Untersuchungen zur Synthese und Reaktivität Siloxanverbrückter Pnikogenverbindungen

Im Rahmen der hier vorliegenden Dissertationsarbeit gelang es, die bicyclischen Pnikogensiloxanverbindungen [E2{(SiMe2)2O}3] mit E = P und As um ihre schwereren Homologe E = Sb und Bi zu komplettieren. Zur Synthese dieser Siloxanverbindungen der schweren Homologen bedurfte es allerdings eines altern...

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Bibliographic Details
Main Author: Bimbös, Christian
Contributors: Hänisch, C. von (Prof. Dr.) (Thesis advisor)
Format: Doctoral Thesis
Published: Philipps-Universität Marburg 2014
Online Access:PDF Full Text
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In the work presented here, it was possible to gain access to the bicyclic siloxane compounds [E2{(SiMe2)2O}3] with Sb and Bi as bridgehead atoms. To synthesize the compounds with the heavier homologues, it was necessary to establish an alternative synthesis strategy to the one used before for with E = P, As. Therefore a heterogeneous reaction path was chosen using the compounds Na3E (E = Sb, Bi) with stoichiometric amounts of the dichlorodisiloxan. This alternative reaction path was also successfully applied for the lighter homologues and so the usage of the toxic gases PH3 and AsH3 was avoided. Through the synthesis of the whole set of these compounds with E = P, As, Sb, Bi as bridgehead atoms, it was possible to systematically compare them by their structural and coordination chemistry properties. Considering some of the structural parameters and their observed changes in the compound series, a slightly reduced LEWIS-basicity of these compounds is discussed. In preliminary studies, some LEWIS-acid-base adducts had been characterized for E = P, and it was possible now to gain access to similar adducts for (E = P, As, Sb) with different Lewis-acids. A structural comparison of the compounds corroborates the assumption of the reduced LEWIS-basicity. In the second part, further investigations on the chemistry of sterically more demanding siloxanes were performed. Initially, these studies were focused on the cyclic five-membered siloxane (HAsSiiPr2)2O. It was possible to synthesize the compound [As6{(SiiPr2)2O}3] (10) by deprotonation and following oxidation reaction. Compound 10 is built by two As3 triangles that are linked by siloxane bridges The synthesis of 10 and the knowledge of other very similar siloxane compounds gave reason to discuss those compounds and to put them into context with the nature of the Si-O bond. In all compounds the inverse relationship between the Si-O Siangles and the Si-O bond lengths can be found A reduction of the negative hyperconjugation is commonly held responsible for this observation. Within the scope of this work, the latter statement was supplemented by the observation of a deshielding effect accompanied with the Si-O-Si angles for the Si nucleus found in the 29Si-NMR spectra. Besides these NMR-spectroscopically inspired studies concerning deshielding effects for the siloxane bridges, the structural motif of compound 10 also gave reason to take a closer look at a possible reaction mechanism to understand the reason for the appearance of the different products formed for E = P and As. As shown there, the cyclic phosphorus compound (HPSiiPr2)2O forms a different product than 10 under the exact same reaction conditions. Hence, an initially postulated reaction mechanism for the reaction of (HPSiiPr2)2O to P4[(SiiPr2)2O]2 is discussed and could further be justified by an interplay of experimental indications and quantum mechanical calculations. Experimental indications for the existence of a reactive species with a P-P double bond were proved as well as a constitutional isomeric P4[(SiiPr2)2O]2 intermediate species. These experimental evidences for the proposed reaction mechanism leading to the formation of the final P4[(SiiPr2)2O]2 species are backed up with quantum chemical calculations. In the context of these calculations, the formation of the framework for E = P according to the postulated reaction path could be supported and energetically comprehended. The application of the eight-membered cyclosiloxane compound [HP(SiiPr2)2O]2 as synthon for the formation of siloxane stabilized oligophosphane compounds is also part of this work. In the line of this investigation it was possible to obtain ring systems with the general formula [P2{(SiiPr2)2O}2{X}n] (with n =1 for X = SiiPr2 (11), (SiMe2)2 (12), PtBu (13), PNEt2 (14) and with n = 2 in the case of X = PPhtBu (15)). Therefore, it was possible to prove the ability of the cyclic compound [HP(SiiPr2)2O]2 to act as such a synthon and to establish this for a concept to build other, more complex systems. In the last part of the work presented here, the question of the influence of the steric demands of the siloxane on reactions and functionalizations with [Li(dme)PH2] from the previous sections was resumed and could be pursued consequently. Besides the reactions of [Li(dme)PH2] with sterically less demanding methyl substituted as well as with sterically more demanding iso-propyl substituted disiloxane already described, the focus was now lying on an asymmetrically substituted disiloxanes. As a product of this reaction, a cyclic eight-membered siloxane ring was obtained as a monoanionic species in the form of a coordination dimer with Li(dme) as bridging ligands. The existence of the asymmetric and monoanionic phosphanylsiloxane cycle of 16 implies the possibility of further functionalizations that could present the basis for intensive investigations on asymmetric cycloheterosiloxanes.