Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270102003955/jz1500sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270102003955/jz1500Isup2.hkl |
CCDC reference: 187923
The first crystal was isolated as a by-product in a synthesis of [Ru(bpy)3](PF6)2. Since this crystal was stained and of low quality, subsequent pure 2,2'-bipyridinium hexafluorophosphate was synthesized according to the method of Milani et al. (1997). Suitable single crystals of (I) were obtained by recrystallization from methanol by slow evaporation of the solvent.
H atoms on C atoms were calculated in their ideal positions and refined using a riding model, with Uiso(H) = 1.2Ueq(C). The H atom attached to N1 was identified in the difference Fourier map and refined freely, but also with with Uiso(H) = 1.2Ueq(N1).
Data collection: IPDS (Stoe & Cie, 1999) Query; cell refinement: IPDS; data reduction: IPDS; program(s) used to solve structure: SIR92 (Altomare et al., 1993); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 1990); software used to prepare material for publication: PLATON.
C10H9N2+·PF6− | F(000) = 608 |
Mr = 302.16 | Dx = 1.806 Mg m−3 |
Monoclinic, P21/c | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -P 2ybc | Cell parameters from 8000 reflections |
a = 6.1072 (4) Å | θ = 3.0–25.8° |
b = 26.0105 (14) Å | µ = 0.32 mm−1 |
c = 7.1380 (5) Å | T = 123 K |
β = 101.416 (9)° | Prism, colourless |
V = 1111.45 (13) Å3 | 0.24 × 0.20 × 0.14 mm |
Z = 4 |
Stoe IPDS diffractometer | 1766 reflections with I > 2σ(I) |
Radiation source: fine-focus sealed tube | Rint = 0.029 |
Graphite monochromator | θmax = 25.8°, θmin = 3.0° |
rotation scans | h = −7→7 |
11182 measured reflections | k = −31→31 |
2093 independent reflections | l = −8→8 |
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.029 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.091 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.05 | w = 1/[σ2(Fo2) + (0.0591P)2 + 0.2024P] where P = (Fo2 + 2Fc2)/3 |
2093 reflections | (Δ/σ)max < 0.001 |
175 parameters | Δρmax = 0.26 e Å−3 |
0 restraints | Δρmin = −0.21 e Å−3 |
0 constraints |
C10H9N2+·PF6− | V = 1111.45 (13) Å3 |
Mr = 302.16 | Z = 4 |
Monoclinic, P21/c | Mo Kα radiation |
a = 6.1072 (4) Å | µ = 0.32 mm−1 |
b = 26.0105 (14) Å | T = 123 K |
c = 7.1380 (5) Å | 0.24 × 0.20 × 0.14 mm |
β = 101.416 (9)° |
Stoe IPDS diffractometer | 1766 reflections with I > 2σ(I) |
11182 measured reflections | Rint = 0.029 |
2093 independent reflections |
R[F2 > 2σ(F2)] = 0.029 | 0 restraints |
wR(F2) = 0.091 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.05 | Δρmax = 0.26 e Å−3 |
2093 reflections | Δρmin = −0.21 e Å−3 |
175 parameters |
Experimental. Data were collected applying an imaging plate system (Stoe) with the following measurement parameters: Detector distance [mm] 70 Phi movement mode Oscillation Phi incr. [degrees] 0.6 Number of exposures 500 Irradiation / exposure [min] 5.00 For a detailed description of the method see: Sheldrick, G·M., Paulus, E. Vertesy, L. & Hahn, F. (1995) Acta Cryst. B51, 89–98. |
Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All e.s.d.'s are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles |
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. |
x | y | z | Uiso*/Ueq | ||
N1 | 0.34527 (19) | 0.10367 (4) | 0.32410 (16) | 0.0202 (3) | |
N2 | 0.31439 (19) | 0.20448 (4) | 0.29431 (16) | 0.0218 (3) | |
C1 | 0.3204 (2) | 0.05270 (5) | 0.32766 (19) | 0.0228 (4) | |
C2 | 0.4682 (2) | 0.02151 (5) | 0.25843 (18) | 0.0225 (4) | |
C3 | 0.6392 (2) | 0.04420 (5) | 0.18499 (18) | 0.0221 (4) | |
C4 | 0.6587 (2) | 0.09728 (5) | 0.18117 (18) | 0.0207 (4) | |
C5 | 0.5079 (2) | 0.12751 (5) | 0.25343 (16) | 0.0184 (4) | |
C6 | 0.5045 (2) | 0.18430 (5) | 0.25800 (17) | 0.0193 (4) | |
C7 | 0.6836 (2) | 0.21390 (5) | 0.22694 (19) | 0.0220 (4) | |
C8 | 0.6626 (2) | 0.26691 (5) | 0.22943 (19) | 0.0246 (4) | |
C9 | 0.4677 (2) | 0.28831 (5) | 0.26513 (19) | 0.0247 (4) | |
C10 | 0.2994 (2) | 0.25554 (5) | 0.29845 (19) | 0.0240 (4) | |
P | 0.00308 (5) | 0.09407 (1) | 0.74709 (4) | 0.0176 (1) | |
F1 | −0.25948 (13) | 0.10382 (3) | 0.71589 (13) | 0.0298 (3) | |
F2 | −0.04013 (15) | 0.04768 (3) | 0.59649 (12) | 0.0294 (3) | |
F3 | 0.01746 (14) | 0.13359 (3) | 0.57551 (11) | 0.0279 (3) | |
F4 | 0.04584 (15) | 0.14050 (3) | 0.89714 (12) | 0.0292 (3) | |
F5 | −0.00984 (15) | 0.05462 (3) | 0.91659 (12) | 0.0286 (3) | |
F6 | 0.26578 (13) | 0.08464 (3) | 0.77507 (13) | 0.0307 (3) | |
H1 | 0.258 (3) | 0.1221 (7) | 0.369 (2) | 0.0240* | |
H1A | 0.20400 | 0.03840 | 0.37660 | 0.0270* | |
H2 | 0.45400 | −0.01410 | 0.26080 | 0.0270* | |
H3 | 0.74110 | 0.02370 | 0.13820 | 0.0260* | |
H4 | 0.77200 | 0.11250 | 0.13050 | 0.0250* | |
H7 | 0.81370 | 0.19850 | 0.20510 | 0.0260* | |
H8 | 0.77820 | 0.28790 | 0.20730 | 0.0300* | |
H9 | 0.44950 | 0.32380 | 0.26690 | 0.0300* | |
H10 | 0.16970 | 0.27000 | 0.32500 | 0.0290* |
U11 | U22 | U33 | U12 | U13 | U23 | |
N1 | 0.0203 (6) | 0.0194 (6) | 0.0222 (6) | 0.0014 (4) | 0.0075 (5) | −0.0007 (4) |
N2 | 0.0216 (6) | 0.0214 (6) | 0.0227 (6) | 0.0013 (4) | 0.0054 (5) | −0.0007 (4) |
C1 | 0.0244 (7) | 0.0213 (7) | 0.0232 (7) | −0.0040 (5) | 0.0061 (5) | 0.0019 (5) |
C2 | 0.0280 (7) | 0.0162 (6) | 0.0225 (6) | −0.0001 (5) | 0.0029 (5) | 0.0001 (4) |
C3 | 0.0251 (7) | 0.0209 (7) | 0.0204 (6) | 0.0031 (5) | 0.0050 (5) | −0.0017 (5) |
C4 | 0.0217 (7) | 0.0204 (7) | 0.0206 (6) | −0.0003 (5) | 0.0058 (5) | 0.0000 (5) |
C5 | 0.0194 (6) | 0.0196 (7) | 0.0158 (6) | −0.0013 (5) | 0.0028 (5) | 0.0003 (4) |
C6 | 0.0207 (7) | 0.0195 (7) | 0.0174 (6) | 0.0013 (5) | 0.0031 (5) | 0.0000 (4) |
C7 | 0.0199 (6) | 0.0209 (7) | 0.0250 (7) | −0.0002 (5) | 0.0040 (5) | 0.0006 (5) |
C8 | 0.0243 (7) | 0.0210 (7) | 0.0273 (7) | −0.0044 (5) | 0.0023 (6) | 0.0008 (5) |
C9 | 0.0285 (7) | 0.0167 (7) | 0.0267 (7) | 0.0009 (5) | −0.0002 (6) | 0.0004 (5) |
C10 | 0.0231 (7) | 0.0211 (7) | 0.0267 (7) | 0.0042 (5) | 0.0026 (6) | −0.0016 (5) |
P | 0.0182 (2) | 0.0139 (2) | 0.0219 (2) | −0.0003 (1) | 0.0068 (2) | 0.0001 (1) |
F1 | 0.0197 (4) | 0.0292 (5) | 0.0420 (5) | 0.0020 (3) | 0.0096 (4) | 0.0020 (3) |
F2 | 0.0366 (5) | 0.0221 (4) | 0.0319 (5) | −0.0055 (3) | 0.0127 (4) | −0.0084 (3) |
F3 | 0.0329 (5) | 0.0236 (4) | 0.0288 (4) | −0.0005 (3) | 0.0103 (4) | 0.0071 (3) |
F4 | 0.0375 (5) | 0.0206 (4) | 0.0301 (5) | −0.0026 (3) | 0.0081 (4) | −0.0064 (3) |
F5 | 0.0371 (5) | 0.0216 (4) | 0.0298 (4) | 0.0014 (3) | 0.0130 (4) | 0.0072 (3) |
F6 | 0.0194 (4) | 0.0278 (5) | 0.0457 (5) | 0.0021 (3) | 0.0084 (4) | 0.0026 (4) |
P—F1 | 1.5949 (9) | C4—C5 | 1.3858 (18) |
P—F2 | 1.6028 (9) | C5—C6 | 1.4777 (18) |
P—F3 | 1.6146 (8) | C6—C7 | 1.3908 (18) |
P—F4 | 1.6009 (9) | C7—C8 | 1.3852 (18) |
P—F5 | 1.6003 (9) | C8—C9 | 1.3826 (18) |
P—F6 | 1.5960 (9) | C9—C10 | 1.3915 (18) |
N1—C1 | 1.3352 (17) | C1—H1A | 0.93 |
N1—C5 | 1.3505 (17) | C2—H2 | 0.93 |
N2—C6 | 1.3453 (17) | C3—H3 | 0.93 |
N2—C10 | 1.3320 (17) | C4—H4 | 0.93 |
N1—H1 | 0.827 (18) | C7—H7 | 0.93 |
C1—C2 | 1.3761 (18) | C8—H8 | 0.93 |
C2—C3 | 1.3894 (18) | C9—H9 | 0.93 |
C3—C4 | 1.3865 (18) | C10—H10 | 0.93 |
F1···C9i | 3.3172 (15) | C6···C9xiii | 3.5528 (18) |
F1···F6ii | 3.0526 (12) | C7···C9xiii | 3.2996 (19) |
F2···C1 | 3.1969 (16) | C7···C8xiii | 3.5632 (19) |
F2···C3ii | 3.1928 (15) | C7···C10xiii | 3.5569 (19) |
F2···C1iii | 3.2260 (16) | C8···C9xiii | 3.5919 (19) |
F2···F2iii | 2.9253 (12) | C8···C7iv | 3.5632 (19) |
F3···N1 | 3.0424 (14) | C8···C6xiii | 3.5469 (18) |
F3···C4ii | 3.3428 (15) | C8···C10xiii | 3.4675 (19) |
F4···C10iv | 3.2616 (15) | C9···C6iv | 3.5528 (18) |
F5···C1v | 3.2151 (16) | C9···C8iv | 3.5919 (19) |
F5···F5vi | 3.0740 (11) | C9···N2xiii | 3.3107 (18) |
F5···C4vii | 3.2263 (16) | C9···C10xiii | 3.4783 (19) |
F5···C3vii | 3.1565 (16) | C9···C7iv | 3.2996 (19) |
F6···F1viii | 3.0526 (12) | C9···F1xiv | 3.3172 (15) |
F6···C2ix | 3.2363 (15) | C10···C10iv | 3.5806 (19) |
F1···H8i | 2.8275 | C10···C7iv | 3.5569 (19) |
F1···H9i | 2.6631 | C10···C10xiii | 3.5806 (19) |
F1···H2iii | 2.6388 | C10···C8iv | 3.4675 (19) |
F2···H1A | 2.3814 | C10···F4xiii | 3.2616 (16) |
F2···H1Aiii | 2.4754 | C10···C9iv | 3.4783 (19) |
F3···H8i | 2.7811 | C2···H3xi | 3.0983 |
F3···H1 | 2.297 (17) | C4···H7 | 2.7916 |
F4···H4vii | 2.6842 | C7···H4 | 2.8051 |
F4···H10iv | 2.5320 | C8···H10viii | 3.0375 |
F4···H8i | 2.6645 | H1···F3 | 2.297 (17) |
F5···H4vii | 2.6792 | H1···N2 | 2.251 (18) |
F5···H3vii | 2.5328 | H1A···F2 | 2.3814 |
F5···H3ix | 2.6927 | H1A···F2iii | 2.4754 |
F6···H9iv | 2.6382 | H2···F6ix | 2.5562 |
F6···H2ix | 2.5562 | H2···F1iii | 2.6388 |
N1···N2 | 2.6345 (15) | H3···F5ix | 2.6927 |
N1···F3 | 3.0424 (14) | H3···F5xii | 2.5328 |
N2···N1 | 2.6345 (15) | H3···C2xi | 3.0983 |
N2···C9iv | 3.3107 (18) | H4···F5xii | 2.6792 |
N2···H1 | 2.251 (18) | H4···F4xii | 2.6842 |
C1···F2 | 3.1969 (16) | H4···C7 | 2.8051 |
C1···F5x | 3.2151 (16) | H4···H7 | 2.3018 |
C1···C2ix | 3.5513 (18) | H7···H4 | 2.3018 |
C1···F2iii | 3.2260 (15) | H7···C4 | 2.7916 |
C2···C1ix | 3.5513 (19) | H8···F3xiv | 2.7811 |
C2···F6ix | 3.2363 (15) | H8···H10viii | 2.4175 |
C2···C3xi | 3.5423 (18) | H8···F1xiv | 2.8275 |
C2···C2ix | 3.5729 (18) | H8···F4xiv | 2.6645 |
C3···C2xi | 3.5423 (18) | H9···F6xiii | 2.6382 |
C3···F2viii | 3.1928 (15) | H9···F1xiv | 2.6631 |
C3···F5xii | 3.1565 (16) | H10···F4xiii | 2.5320 |
C4···F5xii | 3.2263 (16) | H10···C8ii | 3.0375 |
C4···F3viii | 3.3428 (15) | H10···H8ii | 2.4175 |
C6···C8iv | 3.5469 (18) | ||
F4—P—F5 | 90.22 (4) | N1—C5—C4 | 118.07 (12) |
F4—P—F6 | 90.31 (5) | N2—C6—C7 | 123.41 (12) |
F5—P—F6 | 90.32 (5) | C5—C6—C7 | 122.34 (11) |
F1—P—F2 | 89.92 (5) | N2—C6—C5 | 114.25 (11) |
F1—P—F3 | 89.83 (5) | C6—C7—C8 | 118.11 (12) |
F1—P—F4 | 89.99 (5) | C7—C8—C9 | 119.23 (12) |
F1—P—F5 | 90.51 (5) | C8—C9—C10 | 118.48 (12) |
F1—P—F6 | 179.12 (5) | N2—C10—C9 | 123.41 (12) |
F2—P—F3 | 89.83 (4) | N1—C1—H1A | 120.18 |
F2—P—F4 | 179.86 (6) | C2—C1—H1A | 120.25 |
F2—P—F5 | 89.89 (4) | C1—C2—H2 | 120.67 |
F2—P—F6 | 89.78 (5) | C3—C2—H2 | 120.61 |
F3—P—F4 | 90.06 (4) | C2—C3—H3 | 119.88 |
F3—P—F5 | 179.56 (6) | C4—C3—H3 | 119.84 |
F3—P—F6 | 89.34 (5) | C3—C4—H4 | 120.32 |
C1—N1—C5 | 123.91 (11) | C5—C4—H4 | 120.23 |
C6—N2—C10 | 117.32 (11) | C6—C7—H7 | 120.88 |
C1—N1—H1 | 118.9 (13) | C8—C7—H7 | 121.01 |
C5—N1—H1 | 117.2 (13) | C7—C8—H8 | 120.44 |
N1—C1—C2 | 119.56 (12) | C9—C8—H8 | 120.33 |
C1—C2—C3 | 118.72 (12) | C8—C9—H9 | 120.77 |
C2—C3—C4 | 120.28 (12) | C10—C9—H9 | 120.75 |
C3—C4—C5 | 119.45 (12) | N2—C10—H10 | 118.23 |
N1—C5—C6 | 115.86 (11) | C9—C10—H10 | 118.36 |
C4—C5—C6 | 126.06 (11) | ||
C5—N1—C1—C2 | 0.8 (2) | C3—C4—C5—N1 | −0.49 (18) |
C1—N1—C5—C4 | −0.34 (18) | N1—C5—C6—C7 | 166.79 (12) |
C1—N1—C5—C6 | 178.60 (12) | N1—C5—C6—N2 | −13.29 (16) |
C10—N2—C6—C5 | −179.31 (11) | C4—C5—C6—N2 | 165.56 (12) |
C10—N2—C6—C7 | 0.61 (19) | C4—C5—C6—C7 | −14.37 (19) |
C6—N2—C10—C9 | 0.8 (2) | N2—C6—C7—C8 | −1.48 (19) |
N1—C1—C2—C3 | −0.49 (19) | C5—C6—C7—C8 | 178.44 (11) |
C1—C2—C3—C4 | −0.32 (19) | C6—C7—C8—C9 | 0.9 (2) |
C2—C3—C4—C5 | 0.81 (19) | C7—C8—C9—C10 | 0.3 (2) |
C3—C4—C5—C6 | −179.32 (12) | C8—C9—C10—N2 | −1.3 (2) |
Symmetry codes: (i) x−1, −y+1/2, z+1/2; (ii) x−1, y, z; (iii) −x, −y, −z+1; (iv) x, −y+1/2, z+1/2; (v) x, y, z+1; (vi) −x, −y, −z+2; (vii) x−1, y, z+1; (viii) x+1, y, z; (ix) −x+1, −y, −z+1; (x) x, y, z−1; (xi) −x+1, −y, −z; (xii) x+1, y, z−1; (xiii) x, −y+1/2, z−1/2; (xiv) x+1, −y+1/2, z−1/2. |
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1···F3 | 0.827 (18) | 2.297 (17) | 3.0424 (14) | 150.1 (16) |
N1—H1···N2 | 0.827 (18) | 2.251 (18) | 2.6345 (15) | 108.6 (14) |
C1—H1A···F2 | 0.93 | 2.38 | 3.1969 (16) | 146 |
C1—H1A···F2iii | 0.93 | 2.48 | 3.2260 (15) | 138 |
C3—H3···F5xii | 0.93 | 2.53 | 3.1565 (16) | 125 |
C10—H10···F4xiii | 0.93 | 2.53 | 3.2616 (16) | 136 |
Symmetry codes: (iii) −x, −y, −z+1; (xii) x+1, y, z−1; (xiii) x, −y+1/2, z−1/2. |
Experimental details
Crystal data | |
Chemical formula | C10H9N2+·PF6− |
Mr | 302.16 |
Crystal system, space group | Monoclinic, P21/c |
Temperature (K) | 123 |
a, b, c (Å) | 6.1072 (4), 26.0105 (14), 7.1380 (5) |
β (°) | 101.416 (9) |
V (Å3) | 1111.45 (13) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.32 |
Crystal size (mm) | 0.24 × 0.20 × 0.14 |
Data collection | |
Diffractometer | Stoe IPDS diffractometer |
Absorption correction | – |
No. of measured, independent and observed [I > 2σ(I)] reflections | 11182, 2093, 1766 |
Rint | 0.029 |
(sin θ/λ)max (Å−1) | 0.611 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.029, 0.091, 1.05 |
No. of reflections | 2093 |
No. of parameters | 175 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.26, −0.21 |
Computer programs: IPDS (Stoe & Cie, 1999) Query, IPDS, SIR92 (Altomare et al., 1993), SHELXL97 (Sheldrick, 1997), PLATON (Spek, 1990), PLATON.
N1—C1 | 1.3352 (17) | C4—C5 | 1.3858 (18) |
N1—C5 | 1.3505 (17) | C5—C6 | 1.4777 (18) |
N2—C6 | 1.3453 (17) | C6—C7 | 1.3908 (18) |
N2—C10 | 1.3320 (17) | C7—C8 | 1.3852 (18) |
C1—C2 | 1.3761 (18) | C8—C9 | 1.3826 (18) |
C2—C3 | 1.3894 (18) | C9—C10 | 1.3915 (18) |
C3—C4 | 1.3865 (18) | ||
N1—C5—C6 | 115.86 (11) | C5—C6—C7 | 122.34 (11) |
C4—C5—C6 | 126.06 (11) | N2—C6—C5 | 114.25 (11) |
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1···F3 | 0.827 (18) | 2.297 (17) | 3.0424 (14) | 150.1 (16) |
N1—H1···N2 | 0.827 (18) | 2.251 (18) | 2.6345 (15) | 108.6 (14) |
As has been pointed out by Marsh (1995) and by Marsh et al. (2002) in some detail, derivation of space lattices based solely on data collected by conventional automated diffractometers bears some risk. Superstructure arrangements, in which the true structure is based on small but systematic deviations from a more symmetric substructure and that require a unit cell larger than that of the substructure, may go unnoticed. Missing the true translational symmetry will impose an artificially averaged structure upon the subsequent refinement. Nevertheless, the indication of such a mistake in the course of structure refinement may be slight.
While trying to identify a by-product, we came across such an example. Since we failed to identify the compound by comparing its cell parameters, as determined by powder diffractometry, with entries in the Cambridge Structural Database (CSD, Release?; Allen et al., 1983), a structure determination was undertaken. This showed the compound to be 2,2'-bipyridinium hexafluorophosphate, (I), for which a crystal structure was previously described (Milani et al., 1997; CSD reference code NOXXED) in space group Pc, with a unit cell of half the true size [a = 6.1758 (4), b = 13.120 (3) and c = 7.255 (4) Å, and β = 100.74 (3)°; Fig. 2]. \sch
There were several indications of a problem in the original refinement, which unfortunately were ignored. The refinement could only be performed with isotropic displacement parameters of the C atoms. In contrast with that which was claimed in the publication, bond lengths and angles were not in the expected range for aromatic bonds. For instance, while C1—C2 was unusually long (1.500 Å), C2—C3 was far too short (1.210 Å). Also, probably because of intramolecular hydrogen bonding (Table 2), all reported cisoid bipyridinium structures appear to be bent around the midpoint of the C5—C6 bond: the angles on the nitrogen side are considerably smaller than those on the carbon side. For the related 2,2'-bipyridinium tetraphenylborate, angles of 115.4 and 113.6° versus 126.8 and 123.3° were reported (Bakshi et al., 1996; CSD reference code ZUTAT). However, Milani et al. (1997) found angles of 121.5 and 113.3° versus 119.8 and 121.7°. Because of the unrealistically high symmetry imposed in the refinement, the expected kink disappears. The interplanar angle between the pyridine rings in the cation is quite similar in both the previous and the present refinement: Milani reports 12.98°, while we find 13.50°. The torsion around C5—C6 is, in any case, a rather soft parameter, and consequently this interplanar angle adjusts to the packing requirements and varies considerably for the examples included in the CSD (e.g. for 2,2'-bipyridinium tetraphenylborate, a value of 5.27° was observed).
In correcting the structure, significant changes in bond lengths and angles result (Table 1). Refinement with the true translational symmetry cures all the above-mentioned intramolecular shortcomings of the initial refinement. The H atom attached to N1 is clearly visible in the difference map and the assignment of N1 as being protonated is unambiguous when choosing the correct cell.
The most severe warning signal that something was wrong with the original space group assignment came from the ambiguous value for the Flack parameter. Milani et al. (1997) state that an attempt to evaluate the absolute configuration did not produce satisfactory results. Perpendicular to the c axis, the structure consists of layers of bipyridinium cations. In the incorrect small cell, all cations point in the same direction, and thus a large dipole moment would develop in the polar structure. In the correct supercell, however, adjacent rows of molecules point in opposite directions and, of course, no net dipole moment is present in the centrosymmetric space group.
Additionally, the orientation of the anions in both refinements differs significantly (Fig. 2). The orientation in the small cell is clearly an artefact of an averaged orientation. The intermolecular hydrogen bonding (Table 2) is, of course, affected by this.
The crystal-packing pattern is also interesting with respect to the non-covalent intermolecular interactions, which are important for the molecular recognition and self-assembly processes leading to molecular aggregates. Within the above-mentioned bipyridinium layers, the arene moieties are lined up in parallel and adjacent molecules are shifted by 5.1 Å. The intermolecular interaction is controlled by so called `π–π interactions'. This shifted π-stacked arrangement and the short perpendicular distances between the ring centroids and the parallel aromatic plane (3.286 Å) are in line with this view.
Both experimental (Cozzi et al., 1993) and simulation (Jorgensen & Severance, 1990) work shows that most `π–π interactions' are dominated by electrostatics (multipole-multipole). The inherent polarity of aromatic systems stems from the electron-rich core being surrounded by an electron-poor torus of H atoms. This electrostatic description accounts for the energetic preference of shifted π-stacked arrangements observed with the packing of (I). The frequently observed alternative π–π interaction is T-shaped (Desiraju & Gavezzotti, 1989), but this type is not encountered in the current structure.