A zwitterionic viologen disulfonate–hydroquinone charge-transfer crystal: 2,2′-(4,4′-bipyridinium-1,1′-diyl)di(ethanesulfonate)–hydroquinone (1/2)

The structure of the title compound, C₁₄H₁₆N₂O₆S₂·2C₆H₆O₂, consists of 2,2′-(4,4′-bipyridinium-1,1′-diyl)di(ethanesulfon­ate) mol­ecules (with crystallographically imposed twofold symmetry) that are hydrogen bonded to each other, as well as to hydro­quinone mol­ecules, in a complex three-dimensional motif. The orange color of the crystals is indicative of the donor–acceptor interaction between the electron-rich hydro­quinone π-donor and the electron-deficient bipyridinium π-acceptor. The dihedral angle between the bipyridyl planes is 38.31 (11)°. The distance from the centroid of one of the hydro­quinone mol­ecules to the center of the bipyridinium group is 3.653 (3) Å, which is within the range typically observed for molecular complexes exhibiting charge-transfer characteristics.

S1. Comment

Electron donor-acceptor interactions have been recognized as playing a key role in the development of unusual optical, electric and magnetic properties of crystalline materials (Hubig & Kochi, 1995). Since the discovery of neutral-ionic transitions induced by temperature or pressure in mixed π-stacked organic charge-transfer crystals, extensive studies have been carried out on these types of systems (Aoki & Nakayama, 1997; Brocks, 1997). In particular, charge-transfer complexes incorporating viologen [N,N'-bis(substituted)bipyridinium] as the electron-acceptor component have been the subject of much research, due to the potential use of viologen in photochemical energy conversion systems (Jones & Malba, 1985). In addition, the interaction of the viologen moiety with π-electron donors has been extensively exploited in the design of supramolecular assemblies, including rotaxanes, catenanes and molecular shuttles (Asakawa et al., 1996; Simonsen et al., 1998; Hu et al., 1998; Loeb & Wisner, 2000; Willner et al., 1992). The intermolecular interactions of viologen derivatives with electron-rich aromatic systems are therefore of great interest.

Here, we report the crystal structure of the charge-transfer complex 4,4'-bipyridinium-1,1'-bis(2-ethylsulfonate)-bis(hydroquinone), (I). The orange color of these crystals is indicative of the interaction between the hydroquinone π-donor and the bipyridinium π-acceptor components of the complex, which gives rise to charge-transfer electronic transitions. \sch

Fig. 1 shows the molecular structure of the (I), along with the atom-numbering scheme. The two halves of the bipyridinium moiety are related by a twofold axis of symmetry. The dihedral angle between the bipyridyl planes is 38.31 (11)°. The two halves of the hydroquinone molecules are also related by symmetry, the first by an inversion center and the second by a twofold axis, as indicated in Fig. 1.

In previous reports, the packing of viologen crystals in the solid state has been described as a stacking of planar bipyridinium dications and anionic counterions. The anions have close contacts between the N atoms and the neighboring C atoms of the pyridyl rings, and are directed toward this cationic center rather than toward the centroid of the rings (Argay & Kálmán, 1995; Poojary et al., 1994; Russell & Wallwork, 1972). In the zwitterionic bipyridinium described here, equivalent viologen disulfonate molecules pack in an end-to-end manner, as shown in Fig. 2. This packing allows for the sulfonate anions to lie in between two cationic pyridinium units. The intramolecular distance from the N1/C1—C5 centroid to the O1/O2/O3 centroid is 3.964 (3) Å. The corresponding intermolecular distance is 5.146 (3) Å.

The orientation of the nearly planar hydroquinone toward the twisted bipyridinium group is also seen in Fig. 2. It has been suggested that the intermolecular interactions between aromatic systems is more accurately described as a σ-π interaction rather than a π-π interaction (Hunter & Sanders, 1990). This interaction is described primarily as an electrostatic one between the more positive σ framework of one molecule and the electron-rich π system of the other. In the case of (I), the nearly planar hydroquinone is not parallel to the pyridinium rings. This geometry allows for a π-σ interaction between the electron-rich π system of the hydroquinone and the σ framework of each half of the bipyridinium group. The angle formed between the plane of the hydroquinone group and the plane of each bipyridyl ring is 19.16 (13)°. The distance from the centroid of the hydroquinone to the center of the C3—C3' bond is 3.653 (3) Å. Numerous charge-transfer complexes have been prepared, either as molecular crystals or as ion-pair salts. Typically, a close approach of 3.5–3.7 Å of the electron donor and acceptor molecules is observed (Hubig & Kochi, 1995).

As summarized in Table 1, hydrogen bonding is an important intermolecular interaction in the charge-transfer crystal of (I). All O atoms in this crystal act as hydrogen-bond acceptors. Further, the O atoms of the two hydroquinone molecules are also hydrogen-bond donors. Atoms O2 and O3 of the sulfonate group are hydrogen-bonded to the acidic atoms H4 and H5 attached to the O atoms of the hydroquinone molecules. This interaction is indicated in Fig. 1. The remaining sulfonate O atom (O1) is involved in the intermolecular C—H···O hydrogen bond, with the H atom located on atom C1 of the pyridinium ring (Fig. 2).

Two additional C—H···O interactions which fit the criteria of a hydrogen bond, as defined by Desiraju (1996), are observed in this crystal. These are between the hydroquinone O atoms and two acidic C—H H atoms of the ethyl group, as illustrated in Fig. 3. The various hydrogen-bonding interactions result in the formation of the three-dimensional hydrogen-bonded network illustrated in the packing drawing (Fig. 4).

S2. Experimental

4,4'-Bipyridinium-1,1'-bis(2-ethylsulfonate) was prepared as described previously by Vermeulen & Robinson (1996). Hydroquinone was purchased from Fisher. Crystals of (I) were obtained by dissolving 4,4'-bipyridinium-1,1'-bis(2-ethylsulfonate) (0.192 g, 0.516 mol) and guanidinium hydrochloride (0.099 g, 1.03 mol) in deionized water (15 ml). Hydroquinone (0.050 g, 0.454 mol) was dissolved in warm methanol (15 ml). The methanol solution was slowly added to the aqueous solution while stirring. The mixture was aged at room temperature until orange crystals of (I) appeared.

S3. Refinement

The rotational orientations of the hydroxyl groups were determined by the circular Fourier refinement method available in SHELXL97 (Sheldrick, 1997). All H atoms were treated as riding, with O—H distances of 0.82 Å and C—H distances in the range 0.93–0.97 Å.

Computing details

Data collection: MSC/AFC Diffractometer Control Software (Molecular Structure Corporation, 1996); cell refinement: MSC/AFC Diffractometer Control Software; data reduction: PROCESS in TEXSAN (Molecular Structure Corporation, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: LS in TEXSAN and SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP (Johnson, 1965) and PLATON (Spek, 2000); software used to prepare material for publication: TEXSAN, SHELXL97 and PLATON.

Figures

	Fig. 1. The molecular structure and atom-numbering scheme for (I), with displacement ellipsoids at the 50% probability level [symmetry codes: (i) 1 - x, y, 1/2 - z; (ii) 1 - x, y, 3/2 - z; (iii) -x, -y, 1 - z].
	Fig. 2. A view of (I) showing the orientation of the planar hydroquinone with respect to the bipyridinium unit, as well as the C1—H1···O1 intermolecular hydrogen bonds [symmetry codes: (iv) 1 - x, -y, 1 - z; (v) 1/2 - x, 1/2 + y, 3/2 - z; (vi) 1/2 - x, 1/2 - y, 1 - z].
	Fig. 3. A view of (I) showing the hydroquinone O atoms as hydrogen-bond acceptors in C—H···O hydrogen bonds [symmetry codes: (vii) 1/2 - x, 1/2 - y, 1 - z; (viii) x, -y, z - 1/2].
	Fig. 4. The molecular packing and hydrogen bonding in (I), as viewed down the c axis. Black circles indicate O atoms, large shaded circles indicate S atoms and cross-hatched circles indicate N atoms.

4,4'-bipyridinium-1,1'-bis(2-ethylsulfonate)-bis(hydroquinone)

Crystal data

C14H16N2O6S2·2C6H6O2	F(000) = 1240
Mr = 592.62	Dx = 1.514 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71069 Å
Hall symbol: -C 2yc	Cell parameters from 25 reflections
a = 20.006 (5) Å	θ = 13.5–14.8°
b = 11.072 (3) Å	µ = 0.27 mm^−1
c = 11.882 (3) Å	T = 296 K
β = 98.94 (2)°	Prism, orange
V = 2600.2 (12) Å^3	0.40 × 0.24 × 0.22 mm
Z = 4

Data collection

Rigaku AFC-5S diffractometer	1585 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.012
Graphite monochromator	θmax = 25.1°, θmin = 2.1°
ω scans	h = 0→23
Absorption correction: ψ scan (North et al., 1968)	k = 0→13
Tmin = 0.921, Tmax = 0.943	l = −14→13
2370 measured reflections	3 standard reflections every 100 reflections
2301 independent reflections	intensity decay: 0.2%

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.033	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.098	H-atom parameters constrained
S = 1.04	w = 1/[σ^2(Fo^2) + (0.0448P)^2 + 1.6343P] where P = (Fo^2 + 2Fc^2)/3
2301 reflections	(Δ/σ)max < 0.001
183 parameters	Δρmax = 0.22 e Å^−3
0 restraints	Δρmin = −0.28 e Å^−3

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

	x	y	z	Uiso*/Ueq
S1	0.25200 (3)	0.02858 (6)	0.42643 (5)	0.03072 (18)
O1	0.21681 (9)	0.11056 (17)	0.49050 (16)	0.0501 (5)
O2	0.32480 (8)	0.03458 (19)	0.45762 (14)	0.0481 (5)
O3	0.22605 (9)	−0.09374 (16)	0.42906 (15)	0.0461 (5)
O4	0.36208 (9)	0.1026 (2)	0.68028 (17)	0.0602 (6)
O5	0.12228 (9)	−0.11767 (19)	0.55739 (15)	0.0496 (5)
N1	0.32662 (9)	0.21765 (17)	0.25711 (15)	0.0287 (4)
C1	0.36850 (12)	0.2564 (2)	0.3489 (2)	0.0345 (6)
C2	0.43663 (12)	0.2626 (2)	0.3474 (2)	0.0344 (6)
C3	0.46307 (11)	0.2291 (2)	0.25100 (19)	0.0281 (5)
C4	0.41782 (11)	0.1931 (2)	0.15581 (19)	0.0318 (5)
C5	0.35010 (12)	0.1877 (2)	0.16095 (19)	0.0334 (6)
C6	0.25297 (11)	0.2045 (2)	0.2613 (2)	0.0383 (6)
C7	0.23396 (12)	0.0744 (2)	0.28158 (19)	0.0357 (6)
C8	0.43086 (12)	0.1018 (2)	0.7124 (2)	0.0388 (6)
C9	0.45540 (13)	0.1002 (3)	0.8273 (2)	0.0498 (7)
H9	0.4255	0.0990	0.8799	0.060*
C10	0.47568 (14)	0.1004 (3)	0.6352 (2)	0.0483 (7)
C11	0.01358 (12)	−0.0703 (2)	0.5965 (2)	0.0393 (6)
C12	0.06279 (12)	−0.0573 (2)	0.52679 (19)	0.0361 (6)
C13	0.04836 (12)	0.0135 (2)	0.4303 (2)	0.0391 (6)
H1	0.3512	0.2792	0.4140	0.041*
H2	0.4654	0.2894	0.4116	0.041*
H4	0.3530	0.0894	0.6117	0.090*
H5	0.1487	−0.0997	0.5137	0.074*
H4A	0.4337	0.1728	0.0888	0.038*
H5A	0.3201	0.1631	0.0974	0.040*
H6A	0.2274	0.2323	0.1899	0.046*
H6B	0.2409	0.2548	0.3219	0.046*
H7A	0.2582	0.0219	0.2364	0.043*
H7B	0.1860	0.0640	0.2551	0.043*
H10	0.4594	0.0995	0.5575	0.058*
H11	0.0224	−0.1176	0.6618	0.047*
H13	0.0808	0.0231	0.3828	0.047*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
S1	0.0286 (3)	0.0349 (3)	0.0290 (3)	−0.0044 (3)	0.0053 (2)	−0.0020 (3)
O1	0.0548 (12)	0.0504 (12)	0.0493 (11)	0.0010 (9)	0.0210 (9)	−0.0120 (9)
O2	0.0289 (9)	0.0732 (14)	0.0404 (10)	−0.0026 (9)	0.0002 (7)	0.0090 (9)
O3	0.0600 (12)	0.0339 (10)	0.0460 (11)	−0.0116 (9)	0.0136 (9)	−0.0013 (8)
O4	0.0353 (10)	0.0908 (17)	0.0508 (12)	0.0040 (11)	−0.0042 (8)	−0.0136 (12)
O5	0.0353 (10)	0.0697 (14)	0.0440 (11)	−0.0023 (10)	0.0073 (8)	0.0058 (10)
N1	0.0254 (10)	0.0261 (10)	0.0353 (11)	0.0005 (8)	0.0067 (8)	0.0017 (8)
C1	0.0359 (13)	0.0346 (14)	0.0355 (13)	−0.0053 (11)	0.0131 (11)	−0.0089 (11)
C2	0.0311 (12)	0.0393 (14)	0.0332 (13)	−0.0063 (11)	0.0060 (10)	−0.0099 (11)
C3	0.0292 (12)	0.0247 (12)	0.0309 (12)	−0.0006 (10)	0.0068 (9)	0.0008 (10)
C4	0.0308 (13)	0.0370 (14)	0.0289 (12)	0.0027 (10)	0.0091 (10)	−0.0002 (10)
C5	0.0359 (14)	0.0341 (14)	0.0292 (12)	−0.0018 (11)	0.0023 (10)	−0.0004 (11)
C6	0.0227 (11)	0.0456 (17)	0.0471 (15)	0.0024 (11)	0.0069 (10)	0.0093 (12)
C7	0.0289 (12)	0.0465 (15)	0.0305 (12)	−0.0085 (11)	0.0012 (9)	0.0030 (11)
C8	0.0303 (13)	0.0401 (15)	0.0437 (14)	0.0012 (11)	−0.0014 (11)	−0.0043 (12)
C9	0.0389 (15)	0.068 (2)	0.0430 (15)	0.0046 (14)	0.0074 (12)	−0.0158 (14)
C10	0.0418 (15)	0.0623 (19)	0.0379 (14)	−0.0046 (14)	−0.0034 (11)	0.0123 (13)
C11	0.0417 (15)	0.0487 (16)	0.0274 (12)	−0.0116 (12)	0.0053 (10)	0.0004 (11)
C12	0.0318 (13)	0.0462 (16)	0.0298 (12)	−0.0087 (11)	0.0038 (10)	−0.0062 (11)
C13	0.0348 (14)	0.0537 (16)	0.0308 (13)	−0.0123 (12)	0.0115 (10)	−0.0045 (12)

Geometric parameters (Å, º)

S1—O1	1.4373 (18)	C11—C12	1.389 (3)
S1—O2	1.4476 (17)	C12—C13	1.381 (3)
S1—O3	1.4525 (18)	C11—C13^iii	1.382 (4)
S1—C7	1.777 (2)	O5—H5	0.8200
C6—C7	1.518 (3)	O4—H4	0.8200
N1—C6	1.489 (3)	C1—H1	0.9300
N1—C1	1.338 (3)	C2—H2	0.9300
C1—C2	1.367 (3)	C4—H4A	0.9300
C2—C3	1.385 (3)	C5—H5A	0.9300
C3—C4	1.393 (3)	C6—H6A	0.9700
C4—C5	1.367 (3)	C6—H6B	0.9700
N1—C5	1.343 (3)	C7—H7A	0.9700
C3—C3^i	1.481 (4)	C7—H7B	0.9700
O4—C8	1.370 (3)	C9—H9	0.9300
C8—C9	1.377 (4)	C10—H10	0.9300
C8—C10	1.379 (4)	C11—H11	0.9300
C9—C10^ii	1.382 (4)	C13—H13	0.9300
O5—C12	1.364 (3)
O1—S1—O2	113.05 (12)	N1—C5—H5A	119.8
O1—S1—O3	111.80 (11)	C8—O4—H4	109.5
O2—S1—O3	112.70 (12)	C12—O5—H5	109.5
O1—S1—C7	106.88 (12)	N1—C1—H1	119.8
O2—S1—C7	106.24 (11)	C2—C1—H1	119.8
O3—S1—C7	105.53 (11)	C1—C2—H2	119.8
C6—C7—S1	113.99 (18)	C3—C2—H2	119.8
N1—C6—C7	111.82 (19)	C5—C4—H4A	120.0
C1—N1—C5	121.0 (2)	C3—C4—H4A	120.0
C1—N1—C6	120.1 (2)	C4—C5—H5A	119.8
C5—N1—C6	118.9 (2)	N1—C6—H6A	109.3
N1—C1—C2	120.3 (2)	C7—C6—H6A	109.3
C1—C2—C3	120.4 (2)	N1—C6—H6B	109.3
C2—C3—C4	117.7 (2)	C7—C6—H6B	109.3
C3—C4—C5	120.0 (2)	H6A—C6—H6B	107.9
N1—C5—C4	120.5 (2)	C6—C7—H7A	108.8
C2—C3—C3^i	121.6 (2)	S1—C7—H7A	108.8
C4—C3—C3^i	120.7 (2)	C6—C7—H7B	108.8
O4—C8—C9	117.7 (2)	S1—C7—H7B	108.8
O4—C8—C10	122.9 (2)	H7A—C7—H7B	107.7
C10—C8—C9	119.4 (2)	C10^ii—C9—H9	119.9
C10^ii—C9—C8	120.3 (3)	C8—C9—H9	119.9
C9^ii—C10—C8	120.3 (2)	C9^ii—C10—H10	119.8
O5—C12—C11	117.4 (2)	C8—C10—H10	119.8
O5—C12—C13	124.1 (2)	C13^iii—C11—H11	119.7
C11—C12—C13	118.5 (2)	C12—C11—H11	119.7
C13^iii—C11—C12	120.6 (2)	C12—C13—H13	119.5
C12—C13—C11^iii	120.9 (2)	C11^iii—C13—H13	119.5
O1—S1—C7—C6	56.9 (2)	C2—C3—C4—C5	−2.2 (3)
O2—S1—C7—C6	−64.1 (2)	C3—C4—C5—N1	0.4 (4)
O3—S1—C7—C6	176.04 (17)	C1—C2—C3—C3^i	−176.92 (18)
N1—C6—C7—S1	80.2 (2)	C3^i—C3—C4—C5	176.64 (17)
C5—N1—C1—C2	−2.1 (4)	O4—C8—C9—C10^ii	−179.5 (3)
C6—N1—C1—C2	176.5 (2)	C10—C8—C9—C10^ii	1.7 (4)
N1—C1—C2—C3	0.2 (4)	O4—C8—C10—C9^ii	179.7 (3)
C1—N1—C5—C4	1.8 (3)	C9—C8—C10—C9^ii	−1.5 (4)
C6—N1—C5—C4	−176.8 (2)	C13^iii—C11—C12—O5	−178.8 (2)
C1—N1—C6—C7	−99.2 (3)	C13^iii—C11—C12—C13	0.0 (4)
C5—N1—C6—C7	79.4 (3)	O5—C12—C13—C11^iii	178.7 (2)
C1—C2—C3—C4	1.9 (4)	C11—C12—C13—C11^iii	0.0 (4)

Symmetry codes: (i) −x+1, y, −z+1/2; (ii) −x+1, y, −z+3/2; (iii) −x, −y, −z+1.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O4—H4···O2	0.82	1.93	2.740 (3)	171
O5—H5···O3	0.82	1.97	2.773 (3)	164
C1—H1···O1^iv	0.93	2.26	3.120 (3)	154
C6—H6B···O4^iv	0.97	2.59	3.292 (3)	129
C7—H7B···O5^v	0.97	2.56	3.235 (3)	127

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