Redetermination of poly[μ-chlorido-heptachlorido-μ3-l-proline-μ2-l-proline-tetramercury(II)]

The asymmetric unit of the title compound, [Hg4Cl8(C5H9NO2)2]n, consists of four HgCl2 units and two L-proline ligands in the zwitterionic form. In each HgCl2 unit, the HgII ion is strongly bonded to two Cl atoms, and the HgII ions in two of the HgCl2 units are chelated by O atoms of two l-proline ligands, with one strong and one weak Hg—O bond. In the crystal structure, HgCl2 and L-proline units are linked to form an extended chain along the a axis. The chain structure is further stabilized by N—H⋯Cl hydrogen bonds, and the chains are arranged in layers parallel to the ab plane. The structure of the title compound was originally determined by Ehsan, Malik & Haider [(1996). J. Banglad. Acad. Sci. 20, 175] but no three-dimensional coordinates are available.


S1. Comment
During the last few years, organic non-linear optical (NLO) crystals have attracted much interest due to their superior properties over inorganic NLO materials, such as higher susceptibility, faster response and the capability of designing components on the molecular level. However, unlike inorganic NLO crystals, they have not come into wide use, owing to drawbacks such as the difficulty of growing large size perfect single crystals and poor physicochemical stability. Under these circumstances, crystals of metal-organic materials with NLO effects have been developed which are expected not only to retain high NLO effects, but also to minimize some of the shortcomings of pure organic crystals; in other words, they have the advantages of both organic and inorganic crystals in terms of their physicochemical properties. This approach has resulted in their practical use in frequency-doubling of laser radiation (Long, 1995;Jiang & Fang, 1999).
The asymmetric unit consists of four HgCl 2 units and two L-proline zwitterions ( Fig.1). In each HgCl 2 units the metal atom is strongly bonded to two Cl atoms, with Hg-Cl distances in the range 2.255 (7) Å-2.337 (6) Å. These distances are comparable with those observed for ammonium mercury (II) dichloride nitrate (Nockemann & Meyer, 2002) and (Tedmann et al., 2004). Metal atoms in two HgCl 2 units (Hg2 In the crystal structure, the HgCl 2 and L-proline units are linked to form an extended chain along the a axis (Fig.2). The chain structure is further strengthened by N-H···Cl hydrogen bonds ( Table 2). The polymeric chains are arranged into layers parallel to the ab plane (Fig.3). The structure of the title compound was originally determined by Ehsan et al. (1996) but no three-dimensional coordinates are available.

S2. Experimental
The title compound was crystallized at room temperature by slow evaporation of an aqueous solution of L-proline and mercury(II) chloride in a stoichimetric ratio of 1:2.

S3. Refinement
The large anisotropic displacement parameters of atoms C3, C8 and C9 suggested disorder in five-membered rings. But attempts to refine the structure with a disorder model did not improve these parameters. Hence, during the final cycles of refinement the U ij components of atoms C3, C8 and C9 were restrained to approximate isotropic behaviour. The unresolved disorder resulted in poor precision on C-C bond lengths. H atoms were placed in idealized positions and allowed to ride on their parent atoms, with N-H = 0.90 Å and C-H = 0.97 or 0.98 Å and U iso (H) = 1.2U eq (C,N).

Figure 1
The asymmetric unit of the title compound. Displacement ellipsoids are drawn at the 30% probability level. Atom Cl3A is generated by the symmetry operation (1+ x, y, z). Dashed bonds indicate weak interactions.

sup-3
Acta Cryst. (2008). E64, m1048-m1049  Secondary atom site location: difference Fourier map Hydrogen site location: inferred from neighbouring sites H-atom parameters constrained w = 1/[σ 2 (F o 2 ) + (0.0781P) 2 + 1.0577P] where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.75 e Å −3 Δρ min = −2.51 e Å −3 Extinction correction: SHELXL97 (Sheldrick, 2008), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.0066 (5) Absolute structure: Flack (1983), 1736 Friedel pairs Absolute structure parameter: 0.057 (16) Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.