A multifunctional use of bis(methylene)bis(5-bromo-2-hydroxyl salicyloylhydrazone): From metal sensing to ambient catalysis of A3 coupling reactions

Article abstract of DOI:10.1039/d0ra07687b

The potential use of bis(methylene)bis(5-bromo-2-hydroxylsalicyloylhydrazone) as a multifunctional fluorescence sensor for Cu2+, Ni2+, Co2+ and Fe2+ ions was investigated. The optical behaviour shows an increase in an absorption band at 408 nm which can b

Full text of DOI:10.1039/d0ra07687b

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A multifunctional use of bis(methylene)bis(5-
bromo-2-hydroxyl salicyloylhydrazone): from
metal sensing to ambient catalysis of A3 coupling
reactions†
Cite this: RSC Adv., 2020, 10, 40739
a
*
Krisana Peewasan,
and Annie K. Powell
a Marcel P. Merkel, b Olaf Fuhr, bc Christopher E. Anson
*
ab
The potential use of bis(methylene)bis(5-bromo-2-hydroxylsalicyloylhydrazone) as a multifunctional
fluorescence sensor for Cu2+, Ni2+, Co2+ and Fe2+ ions was investigated. The optical behaviour shows an
increase in an absorption band at 408 nm which can be ascribed to the d–d transition (UV-vis) of the
metal ions and a concomitant decrease in fluorescence intensity at 507 nm. The crystallographic analysis
shows the binding site of the sensor to two Cu2+ ions and confirms the stoichiometry of 1 : 2 (ligand to
metal) which is in good agreement with a Job plot analysis. Furthermore the Cu2+-complex catalyses A3
coupling reactions at 1 mol% catalytic loading; chiral propargylamine derivatives were obtained in high
yield after 24 h reaction time under ambient conditions.
Received 8th September 2020
Accepted 20th October 2020
DOI: 10.1039/d0ra07687b
rsc.li/rsc-advances
emission or mass spectroscopy (ICP-AES, ICP-MS).10 However,
the biggest drawbacks of these techniques are expense,
Introduction
complicated sample pre-treatment procedures and high sample
volumes. Thus, alternative methods which provide high selec-
tivity, simplicity and low cost are required such as colorimetric,
uorescence and electrochemical analysis. Through targeted
ligand design metallosensors with high selectivity and sensi-
tivity can be developed. Another important requirement is to
make the system low-cost and atom efficient. To this end,
a number of sensors utilising uorescent dyes both in aqueous
and organic phases has been reported.11–19 Although high
selectivity and sensitivity were achieved, such uorescent
probes require meeting the challenge of multi-step synthesis
and control of pH.
Copper is not only used in biological processes, but is also
well known as catalyst for A3 coupling,20–24 azide–alkyne cyclo-
addition (click-reaction)25,26 and aldol-type reactions.27 Among
these, the A3 coupling reaction is of particular interest, because
chiral propargylamines28 obtained from this reaction are crucial
building blocks in organic chemistry for diastereoselective and
enantioselective components which exhibit biologically activi-
ties and are common in natural products.29
The design and development of chemosensors which provide
high selectivity and sensitivity towards metal ions have attrac-
ted much attention from chemists, biologists and environ-
mental scientists.1,2 Metal ions can play both benecial and
harmful roles depending on their speciation and concentration
in natural systems. Among the 3d metal ions, the detection of
these trace metals in biological systems has been intensively
investigated. It is of particular interest to detect copper (Cu2+
)
because Cu2+ ions are the third most abundant in the human
body aer Fe3+ and Zn2+. The Cu2+ ions play crucial roles in
several biological processes3 such as haemopoiesis as well as
various enzyme-catalysed and redox reactions.4 Although an
average concentration of Cu2+ in the body of 0.1 mM is required
for biological processes high levels lead to various disorders
such as Menkes and Wilson's diseases.5–8 Several techniques are
widely used to detect copper ions, including atomic absorption
spectroscopy (AAS)9 and inductively coupled plasma atomic
aInstitute of Inorganic Chemistry, Karlsruhe Institute of Technology, Engesserstrasse
15, 76131 Karlsruhe, Germany. E-mail: krisana.peewasan@kit.edu; annie.powell@
kit.edu
bInstitute of Nanotechnology, Karlsruhe Institute of Technology Campus North,
Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Karlsruhe,
Germany
We report here the synthesis of the bis(methylene)bis(5-
bromo-2-hydroxyl salicyloylhydrazone) (¼H6L) as a new uo-
rescent metal chemosensor and the application of its copper
complexation for catalysis. H6L was designed as a potentially
decadentate ligand with coordination of ten oxygen or nitrogen
donor atoms. The decadentate coordination sites are provided
by oxygen and nitrogen atoms arranged suitably for complexa-
tion to transition metal ions. Furthermore, the
cKarlsruhe Nano Micro Facility Karlsruhe Institute of Technolog, Hermann-von-
Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
† Electronic supplementary information (ESI) available: Experimental section,
X-ray structure determinations, NMR spectra, IR spectra, UV-vis spectra and
CIFs of complexes. CCDC 2001537. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/d0ra07687b
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salicyloylhydrazone groups introduced in this ligand system can paraformaldehyde (1.2 g) was reuxed in ethanol for 16 h. The
provide uorescence properties making it possible track optical corresponding intermediate (3) was obtained in 68% yield aer
behaviour for various coordination compounds in comparison purication using column chromatography. It was then reacted
with those for the free ligand. The selectivity and sensitivity with 2-hydroxybenzhydrazide (4, 3.48 g, 13.5 mmol) under
towards the metal dications Cu2+, Pb2+, Cd2+, Zn2+, Ni2+, Co2+, reux in ethanol (100 mL) for 16 h. The product bis(methylene)
Fe2+ and Sn2+ in DMSO solution were investigated. Crystallo- bis(5-bromo-2-hydroxyl salicyloylhydrazone) (H6L) was obtained
graphic analysis of the Cu2+-complex reveals the nature of the as a yellow solid (92% yield) aer removal of ethanol and
complexation of the H6L sensor showing a ligand : metal stoi- washing with Et2O.
chiometry of 1 : 2. The [Cu2(H5L)(NO3)2]NO3$0.5H2O$2CH3CN-
complex was also used in a catalytic study for the A3 coupling
Preparation of the copper complex [Cu2(H5L)(NO3)2]
reaction with 1 mol% loading under ambient conditions.
NO3$0.5H2O$2CH3CN
H6L (78 mg, 0.1 mmol) was dissolved in 5 mL of a mixture of
acetonitrile and methanol (1 : 1, % v/v) at room temperature.
Experimental
Materials and method
Then Cu(NO3)2$2.5H2O (56 mg, 2.0 mmol) was added to the
solution and the reaction mixture was stirred at room temper-
ature for 30 min. Aer standing for 7 days, crystals were
collected by ltration (70% yield based on the ligand H6L). The
powder product was characterised using IR, and PXRD (see
ESI†).
All reagents and solvents were used as received from commer-
cial suppliers without further purication. Infrared spectra were
recorded on a PerkinElmer Spectrum GX FT-IR spectrometer in
ATR mode in the range 4000–400 cmꢀ1. The following abbre-
viations were used to describe the peak characteristics: br ¼
broad, sh ¼ shoulder, s ¼ strong, m ¼ medium and w ¼ weak.
NMR spectra of the compounds were measured using a Bruker
Ultrashield plus 500 (500 MHz) and Varian 500 MHz spec-
trometer. 1H- and 13C-measurements were recorded using
deuterated solvents and referenced to tetramethylsilane (TMS)
as an internal standard (d ¼ 0 ppm). The single crystal X-ray
diffraction (SCXRD) data were collected on a STADIVARI
Results and discussion
Design and synthesis of H6L and its optical properties
Ligand H6L was designed to have ten possible coordination
sites from both O and N atoms as well as the potential to exhibit
strong uorescence. The preparation of H6L is versatile and
straightforward, providing the nal product in excellent (92%)
yield via a two step reaction (Scheme 1).
The optical properties of H6L were investigated. A 25 mM
stock solution of H6L (100 mM) in DMSO was prepared for use as
a working solution for a Job plot analysis and uorescence
experiments. The absorption maxima of the bands were found
˚
diffractometer using Ga-Ka, l ¼ 1.34143 A radiation with area
detection using a Dectris Eiger2 R 4M detector. Powder X-ray
diffraction (PXRD) measurements were performed on a STOE
STADI-P diffractometer with Cu-Ka radiation. The optical
properties were investigated using Fluoromax-4 spectrouo-
rometer and Shimadzu UV-24500 in the range 200–800 nm at
room temperature using 1 cm path length cells. The stock
solution of H6L (100 mM) and the corresponding metal ions
solutions (1000 mM) were prepared in DMSO.
Spectral analysis
The UV-vis absorption and uorescence experiments were per-
formed in DMSO. The spectral titration experiments were
accomplished through a stepwise addition of Cu2+ solution (100
mM) to a solution of H6L (25 mM) in DMSO. The absorption
intensity was recorded in the range 200–800 nm reference to
DMSO. The uorescence intensity was recorded at lex/lem ¼ 310
nm/507 nm aer shaking the solution and allowing to stand for
5 min to ensure proper mixing. Both the excitation and emis-
sion slits were set to 10.0 nm. The stoichiometry of the complex
formed between H6L and Cu2+ was determined by preparing
a series of solutions of H6L and Cu2+ at ratios of 1 : 9, 2 : 8, 3 : 7,
4 : 6, 5 : 5, 6 : 4, 7 : 3, 8 : 2, and 9 : 1 and recording the absor-
bance. The plot of [HG] versus [H]/([H] + [G]) was drawn to
determine the stoichiometry using a Job plot analysis.
Preparation of the ligand (H6L)
A mixture of 5-bromosalicylaldehyde (1, 4.20 g, 20 mmol), N,N00-
dimethylethylenediamine (2, 0.88 g, 10 mmol) and Scheme 1 Synthetic route to the ligand H6L.
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Fig.
1 (a) Absorption and (b) fluorescence spectra of H6L. The
measurement was carried out at room temperature, 25 mM in DMSO
and lex ¼ 310 nm, slit; 10 nm/10 nm were used.
at 296 nm, 310 nm, 345 nm and 450 nm, the latter of which can
be attributed to the n / p* transition of the C]N moiety. The
absorption maximum at 310 nm was selected as excitation
wavelength for the uorescence experiment and both the exci-
tation and emission slits were set to 10 nm. Strong uorescence
intensity of the free ligand H6L at 507 nm was observed (Fig. 1).
Use of H6L for Cu2+ sensing
The potential use of H6L to detect Cu2+ ion was investigated by
titration of a 25 mM solution of H6L with a 100 mM solution of
Cu2+ in DMSO. A decrease in intensity of the absorption at
346 nm and an increase in intensity of a new band at 408 nm
were observed on increasing the number of equivalents of Cu2+
.
Fig. 2 (a) UV-vis titration spectra, (b) fluorescence titration spectra (lex
¼ 310 nm, slit; 10 nm/10 nm) of H6L (25 mM) with increasing amount of
Cu2+ in DMSO and (c) Job plot showing a 1 : 2 ratio of H6L to Cu2+
ions.
The band at 408 nm is attributed to a d–d transition. Corre-
sponding uorescence titration experiments of the sensor H6L
with increasing equivalents of Cu2+ show a concomitant
decrease in uorescence intensity at 507 nm of H6L. The uo-
rescence intensity of the free sensor H6L was completely
quenched when 2.0 equivalents of Cu2+ were added. A Job plot
analysis established a 1 : 2 stoichiometry of H6L to Cu2+ (Fig. 2)
with the lower limit of detection (LOD) found to be 0.036 mM,
calculated using the UV-vis absorption data.
but these were much weaker than the absorption at 408 nm
observed for Cu2+ ions. The intensities of these absorptions
show the trend Cu2+ > Ni2+ > Co2+ > Zn2+ > Sn2+ > Fe2+ > Pb2+
Cd2+.
>
The uorescence behaviour of the H6L + M2+ systems showed
similar trends to those of the absorption spectra. The uores-
cence intensity decreases according to Cu2+ > Ni2+ > Co2+ > Fe2+
\ Sn2+ [ Zn2+ [ Pb2+ [ Cd2+ (Fig. 3b) with a dramatic
Chemoselectivity of H6L towards M2+
The response of H6L towards other M2+ ions was investigated.
Absorptions were observed in the 370–450 nm range (Fig. 3a),
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Fig.
4
The structure of [Cu2(H5L)(NO3)2]NO3$0.5H2O$2CH3CN.
Colour codes: green (Cu), blue (N), red (O), black (C) and brown (Br).
The hydrogen bonds are shown as green dashed line. Solvent mole-
cules (MeCN) in the lattice and H atoms which are not involved in
hydrogen bonds are omitted for clarity.
Table 1 Optimisation reaction of A3 coupling reaction
Catalytic
loading
Conversion yield
(%)
Entry
1
2
3
4
5
5 mol%
1 mol%
0.5 mol%
0.1 mol%
1 mol%
95a
92a
80a
78a
95b
Fig. 3 (a) UV-vis spectra of H6L + M2+, (b) fluorescence spectra of H6L
+ M2+ (lex ¼ 310 nm, slit; 10 nm/10 nm) of H6L (25 mM) and M2+ (25 mM)
in DMSO.
Conversion yields were calculated by the integration of 1H NMR at 9.60
ppm and 3.50 ppm aer 16 h reaction time. b The comparison of entry 2
and entry 5 at 1 mol% loading performed over 16 h under the same
reaction conditions performed 24 h, respectively, is shown in bold.
a
break between Fe2+ and Sn2+. Thus Cu2+, Ni2+, Co2+ and Fe2+ are
the best candidates for chemosensing with H6L.
Structural details for [Cu2(H5L)(NO3)2]NO3$0.5H2O$2CH3CN
The positive charges of the two Cu2+ ions are balanced by the
deprotonated ligand at N(6) and three nitrate anions and the
SHAPE30 analysis shows that the best coordination geometry of
Cu(1) is spherical square pyramid with a deviation value 2.463
and the Cu(2) is appear square planar with a deviation value
Fig. 4 shows the structural features of [Cu2(H5L)(NO3)2]NO3-
$0.5H2O$2CH3CN. The coordination sphere of Cu(1) consists of
two-coordinating O atoms from the ligand (O(2) and O(3)), one
nitrogen atom of the ligand (N(2)), one bridging O atom from
one nitrate (O(7)) and one coordinated O atom from another
nitrate (O(10)). The coordination environment of Cu(2) consists
of two O atoms from the ligand (O(4) and O(5)), one nitrogen
atom from the ligand (N(5)) and one bridging O atom from
nitrate (O(7)), thus Cu(1) and Cu(2) are bridged via O(7),
whereas O(10A) only coordinates to Cu(1). Furthermore, N(4) of
the ligand is protonated and a N(4)–H/O(13) intermolecular
hydrogen bond to a nitrate anion is build. O(5) and O(2) form
either intermolecular hydrogen bonds to a water molecule. The
protonated N(3) atom of the ligand form intramolecular
hydrogen bonds to O(3) and O(4). Likewise, N(1) and N(6) build
hydrogen bonds to O(1) and O(6), respectively. The Cu–O
˚
0.246 but has O(10B) making long contact with 2.582 A. The
square-pyramidal coordination sphere of Cu(1) consists of
one N and two O atoms from the ligand (N(2), O(2) and O(3))
and one unidentate bridging O atom O(7) from one nitrate, with
a further O atom O(10) from the other nitrate in the apical
position. The coordination environment of Cu(2) is similar to
˚
Cu(1) although Cu(2)-O(10A) is rather longer at 2.595(4) A, and
the molecule has approximate mirror symmetry. One of the
hydrazine groups (N(1)) is still protonated, while N(6) is
deprotonated and together with the three nitrates balances the
charge. N(4) of the ligand is protonated with N(4)–H/O(13)
forming an intermolecular hydrogen bond to a nitrate anion.
˚
distance ranges from 1.889(2) to 2.595(4) A, whereas the Cu–N
˚
The Cu–O distance ranges from 1.889(2) to 2.595(4) A, whereas
˚
distance is between 1.910(3) and 1.931(3) A. The Cu–Cu distance
˚
the Cu–N distance is between 1.910(3) and 1.931(3) A. O(7)
˚
is 3.5612(7) A and the angle between Cu–O(7)–Cu is 126.62(15).
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Table
2 Extended scope of A3 coupling reaction catalysed by
[Cu2(H5L)(NO3)2]NO3$0.5H2O$2CH3CN
Fig. 5 Possible mechanism for the A3 coupling reaction catalysed by
[Cu2(H5L)(NO3)2]NO3$0.5H2O$2CH3CN.
Catalytic study
The investigation of the potential of [Cu2(H5L)(NO3)2]NO3-
$0.5H2O$2CH3CN as a catalyst for the A3 coupling reaction was
explored. Cyclopentane carboxaldehyde (5), piperidine (6) and
phenylacetylene (7) were chosen as reagents for a model reac-
tion. The catalytic loading was rst investigated with the reac-
tion performed under aerobic conditions at room temperature
for 16 h (Table 1). The efficiency of this catalytic system can be
followed by monitoring the disappearance of the aldehyde
proton signal at 9.60 ppm and the appearance of a new peak of
the quaternary proton of the propargylamine product at
3.50 ppm.
The results show that with 5 mol% catalytic loading the
maximum conversion yield was obtained (95%, entry 1).
However, reducing the catalytic loading to 1 mol% only affects
the conversion yield by 3%. Increasing the reaction time to 24 h
raises the conversion yield to 95% (entry 5). Thus, the catalytic
loading of 1 mol% and reaction time of 24 h were chosen as test
conditions for this system. The scope of the catalytic system was
investigated using various aldehydes, alkynes and amines
(Table 2). The reactions were carried out at room temperature
a
Reaction conditions: 1 mol% catalytic loading, 2.0 mmol aldehyde, 2.2
˚
mmol amine and 2.4 mmol alkyne, 4 A molecular sieves, solvent i-PrOH,
room temperature for 24 h. b No products obtained.
˚
for 24 h in i-PrOH and 4 A molecular sieves were used to trap
forms a unidentate eq,eq-bridge with Cu(1)–O(7)–Cu(2) ¼
water. Aer 24 h, the reaction mixture was ltered through
celite and washed with dichloromethane. The crude products
were puried using column chromatography.
The high efficiency catalysis is clearly seen for the various
combinations of aldehyde and secondary amine, with products
attained in high yield with low catalytic loading, short reaction
time and mild reaction conditions.31–34 However, no products
were obtained when the aromatic and/or aliphatic primary
amines were used. In addition, the stability of the catalyst used
in this reaction was investigated using ESI-TOF mass
ꢁ
˚
126.62(15) and Cu(1)/Cu(2) ¼ 3.5612(7) A. The other nitrate
ligand shows minor disorder between unidentate bridging
(78%) and syn,syn-bridging (22%) modes. SHAPE30 analysis
shows that the best coordination geometry of Cu(1) is spherical
square pyramid with a deviation value 2.463, while Cu(2) is
square planar with a deviation value 0.246, although with a long
˚
contact of 2.582 A to O(10B). A summary of the crystallographic
renement data (Table S1†) and SHAPE analysis (Table S2†) are
given in the ESI.†
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spectrometry. The [Cu2(H5L)(NO3)2]NO3$0.5H2O$2CH3CN was
dissolved in i-PrOH and the change in mass was monitored aer
24 h, 48 h and 72 h (Fig. S27†). Regarding the results from mass
spectroscopy, in solution, one coordinated nitrate was found to
be removed from Cu(1) of the coordination cluster (mass cal. ¼
969.9178 m/z, found ¼ 969.9068 m/z, Fig. S27†) resulting in
a free reactive site on copper. This allows a possible mechanism
for the A3 coupling reaction of this system to be proposed
(Fig. 5). Furthermore, the compound was tested as a catalyst for
click chemistry but without any success.
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Bis(methylene)bis(5-bromo-2-hydroxyl salicyloylhydrazone) can
be used as a multiple functionality ligand. Coordination
complexes formed with this ligand can be used as a highly
selective and sensitive uorescent sensor for Cu2+, Ni2+, Co2+
and Fe2+ ions. The molecular structure of the copper(II) complex
conrms the binding mode of the ligand with the metal to
ligand stoichiometry of 2 : 1 in line with results from a Job plot
analysis. The presence of a d–d band at 408 nm proves that the
copper is in the 2+ state. In addition, the application of the
[Cu2(H5L)(NO3)2]NO3$0.5H2O$2CH3CN for catalysing A3
coupling reactions was found to have high efficiency in
providing a wide range of products in high yields (>80%) with
catalytic loading of 1 mol% and 24 h reaction time. Moreover,
the stability of the catalytic system has been investigated and
the results show the catalyst is stable over 72 h in i-PrOH
solution. Furthermore, we extended the scope of this catalytic
system for copper catalysed azide–alkyne cycloaddition (CuAAC)
using benzyl azide and phenyl acetylene as a model reagent.
However, the expected triazole product was not obtained. This
can be explained by the fact that the Cu2+ is not usually suitable
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Conflicts of interest
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Ed., 2001, 40, 2004–2021.
The authors declare no conict of interest.
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Acknowledgements
We thank the DFG for funding through SFB/TRR 88 “3MET” and
the Helmholtz Association via POF STN. We would like to thank
Prof. Dr Dieter Fenske for collecting the crystallographic data.
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30 M. Llunell, D. Casanova, J. Cirera and S. Alvarez, SHAPE
version 2.1, Universitat de Barcelona, Barcelona, 2013.
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