Selective recognition of Fe3+ and CrO42- ions using a Zn(ii) metallacycle and a Cd(ii) coordination polymer and their heterogeneous catalytic application

Article abstract of DOI:10.1039/c9ce01357a

We report the syntheses and structural characterization of a Zn(ii) metallacycle [Zn₂{(L)₂(DMF)₂(NO₃)₂}]·(NO₃)₂ (1) along with a Cd(ii) coordination polymer [{Cd₁(L)

Full text of DOI:10.1039/c9ce01357a

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PAPER
2−
Selective recognition of Fe3+ and CrO4 ions
using a ZnIJII) metallacycle and a CdIJII)
coordination polymer and their heterogeneous
catalytic application†
Cite this: CrystEngComm, 2019, 21,
7447
Pooja Rani,‡a Anjali Sharma,‡a Ahmad Husain, b Gulshan Kumar,a Harpreet Kaur,b
a
K. K. Bhasina and Girijesh Kumar
*
We report the syntheses and structural characterization of a ZnIJII) metallacycle [Zn2{(L)2IJDMF)2IJNO3)2}]
·(NO3)2 (1) along with a CdIJII) coordination polymer [{Cd1IJL)2IJDMF)2}·(NO3)2]n (2) (where L = N,N′-bis-(3-
pyridyl)terephthalamide; DMF = dimethylformamide) and their utilization as fluorescent probes for highly
selective recognition (turn-off) of Fe3+ and CrO4 ions. Compounds 1 and 2 were prepared by utilizing
2−
ligand L and the nitrate salt of zinc and cadmium, respectively. Single crystal X-ray analysis reveals that 1
adopts an overall ZnIJII) metallacycle architecture, whereas 2 displays a one-dimensional (1D) polymeric
structure. Both 1 and 2 showed high fluorescence stability in aqueous solution and selectively detected
Fe3+ and CrO4 ions with high quenching constants and low detection limits of 2.34 × 106 M−1 and 0.153
2−
μM for Fe3+, 6.72 × 105 and 0.205 μM for CrO4 (for 1); 3.25 × 106 and 0.193 μM for Fe3+, 6.94 × 105 and
2−
Received 28th August 2019,
Accepted 28th October 2019
2−
0.155 μM for CrO4 (for 2). Notably, ligand L itself did not display any type of recognition for any ions in
DMF as well as in aqueous media. Moreover, 1 and 2 were also employed as heterogeneous catalysts for
the ring-opening reaction (ROR) of epoxides with various assorted amines and up to 95% yield (TON, 47.5
and TOF, 11.9 h−1) was observed with perfect regioselectivity in the case of styrene oxide.
DOI: 10.1039/c9ce01357a
rsc.li/crystengcomm
multifunctional materials for the selective recognition of
cations and anions of biological importance in addition to
Introduction
Porous coordination polymers (PCPs) including metal–organic
frameworks (MOFs) have drawn remarkable interest not only
because of their fascinating structural topologies but also due
to their potential applications in diverse fields such as
sorption, optics, magnetism, luminescence, drug delivery,
heterogeneous catalysis and molecular recognition in the past
decades.1–18 There are a number of reports available in the
literature wherein a particular PCP or MOF has been designed
and synthesized for a particular application; however, only a
few reports are available on the design and development of
coordination driven metallacycles that have been used as
their utilization as heterogeneous catalysts for various organic
transformation reactions.19–27 The synthesis of porous
metallacycle structures can be carefully designed to
accomplish specific structures with distinct channels,
topologies, and interesting applications such as molecular
recognition and catalysis.19,28 However, it is still a challenging
task and the foremost goal of contemporary chemists and
biologists to develop efficient synthetic strategies to construct
targeted structures with desired properties. Furthermore, the
internal cavities of these porous framework architectures are
very favorable for “host–guest” chemistry. It is well known
that host–guest interactions play a decisive role in many areas
of chemistry and biology.29,30 For example in our routine life,
various ions play a substantial role either as a prime
requirement for growth or as harmful environmental
pollutants. In principle, the FeIJIII) ion is the vital constituent
of hemoglobin and influences electron transfer and oxygen
metabolism in living organisms.31,32 On the other hand, the
hexa-valent chromium ion is widely used in industry as an
a Department of Chemistry & Centre for Advanced Studies in Chemistry, Panjab
University, Chandigarh-160014, India. E-mail: gkumar@pu.ac.in
b Department of Chemistry, DAV University, Jalandhar, Punjab-144012, India
† Electronic supplementary information (ESI) available: Experimental data for
ligand L, FT-IR and 1H NMR spectra, TGA and DSC plots, PXRD, HRMS,
additional data related to sensing and catalysis, and hydrogen bond tables for L,
metallacycle 1 and coordination polymer 2. CCDC 1921703 and 1921704. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c9ce01357a
important oxidant, and it is
a potentially heavy metal
‡ These authors contributed equally.
pollutant present in water.33,34 Besides this, it causes
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1
permanent damage to human health and other organisms,
even at a small concentration.35 Therefore, the selective
N–H (1593). UV-vis (DMSO), λmax/nm: 285 nm. H NMR (400
MHz, DMSO-d6): δppm 2.89 (s, 3H, Hg, DMF), 2.73 (s, 3H, Hh,
DMF), 7.46 (m, 4H, Hb, J = 8.47 & 4.80 Hz), 7.95 (s, Hi, DMF),
8.15 (d, 8H, Hf), 8.18–8.22 (d, 4H, Hc), 8.24–8.35 (d, 4H, Ha),
9.00 (s, 4H, Hd), 10.68 (s, 4H, He).
detection of Fe3+ and CrO4 ions is very important for the
2−
environment and human health.36–39
Based on the aforementioned advantages, herein, we
report the preparation of a ZnIJII) metallacycle and a CdIJII)
coordination polymer with formulas: [Zn2{(L)2IJDMF)2IJNO3)2}]
·(NO3)2 (1) and [{Cd1IJL)2IJDMF)2}·(NO3)2]n (2). Ligand L,40
1
Synthesis of [{Cd1IJL)2IJDMF)2}·(NO3)2]n (2)
and 2 were well characterized by various spectroscopic and
analytical techniques. Single crystal X-ray analysis reveals that
1 adopts an overall metallacycle architecture, while 2 shows a
1D polymeric structure. We have described 1 and 2 as
fluorescence-based receptors for selective sensing of Fe3+ and
Compound 2 was prepared using a similar scale and protocol
to that adapted for 1, by using CdIJNO3)2·6H2O (0.042 g, 0.18
mmol) in place of ZnIJNO3)2·6H2O. Colourless block-shaped
crystals suitable for single crystal X-ray analysis were
obtained after one week using a similar diffusion technique.
Yield: 0.041 g (52%; based on ligand L); FT-IR (ν cm−1): CO
(1653), N–H (1593). Anal. calcd. For C42H42CdN12O12: C,
49.49%; H, 4.15%; N, 16.49%. Found, C, 49.62%; H, 4.21%;
N, 16.58%. UV-vis (DMSO), λmax/nm: 287 nm. 1H NMR (400
MHz, DMSO-d6): dppm 2.89 (s, 3H, Hg, DMF), 2.73 (s, 3H,
Hh, DMF), 7.45 (dd, 4H, Hb, J = 8.77 & 4.74 Hz), 7.95 (s, Hi,
DMF), 8.15 (d, 8H, Hf), 8.22–8.24 (d, 4H, Hc, J = 7.58 Hz), 8.36
(br, 4H, Ha), 8.99 (d, 4H, Hd), 10.66 (s, 4H, He).
2−
CrO4 ions. Besides this, 1 and 2 were also employed as
heterogeneous catalysts for the ROR of epoxides with various
assorted amines under solvent-free conditions and at room
temperature and promising results were observed thus
confirming that they behave as multifunctional materials.
Experimental section
Materials and methods
The starting materials such as benzene-1,4-dicarboxylic acid
and 3-aminopyridine were purchased from AVRA and Sigma-
Aldrich, respectively, and used as received. The solvents were
of analytical grade and purified whenever necessary as per
the standard literature protocol.41 The IR spectra were
Single crystal X-ray diffraction analysis
Suitable single crystals of L, 1 and 2 were selected for data
collection. The collection was carried out on a Rigaku Super
Nova HyPix3000 diffractometer with monochromatic Mo Kα
radiation (λ = 0.71073 Å). Diffraction data were processed by
CrysalisPro v39.31,42 followed by an empirical absorption
correction with the included Scale3 ABSPACK program.42 The
structures were solved by intrinsic phasing with the SHELXS
structure solution program and refinement by the full-matrix
least-squares method, based on F2 values against all
reflections, was performed with SHELXL-2014/7, using Olex2
GUI.43,44 Details of the crystallographic data collection and
structural solution parameters for 1 and 2 are provided in
Table 1. Selected bond lengths and angles of 1 and 2 are
provided in Table S2 of the ESI.† Meanwhile, Tables S1 and
S3† contain crystallographic data and structural solution
parameters for L and H-bonding and short contact details for
1 and 2, respectively.
recorded with
a Perkin-Elmer FTIR 2000 spectrometer.
Thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) measurement were performed on an SDT
Q600 (V20.9 Build 20) instrument (Artisan Technology Group,
Champaign, IL) under a N2 atmosphere with a heating rate of
10 °C per min. Elemental analysis was carried out using a
Flash EA 1112 series elemental analyzer. The UV-visible
absorbance spectra were recorded using a Shimadzu UV-1800
spectrophotometer. GC and GC-MS studies were performed
using a PerkinElmer Clarus 580 with an Elite-5 column. The
powder X-ray diffraction (PXRD) data were recorded either on
a PANalytical X'PertPRO X-ray diffractometer or on a Bruker
AXS D8 Advance instrument with Cu Kα radiation (λ =
1.54059 Å). For the PXRD pattern, the samples were ground
and subjected to the theta range = 5–50° with a slow scan
rate at room temperature. High-resolution mass spectrometry
(HRMS) data were recorded using a Waters Synapt G2-Si Q
ToF mass spectrometer.
Fluorescence measurements
The fluorescence properties of coordination compounds 1
and 2 were recorded with the help of a Perkin Elmer LS-55
fluorescence spectrophotometer with a slit width of 5 nm.
The solutions of 1 and 2 were prepared by sonicating the
respective sample powder (25 mg in each case) into 100
Synthesis of [Zn2{(L)2IJDMF)2IJNO3)2}]·(NO3)2 (1)
Ligand L (0.025 g, 0.08 mmol) was dissolved in 3 mL of DMF
solution and then ZnIJNO3)2·6H2O (0.054 g, 0.18 mmol) was
added and stirred for 24 h; after that the resulting solution
was filtered off. The resulting solution afforded colourless
block-shaped crystals suitable for single crystal X-ray analysis
after diffusion with diethyl ether after a period of one week.
Yield: 0.042 g (44%; based on ligand L). Anal. calcd. For
C48H56N16O20Zn2: C, 44.08%; H, 4.32%; N, 17.14%. Found, C,
44.15%; H, 4.26%; N, 17.21%. FT-IR (ν cm−1): CO (1653),
mL of deionized water for
1 h at room temperature.
Sonication was done to get a homogeneous suspension.
After sonicating for 1 h, the corresponding fluorescence
spectra
were
recorded.
Steady
state
fluorescence
measurements were done using
a
Fluoromax-4 (Horiba
Jobin Yvon, NJ) and TCSPC SETUP-Fluorocube (Horiba
Jobin-Yvon, NJ).
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Table 1 Crystal data and structure refinement for coordination compounds 1 and 2
Identification code
1
2
CCDC no.
1921703
C48H56N16O20Zn2
1307.82
298(2)
Triclinic
¯
P1
1921704
C42H42CdN12O12
1019.27
298(2)
Monoclinic
P21/n
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
a/Å
b/Å
c/Å
α/°
9.3608(3)
9.9545(4)
17.4674(5)
93.775(3)
10.1917(3)
18.4359(5)
11.7353(3)
90
β/°
102.159(2)
112.230(3)
1453.70(9)
1
1.494
0.913
95.771(2)
90
2193.81IJ10)
2
1.543
0.576
γ/°
Volume/Å3
Z
ρcalc/g cm−3
μ/mm−1
FIJ000)
676.0
1044
Crystal size/mm3
Radiation (Mo Kα/λ)
2θ range for data collection/°
Index ranges
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
Final R indexes [I ≥ 2σ(I)]
Final R indexes [all data]a
Largest diff. peak/hole/e Å−3
0.2 × 0.15 × 0.12
0.71073
6.674 to 51.992
0.15 × 0.14 × 0.1
0.71073
6.712 to 54.832
−11 ≤ h ≤ 11, −11 ≤ k ≤ 12, −21 ≤ l ≤ 21
−13 ≤ h ≤ 12, −23 ≤ k ≤ 22, −14 ≤ l ≤ 15
18 354
20 048
5702 [Rint = 0.0513, Rsigma = 0.0496]
5702/0/392
1.075
R1 = 0.0433, wR2 = 0.1082
R1 = 0.0583, wR2 = 0.1143
0.42/−0.36
4669 [Rint = 0.0680, Rsigma = 0.0829]
4669/0/316
0.973
R1 = 0.0514, wR2 = 0.0960
R1 = 0.1037, wR2 = 0.1110
0.43/−0.41
P
P
P
P
a
R1 = ‖Fo| − |Fc‖/ |Fo|; wR2 = { [wIJ|Fo|2 − |Fc|2)2]/ [wFo4]}1/2
.
General procedure for the ring-opening reactions
monitored by thin-layer chromatography (TLC). After 4 h, the
catalyst was filtered off, and the filtrate was concentrated
under reduced pressure. The crude product was purified by
flash column chromatography on silica gel using a hexane/
ethyl acetate mixture (5 : 1) as the eluent. The products were
The RORs of the epoxides were carried out in oven-dried
glassware under solvent-free and ambient conditions. In a
representative aminolysis reaction, cyclohexene oxide (0.102
mmol) was treated with aniline (0.153 mmol) in the presence
of catalyst 1 or 2 (2 mol%, 0.0204 mmol). However, in the
case of styrene oxide, 0.083 mmol of epoxide was treated with
1
analyzed by GC/GC-MS and H NMR techniques.
aniline (0.120 mmol) in the presence of catalyst 1 or 2 Results and discussion
(0.0166 mmol). The progress of the catalytic reactions was
Synthesis and characterization
The synthesis of ligand L is provided in the ESI.† ZnIJII)
metallacycle 1 and coordination polymer 2 were prepared by
stirring L with the nitrate salt of respective metal ions in
DMF solvent (Scheme 1) in a 1 : 2 mole ratio. Different
combinations were set up in order to get the coordination
polymer or metallacycle with ZnIJII) and CdIJII); however, all
attempts were unsuccessful. Ligand L and coordination
compounds 1 and 2 were well characterized by various
analytical and spectroscopic techniques and details are
provided in the ESI† (Fig. S1–S13). The FT-IR spectra of 1 and
2 show a peak in between 1650 and 1680 cm−1, which
corresponds to the νCO of the amidic (–CONH–) group,
which is also observed in the same region for L and thus
indicates that the amidic group remains uncoordinated. The
UV-vis spectrum of ligand L displays a broad peak in the
range of λmax 280–300 nm, which is ascribed to the π–π* or
n–π* transition of the amidic CO group. Notably, we did
Scheme 1 Preparation of coordination compounds 1 and 2 using
ligand L.
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Fig. 2 (a) ORTEP drawing of 2 (30% probability level), #i x + 1, y, z − 1,
#ii −x + 1, −y + 1, −z + 1, #iii −x + 2, −y + 1, −z. Colour code: C, grey; N,
blue; O, red; Cd, yellow; hydrogen atoms are omitted for clarity of the
structure. (b) H-Bonding interaction of the nitrate anion with the amide
functionality. (c) 1D polymeric chain. (d) 3D H-bonded network
architecture.
Fig. 1 (a) ORTEP drawing of metallacycle 1 (30% probability level), #i 1
− x, 2 − y, 1 − z. Colour code: C, grey; N, blue; O, red; Cd, green;
hydrogen atoms are omitted for clarity of the structure. (b) H-Bonding
interaction of the nitrate anion with the amide functionality. (c) C–H⋯π
interaction based 3D network architecture.
one lattice nitrate ion residing inside the cavity of
metallacycle 1. As shown in Fig. 1a, the ZnIJII) ion displays
not observe any shift in the absorption behaviour of the
amidic CO group after the metalation, which suggests that
in 1 and 2 the metalation of zinc and cadmium ions took
place via the Npyridyl group and the amidic group did not
participate in coordination. Importantly, the 1H NMR
spectrum of 1 and 2 displays a singlet of the amidic-H at δ
10.68 and 10.66 ppm as also observed for L (δ 10.64 ppm,
Fig. S2†), which clearly indicates that the amidic-H atoms
remain intact (Fig. S3 and S4†). This observation is also
proved by single-crystal X-ray diffraction analysis (Fig. 1b and
2b). In addition, the presence of amidic-H atoms along with
the cavities (suggested by single crystal X-ray diffraction
analysis) in 1 and 2 opens the door for us to use these
materials to study “host–guest” chemistry.
a
distorted octahedral geometry, wherein the four
equatorial sites are occupied by two Npyridyl atoms coming
from two different ligands and two oxygen atoms
L
stemming from the chelated nitrate ion, while the apical
sites are occupied by O atoms stemming from coordinated
DMF molecules. The Zn–O bonds fall in the range 2.064IJ2)–
2.281IJ2) Å and Zn–N bonds fall in the range 2.076IJ2)–
2.079IJ2) Å (see Table S2, ESI†). The two Npyridyl atoms
occupy a syn–anti orientation to form a metallacycle with a
Zn⋯Zn separation of 15.361 Å (Fig. 1c) encapsulating the
lattice nitrate anions. The lattice nitrate anions are also
hydrogen bonded to the methyl-CH (2.593 Å) resulting in
the formation of
a
1D chain running along the
crystallographic a-axis, which is further supported by the
C–H⋯π interactions involving the central aryl ring and the
methyl group of DMF as shown in Fig. 1c. The pyridyl
rings also participated in C–H⋯π interactions involving
pyridyl C–H from another molecule and methyl C–H of
DMF (3.257–3.401 Å) extending the network into three-
dimensions. The crystal packing reveals that one of the
amide N–H is hydrogen bonded in a bifurcated mode to
the two O atoms of the nitrate anion, while the third O
atom of the nitrate anion also interacts in a bifurcated
mode with the other amide N–H and the aryl C–H of the
Crystal structure description of [Zn2{(L)2IJDMF)2IJNO3)2}]
·(NO3)2 (1)
The colourless crystals of metallacycle 1 were grown by
diffusion of diethyl ether in a DMF solution of 1. Single
crystal X-ray analysis of compound
1 reveals that it
¯
crystallizes in the triclinic space group P1. The asymmetric
unit of 1 consists of one molecule of L, a ZnIJII) ion, a
chelating nitrate ion, two coordinated DMF molecules, and
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Fig. 3 (a and c) Representation of the fluorescence quenching intensity of the suspension of 1 (2 mL) upon addition of 60 μL of 10 mM solution of
the concerned Mn+ cations (a) or An− anions (c) in H2O. The relative fluorescence intensity of 1 upon addition of 60 μL of 10 mM solution of Fe3+
(b) or CrO42− (d) in H2O. The optical images of the samples recovered after sensing are fixed below the respective cuvette.
Fig. 4 (a and c) Representation of the fluorescence quenching intensity of the suspension of 2 (2 mL) upon addition of 60 μL of 10 mM solution
of the concerned Mn+ cations (a) or An− anions (c) in H2O. The relative fluorescence intensity of 2 upon addition of 60 μL of 10 mM solution of
Fe3+ (b) or CrO42− (d) in H2O. The optical images of the samples recovered after sensing are fixed below the respective vial.
second ligand L of the metallacycle as shown in Fig. 1b.
The cross-section of the metallacycle is found to be ca. 6.73
× 15.36 Å2 (see Fig. 1b and c). The detailed crystallographic
information for metallacycle 1 is given in Tables 1 and S2.†
Crystal structure description of [{Cd1IJL)2IJDMF)2}·(NO3)2]n (2)
Single crystal X-ray analysis of compound 2 reveals that it
crystallizes in the monoclinic space group P21/n. The
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However, there is no complementary amide–amide
interaction present in 2 as the cavity is occupied by the
nitrate ions, which mediate the hydrogen bonding
interactions. Interestingly, these 1D chains are further
interconnected by C–H⋯O interaction (2.46(3) and 2.69(3) Å)
between the COamide and the methyl hydrogen of
coordinated DMF to form 2D sheets that are further
connected by the N–H⋯O interaction involving the amide
N–H and the methyl hydrogen of DMF, extending the
network into the third dimension. This 3D framework
architecture is further supported by moderate C–H⋯π
interaction between the central aryl and one of the terminal
pyridyl rings (2.935 and 3.457 Å). The crystal packing reveals
that the amide N–H is hydrogen bonded to the nitrate anion
in a bifurcated mode as shown in Fig. 2b and d. The cross-
section of the metallacyclic loop is found to be ca. 5.29 ×
16.29 Å2 which is large enough to accommodate the nitrate
Fig. 5 1H NMR spectra of metallacycle 1 before (red trace) and after
(purple trace) the addition of 1 equiv. of K2CrO4 in DMSO-d6.
anions as compared to ca. 3.45
×
16.59 Å2 of
[{CuIJL)2IJH2O)2}IJNO3)2]α (ref. 40) due to ligated DMF
molecules as one of the DMF molecules protrudes into the
cavity (see Fig. 2b and c). The detailed crystallographic
information for 2 is given in Tables 1 and S2 of the ESI.†
Thermal and PXRD analysis
TGA and DSC were employed for both 1 and 2 to probe their
thermal stability, as this is one of the prime requirements for
any organic–inorganic hybrid material, organic building
block and or coordination compound to become a functional
material. TGA of 1 shows a weight loss of 21.42% (observed),
which fitted nicely with ca. 20.66% in the temperature range
of 25–200 °C and the framework is stable up to 300 °C. This
weight loss corresponds to the liberation of two DMF
molecules and two nitrate ions (Fig. S8, ESI†). Compound 2
also exhibits a similar framework stability (up to 310 °C) with
an initial weight loss of 14.14% (calcd, 13.25%) for the
release of one DMF molecule and one nitrate ion in the
temperature range of 25–200 °C (Fig. S8, ESI†). DSC supports
the TGA finding as their particular plots show the respective
losses in the exothermic region (Fig. S9, ESI†). To prove the
purity and crystallinity of the bulk materials, both 1 and 2
were employed for PXRD analysis. Notably, the experimental
PXRD pattern of 1 and 2 matches quite well with that of the
simulated pattern obtained from Mercury 4.0, which shows
the rational crystalline phase purity of the bulk sample (Fig.
S10 and S11, ESI†).
Fig. 6 Recyclability tests of 1 (top) and 2 (bottom) for sensing of
CrO4 ions (representative one).
2−
asymmetric unit of 2 consists of a ligand L, half of a CdIJII)
ion (which sits on an inversion centre), a nitrate ion and a
coordinated DMF molecule. As shown in Fig. 2a, the CdIJII)
ion displays a distorted octahedral geometry, wherein the
four equatorial sites are occupied by two Npyridyl atoms
coming from two different L ligands and two oxygen atoms
stemming from the coordinated DMF molecules, while the
apical sites are occupied by Npyridyl atoms stemming from L
ligands. The Cd–N bonds fall in the range 2.336IJ3)–2.353IJ3) Å
and Cd–O is 2.335(2) Å (Table S2, ESI†). Since the two pyridyl
arms can rotate freely about the amidic core, both anti- and
syn-conformations are possible for L. In the crystal structure
of 2, the two pyridyl nitrogen atoms occupy a syn–anti
orientation to form a cyclic loop with a Cd⋯Cd separation of
16.298 Å, to produce a 1D chain (Fig. 2b) encapsulating the
lattice nitrate anions. A very similar CuIJII) coordination
polymer with the formula [{CuIJL)2IJH2O)2}IJNO3)2]α has been
reported by Dastidar et al.,40 wherein the coordinated solvent
was H2O. Notably, this CuIJII) coordination polymer exhibits
complementary amide–amide hydrogen bonding within the
metallacyclic loop.
2−
Selective recognition of Fe3+ and CrO4 ions
As revealed by the structural analysis, both 1 and 2 contain
several fascinating structural features such as (i) the presence
of uncoordinated amidic-functionality, (ii) labile coordinated
DMF molecules, (iii) channels with dimensions of 6.73 ×
15.36 Å2 and 5.29 × 16.29 Å2, (iv) the presence of nitrate ions
in cavities, which is well-known for H-bonding interaction,
and (v) thermal stability, which encouraged us to use these
compounds as fluorescence-based selective receptors for ions
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Table 2 Aminolysis reaction of cyclohexane oxide and styrene oxide in the presence of catalysts 1 and 2 under heterogeneous and solvent-free
conditions
Yielda [%]
Entry
1.
Oxide
C.O.
Amines
Products
1
TONb,c (TOF, h−1
41.5 (10.4)
)
2
TONb,c (TOF, h−1
42.5 (10.6)
)
83
85
2.
3.
C.O.
C.O.
90
91
45 (11.3)
92
89
46 (11.5)
45.5 (11.4)
44.5 (11.1)
4.
C.O.
62
31 (7.8)
65
32.5 (8.1)
5.
6.
7.
C.O.
C.O
68
71
69
34 (8.5)
59
67
70
29.5 (7.4)
33.5 (8.4)
35 (8.6)
35.5 (8.9)
34.5 (8.6)
C.O.
8.
9.
C.O.
C.O.
58
42
29 (7.3)
21 (5.3)
61
34
30.5 (7.6)
17 (4.3)
10.
11.
S.O.
S.O.
94, 92,d 91e
47 (11.8)
45 (11.3)
91, 88,b 87c
45.5 (11.4)
46.5 (11.6)
90
93
12.
13.
S.O.
S.O.
95
67
47.5 (11.9)
33.5 (8.4)
91
74
45.5 (11.4)
37 (9.3)
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Table 2 (continued)
Yielda [%]
Entry
14.
Oxide
S.O.
Amines
Products
1
TONb,c (TOF, h−1
37.5 (9.4)
)
2
TONb,c (TOF, h−1
32 (8)
)
75
64
15.
16.
S.O.
S.O.
76
64
38 (9.5)
32 (8)
66
58
33 (8.3)
29 (7.3)
17.
18.
S.O.
S.O.
66
45
33 (8.3)
62
36
31 (7.8)
18 (4.5)
22.5 (5.6)
a
Products were quantified by using gas chromatography. C.O. and S.O. stand for cyclohexane oxide and styrene oxide, respectively. Catalyst, 2
b
mol%. Reaction time, 4 h. Turnover number (TON) calculated as = number of moles of reactant consumed/number of moles of catalyst used.
c
Turnover frequency (TOF) calculated as = TON/time of reaction. d Product isolated after the 3rd cycle. e Product isolated after the 5th cycle.
of biological importance. Therefore, the sensing properties of
1 or 2 towards various metal ions such as Mg2+, Ca2+, Cr3+,
Mn2+, Fe3+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, Hg2+,
Sm3+, Gd3+, Tb3+, and Dy3+ (Mn+; n = 2 or 3) and anions such
(except for Fe3+ ions). However, under similar conditions the
addition of an equivalent quantity of Fe3+ ions results in a
great decrease in emission intensities of 1 and 2 (Fig. 3a and
4a). In addition, the titration of a 5 × 10−5 mM solution of
Fe3+ ions with 2 mL of the suspended solution of 1 showed a
regular decrease in fluorescence intensity from 450 nm as we
increased the concentration of Fe3+ ions, as shown in Fig. 3b.
A similar trend is observed in the titration of Fe3+ ions with 2
(Fig. 4b). The relevant Stern–Volmer graph of 1 and 2 shows
a good relationship with the Fe3+ concentration, suggesting a
strong turn-off quenching effect on fluorescence with Ksv
values of 2.34 × 106 M−1 (R2 = 0.9830) and 3.25 × 106 M−1 (R2
= 0.9754), respectively (see the inset of Fig. 3b and 4b).
Notably, Fe3+ afforded a noteworthy turn-off quenching effect
and the intensity was quenched by 94.7% and 96.5% for 1
and 2, respectively, with the concentration of Fe3+ increased
to 6 × 10−4 mM. Finally, it was found that both receptors 1
and 2 feature a high selectivity response towards Fe3+ ions in
aqueous solution.
as N3 , IO4 , I−, H2PO4 , CH3COO−, Cl−, Br−, BF4 , CrO4
,
−
−
−
−
2−
NO3 , NO2 , CO32−, SO32−, SO42−, S2O32−, S2O52−, S2O82−, AsO2−,
−
−
−
−
3−
Cr2O72−, MnO4 , C8H5O4 and PO4 (An−; n = 1–3) were
investigated by recording the emission spectra of the
suspensions of powder 1 and 2 (25 mg) in 100 mL of water at
room temperature and the results are shown in Fig. 3 and 4
(see Fig. S14† for the solvent scan). As shown in Fig. 3a and
4a, receptors 1 and 2 display an emission band at 450 nm
(λex = 300 nm, excitation and emission slits 5 nm) which is
ascribed to the emission of the ligand to metal (L → M)
charge transfer. For the fluorescence measurements, 2 mL of
the concerned host was used with an excitation wavelength
of 300 nm. The detection of the cations and anions was
carried out by the incremental addition of the concerned Mn+
and An− ion solution to the suspended solution of 1 and 2.
Notably, the fluorescence emission intensity does not change
significantly upon addition of 60 μL of 10 mM solution of
Mn+ ions to 2 mL of the suspended solution of 1 and 2
Moreover, we have also scanned a number of biologically
important anions (An−) using the suspension of 1 and 2 for
selective recognition and a decrease in emission intensity
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Fig. 7 (a) ROR of S.O. with aniline in the presence of catalyst 1
(representative one). (b) Catalyst 1 was filtered off after 2 h resulting in
the termination of the catalysis. (c) Re-addition of 1 at 3 h resulted in
the beginning of catalysis leading to completeness. The inset picture
shows the recyclability of metallacycle 1 for five successive catalytic
cycles.
2−
designed Fe3+ and CrO4
fluorescent sensors.45–49 The
detection limit was calculated according to the standard
literature protocol and the details are provided in the ESI.†
The detection limits for Fe3+ were found to be 0.153 and
0.193 μM for 1 and 2, respectively. Meanwhile, for CrO42− they
were found to be 0.205 and 0.155 μM for 1 and 2, respectively
(Fig. S15, ESI†) which are comparable to or better than some
of earlier reported Fe3+ and CrO4 fluorescent sensors.50–54
A
2−
detailed comparative list of CPs/MOFs and or metallacycles
2−
that have been used for the detection of Fe3+ and CrO4
(fluorescence turn-off) is provided in the ESI† (see Table S4
for details).
Scheme 2 The possible mechanism for the ring-opening reaction of
styrene oxide with amines catalyzed by metallacycle 1 (representative
one).
In addition, a competition experiment was performed to
explore the specific capacity of compounds 1 and 2 as
2−
selective receptors for analytes Fe3+ and CrO4 ions. To
explore this, 2 mL suspension of receptor 1 was first mixed
with 60 μL of 10 mM solution of Fe3+ or CrO4 ions (as the
2−
2−
was noticed only with CrO4 ions. In an effort to check the
2−
sensitivity of 1 and 2 towards CrO4 ions, concentration-
case may be) followed by the addition of 60 μL of 10 mM
dependent emission measurements were carried out and the
solution of different Mn+ or An− ions (except for Fe3+ or
2−
results are shown in Fig. 4b. The titration of a 5 × 10−5 mM
CrO4 ions) and there competitive activities were monitored
2−
solution of CrO4 ions with 2 mL suspended solution of 1 or
2 showed a regular decrease in the fluorescence intensity (see
Fig. 3d and 4d) which was almost completely quenched after
addition of excess CrO4 ions. For both cases, the relevant
by measuring their corresponding fluorescence spectrum.
Notably, we did not detect any considerable effect of these
competitive ions on the intensities of host 1 (Fig. S16 and
S17, ESI†), thus indicating the recognition ability of host 1. A
similar trend was also noticed in the competition experiment
of host 2 (Fig. S18 and S19, ESI†). Notably, under identical
conditions ligand L does not show any fluorescence response
or recognition behavior for any ions.
2−
2−
Stern–Volmer graph displays a good relationship with CrO4
concentration, suggesting
a strong quenching effect on
fluorescence with Ksv values of 6.72 × 105 M−1 (R2 = 0.9864)
and 6.94 × 105 M−1 (R2 = 0.9937), respectively (see the inset of
2−
Fig. 3d and 4d). Importantly, CrO4 ions also gave a striking
turn-off quenching effect and the intensity was quenched by
Binding stoichiometry and mechanistic insight for selective
80.5% and 85.8% for
1 and 2, respectively, with the
sensing of Fe3+ and CrO4 ions
2−
2−
concentration of CrO4 increased to 6 × 10−4 mM. It was
found that 1 and 2 feature a high selectivity response towards
CrO4 ions in aqueous solution. Notably, the fluorescence
quenching capacity of 1 and 2 is comparable to those of well-
The binding stoichiometry of hosts 1 and 2 was examined
using the Benesi–Hildebrand (B. H.) plot, which showed a 1 :
1 stoichiometry. Binding constants (Kb) of 3.19 × 10−2 and
2−
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2.34 × 10−2 M−1 (for 1), and 3.77 × 10−2 and 2.43 × 10−2 (for 2)
respective vial), which also show an observable color change
and thus suggest the possible inclusion of analytes Fe3+ and
CrO42− in the host cavity.
2−
for the recognition of Fe3+ and CrO4 ions, respectively, were
determined by the ratio of the intercept and slope in the B.
H. plot. In all cases, the corresponding B. H. plots show a
good linear correlation coefficient (R2) for the 1 : 1
stoichiometry between Fe3+ (or CrO42−) ions and probes 1 and
2, therefore confirming the 1 : 1 stoichiometry (Fig. S20,
ESI†).55 Furthermore, a closer look at the structure of 1 and 2
reveals that the amidic-H atoms are the most prominent
In addition, to confirm the framework stability and
integrity, the PXRD patterns of 1 and 2 recovered from FeCl3
or K2CrO4 aqueous solution are recorded and it is observed
that they are in good agreement with the simulated pattern
(Fig. S28 and S29, ESI†), thereby suggesting that 1 and 2
preserve their basic structural integrity after sensing
experiments and fluorescence quenching is not caused by
decomposition of their crystalline structures.56,59–62 The
inconsistency observed in the peak pattern or generation of
new peaks might be due to the immobilization of few of the
concerned ions in the cavity of the host as indicated by ICP
measurements. The recyclability experiment for sensing of
2−
binding sites for the Fe3+ and CrO4 ions. To get in-depth
information on the binding mode, stoichiometry, stability
and crystallinity of the framework after the sensing study and
the possible fluorescence quenching mechanism of Fe3+ and
2−
CrO4 ions, several experiments were performed. Primarily,
the FT-IR spectra of the original hosts 1 and 2 and the
2−
recovered samples of
1
and 2, from the suspension
CrO4 ions (representative one) with hosts 1 and 2 in
containing FeCl3 and K2CrO4 (Fig. S21 and S22, ESI†), do not
display any obvious change, thereby suggesting that the
framework architecture of the host remains intact even after
the sensing and there is no chemical bonding during this
aqueous media shows that the quenching efficiency of the
concerned host is preserved even after five consecutive cycles
as shown in Fig. 6.65 The FT-IR spectra of the recovered
2−
samples of 1 and 2 after sensing of CrO4 ions match nicely
process, thus indicating the possibility of weak interactions
with those of the fresh samples, thus confirming that the
basic framework structures of 1 and 2 were well maintained
(Fig. S30 and S31, ESI†) during the sensing investigation.
Notably, the samples of 1 and 2 were easily recovered by
centrifuging the suspensions of 1 and 2 after sensing of
2−
between the analytes (Fe3+ or CrO4
)
and amidic
functionality of the host framework.56
1H NMR titration was performed for CrO4 ion sensing
2−
with hosts 1 and 2 to further probe the 1 : 1 complexation.
The 1H NMR spectral differences in CrO4 ion titration for
CrO4 ions and washing several times with deionised water
2−
2−
and methanol.63 ICP measurements of M·FeCl3 and M·CrO4
2−
host 1 are shown in Fig. 5. The most prominent signal for
the amidic proton (–CONH–) of 1 has been shifted upfield by
(M = Zn or Cd) showed that a small amount of Fe3+ (1.3%
and 2.5%) and Cr6+ (1.1% and 1.9%) was present on the
framework of hosts 1 and 2, respectively (Table S5 of the
0.03 ppm by addition of 1 equiv. of CrO4 ion.57 On the
2−
other hand, under similar conditions 2 showed a reasonably
lower shift for the concerned amidic proton (Fig. S23, ESI†).
The rest of the signals for 1 and 2 related to phenyl and
pyridyl moieties almost appear at their relevant places (Fig. 5
and S23†). This change in the chemical shift value could be
attributed to the probable H-bonding interactions between
ESI†),64 thus providing evidence that frameworks 1 and 2
could have an effect on the Fe3+ and CrO4
ions.
2−
Furthermore, these observations also dispel the probability of
2−
exchange of Zn2+ or Cd2+ ions with the guest Fe3+ or CrO4
ions during the course of the sensing study.48,50,65–67 Finally,
2−
the amidic-H and the CrO4 ions. Importantly, this amidic-H
the fluorescence lifetimes of hosts 1 and 2 before and after
2−
2−
did not display any further δ shift upon addition of extra
being immersed in FeCl3 (1·FeCl3) and CrO4 (1·CrO4 )
2−
CrO4
ions, thus confirming the 1 : 1 complexation/
aqueous solutions are found to be almost the same, which
showed that there was no coordination effect between Fe3+ or
interaction. Notably, under identical conditions, we could not
1
2−
record the H NMR spectrum in the presence of the Fe3+ ion,
CrO4 ions on the concerned hosts 1 and 2 (see Fig. S32 and
due to its paramagnetic (d5) nature.57
S33 and Table S6, ESI†).56,66,68 For the high selectivity of 1
2−
High-resolution mass spectrometry (HRMS) analysis was
carried out to detect the stoichiometry using an equimolar
mixture of hosts 1 and 2 and appropriate amounts of FeCl3 and
K2CrO4 in water, which leads to fast fluorescence quenching.
and 2 towards FeCl3 and CrO4 ions, we have argued that (i)
the nitrate ions (5.31 × 5.31 Å2) present in the cavity of 1 and
2 are exchangeable with the guest analytes (FeCl3, 7.23 × 7.23
Å2; CrO42−, 5.94 × 4.79 Å2) which are comparable in size; (ii)
the cavity size of hosts 1 (6.73 × 15.36 Å2) and 2 (5.29 × 16.29
Å2) (see Table S7† for details) also facilitates this exchange
between nitrate ions and guest analytes, thus helping in their
selective sensing. The aforementioned mechanism is well
consistent with those earlier proposed by other research
groups.56,59–62
2−
HRMS of 1·FeCl3 and 1·CrO4 display peaks at m/z 1403.7874
(calcd, 1403.0976) and 1358.9314 (calcd, 1358.1763), which
correspond to [{1·FeCl3}–NO3]+ and [{1·CrO42−}–NO3]+ peaks,
respectively (Fig. S24 and S25, ESI†). In a similar fashion, HRMS
2−
of 2·FeCl3 and 2·CrO4 display peaks at m/z 1119.2415 (calcd,
1119.0616) and 1074.1738 (calcd, 1074.1402), which correspond
to [{2·FeCl3}–NO3]+ and [{2·CrO42−}–NO3]+ peaks, respectively
(Fig. S26 and S27, ESI†) and thus provide supplementary
evidence for the 1 : 1 complexation.57,58 The optical images of 1
and 2 were also recorded before and after the sensing studies
and are illustrated in the inset of Fig. 3 and 4 (below their
Catalytic application of coordination compounds 1 and 2
Both
1 and 2 offered various structural features that
encouraged us to explore their potential application in
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catalysis: (i) the presence of coordinated yet labile DMF
molecules on both ZnIJII) and CdIJII) ions points towards their
facile replacement by the substrateIJs), (ii) a coordination
cage-like structure, which makes the Lewis acidic sites more
open, (iii) the presence of nitrate ions in the cavities of 1 and
2, (iv) the strong Lewis acidic nature of ZnIJII) and CdIJII) ions,
(v) their thermal stability, which reaches up to 320 °C and
thus provides an opportunity to carry out the reactions under
solvent-free conditions, as many catalysts are not stable after
stirring, (vi) the existence of the H-bonded framework
architecture enhances the interaction between substrates,
nucleophiles and the catalyst, and (vii) the insoluble nature
of 1 and 2 in common organic solvents provides an option to
conduct the reactions heterogeneously. From this
perspective, we have tried to utilize the present ZnIJII)
metallacycle (1) and CdIJII) coordination polymer (2) as
heterogeneous catalysts for the RORs of cyclohexene oxide (C.
O.) and styrene oxide (S.O.) with several amines. As is well
known, the ROR of epoxides with amines affords β-amino
alcohols, which are versatile intermediates in the synthesis of
aminolysis of S.O. afforded a single product, out of the two
possible products in all cases, and thus showed a perfect
regioselectivity with catalysts 1 and 2. The asymmetrical
nature of S.O. provides the concerned nucleophile with the
opportunity to open the epoxide ring either through the
benzylic carbon atom or via attack on the less hindered
carbon atom. Interestingly, in all cases we observed that the
product formed by the nucleophilic attack on the benzylic
carbon atom of the epoxide; for instance the relevant regio-
isomer showed the characteristic [M+ − 31] molecular ion
peak in GC-MS studies due to the loss of the CH2OH
fragment (Fig. S34,† representative one).72–74
The 1H NMR spectrum of the product obtained from the
ROR of S.O. and aniline also confirmed the formation of only
one regio-isomer with the –CH2OH group, as we observed
characteristic two sets of a double doublet at δ 3.92–3.96 and
3.73–3.77 ppm for the methylene proton of the –CH2OH
fragment (Fig. S35†). Notably, aminolysis of S.O. with bulkier
2,6-diisopropylaniline and 3,5-dibromoaniline gave 66% and
36% (Table 2, entries 17 and 18) yield, respectively, which are
poorer than those of non-hindered amines. So, based on the
aforementioned noticeable selectivity for a particular regio-
isomer, a mechanism has been proposed and is shown in
Scheme 2. Finally, we did not observe any size selectivity
during the catalytic reaction as both catalysts 1 and 2 contain
a large enough cavity size, where substrates can easily diffuse
and get activated. Importantly, we did not observe any
significant difference in the catalytic activity in terms of TON
and TOF (h−1) of 1 and 2 towards the RORs of epoxides with
amines; this might be due to the presence of a similar
coordination environment around the ZnIJII) and CdIJII) ions
along with the similarities in their framework structure and
a
diverse range of pharmaceutically and industrially
important compounds.69–71 In a usual aminolysis reaction, C.
O. and aniline were mixed in 1 : 1.2 mole ratios and stirred
under heterogeneous and solvent-free conditions at room
temperature in the presence of 2 mol% of catalyst 1 or 2
which afforded the respective β-amino alcohols in good to
excellent yields. The results of all aminolysis reactions are
listed in Table 2. As shown in Table 2, catalysts 1 and 2 could
promote the reaction between C.O. and aniline (entry 1) as
well as substituted anilines (entries 2–9). In addition, the
impact of electronic substituents present on the aniline on
the product yield was assessed with the incorporation of
various electron donating/withdrawing groups at the ortho-
and para positions of the aniline. As expected, the electron-
rich anilines afforded high yields (up to 92%; Table 2, entries
2 and 3) compared to the electron-poor anilines (up to 62%;
Table 2, entries 4–7). This might be due to the enhanced
nucleophilicity of the electron-rich anilines compared to the
electron-poor anilines. A similar trend has been also noticed
in the literature.72–74 The methodology was extended to
aminolysis of C.O. with hindered amines such as 2,6-
diisopropylaniline and 3,5-dibromoaniline.
thus also exhibits
a proof-of-concept of the structure–
function relationship.75
Leaching and control experiments
In order to determine whether the Lewis acidic metal ions
are leaching out or not in the course of the catalytic reaction,
we performed control experiments using metallacycle 1 as a
model catalyst for the ROR of S. O. with aniline. The overall
profile of this control experiment is depicted in Fig. 7. The
solid catalyst 1 was filtered off after 2 h of reaction and
stirring was allowed to continue. As shown in Fig. 7, there
was no aminolysis observed after removal of 1; however the
reaction was again initiated after the re-employment of
catalyst 1 with a gap of 1 h. This simple experiment confirms
that the catalytic sites herein, ZnIJII) ions, do not leach out in
the course of the catalytic reaction. We argued that the
thermal stability (∼20 °C) along with the crystallinity of the
framework structure of 1 might be the reason why leaching
does not occur even after 4 h of stirring under solvent-free
conditions. Further, to prove the importance of 1 and 2 as
catalysts for the aminolysis reaction of epoxides, we also
performed several additional control experiments using metal
salts such as ZnIJNO3)2·6H2O, ZnCl2, ZnIJOAc)2·6H2O, CdIJNO3)2
As anticipated, 2,6-diisopropylaniline afforded only 58%
yield with catalyst 1 (61% with 2, Table 2, entry 8); this might
be due to the hindrance caused by the presence of the bulky
isopropyl group at the ortho-position, though it exhibits the
+I effect. On the other hand, in the case of 3,5-
dibromoaniline, the presence of bromo groups at the
3,5-position not only makes it a poor nucleophile but also
causes hindrance, resulting in only 34% yield (entry 9,
Table 2). The protocol was further extended to the aminolysis
of styrene oxide and up to 95% yield was observed. The
anilines having electron donating substituents gave quite
better yields (Table 2, entries 11 and 12) compared to those
with electron withdrawing substituents (Table 2, entries 13–
17), as also observed in aminolysis of C.O. Notably,
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·6H2O, CdIJOAc)2·2H2O and CdCl2 as catalysts and observed a
maximum product formation of 27% (Table S8, ESI†) under
identical reaction conditions, hence proving the importance
of 1 and 2 as catalysts for the ROR of epoxides with amines.
In addition, to confirm the heterogeneous nature of catalysts
1 and 2, a recyclability test was accomplished using catalysts
1 and 2 for the ROR of S.O. with aniline as a representative
case. Notably, the recovered catalyst was re-used for five
successive cycles without a substantial loss of activity (see
Table 2 and the inset of Fig. 7 for details). Moreover, the
structural integrity and crystallinity of recovered catalysts 1
and 2 were characterized by matching the FT-IR spectra and
PXRD pattern before and after the catalytic reaction (Fig.
S36–S39†). The FT-IR spectra and PXRD pattern of recovered
catalysts 1 and 2 did not reveal any significant differences
and closely match those of the original samples, indicating
that the structural integrity and crystallinity of 1 and 2 are
retained during the catalytic process.
2016IJi)EU-V). The authors also thank Dr. A. P. Singh and Ms.
Neha Gupta, Department of Applied Science, NIT, Delhi, for
helping in the GC/GCMS data analysis. The authors are also
thankful to Ms. Lisha Arora, Ms. Debapriya Das and Dr. S.
Mukhopadhyay, Department of Chemical Sciences, IISER,
Mohali, for helping in fluorescence life time measurements at
the revision stage. The authors are grateful to Prof. P.
Dastidar, Department of Organic Chemistry, IACS Kolkata, for
providing the PXRD facility at the revision stage. The authors
are grateful to the UGC Networking Resource Centre, School of
Chemistry, University of Hyderabad, for providing the NMR,
CHN and XRD powder facilities. We dedicate this paper to
Professor Ram Chand Paul on the occasion of his 100th birthday.
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Acknowledgements
GK thanks the Department of Science and Technology (DST),
Ministry of Science and Technology, New Delhi, for the
Inspire Faculty Award (Ref. No. DST/INSPIRE/04/2015/
001300). The single crystal X-ray diffraction facility at the
Department of Chemistry, Panjab University by DST-FIST,
Government of India, is acknowledged. PR thanks UGC for
the award of a Junior Research Fellowship (Ref. No. 19/06/
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