Dinuclear N-heterocyclic carbene silver complexes: Synthesis, luminescence and catalytic studies

Article abstract of DOI:10.1016/j.molstruc.2018.11.053

A new dibenzimidazole compound (1) and three dibenzimidazolium salts (2a-2c) were prepared. Reactions of these salts with silver(I) oxide in methanol led to dinuclear N-heterocyclic carbene silver complexes (3a-3c). All of the compounds were characterized

Full text of DOI:10.1016/j.molstruc.2018.11.053

Accepted Manuscript
Dinuclear N-heterocyclic carbene silver complexes: Synthesis, luminescence and
catalytic studies
Deniz Demir Atlı, Sibel Gülle
PII:
S0022-2860(18)31366-8
DOI:
https://doi.org/10.1016/j.molstruc.2018.11.053
MOLSTR 25882
Reference:
To appear in: Journal of Molecular Structure
Received Date: 18 October 2018
Revised Date: 16 November 2018
Accepted Date: 17 November 2018
Please cite this article as: D. Demir Atlı, S. Gülle, Dinuclear N-heterocyclic carbene silver complexes:
Synthesis, luminescence and catalytic studies, Journal of Molecular Structure (2018), doi: https://
doi.org/10.1016/j.molstruc.2018.11.053.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Dinuclear N-heterocyclic carbene silver complexes: Synthesis, luminescence and
catalytic studies
Deniz Demir Atlı*, Sibel Gülle
Manisa Celal Bayar University, Faculty of Science and Arts, Chemistry Department,
4
5140, Manisa, Turkey
deniz.demir@cbu.edu.tr
Abstract
A new dibenzimidazole compound (1) and three dibenzimidazolium salts (2a-2c) were
prepared. Reactions of these salts with silver(I) oxide in methanol led to dinuclear N-
heterocyclic carbene silver complexes (3a-3c). All of the compounds were characterized by
1
13
H NMR, C NMR and elemental analyses. Molecular weights of the silver complexes were
confirmed by MALDI-TOF mass spectrometry. Preliminary catalytic studies using all the
silver complexes were performed on three-component coupling reaction of p-formaldehyde,
piperidine and phenylacetylene. Complexes 3a and 3c exhibited good catalytic activities
under neat conditions. The silver complexes showed luminescence properties in CH CN at
3
room temperature.
Keywords: N-heterocyclic carbene, silver, luminescence, coupling reaction.
1
. Introduction
N-heterocyclic carbene (NHC) ligands have drawn great interest in organometallic
chemistry since the discovery of the first stable NHC in 1991 [1]. NHCs with strong σ-
donating and weak π-accepting features are alternative to phosphines. They form stable metal
complexes containing strong metal-carbon bonds [2].
Among the NHC-metal complexes, silver complexes have considerable importance.
The first NHC-Ag(I) complex was isolated in 1993 by Arduengo et al. by the reaction of
AgOTf and a free carbene [3] and since then many NHC silver complexes have been studied
with increased attention. For synthesis, the most frequently used method has been
deprotonation with a silver base such as Ag O, Ag CO and AgOAc [4-6]. NHC-Ag(I)
2
2
3
complexes have been widely used as sources of different metal complexes via
transmetallation [7-14]. These compounds have antimicrobial and anticancer properties [15-
1

ACCEPTED MANUSCRIPT
1
9]. Also, they have been studied as luminescent materials [20-23]. There have been limited
number of reports related to the use of NHC-Ag(I) complexes in catalysis [24-32].
Herein, we report on the synthesis and characterization of three new
dibenzimidazolium salts in which two NHC moieties are connected by 2-methylenepropane-
1
,3-diyl group and their dinuclear dicationic di(NHC) silver complexes. According to our
knowledge, this bridging group which is analog of allyl has not been used for preparing
dibenzimidazolium compounds yet. It is expected that this type of NHC precursors can
generate effective catalysts. We also present luminescent properties and catalytic activities of
Ag(I) complexes in three-component coupling reactions to give a propargylamine derivative.
2
2
. Experimental
.1. Reagents and Instruments
All reactions were carried out under open atmosphere unless stated otherwise. The
reagents and the solvents were obtained from commercial suppliers and used as received.
1
13
THF was distilled under argon over Na/K alloy. H (400 MHz) and C (100 MHz) NMR
spectra were recorded on a Varian VNMRJ spectrometer. Elemental analyses were performed
using a LECO-932 CHNS model analyzer. Molecular weights of the complexes were
measured with Bruker Microflex LT MALDI-TOF mass spectrometer. Thermogravimetric
analysis for 2c was carried out with EXSTAR TG-DTA 7300 instrument.
2
2
.2. Preparation of NHC precursors
.2.1. 1,1′-(2-methylenepropane-1,3-diyl)dibenzimidazole (1)
Benzimidazole (1.182 g, 10 mmol) solution in dry THF (25 mL) was added to NaH
(
0.48 g, 12 mmol) in dry THF (5 mL) under argon atmosphere. The mixture was refluxed for
h. After filtration, 1,1-bis(chloromethyl)ethylene (0.54 mL, 5 mmol) was added to the filtrate
and it was refluxed for 24h. The mixture was filtrated and the solvent was removed. Water
100 mL) was added to the residue and it was extracted with CH Cl (3x20 mL). After drying
1
(
2
2
with Na SO , the solvent was removed and dibenzimidazole compound was recrystallized at
2
4
1
CH Cl /hexane (1/4). Yield: 1.181 g, 82%. H NMR (CDCl ): δ = 7.82 (d, 2H, J = 7.30 Hz,
2
2
3
NCHN), 7.75 (s, 2H, Ar-H), 7.32-7.24 (m, 4H, Ar-H), 7.21-7.17 (m, 2H, Ar-H), 5.24 (s, 2H,
1
3
=
CH ), 4.65 (s, 4H, NCH ) ppm. C NMR (CDCl ): δ = 144.13, 143.30, 138.28, 133.80,
2 2 3
2

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1
5
23.71, 122.91, 120.89, 118.28, 110.04, 47.59 ppm. Anal. Calc. for C H N : C, 74.96; H,
1
8
16
4
.60; N, 19.43. Found: C, 74.62; H, 5.75; N, 19.45%.
.2.2. 1,1′-dibenzyl-3,3′-(2-methylenepropane-1,3-diyl)dibenzimidazolium dibromide
2
(2a)
1
(1 mmol) and benzyl bromide (2 mmol) were stirred in DMF (3 mL) at 80 °C for 24
h. The mixture was cooled to the room temperature. Et O (15 mL) was added and the mixture
2
was filtered. The white solid was recrystallized from EtOH at room temperature. Yield: 89%.
1
H NMR (DMSO-d ): δ = 10.11 (s, 2H, NCHN), 8.02-7.94 (m, 4H, Ar-H), 7.66-7.53 (m, 8H,
6
Ar-H), 7.42-7.34 (m, 6H, Ar-H), 5.80 (s, 4H, NCH ), 5.44 (s, 4H, NCH ), 5.31 (s, 4H, =CH )
2
2
2
1
3
ppm. C NMR (DMSO-d ): δ = 143.48, 136.42, 134.32, 131.71, 131.42, 129.42, 129.19,
6
1
28.88, 127.28, 127.21, 119.62, 114.55, 114.43, 50.46, 49.28 ppm. Anal. Calc. for
C H N Br : C, 60.96; H, 4.81; N 8.89. Found: C, 61.61; H, 4.69; N, 9.08%.
32
30
4
2
2
.2.3.
1,1′-di(3-methoxybenzyl)-3,3′-(2-methylenepropane-1,3-diyl)dibenzimidazolium
dichloride (2b)
Compound 2b was prepared similar to 2a using 3-methoxybenzyl chloride. Yield:
1
8
0%. H NMR (CD OD): δ = 9.86 (s, 2H, NCHN), 7.94-7.88 (m, 4H, Ar-H), 7.69-7.65 (m,
3
4
H, Ar-H), 7.36-7.32 (m, 2H, Ar-H), 7.10-7.04 (m, 4H, Ar-H), 6.96 (dd, 2H, J = 9.16, 2.55
Hz, Ar-H), 5.72 (s, 4H, NCH ), 5.47 (s, 4H, NCH ), 5.32 (s, 2H, =CH ), 3.78 (s, 6H, OCH )
2
2
2
3
1
3
ppm. C NMR (CD OD): δ = 160.44, 142.41, 135.75, 134.34, 131.64, 131.42, 130.20,
3
1
27.23, 127.15, 120.16, 118.89, 114.24, 113.94, 113.79, 113.47, 54.51, 50.74, 49.02 ppm.
Anal. Calc. for C H N O Cl : C, 67.87; H, 5.71; N 9.32. Found: C, 68.46; H, 5.56; N,
3
4
34
4
2
2
9
.60%.
2
.2.4.
1,1′-di(4-methoxybenzyl)-3,3′-(2-methylenepropane-1,3-diyl)dibenzimidazolium
dichloride (2c)
Compound 2c was prepared similar to 2a using 4-methoxybenzyl chloride. Yield:
1
9
5%. H NMR (DMSO-d ): δ = 10.51 (s, 2H, NCHN), 8.06-7.99 (m, 4H, Ar-H), 7.64-7.55
6
(
m, 8H, Ar-H), 6.95-6.92 (m, 4H, Ar-H), 5.75 (s, 4H, NCH ), 5.47 (s, 4H, NCH ), 5.37 (s, 2H,
2 2
1
3
=
CH ), 3.71 (s, 6H, OCH ) ppm. C NMR (DMSO-d ): δ = 159.90, 143.52, 136.46, 131.69,
2 3 6
1
31.28, 130.71, 127.11, 127.08, 126.25, 120.27, 114.70, 114.60, 114.49, 55.64, 49.99, 49.17
3

ACCEPTED MANUSCRIPT
ppm. Anal. Calc. for C H N O Cl ⋅2.25H O: C, 63.59; H, 6.06; N 8.73. Found: C, 63.57; H,
34
34
4
2
2
2
5
.94; N, 8.62%. The presence of hydrate molecule was confirmed by TGA/DTA analysis.
.3. General procedure for preparation of NHC silver complexes
2
Silver(I) oxide (2 mmol) was added to dibenzimidazolium salt (1 mmol) dissolved in
MeOH (20 mL). The mixture was stirred at room temperature for 24 h in dark. The
suspension was filtered through celite. NH PF solution (2.5 mmol) in MeOH (10 mL) was
4
6
added to the filtrate. The mixture was stirred at room temperature for 2 h in dark. After
filtration, the solids were washed with MeOH (2x5 mL) and Et O (2x5 mL). The product was
2
recrystallized from CH CN/Et O (1/3).
3
2
2
.3.1.
Bis[1,1′-dibenzyl-3,3′-(2-methylenepropane-1,3-diyl)dibenzimidazolin-2,2′-
diylidene]disilver(I) bis(hexafluorophosphate) (3a)
1
Yield: 87%. H NMR (DMSO-d ): δ = 7.69-7.67 (m, 8H, Ar-H), 7.39-7.33 (m, 8H,
6
Ar-H), 7.22-7.15 (m, 20H, Ar-H), 5.68 (s, 8H, NCH ), 5.38 (s, 8H, NCH ), 5.02 (s, 4H, =CH )
2
2
2
13
ppm. C NMR (DMSO-d ): δ = 140.22, 136.40, 133.95, 133.59, 129.16, 128.45, 127.59,
6
+
1
1
3
24.89, 124.73, 112.92, 112.87, 52.33, 52.20 ppm. MALDI-TOF MS: [M-PF ] at m/z
6
297.62. Anal. Calc. for C H N Ag P F : C, 53.27; H, 3.92; N, 7.77. Found: C, 52.85; H,
64
56
8
2 2 12
.98; N, 7.03%.
2
.3.2. Bis[1,1′-di(3-methoxybenzyl)-3,3′-(2-methylenepropane-1,3-diyl)dibenzimidazolin-
,2′-diylidene]disilver(I) bis(hexafluorophosphate) (3b)
2
1
Yield: 79%. H NMR (DMSO-d ): δ = 7.69-7.65 (m, 8H, Ar-H), 7.40-7.34 (m, 8H,
6
Ar-H), 7.05 (t, 4H, J = 7.83 Hz, Ar-H), 6.79 (s, 4H, Ar-H), 6.74-6.69 (m, 8H, Ar-H), 5.62 (s,
13
8
H, NCH ), 5.36 (s, 8H, NCH ), 4.97 (s, 4H, =CH ), 3.56 (s, 12H, OCH ) ppm. C NMR
2 2 2 3
(
DMSO-d ): δ = 159.77, 140.35, 137.86, 133.89, 133.65, 130.32, 124.90, 124.75, 119.42,
6
+
1
13.85, 113.18, 112.89, 55.36, 52.17, 52.04 ppm. MALDI-TOF MS: [M-PF ] at m/z
6
1
418.04. Anal. Calc. for C H N O Ag P F : C, 52.25; H, 4.14; N, 7.17. Found: C, 52.45;
68
64
8
4
2 2 12
H, 4.29; N, 7.17%.
4

ACCEPTED MANUSCRIPT
2
.3.3. Bis[1,1′-di(4-methoxybenzyl)-3,3′-(2-methylenepropane-1,3-diyl)dibenzimidazolin-
,2′-diylidene]disilver(I) bis(hexafluorophosphate) (3c)
2
1
Yield: 76%. H NMR (DMSO-d ): δ = 7.65 (t, 8H, J = 6.26 Hz, Ar-H), 7.37-7.29 (m,
6
8
H, Ar-H), 7.18 (d, 8H, J = 8.61 Hz, Ar-H), 6.68 (d, 8H, J = 9.00 Hz, Ar-H), 5.62 (s, 8H,
1
3
NCH ), 5.40 (s, 8H, NCH ), 5.14 (s, 4H, =CH ), 3.57 (s, 12H, OCH ) ppm. C NMR
2
2
2
3
(
DMSO-d ): δ = 159.34, 133.89, 133.42, 129.14, 128.09, 124.75, 124.61, 114.45, 112.91,
6
+
1
12.81, 55.38, 52.41, 51.89 ppm. MALDI-TOF MS: [M-PF ] at m/z 1418.44. Anal. Calc.
6
for C H N O Ag P F : C, 52.25; H, 4.14; N, 7.17. Found: C, 51.95; H, 3.79; N, 7.38%.
6
8
64
8
4
2 2 12
2
.4. General procedure for the three-component coupling reaction
The mixture of p-formaldehyde (30 mg, 1 mmol), NHC-Ag(I) complex (3 mol%),
phenylacetylene (168 µL, 1.5 mmol) and piperidine (120 µL, 1.2 mmol) in the solvent (2 mL)
was stirred at 80 °C for 18h in dark. It was cooled to room temperature and Et O (20 mL)
2
was added. Organic part was dried over MgSO . After filtration and removing the volatile
4
components, propargylic amine compound was obtained by column chromatography on silica
using EtOAc/hexane (1/3).
3
. Results and Discussion
Preparation methods of 1,1′-(2-methylenepropane-1,3-diyl)dibenzimidazole (1) and
the dibenzimidazolium salts (2a-2c) are shown in Scheme 1. Compound 1 was obtained by N-
alkylation of benzimidazole with 1,1-bis(chloromethyl)ethylene in the presence of NaH in
THF. Reaction of dibenzimidazole with benzyl halides gave the dibenzimidazolium salts 2a-c.
The salts are stable to air and moisture. Treatment of 2a-c with 2 equiv. of Ag O in MeOH at
2
room temperature and following the salt metathesis reaction of the formed NHC-Ag(I)
bromide/chloride complexes with NH PF in MeOH at room temperature produced
4
6
hexafluorophosphate complexes (3a-c) (Scheme 2). These silver complexes are non-
hygroscopic and air stable. They are soluble in CH CN, DMSO and DMF and insoluble in
3
MeOH, Et O and hexane.
2
5

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CH2
CH2
CH2
H
N
N
N
N
N
N
N
NaH
THF
2 RX
DMF
+
+
+
N
Cl
Cl
N
N
R
R
X ꢀ
2
1
2a-2c
2
2
2
a: R = CH C H ; X = Br
2
6 5
b: R = CH C H OMeꢀ3; X = Cl
2
6 4
c: R = CH C H OMeꢀ4; X = Cl
2
6 4
Scheme 1. Synthesis of 1 and dibenzimidazolium salts 2a-2c
Scheme 2. Synthesis of dinuclear NHC-Ag(I) complexes 3a-3c
The formulations of the all synthesized compounds were corroborated by elemental
1
13
analysis method. H and C NMR signals were consistent with the expected structures. In the
1
H NMR spectra of the silver complexes (3a-3c), extinction of the signals related to acidic C2
proton of the benzimidazolium salts (2a-2c) is an evidence of deprotonation. The signals for
1
3
carbene carbon of the complexes were not observed in the C NMR spectra. This unusual
situation was ascribed to the fluxional behaviour of the complexes similarly for some NHC-
Ag(I) complexes [33,34]. The molecular formulations of 3a-3c were also established by
+
MALDI-TOF mass spectroscopy. [M-PF ] signals were observed at 1297.62, 1418.04 and
6
1
418.44, respectively.
6

ACCEPTED MANUSCRIPT
3
Three-component coupling reactions of alkynes, aldehydes and amines (A -coupling)
give propargylamines, significant compounds because of being a key intermediate in the
construction of biologically active N-heterocycles. It is known that transition metal salts and
complexes have been efficient catalysts for this reaction [35-40]. However, there have been a
3
limited number of reports related to the catalytic applications of NHC-Ag(I) complexes in A -
coupling reactions [41-47]. Herein, preliminary catalytic studies were carried out using
phenylacetylene, piperidine and p-formaldehyde at 80 °C. The catalytic activity of 3a was
examined in different solvents (Table 1, entries 1-5). Moderate yields were obtained in
acetone and CH CN. Good catalytic activity was observed with 3a and 3c under neat
3
conditions (Table 1, entries 1 and 8). While 3a and 3c complexes bearing benzyl and 4-
methoxybenzyl substituents on the NHC ligand showed almost similar activities (Table 1,
entries 1,2,8,9), lower yields were obtained by 3b complex containing 3-methoxybenzyl
substituent (Table 1, entries 6,7). It was concluded that methoxy group occupying position 3
in compound 3b caused to decrease in activity compared to 3a. It was previously reported that
decreasing of the number of the group on the NHC ligand led to lower catalytic efficiency
[
32].
3
a
Table 1. NHC-Ag(I) catalysed A -coupling reaction
b,c
Entry Catalyst
Solvent
Yield (%)
1
2
3
4
5
6
7
8
9
3a
3a
3a
3a
3a
3b
3b
3c
3c
Neat
81
51
55
25
30
59
44
83
50
CH CN
3
Acetone
DMF
DMSO
Neat
CH CN
3
Neat
CH CN
3
a
Reaction conditions: p-formaldehyde (1.0 mmol), piperidine (1.2 mmol), phenylacetylene (1.5
mmol), catalyst (3 mol %), solvent (2 mL), 18 h, 80 °C, in air.
b
Isolated yields.
Average of two runs.
c
7

ACCEPTED MANUSCRIPT
The fluorescent emission spectra of complexes 3a, 3b and 3c were measured in
CH CN at room temperature as shown in figure 1. The spectra obtained upon excitation at
3
1
0
2
40 nm exhibited maximum emission wavelengths at 484, 484 and 470 nm, respectively. d -
1
0
d interactions of Ag(I) ions are thought to cause luminescence.
7
5
3
1
00
00
00
00
3a
3b
3c
-100 450
460
470
480
Wavelength (nm)
490
500
510
-5
Figure 1. Emission spectra of complexes 3a-3c (1.0 X 10 M in CH CN) at room temperature
3
4
. Conclusions
In conclusion, we prepared three dinuclear NHC silver complexes derived from
1
13
dibenzimidazol-2-ylidenes. Their structures were elucidated by H NMR, C NMR,
elemental analysis and MALDI-TOF mass spectrometry. Preliminary catalytic studies
disclosed that complexes 3a and 3c catalyzed three-component reaction of piperidine,
phenylacetylene and p-formaldehyde with good yields under neat conditions. All the
complexes were found to be luminescent in acetonitrile solutions at room temperature.
Synthesis of new organometallic complexes obtained from these precursors and their catalytic
studies are in progress.
Acknowledgements
This work was supported by Scientific Research Project of Manisa Celal Bayar
University. Project No: 2015/192.
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Highlights
•
•
•
New N-heterocyclic carbene precursors and their dinuclear silver complexes were
synthesized and characterized.
Silver complexes were tested as catalysts in three-component coupling reaction of
piperidine, p-formaldehyde and phenylacetylene.
Silver complexes were observed to be luminescent.