X-ray and NMR spectroscopic characterisation of cyclic titanodiphenylsiloxanes and examination of the hydrolytic stability of their Si-O-Ti bonds

Abstract

Six crystalline titanodiphenylsiloxanes have been synthesised by reaction of diphenylsilanediol (DPSD) with titanium tetraisopropoxide or its complexes with acetylacetonate (acac) as ligand. Two of them show a spirocyclic structure with the formula TiO2[O2Si2(C6H5)(4)](2) A and TiO2[O4Si4(C6H5)(8)](2) B which have already been described in the literature. Two compounds C and D were identified by X-ray analysis to have the same bicyclic structure but different coordinating solvent molecules. Tetrahydrofuran acts as a non-bridging ligand at the Ti atoms in [Ti(acac)0(1.5)](2)[OSi(C6H5)(2)](3)?2C(4)H(8)O C while dioxane acts as a bridging ligand between the Ti atoms of neighbouring molecules of [Ti(acac)O-1.5](2)[OSi(C6H5)(2)](3)?3C(4)H(8)O(2) D. The titanodiphenylsiloxanes E and F were identified by a cyclotetrameric structure and the formulas [Ti(acac)(2)O](2)[OSi(C6H5)(2)](2) and [Ti(acac)(2)O][OSi(C6H5)(2)](3), respectively. The titanodiphenylsiloxanes A-E were characterised by Si-29 and O-17 NMR spectroscopy, IR and time-of-flight mass spectrometry measurements. The hydrolytic stabilities of the Si-O-Ti bonds in the titanodiphenylsiloxanes A-E have been examined mainly by means of Si-29 NMR spectroscopy. The results reveal a strong influence of the structure type of the titanodiphenylsiloxanes on the hydrolytic stability of their Si-O-Ti bonds apart from the hydrolytic conditions (amount of water, Si, Ti and H+ concentration). The hydrolytic stability of the titanodiphenylsiloxanes A-E decreases in the order cyclotetramer (E)> spirocyclo (A, B)> bicycle (C, D). Reasons for the different hydrolytic stability are discussed. The results on the different hydrolytic stabilities of Si-O-Ti bonds can contribute to a better understanding of the synthesis of homogeneous heterometal materials on a molecular scale via the sol-gel process.