Three Polymorphs of Potassium 4-Sulfonatobenzoic Acid

Crystal structures of two coordination isomers of copper(II) 4-sulfo-benzoic acid hexa-hydrate and two mixed silver/potassium 4-sulfo-benzoic acid salts

A reaction of copper(II) carbonate and potassium 4-sulfo­benzoic acid in water acidified with hydro­chloric acid yielded two crystalline products, tetra­aqua­bis­(4-carb­oxy­benzene­sulfonato)­copper(II) dihydrate and hexa­aqua­copper(II) 4-carb­oxy­benzene­sulfonate. A reaction of silver nitrate and potassium 4-sulfo­benzoic acid in water also resulted in two distinct products, an anhydrous silver potassium 4-carb­oxy­benzene­sulfonate salt and a hydrated mixed silver potassium 4-carb­oxy­benzene­sulfonate salt dihydrate.

A reaction of copper(II) carbonate and potassium 4-sulfo­benzoic acid in water acidified with hydro­chloric acid yielded two crystalline products. Tetra­aqua­bis­(4-carb­oxy­benzene­sulfonato)­copper(II) dihydrate, [Cu(O₃SC₆H₄CO₂H)₂(H₂O)₄]·2H₂O, (I), crystallizes in the triclinic space group P with the Cu²⁺ ions located on centers of inversion. Each copper ion is coordinated to four water mol­ecules in a square plane with two sulfonate O atoms in the apical positions of a Jahn–Teller-distorted octa­hedron. The carboxyl­ate group is protonated and not involved in coordination to the metal ions. The complexes pack so as to create a layered structure with alternating inorganic and organic domains. The packing is reinforced by several O—H⋯O hydrogen bonds involving coordinated and non-coordinated water mol­ecules, the carb­oxy­lic acid group and the sulfonate group. Hexa­aqua­copper(II) 4-carb­oxy­benzene­sulfonate, [Cu(H₂O)₆](O₃SC₆H₄CO₂H)₂, (II), also crystallizes in the triclinic space group P with Jahn–Teller-distorted octa­hedral copper(II) aqua complexes on the centers of inversion. As in (I), the carboxyl­ate group on the anion is protonated and the structure consists of alternating layers of inorganic cations and organic anions linked by O—H⋯O hydrogen bonds. A reaction of silver nitrate and potassium 4-sulfo­benzoic acid in water also resulted in two distinct products that have been structurally characterized. An anhydrous silver potassium 4-carb­oxy­benzene­sulfonate salt, [Ag_0.69K_0.31](O₃SC₆H₄CO₂H), (III), crystallizes in the monoclinic space group C2/c. There are two independent metal sites, one fully occupied by silver ions and the other showing a 62% K⁺/38% Ag⁺ (fixed) ratio, refined in two slightly different positions. The coordination environments of the metal ions are composed primarily of sulfonate O atoms, with some participation by the non-protonated carboxyl­ate O atoms in the disordered site. As in the copper compounds, the cations and anions cleanly segregate into alternating layers. A hydrated mixed silver potassium 4-carb­oxy­benzene­sulfonate salt dihydrate, [Ag_0.20K_0.80](O₃SC₆H₄CO₂H)·2H₂O, (IV), crystallizes in the monoclinic space group P2₁/c with the Ag⁺ and K⁺ ions sharing one unique metal site coordinated by two water mol­ecules and six sulfonate O atoms. The packing in (IV) follows the dominant motif of alternating inorganic and organic layers. The protonated carboxyl­ate groups do not inter­act with the cations directly, but do participate in hydrogen bonds with the coordinated water mol­ecules. (IV) is isostructural with pure potassium 4-sulfo­benzoic acid dihydrate.