Bound Species Silica

Cationic palladium(II) complexes are homogeneous catalysts for both intramolecular and inter-molecular hydroamination reactions.267 Palladium species immobilized on silica can be prepared by the simple addition of alkyl- or hydroxopalladium(II) complexes to partially dehydroxylated silica. The silica-bound species are more stable than their molecular precursors and are efficient catalysts for the cyclization of aminoalkynes.268... 

The exact nature of the alkylidenes formed on various oxide surfaces is still uncertain, as is the nature of the alkylidenes responsible for the often observed metathesis activity. Mo(N)(CH2CMe3)3 also has been employed as a precursor to a surface-bound species believed to be of the type Mo(NH)(CHCMe3)(CH2CMe3) (Osurf) [115]. Although the alkylidene carbon atom could not be observed in solid state NMR spectra, which is typical of surface supported alkylidenes, reaction with acetone to give 2,4,4-trimethylpent-2-ene quantitatively confirmed the presence of the reactive neopentylidene complex. Such species would initiate various metathesis reactions when prepared on partially dehydroxylated silica. 

Spectroscopy has not proven to be very conclusive in solving this problem. Similarities between the visible spectrum of the calcined catalyst and that of bulk dichromates have been noted (5,12-14). In the end, however, there is always doubt about the interpretation of spectra because no adequate reference data exist for these surface bound species (76). Krauss and coworkers have carefully studied the luminescence of Cr/silica and concluded that at least a portion of the chromium is present as chromate (75). 

Another type of metal-support site may be created by the reaction of acetylacetone with the mono-amine-bound derivatized silica gels. This type has imino-acac ligands attached to the surface. There are Cu11 species with only one iminoacac chelate in its coordination sphere, and other sites coordinated in a square-planar array by two iminoacac ligands from adjacent surface locations.105,106 These types of supported IMCOS sites also have been used to separate hydrocarbons, ethers, and thioethers. 

Anionic oiigosaccharide species (carboxylic and phosphate derivatives) HPLC weakly basic amino phase bound to silica gel phosphate buffer Baenziger and Natowicz [243]... 

Late metals can also be bound to silica. For example, fMeRhL2(CO)] (L = PMes) reacts with silica surface hydroxyl groups to give a surface-bound Rh(I) species, 932, with release of methane. ... 

Figure 17.3 shows IR spectra for dried silica after outgassing under vacuum of 10 Pa at 150 °C for 3 hours. The intensity of the j/(OH) band of the isolated Si-OH group at 3737 cm becomes weaker upon fluorination, showing the lower concentration of isolated hydroxyl groups. For the highest F contents the /(OH) band of isolated silanol group finally vanishes whereas a small part of the one of H-bounded species (3660 cm ) persists. 

Lochmuller and coworkers used the formation of excimer species to answer a distance between site question related to the organization and distribution of molecules bound to the surface of silica xerogels such as those used for chromatography bound phases. Pyrene is a flat, poly aromatic molecule whose excited state is more pi-acidic than the ground state. An excited state of pyrene that can approach a ground state pyrene within 7A will form an excimer Pyr +Pyr (Pyr)2. Monomer pyrene emits at a wavelength shorter than the excimer and so isolated versus near-neighbor estimates can be made. In order to do this quantitatively, these researchers turned to measure lifetime because the monomer and excimer are known to have different lifetimes in solution. This is also a way to introduce the concept of excited state lifetime. 

Allen, D., and El Rassl, Z. (2004). Capillary electrochromatography with monolithic silica columns III. Preparation of hydrophilic silica monoliths having surface-bound cyano groups chromatographic characterization and application to the separation of carbohydrates, nucleosides, nucleic acid bases and other neutral polar species.. Chromatogr. A 1029, 239—247. 

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