Stepwise Synthesis of Siloxane-Substituted Oligoarsanes and Structural Investigation of Alkaline Earth Metal Derivatives

The reaction between [Li(dme)AsH₂] and (ClSiiPr₂)₂O gives a mixture of two products: the primary diarsanyldisiloxan (H₂AsSiiPr₂)₂O (5) and the cyclic diarsanylsiloxane (HAsSiiPr₂)₂O (6). Through metallation of the mixture, the deprotonated species of both compounds could be obtained and isolated. Further reactions between the two metallated cyclic diarsanylsiloxanes (compounds 9 and 10) and (ClSiiPr₂)₂O both lead to the bicyclic compound As₂[(iPr₂Si)₂O]₂ (11). Subsequent oxidative coupling of 9 or 10 with C₂H₄Br₂ as reagent leads to intermolecular As-As bond formation, yielding the quite remarkable compound As₆[(iPr₂Si)₂O]₃ (12), which features two siloxane-bridged As₃ rings. Compounds 5-12 were characterized by NMR, elemental analysis and X-ray crystallography. Quantum chemical calculations were carried out to investigate the formation of the different products when arsenic (compound 12) is substituted by phosphorus (compound 4) in the final oxidation reaction of the cyclic compounds (HESiiPr₂)₂O. The calculations suggest a reaction path leading to 4 that is in agreement with experimental observations. The theoretical data also provide information that explains the experimental finding that two different products for E = P and E = As are formed under the same oxidative reaction conditions.