omega-Mercapto Functionalized Hafnium and Zirconium Oxoclusters as Nanosized Building Blocks for Inorganic-Organic Hybrid Materials: Synthesis, Characterization and Photo-thiolene Polymerization

Two isostructural mercapto-functionalized zirconium- and hafnium-oxoclusters [M12(µ3-O)8(µ3-OH)8(MP)24.n(MPA), MPA=HS-(CH2)2-C(O)OH; MP = HS-(CH2)2-C(O)O2; M= Zr, Hf; n=4 for Zr, n=5 for Hf] were prepared by reacting 3-mercaptopropionic acid with zirconium and hafnium butoxide, respectively, in an alkoxide:acid 1:7 molar ratio. The two oxoclusters Zr12 and Hf12, which are composed of two Zr6 and Hf6 sub-units bridged by four carboxylate ligands, were thoroughly characterized by single crystal X-ray diffraction, infrared spectroscopy (FT-IR), 1H and 13C nuclear magnetic resonance (NMR) spectroscopy, thermogravimetry (TGA), differential thermoanalysis (DTA) and extended X-ray absorption spectroscopy (EXAFS). The zirconium oxocluster was then embedded in a polymethacrylic matrix by a photothiol-ene polymerization reaction by using different cluster : monomer molar ratios. EXAFS measurements performed on the polymethacrylic-based hybrid materials showed that the structure of the oxocluster is retained even after being embedded in the polymer matrix. Solid-state 13C NMR spectroscopy as well as FT-IR spectroscopy demonstrated that the inorganic-organic hybrid materials are characterized by an almost complete polymerization of methacrylic acid, in which the oxoclusters are embedded in and covalently bound to the polymer matrix.