Guanidinium phenylarsonate–guanidine–water (1/1/2)

In the structure of the title compound, CH6N3 +·C6H6AsO3 −·CH5N3·2H2O, the phenylarsonate anion participates in two R 2 2(8) cyclic hydrogen-bonding interactions, one with a guanidinium cation, the other with a guanidine molecule. The anions are also bridged by the water molecules, one of which completes a cyclic R 5 3(9) hydrogen-bonding association with the guanidinum cation, conjoint with one of the three R 2 2(8) associations about that ion, as well as forming an R 2 1(6) cyclic association with the guanidine molecule. The result is a three-dimensional framework structure.

In the structure of the title compound, CH 6 N 3 + ÁC 6 H 6 -AsO 3 À ÁCH 5 N 3 Á2H 2 O, the phenylarsonate anion participates in two R 2 2 (8) cyclic hydrogen-bonding interactions, one with a guanidinium cation, the other with a guanidine molecule. The anions are also bridged by the water molecules, one of which completes a cyclic R 5 3 (9) hydrogen-bonding association with the guanidinum cation, conjoint with one of the three R 2 2 (8) associations about that ion, as well as forming an R 2 1 (6) cyclic association with the guanidine molecule. The result is a threedimensional framework structure.

Comment
The guanidinium cation has the capacity to form extended hydrogen-bonded structures through its six trigonally disposed H-donor sites. This ability is best illustrated in the host-guest clathrate structures with 4,4'-biphenyldisulfonate  or the supramolecular rosette ribbons with HCO 3 and terephthalic acid (Mak & Xue, 2000). The hydrogen-bonded structures found in the guanidinium salts of carboxylic acids are largely three-dimensional and usually feature cyclic associations involving either two N-H···O carboxyl links [graph set R 2 2 (8) (Etter et al., 1990)] . Some examples of the structures of the guanidinium salts of monocyclic aromatic acids are those with pyromellitic acid (Sun et al., 2002), 3,5-dinitrosalicylic acid (Smith et al., 2001) and phenylacetic acid (Smith & Wermuth, 2010). This last compound has both types of cation-anion interaction but shows an unusual one-dimensional columnar structure. The structure of the guanidinium salt of phenylarsonic acid [benzenearsonic acid (O'Neil, 2001)] has not been previously reported.
Our 2:1 stoichiometric reaction of phenylarsonic acid with guanidinium carbonate aqueous propan-2-ol gave large, highquality crystals of the title compound, the adduct hydrate CH 6 N 3 + C 6 H 6 AsO 3 -. CH 5 N 3 . 2H 2 O (I), the structure of which is reported here.
In (I) the phenylarsonate anion gives two R 2 2 (8) cyclic hydrogen-bonding interactions, one with a guanidinium cation (A), the other with a guanidine molecule (B), in which the second donor H atom is provided by the arsonate O-H group (Fig. 1).
The anions are also bridged by the linked water molecules, one of which (O2W) completes a cyclic R 5 3 (9) hydrogen-bonding association with a guanidinum cation ( Fig. 2) (Table 1). This ring is conjoint with one of the three R 2 2 (8) associations about the cation, whereas with the guanidine molecule there is one R 2 2 (8) and one R 1 2 (6) association, also with O2W. The overall result is a three-dimensional framework structure (Fig. 3).
It is notable that the As environment has a total of eight As-H contacts both inter-and intra-molecular with a range of 2.95 (3)-3.15 (3) Å. Also, one of the H atoms of the guanidine cation (H12B) has no feasibly situated acceptor atom.

Experimental
The title compound was synthesized by heating together under reflux for 10 minutes, 1 mmol of phenylarsonic acid (benzenearsonic acid) and 0.5 mmol of guanidine carbonate in 50% aqueous propan-2-ol. After concentration to ca 30 ml, room temperature evaporation of the hot-filtered solution to moist dryness gave colourless plates of (I) (m.p. 505 K), from which a specimen suitable for X-ray analysis was cleaved.
supplementary materials sup-2 Refinement Hydrogen atoms involved in hydrogen-bonding interactions were located by difference methods and their positional and isotropic displacement parameters were refined. The aromatic H atoms were included in the refinement in calculated positions (C-H = 0.93 Å) and treated as riding, with U iso (H) = 1.2U eq (C). Fig. 1. Molecular configuration and atom naming scheme for the guanidinium cation (A and the guanidine molecule B), the phenylarsonate anion and the two water molecules of solvation in (I). Inter-species hydrogen bonds are shown as dashed lines. Displacement ellipsoids are drawn at the 40% probability level. showing hydrogen-bonding associations as dashed lines. For symmetry codes, see Table 1.

Special details
Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles 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 Rfactors(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Geometric parameters (Å, °)