Dipotassium rhodizonate

Dipotassium rhodizonate, 2K⁺·C₆O₆²⁻, crystallizes in space group Fddd. The rhodizonate anions lie in hexagonal layers connected by the potassium ions, which lie between the planes and connect adjacent layers. The conformation of the rhodizonate ion is distinct from previous observations. The site symmetry of the potassium ions is 2 and the site symmetry of the centroid of the rhodizonate ions is 222.

Comment

The crystal structures of potassium and rubidium rhodizonates were determined by Neumann (1965); however, the coordinates were unavailable in the Cambridge Structural Database (Version 5.25, update of January 2004; Allen, 2002). Rubidium rhodizonate has recently been re-reported by Braga et al. (2001). We wished to compare the cocrystallization behaviour of rhodizonic acid (C₆O₆H₂) with 2,6-di­hydroxy­benzo­quinone (C₆O₄H₄) and tetra­hydroxy­benzo­quinone (C₆O₆H₄). While we have produced crystals containing these mol­ecules (Cowan et al., 2001a,b), so far we have been unable to produce a cocrystal containing rhodizonic acid. We have obtained the potassium salt, (I), as a by-product of this project and redetermined its structure.

Rhodizonic acid is an intriguing mol­ecule used by crime fighters to detect traces of lead produced by gunfire (Bartsch et al., 1996). It is a weak organic acid, which should participate in a variety of different hydrogen bonds, and upon deprotonation to rhodizonate its shape and properties change significantly.

Potassium rhodizonate (Fig. 1) crystallizes in space group Fddd. The metal ion lies between four rhodizonate anions, bonding with eight O atoms (Fig. 2). The rhodizonate anions lie in hexagonal layers connected by the potassium ions, which lie between the planes and connect adjacent layers (Fig. 3). The site symmetry of the potassium ions is 2 and the site symmetry of the centroid of the rhodizonate ions is 222. This structure is incompatable with the cell dimensions and space group, which are availiable in the CSD, of the original structure (Neumann, 1965) and may be a polymorph or a low-temperature phase.

There are four previously published structures containing the rhodizonate ion, in which there are two distinct conformations; the conformation in the present structure is distinct from either of those previously determined. The C—O bond lengths in the rhodizonate ion [1.254 (5) and 1.255 (3) Å] are essentially the same as those observed in the rubidium salt [1.252 (9) and 1.248 (6) Å] ; however, the C—C bond lengths [1.480 (5) and 1.479 (3) Å] are slightly longer than those in the rubidium salt [1.468 (6) and 1.469 (6) Å]. The rhodizonate ion, in contrast with the situation observed in the rubidium salt, is not planar but has a twisted-boat form (r.m.s. deviation from the plane = 0.108 Å); consequently, the molecular symmetry is not D_6h but D₂. It is worth noting that the rubidium salt is not isostructural with the potassium salt (Braga et al., 2001). Lam & Mak (2001a,b) have produced organic cocrystals containing the rhodizonate ion acting as a multi-hydrogen-bond acceptor, in which the rhodizonate ion is smaller with a greater variation in its internal bond lengths; for example, a range of 1.421 (5)–1.458 (5) Å is found in the C—C bonds and 1.234 (4)–1.258 (4) Å in the C=O bonds in bis(tetra-n-butyl­ammonium) rhodizonate tetrakis­(phenyl­urea) clathrate (Lam & Mak, 2001b).

Figure 1 The ions of the title compound, shown with 50% probability displacement ellipsoids. [Symmetry codes: (I)  − x,  − y, z; (II)  − x, y,  − z; (III) x,  − y,  − z.]

Figure 2 The coordination of the potassium ion in the title compound. [Symmetry codes: (I)  − x, 1 − y,  − z; (II)  + x, 1 − y,  + z; (III)  − x, y,  − z; (IV)  − x, − + y,  + z; (V)  + x,  − y,  − z; (VI)  − x, 1 − y,  − z.]

Figure 3 Packing diagram illustrating a layer of rhodizonate ions. The potassium ions connect the anions above and below the plane.

Experimental

Rhodizonic acid dihydrate (approximately 0.5 g), purchased from Aldrich Chemicals, was dissolved in 0.1 M potassium hydro­xide, producing an intense deep-red solution. Crystals suitable for X-ray structure determination were prepared by slow evaporation of the solvent at room temperature.

Crystal data

• 2K⁺·C₆O₆²⁻

• M_r = 246.26

• Orthorhombic, Fddd

• a = 8.426 (2) Å

• b = 12.011 (3) Å

• c = 15.671 (3) Å

• V = 1586.0 (6) ų

• Z = 8

• D_x = 2.063 Mg m⁻³

• Mo Kα radiation

• Cell parameters from 982 reflections

• θ = 12.4–22.1°

• μ = 1.19 mm⁻¹

• T = 100 (2) K

• Block, dark red

• 0.20 × 0.15 × 0.10 mm

Data collection

• Bruker SMART CCD diffractometer

• ω scans

• 3525 measured reflections

• 462 independent reflections

• 392 reflections with I > 2σ(I)

• R_int = 0.049

• θ_max = 27.5°

• h = −10 → 10

• k = −15 → 15

• l = −20 → 20

Refinement

• Refinement on F²

• R[F² > 2σ(F²)] = 0.038

• wR(F²) = 0.093

• S = 1.06

• 462 reflections

• 34 parameters

• w = 1/[σ²(F_o²) + (0.034P)² + 24.4908P] where P = (F_o² + 2F_c²)/3

• (Δ/σ)_max < 0.001

• Δρ_max = 0.47 e Å⁻³

• Δρ_min = −0.47 e Å⁻³

Data collection: SMART-NT (Bruker, 1998); cell refinement: SMART-NT; data reduction: SAINT-NT (Bruker, 1998); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL/PC (Sheldrick, 1999); software used to prepare material for publication: SHELXL97.

Supporting information

Computing details

Data collection: SMART-NT (Bruker, 1998); cell refinement: SMART-NT; data reduction: SAINT-NT (Bruker, 1998); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997).

dipotassium rhodizonate

Crystal data

2K^+·C6O6^2^−	Dx = 2.063 Mg m^−3
Mr = 246.26	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Fddd	Cell parameters from 982 reflections
a = 8.426 (2) Å	θ = 12.4–22.1°
b = 12.011 (3) Å	µ = 1.19 mm^−1
c = 15.671 (3) Å	T = 100 K
V = 1586.0 (6) Å^3	Block, dark red
Z = 8	0.2 × 0.15 × 0.1 mm
F(000) = 976

Data collection

Bruker SMART CCD diffractometer	392 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.049
Graphite monochromator	θmax = 27.5°, θmin = 3.2°
ω scans	h = −10→10
3525 measured reflections	k = −15→15
462 independent reflections	l = −20→20

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	Primary atom site location: structure-invariant direct methods
R[F^2 > 2σ(F^2)] = 0.038	Secondary atom site location: difference Fourier map
wR(F^2) = 0.093	w = 1/[σ^2(Fo^2) + (0.034P)^2 + 24.4908P] where P = (Fo^2 + 2Fc^2)/3
S = 1.06	(Δ/σ)max < 0.001
462 reflections	Δρmax = 0.47 e Å^−3
34 parameters	Δρmin = −0.47 e Å^−3

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
K1	0.3750	0.52405 (7)	0.3750	0.0107 (3)
C1	0.6250	0.6250	0.2194 (2)	0.0115 (8)
O1	0.6250	0.6250	0.29940 (18)	0.0159 (6)
O2	0.3441 (2)	0.61034 (17)	0.21116 (12)	0.0129 (5)
C2	0.4733 (3)	0.6199 (2)	0.17206 (16)	0.0114 (6)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
K1	0.0099 (4)	0.0144 (4)	0.0078 (4)	0.000	0.0011 (4)	0.000
C1	0.0129 (18)	0.0103 (17)	0.0113 (18)	−0.0009 (16)	0.000	0.000
O1	0.0160 (14)	0.0204 (15)	0.0113 (13)	−0.0020 (13)	0.000	0.000
O2	0.0100 (10)	0.0161 (10)	0.0126 (9)	0.0021 (8)	0.0026 (8)	0.0005 (8)
C2	0.0130 (13)	0.0090 (12)	0.0124 (13)	−0.0003 (11)	0.0008 (11)	0.0021 (11)

Geometric parameters (Å, º)

K1—O1	2.7038 (13)	C1—O1	1.254 (5)
K1—O2	2.781 (2)	C1—C2	1.479 (3)
K1—O2^i	2.799 (2)	O2—C2	1.255 (3)
K1—O2^ii	3.016 (2)	C2—C2^iii	1.480 (5)
O1—K1—O1^iv	126.72 (4)	O1^iv—K1—O2^ii	164.86 (5)
O1—K1—O2^v	99.46 (7)	O2^v—K1—O2^ii	126.79 (5)
O1^iv—K1—O2^v	60.09 (6)	O2—K1—O2^ii	83.51 (3)
O1—K1—O2	60.09 (6)	O2^i—K1—O2^ii	79.15 (7)
O1^iv—K1—O2	99.47 (7)	O2^vi—K1—O2^ii	55.69 (7)
O2^v—K1—O2	136.24 (9)	O2^vii—K1—O2^ii	98.63 (8)
O1—K1—O2^i	124.12 (6)	O1—C1—C2	120.11 (16)
O1^iv—K1—O2^i	87.49 (5)	O1—C1—C2^viii	120.11 (16)
O2^v—K1—O2^i	136.21 (7)	C2—C1—C2^viii	119.8 (3)
O2—K1—O2^i	73.01 (7)	C1—O1—K1^viii	115.99 (5)
O1—K1—O2^vi	87.49 (5)	C1—O1—K1	115.99 (5)
O1^iv—K1—O2^vi	124.12 (6)	K1^viii—O1—K1	128.03 (11)
O2^v—K1—O2^vi	73.01 (7)	C2—O2—K1	113.82 (16)
O2—K1—O2^vi	136.21 (7)	C2—O2—K1^i	112.93 (17)
O2^i—K1—O2^vi	109.58 (9)	K1—O2—K1^i	106.99 (7)
O1—K1—O2^vii	164.86 (5)	C2—O2—K1^ix	104.64 (16)
O1^iv—K1—O2^vii	67.63 (4)	K1—O2—K1^ix	135.52 (7)
O2^v—K1—O2^vii	83.51 (3)	K1^i—O2—K1^ix	75.90 (6)
O2—K1—O2^vii	126.79 (5)	O2—C2—C1	120.6 (2)
O2^i—K1—O2^vii	55.69 (7)	O2—C2—C2^iii	119.63 (15)
O2^vi—K1—O2^vii	79.15 (7)	C1—C2—C2^iii	119.77 (16)
O1—K1—O2^ii	67.63 (4)

Symmetry codes: (i) −x+1/2, −y+1, −z+1/2; (ii) x+1/4, y−1/4, −z+1/2; (iii) x, −y+5/4, −z+1/4; (iv) x−1/2, −y+5/4, −z+3/4; (v) −x+3/4, y, −z+3/4; (vi) x+1/4, −y+1, z+1/4; (vii) −x+1/2, y−1/4, z+1/4; (viii) −x+5/4, −y+5/4, z; (ix) x−1/4, y+1/4, −z+1/2.