Molecular Insight for Silica‐Supported Organometallic Chemistry through Transition Metal Silsesquioxanes

Quadrelli, Elsje Alessandra

doi:10.1002/9783527627097.ch14

Cited by 4 publications

(2 citation statements)

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“…Several titanium−silsesquioxane complexes have been synthesized and fully investigated as models of titanium-based heterogeneous catalysts in order to understand their activity at a molecular level. − A recent review reports in detail on the surface organometallic chemistry of titanium species grafted on silica, on the basis of the investigation of molecular models which may exhibit catalytic activities, in the homogeneous phase, comparable to, or in some cases even better than, those of heterogeneous silica-supported titanium catalysts.…”

Section: Introductionmentioning

confidence: 99%

“…These Ti silsesquioxane derivatives catalyze alkene epoxidations by t BuOOH. The tripodal derivative [Cy 7 Si 7 O 12 TiCp] always displays an activity 1 order of magnitude higher, together with a remarkable stability under the reaction conditions . The relevant restriction of these Ti homogeneous epoxidation catalysts, on the basis of models of supported Ti heterogeneous catalysts, is that the use of aqueous hydrogen peroxide instead of an organic hydroperoxide has proved to be almost impossible, as these homogeneous catalysts are unstable under these conditions.…”

Section: Introductionmentioning

confidence: 99%

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Synthesis and Catalytic Activity of Titanium Silsesquioxane Frameworks as Models of Titanium Active Surface Sites of Controlled Nuclearity

et al. 2010

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The synthesis and catalytic activity of some new titanium(IV) complexes with incompletely condensed Cy 6 Si 6 O 7 (OH) 4 and endo-Cy 8 Si 8 O 11 (OH) 2 (Cy = c-C 6 H 11 ) silsesquioxane cages have been carried out with the aim of investigating new Ti silsesquioxane frameworks as molecular models of titanium active sites of catalysts prepared by grafting Ti(IV) precursors on the surface or in a matrix of mesoporous silica. The silsesquioxane Cy 6 Si 6 O 7 (OH) 4 , which bears four potentially reactive OH groups, when reacted with TiCpCl 3 (Cp = η 5 -C 5 H 5 ) affords the new species [Cy 6 Si 6 O 12 Ti 2 (Cp) 2 ], with two titanium atoms connected through one μ-oxygen bridge, as confirmed by a XANES and EXAFS spectroscopic investigation. By reaction of endo-Cy 8 Si 8 O 11 (OH) 2 with [Ti(o-C 6 H 4 O 2 )-(OCH 2 CH 3 ) 2 ] the new complex [Cy 8 Si 8 O 11 O 2 ] 2 Ti was obtained, which exhibits a tetrapodal coordination of Ti, as shown by a X-ray diffraction investigation. The catalytic activity of these two new Ti silsesquioxane complexes was investigated for the catalytic epoxidation of cyclohexene with tert-butyl hydroperoxide and compared with the catalytic activity of the known [Cy 7 Si 7 O 12 TiCp], widely used to model very active tripodal titanium single sites. It appears that a tripodal coordination of Ti sites facilitates the catalytic activity that is in contrast quite low for the two new complexes investigated in this work, probably due to the higher stability toward dissociation to generate a tripodal coordination of the Ti-O-Ti bond and to the steric hindrance of Ti tetrapodal coordination.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Synthesis and Catalytic Activity of Titanium Silsesquioxane Frameworks as Models of Titanium Active Surface Sites of Controlled Nuclearity

et al. 2010

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Imidazolium‐Based Organic–Inorganic Hybrid Silica as a Functional Platform Dramatically Boosts Chiral Organometallics Performance in Asymmetric Catalysis

et al. 2013

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Phase‐transfer featured, imidazolium‐based, organic‐inorganic hybrid silica represents a novel functionalized platform with particularly attractive features in asymmetric catalysis. Herein, we report a chiral organorhodium‐functionalized heterogeneous catalyst. As demonstrated in the studies, it displays comparable or higher catalytic activity and enantioselectivity than its homogeneous counterpart in asymmetric transformations. The superior catalytic performance is attributed to the synergistic effect of the salient imidazolium phase‐transfer character and the confined chiral organorhodium catalytic nature in addition to the merits of mesoporous silica. Furthermore, it is more robust than other silica‐derived heterogeneous systems and can be conveniently recovered and reused at least 10 times without loss of its catalytic efficiency. These features render the catalyst particularly attractive in practice of organic synthesis. The outcomes from the study clearly show that the strategy described here offers a general approach to immobilization of chiral ligand‐derived silane onto the phase‐transfer featured imidazolium‐based organic‐inorganic hybrid silica materials with significant improving catalyst efficiency.

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Surface Organobarium and Organomagnesium Chemistry on Periodic Mesoporous Silica MCM‐41: Convergent and Sequential Approaches Traced by Molecular Models

Michel

König

Törnroos

et al. 2011

Chemistry A European J

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The alkaline earth metal alkyl complexes [Ba(AlEt(4))(2)](n) and Mg(AlMe(4))(2) were directly grafted onto periodic mesoporous silica MCM-41, which had been dehydroxylated at 270 °C (specific surface area a(s): 1023 m(2) g(-1); pore volume V(p): 1.08 cm(3) g(-1); main pore diameter 3.4 nm). Alternatively, barium alkyl surface species were generated by sequential grafting of MCM-41 with Ba[N(SiHMe(2))(2)](2)(thf)(4) and AlEt(3) to yield the hybrid material AlEt(3)@Ba[N(SiHMe(2))(2)](2)(thf)(4)@MCM-41. For a better understanding of the surface chemistry, AlEt(3)@MCM-41 was also accessed. All hybrid materials were analyzed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, elemental analysis, nitrogen physisorption, and solid-state NMR spectroscopy; this clearly revealed distinct surface chemistry for the alkylaluminate-treated materials [Ba(AlEt(4))(2)]@MCM-41 and Mg(AlMe(4))(2)@MCM-41. In an attempt to mimic the surface chemistry, the organometallic precursors were treated with HOSi(OtBu)(3). The reaction of equimolar amounts of {Ba[N(SiHMe(2))(2)](2)}(n) and HOSi(OtBu)(3) produced a mixed silylamido/siloxide cluster of Ba(3)[OSi(OtBu)(3)](3)[N(SiHMe(2))(2)](3) with bridging-only siloxide ligands as well as one bridging and two terminal silylamido ligands. The Schlenk equilibrium was found to govern the [Ba(AlEt(4))(2)](n)-HOSi(OtBu)(3) and Mg(AlMe(4))(2)-HOSi(OtBu)(3) reactions, leading to the isolation of complexes of [Ba(AlEt(4))(2) (toluene)](2) and Mg[OSi(OtBu)(3))](2)(AlMe(3))(2), respectively. Allowing for a donor-induced cleavage of Mg(AlMe(4))(2), the reaction of [MgMe(2)] with one or two equivalents of HOSi(OtBu)(3) was studied. While putative Mg[OSi(OtBu)(3)](Me) and Mg[OSi(OtBu)(3)](2) could not be crystallized from the reaction mixtures, cluster complexes Mg(5)(O)[OSi(OtBu)(3)](5)Me(3) and Mg(4)(OH)(2)[OSi(OtBu)(3)](6) could be unambiguously identified by X-ray crystallography.

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Molecular Insight for Silica‐Supported Organometallic Chemistry through Transition Metal Silsesquioxanes

Cited by 4 publications

References 154 publications

Synthesis and Catalytic Activity of Titanium Silsesquioxane Frameworks as Models of Titanium Active Surface Sites of Controlled Nuclearity

Synthesis and Catalytic Activity of Titanium Silsesquioxane Frameworks as Models of Titanium Active Surface Sites of Controlled Nuclearity

Imidazolium‐Based Organic–Inorganic Hybrid Silica as a Functional Platform Dramatically Boosts Chiral Organometallics Performance in Asymmetric Catalysis

Surface Organobarium and Organomagnesium Chemistry on Periodic Mesoporous Silica MCM‐41: Convergent and Sequential Approaches Traced by Molecular Models

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