1-Carboxymethyl-1′-carboxylatomethyl-3,3′-[p-phenylenebis(oxymethylene)]dipyridinium bromide dihydrate

In the crystal structure of the title salt, C22H21N2O6 +·Br−·2H2O, pairs of betaine molecules are bridged by protons (the bridging proton is disordered), forming strong and symmetrical O—H⋯O hydrogen bonds, leading to an infinite chain along the b axis. The water molecules are linked to the betaine molecule and the bromide ion through O—H⋯O and O—H⋯Br interactions. The central ring, located on an inversion centre, makes dihedral angles of 1.2 (2)° with the outer rings. One of the carboxylic acid groups is deprotonated.

In the crystal structure of the title salt, C 22 H 21 N 2 O 6 + Á-Br À Á2H 2 O, pairs of betaine molecules are bridged by protons (the bridging proton is disordered), forming strong and symmetrical O-HÁ Á ÁO hydrogen bonds, leading to an infinite chain along the b axis. The water molecules are linked to the betaine molecule and the bromide ion through O-HÁ Á ÁO and O-HÁ Á ÁBr interactions. The central ring, located on an inversion centre, makes dihedral angles of 1.2 (2) with the outer rings. One of the carboxylic acid groups is deprotonated.
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Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: PB2040).

Comment
The design and synthesis of substrates for the ultimate preparation of supramolecular species has received much attention in recent years. Double betaines are a class of zwitterionic compounds possessing pairs of carboxylate groups and quaternary ammonium or pyridinium moieties. The carboxylate group is basic, so betaines are good proton acceptors that easily form complexes with Bronsted acids.
The synthesis and crystal structure of 1:2 salt of 1,4-bis(3-picolyloxyl)benzene-N,N ' -diacetic acid with HBr has been reported, here we will describe the preparation and structure of the 1:1 salt.
Experimental 1,4-bis(3-Picolyloxyl)benzene (2.92 g, 10 mmol) was dissolved in methanol (30 ml) to give a light yellow solution, to which ethyl bromoacetate (3 ml, 27 mmol, Aldrich) was added. The resulting solution was refluxed for 3 days. After the methanol was removed by rotary evaporation under reduced pressure, hydrobromic acid (12.5 ml, 4.8% (w/v)) was added to the yellow residue. The mixture was refluxed for 24 h to give a yellow solution. Removal of solvent afforded a light yellow powdery product. Yield: 46%. It was re-crystallized in water to obtain suitable single crystals for X-ray analysis.

Refinement
H atoms in water molecule were located in a difference map. Other H atoms were positioned geometrically and refined using a riding model with C-H = 0.95-0.99 Å and with U iso (H) = 1.2 U eq (C).