Synthesis and characterization of Ag(I) complexes derived from new N-heterocyclic carbenes

Article abstract of DOI:10.14233/ajchem.2019.21877

Two new unsymmetrical imidazolium salts viz., [1-(4-ethylphenyl)-3-propyl-1H-imidazole-3-ium bromide] (3) and [1-(2,6-dimethylphenyl)-3-propyl-1H-imidazole-3-ium bromide] (4) have been synthesized via the reaction of propyl bromide with imidazole derivati

Full text of DOI:10.14233/ajchem.2019.21877

Asian Journal of Chemistry; Vol. 31, No. 5 (2019), 1149-1152
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2019.21877
Synthesis and Characterization of Ag(I) Complexes Derived from New N-Heterocyclic Carbenes
*,
MOHAMMED MUJBEL HASSON , BASIM H. AL-ZAIDI and AHMAD H. ISMAIL
Department of Chemistry, College of Science, Mustansiriyah University, Baghdad, Iraq
Corresponding author: E-mail: hassonmm67@uomustansiriyah.edu.iq
Received: 14 December 2018; Accepted: 30 January 2019;
*
Published online: 28 March 2019;
AJC-19346
Two new unsymmetrical imidazolium salts viz., [1-(4-ethylphenyl)-3-propyl-1H-imidazole-3-ium bromide] (3) and [1-(2,6-dimethylphenyl)-
-propyl-1H-imidazole-3-ium bromide] (4) have been synthesized via the reaction of propyl bromide with imidazole derivatives, [1-(4-
3
ethylphenyl)-1H-imidazole] (1) and [1-(2,6-dimethylphenyl)-1H-imidazole] (2) in absence of solvent. Then two new N-heterocyclic
carbene silver complexes (5 and 6) were prepared through the reaction of imidazoluim salts (3 and 4) as a source of N-heterocyclic
carbene with Ag O by in situ method. These complexes can be used in the future as a transfer agent for preparing other transitional metal
2
carbine complexes (NHCs) via transmetallation method. The formation of these compounds was confirmed by spectral analysis.
Keywords: Imidazolium salts, Heterocyclic carbene, Silver(I) carbene complexes.
complexes were synthesized by this method [21,22]. The most
important application of Ag(NHC)carbene complexes to use
as a metal transferring agents for synthesis of several other
complexes that are faced difficulties to synthesize via normal
methods [23,24].
INTRODUCTION
Imidazolium salts have great attention as a precursor of
N-heterocyclic carbene compounds due to its easy synthesis,
where several methodologies have been achieved for its prepar-
ation [1-7]. By modifying the substituents on N-atoms of imid-
azole compound,variety of imidazole salts can be obtained
and used to prepare many transition metals complexes type
N-heterocyclic carbene complexes (NHCs) under appropriate
conditions [8-11]. The strong donor of σ-bond for these comp-
ounds (N-heterocyclic carbene) make their complexes more
stable as compared to phosphine in addition of its variety
applications depending on the transition element [12-15].
The spark of interest began for these compounds when
crystals were obtained for the first free carbene by Arduengo
et al. [16]. Due to the sensitivity of free carbene compounds
toward air, moisture and heat, as well as the difficulty of syntheis
and stability for a long time [17,18], in situ method as an alter-
native method was utilized to generate carbene by using metal
EXPERIMENTAL
All the chemical reagents used as received without further
purification. Bruker Avance AMX 250, 400 spectrometer has
been used to obtain NMR spectra. Mass spectra have been recorded
in electrospray (ES) mode. FTIR spectra were obtained by using
Jasco FT-IR-660 plus Spectrometer. Elemental analysis of the
complexes were estimated at Elemental Analysis Service
Science Centre, London Metropolitan University. Stuart melting
point (digital) SMP30 apparatus has been used to measure
melting points.
Synthesis of precursors (1 and 2): Both aryl imidazole
derivatives (1and 2) have been synthesized according to reported
methods [25-27].
bases such as silver oxide (Ag
2
O) for the generation of carbene
4-Ethylaniline (6.05 g, 0.05 mol) or 2,6-dimethylaniline
(6.05 g, 0.05 mol) and aqueous glyoxal (30 %, 8.1 mL, 0.05
mol) were dissolved in 25 mL of methanol and the mixture
stirred for 16 h at room temperature, until a thick yellow
through deprotonation of imidazolium and to avoid the isolation
of free carbene which results in the direct use in the process of
complexity [19,20]. By this method, silver complexes (N-hetero-
cyclic carbene) can be obtained under ambient conditions
and does not require harsh conditions. Several silver carbene
mixture was obtained. After that NH
4
Cl (5.4 g, 0.1 mol) and
aqueous formaldehyde (37 %, 8 mL, 0.1 mol) were added.
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4

1
150 Hasson et al.
Asian J. Chem.
The reaction mixture was refluxed for 1 h after addition of
00 mL of methanol. Then added H PO (7 mL, 85 %) in the
mixture while stirring for 10 min and refluxed for 12 h. After
removing the solvent, the obtained residue has been poured
into 100 g of ice and neutralized by aqueous solution of KOH
Br
2
3
4
pressur tube
Br
N
N
N
N
1
45 °C, 16 h
(
4)
Scheme-IV: Synthesis reaction of imidazolium salt (4)
(
40 %), until residue reached pH 9. The product was obtained
by extraction using ethyl acteate (2 × 100 mL) then the organic
layer washed with brine and finally dried by using MgSO
Synthesis of silver complexes: The silver complexes (5
and 6) were obtained from the reaction of imidazoluim salts
4
.
The residue was purified by distillation under vacuum on a
Kugelrohr to obtain white powder (Schemes I and II).
andAg
2
O by in situ method [19,29,30]. Imidazolium salts were
O in a 1:1 molar ratio in dichloromethane and
mixed with Ag
2
stirred for overnight at room temperature. Crude product was
filtered through celite and recrystallized using CH Cl -Et O
to obtain a white solids in high yield (90 and 88 %), respectively
O
MeOH
2
2
2
NH2
N
NH Cl formaldehyd
N
4
O
H3PO4 KOH
(Schemes V and VI).
(
1)
Scheme-I: Synthesis reaction of [1-(4-ethylphenyl)-1H-imidazole] (1)
Br
Br
Ag
Ag O, CH Cl
2
2
2
O
MeOH
N
N
stirring overnight,
room temperature
N
N
N
NH2
NH Cl
formaldehyd
N
4
(
5)
O
H PO
KOH
3
4
(
2)
Scheme-V: Synthesis of silver complex [Ag(NHC)Br] (5)
Scheme-II: Synthesis reaction of 1-(2,6-dimethylphenyl)-1H-imidazole (2)
Br
Synthesis of ligands: Unsymmetrically substituted imida-
zolium salts (3and 4) have been synthesized according to reported
method by using free solvent method [28]. These two salts
Br
Ag
Ag O, CH Cl
2
2
2
N
N
stirring overnight,
room temperature
N
N
1
13
have been characterized by H NMR, C NMR and mass analysis.
(6)
Synthesis of [1-(4-ethylphenyl)-3-propyl-1H-imidazole-
3
-ium bromide] (3): In a pressure tube with stirring bar, a
mixture of precuror 1 (0.688 g, 4 mmol) and propyl bromide
0.49 g, 4 mmol) was stirred at 140 ºC for 16 h and then the
mixture was left overnight at room temperature. The product was
purified by CH Cl -Et O using diffusion purification method
Scheme-VI: Synthesis of silver complex [Ag(NHC)Br] (6)
(
RESULTS AND DISCUSSION
Table-1 represents the physico-chemical properties and
elemental analysis of imidazoluim salts and their silver(I) comp-
lexes.
H NMR analysis: H NMR spectrum of precursor 1 in
CDCl showed three singlet signals at δ (8.3 ppm, 1H), (7.7
ppm, 1H) and (7.1, ppm, 1H) ppm related to imidazole protons.
Doublet signals were noticed at δ (7.3, 2H) and (7.5, 2H) ppm
assigned to protons of benzene ring. The spectrum also revealed
two signals with integration of 2H and 3H assigned, respectively
2
2
2
to gain a white precipitate (Scheme-III).
1
1
Br
3
Br pressur tube
N
N
N
N
145 °C, 16 h
(3)
Scheme-III: Synthesis reaction of imidazolium salt (3)
to (CH
2
) and (CH
3
) of para-substituted ethyl group.
showed a
1
Synthesis of [1-(2,6-dimethylphenyl)-3-propyl-1H-
imidazole-3-ium bromide] (4): In a pressure tube with stirring
bar a mixture of precuror 2 (0.86 g, 5 mmol) and propyl bromide
H NMR spectrum of precursor 2 in DMSO-d
6
singlet signal at (1.9, 6H) ppm, related to two methyl groups.
The signals of imidazole ring were also noticed at (7.1 ppm,
2H) and (7.7 ppm, 1H). Multiplet signals within range (7.2-
(
0.61 g, 5 mmol) was stirred at 145 ºC for 16 h After that the
mixture was left to cool at room temperature and then the
product was purified by CH Cl -Et O using diffusion purifi-
cation method to gain a white precipitate (Scheme-IV).
7.4 ppm, 3H) assigned to three aromatic protons.
1
2
2
2
H NMR spectrum of imidazolium salt (3) in CDCl in
3
comparison to precursor 1 exhibited new signals at (0.95 ppm,
TABLE-1
PHYSICAL PROPERTIES OF IMIDAZOLUIM SALTS AND SILVER COMPLEXES
Elemental analysis (%): Found (calcd.)
Compd.
m.f.
Colour
Yield (%)
m.p. (°C)
C
H
N
3
4
5
6
C
14
C
14
C
14
C
14
H
19
H
19
H
18
H
18
N
2
Br
Br
AgBr
AgBr
White
White
White
White
80
85
90
88
220-222
230-235
148-150
155-158
57.23 (56.96)
57.57 (56.96)
41.95 (41.82)
41.90 (41.82)
6.98 (6.49)
6.89 (6.49)
4.85 (4.51)
4.60 (4.51)
9.85 (9.49)
9.65 (9.49)
7.24 (6.97)
7.10 (6.97)
N
2
N
2
N
2

Vol. 31, No. 5 (2019)
Compound
Synthesis and Characterization of Ag(I) Complexes Derived from New N-Heterocyclic Carbenes 1151
TABLE-2
-
1
SELECTED FTIR DATA (cm ) OF IMIDAZOLUIM SALTS AND SILVER COMPLEXES
ν(C=N)
1465
1470
1480
1488
ν(C-N)
1375
1365
1380
1390
ν(C=C)
1512
1500
1550
1558
ν(C-H) ar.
3117
3070
3180
3190
ν(C-H) aliph.
2985
[C H BrN ] (3)
14 19 2
[
C H BrN ] (4)
2980
2970
2966
1
4
19
2
[
[
Ag(NHC)Br] (5)
Ag(NHC)Br] (6)
t, 3H), (1.95 ppm, m, 2H) and (4.3 ppm, t, 2H) attributed to
CH ), (CH ) and (CH ) of propyl group, respectivelu.As well
Mass analysis: Mass spectrum of both imidazolium salts
(3 and 4) gave base peak at m/z = 215.15, 100% attributed to
(
2
2
3
+
+
as a downfield shift of the imidazole signals from (8.3, 7.7
and 7.1 ppm to 10.9, 8.4 and 8.27 ppm resulted in the formation
of imidazolium salt (3) formation.
the molecular weight of [C14
H
19
2
N ] , [M-Br] .
Mass spectra of silver(I) complexes: Mass spectrum silver
complex (5) showed a mother ion peak at m/z 573.14, 100 %)
1
+
+
H NMR analysis confirmed the formation of imidazolium
for bis-carbene silver fragment [Ag(NHC) H] which was
2
salt (4) in DMSO-d
0.95 ppm, t, 3H), (1.98 ppm, m, 2H) and (4.35 ppm, d, 2H),
with integration 7H, attributed respectively to (CH ), (CH
and (CH ) of propyl group. The signals of imidazole ring were
6
through the appearance of three peaks at
formed in gas state [31]. While mass spectrum of another silver
(
complex (6) gave a mother ion peak at m/z = 401.97, 100 %) can
+
+
2
2
)
be attributed to molecular weight of [C14
H
18AgBrN ] , [M-H] .
2
3
FTIR analysis: FTIR spectra of both imidazolium salts
(3 and 4), showed absorption bands within frequencies ranges
shifted to downfield from (7.4 ppm, s, 1H), (6.9 ppm, d, 2H)
to (9.7ppm, s, 1H), (8.3 ppm, d, 1H) and (8.1 ppm, d, 1H). The
spectrum also showed an additional singlet signal at (2.1 ppm,
s, 6H) related to methyl groups.
-1
3100-3070 and 2995-2885 cm , related to ν(C-H) of aromatic
and aliphatic groups, respectively. The peaks at 1555-1550
-
1
and 1390 cm can be assigned to ν(C=C) and ν(C-N),
13
13
C NMR analysis: C NMR spectrum of imidazolium
salt (3) in CDCl exhibited chemical shifts at δ 135.0, 123.56
respectively.Also the spectra revealed absorption peak in 1470-
-1
3
1465 cm region, which is attributed to the stretching vibrations
of ν(C=N), which slightly shifting in spectra of both silver
complexes upon formation ofAg(I) complexes and generation
of carbene compounds through deprotonation of imidazolium
salts. Table-2 shows the frequencies of key characteristic bands
related to imidazolium salts and their two silver complexes.
and 120.89 ppm assigned to carbon atoms of imidazole ring.
The chemical shift at 121.48, 129.65,?=132 and 146.41) ppm,
assigned to the carbon atoms of benzene ring. In addition the
spectrumshowed chemical shiftsrelated to the propyl and ethyl
groups at (10.6, 23.68, 51.46) ppm and (15.12, 28.24) ppm,
respectively.
Conclusion
13
Three chemical shifts observed in C NMR spectrum of
imidazolium salt (4) in DMSO-d at 10.26, 22.64 and 50.65
We have synthesized new imidazolium salts (3 and 4) as
a precursors of N-heterocyclic carbenes by reaction of N-aryl-
imidazole derivatives (1 and 2) with propyl bromide in absence
of any solvent. Two silver complexes (5 and 6) were synthesized
via the reaction of imidazolium salts with silver oxide by in
situ method under convenient condition. The formation of all
synthesized compounds has been characterized.
6
ppm assigned to propyl carbon atoms. The chemical shift of
two methyl groups appeared at 17.02 ppm, while the chemical
shifts of carbon atoms related to imidazole ring showed at
1
23.24, 130.51 and 137.28 ppm. Similarly, the chemical shifts
related to benzene ring carbon atoms appeared at 123.69,
28.79, 133.54 and 134.64 ppm.
1
1
1
H NMR analysis of silver(I) complexes (5 and 6): H NMR
ACKNOWLEDGEMENTS
spectra of silver(I) complexes (5 and 6) confirmed the disapp-
earance of (NCHN) signals, which was appeared in the spectra
of two imidazolium salts at 10.9 and 9.7 ppm, respectively, as
a result of generation of carbene (NCN) upon coordination
with silver ion. The signals of imidazole ring for complex 5
were shifted from 8.4 and 8.27 ppm to 7.3 and 7.2 ppm due to
the resonance loss of imidazole which resulted in the formation
of complex 5.
The authors are thankful to the Department of Chemistry,
College of Science, Mustansiriyah University, Baghdad, Iraq
for providing laboratory for facilities to the service laboratory
for providing spectra and analytical magnetic susceptibility
measurements. The authors are also grateful to Cardiff University
for NMR and El-MS spectral analysis.
CONFLICT OF INTEREST
The same observation was observed in the spectrum of
complex 6, where the signals of imidazole ring were shifted
from 8.3 and 8.1 ppm to 7.8 and 7.6 ppm for the same reason.
The authors declare that there is no conflict of interests
regarding the publication of this article.
13
C NMR analysis of silver complexes (5 and 6): The
chemical shifts related to N-heterocyclic carbenes appeared in
spectra of imidazolium salts (3 and 4) at (135.0 and 137.24
ppm), respectively. However, the same was found to be disapp-
eared in spectra of silver(I) complexes (5and 6), which confirmed
the formation of both silver complexes. In addition, new chemical
shifts were emerged in spectra of both silver(I) complexes at
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