Formation of 2,4,5-triaryl-4,5-dihydro-1H-imidazoles, (1), from aryl aldehydes. Crystal structures of cis-(1: aryl = pyridin-2-yl), {trans-[(1: aryl = pyridin-2-yl)H]+[OAc]-·3H2O}, {cis-[1: aryl = thien-2-yl]·0.5H2O} and trans-(1: aryl = thien-2-yl)

Article abstract of DOI:10.1016/j.molstruc.2006.11.010

The preparations of cis-2,4,5-triaryl-4,5-dihydro-1H-imidazoles, (aryl = thien-2-yl or pyridin-2-yl) from aryl aldehydes, ammonium chloride and triethylamine in methanol, and their conversions to the trans-isomers are reported. Crystal structures of cis-(1: aryl = pyridin-2-yl), cis-[1: aryl = thien-2-yl·0.5H₂O], trans-(1: aryl = thien-2-yl), and trans-{[(1: aryl = pyridin-2-yl)H]⁺[OAc]^-·3H₂O}, have been determined and compared with related structures.

Full text of DOI:10.1016/j.molstruc.2006.11.010

Journal of Molecular Structure 837 (2007) 274–283
www.elsevier.com/locate/molstruc
Formation of 2,4,5-triaryl-4,5-dihydro-1H-imidazoles, (1),
from aryl aldehydes. Crystal structures of cis-(1: arylDpyridin-2-yl),
{trans-[(1: arylDpyridin-2-yl)H]+[OAc]¡·3H2O}, {cis-[1: arylDthien-2-
yl]·0.5H2O} and trans-(1: arylDthien-2-yl)
Christiane Fernandes a, Adolfo Horn Jr. a, R. Alan Howie b, Jan Schripsema a,
Janet M.S. Skakle b, James L. Wardell c,¤
a Laboratório de Ciências Químicas, Universidade Estadual do Norte Fluminense-UENF, 28013-602, Campos dos Goytacazes, RJ, Brazil
b Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen, AB24 3UE, Scotland, UK
c Departamento de Química Inorgânica, Instituto de Química, Universidade Federal do Rio de Janeiro, 21945-970, Rio de Janeiro, RJ, Brazil
Received 5 October 2006; received in revised form 1 November 2006; accepted 1 November 2006
Available online 14 December 2006
Abstract
The preparations of cis-2,4,5-triaryl-4,5-dihydro-1H-imidazoles, (aryl Dthien-2-yl or pyridin-2-yl) from aryl aldehydes, ammonium
chloride and triethylamine in methanol, and their conversions to the trans-isomers are reported. Crystal structures of cis-(1:
aryl Dpyridin-2-yl), cis-[1: aryl Dthien-2-yl·0.5H2O], trans-(1: aryl Dthien-2-yl), and trans-{[(1: aryl Dpyridin-2-yl)H]+[OAc]¡·3H2O},
have been determined and compared with related structures.
© 2006 Elsevier B.V. All rights reserved.
Keywords: 2,4,5-Triaryl-4,5-dihydro-1H-imidazoles; Crystal structures; cis–trans-isomerisation
1. Introduction
imidazole, cis-(1: Ar DPh), (amarine) [2], see Scheme 1.
Other researchers, in the 19th century, found that cis-(1:
Ar DPh) rearranges under strong basic conditions, e.g.,
using KOBut/HOBut, to the isomeric isoamarine, trans-
2,4,5-triphenyl-4,5-dihydro-1H-imidazole trans-(1: Ar DPh)
[3]. ConWrmations of these now accepted structures came
somewhat later in 1900 [4,5]. During the early period, vari-
ous other authors, including the composer-chemist Boro-
din, showed that these reactions occur for various aryl
aldehydes.
The mechanism of the formation of 1 from starting
reagents, NH3 and ArCHO, was fully established by
Hunter and Sim in 1972 [6]. Reaction of NH3 and PhCHO
gives initially 1,3,5-triphenyl-2,4-diaza-pentadiene, (2:
Ar DPh), which rearranges in the presence of a base, under
kinetic control, mainly to cis-(1: ArDPh) [ca. 96%], but also to
a little trans-(1: ArDPh). Under more strongly basic condi-
tions, cis-(1: ArDPh) rearranges to the trans-(1: ArDPh)
The reaction of an aryl aldehyde with a source of ammo-
nia, which provides 2,4,5-triaryl-4,5-dihydro-1H-imidaz-
oles, 1, has been known for many decades. The history of
the reaction provides a fascinating example of how knowl-
edge and conWrmation of the structure of an organic com-
pound slowly developed over time in the 19th and 20th
centuries.
It was noted as early as 1837 that PhCHO reacted with
NH3 to give a compound now known to be 1,3,5-triphenyl-
2,4-diaza-pentadiene (2: Ar DPh) [1]. A little later, in 1844,
it was reported that heating (2: ArDPh) resulted in cyclisa-
tion and formation of cis-2,4,5-triphenyl-4,5-dihydro-1H-
*
Corresponding author. Tel.: +55 21 2552 0435; fax: +55 21 2552 0435.
E-mail address: j.wardell@abdn.ac.uk (J.L. Wardell).
0022-2860/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.11.010

C. Fernandes et al. / Journal of Molecular Structure 837 (2007) 274–283
275
Ar Dthien-2-yl·0.5H2O] and trans-(1: Ar Dthien-2-yl).
Comparisons are made with related structures, reported
earlier.
2. Experimental
2.1. General
Ir spectra were recorded on a Nicolet FT-IR 760 instru-
ment either in KBr disks or as Wlms. NMR spectra were
recorded on a Jeol Eclipse instrument. C, H, N elemental
analyses were carried out on a Perkin-Elmer 2400 analyser.
Tlcs were carried out using Merck-Kieselgel 60 F254 and
were visualized using iodine. Column chromatography used
Merck-Kieselgel silica gel.
Scheme 1.
2.2. Synthesis of 1
[thermodynamic control]. The pathway of the formation of
cis-1 from 2 proceeds via a “W”-shaped carbanion, derived
from 1,3,5-aryl-2,4-diaza-pentadiene. From orbital symme-
try considerations, a concerted cyclisation of this carbanion
will occur in a disrotatory manner to give the cis-product,
as found [6].
2.2.1. cis-2,4,5-Tri(thien-2-yl)-4,5-dihydro-1H-imidazole,
cis-(1: ArDthien-2-yl)
A solution of thiene-2-carboxaldehyde (3ml, 0.03 mol),
ammonium chloride (3g, 0.06mol) and triethylamine
(8.1ml, 0.06 mol) in MeOH (30 ml) was reXuxed for 26 h.
The solvent was removed in vacuo, the residue was
extracted into CHCl3 and washed successively with brine
and a little isopropanol. The pale yellow solid was recrystal-
lised from EtOH.
Reactive aldehydes, e.g., furfuryl aldehyde and p-chloro-
benzaldehyde, react with liquid ammonia to give com-
pounds cis-1 directly [7]. For less reactive aldehydes, as well
as addition of a strong base such as PhLi [8,9], KNH2 [8] or
hexamethyldisilazane [10], controlled heating of the inter-
mediate 1,3,5-triarylphenyl-2,4-diaza-pentadiene 2 [9,11,12]
has been reported to be necessary to convert 2 to 1. One-pot
syntheses of 1 include the microwave radiation of solvent-
free mixtures of an aldehyde, NH4OAc and alumina [13].
Conversions of cis-1, to trans-1, have been achieved either
using stronger bases, at higher temperatures or longer heating
times than required initially to produce the cis compounds.
DiVerent carbonyl-containing reagents or analogues
have also been used for the formation of 1. Among these
reactions are those between RCO–COR, ArCHO and
NH4OAc in AcOH, [14–16], and between ArCHBNOH
and NaNH2 in reXuxing xylene [17], as well as reduction of
ArCN trimers with Zn/AcOH [18].
A few crystal structures of 1 have been reported. These
include cis-[(1: Ar Dphenyl)·H]+·Cl¡ [11], cis-[(1: ArD4-
tolyl)·H]+·Br¡ [19], cis-(1: Ar Dpyridin-2-yl) [9] and trans-
(1: ArDpyridin-2-yl) [9]. While structural data are
available for the Wrst two compounds, this is not, unfortu-
nately, the case for the other two for which only drawings
of the molecules are accessible.
We were especially interested in preparing heteroaro-
matic derivatives of 1, in particular the pyridin-2-yl and
thien-2-yl derivatives, both for use as ligands in coordina-
tion chemistry and for our studies in supermolecular
arrangements. An earlier report had indicated the use of
2,4,5-tri(pyridin-2yl)imidazoline, as ligands for Ni(II),
Cu(II), and Zn(II) [20].
Anal. Calc. for C15H15N2S3. C, 56.93; H, 3.82; N, 8.84.
Found: C, 56.3; H, 3.94; N, 8.42%. H NMR (CDCl3,
400 MHz): ꢀ: 7.50 (d, J D3.6Hz, 1H), 7.49 (d,
J D3.6 Hz,1H), 7.12 (dd, J D3.6,5.1 Hz, 1H), 7.06 (dd,
J D1.0 Hz, 5.0Hz, 2H), 6.80 (dd, J D3.5Hz, 5.0 Hz, 2H), 6.74
(s.br, 2H), 5.7 (s.br, 1H), 5.5 (s.br, 2H).
1
13C NMR (CDCl3, 100 MHz): ꢀ: 159.4, 142.0, 132.9,
129.5, 128.2, 127.5, 126.3, 125.3, 124.8, 99.7, 62.8.
Further recrystallisations from ethanol were required to
give suitable crystals for crystallography. The crystals used
in the structure determination were found to be the hemi-
hydrate.
2.2.2. trans-2,4,5-Tri(thien-2-yl)-4,5-dihydro-1H-imidazole,
trans-(1: ArDthien-2-yl)
cis-2,4,5-Tri(thien-2-yl)-4,5-dihydro-IH-imidazole,(20 mg)
was dissolved in a solution of ButOK (30 mg in ButOH
(5 ml). After stirring the reaction solution for 2 h at room
temperature, water (1 ml) and CH2Cl2 (1 ml) were added,
the organic layer was collected, dried over MgSO4and
rotary evaporated. The residue was dissolved in EtOH
and left at room temperature. Crystals of anhydrous
trans-2,4,5-tri(thien-2-yl)-4,5-dihydro-IH-imidazole, trans-
(1:Ar Dthien-2-yl) slowly were deposited. m.p. 130–
133 °C.
1H NMR (CDCl3, 200 MHz) for trans-(1: Ar Dthien-2-
yl): ꢀ: 8.61(m, 3H), 8.31 (d, J D7.9 Hz), 7.79 (dt, J D1.6,
7.7Hz, 1H), 7.68 (dt, J D1.8Hz, 6.5 Hz, 2H), 7.49 (d,
J D7.7 Hz, 2H), 7.38 (m, 1H), 7.22 (m, 2H), 6.80 (s.br, 1H),
5.54 (s, 2H) [9] 13C NMR (CDCl3, 50 MHz) trans-
We now wish to report the structures of cis-(1: ArDpyridin-
2-yl), trans-{[(1: ArDpyridin-2-yl)H]+[OAc] ¡·3H2O}, cis-[1:

276
C. Fernandes et al. / Journal of Molecular Structure 837 (2007) 274–283
(1: Ar Dthien-2-yl): ꢀ: 163.2, 161.7, 149.6, 148.8, 136.6,
2.2.4. trans-2,4,5-Tri(pyridin-2-yl)-4,5-dihydro-1H-
imidazole, trans-(1: Ar Dpyridin-2-yl)
125.5, 123.1, 122.4, 121.5, 73.5 [9].
This was obtained similarly to trans-2,4,5-tri(thien-2-yl)-
4,5-dihydro-1H-imidazole, (1: Ar Dthien-2-yl), but at the
1 mmol scale, m.p. 130–132 °C; lit m.p. 129–131 °C [9].
2.2.3. cis-2,4,5-Tri(pyridin-2-yl)-4,5-dihydro-1H-imidazole,
cis-(1: Ar Dpyridin-2-yl)
A
solution of pyridine-2-carboxaldehyde (10ml,
0.093 mol), ammonium chloride (5.0 g, 0.093 mol) and tri-
ethylamine (13ml, 0.093mol) in MeOH (30ml) was stirred
at room temperature for 24h. The solvent was removed in
vacuo, extracted into CHCl3, washed with water, dried
over MgSO4 and rotary evaporated. The residue was
recrystallised from EtOH, m.p 145°C; lit m.p. 141–143 °C
for cis-2,4,5-tri(pyridin-2-yl)-4,5-dihydro-1H-imidazole [9].
Spectral data agreed with those reported by Larter et al. [9].
Suitable crystals of anhydrous cis-2,4,5-tri(pyridin-2-yl)-
4,5-dihydro-1H-imidazole for X-ray study were grown
from MeCN solution.
2.2.5. trans-2,4,5-Tri(pyridin-2-yl)-4,5-dihydro-1H,3H-
imidazolium acetate.trihydrate, [trans-(1: ArDpyridin-2-
yl)H]+[OAc]¡·3H2O
cis-2,4,5-Tri(pyridin-2-yl)-4,5-dihydro-1H-imidazole, cis-
(1: Ar Dpyridin-2-yl) (0.20 g), was added to ethyl acetate
(15ml) and the solution heated to cause dissolution. On
leaving the solution at room temperature, pale yellow crys-
tals of rac-trans-2,4,5-tri(pyridin-2-yl)-4,5-dihydro-1H,3H-
imidazolium acetate·trihydrate were slowly deposited.
1H NMR (DMSO-d6, 400 MHz) for trans-[(1:
aryl Dpyridin -2-yl)H·AcOH·3H2O]: ꢀ: 8.68 (d, J D1.6Hz,
Table 1
Crystal data and structure reWnementa
Compound
Empirical formula
cis-(1: R DPy)
trans-(1: R DPy)H
cis-(1: R DTp)
trans-(1: R DTp)
trans-(1: R DTp)
C18H15N5
(C18H16N5)+(C2H3O2) 2(C15H12N2S3)·H2O
¡·3H2O
C15H12N2S3
C15H12N2S3
Temperature (K)
Formula weight
Crystal system, space
group
120(2)
301.35
Monoclinic, P21/c
120(2)
415.45
Monoclinic, P21/n
120(2)
650.91
Triclinic, P-1
120(2)
316.45
Orthorhombic, Pbca
291(2)
316.45
Orthorhombic, Pbca
Unit cell dimensions
a (Å)
b (Å)
c (Å)
ꢁ (°)
8.9544(3)
15.1425(5)
11.4455(3)
90
101.594(2)
90
1520.25(8)
4, 1.317
0.083
15.1297(5)
9.2230(1)
15.1308(4)
90
93.9585(10)
90
2106.33(9)
4, 1.310
0.096
9.9207(3)
12.3200(3)
14.0979(4)
104.0955(16)
110.4382(12)
95.5952(17)
1534.09(7)
2, 1.409
9.6867(2)
29.4008(6)
31.5991(7)
90
90
90
8999.3(3)
24, 1.401
0.484
9.8305(5)
9.9464(5)
31.9543(16)
90
90
90
3124.4(3)
8, 1.345
0.465
ꢂ (°)
ꢃ (°)
Volume (Å3)
Z, Dcalc (Mg/m3)
Absorption coeYcient
0.478
(mm¡1
F(000)
)
632
880
676
3936
1312
Crystal size (mm)
ꢄ range for data
collection (°)
0.36 £0.18 £0.05
0.50 £0.30 £0.14
0.30 £0.20 £0.10
0.40 £0.20 £0.10
0.30 £0.25 £0.20
2.97–27.69
2.96–27.47
3.10–27.56
2.93–27.51
2.43–23.24
Index ranges
¡11 6 h 6 11
¡19 6 k 6 19
¡14 6 l 6 14
19901/3493
¡19 6 h 6 19
¡10 6 k 6 11
¡19 6 l 6 19
23623/4767
¡12 6 h 6 12
¡15 6 k 6 16
¡18 6 l 6 17
28688/7007
¡12 6 h 6 12
¡38 6 k 6 38
¡40 6 l 6 40
65725/10255
[R(int) D0.0605]
6505
¡10 6 h 6 10
¡10 6 k 6 11
¡29 6 l 6 35
ReXections collected/
unique
ReXections observed
[I > 2sigma(I)]
13916/2238 [R(int) D0.0327]
[R(int) D0.0541]
[R(int) D0.0447]
[R(int) D0.0642]
2698
3631
5303
1582
Max. and min.
transmission
1.0000 and 0.7892
3493/0/268
1.000 and 0.8947
4767/0/305
0.9867 and 0.9323
7007/280/497
1.0000 and 0.6641
10255/36/590
0.9280 and 0.7351
2238/30/218
Data/restraints/
parameters
Goodness-of-Wt on F2
Final R indices
[I > 2sigma(I)]
1.089
1.022
1.011
1.062
1.043
R1 D0.0470
wR2 D0.1067
R1 D0.0664
wR2 D0.1148
R1 D0.0413
wR2 D0.1031
R1 D0.0619
wR2 D0.1135
0.0122(15)
R1 D0.0433
wR2 D0.1082
R1 D0.0646
wR2 D0.1188
R1 D0.0561
wR2 D0.1305
R1 D0.1078
wR2 D0.1535
R1 D0.0619 wR2 D0.1767
R indices (all data)
R1 D0.0839 wR2 D0.1951
Extinction coeYcient
Largest diV. peak and
hole (e/Å3)
0.214 and ¡0.231
0.296 and ¡0.232
0.604 and ¡0.613
0.513 and ¡0.498
0.341 and ¡0.402
a
In all cases the X-ray wavelength was 0.71073 Å, correction for absorption was semi-empirical from equivalent reXections and the reWnement method
was full-matrix least-squares on F2.

C. Fernandes et al. / Journal of Molecular Structure 837 (2007) 274–283
277
1H), 8.54 (m, 2H), 8.14 (d, J D7.6Hz), 7.95 (t, J D7.6Hz,
1H), 7.78 (t, 7.6Hz, 2H), 7.56 (m, 1H), 7.45 (d, J D7.6Hz,
2H), 7.30 (m, 2H), 5.25 (s, 2H), 3.4 (s.br, 1H). 13C NMR
(DMSO-d6, 100 MHz) trans-[(1: Ar Dpyridin -2-yl).
AcOH·3H2O]: ꢀ: 172.0, 162.4, 162.0, 149.0, 148.8, 148.2,
136.9, 136.8, 125.6, 122.5. 122.4, 121.5, 21.0.
COLLECT and DENZO [24] programs. Correction for
absorption, in all cases by comparison of the intensities of
equivalent reXections, was applied using the program SAD-
ABS version 2.10 [25] in the case of cis-(1: Ar Dpyridin-2-
yl), SADABS version 2.03 [26] in the case of trans-(1:
ArDthien-2-yl) at 291(2)K and the program SORTAV [27]
in the other cases. In the case of [trans-(1: ArDthien-2-
yl)H]+[OAc]¡·3H2O, correction for extinction was applied
in the standard SHELXL-97 [28] manner. The program
ORTEP-3 for Windows [29] has been used in the prepara-
tion of the Figures and programs SHELXL-97 and PLA-
TON [30] in the calculation of molecular geometry and
intermolecular contacts.
2.3. X-ray crystallography
2.3.1. Data collection
In a preliminary experiment intensity data were obtained
for trans-(1: Ar Dthien-2-yl) with Mo Kꢀ radiation at
291(2) K by means of a Bruker SMART area detector
diVractometer under the control of the programs SMART
[21] and, for data reduction and cell reWnement, SAINT
[22]. Thereafter intensity data for all four compounds were
obtained at 120(2)K with Mo Kꢀ radiation by means of the
Enraf-Nonius KappaCCD area detector diVractometer of
the EPSRC National crystallographic service at the Uni-
versity of Southampton. Data collection was carried out
under the control of the program COLLECT [23] and data
reduction and unit cell reWnement were achieved with the
2.3.2. Structure solution and reWnement
All structures were solved by direct methods using
SHELXS-97 [31] and completed and fully reWned by means
of the program SHELXL-97 [28]. In the Wnal stages of
reWnement hydrogen atoms were introduced into the struc-
tural models. Except for trans-(1: Ar Dthien-2-yl) at 291 K
when they were treated in the same manner as hydrogen
attached to carbon, hydrogen atoms attached to nitrogen
C56
a
C26
b
N51
N21
C43
N21
C26
N1
C52
N3
O4
C42
C22
C5
C4
C2
C22
N1
N41
C4
O1
C2
O3
C5
C42
N41
N3
C46
C60
O2
C52
O5
N51
C56
C61
C55
c
d
S51a
C54a
C53a
N1a
S51
C2a
C22a
C5a
C4a
N1
C52
C42a
C25a
C5
C4
N3a
C22
S21a
S41a
C45b
O1
S21b
C45a
C2
C44b
C25
S21
N3
N3b
S41b
C25b
C42
C4b
C5b
C2b
N1b
S51b
S41
C24b
C52b
C53b
C45
Fig. 1. (a) The molecule of cis-(1: Ar Dpyridin-2-yl). (b) The asymmetric unit in the structure of [trans-(1: Ar Dpyridin-2-yl)H]+[OAc]¡·3H2O. (c) The
asymmetric unit in the structure of cis-(1: Ar Dthien-2-yl)·0.5H2O. (d) Molecule A of trans-(1: Ar Dthien-2-yl). Ellipsoids are shown at the 50% probabil-
ity level. Hydrogen atoms, where shown, are drawn as small spheres of arbitrary radii. For clarity selected non-hydrogen atoms of aryl groups are labelled.

278
C. Fernandes et al. / Journal of Molecular Structure 837 (2007) 274–283
or oxygen atoms were placed initially in positions deter-
mined by peaks in diVerence maps and then their coordi-
nates and isotropic displacement parameters were reWned in
the usual manner. This same procedure was applied to the
hydrogen atoms attached to carbon in the case of cis-(1:
ArDpyridin-2-yl). Otherwise hydrogen atoms attached to
carbon were placed in calculated positions and reWned with
a riding model. The asymmetric unit of [trans-(1:
ArDpyridin-2-yl)H]+[OAc]¡·3H2O comprises, in addition
to the cationic imidazole, an acetate anion [C(60), C(61),
O(1) and O(2)] and three water molecules [O(3)–O(5)]. The
asymmetric unit of cis-(1: ArDthien-2-yl) contains, in addi-
tion to a molecule of water [O(1)], two imidazole molecules
which are distinguished by suYx A and B. The asymmetric
unit of trans-(1: Ar Dthien-2-yl) at 291(2)K is a single mol-
ecule but at 120(2)K comprises three imidazole molecules
distinguished by suYx A, B and C. Three thiophenyl groups
in cis-(1: ArDthien-2-yl) and at both 120(2)K and 291(2)K
one in trans-(1: Ar Dthien-2-yl) are disordered over two
positions related to one another by rotation through 180°
about the bond connecting them to the imidazole ring. In
such cases all of the ring atoms except that which bonds
directly to the imidazole ring occur in the structural models
in pairs as, for cis-(1: Ar Dthien-2-yl), S41A/S41C, S21B/
S21D and S41B/S41D with occupancies within each pair as
0.681(4)/0.329(4), 0.550(4)/0.450(4) and 0.670(4)/0.330(4),
respectively. In trans-(1: Ar Dthien-2-yl) at 120(2)K the
disordered pair is of the form S51C/S51D with occupancies
0.700(4)/0.300(4) while at 291(2) K it is of the form S51/
S51A with occupancies 0.647(11)/0.353(11). Successful
reWnement of the disordered rings required their bond
lengths and bond angles to be restrained to match those of
an ordered thienyl ring. The minor components of the dis-
order have been ignored in the preparation of the Figures
and Tables.
Crystal data and structure reWnement details are listed in
Table 1.
3. Results and discussion
3.1. Synthesis
Our synthesis involved reaction of an aryl aldehyde,
NH4Cl and NEt3 in methanol for ca. 24h. The combination
of NH4Cl and NEt3 provided both the ammonia for the
reaction and a suYciently strong basic media for the con-
version of the aldehyde to cis-(1: Ar D2-pyridinyl, 2-thienyl
or Ph) in a one-pot reaction. Isomerisation of cis-1 to trans-
1 were generally achieved by heating in ButOH solution in
the presence of KOBut, while, in addition, for trans-(1:
aryl D2-pyridinyl), isomerisation was achieved on heating
an ethyl acetate solution. The product isolated from the
Table 2
Selected geometric parameters (Å, °) for various 1
cis-(1:
R Dpy)
trans-(1:
R DPy)H
cis-(1: R DTp)
Mol. A Mol. B
trans-(1: R DTp) at 120(2) K
Mol. A Mol .B Mol.C
trans- (1:
cis- (1:
cis-(1:
R DTp) at RDPh)Ha,b R Dp-
291(2) K
tolyl) Ha,c
N(1)–C(2)
N(1)–C(5)
C(2)–N(3)
N(3)–C(4)
C(4)–C(5)
C(2)–C(22)
C(4)–C(42)
C(5)–C(52)
1.369(2)
1.3142(17)
1.4784(16)
1.3211(17)
1.4671(17)
1.5676(18)
1.4754(18)
1.5181(18)
1.5150(19)
1.376(3)
1.470(3)
1.288(3)
1.479(3)
1.569(3)
1.460(3)
1.498(3)
1.495(3)
1.369(3)
1.476(3)
1.299(3)
1.480(3)
1.569(3)
1.457(3)
1.500(3)
1.495(3)
1.359(4)
1.369(4)
1.368(4)
1.357(5)
1.443(5)
1.285(5)
1.479(5)
1.572(6)
1.466(6)
1.496(6)
1.488(6)
1.3135
1.4707
1.3168
1.4751
1.5638
1.4750
1.5110
1.5131
1.3575
1.4824
1.3308
1.4859
1.5782
1.4775
1.5154
1.5320
1.457(2)
1.287(2)
1.471(2)
1.584(2)
1.482(2)
1.501(2)
1.515(2)
1.463(4)
1.299(4)
1.477(4)
1.570(4)
1.456(4)
1.486(4)
1.494(4)
1.470(4)
1.292(4)
1.491(4)
1.554(4)
1.465(4)
1.493(4)
1.497(4)
1.458(4)
1.281(4)
1.482(4)
1.568(4)
1.452(4)
1.492(4)
1.499(4)
C(2)–N(1)–C(5) 107.98(13) 110.80(11)
N(1)–C(2)–N(3) 117.18(14) 112.88(12)
C(2)–N(3)–C(4) 106.25(13) 111.21(11)
N(3)–C(4)–C(5) 105.59(12) 102.00(10)
N(1)–C(5)–C(4) 100.10(12) 102.17(10)
105.68(17)
116.93(19)
106.64(17)
103.55(16)
100.88(16)
¡26.8(3)
105.70(17) 108.4(2)
116.84(19) 116.6(3)
106.74(17) 106.0(2)
103.08(16) 105.3(2)
101.16(16) 100.0(2)
107.0(2)
108.3(2) 109.1(3)
116.7(3) 116.6(4)
107.2(2) 106.4(3)
104.8(2) 104.9(3)
101.4(2) 100.5(3)
111.86
107.64
116.7(3)
105.9(2)
104.6(2)
100.6(2)
¡92.6(3)
111.21
110.88
100.97
100.43
¡23.49
108.82
114.48
101.27
102.69
¡26.19
C(42)–C(4)–
C(5)–C(52)
Q(2)d (Å)
16.77(18) 129.38(12)
¡30.0(3)
94.6(3)
¡102.8(3)
99.1(5)
0.166(2)
144.5(6)
0.095(1)
118.9(8)
0.246(2)
318.9(5)
Envelope
On C(5)
13.86(4)
74.9(3)
0.252(2)
314.7(5)
Twist on
C(4)–C(5)
11.7(9)
0.186(3)
313.7(9)
Twist on
C(4)–C(5)
4.8(2)
0.220(3)
136.9(8)
Envelope
on C(5)
14.6(2)
83.80(10)
73.91(10)
0.125(3)
0.154(4)
0.211
301.63
Twist on
0.223
300.53
Twist on
ꢅ(2)d (°)
138.6(14) 313.8(16)
Closest
Envelope
On C(5)
9.53(9)
66.40(4)
77.09(5)
Twist on
C(4)–C(5)
16.54(8)
82.49(4)
80.80(5)
Envelope
on C(5)
6.9(2)
Twist on
C(4)-C(5)
8.2(4)
descriptor
Dihedral(2)e (°)
Dihedral(4)e (°)
Dihedral(5)e (°)
a
C(4)–C(5) C(4)–C(5)
22.68
81.41
58.23
21.36
49.35
80.67
78.0(4)
77.45(8)
76.64(11)
84.35(11)
85.59(11) 87.1(2)
62.0(4) 84.8(7)
72.53(9)
Su’s are lacking in the cif format coordinate data extracted from the Cambridge Structural Database [33], through the agency of the EPSRC Chemical
Database Service at Daresbury [34], and also, therefore, in quantities derived from them.
b
Ref. [11].
Ref. [19].
c
d
Pucker parameters as deWned by Cremer and Pople [32].
Dihedral(n) is the angle between the least-squares planes of the imidazole nucleus and the aryl substituent at C(n).
e

C. Fernandes et al. / Journal of Molecular Structure 837 (2007) 274–283
279
latter was found to be a hydrated acetic acid salt. The for-
mation of the acetate salt clearly results from either acetic
acid contamination of the solvent or, less probably, from
hydrolysis of the ethyl acetate.
While solid 1 had localised structures, with distinct
NH and N sites in the imidazole ring (see below) NMR
spectra in solution indicated 2-fold symmetrical rings as
a result of delocalisation of the hydrogen over the two
ring nitrogen sites.
When NaBH4 was added as an additional reagent to the
thien-2-ylcarboxaldehyde, NH4Cl. NEt3, NaBH4 reaction
mixture, other products, in addition to cis-(1: Ar Dthien-2-
yl), were indicated. The major one of these was isolated by
chromatography and shown to be tris(thien-2-ylm-
ethyl)amine.
3.2. Crystal structures
To obtain suYciently good crystals for X-ray structure
determinations, samples were recrystallised from diVerent
solutions. Thus obtained and used in the X-ray analyses were
cis-(1: ArDpyridin-2-yl), from MeCN solution, cis-[1:
ArDthien-2-yl·0.5H2O] and trans-(1: ArDthien-2-yl) both
yl)H]+[OAc]¡·3H2O} which was isolated from EtOAc.
A limitation in this being a general reaction for arylalde-
hydes was provided by 2-hydroxybenzaldehyde, which gave
3 on reaction with ammonium chloride and triethylamine
in methanol solution.. This will be reported on elsewhere.
The atom arrangements and numbering schemes for
cis-(1: Ar Dpyridin-2-yl), [trans-(1: Ar Dpyridin-2-
yl)H]+[OAc]¡·3H2O, trans-(1: Ar Dthien-2-yl)·0.5H2O and
molecule A of trans-(1: Ar Dthien-2-yl), all at 120 K, are
shown in Fig. 1. The labelling of only selected atoms of
the aryl substituents is possible because their carbon
atoms are labelled in strict cyclic order. These examples
indicate just how similar, apart from the disposition of the
hydrogen atoms attached to C(4) and C(5), the cis and
trans molecules are. All of the imidazoles in the structures
whose reWnements are reported in this paper are asym-
metric and can exist as enantiomeric pairs. For the cis spe-
cies the enantiomers diVer with regard to which of the two
nitrogen atoms in the imidazole ring is protonated and
the 4R, 5S or 4S, 5R conWgurations at C(4) and C(5) are
irrelevant. For the trans species the converse is true. Now
it is the 4R, 5R or 4S, 5S conWgurations at C(4) and C(5)
from
EtOH
and
trans-{[(1:
ArDpyridin-2-
Table 3
Hydrogen-bond parameters (Å, °) for various 1
D–H
H,A
D,A
D–H,A
a. cis-(1: Ar Dpyridin-2-yl)
N(1)–H(1),N(41i)
C(4)–H(4),N(21ii)
0.90(2)
0.984(16)
2.37(2)
2.525(16)
3.1526(19)
3.331(2)
145.1(16)
139.0(12)
Symmetry codes (i) x, ¡y + 1/2, z ¡1/2; (ii) x, ¡y + 1/2; z + 1/2
b. [trans-(1: Ar Dpyridin-2-yl)H]+[OAc]¡·3H2O
N(1)–H(1),O(1)
0.894(19)
1.82(2)
1.84(2)
1.84(2)
1.85(3)
1.80(2)
2.09(3)
1.88(3)
1.84(3)
2.7140(15)
2.7111(16)
2.7548(17)
2.7666(15)
2.6928(16)
2.9832(17)
2.7684(16)
2.7060(17)
178.9(18)
166.0(17)
170(2)
169(2)
175(2)
172(2)
173(2)
176(2)
N(3)–H(3),O(2i)
O(3)–H(3A),O(4)
O(3)–H(3B),O(1)
O(4)–H(4A),O(5ii)
O(4)–H(4B),N(41iii)
O(5)–H(5A),O(2iv)
O(5)–H(5B),O(3)
0.886(19)
0.92(2)
0.93(3)
0.89(2)
0.90(3)
0.89(3)
0.87(3)
Symmetry codes (i) ¡x + 1/2, y ¡1/2, ¡z + 1/2; (ii) ¡x + 1/2, y + 1/2, ¡z ¡1/2; (iii) ¡x + 1/2, y + 1/2, ¡z + 1/2; (iv) ¡x + 1, ¡y + 2, ¡z
c. cis-(1: Ar Dthien-2-yl)·0.5H2O
O(1)–H(1W),N(3A)
O(1)–H(2W),(N3B)
N(1A)–H(1A),O(1i)
N(1B)–H(1B),O(1ii)
C(5A)–H(5A),S(21Ai)
0.80(3)
0.79(3)
0.88(3)
0.88(3)
1.00
2.00(3)
2.01(3)
2.06(3)
1.99(3)
2.87
2.791(2)
2.793(2)
2.892(2)
2.845(2)
3.852(2)
170(3)
171(3)
158(2)
163(3)
169
Symmetry codes (i) ¡x + 1, ¡y + 1, ¡z; (ii) ¡x + 1, ¡y, ¡z
d. trans-(1: Ar Dthien-2-yl) at 120(2) K
N(1A)–H(1A),N(3B)
0.906(10)
2.020(17)
1.989(13)
2.036(14)
2.859(3)
2.876(3)
2.913(3)
153(3)
166(3)
164(3)
N(1B)–H(1B),N(3Ci)
N(1C)–H(1C),N(3A)
0.907(10)
0.901(10)
Symmetry code (i) ¡x + 3/2, y ¡1/2, z
e. trans-(1: Ar Dthien-2-yl) at 291(2) K
N(1)–H1,N(3i)
0.90
2.07
2.918(5)
156
Symmetry code (i) ¡x + 1/2, y ¡1/2, z

280
C. Fernandes et al. / Journal of Molecular Structure 837 (2007) 274–283
which distinguish between the enantiomers and the pro-
tonation of the imidazole nitrogen atoms is now irrele-
vant. As a consequence the structures described in this
paper, all of which are centrosymmetric, must be racemic.
The known structures of cis-[(1: Ar Dphenyl)·H]+·Cl¡ [11]
and cis-[(1: Ar D4-tolyl)·H]+·Br¡ [19], which are also cen-
trosymmetric, are not racemic on this basis because, in
both cases, both nitrogen atoms of the imidazole nucleus
of the cation are protonated.
Selected geometric parameters for all of the structures
whose structure determinations are described in this paper,
and for two related structures, are given in Table 2. No
bond length or bond angle data is given for the aryl substit-
uents, which are not discussed in detail in this paper,
because they are not unusual in any way, are comparatively
poorly determined and, for some of the thienyl groups, sub-
ject to the eVects of severe disorder. For the neutral mole-
cules the N(1)–C(2) bond distances are in the range
Table 4
Parameters (Å, °) for selected ꢁ,ꢁ interactions in various 1
Cg(I),Cg(J)
Cg,Cg
ꢁ
ꢂ
ꢃ
CgIperp
3.387
CgJperp
3.465
a. cis-(1: Ar Dpyridin-2-yl)
Cg(3),Cg(4)
3.814
39.75
24.70
27.36
Ring(3) with Cg(3) deWned by N(41)/C(42)–C(46); ring(4) with Cg(4) deWned by N(51)/C(52)–C(56)
Intramolecular contact
b. [trans-(1: Ar Dpyridin-2-yl)H]+[OAc]¡·3H2O
Cg(2),Cg(3i)
3.822
15.81
17.50
10.74
3.755
3.743
3.645
3.743
Ring(2) with Cg(2) deWned by N(21)/C(22)–C(26); ring(3) with Cg(3) deWned by N(41)/C(42)–C(46)
Symmetry code (i) ¡x + 1/2, y ¡1/2, ¡z +1/2
c. cis-(1: Ar Dthien-2-yl)·0.5H2O
Cg(2),Cg(2iii)
3.999
0.0
20.61
20.61
Ring(2) deWned by S(21B)/C(22B)–C(25B)
Symmetry code (iii) ¡x, ¡y, ¡z
d. trans-(1: Ar Dthien-2-yl) at 120(2) K – no signiWcant ꢁ,ꢁ interaction observed
e. trans-(1: Ar Dthien-2-yl) at 291(2) K – no signiWcant ꢁ,ꢁ interaction observed
Note. Cg,Cg is the distance between ring centroids (Å), CgIperp is the perpendicular distance of Cg(I) from ring J (Å) and CgJperp the perpendicular dis-
tance of Cg(J) from ring I (Å). The angles in (°) are ꢁ – the dihedral angle between the planes, ꢂ – the angle between vectors Cg,Cg and CgIperp
,
and ꢃ – the angle between vectors Cg,Cg and CgJperp
.
Table 5
Parameters (Å, °) for selected C–H,ꢁ interactions in various 1
C–H,Cg
H,Cg
Hperp
ꢃa
C–H,Cg
C,Cg
a. cis-(1: Ar Dpyridin-2-yl)
C(24)–H(24),Cg(3iii)
C(46)–H(46),Cg(4ii)
3.046
3.297
2.736
2.953
26.10
26.38
140.00
131.26
3.869
4.029
Ring(3) with Cg(3) deWned by N(41)/C(42)–C(46); ring(4) with Cg(4) deWned by N(51)/C(52)–C(56)
Symmetry codes (ii) x, ¡y + 1/2; z + 1/2; (iii) ¡x, ¡y, ¡z + 2
b. [trans-(1: Ar Dpyridin-2-yl)H]+[OAc]¡·3H2O
C(24)–H(24),Cg(4v)
3.166
3.147
2.902
2.870
23.58
24.18
118.70
119.49
3.717
3.707
C(25)–H(25),Cg(4v)
Ring(4) with Cg(4) deWned by N(51)/C(52)–C(56)
Symmetry code (v) x ¡1/2, ¡y + 3/2, z ¡1/2
c. cis-(1: Ar Dthien-2-yl)·0.5H2O
C(5A)–H(5A),Cg(1i)
2.643
2.751
2.622
2.705
7.17
10.57
152.51
153.44
3.560
3.673
C(5B)–H(5B),Cg(2ii)
Ring(1) with Cg(1) deWned by S(21A)/C(22A)–C(25A); ring(2) with Cg(2) deWned by S(21B)/C(22B)–C(25B)
Symmetry codes (i) ¡x + 1, ¡y + 1, ¡z; (ii) ¡x + 1, ¡y, ¡z
d. trans-(1: Ar Dthien-2-yl) at 120(2) K
C(45A)–H(45A),Cg(1ii)
C(45B)–H(45B),Cg(2iii)
C(45C)–H(45C),Cg(3iii)
2.683
2.796
2.727
2.670
2.794
2.714
5.53
2.22
5.60
139.15
136.65
148.77
3.457
3.547
3.573
Ring(1) deWned by S(21A)/C(22A)–C(25A); ring(2) deWned by S(21B)/C(22B)–C(25B) and ring(3) deWned by S(21C)/C(22C)–C(25C). Symmetry codes (ii)
x ¡1, y, z; (iii) x + 1, y, z.
e. trans-(1: Ar Dthien-2-yl) at 291(2) K
C(45)–H(45),Cg(1i)
2.856
2.856
0.34
145.33
3.658
Symmetry code (i) x + 1, y, z
a
ꢃ is the angle at the hydrogen atom between the vectors H,Cg and Hperp
.

C. Fernandes et al. / Journal of Molecular Structure 837 (2007) 274–283
281
1.359(4)–1.376(3) Å and always signiWcantly greater than
O3
a
the C(2)–N(3) bond distances which are in the range
1.281(4)–1.299(3) Å. In the cations the N(1)–C(2) and
C(2)–N(3) distances are very similar within each cation and
fall in the range 1.3142(17)–1.3575 Å. Otherwise the bond
distances are very similar throughout the series with the
C(4)–C(5) bond distances, in the range 1.554(4)–1.584(2) Å,
always the longest.
b
O4
O4
O2
C61
O2
O5
O4
O4
O3
O3
C61
O1
O5
O5
O5
a
O4
The intra-ring bond angles are largely consistent
throughout the entire series. The N(1)–C(2)–N(3) angle,
however, in the range 116.6(3)–117.18(4)° is larger in the
neutral molecules than in the cations where the corre-
sponding range is 108.82–112.88(12)°. The converse is true
for the C(2)–N(3)–C(4) angle which falls in the ranges
105.9(2)–107.2(2)° and 110.88–114.48° for the neutral mol-
ecules and the cations, respectively. None of the parame-
ters discussed so far correlate with the cis or trans
disposition of the hydrogen atoms attached to C(4) and
C(5). The values of the C(42)–C4–C(5)–C(52) torsional
angle clearly do, being in the ranges, in terms of magni-
tude, of 16.77(18)–30.0(3)° and 92.6(3)–129.38(12)° for the
cis and trans cases, respectively. As expected the imidazole
rings are non-planar and this is expressed in terms of the
Cremer and Pople [32] pucker parameters. The departure
from planarity is always one or other of two closely
related types. Despite this, calculation of dihedral angles
between least-squares planes provides a convenient, if
somewhat approximate, means of assessing the disposi-
tion of the aryl substituents relative to the imidazole ring.
The values for the substituent on C(2) occur in two
ranges, in the range 4.8(2)–14.6(2)° for the neutral
molecules where the aryl group is conjugated with the
C(2)–N(3) double bond and 16.54(8)–22.68° for the cat-
ions where conjugation is absent. For the substituents on
C(4) or C(5) the values, determined by the steric require-
ments of the substituents and the packing of the mole-
cules, are greater and in the much wider range of 49.35–
85.59(11)°. The dihedral angles provide the most obvious
diVerences between molecules A and B and molecules A, B and
O3
o
c
b
C55
C26
C44
C46
b
C23
O2
C55
N21
N41
O1
C46
C54
O1
C56
C23
C25
O2
a
O1
C45
o
C45
c
N51
N21
Fig. 3. Hydrogen bonds (dashed lines) in (a) the layer of acetate anions
and water molecules and (b), along with C–H,ꢁ contacts, in the layer of
imidazole cations in [trans-(1: Ar Dpyridin-2-yl)H]+[OAc]¡·3H2O. Ellip-
soids are drawn at the 50% probability level and the hydrogen atoms
involved in the intermolecular contacts are shown as small spheres of arbi-
trary radii. Selected atoms are labelled.
C in the asymmetric units of cis-(1: ArDthien-2-yl)·0.5H2O and
trans-(1: ArDthien-2-yl) at 120(2), respectively.
Hydrogen-bonds (Table 3), ꢁ,ꢁ contacts (Table 4) and
C–H,ꢁ interactions (Table 5) all contribute to the intermo-
lecular connectivity in the structures whose reWnements are
o
c
a
H24
H46
H24
H4
H1
H1
H1
b
H24
Fig. 2. Intermolecular connectivity (dashed lines) within a layer of molecules of cis-(1: Ar Dpyridin-2-yl). Ellipsoids are drawn at the 50% probability level
and the hydrogen atoms involved in the intermolecular contacts are shown as small spheres of arbitrary radii. Selected atoms are labelled.

282
C. Fernandes et al. / Journal of Molecular Structure 837 (2007) 274–283
a
b
S51a
S51a
S21a
N3a
H5a
N3a
O1
S21a
N3b
S41a
O1
S51a
H1a
S21b
N3a
S41b
S41a
S41a
S21a
S51b
Fig. 4. Hydrogen-bonding in cis-(1: Ar Dthien-2-yl)·0.5H2O showing (a) the formation of centrosymmetric dimers exempliWed for the case of molecule A
and (b) type B and type A dimers alternating in a hydrpgen bonded chain propagated in the direction of b. In (b), for clarity, the carbon atoms of the thie-
nyl groups are shown as point atoms and the bonds as thin lines. Selected atoms are labelled and in (b) they apply to the molecules at the front of the
image. Ellipsoids are drawn at the 50% probability level and the hydrogen atoms which are present are shown as small spheres of arbitrary radii.
H45a
H45c
H45b
c
b
o
a
H45b
H45c
H45a
Fig. 5. A layer of molecules in trans-(1: Ar Dthien-2-yl). Dashed lines running across the page represent hydrogen-bonds while those running up and down
represent C–H,ꢁ contacts. Ellipsoids are drawn at the 50% probability level and the hydrogen atoms which are present are shown as small spheres of
arbitrary radii.
described in this paper. In cis-(1: ArDpyridin-2-yl), Fig. 2,
the hydrogen-bonds of the form N(1)–H(1),N(41) and
C(4)–H(4),N(21) and the C–H,ꢁ contact of the form
C(46)–H(46),Cg(4) interconnect the molecules to form zig-
zag chains propagated in the direction of c. Two of these, one
each centred on yD1/4 and 3/4, are shown in Fig. 2. The
C–H,ꢁ contact of the form C(24)–H(24),Cg(3) intercon-
nects the chains to form layers, one unit cell thick, parallel to
(1 0 0). There is no signiWcant connectivity between the layers.
In [trans-(1: ArDpyridin-2-yl)H]+[OAc]¡·3H2O the ace-
tate anions and the water molecules are connected by
hydrogen-bonds to form a two dimensional network in a
layers parallel to (¡1 0 1) (Fig. 3a). These layers alternate
with layers of imidazole cations (Fig. 3b). Connectivity
between the layers of anions and cations, which completes
the three dimensional connectivity of the structure, is
achieved by hydrogen bonds of the form N(1)–H(1),O(1)
and N(3)–H(3),O(2) with the imidazole as donor and ace-
tate as acceptor and O(4)–H(4B),N(41) where water mol-
ecule O(4) is the donor species. The imidazole/acetate
hydrogen-bonding creates chains propagated in the direct
of b examples of which appear on either side of Fig. 3b.
Connectivity between the chains within the imidazole layer,
as distinct from that provided through the anion layer by

C. Fernandes et al. / Journal of Molecular Structure 837 (2007) 274–283
283
hydrogen-bonding, is provided by contacts of the form
References
C(24)–H(24),Cg(4) and C(25)–H(25),Cg(4).
[1] M.A. Laurent, Liebigs Ann. Chem. 21 (1837) 130.
[2] M.A. Laurent, C. R. Acad. Sci. Paris 19 (1844) 353.
[3] (a) Ekman, Liebigs Ann. Chem. 112 (1839) 151;
The water molecule has a particularly signiWcant role in
the structure of cis-(1: Ar Dthien-2-yl)·0.5H2O. First it par-
ticipates in hydrogen-bonds which create centrosymmetric
dimers comprising pairs of type A molecules (Fig. 4a) and,
separately, virtually identical dimers of type B molecules.
Second it links the dimers, alternating in type, to form
chains propagated in the direction of b (vertically up the
page in Fig. 4b). There is no signiWcant intermolecular con-
tact between the chains formed in this way.
(b) , see alsoH.L. Snape, A. Brooke, Trans. Chem. Soc. (London) 77
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In the structure of trans-(1: ArDthien-2-yl) at both
120(2)K and 291(2)K the molecules occur in well deWned
layers parallel to (0 0 1) (Fig. 5) within which hydrogen-
bonds of the form N(1)–H(1),N(3) (Table 3d and e) con-
nect the molecules to form chains propagated in the direction
of b and the C–H,ꢁ contacts given in Table 5d and e con-
nect the chains, related to one another by cell translation, in
the direction of a and complete the connectivity within the
layer. In neither case is there signiWcant interaction between
molecules in adjacent layers. The single molecule in the asym-
metric unit of the structure of trans-(1: ArDthien-2-yl) at
291K is simply the average over the three molecules in the
asymmetric unit of the structure of the same compound at
120K. The 291K structure has a number of deWciencies
including an impossibly short intermolecular C–C contact
and is clearly somewhat approximate.
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These four examples reveal a remarkable variation in the
packing and intermolecular connectivity between molecules
which are not, in many respects as noted earlier, dramati-
cally diVerent from one compound to another.
Acknowledgements
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JLW wishes to thank CNPq, Brazil for Wnancial support.
The authors thank the EPSRC, UK, for access to both the
X-ray crystallographic service at the University of South-
ampton, for low temperature data collection, and the
Chemical Database Service at Daresbury.
Appendix A. Supplementary data
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Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.
2006.11.010.