Disodium rhodizonate: a powder diffraction study

The crystal structure of the title compound, disodium 1,2,3,4-tetra­oxo-5-cyclo­hexene-5,6-diolate, 2Na⁺·C₆O₆^2-, is built up of alternating layers of hexa­gonally packed Na⁺ cations and rhodizonate dianions. The rhodizon­ate dianion has a slightly twisted conformation.

Key indicators

• Powder synchrotron study
• T = 298 K
• Mean [sigma](C-C) = 0.001 Å
• R factor = 0.000
• wR factor = 0.000
• Data-to-parameter ratio = 0.0

checkCIF/PLATON results

No syntax errors found



Alert level A
PLAT080_ALERT_2_A Maximum Shift/Error ............................       2.25

Author Response: For structure solutions out of powder data this shift lies, due to correlations, within the expected area.

Alert level C
PLAT040_ALERT_1_C No H-atoms in this Carbon Containing Compound ..          ?
PLAT042_ALERT_1_C Calc. and Rep. MoietyFormula Strings Differ ....          ?
PLAT125_ALERT_4_C No _symmetry_space_group_name_Hall Given .......          ?
PLAT141_ALERT_4_C su on a - Axis Small or Missing (x 100000) .....          5 Ang.
PLAT142_ALERT_4_C su on b - Axis Small or Missing (x 100000) .....          6 Ang.
PLAT143_ALERT_4_C su on c - Axis Small or Missing (x 100000) .....          3 Ang.
PLAT430_ALERT_2_C Short Inter D...A Contact  O5     ..  O5      ..       2.89 Ang.

   1 ALERT level A = In general: serious problem
   0 ALERT level B = Potentially serious problem
   7 ALERT level C = Check and explain
   0 ALERT level G = General alerts; check

   2 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   2 ALERT type 2 Indicator that the structure model may be wrong or deficient
   0 ALERT type 3 Indicator that the structure quality may be low
   4 ALERT type 4 Improvement, methodology, query or suggestion

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S1. Comment

Rhodizonic acid and its salts, especially the sodium and potassium salts, have been used, among others, in combination with tartaric acid as a determining reagent for gunpowder, the so called `rhodizonate staining technique' in forensic medicine (Marty et al., 2002; Bartsch et al., 1996). Academic interest has raised the question of whether oxocarbon dianions, C_nO_n²⁻, with their prototype C₆O₆²⁻ (rhodizonate), are aromatic or not (West, 1980; Braga et al., 2001). Recent studies of the solid-state structure of the rhodizonate dianion have showed that planar (2Rb⁺·C₆O₆²⁻; Braga et al., 2001) as well as twisted boat-like (2 K⁺·C₆O₆²⁻; Cowan & Howard, 2004) conformations are possible. Here, we report the crystal structure determination of the homologue 2Na⁺·C₆O₆²⁻, (I), from synchrotron powder diffraction data.

Disodium rhodizonate (Fig. 1) is isostructural with dipotassium rhodizonate (Cowan & Howard, 2004). Layers of hexagonally packed cations alternate with layers of equally packed anions, as shown in Fig. 2, where the cations lie a/4 above the anions. The C—O bond lengths [1.250 (2) and 1.253 (1) Å] are similar to those of the higher homologues [K: 1.254 (5) and 1.255 (3) Å; Rb: 1.252 (9) and 1.255 (3) Å], whereas the C—C bond lengths [1.451 (2) and 1.466 (1) Å] are closer to those observed in the rubidium salt [1.468 (6) and 1.469 (6) Å] rather than those in the potassium salt [1.480 (5) and 1.479 (3) Å].

Fig. 3 shows the environment of the alkali metal. Like K₂C₆O₆, the alkali metal in (I) is coordinated by eight O atoms from four different rhodizonate dianions, leading to a distorted quadratic antiprismatic coordination. Fig. 4 displays the slightly twisted conformation of the C₆O₆²⁻ anion (r.m.s. deviation from the mean plane is 0.113 Å).

S2. Experimental

Disodium rhodizonate (97%, Aldrich) was dried in vacuo at 423 K over P₄O₁₀ for 3 d.

S3. Refinement

Powder diffraction data were collected at the high-resolution powder diffractometer at beamline ID31 at the European Synchrotron Radiation Facility (ESRF). An Si 111 reflection was used to select an X-ray energy of 30.99 keV. The size of the beam was adjusted to 2 × 0.6 mm² using slits. The wavelength was determined to 0.40009 (5) Å from a silicon standard. The sample of (I) was contained in a 0.7 mm lithium borate glass (glass No. 50) capillary. The sample was rotated around θ in order to improve randomization of the crystallites. The diffracted beam was analyzed with a nine-crystal analyzer stage [nine Ge(111) crystals separated by 2° intervals] and detected with nine Na(Tl)I scintillation counters simultaneously. The incoming beam was monitored by an ion-chamber for normalization of the decay of the primary beam. 15 min scans were taken at T = 298 K in continuous mode for 1 h each, and later normalized and converted to step-scan data from −11.0 to 32.92° 2θ in steps of 0.002°. To minimize the effect of decomposition due to radiation damage, only identical scans were summed. Data reduction of the powder diffraction pattern of (I) was performed using the GUFI program (Dinnebier & Finger, 1998). Indexing using ITO (Visser, 1969) led to an orthorhombic unit cell. The space group assignment of Fddd was made according to the extinction rules, which could later be confirmed by Rietveld refinements (Rietveld, 1969). The number of formula units per unit cell (Z = 8) followed directly from volume increments. The peak profiles and precise lattice parameters were determined by Le Bail fits (Le Bail et al., 1988) using the program GSAS (Larson & Von Dreele, 1994). The background was modelled manually using GUFI. The peak profile was described by a pseudo-Voigt function (Thompson et al., 1987) in combination with a special function that accounts for the asymmetry due to axial divergence (Finger et al., 1994). The powder pattern of (I) exhibits some anisotropic peak broadening caused by lattice strain. The phenomenological strain model of Stephens (1999), as implemented in GSAS, was used to model the anisotropy of the full width at half maximum. Starting values for the unconstrained Rietveld refinement were taken from the isostructural compound 2 K⁺·C₆O₆²⁻ (Cowan & Howard, 2004). The background and starting values for the peak profile were taken from the corresponding LeBail fit. The Rietveld refinement converged satisfactorily (Fig. 5).

Computing details

Data collection: Please complete; cell refinement: GSAS (Larson & Von Dreele, 1994); data reduction: GUFI (Dinnebier & Finger, 1998); program(s) used to solve structure: GSAS; program(s) used to refine structure: GSAS; molecular graphics: ATOMS (Dowty, 2002); software used to prepare material for publication: GSAS.

Figures

	Fig. 1. A view of (I), showing the atom-labelling scheme. [Symmetry codes: (i) −x + 1/4, −y + 1/4, z; (ii) x, −y + 1/4, −z + 1/4; (iii) −x + 1/4, y, −z + 1/4; (iv) x + 1/4, y + 1/4, −z].
	Fig. 2. A view along [100]. Na^+ (grey) lies a/4 above C6O6^2− C atoms are denoted by black and O atoms by white.
	Fig. 3. The distorted quadratic antiprism of O (white) around Na (grey).
	Fig. 4. The twisted conformation of the rhodizonate dianion.
	Fig. 5. The Rietveld plot for (I). Shown are the observed pattern (diamonds), the best Rietveld-fit profile (line), and the difference curve between observed and calculated profile in an additional window below. The high-angle part of the plot is enlarged by a factor of five.

Disodium 1,2,3,4-tetraoxo-5-cyclohexene-5,6-diolate

Crystal data

2Na^+·C6O6^2^−	F(000) = 848
Mr = 214.04	Dx = 2.182 Mg m^−3
Orthorhombic, Fddd	? radiation, λ = 0.400094 Å
a = 11.48349 (5) Å	µ = 0.0 mm^−1
b = 14.32080 (6) Å	T = 298 K
c = 7.92477 (3) Å	Particle morphology: block
V = 1303.25 (2) Å^3	colourless
Z = 8	cylinder, 2 × 0.3 mm

Data collection

ID31 at ESRF diffractometer	Scan method: continuous
Specimen mounting: 0.7 mm lithium borate glass capillary	2θmin = 2°, 2θmax = 26°, 2θstep = 0.001°
Data collection mode: transmission

Refinement

Refinement on Inet	Profile function: pseudo-Voigt
Least-squares matrix: full	28 parameters
Rp = 0.058	0 restraints
Rwp = 0.078	0 constraints
Rexp = 0.060	Weighting scheme based on measured s.u.'s
R(F^2) = 0.08970	(Δ/σ)max = 2.25
χ^2 = 1.690	Background function: GSAS Background function number 2 with 4 terms. Cosine Fourier series 1: 0.00000 2: 0.00000 3: 0.00000 4: 0.00000
24000 data points	Preferred orientation correction: none

Special details

Refinement. CW Profile function number 4 with 18 terms has been used in the refinement. Pseudo-Voigt profile coefficients as parameterized in Thompson, Cox & Hastings (1987), asymmetry correction of Finger, Cox & Jephcoat (1994). Microstrain broadening by Stephens (1999). #1(GU) = 0.000 #2(GV) = 0.000 #3(GW) = 0.000 #4(GP) = 0.000 #5(LX) = 0.016 #6(ptec) = 0.00 #7(trns) = 0.00 #8(shft) = 0.0000 #9(sfec) = 0.00 #10(S/L) = 0.0013 #11(H/L) = 0.0020 #12(eta) = 0.8260 #13(S400) = 9.4E-03 #14(S040) = 3.8E-04 #15(S004) = 1.0E-02 #16(S220) = 2.2E-02 #17(S202) = 1.1E-02 #18(S022) = 3.4E-03 Peak tails are ignored where the intensity is below 0.0020 times the peak Aniso. broadening axis 0.0 0.0 1.0

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

	x	y	z	Uiso*/Ueq
Na1	0.47634 (6)	0.125	0.125	0.0268 (3)*
C2	0.125	0.22737 (11)	0.125	0.0181 (6)*
C3	0.12995 (16)	0.17552 (8)	0.28431 (12)	0.0214 (4)*
O4	0.125	−0.06472 (8)	0.125	0.0270 (4)*
O5	0.14364 (7)	0.03213 (5)	−0.17135 (9)	0.0274 (3)*

Geometric parameters (Å, º)

C2—C3	1.4658 (11)	C2—O4^i	1.2509 (17)
C3—C3^i	1.451 (2)	C3—O5^ii	1.2537 (11)
C3^iii—C2—C3	119.13 (16)	C3^i—C3—O5^ii	119.48 (7)
C2—C3—C3^i	120.13 (8)	C2—C3—O5^ii	120.39 (11)
C3—C2—O4^i	120.44 (8)

Symmetry codes: (i) −x+1/4, −y+1/4, z; (ii) x, −y+1/4, −z+1/4; (iii) −x+1/4, y, −z+1/4.