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-<i>N</i>,<i>N'</i>-bis(3-<i>tert</i>-butyl-5-<i>m</i>-dicarboxybenzo-<i>o</i>-hydroxybenzyl)) and H₆L2a (1,2-diphenylethylenediamine-<i>N</i>,<i>N'</i>-bis(3-<i>tert</i>-butyl-5-<i>m</i>-diacarboxybenzo-<i>o</i>-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 (<b>1</b>) (1,2-cyclohexanediamine-<i>N</i>-(3-<i>tert</i>-butyl-5-<i>m</i>-dicarboxybenzo-<i>o</i>-hydroxybenzyl)) and [Cu₃(L2b)₂H₂O]·6DMF·12H₂O (<b>2</b>) (1,2-diphenylethylenediamine-<i>N</i>-(3-<i>tert</i>-butyl-5-<i>m</i>-diacarboxybenzo-<i>o</i>-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 <b>1</b> is attributed to its unique cavity structure, while these channels are occupied by the phenyl moieties of the ligands in compound <b>2</b>.