AIEE active perylene bisimide supported mercury nanoparticles for synthesis of amides via aldoximes/ketoximes rearrangement

Article abstract of DOI:10.1039/c4cc10273h

Aggregates of perylene bisimide (PBI) derivative 3 serve as reactors and stabilizers for the preparation of mercury nanoparticles (HgNPs) which exhibit excellent catalytic efficiency in the conversion of aldoximes/ketoximes into primary/secondary amides via Beckmann rearrangement. This journal is

Full text of DOI:10.1039/c4cc10273h

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AIEE Active Perylene Bisimide Supported Mercury
Nanoparticles For Synthesis of Amides via
Aldoximes/Ketoximes Rearrangement
Cite this: DOI: 10.1039/x0xx00000x
Sandeep Kaur, Manoj Kumar and Vandana Bhalla*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/.
Aggregates of perylene bisimide (PBI) derivative 3 serve as interested in the development of supramolecular aggregates which
reactors and stabilizer for the preparation of mercury could serve as “not quenched” reactors for generation of metal
nanoparticles (HgNPs) which exhibit excellent catalytic nanoparticles. For this purpose, perylene bisimide (PBI) is a scaffold of
efficiency in the conversion of aldoximes/ketoximes into our choice due to its high fluorescence quantum yield, high
primary/secondary amides via Beckmann rearrangement.
photostability and chemical inertness.6 The utilization of PBI scaffold
for the preparation of fluorescent probes is scarce because of the
notorious effect of aggregationꢀcaused quenching due to its strong
interꢀmolecular πꢀπ stacking and attractive dipoleꢀdipole interactions in
aqueous media.7 To weaken the strength of interꢀmolecular πꢀπ
stacking, we planned to decorate PBI with bulky groups at bay
positions and hence designed and synthesized perylene bisimide
Mercury is highly toxic environmental and industrial pollutant.1
Accumulation of mercury in the human body can affect the central
nervous system, endocrine system, renal system and reproductive
system.1 Therefore, the development of new methods for the
removal/detoxification of mercury is of paramount importance. For
detoxification of mercury, trapping in its nanoꢀform has been suggested
as useful method.2 In fact, mercury in its nanoꢀform is more
advantageous as plasmon resonance absorption of nanoparticles can be
utilized in photonics and sensing.3 Despite the importance of mercury
nanoparticles, there is paucity of investigations on mercury in its nanoꢀ
forms. Recently, mercury nanodrops have been fabricated within PVA
films which were then condensed to nanocrystals in cryoꢀTEM
experiment.3 However, thermal annealing was needed to carry out
reduction of mercury ions. Another report shows utilization of
Enterobacter sp. strain for in situ generation of mercury nanoparticles
(HgNPs). However, preparation of seed culture requires longer time
and extensive efforts.2 Thus development of a simple, fast and facile
method for the generation of HgNPs is still a challenge.
derivative
3 in which PBI scaffold is linked to naphthyl groups through
phenyl spacer and rotatable CꢀC and CꢀN bonds. We envisaged that
phenyl groups with rotatable CꢀC and CꢀN will impart aggregation
induced emission enhancement (AIEE) characteristics to the molecule
in the aqueous media and the presence of naphthyl groups bearing polar
hydroxyl group may impart restriction to the parallel faceꢀtoꢀface
intermolecular interactions in its aggregated state.8 Further, due to the
presence of soft imino linkages, aggregates of derivative
affinity towards mercury ions9 and thus could act as reactors and
stabilizer for generation of HgNPs. To our pleasure, derivative
3 may exhibit
3
exhibited aggregation induced emission enhancement (AIEE)
characteristics and formed fluorescent aggregates in aqueous media.
Interestingly, fluorescent aggregates of derivative
3 served as reactors
Recently, from our laboratory we have reported the utilization of
and stabilizer for the preparation of HgNPs and showed HgNPs
mediated emission enhancement, thus, displaying “not quenched”
response towards nanoparticles. Further, in situ generated HgNPs
showed excellent catalytic efficiency in the conversion of
aldoximes/ketoximes into primary/secondary amides. The work
presented in this manuscript has many advantages; first, it presents a
simple design for the preparation of AIEE active PBI derivative which
is unprecedented. Recently, Tang et al. reported tetraphenylethenylꢀ
modified perylene bisimide derivatives having aggregation induced
emission enhancement characteristics (AIEE).7a Their strategy of
development of AIE active PBI derivative involved incorporation of
supramolecular
assemblies
of
pentacenequinone
and
hexaphenylbenzene derivatives as reactors and stabilizer for in situ
generation of gold, iron oxide and copper nanoparticles.4 However, in
all these reports, except for the generation of iron oxide nanoparticles,
generation of gold and copper nanoparticles was accompanied by
quenching of emission intensity.4 The quenching probes generally have
interference by fluctuations in background fluorescence resulting in low
sensitivity and reliability.5 Therefore, preparation of fluorescent probes
showing “no quenching response” is of paramount importance due to
their better sensitivity and reliability.5 Keeping this in mind, we were
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DOI: 10.1039/C4CC10273H
AIE active tetraphenylethenyl groups in to the PBI scaffold. In case of observed with increasing fraction of triethyleneglycol (TEG) (Fig. S5,
derivative , we were successful in introducing AIEE characteristics in ESI†). Further, the temperature dependent fluorescence spectra also
3
the molecule without having any additional AIE active unit. Second, to showed the decrease in emission intensity with the rise in temperature
the best of our knowledge, this is the first report where HgNPs from 25 to 75°C (Fig. S6, ESI†). The viscosity and temperature
mediated aggregation induced emission enhancement of the PBI dependent studies suggest that restriction of intramolecular rotation
derivative has been demonstrated. Third, for the first time, a simple, (RIR) also plays a crucial role in emission enhancement of derivatives
fast, reductant free and convenient approach for development of stable 3 14
HgNPs at room temperature in aqueous media has been presented and the life time of derivative
the method being reported in the present manuscript for the preparation fluorescence study of derivative
.
We also carried out time resolved fluorescence study to determine
in the aggregated state. The timeꢀresolved
in H2O/THF (1:1, v/v) solvent
3
3
of HgNPs is better in comparison to the other methods reported in the mixture (Fig. S7, ESI†) shows longer life time when monitored at 530
literature (Table S1, ESI†). Fourth, the catalytic efficiency of in situ nm which suggests the presence of aggregated species (Table S4,
generated nanoparticles in the aldoximes/ketoximes Beckmann ESI†). Further, the concentration dependent 1H NMR studies of
rearrangement reaction has been demonstrated.
PBI derivative 1b was synthesized by SuzukiꢀMiyaura coupling of hydroxyl and imino protons, respectively and average upfield shift of
aniline boronic ester 2a9a with 1a10
Further, condensation of derivative 0.12 ppm for aromatic protons (Fig. S8, ESI†). The downfield shift of
1b with βꢀhydroxy naphthaldehyde, 2b furnished derivative in 86% hydroxyl and imino protons suggests intermolecular hydrogen bonding
yield (Scheme 1). The structures of derivatives 1b and were between the PBI molecules. The upfield shift of aromatic protons
confirmed from their spectroscopic and analytical data (Fig. S26ꢀS33, indicates the presence of πꢀπ interactions between the adjacent PBI
derivative 3 showed average downfield shift of 0.30 and 0.21 ppm for
.
3
3
ESI†).
molecules. On the basis of all these studies, we propose that PBI
molecules form πꢀπ stacked units which interconnect through hydrogen
bonding to form closely packed fluorescent supramolecular assemblies.
The dense packing arrangement of PBI molecules disfavors the
structural relaxation pathways in excited state that are known for
formation of nonꢀfluorescent aggregates of PBI derivatives.15 We
believe that dense packing of PBI molecules in aggregated state,
aggregation driven growth of the aggregates and restriction in the
intramolecular rotation of the rotors attached to the perylene core are
2a
Scheme 1. Synthesis of PBI based derivative 3: (i) K2CO3 (aq.), PdCl2(PPh3)2,
THF, 80°C; (ii) dry THF : methanol (4 : 6), 80°C.
the main reasons for the AIEE phenomena in case of derivative
The presence of imine and hydroxyl functionalities in the derivative
prompted us to investigate its sensing behaviour toward di erent metal
ions (Cd2+, Ba2+, Hg2+, Fe3+, Ni2+, Zn2+, Cu2+, Pb2+, Ca2+, Co2+, Ag+, Na+,
K+, Li+, Pd2+, Cr3+, and Al3+) as their perchlorate/chloride/or both
perchlorate and chloride salts by UVꢀvis and fluorescence spectroscopy
in H2O/THF (1:1, v/v). Among the various metal ions tested (Fig. S9Aꢀ
3.
3
The fluorescence spectrum of compound
3
in THF exhibits weakly
ff
emissive band at 530 and a smaller peak at 570 nm, when excited at
485 nm (Fig. S1, ESI†).11 Upon addition of water fraction up to 50%
(volume fraction) to the THF solution of 3, a slight red shift of 5 nm in
the emission band at 530 nm is observed. This red shifting of the
emission band is accompanied by enhancement in the emission
intensity (Fig. S1, ESI†). In addition, fluorescence enhancement was
also observed in the emission band at 570 nm. However, addition of
B, ESI†) aggregates of derivative
Hg2+ ions in the UVꢀvis spectra. Upon addition of Hg2+ ions (18 equiv.)
to the solution of derivative in mixed aqueous media (H2O/THF, 1:1),
3 show selective response towards
3
more than 50% water fraction to solution of derivative
3 resulted in the
two isobestic points are observed at 374 and 507 nm, respectively (Fig.
S10, ESI†). Formation of these isobestic points suggests existence of
more than one species in the solution. Further, a new absorption band at
290 nm corresponding to the formation of HgNPs was observed.3,16
(Fig. S10, ESI†). The intensity of this band increases with time (Fig.
S10, ESI†). These spectral changes are accompanied by the naked eye
color change from colourless to pink colour (inset, Fig. S10, ESI†). The
rate constant for the formation of HgNPs was found to be 3.84×10ꢀ3
Secꢀ1 (Fig S11, ESI†). In the fluorescence spectra, upon addition of
decrease in fluorescence emission intensity of both the emission band
(Fig. S2, ESI†). This phenomenon is often observed in derivatives with
AIEE properties as after the aggregation only the molecules on the
surface of aggregates emit light and contribute to the fluorescent
intensity upon excitation and this leads to a decrease in emission
intensity.12 The dynamic light scattering (DLS) studies clearly show
average diameter of the particles around 550 nm, 960 nm and 1400 nm
in 10%, 30%, and 50% H2O/THF solvent mixture respectively (Fig. S3,
ESI†). The DLS studies suggest that the aggregation driven growth is
one of the reasons for the emission enhancement.13 The polarized
Hg2+ ions (0ꢀ18 equiv.) to the solution of derivative
3 (H2O/THF, 1:1),
fluorescence enhancement along with broadening of the spectra is
observed (Fig. 1A). To rule out the possibility of hydrolysis of
optical microscopic (POM) image of the derivative
3 shows
birefringence at room temperature, thus, indicating presence of ordered
structure (Fig. S4(A), ESI†). The scanning electron microscopy (SEM)
derivative
studies of compound 1b (one of the products in case of hydrolysis of
derivative ) in mixed aqueous media and it was found to be nonꢀ
fluorescent (Fig. S12, ESI†) while derivative is highly fluorescent in
presence of Hg2+ ions due to its AIEE characteristics (Fig. S12, ESI†).
These studies clearly indicate that derivative containing imine
3
in the presence of Hg2+ ions, we carried out fluorescence
image of derivative
3 in H2O/THF (1:1, v/v) mixture shows the
3
presence of interconnected rods. (Fig. S4(B), ESI†). To get insight into
the AIEE phenomena, effect of variations in the viscosity and
3
temperature on fluorescence behaviour of derivatives
3
was also
was
3
investigated. The increase in fluorescence intensity of derivative
3
linkages do not undergo hydrolyses in the presence of Hg2+ ions.
2 | J. Name., 2012, 00, 1-3
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(A)
aggregates of derivative
3
for the formation of HgNPs. In the next part
(B)
Hg2+ (Equiv.)
18
of our work, we were interested in investigating the catalytic efficiency
of in situ generated HgNPs in Beckmann rearrangement for formation
of amides. Amide formation reactions are one of the most important
reactions in the organic chemistry due to their utility in pharmaceuticals
0
(C)
(C)
and natural products.17 Beckmann rearrangement constitutes
a
fundamental tool in organic synthesis for preparation of secondary
amides.18 This rearrangement, under classical conditions, requires harsh
conditions such as large amount of a strong acid, high temperature,
longer reaction time and is not suitable for preparation of primary
amides.19 Beckmann rearrangement of aldoximes under classical
reaction conditions lead to the formation of nitriles. Although some
examples of transformation of aldoximes to primary amides using
stoichiometric amounts of costly and very reactive reagents have been
reported yet till date, development of new approaches to carry out this
transformation remains a challenge. To our pleasure, in presence of in
situ generated HgNPs, we were successful in carrying out Beckmann
rearrangement reactions of aldoximes as well as ketoximes to
respective amides smoothly. Transformation of various aldoximes (4a-
f) to corresponding primary amides (5a-f) was carried out in the
presence of in situ generated HgNPs (50ꢀ100 ꢁl, H2O/THF, 1:1, v/v) in
toluene (Table 1) (Scheme 2). On the other hand, the Beckmann
rearrangement reactions of various ketoximes (6a-g) to synthesize
corresponding secondary amides (7a-g) (Table S7, ESI†) were carried
out in the presence of in situ generated HgNPs (50 ꢁl, H2O/THF, 1:1,
v/v) in acetonitrile at room temperature (Scheme 2, ESI†). The scope of
rearrangement reactions of aldoximes/ketoximes was further explored
by using different substrates bearing electronꢀdonating or electron
withdrawing groups at the paraꢀposition of the phenyl ring. The
structures of all the products were characterised by their m.p., 1H NMR
and 13C NMR spectra (Fig. S34ꢀS93 in ESI†). In case of aldoximes
having electron donating groups at the paraꢀposition of the phenyl
groups (4c-d) reactions were complete in shorter time and at room
temperature whereas in other cases higher reaction time and high
temperature was required. Similar trends were observed in
248.7
270.6
226.8
Fig. 1 (A) Fluorescence spectra of compound
3
(5 µM) showing the response to
ered with HEPES;
pH = 7.05, λex = 485 nm. (B) TEM image of HgNPs. Scale bar 10 nm and (C)
the Hg2+ ion (0ꢀ18 equiv.) in H2O/THF (1:1, v/v) mixture bu
ff
showing the interplanar spacing.
Further, the excitation spectrum of the emission at 570 nm showed peak
at 309 nm which suggests that HgNPs are also contributing to the
emission enhancement at 570 nm (Fig. S13, ESI†).3 The changes in
absorption and emission spectra suggest interaction of mercury ions
with aggregates of derivative 3, and their subsequent reduction to zeroꢀ
valent mercury. The calculated detection limit was found to be 7.15 nM
for Hg2+ ions (Fig. S14, ESI†). Under the same conditions as used for
Hg2+ ions, we also tested the binding behaviour of aggregates of
derivative
3
toward other metal ions such as Cd2+, Ba2+, Fe3+, Ni2+,
Zn2+, Cu2+, Pb2+, Ca2+, Co2+, Ag+, Na+, K+, Li+, Pd2+, Cr3+, and Al3+ ions
but no significant change in fluorescence intensity was observed (Fig.
S15AꢀB, ESI†).
We also carried out time resolved fluorescence studies of derivative
in the presence of mercury ions. The timeꢀresolved fluorescence studies
of derivative in H2O/THF (1:1, v/v) solvent mixture in the presence of
3
3
mercury ions show longer life times when monitored at 570 nm (Fig.
S16, ESI†). The longer life time suggests the presence of aggregated
species in the presence of mercury ions (Table S5, ESI†). The TEM
images of aggregates of derivative
3 in the presence of mercury ions
showed the presence of spherical shaped nanoparticles (Fig. 1B). It
revealed the interplanar spacing3 of 2.706 Å (Fig. 1C). The DLS studies
showed the formation of HgNPs having average diameter in the range
of 5ꢀ20 nm (Fig. S17, ESI†). The POM image of the derivative
3
containing HgNPs shows birefringence at room temperature (Fig.
S18A, ESI†). This study suggests the presence of aggregates having
ordered morphology. Further, confocal microscopy image of compound
Toluene,
R. T. to 85 °C
Scheme 2. Catalytic activity of HgNPs in synthesis of primary amides from
3
in the H2O/THF (1:1, v/v) solvent mixture in the presence of mercury
corresponding aldoximes.
ions (Fig. S18B, ESI†) clearly indicates the presence of luminescent
particles.
Table 1. HgNPs catalysed Beckmann rearrangement of various aldoximes to
corresponding primary amides
To get insight into the mechanism of formation of HgNPs, we slowly
evaporated the solution of aggregates of derivative 3 containing HgNPs.
After two days, precipitates were observed which were filtered and
washed with THF. The 1H NMR of the residue obtained after
evaporation of THF solution showed the upfield shifting of signals
corresponding to aromatic protons (Fig. S19 and Table S6, ESI†). We
propose that upon addition of mercury ions to the solution of aggregates
of derivative
channels, interact with imine and hydroxyl moieties and get reduced.
Thus, the aggregates of derivative function as a reducing agent as
well as a stabilizing agent for the preparation of HgNPs. We also
carried out UVꢀvis studies of derivative towards mercury ions in THF.
3, the mercury ions enter the network of interconnected
3
3
However, the absorption spectrum does not indicate formation of
nanoparticles (Fig. S20, ESI†). These studies show the importance of
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VJieowuArrtnicalelONnlainme e
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transformation of ketoximes to secondary amides. Actually, in the
presence of electronsꢀdonating groups, phenyl ring becomes more
electron rich which makes the migration faster.21 Additionally, it was
found that in situ generated HgNPs could be reused upto five times in
case of 6b without significant loss of their catalytic activity (Fig S21,
ESI†). We also monitored the rearrangement reactions of compounds
6b and 6c using in situ generated HgNPs by UVꢀvisible spectra (Fig
S22, 24 ESI†) and the rate constants for the formation of products 7b
and 7c were found to be 4.96×10ꢀ3 and 2.19×10ꢀ3 secꢀ1 respectively (Fig
S23, 25 ESI†). A plausible mechanism for the transformation of
aldoximes and ketoximes to respective amides is given in schemes 3ꢀ4,
ESI†.18,22 The efficiency of the in situ generated HgNPs has been tested
for the rearrangement reaction of 4ꢀmethoxyacetaphenoxime (6b) with
various amounts of catalyst, down to 10 ppm of mercury (Table S8,
ESI†). When the quantities of HgNPs is 0.5 mole %, the yield is 98% in
20 min against 0% when the reaction is conducted without HgNPs. The
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8
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also carried out the rearrangement reactions of 1,10ꢀphenanthrolineꢀ3,8ꢀ
dicarbaldehyde oxime and 2,3,4,5ꢀtetraphenylcyclopentaꢀ2,4ꢀdienꢀ1ꢀone
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naphthyl moieties which successfully overcomes the notorious effect of 17 Polyesters and Polyamides, ed. B. L. Deopura, B. Gupta, M. Joshi
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aldoximes/ketoximes to primary/secondary amides
.
9
We are thankful to DST, CSIR and UGC for financial support.
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details, NMR and Mass Spectra,]. See DOI: 10.1039/c000000x/
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