4-Carb­oxy­piperidinium 1-carb­oxy­cyclo­butane-1-carboxyl­ate

The title salt, C₆H₁₂NO₂⁺·C₆H₇O₄^- or ISO⁺·CBDC^-, is an ionic ensemble assisted by hydrogen bonds. The amino acid moiety (ISO or piperidine-4-carb­oxy­lic acid) has a protonated ring N atom (ISO⁺ or 4-carb­oxy­piperidinium), while the semi-protonated acid (CBDC^- or 1-carb­oxy­cyclo­butane-1-car­box­yl­ate) has the negative charge residing on one carboxyl­ate group, leaving the other as a neutral -COOH group. The -⁺NH₂- state of protonation allows the formation of a two-dimensional crystal packing consisting of zigzag layers stacked along a separated by van der Waals distances. The layers extend in the bc plane connected by a complex network of N-H...O and O-H...O hydrogen bonds. Wave-like ribbons, constructed from ISO⁺ and CBDC^- units and described by the graph-set symbols C₃³(10) and R₃³(14), run alternately in opposite directions along c. Inter­calated between the ribbons are ISO⁺ cations linked by hydrogen bonds, forming rings described by the graph-set symbols R₆⁶(30) and R₄²(18). A detailed analysis of the structures of the individual components and the intricate hydrogen-bond network of the crystal structure is given.

S1. Comment

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 pK_a (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.

Related literature

For related literature, see: Aakeröy et al. (2001, 2006); Adams et al. (2006); Allen (2002); Ballabh et al. (2003, 2008); Bernstein et al. (1995); Blagden et al. (2007); Cremer & Pople (1975); Delgado et al. (2001); Jones & Davey (2005); Lemmerer et al. (2008); Luger & Bülow (1983); Mora et al. (2005); Morissette et al. (2004); Rogowska et al. (2006); Santarsiero (1990); Shan & Zaworotko (2008); Tiekink & Vittal (2006); Trask et al. (2005); Trivedi et al. (2003, 2006); Ávila et al. (2004).

S2. Experimental

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.

S3. Refinement

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 U_iso(H) = 1.2U_eq(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.

Computing details

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).

Figures

	Fig. 1. The asymmetric unit of ISO^+.CBDC^-, showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level.
	Fig. 2. The crystal packing of ISO^+.CBDC^-, showing (a) a view in the ab plane of the zigzag layers stacking along a, and (b) one layer in the bc plane described by the combination of graph-set symbols R3^3(14), R6^6(29) and R4^2(18).
	Fig. 3. (a) A homomeric CBDC chain, linked by O4—H4···O5^iv hydrogen bonds [symmetry code: (iv) x, -y + 1/2, z - 1/2], with the graph-set symbol C(6), displaying the cyclobutane rings oriented towards the same side of the chain. (b) The double head-to-tail structure, graph-set symbol R4^2(18), formed by two ISO^+ cations and two CBDC^- anions. It is similar to that observed in (c) cis-4-ammoniocyclohexanecarboxylate hemihydrate (Ávila et al., 2004). Owing to the flexibility of the pendant ^+NH3 group, direct interaction of the amino acid molecules is allowed, while in ISO^+.CBDC^- the amino acid requires two bridging CBDC^- anions to form the R4^2(18) ring. [Some atom labels appear twice; please supply symmetry codes for the non-unique positions]

4-Carboxypiperidinium 1-carboxycyclobutane-1-carboxylate

Crystal data

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) Å^3	Lamellar, colourless
Z = 4	0.3 × 0.2 × 0.1 mm

Data collection

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

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.050	H-atom parameters constrained
wR(F^2) = 0.139	w = 1/[σ^2(Fo^2) + (0.0657P)^2 + 0.6501P] where P = (Fo^2 + 2Fc^2)/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+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.028 (3)

Special details

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.

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

	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*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
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)

Geometric parameters (Å, º)

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)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1A···O6^i	0.90	1.96	2.816 (2)	158
N1—H1B···O3^ii	0.90	2.01	2.899 (2)	169
O2—H2···O6^iii	0.82	1.82	2.627 (2)	170
O4—H4···O5^iv	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.