Bisphosphonate-modified boat-shaped polyoxoniobates for efficient catalysis of knoevenagel condensation

Abstract

Three novel inorganic-organic hybrid polyoxoniobates (PONbs), H₂₇K₂Na₆(H₂O)₁₇[K(H₂O)(Nb₆O₁₉)₂(Nb₆O₂₀)(RA)₂]·13H₂O (1, H₅RA = C₇H₁₁NO₇P₂, risedronic acid), H₂₁(CN₃H₆)₁₂[K₃(H₂O)(Nb₆O₁₉)₂(Nb₆O₂₀)(ZA)₂]·5H₂O (2, H₅ZA = C₅H₁₀N₂O₇P₂, zoledronic acid, CN₃H₆ = guanidine) and H₂₄(CN₃H₆)₇K₂(H₂O)₄[K₃(H₂O)₂(Nb₆O₁₉)₂(Nb₆O₂₀)(EA)₂]·8H₂O (3, H₅EA = C₂H₈O₇P₂, etidronic acid, CN₃H₆ = guanidine) were successfully synthesized. The polyoxoanion clusters are in a small boat shape and consist of {(Nb₆O₁₉)₂(Nb₆O₂₀)} unit and two deprotonated RA⁵⁻, ZA⁵⁻, and EA⁵⁻ ligands, respectively. Furthermore, PONb 1 can be used as an effective heterogeneous catalyst for the Knoevenagel condensation reaction with high yields and good substrate compatibility under optimal conditions. In addition, PONb 1 also has good catalytic activity in gram-scale reactions.