Cadmium Metal-Organic Frameworks Based on Ditopic Triazamacrocyclic Linkers: Unusual Structural Features and Selective CO2 Capture

Two three-dimensional cadmium metal organic frameworks with general formula [Cd-2(L-1)(H2O)(3)](NO3)(0.7)(HCOO)(0.2)Br-0.1 (Cd2L1, L-1 = 1,4,7-tris(4-carboxybenzy1)-1,4,7-triazacyclononane) and Cd(HL2)(H2O)(2) (CdL2, L-2 = 1,4,7-tris(3-(4-benzoate)prop-2-yn-1-yl)-1,4,7-triazacydononane) based on 1,4,7-triazacyclononane N-functionalized by different arylcarboxylic acids were prepared under solvothermal conditions and characterized by single crystal X-ray analysis and porosity measurements. The crystal structure of Cd2L1 reveals a cationic net with a bcs topology,. and nodes are constituted by dinuclear cadmium complexes, in which each cadmium atom adopts a hexacoordinated environment involving both the carboxylate and the cyclic amine. In contrast, CdL2 displays a 2-fold interpenetrated structure with a pcu topology. In this net, the node is a mononuclear complex in which the Cd atom exhibits a seven coordination geometry. Both materials show a high permanent porosity and good CO2 adsorption properties with a high selectivity over N-2 and CH4. The adsorption capacity and selectivity for CO2 were calculated from a multisite Langmuir isotherm model and the ideal adsorbed solution theory, which gave insights into the nature of solid gas interactions and showed the influence of interpenetration or polarity of the charged framework on their adsorption properties.