solid-state (SS) NMR spectroscopic techniques,
as well as diffuse reflectance FTIR, elemental analysis, and reaction
stoichiometry. A key 1JSiH SSNMR
measurement reveals that the grafted sites most closely resemble Ln·THF
adducts, suggesting that siloxane coordination occurs in grafted compounds.
These species catalyze the hydroamination/bicyclization of aminodialkenes,
and both solution-phase and interfacial conditions provide the bicyclized
product with equivalent cis:trans ratios. Similar diastereoselectivities
mediated by catalytic sites under the two conditions suggest similar
effective environments.