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Cyclosiloxanes Containing a Gallium Atom as Ring Member from the Reaction of tert-Butoxygallane with Di- and Trisiloxanediols

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journal contribution
posted on 21.12.2001, 00:00 by Michael Veith, Heidi Vogelgesang, Volker Huch
tert-Butoxygallane, [tBuOGaH2]2, reacts with tetraphenyl-1,3-disiloxanediol, HO(Ph2)Si−O−Si(Ph2)OH, with hydrogen elimination to form the bicyclic compound [(OSiPh2OSiPh2O)[Ga(H)]2(OtBu)2] (1). The skeleton of molecule 1 consists of a four-membered Ga2O2 ring to which a five-membered O−Si−O−Si−O bridge is connected through Ga−O bonds. While the gallium atoms each have a hydrogen atom as further substituent, the oxygen atoms of the four-membered ring have tert-butyl substituents and the silicon atoms each bear two phenyl groups. When the reactant ratio is changed to [tBuOGaH2]2:1,3-disiloxanediol = 1:2, two new products are formed in almost equal amounts:  [(OSiPh2OSiPh2OSiPh2O)GaH]2 (2) and [(OSiPh2OSiPh2OSiPh2O)GaOtBu]2 (3). Both compounds are centrosymmetric, forming either a central Ga2O2 ring to which two eight-membered GaO4Si3 rings are fused with two common GaO edges (2) or a central Ga2O2 ring, the gallium corners serving as spiro centers for again eight-membered GaO4Si3 rings (3). In comparison to 1, molecules 2 and 3 contain Si−O chains with one further SiO unit and have lost either tert-butyl alcohol (2) or hydrogen (3) with respect to 1. Compound 2 can also be obtained when, instead of tetraphenyl-1,3-disiloxanediol, hexaphenyl-1,5-trisiloxanediol (HO(Ph2)SiOSiPh2OSi(Ph2)OH) is reacted with tert-butoxygalane. The central Ga2O2 rings in molecules 2 and 3 originate from Lewis acid and base interactions but are of different stability. In 2, the ring is formed by oxygen atoms bearing a silicon substituent and therefore should have reduced basicity:  consequently, 2 can be split (into halves) by simple reaction with triethylamine, the nitrogen atom serving as a donor to gallium. The product of this reaction is an eight-membered Si3O4Ga ring with hydrogen and triethylamine as ligands at gallium and two phenyl groups on each of the silicon atoms and has the formula [(OPh2SiOSiPh2OSiPh2O)GaH(NEt3)] (4). Further change of the stoichiometry between tert-butoxygallane and the 1,5-trisiloxanediol in favor of the silicon compound again has a consequence for the reaction products. Using NEt3 as crystallization promotor gave [(OSiPh2OSiPh2OSiPh2O)2Ga·HNEt3] (5). In this compound a spiro-connected gallium atom is a member of two GaO4Si3 rings. The triethylamine group is coordinating a proton which forms a bridge between an oxygen atom of the rings and nitrogen. Compound 5 can also be obtained from 2 using a dilute solution of water in THF. Compounds 15 have been fully characterized by 1H, 13C, 29Si NMR and IRspectroscopy and have been the subject of single-crystal X-ray diffraction studies.