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

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(Ph 2 )SiOSiPh 2 OSi(Ph 2 )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, 13 C, 29 Si NMR and IRspectroscopy and have been the subject of single-crystal X-ray diffraction studies.