Conformational Isomerism of 1,2-Bis(imidazol-1′-yl)ethane Affected by the Orientation of Functional Carboxylate Groups in Cadmium and Cobalt Helical Coordination Polymers

Four helical coordination polymers, [Cd(anti-bime)(1,3-pda)]_n (1), [Cd(gauche-bime)(1,4-pda)]_n (2), [Co(anti-bime)(1,2-pda)]_n (3), and [Co(gauche-bime)(1,4-pda)]_n (4), have been solvothermally prepared by the assembly of mixed flexible ligands, bime and 1,3-H₂pda, 1,4-H₂pda, 1,2-H₂pda with Cd(NO₃)₂·4H₂O and Co(NO₃)₂·6H₂O, respectively (bime = 1,2-bis(imidazol-1′-yl)ethane, 1,3-H₂pda = 1,3-phenylenediacetic acid, 1,4-H₂pda = 1,4-phenylenediacetic acid, 1,2-H₂pda = 1,2-phenylenediacetic acid). Single-crystal X-ray diffraction analyses reveal that all of the complexes possess helical structures owing to the use of mixed flexible ligands; the conformations of flexible bime are entirely different, anti-bime occurs in 1 and 3, while gauche-bime occurs in 2 and 4. The fluorescent properties of 1 and 2 suggest that the intense broad photoluminescence emission at 387 nm for 1 and 389 nm for 2, 3, and 4 exhibit similar weak antiferromagnetic coupling between the Co²⁺ centers.