Two new calix[4]resorcinarene-based coordination cages adjusted by metal ions for the Knoevenagel condensation reaction

Abstract

Two new calix[4]resorcinarene-based coordination cages, namely, [Zn₄(TPC4R)(PDC)₄]·2DMF·6H₂O (1-Zn) and [In₁₁(TPC4R)₂(PDC)₁₆(μ₂-OH)₂(H₂O)₂]·[(CH₃)₂NH₂]·8DMF·20H₂O·EtOH (2-In), have been synthesized via solvothermal reactions (TPC4R = tetra(2-(4H-pyrazol-3-yl)pyridine)calix[4]resorcinarene, H₂PDC = 3,5-pyridinedicarboxylic acid, DMF = N,N′-dimethylformamide). By carefully tuning different metal ions, two structurally different cages 1-Zn and 2-In were achieved. The former shows a bowl-shaped structure, while the latter features a dumbbell-like structure. After activation, they exhibited unsaturated Zn(II) or In(III) Lewis acid sites and the free nitrogen Lewis base sites of the PDC²⁻. Therefore, they were employed as catalysts for the Knoevenagel condensation reaction in the absence of a solvent. Particularly, 1-Zn featured high structural stability and enhanced the catalytic activity.