The coordination complex structures and hydrogen bonding in the three-dimensional alkaline earth metal salts (Mg, Ca, Sr and Ba) of (4-aminophenyl) arsonic acid

Abstract

(4-Amino­phen­yl)arsonic acid (p-arsanilic acid) is used as an anti­helminth in veterinary applications and was earlier used in the monosodium salt dihydrate form as the anti­syphilitic drug atoxyl. Examples of complexes with this acid are rare. The structures of the alkaline earth metal (Mg, Ca, Sr and Ba) complexes with (4-amino­phen­yl)arsonic acid (p-arsanilic acid) have been determined, viz. hexa­aqua­magnesium bis­[hydrogen (4-amino­phen­yl)arsonate] tetra­hydrate, [Mg(H2O)6](C6H7AsNO3)·4H2O, (I), catena-poly[[[di­aqua­calcium]-bis­[μ2-hydrogen (4-amino­phen­yl)arsonato-κ2O:O′]-[di­aqua­calcium]-bis­[μ2-hydrogen (4-amino­phen­yl)arsonato-κ2O:O]] dihydrate], {[Ca(C6H7AsNO3)2(H2O)2]·2H2O}n, (II), catena-poly[[tri­aqua­strontium]-bis­[μ2-hydrogen (4-amino­phen­yl)arsonato-κ2O:O′]], [Sr(C6H7AsNO3)2(H2O)3]n, (III), and catena-poly[[tri­aqua­barium]-bis­[μ2-hy­drogen (4-amino­phen­yl)arsonato-κ2O:O′]], [Ba(C6H7AsNO3)2(H2O)3]n, (IV). In the structure of magnesium salt (I), the centrosymmetric octa­hedral [Mg(H2O)6]2+ cation, the two hydrogen p-arsanilate anions and the four water mol­ecules of solvation form a three-dimensional network structure through inter-species O—H and N—H hydrogen-bonding inter­actions with water and arsonate O-atom and amine N-atom acceptors. In one-dimensional coordination polymer (II), the distorted octa­hedral CaO6 coordination polyhedron comprises two trans-related water mol­ecules and four arsonate O-atom donors from bridging hydrogen arsanilate ligands. One bridging extension is four-membered via a single O atom and the other is eight-membered via O:O′-bridging, both across inversion centres, giving a chain coordination polymer extending along the [100] direction. Extensive hydrogen-bonding involving O—H...O, O—H...N and N—H...O inter­actions gives an overall three-dimensional structure. The structures of the polymeric Sr and Ba complexes (III) and (IV), respectively, are isotypic and are based on irregular MO7 coordination polyhedra about the M2+ centres, which lie on twofold rotation axes along with one of the coordinated water mol­ecules. The coordination centres are linked through inversion-related arsonate O:O′-bridges, giving eight-membered ring motifs and forming coordination polymeric chains extending along the [100] direction. Inter-chain N—H...O and O—H...O hydrogen-bonding inter­actions extend the structures into three dimensions and the crystal packing includes π–π ring inter­actions [minimum ring centroid separations = 3.4666 (17) Å for (III) and 3.4855 (8) Å for (IV)].