Phospho­ric acid imidazolium dihydrogenphosphate

The title compound, H₃PO₄·C₃H₅N₂⁺·H₂PO₄^-, was synthesized during an investigation of the inter­calation of imidazole into a series of VOPO₄ compounds. The asymmetric unit contains two phosphate species, which are disordered H₂PO₄^- and H₃PO₄, and one imidazolium cation, and has significant hydrogen bonding.

Key indicators

• Single-crystal X-ray study
• T = 296 K
• Mean [sigma](P-O) = 0.002 Å
• Disorder in main residue
• R factor = 0.038
• wR factor = 0.079
• Data-to-parameter ratio = 14.1

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PLAT042_ALERT_1_C Calc. and Rep. MoietyFormula Strings Differ ....          ?
PLAT301_ALERT_3_C Main Residue  Disorder .........................      25.00 Perc.

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              O---O distances (~2.5 \%A), making them hydrogen bonds (Table 2). The
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S1. Comment

Recently, there has been much interest in vanadium phosphate-based compounds, due mostly to their potential application as battery materials, electrochromic displays and catalysts (Padhi et al., 1997; Song et al., 2005; Whittingham, 2004; Whittingham et al., 2005). Since the early 1980 s, a new class of ionic liquids, based on imidazole (Imd), has also attracted great interest due to its potential commercial and environmental advantages (Seddon, 1997). These ionic liquids are solvent-free, allowing them, potentially, to behave in a very different manner from conventional ionic electrolytic solutions. They are also capable of operating over a wide range of temperature, from ambient to well over 373 K. The possible use of an imidazolium salt, ImdX, mixed with the corresponding lithium salt, LiX, could be very attractive for advanced lithium batteries. Such a battery might contain a lithium-containing anode material, an imidazolium electrolyte and a cathode containing a layered-structure vanadium oxide or phosphate. However, a concern with such a system is the possible intercalation of the imidazolium cations between the layers of the oxide/phosphate structures. Such intercalation compounds have not previously been reported. While studying these possible reactions, we accidentally synthesized the title compound, (I), containing only imidazolium cations and phosphate anions. This paper describes the crystal structure of this compound.

Colourless crystals of (I) were found to crystallize in the monoclinic system. The structure is of an ionic type with hydrogen bonding (Figs. 1 and 2). It contains two phosphate groups and one imidazolium cation. Some of the H atoms are disordered between the two phosphate groups so they alternate between H₂PO₄⁻ and H₃PO₄. This disorder provides H atoms for all the short O—O distances (~2.5 Å), making them hydrogen bonds (Table 2). The imidazolium cation is also disordered between two orientations in about a 3:1 ratio. The strong hydrogen bonding in this structure is in addition to the electrostatic interactions between the phosphate anions and the imidazolium cations. Both PO₄ tetrahedra (Table 1) have two short P—O distances in the range 1.50–1.51 Å, while the other two lie between 1.54 and 1.56 Å.

S2. Experimental

Several approaches have been used for the synthesis of VOPO₄·2H₂O, the starting material in this study. The conventional approach involves the prolonged refluxing of V₂O₅ in aqueous phosphoric acid (Ladwig, 1965). A more rapid method uses a sonochemical approach (Park et al., 2001). In order to optimize the possible reactions, we decided to make a highly porous phosphate of formula VOPO₄·2H₂O by reacting foam gel V₂O₅ (Chandrappa et al., 2002) with phosphoric acid. The title compound was isolated from a mixture of excess imidazole and the above VOPO₄·2H₂O in a 3:1 molar ratio. This mixture was allowed to stand for 30 d under ambient conditions. Crystals began to grow at the surface of a greenish solid precipitate in the beaker. The title compound was obtained by careful removal of a single-crystal from the mixture.

S3. Refinement

H atoms of the phosphate groups were located from a difference Fourier map and refined independently. On introducing partially occupied H atoms into the PO₄ groups, refinement of the H-atom positions was constrained, but the U_iso(H) values were refined freely. The H atoms of the imidazolium ions were refined within a rigid-body model riding on their parent C or N atoms, with C—H = 0.93 Å and N—H = 0.86 Å, and with U_iso(H) = 1.2U_eq(C,N). [Please check added text]

Computing details

Data collection: SMART (Bruker, 1999); cell refinement: SAINT (Bruker, 1999); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 2003); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. A view of the molecular ions in (I), showing the numbering scheme employed. Displacement ellipsoids are shown at the 50% probability level and H atoms are displayed with arbitrarily small radii.
	Fig. 2. A view showing the packing of the moloecular ions in (I).

Phosphoric acid imidazolium dihydrogenphosphate

Crystal data

H3PO4·C3H5N2^+·H2PO4^−	F(000) = 544
Mr = 264.07	Dx = 1.749 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 3640 reflections
a = 9.138 (4) Å	θ = 2.6–31.3°
b = 14.605 (6) Å	µ = 0.46 mm^−1
c = 7.516 (3) Å	T = 296 K
β = 91.447 (6)°	Prism, colourless
V = 1002.8 (7) Å^3	0.36 × 0.28 × 0.24 mm
Z = 4

Data collection

Bruker APEX CCD area-detector diffractometer	2291 independent reflections
Radiation source: fine-focus sealed tube	1895 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.025
Detector resolution: 8.33 pixels mm^-1	θmax = 27.5°, θmin = 2.2°
ω scans	h = −11→8
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	k = −16→18
Tmin = 0.802, Tmax = 0.895	l = −9→9
6101 measured reflections

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.038	H-atom parameters constrained
wR(F^2) = 0.079	w = 1/[σ^2(Fo^2) + (0.003P)^2 + 1.182P] where P = [-max(Fo^2,0) + 2Fc^2]/3
S = 1.00	(Δ/σ)max < 0.001
2291 reflections	Δρmax = 0.28 e Å^−3
162 parameters	Δρmin = −0.36 e Å^−3
40 restraints	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0192 (9)

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	Occ. (<1)
N1	0.1910 (4)	−0.0147 (3)	0.6095 (6)	0.0482 (12)	0.775 (5)
H1	0.1425	−0.0293	0.7019	0.058*	0.775 (5)
C2	0.2814 (6)	0.0540 (3)	0.5987 (6)	0.0496 (11)	0.775 (5)
H2	0.3060	0.0947	0.6899	0.059*	0.775 (5)
N3	0.3314 (4)	0.0557 (3)	0.4369 (6)	0.0530 (11)	0.775 (5)
H3	0.3925	0.0948	0.3963	0.064*	0.775 (5)
C4	0.2694 (7)	−0.0156 (5)	0.3435 (7)	0.0698 (17)	0.775 (5)
H4	0.2844	−0.0299	0.2248	0.084*	0.775 (5)
C5	0.1858 (6)	−0.0593 (3)	0.4521 (8)	0.0580 (12)	0.775 (5)
H5	0.1321	−0.1118	0.4260	0.070*	0.775 (5)
N1A	0.3153 (16)	0.0060 (13)	0.362 (2)	0.0482 (12)	0.225 (5)
H1A	0.3587	0.0205	0.2663	0.058*	0.225 (5)
C2A	0.311 (2)	0.0571 (13)	0.516 (4)	0.0580 (12)	0.225 (5)
H2A	0.3574	0.1124	0.5406	0.070*	0.225 (5)
N3A	0.226 (3)	0.0103 (17)	0.617 (3)	0.0698 (17)	0.225 (5)
H3A	0.2038	0.0251	0.7234	0.084*	0.225 (5)
C4A	0.179 (2)	−0.0629 (15)	0.533 (3)	0.0530 (11)	0.225 (5)
H4A	0.1091	−0.1034	0.5744	0.064*	0.225 (5)
C5A	0.241 (2)	−0.0697 (12)	0.391 (2)	0.0496 (11)	0.225 (5)
H5A	0.2357	−0.1201	0.3153	0.059*	0.225 (5)
P1	0.01161 (6)	0.16289 (3)	0.03493 (7)	0.02685 (15)
O11	0.06306 (18)	0.15451 (10)	−0.15976 (19)	0.0386 (4)
H11	0.0805	0.2056	−0.1990	0.036 (9)*	0.70 (4)
O12	−0.11891 (17)	0.22935 (11)	0.0439 (2)	0.0476 (4)
H12	−0.1893	0.2077	−0.0114	0.074 (11)*
O13	0.12885 (16)	0.20326 (11)	0.1534 (2)	0.0355 (4)
H13	0.0916	0.2374	0.2261	0.036 (9)*	0.30 (4)
O14	−0.03690 (19)	0.06759 (10)	0.0857 (2)	0.0446 (4)
H14	0.0001	0.0301	0.0192	0.055 (17)*	0.50
P2	0.50326 (6)	0.16567 (4)	0.03353 (7)	0.02883 (15)
O21	0.55744 (18)	0.15440 (11)	0.2303 (2)	0.0398 (4)
H21	0.5776	0.2048	0.2722	0.056 (9)*
O22	0.37996 (17)	0.23818 (11)	0.0257 (2)	0.0412 (4)
H22	0.3072	0.2179	0.0738	0.084 (12)*
O23	0.62265 (16)	0.20105 (11)	−0.0815 (2)	0.0386 (4)
O24	0.43963 (19)	0.07395 (11)	−0.0202 (2)	0.0475 (4)
H24	0.5002	0.0336	0.0002	0.056 (18)*	0.50

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.044 (2)	0.058 (3)	0.0428 (17)	−0.0056 (16)	0.0130 (15)	0.0116 (19)
C2	0.058 (2)	0.051 (2)	0.0405 (18)	−0.0082 (17)	0.0060 (16)	−0.0091 (17)
N3	0.062 (2)	0.055 (2)	0.0429 (19)	−0.0234 (15)	0.0087 (15)	0.0090 (17)
C4	0.079 (4)	0.087 (4)	0.044 (2)	−0.025 (3)	0.018 (2)	−0.024 (2)
C5	0.067 (3)	0.0396 (19)	0.068 (3)	−0.0160 (17)	0.007 (3)	−0.007 (2)
N1A	0.044 (2)	0.058 (3)	0.0428 (17)	−0.0056 (16)	0.0130 (15)	0.0116 (19)
C2A	0.067 (3)	0.0396 (19)	0.068 (3)	−0.0160 (17)	0.007 (3)	−0.007 (2)
N3A	0.079 (4)	0.087 (4)	0.044 (2)	−0.025 (3)	0.018 (2)	−0.024 (2)
C4A	0.062 (2)	0.055 (2)	0.0429 (19)	−0.0234 (15)	0.0087 (15)	0.0090 (17)
C5A	0.058 (2)	0.051 (2)	0.0405 (18)	−0.0082 (17)	0.0060 (16)	−0.0091 (17)
P1	0.0265 (3)	0.0219 (3)	0.0323 (3)	−0.0002 (2)	0.0049 (2)	−0.0022 (2)
O11	0.0562 (10)	0.0293 (8)	0.0306 (8)	−0.0045 (7)	0.0063 (7)	0.0009 (6)
O12	0.0279 (8)	0.0365 (9)	0.0779 (12)	0.0062 (7)	−0.0074 (8)	−0.0179 (8)
O13	0.0270 (8)	0.0414 (9)	0.0382 (8)	0.0037 (7)	0.0021 (6)	−0.0102 (7)
O14	0.0619 (11)	0.0266 (8)	0.0463 (9)	−0.0061 (8)	0.0240 (8)	−0.0009 (7)
P2	0.0243 (3)	0.0273 (3)	0.0349 (3)	0.0023 (2)	0.0015 (2)	−0.0013 (2)
O21	0.0483 (10)	0.0353 (9)	0.0358 (8)	−0.0007 (7)	0.0000 (7)	0.0004 (7)
O22	0.0276 (8)	0.0324 (8)	0.0638 (11)	0.0055 (7)	0.0090 (7)	0.0072 (7)
O23	0.0247 (7)	0.0526 (10)	0.0386 (8)	0.0039 (7)	0.0037 (6)	0.0047 (7)
O24	0.0438 (10)	0.0326 (9)	0.0655 (11)	−0.0007 (8)	−0.0102 (8)	−0.0090 (8)

Geometric parameters (Å, º)

N1—C2	1.303 (5)	C5—H5	0.9300
N1—C5	1.351 (6)	P1—O13	1.4965 (16)
N1—H1	0.8600	P1—O14	1.5125 (16)
C2—N3	1.311 (4)	P1—O12	1.5405 (16)
C2—H2	0.9300	P1—O11	1.5530 (16)
N3—C4	1.369 (6)	P2—O23	1.5010 (16)
N3—H3	0.8600	P2—O24	1.5112 (17)
C4—C5	1.299 (7)	P2—O22	1.5464 (16)
C4—H4	0.9300	P2—O21	1.5565 (17)
C2—N1—C5	109.0 (3)	O13—P1—O11	111.24 (9)
C2—N1—H1	125.5	O14—P1—O11	105.22 (9)
C5—N1—H1	125.5	O12—P1—O11	110.11 (10)
N1—C2—N3	108.0 (3)	P1—O11—H11	109.5
N1—C2—H2	126.0	P1—O12—H12	109.5
N3—C2—H2	126.0	P1—O13—H13	109.5
C2—N3—C4	108.2 (3)	P1—O14—H14	109.5
C2—N3—H3	125.9	O23—P2—O24	115.57 (10)
C4—N3—H3	125.9	O23—P2—O22	106.37 (9)
C5—C4—N3	107.1 (4)	O24—P2—O22	108.77 (10)
C5—C4—H4	126.5	O23—P2—O21	111.44 (9)
N3—C4—H4	126.5	O24—P2—O21	105.72 (10)
C4—C5—N1	107.8 (4)	O22—P2—O21	108.82 (9)
C4—C5—H5	126.1	P2—O21—H21	109.5
O13—P1—O14	115.00 (10)	P2—O22—H22	109.5
O13—P1—O12	105.62 (9)	P2—O24—H24	109.5
O14—P1—O12	109.67 (10)
C5—N1—C2—N3	1.5 (5)	N3—C4—C5—N1	2.0 (7)
N1—C2—N3—C4	−0.3 (5)	C2—N1—C5—C4	−2.2 (6)
C2—N3—C4—C5	−1.1 (6)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···O14^i	0.86	1.97	2.827 (4)	177
N3—H3···O21	0.86	2.16	2.986 (4)	160
O11—H11···O13^ii	0.82	1.79	2.587 (2)	162
O12—H12···O23^iii	0.82	1.79	2.554 (2)	155
O13—H13···O11^iv	0.82	1.82	2.587 (2)	155
O14—H14···O14^v	0.82	1.67	2.461 (3)	162
O21—H21···O23^iv	0.82	1.80	2.602 (2)	165
O22—H22···O13	0.82	1.76	2.561 (2)	164
O24—H24···O24^vi	0.82	1.67	2.441 (4)	156

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