Mercury halide coordination polymers exhibiting reversible structural transformation

Solvothermal reactions of HgX 2 salts with the new bis-pyridyl-bis-amide ligand 2,2′-(1,3-phenylene)-bis( N -(pyridin-3-yl)acetamide) (1,3-pbpa) in acetonitrile afforded the complexes [Hg(1,3-pbpa)X 2 ] n (X = Cl, 1 ; Br, 2 ; I, 3 ), while the complexes [Hg(1,3-pbpa)X 2 ·MeCN] n (X = Br, 4 ; I, 5 ) were obtained by layering solutions of HgX 2 and 1,3-pbpa at room temperature. Complexes 1 and 2 are isostructural one-dimensional (1D) helical chains different from the 1D helical chain 3 in spans, which are 18.0, 18.3 and 29.1 Å, respectively. Upon solvent adoption and removal, complexes 2 and 3 undergo reversible structural transformation into the 1D mesohelical chains 4 and 5 with spans of 29.1 and 29.9 Å, respectively. Pyridyl ring rotation and amide group reorientation are proposed for the structural transformation accompanied by a simultaneous change in luminescence. The structural transformation in 2 and 4 represents a unique example of elastic 1D helical chains that show stretching during the process. The roles of halide anions in the structural changes and the luminescence properties of 1–5 are also discussed.