A Hollowed-Out Heterometallic Cluster for Catalytic Knoevenagel Condensation

Lanthanide-containing clusters are synthetically challenging and with significant chemical and materials applications. Herein, two isostructural heterometallic clusters of the formula (NO3)(12)@[Ln(132)-Ni-78(OH)(292)(IDA)(48)(CH3COO)(96)( NO3)(12)(H2O)(78)]Cl-44 center dot xH(2)O center dot yCH(3)OH (IDA= iminodiacetate; Ln=Gd 1, x= 110, y= 0; Ln= Eu 2, x= 95, y= 40) were obtained via co-hydrolysis of Ln(3+) (Gd3+ or Eu3+) and Ni2+ in the presence of iminodiacetate (IDA). Crystallographic studies show that each features a truncated tetrahedral core of Ln(132)Ni(78) within which a void of 1.1 nm in diameter; connecting the central cage and its exterior are four trumpet-like passageways surface-decorated with dinuclear units of [Gd(mu(3)-OH)(2)Gd]. Mass spectroscopic analyses indicate that both clusters maintained their structural integrity in aqueous solution, with cryoelectron microscopy providing the most convincing visual evidence in support of the cluster's solution stability. Size-selective Knoevenagel condensation, believed to occur in the passageways on the basis of experimental and molecular modeling results, was achieved in the presence of 1. The application of 1 as a uniquely structured molecular reactor and a recyclable heterogeneous catalyst was further illustrated by the one-pot three-component synthesis of biologically and pharmaceutically significant 4H-pyran derivatives.