Increased Lewis Acidity in Hafnium-Substituted Polyoxotungstates

✍ Scribed by Cécile Boglio; Kevin Micoine; Pauline Rémy; Bernold Hasenknopf; Serge Thorimbert; Emmanuel Lacôte; Max Malacria; Carlos Afonso; Jean-Claude Tabet

• Publisher: John Wiley and Sons
• Year: 2007
• Tongue: English
• Weight: 191 KB
• Volume: 13
• Category: Article
• ISSN: 0947-6539
• DOI: 10.1002/chem.200700010

✦ Synopsis

Abstract

Monolacunary polyoxotungstates [α~1~‐P~2~W~17~O~61~]^10−^ and [α‐PW~11~O~39~]^7−^ react with HfCl~4~ to yield [α~1~‐HfP~2~W~17~O~61~]^6−^ and [α‐Hf(OH)PW~11~O~39~]^4−^, isolated as organo‐soluble tetrabutylammonium (TBA) salts. Subsequent analyses, including mass spectrometry, show that they are stronger Lewis acids than (TBA)~5~H~2~[α~1~‐YbP~2~W~17~O~61~]. The new polyoxotungstates catalyze Lewis acid mediated organic reactions, such as Mukaiyama aldol and Mannich‐type additions. In particular, reactions with aldehydes, which were impossible with lanthanide polyoxotungstates, are made possible. Thus these modifications of the polyoxometalate composition allowed fine tuning of the Lewis acidity. The catalysts could be easily recovered and reused.