Periodic bridging organic groups

Bridging organic groups are often featured as a mechanism of transfer in this area of the periodic table. Equations (23) and (24) (L = PPhs)... 

Figure 3.7 Schematic of the incorporation of organic groups in the pore walls of mesoporous silica using bridged organosilane precursors (top). Schematic of the structure of a phenylene-bridged hybrid mesoporous organosilica with both atomic and mesoporous periodicity (bottom) [36],...

One-pot synthesis [11-14], in addition to the surface modification, is widely used for the preparation of framework-modified materials known as periodic mesoporous organosilicas (PMOs) [15]. They are synthesized by hydrolysis and condensation of bis(trialkoxysilyl) organic precursors and related compounds in the presence of both ionic and nonionic templates. In contrast to the conventional OMOs that possess surface organic groups [2, 8-14], the PMO framework contains Si-R-Si linkages (where R is an organic bridging spacer) [15]. 

The Exxon catalyst is a derivative of cyclopentadiene, indene, or fluorene which may be bridged, and which may also be substituted with up to five different atoms or radicals. This organic group is bonded to a metal atom from Group IV b of the Periodic Table, titanium, zirconium or hafnium, which has been activated by addition of a strong Lewis acid, to give a cationic structure. 

Carbon-13 chemical shifts of transition metal carbonyls [471, 473] decrease as one proceeds down a given group of the periodic table, as demonstrated for the triad Cr(CO)6, Mo(CO)e, and W(CO)e in Table 4.71. Substitution of CO by cyclopentadienide and other organic ligands deshields the carbonyl carbon nuclei. This is exemplified by iron pentacarbonyl (211 ppm) in comparison to cyclopentadienylirondicarbonyl-dimer, which displays one avaraged carbonyl signal ( 240 ppm) at room temperature [488] due to rapid cis-trans isomerization and intramolecular bridged-terminal carbonyl interconversion ... 

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