Crystal and molecular structures of 1:2 crystalline salts of two isomeric double betaines with hydrobromic acid and nitric acid

Four 1:2 crystalline salts of the isomeric flexible double betaines 1,4-bis(2-picolyloxyl)benzene-N,N0-diacetate (L1) and 1,4-bis(3- picolyloxyl)benzene-N,N0-diacetate (L2) with HBr and HNO3, [H2L1Br2] 1, [H2L1(NO3)2] 2, [H2L2Br2] 3 and [H2L2(NO3)2]·2H2O 4, have been synthesized and characterized by single-crystal X-ray analysis. They are all hydrogen-bonded networks consolidated by strong intermolecular interactions involving the bromide or nitrate ion, together with p–p stacking between the betaine molecules. The isostructural bromide–nitrate pair of compounds 1 and 2 provide an interesting corollary to the well-known methyl-chloro and benzene– thiophene exchange rules in crystal engineering. q 2003 Elsevier B.V. All rights reserved.