Polyoxometalate (POM)-based, multi-functional, inorganic-organic, hybrid compounds: syntheses and molecular structures of silanol- and/or siloxane bond-containing species grafted on mono- and tri-lacunary Keggin POMs

Using 3-mercaptopropyltrimethoxysilane (HS(CH?)?Si(OMe)?) as a silane-coupling agent (SCA), mono- and tri-lacunary Keggin polyoxometalate (POM)-based, multi-functional, inorganic-organic, hybrid compounds, (Et?N)?α-PW??O??{(HS(CH?)?Si)?O}] EtN-1 (the 1?:?2 complex of a POM unit and organosilyl groups), (Bu?N)?A-PW?O??(HS(CH?)?SiOH)?] BuN-2 (the 1?:?3 complex) and (Bu?N)?A-α-PW?O??(HS(CH?)?SiO)?(Si(CH?)?SH)] BuN-3 (the 1?:?4 complex) were synthesized and unequivocally characterized by elemental analysis, thermogravimetric and differential thermal analyses (TG/DTA), FTIR, solid-state (2?Si and 31P) CPMAS NMR, solution (2?Si, 31P, 1H and 13C) NMR, and X-ray crystallography. Note: The moieties of their polyoxoanions are abbreviated simply as 1-3, respectively.] The X-ray molecular structures of EtN-1 and BuN-3 were determined. In EtN-1, two organic groups connected through a siloxane bond (-Si-O-Si- bond) were grafted on a mono-lacunary site of a Keggin POM, whereas in BuN-3 four organic groups connected through siloxane bonds were grafted on a tri-lacunary site of a Keggin POM. In BuN-2, three organic groups were grafted in the form of silanol (-SiOH) on a tri-lacunary site, i.e., in BuN-2 there was no siloxane bond. BuN-3 was synthesized as BuN-3a and BuN-3b by two methods, respectively; (1) BuN-3a was obtained by a 1?:?1 molar-ratio reaction of BuN-2 and an SCA in CH?CN, and (2) BuN-3b was prepared by a direct 1?:?4 molar-ratio reaction of a tri-lacunary Keggin POM and SCA in water-CH?CN. X-Ray crystallography revealed that BuN-3a is the same as BuN-3b. It is probable that BuN-2 is an intermediate in the formation of BuN-3. Terminal -SH groups in 1-3, as well as -OH groups in 2, can be utilized for immobilization of POMs and, also, as building blocks for the formation of novel hybrid compounds.