DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1997, Pages 137–138 137 Grafting of versatile lanthanide silylamide precursors onto mesoporous MCM-41† Reiner Anwander* and Rainer Roesky Institut für Technische Chemie I, Universität Stuttgart, D-70550 Stuttgart, Germany Chemical anchoring of organometallic lanthanide silylamides onto the internal walls of dehydrated MCM-41 in n-hexane at ambient temperature was monitored by FTIR spectroscopy and nitrogen adsorption/desorption. In the past decade surface organometallic chemistry 1 has been challenged by the discovery of the intriguing class of mesoporous support materials composed of, for example, aluminosilicates and metal oxides.2,3 Like the commonly used silica and alumina supports 4 the new systems are capable of surface reactions via terminal silanol groups.However, the prevailing structural patterns such as hexagonally arranged, uniform mesopores ensure a more detailed characterization by means of nitrogen adsorption/desorption, X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM).5 Furthermore, the interplay of the proposed readily tunable pore radius (20–100 Å) and the accommodation of both bulky metal complexes and reactants might even allow substrate transformations in a regime of stereo- and shape-selectivity.6 Given the current high interest in the field of ‘stereoselective catalysis by organolanthanide complexes’ 7 we anticipated the grafting of such reactive moieties onto mesoporous material MCM-41 via formation of a thermodynamically stable lanthanide– siloxide s bond.8 MCM-41-derived hybrids containing maingroup- 9 and d-transition-organometallics 10 have recently been described.Herein, we report our initial results on a heterogeneously performed silylamide route employing MCM-41 1 and amides of type [Nd{N(SiMe3)2}3] 2 and [Nd{N(SiHMe2)2}3(thf)2] (thf = tetrahydrofuran) 3. Such highly soluble, monomeric silylamides are known to act as key precursors in homogeneous, protolytic exchange reactions under mild conditions.11 Dehydrated MCM-41 was synthesized according to the literature employing [N(C14H29)Me3]Br as a templating agent.2 After calcination (N2: 500 8C, 5 h, heating rate 1.5 8C min21; air: 500 8C, 5 h) and dehydration (1025 Torr: 280 8C, 4 h, heating rate 1 8C min21),‡ 1 was characterized by XRD, nitrogen adsorption and desorption isotherms (BET surface area: 1005 m2 g21; pore diameter: 26 Å (desorption); pore volume: 0.78 cm3 g21), elemental analysis (ICP, Si :Al ª 18), and IR spectroscopy [n(OH): 3695 cm21].12 The grafting procedure involves the addition of silylamide desolved in n-hexane to a suspension of MCM-41 in n-hexane within a period of 5 min (Scheme 1).Upon stirring for 2 h, followed by several n-hexane washings, the resulting bluish hybridic materials were dried under vacuum for at least 5 h.§ This way, approximately 1 mmol of silylamide per g of 1 could be loaded.However, quantitative gas chromatography allowed the detection of only 0.3–0.4 mmol of released silylamine ligand probably due to retention in the mesopores by metal complexation or silylation reaction.¶ † Non-SI unit employed: Torr ª 133.322 Pa. ‡ Dehydration at temperatures >350 8C led to partial collapse of the mesopores as indicated by nitrogen adsorption and desorption isotherms (recorded on a micromeritics ASAP 2000). The consumption of all terminal silanol groups and the disappearance of the SiO mode at 980 cm21 (strained Si–O]Si bonds) were unequivocally proven by Fourier-transform IR spectroscopy (Fig. 1). Type-3 silylamide was employed not only for reasons of changed reactivity during immobilization and following ligand exchange reactions, but also for its unique behaviour as an IR probe. Two n(Si]H) stretches appear in the spectra of hybrid material 5. The lower energy one at 2058 cm21 can be assigned to metal bonded amide moieties.11 Independently performed reactions of the silylamines HN(SiMe3)2 and HN(SiHMe2)2 with 1 in n-hexane revealed formation of silylated 6a and 6b under these mild conditions and allowed the assignment of the Si]H stretching frequency at 2144 cm21 to 6b-analogous ‘OSiHMe2’ moieties.The competing silylation reaction is in agreement with the findings from quantitative gas chromatography and points out a self-supported spacing of the lanthanide centres by steric restrictions or diffusion-controlled reactions within the mesopores.For comparison, silylation reactions along the homogeneously performed silylamide route are observed only when an excess of more acidic alcohols such as HOCH(CF3)2 14 or HOSiBut 3 15 are employed. Calculations from the elemental analysis of 6a reveal that approximately 13% of the MCM-41 silicon atoms are carrying reactive sites available for silylation.|| Also, elemental analyses of the hybrid materials 4 and 5 favour the formation of ‘MCM-41] O2Ln[N(SiMe3)2]’ over bis(amide) moieties. Scheme 1 Possible surface species of the immobilization of neodymium silylamides on MCM-41: (i) hexane, room temperature, 20 h; thf is not shown for 5 § All manipulations were performed in a dinitrogen-filled glove box (MB Braun MB150B-G-II).The initial grafting runs were carried out with coloured silylamide solutions of neodymium to visualize completeness of the reactions (titration). Prolonged reaction times (>24 h) gave no further immobilization.Also, after prolonged Soxhlet extractions with thf only traces of what we assume to be a non-chemically anchored complex were isolated. X-Ray diffraction spectra of airexposed hybrid materials showed the characteristic pattern of 1, however, with decreased intensity. For 1 (stirred in n-hexane, evacuated for 5 h, N2-filled) (Found: C, 0.28; H, 0.27; N, 0.41%. Synthesis of 4 from 2: 2 (0.300 g, 0.48 mmol) in n-hexane (10 cm3) was added to 1 (0.368 g) in n-hexane (10 cm3) over 5 min.The mixture was stirred for 20 h, 4 was separated by centrifugation, washed with nhexane (20 cm3) and dried in vacuo for at least 5 h (Found: C, 9.45; H, 2.45; N, 1.95; Nd, 11.1%). IR (Nujol): 831s, 771m, 661m, 604m cm21 [N(SiMe3)2]. Synthesis of 5 from 3: as above using 3 (0.330 g, 0.48 mmol) and 1 (0.248 g) (Found: C, 10.0; H, 2.35; N, 2.00; Nd, 8.8%). IR (Nujol): 2144m, 2058m, 902s, 836s, 681m, 626m cm21 [N(SiHMe2)2].¶ Hexamethyldisilazane was thoroughly studied as a trimethylsilylating agent for silica (gels).13138 J. Chem. Soc., Dalton Trans., 1997, Pages 137–138 Nitrogen adsorption and desorption isotherms of 4 and 5 clearly demonstrate the filling of the mesopores (Fig. 2). The host-characteristic type-IV isotherm (1) is replaced by type-I isotherms (4, 5) indicating the presence of microporous materials. The original pore volume of 0.78 cm3 g21 is reduced to approximately 0.30 cm3 g21 (after activation at 100 8C under high vacuum). 16 The silylated materials still display type-IV isotherms and the pore characteristics reveal effective pore-size engineering [BET surface area: ª840 m2 g21 (6a, 6b); pore diameter (desorption): 21 Å (6a, 6b); pore volume: 0.53 cm3 g21 (6a), 0.54 cm3 g21 (6b)]. Further evidence for a competing silylation reaction stems from the reaction of monomeric [Nd(NPri 2)3(thf)] 17 7 with 1.* Evaluation of the free isopropylamine results in complete desorption at T < 150 8C under high vacuum.As a result, approximately 2 mmol of 7 can be immobilized to yield hybrid material 8 [17.3% Nd (m/m) detected by ICP analysis]. The pore volume is decreased to 0.10 cm3 g21 (Fig. 2). All anchored amide complexes can be ‘degrafted’ as alkoxide complexes by treatment with HOC(CF3)3 (pKa 5.7) 18–thf solutions. The support residues display nitrogen adsorption and desorption isotherms which are similar to the silylated materials (4, 5) and to original 1 (8).A direct conclusion of these findings is that by varying the ratio of silylamide to silylamine different metal loadings will be available by simultaneous consumption of all terminal silanol groups, and hence the spacing of the metal centres will be effected. Preliminary ligand exchange reactions with less acidic alcohols such as ethanol and binaphthol (stoichiometric) reveal that we can direct future investigations towards the generation of both unsaturated lanthanide centres (small ligand chemistry) Fig. 1 FTIR spectra (Nujol). MCM-41 (1), [Nd{N(SiHMe2)2}3(thf)2] (3), MCM-41 + [Nd{N(SiHMe2)2}3(thf)2] (5), MCM-41 + HN(SiHMe2) 2 (6b, 1023 Torr, 2 h, 250 8C) * Synthesis of 6a from 1: to a suspension of 1 (0.490 g) in n-hexane was added HN(SiMe3)2 (excess, 0.5 cm3). After stirring for 20 h the unreacted silylamine and n-hexane were removed in vacuo. Material 6a was dried in vacuo for at least 5 h at 20 8C, then heated at 250 8C under high vacuum for 2 h (Found: C, 6.50; H, 1.60%).IR (Nujol): 864s, 846s, 757m, 563m cm21 [N(SiMe3)2]. Synthesis of 6b from 1: as above using 1 (0.247 g) and HN(SiHMe2)2 (excess, 0.3 cm3) (Found: C, 5.15; H, 1.50%). IR (Nujol): 2145m, 904s, 837s, 773m, 629m cm21 [N(SiHMe2)2]. Synthesis of 8 from 7: 7 (0.185 g, 0.36 mmol) was added to 1 (0.342 g) in n-hexane (10 cm3). The mixture was stirred for 2 h and additional 7 (0.182 g, 0.35 mmol) added.More 7 (0.104 g, 0.20 mmol) was added and the solution stirred for 20 h, 8 was separated by centrifugation, the residue washed with n-hexane (20 cm3) and dried in vacuo for at least 5 h (Found: C, 14.50; H, 2.65; N, 2.20; Nd, 17.3%). IR (Nujol): 918m, 823m, 799m cm21 (NPri 2). Only small amounts of silylamide complexes could be grafted on pre-silylated material, probably reflecting some nonsilylated bridging OH-moieties. and ‘immobilized chirality’, two main features of organolanthanides and stereoselective catalysis.Acknowledgements We thank the Deutsche Forschungsgemeinschaft (DFG) and Hoechst AG for financial support of this research project and the DFG for the award of a fellowship to R. A. Additionally, generous support from Professor J. Weitkamp is gratefully acknowledged. 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