Smith, G., Wermuth, U. D., and Healy, P. C., "Hydrogen Bonding in Proton-Transfer Compounds of 5-Sulfosalicylic Acid with Ortho-Substituted Monocyclic Heteroaromatic Lewis Bases," J. Chem. Crystallogr., 2006, 36, 841-849. Copyright 2006 Springer Hydrogen bonding in proton-transfer compounds of 5-sulfosalicylic acid with ortho- substituted monocyclic heteroaromatic Lewis bases

The crystal structures of the 1:1 proton-transfer compounds of 5- sulfosalicylic acid with the ortho-substituted monocyclic heteroaromatic Lewis bases, 2-aminopyridine, 2-hydroxypyridine and 2-aminopyrimidine, viz. 2-aminopyridinium 5-sulfosalicylate (1), 2-hydroxypyridinium 5-sulfosalicylate monohydrate (2) and 2- aminopyrimidinium 5-sulfosalicylate monohydrate (3) have been determined and their hydrogen-bonding patterns described. All compounds are monoclinic, space group P21/c, with Z=4 in cells with dimensions a=7.898(5), b=11.159(11), c=14.912(7) A, β=96.849(11)° (1);=7.260(2), b=15.292(3), c=12.615(2) A, β=102.45(5)° (2) and a=7.0430(7), b=12.1871(16), c=16.2825(12) A, β=101.364(7)° (3). All three compounds show some molecular disorder, in 1 within the cation species and with both 2 and 3, a similar rotational disorder in the anion sulfonate group. Hydrogen bonding in all three compounds together with significant cation-anion or cation-cation inter-ring π-π interactions generate three-dimensional layered polymer structures. Introduction, results and discussion The substituted salicylic acid 3-carboxy-4-hydroxybenzenesulfonic acid (5- sulfosalicylic acid, 5-SSA) is a particularly strong acid which is capable of protonating water and several hydrated structures of the acid are known, e.g. the dihydrate and dideuterate,1 - 3 the trihydrate,4 and the pentahydrate,5 in which various protonated poly-aqua oxonium species e.g. H7O3 + have been identified.4 This property is also considered to be the reason for the unusual conductivity properties of the acid and many of its compounds. The acid strength also means that Lewis bases are readily protonated by 5-SSA and because of the presence of the additional interactive carboxylic acid and phenolic substituent groups the acid has potential for structure extension through hydrogen bonding, making it a useful synthon for crystal engineering purposes.