Strong phenyl-perfluorophenyl π-π stacking and C-H?F-C hydrogen bonding interactions in the crystals of the corresponding aromatic aldimines

Article abstract of DOI:10.1016/j.tetlet.2005.01.183

The X-ray diffraction analysis of two co-crystals, 1?2 (aldimines 1 and 2) and 3?4 (aldimines 3 and 4), reveals that there are strong phenyl-perfluorophenyl π-π stacking and intermolecular hydrogen bonding interactions. The new perfluoroaryl-aryl face-to-face interaction of the crystalline aldimines provides a design motif for a new class of self-assembling system.

Full text of DOI:10.1016/j.tetlet.2005.01.183

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 2713–2716
Strong phenyl–perfluorophenyl p–p stacking and
C–Hꢀ ꢀ ꢀF–C hydrogen bonding interactions in the crystals of
the corresponding aromatic aldimines
*
Shizheng Zhu, Shifa Zhu, Guifang Jin and Zhanting Li
*
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
54 Fenglin Lu, Shanghai 200032, PR China
3
Received 2 September 2004; revised 20 January 2005; accepted 21 January 2005
Abstract—The X-ray diffraction analysis of two co-crystals, 1Æ2 (aldimines 1 and 2) and 3Æ4 (aldimines 3 and 4), reveals that there are
strong phenyl–perfluorophenyl p–p stacking and intermolecular hydrogen bonding interactions. The new perfluoroaryl–aryl face-to-
face interaction of the crystalline aldimines provides a design motif for a new class of self-assembling system.
Ó 2005 Elsevier Ltd. All rights reserved.
5
Noncovalent interactions between aromatic units play a
significant role in determining the structures and proper-
ties of molecular assemblies in biology, chemistry, and
state to lead to cyclobutane products. Many aryl–per-
fluoroaryl stackings have also been observed at low tem-
6
perature. Here we first report the face-to-face and head-
to-tail or head-to-head stacking between the phenyl and
perfluorophenyl rings in the co-crystals of aldimines
1
materials science. Although a considerable amount of
work studying the stacking of aromatic rings has con-
centrated on phenyl–phenyl interaction, there is a grow-
ing interest in the interactions between aryl and
1
2
1
2
Ar CH@NAr (Ar , Ar = C H , C F ) at room
6
5
6 5
temperature.
2
–6
perfluoroaryl units. Patrick and Prosser first reported
that a 1:1 mixture of benzene (mp = 5.5 °C) and hexafluo-
1
2
1
2
Aldimines Ar CH = NAr (Ar , Ar = C H , C F ) were
prepared from the corresponding amines and aldehydes
in high yields (Scheme 1).
6
5
6 5
robenzene (mp = 4 °C) forms a complex, that melts at
3
2
4 °C. In the past decade, the intermolecular interac-
tions in hydroaromatic and perfluoroaromatic mole-
cules, such as aryl–perfluoroaryl stacking synthon (Ar–
After a benzene and dichloromethane solution of an
equal molar mixture 1 and 2 was slowly evaporated,
the 1:1 co-crystals 1Æ2 were isolated as a colorless crystal-
line solid that is stable in air at room temperature. The
strength of the aromatic p–p stacking interactions was
revealed by the melting point increase of the co-crystals
1Æ2 (98–100 °C) compared with the pure starting materi-
als 1 and 2 (51–56 °C, and 88–90 °C). The structure and
F
Ar ), C–Hꢀ ꢀ ꢀF, C–Fꢀ ꢀ ꢀp, and Fꢀ ꢀ ꢀF interactions have
been the focus of structural, photophysical, topochemi-
cal, and differential scanning calorimeter (DSC) stud-
4
–6
ies.
Coates et al. reported the alternating stacking
interactions of monoolefines and diolefines substituted
with phenyl and perfluorophenyl groups, and can under-
go photochemically induced [2+2] reactions in the solid
F
F
F
F
F
F
F
F
F
N
F
F
F
N
F
F
F
F
N
N
F
F
F
F
o
o
o
o
1
(mp = 51-56 C)
2 (mp = 88-90 C)
3 (mp = 115-117 C)
4 (mp = 114-115 C)
Keywords: Non-covalent interactions; p–p Stacking; Hydrogen bond.
*
Corresponding authors. Tel.: +86 21 54925185; fax: +86 21 64166128; e-mail: zhusz@mail.sioc.ac.cn
0
040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.01.183

2714
S. Zhu et al. / Tetrahedron Letters 46 (2005) 2713–2716
Ar
Ar
only ꢁ7° and ꢁ4°, respectively. The mean perpendicular
˚
rophenyl ring is 3.44 A, which indicates that there are
EtOH
r.t.
neat
140 oC,2 h C
PhNH
2
+ArCHO
N
C F NH +ArCHO
N
6 5
F
6
5
2
distance of the phenyl ring to the neighboring pentafluo-
Ph
Ar = Ph, C
6 5
F
significant intermolecular p–p interactions between the
2
–6
phenyl rings and the pentafluorophenyl rings.
The
Scheme 1.
structure of the complex 1Æ2 is further stabilized by
Hꢀ ꢀ ꢀF hydrogen bonding (C20A–Hꢀ ꢀ ꢀF5, as shown by
the dashed lines in Fig. 1b) between 1 and 2 from the
stacking layer of two adjacent columns. The Hꢀ ꢀ ꢀF dis-
the packing arrangement of 1 and 2 in the co-crystals
were determined by single-crystal X-ray diffraction
analysis.
˚
tance is 2.59 A, and the C–Hꢀ ꢀ ꢀF angle is 154.6°. There
4
are also numerous weak C–Hꢀ ꢀ ꢀF interactions between
8
As can been seen in Figure 1, co-crystal 1Æ2 stacks in
the phenyl rings and pentafluorophenyl rings in the
˚
vertical columns in an alternating fashion, face-to-face
and head-to-tail, which could reduce the repulsion force
of the lone pair electrons of the nitrogen atom in the imi-
nes. The alternating arrangement can be attributed to a
quadrupolar interaction between electron-rich and elec-
neighboring column molecules (ꢁ2.80 A, 146°), which
also can stabilize the co-crystals. The shortest intermole-
˚
cular H3ꢀ ꢀ ꢀF3 hydrogen bond distance is 2.720 A. All
of these interactions cause the elevation of the melting
point of the co-crystal.
7
tron-deficient aromatic rings. The torsional angle of the
two phenyl rings in imines 1 is ꢁ36°, which is less than
that of imines 2 (ꢁ44°). The phenyl rings in the imines 1
and the pentafluorophenyl rings in the imines 2 are
almost parallel to each other, with dihedral angles of
Slow evaporation of an equal molar solution of imines 3
and 4 in hexane also afforded a colorless co-crystal with
melting point of 110–112 °C. This is lower than those of
pure starting compounds. We propose that there could
Figure 1. (a) Molecular structure of the complex 1Æ2. (b) Molecular packing map of 1Æ2. The hydrogen bonding between the fluorine atoms and the
hydrogen atoms is indicated using dashed lines.

S. Zhu et al. / Tetrahedron Letters 46 (2005) 2713–2716
2715
˚
be two possible stacking patterns, head-to-head or head-
to-tail, based on the structures of imines 3 and 4. The p–
p interactions would dominate in the former stacking
manner, however, there would also be lone pair elec-
trons repulsion interactions between the nitrogen atoms,
which are oriented in the same direction. If stacking in
the latter manner, lone pair electrons repulsion could
be avoided, but there would be electrostatic repulsion
interactions between the neighboring aromatic rings.
phenyl rings is ꢁ3.4 A, which is close to that of 1Æ2
˚
(ꢁ3.44 A). The packing map data of the 3Æ4 co-crystal
reveals that there is no intermolecular hydrogen bonding
between the imines C–Hꢀ ꢀ ꢀF, but that there are numer-
ous weak C–Hꢀ ꢀ ꢀ F interactions between the phenyl
rings and pentafluorophenyl rings in the neighboring
˚
column molecules (ꢁ2.70 A, ꢁ131°). The shortest inter-
˚
molecular H22ꢀ ꢀ ꢀF5 hydrogen bond distance is 2.749 A.
The electrostatic repulsion of the lone pair electrons of
the imine nitrogen atoms and the absence of intermolec-
ular hydrogen bonding may account for the lower melt-
ing point of the 3Æ4 co-crystal than those of pure starting
molecules 3 or 4 (Fig. 2).
8
X-ray analysis of complex 3Æ4 shows that the crystals
are stacked in the head-to-head, face-to-face fashion,
which permits the phenyl rings and pentafluorophenyl
rings to stack alternatingly. The electrostatic attraction
interactions of the aromatic rings are enhanced by the
lone pair electron repulsion. The torsion angles of imi-
nes 3 and 4 are ꢁ48°. Due to the neighboring phenyl–
perfluorophenyl face-to-face orientation, the corre-
sponding two aromatic rings are parallel each other,
with a dihedral angle only ꢁ4°. The mean perpendicular
distance of the phenyl rings to the neighboring perfluoro-
To conclude, X-ray diffraction analysis of two closely re-
lated crystal structures of phenyl and perfluorophenyl
aldimines 1 and 2 at room temperature disclosed that
there exist significant p–p stacking interactions between
the phenyl and perfluorophenyl rings. The molecules are
stacked in vertical columns, face-to-face and head-to-
tail. The phenyl and perfluorophenyl rings, which stack
Figure 2. (a) Molecular structure of complex 3Æ4. (b) Molecular packing diagram of the 3Æ4 co-crystal. The fluorine atoms are green, and the nitrogen
atoms are azure. The intermolecular C–Hꢀ ꢀ ꢀF–C interactions are shown using dashed lines.

2716
S. Zhu et al. / Tetrahedron Letters 46 (2005) 2713–2716
alternatingly, p–p stacking interactions, electrostatic
attractions and intermolecular hydrogen bonding in
the co-crystals of 1Æ2, work together to increase its melt-
ing point. In complex of 3Æ4 the p–p stacking interac-
tions dominated the electrostatic repulsions of the lone
pair of the imines nitrogen atoms, which forces the mole-
cules to align in a head-to-head manner. The co-crys-
tals 3Æ4, however, have a lower melting point than the
corresponding starting materials, which could be attrib-
uted to the electrostatic repulsion interactions and the
absence of intermolecular hydrogen bonds. The work re-
ported here provides strong evidence for the power of
fluoroaryl–aryl face-to-face interactions as a design mo-
tif for a new class of self-assembling systems. The poten-
tial applications of these new fluoroaldimines structures
are the subject of the on-going investigations.
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References and notes
8. Crystal data: 1Æ2 (1:1): CCDC No. 241887 C H F N ,
2
6
12 10
2
M = 542.38; monoclinic, spacegroup C2/c, a = 26.736(2),
˚
1
2
. Hunter, C. A. K.; Lawson, R.; Perkins, J.; Urch, C. J. J.
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b = 7.4098(6), c = 25.102(2) A, a = 90, b = 114.213(2),
˚
3
3
c = 90°, V = 4535.3(7) A , Z = 8, D
c
= 1.589 Mg/m , l =
5248 [R(int) = 0.0744] unique reflections,
ꢂ1
0.152 mm
,
1
982, 38, 1966–1972; (b) Williams, J. H.; Cockcroft, J. K.;
Fitch, A. N. Angew. Chem., Int. Ed. Engl. 1992, 31, 1655–
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999, 38, 2741–2745; (e) Naae, D. G. Acta Crystallogr. Sect.
F(000) = 2176, final R = 0.0472, wR = 0.1011, [I > 2d(I)].
1
2
12 10 2
3 Æ 4 (1:1): CCDC No. 241888 C26H F N , M = 542.38;
˚
triclinic, P1, a = 6.142(4), b = 7.556(5), c = 12.347(8) A,
1
˚
3
3
ꢂ1
,
V = 560.6(6) A, Z = 1, D
2848 [R(int) = 0.0900] unique reflections, F(000) = 272,
final = 0.0521, wR = 0.1208, [I > 2d(I)]. Intensity
data was collected at 293(2) K with Bruker P4
= 1.607 Mg/m , l = 0.154 mm
c
(
R
1
2
1
a
B 1979, 35, 2765–2768; (f) Renak, M. L.; Bartholomew, G.
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four-circle diffractometer; with equipped a graphite mono-
˚
chromator, and Mo Ka radiation [k (Mo Ka) = 0.71073 A)].
A total of 13,276 and 3311 independent reflections were
measured in range 1.69 < h < 28.35° and 1.88 < h < 28.30°,
respectively. The structures were solved by directed meth-
ods and expanded using Fourier techniques. The non-
hydrogen atoms were refined anisotropically. Hydrogen
atoms were included but not refined. The final cycle of
6
096; (i) Beck, C. M.; Burdeniuc, J.; Crabtree, R. H.;
Rheingold, A. L.; Yap, G. P. A. Inorg. Chim. Acta 1998,
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999, 1027–1028; (k) Lork, E.; Mews, R.; Shakirov, M. M.;
^
2
2
full matrix least-square refinement was based on F .
All calculations were performed using the program
1
SHELX-97.