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

Abstract

Using 3-mercaptopropyltrimethoxysilane (HS(CH₂)₃Si(OMe)₃) as a silane-coupling agent (SCA), mono- and tri-lacunary Keggin polyoxometalate (POM)-based, multi-functional, inorganic–organic, hybrid compounds, (Et₄N)₃[α-PW₁₁O₃₉{(HS(CH₂)₃Si)₂O}] EtN-1 (the 1 : 2 complex of a POM unit and organosilyl groups), (Bu₄N)₃[A-PW₉O₃₄(HS(CH₂)₃SiOH)₃] BuN-2 (the 1 : 3 complex) and (Bu₄N)₃[A-α-PW₉O₃₄(HS(CH₂)₃SiO)₃(Si(CH₂)₃SH)] BuN-3 (the 1 : 4 complex) were synthesized and unequivocally characterized by elemental analysis, thermogravimetric and differential thermal analyses (TG/DTA), FTIR, solid-state (²⁹Si and ³¹P) CPMAS NMR, solution (²⁹Si, 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 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.