A pillar-layer strategy to construct 2D polycatenated coordination polymers for luminescence detection of Cr₂O₇^2- and CrO₄^2- in aqueous solution

Six coordination polymers 1-6 were constructed using a rod and loop strategy, in which the H₂L ligand served as the loop and the N-donor ligands (bpp, 1,4-bimb, 1,3-bimb, bpb, bpdp and bpeb; bpp = 1,3-di(pyridin-4-yl)propane; 1,4-bimb = 1,4-bis((1H-imidazol-1-yl)methyl)benzene; 1,3-bimb = 1,3-bis((1H-imidazol-1-yl)methyl)benzene; bpb = 1,4-di(pyridin-4-yl)benzene; bpdp = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene; bpeb = 1,4-bis[2-(4-pyridyl)ethenyl]benzene) acted as the rods. All the complexes except 6 possessed binuclear secondary building units (SBUs) and displayed the same topologies. In compounds 1-4, two paddle-wheel Zn₂ units were linked by two L ligands to form rhombic [Zn₄L₂] units and these units propagated into a chain-like motif. The N-donor ligands supported the chain-like structure and expanded into 2D layers. Three layers were interpenetrated forming a 2D + 2D → 2D polycatenation backbone. The 2D polycatenation layers were further packed into a 3D structure by hydrogen bonds and unconventional interactions. Compound 6 showed a 3D pillar-layer structure consisting of a four-fold interpenetrated network. This coordination polymer had an accessible volume of 38% for the fully desolvated structure and exhibited strong fluorescence that was selectively quenched by Cr^VI ions in aqueous solution at low concentrations.