Construction of 3D homochiral metal–organic frameworks (MOFs) of Cd(ii): selective CO2 adsorption and catalytic properties for the Knoevenagel and Henry reaction

Abstract

Two new homochiral metal–organic frameworks of Cd(II), [{Cd₂(L-glu)₂(bpe)₃(H₂O)}·2H₂O] (1) and [{Cd₃(L-glu)₂(bpe)₃(H₂O)}·2NO₃·H₂O] (2) (where, L-glu = L-glutamate dianion, and bpe = 1,2-bis(4-pyridyl)ethylene) have been synthesized solvothermally by employing two different temperatures. Single crystal X-ray diffraction studies revealed that both 1 and 2 are homochiral and possess a 3D pillar-layered framework structure having 4,8- and 8,10-connected binodal nets with vertex symbols of {3^2.4.5^3}{3^4.4^6.5^10.6^8} and {3^11.4^28.5^5.6}2{3^8.4^18.5.6}, respectively. Interestingly, solvothermal synthesis carried out at 100 °C resulted in a 3D framework, 1 which features large rectangular 1D channels with a dimension of ∼10.38 × 4.44 Ų decorated with pendant –NH₂ groups. Whereas, increasing the temperature of the reaction to 120 °C led to a non-porous highly connected 3D framework, 2 in which the –NH₂ group of the L-glu ligand is coordinated to a Cd(II) node. Gas (N₂, CO₂, H₂ and Ar) uptake studies on the dehydrated framework of 1 revealed excellent selectivity for CO₂ over other gases at 273 K with a high isosteric heat of adsorption (Q_st) value of 40.8 kJ mol⁻¹. The high selectivity for CO₂ gas has been attributed to the stronger interaction of CO₂ with the basic –NH₂ functionalized pore surface of compound 1. Furthermore, 1 acts as a very good recyclable catalyst for the carbon–carbon bond forming reactions, such as the Knoevenagel condensation and Henry reaction of benzaldehydes. Moreover, the catalyst can be easily separated from the reaction mixture and reused in four consecutive cycles without significant loss of catalytic activity and structural rigidity.