New Polycyclic Aluminopolysiloxanes

Abstract

When diphenyldihydroxysilane, Ph₂Si(OH)₂, is allowed to react with the aluminiumalkoxydihydride (tBu-O)AlH₂, the polycyclic compound [(Ph₂Si)₂O₃]₄Al₄(OH)₄, 1, is formed. The compound is made up of five eight membered cycles, the central one originating from formal donor-acceptor bonds between OH groups and Al atoms. The structure of 1 can be modified by reacting it with Lewis bases like Et₃N, Et₂O or C₅H₅N; in each case the basic structure of the polycycle is conserved, however serious changes are observed in the central Al₄O₄ ring depending upon the basicity (the hydrogen withdrawing properties) of the oxygen or nitrogen atom. It is intriguing that the psendo host-guest behaviour of 1 is not the same for different bases and the ‘shell’ or ‘basket’ formed by the multiple phenyl groups and the shape of the Al-O-Si-skeleton is functioning as a selective trap such that the four OH groups accomodate two, three and four molecules of Et₃N (2), Et₂O (3) and C₅H₃N (4), respectively. The spatial consideration of the central part of the molecular unit for the incoming base i.e. an opening or closing effect is subjected to the steric requirements of the base employed. The replacement of hydrogen atoms in 1 by lithium atoms results in further cyclisation of the molecule to yield a higher polycyclic compound, [(Ph₂Si)₂O₃]₄O₄Al₄Li₄, which can be isolated as a tetrakis(diethylether) adduct (5) or as a tris(diethylether) bis(ammonia) adduct (6). Nevertheless the connection of the atoms in 1 is retained in compounds 5 and 6. If the organic bases attacking 1 are replaced by the smaller and more acidic water molecule the new compound 7, [(Ph₂Si)₂O₃]₆Al₆(OH)₃Al(OH)₆*3OEt₂, is formed, the structure of which is completely different from 1, 2, 3 or 4. In this case the starting molecule has been rearranged by the influence of water.

Keywords::

• alumosiloxanes
• molecular polycycycles with Si, O, Al
• host-guest chemistry
• proton acidity of aluminiumhydroxyl groups