Synthesis of metal-organic rectangles decorated with primary amide groups for the synergistic Lewis acid and Bronsted base heterogeneous catalysis

Abstract

This work reports the one-pot self-assembly of two Zn/Cd-based molecular rectangles (1 and 2, respectively) with a new ligand containing a primary amide group, 2-(tert-butyl(pyridin-2-ylmethyl) amino) acetamide (bpaa), under ambient conditions. Based on the single crystal X-ray structures of 1 and 2, molecular rectangles have an average dimension of ~7.9 x ~7.2 Å. Both 1 and 2 are thermally stable up to 275 °C and their chemical resistance towards solvents (methanol and water), and acidic and basic environments (pH: 5, 7.4 and 9) monitored by powder X-ray diffraction is remarkable. The synergistic presence of the Lewis acidic metal centers and the Bronsted basic nature of the free -NH2 groups in 1 and 2 has powered these to be very efficient (catalyst: 1.5 mol%; time: 3 h) and recyclable catalysts for making multipurpose coumarin-3-carboxylic acids via the Knoevenagel-intramolecular tandem cyclization reaction of Meldrum’s acid with salicylaldehyde under green conditions (solvent: methanol; temperature: 25 °C). In addition to a broad substrate scope, the mechanistic pathway has been supported by an experimental proof where the interaction of free -NH2 groups in 1 and 2 with Meldrum’s acid and that of the Lewis acid metal centers with salicylaldehyde is evident from the fluorescence quenching of the catalysts that is not observed for other catalysts not having such groups. Our finding provides an encouraging strategy for utilizing such combination of active sites for other multi-component organic transformations.