Crystal structures and hydrogen bonding in the isotypic series of hydrated alkali metal (K, Rb and Cs) complexes with 4-aminophenylarsonic acid

p-Arsanilic acid forms an isotypic set of three compounds with the alkali metals K, Rb and Cs, in which the primary layered coordination polymeric structures have similar asymmetric units comprising two independent and different metal cations and a bridging water molecule which lie within crystallographic mirror planes parallel to (100). The layers are linked across [100] through amine N—H⋯O hydrogen bonds to arsonate and water O-atom acceptors, giving overall three-dimensional network structures.

In an attempt to complete the structures of the alkali metal series of p-arsanilate salts, our reaction of the acid with potassium carbonate, rubidium carbonate and caesium carbonate in ethanol/water resulted in the formation of the crystalline hydrated salts with general formula [M + 2 (C 6 H 7 AsNO 3 ) À 2 Á3H 2 O]. Compounds (I) (M = K), (II) (Rb) and (III) (Cs) and their crystal structures are reported herein. However, suitable crystals of the Li analogue were not obtained to allow its crystal structure determination.

Structural commentary
The structures of the three title compounds [(I), (II) and (III)] form an isotypic series, with the asymmetric units in each comprising two independent and different metal complex cations (M1 and M2), which lie on crystallographic mirror planes that also contain one of the coordinating water molecules (O2W), with the hydrogen p-arsanilate ligands and the second water molecules (OW1, O1W ii ) [symmetry code: (ii) Àx + 1, Ày, z] lying across the mirror plane (Figs. 1, 2 and 3, respectively). In all three examples, the M2 cation is fivecoordinate, while with M1, the coordination spheres progress from five-coordinate in (I) to eight-coordinate in (II) and (III). The overall M-O bond length ranges are 2.694 (5)-3.009 (7) Å (K) ( Table 1), 2.818 (4)-3.246 (4) Å (Rb) ( Table 2) and 2.961 (9)-3.400 (10) Å (Cs) ( Table 2). The amine N atom is not involved in bonding to the metal, as is the case in a number of other p-arsanilate complexes, e.g. with Zn . The M1O 5 polyhedra in all three structures comprise four bridging arsonate O atoms and the 2 bridging water molecule (O2W) (Tables 1, 2 and 3). The second M2O 5 polyhedron in (I) comprises the bridging O11 and O11 ii The molecular configuration and atom numbering scheme for the complex unit in (I). The metal cations (K1 and K2) and the water molecule (O2W) lie on a mirror plane with mirror-related atoms indicated by symmetry code (ii) Àx + 1, Ày, z + 1 2 . For other codes, see Table 1. Non-H atoms are shown as displacement ellipsoids at the 40% probability level.

Figure 2
The molecular configuration and atom numbering scheme for the complex unit in the isotypic structure (II). The metal cations (Rb1 and Rb2) and the water molecule (O2W) also lie on a mirror plane. For symmetry codes, see Table 2. Non-H atoms are shown as displacement ellipsoids at the 40% probability level.

Figure 3
The molecular configuration and atom numbering scheme for the complex unit in the isotypic structure (III). The metal cations (Cs1 and Cs2) and the water molecule (O2W) also lie on a mirror plane. For symmetry codes, see Table 3. Non-H atoms are shown as displacement ellipsoids at the 40% probability level. donors, the 2 -O2W i [symmetry code: (i) Àx + 1, Ày + 2, z + 1 2 ] donor and two monodentate water molecules (O1W and O1W i ) ( Table 1).
With (II) and (III), the irregular M2O 8 coordination sphere comprises all bonds mentioned in the description of the K complex (I), and in addition, the Rb and Cs bond length expansion allows further coordination sites through additional bridging bonds to both of the water molecules (two through O1W and one through O2W), (Tables 2 and 3 In all structures, two-dimensional coordination polymeric complex structures are generated, with the layers lying in the mirror planes parallel to (100). Fig. 4 shows the basic makeup of the layer in (I) while those for (II) or (III) are shown in Fig. 5. The water molecule O2W provides hydrogen-bonding links across the mirror plane to arsonate O13 acceptors (Tables 4, 5 and 6).

Figure 4
A partial expansion of the two-dimensional coordination polymeric sheet structure of (I), which extends across the mirror plane parallel to (100). Aromatic H atoms are omitted. For symmetry codes, see Table 1.

Figure 5
A partial expansion of the two-dimensional coordination polymeric sheet structure of (II) [or (III)], which extends across the mirror plane parallel to (100). Aromatic H atoms have been omitted.
(a trihydrate) differs from the current isotypic set in having significantly different coordination spheres, also lacking the mirror symmetry of the primary polymeric layers in (I)-(III). With these, the N4 amino group acts as an acceptor to an O1W hydrogen bond. The water molecule O1W also forms a hydrogen bond with O11 vi [symmetry code: (vi) x, Ày + 2, z + 1 2 ] in (I), but not in (II) or (III). The protonated p-arsanilate O atom (O12) forms an intra-layer hydrogen bond with an O11 acceptor, giving overall three-dimensional network structures in all cases (Figs. 6 and 7). Noassociations are present in the structures.

Synthesis and crystallization
Compounds (I)-(III) were synthesized by heating together for 5 min, 1 mmol quantities of 4-aminophenylarsonic acid and 0.5 mmol of either K 2 CO 3 [for (I)], Rb 2 CO 3 [for (II)] or Cs 2 CO 3 [for (III)], in 20 ml of 50% ethanol/water (v/v). Room temperature evaporation of the solutions gave colourless crystal plates of the title compounds from which specimens were cleaved for the X-ray analyses.

(I) Poly[di-µ 3 -4-aminophenylarsonato-tri-µ 2 -aqua-dipotassium]
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.50 e Å −3 Δρ min = −0.84 e Å −3 Absolute structure: Flack (1983), 1281 Friedel pairs Absolute structure parameter: 0.03 (2) 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 esds 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 > 2sigma(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.

(II) Poly[di-µ 3 -4-aminophenylarsonato-tri-µ 2 -aqua-dirubidium]
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.003 Δρ max = 0.70 e Å −3 Δρ min = −0.46 e Å −3 Extinction correction: SHELXL97, Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.00306 (19) Absolute structure: Flack (1983), 1309 Friedel pairs Absolute structure parameter: −0.008 (12) 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 esds 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 > 2sigma(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. 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 > 2sigma(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.