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

Abstract

tert-Butoxygallane, [tBuOGaH 2 ] 2 , reacts with tetraphenyl-1,3-disiloxanediol, HO(Ph 2 )Si-O-Si(Ph 2 )OH, with hydrogen elimination to form the bicyclic compound [(OSiPh 2 OSiPh 2 O)-[Ga(H)] 2 (OtBu) 2 ] (1). The skeleton of molecule 1 consists of a four-membered Ga 2 O 2 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 [tBuOGaH 2 ] 2 :1,3-disiloxanediol = 1:2, two new products are formed in almost equal amounts: [(OSiPh 2 OSiPh 2 OSiPh 2 O)GaH] 2 (2) and [(OSiPh 2 OSiPh 2 OSiPh 2 O)GaOtBu] 2 (3). Both compounds are centrosymmetric, forming either a central Ga 2 O 2 ring to which two eight-membered GaO 4 Si 3 rings are fused with two common GaO edges (2) or a central Ga 2 O 2 ring, the gallium corners serving as spiro centers for again eight-membered GaO 4 Si 3 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)SiOSiPh 2 OSi(Ph2)OH) is reacted with tert-butoxygalane. The central Ga 2 O 2 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 Si 3 O 4 Ga ring with hydrogen and triethylamine as ligands at gallium and two phenyl groups on each of the silicon atoms and has the formula (OPh 2 SiOSiPh 2 OSiPh 2 O)GaH(NEt 3 )] (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 NEt 3 as crystallization promotor gave [(OSiPh 2 OSiPh 2 OSiPh 2 O) 2 Ga-HNEt 3 ] (5). In this compound a spiro-connected gallium atom is a member of two GaO 4 Si 3 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 1-5 have been fully characterized by 1 H, 1 3 C, 2 9 Si NMR and IRspectroscopy and have been the subject of single-crystal X-ray diffraction studies.