catena-Poly[[[dichlorido(pyridin-1-ium-3-yl)arsenic(III)]-μ-chlorido] monohydrate]

The crystal structure of the title compound, {[AsCl3(C5H5N)]·H2O}n, is characterized by polymeric chains consisting of alternating arsenic and chlorine atoms running parallel to the a axis. O—H⋯Cl and N—H⋯O hydrogen bonds mediated by non-coordinating water molecules assemble these chains into a three-dimensional framework. The AsIII atom, the atoms of the pyridinium ring and the water O atom have m site symmetry and the bridging Cl atom has site symmetry 2. This is the first reported organotrichloroarsenate(III) in which arsenic adopts a ψ-octahedral fivefold coordination.

The crystal structure of the title compound, {[AsCl 3 (C 5 H 5 N)]Á-H 2 O} n , is characterized by polymeric chains consisting of alternating arsenic and chlorine atoms running parallel to the a axis. O-HÁ Á ÁCl and N-HÁ Á ÁO hydrogen bonds mediated by non-coordinating water molecules assemble these chains into a three-dimensional framework. The As III atom, the atoms of the pyridinium ring and the water O atom have m site symmetry and the bridging Cl atom has site symmetry 2. This is the first reported organotrichloroarsenate(III) in which arsenic adopts a -octahedral fivefold coordination.

catena-Poly[[[dichlorido(pyridin-1-ium-3-yl)arsenic(III)]-µ-chlorido] monohydrate] Lukas Reck and Wolfgang Schmitt Comment
In the course of our work on arylarsonate functionalized metal oxide clusters (see Breen, Clérac et al. (2012),  and references therein for vanadium oxide clusters, Onet et al. (2011) for molybdenum oxide clusters and Zhang et al. (2012) for manganese oxide clusters) we prepared the title compound, whose crystal structure had until now not been determined. In general, the structural chemistry of organohaloarsenates(III) has not been the object of intense study, the only relevant peer-reviewed publication known to us being the contribution by Grewe et al. (1998), which is summarized below. The homologous organohalobismuthates(III) and organohaloantimonates(III) have been more widely studied.
Organodichloroarsines compounds are known to act as soft Lewis acids and form adducts with chloride anions, e.g.
In the title compound, the acidity of the aryldichloroarsine is enhanced by the electron-withdrawing, positively charged pyridinio substituent, so that a ψ-octahedral structure similar to the one encountered in [RBr 2 As(µ-Br 2 )AsBr 2 R] 2or [PhCl 2 Sb(µ-Cl) 2 SbCl 2 Ph] 2becomes possible: As is coordinated axially to the organic substituent and equatorially to four chloride ligands, two bridging and two terminal. The terminal chloride ligands are located cis to one another and connected to As by short bonds, while the bridging chloride ligands are much more distant from the As centre. In contrast to in the compounds cited above, these bridging chloride ligands do not link to the same organoarsenic unit, but to two different arsenic atoms, thus forming a polymeric chain running along the crystallographic a axis (cf. Figure 1).  Figure 2). It is the lack of centrosymmetry in the hydrogen bonding pattern that accounts for the non-centrosymmetric crystal structure and space group.
The arsenic atom, the pyridinium ring and the oxygen atom of the water molecule all lie in the same mirror plane (Wyckoff position 2 b). The displacement ellipsoids on the pyridinium ring suggest a slight disorder about this mirror plane, but refining this disorder did not improve the quality of the structural model significantly. The bridging chlorine atom (Cl2) lies on a twofold axis (Wyckoff position 2a).

Experimental
The synthesis reported by Binz & von Schickh (1936) was slightly modified as follows: 3-aminopyridine (20 mmol, 1.882 g) was dissolved in conc. HCl (32%, 15 ml) and As 2 O 3 (20 mmol, 3.957 g) and CuCl (2 mmol, 0.198 g) were added to the solution. The solution was cooled in an ice bath to -5 °C. NaNO 2 (30 mmol, 2.07 g) was dissolved in water (3 ml) and slowly added to the solution while keeping the temperature below 0 °C. The reaction mixture was then stirred at 35 °C for 8 h. After cooling to r.t. the precipitate was filtered off and washed with 1-molar HCl solution (2 × 20 ml). The filtrate was cooled to -20 °C and after 7 days the product was obtained in the form of large colourless needles.

Refinement
H atoms on the pyridine aromatic ring were positioned geometrically and refined using a riding model with C-H distances constrained to 0.95 Å and the N-H distance constrained to 0.88 Å. U iso for these hydrogen atoms was constrained to U iso (H) = 1.2 U eq (C or N). Restraints were applied to the O1-H1 distance and the H1-O1-H1 angle and U iso was constrained to U iso (H1) = 1.5 U eq (O1). The anisotropic displacement parameters of the atoms in the aromatic ring indicate a slight disorder around the mirror plane, but refining this disorder did not improve the structural model significantly.

catena-Poly[[[dichlorido(pyridin-1-ium-3-yl)arsenic(III)]-µ-chlorido] monohydrate]
Crystal data [AsCl 3 (C 5  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 1.14 e Å −3 Δρ min = −1.28 e Å −3 Absolute structure: Flack (1983), 529 Friedel pairs Flack parameter: 0.006 (12) Special details Experimental. R(int) was 0.0767 before and 0.0385 after correction. The Ratio of minimum to maximum transmission is 0.7260. The λ/2 correction factor is 0.0015. 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.  (7)