Identification of radical structures on 1-pentamethylbenzyl-3-ethylimidazoliumsilver(I)bromide and 1,3-bis(pentamethylbenzyl)-4,5-dimethylbenzimidazoliumsilver(I)bromide exposed to gamma rays: an EPR study

Article abstract of DOI:10.1002/mrc.4479

1-Pentamethylbenzyl-3-ethylimidazoliumsilver(I)bromide and 1,3-bis(pentamethylbenzyl)-4,5dimethylbenzimidazoliumsilver(I)bromide and their Ag⁺ complexes were synthesized and their polycrystal forms were produced by recrystallization in dichloromethane/Et₂O solvent system. Structural determinations were carried out by ¹H NMR and ¹³C NMR with a Varian 400 NMR system using tetramethylsilane as internal standard and CDCl₃ as solvent. The disappearance of acidic N-heterocyclic carbene proton showed the formation of Ag(I) complexes. Also, elemental analyses were carried out. Electron paramagnetic resonance (EPR) measurements were performed to determine the formed radical structure on the samples irradiated at the room temperature for 72 h by using ⁶⁰Co-source with dose rate of 0.680 kGy. The EPR measurements were carried out in the temperature range of 200 K–450 K. Identical radicals were determined on the irradiated compounds. It was observed that the shapes of the spectra of the samples were independent of the temperature but, the resonance line intensities changed linearly with the temperature. Also, it was detected that the free radical on the 1-pentamethylbenzyl-3-ethylimidazoliumsilver(I)bromide is not stable compared to that on the 1,3-bis(pentamethylbenzyl)-4,5dimethylbenzimidazoliumsilver(I)bromide. Copyright

Full text of DOI:10.1002/mrc.4479

Research article
Received: 20 May 2015
Revised: 24 June 2016
Accepted: 29 June 2016
Published online in Wiley Online Library: 24 July 2016
(wileyonlinelibrary.com) DOI 10.1002/mrc.4479
Identification of radical structures on 1-
pentamethylbenzyl-3-ethylimidazoliumsilver(I)
bromide and 1,3-bis(pentamethylbenzyl)-4,5-
dimethylbenzimidazoliumsilver(I)bromide
exposed to gamma rays: an EPR study
Yusuf Ceylan,a Keziban Usta,b* Ahmet Kunduracioglu,c Ayhan Ustad
and Bekir Cetinkayae
1-Pentamethylbenzyl-3-ethylimidazoliumsilver(I)bromide and 1,3-bis(pentamethylbenzyl)-4,5dimethylbenzimidazoliumsilver(I)
bromide and their Ag+ complexes were synthesized and their polycrystal forms were produced by recrystallization in
1
dichloromethane/Et2O solvent system. Structural determinations were carried out by H NMR and 13C NMR with a Varian 400
NMR system using tetramethylsilane as internal standard and CDCl3 as solvent. The disappearance of acidic N-heterocyclic
carbene proton showed the formation of Ag(I) complexes. Also, elemental analyses were carried out. Electron paramagnetic res-
onance (EPR) measurements were performed to determine the formed radical structure on the samples irradiated at the room
temperature for 72h by using 60Co-source with dose rate of 0.680 kGy. The EPR measurements were carried out in the temperature
range of 200 K–450 K. Identical radicals were determined on the irradiated compounds. It was observed that the shapes of the
spectra of the samples were independent of the temperature but, the resonance line intensities changed linearly with the temper-
ature. Also, it was detected that the free radical on the 1-pentamethylbenzyl-3-ethylimidazoliumsilver(I)bromide is not stable
compared to that on the 1,3-bis(pentamethylbenzyl)-4,5dimethylbenzimidazoliumsilver(I)bromide. Copyright © 2016 John Wiley
& Sons, Ltd.
Keywords: EPR; irradiation effect; chemical synthesis; NHC complexes
that the majority of Ag was presented in diamagnetic Ag+1 form. In
Introduction
the study of Arcon et al.,[13] the sample containing Br was investi-
N-heterocyclic carbene (NHC) salts (mainly imidazolium and
benzimidazoliıum salts) have given a new route to chemists since
1968. These versatile compounds both as metal free compounds
and as their transition metal complexes have got many properties,
making them attractive for researchers.[1] Their antimicrobial,
antihelmintic, antifungal and anticancer activities and potentials
in catalytic applications made them noteworthy compounds. NHC
salts have three main parts and can be changed for gaining differ-
ent properties. So NHC salts and their metal complexes can be
fine-tuned for the most suitable properties (Scheme 1).
gated and observed that EPR signals were temperature dependent
and the signals exhibited a dramatic narrowing.
It is known that irradiation changes a number of physical and
chemical properties of the irradiated materials such as electrical
resistivity, magnetic susceptibility, etc..[14,15] The purpose of this
*
Correspondence to: Keziban Usta, Alanya Alaaddin Keykubat University,
Department of Metallurgical and Materials Engineering, Alanya, Antalya,
Turkey. E-mail: kezibanusta09@gmail.com
It has been known for a long time that silver(I)NHC complexes
have antimicrobial activity.[2] In recent studies, these complexes
have been examined for their catalytic activity. And during these
studies, some research groups have achieved promising results.[3]
These versatile compounds have exhibited very good results in
anti-tumor (anticancer) activities in the last decade.[4,5]
In recent years, electron paramagnetic resonance (EPR)
technique has been widely used for characterization of various
materials.[6–11] Researchers have investigated compounds contain-
ing Ag and Br by using EPR method. For instance, Samokhvalov
et al.[12] have studied surface characterization of Ag and have found
a Selcuk University, Faculty of Science, Department of Physics Konya, Turkey
b Alanya Alaaddin Keykubat University, Department of Metallurgical and Materials
Engineering Antalya, Turkey
c
Pamukkale University, Vocational College, Department of Chemistry Denizli,
Turkey
d Alanya Alaaddin Keykubat University, Department of Mechanical Engineering
Antalya, Turkey
e Ege University, Faculty of Science, Department of Chemistry Izmir, Turkey
Magn. Reson. Chem. 2016, 54, 864–869
Copyright © 2016 John Wiley & Sons, Ltd.

ESR study on NHC complexes
synthesized according to the route in Scheme 2.[16] In a 50 ml
Schlenk tube, 2.41 g (0.01 mole) of penta methyl benzylbromide
was dissolved in 20 ml of toluene. Stirring and heating at 50°C
0.96 g (0.01 mole) of 1-ethylimidazole was added. The reaction mix-
ture was stirred in constant temperature for 2 h. A white dense milk-
iness was observed. Toluene was distilled off under vacuum and a
dirty-white solid was obtained. This solid was washed and
redissolved in 1 ml of dichloromethane (DCM) and hexane was
added to the solution until it became cloudy. The white substance
achieved after 24h of crystallization was dried under vacuum.
Yield = 2.8 g (83%), mp = 135–137 °C 1H NMR (400MHz, CDCl3,
25°C) δ = 10.35 (s, 1H, CcarbeneH), 7.26–6.85 (s, 2H, backbones),
2.24–2.19 (s, 15H, Me5Ph Hs), 1.61 (t, 3H, N―CH2―CH3), 4.50 (q,
2H, NCH2―CH3), 5.65 (s, 2H, N―CH2―Bzyl), 13C NMR δ = 137.32
(Ccarbene), 125.2 and 120.81 (backbones) 77.38–76.75(CDCl3–sol-
vent) 49.10–45.47 (N―C*H2―CH3) 17.24–15.62 (Bzl―(CH3) and
(N―CH2―C*H3), 133.78–133.56 (Bzl-Arom).
Scheme 1. The ‘property forming’ sites of a NHC salt and NHC-M complex
(CAÀ = Counter Anion).
study is to give information about the radical structures of
1-pentamethylbenzyl-3-ethylimidazoliumsilver(I)bromide
plex.1) and 1,3-bis(pentamethylbenzyl)-4,5 dimethylbenzimidazo-
liumsilver(I)bromide (Complex 2) by means of EPR.
(Com-
The synthesis of 1,3-bis(pentamethylbenzy)-4,5-
dimethylbenzimidazoliumbromide (Salt 2)
Instead of the conventional method with strong base, a more prac-
tical way can be used. To perform this, DMF (N-N′-dimethyl formam-
ide) was used as both a solvent and basic reagent (Scheme 2);
1.46 g (0.01 mole) of 4,5-dimethylbenzimidazole was dissolved in
10ml of DMF in a 50 ml Schlenk type tube stirring at 70 °C. After stir-
ring for 30 min, two eq. of pentamethyl benzylbromide (4.82 g or
0.02 mole) was added and the reaction mixture was stirred at con-
stant temperature for 10h. Solvent was distilled off under vacuum
and then a dirty-white yellowish solid was obtained. This solid
was washed with hexane (5 ml ×3 times) and dried under vacuum.
No further recrystallization was needed. Yield 4.5g (86%),
mp = 208–210 °C, 1H NMR δ = 9.56 (s, 1 H, C carbene), 8.07 (s, 2 H, ar-
omatic part of benzimidazol), 5.7 (s, 4 H, N―CH2―), 2.15 (s, 30 H,
Me5), 2,61 (s, 6 H, back methyls), 13C NMR δ = 137.70(Ccarbene),
Experimental
All solvents and reagents were commercially obtained from the
suppliers and were used without further purification. Melting points
were measured using an Electrothermal 9100 melting point detec-
1
tion apparatus. H NMR and 13C NMR spectra were referenced to
tetramethylsilane (TMS) at 0.00 ppm as internal standard solution
and recorded at room temperature using a Varian 400 MHz
spectrometer.
The synthesis of 1-pentamethylbenzyl-3-
ethylimidazoliumbromide (Salt 1)
125.06
(backbones)
77.34–76.70(CDCl3–solvent)
47.74
Pentamethylbenzylbromide and 1-ethylimidazole were synthe-
sized according to conventional methods in literature[16] (Scheme
2). Salt 1-pentamethylbenzyl-3-ethylimidazoliumbromide was
(N―C*H2―Bzl) 20.81–16.94 (Bzl―(CH3), 133.32–130.54 (Bzl-Arom)
The synthesis of silver(I)NHC complexes
Both complexes have been synthesized by reaction between
1 mmol of the corresponding NHC salt and 0.5mmol (138 mg) of
Ag2CO3 (Scheme 2). Ag2CO3 was used in such reactions as both a
Ag+ ion supplier and basic salt for stripping the acidic proton of
the carbene. Both of the complexes have been recrystallized in
DCM/Et2O in 24 h. For both complexes, the formation of complexes
was monitored by 1H NMR spectra, 13C NMR spectra and elemental
analysis methods.
The NMR data are: For complex 1, ,1H NMR δ = 7.26–6.91 (s,
2 H, backbones), 2.26–2.19 (s, 15H, (benzylic methyl Hs), 1.44
(t, 3H, methyl of ethyl group), 4.15 (q, 2H, methylene of ethyl
group), 5.34 (s, 2H, ethylene of benze group). 13C NMR
δ = 186.08(Ccarbene), 125.24 and 121.68 (backbones) 77.39–
76.75(CDCl3–solvent) 46.78 and 45.37 (N―C*H2―Bzl/
N―C*H2―CH3) 17.24–15.46 (Bzl―(CH3) and ―CH2―CH3 of
ethyl), 137.31–133.54 (Bzl-Arom). Elemental Analysis(Theo/
Exp.) = C, 60.47/61,05; H, 6.61/6,84; N, 4.27/4.46).; For complex
1
2, , H NMR δ = 8.03 (s, 2 H, aromatic part of benzimidazol),
5.59 (s, 4 H, methylenes of benzylic groups), 2.06–2.33 (s, 30 H,
methyls of penta methyl benzenes), 2,55 (s, 6 H, methyls of
benzimidazol). 13C NMR δ = 182.28(Ccarbene), 125.02(back-
bones) 77.38–76.75(CDCl3–solvent) 47.69 (N―C*H2―Bzl)
Scheme 2. Synhesis routes for Salt 1, Complex 1 (a); Salt 2, Complex 2 (b)
i = KOH, Acetone, 1 eq. EtBr, 24 h. reflux ii = Toluene, pentamethyl
benzylbromide 45–50 °C, 3–4 h. iii = 0.5 eq. Ag2CO3, DCM, 24 h. iv = DMF,
pentamethyl benzylbromide 10–12 h. 50–60 °C.
Magn. Reson. Chem. 2016, 54, 864–869
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Y. CEYLAN ET AL.
20.77–16.85 (Bzl―(CH3)), 137.14–130.45 (Bzl-Arom). Elemental
Analysis: (Theo/Exp.)= C, 45.87/46.16; H, 5.66/5.42; N, 6.29/6.74
Results and Discussion
Chemical analysis
1H NMR and 13C NMR data for the compounds were recorded in
CDCl3 for all compounds. From the 1H NMR spectra of salt 1 the sin-
glet peak was observed at 10.35 and it was assigned to acidic pro-
ton of the carbene ring. The two peaks observed at 4.50 (m, 2H),
5.65 (s, 2H) ppm were assigned to CH2 protons bridging the ring
to other groups. The singlet at 2.23 ppm was assigned to methyl
protons on the benzene ring. The methyl group of the tail gave a
triplet at 1.61 ppm. The protons of backbone of the ring were sep-
arately observed at 6.85–7.59ppm. Because of asymmetry of the
molecule, a difference was seen (Fig. 1). From the 13C NMR spectra
of salt 1 the most significant peaks were interpreted as the peak
observed at 137.32ppm shows the carbene carbon which is highly
diagnostic for NHCs; also the peaks at 125.20 and 133.60 refer to the
EPR measurements
The polycrystal forms of complex 1 and complex 2 samples were
irradiated for 72 h by using 60Co-gamma ray source with dose rate
of 0.680 kGy. The measurements were performed by X-band EPR
spectrometer in Saraykoy laboratory of TAEK (Turkish Atomic
Energy Authority). In the measurements, the spectrometer was set
as follows: the microwave power was 5 mW, the modulation fre-
quency was 100 kHz and modulation amplitude was 0.30G. The
high and low temperature measurements were carried out
between 200 K and 450 K by using temperature control unit of
the spectrometer. For verification of the values obtained from the
recorded spectra, WINEPR simulation program was used.
Figure 1. 1H-NMR of salt 1.
Figure 2. 1H-NMR of complex 1.
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ESR study on NHC complexes
Figure 3. a. 1H-NMR of salt 2. b 1H-NMR of complex 2.
backbone carbon atoms and 49.10 ppm shows the benzylic
―CH2― carbon, which is attached to a phenyl.
The most important detail on the 1H NMR spectra is the disap-
pearance of the peak at 10.35 as the complexation reaction forms
the Ag(I)NHC complex. Other peaks have remained in the spectra.
The biggest change was observed on the ring protons and bridging
protons. The two peaks observed at 4.15 (m, 2H) and 5.34 (s, 2H)
ppm were assigned to CH2 protons bridging the ring to other
groups. The singlet at 2.20 ppm was assigned to methyl protons
on the benzene ring. The methyl group of the tail was observed
as a triplet at 1.44 ppm. The protons of backbone of the ring were
observed at 6.57–6.91 ppm separately. It can be noticed from the
values that the signals have shifted in different amounts (Fig. 2).
Figure 4. Molecular structures of complex 1 and complex 2.
Figure 5. The EPR spectra at 260 K (a), 320 K (b) for complex 2.
Magn. Reson. Chem. 2016, 54, 864–869
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Y. CEYLAN ET AL.
most considerable differences were in the protons, neighboring
the main ring of the NHC. The differences reach 0.37 ppm. The hy-
drogen protons on the main body and the benzene ring showed
minor differences (0.04 ppm or less). Also similar observations were
seen in the 13C NMR spectra. For example, the peak observed at
177.90 ppm shows that the carbene carbon atom has bonded to
the Ag(I) ion to form a complex. In 13C NMR, the other peaks
changed slightly.
For salt 2 and complex 2, the same analysis was carried out using
1H and 13C NMR and elemental analysis as well. For salt 2 the most
1
significant peak in the H NMR spectrum is at 9.26 ppm, which
shows acidic proton of the carbene. The other important peaks
are 7.26 for backbones, 5.68 for the protons neighboring to N1
and Ph in benzylic groups. A major number of peaks were massed
between 2.61 and 2.15ppm which refers to the methyl groups on
benzene rings. After complex formation the most remarkable
change was the disappearance of the peak at 9.26 ppm. Other
peaks changed slightly (about 0.05 ppm) (Fig. 3a,b).
Figure 6. The molecular structures (a) and radical structures (b) for
complex 2 (A) and complex 1 (B).
In 13C NMR spectra, the most remarkable peaks are examined in a
similar way to salt 1 and complex 1. The most striking peaks are the
ones at 137.70 ppm and 125.06ppm, those show carbene carbon
and backbone carbons respectively. After complex formation a
peak at 182.28ppm appeared. The other peaks changed their value
up to 0.20 ppm as well.
EPR analysis
Complex 1 and complex 2 (Fig. 4) were irradiated for 72 h. EPR ex-
periments were performed to determine change in the structures.
It was observed that the EPR spectra of the complex 2 are generally
similar to those of complex 1. The measurements were performed
in the temperature range of 200 K – 450 K. A temperature depen-
dence of the number of EPR lines was not observed for both com-
pounds, but the line intensities changed with temperature (Fig. 5).
From analysis of the spectra and the molecular structures, it was
determined that chemical reaction of both samples with gamma
rays gave rise to one radical type; these radicals were similar for
both compounds and also the produced unpaired electrons were
mainly localized on the Ag atom. Considering the spectra, the EPR
line splits into a doublet of 1:1 intensity ratios because of Ag nu-
cleus, then each line splits twice into 1:1:1 intensity ratios because
of the magnetic unequivalence of the nitrogen atoms. Then each
line splits into 1:1 intensity ratios twice because of the magnetically
inequivalent hydrogen protons. But, because of the overlapping,
these splittings were not observed clearly in the spectra. According
Figure 7. For complex 2, the experimental EPR spectra (a) and its
simulations (b).
The most important difference between two spectra was disap-
pearance of the peak in 10.35 ppm of acidic proton in the salt. This
shows the formation of NHC―Ag (I) complex from the NHC salt.
Other peaks in the spectra have shifted to different values. The
Figure 8. Temperature dependence of the unpaired spin concentration between 200 K and 450 K for complex 2 (a) and complex 1 (b).
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Magn. Reson. Chem. 2016, 54, 864–869

ESR study on NHC complexes
to analysis based on the results obtained from the spectra and the
molecular structures of the samples, the effect of the gamma irradi-
ation on the molecular structure was attributed as shown in Fig. 6.
In addition, these obtained results were verified by a computer
simulation using the code WINEPR (Fig. 7). The hyperfine parame-
ters and g-factor values were found for both samples as follows:
g = 2.0134, AAg = 15.30 Gauss (G), AN1 = 9.05 G, AN2 = 11.62 G,
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The radical structures produced by irradiation were investigated by
using the EPR method. The obtained EPR spectra supported the
presence of one type of radical structure on both 1-
pentamethylbenzyl-3-ethylimidazoliumsilver(I)bromide
(com-
plex.1) and 1,3-bis(pentamethylbenzyl)-4,5 dimethylbenzimidazo-
liumsilver(I)bromide (complex 2). It was understood from change
of color that the irradiation produced color centers on complex 2.
According to the obtained results, it can be said that Complex2
was less resistant to irradiation than complex 1. We hope that the
results reported, (i.e. characterization of the material irradiated by
using the EPR technique) will contribute to the understanding of
the such materials in chemistry, physics, etc.
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Supporting information
Acknowledgements
Additional supporting information may be found in the online ver-
sion of this article at the publisher’s web site.
This study was partially supported by Alanya Alaaddin Keykubat
University and the Scientific Research Projects Coordination Cen-
ters of Pamukkale, and Selcuk Universities –Turkey.
Magn. Reson. Chem. 2016, 54, 864–869
Copyright © 2016 John Wiley & Sons, Ltd.
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