Crystal structure resolution of two different chlorhexidine salts

Two salts of the chlorhexidine di-cation (H2CHx2+) – (H2CHx)(SO4)·3H2O and (H2CHx)(CO3)·4H2O – have been synthesised and characterised crystallographically.


Introduction
Chlorhexidine (I, CHx) is a chemical disinfectant and antiseptic with a broad spectrum of action; it is active against Gram positive and Gram negative bacteria, as well as fungi [1]. It is a symmetrical bisbiguanidine, which is a class of chemically related compounds studied for their bactericidal properties.
[1], [2] In the last 60 years chlorhexidine has been used as an antiseptic for mucous membranes, skin and wounds, or as a preservative in pharmaceutical formulations of ophthalmic products [2]. Due to its low solubility and ability to form micelles in solution [3], chlorhexidine does not crystallize easily, however, three salts of the chlorhexidine dication with anionic calixarenes were characterised crystallographically and reported in 2008 [4].
Herein we report the crystallographic characterisation of two salts of the chlorhexidine di-cation, namely (H 2 CHx)(SO 4  Crystals were prepared by reaction of neutral chlorhexidine and sodium sulfate or potassium carbonate in aqueous ethanol, as part of a series of attempts to generate chlorhexidine-containing coordination complexes. Full crystallographic details of both salts are presented in Table 1. Structurally, the two salts are very similar to one another (Fig. 1), with small variations in the conformation of the hexyl chains and intermolecular hydrogen-bonding due to the different arrangement of oxygen atoms between the tetrahedral SO 4 2À and the trigonal planar CO 3 2À anions, and different protonation sites along the chlorhexidine moiety. The differences in protonation site may be seen by a comparison of Fig. 2a  intra-chain hydrogen bonds with two non-adjacent chlorhexidine cations that both have the same conformation (either left-or righthanded). When viewed along the a-axis, the carbonate anions sit almost aligned with the two sides of the U-shape, and so are only able to interact with the chlorhexidine cations along one side of the U-shape. This generates a two-dimensional hydrogen-bonded framework of CO 3 2À anions and H 2 CHx 2þ cations that extends parallel to the ac-plane (Fig. 5). Water molecules of crystallisation occupy the remaining space between the coils and form hydrogen bonds with both the oxyanions and the chlorhexidine cations so that both compounds contain a complex three-dimensional hydrogen-bonded network. A list of the hydrogen bonds found in both compounds presented in Supporting Information.

Conclusions
The carbonate and sulfate salts of the chlorhexidine di-cation have been synthesised and characterised crystallographically. Investigations showed that the two salts are structurally similar to each other, although differences in the conformations of the hexyl

Experimental
All reagents for synthesis were purchased from Sigma-Aldrich, Fluka, and ABCR and were used without further purification.

Crystallography
Crystals were coated with a protective oil and mounted on a loop crystal mount. Crystallographic data were collected on a Rigaku Cu MM007 HF (dual port) high brilliance generator using a Dectris Pilatus P100 detector and Oxford DTC LT system. Absorption corrections were applied using multi-scan methods [5]. Structure solutions were obtained using SHELXS-97 and refined by full matrix on F 2 using SHELXL-97 [6] and SHELXL-2014 [7] as part of the WinGX suite [8]. All full occupancy non-hydrogen atoms were refined with anisotropic thermal displacement parameters. Aromatic and aliphatic hydrogen atoms were included at their geometrically estimated positions. Hydrogen atoms belonging to full occupancy water molecules of crystallisation were assigned, fixed at a distance of 0.9 Å from the oxygen atom and 1.47 Å from the second hydrogen atom of the water molecule, and their thermal parameters linked to those of the oxygen atom to which they are bound. Similarly, the amine and imine hydrogen atoms were assigned, fixed at a distance of 0.88 Å from the nitrogen atom and (where appropriate) 1.52 Å from the second hydrogen atom of the protonated imine group, and their thermal parameters linked to the nitrogen atom to which they are bound. Crystallographic data are presented in Table 1.