Silica‐Supported Zirconium Complexes and their Polyoligosilsesquioxane Analogues in the Transesterification of Acrylates: Part 1. Synthesis and Characterization

Abstract

AbstractVarious silica‐supported acetylacetonate and alkoxy zirconium(IV) complexes have been prepared and characterized by quantitative chemical measurements of the surface reaction products, quantitative surface microanalysis of the surface complexes, in situ infrared spectroscopy, CP‐MAS 13C NMR spectroscopy and EXAFS. The complex (SiO)Zr(acac)3 (acac=acetylacetonate ligand) (1) can be obtained by reaction of zirconium tetraacetylacetonate [Zr(acac)4] with a silica surface previously dehydroxylated at 500 °C. The complexes (SiO)3Zr(acac) (2) and (SiO)3Zr(O‐n‐Bu) (n‐Bu=butyl ligand) (3) can be synthesized by reaction of (SiO)3ZrH with, respectively, acetylacetone and n‐butanol at room temperature. The spectroscopic data, including EXAFS spectroscopy, confirm that in compound 1 the zirconium is linked to the surface by only one SiOZr bond whereas in the case of compounds 2 and 3 the zirconium is linked to 3 surface oxygen atoms which are sigma bonded. EXAFS data indicate also that the acetylacetonate ligands behave as chelating ligands leading to a hepta‐coordination around the zirconium atom in 1 and a penta‐coordination in 2. In order to provide a molecular analogue of 1, the synthesis of the following polyoligosilsesquioxane derivative (c‐C5H9)7Si8O12(CH3)2Zr(acac)3 (1′) was achieved. The compound 1′ is obtained by reacting (c‐C5H9)7Si8O11(CH3)2(OH), 4, with an equimolecular amount of Zr(acac)4. In the same manner, syntheses of complexes (c‐C5H9)7Si7O12Zr(acac) (2′) and of (c‐C5H9)7Si7O12Zr(O‐n‐Bu) (3′) were achieved by reaction of the unmodified trisilanol, (c‐C5H9)7Si7O9(OH)3, with respectively Zr(acac)4 and Zr(O‐n‐Bu)4 at 60 °C in tetrahydrofuran. Compounds 1′, 2′ and 3′ can be considered as good models of 1, 2 and 3 since their spectroscopic properties are comparable with those of the surface complexes. The synthetic results obtained will permit us to study the catalytic properties of these surface complexes and of their molecular analogues with the ultimate goal of delineating clear structure‐activity relationships.