supplementary materials

The rational engineering and controlled preparation of novel coordination complexes are currently of great interest in coordination and supramolecular chemistry not only because of their intriguing structural diversities but also their potential applications as functional materials (Chen, Kang & Su, 2006; Fang et al., 2009; Liu et al., 2007; Ma et al., 2009; Tranchemontagne et al., 2009; Uemura et al., 2009; Xue et al., 2008). One of the most successful strategies for constructing such complexes has been the assembly reaction of different metal ions (as nodes) with well designed organic ligands (as building blocks), which, so far, has been at an evolutionary stage with the current focus mainly on understanding the factors to determine the crystal packing as well as exploring relevant potential properties (Kitagawa et al., 2004). N-containing heterocyclic ligands are extensively used as bridging ligands in coordination and metallosupramolecular chemistry (Batten et al., 2002; Chen et al., 2007; Culp et al., 2008; Kaim, 1983; Leininger et al., 2000; Richardson & Steel, 2003; Steel, 2005). For examples, the use of 4,4'-bipyridine (4bpy) and pyrazine (pyz) has resulted in a large number of extended assemblies including helical networks as well as diamondoid, honeycomb, square-grid, ribbon, grid, T-shaped, Ladder, brick wall, and octahedral frameworks (Arpi et al., 2006; Chen, Wang et al., 2006; Choi et al., 2009; Derossi et al., 2007; Du et al., 2007; Liu et al., 2006; Li et al., 2008; Ramírez et al., 2009; Qiu et al., 2008).

In comparison with 4bpy and pyz ligands (Fig. 3), however, 1,4-diazabicyclo[2.2.2]octane (daco) acts as a flexible N-containing heterocyclic bridging ligand with a skeletal separation of ca6 Å between two N donors, shorter than that of ca 7 and 11 Å for 4bpy and pyz, respectively (Dybtsev et al., 2004, Li et al., 2006; Rao & Rao, 2007; Steel, 2005). As part of a study on the effect of the space between two N donors or the skeletal rigidity in linear N-containing heterocyclic ligands on the self-assembly of coordination complexes, we have chosen to use 1,4-diazabicyclo[2.2.2]octane (daco) instead of 4bpy or pyz to construct the title compound, a new one-dimensional Hg^II coordination polymer.

To explore the coordination possibility of a series of N-containing heterocyclic linear ligands, Xie & Wu (2007) as well as Nockemann & Meyer (2004) have selected 4,4'-bipyridine (4bpy) and pyrazine (pyz) to react with Hg^II atom, obtaining two different two-dimensional neutral networks, [HgCl₂(4bpy)]_n (A) and [HgCl₂(pyz)]_n (B), respectively (Fig. 4). In these two structures, the Hg atoms are both in octahedral coordination environments, coordinated by four µ₂-Cl atoms and two µ₂-4bpy for A as well as four µ₂-Cl atoms and two µ₂-pyz for B in trans positions. The relevant one-dimensional chain motifs bridged by µ₂-4bpy or µ₂-pyz in complexes A and B are further interlinked to form the two-dimensional layers via the µ₂-Cl atoms. In this contribution, however, when we used HgCl₂ to react with 1,4-diazabicyclo[2.2.2] octane (daco) under a conventional solution diffusion condition, the title one-dimensional coordination polymer, [Hg₂Cl₄(daco)]_n, was produced.

The crystal structure of the title compound consists of one-dimensional (1D) neutral [Hg₂Cl₄(daco)]_n chains (Fig. 1). The Hg^II center is four-coordinated, by three Cl atoms (one terminal Cl and two µ₂-Cl atoms) and one N-atom donor from one daco ligand, with a distorted tetrahedral geometry. Two µ₂-Cl atoms bridge adjacent Hg^II centers to form one planar four-membered rings composed of Hg1–Cl1–Hg1ⁱ–Cl1ⁱⁱ with a non-bonding Hg(1)···Hg(1ⁱ) separation of 4.067 (1) Å [symmetry codes (i) = x, –y, 1–z; (ii) = 1–x, –y, 1–z]. Each daco ligand takes a µ₂-bridging coordination mode to connect the adjacent Hg^IIcenters, generating a one-dimensional chain running along the [001] direction. The Hg–N and Hg–Cl bond distances as well as the bond angles around each Hg^II center are within the expected range for such complexes (Orpen et al., 1989; Wang et al., 2007). Moreover, adjacent 1D [Hg₂Cl₄(daco)]_n chains are arranged into two-dimensional layers, running parallel to the (100) plane, by interchain C–H···Cl hydrogen-bonding interactions between the daco ligands and the terminal Cl atoms (Fig. 2 and Table 1).

Thus, in comparison with the previous findings, the present work reveals that the space between two N donors, or the skeletal rigidity of N-containing heterocyclic linear ligands, could play an important role in the final structure of the relate coordination complexes. This fact may offer the means to construct new coordination architectures with potentially useful properties by varying of the space between the two N donors, or the rigidity of skeleton of N-containing heterocyclic ligands. It is noteworthy that mercury(II) metal halide-daco materials are relatively rare, and to the best of our knowledge, only one halide-daco complex has been reported previously, viz. HgI₂-daco (Pickardt et al., 1995). This complex, [HgI₂(daco)]_n, also exhibits a zigzag one-dimensional structure, but here the I-atoms are all terminally coordinated to the metal centers, while in the title complex one of the Cl atoms act as a bridging atom.