Crystal structure of lead(II) tartrate: a redetermination

The redetermination of the crystal structure of lead tartrate from crystals grown in a gel medium confirmed the previous powder X-ray diffraction study in the space group P212121 with higher precision. Contradictions in the literature regarding space group and water content could be clarified.


Chemical context
Crystal growth in gels (Henisch, 1970) is a convenient method to obtain single crystals of high quality from compounds with rather low solubility products. Therefore gel growth was the method of choice for single-crystal growth of the low-soluble fluorophosphate BaPO 3 F. This compound is interesting insofar as the polycrystalline material (prepared by fast precipitation) has orthorhombic symmetry whereas single crystals grown slowly in a gel have monoclinic symmetry. Both the orthorhombic and monoclinic BaPO 3 F phases belong to the same order-disorder (OD) family and can be derived from the baryte (BaSO 4 ) structure type by replacing the SO 4 2À anions with isoelectronic PO 3 F 2À anions in two orientations (Stö ger et al., 2013). The same baryte-type structure has been reported for PbPO 3 F on the basis of similar lattice parameters and systematic absences of reflections (Walford, 1967). However, structural details were not determined at that time. In analogy with the barium compound, it was intended to grow crystals of lead fluorophosphate in a gel medium. In order to take into account the somewhat lower solubility of PbPO 3 F in comparison with BaPO 3 F (Lange, 1929), crystal-growth experiments were performed with lead salts in ammoniacal tartrate solutions to produce a soluble, poorly dissociated lead tartrate complex which lowers the concentration of Pb 2+ to such an extent that its direct precipitation is prevented. In fact, after some days colourless single crystals appeared in the gel medium that, on the basis of unit-cell determinations, turned out to be lead tartrate, [Pb(C 4 H 4 O 6 )]. The structure of this compound was originally solved and refined from laboratory X-ray powder diffraction data in space group P2 1 2 1 2 1 (De Ridder et al., 2002). However, some years later it was reported that gel-grown lead tartrate crystallizes as a dihydrate (Lillybai & Rahimkutty, 2010) or in a different space group (Pna2 1 ; Labutina et al., 2011). Motivated by these disagreements, it was decided to re-investigate the crystal structure of gel-grown lead tartrate on the basis of single-crystal diffraction data for an unambiguous determination of the space group and the composition, and to obtain more precise results compared to the powder refinement.

Structural commentary
The present study confirms in principle the results of the previous powder X-ray diffraction study and reveals the determination of the absolute structure (Flack parameter 0.003 (7); Flack, 1983) and all non-H atoms refined with anisotropic displacement parameters. In comparison with the powder study, the higher precision and accuracy of the present model is, for example, reflected by the notable differences in the Pb-O bond lengths determined in the two studies (Table 1). An important result of the present study is that neither a different space group nor a different content in terms of an incorporation of water into the structure could be found on the basis of the single-crystal data.
The Pb 2+ cation has a coordination number of eight considering a cut-off value of 3 Å for the ligating oxygen atoms. The coordination polyhedron is considerably distorted (Fig. 1), with Pb-O distances in the range 2.472 (2)-3.004 (2) Å ( Table 1). The resulting bond-valence sum (Brown, 2002) of 1.75 valence units, using the parameters of Krivovichev & Brown (2001) for the Pb-O bonds, is reasonably close to the expected value of 2.0 valence units. Bond lengths and angles within the tartrate anion are in normal ranges.

Figure 2
The crystal packing of the title compound in projection along [100]. Only complete tartrate anions are shown. O-HÁ Á ÁO hydrogen bonds are shown in blue (see Table 2 for details). Pb-O bonds have been omitted for clarity. Colour code: Pb green, C grey, O red, H white.

Table 2
Hydrogen-bond geometry (Å , ). (3) 169 (4) Each Pb 2+ cation is bonded to five tartrate anions (three chelating and two in a monodentate fashion, Fig. 1) while each tartrate anion links four Pb 2+ cations, leading to a threedimensional framework. O-HÁ Á ÁO hydrogen bonds (Table 2) between the hydroxy groups of one tartrate anion and the carboxylate O atoms of adjacent tartrate anions stabilize this arrangement. Since no solvent-accessible voids were observed in the crystal structure, an incorporation of water molecules as reported by Lillibay & Rahimkutty (2010) is impossible.

Database survey
Tartaric acid and its salts or coordination compounds have been structurally examined in great detail. The current release of the CSD (Version 5.35 with all updates; Groom & Allen, 2014) revealed 644 entries, including the pure acid, co-crystals, compounds with the hydrogen tartrate anion and compounds with the tartrate anion.

Synthesis and crystallization
Commercially available gelatin was dissolved in hot water. The solution (50 ml) was cooled to about 300 K and 300 mg of (NH 4 ) 2 (PO 3 F)(H 2 O), prepared according to Schü lke & Kayser (1991), were dissolved in the still liquid solution that was filled in a large test tube. After initiation of gelling, a second neutral gel layer was put on top of the first gel layer. After the neutral gel had set, an aqueous solution consisting of Pb(NO 3 ) 2 (30 mg) and sodium potassium tartrate (250 mg) was poured over the second gel layer. After three weeks, colourless single crystals of lead(II) tartrate, mostly with a block-like form, could be isolated. PbPO 3 F in the form of polycrystalline material was also present in the reaction mixture as revealed by powder X-ray diffraction measurements.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Atom labelling and starting coordinates for the refinement were taken from the previous powder diffraction study (De Ridder et al., 2002). H atoms bonded to C atoms were placed in calculated positions and refined as riding atoms, with C-H = 0.98 Å and with U iso (H) = 1.2U eq (C  Computer programs: APEX2 and SAINT-Plus (Bruker, 2013), SHELXS97 and SHELXL97 (Sheldrick, 2008), ATOMS (Dowty, 2006) and publCIF (Westrip, 2010).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.