Silica-mediated synthesis

When a slurry of Si02, [Re(C0)30H]4, excess K2CO3 and water is stirred at 25 °C and evaporated to dryness, K[Re2(CO)6( i-OH)3] is formed in quantitative yield [65]. However, the Si02 surface does not play an important role in this conversion because the same complex can be obtained quantitatively by reaction in water of ]Re(C0)30H]4 with K2CO3 or with [NEt4]OH [65]. Nonetheless, the investigation of this silica-mediated synthesis was a springboard for a new convenient synthesis in solution of [Re2(CO)6( i-OH)3]", a complex previously prepared by more complex reactions [66]. 

Ni and Co or of oxophilic metals, for example. Re, is still poorly studied the surface-mediated synthesis of bimetallic carbonyl clusters is limited to a few examples the surface-mediated synthesis of metal compounds without carbonyl ligands has just begun with the silica-mediated synthesis of [RhH2(PMe3)4] by treatment of bis (allyl) rhodium with PMe3 followed by H2 [121] the silica-mediated synthesis of tantalum clusters has been investigated recently but the products were not extracted from the surface-for example, treatment of silica physisorbed Ta(CH2Ph)5 in H2 at 523 K for 20 h led to tri-tantalum clusters, as shown by EXAFS spectroscopy [122]. 

Treatment of silica-supported [Os(CO)3Cl2]2 (2-5% w/w) of Os relative to Si02) with a slurry of K2CO3 (molar ratio K2CO3/OS = 20 1) in CH2CI2, followed by evaporation of the solvent and reaction with CO (1 atm) at 265 °C for 24 h affords K2[Os5C(CO)h] which can be extracted with CH3CN (74% yield)J This silica-mediated synthesis is much more efficient than the best known synthesis in solutionj ... 

A. Agarwal, S. Rani, and Y. D. Vankar, Protic acid (HC104 supported on silica gel)-mediated synthesis of 2, 3-unsaturated-O-glucosides and a chiral furan diol from 2,3-glycals, J. Org. Chem., 69 (2004) 6137-6140. 

Finally, the surface-mediated synthesis of ruthenium carbonyl complexes has also been used to prepare supported ruthenium particles. Using silica as a reaction medium and conventional salts, apart from Ru3(CO)i2, mononuclear Ru(CO)j, and high nuclearity carbonyl-derived species can be obtained by CO reductive carbonylation [127, 128]. This opens new routes to preparing tailored supported ruthenium particles. 

Figure 15.1 Ordered nanoscale structures of mesoporous materials such as FSM-16 and HMM-1 (Et-HMM-1) as the silica and organosilica templates for surface-mediated synthesis of metal/alloy nanowires and nanoparticles.

This chapter focuses on silica synthesis via the microemulsion-mediated alkoxide sol-gel process. The discussion begins with a brief introduction to the general principles underlying microemulsion-mediated silica synthesis. This is followed by a consideration of the main microemulsion characteristics believed to control particle formation. Included here is the influence of reactants and reaction products on the stability of the single-phase water-in-oil microemulsion region. This is an important issue since microemulsion-mediated synthesis relies on the availability of surfactant/ oil/water formulations that give stable microemulsions. Next is presented a survey of the available experimental results, with emphasis on synthesis protocols and particle characteristics. The kinetics of alkoxide hydrolysis in the microemulsion environment is then examined and its relationship to silica-particle formation mechanisms is discussed. Finally, some brief comments are offered concerning future directions of the microemulsion-based alkoxide sol-gel process for silica. 

Babu ARS, Raghunathan R (2007) Ti02-silica mediated one pot three component 1, 3-dipo-lar cycloaddition reaction a facile and rapid synthesis of dispiro acenaphthenone/oxindole [indanedione/oxindole] pyrroloisoquinoline ring systems. Tetrahedron 63 8010-8016... 

Macromonomer-Mediated Synthesis of Polymer-Silica Colloidal Clusters... 

The available reports on microemulsion-mediated synthesis of silica [56-83] are based almost exclusively on the alkoxide sol-gel method only one exception has been found [70]. The first attempt to extend the alkoxide sol-gel process to microemulsion systems was reported by Yanagi et al. in 1986 [56]. Since then, additional papers have appeared from different research groups. In most cases the microemulsion-plus-reactant protocol was used ... 

A variant of the methods described above was reported by Jafelicci et al. [272] who prepared a microemulsion by addition of water or 3M nitric acid solution into a solution of NaAOT in heptane under sonication. A dilute solution of sodium silicate was added to this microemulsion and the system further sonicated. The product particles were gathered by centrifugation. The spherical silica particles (hollow under specific conditions) thus obtained had a size range of 1-10 pm. It is noteworthy that though the authors claim to have prepared particles via microemulsions, the large size of the particles does not indicate them to be a product of microemulsion-mediated synthesis in nanoreactors (see Section 5.3.1). 

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