The first mesomorphic benzimidazolium-based silver(I)-N-heterocyclic dicarbene complexes: Synthesis, characterization and phase properties

Article abstract of DOI:10.1016/j.molliq.2018.12.075

The first mesomorphic benzimidazolium based mononuclear silver (I) N-heterocyclic dicarbene complexes, Ag(I)-NHC were prepared via in situ deprotonation of 1-octadecyl-3-alkyl/arylbenzimidazolium salts with silver oxide (Ag₂O). Both series of Ag (I)-NHC complexes contained a C18 alkyl chain on one N atom and differed in term of the substituent (alkyl and substituted benzyl moiety) on the other N atom in the benzimidazolium ring. All the synthesized compounds were characterized by elemental analysis, Fourier-transform infrared, ¹H and ¹³C NMR spectroscopy. Cation geometry, anion and alkyl chain length were found to be essential parameters for mesophase formation in the salts and Ag(I)-NHC complexes in this study. All dialkylated salts except the one with the shortest chain, n = 10 were smectogenic. On the other hand, a large width-to-length ratio of the cationic core in the benzyl analogues had a negative effect on the formation and stability of mesophase. No mesophases were observed for these compounds. Wider mesophase range was seen with decrease in anion size and elongation of alkyl chains. A metastable SmA phase was observed in two Ag(I)-NHC complexes. The minimum chain length required for compounds with different anions to exhibit liquid crystal (LC) properties was addressed. Mesophase was observed for bromide salts with C12 onwards while C14 is the minimum chain length required for the hexaflurophosphate containing Ag(I)-NHC complexes to display LC properties. All Ag(I)-NHC complexes except the one bearing a cyano group could be considered ionic liquids with melting/clearing temperatures below 100 °C.

Full text of DOI:10.1016/j.molliq.2018.12.075

Accepted Manuscript
The first mesomorphic benzimidazolium-based silver(I)-N-
heterocyclic dicarbene complexes: Synthesis, characterization and
phase properties
Choon Wan Yeap, Rosenani A. Haque, Wan Sinn Yam, Mohd.R.
Razali
PII:
S0167-7322(18)32959-3
DOI:
https://doi.org/10.1016/j.molliq.2018.12.075
MOLLIQ 10151
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
7 June 2018
13 December 2018
13 December 2018
Please cite this article as: Choon Wan Yeap, Rosenani A. Haque, Wan Sinn Yam, Mohd.R.
Razali , The first mesomorphic benzimidazolium-based silver(I)-N-heterocyclic dicarbene
complexes: Synthesis, characterization and phase properties. Molliq (2018),
https://doi.org/10.1016/j.molliq.2018.12.075
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ACCEPTED MANUSCRIPT
The First Mesomorphic Benzimidazolium-based Silver(I)-N-Heterocyclic
Dicarbene Complexes: Synthesis, Characterization and Phase Properties
Choon Wan Yeap, Rosenani A. Haque, Wan Sinn Yam‡ and Mohd. R. Razali
School of Chemical Sciences, Universiti Sains Malaysia, 11800 USM Penang, Malaysia.
‡
Corresponding email: wansinn@usm.my
The first mesomorphic benzimidazolium based mononuclear silver(I) N-heterocyclic
dicarbene complexes, Ag(I)-NHC were prepared via in situ deprotonation of 1-octadecyl-
3
-alkyl/arylbenzimidazolium salts with silver oxide (Ag O). Both series of Ag(I)-NHC
2
complexes contained a C18 alkyl chain on one N atom and differed in term of the
substituent (alkyl and substituted benzyl moiety) on the other N atom in the
benzimidazolium ring. All the synthesized compounds were characterized by elemental
1
13
analysis, Fourier-transform infrared, H- and C-NMR spectroscopy. Cation geometry,
anion and alkyl chain length were found to be essential parameters for mesophase
formation in the salts and Ag(I)-NHC complexes in this study. All dialkylated salts
except the one with the shortest chain, n =10 were smectogenic. On the other hand, a
large width-to-length ratio of the cationic core in the benzyl analogues had a negative
effect on the formation and stability of mesophase. No mesophases were observed for
these compounds. Wider mesophase range was seen with decrease in anion size and
elongation of alkyl chains. A metastable SmA phase was observed in two Ag(I)-NHC
complexes. The minimum chain length required for compounds with different anions to
exhibit liquid crystal (LC) properties was addressed. Mesophase was observed for
bromide salts with C12 onwards whilst C14 is the minimum chain length required for the
hexaflurophosphate containing Ag(I)-NHC complexes to display LC properties. All
1

ACCEPTED MANUSCRIPT
Ag(I)-NHC complexes except the one bearing a cyano group could be considered ionic
liquids with melting/clearing temperatures below 100 °C.
Keywords: Silver(I)-N-heterocyclic dicarbene complexes, smectogenic, in situ
deprotonation, benzimidazolium salts, ionic liquids
1
. Introduction
Research interests in the studies of metal containing ionic liquid crystals (MILCs) has
grown rapidly due to their unique properties such as colour, structural motifs, and
magnetism that cannot be obtained from conventional ionic liquid crystals [1-3].
Numerous reports on MILCs had been documented but work on the phase properties of
benzimidazolium based carbene complexes is limited. N-heterocyclic carbenes have
strong electron-donating ability and they often form stable complexes with metal ions [4].
The first mesomorphic dialkylbenzimidazol-2-ylidene Au(I) dicarbene complexes were
obtained via the phase-transfer catalysis technique [5]. Thermally stable liquid crystalline
Pd(II) dicarbene complexes were also reported [6]. We are interested in the chemistry of
Ag(I). Ag(I) compounds are used in medicine [7], as transmetalation agents [8],
photoactive materials [9], and show superior catalytic activities[10]. Our investigation on
Ag(I) dicarbene complexes showed that these compounds are potential anticancer agents
[
11-13]. Liquid crystals are self-organized and functional soft materials with ordering and
mobility at nanometric level. They respond to various external stimuli such as light, heat,
electric and magnetic fields as well as the changes in chemical environment. Owing to
their dynamic nature, they are good candidates for the construction of stimuli-responsive,
2

ACCEPTED MANUSCRIPT
multifunctional materials in controlled drug delivery. Hence, we are tempted to study the
LC properties of Ag(I)-NHC complexes. We have attempted to investigate the essential
parameters for mesophase formation and this has prompted us to conduct a systematic
structure-property study on 1,3-disubstitutedbenzimidazol-2-ylidene silver (I) carbene
complexes by varying the cation geometry, anion and chain length. Herein, we report the
synthesis and phase behavior of 3-alkyl/aryl-1-octadecyl benzimidazolium halides and
the corresponding Ag(I) dicarbene complexes. MILCs containing carbene can be
obtained using various methods, for example, (i) reaction of free NHCs with metal
precursors [14]; (ii) reaction of azolium salts with suitable basic transition metal salts or
metal precursors under basic phase transfer catalysis (PTC) conditions [15]; (iii) reaction
of electron-rich olefin dimers with organometallic fragments [16]; and (iv)
transmetalation with metal-NHCs [17]. In this study, the Ag(I) dicarbene complexes
were prepared using in situ deprotonation of azolium salts with silver oxide (Ag O).
2
2
. Experimental
2
.1 Materials and instruments
All chemicals and solvents were of analytical grade and used as received. Elemental
analyses were carried out using a Perkin Elmer 2400 LS Series CHNS/O analyzer. FTIR
1
13
spectra were recorded on a Perkin Elmer 2000 spectrometer. H- and C- NMR spectral
analyses were recorded on a Bruker-Avance 500 MHz spectrometer. Melting points were
taken using a Stuart Scientific SMP-1 (UK) instrument. Phase transition temperatures and
enthalpy change values of the 1,3-disubstitutedbenzimidazolium salts (carbene precursors)
and their metal complexes were determined using a Perkin Elmer Pyris-1 differential
3

ACCEPTED MANUSCRIPT
-1
scanning calorimeters (DSC) at a heating and cooling rate of 5°Cmin . Liquid crystalline
textures of the compounds were studied using a BX53 Olympus microscope equipped
with a T95 LinkPad controller and a THMS600 hotstage.
Scheme 1: The preparation of 1,3-disubstituted benzimidazol-2-ylidenes (1-8) and their
Ag(I)-NHC complexes (9-15).
2
.2 Synthesis of N-octadecyl benzimidazole
N-Octadecyl benzimidazole was prepared according to the reported method with some
modifications [18]. A mixture of benzimidazole (0.50 g, 4.23 mmol) and KOH (1.5
equiv.) in MeCN (20 ml) was stirred for 30 minutes in a round bottom flask. 1-
Bromooctadecane (1 equiv.) was then added and the reaction mixture was stirred at room
temperature overnight. The product was extracted from the reaction mixture with n-
hexane (4 x 20 ml). The n-hexane layer was then collected and dried over MgSO
N-octadecyl benzimidazole.
4
to yield
4

ACCEPTED MANUSCRIPT
-1
Yield: 1.33g (84%). Melting point: 49-52 °C. FT-IR (KBr), ν/cm : 2917, 2850 (Caliph-H);
1
1
0
496 (C=N); 1612, 1471 (C=C) ; 1199 (C-N). H NMR (500 MHz, CD OD), δ/ppm:
ar
3
.89 (t, J = 7 Hz, 3H, CH ); 1.24-1.32 (m, 30H, 15 x CH ); 1.87 (quin, J = 7.1 Hz, 2H,
3
2
CH ); 4.27 (t, J = 7 Hz, 2H, N-CH ); 7.26 (td, J = 7.5, 1 Hz, 1H, Ar-H); 7.31 (td, J = 7.5,
2
2
1
Hz, 1H, Ar-H); 7.54 (d, J = 8 Hz, 1H, Ar-H); 7.67 (d, J = 8 Hz, 1H, Ar-H); 8.13 (s, 1H,
1
3
NCHN). C NMR (125 MHz, CD OD), δ/ppm: 144.63 (NCHN); 144.10, 134.98, 124.17,
3
1
3
23.44, 120.18, 111.43 (Ar-C); 46.01 (N-CH ); 33.09, 30.86, 30.80, 30.77, 30.75, 30.70,
2
0.59, 30.55, 30.48, 30.18, 27.74, 23.75 (CH ); 14.46(CH ). Signals of the methylene
2
3
carbons were overlapped due to similar chemical environment these carbons experience.
Similar peak pattern was observed for the 3-alkyl/aryl-1-octadecyl-benzimidazolium
bromides and Ag(I)-NHC complexes.
2
.3 Synthesis of 3-alkyl/aryl-1-octadecyl-benzimidazolium halides (1-8)
3
-Alkyl/Aryl-1-octadecyl-benzimidazolium halides (1-8) were prepared from N-
octadecyl benzimidazole according to the reported methods with some modifications [18,
9]. N-Octadecyl benzimidazole (0.30 g, 0.81 mmol) and the corresponding 1-
1
bromoalkane (1-5) (10 quiv.) in neat condition or benzyl halides (6-8) (1.5 equiv.) in 1,4-
dioxane were added in a round bottom flask and the reaction mixture was refluxed for
two days. The mixture was cooled to room temperature and the solid obtained was
collected and recrystallized from ethyl acetate to give a white or cream-color solid.
1
(white solid): Yield: 0.47 g (96 %). Melting point: 108-110 °C. Anal. calcd for
C H N Br·H O: C, 68.93; H, 10.74; N, 4.59. Found: C, 68.63; H 10.84; N, 4.61. FT-IR
35
63
2
2
-1
(
KBr), ν/cm : 3494, 3426 (O-H); 3058 (C -H); 2915, 2849 (Caliph-H); 1564 (C=N); 1621,
ar
5

ACCEPTED MANUSCRIPT
1
1
470 (C=C) ; 1211 (C-N). H NMR (500 MHz, CD OD), δ/ppm: 0.89 (t, J = 6.8 Hz, 3H,
ar
3
CH ); 0.89 (t, J = 6.8 Hz, 3H, CH ); 1.27-1.39 (m, 44H, 22 x CH ); 2.03 (quint, J = 7 Hz,
3
3
2
4
H, 2 x CH ); 4.56 (t, J = 7.25 Hz, 4H, 2 x N-CH ); 7.72-7.75 (m, 2H, Ar-H); 8.00-8.03
2 2
1
3
(
m, 2H, Ar-H); 9.67 (s, 1H, NCHN). C NMR (125 MHz, CD OD), δ/ppm: 142.68
3
(
NCHN); 133.00, 128.33, 114.67 (Ar-C); 48.58 (N-CH ); 33.09, 33.05, 30.78, 30.73,
2
3
1
0.62, 30.60, 30.56, 30.55, 30.48, 30.42, 30.21, 30.19, 30.14, 27.54, 23.75, 23.73 (CH2);
4.46 (CH3).
2
(cream-colour solid): Yield: 0.45 g (86 %). Melting point: 112-113. Anal. calcd for
C H N Br·H O: C, 69.67; H, 10.90; N, 4.39. Found: C, 69.78; H 11.04; N, 4.25. FT-IR
37
67
2
2
-1
(
KBr), ν/cm : 3494, 3427 (O-H); 3059 (C -H); 2915, 2849 (Caliph-H); 1565 (C=N), 1621,
ar
1
1
470 (C=C) ; 1216 (C-N). H NMR (500 MHz, CD OD), δ/ppm: 0.89 (t, J = 7 Hz, 3H,
ar
3
CH ); 0.89 (t, J = 7 Hz, 3H, CH ); 1.27-1.39 (m, 48H, 24 x CH ); 2.03 (quint, J = 7 Hz,
3
3
2
4
H, 2 x CH ); 4.56 (t, J = 7.25 Hz, 4H, 2 x N-CH ); 7.72-7.75 (m, 2H, Ar-H); 8.00-8.03
2 2
1
3
(
m, 2H, Ar-H); 9.67 (s, 1H, NCHN). C NMR (125 MHz, CD OD), δ/ppm: 142.68
3
(
NCHN); 133.00, 128.34, 114.68 (Ar-C); 48.59 (N-CH ); 33.08, 30.79, 30.77, 30.63,
2
3
0.62, 30.56, 30.48, 30.19, 30.18, 30.14, 27.53, 23.74 (CH ); 14.46 (CH ).
2 3
3
(cream-colour solid): Yield: 0.41 g (76 %). Melting point: 114-116. Anal. calcd for
C H N Br·1.5H O: C, 69.40; H, 11.05; N, 4.15. Found: C, 69.82; H 11.02; N, 3.98. FT-
39
71
2
2
-1
IR (KBr), ν/cm : 3494, 3426 (O-H); 3059 (C -H); 2915, 2850 (Caliph-H); 1564 (C=N);
ar
1
1
6
621, 1470 (C=C) ; 1214 (C-N). H NMR (500 MHz, CD OD), δ/ppm: 0.89 (t, J = 7 Hz,
ar
3
H, 2 x CH ); 1.27-1.39 (m, 52H, 26 x CH ); 2.03 (quint, J = 7 Hz, 4H, 2 x CH ); 4.56 (t,
3
2
2
6

ACCEPTED MANUSCRIPT
J = 7.25 Hz, 4H, 2 x N-CH ); 7.72-7.75 (m, 2H, Ar-H); 8.00-8.04 (m, 2H, Ar-H); 9.68 (s,
2
1
3
1
1
2
H, NCHN). C NMR (125 MHz, CD OD), δ/ppm: 142.69 (NCHN); 133.00, 128.34,
3
14.68 (Ar-C); 48.57 (N-CH ); 33.09, 30.80, 30.75, 30.63, 30.57, 30.49, 30.18, 30.15,
2
7.53, 23.75 (CH ); 14.47 (CH ).
2
3
4
(cream-colour solid): Yield: 0.48 g (85 %). Melting point: 100-102. Anal. calcd for
C H N Br·1.5H O: C, 70.05; H, 11.18; N, 3.98. Found: C, 70.11; H 11.25; N, 3.81. FT-
41
75
2
2
-1
IR (KBr), ν/cm : 3469 (O-H); 3042 (C -H); 2918, 2850 (Caliph-H); 1564 (C=N); 1614,
ar
1
1
466 (C=C) ; 1217 (C-N). H NMR (500 MHz, CD OD), δ/ppm: 0.88 (t, J = 7 Hz, 6H, 2
ar
3
x CH ); 1.26-1.38 (m, 56H, 28 x CH ); 2.02 (quint, J = 7 Hz, 4H, 2 x CH ); 4.54 (t, J =
3
2
2
7
.25 Hz, 4H, 2 x N-CH ); 7.71-7.75 (m, 2H, Ar-H); 7.96-7.99 (m, 2H, Ar-H); 9.69 (s, 1H,
2
1
3
NCHN). C NMR (125 MHz, CD OD), δ/ppm: 142.55 (NCHN); 132.97, 128.54, 114.70
3
(
Ar-C); 48.71 (N-CH ); 33.13, 30.84, 30.82, 30.79, 30.68, 30.61, 30.53, 30.29, 30.20,
2
2
7.62, 23.80 (CH ); 14.67 (CH ).
2 3
5
(cream-colour solid): Yield: 0.44 g (75 %). Melting point: 115-116. Anal. calcd for
C H N Br·H O: C, 71.53; H, 11.30; N, 3.87. Found: C, 71.37; H 11.43; N, 3.70. FT-IR
43
79
2
2
-
1
(
(
(
KBr), ν/cm : 3430 (O-H); 2917, 2850 (C -H); 1561 (C=N); 1618, 1467 (C=C) ; 1213
aliph ar
1
C-N). H NMR (500 MHz, CD OD), δ/ppm: 0.87 (t, J = 7 Hz, 6H, 2 x CH ); 1.24-1.37
3
3
m, 60H, 30 x CH ); 2.02 (quint, J = 7 Hz, 4H, 2 x CH ); 4.54 (t, J = 7.25 Hz, 4H, 2 x N-
2
2
1
3
CH ); 7.71-7.74 (m, 2H, Ar-H); 7.92-7.94 (m, 2H, Ar-H); 9.74 (s, 1H, NCHN). C NMR
2
(
125 MHz, CD OD), δ/ppm: 142.47 (NCHN); 132.93, 128.68, 114.71 (Ar-C); 48.83 (N-
3
7

ACCEPTED MANUSCRIPT
CH ); 33.16, 30.89, 30.86, 30.82, 30.72, 30.65, 30.56, 30.40, 30.24, 27.70, 23.85 (CH );
2
2
1
4.84 (CH3).
6
(white solid): Yield: 0.34 g (82 %). Melting point: 105-108 °C. Anal. calcd for
C H N Cl·H O: C, 74.59; H, 9.97; N, 5.43. Found: C, 74.43; H 10.03; N, 5.37. FT-IR
32
49
2
2
-1
(
KBr), ν/cm : 3425 (O-H); 3033 (C -H); 2922, 2851 (Caliph-H); 1565 (C=N); 1615, 1470
ar
1
(
C=C) ; 1228 (C-N). H NMR (500 MHz, DMSO-d ), δ/ppm: 0.83 (t, J = 7 Hz, 3H, CH );
ar
6
3
1
.21-1.30 (m, 30H, 15 x CH ); 1.92 (quint, J = 7 Hz, 2H, CH ); 4.51 (t, J = 7.25 Hz, 2H,
2 2
N-CH ); 5.78 (s, 2H, N-CH -Ar); 7.36-7.42 (m, 1H, Ar-H); 7.39 (d, J = 7.5 Hz, 2H, Ar-
2
2
H); 7.51 (d, J = 7 Hz, 2H, Ar-H); 7.63-7.68 (m, 2H, Ar-H); 7.95 (dd, J = 7, 1 Hz, 1H, Ar-
1
3
H); 8.09 (dd, J = 7.25, 1.25 Hz, 1H, Ar-H); 10.09 (s, 1H, NCHN). C NMR (125 MHz,
DMSO-d ), δ/ppm: 142.85 (NCHN); 134.52, 131.77, 131.30, 129.44, 129.18, 128.66,
6
1
2
27.20, 127.15, 114.33 (Ar-C); 50.35 (N-CH -Ar); 47.31 (N-CH ); 31.72, 29.45, 29.43,
2
2
9.33, 29.28, 29.13, 28.85, 26.21, 22.53 (CH ); 14.38 (CH ).
2
3
7
(white solid): Yield: 0.37 g (80 %). Melting point: 118-119 °C. Anal. calcd for
C H N Br·0.25H O: C, 70.75; H, 9.26; N, 5.00. Found: C, 70.84; H 9.22; N, 4.82. FT-
33
51
2
2
-1
IR (KBr), ν/cm : 3435 (O-H); 3037 (C -H); 2918, 2850 (Caliph-H); 1557 (C=N); 1615,
ar
1
1
3
461 (C=C) ; 1216 (C-N). H NMR (500 MHz, DMSO-d ), δ/ppm: 0.85 (t, J = 6.75 Hz,
ar
6
H, CH ); 1.22-1.31 (m, 30H, 15 x CH ); 1.92 (quint, J = 6.75 Hz, 2H, CH ); 2.29 (s, 3H,
3
2
2
Ar-CH ); 4.52 (t, J = 7.25 Hz, 2H, N-CH ); 5.73 (s, 2H, N-CH -Ar); 7.22 (d, J = 8 Hz,
3
2
2
2
1
H, Ar-H); 7.42 (d, J = 7.5 Hz, 2H, Ar-H); 7.63-7.69 (m, 2H, Ar-H); 7.97 (d, J = 8 Hz,
1
3
H, Ar-H); 8.11 (d, J = 8 Hz, 1H, Ar-H); 9.99 (s, 1H, NCHN). C NMR (125 MHz,
8

ACCEPTED MANUSCRIPT
DMSO-d ), δ/ppm: 142.71 (NCHN); 138.64, 131.49, 131.77, 131.28, 129.97, 128.75,
6
1
2
27.14, 127.10, 114.38, 114.33 (Ar-C); 50.18 (N-CH -Ar); 47.28 (N-CH ); 31.74, 29.48,
2 2
9.35, 29.30, 29.15, 28.87, 26.21, 22.54 (CH ); 21.16 (Ar-CH ); 14.40 (CH ).
2
3
3
8
(white solid): Yield: 0.39 g (82 %). Melting point: 152-154 °C. Anal. calcd for
C H N Br·H O: C, 67.79; H, 8.61; N, 7.18. Found: C, 67.99; H 8.49; N, 6.99. FT-IR
33
48
3
2
-1
(
KBr), ν/cm : 3461, 3411 (O-H); 3036 (C -H); 2918, 2850 (Caliph-H); 2231 (C≡N); 1561
ar
1
(
C=N); 1610, 1466 (C=C) ; 1216 (C-N). H NMR (500 MHz, DMSO-d ), δ/ppm: 0.85 (t,
ar
6
J = 6.75 Hz, 3H, CH ); 1.23-1.33 (m, 30H, 15 x CH ); 1.94 (quint, J = 7 Hz, 2H, CH );
3
2
2
4
.52 (t, J = 7.25 Hz, 2H, N-CH ); 5.91 (s, 2H, N-CH -Ar); 7.64-7.69 (m, 2H, Ar-H); 7.69
2 2
(d, J = 8 Hz, 2H, Ar-H); 7.91 (d, J = 8.5 Hz, 2H, Ar-H); 7.92 (d, J = 8.5 Hz, 1H, Ar-H);
1
3
8
.14 (d, J = 8 Hz, 1H, Ar-H); 10.03 (s, 1H, NCHN). C NMR (125 MHz, DMSO-d ),
6
δ/ppm: 143.24 (NCHN); 139.97, 133.31, 131.82, 131.31, 129.50, 127.30, 127.20, 114.45,
1
2
14.20, 111.94 (Ar-C); 118.85 (Ar-CN); 49.76 (N-CH -Ar); 47.38 (N-CH ); 31.75, 29.49,
2
2
9.39, 29.30, 29.16, 28.92, 28.84, 26.24, 22.55 (CH ); 14.40 (CH ).
2
3
2
.4
Synthesis
of
3-alkyl/aryl-1-octadecyl-benzimidazol-2-ylidene
silver(I)
hexafluorophosphates (9-15)
The mononuclear Ag(I)-NHC complexes (9-15) were prepared via in situ deprotonation
reaction between the 3-alkyl/aryl-1-octadecyl-benzimidazolium halides (1-4, 6-8) and
silver oxide, Ag O according to the reported techniques [19, 20]. A mixture of the
2
corresponding 3-alkyl/aryl-1-octadecyl-benzimidazolium halides (0.66 mmol) and Ag O
2
(1.0 equiv.) in methanol (25 mL) was stirred in the dark at room temperature for two days.
9

ACCEPTED MANUSCRIPT
The reaction mixture was then filtered through Celite to remove unreacted Ag O. To the
2
clear filtrate, KPF (2.0 equiv.) was added. The mixture was stirred at room temperature
6
for 3 h and allowed to stand overnight. The solvent was then removed under reduced
pressure, and the resultant powder was washed with distilled water to remove unreacted
KPF and air dried. The powder was then recrystallized from methanol to give a white
6
powder.
9
: Yield: 0.22 g (53 %). Melting point: 83-85 °C. Anal. calcd for
C H AgF N P.2.5H O: C, 63.71; H, 9.85; N, 4.25. Found: C, 63.87; H, 9.98; N, 4.44.
70
124
6
4
2
-1
1
FT-IR (KBr), ν/cm : 2921, 2852 (C -H); 1466 (C=C) ; 1409 (C=N); 1036 (C-N). H
aliph
ar
NMR (500 MHz, Acetone-d ), δ/ppm: 0.84 (t, J = 7 Hz, 6H, 2 x CH ); 0.89 (t, J = 7 Hz,
6
3
6
8
H, 2 x CH ); 1.23-1.32 (m, 72H, 36 x CH ); 1.38-1.44 (m, 8H, 4 x CH ); 1.48-1.54 (m,
3 2 2
H, 4 x CH ); 2.11 (quint, J = 7.5 Hz, 8H, 4 x CH ); 4.72 (t, J = 7.25 Hz, 8H, 4 x N-CH -
2
2
2
1
3
)
; 7.55-7.57 (m, 4H, 4 x Ar-H); 7.89-7.91 (m, 4H, 4 x Ar-H). C NMR (125 MHz,
Acetone-d ), δ/ppm: 187.71 (Ag-C); 133.76, 124.38, 112.15 (Ar-C); 49.30 (N-CH );
6
2
3
1.75, 31.73, 30.52, 30.49, 29.52, 29.48, 29.46, 29.25, 29.19, 29.18, 26.86, 22.44, 22.41
(
CH ); 13.46, 13.45 (CH ).
2
3
1
0: Yield: 0.28 g (54 %). Melting point: 78-80 °C. Anal. calcd for
C H AgF N P.2.5H O: C, 64.60; H, 10.04; N, 4.07. Found: C, 64.61; H, 9.30; N, 4.36.
74
132
6
4
2
-1
1
FT-IR (KBr), ν/cm : 2920, 2851 (C -H); 1467 (C=C) ; 1404 (C=N); 1014 (C-N). H
aliph
ar
NMR (500 MHz, Acetone-d ), δ/ppm: 0.87 (t, J = 7 Hz, 6H, 2 x CH ); 0.89 (t, J = 7 Hz,
6
3
6
8
H, 2 x CH ); 1.23-1.29 (m, 80H, 40 x CH ); 1.39-1.44 (m, 8H, 4 x CH ); 1.48-1.54 (m,
3 2 2
H, 4 x CH ); 2.11 (quint, J = 7.5 Hz, 8H, 4 x CH ); 4.73 (t, J = 7.25 Hz, 8H, 4 x N-CH -
2
2
2
1
0

ACCEPTED MANUSCRIPT
1
3
)
; 7.55-7.58 (m, 4H, 4 x Ar-H); 7.90-7.91 (m, 4H, 4 x Ar-H). C NMR (125 MHz,
Acetone-d ), δ/ppm: 188.53, 186.98 (d, Ag-C); 133.76, 124.40, 112.16 (Ar-C); 49.31 (N-
6
CH -); 31.75, 30.52, 30.50, 29.53, 29.50, 29.28, 29.19, 26.88, 22.44 (-CH -); 13.47 (-
2
2
CH3).
1
6
2
1: Yield: 0.17 g (42 %). Melting point: 81-84 °C. Anal. calcd for C H AgF N P: C,
7
8
140
6
4
-1
7.55; H, 10.18; N, 4.04. Found: C, 67.84; H, 10.29; N, 3.95. FT-IR (KBr), ν/cm : 2919,
1
851 (Caliph-H); 1467 (C=C) ; 1405 (C=N); 1124 (C-N). H NMR (500 MHz, Acetone-
ar
d ), δ/ppm: 0.88 (t, J = 7 Hz, 6H, 2 x CH ); 0.89 (t, J = 7 Hz, 6H, 2 x CH ); 1.23-1.29 (m,
6
3
3
8
7
7
8H, 44 x CH ); 1.38-1.44 (m, 8H, 4 x CH ); 1.48-1.54 (m, 8H, 4 x CH ); 2.11 (quint, J =
2 2 2
.5 Hz, 8H, 4 x CH ); 4.73 (t, J = 7 Hz, 8H, 4 x N-CH -); 7.55-7.57 (m, 4H, 2 x Ar-H);
2
2
1
3
.89-7.91 (m, 4H, 2 x Ar-H). C NMR (125 MHz, Acetone-d ), δ/ppm: 188.56, 187.00
6
(d, Ag-C, J(109Ag-C) = 207 Hz, J (107Ag-C) = 183 Hz); 133.76, 124.40, 112.16 (Ar-C); 49.31
(
N-CH -); 31.75, 30.49, 29.53, 29.49, 29.25, 29.19, 26.88, 22.44 (-CH -); 13.47 (-CH ).
2
2
3
1
6
2
2: Yield: 0.22 g (44 %). Melting point: 99-101 °C. Anal. calcd for C H AgF N P: C,
82 148 6 4
-1
8.26; H, 10.34; N, 3.88. Found: C, 68.19; H, 10.27; N, 3.85. FT-IR (KBr), ν/cm : 2919,
1
851 (Caliph-H); 1467 (C=C) ; 1405 (C=N); 1144 (C-N). H NMR (500 MHz, Acetone-
ar
d ), δ/ppm: 0.89 (t, J = 7 Hz, 12H, 4 x CH ); 1.23-1.29 (m, 96H, 48 x CH ); 1.39-1.44 (m,
6
3
2
8
H, 4 x CH ); 1.47-1.53 (m, 8H, 4 x CH ); 2.11 (quint, J = 7.5 Hz, 8H, 4 x CH ); 4.73 (t,
2 2 2
J = 7 Hz, 8H, 4 x N-CH -); 7.55-7.57 (m, 4H, 4 x Ar-H); 7.89-7.91 (m, 4H, 4 x Ar-H).
2
1
3
C NMR (125 MHz, Acetone-d ), δ/ppm: 188.62, 187.07 (d, Ag-C, J
= 206 Hz, J
6
(109Ag-C)
1
1

ACCEPTED MANUSCRIPT
=
181 Hz); 133.82, 124.40, 112.11 (Ar-C); 49.35 (N-CH -); 31.69, 30.51, 29.45,
(107Ag-C)
2
2
9.41, 29.28, 29.20, 26.84, 22.35 (-CH -); 13.34 (-CH ).
2 3
1
3: Yield: 0.32 g (56%). Melting point: 110-113 °C. Anal.
calcd for
C H AgF N P.H O: C, 64.47; H, 8.28; N, 4.70. Found: C, 64.38; H, 8.26; N, 4.60. FT-
64
96
6
4
2
-1
IR (KBr), ν/cm : 2919, 2850 (C -H); 1467 (C=C) ; 1401 (C=N); 1129 (C-N).
aliph
ar
1
H NMR (500 MHz, Acetone-d ), δ/ppm: 0.89 (t, J = 7 Hz, 6H, 2 x CH ); 1.22-1.35 (m,
6
3
5
6H, 28 x CH ); 1.43 (quint, J = 7.5 Hz, 4H, 2 x CH ); 2.03 (quint, J = 7.25 Hz, 4H, 2 x
2 2
CH ); 4.65 (t, J = 7.25 Hz, 4H, 2 x N-CH -); 5.86 (s, 4H, 2 x N-CH -Ar); 7.33-7.38 (m,
2
2
2
2
7
8
H, 2 x Ar-H); 7.36 (d, J = 7.5 Hz, 4H, 4 x Ar-H); 7.42 (d, J = 7.5 Hz, 4H, 4 x Ar-H);
.48 (td, J = 7.5, 1 Hz, 2H, 2 x Ar-H); 7.53 (td, J = 7.5, 1 Hz, 2H, 2 x Ar-H); 7.75 (d, J =
1
3
Hz, 2H, 2 x Ar-H); 7.88 (d, J = 8 Hz, 2H, 2 x Ar-H). C NMR (125 MHz, Acetone-d ),
6
δ/ppm: 188.97 (Ag-C); 136.40, 133.98, 133.83, 129.00, 128.26, 127.29, 124.45, 112.36,
1
2
12.15 (Ar-C); 52.30 (N-CH -Ar); 49.41 (N-CH -); 31.75, 30.40, 29.52, 29.46, 29.19,
2
2
9.17, 26.78, 22.44 (-CH -); 13.47 (-CH ).
2
3
1
4: Yield: 0.20 g (38 %). Melting point: 94-96 °C. Anal. calcd for
C H AgF N P.0.5H O: C, 65.44; H, 8.40; N, 4.63. Found: C, 65.38; H, 8.49; N, 4.55.
66
100
6
4
2
-1
1
FT-IR (KBr), ν/cm : 2916, 2850 (C -H); 1470 (C=C) ; 1401 (C=N); 1021 (C-N). H
aliph
ar
NMR (500 MHz, Acetone-d ), δ/ppm: 0.89 (t, J = 7 Hz, 6H, 2 x CH ); 1.21-1.33 (m, 56H,
6
3
2
2
8 x CH ); 1.41 (quint, J = 7.5 Hz, 4H, 2 x CH ); 2.02 (quint, J = 7 Hz, 4H, 2 x CH );
2 2 2
.27 (s, 6H, 2 x Ar-CH ); 4.64 (t, J = 7.25 Hz, 4H, 2 x N-CH -); 5.79 (s, 4H, 2 x N-CH -
3
2
2
Ar); 7.17 (d, J = 8 Hz, 4H, 4 x Ar-H); 7.31 (d, J = 8 Hz, 4H, 4 x Ar-H); 7.48 (td, J = 7.5,
1
2

ACCEPTED MANUSCRIPT
1
Hz, 2H, 2 x Ar-H); 7.52 (td, J = 7.5, 1 Hz, 2H, 2 x Ar-H); 7.77 (d, J = 7.5 Hz, 2H, 2 x
1
3
Ar-H); 7.88 (d, J = 8 Hz, 2H, 2 x Ar-H). C NMR (125 MHz, Acetone-d ), δ/ppm:
6
1
1
89.45, 187.88 (d, Ag-C, J(109Ag-C) = 209 Hz, J (107Ag-C) = 183 Hz); 137.99, 133.97, 133.79,
33.35, 129.60, 127.38, 124.43, 112.37, 112.11 (Ar-C); 52.05 (N-CH -Ar); 49.38 (N-
2
CH -); 31.74, 30.38, 29.50, 29.46, 29.17, 29.16, 26.76, 22.43 (-CH -); 20.20 (Ar-CH );
2
2
3
1
3.46 (-CH3).
1
5: Yield: 0.17 g (34 %). Melting point: 106-108 °C. Anal. calcd for
C H AgF N P·H O: C, 63.80; H, 7.79; N, 6.76. Found: C, 63.93; H, 7.67; N, 6.66. FT-
66
94
6
6
2
-1
IR (KBr), ν/cm : 2917, 2850 (C -H); 2230 (C≡N); 1467 (C=C) ; 1401 (C=N); 1195
aliph
ar
1
(
C-N). H NMR (500 MHz, Acetone-d ), δ/ppm: 0.89 (t, J = 7 Hz, 6H, 2 x CH ); 1.23-
6
3
1
4
.34 (m, 56H, 28 x CH ); 1.43 (quint, J = 7.4 Hz, 4H, 2 x CH ); 2.05 (quint, J = 7.5 Hz,
2 2
H, 2 x CH ); 4.68 (t, J = 7 Hz, 4H, 2 x N-CH -); 6.00 (s, 4H, 2 x N-CH -Ar); 7.47-7.50
2
2
2
(
m, 2H, 2 x Ar-H); 7.53-7.57 (m, 2H, 2 x Ar-H); 7.56 (d, J = 8.5 Hz, 4H, 4 x Ar-H); 7.71
d, J = 8 Hz, 2H, 2 x Ar-H); 7.76 (d, J = 8 Hz, 4H, 4 x Ar-H); 7.92 (d, J = 8 Hz, 2H, 2 x
(
1
3
Ar-H). C NMR (125 MHz, Acetone-d ), δ/ppm: 188.84, 188.37 (d, Ag-C); 141.75,
6
1
33.88, 133.82, 132.75, 127.93, 124.70, 124.67, 112.32, 112.22, 112.04 (Ar-C); 117.93
(
Ar-CN); 51.78 (N-CH -Ar); 49.48 (N-CH -); 31.75, 30.43, 29.52, 29.48, 29.21, 29.19,
2
2
2
6.82, 22.44 (-CH -); 13.47 (-CH ).
2
3
3
.0 Results and discussion
1
3

ACCEPTED MANUSCRIPT
Synthesis of N-octadecyl benzimidazole, 3-alkyl-1-octadecyl-benzimidazolium halides
(1-8) and the Ag(I)-NHC complexes (9-15) were carried out according to the reported
methods with modifications [18, 21]. The Ag(I)-NHC complexes were prepared via in
situ deprotonation of the corresponding 1,3-disubstituted benzimidazol-2-ylidenes and
silver oxide (1:1, w/w) in methanol at room temperature and stirred for two days (Scheme
1
). After the completion of reaction, all complexes containing the bromide ions were
converted to the corresponding PF homologues via in-situ metathesis reaction for the
6
same reason as mentioned in our previously reported work [19]. However, all attempts
(including prolonged reaction time, varying molar ratio and addition of a base) to
synthesize Ag(I)-NHC complex from compound 5 failed as evident from the presence of
1
the acidic proton signal, N-CH-N of the benzimidazolium ring in the H-NMR spectra.
Failure to convert 5 into a silver(I)-NHC complex may be attributed to the lability of the
complex which degraded in the presence of a strong base.
3
.1 FTIR analysis
From the IR spectra of all 1,3-disubstituted benzimidazolium salts, the presence of water
-1
was apparent based on the observation of strong bands within 3411-3494 cm [22]. The
-1
C-H aromatic stretching was observed within 3033-3059 cm while the aliphatic
-1
C sp3-H stretching was observed within 2849-2922 cm . Aromatic C=C stretching could
-
1
-1
-1
be observed at 1617 cm and 1467 cm . The bands observed at 1557-1565 cm were
attributed to the C=N stretching in the salts. The C-N stretching in the 1,3-disubstituted
-1
benzimidazolium salts were observed within 1211 to 1217 cm . Upon replacing the alkyl
chains with the substituted benzyl moieties, no significant frequency shifts were seen for
1
4

ACCEPTED MANUSCRIPT
the diagnostic bands: C-H aromatic, C -H aliphatic, C=C aromatic, C=N and C-N
sp3
stretching except for the one bearing the cyano group which showed a sharp band around
-1
2
231 cm assignable to the ν(C≡N) stretching. Upon complexation with silver ions, C=N
-1
stretching within 1557-1565 cm in the 1,3-disubstituted benzimidazolium salts was
-1
-1
shifted to 1401 cm -1409 cm . Similarly, C-N stretching in the salts was shifted from
-1
-1
-1
1
211-1228 cm to 1014 cm -1195 cm . This could probably be due to an increase in
mass when H atom was replaced by the heavier silver ion upon complexation, thus
causing the C=N and C-N bond to vibrate at a lower frequency. However, the C≡N
stretching frequency remained unchanged after complexation, thus suggesting that the
nitrile group does not coordinate directly with the silver centers [23, 24].
1
13
3
.2 H- and C- NMR analysis
A sharp singlet within δ 9.67-10.09 ppm could be observed in the H-NMR spectra of all
,3-dialkylbenzimidazolium salts, 1-5. This signal could be ascribed to the acidic proton
NCHN) in the benzimidazolium salts which conformed to our previous findings [19].
1
1
(
Resonances of the aromatic protons were observed as two multiplets within δ 7.71-7.75
ppm and δ 7.92-8.04 ppm. Shifting of triplets from δ 4.27 ppm in N-octylbenzimidazole
(1-monosubstituted) to δ 4.54-4.56 ppm was an indication of alkylation on the other N
atom (1,3-disubstituted) of the benzimidazolium salts. The protons in the long alkyl
chains could be observed as triplets (terminal -CH ) at δ 0.83-0.90 ppm, multiplets
3
(
methylene –CH -) at δ 1.20-1.39 ppm and quintets (methylene –CH -) at δ 1.92-2.03
2
2
ppm, respectively.
1
5

ACCEPTED MANUSCRIPT
Upon replacing the alkyl chains with the substituted benzyl moiety, 6-8, the aromatic
protons of the benzimidazolium core were observed as multiplets within δ 7.63-7.69 ppm,
two doublets at δ 7.92-7.97 ppm and δ 8.09-8.14 ppm. The singlet at δ 5.73-5.91 ppm
was ascribed to the benzyl protons. In addition, the aromatic and methyl protons of the
benzyl substituents gave rise to two doublets within δ 7.22-7.91 ppm (benzene, δ 7.509
ppm and δ 7.391 ppm; methylbenzene, δ 7.421 ppm and δ 7.217 ppm; benzonitrile,
δ 7.694 ppm and δ 7.906 ppm) and a singlet at δ 2.29 ppm, respectively. The formation of
the Ag(I)-NHC complexes was evident from the disappearance of the acidic proton signal
(NCHN) (δ 9.67-10.09 ppm) in the benzimidazolium rings. Other proton resonances were
seen in the expected regions with no any significant frequency shifts.
1
3
The formation of Ag(I)-NHC complexes could be further confirmed using the C-NMR
spectroscopy. The signal observed within δ 139.97-142.69 ppm in the 1,3-disubstituted
benzimidazolium salts could be ascribed to the acidic carbon NCHN of the
benzimidazolium rings. The signals within δ 114.67-133.00 ppm were assigned to the
aromatic carbons in the benzimidazolium rings. The signal at δ 14.46-14.84 ppm was
attributed to the methyl carbon of the alkyl chains while signals within δ 23.73-33.16
ppm and δ 48.57-48.83 ppm could be assigned to the methylene carbons of the long alkyl
chains. Signals of the methylene carbons in all complexes were overlapped due to similar
chemical environment these carbons experience.
Aromatic, benzyl, methyl and nitrile carbons in the substituted benzyl analogues
resonated at δ 111.94-143.24 ppm, 49.76-50.35 ppm, 21.16 ppm and 118.85 ppm. Other
1
6

ACCEPTED MANUSCRIPT
carbon signals were observed within similar regions as those discussed for alkyl
substituted Ag(I)-NHC complexes, except that of the benzyl carbon which was shifted
downfield as it is positioned between two electron withdrawing nitrogen atom and
benzene ring.
Upon complexation, an additional singlet or two doublets ascribable to the Ag-C signal
was seen within 187.00-189.84 ppm but the acidic carbon NCHN signal was missing.
The splitting patterns of the Ag-C signal was found to depend on the fluxional behavior
of the silver(I)-NHC complexes [8, 25]. The presence of two doublets was resulted from
1
07
109
the bonding of the carbene carbon to Ag and Ag, respectively, while the singlet
indicated the lack of coupling between the two elements. Since silver has two almost
107
109
equally abundant spin 1/2 nuclei (52% of Ag and 48% of Ag), the carbene carbon of
a silver(I)-NHC complex should be a doublet of doublets if the coupling happens slowly
on the NMR time scale. If the process is fast, this carbene carbon signal would be a
singlet. According to Wang and Lin, silver(I)-NHC complexes that are static or having
slow dynamic behaviour will show better Ccarbene coupling to silver as evident from NMR
analysis at low temperature. As the dynamic behaviour of silver(I)-NHC increases at high
temperature on the NMR time scale, the Ccarbene resonances will become broader and
eventually coalesced into a singlet [15]. They observed that in the silver(I)-NHC complex,
[
Ag(Et -Bimy) ][AgBr2], the carbene carbon was a singlet while that in [Ag(Et -
2 2 2
Bimy) ]PF were doublets. Similar observation was reported by Chianese and co-workers
2
6
in which the carbene carbon in methylsubstituted silver(I)-NHC complex,
Ag(C H N ) ]PF resonated as a singlet [26]. In this study, the fluxional behavior of the
[
7
12 2 2
6
1
7

ACCEPTED MANUSCRIPT
silver(I)-NHC complexes is not clear. The fluxional behavior of the compounds could be
temperature dependent, of which it could be investigated using the solution-state
variable-temperature (VT) NMR spectroscopy. For example, the reported carbene carbon
peak of a bithiophenesubstituted silver(I)-NHC complex, C H N S AgCl was a broad
3
9
36
2 4
singlet at room temperature and a doublet of doublets at −50 °C [27].
3
.3 Mesomorphic properties
The phase transitional properties of the bromide salts (carbene precursors) (1-8) and
Ag(I)-NHC complexes (9-15) were investigated by differential scanning calorimetry
(DSC) (without pretreatment of the samples) and polarizing optical microscopy (POM).
Table 1 summarizes the phase transition temperatures and enthalpy change values of 1-15.
Representative DSC traces of a bromide salt, 4 and Ag(I)-NHC complex 11 are shown in
Figure 1. Crystal polymorphism was seen in almost all the salts and metal complexes.
Figure 1: DSC trace of salt 4 (left) and complex 11 (right).
The melting temperatures, T of 2-5 increased slightly and varied in a narrow range (125
m
-
128 °C) with elongation of the aliphatic chains. This suggested that the melting points
1
8

ACCEPTED MANUSCRIPT
are mainly governed by cation-anion interaction. The clearing temperatures, T however,
c,
showed a larger dependence on chain length which indicated that hydrophobic
interactions dominate during mesophase-isotropization transition. It increased from
1
37 °C to 146 °C as the aliphatic chain lengthened, thus leading to wider mesophase
range. This is in good agreement with the trend in T and T reported for symmetrical
m
c
and unsymmetrical 1,3-dialkylated imidazolium analogues [28-36]. However, wider
mesophase range was reported for the imidazolium salts. The extended benzene ring in
the benzimidalolium salts appeared to have a destabilizing effect on mesophase due to a
large breadth-to-length ratio. The absence of a mesophase in 1 could be due to
insufficient van der Waals forces between the aliphatic chains to hold the ions to undergo
ordered thermal motion [4, 37]. The relatively large ΔH on melting indicated a
considerable structural change: the breakup of the ordered, three-dimentional crystal
lattice. The small ΔH at isotropization suggested a partially ordered state. It increased
with alkyl chain length and indicated the breakup of van der Waals interaction between
the chains. On the other hand, only endotherms characteristic of crystal-isotroptic
transition were observed in compounds 6-8, and the glassy transition was seen in 7 upon
cooling the isotropic melt (Fig. 2). Commonly observed in ionic liquids, this supercooled
state is caused by slow diffusion of ions and insufficient crystal nucleation that suppress
the crystallization of highly viscous liquids [38, 39].
1
9

ACCEPTED MANUSCRIPT
Figure 2: A DSC trace of 7 showing glassy state.
-1
Table 1: Phase transition temperatures (ºC) and enthalpy change values (kJ mol in
-1
parentheses) of 1-15 determined by DSC at a heating and cooling rate of 5 °C min .
-1
Transition temperatures ,ºC (ΔH, kJmol )
Compound
Heating
Cooling
I 109.6(-15.7) Cr
I 122.2(-1.5) SmA
[
[
C bimC ][Br]; n = 10
1
8
n
Cr 120.5(14.4) I
(1)
C bimC ][Br]; n = 12
1
8
n
(2)
Cr 49.1(4.1) Cr 124.8(7.8) I 112.5(15.2) Cr1
1
4
2.0(3.8) Cr
[
[
[
[
C bimC ][Br]; n = 14
Cr 56.6(5.9) Cr 69.5(6.0) Cr2 I 134.3(-1.07) SmA
1
8
n
1
(3)
125.9(14.6) SmA 136.8(1.07) 114.7(-18.3) Cr 52.2(-
1
I
22.5) Cr
C bimC ][Br]; n = 16
Cr 84.9(4.0) Cr 102.2(36.3)
I 140.2(-2.8) SmA
1
8
n
1
(4)
Cr 126.4(10.8) SmA
112.2(-13.7) Cr 73.2(-
2
1
1
41.5(2.6) I
22.2) Cr
C bimC ][Br]; n = 18
I 144.7(-3.1) SmA
115.5(-12.4) Cr1
1
8
n
Cr 127.9(97.6) SmA
(5)
1
46.1(2.6) I
1
07.1(-73.8) Cr
C bimCH C H ][Cl];
I 90.5(-5.5) Cr 53.4(-
5.6) Cr
1
8
2
6
5
1
Cr 61.6(7.3) Cr 108.3(11.9) I
1
(6)
[
C bimCH C H CH ][Br]; Cr 105.3(13.3) Cr 118.4(3.7) Glassy state, Tg
1
8
2
6
4
3
1
-1
(7)
I
13.8(0.7 kJ(mol.°C) )
[
C bimCH C H CN][Br];
1
8
2
6
4
Cr 170.2(20.7) I
I 161.2(-21.0) Cr
(8)
[
[
[
Ag(C bimC ) ][PF ]; n = 10
18 n 2 6
Cr 35.4(6.97) Cr 80.8(49.4) I I 45.5(9.5) Cr
1
(9)
Ag(C bimC ) ][PF ]; n = 12
1
8
n 2
6
Cr 57.3(73.4) I
I 48.7(4.51) Cr
(10)
Ag(C bimC ) ][PF ]; n = 14 Cr 50.5(27.7) Cr 75.4(79.3) I I 70.7(4.4) SmA
1
8
n 2
6
1
2
0

ACCEPTED MANUSCRIPT
(11)
50.4(36.6) Cr
[
[
Ag(C bimC ) ][PF ]; n = 16
I 80.7(5.4) SmA
72.9(102.5) Cr
1
8
n 2
6
Cr 97.6(106.9) I
(12)
Ag(C bimCH C H ) ][PF ];
1
8
2
6
5 2
6
Cr 91.0(4.2) Cr 106.5(53.5) I I 48.1(-76.5) Cr
1
(13)
[
Ag(C bimCH C H CH ) ]
1
8
2
6
4
3 2
Cr 50.7(36.0) Cr 74.3(56.7)
Glassy state, T 5.2(0.5
kJ(mol.°C) )
1
g
[
PF6];
-1
Cr 93.4(34.4) I
1
(14)
[
Ag(C bimCH C H CN) ]
1
8
2
6
4
2
Cr 67.4(70.6) Cr 105.7(87.4)
I
1
[
PF6];
I 43.2(-3.7) Cr
(15)
Note: C bimC : 3-alkyl-1-octadecyl-benzimidazolium ; Cr, Cr : crystal phase; SmA:
1
8
n
1
smectic A phase; I: isotropic liquid
Observation under crossed polarizers indicated that all the 1,3-dialkylated
benzimidazolium salts (2-5), except 1 exhibited a smectic A phase that is characterized
by fan texture. Contrary to neutral LCs, rigid cores are not essential in inducing
mesophases in ionic compounds. Driving forces for mesophase formation in ILCs are
ionic head groups, hydrogen bonding between cation and anion and van der Waals
hydrophobic interactions between long alkyl chains [37] .
The 1,3-dialkylated salts reported herein have well defined intramolecular, incompatible
segments of polar aromatic cores and non-polar aliphatic side chains. Aggregation of
compatible parts, microsegregation of incompatible parts, and minimization of volume in
bulk of these ionic mesogens had led to the formation of a thermotropic lamellar phase.
The ionic nature of the head groups resulted in non-covalent attraction between the polar
head groups and the glass surface had created a homeotropic alignment with the
hydrophobic tails arranged perpendicular to and away from the glass surface, hence dark
domains were seen [37, 40]. The coexistence of the dark domain and fan texture from
2
1

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cooling the isotropic melt confirmed the presence of a SmA phase (Figure 3). The
smectic phase was stabilised on elongation of the aliphatic chains as evident from the
DSC analysis in which increased thermal mesomorphic range was recorded starting from
2
(n = 12) to 5 (n = 18).
a)
b)
Figure 3: The fan texture observed for (a) 4 at 134 °C and (b) 11 at 67 °C upon cooling
magnification of x50). The dark domain was observed spontaneously from isotropic
(
melt.
To understand the influence of the number and length of aliphatic chains on
mesomorphism, the methyl chain in the documented 1-methyl-3-alkyl-benzimidazolium
bromides was substituted with a octadecyl chain in this study [19]. When comparing with
their methyl counterparts, higher melting temperatures but lower clearing temperatures
(decreased phase stability) were recorded for the salts with two long aliphatic chains. The
former could be associated with the packing efficiency of the molecules with two long
chains and the latter could be rationalized in term increased flexibility which is more
important over van der Waals forces during clearing. A similar trend was seen in the
corresponding imidazolium analogues wherein smaller mesomorphic range was reported
2
2

ACCEPTED MANUSCRIPT
for imidazolium salts with two long aliphatic chains [41]. However, regardless of the
identity of anions, the 1,3-dialkylimidazolium analogues exhibited a wider mesophase
range as compared to that of the dialkylbenzimidazolium salts [28, 29]. The reported
dialkylimidazolium bromides and chlorides have much lower melting but higher clearing
temperatures, of which some are room temperature ILCs [28]. This implied that an
increase in breadth-to-length ratio of the benzimidazolium core has a destabilizing effect
on mesophase. The minimum chain length to induce a mesophase in the 1,3-
dialkylimidazolium halides is C8 whilst that for the corresponding benzimidazolium
halides is C12 (present study, [41]). When comparing the mesophase stablity of the
bromide salts with that of the reported chloride analogues, wider temperature range
(
lower T and higher T ) was reported for the latter. This could be explained by the
m c
presence of stronger hydrogen bonds between the chloride ion and the benzimidazolium
core [41].
The presence of bulky cation in the disubstituted benzyl analogues is unfavourable for the
formation of mesophases. Compounds 6-8 were non-mesogenic due to a large width-to-
length ratio of the benzimidazolium core. In order to comprehend the change in phase
behavior, the molecular shapes and arrangements of these salts were deduced from the
reported analogous crystal structures that bear close structural resemblance 42]. In the
disubstituted benzyl salts, due to steric hindrance, the bulky benzyl moiety is almost
orthogonal to the benzimidazolium plane with a large dihedral angle (84.6 °). The loss of
co-planarity of the rigid cores in these molecules is destructive to the effective molecular
packing that is essential in mesophase stabilitization and this was reflected in the loss of
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3

ACCEPTED MANUSCRIPT
LC properties in these salts. The melting temperatures of the salts are substituents
dependent. Contrary to dialkylated analogues, higher melting temperatures were recorded
for the bromide salts, 7 and 8 than that of the chloride salt, 6, thus indicating that core-
core interactions are more important than hydrogen bonding between cation and anion.
Effect of different anions on the mesomorphic properties was also observed. A notable
change in the transitional properties and phase behaviour was observed for the Ag(I)-
NHC complexes (9-15) when the bromide ion in the ligands was replaced with
hexafluorophosphate ion. The hexafluorophosphate ion-containing complexes were
prepared due to ease of handling as they are less heat sensitive and more stable compared
to the bromide analogues. Again, the mesomorphism of the Ag(I)-NHC complexes was
cation, anion and chain length dependent. The large hexafluorophosphate anion is
expected to give the dual effects of lowering the transitional temperatures and reducing
the tendency of liquid crystalline behaviour. This was indeed true in the present study,
with only two Ag(I)-NHC complexes, 11 and 12 showing metastable monotropic
smectic A phase which resulted from weak Coulombic interactions and attraction
between Ag(I) ions and σ electron pair of the acidic carbon (N-C-N) in the ligands that
are not strong enough to sustain the ions undergoing ordered thermal motion [43, 44].
The effect of anion was further illustrated in reduced phase stability of the dialkylated
Ag(I)-NHC complexes as compared to that of the reported Au(I)-NHC and Pd(II)-NHC
analogues having bromide and chloride as the counterion, respectively. The ease of anion
-
to form liquid crystals followed the order Cl  Br  PF [5, 6].
6
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4

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It is interesting to note that contrary to 1,3-dialkylimidazolium salts of which the
mesophase is destabilized by the benzimidazolium core, the role of the latter is reversed
upon complexation wherein the presence of the extended benzene ring is essential to
induce
mesomorphism
in
the
metal-NHC
complexes.
Stable
N-N’-
dialkylbenzimidazolium based Pd(II) and Au(I)-NHC complexes were reported and in
this study, metastable Ag(I)-NHC complexes are obtained. Hitherto reported 1,3-
dialkylimidazolium Ag(I)- and Au(I)-NHC complexes are, however, non-mesomorphic,
probably due to insufficient core-core interactions [45]. LC behavior was observed in: (i)
Ag(I) and Au(I)-NHC complexes bearing a long alkyl chain and a pendent hydroxyl
-
-
group or N-amido moiety on the C1 and C3 positions in the core structure with Br / BF /
4
-
NO counter ions [44-46], and (ii) Au(I)-NHC complexes with substituted imidazolium
3
moieties [47]. The presence of hydrogen bonding in these functionalized metal-NHC
complexes is believed to compensate for the lack of core-core interactions to induce LC
behaviour. On the other hand, the 1,3-dialkylimidazolium based Pd(II)-NHC complexes
were smectogenic, probably due to enhanced Coulombic interactions of the tetradentate
Pd metal and two chloride ions that stabilized the mesophase [6].
The coordination of the Ag(I) ions to the ligands increased the width-to-length ratio of
the molecules that resulted in generally much lower T for 9-15 and T for 11 and 12.
m
c
Contrary to the bromide salts, larger temperature dependence of T was seen for 11 and
m
1
2. An increase in the melting and clearing temperatures was observed as the terminal
aliphatic chains lengthened. They, however, had lower melting and clearing temperatures
than that of the bromide salts of equivalent chain length. This may be attributed to
2
5

ACCEPTED MANUSCRIPT
absence of potential hydrogen bond donors and acceptors in the complexes coupled with
the anion [48, 49]. The benzyl containing Ag(I)-NHC complexes were non-mesogenic
due to similar reason as described for the corresponding bromide salts. This was in
agreement with that reported for Ag(I) complexes of N,N-dialkylbenzimidazoles [50].
Different anions required different minimum alkyl chain length to induce LC properties
in the bromide salts and Ag(I)-NHC complexes. The minimum alkyl chain length
required for the bromide salts to exhibit LC properties is C12 and that for the Ag(I)-NHC
complexes is C14. Possibly, the reduced cation-anion interaction due to the larger
hexafluorophosphate ion is compensated by enhanced hydrophobic interactions between
long alkyl chains [4].
It is noteworthy to mention that all Ag(I)-NHC complexes except 15 can be considered as
ionic liquids with melting/clearing temperatures below 100 °C.
4
. Conclusion
Eight 1,3-disubstituted salts and the corresponding Ag(I) dicarbene complexes,
respectively were prepared and characterized. The mesomorphism and transitional
temperatures of these compounds were investigated by DSC and POM. The cation
geometry, anion and terminal aliphatic chains were found to be essential parameters for
mesophase formation. Higher phase stability was recorded for dialkylated salts than that
of the corresponding Ag(I) dicarbene complexes due to anion effect. The large
hexafluorophosphate counterion led to weak Coulombic interactions which was
2
6

ACCEPTED MANUSCRIPT
insufficient to sustain the ions undergoing ordered motion, hence, only metastable SmA
phase was seen in two Ag(I)-NHC complexes. The number and length of the aliphatic
chains were found to influence the transitional temperatures and mesophase stability. All
dialkylated salts except the shortest with n =10 were smectogenic. The benzyl containing
compounds, however, were non-mesogenic due to large width-to-length ratio and loss in
co-planarity in the core structure that is destructive to mesophase stabilization. The
minimum alkyl chain length required for the bromide salts to exhibit LC properties is
C12 and that for the Ag(I)-NHC complexes is C14. All Ag(I)-NHC complexes except the
one bearing the cyano group could be considered ionic liquids with melting/clearing
temperatures below 100 °C.
5
. Acknowledgment
The authors would like to thank Universiti Sains Malaysia for the research facilities and
financial support through the RUI research grants 1001/PKIMIA/6316044 and
1
001/PKIMIA/8011094. C. W. Yeap wishes to thank the Ministry of Higher Education
Malaysia for the MyPhd sponsorship.
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