A series of homoleptic rare earth silazido compounds, their solvent adducts, and their silica-grafted derivatives were prepared to compare spectroscopic and catalytic features of related single site catalytic precursors under homogeneous and interfacial conditions. Trivalent tris(silazido) rare earth compounds Ln{N(SiHMe2)tBu}3 (Ln = Sc, Y, Lu) form in high yield by reactions of [LiN(SiHMe2)tBu] and LnCl3. NMR, IR spectroscopy, and single-crystal X-ray diffraction reveal bridging Ln←H–Si motifs. Solvent coordinated, structurally-characterized, distorted tetrahedral complexes retain the Ln←H–Si interactions. The spectroscopic features characterizing the secondary interactions (νSiH, 1JSiH) change from values for the silazane HN(SiHMe2)tBu following the trends Lu &gt; Y &gt; Sc and “solvent free” ~ Ln●Et2O &gt; Ln●THF. The lability of the etherial ligand follows the same pattern, with Et2O readily dissociating from Y●Et2O while THF is displaced only during surface grafting reactions. Scandium and yttrium Y●THF were grafted onto mesoporous silica nanoparticles (MSN) pre-treated under vacuum at 550 °C (MSN550) or 700 °C (MSN700) to give Ln{N(SiHMe2)tBu}n@MSN (Ln = Y, Sc) along with THF and protonated silazido as HN(SiHMe2)tBu and H2NtBu. The surface species were characterized by multinuclear, multidimensional SSNMR spectroscopic techniques, as well as diffuse reflectance FTIR, elemental analysis, and reaction stoichiometry. These data indicate that a mixture of monopodal (≡SiO–Ln{N(SiHMe2)tBu}2) and bipodal species ((≡SiO–)2LnN(SiHMe2)tBu) is obtained from MSN550, whereas MSN700 primarily provides the monopodal surface species. Moreover, a 1JSiH SSNMR measurement reveals that the Ln←H–Si interaction is retained in surface species, and comparison of the values show that grafted sites most closely resemble four-coordinate THF adducts (i.e., 2●THF). Thus, we suggest that surface siloxane coordination occurs in grafted species.

Surface-supported and solution phase homoleptic rare earth silazido catalysts

253rd National Meeting (2017)

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