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

The structures of the alkali metal (K, Rb and Cs) complex salts with 4-amino-phenyl-arsonic acid (p-arsanilic acid) manifest an isotypic series with the general formula [M2(C6H7AsNO3)2(H2O)3], with M = K {poly[di-μ3-4-amino-phenyl-arsonato-tri-μ2-aqua-dipotassium], [K2(C6H7AsNO3)2(H2O)3], (I)}, Rb {poly[di-μ3-4-amino-phenyl-arsonato-tri-μ2-aqua-dirubidium], [Rb2(C6H7AsNO3)2(H2O)3], (II)}, and Cs {poly[di-μ3-4-amino-phenyl-arsonato-tri-μ2-aqua-dirubidium], [Cs2(C6H7AsNO3)2(H2O)3], (III)}, in which the repeating structural units lie across crystallographic mirror planes containing two independent and different metal cations and a bridging water mol-ecule, with the two hydrogen p-arsanilate ligands and the second water mol-ecule lying outside the mirror plane. The bonding about the two metal cations in all complexes is similar, one five-coordinate, the other progressing from five-coordinate in (I) to eight-coordinate in both (II) and (III), with overall M-O bond-length ranges of 2.694 (5)-3.009 (7) (K), 2.818 (4)-3.246 (4) (Rb) and 2.961 (9)-3.400 (10) Å (Cs). The additional three bonds in (II) and (III) are the result of inter-metal bridging through the water ligands. Two-dimensional coordination polymeric structures with the layers lying parallel to (100) are generated through a number of bridging bonds involving the water mol-ecules (including hydrogen-bonding inter-actions), as well as through the arsanilate O atoms. These 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.