A Hollowed-Out Heterometallic Cluster for Catalytic Knoevenagel Condensation

Abstract

Lanthanide-containing clusters are synthetically challenging and with significant chemical and materials applications. Herein, two isostructural heterometallic clusters of the formula (NO₃)₁₂@[Ln₁₃₂Ni₇₈(OH)₂₉₂(IDA)₄₈(CH₃COO)₉₆(NO₃)₁₂(H₂O)₇₈]Cl₄₄⋅xH₂O⋅yCH₃OH (IDA=iminodiacetate; Ln=Gd 1, x=110, y=0; Ln=Eu 2, x=95, y=40) were obtained via co-hydrolysis of Ln³⁺ (Gd³⁺ or Eu³⁺) and Ni²⁺ in the presence of iminodiacetate (IDA). Crystallographic studies show that each features a truncated tetrahedral core of Ln₁₃₂Ni₇₈ 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(μ₃-OH)₂Gd]. 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.