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Dedicated to Professor Gerd-Volker Röschenthaler on the occasion on his 60th birthday Numerous industrially and commercially important processes such as the production of polyolefins are catalyzed by transitionmetal complexes immobilized on silica surfaces.[1] However, due to the heterogeneous nature of these catalysts, full characterization of the catalytically active species is often difficult, and little is known about the reaction mechanisms and the nature of the metal-silica interactions. Surface organometallic chemistry (SOMC), developed by Basset and co-workers, is one suitable approach towards a better understanding of heterogeneous catalytic processes.[2] A different approach uses suitable well-defined homogeneous model systems that provide a coordination environment similar to that of silica surfaces. In recent years, polyhedral metallasilsesquioxanes have been promoted as molecular model compounds which to some extent can provide insight into the scenarios occurring on metal-modified silica surfaces.[3] Feher et al. have established that the incompletely condensed silsesquioxane derivative Cy 7 Si 7 O 9 (OH) 3 (1, Cy = cyclohexyl, Scheme 1) shares structural similarities with crystalline forms of silica such as b-cristobalite and b-tridymite.[4] Fully metalated silsesquioxane derivatives of the type Cy 7 Si 7 O 9 (OM) 3 (M = Li, Na, K) would constitute highly desirable precursors for the construction of catalyst model compounds, including novel heterobimetallic species. However, such alkali metal derivatives of 1 were unknown until recently, and structural information on such materials is still lacking.There have been contrasting reports in the literature concerning the metalation of 1 by alkali metal reagents. It was reported by Feher et al. that treatment of 1 with three equivalents of NaOtBu resulted in complete breakdown of the silsesquioxane cage.[3a] In contrast, Aspinall and coworkers more recently succeeded in synthesizing Cy 7 Si 7 O 9 (OLi) 3 by the reaction of 1 with n-butyllithium.[5]Cy 7 Si 7 O 9 (OLi) 3 was isolated in virtually quantitative yield as an amorphous, air-stable (!) solid of unknown structure. We report here that with the proper choice of deprotonating agents well-defined, crystalline alkali metal silsesquioxanes are readily available. These represent excellent precursors for the preparation of unprecedented catalyst model compounds.We found that alkali metal bis(trimethylsilyl)amides are the reagents of choice for achieving smooth and high-yield deprotonation of the incompletely condensed silsesquioxane 1. Treatment of 1 with LiN(SiMe 3 ) 2 in diethyl ether/THF afforded the crystalline lithium silsesquioxane dimer [{(c-C 6 H 11 ) 7 Si 7 O 12 } 2 Li 6 (C 4 H 8 O) 2 ]·C 4 H 10 O (2) in 93 % yield Scheme 1. Synthesis of the metalated silsesquioxanes 2-4 (Cy = cyclohexyl).

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The first metallasilsesquioxanes comprising potassium and uranium have been synthesized and structurally characterized by single-crystal X-ray diffraction. (CySiO)K(DME) (2; Cy = cyclohexyl) is a centrosymmetric dimer, in which the two silsesquioxide ligands are interconnected by μ- and μ-bridging siloxide moieties. (CySiO)U (3) represents the first metallasilsesquioxane complex of an actinide element, featuring a U atom that is coordinated by two tridentate silsesquioxide ligands in a distorted octahedral fashion. The different structural effects of the large metal atomic radii are discussed.

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