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 ] (I^x -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, I^Si -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 ] (I^Si -PE) was successfully synthesized. In particular, these multidentate alkoxy groups showed high resistance against hydrolysis. Moreover, by reacting I^Si -PE with trivacant lacunary Keggin-type POMs, the dimeric inorganic-organic-inorganic hybrid structures TBA_n H_8-n [(A-α-SiW₉ O₂₈ ){(OCH₂ )₂ C(CH₂ O)₂ }₃ (A-α-XW₉ O₂₈ )] (II^SiX -PE; X=Si, Ge) with the same or different heteroatoms (Si and Ge) were successfully synthesized.