New phenoxymethyl substituted mesoionic triazolium salts: Synthesis and structural characterisation

Article abstract of DOI:10.1016/j.molstruc.2018.10.095

A new set of mesoionic 1,2,3-triazolium salts functionalised by para substituted phenoxymethyl side groups were prepared using the ‘click’ Cu catalyzed [3 + 2] cycloaddition of organic azides and terminal alkynes. Four previously unreported neutral triazoles (1–4) were first isolated en route to formation of the salts (5–8). All the compounds were fully characterised (¹H, ¹³C NMR; IR; HR-MS; EA) and representative neutral triazoles and salts were structurally characterised by single crystal X-ray diffraction (SCXRD). Full examination of structural characteristics of the neutral compounds and the salts were presented by detailed analysis of multinuclear NMR data and SCXRD. As metal-free catalysts for the transfer hydrogenation of ketones to alcohols in isopropanol as solvent and hydrogen source, salts 5–8 showed moderate activities for the transformation of acetophenone to 1-phenyl alcohol.

Full text of DOI:10.1016/j.molstruc.2018.10.095

Journal of Molecular Structure 1179 (2019) 100e107
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
New phenoxymethyl substituted mesoionic triazolium salts: Synthesis
and structural characterisation
*
George Dhimba, Nasir S. Lawal, Muhammad D. Bala
School of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X54001, Durban 4000, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 August 2018
Received in revised form
A new set of mesoionic 1,2,3-triazolium salts functionalised by para substituted phenoxymethyl side
groups were prepared using the ‘click’ Cu catalyzed [3 þ 2] cycloaddition of organic azides and terminal
alkynes. Four previously unreported neutral triazoles (1e4) were first isolated en route to formation of
the salts (5e8). All the compounds were fully characterised ( H, C NMR; IR; HR-MS; EA) and repre-
sentative neutral triazoles and salts were structurally characterised by single crystal X-ray diffraction
26 October 2018
1
13
Accepted 31 October 2018
Available online 1 November 2018
(
SCXRD). Full examination of structural characteristics of the neutral compounds and the salts were
presented by detailed analysis of multinuclear NMR data and SCXRD. As metal-free catalysts for the
transfer hydrogenation of ketones to alcohols in isopropanol as solvent and hydrogen source, salts 5e8
showed moderate activities for the transformation of acetophenone to 1-phenyl alcohol.
© 2018 Elsevier B.V. All rights reserved.
Keywords:
Triazolium salt
Click chemistry
X-ray diffraction
Metal-free catalyst
1. Introduction
Also, in an effort to develop much cheaper, sustainable and
environmentally benign catalyst systems, we recently reported a
Since the versatile “green” concept of “click” reaction was first
coined and defined by Kolb, Finn and Sharpless in 2001, the
chemistry of mesoionic 1,2,3-triazolium salts has evolved over the
years [1]. This is due to the possibility of functionalizing the triazole
ring with a wide range of wingtip (N1 and C4) substituents, thereby
providing stability and extending the scope of triazolium N-het-
erocyclic chemistry [2]. This opportunity makes it possible for fine-
tuning the triazole ring to suit a variety of chemical applications
including use as ionic liquids, as organocatalysts and as ligands for
organometallic synthesis [3]. Compared with imidazolium salts,
triazolium salts tend to be more stable, because the former are
inherently limited by their acidic C(2)eH proton which is very
susceptible to deprotonation in the presence of strong bases,
thereby generating reactive carbenes, the well-publicized N-het-
erocyclic carbenes (NHC) [4]. In addition, triazolium ligands are
gradually been accepted as alternative ligands to the imidazolium
variants in organometallic chemistry and homogeneous catalysis
set of imidazolium based organocatalysts for the transfer hydro-
genation (TH) of ketones [8]. Herein, we report new phenox-
ymethyl substituted triazolium salts synthesized via Cu(І) catalyzed
azide/alkyne cycloaddition, for use as potential sources of NHC li-
gands and also demonstrated their potential as organocatalyst in
TH of acetophenone.
2. Experimental
All chemicals and reagents were of reagent grade and were used
as purchased. All NMR spectra were recorded in deuterated sol-
III
vents using a Bruker Ultra-Shield™ spectrometer AVANCE oper-
ating at a frequency of 400 MHz and ambient temperature.
Chemical shifts were recorded as
d
values in reference to SiMe
3
at
ꢀ
1
0.00 parts per million (ppm) at 25 C. H NMR were reported as:
chemical shift (d, ppm) and referenced to the solvent peak CDCl
3
(
7.26 ppm); multiplicity and number of protons are presented in
13
[
5,6]. This is due to the ease of their synthesis via the highly effi-
parentheses. The proton decoupled C NMR was conducted to
obtain the carbon skeleton of the triazoles and was presented as:
cient route of ‘click’ [3 þ 2] cycloaddition reactions which has also
expanded their scope as ionic liquids, anti-microbial agents and
materials [7].
chemical shift (
77.16 ppm) with the specific carbon indicated in parentheses. The
IR spectra were recorded on a Perkin Elmer Attenuated Total
d, ppm) and referenced to the solvent peak CDCl
3
(
ꢁ1
Reflectance (ATR) spectrophotometer in the 4000-400 cm region.
Melting point measurements were performed using a Stuart Sci-
entific melting point apparatus. The mass-to-charge ratio (m/z) of
*
Corresponding author.
E-mail address: bala@ukzn.ac.za (M.D. Bala).
https://doi.org/10.1016/j.molstruc.2018.10.095
022-2860/© 2018 Elsevier B.V. All rights reserved.
0

G. Dhimba et al. / Journal of Molecular Structure 1179 (2019) 100e107
101
the compounds was determined using a Bruker Micro TOF-Q11
mass spectrometry with electron spray ionisation (ESI) and a
sample concentration of approximately 1 ppm.
CH
3
) 13C NMR (100 MHz, CDCl
3
, ppm):
d
157.3, 143.7, 132.3, 122.4,
ꢁ1
116.6, 113.4, 62.3, 50.2, 32.2, 19.7, 13.4; IR (ATR, cm ): 3145, 3107,
3062, 2957, 2874, 2431, 1916, 1890, 1579, 1592, 14911289, 1251.
HRMS calcd for C13
Anal. Found (calcd) for C13
13.15); H, 4.80 (5.20).
H
16BrN
3
NaO 333.0380. Found 332.0380. Elem.
16 3
H N
BrO: C, 50.47 (50.34); N, 14.23
2
.1. General procedure for the synthesis of 1,2,3-triazole based
(
compounds 1-8
Scheme 1 illustrates the typical route for the synthesis of the
,2,3-triazoles 1e4 and their corresponding salts 5e8. Thus, the
general procedure for the synthesis of 1e4 is described: In a 100 ml
round bottomed flask containing acetone (30 ml) was transferred
p-nitro phenol (1.500 g, 10.8 mmol), propagyl bromide (1.28 g,
2.1.3. Ethyl-2-(4-((4-nitrophenoxy)methyl)-1H-1,2,3-triazol-1-yl)
acetate (3)
1
White crystalline solid, 47% yield, mp 110e112 C. 1H NMR
ꢀ
(400 MHz, CDCl
C]CH) 7.10e7.07 (q, 2H, Ar), 5.34 (s, 2H, CH
3
, ppm):
d
8.22e8.20 (q, 2H, Ar) 7.81 (s, 1H, triazole,
), 5.19 (s, 2H, CH ),
C NMR (100 MHz,
d 166.0, 163.0, 162.5, 143.2, 141.9, 125.8, 124.4, 115.0,
2
2
13
1
0.8 mmol) and K
2
CO
3
(2.99 g, 21.6 mmol). The mixture was stirred
4.31e4.26 (q, 2H, CH
CDCl , ppm):
62.6, 50.9, 14.0, IR (ATR, cm ): 3462, 3263, 2996, 2907, 2451, 1935,
1738, 1607, 1592, 1521, 1343, 1243; HRMS calcd for C13
2 3
), 1.33e1.29 (t, 3H, CH )
under reflux for 18 h, then cooled to room temperature and filtered
over a pad of Celite. The crude filtrate was concentrated under
reduced pressure and added to 1 mol equivalent of in situ generated
organic azide. Then aqueous solutions of CuBr (5%), sodium ascor-
3
ꢁ1
H
14
N
4
329.0862. Found 329.0869. Elem. Anal. Found (calcd) for C13
C, 52.98 (52.58); N, 18.34 (18.29); H, 4.61 (4.28).
H
14
N
4
:
bate (10%) and 5 ml H
further 18 h at room temperature. The resulting suspension was
partitioned between aqueous NH OH/EDTA (100 ml) and CH Cl
100 ml). The organic layer was separated and washed with H
100 ml), then dried over MgSO . All volatiles were removed under
2
O were added. The mixture was stirred for a
4
2
2
2
.1.4. Ethyl-2-(4-((4-bromophenoxy)methyl)-1H-1,2,3-triazol-1-yl)
acetate (4)
White solid, 38% yield, mp 147e148 C. 1H NMR (400 MHz,
CDCl , ppm): 7.75 (s, 1H, triazole, C]CH), 7.36e7.36 (q, 2H, Ar)
.90e6.87 (t, 2H, Ar), 5.21 (s, 2H, CH ), 5.16 (s, 2H, CH ), 4.30e4.25
C NMR (100 MHz, CDCl , ppm):
66.1, 157.2, 144.3, 132.3, 124.1, 116.6, 113.5, 62.5, 62.1, 50.9, 14.0 IR
(
(
2
O
ꢀ
4
reduced pressure to yield solids that were characterised and
confirmed as products 1e4.
In a typical N-alkylation procedure (Scheme 1), 0.500 g of a
triazole (1e4) was added into a round bottomed flask, followed by
3
d
6
2
2
13
(q, 2H, CH
2
) 1.32e1.28 (t, 3H, CH
3
)
3
1
ꢁ1
1
0 ml of acetonitrile and 1.2 equiv. of methyl iodide, the reaction
(ATR, cm ); 3152, 3098, 2939, 2959, 1892, 1745, 1490, 1240; HRMS
ꢀ
was stirred under reflux at 80 C for 18 h. Excess methyl iodide and
calcd for C13
Found (calcd) for C13
H, 3.99 (4.15).
H
14BrNaO
3
N
362.0116. Found 362.0110. Elem. Anal.
BrO : C, 47.20 (47.90); N, 12.96 (12.35);
solvent were removed by vacuum distillation. The crude salt was
then precipitated with dry ethyl acetate followed by filtration to
correspondingly afford triazolium salts (5e8) in moderate to high
yields.
H
14
3
3
2
.1.5. 1-Butyl-3-methyl-4-((4-nitrophenoxy)methyl)-1H-1, 2, 3-
triazol-3-ium iodide (5)
Pale yellow solid, 86% yield, mp 129e130 C. H NMR (400 MHz,
CDCl , ppm): 9.57 (s, 1H, triazole, C]CH), 8.18e8.16 (d, 2H, Ar)
7.27e7.24, (d, 2H, Ar) 5.87, (s, 2H, CH ), 4.72e4.51 (t, 2H, CH ), 4.51
ꢀ
1
2
.1.1. 1-Butyl-4-((4-nitrophenoxy)methyl)-1H-1, 2, 3-triazole (1)
ꢀ
1
Yellow crystals, 62.6% yield, mp 64e66 C. H NMR (400 MHz,
CDCl , ppm): 8.22e8.20 (q, 2H, Ar) 7.63 (s, 1H, triazole, C]CH),
.10e7.07 (m, 2H, Ar) 5.31 (s, 2H, CH ), 4.40e4.36 (t, 2H, CH
) 1.42e1.32 (m, 2H, CH ) 0.98e0.94 (t, 3H,
C NMR (100 MHz, CDCl , ppm): 163.1, 142.7, 141.9, 125.9,
d
3
3
d
2
2
7
1
CH
1
2
2
)
(s, 3H, NCH ), 2.10e2.02 (m, 2H, CH ), 1.50e1.40 (m, 2H, CH )
3
2
2
13
.95e1.87 (q, 2H, CH
2
2
1.00e0.97 (t, 3H, CH ); C NMR (100 MHz, CDCl , ppm):
d
161.4,
3
3
13
3
)
3
d
142.6, 139.0, 131.4, 126.1, 115.4, 59.9, 54.4, 40.1, 31.2, 19.4, 13.3; IR
ꢁ1
ꢁ1
22.7, 114.8, 62.5, 50.3, 32.2, 19.7, 13.4; IR (ATR, cm ): 3150, 3086,
(ATR, cm ) 3085, 2998, 2889, 1760, 1747, 1605, 1589, 1509, 1490,
þ
2
956, 2438, 2119, 1979, 1745, 1660, 1552, 1593,1512, 1496, 1465;
1339, 1235, 1110, 1012, 997, 850; LRMS calcd for [C14H19N O ]
4
3
HRMS calcd for C13
Anal. Found (calcd) for C13
H
16
N
4
NaO
3
299.1120. Found 299.1114. Elem.
: C, 56.80 (56.51); N, 21.51
291.1457. Found 291.1405. Elem. Anal. Found (calcd) for
C14H19IN O : C, 39.91 (40.21); H, 13.22 (13.40); N, 4.53 (4.8).
H
16
N
4
O
3
4
3
(
21.28); H, 5.04 (5.84).
2
.1.6. 4-((4-Bromophenoxy) methyl)-1-butyl-3-methyl-1H-1, 2, 3-
triazol-3-ium iodide (6)
White solid, 96% yield, mp 179e181 C. 1H NMR (400 MHz,
CDCl , ppm): 9.54 (s, 1H, triazole, C]CH), 7.43e7.40 (d, 2H, Ar)
7.00e6.98, (d, 2H, Ar) 5.63, (s, 2H, CH ), 4.69e4.65 (t, 2H, CH ), 4.44
(s, 3H, NCH ), 2.07e1.99 (m, 2H, CH ), 1.46e1.38 (m, 2H, CH
2
.1.2. 4-((4-Bromophenoxy)methyl)-1-butyl-1H-1,2 3-triazole (2)
ꢀ
1
ꢀ
White solid, 56% yield, mp 66e67 C. H NMR (400 MHz, CDCl
ppm): 7.57 (s, 1H, triazole, C]CH) 7.40e7.36 (m, 2H, Ar)
.90e6.86 (m, 2H, Ar) 5.18 (s, 2H, CH2) 4.38e4.34 (t, 2H, CH
.93e1.86 (m, 2H, CH ) 1.41e1.32 (m, 2H, CH ) 0.98e0.94 (t, 3H,
3
,
d
3
d
6
1
2
)
2
2
2
2
3
2
2
)
Scheme 1. Click chemistry route to the synthesis of neutral 1,2,3-triazoles (1e4) and corresponding salts (5e8).

102
G. Dhimba et al. / Journal of Molecular Structure 1179 (2019) 100e107
1
1
.00e0.96 (t, 3H, CH3); 13C NMR (100 MHz, CDCl
39.5, 132.8, 131.2, 116.8, 116.0, 59.4, 54.3, 39.8, 31.2, 19.4, 13.3; IR
3
, ppm):
d
155.8,
compounds were obtained via standard copper(I)-catalyzed
alkyne-azide cycloaddition (CuAAC) “click” reactions of in situ
generated organic azides with terminal alkynes obtained from the
reaction of propagyl bromide and para substituted phenols
(Scheme 1). Due to safety concerns associated with the handling
and isolation of azides bearing short-chain low molecular weight
alkyl groups, isolation of the organic azides was avoided. They are
well-known to be highly energetic and potentially explosive at high
concentrations [13] hence the cycloaddition reactions were con-
ducted in situ with crude concentrates of previously generated al-
kynes. The triazoles were isolated in moderate yields and were N-
alkylated in a simple and straightforward protocol depicted in
Scheme 1. The Quaternisation step usually conducted in polar
aprotic solvents such as acetonitrile was achieved under mild
conditions without the need for an inert atmosphere or any special
precautions affording moderate to high yields of salts 5e8 [14,15].
All new compounds were fully characterised by spectroscopic
and analytical techniques. Well-resolved NMR spectra were ob-
tained for the triazoles and corresponding N-quarternised salts. For
instance, the proton NMR spectrum of 1 showed the typical triazole
fingerprint proton pattern at 7.63 ppm (Fig. 1), while the proton
ꢁ
1
(
ATR, cm ) 3084, 2987, 2889, 2185, 1759, 1582, 1487, 1367, 1231,
þ
1
3
(
158, 1021, 813; LRMS calcd for [C14
24.0726. Elem. Anal. Found (calcd) for C14
37.19); H, 9.17 (9.29); N, 4.18 (4.24).
H19BrN
3
O] 324.0711 Found
3
H19BrIN O: C, 36.72
2.1.7. 1-(3-Ethoxy-2-oxopropyl)-3-methyl-4-((4-nitrophenoxy)
methyl)-1H-1, 2, 3-triazol-3-ium iodide (7)
Pale yellow solid, 53% yield, mp 127e128 C. H NMR (400 MHz,
CDCl , ppm): 9.58 (s, 1H, triazole C]CH), 8.24e8.22 (d, 2H, Ar)
.22e7.20 (d, 2H, Ar) 5.80 (s, 2H, CH ), 5.763 (s, 2H, CH ) 4.50 (s, 3H,
NCH ),4.35e4.29 (q, 2H, CH ), 1.36e1.33 (t, 3H, CH
100 MHz, CDCl , ppm): 164.2, 161.3, 142.8, 138.8, 133.0, 126.2,
15.2, 63.7, 59.7, 54.5, 40.1,14; IR (ATR, cm ) 3453, 3041,2975,1919,
749, 1911, 1590, 1511, 1339, 1224, 1109, 1011, 850; LRMS calcd for
ꢀ
1
3
d
7
2
2
1
3
3
2
3
); C NMR
(
1
1
3
d
ꢁ
1
þ
[C
14
17
H N
4
O
5
]
321.1199 Found 321.1208. Elem. Anal. Found (calcd)
for C14H17IN O : C, 37.93 (37.52); N, 11.82 (12.50); H, 3.53 (3.82).
4 5
2.1.8. 4-((4-Bromophenoxy)methyl)-1-(2-ethoxy-2-oxoethyl)-3-
methyl-1H-1, 2, 3-triazol-3-ium iodide (8)
White solid, 51% yield, mp 124e126 C. 1H NMR (400 MHz,
CDCl , ppm): 9.55 (s, 1H, triazole C]CH), 7.44e7.42 (d, 2H, Ar)
.96e6.94 (d, 2H, Ar) 5.77 (s, 2H, CH ), 5.57 (s, 2H, CH ) 4.45 (s, 3H,
NCH ), 4.34e4.28 (q, 2H, CH ), 1.36e1.32 (t, 3H, CH
100 MHz, CDCl , ppm): 164.7, 155.8, 139.3, 132.8, 132.7, 116.7,
ꢀ
decoupled C NMR spectrum showed the typical triazole ring C(5)
resonance signal at 123 ppm (see the ESI), which is consistent with
reported literature assignments for triazoles [16].
13
3
d
6
2
2
1
3
3
2
3
); C NMR
Quaternisation of 1 to the corresponding triazolium salt (5) was
confirmed by the appearance of a singlet signal at 4.51 ppm which
integrated to 3 protons for methyl alkylation. Downfield shift of the
C(5)eH proton to 9.57 ppm also confirmed successful N-alkylation
(Fig. 2) [17,18]. Downfield shifts in carbon resonance positions was
(
3
d
ꢁ
1
6
2
3.6, 59.3, 54.5, 40.0, 14; IR (ATR, cm ) 3443, 3100, 2985, 2914,
869, 1881, 1757, 1581, 1489, 1230, 1157, 1006, 815; LRMS calcd for
þ
[
(
(
C
14
H17BrN
3
O
3
]
354.0453 Found 354.0399. Elem. Anal. Found
: C, 34.34 (34.88); H, 8.59 (8.72); N, 3.44
13
calcd) for C14
3.55).
H17BrIN
3
O
3
also observed in the C NMR spectra of all the salts when
compared to corresponding neutral triazoles with an average of
5
ppm shift in resonance positions (see ESI). For all the salts, the
2
.2. X-ray structure determination
selective alkylation and quaternisation of the more nucleophilic
N(3) was observed and is confirmed by 2D He H NOESY NMR
analysis [19,20].
1
1
Single crystals were selected and glued onto the tip of a glass
fiber, mounted in a stream of cold nitrogen at 173 K and centered in
the X-ray beam using a video camera. Intensity data were collected
on a Bruker APEX II CCD area detector diffractometer with graphite
Generally, NOE cross peaks are caused by cross relaxation be-
tween adjacent protons that are spatially in close proximity
(approximately less than 5 Å apart) [21]. For example the NOESY
spectrum of 7 (Fig. 3) showed NOE correlations between the methyl
protons bonded to N(3) and methylene protons bonded to the
phenoxy ring, suggesting that they are in close proximity, a result
that is in harmony with findings on similar compounds reported in
the literature [17].
monochromated Mo Ka radiation (50 kV, 30 mA) using the APEX 2
data collection software. The collection method involved
u
-scans of
ꢀ
width 0.5 and 512 ꢂ 512 bit data frames. Data reduction was
carried out using the program SAINT þ while face indexed and
multi-scan absorption corrections were made using SADABS. The
structures were solved by direct methods using SHELXS [9]. Non-
hydrogen atoms were first refined isotropically followed by aniso-
tropic refinement by full matrix least-squares calculations based on
Calculated m/z values for all the compounds correlated well
with observed values, while the fragmentation patterns showed
þ
þ
peaks at [M] (100% abundance) and [Mþ1] (1.1% abundance)
2
12
13
F using SHELXS. Hydrogen atoms were first located in the differ-
which are attributed to the C and C isotopes respectively. As
constituted, the MS of triazoles 2 and 4 and the corresponding
triazolium salts 6 and 8 showed the presence of the bromide sub-
stituent. Bromine has two isotopes, Br and Br in approximately
1:1 abundance ratio, compounds containing one bromine atom will
show two peaks in the molecular ion region, depending on which
ence map then positioned geometrically and allowed to ride on
their respective parent atoms. Diagrams were generated using
SHELXTL, PLATON [10] and ORTEP-3 [11]. Crystallographic data for
the structures in this article have been deposited with the Cam-
bridge Crystallographic Data Centre, CCDC 1541766e1541769 for 1,
7
9
81
7
9
3
, 6, and 8 respectively. These data can be obtained free of charge at
bromine isotope the molecular ion contains [22]. The Br isotope
showed a base peak at [M] whereas those of Br were observed at
þ
81
http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12 Union Road Cambridge
CB2 1EZ, UK; Fax: þ44e1223/336-033; E-mail: deposit@ccdc.cam.
ac.uk).
þ
[M þ2] . Important IR bands observed for the triazoles corre-
ꢁ
1
sponded to the CeH stretch (3000-2850 cm ), ¼CeH stretch
ꢁ1
ꢁ1
(3100-3000 cm ), C]C stretch (1680-1640 cm
)
and C]O
ꢁ
1
ꢁ1
stretch (1750-1735 cm ). An absorption band at 1254-1131 cm
was assigned to N]N of the neutral 1,2,3-triazoles [23]. The ob-
tained EA results of the triazoles and corresponding salts correlated
well to theoretical values within acceptable limits of deviation.
3
. Results and discussion
The strategy adopted for the synthesis of the disubstituted tri-
azoles allowed for changes in the types of substituents at positions
and 4 (so-called wingtip) of the heterocyclic ring aimed at
1
3.1. Single crystal structural analysis
probing electronic and steric influences. We adopted a procedure
reported by Luo et al., [12] to synthesize four new triazoles 1e4. The
Structural data of representative compounds 1, 3, 6, and 8, were

G. Dhimba et al. / Journal of Molecular Structure 1179 (2019) 100e107
103
Fig. 1. 1H NMR spectrum of 1.
Fig. 2. 1H NMR spectrum of 5.
Fig. 3. NOESY NMR spectrum of 7.

104
G. Dhimba et al. / Journal of Molecular Structure 1179 (2019) 100e107
ꢀ
Fig. 4. ORTEP representation of the structure of 1 with the displacement ellipsoids drawn at the 50% probability level. Selected bond lengths (Å) and angles ( ) N1eN4, 1.338(6);
N1eC4 1.475(7); N1eC5 1.342(6); N4eN2 1.312(7); N4eN1eC5 110.7(4); N2eN4eN1 107.9(4); N2eC10T-C5 108.2(40).
respectively. The crystal structures of the neutral triazole com-
pounds 1 (Figs. 4) and 3 (Fig. 6) illustrated structural compositions
consistent with the characterisation data discussed above with
both compounds crystallizing in the P-1 space group of the triclinic
crystal system [24]. All bond angles and lengths are within limits
reported for related azolium compounds [25].
If the phenoxymethyl and straight chain (butyl group in 1 and
ethyl acetate in 3) wingtip substituents are respectively assumed as
the ‘head’ and ‘tail’ of each molecule, then the packing arrangement
of molecules in crystals of the neutral triazole compounds 1 and 3
may be described in terms of the interactions between these two
Fig. 5. Packing arrangement in a crystal of 1.
moieties around each triazole unit. Interestingly, the packing
arrangement in arrays of compound 1 (Fig. 5) is composed of
determined from crystals grown in acetone/hexane solvent mix-
tures with atom numbering schemes depicted in Figs. 3e6
ꢀ
Fig. 6. ORTEP representation of the structure of 3 with the displacement ellipsoids drawn at the 50% probability level. Selected bond lengths (Å) and angles ( ) N2eN4 1.342(3);
N2eC7 1.3556(19); N2eC8 1.458(2); N4eN3 1.316(16); N3eC6 1.365(3); C6eC5 1.4931(16); C6eC7 1.368(3); N4eN2eC7 111.38(15); N3eN4eN2 107.3 (2); N2eC7eC6 103.9(2).

G. Dhimba et al. / Journal of Molecular Structure 1179 (2019) 100e107
105
covalent interaction that occurs when net attractive forces be-
tween a positively polarized halogen atom, frequently bromine or
iodine exists in close proximity to a Lewis base [26].
Crystal packing in molecules of 6 and 8 are shown in Figs. 9 and
11 respectively. In compound 6, a repeating zig-zag pattern is
observed with the phenoxymethyl substituents pointing away from
each other in order to minimize steric repulsions due to the many
conformations of the phenyl rings as a result of free rotation along
the C3eO2eC2 ethereal bond. The ethyl acetate tail ends of 8
packed in a similar fashion to the butyl N-substituent in 1 hence
affording related packing patterns. It is also noteworthy that the
quaternisation of the neutral triazoles to N-alkylated triazolium
salts bearing delocalized positive charges has very little effect on
the NeN bond lengths which are only slightly shorter in 6 and 8
when compared to 1 and 3.
3.2. Catalytic transfer hydrogenation
Fig. 7. Packing arrangement in a crystal of 3.
In view of growing global concerns for the health of the envi-
ronment and the need for the development of sustainable pro-
cesses, we envisioned that triazolium based organocatalysts will
share similarities in catalytic performance to related imidazolium
analogues reported to be active in catalytic transfer hydrogenation
repeating blocks of tail-to-tail then head-to-head units. This is a
very efficient packing pattern that allows for minimal inter-
molecular and steric repulsion between neighboring moieties.
However, an arrangement composed of only regularly alternating
head-to-tail layers is observed in the packing of 3 shown in Fig. 7
(CTH) reactions [8]. Also, the stability and resistance of triazolium
C(4,5) protons to deprotonation by bases is well-documented
ꢀ
with adjacent layers related through 180 rotation. This type of
packing pattern is common in molecules with bulky side groups
and helps to reduce inter-molecular repulsion and steric strain. In
addition, this pattern is differentiated from that of compound 1 by
the absence of intermolecular
phenoxymethyl moiety.
p-p interactions involving the N-
The triazolium salts 6 (Fig. 8) and 8 (Fig. 10) respectively belong
to the monoclinic (space group C2/c) and triclinic (space group P-1)
crystal systems. Interestingly, the structures of 6 and 8 showed
ꢀ
non-classical long-range inter-halogen contacts at an angle of 180
separated by 3.735 and 3.692 Å respectively. This is a weak non-
Fig. 9. Packing arrangement in a crystal of 6.
ꢀ
Fig. 8. ORTEP representations of the structure of 6 with the displacement ellipsoids drawn at the 50% probability level. Selected bond lengths (Å) and angles ( ) N1eN2 1.312 (13);
N2eN3 1.323 (10); N3eC2 1.354 (11); C2eC1 1.396 (13); C1eN1 1.353 (11); N1eC3 1.498 (11); N1eN2eN3 104.4 (8); N2eN3eC2 112.7 (8); C3eN1eN2 113.6 (8).

106
G. Dhimba et al. / Journal of Molecular Structure 1179 (2019) 100e107
ꢀ
Fig. 10. ORTEP representations of the structure of 8 with the displacement ellipsoids drawn at the 50% probability level. Selected bond lengths (Å) and angles ( ) N1eN2 1.326 (11);
N2eN3 1.314 (11); N3eC6 1.370 (11); C6eC7 1.490 (12); C6eC5 1.376 (14); C5eN1 1.382 (11); N1eN2eN3 105.1 (7); N2eN3eC6 113.2 (8); C5eN1eN2 111.4 (8).
Table 1
Transfer hydrogenation of acetophenone catalyzed by triazolium salts 5e8.
Entry
Organocatalyst
Conversion (%)a
1
2
3
4
5
6
7
8
56
41
65
58
ꢀ
Reactions were conducted in air at 82 C using acetophenone (2.2 mmol), KOH
(
0.112 g) dissolved in 10 ml isopropanol.
Conversion was determined by GC analysis after 12 h. No reaction was recorded
a
in the absence of KOH.
results presented in Table 1, entries 1e4 showed that all the salts
were moderately active in the CTH protocol resulting in the con-
version of acetophenone to phenylethanol with organocatalyst 7
showing the best conversion rate of 65%. In summary, triazolium
salts bearing electron withdrawing N,N-substituents produced
higher conversions to 1-phenylethanol, which we attributed to
improved ability of the quarternised azolium salt to interact with
the nucleophilic ketone substrate hence promoting reduction of the
C]O bond.
After scanning a range of ketones, catalyst 7 was shown to be
active for aromatic ketones bearing activating (electron with-
drawing) groups para to the carbonyl, but poor for aliphatic ketones
and aromatic ketones bearing sterically encumbering groups ortho
to the C]O group.
Fig. 11. Packing arrangement in a crystal of 8.
4. Conclusion
which is in stark contrast to the susceptibility of the acidic imida-
zolium C(2) proton to attack under such conditions [4]. As a follow-
up to our previous report on the use of imidazolium salts as orga-
nocatalysts for CTH, we hypothesized that triazolium salts will thus
possess added advantages for CTH reactions that are promoted by
inorganic bases such as KOH [8]. Hence, based on well-established
procedures and techniques, we ran all the salts (5e8) on aceto-
phenone as the model substrate at varying temperatures. From the
In summary, a new series of azolium ionic compounds were
prepared and the molecular structures of representative neutral
1,2,3-triazoles and corresponding salts were determined by a va-
riety of techniques including single crystal XRD. The reported
compounds may be used as sources of mesoionic carbene ligands
for the synthesis of organometallic complexes or may be used as
metal free organocatalysts for the activation of small molecules.

G. Dhimba et al. / Journal of Molecular Structure 1179 (2019) 100e107
Technol. 3 (2013) 235e243.
107
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