Pradhumna Mahat Chhetri

Hg(II) Coordination Polymers Based on N,N'-bis(pyridine-4-yl)formamidine

Reactions of N,N’-bis(pyridine-4-yl)formamidine (4-Hpyf) with HgX2 (X = Cl, Br, and I) afforded the formamidinate complex {[Hg(4-pyf)2]·(THF)}n, 1, and the formamidine complexes {[HgX2(4-Hpyf)]·(MeCN)}n (X = Br, 2; I, 3), which have been structurally characterized by X-ray crystallography. Complex 1 is a 2D layer with the {44·62}-sql topology and complexes 2 and 3 are helical chains. While the helical chains of 2 are linked through N–H···Br hydrogen bonds, those of 3 are linked through self-complementary double N–H···N hydrogen bonds, resulting in 2D supramolecular structures. The 4-pyf- ligands of 1 coordinate to the Hg(II) ions through one pyridyl and one adjacent amine nitrogen atoms and the 4-Hpyf ligands of 2 and 3 coordinate to the Hg(II) ions through two pyridyl nitrogen atoms, resulting in new bidentate binding modes. Complexes 1–3 provide a unique opportunity to envisage the effect of the halide anions of the starting Hg(II) salts on folding and unfolding the Hg(II) coordination polymers. Density function theory (DFT) calculation indicates that the emission of 1 is due to intraligand π→π * charge transfer between two different 4-pyf- ligands, whereas those of 2 and 3 can be ascribed to the charge transfer from non-bonding p-type orbitals of the halide anions to π * orbitals of the 4-pyf- ligands (n→π *). The gas sorption properties of the desolvated product of 1 are compared with the Cu analogues to show that the nature of the counteranion and the solvent-accessible volume are important in determining their adsorption capability.

Mercury halide coordination polymers exhibiting reversible structural transformation

Solvothermal reactions of HgX2 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)X2]n (X = Cl, 1; Br, 2; I, 3), while the complexes [Hg(1,3-pbpa)X2·MeCN]n (X = Br, 4; I, 5) were obtained by layering solutions of HgX2 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 A, 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 A, 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.