catena-Poly[[chloridomercury(II)]-μ-1,4-diazabicyclo[2.2.2]octane-κ2 N:N′-[chloridomercury(II)]-di-μ-chlorido]

In the title coordination polymer, [Hg2Cl4(C6H12N2)]n, each HgII center within the chain is four-coordinated by one terminal Cl atom, two bridging μ2-Cl atoms, and one N-atom donor from a μ2-1,4-diazabicyclo[2.2.2]octane (μ2-daco) ligand in a distorted tetrahedral geometry. The daco ligand acts as an end-to-end bridging ligand and bridges adjacent HgII centers, forming a chain running along [001]. Weak C—H⋯Cl hydrogen-bonding interactions link the chains into a three-dimensional network. Comparison of the structural differences with previous findings suggests that the space between the two N donors, as well as the skeletal rigidity in N-heterocyclic linear ligands, may play an important role in the construction of such supramolecular networks.

In the title coordination polymer, [Hg 2 Cl 4 (C 6 H 12 N 2 )] n , each Hg II center within the chain is four-coordinated by one terminal Cl atom, two bridging 2 -Cl atoms, and one N-atom donor from a 2 -1,4-diazabicyclo[2.2.2]octane ( 2 -daco) ligand in a distorted tetrahedral geometry. The daco ligand acts as an end-to-end bridging ligand and bridges adjacent Hg II centers, forming a chain running along [001]. Weak C-HÁ Á ÁCl hydrogen-bonding interactions link the chains into a three-dimensional network. Comparison of the structural differences with previous findings suggests that the space between the two N donors, as well as the skeletal rigidity in Nheterocyclic linear ligands, may play an important role in the construction of such supramolecular networks.

Comment
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).
In comparison with 4bpy and pyz ligands (Fig. 3), however, 1,4-diazabicyclo[2.2.2]octane (daco) acts as a flexible Ncontaining 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., 2006Rao & 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, [HgCl 2 (4bpy)] n (A) and [HgCl 2 (pyz)] n (B), respectively (Fig. 4). In these two structures, the Hg atoms are both in octahedral coordination environments, coordinated by four µ 2 -Cl atoms and two µ 2 -4bpy for A as well as four µ 2 -Cl atoms and two µ 2 -pyz for B in trans positions. The relevant one-dimensional chain motifs bridged by µ 2 -4bpy or µ 2 -pyz in complexes A and B are further interlinked to form the two-dimensional layers via the µ 2 -Cl atoms. In this contribution, however, when we used HgCl 2 to react with 1,4-diazabicyclo[2.2.2] octane (daco) under a conventional solution diffusion condition, the title one-dimensional coordination polymer, [Hg 2 Cl 4 (daco)] n , was produced.
The Hg II center is four-coordinated, by three Cl atoms (one terminal Cl and two µ 2 -Cl atoms) and one N-atom donor from one daco ligand, with a distorted tetrahedral geometry. Two µ 2 -Cl atoms bridge adjacent Hg II centers to form one supplementary materials sup-2 planar four-membered rings composed of Hg1-Cl1-Hg1 i -Cl1 ii with a non-bonding Hg(1)···Hg(1 i ) separation of 4.067 (1) Å [symmetry codes (i) = x, -y, 1-z; (ii) = 1-x, -y, 1-z]. Each daco ligand takes a µ 2 -bridging coordination mode to connect the adjacent Hg II centers, 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 [Hg 2 Cl 4 (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. HgI 2 -daco (Pickardt et al., 1995). This complex, [HgI 2 (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.

Refinement
H atoms were included in calculated positions and treated as riding atoms: C-H = 0.97 Å, with U iso (H) = 1.2 U eq (C).

Figures
Fig . 1. The one-dimensional molecular structure of the title complex, propagating in the [001] direction. Displacement ellipsoids are drawn at the 30% probability level. The atoms labeled with the suffixes A and B are generated by the symmetry operations (x, -y, -z + 1; -x + 1, -y, -z + 1), respectively.