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DaltonTransactions
COMMUNICATION
Cite this: Dalton Trans., 2013, 42, 2366
Received 4th October 2012,Accepted 17th December 2012
DOI: 10.1039/c2dt32818f
www.rsc.org/dalton
Selenium containing imidazolium salt in designingsingle source
precursors for silver bromide and selenidenano-particles
Hemant Joshi, Kamal Nayan Sharma, Ved Vati Singh, Pradhumn Singh
andAjai Kumar Singh*
The AgBr and Ag2Se nanoparticles (NPs) have been synthesized
for the first time from two single source precursors
([Ag2(L)2Br2]
(1) and [Ag(LHBr)2]BF4 (2) respectively) designed using the
same
ligand 3-benzyl-1-(2-phenylselanyl-ethyl)-3H-imidazolium
bromide
(L). The ODEODAOA (1 : 1 : 2) and TOPOA (1 : 2) are most
suitable solvents for thermolysis of 1 and 2 respectively,
resulting
in the NPs. The composition of the solvent used in
thermolysis
affects the purity of NPs. The bonding of L in 1 is unique, as
it
has a pre-carbene site intact.
There is considerable current interest in nanocrystals of
metalchalcogenides having optical and optoelectronic
properties.They have been investigated for applications in solar
cells,1
light-emitting diodes,2 thin-film transistors,3 and
biologicalimaging.4 Nanocrystals of silver halides and
chalcogenides areof specific interest due to their existing and
envisaged appli-cations. Silver bromide NPs have been reported to
interactwith polymer composites, which show antimicrobial
activity.5
A visible light photocatalyst has been designed using nano-sized
silver bromide.6 Photocytotoxicity of AgBr based visiblelight
photocatalytic nanoparticles has been reported in vitro aswell as
in vivo.7 The AgBr nanoparticles templated on apolymer surface have
been used for olefin separation.8 Nano-sized silver selenide
showing some interesting and usefulproperties exists in two phases,
orthorhombic and cubic. Itspotential for several applications, such
as solid electrolyte insecondary batteries9 (photochargable) and
photosensitizer inphotographic films or thermochromic materials,
has beenrecognized. The Ag2Se as a solid electrolyte has been
reportedof interest as an additive in network glasses, such as
chalcogenides and oxides. The resulting composite glassesshow
high electrical conductivities relevant for applications insensors,
batteries and displays.10 In spite of potential fordiverse
applications, the synthesis of high-quality AgBr andAg2Se
nano-particles (NPs) is less explored, particularly if wecompare
with those of other semiconducting materials.11 Themethods used so
far for their preparation suffer from one ormore of the following
limitations. (i) Nano-particles formedhave size variation over a
wide range. (ii) Two separate sourcesare used for each Ag and Br or
Se. (iii) A sophisticated physicaltechnique is employed. Important
methods reported to syn-thesize AgBr and Ag2Se NPs are as follows.
The microemulsifi-cation of bulk AgBr in CTAB
(hexadecyltrimethylammoniumbromide) has resulted in nano-sized
AgBr.12 The solidsolidreaction13 has been employed to generate AgBr
NPs. In AgBrpolymer composites, nanosized AgBr is generated
frombromide of an ionic polymer and Ag+ provided by a
silver(I)salt, but AgBr nanoparticles thus formed are implanted on
thepolymer surface.5 The preparation of quantum dots of AgBr
viaelectroporation of vesicles has been reported.14 The
bio-mimicked reduction of selenite with glutathione (GSH) toGSSeH
followed by a reaction with a Ag+alanine complex gaveAg2Se quantum
dots.
15 The reaction of AgNO3 and Se at atemperature of 150 C formed
quantum dots of Ag2Se.
16 Theother reports on the preparation of Ag2Se nanoparticles
arebased on vapor-phase growth,17 and sonochemical18 methods.The
nanowires,9 and nanoscale dendrites19 of Ag2Se have alsobeen
reported. The synthesis of AgBr and Ag2Se NPs withoutthe use of
exotic techniques, extreme conditions and dualsources continues to
be a current challenge. The synthesis ofAgBr and Ag2Se NPs by
thermolysis of single source precursors(SSPs) at moderate
temperature is not in our knowledge. TheSSPs are often less noxious
than multiple ones because of anintrinsic control of reactivity and
stoichiometry.20 As organo-selenium ligands have been used to
design catalysts forvarious chemical transformations21 and
single-source precur-sors (SSPs) for various metal selenides,20b it
was thereforethought worthwhile to design a selenium containing
imid-azolium salt (L) and its two silver complexes [Ag2(L)2Br2] (1)
and
Electronic supplementary information (ESI) available: NMR
spectra (Fig. S2S14), mass spectra (Fig. S15S18), crystal data and
refinement parameters(Table T1), bond lengths and angles (Table
T2), SEMEDX images (Fig. S19S21),PXRD (Fig. S22), and TEM image
(Fig. S23). CIF for 1. CCDC 885040. For ESI andcrystallographic
data in CIF or other electronic format see
DOI:10.1039/c2dt32818f
Department of Chemistry, Indian Institute of Technology Delhi,
New Delhi 110016,
India. E-mail: [email protected],
[email protected];
Fax: +91-01-26581102; Tel: +91-011-26591379
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[Ag(LHBr)2]BF4 (2) [by variation in counter anions]. The 1 and2
on thermolysis at 200 C have formed AgBr and Ag2Se NPsrespectively,
thus behaving as SSPs. Technically 1 and 2 maybe considered
different but essentially the different bondingmode of L in 2 is
triggered by removing Br with BF4
only.Thus SSPs for AgBr/Ag2Se in essence are not different.
Theseresults are reported in this communication in detail. For
thefirst time two silver(I) complexes synthesized using the
sameselenium containing imidazolium salt have been found tofunction
as SSPs for two different nanoparticles viz. of AgBrand Ag2Se.
One-pot synthesis of AgBr and Ag2Se NPs respectively fromsingle
source precursors, Ag(I) complexes 1 and 2, is summar-ized in
Scheme 1. For the synthesis of 1 and 2 all reactionswere carried
out at room temperature, under a nitrogen atmos-phere and in the
absence of light (see ESI for details). TheAg(I) complex 1 was
prepared by treating L dissolved in dichloro-methane directly with
solid Ag2O at room temperature. Forthe synthesis of 2, the L was
first treated with AgBF4 and a pre-cipitate of AgBr was filtered
off. The filtrate on treatment withsolid Ag2O at room temperature
resulted in the desiredcomplex 2. The 1H, 13C{1H} and 77Se{1H} NMR
spectra of Land their complexes 1 and 2 have been found consistent
withtheir molecular structures. All spectra of three species
aregiven in the ESI (Fig. S2S14). The assignments of signals
in13C{1H} NMR of L are based on HMQC experiments (Fig. S7 inESI).
The signals observed in 1H and 13C{1H} NMR spectra ofL at 10.09
(carbenic proton attached to a C atom of NvCN)and 136.1 ppm (C atom
of NvCN) respectively are at higherfrequency (2.71 and 5.3 ppm
respectively) with respect tothose of imidazole A (Fig. S2, S3, S5
and S6 in ESI). The posi-tion of the carbenic proton signal at
10.09 ppm in the 1H NMRspectrum is within the range reported for
the position of suchproton in the spectra of other imidazolium
salts ( 9.012.0),22
and indicates its high acidity. The signal in the 77Se{1H}
NMRspectrum of L appears at lower frequency (4.8 ppm) withrespect
to that of A. In the 1H NMR spectrum of complex 1, asignal at 8.8
ppm appears due to NvCHN whereas in thespectra of 2 there is no
such signal, indicating deprotonationof carbenic hydrogen.23 In
77Se NMR spectra of 1 and 2(Fig. S11 and S14, in ESI) the signal is
at a lower frequency(13.1 and 15.8 ppm respectively) with respect
to that of the freeL (Fig. S8, in ESI). This suggests almost a
similar electronicenvironment for Se in both the complexes. In the
13C{1H}NMR spectrum of 2, an NvCN signal appears at 179.6 ppm,
a typical position for carbene bonded to silver.23 In the
spectraof 1 no such signal appears indicating the absence of
carbenebonded to silver.24 The mass spectral data of 1 and 2 were
alsofound consistent with their structures (see ESI
forassignments).
The single crystal structure of 1 has been solved by
X-raydiffraction. The 1 is homo-bimetallic (bromo-bridged)
innature. It is interesting to note that Ag is bonded to
seleniumand not to an available carbene donor site. To the best of
ourknowledge this is a unique bonding state shown with Ag by
acarbene functionalized with a soft donor site (presentlyselenium)
and most probably complex 1 appears to be firstexample of this
type. In the crystal of 1, AgAg distance,3.1179(8) , indicates
argentophilic interaction.25 As AgBrbridges are not symmetrical but
compensatory type, facilita-ting silversilver interaction (Fig. 1).
However the attempts tocrystallize complex 2 were not successful.
On the basis of spec-tral data ligand L in 2 appears to be bonded
with Ag throughSe and a carbene site.
High-quality surfactant capped NPs of AgBr and Ag2Sehaving size
in a narrow range have been prepared in goodyields for the first
time by one pot synthesis using silver(I)complexes 1 and 2
respectively as single-source precursors.The NPs of both AgBr and
Ag2Se are stable in air and also oncentrifugation up to 5800 rpm in
ethanol. The combination ofdifferent capping ligands and solvents
has been reported toplay a significant role in affecting the
dispersity of NPs.11c
However, the effect of a solvent system on composition of NPsis
not in our knowledge. We have found probably for the firsttime that
chemically different NPs are obtained from 1 (usedas SSP) on
changing the combination of solvents during theirsynthesis.
Thermolysis (at 200 C) of 0.1 mmol of complex 1 in1-octadecene
(ODE) : octadecylamine (ODA) : oleic acid (OA)(v : v ratio 1 : 1 :
2) results in nearly 100% pure AgBr NPs.However, a mixed phase of
NPs of Ag (5.2%), AgBr (67.3%) andAg2Se (27.4%) has been obtained
when thermolysis of 1 wascarried out in a mixture (1 : 2) of
trioctylphosphine (TOP) andOA under similar conditions (Fig. S22
and S23, see in ESI).Thermolysis of 0.1 mmol of complex 2 in a TOP
+ OA mixture(1 : 2), at 200 C for 60 min gives NPs of Ag2Se only.
It has beenreported recently that NPs of different phases of
palladiumselenide are obtained when Pd(II) complexes of
different
Scheme 1 Synthesis of AgBr and Ag2Se NPs by using SSPs.
Fig. 1 ORTEP diagram of 1 with 30% probability ellipsoids;
hydrogen atomsare omitted for clarity. Key bond lengths ():
Se(1)Ag(1) 2.663(6), Br(1)Ag(1)2.662(6), Br(2)Ag(1) 2.650(6). Bond
angles (): Br(1)Ag(1)Se(1) 124.7(2),Se(1)Ag(1)Br(2) 104.9(2),
Br(1)Ag(1)Br(2) 111.9(2).
Dalton Transactions Communication
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ligands are used as SSPs.26 In the present case
chemicallydifferent NPs are obtained on thermolysis under similar
reac-tion conditions of Ag(I) complexes (1 and 2) designed usingthe
same ligand L as SSPs. The two silver complexes differ onlysomewhat
in bonding mode of L and the anion.
The NPs were characterized by powder X-ray, SEM-EDX andTEM. In
Fig. 2 and 3 powder X-ray diffraction (PXRD) patternsof presently
synthesized NPs of AgBr and Ag2Se are shown andthey are sharp in
nature. The standard PXRD patterns are ineach figure below the
experimental ones. Each of the PXRDpatterns was found consistent
with that of the correspondingstandard phase of the same
composition. The comparison ofpresent PXRD patterns with those of
standard phases suggeststhat the phase of present AgBr NPs has a
cubic structure(JCPDS 79-0149) and structure of the phase of Ag2Se
NPs isorthorhombic (JCPDS 24-1041).
The morphology of presently prepared AgBr and Ag2Se NPswas
examined with TEM recorded at room temperature. TheNPs (Fig. 4 and
5) obtained from 1 and 2 are spherical inshape and 80% of them have
size in the ranges 2540 and1525 nm respectively (see the size
distribution given in Fig. 4and 5). The energy-dispersive X-ray
spectra from SEM(Fig. S19S21, in ESI) indicate the presence of Ag
and Bratoms in NPs obtained from 1 and Ag and Se atoms in
NPsobtained from 2.
The UV-visible spectrum (Fig. 6) of AgBr NPs recorded intoluene
has a peak at 308 nm (band gap 4.10 eV) whereasAg2Se NPs show a
peak in the near IR region (Fig. 7) at1018 nm (band gap 1.21 eV)
(also in toluene). These values areconsistent with earlier
reports.27 The existence of a band inthe near infra-red (NIR)
region in the case of Ag2Se NPs is very
significant in the context of using them in sensors or
biomedi-cal imaging in living tissues. Near IR radiations solve
auto-fluorescence problem by reducing the fluorescence
Fig. 3 PXRD pattern of Ag2Se NPs.
Fig. 2 PXRD pattern of AgBr NPs.
Fig. 4 TEM images and distribution of AgBr NPs.
Fig. 5 TEM images and size distribution of Ag2Se NPs.
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background, which makes NPs absorbing in NIR promisingcandidates
for biomedical applications as interference ismuch less from blood
or tissues.
Conclusion
3-Benzyl-1-(2-phenylselanyl-ethyl)-3H-imidazolium bromide
(L)easily forms complexes [Ag2(L)2Br2] (1) and [Ag(LHBr)2]BF4(2),
latter by minor variation in the synthetic procedure usedfor 1,
which changes the counter anion. The 1 and 2 on thermo-lysis [in
ODEODAOA (1 : 1 : 2) and TOPOA (1 : 2) respect-ively] give AgBr and
Ag2Se nano-particles, acting as singlesource precursors. In 1 the L
shows a unique bonding modewith Ag(I) as it coordinates through a
soft donor site Se onlyand the carbene donor site remains
uncoordinated. It is forthe first time that two silver(I) complexes
synthesized using thesame selenium containing imidazolium salt have
been foundto function as SSPs for two different types of
nanoparticles.The composition of solvent mixtures has been found to
affectthe composition of NPs significantly.
Acknowledgements
The authors thank Council of Scientific and IndustrialResearch
(CSIR), New Delhi, India for the project no. 01(2421)10/EMR-II and
JRF/SRF to KNS. Department of Science andTechnology (India) is
acknowledged for research projects(SR/WOS-A/CS-22/2009 and
SR/S1/IC-40/2010), and financial
support for single crystal X-ray (FIST) and HR-TEM
(NSIT)facilities at IIT Delhi. HJ thanks University Grants
Commission(India) for JRF.
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