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 (NO ₃ ) ₁₂ @[Ln ₁₃₂ Ni ₇₈ (OH) ₂₉₂ (IDA) ₄₈ (CH ₃ COO) ₉₆ (NO ₃ ) ₁₂ (H ₂ O) ₇₈ ]Cl ₄₄ ⋅xH ₂ O⋅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⁺ (Gd ³⁺ or Eu ³⁺ ) and Ni ²⁺ 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(μ ₃ -OH) ₂ Gd]. Mass spectroscopic analyses indicate that both clusters maintained their structural integrity in aqueous solution, with cryo-electron 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.
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