Engineering a Nanoscale Primary Amide-Functionalized 2D Coordination Polymer as an Efficient and Recyclable Heterogeneous Catalyst for the Knoevenagel Condensation Reaction

Abstract

This work reports the design, structural characterization, and catalytic behavior of the first example of primary amide-functionalized coordination polymers (CPs), namely, {[Cd2(2-bpbg)(fum)2(H2O)2]·8.5H2O}n (1) (where 2-bpbg = N,N′-bis(2-pyridylmethyl)-1,4-diaminobutane-N,N′-diacetamide and fum = fumarate). CP 1 is synthesized from a one-pot self-assembly of starting materials in methanol under ambient conditions in excellent yield and purity, allowing an easy access to multigram quantities of it within few hours. As an example, CP 1 was used as a highly efficient heterogeneous catalyst in the carbon–carbon bond-forming Knoevenagel condensation reaction for the conversion of benzaldehyde to benzylidene malononitrile. CP 1 possesses both Lewis acidic and Brønsted basic character for the presence of unsaturated metal sites and primary amide groups, respectively, making it a highly competent bifunctional catalyst for such type of reactions. Surprisingly, on the one hand, 100% conversion was observed using only 2 mol % catalyst within 1 h at 27 °C in methanol. On the other hand, 2 mol % and 3 mol % catalyst loadings but without a solvent gives 93% and 100% conversions, respectively, in 1 h at 27 °C. CP 1 is far better than those reported in the literature. To prove the uniqueness and efficiency of the primary amide-based ligand, a similar compound with a pyridyl-based ligand was also synthesized, {[Cd2(tpbn)(fum)2]·6H2O}n (2) (where tpbn = N′,N′,N″,N″-tetrakis(pyridin-2-ylmethyl)butane-1,4-diamine). With CP 2 under the same catalyst loading and conditions (2 mol %, 27 °C, 1 h), only 28% conversion was observed. This demonstrates that selective heterogeneous catalytic properties of 1 over 2 are due to the presence of the primary amide moieties and open metal sites. Moreover, CP 1 can easily be separated from the reaction mixture and reused for five consecutive cycles without significant loss of its activity. Both 1 and 2 were fully characterized by elemental analysis, infrared spectroscopy, thermogravimetric analysis, and single-crystal and powder X-ray diffraction. These crystallize in the triclinic P1̅ space group, showing their isostructural nature and three-connected, uninodal {63} honeycomb net topology.