Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270111054977/sf3160sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270111054977/sf3160Isup2.hkl | |
Chemical Markup Language (CML) file https://doi.org/10.1107/S0108270111054977/sf3160Isup3.cml |
CCDC reference: 867029
The multicomponent title crystal was prepared by mixing piperidine-2-carboxylic acid (0.0809 g, Aldrich, 98%) and cyclobutane-1,1-dicarboxylic acid (0.0702 g, Aldrich, 99.8%) in a 1:1 molar ratio. The regents were ground separately with an agate pestle and mortar, and dissolved in ethanol (5 ml). The two solutions were mixed and placed in a reflux system for a period of 3 h at a constant temperature of 343 K. Colourless lamellar crystals of (I) of approximately 0.9 × 1.2 mm were obtained by slow evaporation of the reflux solution.
Thermal analysis of (I) was performed using a Perkin–Elmer TGA7 coupled with a DSC console. Samples were heated from 298 to 673 K at a rate of 10 K min-1 under a nitrogen flux of 100 ml min-1. The sample consisted of a mixture of pure CBDC (m.p. 467.5 K) and the multicomponent ISO+.CBDC- crystal (m.p. 555.3 K); no trace of pure ISO (m.p. 570.5 K) was found.
All H atoms were placed in calculated positions and refined using a riding model, with C—H = 0.97, N—H = 0.90 and O—H = 0.82 Å [Please confirm added text], and with Uiso(H) = 1.2Ueq(parent atom). The low completeness ratio is due to the experimental set-up, whereby the equipment has a χ circle and an added area detector (four-circle diffractometer modified with a CCD detector). This precludes the collection of some regions of the reciprocal-lattice space and lowers the completeness. In order to compensate, additional redundant data were measured.
Data collection: CrystalClear (Rigaku, 2000); cell refinement: CrystalStructure (Rigaku/MSC, 2004); data reduction: CrystalStructure (Rigaku/MSC, 2004); program(s) used to solve structure: SIR2008 (Burla et al., 2007); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 1998); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008), PLATON (Spek, 2009) and publCIF (Westrip, 2010).
C6H12NO2+·C6H7O4− | F(000) = 584 |
Mr = 273.28 | Dx = 1.390 Mg m−3 |
Monoclinic, P21/c | Melting point: 555.3 K |
Hall symbol: -P 2ybc | Mo Kα radiation, λ = 0.71073 Å |
a = 7.027 (2) Å | Cell parameters from 2561 reflections |
b = 20.776 (5) Å | θ = 2.0–27.7° |
c = 8.949 (2) Å | µ = 0.11 mm−1 |
β = 91.06 (1)° | T = 293 K |
V = 1306.2 (6) Å3 | Lamellar, colourless |
Z = 4 | 0.3 × 0.2 × 0.1 mm |
Rigaku AFC-7S Mercury diffractometer | 2561 independent reflections |
Radiation source: normal-focus sealed tube | 2026 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.037 |
Detector resolution: 14.6306 pixels mm-1 | θmax = 27.7°, θmin = 2.0° |
ω scans | h = −8→8 |
Absorption correction: multi-scan (REQAB; Jacobson, 1998) | k = −24→20 |
Tmin = 0.970, Tmax = 0.986 | l = −10→10 |
14962 measured reflections |
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)] = 0.050 | H-atom parameters constrained |
wR(F2) = 0.139 | w = 1/[σ2(Fo2) + (0.0657P)2 + 0.6501P] where P = (Fo2 + 2Fc2)/3 |
S = 1.04 | (Δ/σ)max < 0.001 |
2561 reflections | Δρmax = 0.41 e Å−3 |
173 parameters | Δρmin = −0.30 e Å−3 |
0 restraints | Extinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
Primary atom site location: structure-invariant direct methods | Extinction coefficient: 0.028 (3) |
C6H12NO2+·C6H7O4− | V = 1306.2 (6) Å3 |
Mr = 273.28 | Z = 4 |
Monoclinic, P21/c | Mo Kα radiation |
a = 7.027 (2) Å | µ = 0.11 mm−1 |
b = 20.776 (5) Å | T = 293 K |
c = 8.949 (2) Å | 0.3 × 0.2 × 0.1 mm |
β = 91.06 (1)° |
Rigaku AFC-7S Mercury diffractometer | 2561 independent reflections |
Absorption correction: multi-scan (REQAB; Jacobson, 1998) | 2026 reflections with I > 2σ(I) |
Tmin = 0.970, Tmax = 0.986 | Rint = 0.037 |
14962 measured reflections |
R[F2 > 2σ(F2)] = 0.050 | 0 restraints |
wR(F2) = 0.139 | H-atom parameters constrained |
S = 1.04 | Δρmax = 0.41 e Å−3 |
2561 reflections | Δρmin = −0.30 e Å−3 |
173 parameters |
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. |
x | y | z | Uiso*/Ueq | ||
O1 | 0.3105 (3) | 0.45517 (9) | 0.3477 (2) | 0.0730 (6) | |
O2 | 0.3932 (3) | 0.52748 (8) | 0.1822 (2) | 0.0677 (6) | |
H2 | 0.3108 | 0.5485 | 0.2236 | 0.102* | |
N1 | 0.8441 (2) | 0.39203 (8) | 0.4046 (2) | 0.0388 (4) | |
H1A | 0.8889 | 0.3924 | 0.4995 | 0.047* | |
H1B | 0.9243 | 0.3684 | 0.3497 | 0.047* | |
C1 | 0.4092 (3) | 0.47147 (10) | 0.2462 (2) | 0.0346 (5) | |
C2 | 0.5612 (3) | 0.42975 (9) | 0.1768 (2) | 0.0328 (5) | |
H2A | 0.5289 | 0.4251 | 0.0704 | 0.039* | |
C3 | 0.7596 (3) | 0.46024 (10) | 0.1884 (2) | 0.0366 (5) | |
H3A | 0.8452 | 0.4371 | 0.1237 | 0.044* | |
H3B | 0.7525 | 0.5044 | 0.1539 | 0.044* | |
C4 | 0.8390 (3) | 0.45916 (10) | 0.3460 (2) | 0.0383 (5) | |
H4A | 0.9667 | 0.4769 | 0.3476 | 0.046* | |
H4B | 0.7606 | 0.4856 | 0.4095 | 0.046* | |
C5 | 0.6510 (3) | 0.36138 (11) | 0.4002 (3) | 0.0430 (6) | |
H5A | 0.5677 | 0.3843 | 0.4672 | 0.052* | |
H5B | 0.6612 | 0.3172 | 0.4347 | 0.052* | |
C6 | 0.5663 (3) | 0.36231 (9) | 0.2447 (3) | 0.0386 (5) | |
H6A | 0.4377 | 0.3454 | 0.2473 | 0.046* | |
H6B | 0.6402 | 0.3343 | 0.1814 | 0.046* | |
O3 | 0.0798 (2) | 0.19890 (6) | 0.74633 (17) | 0.0408 (4) | |
O4 | −0.0712 (2) | 0.25839 (6) | 0.57409 (16) | 0.0394 (4) | |
H4 | −0.1072 | 0.2231 | 0.5441 | 0.059* | |
O5 | −0.1439 (2) | 0.34122 (7) | 0.92241 (16) | 0.0410 (4) | |
O6 | −0.1161 (2) | 0.40041 (6) | 0.71776 (15) | 0.0377 (4) | |
C7 | 0.0368 (3) | 0.25099 (8) | 0.6931 (2) | 0.0285 (4) | |
C8 | −0.0638 (3) | 0.35515 (8) | 0.8022 (2) | 0.0283 (4) | |
C9 | 0.1083 (2) | 0.31502 (8) | 0.7556 (2) | 0.0275 (4) | |
C10 | 0.2733 (3) | 0.30732 (10) | 0.8709 (2) | 0.0387 (5) | |
H10A | 0.2769 | 0.3405 | 0.9470 | 0.046* | |
H10B | 0.2813 | 0.2648 | 0.9153 | 0.046* | |
C11 | 0.4144 (3) | 0.31829 (12) | 0.7441 (3) | 0.0497 (6) | |
H11A | 0.4669 | 0.2789 | 0.7040 | 0.060* | |
H11B | 0.5144 | 0.3488 | 0.7689 | 0.060* | |
C12 | 0.2515 (3) | 0.34698 (9) | 0.6475 (3) | 0.0360 (5) | |
H12A | 0.2450 | 0.3302 | 0.5464 | 0.043* | |
H12B | 0.2476 | 0.3936 | 0.6478 | 0.043* |
U11 | U22 | U33 | U12 | U13 | U23 | |
O1 | 0.0696 (13) | 0.0659 (12) | 0.0850 (14) | 0.0322 (10) | 0.0405 (11) | 0.0287 (10) |
O2 | 0.0808 (13) | 0.0492 (10) | 0.0745 (13) | 0.0349 (9) | 0.0353 (11) | 0.0244 (9) |
N1 | 0.0317 (9) | 0.0466 (10) | 0.0380 (10) | 0.0109 (7) | −0.0023 (7) | 0.0014 (7) |
C1 | 0.0325 (11) | 0.0370 (11) | 0.0342 (11) | 0.0057 (8) | −0.0010 (9) | 0.0049 (8) |
C2 | 0.0318 (10) | 0.0350 (10) | 0.0316 (11) | 0.0057 (8) | −0.0009 (8) | −0.0018 (8) |
C3 | 0.0341 (11) | 0.0339 (10) | 0.0420 (12) | 0.0013 (8) | 0.0082 (9) | 0.0033 (8) |
C4 | 0.0300 (10) | 0.0384 (11) | 0.0464 (13) | −0.0009 (8) | 0.0009 (9) | −0.0050 (9) |
C5 | 0.0357 (11) | 0.0408 (12) | 0.0526 (14) | 0.0056 (9) | 0.0047 (10) | 0.0163 (9) |
C6 | 0.0312 (11) | 0.0313 (11) | 0.0534 (13) | 0.0019 (8) | 0.0007 (9) | −0.0030 (9) |
O3 | 0.0469 (9) | 0.0261 (8) | 0.0493 (9) | 0.0009 (6) | −0.0012 (7) | 0.0056 (6) |
O4 | 0.0420 (8) | 0.0314 (7) | 0.0443 (9) | 0.0007 (6) | −0.0130 (7) | −0.0046 (6) |
O5 | 0.0404 (8) | 0.0440 (8) | 0.0387 (9) | 0.0148 (6) | 0.0066 (7) | 0.0074 (6) |
O6 | 0.0410 (8) | 0.0329 (8) | 0.0391 (8) | 0.0118 (6) | 0.0024 (6) | 0.0066 (6) |
C7 | 0.0242 (9) | 0.0277 (10) | 0.0337 (11) | 0.0009 (7) | 0.0060 (8) | 0.0003 (7) |
C8 | 0.0281 (10) | 0.0257 (9) | 0.0310 (11) | 0.0024 (7) | −0.0032 (8) | −0.0011 (7) |
C9 | 0.0248 (9) | 0.0253 (9) | 0.0323 (10) | 0.0010 (7) | −0.0001 (8) | 0.0022 (7) |
C10 | 0.0316 (11) | 0.0383 (11) | 0.0458 (13) | −0.0003 (8) | −0.0070 (9) | −0.0002 (9) |
C11 | 0.0257 (11) | 0.0516 (14) | 0.0718 (17) | −0.0041 (9) | −0.0011 (11) | 0.0119 (11) |
C12 | 0.0317 (11) | 0.0285 (10) | 0.0481 (12) | −0.0030 (8) | 0.0054 (9) | 0.0035 (8) |
O3—C7 | 1.218 (2) | C6—C5 | 1.503 (3) |
O4—C7 | 1.306 (2) | C6—C2 | 1.527 (3) |
O5—C8 | 1.257 (2) | C6—H6A | 0.9700 |
O6—C8 | 1.257 (2) | C6—H6B | 0.9700 |
O4—H4 | 0.8200 | C1—C2 | 1.518 (3) |
C7—C9 | 1.524 (3) | C10—C11 | 1.537 (3) |
N1—C4 | 1.490 (3) | C10—H10A | 0.9700 |
N1—C5 | 1.499 (3) | C10—H10B | 0.9700 |
N1—H1A | 0.9000 | C2—C3 | 1.533 (3) |
N1—H1B | 0.9000 | C2—H2A | 0.9800 |
O1—C1 | 1.201 (3) | C3—C4 | 1.507 (3) |
O2—C1 | 1.301 (2) | C3—H3A | 0.9700 |
O2—H2 | 0.8200 | C3—H3B | 0.9700 |
C8—C9 | 1.533 (2) | C5—H5A | 0.9700 |
C9—C10 | 1.546 (3) | C5—H5B | 0.9700 |
C9—C12 | 1.557 (3) | C4—H4A | 0.9700 |
C12—C11 | 1.541 (3) | C4—H4B | 0.9700 |
C12—H12A | 0.9700 | C11—H11A | 0.9700 |
C12—H12B | 0.9700 | C11—H11B | 0.9700 |
C7—O4—H4 | 109.5 | C11—C10—H10A | 113.9 |
O3—C7—O4 | 124.03 (17) | C9—C10—H10A | 113.9 |
O3—C7—C9 | 123.68 (17) | C11—C10—H10B | 113.9 |
O4—C7—C9 | 112.27 (15) | C9—C10—H10B | 113.9 |
C4—N1—C5 | 111.88 (15) | H10A—C10—H10B | 111.1 |
C4—N1—H1A | 109.2 | C1—C2—C6 | 111.88 (16) |
C5—N1—H1A | 109.2 | C1—C2—C3 | 112.49 (16) |
C4—N1—H1B | 109.2 | C6—C2—C3 | 109.74 (16) |
C5—N1—H1B | 109.2 | C1—C2—H2A | 107.5 |
H1A—N1—H1B | 107.9 | C6—C2—H2A | 107.5 |
C1—O2—H2 | 109.5 | C3—C2—H2A | 107.5 |
O6—C8—O5 | 123.78 (17) | C4—C3—C2 | 112.20 (17) |
O6—C8—C9 | 117.81 (16) | C4—C3—H3A | 109.2 |
O5—C8—C9 | 118.41 (15) | C2—C3—H3A | 109.2 |
C7—C9—C8 | 108.58 (14) | C4—C3—H3B | 109.2 |
C7—C9—C10 | 113.09 (15) | C2—C3—H3B | 109.2 |
C8—C9—C10 | 117.39 (16) | H3A—C3—H3B | 107.9 |
C7—C9—C12 | 110.90 (15) | N1—C5—C6 | 111.20 (17) |
C8—C9—C12 | 117.34 (15) | N1—C5—H5A | 109.4 |
C10—C9—C12 | 88.45 (15) | C6—C5—H5A | 109.4 |
C11—C12—C9 | 88.20 (15) | N1—C5—H5B | 109.4 |
C11—C12—H12A | 114.0 | C6—C5—H5B | 109.4 |
C9—C12—H12A | 114.0 | H5A—C5—H5B | 108.0 |
C11—C12—H12B | 114.0 | N1—C4—C3 | 110.44 (16) |
C9—C12—H12B | 114.0 | N1—C4—H4A | 109.6 |
H12A—C12—H12B | 111.2 | C3—C4—H4A | 109.6 |
C5—C6—C2 | 112.71 (17) | N1—C4—H4B | 109.6 |
C5—C6—H6A | 109.1 | C3—C4—H4B | 109.6 |
C2—C6—H6A | 109.1 | H4A—C4—H4B | 108.1 |
C5—C6—H6B | 109.1 | C10—C11—C12 | 89.38 (15) |
C2—C6—H6B | 109.1 | C10—C11—H11A | 113.7 |
H6A—C6—H6B | 107.8 | C12—C11—H11A | 113.7 |
O1—C1—O2 | 122.60 (19) | C10—C11—H11B | 113.7 |
O1—C1—C2 | 124.66 (18) | C12—C11—H11B | 113.7 |
O2—C1—C2 | 112.73 (17) | H11A—C11—H11B | 111.0 |
C11—C10—C9 | 88.75 (16) |
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1A···O6i | 0.90 | 1.96 | 2.816 (2) | 158 |
N1—H1B···O3ii | 0.90 | 2.01 | 2.899 (2) | 169 |
O2—H2···O6iii | 0.82 | 1.82 | 2.627 (2) | 170 |
O4—H4···O5iv | 0.82 | 1.74 | 2.522 (2) | 159 |
Symmetry codes: (i) x+1, y, z; (ii) x+1, −y+1/2, z−1/2; (iii) −x, −y+1, −z+1; (iv) x, −y+1/2, z−1/2. |
Experimental details
Crystal data | |
Chemical formula | C6H12NO2+·C6H7O4− |
Mr | 273.28 |
Crystal system, space group | Monoclinic, P21/c |
Temperature (K) | 293 |
a, b, c (Å) | 7.027 (2), 20.776 (5), 8.949 (2) |
β (°) | 91.06 (1) |
V (Å3) | 1306.2 (6) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.11 |
Crystal size (mm) | 0.3 × 0.2 × 0.1 |
Data collection | |
Diffractometer | Rigaku AFC-7S Mercury diffractometer |
Absorption correction | Multi-scan (REQAB; Jacobson, 1998) |
Tmin, Tmax | 0.970, 0.986 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 14962, 2561, 2026 |
Rint | 0.037 |
(sin θ/λ)max (Å−1) | 0.653 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.050, 0.139, 1.04 |
No. of reflections | 2561 |
No. of parameters | 173 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.41, −0.30 |
Computer programs: CrystalClear (Rigaku, 2000), CrystalStructure (Rigaku/MSC, 2004), SIR2008 (Burla et al., 2007), DIAMOND (Brandenburg, 1998), SHELXL97 (Sheldrick, 2008), PLATON (Spek, 2009) and publCIF (Westrip, 2010).
O3—C7 | 1.218 (2) | C9—C10 | 1.546 (3) |
O4—C7 | 1.306 (2) | C9—C12 | 1.557 (3) |
O5—C8 | 1.257 (2) | C12—C11 | 1.541 (3) |
O6—C8 | 1.257 (2) | C6—C5 | 1.503 (3) |
C7—C9 | 1.524 (3) | C6—C2 | 1.527 (3) |
N1—C4 | 1.490 (3) | C1—C2 | 1.518 (3) |
N1—C5 | 1.499 (3) | C10—C11 | 1.537 (3) |
O1—C1 | 1.201 (3) | C2—C3 | 1.533 (3) |
O2—C1 | 1.301 (2) | C3—C4 | 1.507 (3) |
C8—C9 | 1.533 (2) | ||
O3—C7—O4 | 124.03 (17) | C11—C12—C9 | 88.20 (15) |
O3—C7—C9 | 123.68 (17) | C5—C6—C2 | 112.71 (17) |
O4—C7—C9 | 112.27 (15) | O1—C1—O2 | 122.60 (19) |
C4—N1—C5 | 111.88 (15) | O1—C1—C2 | 124.66 (18) |
O6—C8—O5 | 123.78 (17) | O2—C1—C2 | 112.73 (17) |
O6—C8—C9 | 117.81 (16) | C11—C10—C9 | 88.75 (16) |
O5—C8—C9 | 118.41 (15) | C1—C2—C6 | 111.88 (16) |
C7—C9—C8 | 108.58 (14) | C1—C2—C3 | 112.49 (16) |
C7—C9—C10 | 113.09 (15) | C6—C2—C3 | 109.74 (16) |
C8—C9—C10 | 117.39 (16) | C4—C3—C2 | 112.20 (17) |
C7—C9—C12 | 110.90 (15) | N1—C5—C6 | 111.20 (17) |
C8—C9—C12 | 117.34 (15) | N1—C4—C3 | 110.44 (16) |
C10—C9—C12 | 88.45 (15) | C10—C11—C12 | 89.38 (15) |
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1A···O6i | 0.90 | 1.96 | 2.816 (2) | 158.1 |
N1—H1B···O3ii | 0.90 | 2.01 | 2.899 (2) | 169.0 |
O2—H2···O6iii | 0.82 | 1.82 | 2.627 (2) | 169.9 |
O4—H4···O5iv | 0.82 | 1.74 | 2.522 (2) | 159.0 |
Symmetry codes: (i) x+1, y, z; (ii) x+1, −y+1/2, z−1/2; (iii) −x, −y+1, −z+1; (iv) x, −y+1/2, z−1/2. |
Cocrystallization of organic compounds is serendipitous because the resulting multicomponent crystal could be a cocrystal, in which the different components are neutral species, or a salt, in which the components are charged species (Morissette et al., 2004). Many factors appear to influence the formation of either one or the other. Some strategies for the preparation of these materials are described by Tiekink & Vittal (2006) comprising aspects such as a preparation method such as recrystallization, growth from the melt, grinding etc. (Blagden et al., 2007); solvent choice and solubility (Jones & Davey, 2005); and the chemistry of functional groups and pKa (Trask et al., 2005). In addition, some authors consider necessary the identification of hierarchical best donor/best acceptor synthons (Aakeröy et al., 2001), or scrutinize the tendency to maximize noncovalent interactions among components (Aakeröy et al., 2006) and the charge-assisted ensemble of hydrogen bonds around the charged species in salts (Adams et al., 2006). Hence, predictability has been difficult up to now. However, some homomeric or heteromeric synthons such as amide/amide (Aakeröy et al., 2001), carboxylic acid/aminopyrimidine (Shan & Zaworotko, 2008) and amide/pyridine (Lemmerer et al., 2008) have been used to prepare multicomponent crystals. The Cambridge Structural Database (Version 5.32?; CSD; Allen, 2002) shows 220 cocrystals and salts having at least one amino acid component. This makes them reliable supramolecular reagents (Aakeröy et al., 2006; Rogowska et al., 2006) with the bonus of having the amino group in different states of protonation, viz. +NH3, +NH2 and +NH, which provides a way of building three-, two- or one-dimensional supramolecular motifs. In this study, we used isonipecotic acid (piperidine-4-carboxylic acid, ISO), an amino acid that, alone or as a hydrate, shows extended head-to-tail motifs based on N—H···O hydrogen bonds (Delgado et al., 2001; Mora et al., 2005), mixed with 1,1-cyclobutane-1,1-dicarboxylic acid (CBDC) (Santarsiero, 1990), in order to prepare the 1:1 multicomponent crystal structure ISO+.CBDC-, (I).
Salt (I) is an ionic ensemble assisted by hydrogen bonds. The asymmetric unit (Fig. 1) consists of one ISO+ cation with a positive charge residing on atom N1 and a CBDC- anion acting as a semicarboxylate ion, i.e. 1-carboxycyclobutane-1-carboxylate [for the carboxyl group, C7—O3 and C7—O4 are 1.218 (2) and 1.306 (2) Å, respectively; for the carboxylate group, C8—O5 and C8—O6 are both 1.257 (2) Å]. A restricted search of the CSD (R factor less than 0.05) showed five multicomponent crystal structures displaying CBDC- as a semicarboxylate ion [with imidazolium (CSD refcode EQUXOD; Ballabh et al., 2003), benzimidazolium (EQUXUJ; Ballabh et al., 2003), dibenzylammonium (MEFRAR; Trivedi et al., 2006), 2-phenylimidazolium (VARHIG; Trivedi et al., 2003) and 1-butanaminium (TOKSUI; Ballabh et al., 2008)]. On the other hand, for ISO+, C1—O2 and C1—O1 are 1.301 (2) and 1.201 (3) Å, respectively. This finding, along with the localization in the difference Fourier map of two H atoms linked to atom N1, shows that the amino acid is not a zwitterion, but a positive ion.
The piperidinium ring in the ISO+ cation of (I) adopts the most stable chair conformation (Cremer & Pople, 1975). The orientation of the –COOH group in ISO+ is axial (Luger & Bülow, 1983), contrasting with the 11 ISO structures found in a restricted search of the CSD (R factor less than 0.05) in which the orientation is equatorial. The torsion angles O1—C1—C2–C6 and O2—C1—C2—C3 are 3.6 (3) and 60.7 (2)°, respectively. These torsion angles vary significantly in all the reported structures. Mora et al. (2005) attributed this effect to the required rotation of this group to optimize the formation of hydrogen bonds with neighbouring molecules. The cyclobutane ring of the CBDC- anion of (I) is slightly puckered, with an internal torsion angle C12—C9—C10—C11 = 17.1 (2)°.
Table 2 displays the geometry of all the relevant hydrogen bonds observed in (I). Fig. 2(a) shows how the crystal structure packs along a in the form of zigzag layers as viewed in the ab plane, also seen in the structure of the diacidic form of CBDC (CBUTCA01; Santarsiero, 1990). Fig. 2(b) shows one of the layers viewed in the bc plane, which can be described by a combination of graph-set symbols (Bernstein et al., 1995). (i) Wave-like ribbons described by the second-order graph-set symbol R33(14) are related by c-glide planes and run alternately in opposite directions along c; these ribbons are formed by double chains of molecules, one of intercalated ···ISO–CBDC–CBDC–ISO··· chains described by the graph-set symbol C33(10) and one of only CBDC chains described by the graph-set symbol C(6). (ii) These ribbons are linked through ISO+ cations related to each other by inversion centres, forming two rings with second-order graph-set symbols R66(29) and R42(18). Fig. 3(a) shows how, in the homomeric CBDC chains, graph-set symbol C(6), the cyclobutane ring is oriented towards the same side; this contrasts with the previously reported CBDC multicomponent crystals, all having an alternating orientation of the cyclobutane ring in these chains [EQUXOD (Ballabh et al., 2003), VARHIG (Trivedi et al., 2003) and MEFRAR (Trivedi et al., 2006)]. The ring described by graph-set symbol R42(18) forms a double head-to-tail structure (Fig. 3b), which has been observed in other amino acids (Ávila et al., 2004); for cis-4-aminocyclohexanecarboxylic acid, the terminal amino group hangs outside the cyclohexane ring, providing additional flexibility to this group and enabling its linking through hydrogen bonds to another amino acid unit to form the double head-to-tail structure shown in Fig. 3(c). In contrast, the amino group in (I) does not have such flexibility because the N atom is incorporated in the pipridinium ring, thus requiring the help of two CBDC- anions to form the R42(18) ring.