Solvothermal reactions of HgX2 with 2,2′-(1,2-phenylene)-bis­(N-pyridin-3-yl)­acetamide, L, in ethanol afforded [Hg­(L)­X2] n (X = Cl, 1; Br, 2; I, 3), which are isostructural one-dimensional (1D) zigzag chains, while layering reactions of a ethanoic solution of L with a methanoic solution and an acetonitrile solution of HgI2, respectively, gave 1D helical chains [Hg­(L)­I2·MeOH] n , 4, and [Hg­(L)­I2·MeCN] n , 5. In marked contrast to 1 and 2, the iodide-containing 3 is able to exhibit reversible structural transformation with 4 and 5 by adsorption and desorption of methanol and acetonitrile, suggesting the importance of N–H---X and Hg---X interactions in the evaluation of structural transformation. Moreover, complexes 4 and 5 exhibit reversible crystal to crystal transformation triggered by solvent exchange. Complexes 3–5 represent a unique example that the solvents show a significant effect on folding and unfolding of the HgI2 single-stranded helical coordination polymers.

The structural transformation in 1D helical and mesohelical Hg(ii) chains which demonstrate elastic stretching during the process is investigated.

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Reactions of N,N'-bis(pyridine-4-yl)formamidine (4-Hpyf) with HgX 2 (X = Cl, Br, and I) afforded the formamidinate complex {[Hg(4-pyf) 2 ]¨(THF)} n , 1, and the formamidine complexes {[HgX 2 (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 {4 4¨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 πÑπ * 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 (nÑπ *). 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.

Four coordination polymers constructed from the flexible isomeric bis-pyridyl-bis-amide (bpba) ligands, N,N′-di­(3-pyridyl)­suberoamide (L1 ) and N,N′-di­(4-pyridyl)­suberoamide (L2 ), and the auxiliary 1,4-naphthalenedicarboxylic acid (1,4-H2NDC), including [Co­(L1 )1.5(1,4-NDC)­(H2O)] n , 1, [Co3(L1 )1.5(1,4-NDC)3(EtOH)] n , 2, {[Co­(L2 )1.5(1,4-NDC)]­·H2O} n , 3, and {[Co­(L2 )0.5(1,4-NDC)]­·EtOH} n , 4, have been synthesized and structurally characterized by using single crystal X-ray crystallography. Complex 1 forms a two-dimensional layer with double edges and 2 exhibits a unique three-dimensional (3D) self-catenated framework with the (48.66.8)-6T60 topology, while 3 displays a 3D framework with the highest 5-fold interpenetration for the bnn topology and 4 shows a 3D 2-fold interpenetrated framework with the pcu topology. The donor atom positions of the L1 and L2 ligands and the identity of the solvents play important roles in determining the structural diversity. Moreover, irreversible structural transformation from 2 to 1 has been verified by immersing crystals of 2 into water. The coordination ability of the CH3CH2OH molecule in 2 and the thermal stability of 1 govern the irreversible structural transformation.