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We illustrate, combining experimental and theoretical results, that the five-membered chelate ring is able to adapt itself to a great variety of metal ions.

Lanthanoid-containing polyoxometalates (Ln-POMs) have been developed as effective and robust catalysts due to their Lewis acid–base active sites including the oxygen-enriched surfaces of POM and the unique 4f. electron configuration of Ln. As an extension of our interest in Ln-POMs, a series of as-synthesized nanocatalysts K15[Ln(BW11O39)2] (Ln-B2W22, Ln = La, Ce, Nd, Sm, Gd, and Er) synthesized and fully characterized using different techniques. The Ln3+ ion with a big ionic radius was chosen as the Lewis acid center which is sandwiched by two mono-lacunary Keggin [BW11O39]9− units to form Ln-containing sandwiched type cluster. Consequently, the catalytic activity of nanocatalysts with different Ln was examined in the synthesis of bioactive isatin derivatives and compared under the same optimized reaction conditions in terms of yields of obtained products, indicating the superiority of the nano-Gd-B2W22 in the aforementioned simple one-pot reaction. The effects of different dosages of nanocatalyst, type of solvent, reaction time, and reaction temperature in this catalytic system were investigated and the best results were obtained in the presence of 10 mol% of nano-Gd-B2W22 in water for 12 min at the reflux condition.

Keggin‐type heteropolyacid, H5BW12O40 (BWA) with a higher negative charge and stronger Brønsted acidity comparing to Si and P derivatives was used as an efficient, green, and reusable catalyst in a three‐component reaction involving the cyclocondensation of various β‐dicarbonyl compounds, differently substituted aromatic aldehydes and malononitrile in EtOH/H2O for the facile, clean, and high yielding synthesis of 4H‐pyrans. All reactions were completed in short times and the products were obtained in good to excellent yields. The reaction medium could be recycled and reused several times without any loss of efficiency.

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