Crystal structure of hexaaquanickel(II) bis{2-[(5,6-dihydroxy-3-sulfonatoquinolin-1-ium-7-yl)oxy]acetate} dihydrate

In the packing of the title compound, Ni(H2O)6 is acting as a glue between neighbouring zwitterionic quinoline derivatives which are not directly complexing with NiII.


Chemical context
Quinoline and its derivatives have been of great interest due to their interesting biochemical activities. Quinine, cinchonine, chloroquine, plasmoquine and acriquine, for instance, are known to be able to cure malaria (Foley & Tilley, 1998;Długosz & Duś, 1996;Nayyar et al., 2006). Complexes of quinoline-containing organic compounds with transition metals are also known for their wide variety of structures and profound biochemical activities which allow them to act as antibacterial and anti-Alzheimer agents (Deraeve et al., 2008) and as cures for many types of cancers such as cervical cancer, lung cancer and breast cancer (Yan et al., 2012;Daniel et al., 2004). These complexes, therefore, have been synthesized and investigated intensively (Kitanovic et al., 2014). by IR or 1 H NMR spectroscopic methods. The spectroscopic data are different from those obtained for free QOH and in favour of a deprotonated carboxylic acid group, but give no indication about a possible complex formation. X-ray diffraction now shows that QOH is not complexing directly with Ni II .

Structural commentary
The structure determination shows that the carboxyl group of QOH is deprotonated and the anion is present in its zwitterionic form (Fig. 1), which was also observed for Q (Dinh et al., 2012). The best plane through the quinoline ring (r.m.s. deviation = 0.009 Å ) makes an angle of 15.29 (19) with the carboxylate plane. The sulfonate group at the 3-position occurs in two orientations with occupancy factors of 0.655 (5) and 0.345 (5). QOH, however, is not acting as a ligand for Ni II , which occurs as a hexaaqua complex. This [Ni(H 2 O) 6 ] 2+ is located about an inversion center and has an octahedral volume of 11.629 Å 3 with Ni-O bond lengths between 2.034 (3) and 2.106 (2) Å .

Supramolecular features
The hexaaquanickel(II) cation plays the role of glue in the crystal packing. In total, it interacts with eight QOH moieties and two water molecules through O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonding (Table 1, Fig. 2).

Database survey
A search of the Cambridge Structural Database (Version 5.36; last update May 2015; Groom & Allen, 2014) for quinoline derivatives gives 3040 hits of which 529 are protonated at the nitrogen atom. Searching for quinoline derivatives bearing a sulfonate group results in 30 hits for substitution at the 5position, 3 hits at the 8-position, 2 hits at the 7-position and two structures have a sulfonate group at the 3-position [CSD refcodes BAPBOK (Skrzypek & Suwinska, 2002) and HIVHUQ (Skrzypek & Suwinska, 2007)]. As for the title compound, these two structures occur in the zwitterionic form, but do not show disorder in the sulfonate group.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms H2B, H3B, H4B, H14, H29A and H29B were located in difference Fourier maps. All other H atoms were placed at idealized positions and refined in riding mode, with C-H distances of 0.95 (aromatic) and 0.99 Å (methylene), and O-H distances of 0.84 Å . The H atoms of water molecule O29 were refined with an O-H distance restraint of 0.85 Å and HÁ Á ÁH distance restraint of 1.39 Å . For all H atoms, U iso (H) values were assigned as 1.2U eq of the parent atoms (1.5U eq for H22 and H23). The SO 3 group is disordered over two positions, the occupancy ratio refines to 0.655 (5):0.345 (5) for part 1 (O6, O7, 08) and part 2 (O9, O10, O11), respectively.  SHELXL (Sheldrick, 2008); molecular graphics: Olex2 (Dolomanov et al., 2009); software used to prepare material for publication: Olex2 (Dolomanov et al., 2009).