omega-Mercapto-functionalized hafnium- and zirconium-oxoclusters as nanosized building blocks for inorganic-organic hybrid materials: synthesis, characterization and photothiol-ene polymerization

Two isostructural mercapto-functionalized zirconium-and hafnium-oxoclusters [M-12(mu(3)-O)(8)(mu 3-OH) (8)(MP)(24)center dot n(MPA), MPA = HS-(CH2)(2) -C(O)OH; MP = HS -(CH2)(2)-C(O)O-; 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), H-1 and C-13 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 C-13 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.