Crystal structures of 2∶1 inclusion complexes of xylidine isomers with 1,1,6,6-tetraphenylhexa-2,4-diyne-1,6-diol: the importance of weak intermolecular interactions in crystal stability

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Abstract

Molecular structures of inclusion complexes of the title host with 2,5-, 2,6- and 3,4-xylidines were determined by X-ray analysis. In addition to the relatively strong hydrogen bonds, intermolecular C–H⋯π, N–H⋯π and π⋯π interactions were found in the structures. The importance of these weak interactions is discussed in relation to the stability of the inclusion complexes.

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Introduction

The diacetylenediol 1 has been extensively used as an excellent host molecule in host–guest chemistry because of its facile formation of stable inclusion complexes with various kinds of guest molecules.¹ Because the ease of crystallization of its inclusion complexes is delicately influenced by the structure of the guest molecules, this host allows us to separate a single isomer from a mixture of multiple isomers by preferential crystallization. Isomers of picolines (methylpyridines)² and lutidines (dimethylpyridines)³ were successfully separated by this method using host 1, although the separation by other ordinary methods was far from practical. During the application of this method to the separation of xylidine (dimethylaniline) isomers (2), we were able to obtain X-ray structures of the inclusion complexes of the three isomers. The systematic analyses of these data reveal that weak intermolecular interactions such as X–H⋯Y (X, Y⊕=⊕N or O), C–H⋯π and π⋯π interactions play an important role in the stability of the inclusion complexes as often claimed in the crystal engineering.^4–7 We report here the relationship between crystal stability and structure in the inclusion complexes based on X-ray analysis.

Results and discussion

Three isomers, 2,5-, 2,6- and 3,4-xylidines (2a, 2b and 2c), were used. The treatment of host 1 with a large excess of xylidine afforded the corresponding inclusion complex (2∶1 guest–host ratio) as colorless prisms.⁸ When 1 was treated with a 1∶1 mixture of two xylidine isomers, the inclusion complex of each isomer could be readily separated by fractional crystallization from methanol. For example, in the case of a mixture of 2b and 2c, the complex of 2c crystallized more easily than that of 2b, and the subsequent distillation of the separated crystals on heating gave the free xylidines in more than 99% purity. According to the competition experiments of all the possible combinations, the ease of crystallization of the inclusion complexes was established in the following order, 1•2c⊕>⊕1•2a⊕>⊕1•2b.⁹

The X-ray structures of the inclusion complexes of 2a, 2b and 2c are shown in Figs. 1–3, respectively.¹⁰ Selected structural parameters in the host molecules are as follows. 2a: O–C1⊕=⊕1.428(2), C1–C2⊕=⊕1.484(2), C2–C3⊕=⊕1.199(2), C3–C3*⊕=⊕1.379(3) Å; O–C1–C2⊕=⊕109.0(1), C1–C2–C3⊕=⊕176.9(2), C2–C3–C3*⊕=⊕179.0(3)°. 2b: O–C1⊕=⊕1.424(2), C1–C2⊕=⊕1.490(3), C2–C3⊕=⊕1.202(3), C3–C3*⊕=⊕1.382(4) Å; O–C1–C2⊕=⊕109.0(2), C1–C2–C3⊕=⊕178.5(2), C2–C3–C3*⊕=⊕179.3(3)°. 2c: O–C1⊕=⊕1.428(2), C1–C2⊕=⊕1.487(2), C2–C3⊕=⊕1.208(3), C3–C3*⊕=⊕1.383(4) Å; O–C1–C2⊕=⊕108.6(1), C1–C2–C3⊕=⊕172.5(2), C2–C3–C3*⊕=⊕179.3(2)°. These structural data, as well as those of the aromatic moieties in the host and guest molecules, are almost in agreement with standard values.¹¹ An exception is the small bond angle at the acetylenic carbon C2 in 2c (172.5°), this bending deformation leading to a shallow zig-zag structure for the diacetylenic axis.¹² The two OH groups in the host are anti about the diacetylenic axis in each complex.

Fig. 1 ORTEP drawing of the 2∶1 complex of 2a with 1 with thermal ellipsoids at 50% probability level. Click image or fig1.htm to access a 3D representation.

Fig. 2 ORTEP drawing of the 2∶1 complex of 2b with 1 with thermal ellipsoids at 50% probability level. Click image or fig2.htm to access a 3D representation.

Intermolecular contacts in these inclusion complexes are shown in Table 1. The relatively strong O–H⋯N hydrogen bond between the host and guest molecules are present throughout.¹³ It is notable that the N–H⋯O and π⋯π interactions are found only in the complex of 2c. The latter works between phenyl groups to connect one host molecule to another successively in a linear fashion [3D structure ofFig. 3]. Each host column is bridged to the next by guest molecules via a hydrogen bonded network (O–H⋯N–H⋯O–H). In contrast, significant interactions other than the O–H⋯N are weak C–H⋯π or N–H⋯π interactions in the other complexes: five and three sets of these interactions are found in 1•2a and 1•2b, respectively. These analyses indicate that the strength of intermolecular interactions decreases in the complex order 2c⊕>⊕2a⊕>⊕2b, being the same as the order of stability of the inclusion complexes as viewed by competition experiments.

Fig. 3 ORTEP drawing of the 2∶1 complex of 2c with 1 with thermal ellipsoids at 50% probability level. Click image or fig3.htm to access a 3D representation.

Table 1 Significant intermolecular interactions (interatomic distances in Å) found in the X-ray structures of 1•2a, 1•2b and 1•2c^a

Interactions^b	1•2a	1•2b	1•2c
a Symmetry operators are given for the symmetry related atoms from the original ones. b Interatomic distances within 2.9, 3.1 and 3.6 Å are selected for the X–H⋯Y (X and Y⊕=⊕O or N), X–H⋯π (X⊕=⊕C or N) and π···π interactions, respectively. (H) and (G) denote that the interacting atoms belonging to host and guest molecules, respectively. c Bond angles of X–H⋯Y (X and Y⊕=⊕O or N) in °.
O–H(H)⋯N(G)	1.98 (H1⋯N) 171^c	1.90 (H1⋯N) 156^c	1.96 (H1⋯N) 163^c
N–H(G)⋯O(H)	—	—	2.45 (H12⋯O(2⊕−⊕x, −y, 2⊕−⊕z)) 155^c
N–H(G)⋯π(H)	2.89 (H13⋯C11(1⊕−⊕x, −y, −z)	2.86 (H12⋯C6(x, y, 1⊕+⊕z))	—
C–H(H)⋯π(G)	—	—	2.90 (H3⋯C20(x, y, 1⊕−⊕z))
C–H(G)⋯π(G)	2.88 (H14⋯C18(3/2⊕−⊕x, y⊕−⊕1/2, –z))	—	3.05 (H19⋯C19(2⊕−⊕x, 1⊕−⊕y, 3⊕−⊕z)) 3.03 (H21⋯C18(2⊕−⊕x, 1⊕−⊕y, 3⊕−⊕z))
C–H(G)⋯π(H)	2.96 (H16⋯C5) 3.03 (H18⋯C13(1⊕−⊕x, −y, −z))	3.02 (H16⋯C5(x⊕−⊕1, y, z))	—
C–H(H)⋯π(H)	3.01 (H6⋯C7(1/2⊕−⊕x, y⊕−⊕1/2, 1/2⊕−⊕z))	2.90 (H9⋯C9(x⊕−⊕1, y, z))	—
π(H)⋯π(H)	—	—	3.41 (C14⋯C14(1⊕−⊕x, −y, 1⊕−⊕z))

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We consider that, while the initial organization of the host and guest molecules occurs mainly via O–H⋯N hydrogen bonds, other weaker interactions play an important role in the stability of inclusion crystals of host 1 with the xylidine guests. Structural differences in guest molecules including the steric crowdedness of the NH₂ moieties influence the mode and strength of the weak interactions, depending on the fitting of their shapes to the cavities constructed by the host network. The idea is essential for the evaluation of matching or mismatching of host–guest inclusion phenomena.

Crystallographic studies

X-Ray data were collected on a Rigaku RAXIS-IV imaging plate diffractometer with Mo-Kα radiation to a maximum 2θ of 55.1° at room temperature. A total of n⊕×⊕6.00° oscillation images was collected, each being exposed for 70.0 min. The reflection data were corrected for Lorentz-polarization effects and secondary extinction. The structures were solved by direct methods and refined by full-matrix least-squares methods by using a teXsan program. All observed reflections (I⊕>⊕3.0σ(I)) were used for the refinement. Non-hydrogen and hydrogen atoms were refined anisotropically and isotropically, respectively. The function minimized was Σ[w(|F_o|⊕−⊕|F_c|)²], where w⊕=⊕[σ_c²|F_o|⊕+⊕(p²/4)|F_o|²]⁻¹. Other data are listed in Table 2.

Table 2 Crystal data for 1•2a, 1•2b and 1•2c^a

Properties	1•2a	1•2b	1•2c
a Click b009083m.txt for full crystallographic data (CCDC nos. 152851–152853).
Formula	(C30H22O2)0.5·(C8H11N)	(C30H22O2)0.5·(C8H11N)	(C30H22O2)0.5·(C8H11N)
M	328.43	328.43	328.43
Crystal system	Monoclinic	Triclinic	Triclinic
Space group	P21/n (no. 14)	P 1̄ (no. 2)	P 1̄ (no. 2)
a/Å	15.529(3)	8.713(1)	12.503(7)
b/Å	5.8476(7)	14.265(8)	13.249(3)
c/Å	20.222(3)	7.838(6)	6.038(2)
α/°	90	106.59(8)	94.53(3)
β/°	90.91(1)	96.77(3)	96.86(4)
γ/°	90	90.00(2)	69.04(4)
V/Å^3	1836.0(4)	926.5(9)	926.7(7)
Z	4	2	2
μ (Mo-Kα)/cm^−1	0.72	0.71	0.71
D c/g cm^−3	1.188	1.177	1.177
Images collected (n⊕×⊕6.00°)	10	24	21
Measured reflections	2860	3765	3685
Number of observations (I⊕>⊕3.0σ(I))	2222	2900	3161
No. variables	315	315	315
p factor	0.200	0.200	0.183
R	0.053	0.057	0.073
R w	0.092	0.105	0.138
Goodness of fit	1.29	1.22	1.49
Maximum and minimum residual electron density/e Å^−3	0.17/−0.23	0.27/−0.32	0.32/−0.35

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Acknowledgements

This work was partly supported by a special fund from the Japan Private School Promotion Foundation. We thank Dr Haruo Akashi for his assistance of X-ray data analysis.

Notes and references

1. For the applications of host 1 and related diol compounds, see: F. Toda, in Comprehensive Supramolecular Chemistry, ed. D. D. MacNicol, F. Toda and R. Bishop, Elsevier, Oxford, UK, 1996, vol. 6, ch. 15..
2. K. Dohi, K. Tanaka and F. Toda, Nippon Kagaku Kaishi, 1986, 927.
3. M. Caira, L. R. Nassimbeni, F. Toda and D. Vujovic, J. Chem. Soc., Perkin Trans. 2, 1999, 2681.
4. G. R. Desiraju, Crystal Engineering: The Design of Organic Solids, Elsevier, Amsterdam, 1989..
5. G. R. Desiraju, ref. 1, ch. 1..
6. G. R. Desiraju and T. Steiner, The Weak Hydrogen Bond, Oxford University Press, Oxford, UK, 1999..
7. M. Nishio, M. Horota and Y. Umezawa, The CH/π Interactions, Wiley-VCH, New York, 1998..
8. All the complexes afforded satisfactory analysis data. Melting points are 108–109, 94–95 and 102–104 °C for the complexes of 2a, 2b, and 2c, respectively..
9. Details of the competition experiments will be reported in due course..
10. In each complex, a unique asymmetric unit consists of one half of a host molecule and one guest molecule. Atoms with starred numbers are initiated by a symmetric operation..
11. For example, the X-ray data of 3,4-dimethylaniline.S. -Q. Dou, N. Weiden and A. Weiss, Acta Chim. Hung., 1993, 130, 497..
12. In the Cambridge Structual Database, 15 structures were found for compounds containing 1,1,6,6-tetraphenylhexa-2,4-diyne-1,6-diol molecules. Among 20 independent values of the bond angle at C2, the free host (173.9°) and the inclusion complex with acetone (173.8°) have values significantly smaller than 180°.D. R. Bond, L. Johnson, L. R. Nassimbeni and F. Toda, J. Solid State Chem., 1991, 92, 68..
13. IR spectra clearly support the presence of the hydrogen bond: the ν_OH bands are shifted from 3532 cm⁻¹ (free host) to ca. 3200 cm⁻¹ by the complexation..

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