Tripods, molecular

Piotrowiak P., Galoppini E., Wei Q., Meyer G. J. and Wiewior P. (2003), Subpico-seeond photoinduced charge injeetion from molecular tripods into mesoporous Ti02 over the distance of 24 Angstroms , J. Am. Chem. Soc. 125, 5278-5279. 

Sub-picosecond photo-induced charge injection from molecular tripods into mesoporous Ti02 has been achieved over the distance of 24 A (Scheme 70) 186 three-point attachment to the Ti02 surface and the rigidity of the spacer in molecules of such remarkable architecture as (123) facilitate control of the distance and orientation of the sensitizer with respect to the surface. Details of PET in two dendrimer system have been reported. The first contains a perylenediimide as an acceptor core, a rigid second generation polyphenylene... 

Figure 3. The molecular structure of the oxygen tripod ligand [Cp 3Cr3(jX3-0)(p2-OH)3) Q ...

The effect of molecular geometry can often be evaluated in a straightforward manner. Consider the tetrahedral CH4 molecule, which will be shown as having one C-H bond pointing "up" and the other three forming a tripod-like base ... 

Metalated container molecules can be viewed as a class of compounds that have one or more active metal coordination sites anchored within or next to a molecular cavity (Fig. 2). A range of host systems is capable of forming such structures. The majority of these compounds represent macrocyclic molecules and steri-cally demanding tripod ligands, as for instance calixarenes (42), cyclodextrins (43,44), and trispyrazolylborates (45-48), respectively. In the following, selected types of metalated container molecules and their properties are briefly discussed and where appropriate the foundation papers from relevant earlier work are included. Porphyrin-based hosts and coordination cages with encapsulated metal complexes have been reviewed previously (49-53) and, therefore, only the most recent examples will be described. Thereafter, our work in this field is reported. 

The higher conversion in the presence of Ti-beta is probably a result of the higher temperature (343 v.v. 323 K). Diffusional constraints cannot account for the observed differences in selectivity. Ti-beta and TS-1 are distinctly more selective than the mesoporous material. Recalling that tetrapodal titanium sites are more predominant in the former two molecular sieves although tripodal titanium sites are the major surface species over the latter mesoporous material (Section II), we infer that the data indicate that high epoxidation selectivity is probably correlated with the presence of tetrapodal structures in these two molecular sieves. This correlation is discussed in Section VI. 

The majority of the titanium ions in titanosilicate molecular sieves in the dehydrated state are present in two types of structures, the framework tetrapodal and tripodal structures. The tetrapodal species dominate in TS-1 and Ti-beta, and the tripodals are more prevalent in Ti-MCM-41 and other mesoporous materials. The coordinatively unsaturated Ti ions in these structures exhibit Lewis acidity and strongly adsorb molecules such as H2O, NH3, H2O2, alkenes, etc. On interaction with H2O2, H2 + O2, or alkyl hydroperoxides, the Ti ions expand their coordination number to 5 or 6 and form side-on Ti-peroxo and superoxo complexes which catalyze the many oxidation reactions of NH3 and organic molecules. 

Significant progress has been achieved in the preceding few years in the study of titanosilicate molecular sieves, especially TS-1, TS-2, Ti-beta, and Ti-MCM-41. In the dehydrated, pristine state most of the Ti4+ ions on the surfaces of these materials are tetrahedrally coordinated, being present in either one of two structures a tetrapodal (Ti(OSi)4) or a tripodal (Ti(OSi)3OH) structure. The former predominates in TS-1, TS-2, and Ti-beta, and the latter is prominent in Ti-MCM-41. The Ti ions are coordinatively unsaturated and act as Lewis acid sites that coordinatively bind molecules such as H20, NH3, CH3CN, and H202. Upon interaction with H202 or H2 + 02, the Ti ions form titanium oxo species. Spectroscopic techniques have been used to identify side-bound hydroperoxo species such as Ti(02H) and superoxo structures such as Ti(02 ) on these catalysts. 

Among several families of compounds used as molecular models for such surface silanolic grafting sites (mono or poly-silanols [10-12], heteropolyanions [13, 14], calixarenes [15, 16], tripodal amido complexes [17], etc.) some silsesquioxanes [18-23] can be considered among the most convincing molecular analogues of silica surface silanols. 

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