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 center dot Et2O, 2 center dot THF, and 3 center dot THF. The change in spectroscopic features characterizing the secondary interactions (v(s1H), (1)J(siH)) from the unactivated SiH in the silazane HN(SiHMe2)tBu follows the trend 3 > 2 > 1 approximate to 2 center dot THF approximate to 3 center dot THF. Ligand lability follows the same pattern, with Et2O readily dissociating from 2 center dot Et2O while THF is displaced only during surface grafting reactions. I and 2 center dot 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 H(2)NtBu. 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 (1)J(SiH) 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.