The 1:1 adduct of hexa­methyl­ene­tetramine (HMT) with racemic trans-1,2-cyclo­hexane­di­carboxylic acid (CDA)

Hexa­methyl­ene­tetramine and rac-trans-1,2-cyclo­hexane­di­carboxylic acid crystallize in a 1:1 ratio as a neutral molecular adduct, C₆H₁₂N₄·C₈H₁₂O₄. Two di­carboxylic acid mol­ecules and two tetr­amine mol­ecules form a hydrogen-bonded ring, in the shape of a rhombus, which lies on a crystallographic twofold axis bisecting the two diacid mol­ecules. The O-H...N hydrogen bonds have lengths 2.6808 (19) and 2.6518 (19) Å, and, in each ring, both acid mol­ecules have the same handedness.

S1. Comment

Hexamethylenetetramine (C₆H₁₂N₄, also known as hexamine and urotropine, and abbreviated as HMT) forms one-, two- and three-dimensional structures with dicarboxylic acids and phenols (Gaillard et al., 1996, 1998; Hostettler et al., 1999; Lough et al., 2000; Coupar Ferguson et al., 1997; Coupar Glidewell & Ferguson, 1997; Mak, 1965; Jordan & Mak, 1970). These self-assembled co-crystalline molecular adducts of HMT with dicarboxylic acids are usually interconnected by hydrogen bonds, with the N atoms of HMT acting as acceptors and the carboxyl H atoms as donors. Similarly, HMT with alkanedioic acids [HOOC-(CH₂)_n-COOH] forms 1:1 crystalline adducts with rich phase diagrams. Trimesic acid (benzene-1,3,5-tricarboxylic acid), with its three carboxylic acid groups, has been extensively used in the creation of highly ordered co-crystalline solids. Shan et al. (2003) used a range of bases, including HMT, with cyclohexane-1,3-cis,5-cis-tricarboxylic acid to form supramolecular motifs. In the present study, we used racemic 1,2-trans-cyclohexanedicarboxylic acid, CDA, with the possibility of double deprotonation, to bind with HMT to form similar co-crystalline solids. Pale-yellow rectangular prisms of the title adduct, (I), resulted by mixing hot ethanol solutions of HMT and CDA (in 1:1 molar ratio) and evaporating the mixture at room temperature. Unlike in other systems involving HMT and dicarboxylic acids, we have found only one stoichiometric co-crystal of HMT with CDA. In this report, we present the crystal structure of the adduct, (I). \sch

The asymmetric unit of the co-crystal of CDA and HMT consists of one HMT molecule in a general position and two CDA molecules lying on a crystallographic twofold axis. The one independent COOH group on each CDA molecule forms a hydrogen bond with an N atom on the HMT molecule, and the twofold axis thus forms a hydrogen-bonded rhombus with formula CDA₂HMT₂ (Fig. 1). Details of the hydrogen bonds, which are essentially linear, are given in Table 2. Although the crystal is racemic, both CDA molecules in any given hydrogen-bonded rhombus have the same handedness.

There is a mismatch in number between the two strong hydrogen-bond donors on CDA and the total of six acceptors (four N on HMT and two C═O on CDA), so formation of the hydrogen-bonded rhombus precludes any extended hydrogen-bonding network. In the absence of such a network, weaker C—H···O and C—H···N interactions involve the remaining acceptors. These are also detailed in Table 2, which shows that all C—H donors are HMT CH₂ groups. The two excess HMT N atoms each accept one C—H···N interaction, and each CDA C═O accepts two C—H···O interactions. These weaker interactions allow the perching of two HMT molecules above and below each rhombus. This results in alternating layers of HMT and CDA molecules each 1/4 translation along the polar c axis, as shown in Fig. 2.

The space group for (I) is uncommon, being reported for approximately 0.1% of the entries in the Cambridge Structural Database (Version?; Allen, 2002). In their analysis of space-group frequencies, Brock & Dunitz (1994) found that twofold axes in this space group are usually occupied, with 18 of 24 structures in their sample having Z' = 1/2.

S2. Experimental

All reagents and solvents used were commercially available and were used as received without further purification. An ethanol solution (15 ml) of CDA (0.172 g, 1.0 mmol) was added to a stirred solution (5 ml) of HMT (0.100 g, 1.0 mmol) and the reaction mixture stirred for 1 hr. The resulting colorless solution was allowed to stand in air at room temperature for 2 d, yielding colorless crystals of (I) suitable for X-ray analysis. IR spectra for the product were obtained in the 4000–400 cm⁻¹ range in KBr pellets on a Nicolet Nexus 670 F T—IR spectrophotometer. (The –OH stretching frequency ν_OH around 3560 cm⁻¹ is very broad compared with the sharp –OH band in the spectrum for CDA. Similarly, the sharp bands of –CN stretching and –NCN bond + ring deformation around 1440 cm⁻¹ and 1540 cm⁻¹ are also found to become one broad band in the adduct).

S3. Refinement

The absolute structure could not be determined, and Friedel equivalents were averaged. H atoms on C atoms were treated as riding in idealized positions, with C—H distances in the range 0.95–0.99 Å, depending on atom type. The coordinates of H atoms on O atoms were refined. Displacement parameters for all H atoms were assigned as U_iso(H) = 1.2U_eq of the attached atom.

Computing details

Data collection: COLLECT (Nonius, 2000); cell refinement: HKL SCALEPACK (Otwinowski & Minor 1997); data reduction: HKL DENZO (Otwinowski & Minor 1997) and SCALEPACK; program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The hydrogen-bonded ring of (I), showing the atom-numbering scheme and with displacement ellipsoids at the 50% probability level. Unlabelled atoms are related to labelled ones by a horizontal twofold axis (1 − x, 1 − y, z).
	Fig. 2. A stereoview of the unit cell of (I). The origin is top center (background), with c vertical and b nearly horizontal.

Hexamethylenetetramine–rac-trans-1,2-cyclohexanedicarboxylic acid (1:1)

Crystal data

C6H12N4·C8H12O4	F(000) = 1344
Mr = 312.37	Dx = 1.296 Mg m^−3
Orthorhombic, Aba2	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: A 2 -2ac	Cell parameters from 3171 reflections
a = 11.990 (3) Å	θ = 2.5–33.1°
b = 11.814 (3) Å	µ = 0.10 mm^−1
c = 22.606 (6) Å	T = 102 K
V = 3202.1 (14) Å^3	Rectangular prism, colorless
Z = 8	0.25 × 0.25 × 0.12 mm

Data collection

Nonius KappaCCD (with Oxford Cryostream) diffractometer	2652 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.033
Graphite monochromator	θmax = 33.1°, θmin = 2.5°
ω scans with κ offsets	h = −18→18
17655 measured reflections	k = −18→18
3119 independent reflections	l = −34→34

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.041	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.097	H atoms treated by a mixture of independent and constrained refinement
S = 1.04	w = 1/[σ^2(Fo^2) + (0.0511P)^2 + 0.5607P] where P = (Fo^2 + 2Fc^2)/3
3119 reflections	(Δ/σ)max < 0.001
205 parameters	Δρmax = 0.32 e Å^−3
1 restraint	Δρmin = −0.27 e Å^−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	0.43824 (11)	0.29207 (10)	0.22175 (6)	0.0225 (3)
H1O	0.396 (2)	0.275 (2)	0.2526 (13)	0.034*
O2	0.36053 (11)	0.46191 (11)	0.23555 (6)	0.0247 (3)
O3	0.30918 (11)	0.39861 (12)	0.50944 (6)	0.0252 (3)
H3O	0.296 (2)	0.352 (2)	0.4769 (13)	0.038*
O4	0.48849 (11)	0.35855 (12)	0.49274 (6)	0.0272 (3)
N1	0.11509 (11)	0.23047 (12)	0.34622 (7)	0.0154 (3)
N2	0.24139 (12)	0.07814 (11)	0.37716 (6)	0.0151 (3)
N3	0.30955 (11)	0.22983 (11)	0.31311 (6)	0.0136 (2)
N4	0.26459 (11)	0.26894 (11)	0.41666 (6)	0.0148 (3)
C1	0.12582 (13)	0.10846 (13)	0.36051 (8)	0.0159 (3)
H1A	0.0750	0.0896	0.3936	0.019*
H1B	0.1030	0.0631	0.3257	0.019*
C2	0.31559 (14)	0.10774 (13)	0.32809 (8)	0.0155 (3)
H2A	0.2950	0.0625	0.2929	0.019*
H2B	0.3933	0.0882	0.3389	0.019*
C3	0.33904 (13)	0.29520 (14)	0.36651 (8)	0.0153 (3)
H3A	0.3344	0.3771	0.3575	0.018*
H3B	0.4170	0.2779	0.3778	0.018*
C4	0.14829 (13)	0.29488 (14)	0.39864 (8)	0.0157 (3)
H4A	0.0971	0.2767	0.4317	0.019*
H4B	0.1417	0.3768	0.3902	0.019*
C5	0.19139 (14)	0.25643 (15)	0.29739 (7)	0.0168 (3)
H5A	0.1852	0.3377	0.2872	0.020*
H5B	0.1696	0.2120	0.2621	0.020*
C6	0.27195 (15)	0.14611 (14)	0.42904 (7)	0.0176 (3)
H6A	0.3491	0.1271	0.4412	0.021*
H6B	0.2216	0.1271	0.4623	0.021*
C7	0.42306 (13)	0.39993 (14)	0.20842 (7)	0.0153 (3)
C8	0.48856 (13)	0.43654 (14)	0.15431 (7)	0.0145 (3)
H8	0.5615	0.3955	0.1540	0.017*
C9	0.42250 (16)	0.40338 (16)	0.09842 (8)	0.0212 (3)
H9A	0.4095	0.3206	0.0983	0.025*
H9B	0.3490	0.4416	0.0988	0.025*
C10	0.48636 (19)	0.43710 (19)	0.04264 (8)	0.0298 (4)
H10A	0.5564	0.3930	0.0403	0.036*
H10B	0.4409	0.4188	0.0074	0.036*
C11	0.41698 (14)	0.40626 (14)	0.52135 (7)	0.0152 (3)
C12	0.44029 (13)	0.47767 (14)	0.57575 (7)	0.0139 (3)
H12	0.3881	0.5437	0.5758	0.017*
C13	0.41838 (15)	0.40650 (14)	0.63174 (8)	0.0189 (3)
H13A	0.3399	0.3803	0.6319	0.023*
H13B	0.4672	0.3389	0.6315	0.023*
C14	0.44094 (17)	0.47615 (18)	0.68738 (8)	0.0252 (4)
H14A	0.3867	0.5392	0.6896	0.030*
H14B	0.4304	0.4278	0.7227	0.030*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0279 (6)	0.0198 (6)	0.0198 (6)	0.0013 (5)	0.0113 (5)	0.0032 (5)
O2	0.0316 (7)	0.0214 (6)	0.0212 (6)	0.0000 (5)	0.0127 (5)	−0.0015 (5)
O3	0.0177 (6)	0.0355 (7)	0.0225 (7)	−0.0035 (5)	−0.0006 (5)	−0.0146 (6)
O4	0.0211 (6)	0.0379 (8)	0.0226 (7)	0.0008 (6)	0.0022 (5)	−0.0154 (6)
N1	0.0120 (6)	0.0165 (6)	0.0177 (6)	−0.0005 (5)	−0.0001 (5)	0.0002 (5)
N2	0.0175 (6)	0.0117 (5)	0.0162 (6)	−0.0004 (5)	0.0014 (5)	0.0013 (5)
N3	0.0143 (6)	0.0135 (6)	0.0129 (6)	0.0000 (5)	0.0016 (5)	0.0005 (5)
N4	0.0139 (6)	0.0171 (6)	0.0134 (6)	−0.0030 (5)	−0.0001 (5)	−0.0037 (5)
C1	0.0147 (7)	0.0139 (6)	0.0191 (8)	−0.0040 (5)	0.0009 (6)	−0.0024 (6)
C2	0.0153 (7)	0.0128 (6)	0.0185 (7)	0.0018 (6)	0.0045 (6)	−0.0027 (6)
C3	0.0131 (6)	0.0158 (7)	0.0170 (7)	−0.0045 (5)	0.0020 (6)	−0.0011 (6)
C4	0.0120 (6)	0.0144 (6)	0.0208 (8)	−0.0012 (6)	0.0030 (6)	−0.0047 (6)
C5	0.0157 (7)	0.0202 (7)	0.0145 (7)	0.0001 (6)	−0.0020 (6)	0.0048 (6)
C6	0.0217 (8)	0.0178 (7)	0.0133 (7)	−0.0008 (6)	−0.0007 (6)	0.0036 (6)
C7	0.0167 (7)	0.0193 (7)	0.0098 (7)	−0.0043 (6)	0.0009 (6)	−0.0025 (5)
C8	0.0162 (7)	0.0175 (7)	0.0097 (6)	−0.0029 (6)	0.0032 (5)	−0.0012 (6)
C9	0.0241 (8)	0.0262 (8)	0.0134 (7)	−0.0110 (7)	−0.0007 (6)	−0.0030 (7)
C10	0.0410 (11)	0.0383 (11)	0.0100 (7)	−0.0138 (9)	0.0033 (7)	−0.0049 (7)
C11	0.0188 (7)	0.0154 (7)	0.0115 (7)	−0.0036 (6)	0.0003 (5)	−0.0002 (5)
C12	0.0140 (6)	0.0162 (7)	0.0114 (6)	−0.0019 (6)	0.0012 (5)	−0.0019 (5)
C13	0.0220 (8)	0.0227 (7)	0.0122 (7)	−0.0066 (7)	0.0024 (6)	0.0004 (6)
C14	0.0309 (10)	0.0329 (10)	0.0117 (7)	−0.0088 (8)	0.0040 (7)	−0.0007 (7)

Geometric parameters (Å, º)

O1—C7	1.322 (2)	C4—H4B	0.9900
O1—H1O	0.88 (3)	C5—H5A	0.9900
O2—C7	1.214 (2)	C5—H5B	0.9900
O3—C11	1.323 (2)	C6—H6A	0.9900
O3—H3O	0.93 (3)	C6—H6B	0.9900
O4—C11	1.213 (2)	C7—C8	1.516 (2)
N1—C4	1.464 (2)	C8—C8^i	1.524 (3)
N1—C5	1.466 (2)	C8—C9	1.542 (2)
N1—C1	1.483 (2)	C8—H8	1.0000
N2—C2	1.464 (2)	C9—C10	1.528 (3)
N2—C6	1.468 (2)	C9—H9A	0.9900
N2—C1	1.480 (2)	C9—H9B	0.9900
N3—C3	1.476 (2)	C10—C10^i	1.522 (4)
N3—C2	1.483 (2)	C10—H10A	0.9900
N3—C5	1.494 (2)	C10—H10B	0.9900
N4—C3	1.476 (2)	C11—C12	1.517 (2)
N4—C6	1.481 (2)	C12—C12^i	1.526 (3)
N4—C4	1.485 (2)	C12—C13	1.542 (2)
C1—H1A	0.9900	C12—H12	1.0000
C1—H1B	0.9900	C13—C14	1.527 (3)
C2—H2A	0.9900	C13—H13A	0.9900
C2—H2B	0.9900	C13—H13B	0.9900
C3—H3A	0.9900	C14—C14^i	1.524 (4)
C3—H3B	0.9900	C14—H14A	0.9900
C4—H4A	0.9900	C14—H14B	0.9900
C7—O1—H1O	108.4 (17)	N4—C6—H6A	109.3
C11—O3—H3O	111.2 (17)	N2—C6—H6B	109.3
C4—N1—C5	109.34 (13)	N4—C6—H6B	109.3
C4—N1—C1	107.78 (13)	H6A—C6—H6B	107.9
C5—N1—C1	108.26 (13)	O2—C7—O1	123.45 (15)
C2—N2—C6	108.84 (13)	O2—C7—C8	123.72 (15)
C2—N2—C1	108.56 (13)	O1—C7—C8	112.79 (14)
C6—N2—C1	107.73 (13)	C7—C8—C8^i	111.95 (11)
C3—N3—C2	108.08 (13)	C7—C8—C9	108.82 (13)
C3—N3—C5	108.16 (12)	C8^i—C8—C9	110.02 (12)
C2—N3—C5	107.77 (12)	C7—C8—H8	108.7
C3—N4—C6	108.37 (13)	C8^i—C8—H8	108.7
C3—N4—C4	108.30 (13)	C9—C8—H8	108.7
C6—N4—C4	108.07 (12)	C10—C9—C8	110.65 (14)
N2—C1—N1	111.84 (12)	C10—C9—H9A	109.5
N2—C1—H1A	109.2	C8—C9—H9A	109.5
N1—C1—H1A	109.2	C10—C9—H9B	109.5
N2—C1—H1B	109.2	C8—C9—H9B	109.5
N1—C1—H1B	109.2	H9A—C9—H9B	108.1
H1A—C1—H1B	107.9	C10^i—C10—C9	111.24 (15)
N2—C2—N3	112.06 (12)	C10^i—C10—H10A	109.4
N2—C2—H2A	109.2	C9—C10—H10A	109.4
N3—C2—H2A	109.2	C10^i—C10—H10B	109.4
N2—C2—H2B	109.2	C9—C10—H10B	109.4
N3—C2—H2B	109.2	H10A—C10—H10B	108.0
H2A—C2—H2B	107.9	O4—C11—O3	123.38 (16)
N3—C3—N4	111.92 (12)	O4—C11—C12	124.08 (16)
N3—C3—H3A	109.2	O3—C11—C12	112.51 (14)
N4—C3—H3A	109.2	C11—C12—C12^i	111.41 (11)
N3—C3—H3B	109.2	C11—C12—C13	109.31 (13)
N4—C3—H3B	109.2	C12^i—C12—C13	110.39 (11)
H3A—C3—H3B	107.9	C11—C12—H12	108.6
N1—C4—N4	111.74 (13)	C12^i—C12—H12	108.6
N1—C4—H4A	109.3	C13—C12—H12	108.6
N4—C4—H4A	109.3	C14—C13—C12	110.61 (14)
N1—C4—H4B	109.3	C14—C13—H13A	109.5
N4—C4—H4B	109.3	C12—C13—H13A	109.5
H4A—C4—H4B	107.9	C14—C13—H13B	109.5
N1—C5—N3	111.63 (13)	C12—C13—H13B	109.5
N1—C5—H5A	109.3	H13A—C13—H13B	108.1
N3—C5—H5A	109.3	C14^i—C14—C13	111.33 (13)
N1—C5—H5B	109.3	C14^i—C14—H14A	109.4
N3—C5—H5B	109.3	C13—C14—H14A	109.4
H5A—C5—H5B	108.0	C14^i—C14—H14B	109.4
N2—C6—N4	111.73 (13)	C13—C14—H14B	109.4
N2—C6—H6A	109.3	H14A—C14—H14B	108.0
C2—N2—C1—N1	−58.26 (17)	C2—N3—C5—N1	58.89 (17)
C6—N2—C1—N1	59.46 (17)	C2—N2—C6—N4	58.40 (17)
C4—N1—C1—N2	−59.52 (17)	C1—N2—C6—N4	−59.14 (17)
C5—N1—C1—N2	58.64 (17)	C3—N4—C6—N2	−58.21 (17)
C6—N2—C2—N3	−58.47 (17)	C4—N4—C6—N2	58.95 (18)
C1—N2—C2—N3	58.53 (16)	O2—C7—C8—C8^i	26.6 (2)
C3—N3—C2—N2	58.10 (17)	O1—C7—C8—C8^i	−155.58 (16)
C5—N3—C2—N2	−58.57 (17)	O2—C7—C8—C9	−95.17 (19)
C2—N3—C3—N4	−58.10 (16)	O1—C7—C8—C9	82.61 (17)
C5—N3—C3—N4	58.32 (16)	C7—C8—C9—C10	−179.13 (16)
C6—N4—C3—N3	58.38 (17)	C8^i—C8—C9—C10	57.9 (2)
C4—N4—C3—N3	−58.62 (16)	C8—C9—C10—C10^i	−56.1 (3)
C5—N1—C4—N4	−58.42 (17)	O4—C11—C12—C12^i	23.5 (2)
C1—N1—C4—N4	59.04 (16)	O3—C11—C12—C12^i	−158.52 (16)
C3—N4—C4—N1	58.22 (16)	O4—C11—C12—C13	−98.8 (2)
C6—N4—C4—N1	−58.98 (17)	O3—C11—C12—C13	79.21 (17)
C4—N1—C5—N3	58.12 (17)	C11—C12—C13—C14	−179.90 (15)
C1—N1—C5—N3	−59.04 (17)	C12^i—C12—C13—C14	57.2 (2)
C3—N3—C5—N1	−57.73 (17)	C12—C13—C14—C14^i	−56.0 (2)

Symmetry code: (i) −x+1, −y+1, z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1O···N3	0.88 (3)	1.80 (3)	2.6808 (19)	175 (3)
O3—H3O···N4	0.93 (3)	1.72 (3)	2.6518 (19)	177 (3)
C1—H1A···O4^ii	0.99	2.54	3.435 (2)	150
C1—H1B···O2^iii	0.99	2.40	3.317 (2)	153
C2—H2A···O2^iii	0.99	2.56	3.436 (2)	147
C3—H3A···N2^iv	0.99	2.58	3.487 (2)	152
C3—H3B···N1^v	0.99	2.48	3.355 (2)	147
C4—H4A···O4^ii	0.99	2.48	3.388 (2)	152

Symmetry codes: (ii) x−1/2, −y+1/2, z; (iii) −x+1/2, y−1/2, z; (iv) −x+1/2, y+1/2, z; (v) x+1/2, −y+1/2, z.