1,10-Phenanthrolin-1-ium hydrogen (S,S)-tartrate trihydrate and a correction

The title structure, C12H9N2 +·C4H5O6 −·3H2O, shows that one of the protons of d-tartaric acid has been transferred to 1,10-phenanthroline. The d-hydrogen tartrate anions are joined together in a head-to-tail fashion via a short hydrogen bond with donor–acceptor distance of 2.4554 (12) Å, unsymmetrical O—H distances of 1.01 (4) Å and 1.45 (4) Å, and a 174 (4)° O—H—O bond angle. The phenanthrolinium rings are π-stacked with an average separation of 3.58 (11) Å. The structural report corrects a previous report in the literature [Wang et al. (2006 ▶). Acta Cryst. E62, o2508–o2509] of the isostructural l-hydrogen tartrate enantiomer in which the proton transfer and short hydrogen bond were missed.

MMO thanks the Department of Chemistry, University of California, Davis, for the purchase of the X-ray diffractometer.

Comment
Tartaric acid is a colorless, diprotic organic acid that occurs naturally in many plants, particularly grapes, bananas, and tamarinds, and is one of the main acids found in wine. It is added to other foods to give a sour taste, and is used as an antioxidant.
Many proton transfer compounds of tartaric acid and various bases have been reported, for example, (Paixão et al., 1999;Bai et al., 2005;Zhang et al., 2006;Suresh et al., 2006;Wang et al., 2008;Su et al., 2009). The title structure contains a cation of protonated 1,10-phenanthroline, an anion of mono-deprotonated D-tartaric acid, and three water molecules ( Fig.   1). Thus, the crystal structure shows that one of the protons of the tartaric acid carboxylic groups has been transferred to one of the nitrogen atoms of the 1,10-phenanthroline molecule. A portion of the hydrogen bonding motif involving the anions, cations and water molecules is presented in Fig. 2; details of the O-H···O and N-H···O hydrogen bonds are given in Table   1. Fig. 2 also shows how the 1,10-phenanthrolinium rings are π-stacked such that they are perpendicular to the chain of tartrate anions that run along the a axis. The average perpendicular distance between the plane of N1/N2/C1/C2/C3/C4/C5/ C6/C7/C8/C9/C10/C11/C12 and the 14 atoms at ii = 1/2 + x, 3/2 -y, 1 -z of the stacked phenanthroline ring is 3.58 (11) Å. The tartrate anions are connected head-to-tail by a short hydrogen bond between H2A, bonded to O2, and O6 i of the anion at i = x -1, y, z. The O2-H2A distance is 1.01 (4) Å, H2A-O6 i is 1.45 (4) Å and O2···O6 i is 2.4554 (12) Å. The O2-H2A-O6 i angle is 174 (4)°. Electrostatic considerations, together with the use of resonance structures, could be used to explain the short hydrogen bond. Additionally, the existence of a number of supporting hydrogen bonds could be a factor, and these are depicted in Fig. 3. A similar head-to-tail arrangement with a short donor-acceptor distance is seen in some other hydrogen tartrate structures (Paixão et al., 1999;Zhang et al., 2006). The geometry of the hydrogen atom, H2A, that is involved in the short hydrogen bond has larger standard uncertainties than other hydrogen atoms in the structure. The larger uncertainty can be accounted for by examination of a plot of difference electron density ( Fig. 4) (EDEN, Sheldrick (2008)), which shows that H2A resides in a shallow potential well that has a single minimum close to O2 but tails off towards O6 i .
A previous structural determination of the isostructural L-tartrate enantiomer (II) (Wang et al., 2006) missed the proton transfer and identified the compound as 1,10-phenanthroline (2R,3R)-tartaric acid. We have examined their data and confirmed that the proton transfer did occur, and refinement of the structure using the model of the title compound results in lower R values. Interestingly, a subsequent paper on the quinoline analog by Smith et al., 2006, expressed surprise that the proton in (II) was not transferred: "···the absence of transfer in the L-tartaric acid-1,10-phenanthroline compound reported by Wang et al. (2006) when compared with the structurally similar [quinolinium] is not understood, considering that the pK a value for 1,10-phenanthroline (4.86) is very close to that of quinoline (4.81)." We note that, in the structure of (II), the details of the short hydrogen bond are not revealed because tartaric acid O-H distance restraints of 0.82 (1) Å were applied, and also because the acceptor O atom has the misplaced H atom. In the refinement of the title compound, hydrogen atoms were freely refined. In (II), data were collected at 293 (2) K, and the data/parameter ratio is 7.34. In the title compound, data were collected at 90 (2) K, and the data/parameter ratio is 10.03.

supplementary materials sup-2 Experimental
The reaction between solutions of D-tartaric acid (7 mg, 1 mmol) in water (10 ml) and 1,10-phenanthroline (9 mg, 1 mmol) in methanol (5 ml) in a 1:1 molar ratio gave colorless rod crystals after slow evaporation of the solvent at room temperature.

Refinement
Friedel opposites were merged. The absolute configuration followed from the use of D-tartaric acid as a starting material.
Hydrogen atoms were located in a difference Fourier map and freely refined.