Structural Modulation of Cu(II)-Based Coordination Polymers by Schiff Base Ligands and Electrochemical Sensing for Heavy-Metal Ions

Retaining the topology of the framework and the metal nodes while judiciously modifying the organic linker constituents presents an opportune avenue to systematically investigate the structure-activity relationship of coordination polymers (CPs). Herein, two functionalized Schiff base ligands, H₆L1a (1,2-cyclohexanediamine-N,N'-bis(3-tert-butyl-5-m-dicarboxybenzo-o-hydroxybenzyl)) and H₆L2a (1,2-diphenylethylenediamine-N,N'-bis(3-tert-butyl-5-m-diacarboxybenzo-o-hydroxybenzyl)), were designed and prepared. In the two ligands, the amine moiety connecting the Schiff base is either 1,2-diaminocyclohexane or the more sterically hindered diphenylethylenediamine, which directly influences the pore distribution in the subsequently synthesized crystalline materials. Their corresponding homologous Schiff base CPs [Cu₃(L1b)₂H₂O]·7DMF·16H₂O (1) (1,2-cyclohexanediamine-N-(3-tert-butyl-5-m-dicarboxybenzo-o-hydroxybenzyl)) and [Cu₃(L2b)₂H₂O]·6DMF·12H₂O (2) (1,2-diphenylethylenediamine-N-(3-tert-butyl-5-m-diacarboxybenzo-o-hydroxybenzyl)) were synthesized successfully by in situ ligand transformation reactions. Two compounds have similar structures and the same active Cu centers, providing a good model to investigate the structure-activity relationship of crystalline materials. Experiments showed that both compounds exhibit excellent electrocatalytic sensing performance toward heavy-metal ions. The result analysis illustrated that the superior performance of compound 1 is attributed to its unique cavity structure, while these channels are occupied by the phenyl moieties of the ligands in compound 2.