Crystal structure of zwitterionic 3,3′-[1,1′-(butane-1,4-diyl)bis(1H-imidazol-3-ium-3,1-diyl)]bis(propane-1-sulfonate) dihydrate

A known N-heterocyclic carbene precursor, zwitterionic 3,3′-[1,1′-(butane-1,4-diyl)bis(1H-imidazol-3-ium-3,1-diyl)]bis(propane-1-sulfonate) crystallized as its dihydrate and was characterized by single-crystal X-ray diffraction, revealing point group symmetry for the zwitterionic molecule.


Chemical context
Imidazolium salt-based ionic liquids are versatile because of their unique properties and their use as green solvents, replacing volatile or toxic organic solvents (De et al., 2019). Moreover, they are very often used as reaction media, or -the water-immiscible ones -for extraction. X-ray crystallographic studies of several crystalline imidazolium salts have been described. However, examples of zwitterionic imidazolium salts are limited in the literature, and only a few examples of zwitterionic imidazolium sulfonates, with their crystal structures determined, have been reported to date. The introduction of hydrophilic substituents (e.g. sulfonate groups) made possible the synthesis of water-soluble metal complexes, and subsequently, a range of catalytic applications (Kohmoto et al., 2012).

Structural commentary
The di-N-heterocyclic carbene precursor 1 crystallizes as a dihydrate, with one half of the molecule and one water molecule of crystallization being present in the asymmetric unit. The other half of the molecule is generated by the application of inversion symmetry (symmentry operation: 1 À x, Ày, Àz). No molecules of the solvent, DMF, from which the crystals were obtained, are built into the lattice.

Supramolecular features
The water molecules bridge adjacent zwitterionic molecules through hydrogen bonds of medium strength with sulfonate O atoms as acceptor groups into ribbons aligned parallel to [001] 1354 Udvardy et al. The molecular structure of 1 showing the atom labelling and displacement ellipsoids drawn at the 50% probability level. The asymmetric unit of 1 is given in darker colours, and the symmetry-generated part (symmetry code: 1 À x, Ày, Àz) of the molecule is given in lighter colours. The water molecule is also shown.

Synthesis and crystallization
Compound 1 was synthesized according to the method of Papini et al. (2009). 4.4 mmol of 1,3-propanesultone were slowly added to a solution of 2 mmol of 1,1 0 -(butane-1,4-diyl)di-1H-imidazole in 30 ml of acetone at 273 K. Then the mixture was left to warm to room temperature and stirred for 5 d. The solvent was evaporated and the resulting white solid was recrystallized from methanol affording 1 as a white powder (yield: 617 mg, 71%). Analytical data: 13  . For recrystallization, 1 was suspended in DMF and heated to approximately 373 K, then filtered and left overnight to slowly cool down to room temperature. Single crystals, suitable for X-ray analysis, were obtained as colourless prisms after storing the solution in open glass vials in a refrigerator at 278 K. A possible source of water is the employed DMF, which is hygroscopic and easily adsorbs water from a humid atmosphere. The same type of prismatic crystals were also grown from hot water, revealing a very similar unit cell. However, these crystals were of poor quality, and the best R int value was very high, 0.19.

Refinement
Crystal data, and details of data collection and structure refinement are summarized in Table 2. Hydrogen atoms of the zwitterionic molecules were placed at idealized positions and refined using a riding model. The positions of hydrogen atoms of the water molecule were discernible in a difference-Fourier map. They were refined with a fixed bond length of 0.85 Å and U iso (H) = 1.5U eq (O).

3,3′-[1,1′-(Butane-1,4-diyl)bis(1H-imidazol-3-ium-3,1-diyl)]bis(propane-1-sulfonate) dihydrate
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.