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

Abstract

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(SiHMe 2 )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 solventcoordinated 2·Et 2 O, 2·THF, and 3·THF. The change in spectroscopic features characterizing the secondary interactions (ν SiH , 1 J SiH ) from the unactivated SiH in the silazane HN(SiHMe 2 )tBu follows the trend 3 > 2 > 1 ≈ 2·Et 2 O > 2·THF ≈ 3· THF. Ligand lability follows the same pattern, with Et 2 O readily dissociating from 2·Et 2 O while THF is displaced only during surface grafting reactions. 1 and 2·THF graft onto mesoporous silica nanoparticles (MSN) to give Ln{N(SiHMe 2 )tBu} n @MSN (Ln = Sc (1@MSN), Y (2@MSN)) along with THF and protonated silazido as HN(SiHMe 2 )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.

Silica-Supported Organolanthanum Catalysts for C–O Bond Cleavage in Epoxides

“…19 The oxophilic lanthanide centers also form strong bonds to oxygen in metal oxides, which is a useful property for attaching these sites onto supports in heterogeneous catalysts. [20][21][22][23][24][25][26][27][28][29] Homoleptic compounds in particular, such as La(CH2Ph)3THF3 or La{CH(SiMe3)2}3, [30][31] have potential in surface organometallic chemistry (SOMC). 26,32 For example, lanthanum benzyl grafted onto silica is a catalyst for styrene and ethylene polymerization and alkyne dimerization.…”

“…20,22 In Y{N(SiHMe2)tBu}3, for example, the averaged 1 responds to the presence (145 Hz) or absence (124 Hz) of a THF donor; the value in the THF-free silica-grafted surface species (141 Hz) suggests an additional interaction with surface siloxane as a 2 edonor. 29 Thus, the NMR properties of SiH groups in the lanthanide alkyl species Ln{C(SiHMe2)3}3 might also facilitate assignment of surface structures formed in grafting reactions. In the present study, we have pursued the thermally stable, easily synthesized compound La{C(SiHMe2)3}3 as reactant for producing organolanthanum compounds in SOMC on mesoporous silica nanoparticles (MSNs).…”

“…SSNMR experiments were performed on a 600 MHz (14.1 T) Varian NMR spectrometer, equipped with 1.6-mm and 3.2-mm magic angle spinning (MAS) probes, and a 400 MHz (9.4 T) Varian NMR spectrometer, equipped with 3.2-mm and 5.0-mm MAS probes. 1D and 2D experiments were carried out on 1 H, 11 B, 13 C and 29 Si nuclei, and included 1D DPMAS, 1D CPMAS, 2D HETCOR and idHETCOR, 2D J-resolved spectroscopy, 44 and 2D MQMAS. 45 The abbreviations refer to direct polarization (DP), cross polarization (CP), directly and indirectly detected 29 Si, to the indirect (vertical) dimension.…”

“…1D and 2D experiments were carried out on 1 H, 11 B, 13 C and 29 Si nuclei, and included 1D DPMAS, 1D CPMAS, 2D HETCOR and idHETCOR, 2D J-resolved spectroscopy, 44 and 2D MQMAS. 45 The abbreviations refer to direct polarization (DP), cross polarization (CP), directly and indirectly detected 29 Si, to the indirect (vertical) dimension. [46][47] To minimize the possibility of oxygen contamination, the samples were packed in zirconia rotors in a glovebox under nitrogen atmosphere, and the experiments were carried out using nitrogen to propel the MAS rotors.…”

“…The chemical shifts for 1 H, 13 La loading and podality. The La loadings and surface speciations (ratios of monopodal sites to bipodal sites, where podality is defined as the number of X-type surface silanoate ligands) 50 in 1@MSN550 and 1@MSN700 were evaluated by mass balance of grafting (in situ solution-state NMR, method I), quantification of surface alcoholysis reactions (in situ solution-state NMR, method II), and elemental analysis combined with quantitative 29 Si SSNMR (method III). The data obtained from these three methods are summarized in Table 1.…”