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.

Abstract

Single crystals of poly[μ₄-tartrato-κ⁶ O ¹,O ³:O ^1′:O ²,O ⁴:O ^4′-lead], [Pb(C₄H₄O₆)]_n, were grown in a gel medium. In comparison with the previous structure determination of this compound from laboratory powder X-ray diffraction data [De Ridder et al. (2002 ▸). Acta Cryst. C58, m596–m598], the redetermination on the basis of single-crystal data reveals the absolute structure, all atoms with anisotropic displacement parameters and a much higher accuracy in terms of bond lengths and angles. It could be shown that a different space group or incorporation of water as reported for similarly gel-grown lead tartrate crystals is incorrect. In the structure, each Pb²⁺ cation is bonded to eight O atoms of five tartrate anions, while each tartrate anion links four Pb²⁺ cations. The resulting three-dimensional framework is stabilized by O—H⋯O hydrogen bonds between the OH groups of one tartrate anion and the carboxyl­ate O atoms of adjacent anions.

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 BaPO₃F. 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 BaPO₃F phases belong to the same order–disorder (OD) family and can be derived from the baryte (BaSO₄) structure type by replacing the SO₄ ²⁻ anions with isoelectronic PO₃F²⁻ anions in two orientations (Stöger et al., 2013 ▸). The same baryte-type structure has been reported for PbPO₃F 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 PbPO₃F in comparison with BaPO₃F (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 Pb²⁺ 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(C₄H₄O₆)]. The structure of this compound was originally solved and refined from laboratory X-ray powder diffraction data in space group P2₁2₁2₁ (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 (Pna2₁; 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.

Table 1. Comparison of the PbO bond lengths () in the current and the previous (De Ridder et al., 2002 ▸) refinements of lead tartrate.

For the previous refinement: a = 7.99482(3), b = 8.84525(4), c = 8.35318(4).

Bond	current refinement	previous refinement
PbO1i	2.472(2)	2.859(12)
PbO5ii	2.482(2)	2.398(11)
PbO6iii	2.594(2)	2.575(12)
PbO3i	2.5972(17)	2.637(9)
PbO4iv	2.6878(19)	2.649(11)
PbO4iii	2.7866(19)	2.847(12)
PbO2iv	2.935(2)	2.975(13)
PbO1	3.004(2)	2.754(12)

Symmetry codes: (i) x , y, z ; (ii) x+, y, z ; (iii) x, y , z+; (iv) x , y+, z.

The Pb²⁺ 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 1.

Coordination environment of the Pb²⁺ cation in the title compound, with atom labelling (for symmetry codes refer to Table 1 ▸). Displacement ellipsoids are drawn at the 50% probability level.

Packing features  

In the crystal structure, the Pb²⁺ cations are arranged in hexa­gonally packed rows extending parallel to [100] (Fig. 2 ▸). Each Pb²⁺ cation is bonded to five tartrate anions (three chelating and two in a monodentate fashion, Fig. 1 ▸) while each tartrate anion links four Pb²⁺ cations, leading to a three-dimensional framework. O—H⋯O 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.

Figure 2.

The crystal packing of the title compound in projection along [00]. Only complete tartrate anions are shown. O—H⋯O 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 (, ).

DHA	DH	HA	D A	DHA
O3H3OO2i	0.84(1)	2.02(4)	2.646(3)	131(5)
O4H4OO6ii	0.85(1)	1.79(1)	2.618(3)	169(4)

Symmetry codes: (i) ; (ii) .

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 (NH₄)₂(PO₃F)(H₂O), prepared according to Schülke & 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(NO₃)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. PbPO₃F 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). Hydroxyl H atoms were found from difference Fourier maps and refined with an O—H distance restraint of 0.85 (1) Å and with U _iso(H) = 1.2U _eq(O). The highest and lowest remaining electron densities are found 0.59 and 0.49 Å, respectively, from the Pb atom and are caused by truncation effects. No other electron densities attributable to additional atoms could be found, ruling out an incorporation of water mol­ecules. Refinements in space group Pna2₁ as suggested by Labutina et al. (2011 ▸) led to unreasonable models.

Table 3. Experimental details.

Crystal data
Chemical formula	[Pb(C4H4O6)]
M r	355.26
Crystal system, space group	Orthorhombic, P212121
Temperature (K)	296
a, b, c ()	7.9890(2), 8.8411(3), 8.3434(2)
V (3)	589.31(3)
Z	4
Radiation type	Mo K
(mm1)	28.61
Crystal size (mm)	0.15 0.15 0.09
Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Bruker, 2013 ▸)
T min, T max	0.099, 0.183
No. of measured, independent and observed [I > 2(I)] reflections	81742, 4512, 3836
R int	0.046
(sin /)max (1)	0.971
Refinement
R[F 2 > 2(F 2)], wR(F 2), S	0.021, 0.042, 1.06
No. of reflections	4512
No. of parameters	108
No. of restraints	2
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
max, min (e 3)	3.20, 2.26
Absolute structure	Flack (1983 ▸), 1959 Friedel pairs
Absolute structure parameter	0.003(7)

Computer programs: APEX2 and SAINT-Plus (Bruker, 2013 ▸), SHELXS97 and SHELXL97 (Sheldrick, 2008 ▸), ATOMS (Dowty, 2006 ▸) and publCIF (Westrip, 2010 ▸).

Supplementary Material

CCDC reference: 1039408

Additional supporting information: crystallographic information; 3D view; checkCIF report

supplementary crystallographic information

Crystal data

[Pb(C4H4O6)]	F(000) = 632
Mr = 355.26	Dx = 4.004 Mg m−3
Orthorhombic, P212121	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2ac 2ab	Cell parameters from 9854 reflections
a = 7.9890 (2) Å	θ = 3.4–40.9°
b = 8.8411 (3) Å	µ = 28.61 mm−1
c = 8.3434 (2) Å	T = 296 K
V = 589.31 (3) Å3	Block, colourless
Z = 4	0.15 × 0.15 × 0.09 mm

Data collection

Bruker APEXII CCD diffractometer	4512 independent reflections
Radiation source: fine-focus sealed tube	3836 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.046
ω– and φ–scans	θmax = 43.7°, θmin = 3.4°
Absorption correction: multi-scan (SADABS; Bruker, 2013)	h = −15→15
Tmin = 0.099, Tmax = 0.183	k = −17→17
81742 measured reflections	l = −16→16

Refinement

Refinement on F2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.021	H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.042	w = 1/[σ2(Fo2) + (0.0155P)2 + 1.1508P] where P = (Fo2 + 2Fc2)/3
S = 1.06	(Δ/σ)max = 0.007
4512 reflections	Δρmax = 3.20 e Å−3
108 parameters	Δρmin = −2.26 e Å−3
2 restraints	Absolute structure: Flack (1983), 1959 Friedel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: −0.003 (7)

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 F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Pb1	−0.342448 (9)	0.001349 (17)	−0.054104 (9)	0.01396 (2)
O1	−0.0806 (2)	0.1027 (2)	0.1809 (3)	0.0159 (3)
O2	0.1001 (3)	0.1758 (3)	−0.0066 (2)	0.0180 (4)
O3	0.1564 (2)	−0.0126 (3)	0.3659 (2)	0.0147 (3)
O4	0.2220 (2)	0.3195 (2)	0.3069 (2)	0.0123 (3)
O5	0.5788 (3)	0.0785 (3)	0.3132 (3)	0.0215 (4)
O6	0.4746 (3)	0.2388 (3)	0.4968 (3)	0.0244 (5)
C1	0.0635 (3)	0.1138 (3)	0.1249 (3)	0.0103 (3)
C2	0.2130 (3)	0.0566 (3)	0.2235 (3)	0.0100 (3)
H2	0.2764	−0.0171	0.1601	0.012*
C3	0.3222 (3)	0.1966 (3)	0.2538 (3)	0.0101 (3)
H3	0.3686	0.2263	0.1497	0.012*
C4	0.4706 (3)	0.1686 (3)	0.3642 (3)	0.0128 (4)
H3O	0.224 (4)	−0.019 (6)	0.443 (4)	0.029 (12)*
H4O	0.144 (4)	0.288 (5)	0.367 (4)	0.015 (10)*

Atomic displacement parameters (Ų)

	U11	U22	U33	U12	U13	U23
Pb1	0.01063 (3)	0.01446 (3)	0.01679 (3)	0.00067 (7)	−0.00006 (2)	0.00120 (7)
O1	0.0081 (6)	0.0230 (9)	0.0167 (8)	0.0000 (6)	−0.0005 (6)	0.0032 (7)
O2	0.0155 (8)	0.0274 (10)	0.0111 (7)	0.0026 (8)	−0.0009 (6)	0.0057 (7)
O3	0.0116 (5)	0.0192 (9)	0.0134 (6)	0.0001 (8)	0.0009 (5)	0.0072 (7)
O4	0.0092 (6)	0.0113 (7)	0.0165 (8)	0.0011 (5)	0.0001 (6)	0.0004 (6)
O5	0.0100 (7)	0.0277 (11)	0.0267 (11)	0.0073 (7)	−0.0009 (7)	−0.0005 (8)
O6	0.0252 (10)	0.0242 (10)	0.0237 (10)	0.0083 (9)	−0.0156 (9)	−0.0090 (8)
C1	0.0081 (8)	0.0120 (8)	0.0108 (8)	−0.0009 (6)	−0.0017 (6)	−0.0006 (6)
C2	0.0081 (7)	0.0121 (8)	0.0098 (8)	0.0003 (7)	0.0004 (6)	0.0011 (6)
C3	0.0067 (8)	0.0129 (8)	0.0107 (8)	0.0005 (6)	−0.0001 (6)	0.0011 (6)
C4	0.0075 (7)	0.0137 (9)	0.0172 (10)	−0.0003 (7)	−0.0017 (7)	0.0003 (7)

Geometric parameters (Å, º)

Pb1—O1i	2.472 (2)	O3—H3O	0.843 (10)
Pb1—O5ii	2.482 (2)	O4—C3	1.420 (3)
Pb1—O6iii	2.594 (2)	O4—H4O	0.845 (10)
Pb1—O3i	2.5972 (17)	O5—C4	1.250 (3)
Pb1—O4iv	2.6878 (19)	O6—C4	1.270 (3)
Pb1—O4iii	2.7866 (19)	C1—C2	1.536 (3)
Pb1—O2iv	2.935 (2)	C2—C3	1.535 (3)
Pb1—O1	3.004 (2)	C2—H2	0.9800
O1—C1	1.246 (3)	C3—C4	1.522 (3)
O2—C1	1.261 (3)	C3—H3	0.9800
O3—C2	1.411 (3)
O1i—Pb1—O5ii	72.93 (7)	C1—O1—Pb1v	126.48 (16)
O1i—Pb1—O6iii	74.36 (7)	C2—O3—Pb1v	120.65 (14)
O5ii—Pb1—O6iii	99.96 (9)	C2—O3—H3O	118 (3)
O1i—Pb1—O3i	62.88 (6)	Pb1v—O3—H3O	115 (3)
O5ii—Pb1—O3i	135.70 (7)	C3—O4—Pb1vi	108.28 (13)
O6iii—Pb1—O3i	71.85 (8)	C3—O4—H4O	111 (3)
O1i—Pb1—O4iv	64.23 (6)	Pb1vi—O4—H4O	121 (3)
O5ii—Pb1—O4iv	69.82 (7)	C4—O5—Pb1vii	128.07 (19)
O6iii—Pb1—O4iv	138.58 (7)	C4—O6—Pb1viii	125.98 (18)
O3i—Pb1—O4iv	87.75 (6)	O1—C1—O2	125.1 (2)
O1i—Pb1—O4iii	122.21 (6)	O1—C1—C2	119.4 (2)
O5ii—Pb1—O4iii	82.70 (7)	O2—C1—C2	115.4 (2)
O6iii—Pb1—O4iii	59.20 (6)	O3—C2—C3	113.16 (19)
O3i—Pb1—O4iii	123.07 (6)	O3—C2—C1	110.14 (19)
O4iv—Pb1—O4iii	148.738 (15)	C3—C2—C1	105.30 (18)
O1i—Pb1—O2iv	118.51 (7)	O3—C2—H2	109.4
O5ii—Pb1—O2iv	119.10 (7)	C3—C2—H2	109.4
O6iii—Pb1—O2iv	140.78 (8)	C1—C2—H2	109.4
O3i—Pb1—O2iv	81.74 (7)	O4—C3—C4	111.99 (19)
O4iv—Pb1—O2iv	65.93 (6)	O4—C3—C2	110.38 (18)
O4iii—Pb1—O2iv	119.18 (6)	C4—C3—C2	114.25 (19)
O1i—Pb1—O1	150.22 (4)	O4—C3—H3	106.6
O5ii—Pb1—O1	77.58 (7)	C4—C3—H3	106.6
O6iii—Pb1—O1	115.47 (6)	C2—C3—H3	106.6
O3i—Pb1—O1	145.98 (6)	O5—C4—O6	126.2 (3)
O4iv—Pb1—O1	101.71 (6)	O5—C4—C3	115.9 (2)
O4iii—Pb1—O1	56.54 (5)	O6—C4—C3	117.9 (2)
O2iv—Pb1—O1	72.91 (6)

Symmetry codes: (i) −x−1/2, −y, z−1/2; (ii) −x+1/2, −y, z−1/2; (iii) −x, y−1/2, −z+1/2; (iv) x−1/2, −y+1/2, −z; (v) −x−1/2, −y, z+1/2; (vi) x+1/2, −y+1/2, −z; (vii) −x+1/2, −y, z+1/2; (viii) −x, y+1/2, −z+1/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O3—H3O···O2vii	0.84 (1)	2.02 (4)	2.646 (3)	131 (5)
O4—H4O···O6ix	0.85 (1)	1.79 (1)	2.618 (3)	169 (4)

Symmetry codes: (vii) −x+1/2, −y, z+1/2; (ix) x−1/2, −y+1/2, −z+1.