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(CH2)(3)Si(OMe)(3)) as a silane-coupling agent (SCA), mono-and tri-lacunary Keggin polyoxometalate (POM)-based, multi-functional, inorganic-organic, hybrid compounds, (Et4N) (3)[alpha-PW11O39{(HS(CH2)(3)Si)(2)O}] EtN-1 (the 1 : 2 complex of a POM unit and organosilyl groups), (Bu4N)(3)[A-PW9O34(HS(CH2)(3)SiOH)(3)] BuN-2 (the 1 : 3 complex) and (Bu4N)(3)[A-alpha-PW9O34(HS(CH2)(3)SiO)(3)(Si(CH2)(3)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 (Si-29 and P-31) CPMAS NMR, solution (Si-29, P-31, H-1 and C-13) 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 CH3CN, and (2) BuN-3b was prepared by a direct 1 : 4 molar-ratio reaction of a tri-lacunary Keggin POM and SCA in water-CH3CN. 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.