Silsesquioxanes Metallasilsesquioxanes

The incompletely condensed silsesquioxanes can be considered as intermediates in the synthesis of the whole family of silsesquioxanes, but at the same time they are the building blocks (e.g., initiators) for the new polymeric architectures.66 67 The importance of this class of materials in model studies of silica surfaces and silicate defects,68 and in creating various oligosilanols with novel frameworks,69 70 makes them the subject of extensive research.71 The use of the incompletely condensed silsesquioxanes in forming dimeric silsesquioxanes and metallasilsesquioxanes in attempts to design new routes to the well-defined Si-O assemblies has recently been reviewed in a well-illustrated research report.72... 

The fact that silsesquioxane molecules like 2-7 contain covalently bonded reactive functionalities make them promising monomers for polymerization reactions or for grafting these monomers to polymer chains. In recent years this has been the basis for the development of novel hybrid materials, which offer a variety of useful properties. This area of applied silsesquioxane chemistry has been largely developed by Lichtenhan et al With respect to catalysis research, the chemistry of metallasilsesquioxanes also receives considerable current interest. As mentioned above, incompletely condensed silsesquioxanes of the type R7Si70g(0H)3 (2-7, Scheme 4) share astonishing structural similarities with p-tridymite and p-cristobalite and are thus quite realistic models for the silanol sites on silica surfaces. Metal... 

The preparation of the heptanorbornyl silsesquioxane trisilanol 5 has been reported by Maschmeyer et al It is formed in admixture with the corresponding tetrasilanol. The norbornyl-substituted species still await further exploration as precursors in metallasilsesquioxane chemistry. The same applies for the isobutyl and isooctyl derivatives 6 and 7, respectively, which have been propagated in the patent literature by Lichtenhan and Abbenhuis. The application of high-speed experimentation techniques to optimize the preparation of silsesquioxanes as precursors for Ti catalysts has been reported by Maschmeyer et al ... 

No fully characterized metallasilsesquioxane derivatives of Mn, Tc or Re have been reported until now. It has been reported that reactions of 2 or 3 with rhe-nium(VII) precursors are complicated and often result in dehydration of the silsesquioxane cage without incorporation of rhenium.Recently it has been found... 

The first iron-containing silsesquioxanes which appeared in the literature were compounds containing ferrocenyl units as side-groups.However, these are not within the scope of this review as iron is not part of the metallasilsesquioxane skeleton. Meanwhile, several ferrasilsesquioxane complexes have been synthesized. The first iron(III) compound of this type was prepared in our laboratory according to Scheme 56. ° In 161, the coordination sphere of iron is completed by TMEDA (N,N,N, N -tetramethylethylenediamine) as a chelating amine ligand. Pale yellow, crystalline 161 was isolated in 80% yield and structurally characterized by X-ray diffraction. This compound was later used by Maxim et to prepare iron... 

Apparently real metallasilsesquioxanes of cobalt, rhodium or iridium have thus far not been reported in the literature. However, several silsesquioxane ligands containing donor-substituted side chains as well as related silsesquioxane dendri-mers have been found to form complexes with cobalt and rhodium complex frag-ments.i°3.ii4 ns... 

While apparently no metallasilsesquioxane complexes of nickel and palladium have yet been prepared, several literature reports have appeared on platinum compounds containing silsesquioxane ligands. Abbenhuis reported the synthesis and characterization of three platinum(II) complexes stabilized by the chelating diphosphine ligand dppe (= 1,2-bis(diphenylphosphino)ethane) as outlined in Scheme 63. ... 

Another strategy for synthesizing heteroatom-substituted silicate cages is to introduce the heteroatom after pre-assembly of the silicate fragment. Examples of this approach are given in the contribution of F. Edelmann (Chapter 29), which describes the preparation of various metallasilsesquioxanes by reacting incompletely condensed silsesquioxanes with metal salts. 

The first metallasilsesquioxanes incorporating lanthanides were described in 1994 by Herrmann et alJ More recently, several Ln silsesquioxanes resulting from reactions of 1 with lanthanide silylamides and aryloxides have been reportedJ Our own efforts in this field resulted in the isolation of a novel Ce(lV) silsesquioxane complex as well as the structural characterization of a bimetallic Yb/Li derivative.Treatment of [Ce N(SiMe3)2 3] or anhydrous CeCls with two equivalents of Cy8Si80n(0H)2 (42) in diethyl ether in the presence of an excess of pyridine exclusively afforded the diamagnetic complex (Cy8Si80]3)2Ce(py)3 (43, Scheme 29.11). Quite surprisingly, in both cases cerium was oxidized to the tetravalent oxidation state. 

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