research communications\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

ISSN: 2056-9890

Hydrogen-bonded co-crystal structure of benzoic acid and zwitterionic L-proline

a764 Natural Sciences Complex, Buffalo, 14260-3000, USA, b730 Natural Sciences Complex, Buffalo, 14260-3000, USA, and c771 Natural Sciences Complex, Buffalo, 14260-3000, USA
*Correspondence e-mail:

Edited by A. J. Lough, University of Toronto, Canada (Received 3 November 2016; accepted 1 February 2017; online 14 February 2017)

The title compound [systematic name: benzoic acid–pyrrolidin-1-ium-2-carboxyl­ate (1/1)], C7H6O2·C5H9NO2, is an example of the application of non-centrosymmetric co-crystallization for the growth of a crystal containing a typically centrosymmetric component in a chiral space group. It co-crystallizes in the space group P212121 and contains benzoic acid and L-proline in equal proportions. The crystal structure exhibits chains of L-proline zwitterions capped by benzoic acid mol­ecules which form a C(5)[R33(11)] hydrogen-bonded network along [100]. The crystal structure is examined and compared to that of a similar co-crystal containing L-proline zwitterions and 4-amino­benzoic acid.

1. Chemical context

Non-centrosymmetric materials are of particular importance in the field of materials chemistry for the large number of symmetry-dependent properties they can possess, including circular dichroism, pyroelectricity, and non-linear optical behavior (Halasyamani & Poeppelmeier, 1998[Halasyamani, P. S. & Poeppelmeier, K. R. (1998). Chem. Mater. 10, 2753-2769.]; McMillen et al., 2012[McMillen, C. D., Stritzinger, J. T. & Kolis, J. W. (2012). Inorg. Chem. 51, 3953-3955.]; Aitken et al., 2009[Aitken, J. A., Lekse, J. W., Yao, J.-L. & Quinones, R. (2009). J. Solid State Chem. 182, 141-146.]). While purposefully engineering these materials can be difficult, one method for eliminating centrosymmetry in crystalline materials is co-crystallization with an enanti­opure chiral compound (Kwon et al., 2006[Kwon, S.-J., Kwon, O. P., Jazbinsek, M., Gramlich, V. & Günter, P. (2006). Chem. Commun. pp. 3729-3731.]). In this way, provided that the chiral compound is not capable of racemization, the potential point groups are limited only to those which are chiral, and therefore non-centrosymmetric. The amino acid proline plays an important role in determining the structure of proteins, due to its structural rigidity. Proline has also been shown to be a good candidate for synthesizing non-centrosymmetric co-crystals. In fact, Timofeeva et al. (2003[Timofeeva, T. V., Kuhn, G. H., Nesterov, V. V., Nesterov, V. N., Frazier, D. O., Penn, B. G. & Antipin, M. Y. (2003). Cryst. Growth Des. 3, 383-391.]) reported success co-crystallizing di­cyano­vinyl aromatic compounds with L-proline while the same compounds would grow neat crystals when co-crystallization with L-tartaric acid was attempted.

[Scheme 1]

2. Structural commentary

L-proline zwitterion (LP) and benzoic acid (BA) co-crystallize in the chiral space group P212121 with one mol­ecule of L-proline and one mol­ecule of benzoic acid in the asymmetric unit, shown in Fig. 1[link]. The L-proline exists in its zwitterionic form within the lattice while the carb­oxy­lic acid group of the benzoic acid mol­ecules remain protonated. Although the Flack parameter could not be used to unambiguously assign the absolute configuration, the enanti­omer was reliably assigned by reference to an unchanging chiral centre in the synthetic procedure.

[Figure 1]
Figure 1
The asymmetric unit of the title compound, showing the atom-naming scheme. Displacement ellipsoids are shown at the 50% probability level.

3. Supra­molecular features

In this structure, each LP hydrogen bonds with four other LP mol­ecules and one BA. The LP hydrogen bonding forms 1D chains along [100] via (carboxyl­ate) O⋯H—N (pyrollium) inter­actions in a C(5)[[R_{3}^{3}](11)] motif (Table 1[link]). The BA mol­ecules act as capping groups and hydrogen bond to each of the LP carboxyl­ates through O—H⋯O (carboxyl­ate) inter­actions. The complete BA–LP chains, as shown in Fig. 2[link], propagate along [100] and are approximately contained in (021) and (0[\overline{2}]1). These chains are held together by edge–face-type ππ stacking between adjacent BA mol­ecules approximately along [010], with a ring-centroid to ring-centroid distance of 4.8451 (16) Å.

Table 1
Hydrogen-bond geometry (Å, °)

N1—H1A⋯O2i 0.91 1.92 2.751 (3) 151
N1—H1B⋯O1 0.91 2.18 2.679 (3) 114
N1—H1B⋯O1ii 0.91 2.08 2.782 (2) 133
C4—H4⋯O3iii 1.00 2.30 3.192 (3) 147
O4—H4A⋯O2iv 0.84 1.76 2.595 (2) 173
Symmetry codes: (i) x-1, y, z; (ii) [x-{\script{1\over 2}}, -y+{\script{3\over 2}}, -z+1]; (iii) [-x+{\script{1\over 2}}, -y+1, z-{\script{1\over 2}}]; (iv) [-x+{\script{3\over 2}}, -y+1, z+{\script{1\over 2}}].
[Figure 2]
Figure 2
Diagram illustrating the hydrogen-bonding inter­actions in BA–LP co-crystal.

4. Database survey

Recently, the co-crystal structure of LP and para-amino­benzoic acid (PABA) was reported (Athimoolam & Natarajan, 2007[Athimoolam, S. & Natarajan, S. (2007). Acta Cryst. C63, o283-o286.]). While the structure of BALP retains some structural similarities with the PABALP co-crystal, due to the absence of one hydrogen-bonding moiety, the amino group, the structure of BALP (Fig. 3[link]) also exhibits some important differences when compared to that of PABALP. The head-to-tail LP chains in PABALP are similar to those in BALP, though instead of two chains hydrogen-bonded together to form rings, the chains hydrogen bond to form a continuous 2D sheet of LP mol­ecules. Much like BALP, the PABA mol­ecules hydrogen bond to the periphery of the LP chains; however, this crystal incorporated water into the lattice and it is to these water mol­ecules that the PABA mol­ecules are bound. The major difference between the two structures is the presence of the hydrogen-bond donating group at the 4-position of the PABA mol­ecules. This moiety allows the PABA mol­ecules to bridge the LP chains in PABA--LP, a supra­molecular feature absent in the title compound. The result of the lack of para-substitution and water in the lattice is that BALP forms a hydrogen-bonding network which extends in only one dimension, instead of the three-dimensional network of PABALP.

[Figure 3]
Figure 3
Diagram illustrating the hydrogen bonding network of LP in the previously reported PABA–LP co-crystal (left) and view of the PABA hydrogen-bonding network in the previously reported co-crystal (right).

5. Synthesis and crystallization

Solid BA (10.1 mg, 9.01 × 10 −2 mmol) and LP (9.3 mg, 8.08 × 10−2 mmol) were added to a 25 ml scintillation vial. To this was added approximately 8 ml of ethanol followed by sonication until all solutes were fully dissolved. The loosely capped vial was then placed on an open shelf. After three weeks, colorless needle-shaped crystals of the title compound suitable for single-crystal X-ray diffraction measurements were obtained.

6. Refinement

The crystal, data collection, and refinement details are listed in Table 2[link]. The positions of the carboxyl­ate and pyrollium hydrogen atoms were determined from the Fourier difference map, and all other hydrogen atoms were placed in idealized positions with C—H bond lengths set to 0.93 and 0.97 Å for aryl and alkyl hydrogen atoms, respectively. These hydrogen atoms were refined using a riding model with Uiso(H) = 1.5Ueq(O) for the carb­oxy­lic acid proton on the BA mol­ecules and Uiso(H) = 1.2Ueq in all other cases. No other constraints were applied to the refinement model.

Table 2
Experimental details

Crystal data
Chemical formula C5H9NO2·C7H6O2
Mr 237.25
Crystal system, space group Orthorhombic, P212121
Temperature (K) 90
a, b, c (Å) 5.6993 (7), 12.0762 (13), 16.6839 (19)
V3) 1148.3 (2)
Z 4
Radiation type Mo Kα
μ (mm−1) 0.10
Crystal size (mm) 0.1 × 0.01 × 0.01
Data collection
Diffractometer Bruker SMART APEXII area detector
Absorption correction Multi-scan (SADABS; Bruker, 2013[Bruker (2013). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.])
Tmin, Tmax 0.619, 0.746
No. of measured, independent and observed [I > 2σ(I)] reflections 13711, 2880, 2375
Rint 0.069
(sin θ/λ)max−1) 0.670
R[F2 > 2σ(F2)], wR(F2), S 0.043, 0.086, 1.06
No. of reflections 2880
No. of parameters 155
H-atom treatment H-atom parameters constrained
Δρmax, Δρmin (e Å−3) 0.20, −0.18
Absolute structure Flack x determined using 824 quotients [(I+)−(I)]/[(I+)+(I)] (Parsons et al., 2013[Parsons, S., Flack, H. D. & Wagner, T. (2013). Acta Cryst. B69, 249-259.])
Absolute structure parameter 0.5 (8)
Computer programs: APEX2 and SAINT (Bruker, 2013[Bruker (2013). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.]), olex2.solve (Bourhis et al., 2015[Bourhis, L. J., Dolomanov, O. V., Gildea, R. J., Howard, J. A. K. & Puschmann, H. (2015). Acta Cryst. A71, 59-75.]), SHELXL2014 (Sheldrick, 2015[Sheldrick, G. M. (2015). Acta Cryst. C71, 3-8.]) and OLEX2 (Dolomanov et al., 2009[Dolomanov, O. V., Bourhis, L. J., Gildea, R. J., Howard, J. A. K. & Puschmann, H. (2009). J. Appl. Cryst. 42, 339-341.]).

Supporting information

Computing details top

Data collection: APEX2 (Bruker, 2013); cell refinement: SAINT (Bruker, 2013); data reduction: SAINT (Bruker, 2013); program(s) used to solve structure: olex2.solve (Bourhis et al., 2015); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

Benzoic acid–pyrrolidin-1-ium-2-carboxylate (1/1) top
Crystal data top
C5H9NO2·C7H6O2Dx = 1.372 Mg m3
Mr = 237.25Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, P212121Cell parameters from 2011 reflections
a = 5.6993 (7) Åθ = 3.4–24.7°
b = 12.0762 (13) ŵ = 0.10 mm1
c = 16.6839 (19) ÅT = 90 K
V = 1148.3 (2) Å3Needle, colourless
Z = 40.1 × 0.01 × 0.01 mm
F(000) = 504
Data collection top
Bruker SMART APEXII area detector
2880 independent reflections
Radiation source: microfocus rotating anode, Incoatec Iµs2375 reflections with I > 2σ(I)
Mirror optics monochromatorRint = 0.069
Detector resolution: 7.9 pixels mm-1θmax = 28.4°, θmin = 2.1°
ω and φ scansh = 77
Absorption correction: multi-scan
(SADABS; Bruker, 2013)
k = 1516
Tmin = 0.619, Tmax = 0.746l = 2222
13711 measured reflections
Refinement top
Refinement on F2Hydrogen site location: inferred from neighbouring sites
Least-squares matrix: fullH-atom parameters constrained
R[F2 > 2σ(F2)] = 0.043 w = 1/[σ2(Fo2) + (0.0344P)2 + 0.0827P]
where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.086(Δ/σ)max < 0.001
S = 1.06Δρmax = 0.20 e Å3
2880 reflectionsΔρmin = 0.18 e Å3
155 parametersAbsolute structure: Flack x determined using 824 quotients [(I+)-(I-)]/[(I+)+(I-)] (Parsons et al., 2013)
0 restraintsAbsolute structure parameter: 0.5 (8)
Primary atom site location: iterative
Special details top

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. 1. Fixed Uiso At 1.2 times of: All C(H) groups, All C(H,H) groups, All N(H,H) groups At 1.5 times of: All O(H) groups 2.a Ternary CH refined with riding coordinates: C4(H4) 2.b Secondary CH2 refined with riding coordinates: N1(H1A,H1B), C1(H1C,H1D), C2(H2A,H2B), C3(H3A,H3B) 2.c Aromatic/amide H refined with riding coordinates: C7(H7), C8(H8), C9(H9), C10(H10), C11(H11) 2.d Idealised tetrahedral OH refined as rotating group: O4(H4A)

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
O10.6846 (3)0.69609 (14)0.45367 (10)0.0186 (4)
O20.8438 (3)0.62955 (13)0.34146 (9)0.0131 (4)
N10.2560 (3)0.72586 (15)0.39137 (11)0.0118 (4)
C10.1825 (5)0.8393 (2)0.36504 (13)0.0161 (5)
C20.2381 (4)0.8397 (2)0.27615 (13)0.0161 (5)
C30.4714 (4)0.7775 (2)0.27268 (14)0.0149 (5)
C40.4481 (4)0.68629 (19)0.33630 (13)0.0110 (5)
C50.6756 (4)0.66858 (18)0.38222 (14)0.0119 (5)
O30.2945 (3)0.46855 (14)0.70526 (9)0.0174 (4)
O40.6289 (3)0.37449 (14)0.68623 (9)0.0173 (4)
C60.3977 (4)0.41409 (19)0.57268 (14)0.0125 (5)
C70.1924 (5)0.4564 (2)0.54035 (14)0.0183 (5)
C80.1538 (5)0.4548 (2)0.45852 (14)0.0201 (6)
C90.3232 (5)0.41022 (19)0.40809 (14)0.0196 (6)
C100.5282 (5)0.3668 (2)0.44000 (15)0.0200 (6)
C110.5651 (5)0.3678 (2)0.52216 (14)0.0162 (5)
C120.4324 (4)0.42155 (19)0.66114 (14)0.0132 (5)
Atomic displacement parameters (Å2) top
O10.0133 (9)0.0308 (10)0.0117 (8)0.0002 (8)0.0019 (7)0.0037 (7)
O20.0085 (9)0.0172 (8)0.0135 (8)0.0008 (7)0.0004 (7)0.0001 (7)
N10.0101 (11)0.0146 (9)0.0106 (9)0.0001 (8)0.0006 (8)0.0010 (8)
C10.0155 (13)0.0137 (11)0.0191 (12)0.0018 (12)0.0015 (10)0.0011 (9)
C20.0186 (14)0.0141 (11)0.0155 (12)0.0045 (11)0.0008 (10)0.0015 (9)
C30.0139 (13)0.0181 (12)0.0127 (12)0.0029 (11)0.0020 (10)0.0016 (10)
C40.0108 (12)0.0128 (11)0.0094 (11)0.0004 (10)0.0011 (9)0.0010 (9)
C50.0112 (12)0.0116 (10)0.0129 (11)0.0026 (11)0.0005 (10)0.0022 (9)
O30.0169 (10)0.0200 (9)0.0152 (8)0.0022 (8)0.0045 (7)0.0013 (7)
O40.0186 (10)0.0231 (9)0.0101 (8)0.0053 (8)0.0017 (7)0.0015 (7)
C60.0116 (12)0.0116 (11)0.0142 (11)0.0024 (10)0.0007 (9)0.0017 (9)
C70.0157 (13)0.0174 (12)0.0216 (13)0.0029 (11)0.0018 (11)0.0008 (10)
C80.0196 (14)0.0169 (12)0.0237 (13)0.0018 (11)0.0068 (12)0.0044 (11)
C90.0272 (15)0.0168 (12)0.0147 (12)0.0033 (12)0.0059 (11)0.0017 (9)
C100.0241 (14)0.0197 (13)0.0164 (12)0.0006 (12)0.0005 (11)0.0027 (11)
C110.0160 (13)0.0153 (11)0.0172 (12)0.0028 (11)0.0010 (10)0.0011 (10)
C120.0138 (13)0.0111 (11)0.0146 (11)0.0032 (10)0.0026 (10)0.0027 (9)
Geometric parameters (Å, º) top
O1—C51.239 (3)C4—C51.521 (3)
O2—C51.266 (3)O3—C121.217 (3)
N1—H1B0.9100O4—C121.324 (3)
N1—C11.498 (3)C6—C71.386 (3)
N1—C41.507 (3)C6—C111.391 (3)
C1—H1C0.9900C6—C121.492 (3)
C1—C21.517 (3)C7—C81.383 (3)
C2—H2B0.9900C8—C91.389 (4)
C2—C31.528 (3)C9—H90.9500
C3—H3A0.9900C9—C101.386 (4)
C3—C41.536 (3)C10—C111.387 (3)
H1A—N1—H1B108.4C5—C4—C3112.05 (19)
C1—N1—H1B110.0O1—C5—O2125.8 (2)
C1—N1—C4108.32 (18)O1—C5—C4118.9 (2)
C4—N1—H1A110.0O2—C5—C4115.28 (19)
N1—C1—H1C111.1C7—C6—C11119.5 (2)
N1—C1—H1D111.1C7—C6—C12118.3 (2)
N1—C1—C2103.39 (18)C11—C6—C12122.2 (2)
C2—C1—H1C111.1C8—C7—C6120.8 (2)
C1—C2—H2B111.3C7—C8—C9119.6 (3)
C1—C2—C3102.53 (19)C9—C8—H8120.2
C3—C2—H2A111.3C10—C9—C8120.0 (2)
C2—C3—H3B110.8C9—C10—C11120.3 (2)
C2—C3—C4104.55 (19)C11—C10—H10119.9
C4—C3—H3A110.8C10—C11—C6119.9 (2)
N1—C4—C3104.87 (18)O3—C12—O4123.7 (2)
N1—C4—H4109.6O3—C12—C6122.7 (2)
N1—C4—C5110.90 (18)O4—C12—C6113.5 (2)
N1—C1—C2—C339.6 (2)C7—C6—C11—C101.6 (4)
N1—C4—C5—O16.0 (3)C7—C6—C12—O34.2 (3)
N1—C4—C5—O2176.36 (18)C7—C6—C12—O4177.2 (2)
C1—N1—C4—C34.2 (2)C7—C8—C9—C100.7 (4)
C1—N1—C4—C5117.0 (2)C8—C9—C10—C110.1 (4)
C1—C2—C3—C437.3 (2)C9—C10—C11—C61.0 (4)
C2—C3—C4—N120.6 (2)C11—C6—C7—C81.0 (4)
C2—C3—C4—C5141.01 (19)C11—C6—C12—O3174.9 (2)
C3—C4—C5—O1110.8 (2)C11—C6—C12—O43.7 (3)
C3—C4—C5—O266.8 (3)C12—C6—C7—C8178.1 (2)
C4—N1—C1—C227.4 (2)C12—C6—C11—C10177.5 (2)
C6—C7—C8—C90.1 (4)
Hydrogen-bond geometry (Å, º) top
N1—H1A···O2i0.911.922.751 (3)151
N1—H1B···O10.912.182.679 (3)114
N1—H1B···O1ii0.912.082.782 (2)133
C4—H4···O3iii1.002.303.192 (3)147
O4—H4A···O2iv0.841.762.595 (2)173
Symmetry codes: (i) x1, y, z; (ii) x1/2, y+3/2, z+1; (iii) x+1/2, y+1, z1/2; (iv) x+3/2, y+1, z+1/2.


Both authors contributed equally to this report.


This material is based upon work supported by the National Science Foundation under grant No. DMR-1455039.

Funding information

Funding for this research was provided by: National Science Foundation (award No. DMR-1455039).


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