A 3D Ag18 Cluster-Based Metal-Organic Framework as a Heterogeneous Catalyst for CO2 Cycloaddition and Knoevenagel Condensation

The rational design of three-dimensional (3D) metal-organic frameworks (MOFs) based on high-nuclearity silver clusters with well-defined structures and multiple catalytic functions remains a significant pursuit in synthetic materials science. Herein, we constructed a new 3D silver-cluster-based MOF, formulated as [(CrO4)@Ag18(tBuC≡C)12(BPDC)4·6CH3OH·H2O]n (Ag18BPDC, BPDC = 4,4′-biphenyldicarboxylate), which features an Ag18 cluster as the cluster nodes and further extended by four BPDC linkers, resulting in a 3D diamondoid (dia) topological network. The framework exhibits excellent solvent and thermal stability, with accessible Lewis acidic Ag+ sites confirmed by N-methylacridone (NMA) fluorescence probing. Ag18BPDC functions as an efficient heterogeneous catalyst for both CO2 cycloaddition with epoxides and Knoevenagel condensation of aldehydes with malononitrile under mild conditions, showing high activity, broad substrate scope, and good recyclability. Mechanistic studies suggest that substrate activation originates from the Lewis acidity of Ag+, while the porous architecture enhances reaction efficiency through pore confinement and substrate enrichment. This work provides a new paradigm for constructing multifunctional 3D MOF catalysts based on high-nuclearity silver clusters.