Dominant role of C-Bra??N halogen bond in molecular self-organization. Crystallographic and quantum-chemical study of schiff-base-containing triazoles

Article abstract of DOI:10.1021/jp049742v

Three Schiff-base-containing triazole derivatives have been synthesized and investigated by X-ray crystallography. Structural motifs (zigzag or linear chains) formed in the solid state are stabilized by C-Bra??N halogen bond interactions, while hydrogen bonds do not seem to play a significant role in the molecular self-organization of studied compounds. Quantum-chemical studies confirm that these interactions are sufficient to account for both the arrangement of structural motifs and very short Bra??N distances observed experimentally. It is also shown that Bra??N interactions can lead to generally shorter donor-acceptor distances than Cla??N contacts.

Full text of DOI:10.1021/jp049742v

J. Phys. Chem. B 2004, 108, 12327-12332
12327
Dominant Role of C-Br‚‚‚N Halogen Bond in Molecular Self-Organization.
Crystallographic and Quantum-Chemical Study of Schiff-Base-Containing Triazoles
Sławomir Berski, Zbigniew Ciunik, Krzysztof Drabent,* Zdzisław Latajka, and
Jarosław Panek*
Faculty of Chemistry, UniVersity of Wrocław, ul. F. Joliot-Curie 14, 50-383 Wrocław, Poland
ReceiVed: January 18, 2004; In Final Form: May 29, 2004
Three Schiff-base-containing triazole derivatives have been synthesized and investigated by X-ray crystal-
lography. Structural motifs (zigzag or linear chains) formed in the solid state are stabilized by C-Br‚‚‚N
halogen bond interactions, while hydrogen bonds do not seem to play a significant role in the molecular
self-organization of studied compounds. Quantum-chemical studies confirm that these interactions are sufficient
to account for both the arrangement of structural motifs and very short Br‚‚‚N distances observed
experimentally. It is also shown that Br‚‚‚N interactions can lead to generally shorter donor-acceptor distances
than Cl‚‚‚N contacts.
Introduction
have prepared 4-(4-H-1,2,4-triazol-4-yl)-n-Br-phenylmeth-
animine where n ) 4 (1) or n ) 3 (2) and 4-(4-H-1,2,4-triazol-
Weak intermolecular interactions play important roles in many
chemical phenomena, such as molecular recognition, confor-
mational transformation, and molecular packing in crystals.1
Hydrogen bonding is the most frequently used tool to assemble
organic molecules in solid, liquid, or gas phases.2 On the other
hand, it is well-known that carbon-bound halogen atoms can
act as electron acceptors and form short contacts with atoms
containing lone pairs (e.g., nitrogen, phosphorus, oxygen, and
sulfur).3 This electron donor-acceptor interaction has been
called “halogen bonding”4 to emphasize its similarity with
hydrogen bonding. The highly directional character of this
interaction as well as the interpenetrating of van der Waals
volumes of halogen and nitrogen atoms has been reported. This
noncovalent interaction can be strong enough to control the
aggregation of organic molecules.3-7
4-yl)-4-Br-phenylmethylmethanimine, (3) (Figure 1). Detailed
analysis of the crystal structures of 1-3 was performed. Due
to the fact that the structures of molecules 1-3 seem to be
stabilized by the C-Br‚‚‚N interaction, it was interesting to
characterize them by means of advanced theoretical methods:
the topological analysis of the electron density11 and SAPT
interaction energy decomposition scheme.12 The urge for
theoretical support is partially motivated by recent work of
Dance,13 who pointed out that the distance criterion need not
always be indicative of the presence of attractive forces between
atoms. Indeed, close contact between atoms may be energetically
destabilizing, but when forced by several stronger interactions
may yield a net gain in stability.
Experimental Section
Recently, particular attention has been focused on the
interaction between haloperfluorocarbons and aromatic dinitro-
gen hydrocarbons3a,5 and between organic nitriles and halogen
atoms.6b,7 On the other hand, studies on the nitrogen-halogen
interactions involving nitrogen-rich heterocycles are relatively
rare.8-10 These investigations concern crystal structure studies
on aromatic compounds containing the NdC-X subunit where
X ) Cl or Br. In the structures of bromotetrazole9 and 3-chloro-
1,2,4-triazole,10 both the N‚‚‚X intermolecular interactions and
strong N-H‚‚‚N hydrogen bonds (D‚‚‚A distance equal to 2.757
Å and 2.865 Å, respectively) are responsible for the formation
of two-dimensional arrays of molecules. Presumably, the
hydrogen bonds, which must be much stronger than N‚‚‚X
interactions, are of greater importance in determining the crystal
packing. At this point it is necessary to mention that weak
N‚‚‚X-C interactions are observed in binary structures,8c,d while
their presence in crystals of a single chemical species is less
common.8b Our aim is to design and prepare such a unary crystal
with distinct CdN‚‚‚X-C halogen bonds. To prevent the
formation of strong hydrogen bonds between heterocycles, we
Physical Measurements. Infrared spectra were recorded as
Nujol mulls and KBr pellets on a Bruker IFS 113V FT-IR
spectrometer. Elemental analyses were carried out at the
Microanalytical Laboratory of this university. 1H NMR spectra
were recorded on a Bruker AM300 spectrometer in CD3CN.
ESI MS were performed on a mass spectrometer Finnigan Mat
type TSQ700.
Starting Materials. Commercially available solvents, hy-
drazine monohydrate, formic acid, 4-bromobenzaldehyde, 3-bro-
mobenzaldehyde (Fluka), and 4-bromoacetophenon (Aldrich)
were used without further purification. The 4-amino-1,2,4-
triazole was prepared according to literature method.14
Synthesis of 4-(4-H-1,2,4-triazol-4-yl)-4-Br-phenylmeth-
animine, (1). Ethanolic solution (30 mL) of 4-bromobenzalde-
hyde (3.70 g, 20 mM) was added to a warm ethanolic solution
(25 mL) of 4-amino-1,2,4-triazole (1.69 g, 20 mM) and the
resulting solution was refluxed for 4 h. Upon standing overnight
at the room temperature, the solution deposited white crystals
that were isolated by filtration, washed with a small amount of
ethanol and diethyl ether, and dried under vacuum. For 1: yield
2,1 g, 41.8%, Mp: 197 °C. MS (m/e) 251.1. Anal. calcd. for
C9H7N4Br: C, 43.05; H, 2.81; N, 23.31; Br, 31.82. Found: C,
* Corresponding
wchuwr.chem.uni.wroc.pl.
authors:
Krzysztof
Jaroslaw
Drabent,
Panek,
drab@
jarek@
elrond.chem.uni.wroc.pl (phone +48 71 3757246, fax +48 71 3282348).
10.1021/jp049742v CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/27/2004

12328 J. Phys. Chem. B, Vol. 108, No. 33, 2004
Berski et al.
Figure 2. Top: the chain of molecules linked by the C-Br‚‚‚N bonds
in 1. Middle: two chains of molecules linked by the C-Br‚‚‚N bonds
in 2. Weak C-H‚‚‚Br interchain interactions are also shown. Bottom:
two neighbor chains of molecules linked by the C-Br‚‚‚N bonds in 3.
ment details, while the most important features of the structures
are presented in Figures 1-2. All measurements were performed
on a Kuma KM4CCD κ-axis diffractometer with graphite-
monochromated MoKR radiation at 100 K using an Oxford
Cryosystem adapter. Crystals were positioned at 65 mm from
the KM4CCD camera. Using a counting time of 20 s, 612
frames were measured at 0.75°. The data were corrected for
Lorentz and polarization effects. An analytical absorption
correction was also applied.15 Data reduction and analysis were
carried out with the Oxford Diffraction Poland (formerly Kuma
Diffraction Wrocław, Poland) programs. All structures were
solved by direct methods (program SHELXS9716) and refined
by the full-matrix least-squares method on all F2 data using the
SHELXL9717 program. Non-hydrogen atoms were refined with
anisotropic displacement parameters; hydrogen atoms were
included from geometry of molecules or ∆F maps and were
refined with isotropic displacement parameters.
Computational Methodology. Large number of atoms in the
crystallographic unit cell prevents performing detailed ab initio
calculations by using the periodic Hartree-Fock method.
Therefore, several models of increasing simplification have been
chosen for our computational study. First, model dimers of 1,
2, and 3 have been chosen with geometrical structures taken
from the bulk solid. The assumed complexes named 4, 5, and
6, respectively, are presented in Figure 4. Geometry optimization
using the LANL2DZ approximation18 was carried out for 4, 5,
and 6 at the density-functional B3LYP19-21 computational level
with the Gaussian 98 program.22 DFT theory is nowadays
routinely applied to organic systems, which is also the case for
halogen-bonded intermolecular complexes.3b,c,8c However, when
dispersion energy is significant, one must treat its results with
care. Therefore, more involved computational models will also
be invoked further in this study as tools for assessing the
accuracy of the DFT approach for the systems under consid-
eration. To obtain a more detailed insight into the nature of the
C-X‚‚‚N (X ) Cl, Br, and I) molecular interaction additional
calculations have been performed for model complexes consist-
ing of 4-amino-4H-1,2,4-triazole and 4-X-benzaldehyde (X )
Figure 1. Numbering scheme and overall conformation of 1-3.
1
43.02; H, 2.74; N, 22.04; Br, 31.96. H NMR (CD3CN, RT,
[ppm] (multiplicity, assignment)): 8.76 (s, C-Him), 8.69 (s,
2C-Htr), 7.73 (4H, m, o-, m-Ph).
Synthesis of 4-(4-H-1,2,4-triazol-4-yl)-3-Br-phenylmeth-
animine, (2). The compound 2 was prepared in a similar manner
by using 3-bromobenzaldehyde instead of 4-bromobenzalde-
hyde. For 2: yield 2.04 g, 40.6%, Mp: 185-188 °C. MS (m/e)
251.1. Anal. calcd. for C9H7N4Br: C, 43.05; H, 2.81; N, 23.31;
1
Br, 31.82. Found: C, 43.26; H, 2.82; N, 22.30; Br, 32.00. H
NMR (CD3CN, RT, [ppm] (multiplicity, assignment)): 9.03 (s,
C-Him), 8.74 (s, 2C-Htr), 8.09-7.49 (4H, m, o-,o′-, m-, p-Ph).
Synthesis of 4-(4-H-1,2,4-triazol-4-yl)-4-Br-phenylmeth-
ylmethanimine, (3). Ethanolic solution (30 mL) of 4- bromo-
acetophenon (3.98 g, 20 mM) was added to a warm ethanolic
solution (25 mL) of 4-amino-1,2,4-triazole (1.69 g, 20 mM)
and the resulting solution was refluxed for 4 h, then the reaction
mixture was cooled to the room temperature. Subsequently, the
volume of the reaction mixture was reduced to 5 mL on a rotary
evaporator resulting in precipitation of a white solid. The solid
was filtered off and then dissolved in diethyl ether. The solution
was allowed to stand overnight in a refrigerator, resulting in
formation of well-shaped colorless crystals that were separated
by filtration, washed with a small amount of diethyl ether, and
dried in air. For 3: yield 1.59 g, 30.0%, Mp: 76-78 °C. MS
(m/e) 265.0. Anal. calcd. for C10H9N4Br: C, 45.31; H, 3.42; N,
21.13; Br, 30.14. Found: C, 45.86; H, 3.65; N, 19.16; Br, 30.21.
1H NMR (CDCl3, RT, [ppm] (multiplicity, assignment)): 8.29
(s, 2C-Htr), 7.71 (4H, dd, o-, m-Ph, J ) 8.6 Hz), 2.39 (3H,
CH3)
Crystallographic Data Collection and Structure Deter-
mination. Crystal data32 for 1, 2, and 3 are given in Tables
S1-S3 of the Supplementary Information, together with refine-

C-Br‚‚‚N Bond in Molecular Self-organization
J. Phys. Chem. B, Vol. 108, No. 33, 2004 12329
Figure 5. Model dimers of 7, 8, and 9.
Figure 6. Dimers 10 and 11, the smallest models for N‚‚‚X interactions.
interactions for 7-9 structures were investigated with Bader’s
Atoms in Molecules analysis of electron density11 using
AIMPAC23 and AIM200024 programs. Finally, the simplest
models of C-X‚‚‚N interaction were constructed as complexes
of methanimine with chloro- and bromomethanimine (denoted
10 and 11 respectively - see Figure 6). The reason for the high
simplification of models is the necessity to treat them with more
sophisticated theoretical methods. Their structures were fully
optimized at MP2 and MP4(SDQ) correlated ab initio levels
using a series of three basis sets (6-31G(d, p), aug-cc-pVDZ
and mixed basis set with aug-cc-pVTZ on interacting N‚‚‚X
atoms and aug-cc-pVDZ for the remaining atoms). The mixed
basis set was also applied in B3LYP DFT geometry optimiza-
tions of 10 and 11. The Gaussian 98 package was used for this
part of the calculations. Interaction energy for N-Cl and N-Br
systems (10, 11) and mixed basis set (cc-pVTZ for C-X‚‚‚N,
cc-pVDZ for the rest of the molecule) was partitioned according
to symmetry-adapted perturbation theory12 (SAPT) using
SAPT2002 program.25
Figure 3. Weak intermolecular interactions in 1, 2, and 3 (top, middle,
and bottom respectively). The dark molecules are linked via the
Br‚‚‚N interactions. The open dashed lines represent weak intermo-
lecular CH‚‚‚A (A ) N, Br) interactions. Lengths of the hydrogen bonds
are no longer than the sum of respective van der Waals radii.29
Results and Discussion
X-ray Crystal and Molecular Structure. The numbering
scheme and overall conformation of the studied compounds are
shown in Figure 1. Bond lengths, valence, and torsion angles
in studied compounds are presented in Tables S1-S3 of the
Supplementary Information.
In principle, the aroyl Schiff bases of 4-amino-1,2,4-triazoles
show a tendency to retain the planar conformation with a rather
flat surface of potential energy as calculated for rotation around
the N(3)-N(4) and C(1)-C(2) bonds.26 A common feature for
this group of compounds is the tendency for creating hydrogen
bond systems with the C-H groups as donors [C(8) and/or C(9)]
and nitrogen atoms [N(1) and/or N(2)] or phenyl rings as
acceptors. These interactions control molecular arrangement in
all reported crystal structures of the aroyl Schiff bases of
4-amino-1,2,4-triazoles.27,28
Figure 4. Model dimers 4, 5, and 6 taken from crystals of compounds
1, 2, and 3.
The molecular structures of 1 and 2 are essentially planar
with angles between the best planes calculated for phenyl and
triazole rings of 13.7(2) and 7.6(2)°, respectively. The presence
of a methyl group at the C(1) atom changes the geometry of 3
toward a twisted conformation. In a crystal of 3, two symmetry-
Cl, Br, I) abbreviated as 7, 8, and 9 respectively (see Figure 5).
Geometry optimization with C-X‚‚‚N linearity and overall
system planarity constraint was carried out for these structures
at the same level of theory as mentioned above. C-X‚‚‚N

12330 J. Phys. Chem. B, Vol. 108, No. 33, 2004
Berski et al.
TABLE 1: Lengths (Å) and Angles (°) of the C-Br‚‚‚N
Interactions in Crystals of 1, 2, and 3
TABLE 3: Selected Results of Geometrical Optimization of
the Model Complexes 4-9
C-Br‚‚‚N
Br‚‚‚N
C-Br‚‚‚N
parameter
4 -Br 5 -Br 6 -Br 7 -Cl 8 -Br 9 -I
compound 1
R(X‚‚‚N) [Å]
3.150 3.147 3.153 3.260 3.129 3.089
C(5)-Br(1)‚‚‚N(1i)
compound 2
3.240(4)
3.192(4)
172.38(13)
168.94(13)
F(rc) [e a0
]
---
---
--- 0.007 0.011 0.015
-3
∆E [kcal mol-1
] -1.28 -1.18 -1.25 0.38 -1.27 -3.37
a
C(6)-Br(1)‚‚‚N(1ii)
compound 3
a Bonding energy (relative to isolated monomers) corrected for BSSE,
computed at B3LYP level with LANL2DZ pseudopotential and basis
set.
C(5)-Br(1)‚‚‚N(52iii)
C(55)-Br(2)‚‚‚N(2iV)
3.199(3)
3.324(3)
164.40(11)
161.35(12)
Symmetry codes: (i) -1.5 + x, 0.5 - y, 0.5 + z; (ii) 2 + x, y, 1 +
z; (iii) 1 + x, y, 1 + z; (iv) 1 + x, y, z.
is one more point to this part of the discussion. One can argue
that the role of a single but weak halogen bond must be surely
masked by other, generally weaker, but more numerous, van
der Waals interactions (in this case, mainly C-H‚‚‚H). However,
we would like to point out the directionality of the halogen
bond3c as opposed to general nondirectional dispersion forces.
The presence of a halogen bond is often a sufficient impulse
for directional molecular aggregation,3-7 as witnessed in our
case by characteristic zigzag or chain motifs.
TABLE 2: Geometry of Weak Intermolecular CH‚‚‚A
Interactions in Crystals of 1, 2, and 3
D-H‚‚‚A
D‚‚‚A [Å]
H‚‚‚A [Å]
D-H‚‚‚A [°]
compound 1
C(1)-H(1)...N(2i)
C(4)-H(4)...Br(1ii)
C(8)-H(8)...N(2i)
C(9)-H(9)...Br(1iii)
compound 2
3.463(5)
3.954(4)
3.654(6)
3.931(4)
2.50(5)
3.07(5)
2.71(5)
3.01(5)
164(4)
158(4)
162(4)
167(4)
C(4)-H(4)...Br(1iV)
C(1)-H(1)...N(2V)
compound 3
3.857(4)
3.529(7)
2.91(6)
2.56(7)
157(5)
159(5)
Computational Results and Discussion
Crystal structures of 1, 2, and 3 seem to be stabilized by the
C-Br‚‚‚N interaction. Theory should help explain whether such
an interaction alone can be responsible for the observed
structural motifs and distance ranges. As described in the
Computational Methodology section, we start by examining
dimers that are cut out of the experimental crystal structure,
systems 4-6 (Figure 4). Results of the geometry optimization
(Table 3) show that the experimental Br‚‚‚N distance is closely
reproduced by the DFT approach. The complexes rearrange only
slightly during the optimization; they attain more planar
structures and linear C-Br‚‚‚N arrangement as compared to the
experiment. Bonding energies of 4-6 are ca. -1.2 kcal/mol.
In the case of such a low stabilization, the performance of DFT
methods should be verified, which is one of the aims of the
following theoretical studies. The Br‚‚‚N interaction region was
investigated in the framework of AIM theory and in all of the
4-6 dimers a bond critical point (BCP) accompanied by the
bond path linking the N and Br atoms were found. According
to Bader’s criteria11a for the definition of a chemical bond, this
signifies the presence of bonding interaction, especially in the
view of the fact that there are no other close contacts in the
4-6 systems. However, the electron density at the BCP (F(rc))
is small (ca. 0.01 e a0-3) and its Laplacian is positive, which
indicates weak interaction (similar to weak hydrogen bonds).
We can conclude that for 4-6 the B3LYP functional is able to
reproduce, at least semiquantitatively, the geometry (bond length
and directionality) of the Br‚‚‚N interaction, and that this
interaction is strong and directional enough to arrange the gas-
phase 4-6 dimers similarly to 1-3. This does not exclude the
presence of other forces in the crystals of 1-3, but combined
with the analysis given above indicates the importance of the
halogen bond for the crystal structure of the studied systems.
Now let us proceed to verify the accuracy of the DFT theoretical
model in more detail.
C6-H6...N1(Vi)
C9-H9...N51(Vii)
3.454(4)
3.468(5)
3.535(5)
3.717(4)
3.722(4)
2.54(5)
2.56(5)
2.69(5)
3.04(5)
3.03(4)
174(4)
164(4)
144(4)
124(3)
126(3)
C54-H54...N4(Viii
)
C56-H56...Br1(ix)
C57-H57...Br1(ix)
See Figure 3 for graphical representation. Symmetry codes: (i) 2.5
- x, 0.5 + y, -0.5 - z; (ii) -x, 1 - y, -z; (iii) 1 - x, -y, -z; (iv)
1 - x, -0.5 + y, 1 - z; (v) -2 - x, -0.5 + y, -z; (vi) 1 + x, y, z;
(vii) x, y, 1 + z; (viii) 1 - x, -y, 1 - z; (ix) x, y, -1 + z.
independent molecules are observed with different interplanar
angles equal to 45.37(12) and 68.06(14)°. The bond lengths and
valence angles observed in the structures are in the normal
ranges. Surprisingly, the detailed analysis of intermolecular
interactions for all compounds under investigations showed, in
the absence of significant short C-H‚‚‚N interactions, zigzag
(in 1 and 2) or linear (in 3) chains of molecules (Figure 2) held
together by the weak attractive intermolecular C-Br‚‚‚N
interactions with Br‚‚‚N distances in the range of 3.192(4)-
3.324(3) Å (Table 1). Within the estimated error (3 esd’s), these
are shorter than the sum of the van der Waals radii (3.44 Å).29
The short C-Br‚‚‚N contacts are nearly linear with the
C-Br‚‚‚N angles in the range of 160.17(11)-172.38(13)°. The
geometrical parameters found in this study are comparable to
other recently reported Br‚‚‚N(sp2) contacts that exhibit high
directional penetrations of electronegative atoms into the van
der Waals volumes of halogen atoms.6b There are, however,
also some weak, near-linear C-H‚‚‚A interactions (Table 2,
Figure 3) that can influence the arrangement of 1-3 molecules
in the solid phase. Of these, we think that the C-H‚‚‚N pincer
interactions could in principle constitute additional forces
directing molecular aggregation. The N‚‚‚H distances ranging
from 2.50 to 2.71 Å are found in 1-3. However, the pincer
C-H‚‚‚N interaction is present only in 1 and 2. In the case of
3, the C-H‚‚‚N bond is rather more likely to control the tilt of
the phenyl ring with respect to the molecular backbone, while
the C-Br‚‚‚N interactions control formation of infinite chains.
As is evident from Figure 3, even in case of 1 and 2 the
C-H‚‚‚N interactions do not act in close synergy with halogen
bonds, while C-H‚‚‚Br seems to cooperate with C-Br‚‚‚N. We
suggest, therefore, that the halogen bonds are primary sources
of the particular arrangement of investigated structures. There
Model systems of 7-9 (Figure 5) enable us to study the
influence of halogen atoms on C-X‚‚‚N interaction (Table 3).
This will enable us to define the systems for which the B3LYP/
LANL2DZ level is applicable. Interestingly, the X‚‚‚N distance
increases with rising electronegativity of the X atom implying
that the more polarizable the atom, the stronger the X‚‚‚N
interaction. This conclusion is supported by the magnitude of
the interaction energy ∆E. The complex 7 with the Cl atom is
slightly unstable (signifying failure of the DFT approach), but

C-Br‚‚‚N Bond in Molecular Self-organization
J. Phys. Chem. B, Vol. 108, No. 33, 2004 12331
TABLE 4: Parameters of C-X‚‚‚N Bridge and Binding
predicted by DFT with respect to the ab initio results. The
simplified systems 10 and 11 do not contain aromatic rings,
which increase polarization effects and help the DFT method
to perform well for 4-9. The stability of chlorine-containing
10 computed by DFT is particularly too small, supporting our
earlier notion. Interaction energy partitioning with the SAPT
scheme indicates that virtually all stabilization energy in
complex 10 and 11 is due to the dispersion contribution, while
other major components (polarization, induction, and exchange
energies) basically cancel each other. However, these compo-
nents are responsible for the directionality of the halogen bond.
DFT recovers most of these components and therefore it is
successful for systems containing heavier, easily polarizable
halogen atoms (Br or I). The SAPT results are consistent with
the observed strength of C-X‚‚‚N interactions, which can be
easily dominated by hydrogen bonds and only rarely are able
to become the dominant factors in shaping crystal structures.
Our analysis indicates that the crystals of 1, 2, and 3 belong to
the group of structures with internal arrangement directly
governed by the N‚‚‚Br interactions.
Energies of 10 and 11 Systems
ab initio
ab initio
DFT
DFT
parameter
10 -Cl
11 -Br
10 -Cl 11 -Br
X‚‚‚N [Å]a
3.098-3.245
3.022-3.161
169.2-170.0
-2.201
3.386 3.304
171.3 175.4
-0.227 -0.726
--- ---
<(C-X‚‚‚N) [0] 165.0-170.2
E [kcal/mol]b
DISP [kcal/mol]c -2.146
-1.551
E
-3.078
a Ab initio: Range of results from different basis sets (6-31G(d, p),
aug-cc-pVDZ, and mixed basis set: aug-cc-pVTZ for N‚‚‚X, aug-cc-
pVDZ for the rest of the complex) at MP2 and MP4(SDQ) level with
frozen-core or all-electron correlation included. DFT: Results obtained
with B3LYP DFT functional and the mixed basis set. b Bonding energy
corrected for BSSE, computed with mixed basis set (see above) at both
the all-electron MP2 level and B3LYP DFT functional. c Dispersion
energy calculated within SAPT approximation and mixed basis set:
cc-pVTZ for N‚‚‚Cl-C, cc-pVDZ for the rest of the complex.
the observed order of energy values is consistent with decreasing
X‚‚‚N distance. Moreover, the stabilization rises monotonically
for 8 and 9. This sequence of energies was theoretically observed
by Valerio et al.3b for the simple model of CF3X‚‚‚NH3 (X )
Cl, Br, I), where DFT interaction energies ranged from -2.3
kcal/mol for chlorine species to -6.4 for iodine system. The
lack of stability found for 7 should not suggest that there is no
stabilizing C-Cl‚‚‚N interaction that would be in contradiction
to experimental proofs.5a,30 One should remember that the results
presented here are based on the model systems and describe
only the gas-phase heterodimer. However, we note that inter-
molecular X‚‚‚N distances in 7 and 8 are similar to those found
both in earlier literature studies3-10 and in compounds 1 to 3.
Thus, we suggest that the B3LYP functional can fail for the
Cl‚‚‚N halogen bond, while it can provide a reasonable
description of the bonding for heavier halogens. We will return
to this assumption in the paragraph dealing with SAPT results.
The topological analysis of the electron density11a,31 for 7-9
shows a picture similar to that already obtained for the 4-6
systems. Electron densities at the bond critical point increase
from 7 to 9, but their magnitudes (ca. 0.01 e a0-3) and positive
sign of a Laplacian confirm that the bonding is of the closed-
shell, noncovalent type. However, the complex formation
together with calculated geometrical and electron density
parameters, and finally the presence of bond critical points
accompanied by proper bond paths, indicate strongly that the
short distances found experimentally in 1-3 are not forced
contacts, but rather proper manifestations of “halogen bonding”.
The final test to confirm this thesis involved MP2 and MP4
perturbational computations, schemes that are regarded as less
error-prone than the density functional theory in case of weak
interactions. Selected results of the investigations on two simple
dimers 10 and 11 (Figure 6) are shown in Table 4. These
systems are probably the simplest to exhibit C-X‚‚‚N interac-
tions and had been used previously for theoretical modeling of
halogen bonding.8 It was seen in our results that bromine
systems can indeed form shorter N‚‚‚X distances than chlorine-
containing species, as observed already for compounds 7-9.
The directionality of the “halogen bonding” is seen in the range
of calculated C-X‚‚‚N angles, which are very close to the linear
arrangement and are consistent among various methods used,
while the calculated N‚‚‚X distances are ca. 0.1 Å shorter than
found experimentally (cf. Table 1). Note that these distances
are rather strongly dependent on the level of theory applied
(basis set, correlation scheme), but the largest separations, which
are closest to the experimental findings, were obtained with the
best computational methods. The DFT bond lengths are larger
by 0.2 Å, which is consistent with much smaller stabilization
Conclusions
This paper describes synthesis, structural characterization, and
theoretical investigations on a series of 4-(4-H-1,2,4-triazol-4-
yl)-bromophenylmethanimines (1, 2, and 3). The most striking
feature of their unary crystal structures is the lack of hydrogen
bonds and the presence of short intermolecular N‚‚‚Br contacts
(3.19-3.32 Å). Planarity of these molecules is retained only in
the absence of a bulky methyl substituent at the imine carbon
atom. Theoretical calculations were used to get insight into the
importance of N‚‚‚Br interactions. The presence of bond critical
points between the N and Br atoms in 4, 5, and 6 dimers implies
that those atoms are in contact. The calculations on compounds
7-9 show that the N‚‚‚X separation decreases with growing
polarizability of the halogen atom. This notion is supported by
calculations for the 10 and 11 models. SAPT interaction energy
partitioning scheme, employed for 10 and 11, explains their
stability in terms of the dispersion forces dominating over
repulsion terms. Theoretical description of clearly defined,
directional interactions of C-X‚‚‚N type in gas-phase calcula-
tions indicates that these forces play a decisive role in crystal
structures of compounds 1-3.
Acknowledgment. S.B. would like to thank to Dr Krzysztof
Mierzwicki for help with problems of geometry optimizations.
Authors acknowledge the Wrocław Supercomputing Center for
providing the computational time, and wish to express their
thanks to Ms Magdalena Ozarowski for linguistic advice.
Supporting Information Available: Tables containing
crystal data and structure refinement, atomic coordinates and
equivalent isotropic displacement parameters, bond lengths and
angles, anisotropic displacement parameters, hydrogen coordi-
nates and isotropic displacement parameters, and torsion angles
for compounds 1, 2, and 3. This material is available free of
charge via the Internet at http://pubs.acs.org.
References and Notes
(1) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond; Oxford
Science Publ.: Oxford, 1999.
(2) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford
University Press: Oxford, 1997.
(3) (a) Metrangolo, P.; Resnati, G. Chem. Eur. J. 2001, 7, 2511-2519.
(b) Valerio, G.; Raos, G.; Meille, S. V.; Metrangolo, P.; Resnati, G. J. Phys.
Chem. A 2000, 104, 1617-1620. (c) Romaniello, P.; Lelj, F. J. Phys. Chem.
A 2002, 106, 9114-9119.

12332 J. Phys. Chem. B, Vol. 108, No. 33, 2004
Berski et al.
(4) (a) Bent, H. A. Chem. ReV. 1968, 68, 587-648. (b) Legon, A. C.
Angew. Chem. 1999, 111, 2850-2880; Angew. Chem., Int. Ed. Engl. 1999,
38, 2687-2714.
(5) (a) Corradi, E.; Meille, S. V.; Messina, M. T.; Metrangolo, P.;
Resnati, G. Angew. Chem., Int. Ed. 2000, 39, 1782-1786. (b) Messina, M.
T.; Metrangolo, P.; Navarrini, W.; Radice, S.; Resnati, G.; Zerbi, G. J. Mol.
Struct. 2000, 524, 87-94.
(6) (a) Imakubo, T.; Maruyama, T.; Sawa, H.; Kobayashi, K. Chem.
Commun. 1998, 2021-2022. (b) Lommerse, J. P. M.; Stone, A. J.; Taylor,
R.; Allen, F. H. J. Am. Chem. Soc. 1996, 118, 3108-3116.
(7) (a) Bond, A. D.; Griffiths, J.; Rawson, J. M.; Hulliger, J. Chem.
Commun. 2001, 2488-2489. (b) Devic, T.; Bertran, J. N.; Domercq, B.;
Canadell, E.; Avarvari, N.; Auban-Senzier, P.; Fourmigue, M. New J. Chem.
2001, 25, 1418-1424. (c) Desiraju, G. R.; Harlow, R. L. J. Am. Chem.
Soc. 1989, 111, 1, 6757-6764.
(22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M.
W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.9; Gaussian,
Inc.: Pittsburgh, PA, 1998.
(23) Bader, R. F. W. AIMPAC, Suite of programs for the Theory of
Atoms in Molecules; McMaster University: Hamilton, Ontario, Canada,
1991.
(8) (a) Xu, K.; Ho, D. M.; Pascal, R. A., Jr. J. Am. Chem. Soc. 1994,
116, 105-110. (b) Ba´thori, N.; Biha´tsi, L.; Bombicz, P.; Czugler, M. Cryst.
Eng. Comm. 2003, 5, 42-44. (c) Bianchi, R.; Forni, A.; Pilati, T. Chem.
Eur. J. 2003, 9, 1631-1638. (d) DeSantis, A.; Forni, A.; Liantonio, R.;
Metrangolo, P.; Pilati, T.; Resnati, G. Chem. Eur. J. 2003, 9, 3974-3983.
(9) Ansell, G. B. J. Chem. Soc., Perkin Trans. 1973, 2, 2036-2038.
(10) Claramunt, R. M.; Lopez, C.; Garcia, M. A.; Otero, M. D.; Torres,
M. R.; Pinilla, E.; Alercon, S. H.; Alkorta, I.; Elguero, J. New J. Chem.
2001, 25, 1061-1068.
(24) AIM2000; University of Applied Sciences, Dept of Mathematics
and Technology; Bielefeld; http://gauss.fh-bielefeld.de/aim2000.
(25) Bukowski, R.; Cencek, W.; Jankowski, P.; Jeziorski, B.; Jeziorska,
M.; Kucharski, S. A.;. Misquitta, A. J.; Moszynski, R.; Patkowski, K.;
Rybak, S.; Szalewicz, K.; Williams, H. L.; Wormer, P. E. S. SAPT2002:
An Ab Initio Program for Many-Body Symmetry-Adapted Perturbation
Theory Calculations of Intermolecular Interaction Energies; University of
Delaware and University of Warsaw.
(11) (a) Bader, R. F. W. Atoms in Molecules - A Quantum Theory;
Oxford University Press: Oxford, 1990. (b) Bader, R. F. W. J. Phys. Chem.
A 1998, 102, 7314-7323.
(26) Ciunik, Z.; Drabent, K.; Szterenberg L. J. Mol. Struct. 2002, 641,
175-182.
(27) Zhu, D.-R.; Xu, Y.; Liu, Y.-J.; Song, Y.; Zhang, Y.; You, X.-Z.
Acta Crystallogr., Sect. C 2000, 56, 242-243.
(12) Jeziorski, B.; Moszynski, R.; Szalewicz, K. Chem. ReV. 1994, 94,
1887-1930.
(28) Raj, S. S. S.; Fun, H.-K.; Zhu, D.-R.; Jian, F.-F.; Zhang, K.; You,
X.-Z. Acta Crystallogr., Sect. C 1999, 55, 1526-1528.
(13) Dance, I. New J. Chem. 2003, 27, 22-27.
(14) Herbst, R. M.; Garrison, J. A. J. Org. Chem. 1953, 18, 872-877.
(15) CrysAlis RED 1.166 package; Oxford Diffraction: Poland Sp z
o.o., 2001.
(29) Bondi, A. J. Phys. Chem. 1964, 68, 441-454.
(30) Shekhar Reddy, D.; Panneerselvam, K.; Pilati, T.; Desiraju, G. R.
J. Chem. Soc. Chem. Commun. 1993, 8, 661-662.
(16) Sheldrick, G. M. SHELXS97, Program for solution of crystal
structures; University of Go¨ttingen: 1997.
(17) Sheldrick, G. M. SHELXL97, Program for crystal structure
refinement; University of Go¨ttingen: 1997.
(18) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299-310.
(19) Becke, A. D. J. Chem. Phys. 1993, 98, 1372-1377.
(20) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
(21) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785-789.
(31) Bader, R. F. W.; Nguyen-Dang, T. T.; Tal, Y. Rep. Prog. Phys.
1981, 44, 893-948.
(32) CCDC contains the supplementary crystallographic data for this
paper under the following deposit numbers: 211333 for compound 1,
211334 for 2, and 211335 for 3. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union
Road, Cambridge CB2 1EZ, United Kingdom; fax: +44 1223 336033;
e-mail: deposit@ccdc.cam.ac.uk.