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 ]₂A and TiO₂ [O₄Si₄(C₆H₅)₈ ]₂B 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)O_1.5 ]₂ [OSi(C₆H₅)₂ ]₃ ·2C₄H₈O 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 ·3C₄H₈O₂D. The titanodiphenylsiloxanes E and F were identified by a cyclotetrameric structure and the formulas [Ti(acac)₂O]₂ [OSi(C₆H₅)₂ ]₂ and [Ti(acac)₂O][OSi(C₆H₅)₂ ]₃ , respectively. The titanodiphenylsiloxanes A–E were characterised by ²⁹Si and ¹⁷O 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 ²⁹Si 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)>bicyclo (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.