Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S1600536807022106/bt2355sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S1600536807022106/bt2355Isup2.hkl |
CCDC reference: 651427
Compound (I) was synthesized by heating 1 mmol quantities of citric acid and quinoline in 50 ml of 2-propanol for 10 min under reflux. Colourless needles (m.p. 403 K) were obtained after partial room-temperature evaporation of solvent.
Hydrogen atoms potentially involved in hydrogen-bonding interactions were located by difference methods and their positional and isotropic displacement parameters were refined. Other H atoms were included in the refinement at calculated positions [C—H (aromatic) = 0.95 Å and C–H (aliphatic) = 0.99 Å] using a riding-model approximation with Uiso(H) = 1.2Ueq(C).
The structures of the quinolinium carboxylates and sulfonates are not prevalent in the crystallographic literature and most of the examples are 1:1 salts, mainly with the aromatic acids e.g. 5-sulfosalicylic acid (Smith et al., 2004), 3,5-dinitrosalicylic acid (Smith, Wermuth & White, 2006), and picrylsulfonic acid (Smith, Wermuth & Healy, 2006). These compounds often feature π-associated cation-anion stacks with peripheral hydrogen bonding giving three-dimensional framework structures. With the quinolinium salts of the aliphatic carboxylic acids, most examples are anhydrous acid salts of polyprotic analogues, e.g. fumaric acid (Shan et al., 2003) and L-tartaric acid (Smith et al., (2006). The 1:1 stoichiometric reaction of quinoline with citric acid in isopropyl alcohol was expected to give a similar acid citrate and this was confirmed in the structure determination of C9H8N+ C6H7O7- (I), reported here.
Figure 1 shows the quinolinium cation and the dihydrogen citrate anion in which one of the β-carboxylic acid groups rather than the α-group has lost the proton. It is more usual for the α-group to be associated with the first dissociation constant (Tapscott, 1982) and is seen in typical structures such as sodium dihydrogen citrate (Glusker et al., 1965), and sildenafil dihydrogen citrate (Yathirajan et al., 2005). In (I), this results in the generation a chiral centre at C31 in the anion species but these form a racemete in the centrosymmetric crystal. These anions form a convoluted two-dimensional hydrogen-bonded substructure through head-to-tail carboxylic acid···carboxylate interactions, one linear, the other three-centred cyclic [R21(4)] (Table 1). The partially overlapping quinolinium cations [C5–C10: minimum ring centroid and perpendicular separations of 3.840 (1) and 3.560 (1) Å respectively] form π-associated stacks which extend down the a cell direction. The anion substructures accommodate these stacks (Fig. 2). which are linked to the anionic substructure by symmetric three-centre R21(5) N+H···O(carboxyl, hydroxyl) hydrogen-bonding associations. The result is a three-dimensional framework structure which in addition has 66.8 Å3 potential solvent accessible voids.
The conformation of the dihydrogen citrate anions is maintained by the presence of an intramolecular hydroxyl–carboxyl hydrogen bond [O31—H···O12, 2.6337 (13) Å].
There is a similarity between the structure of the title compound and those of the quinolinium hydrogen salts of fumaric acid (Shan et al., 2003) and L-tartaric acid (Smith, Wermuth & White, 2006).
For related literature, see: Glusker et al. (1965); Smith et al. (2004, 2007); Smith, Wermuth & Healy (2006); Tapscott (1982); Yathirajan et al. (2005).
Data collection: SMART (Bruker, 2000); cell refinement: SMART; data reduction: SAINT (Bruker, 1999); 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: PLATON.
C9H8N+·C6H7O7− | F(000) = 672 |
Mr = 321.28 | Dx = 1.390 Mg m−3 |
Monoclinic, P21/n | Melting point: 403 K |
Hall symbol: -P 2yn | Mo Kα radiation, λ = 0.71073 Å |
a = 7.5202 (6) Å | Cell parameters from 4204 reflections |
b = 11.9267 (10) Å | θ = 2.9–27.5° |
c = 17.1484 (14) Å | µ = 0.11 mm−1 |
β = 93.217 (1)° | T = 130 K |
V = 1535.6 (2) Å3 | Cut block, colourless |
Z = 4 | 0.50 × 0.35 × 0.30 mm |
Bruker SMART CCD detector diffractometer | 2695 independent reflections |
Radiation source: sealed tube | 2431 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.020 |
φ and ω scans | θmax = 25.0°, θmin = 2.1° |
Absorption correction: multi-scan (SADABS; Bruker, 1999) | h = −8→8 |
Tmin = 0.95, Tmax = 0.97 | k = −12→14 |
7847 measured reflections | l = −20→20 |
Refinement on F2 | Secondary atom site location: difference Fourier map |
Least-squares matrix: full | Hydrogen site location: inferred from neighbouring sites |
R[F2 > 2σ(F2)] = 0.032 | H atoms treated by a mixture of independent and constrained refinement |
wR(F2) = 0.092 | w = 1/[σ2(Fo2) + (0.0491P)2 + 0.223P] where P = (Fo2 + 2Fc2)/3 |
S = 1.07 | (Δ/σ)max < 0.001 |
2695 reflections | Δρmax = 0.18 e Å−3 |
225 parameters | Δρmin = −0.15 e Å−3 |
0 restraints | Extinction correction: SHELXL97, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
Primary atom site location: structure-invariant direct methods | Extinction coefficient: 0.0085 (16) |
C9H8N+·C6H7O7− | V = 1535.6 (2) Å3 |
Mr = 321.28 | Z = 4 |
Monoclinic, P21/n | Mo Kα radiation |
a = 7.5202 (6) Å | µ = 0.11 mm−1 |
b = 11.9267 (10) Å | T = 130 K |
c = 17.1484 (14) Å | 0.50 × 0.35 × 0.30 mm |
β = 93.217 (1)° |
Bruker SMART CCD detector diffractometer | 2695 independent reflections |
Absorption correction: multi-scan (SADABS; Bruker, 1999) | 2431 reflections with I > 2σ(I) |
Tmin = 0.95, Tmax = 0.97 | Rint = 0.020 |
7847 measured reflections |
R[F2 > 2σ(F2)] = 0.032 | 0 restraints |
wR(F2) = 0.092 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.07 | Δρmax = 0.18 e Å−3 |
2695 reflections | Δρmin = −0.15 e Å−3 |
225 parameters |
Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su'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 | ||
O31 | 0.69503 (12) | 0.31070 (7) | 0.07462 (5) | 0.0428 (3) | |
O11 | 0.44066 (13) | 0.60432 (9) | 0.13920 (7) | 0.0622 (4) | |
O12 | 0.40951 (11) | 0.43148 (8) | 0.09764 (6) | 0.0472 (3) | |
O51 | 0.85983 (16) | 0.25801 (10) | 0.30537 (6) | 0.0627 (4) | |
O52 | 0.98000 (17) | 0.22265 (10) | 0.19299 (6) | 0.0688 (4) | |
O61 | 1.08487 (11) | 0.46033 (8) | 0.13149 (5) | 0.0455 (3) | |
O62 | 1.01275 (11) | 0.35650 (8) | 0.02589 (5) | 0.0431 (3) | |
C11 | 0.50345 (15) | 0.51775 (10) | 0.11240 (7) | 0.0350 (4) | |
C21 | 0.69856 (15) | 0.51305 (10) | 0.09419 (7) | 0.0347 (3) | |
C31 | 0.78566 (15) | 0.39947 (10) | 0.11505 (7) | 0.0332 (3) | |
C41 | 0.78738 (17) | 0.38158 (11) | 0.20312 (7) | 0.0398 (4) | |
C51 | 0.88543 (17) | 0.27801 (11) | 0.23168 (8) | 0.0427 (4) | |
C61 | 0.97426 (15) | 0.40111 (10) | 0.08600 (7) | 0.0346 (3) | |
N1 | 0.82320 (16) | 0.18706 (10) | −0.05817 (7) | 0.0497 (4) | |
C2 | 0.8459 (2) | 0.22022 (14) | −0.13027 (9) | 0.0562 (5) | |
C3 | 0.8327 (2) | 0.14309 (16) | −0.19125 (9) | 0.0625 (6) | |
C4 | 0.7987 (2) | 0.03385 (16) | −0.17563 (9) | 0.0628 (6) | |
C5 | 0.7391 (2) | −0.11459 (14) | −0.07793 (11) | 0.0498 (5) | |
C6 | 0.7164 (2) | −0.14220 (15) | −0.00272 (12) | 0.0689 (6) | |
C7 | 0.7289 (2) | −0.06036 (14) | 0.05581 (11) | 0.0655 (6) | |
C8 | 0.7652 (2) | 0.04851 (13) | 0.03880 (9) | 0.0572 (5) | |
C9 | 0.78666 (18) | 0.07852 (11) | −0.03881 (8) | 0.0443 (4) | |
C10 | 0.77402 (18) | −0.00213 (13) | −0.09901 (9) | 0.0646 (6) | |
H51 | 0.931 (3) | 0.203 (2) | 0.3252 (13) | 0.080 (8)* | |
H21A | 0.76380 | 0.57310 | 0.12360 | 0.0420* | |
H21B | 0.70910 | 0.52770 | 0.03780 | 0.0420* | |
H31 | 0.586 (3) | 0.3302 (15) | 0.0746 (10) | 0.064 (5)* | |
H41A | 0.84280 | 0.44780 | 0.22940 | 0.0480* | |
H41B | 0.66280 | 0.37690 | 0.21870 | 0.0480* | |
H61 | 1.208 (3) | 0.4490 (17) | 0.1150 (12) | 0.088 (6)* | |
H1 | 0.832 (3) | 0.2372 (17) | −0.0209 (12) | 0.074 (6)* | |
H2 | 0.87120 | 0.29670 | −0.14060 | 0.0680* | |
H3 | 0.84730 | 0.16670 | −0.24340 | 0.0750* | |
H4 | 0.79150 | −0.01900 | −0.21710 | 0.0750* | |
H5 | 0.73150 | −0.17110 | −0.11700 | 0.0780* | |
H6 | 0.69180 | −0.21770 | 0.01050 | 0.0830* | |
H7 | 0.71170 | −0.08110 | 0.10830 | 0.0790* | |
H8 | 0.77580 | 0.10320 | 0.07910 | 0.0690* |
U11 | U22 | U33 | U12 | U13 | U23 | |
O31 | 0.0297 (5) | 0.0399 (5) | 0.0592 (6) | −0.0029 (4) | 0.0066 (4) | −0.0124 (4) |
O11 | 0.0347 (5) | 0.0547 (6) | 0.0974 (8) | 0.0032 (5) | 0.0045 (5) | −0.0326 (6) |
O12 | 0.0253 (5) | 0.0451 (5) | 0.0711 (6) | −0.0022 (4) | 0.0030 (4) | −0.0105 (4) |
O51 | 0.0738 (7) | 0.0627 (7) | 0.0537 (6) | 0.0255 (6) | 0.0220 (5) | 0.0206 (5) |
O52 | 0.0789 (8) | 0.0706 (7) | 0.0585 (6) | 0.0392 (6) | 0.0184 (6) | 0.0070 (5) |
O61 | 0.0257 (5) | 0.0609 (6) | 0.0498 (5) | −0.0017 (4) | 0.0023 (4) | −0.0123 (4) |
O62 | 0.0353 (5) | 0.0516 (6) | 0.0434 (5) | −0.0009 (4) | 0.0107 (4) | −0.0081 (4) |
C11 | 0.0282 (6) | 0.0404 (7) | 0.0360 (6) | 0.0017 (5) | −0.0021 (5) | −0.0018 (5) |
C21 | 0.0284 (6) | 0.0361 (6) | 0.0396 (6) | −0.0019 (5) | 0.0014 (5) | 0.0006 (5) |
C31 | 0.0260 (6) | 0.0348 (6) | 0.0389 (6) | −0.0006 (5) | 0.0036 (5) | −0.0034 (5) |
C41 | 0.0353 (7) | 0.0429 (7) | 0.0419 (7) | 0.0068 (5) | 0.0097 (5) | 0.0026 (5) |
C51 | 0.0393 (7) | 0.0436 (7) | 0.0459 (7) | 0.0043 (6) | 0.0079 (5) | 0.0036 (6) |
C61 | 0.0278 (6) | 0.0377 (6) | 0.0385 (6) | 0.0021 (5) | 0.0027 (5) | 0.0005 (5) |
N1 | 0.0546 (7) | 0.0443 (7) | 0.0497 (7) | 0.0104 (5) | −0.0025 (5) | −0.0084 (6) |
C2 | 0.0492 (9) | 0.0582 (9) | 0.0615 (9) | 0.0132 (7) | 0.0050 (7) | 0.0075 (7) |
C3 | 0.0566 (10) | 0.0827 (12) | 0.0487 (8) | 0.0153 (8) | 0.0072 (7) | 0.0000 (8) |
C4 | 0.0558 (9) | 0.0776 (12) | 0.0550 (9) | 0.0078 (8) | 0.0032 (7) | −0.0235 (8) |
C5 | 0.0378 (7) | 0.0543 (9) | 0.0575 (8) | 0.0062 (6) | 0.0034 (6) | −0.0165 (7) |
C6 | 0.0579 (10) | 0.0494 (9) | 0.1007 (14) | 0.0040 (7) | 0.0172 (9) | 0.0010 (9) |
C7 | 0.0676 (11) | 0.0598 (10) | 0.0701 (10) | 0.0139 (8) | 0.0130 (8) | 0.0081 (8) |
C8 | 0.0667 (10) | 0.0535 (9) | 0.0513 (8) | 0.0139 (7) | 0.0023 (7) | −0.0055 (7) |
C9 | 0.0384 (7) | 0.0441 (8) | 0.0502 (8) | 0.0100 (6) | −0.0003 (6) | −0.0074 (6) |
C10 | 0.0538 (9) | 0.0512 (9) | 0.0894 (13) | 0.0006 (7) | 0.0090 (8) | −0.0255 (8) |
O31—C31 | 1.4188 (15) | C21—H21A | 0.9900 |
O11—C11 | 1.2347 (16) | C41—H41B | 0.9900 |
O12—C11 | 1.2656 (15) | C41—H41A | 0.9900 |
O31—H31 | 0.85 (2) | C2—C3 | 1.392 (2) |
O51—C51 | 1.3108 (17) | C3—C4 | 1.357 (3) |
O52—C51 | 1.1979 (18) | C4—C10 | 1.404 (2) |
O51—H51 | 0.90 (2) | C5—C10 | 1.418 (2) |
O61—C61 | 1.3140 (15) | C5—C6 | 1.351 (3) |
O62—C61 | 1.2095 (15) | C6—C7 | 1.399 (3) |
O61—H61 | 0.99 (2) | C7—C8 | 1.362 (2) |
N1—C9 | 1.3681 (18) | C8—C9 | 1.396 (2) |
N1—C2 | 1.318 (2) | C9—C10 | 1.410 (2) |
N1—H1 | 0.88 (2) | C2—H2 | 0.9500 |
C11—C21 | 1.5182 (16) | C3—H3 | 0.9500 |
C21—C31 | 1.5381 (17) | C4—H4 | 0.9500 |
C31—C41 | 1.5246 (17) | C5—H5 | 0.9500 |
C31—C61 | 1.5295 (16) | C6—H6 | 0.9500 |
C41—C51 | 1.5063 (18) | C7—H7 | 0.9500 |
C21—H21B | 0.9900 | C8—H8 | 0.9500 |
C31—O31—H31 | 103.4 (12) | C51—C41—H41A | 109.00 |
C51—O51—H51 | 112.3 (14) | C51—C41—H41B | 109.00 |
C61—O61—H61 | 109.1 (12) | H41A—C41—H41B | 108.00 |
C2—N1—C9 | 123.35 (13) | N1—C2—C3 | 119.92 (15) |
C9—N1—H1 | 118.5 (13) | C2—C3—C4 | 119.50 (15) |
C2—N1—H1 | 118.1 (13) | C3—C4—C10 | 120.94 (15) |
O11—C11—O12 | 122.24 (11) | C6—C5—C10 | 120.61 (16) |
O11—C11—C21 | 120.11 (11) | C5—C6—C7 | 120.51 (16) |
O12—C11—C21 | 117.64 (10) | C6—C7—C8 | 121.17 (17) |
C11—C21—C31 | 112.87 (10) | C7—C8—C9 | 118.94 (15) |
C41—C31—C61 | 111.59 (10) | N1—C9—C10 | 118.29 (13) |
O31—C31—C41 | 110.90 (10) | C8—C9—C10 | 121.08 (13) |
O31—C31—C21 | 110.91 (9) | N1—C9—C8 | 120.62 (13) |
O31—C31—C61 | 106.21 (9) | C5—C10—C9 | 117.68 (14) |
C21—C31—C41 | 109.48 (10) | C4—C10—C5 | 124.32 (15) |
C21—C31—C61 | 107.67 (9) | C4—C10—C9 | 118.00 (14) |
C31—C41—C51 | 114.43 (11) | N1—C2—H2 | 120.00 |
O51—C51—C41 | 111.53 (11) | C3—C2—H2 | 120.00 |
O51—C51—O52 | 123.89 (13) | C2—C3—H3 | 120.00 |
O52—C51—C41 | 124.54 (13) | C4—C3—H3 | 120.00 |
O61—C61—C31 | 112.41 (10) | C3—C4—H4 | 120.00 |
O61—C61—O62 | 124.66 (11) | C10—C4—H4 | 120.00 |
O62—C61—C31 | 122.86 (11) | C6—C5—H5 | 120.00 |
C11—C21—H21A | 109.00 | C10—C5—H5 | 120.00 |
C11—C21—H21B | 109.00 | C5—C6—H6 | 120.00 |
H21A—C21—H21B | 108.00 | C7—C6—H6 | 120.00 |
C31—C21—H21A | 109.00 | C6—C7—H7 | 119.00 |
C31—C21—H21B | 109.00 | C8—C7—H7 | 119.00 |
C31—C41—H41A | 109.00 | C7—C8—H8 | 121.00 |
C31—C41—H41B | 109.00 | C9—C8—H8 | 121.00 |
C2—N1—C9—C10 | 0.5 (2) | C31—C41—C51—O52 | 11.94 (19) |
C9—N1—C2—C3 | 0.1 (2) | C31—C41—C51—O51 | −170.43 (11) |
C2—N1—C9—C8 | 179.57 (14) | N1—C2—C3—C4 | −0.8 (2) |
O12—C11—C21—C31 | −40.13 (15) | C2—C3—C4—C10 | 1.0 (2) |
O11—C11—C21—C31 | 141.26 (12) | C3—C4—C10—C9 | −0.4 (2) |
C11—C21—C31—C41 | −64.53 (13) | C3—C4—C10—C5 | −179.63 (15) |
C11—C21—C31—C61 | 173.98 (10) | C6—C5—C10—C9 | 0.9 (2) |
C11—C21—C31—O31 | 58.17 (13) | C10—C5—C6—C7 | −0.7 (2) |
O31—C31—C41—C51 | 62.88 (13) | C6—C5—C10—C4 | −179.79 (15) |
O31—C31—C61—O61 | −164.52 (10) | C5—C6—C7—C8 | −0.4 (2) |
C21—C31—C61—O61 | 76.61 (12) | C6—C7—C8—C9 | 1.3 (2) |
C21—C31—C61—O62 | −100.34 (13) | C7—C8—C9—C10 | −1.0 (2) |
C41—C31—C61—O61 | −43.55 (14) | C7—C8—C9—N1 | 180.00 (14) |
C21—C31—C41—C51 | −174.41 (10) | N1—C9—C10—C4 | −0.4 (2) |
C61—C31—C41—C51 | −55.31 (14) | N1—C9—C10—C5 | 178.95 (13) |
C41—C31—C61—O62 | 139.50 (12) | C8—C9—C10—C4 | −179.42 (14) |
O31—C31—C61—O62 | 18.52 (15) | C8—C9—C10—C5 | −0.10 (19) |
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1···O31 | 0.88 (2) | 2.17 (2) | 2.9215 (15) | 144 (2) |
N1—H1···O62 | 0.88 (2) | 2.10 (2) | 2.8219 (15) | 140 (2) |
O51—H51···O11i | 0.90 (2) | 1.62 (2) | 2.5208 (16) | 180 (3) |
O31—H31···O12 | 0.85 (2) | 1.85 (2) | 2.6337 (13) | 151.4 (17) |
O61—H61···O11ii | 0.99 (2) | 2.57 (2) | 3.1758 (14) | 119.6 (14) |
O61—H61···O12ii | 0.99 (2) | 1.58 (2) | 2.5641 (12) | 174 (2) |
C7—H7···O51i | 0.95 | 2.50 | 3.314 (2) | 144 |
C8—H8···O31 | 0.95 | 2.55 | 3.2358 (18) | 129 |
C21—H21B···O12iii | 0.99 | 2.49 | 3.4087 (16) | 154 |
Symmetry codes: (i) −x+3/2, y−1/2, −z+1/2; (ii) x+1, y, z; (iii) −x+1, −y+1, −z. |
Experimental details
Crystal data | |
Chemical formula | C9H8N+·C6H7O7− |
Mr | 321.28 |
Crystal system, space group | Monoclinic, P21/n |
Temperature (K) | 130 |
a, b, c (Å) | 7.5202 (6), 11.9267 (10), 17.1484 (14) |
β (°) | 93.217 (1) |
V (Å3) | 1535.6 (2) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.11 |
Crystal size (mm) | 0.50 × 0.35 × 0.30 |
Data collection | |
Diffractometer | Bruker SMART CCD detector |
Absorption correction | Multi-scan (SADABS; Bruker, 1999) |
Tmin, Tmax | 0.95, 0.97 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 7847, 2695, 2431 |
Rint | 0.020 |
(sin θ/λ)max (Å−1) | 0.595 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.032, 0.092, 1.07 |
No. of reflections | 2695 |
No. of parameters | 225 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.18, −0.15 |
Computer programs: SMART (Bruker, 2000), SMART, SAINT (Bruker, 1999), SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), PLATON (Spek, 2003), PLATON.
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1···O31 | 0.88 (2) | 2.17 (2) | 2.9215 (15) | 144 (2) |
N1—H1···O62 | 0.88 (2) | 2.10 (2) | 2.8219 (15) | 140 (2) |
O51—H51···O11i | 0.90 (2) | 1.62 (2) | 2.5208 (16) | 180 (3) |
O31—H31···O12 | 0.85 (2) | 1.85 (2) | 2.6337 (13) | 151.4 (17) |
O61—H61···O11ii | 0.99 (2) | 2.57 (2) | 3.1758 (14) | 119.6 (14) |
O61—H61···O12ii | 0.99 (2) | 1.58 (2) | 2.5641 (12) | 174 (2) |
Symmetry codes: (i) −x+3/2, y−1/2, −z+1/2; (ii) x+1, y, z. |
The structures of the quinolinium carboxylates and sulfonates are not prevalent in the crystallographic literature and most of the examples are 1:1 salts, mainly with the aromatic acids e.g. 5-sulfosalicylic acid (Smith et al., 2004), 3,5-dinitrosalicylic acid (Smith, Wermuth & White, 2006), and picrylsulfonic acid (Smith, Wermuth & Healy, 2006). These compounds often feature π-associated cation-anion stacks with peripheral hydrogen bonding giving three-dimensional framework structures. With the quinolinium salts of the aliphatic carboxylic acids, most examples are anhydrous acid salts of polyprotic analogues, e.g. fumaric acid (Shan et al., 2003) and L-tartaric acid (Smith et al., (2006). The 1:1 stoichiometric reaction of quinoline with citric acid in isopropyl alcohol was expected to give a similar acid citrate and this was confirmed in the structure determination of C9H8N+ C6H7O7- (I), reported here.
Figure 1 shows the quinolinium cation and the dihydrogen citrate anion in which one of the β-carboxylic acid groups rather than the α-group has lost the proton. It is more usual for the α-group to be associated with the first dissociation constant (Tapscott, 1982) and is seen in typical structures such as sodium dihydrogen citrate (Glusker et al., 1965), and sildenafil dihydrogen citrate (Yathirajan et al., 2005). In (I), this results in the generation a chiral centre at C31 in the anion species but these form a racemete in the centrosymmetric crystal. These anions form a convoluted two-dimensional hydrogen-bonded substructure through head-to-tail carboxylic acid···carboxylate interactions, one linear, the other three-centred cyclic [R21(4)] (Table 1). The partially overlapping quinolinium cations [C5–C10: minimum ring centroid and perpendicular separations of 3.840 (1) and 3.560 (1) Å respectively] form π-associated stacks which extend down the a cell direction. The anion substructures accommodate these stacks (Fig. 2). which are linked to the anionic substructure by symmetric three-centre R21(5) N+H···O(carboxyl, hydroxyl) hydrogen-bonding associations. The result is a three-dimensional framework structure which in addition has 66.8 Å3 potential solvent accessible voids.
The conformation of the dihydrogen citrate anions is maintained by the presence of an intramolecular hydroxyl–carboxyl hydrogen bond [O31—H···O12, 2.6337 (13) Å].