Silsesquioxane transition metal substitution

Spin-Spin Interactions in Silsesquioxanes and Transition Metal Substitution... 

The transition metal substitution does not import a further strengthening of the spin-spin interactions in the silsesquioxanes. This is shown here for the dicoordinate structure (Figure 30.5). 

The properties of siloxide as ancillary ligand in the system TM-O-SiRs can be effectively utilized in molecular catalysis, but predominantly by early transition metal complexes. Mono- and di-substituted branched siloxy ligands (e.g., incompletely condensed silsesquioxanes) have been employed as more advanced models of the silanol sites on silica surface for catalytically active centers of early TM (Ti, W, V) that could be effectively used in polymerization [5], metathesis [6] and epoxidation [7] of alkenes as well as dehydrogenative coupling of silanes [8]. 

The value is consistant with the previous considerations on the 1.2- to 1.4-biradical structures, with fairly small S-T energy separations. It indicates that in the TM-substituted silsesquioxanes the transition metal fragment can be readily oxidized at the metal center, but a delocalization over the cube does not take place. More generally, spin-delocalization is interrupted by the electronegative atoms linking the silicon centers. A similar phenomenon is observed for the spin-spin interactions in the resulting cubes with phosphaneiminato ligands. 

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