Alkoxides of Trivacant Lacunary Polyoxometalates.

Lacunary polyoxometalates (POMs) are useful rigid multidentate inorganic ligands to construct multinuclear metal oxo clusters and inorganic-organic hybrid materials. However, the use of multivacant lacunary POMs often suffers from their undesired condensation reactions at the highly reactive vacant sites. Herein, the introduction of alkoxy protecting groups into the vacant sites of lacunary POMs in organic media is reported for the first time. By reacting organic solvent-soluble trivacant lacunary Keggin-type POMs with methanol, six methoxy groups were successfully introduced into the vacant sites, resulting in the formation of the alkoxides TBA4 [A-α-XW9 O28 (OCH3 )6 ] (Ix -OMe; X=Si, Ge). These methoxy groups could protect the reactive vacant sites and suppress the undesired dimerization reaction into the corresponding α-Dawson-type POMs. Since the introduced monodentate methoxy groups could easily be dissociated by hydrolysis, ISi -OMe could be utilized as the precursor for the synthesis of metal-substituted POMs. In addition, by using pentaerythritol as the multidentate alkoxy ligand, the monomeric alkoxide TBA4 [A-α-SiW9 O28 {(OCH2 )2 C(CH2 OH)2 }3 ] (ISi -PE) was successfully synthesized. In particular, these multidentate alkoxy groups showed high resistance against hydrolysis. Moreover, by reacting ISi -PE with trivacant lacunary Keggin-type POMs, the dimeric inorganic-organic-inorganic hybrid structures TBAn H8-n [(A-α-SiW9 O28 ){(OCH2 )2 C(CH2 O)2 }3 (A-α-XW9 O28 )] (IISiX -PE; X=Si, Ge) with the same or different heteroatoms (Si and Ge) were successfully synthesized.