Crystal engineering using very short and linear C(sp)-H···N hydrogen bonds: Formation of head-to-tail straight tapes and their assembly into nonlinear optical polar crystals

Article abstract of DOI:10.1039/b103689k

The crystallization of 4-ethynylpyridine (1) and 4-(4-ethynylphenyl)ethynylpyridine (2) leads to C(sp)-H···N hydrogen bonded straight tapes that further assemble into polar crystals, in the case of 2, and show intense powder SHG response, 8 times more efficient than crystalline urea.

Full text of DOI:10.1039/b103689k

View Article Online / Journal Homepage / Table of Contents for this issue
Crystal engineering using very short and linear C(sp)–H…N hydrogen
bonds: formation of head-to-tail straight tapes and their assembly into
nonlinear optical polar crystals
Masakazu Ohkita,*a Takanori Suzuki,a Keitaro Nakatanib and Takashi Tsuji*a
a Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan.
E-mail: ohkita@sci.hokudai.ac.jp and tsuji@sci.hokudai.ac.jp
b Départment de Chimie, Ecole Normale Supérieure de Cachan, 61, Avenue du Préridant Wilson,
94235Cachan Cedex, France
Received (in Cambridge, UK) 24th April 2001, Accepted 26th June 2001
First published as an Advance Article on the web 19th July 2001
The crystallization of 4-ethynylpyridine (1) and 4-(4-ethy-
nylphenyl)ethynylpyridine (2) leads to C(sp)-H…N hydro-
gen bonded straight tapes that further assemble into polar
crystals, in the case of 2, and show intense powder SHG
response, 8 times more efficient than crystalline urea.
contacts reported.5 The C(sp)–H–N angle is 180° and the
molecules in the tape are located on a crystallographic two-fold
axis. The head-to-tail polar tapes of 1 are arranged in
antiparallel fashion in the crystal, resulting in a centrosym-
metric packing with space group C2/c.
The crystal structure analysis‡ for 2§ also shows the
formation of a tape structure through very short and linear
C(sp)–H…N contacts (Fig. 2). Molecule 2 is almost planar in
the crystal with the largest deviation of 0.21 Å from the least-
square plane; the dihedral angle between the two aromatic rings
is 19.0°. Interestingly, further packing analysis reveals that the
unit cell of the crystal contains eight molecules of 2 in a non-
centrosymmetric packing with space group Fdd2. Since the
majority of achiral organic compounds tend to pack into
centrosymmetric crystals,3 this observation is rather unusual.
Moreover, the dipole moments of the molecules are arranged
perfectly in a parallel orientation in the crystal and, therefore,
the vector parts of the first hyperpolarizabilities of the
molecules are directed in a completely parallel orientation,
which make this compound attractive for second-order NLO
materials. In fact, the crystals of 2 show a strong second-
harmonic generation (SHG) signal, 8 times more intense than
that of crystalline urea, in the Kurtz powder test at 1907 nm.
With this finding, we explored the SHG response in related
compounds and found that the crystals of 4-(4-ethynylphenyl)-
ethynylbenzonitrile (3)¶ also exhibit an intense SHG signal, 16
times more efficient than crystalline urea, in the Kurtz powder
test at 1907 nm. We surmise that molecules of 3 are also
arranged linearly in the crystal, in a head-to-tail fashion directed
by the C(sp)–H…N hydrogen bond.6 Although it is not clear at
present what factors are responsible for the polar organization
of 2 and 3, it can be pointed out that the p-phenylene units in 2
and 3, which is absent in 1, should play an important role in the
polar assembly process.∑
The aims of crystal engineering are to design crystal structures
of molecular solids with specific topological features, chemical
function, or physical properties.1 One area of particular
endeavor in this field is the design of non-centrosymmetric
polar crystals2 because of their importance for physical
properties of bulk assemblies such as non-linear optical (NLO)
activity.3 In this study we introduce a new class of polar crystals
in which the molecules are directed in a completely parallel
orientation through C(sp)–H…N hydrogen bonds. Here we
report the X-ray crystal structures of 4-ethynylpyridine (1) and
4-(4-ethynylphenyl)ethynylpyridine (2) as well as NLO proper-
ties of the polar crystals of 2.
The crystal structural analysis† for 14 revealed the formation
of a linear tape structure formed by very short and linear C(sp)–
H…N contacts (Fig. 1); the H…N distance (2.33 Å) found in the
structure is about 0.4 Å shorter than the sum of their van der
Waals (vdW) radii (2.75 Å) and is one of the shortest C–H…N
In conclusion, we have found a unique assembly of directed
polar crystals based on the C(sp)–H…N hydrogen bond, which
has been little exploited in crystal engineering so far. The
present results clearly demonstrate that the C–H…N weak
Fig. 1 Packing arrangement of 1 in the crystal. Short C(sp)–H…N contacts
are shown by dotted lines; H…N 2.33 Å, C…N 3.28 Å, C–H–N 180°.
Fig. 2 Packing arrangement of 2 in the crystal. Short C(sp)–H…N contacts
are shown by dotted lines; H…N 2.32 Å, C…N 3.27 Å, C–H–N 180°.
1454
Chem. Commun., 2001, 1454–1455
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b103689k

View Article Online
(1 H, s), 7.49 (4 H, s), 7.60 (2 H, AAABBA) and 7.64 (2 H, AAABBA); dC (75
hydrogen bond is capable of not only constructing well-defined
crystal structures but also inducing functional properties (SHG
response) to the resulting bulk assemblies.
This work was supported in part by a Grant-in-Aid for
Scientific Research (No. 12640508) from the Ministry of
Education, Science, Sports and Culture of Japan. We thank
Professor Tamotsu Inabe (Hokkaido University) for the use of
X-ray analytical facilities.
MHz, CDCl3) 79.45, 82.98, 89.48, 93.02, 111.73, 118.41, 122.58, 122.81,
127.82, 131.62, 132.56, 132.16 and 133.13; m/z (FD) 227 (M+, 100%).
∑ Connection of the tapes of 2 by C(sp2)–H…p contacts between the
pyridine a-proton and the terminal acetylene moiety (H…p centroid
distance 2.86 Å) might play a significant role in the polar assembly process.
Although there are face-to-face overlaps between the pyridine ring and the
p-phenylene unit of 2 in the crystal, the interplanar distance (3.80 Å) beyond
the sum of vdW radii (3.40 Å), so that the p-stacking interaction would play
a less important role in determining the packing.
Notes and references
† Crystal data for 1: C7H5N, M = 103.12, colorless rod, 0.60 3 0.20 3 0.20
mm, monoclinic, space group C2/c, a = 9.800(5), b = 8.684(5), c =
7.334(4) Å, b = 116.90(4)°, V = 556.6(6) Å3, Z = 4, rcalcd = 1.231 g
cm23, T = 193 K, Mo-Ka radiation. A total of 577 unique reflections
(2qmax = 54.2°) were collected, of which 456 observed reflections [I >
3s(I)] were used in the structure solution (direct methods) and refinement
(full-matrix least-squares) to give final R = 0.085 and Rw = 0.114. Residual
electron density is 0.42 e Å23. CCDC 153071. See http://www.rsc.org/
suppdata/cc/b1/b103689k/ for crystallographic data in .cif or other for-
mat.
‡ Crystal data for 2: C15H9N, M = 203.24, colorless prism, 0.20 3 0.20 3
0.15 mm, orthorhombic, space group Fdd2, a = 17.491(1), b = 7.748(1),
c = 15.5880(9) Å, V = 2112.5(4) Å3, Z = 8, rcalcd = 1.278 g cm23, T =
123 K, Mo-Ka radiation. A total of 595 unique reflections (2qmax = 55°)
were collected, of which 513 observed reflections [I > 3s(I)] were used in
the structure solution (direct methods) and refinement (full-matrix least-
squares) to give final R = 0.056 and Rw = 0.072. Residual electron density
is 0.30 e Å23. CCDC 164007.
§ Compound 2 was prepared by successive Sonogashira coupling of
4-bromoiodobenzene with 1 and trimethylsilylacetylene followed by
desilylation using Bu4NF. Spectroscopic data for 2; mp 180–181 °C
(Found: M+, 203.0733. C15H9N requires M, 203.0735); nmax (KBr)/cm21
3148, 2216, 2088, 1592, 1502, 1408 and 838; dH (300 MHz, CDCl3) 3.21 (1
H, s), 7.38 (2 H, AAAXXA), 7.50 (4 H, s) and 8.61 (2 H, AAAXXA); dC (75
MHz, CDCl3) 79.53, 83.06, 88.49, 93.27, 122.55, 123.02, 125.56, 131.17,
131.81, 132.24 and 149.89; m/z (FD) 203 (M+, 100%).
¶ Compound 3 was prepared by successive Sonogashira coupling of
4-bromoiodobenzene with 4-ethynylbenzonitrile and trimethylsilylacety-
lene followed by desilylation using Bu4NF. Spectroscopic data for 3; mp
196–198 °C (Found: M+, 227.0753. C17H9N requires M, 227.0735); nmax
(KBr)/cm21 3236, 2212, 1600, 1504 and 840; dH (300 MHz, CDCl3) 3.21
1 G. R. Desiraju, Crystal Engineering: The Design of Organic Solids,
Elsevier, Amsterdam, 1989; J.-M. Lehn, Supramolecular Chemistry,
VCH, Weinheim, 1995; G. R. Desiraju and T. Steiner, The Weak
Hydrogen Bond in Structural Chemistry and Biology, Oxford University
Press, Oxford, 1999; M. J. Calhorda, Chem. Commun., 2000, 801.
2 B. Gong, C. Zheng, H. Zeng and J. Zhu, J. Am. Chem. Soc., 1999, 121,
9766; J. Kawamata, K. Inoue and T. Inabe, Bull. Chem. Soc. Jpn., 1998,
71, 2777; K. D. M. Harris and M. D. Hollingsworth, Nature, 1989, 341,
19 and references cited therein.
3 R. W. Boyd, Nonlinear Optics, Academic Press, New York, 1992; Novel
Optical Materials and Applications, Eds. I.-C. Khoo, F. Simoni and C.
Umeton, John Wiley, New York, 1997; Nonlinear Optical Properties of
Organic Molecules and Crystals, Eds. D. S. Chemla and J. Zyss,
Academic Press, Boston, 1987; Molecular Nonlinear Optics: Materials,
Physics and Devices, Ed. J. Zyss, Academic Press, Boston, 1994.
4 N. R. Champness, A. N. Khlobystov, A. G. Majuga, M. Schröder and
N. V. Zyk, Tetrahedron Lett., 1999, 40, 5413.
5 Examples for the C–H…N interaction: R. Taylor and O. Kennard, J. Am.
Chem. Soc., 1982, 104, 5063; D. S. Reddy, B. S. Goud, K. Panneerselvam
and G. R. Desiraju, J. Chem. Soc., Chem. Commun., 1993, 663; D. S.
Reddy, D. C. Craig and G. R. Desiraju, J. Am. Chem. Soc., 1996, 118,
4090; F. A. Cotton, L. M. Daniels, G. R. Jordan, IV and C. A. Murillo,
Chem. Commun., 1997, 1673; M. Mascal, Chem. Commun., 1998, 303;
A. N. M. M. Rahman, R. Bishop, D. C. Craig and M. L. Scudder, Chem.
Commun., 1999, 2389; T. Steiner and G. R. Desiraju, Chem. Commun.,
1998, 891; V. R. Thalladi, A. Gehrke and R. Boese, New J. Chem., 2000,
24, 463.
6 For the crystal structures of related SHG active cyano compounds: G. R.
Desiraju and R. L. Harlow, J. Am. Chem. Soc., 1989, 111, 6757; P. J.
Langley, J. Hulliger, R. Thaimattam and G. R. Desiraju, New. J. Chem.,
1998, 1307.
Chem. Commun., 2001, 1454–1455
1455