Synthesis of a series of pyridine-functionalised benzoimidazole-based N-heterocyclic carbene ligands

Article abstract of DOI:10.3184/174751912X13282044479441

A series of pyridine-functionalised bis(benzoimidazolium) dichlorides, dibromides and dihexafluorophosphate with different alkyl bridged length (n = 2-4) were synthesised in high yield. All the compounds are new and fully characterised by ¹H and ¹³C NMR, IR and elemental analyses.

Full text of DOI:10.3184/174751912X13282044479441

JOURNAL OF CHEMICAL RESEARCH 2012
RESEARCH PAPER 111
FEBRUARY, 111–113
Synthesis of a series of pyridine-functionalised benzoimidazole-based
N-heterocyclic carbene ligands
Jin Guoa, Pan Hea, Long Yanga, Xiang Liua, Lanlan Lva, Yanhui Shia,b*, and Changsheng Caoa
aSchool of Chemistry and Chemical Engineering and Jiangsu Key laboratory of Green Synthetic Chemistry for Functional Materials, Xuzhou
Normal University, Xuzhou, Jiangsu 221116, P. R. China
bState Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing, Jiangsu 210093, P. R. China
A series of pyridine-functionalised bis(benzoimidazolium) dichlorides, dibromides and dihexafluorophosphate
with different alkyl bridged length (n = 2–4) were synthesised in high yield. All the compounds are new and fully
characterised by 1H and 13C NMR, IR and elemental analyses.
Keywords: N-heterocyclic carbene ligand, pyridine-functionalised, tetradentate, benzoimidazole
The chemistry of N-heterocyclic carbenes (NHCs) and their
complexes has been investigated with great intensity in recent
years, and many publications and reviews relating to NHC
complexes have been published.1–5 Easy preparation and modi-
fication on NHCs may have helped their great development.
Various donor functions, such as containing N, O, S or P donor
atom can be introduced at the nitrogen atom of the imidazole
in a simple manner, and a donor-functionalised NHC can act
as a polydentate ligand upon coordination to a metal centre.
Various complexes with donor-functionalised NHC and their
use in catalysis have been explored.6,7 We have developed
an interest in exploring the utility of N-heterocyclic carbenes
in chemical catalysis,8–10 and reported the synthesis of non-
functionalised N-heterocyclic carbene precursors based on
imidazole and triazole.11–13 As an extension to our work on
O-functionalised NHC precursors based on imidazole and
benzoimidazole,14 we now report the preparation of a series of
pyridine-functionalised tetradentate N-heterocyclic carbene
precursors based on benzoimidazole.
When the saturated aqueous solution of NH4PF6 was
added into the aqueous solution of the bis(benzoimidazolium)
dichlorides or dibromides, their corresponding dihexafluoro-
phosphate salts precipitated out quantitatively (Scheme 1).
This showed that both bisimidazolium dichloride and dibro-
mide salts can be easily converted to dihexafluorophosphate
salts in near quantitative yield. All the compounds are new, so
they are fully characterised by NMR spectroscopy and give
satisfactory elemental analyses.
In conclusion, we have demonstrated an easy process to pre-
pare a series of pyridine-functionalised bis(benzoimidazolium)
dichlorides, dibromides and dihexafluorophosphate with
different alkyl bridged length (n = 2–4) in high yield.
Experimental
All reagents were commercially available and were used without
further purification. 1H NMR spectra were recorded on a Bruker DPX
400 MHz spectrometer at room temperature and referenced to the
1
residual H signals of the solvent. Coupling constants J are given in
1-(Pyridin-2-ylmethyl)-1H-benzoimidazole was synthesised
by the nucleophilic substitution reaction of 2-(chloromethyl)
pyridine with benzoimidazole in 30% NaOH aqueous solution
with TBAB (tetra-n-butylammonium bromide) as a phase tran-
sition catalyst in 92% yield (Scheme 1). The alkane bridged
pyridine-functionalised bisbenzoimidazolium dichlorides or
dibromides were synthesised by direct reaction of dialkyl
halides with the pyridine-functionalised benzoimidazole in
toluene at an elevated temperature in good yields, ranging
from 80 to 89%. As the alkyl bromides are more reactive than
alkyl chlorides, the reaction temperature for bromides is 10
degree lower (120 °C) than that for chlorides (130 ºC). All the
reactions can be completed within 5 h.
Hz. IR spectra were recorded on KBr pellets on a FTIR-Tensor 27
spectrometer. Melting points were detected by microscope melting
point apparatus. Elemental analyses were performed on a EuroVektor
Euro EA-300 elemental analyser.
1-(Pyridin-2-ylmethyl)-1H-benzoimidazole:15 To the mixture of
1H-benzoimidazole (10 mmol, 1.181 g), tetra-n-butylammonium
bromide (TBAB) (0.05 g) and 30% NaOH aqueous solution (10 mL)
in a 50 mL of round-bottom flask was added 2-(chloromethyl)pyridine
(12 mmol, 1.531 g). The reaction mixture was stirred at room tem-
perature for 2 h, while a large amount of light yellow solid appeared.
Water (20 mL) was added to the slurry, and the solid was filtered and
washed with plenty of water to give the product as a white solid in
1
92% yield (1.92 g). M.p. 119–120 ºC. H NMR (400 MHz, CDC13)
δ = 8.61 (d, J = 4.0 Hz, 1H, o-C5H5N), 8.06 (s, 1H, NCHN), 7.83 (dd,
Scheme 1 Synthesis of alkyl-bridged bisimidazolium dichlorides.
* Correspondent. E-mail: yhshi_2000@126.com

112 JOURNAL OF CHEMICAL RESEARCH 2012
J = 1.6 Hz, J = 6.8 Hz, 1H, m-C6H4), 7.59 (dt, J = 1.6 Hz, J = 7.6 Hz,
1H, p-C5H5N), 7.31–7.33 (m, 1H, m-C6H4), 7.26–7.30 (m, 2H,
o-C6H4), 7.21–7.24 (m, 1H, m-C5H5N), 6.90 (d, J = 7.6 Hz, 1H,
m-C5H5N), 5.49 (s, 2H, NCH2C).
763, 558 cm−1. 1H NMR (400 MHz, D2O) δ = 8.39 (d, J = 4.8 Hz, 2H,
C5H5N), 7.89 (t, J = 7.6 Hz, 2H, C6H4), 7.81–7.79 (m, 2H, C6H4),
7.63–7.58 (m, 6H, C6H4, C5H5N), 7.54 (d, J = 8.0 Hz, 2H, C5H5N),7.40
(t, J = 5.6 Hz, 2H, C5H5N), 5.72 (s, 4H, NCH2C), 4.77 (t, J = 7.2 Hz,
4H), 2.91-2.84 (m, 2H, CH2). 13C NMR (100 MHz, D2O): δ = 151.5,
149.5, 138.9, 131.3, 131.2, 127.6, 127.5, 124.6, 123.6, 113.7, 113.1,
51.5, 44.6, 28.0. Anal. Calcd for C29H28Br2N6 (620.38 g mol−1): C,
56.14; H, 4.55; N, 13.55. Found: C, 55.88; H, 4.45; N, 13.66%.
1,1′-Di-(pyridin-2-ylmethyl)-4,4′-(1,4-butanediyl)bisbenzoimidazo
lium dibromide (6): To a 10 mL heavy wall pressure tube was added
1-(pyridin-2-ylmethyl)-1H-benzoimidazole (5.0 mmol, 1.046 g), 1,4-
dibromobutane (2.5 mmol, 0.540 g) and toluene (1 mL). The reaction
mixture was heated at 120 ºC for 3 h, while a large amount of white
solid appeared. The solid was filtered and washed with dichlorometh-
ane to give the product as white solids in 82% yield (1.29 g). M.p.
276–277 °C. IR (KBr): 3433, 2170, 1638, 1454, 1401, 1076, 986, 848,
747, 558 cm−1. 1H NMR (400 MHz, D2O) δ = 8.34 (d, J = 4.8 Hz, 2H,
C5H5N), 7.86 (t, J = 7.6 Hz, 2H, C6H4), 7.59–7.53 (m, 6H, C6H4,
C5H5N), 7.46 (t, J = 8.4 Hz, 2H, C6H4), 7.38–7.31 (m, 4H, C5H5N),
5.74 (s, 4H, NCH2C), 4.56 (s, 4H, NCH2), 2.02 (s, 4H, CH2). 13C NMR
(100 MHz, D2O) δ = 151.6, 149.4, 138.8, 131.2, 130.9, 127.3, 127.0,
124.5, 123.5, 113.6, 112.9, 51.3, 46.7, 24.8. Anal. Calcd for
C30H30Br2N6 (634.41 g mol−1): C, 56.80; H, 4.77; N, 13.25. Found: C,
56.53; H, 4.69; N, 13.37%.
1,1′-Di-(pyridin-2-ylmethyl)-4,4′-(1,2-ethanediyl)bisbenzoimidazol
ium dichloride (1): To a 10 mL heavy wall pressure tube was added
1-(pyridin-2-ylmethyl)-1H-benzoimidazole (5 mmol, 1.046 g), 1,2-
dichloroethane (2.5 mmol, 0.247 g) and toluene (1 mL). The reaction
mixture was heated at 130 ºC for 3.5 h, while a large amount of white
solid appeared. The solid was filtered and washed with DCM to give
the product as white solids in 84% yield (1.09 g). M.p. > 300 ºC. IR
(KBr): 3457, 2955, 1628, 1573, 1560, 1435, 1183, 1090, 997, 761,
1
569 cm−1. H NMR (400 MHz, D2O) δ = 8.32 (d, J = 4.0 Hz, 2H,
C5H5N), 7.77 (t, J = 7.6 Hz, 2H, C6H4), 7.67 (d, J = 8.4 Hz, 2H, C6H4),
7.50 (t, J = 7.6 Hz, 2H, C5H5N), 7.37–7.25 (m, 8H, C6H4, C5H5N), 5.70
(s, 4H, NCH2C), 5.26 (s, 4H, NCH2). 13C NMR (100 MHz, D2O) δ =
151.2, 149.6, 139.0, 131.0, 128.0, 127.8, 124.7, 123.5, 113.9, 113.2,
111.8, 51.6, 46.6. Anal. Calcd for C28H26Cl2N6 (517.45 g mol−1): C,
64.99; H, 5.06; N, 16.24. Found: C, 64.71; H, 5.15; N, 16.45%.
1,1′-Di-(pyridin-2-ylmethyl)-4,4′-(1,3-propanediyl)bisbenzoimidaz
olium dichloride (2): To a 10 mL heavy wall pressure tube was added
1-(pyridin-2-ylmethyl)-1H-benzoimidazole (10 mmol, 2.092 g), 1,3-
dichloropropane (5 mmol, 0.565 g) and toluene (2 mL). The reaction
mixture was heated at 130 ºC for 3.5 h, while a large amount of white
solid appeared. The solid was filtered and washed with DCM to give
the product as white solids in 86% yield (2.30 g). M.p. > 300 ºC.
IR (KBr): 3395, 2967, 2361, 2342, 1560, 1438, 762, 669, 649 cm−1.
1H NMR (400 MHz, D2O) δ = 8.38 (d, J = 4.8 Hz, 2H, C5H5N), 7.88
(t, J = 7.6 Hz, 2H, C6H4), 7.79 (d, J = 8.4 Hz, 2H, C6H4), 7.62–7.53 (m,
8H, C6H4, C5H5N), 7.39 (t, J = 5.6 Hz, 2H, m-C5H5N), 5.72 (s, 4H,
1,1′-Di-(pyridin-2-ylmethyl)-4,4′-(1,2-ethanediyl)bisbenzoimidazol
ium dihexafluorophosphate (7): To compound 1 (2.0 mmol, 1.033 g)
or 4 (2.0 mmol, 1.213 g) dissolved into water (6 mL) in a 50 mL
beaker was added dropwise saturated NH4PF6 aqueous solution
(30 mL). A large amount of white solid appeared. The solid was
filtered and washed with water to give the product as a white solid in
99% yield (1.46 g). M.p. 228–230 °C. IR (KBr): 3346, 1675, 1635,
NCH2C), 4.76 (t, J = 7.2 Hz, 4H, NCH2), 2.90-2.83 (m, 2H, CH2). 13
C
NMR (100 MHz, D2O) δ = 151.5, 149.4, 138.9, 131.2, 131.1, 127.6,
127.5, 124.6, 123.5, 113.7, 113.1, 51.4, 44.5, 28.0. Anal. Calcd for
C29H28Cl2N6 (531.48 g mol−1): C, 65.54; H, 5.31; N, 15.81. Found: C,
65.31; H, 5.22; N, 15.93%.
1
1402, 1190, 1077, 986, 848, 765, 558 cm−1. H NMR (400 MHz,
DMSO-d6) δ = 9.87 (s, 2H, NCHN), 8.47 (d, J = 4.4 Hz, 2H, C5H5N),
7.93–7.88 (m, 4H, C6H4), 7.81 (t, J = 8.4 Hz, 2H, C5H5N), 7.62–7.56
(m, 4H, C6H4), 7.46 (t, J = 7.6 Hz, 2H, C5H5N), 7.39 (t, J = 6.4 Hz,
2H, C5H5N), 5.88 (s, 4H, NCH2C), 5.20 (s, 4H, NCH2). 13C NMR
(100 MHz, DMSO-d6) δ = 152.5, 149.5, 143.5, 137.5, 131.0, 130.9,
126.9, 126.6, 123.7, 122.7, 113.8, 113.0, 50.9, 45.8. Anal. Calcd for
C28H26F12N6P2 (736.47 g mol−1): C, 45.66; H, 3.56; N, 11.41. Found:
C, 45.46; H, 3.48; N, 11.60%.
1,1′-Di-(pyridin-2-ylmethyl)-4,4′-(1,3-propanediyl)bisbenzoimidaz
olium dihexafluorophosphate (8): To compound 2 (2.0 mmol,
1.063 g) or 5 (2.0 mmol, 1.241 g) dissolved into water (6 mL) in a
50 mL beaker was added dropwise saturated NH4PF6 aqueous (30 mL)
solution. A large amount of white solid appeared. The solid was
filtered and washed with water to give the product as a white solid in
99% yield (1.49 g). M.p. 235–237 °C. IR (KBr): 3431, 3334, 3465,
1662, 1638, 1615, 1572, 1455, 1401, 1190, 892, 841, 757, 558 cm−1.
1H NMR (400 MHz, DMSO-d6) δ = 9.96 (s, 2H, NCHN), 8.48 (d,
J = 4.0 Hz, 2H, C5H5N), 8.11 (d, J = 8.0 Hz, 2H, C6H4), 7.98 (d,
J = 7.6 Hz, 2H, C6H4), 7.91 (t, J = 7.2 Hz, 2H, C5H5N), 7.72-7.65 (m,
6H, C5H5N, C6H4), 7.40–7.37 (m, 2H, C5H5N), 5.93 (s, 4H, NCH2C),
4.73 (t, J = 6.8 Hz, 4H, NCH2), 2.67–2.63 (m, 2H, CH2). 13C NMR
(100 MHz, DMSO-d6) δ = 152.8, 149.5, 143.1, 137.5, 131.3, 131.0,
126.8, 126.6, 123.7, 122.7, 113.8, 113.6, 50.8, 43.9, 28.0. Anal. Calcd
for C29H28F12N6P2 (750.50 g mol−1): C, 46.41; H, 3.76; N, 11.20.
Found: C, 46.29; H, 3.80; N, 11.37%.
1,1′-Di-(pyridin-2-ylmethyl)-4,4′-(1,4-butanediyl)bisbenzoimidazo
lium dichloride (3): To a 10 mL heavy wall pressure tube was added
1-(pyridin-2-ylmethyl)-1H-benzoimidazole (2 mmol, 0.419 g), 1,4-
dichlorobutane (1 mmol, 0.113 g) and toluene (1 mL). The reaction
mixture was heated at 130 ºC for 5 h, while a large amount of white
solid appeared. The solid was filtered and washed with dichlorometh-
ane to give the product as white solids in 80% yield (0.41 g). M.p.
275–277 ºC. IR (KBr): 3415, 3130, 3039, 2361, 2341, 1616, 1591,
1
1561, 1481, 1428, 1178, 997, 770, 649 cm−1. H NMR (400 MHz,
D2O) δ = 8.35 (d, J = 4.4 Hz, 2H, C5H5N), 7.87 (t, J = 7.2 Hz, 2H,
C6H4), 7.56 (t, J = 7.6 Hz, 6H, C6H4, C5H5N), 7.47 (t, J = 7.6 Hz, 2H,
C5H5N), 7.39–7.32 (m, 4H, C6H4), 5.75 (s, 4H, NCH2C), 4.58 (s, 4H,
NCH2), 2.03 (s, 4H, CCH2C). 13C NMR (100 MHz, D2O) δ = 151.6,
149.3, 138.8, 131.2, 130.7, 127.3, 127.0, 124.5, 123.5, 113.5, 112.9,
51.3, 46.7, 24.6. Anal. Calcd for C30H30Cl2N6 (545.51 g mol−1): C,
66.05; H, 5.54; N, 15.41. Found: C, 65.78; H, 5.63; N, 15.62%.
1,1′-Di-(pyridin-2-ylmethyl)-4,4′-(1,2-ethanediyl)bisbenzoimidazol
ium dibromide (4): To a 10 mL heavy wall pressure tube was added
1-(pyridin-2-ylmethyl)-1H-benzoimidazole (10 mmol, 2.093 g), 1,2-
dibromoethane (5 mmol, 0.939 g) and toluene (2 mL). The reaction
mixture was heated at 120 ºC for 3 h, while a large amount of white
solid appeared. The solid was filtered and washed with DCM to give
the product as white solids in 87% yield (2.65 g). M.p. >300 ºC. IR
(KBr): 3441, 2170, 1661, 1453, 1402, 1184, 1077, 986, 848, 759, 558
cm−1. 1H NMR (400 MHz, D2O) δ = 8.37 (d, J = 4.4 Hz, 2H, C5H5N),
7.82 (t, J = 7.6 Hz, 2H, C6H4), 7.72 (d, J = 8.4 Hz, 2H, C6H4), 7.55 (t,
J = 6.8 Hz, 2H, C5H5N), 7.42–7.29 (m, 8H, C5H5N, C6H4), 5.74 (s, 4H,
NCH2C), 5.31 (s, 4H, NCH2). 13C NMR (100 MHz, D2O) δ = 151.2,
149.6, 139.0, 131.0, 128.0, 127.8, 124.7, 123.6, 113.9, 111.8, 51.6,
46.6. Anal. Calcd for C28H26Br2N6 (606.35 g mol−1): C, 55.46; H, 4.32;
N, 13.86. Found: C, 55.31; H,4.39; N, 13.93%.
1,1′-Di-(pyridin-2-ylmethyl)-4,4′-(1,3-butanediyl)bisbenzoimidazo
lium dihexafluorophosphate (9): To compound 3 (2.0 mmol, 1.091 g)
or 6 (2.0 mmol, 1.269 g) dissolved into water (6 mL) in a 50 mL
beaker was added dropwise saturated NH4PF6 aqueous solution
(30 mL). A large amount of white solid appeared. The solid was
filtered and washed with water to give the product as a white solid in
99% yield (1.51 g). M.p. 208–209 °C. IR (KBr): 3433, 2170, 1638,
1
1454, 1401, 1076, 987, 846, 764, 558 cm−1. H NMR (DMSO-d6,
400 MHz) δ = 9.94 (s, 2H, NCHN), 8.41 (d, J = 4.4 Hz, 2H, C5H5N),
8.10–8.08 (m, 2H, C6H4), 7.97–7.88 (m, 4H, C5H5N, C6H4), 7.68–7.65
(m, 6H, C5H5N, C6H4), 7.37–7.34 (m, 2H, C5H5N), 5.92 (s, 4H,
NCH2C), 4.65 (s, 4H, NCH2), 2.03 (s, 4H, CH2). 13C NMR (100 MHz,
DMSO-d6): δ = 152.8, 149.5, 143.1, 137.5, 131.3, 130.9, 126.8,
126.5, 123.6, 122.7, 113.8, 113.7, 50.7, 46.1, 25.4. Anal. Calcd for
C30H30P2F12N6 (764.53 g mol−1): C, 47.13; H, 3.96; N, 10.99. Found:
C, 46.88; H, 3.90; N, 11.15%.
1,1′-Di-(pyridin-2-ylmethyl)-4,4′-(1,3-propanediyl)bisbenzoimidaz
olium dibromide (5): To a 10 mL heavy wall pressure tube was added
1-(pyridin-2-ylmethyl)-1H-benzoimidazole (5.0 mmol, 1.046 g), 1,3-
dibromopropane (2.5 mmol, 0.505 g) and toluene (1 mL). The reac-
tion mixture was heated at 120 ºC for 3 h, while a large amount of
white solid appeared. The solid was filtered and washed with DCM to
give the product as a white solid in 89% yield (1.38 g). M.p. 147–148
°C. IR (KBr): 3433, 2170, 1660, 1454, 1401, 1190, 1077, 987, 847,

JOURNAL OF CHEMICAL RESEARCH 2012 113
3
S. Díez-González, N. Marion and S.P. Nolan, Chem. Rev., 2009, 109,
3612.
We gratefully acknowledge Qing Lan Project of Jiangsu
Education Committee (08QLT001 and 08QLD006), Scientific
Research Foundation (SRF) for the Returned Overseas
Chinese Scholars (ROCS), State Education Ministry (SEM),
the Priority Academic Program Development of Jiangsu
Higher Education Institutions (PAPD), 333 project for the
cultivation of high-level talents (33GC10002) and National
Natural Science Foundation of China (NSFC) (21071121 and
21172188) for financial support of this work.
4
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Received 19 December 2011; accepted 16 January 2012
Paper 1101050 doi: 10.3184/174751912X13282044479441
Published online: 23 February 2012
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