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.