Insights into the Structure of Keggin-Type Polyoxometalate-Based Organic–Inorganic Hybrid Materials: The Actual Ratio of Organic Cations to Heteropolyanions

Polyoxometalate (POM)-based organic-inorganic hybrid materials possess versatile properties and applications; however, the ratios of organic cations to POM anions still remain to be solved. In this work, 14 POM-based organic-inorganic hybrid materials were synthesized by the precipitation, hydrothermal, and solvent-evaporation methods. These hybrid materials consisted of a wide range of quaternary ammonium and imidazolium cations with different alkyl chains and different Keggin-type heteropolyanions [i.e., phosphotungstic ([PW12O40]3-), phosphomolybdic ([PMo12O40]3-), silicotungstic ([SiW12O40]4-), and silicomolybdic ([SiMo12O40]4-) anions]. Their compositions and structures were characterized complementarily by elemental analysis, powder X-ray diffraction, single-crystal X-ray diffraction, and Fourier transform infrared spectroscopy. The actual ratios of organic cations to heteropolyanions of [PW12O40]3-, [PMo12O40]3-, [SiW12O40]4-, and [SiMo12O40]4- were found to always be 3:1, 3:1, 4:1, and 4:1, respectively, independent of the organic cations, synthesis methods, and reaction parameters. This finding demonstrates that the organic cations completely substituted the protons of the heteropolyacid precursors in the hybrid materials, which thus hardly possessed Brønsted acidity probed by the pyridine adsorption and cellulose hydrolysis reaction. Such complete substitution of the protons arose apparently from the strong noncovalent interactions between the organic cations and heteropolyanions (such as electrostatic and C-H···O interactions) in the POM-based hybrid materials.