1-(4-Dimethylaminobenzyl)-2-(4-dimethylaminophenyl)-benzimidazole: Synthesis, X-ray crystallography and density functional theory calculations

Article abstract of DOI:10.1016/j.molstruc.2006.02.022

The product of the reaction between o-phenylenediamine and 4-dimethylaminobenzaldehyde, mp=179°, is unambiguously shown to be 1-(4-dimethylaminobenzyl)-2-(4-dimethylaminophenyl)-benzimidazole by X-ray crystallography. It crystallises in the monoclinic spa

Full text of DOI:10.1016/j.molstruc.2006.02.022

Journal of Molecular Structure 794 (2006) 244–250
www.elsevier.com/locate/molstruc
1-(4-Dimethylaminobenzyl)-2-(4-dimethylaminophenyl)-benzimidazole:
Synthesis, X-ray crystallography and density functional theory calculations
Iran Sheikhshoaie a, Ferdinand Belaj b, Walter M.F. Fabian b,
*
a Chemistry Department, Faculty of Science, University of Kerman, P.O. Box 76135-133, Kerman, Iran
b
¨ ¨
Institut fur Chemie, Karl-Franzens Universitat Graz, Heinrichstr. 28, A-8010 Graz, Austria
Received 7 February 2006; received in revised form 15 February 2006; accepted 16 February 2006
Available online 24 March 2006
Abstract
The product of the reaction between o-phenylenediamine and 4-dimethylaminobenzaldehyde, mpZ1798, is unambiguously shown to be 1-(4-
dimethylaminobenzyl)-2-(4-dimethylaminophenyl)-benzimidazole by X-ray crystallography. It crystallises in the monoclinic space group P21/c,
˚
˚
˚
with aZ18.327(5) A, bZ6.318(15) A, cZ18.204(4) A, bZ110.73(2)8, R1Z0.0647. Quantum chemical calculations [density functional theory,
B3LYP/6-31G(d)] are used to propose a reaction mechanism for the rearrangement of the initially formed bis-Schiff base via a two-step process
involving cyclization (TS1, DGsZ32 kcal mol–1) to a tetrahedral intermediate (3, DGreactZ30 kcal mol–1). A formal (1,3)-shift (TS2, DGsZ
38 kcal mol–1 with respect to 3) then yields in a strongly exothermic reaction (DGreactZK17 kcal mol–1) the rearranged product, 1-(4-
dimethylaminobenzyl)-2-(4-dimethylaminophenyl)-benzimidazole (2).
q 2006 Elsevier B.V. All rights reserved.
Keywords: X-ray; Schiff base rearrangement; Density functional calculations; Mechanism
1. Introduction
aminobenzylidene)-benzene-1,2-diamine 1, RZN(CH3)2, by
reaction of o-phenylenediamine with p-dimethylaminobenzal-
dehyde, a colourless compound, mpZ1798, was obtained. On
Aromatic Schiff bases form complexes with a variety of
metal ions [1] with a number of possible applications, e.g. as
liquid crystal or non-linear optic materials, as catalysts in
stereoselective organic transformations and multielectron
redox reactions, and for analytical applications [1–7]. Of
special interest are multidentate Schiff base ligands, e.g. those
derived by reaction of aromatic aldehydes with o-phenylene-
diamines (1 in Scheme 1). Surprisingly, although formation of
such bis-Schiff bases by the ‘standard’ synthetic procedure (see
Section 2) has been described by several authors [8–14], others
have instead observed cyclisation to benzimidazole derivatives
[15,16]. Such benzimidazoles, which also can be obtained by
microwave-assisted condensation of aromatic aldehydes and
o-phenylenediamines in the presence of montmorillonite
catalysts [17] or indium-mediated reductive coupling of
o-nitroaniline with aromatic aldehydes [18], easily form-like
their Schiff base isomers-complexes with a number of metal
cations [19]. In an attempt to prepare N,N0-bis(4-dimethyl-
1
the basis of its H NMR spectrum the Schiff base structure
immediately could be ruled out (see Section 2). On the other
hand, however, the melting point significantly differed from
that reported for the also possible benzimidazole derivative 2
(252–2558 [15,17]). However, a value of 1688 for the mp of this
very same compound can also be found in Ref. [18]. Thus, the
identities of the products formed by reacting o-phenylene-
diamine with aromatic aldehydes are still not completely clear.
Consequently, in the present paper we show unambiguously by
X-ray crystallography that despite this difference in melting
points, the compound obtained by us is 1-(4-dimethylamino-
benzyl)-2-(4-dimethylaminophenyl)benzimidazole 2. In
addition, by density functional theory computations [B3LYP/
6-31G(d)], we propose a detailed reaction mechanism
for the formation of 2 from o-phenylenediamine and
4-dimethylaminobenzaldehyde.
2. Experimental section
*
Corresponding author. Tel.: C43 316 380 8636; fax: C43 316 380 9840.
E-mail address: walter.fabian@uni-graz.at (W.M.F. Fabian).
URL: http://www.uni-graz.at/walter.fabian (W.M.F. Fabian).
2.1. Synthesis of 2
A mixture of 1,2-phenylenediamine (1 mmol), and
4-dimethylaminobenzaldehyde (2 mmol) in hot ethanol
0022-2860/$ - see front matter q 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.02.022

I. Sheikhshoaie et al. / Journal of Molecular Structure 794 (2006) 244–250
245
2.2. X-ray diffraction data of 2
N(CH3)2 (CH3)2N
All the measurements were performed using graphite-
monochromatised Mo Ka radiation at 100 K: C24H26N4, Mr
N
N
˚
370.49, monoclinic, space group P21/c, aZ18.327(5) A, bZ
˚
˚
6.3188(15) A, cZ18.204(4) A, bZ110.73(2)8, VZ
N
N
3
1971.6(9) A , ZZ4, dcalcZ1.248 g cmK3, mZ0.075 mmK1
.
˚
N(CH3)2
A total of 4423 reflections were collected (QmaxZ25.308),
from which 3591 were unique (RintZ0.0421), with 2364
having IO2s(I). The structure was solved by direct methods
(SHELXSK97 [21]) and refined by full-matrix least-squares
techniques against F2 (SHELXL-97 [22]). The non-hydrogen
atoms were refined with anisotropic displacement parameters
without any constraints. The H atoms of the CH2 group were
refined with common isotropic displacement parameters for the
H atoms and idealized geometry with approximately tetra-
N(CH3)2
1
2
Scheme 1. Structures of the o-phenylendiamine-4-dimethylaminobenzaldehyde
bis-Schiff base 1 and the rearrangement product 1-(4-dimethylaminobenzyl)-
2-(4-dimethylaminophenyl)-benzimidazole 2.
(30 mL) was stirred and refluxed for 4 h, the formed yellow
precipitate was filtered and washed sparingly with hot ethanol
several times to give 0.23 mg (63%) of 2: mpZ1798; The
assignment of NMR resonances has been done by the
heteronuclear multiple-bond connectivity (HMBC) procedure
˚
hedral angles and C–H distances of 0.99 A. The H atoms of the
phenyl rings were put at the external bisector of the C–C–C
˚
angle at a C–H distance of 0.95 A and common isotropic
displacement parameters were refined for the H atoms of the
same phenyl group. The H atoms of the methyl group C28 are
disordered over two orientations and were refined with site
occupation factors of 0.5 at two positions rotated from each
other by 608. The H atoms of the methyl groups were refined
with common isotropic displacement parameters for the H
atoms of the same group and idealized geometry with
tetrahedral angles, enabling rotation around the X–C bond,
1
[20], the atom numbering corresponds to that in Fig. 1: H
NMR (500 MHz, CDCl3) d 2.95 (6H, s, N14(CH3)2), 3.02 (6H,
s, N24(CH3)2), 5.39 (2H, s, CH2), 6.65 (2H, d, JZ8.68 Hz,
C13(15)–H), 6.71 (2H, d, JZ8.84 Hz, C23(25)–H), 7.00 (2H,
d, JZ8.68 Hz, C12(16)–H), 7.14 (1H, t, JZ7.88 Hz, C6–H),
7.18 (1H, d, JZ7.38 Hz, C7–H), 7.24 (1H, t, JZ7.99 Hz, C5–
H), 7.63 (2H, d, JZ8.84 Hz, C22(26)–H), 7.82 (1H, d, JZ
7.98 Hz, C4–H); 13C NMR (500 MHz, CDCl3) d 40.0
(C27,C28), 40.3 (C17,C18), 47.9 (C10), 110.2 (C7), 111.7
(C23, C25), 112.6 (C13, C15), 116.9 (C21), 119.0 (C4), 122.1
(C5,C6), 124.1 (C11), 126.7 (C12, C16), 130.2 (C22, C26),
136.2 (C8), 142.9 (C9), 149.9 (C14), 151.0 (C24), 155.0 (C2);
Anal. Calcd for C24H26N4: C, 77.83; H, 7.02; N, 15.13. Found:
C, 77.02; H, 7.98; N, 15.11. Single crystals of 2 were obtained
from a solution of CHCl3/ethylacetate (1:1, mpZ182.28).
˚
and C–H distances of 0.98 A. For 265 parameters final R
indices of RZ0.0647 and wR2Z0.1324 (GOFZ1.034) were
obtained. The largest peak in a difference Fourier map was
0.202 e AK3. Crystal data and structure refinement for 2 are
˚
provided in Table 1. CCDC 297274 contains the supplemen-
tary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/conts/
retrieving.html (or from the Cambridge Crystallographic
Fig. 1. Stereoscopic ORTEP [28] plot of 2 showing the atomic numbering scheme. The probability ellipsoids are drawn at the 50% probability level. The H atoms of
the methyl group C28 are disordered over two sites.

246
I. Sheikhshoaie et al. / Journal of Molecular Structure 794 (2006) 244–250
Table 1
Bulk solvent effects were estimated by B3LYP/6-31G(d) single
Crystal data and structure refinement for 2
point calculations using the polarizable continuum method
(IEF-PCM [26]) with ethanol as solvent. The electronic
structures of the various minima and TSs were analyzed with
the aid of the NBO method [27].
Crystal data
Identification code
Empirical formula
Formula weight
400a
C24H26N4
370.49
Crystal description
Crystal size
Needle, colourless
0.40!0.38!0.22 mm
Monoclinic, P21/c
3. Results and discussion
Crystal system, space group
3.1. X-ray crystal structure
Unit cell dimensions
˚
18.327(5) A
a
˚
6.3188(15) A
˚
18.204(4) A
b
The crystal structure analysis of 2 showed that the compound
is 1-(4-dimethylaminobenzyl)-2-(4-dimethylaminophenyl)-ben-
zimidazole. Fig. 1 shows a stereoscopic ORTEP plot [28] of 2
with the atom numbering scheme; the packing of 2 is depicted in
Fig. 2. Selected bond lengths, bondangles and dihedral angles are
listed in Table 2. Also given there are the corresponding
calculated [B3LYP/6-31G(d)] geometric parameters. Whereas
the phenyl ring bonded to C2 is only slightly twisted out of the
plane of the benzimidazole ring [the angle between their least-
squares planes is 24.01(11)8, t1Zt (N1–C2–C21–C26)ZK
22.68], the benzyl group is oriented almost perpendicular to the
benzimidazole ring [87.83(11)8, t2Zt (C8–N1–C10–C11)ZK
84.58] (Fig. 1). The dimethylamino groups are nearly co-planar to
the phenyl rings they are bonded to [the angles between their
least-squares planes are 8.76(17)8 for N14 and 13.49(17)8 for
N24; t3Zt (C15–C14–N14–C18)Z12.68, t4Zt (C15–C14–
N14–C17)Z166.68; t5Zt (C25–C24–N24–C28Z18.88, t6Zt
(C25–C24–N24–C27)ZK170.08] and essentially trigonal as
evidenced by the corresponding sum of angles (w1Z354.98 for
N14; w2Z354.5 for N24). The corresponding calculated
[B3LYP/6-31G(d)] structural parameters are t1ZK33.78,
t2ZK81.48, t3Z11.68, t4Z170.08, t5Z9.18, t6Z172.48,
w1Z359.48, w2Z357.88. The T shaped molecules are packed in
a herringbone pattern (see Fig. 2).
c
b
110.73(2)8
3
˚
Volume
1971.6(9) A
Z
4
Calculated density
F(000)
1.248 Mg/m3
792
0.075 mmK1
None
Linear absorption coefficient m
Absorption correction
Unit cell determination
18.12!Q!21.368
42 reflections used at 100 K
Data collection
Temperature
100 K
Diffractometer
Stoe
Radiation source
Fine-focus sealed tube
˚
Mo Ka, 0.71069 A
Radiation and wavelength
Monochromator
Graphite
Scan type
uK2Q scans
3 every 100 reflections
9.4%
Standard reflections
Intensity decay
Theta range for data collection
Index ranges
2.71–25.308
0%h%22, K7%k%1, K21%l%20
4423/3591
Reflections collected/unique
Significant unique reflections
R(int), R(sigma)
2364 with IO2s(I)
0.0421, 0.0874
99.9%
Completeness to QZ25.308
Refinement
Refinement method
Data/parameters/restraints
Goodness-of-fit on F2
Final R indices [IO2s(I)]
R indices (all data)
Extinction expression
Weighting scheme
Full-matrix least-squares on F2
3591/265/0
1.034
R1Z0.0647, wR2Z0.1156
R1Z0.1069, wR2Z0.1324
None
3.2. Quantum chemical calculations
For the formation of 2 by reaction of o-phenylenediamine
with 4-diaminobenzaldehyde
wZ1=½s2ðFo2ÞCðaPÞ C
2
a mechanism involving
bPꢀ where PZðFo2 C2Fc2Þ=3
a, b 0.0236, 0.7910
0.000
cyclisation of the initially formed bis-Schiff base 1
[RZN(CH3)2] to a zwitterionic intermediate 3, followed by
1,3-H shift (Scheme 2), has been proposed [16]. In the
following, a detailed mechanistic study of this reaction
sequence using quantum chemical methods [B3LYP/6-
31G(d)] density functional theory calculations] is presented.
To assess the appropriateness of the chosen model chemistry,
selected structural parameters calculated by the B3LYP/6-
31G(d) procedure for compound 2 are compared with the
corresponding experimental data in Table 2 (for atom
numbering, see Fig. 1). Agreement is generally very good,
with the largest deviations between experimental X-ray and
calculated data found for the C2–N1–C10–C11 torsional angle,
1008 (X-ray) vs. 1238 (calcd). Intermolecular interactions in the
crystal (see Fig. 2 for an ORTEP plot of the packing of 2) easily
can account for this discrepancy. Thus, we are confident that
the chosen computational procedure will be of sufficient
reliability.
Weighting scheme parameters
Largest D/s in last cycle
3
˚
0.202 and K0.255 e/A
Largest difference peak and hole
Structure solution program
Structure refinement program
SHELXSK97 Sheldrick [21]
SHELXL-97 Sheldrick [22]
Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax:
C44 1223 336033).
2.3. Computational details
All density functional theory (Becke’s hybrid HF-DFT
procedure [23] with the Lee-Yang-Parr [24] correlation
functional [B3LYP/6-31G(d)]) calculations were done with
GAUSSIAN 03 [25]. Geometries were completely optimized and
characterized as true minima or transition states (TSs) by
frequency calculations. For transition states, in addition
intrinsic reaction coordinate calculations have been performed.

I. Sheikhshoaie et al. / Journal of Molecular Structure 794 (2006) 244–250
247
Fig. 2. Stereoscopic ORTEP [28] plot of the packing of 2. The atoms are drawn with arbitrary radii.
A detailed conformational analysis with respect to rotation
around the aryl-N formal single bonds resulted in four distinct
structures 1a–d, characterized by the respective torsional
angles C4–C9–N3–C2 and C7–C8–N1–C10 (Table 3, the atom
numbering corresponds to that given in Fig. 1 for compound 2;
the conformation shown in Scheme 2 corresponds approxi-
mately to that of 1d). Conformer 1d, where the reacting
centres, N1 and C2, are in an arrangement particularly
favourable for cyclisation, is the lowest one among 1a–1d.
The reaction commences by attack of the nitrogen lone pair
lp(N1) at C2 with a concomitant 908 rotation (breaking) of the
C2aN3 double bond. In the cyclisation transition state TS1 the
phenyl ring on C2 is almost perpendicular to the plane formed
by N1–C2–N3, t(C9–N3–C2–C21)Z948 (Table 3). The newly
forming N1–C2 single bond is already quite short, r(N1–C2)Z
˚
˚
1.823 A, compared to 2.863, 1.532, 1.503 and 1.397 A in 1d, 3,
TS2, and 2, respectively. Calculated structures of TS1 and TS2
are shown in Fig. 3.
The NBO analysis indicates the presence of a s-type N1–C2
single bond with an occupancy of 1.756; that of the
corresponding antibonding s* is 0.268. The electron pair of
the C2aN3 double bond in 1d becomes a partial N3aC9
p-bond (occupancyZ1.869; that of the corresponding anti-
bonding p* orbital is 0.495) in TS1 and the p-type lone pair
lp(N3) in the cyclic intermediate 3. The barrier for this first step
is ca. 30 kcal molK1, substantially higher than those normally
found in pseudopericyclic reactions [29–32]. Although
formally there is one orbital disconnection, i.e. a switch from
Table 2
Selected geometric parameters of 2 (bond lengths in A, bond angles and dihedrals in 8) obtained by X-ray crystallography and B3LYP/6-31G(d) calculations
˚
X-ray
1.380(3)
B3LYP
X-ray
B3LYP
N1–C2
1.397
1.388
1.457
1.320
1.468
1.382
1.414
1.521
1.391
1.453
1.453
1.386
1.453
1.452
106.2
128.9
123.7
112.6
122.9
124.6
105.7
N1–C8–C7
131.6(3)
105.4(2)
123.0(3)
110.5(2)
129.7(3)
119.7(3)
114.1(2)
119.3(3)
118.1(3)
117.5(2)
121.0(2)
121.5(2)
116.9(2)
99.5(3)
132.1
105.4
122.6
110.2
130.0
119.8
115.0
119.2
119.2
118.1
119.6
119.5
118.7
112.8
K18.5
K10.8
11.6
N1–C8
1.386(3)
1.457(3)
1.331(3)
1.473(3)
1.393(3)
1.398(4)
1.524(4)
1.390(3)
1.445(4)
1.452(4)
1.379(3)
1.440(4)
1.443(3)
N1–C8–C9
N1–C10
C7–C8–C9
C2–N3
N3–C9–C8
C2–C21
N3–C9–C4
N3–C9
C8–C9–C4
C8–C9
N1–C10–C11
C10–C11
C14–N14
N14–C17
N14–C18
C24–N24
N24–C27
N24–C28
C2–N1–C8
C2–N1–C10
C8–N1–C10
N3–C2–N1
N3–C2–C21
N1–C2–C21
C2–N3–C9
C14–N14–C17
C14–N14–C18
C17–N14–C18
C24–N24–C27
C24–N24–C28
C27–N24–C28
C2–N1–C10–C11
N1–C10–C11–C12
C13–C14–N14–C17
C15–C14–N14–C18
N3–C2–C21–C22
C23–C24–N24–C27
C25–C24–N24–C28
106.9(2)
K9.2(4)
K14.2(4)
12.6(4)
130.1(2)
122.9(2)
112.5(2)
121.7(2)
125.7(2)
104.7(2)
K19.5(4)
7.6(4)
K30.7
K8.3
9.1
18.8(4)

248
I. Sheikhshoaie et al. / Journal of Molecular Structure 794 (2006) 244–250
N(CH3)2
(CH3)2N
10
7
N(CH3)2
8
9
N
1
TS1
6
5
TS2
2
+
-
N
N
N
2
4
3
H
N(CH3)2
1
3
Scheme 2. Reaction mechanism for the rearrangement 1/2.
the p-system comprising C2–N3–C9–C8 to the orthogonal
s-type lone pair on N1-the prerequisite for a pseudopericyclic
reaction to occur-formation of the N1–C2 single bond requires
rotation of the C2aN3 double bond with a concomitant cost in
energy. Furthermore, formation of the intermediate 3 is
strongly endothermic/endergonic (DGreactZC30 kcal mol–1)
and 3 is barely stable with a barrier for ring opening of DGsZ
2 kcal mol–1. Thus, the reverse reaction, ring opening of the
intermediate 3, meets the criteria for low-barrier pseudoper-
icyclic reactions [31,32]. Surprisingly, despite the formal
zwitterionic character of 3, polar solvents (EtOH) do not
appreciably stabilise this structure (Table 3). Although,
according to the natural population analysis (NPA) there is a
decrease and increase of negative charge on N1 and N3,
respectively, in 3 [K0.3082 (N1), K0.5417 (N3)] as compared
to 1 [K0.4601 (N1), K0.4663 (N3)], actually the charge
separation is considerably smaller than suggested by the
zwitterionic formula of Scheme 2. The N1–C10 bond is quite
(1,3)-Sigmatropic rearrangements would involve antarafa-
cial movements of the hydrogen atom and are seldom observed
in thermal reactions [34]. However, as described above, the
double bond character of the N1–C10 bond in 3 is very small
and, thus, the usual frontier orbital (orbital symmetry) rules
will not apply. Viewing 3 as an immonium ion, C10 is expected
to have partial carbenium ion character [q(C10)NPAZ0.05],
and all pericyclic reactions are allowed. Just counting
electrons, yields six electrons involved in the rearrangement;
thus, it is suprafacially allowed. Furthermore, transition state
TS2 has some unusual features: (i) the moving hydrogen atom
is still quite strongly bound to C2, bond orderZ0.640, r(C2–
˚
H)Z1.238 A and an occupancy of 1.744 of the C2–H NBO. (ii)
Despite the planar arrangement of the N1 substituents in both
intermediate 3 and product 2, in TS2 N1 is strongly
pyramidalised (sum of anglesZ3168) with the aryl rings at
N1 and C2 pointing in opposite directions (t(C10–N1–C2–
C21)ZK103.38). As a consequence of this distortion, the H
atom approaches C10 suprafacially in contrast what is expected
for a formal 1,3 H-shift. In fact, the four atoms involved in this
rearrangement, H–C2–N1–C10, lie in a common plane forming
a distorted rectangle (Fig. 3). Thus, TS2 shows some
˚
˚
short, r(N1–C10)Z1.330 A, compared to 1.285 A in 1d and
˚
1.457 A in 2, but the NBO analysis does not indicate a
substantial double bond character. Yet, nitrogen N1 is trigonal
and the pendant moiety, 4-(CH3)2N–C6H4–CH, is planar
[t(C7–C8–N1–C10)ZK5.88,
t(C8–N1–C10–C11)Z
K171.98]. Apparently, 3 is characterized by a rather
delocalised, polymethine-like electronic structure of the N3–
C6H4–N1–C10H–C6H4–N(CH3)2 moiety without significant
charge separation. Rearrangement of the intermediate 3 to the
final product 2 via a formal 1,3-H shift is found to be the rate
determining step and has a quite substantial barrier (TS2,
Table 3), in line with the reaction conditions, i.e. refluxing
EtOH. It is, however, strongly exothermic/exergonic (DGreact
ZK16 kcal mol–1 in EtOH). Thus, although ring-opening of 3
back to the starting material has an almost negligible Gibbs free
activation energy (DGsZ3 kcal mol–1 in EtOH), the title
compound 1-(4-dimethylaminobenzyl)-2-(4-dimethylamino-
phenyl)-benzimidazole 2 is the product formed under
thermodynamic control. As a consequence, one would expect
formation of the bis-Schiff base under milder reaction
conditions. For instance, formation of 4,5-dichloro-N,N0-bis-
(4-diethylamino-benzylidene)-benzene-1,2-diamine has been
observed in the room temperature reaction of 4,5-dichloro-
benzene-1,2-diamine with 4-(diethylamino)salicylaldehyde
[33].
Table 3
B3LYP/6-31G(d) calculated relative Gibbs’ free energies (kcal mol–1) in the
gas phase and ethanol solution and selected geometrical parameters for
reactants 1a–1d, transition states TS1 and TS2, intermediate 3 and product 2 of
the Schiff base/benzimidazole rearrangement
DG (gas phase)
DG (EtOH)
1a
1b
1c
0.0
1.7
0.0
3.2
44.1a
K129.0a
K131.7a
146.4a
K44.4b
K129.0b
K38.5b
K38.2b
0.5
1.6
1d
TS1
3
K1.1
31.6
0.0
31.2
28.6
66.3
K15.7
298c
1.823d, 94.1e
29.6
TS2
2
67.7
900c
1.238f, 1.705g
K16.9
a
t(C4–C9–N3–C2).
b
c
d
e
f
t(C7–C8–N1–C10).
Imaginary frequency (cmK1).
r(N1–C2).
t(C9–N3–C2–C21).
r(C2–H).
g
r(C10–H).

I. Sheikhshoaie et al. / Journal of Molecular Structure 794 (2006) 244–250
249
˚
Fig. 3. Calculated [B3LYP/6-31G(d)] structures and pertinent structural data (distances in A, angles in 8) of transition states TS1 and TS2.

250
I. Sheikhshoaie et al. / Journal of Molecular Structure 794 (2006) 244–250
[4] C.-M. Che, J.-S. Huang, Coord. Chem. Rev. 242 (2003) 97.
[5] E. Tsuchida, K. Oyaizu, Coord. Chem. Rev. 237 (2003) 213.
¨
[6] K. Binnemans, C. Gorller-Walrand, Chem. Rev. 102 (2002) 2303.
resemblance to the pseudopericyclic 1,3-shifts observed in
a-oxo-ketenes and related heterocumulenes (lone-pair-LUMO-
mediated pericyclic reactions [35,36]). (iii) The rather high
barrier calculated for the rearrangement 3/2 can be attributed
to the substantial geometrical reordering accompanying this
transformation. (iv) The moving hydrogen atom carries a
positive charge (C0.248) which is comparable to that in the
intermediate 3 (C0.250) and TS1 (C0.232). (v) The carbon
atom C10, which becomes sp3-hybridised in the product is still
trigonal in TS2 (sum of anglesZ3588). (vi) Although there is
some interaction between C10 and H15 via lp(C10), or, more
precisely, the bonding and antibonding linear combinations of
lp(N2) and lp(C10) with s(C2–H) and s*(C2–H),
respectively, the C10–H bond is still quite weak, bond
[7] S.K. Menon, S.K. Jogani, Y.K. Agrawal, Rev. Anal. Chem. 19
(2000) 361.
[8] M.H. Mashhadizadeh, I. Sheikhshoaie, S. Saeid-Nia, Sens. Actuators,
B94 (2003) 241.
[9] I. Sheikhshoaie, J. Coord. Chem. 56 (2003) 463.
[10] R.M. Mahmoud, A.M. El-Nady, F.A. Adam, M.A. El-Taher, Bull. Fac.
Sci., Assiut Univ. 19 (1990) 35.
[11] M.R. Mahmoud, A.M. El-Nady, F.A. Adam, M.A. El-Taher, J. Chin.
Chem. Soc. 37 (1990) 479.
[12] F.A. Adam, M.A. El-Taher, M.R. Mahmoud, Chem. Scr. 29 (1989) 161.
[13] A.A. El-Samahy, A.M. Abdel-Mawgoud, E.A. Abou Gharib, N.M. Ismail,
Bull. Soc. Chim. Fr. (1984) 175.
[14] J. Kolsek, N. Novak, M. Perpar, Z. Anal. Chem. 159 (1957) 113.
[15] N.V.S. Rao, C.V. Ratman, Proc. Indian Acad. Sci. 43A (1956) 173.
[16] C.V.C. Rao, V. Veeranagaiah, K.K. Reddy, N.V.S. Rao, Indian J. Chem.
17B (1979) 566.
˚
order!0.2, r(C10–H)Z1.705 A.
[17] S. Perumal, S. Mariappan, S. Selvaraj, ARKIVOC (2004) 46.
[18] B.H. Kim, R. Han, J.S. Kim, Y.M. Jun, W. Baik, B.M. Lee, Heterocycles
63 (2004) 41.
4. Conclusion
X-ray crystallography shows that the compound obtained in
the reaction of o-phenylenediamine with 4-dimethylamino-
benzaldehyde in refluxing EtOH is 1-(4-dimethylaminoben-
zyl)-2-(4-dimethylaminophenyl)-benzimidazole 2 rather than
N,N0-bis(4-dimethylaminobezylidene)-benzene-1,2-diamine 1.
The benzyl group is almost perpendicular oriented with respect
to the heterocyclic ring system. In contrast, the 2-aryl moiety is
nearly coplanar. 1H and 13C NMR spectra have been
completely assigned by the heteronuclear multiple-bond
connectivity (HMBC) procedure [20]. The melting point of
this compound is mpZ1828, much closer to that reported in
[18], mpZ1688, than those given in other previous reports
(252–2558 [15,17]). Thus, the present results allow to establish
the true identity of the compounds obtained by reaction of
o-phenylenediamine with 4-dimethylaminobenzaldehyde.
Density functional theory calculations [B3LYP/6-31G(d)]
indicate facile cyclisation of the initially formed Schiff base
1 to a tetrahedral intermediate 3. Rearrangement of this
intermediate has a rather high barrier, the product of this
rearrangement, 1-(4-dimethylaminobenzyl)-2-(4-dimethylami-
nophenyl)-benzimidazole 2, however, is considerably more
stable than 1. Despite the formal zwitterionic character of 3,
polar solvents are calculated (polarizable continuum model) to
have only a marginal stabilising effect. Analysis of the
electronic structures of the various minima and transition
states involved in the reaction 1/2 with the aid of the NBO
method helps to rationalise the unusual features of this process,
especially those of TS2. Here, in contrast to what one would
expect for a formal (1,3)-sigmatropic rearrangement, move-
ment of the hydrogen occurs in a suprafacial manner.
[19] H.B. Mallesh, G. Krishnamurthy, N. Shashikala, Asian J. Chem. 16
(2004) 1439.
[20] A. Bax, M.F. Summers, J. Am. Chem. Soc. 108 (1986) 2093.
[21] G.M. Sheldrick, SHELXS–97, Program for the Solution of Crystal
¨
Structures, University of Gottingen, Germany 1997.
[22] G.M. Sheldrick, SHELXL–97, Program for the Refinement of Crystal
¨
Structures, University of Gottingen, Germany 1997.
[23] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
[24] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.
[25] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R.
Cheeseman, J.A. Montgomery, Jr., T. Vreven, K.N. Kudin, J.C. Burant,
J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi,
G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara,
K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O.
Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross,
V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O.
Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala,
K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G.
Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K.
Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q.
Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A.
Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith,
M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W.
Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople,
GAUSSIAN 03, Revision B.04, Gaussian Inc., Wallingford CT, 2004.
[26] J. Tomasi, B. Mennucci, E. Cances, J. Mol. Struct. (Theochem) 464
(1999) 211.
[27] E.D. Glendening, A.E. Reed, J.E. Carpenter, F. Weinhold, NBO Version
3.1 1988.
[28] C.K. Johnson, ORTEP, Report ORNL-3794, Oak Ridge National
Laboratory, Tennessee, USA 1965.
[29] J.A. Ross, R.P. Seiders, D.M. Lemal, J. Am. Chem. Soc. 98 (1976) 4325.
[30] L.A. Burke, J. Elguero, G. Leroy, M. Sana, J. Am. Chem. Soc. 98 (1976)
1685.
[31] D.M. Birney, S. Ham, G.R. Unruh, J. Am. Chem. Soc. 119 (1997) 4509.
[32] D.M. Birney, P.E. Wagenseller, J. Am. Chem. Soc. 116 (1994) 6262.
[33] S. Di Bella, I. Fragala, I. Ledoux, J. Zyss, Chem. Eur. J. 7 (2001) 3738.
[34] I. Fleming, Frontier Orbitals and Organic Chemical Reactions, Wiley,
Chichester, 1978, p. 99.
References
[1] D.A. Atwood, M.J. Harvey, Chem. Rev. 101 (2001) 37.
[2] N. Hoshino, Coord. Chem. Rev. 174 (1998) 77.
[3] P.G. Cozzi, Chem. Soc. Rev. 33 (2004) 410.
[35] R. Koch, M.W. Wong, C. Wentrup, J. Org. Chem. 61 (1996) 6809.
[36] L. George, K.-P. Netsch, G. Penn, G. Kollenz, C. Wentrup, Org. Biomol.
Chem. 4 (2006) 558.