A Functional Zn(II) Metallacycle Formed from an N-Heterocyclic Carbene Precursor: A Molecular Sensor for Selective Recognition of Fe3+ and IO4- Ions

Article abstract of DOI:10.1021/acs.inorgchem.7b00098

We have reported the synthesis and structural characterization of a unique Zn(II) metallacycle (1) and its utilization as a fluorescent probe for the shape-specific selective recognition (turn-off) of Fe³⁺ and IO₄^- ions. The relevant Stern-Volmer graphs indicate that the recognitions of Fe³⁺ and IO₄^- ions are examples of diphasic and monophasic quenchings, respectively. The title metallacycle has been prepared by the reaction of a novel N-heterocyclic carbene precursor, 1,3-bis(2,6-diisopropyl-4-(pyridin-4-yl)phenyl)-1H-imidazol-3-ium chloride/bromide (L), and zinc(II) chloride salt. Notably, the ligand itself did not show any type of recognition for any ions. DFT calculations were performed on L and metallacycle 1 using the geometric parameters, obtained from their single-crystal X-ray diffraction data, to understand the electronic structures of the ligand and macrocycle. The detection limit for the recognition of the Fe³⁺ ion was determined to be 2.5 × 10^-6 mol/L, and that for IO₄^- ion was found to be 6.3 × 10^-5 mol/L.

Full text of DOI:10.1021/acs.inorgchem.7b00098

Article
pubs.acs.org/IC
A Functional Zn(II) Metallacycle Formed from an N‑Heterocyclic
Carbene Precursor: A Molecular Sensor for Selective Recognition of
Fe3+ and IO4 Ions
−
Girijesh Kumar,† Ramu Guda,‡ Ahmad Husain,§ Ramakrishna Bodapati,∥ and Samar K. Das*,∥
†Department of Chemistry & Centre for Advanced Studies in Chemistry, Panjab University, Chandigarh 160014, India
‡Department of Chemistry, Kakatiya University, Warangal 506009, India
§Department of Chemistry, DAV University, Jalandhar, Punjab 144012, India
∥School of Chemistry, University of Hyderabad, Central University P.O., Hyderabad 500046, India
S
* Supporting Information
ABSTRACT: We have reported the synthesis and structural
characterization of a unique Zn(II) metallacycle (1) and its
utilization as a fluorescent probe for the shape-specific selective
recognition (turn-off) of Fe3+ and IO4 ions. The relevant
−
Stern−Volmer graphs indicate that the recognitions of Fe3+
−
and IO4 ions are examples of diphasic and monophasic
quenchings, respectively. The title metallacycle has been
prepared by the reaction of a novel N-heterocyclic carbene
precursor, 1,3-bis(2,6-diisopropyl-4-(pyridin-4-yl)phenyl)-1H-
imidazol-3-ium chloride/bromide (L), and zinc(II) chloride
salt. Notably, the ligand itself did not show any type of
recognition for any ions. DFT calculations were performed on
L and metallacycle 1 using the geometric parameters, obtained
from their single-crystal X-ray diffraction data, to understand
the electronic structures of the ligand and macrocycle. The
detection limit for the recognition of the Fe3+ ion was
determined to be 2.5 × 10−6 mol/L, and that for IO4 ion was found to be 6.3 × 10−5 mol/L.
−
of the prime constituents in physiological processes occurring
in the human body as well as in other biological systems. This is
because the Fe3+ ion is the key component of the hemoglobin
and is responsible for oxygen metabolism, oxygen uptake, and
electron transfer in living organisms.14,15 A deficiency of iron
leads to the extremely familiar disease anemia.16 On the other
hand, an excess of iron can damage DNA, proteins, lipids, and
other cellular components. Thus, sensing iron cations, using a
molecular receptor, is a challenging task. Likewise, the
INTRODUCTION
■
Over the past few decades, the design and synthesis of
receptors for the molecular recognition of ions have drawn
tremendous attention from the chemistry community due to
their fascinating host−guest interactions that play a decisive
role in many areas of chemistry and biology.1−3 In our daily life,
ions play a significant role either as an essential requirement for
growth or as harmful environmental pollutants.4,5 From this
perspective, selective molecular sensing of the ions is still one of
the foremost goals for contemporary biologists and chemists. In
order to achieve this goal, wide varieties of framework-
containing materials, collectively known as coordination
polymers (CPs), have widely been used in recent years as
fluorescent probes for the selective detection of cations and
anions.6,7 Even though N-heterocyclic carbenes (NHCs) have
been known for almost five decades and used as the key ligands
in the synthesis of a variety of organometallic complexes that
are capable of catalyzing various organic transformation
reactions,8−10 merely a few reports are available where NHC-
based metal complexes were used as fluorescence-based
receptors/probes for the recognition of biologically relevant
periodate (IO4 ) anion is a 2e− oxidant in acidic as well as
−
alkaline media and is responsible for the oxidation of L-serine,
which is a significant nonessential amino acid having the −OH
functionality and is responsible for fat metabolism, growth of
tissue, and the immune system.17 Therefore, molecular
recognition of the periodate anion is an important assignment
for a synthetic chemist.
Herein, we have reported the design and synthesis of the
novel cationic ligand 1,3-bis(2,6-diisopropyl-4-(pyridin-4-yl)-
phenyl)-1H-imidazol-3-ium chloride/bromide (L) having two
extended pyridyl groups along with the imidazolium moiety
−
ions: for example, Fe3+ and periodate (IO4 ) ions, the ones
Received: January 12, 2017
reported here.11−13 Neutral iron and its cationic form are some
© XXXX American Chemical Society
A
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Scheme 1. Synthetic Route for the Zn(II) Metallacyle 1 Using Ligand L
Figure 1. (a) Perspective view of L ligand. The ellipsoids were drawn at their probability level of 30%; hydrogen atoms are omitted for clarity. Color
code: gray, C; red, O; blue, N; green, Cl; yellow, Br. (b) Molecular diagram of metallacycle 1 showing two independent coordination units in the
lattice. (c) Packing diagram of metallacycle 1 viewed along the crystallographic a axis showing well-defined cavities, occupied by the lattice water
molecules and chlorine counterions. (d) Rods passing through the cavity of the metallacycle 1.
and its Zn(II) metallacycle (1). We have described compound
1 as a fluorescence-based receptor for selective sensing of Fe3+
and IO4 ions. We also have performed density functional
positions (Figure S10 in the Supporting Information).
Importantly, the singlet of the carbene-H, appearing at δ
−
1
10.39 ppm in the H NMR spectrum of ligand L, completely
theory (DFT) calculations to corroborate this host−guest
disappears within 4 h after the base treatment, as observed in
the 1H NMR spectrum of the in situ generated species (Figure
S19 in the Supporting Information). This clearly indicates the
generation of carbene species during the course of the reaction.
Interestingly however, after the metalation, this carbene center
again becomes protonated, as confirmed by NMR spectral
studies as well as single-crystal X-ray diffraction analysis (Figure
1b and Figures S10−S13 in the Supporting Information).
However, we were not able to determine the probable reason
for this reverse protonation as well as the source of the
concerned hydrogen atom. In addition, the presence of a
hydrogen atom at the carbene center makes it unavailable for
metal coordination and therefore only appended Npyridyl atoms
of L are readily available for the anchoring of the Zn(II) ions, as
shown in Figure 1b and Scheme 1.
recognition.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The synthesis of ligand
L is provided in the Experimental Section, while syntheses of
precursors for the ligand L are provided in the experimental
section of the Supporting Information. The details of the
characterization of the precursors as well as ligand L are also
provided in Figures S1−S9 in the Supporting Information. The
Zn(II) metallacycle 1 was prepared by the reaction of ligand L
with zinc chloride (anhydrous) under an N2 atmosphere in the
presence of potassium tert-butoxide as base in an anhydrous
DMF solvent (Scheme 1) and was well characterized by various
analytical and spectroscopic techniques (Figures S10−S18 in
the Supporting Information).
Crystal Structure Analysis. Crystals of both ligand L and
metallacycle 1 have been analyzed crystallographically, and
details of the data collection and structure refinement are given
1
The H NMR spectrum of 1 clearly shows signals for the
carbene-H, aromatic, and methyl protons at their respective
B
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Table 1. Crystal Data and Structure Refinement Details for Ligand L and Zn(II) Metallacycle 1
L
1
empirical formula
formula wt
temp (K)
cryst syst
space group
a (Å)
C34H49Br0.66Cl0.34N4O3
C111H128N12O2Cl11Zn4
2313.68
662.708
173(2)
100(2)
orthorhombic
triclinic
Pbca
P1
̅
18.9774(8)
15.443(12)
16.958(13)
16.998(13)
65.686(9)
65.686(9)
73.124(9)
3783(5)
b (Å)
16.3898(4)
c (Å)
23.4059(7)
α (deg)
β (deg)
90
90
γ (deg)
V (Å3)
90
7280.1(4)
Z
8
1
ρcalc (g/cm3)
1.198
1.015
μ (mm−1
)
1.635
0.861
F(000)
2767
1199
cryst size (mm3)
0.2 × 0.18 × 0.15
Cu Kα (λ = 1.54184)
3.777−71.911
0.2 × 0.15 × 0.13
Mo Kα (λ = 0.71073)
3.062−42.52
100
radiation
2θ range for data collection (deg)
completeness to θ (%)
index ranges
99.9
−22 ≤ h ≤ 23, −10 ≤ k ≤ 19, −24 ≤ l ≤ 28
20154
−15 ≤ h ≤ 15, −17 ≤ k ≤ 17, −17 ≤ l ≤ 17
54954
no. of rflns collected
no. of indep rflns
7002 (Rint = 0.0456)
7002/423/444
8432 (Rint = 0.0988)
8432/991/866
no. of data/restraints/params
goodness of fit on F2
final R indexes (I > 2σ(I))
final R indexes (all data)
1.017
1.531
R1 = 0.0714, wR2 = 0.2042
R1 = 0.1153, wR2 = 0.2384
0.99/−0.420
R1 = 0.1302, wR2 = 0.3708
R1 = 0.1759, wR2 = 0.3992
1.50/−0.72
largest diff peak/hole (e Å−3
)
a
R1 = Σ∥Fo| − |Fc∥/Σ|Fo|; wR2 = {Σ[w(|Fo|2 − |Fc|2)2]/Σ[wFo4]}1/2
.
in Table 1. Details of selected bond lengths and bond angles are
provided in Table S1 in the Supporting Information. Colorless
crystals of ligand L, suitable for single-crystal X-ray diffraction
analysis, were obtained from an aqueous solution. The ligand L
crystallizes in a centrosymmetric orthorhombic unit cell with
the space group Pbca, and its asymmetric unit consists of one
ligand unit, one free halogen (Cl/Br), and three lattice
occluded water molecules. In the crystal structure of L,
occupational disorder between bromine and chlorine was
found on the halogen position. The position of the halogen site
was partially occupied by bromine (66.3% occupancy) and
chlorine (33.7% occupancy).18−21 The torsion angles between
the central imidazole ring and two phenyl rings were found to
be 89.2 and 100.6°. The phenylpyridyl moieties are significantly
nonplanar, as is evident from the dihedral angles of 28.1 and
29.1° involving the terminal pyridyl rings and phenyl spacer.
Moreover, the lattice water molecules and halide ions
accumulate to form hydrophilic and hydrophobic layers, viewed
down the crystallographic c axis (Figure S5 in the Supporting
Information). The charge of the framework of L is balanced by
halide anion present in the crystal lattice. High-resolution mass
spectrometry (HRMS) analysis also proved the presence of
both Cl− and Br− as counter ions: for instance, the peak at m/z
ca. 666.60 is nicely fitted with the found peak m/z 666.36 ([M
+ 2]+) (Figure S6 in the Supporting Information).
than that of Cl− and their respective peaks appear at 1.60 and
2.65 keV, respectively, in the EDX spectrum (Figures S7 and S8
in the Supporting Information).
The colorless crystals of metallacycle 1 were grown by
diffusion of diethyl ether in a DMF solution of 1. The crystals
crystallize in a centrosymmetric triclinic unit cell with the space
group P1. In this case, the asymmetric unit is composed of two
̅
crystallographically independent coordination complex compo-
nents, overall consisting of two Zn(II) metal centers, two L
ligands, five chloride ions coordinated to two Zn(II) ions, a
half-occupied lattice chloride ion, and one lattice water
molecule (considering occupancies of zinc and chloride ions).
In the crystal structure, component I, [Zn2(L)Cl6], is
composed of a disordered coordination complex localized
over almost two positions comprising one L ligand coordinated
to two Zn(II) ions via an Npyridyl atom on either side. Zn(II)
centers adopt a tetrahedral coordination environment by
coordinating to the pyridyl moiety of L ligand and three
chloride ions.
On the other hand component II, [Zn2(L)2Cl4], adopts an
overall dimeric 40-membered metallacyclic structure with a
Zn- - -Zn separation of 20.052 Å and comprised of two L
ligands coordinated to two Zn(II) atoms via an Npyridyl atom on
either side. The Zn(II) centers adopt a tetrahedral coordination
environment by coordinating to two Npyridyl atoms stemming
from two different L ligands and two chloride ions each. The
charge of the coordination framework is balanced by
encapsulated Cl− ion, present in the cavity, along with two
lattice water molecules as shown in Figure 1b (see also Scheme
1). The cross-section of the metallacycle is found to be 9.066 ×
Furthermore, to probe the counterion composition (occupa-
tional disorder) of ligand L, scanning electron microscopy
(SEM) with EDX (energy dispersive X-ray spectroscopy) and
element mapping analysis was carried out, which clearly
indicated that the percent composition of Br− ion is greater
C
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Figure 2. Calculated molecular orbitals of ligand L (left) and metallacycle 1 (right) in the gas phase (isovalue 0.02).
20.052 Å2 (Table S2 in the Supporting Information). The
crystal structure of 1 displays 16.7% potential solvent accessible
voids (SAVs), as suggested by a PLATON analysis.22 Notably,
SAVs exist between the pair of metallacycle dimers stacking on
top of each other running parallel to the crystallographic a axis
(Figure S15 in the Supporting Information). The XRD powder
analysis of 1 shows the crystallinity and phase purity of the bulk
sample (Figure S16 in the Supporting Information).
Thermal Analysis and Microscopy. Thermal stability of
the organic, inorganic, or organic−inorganic hybrid materials is
one of the essential parameters for their use as functional
materials. In this regard, thermogravimetric analysis (TGA) was
employed for both ligand L as well as metallacycle 1. TGA of L
shows a weight loss of 7.8% (observed), which fitted nicely with
the ca. 8.18% in the temperature range of 50−180 °C. This
weight loss corresponds to the liberation of three water
molecules (Figure S9 in the Supporting Information).
However, metallacycle 1 exhibits stability up to 320 °C with
the first weight loss of 18.19% (ca. 18.43%) for the release of
two water molecules and 11 chloride atoms in the temperature
range of 25−225 °C (Figure S17 in the Supporting
Information). SEM analyses of crystals of L and metallacycle
1 exhibit rough surface morphology (Figures S7 and S18 in the
Supporting Information).
relevant cations (Mn+; n = 1−3) such as Mg2+, Ca2+, Cr3+,
Mn2+, Fe3+, Fe2+, Co2+, Ni2+, Cd2+, Zn2+, Ag+, Pb2+, Ru3+, Sm3+,
Eu3+, Gd3+, Tb3+, Dy3+, and Er3+ and anions (An−; n = 1−3)
such as SCN−, N3 , IO4 , I−, H2PO4 , HCO3 , CH3COO−,
−
−
−
−
Cl−, Br−, BF4 , ClO4 , Cr2O72−, CO32−, SO32−, SO42−, S2O5
,
−
−
2−
S2O82−, and PO43− (as their Na+ or K+ salts) using metallacyle 1
as receptor was exploited in DMF at 25 °C. As shown in Figure
3, the receptor 1 displays an emission at 399.7 nm (λex = 320
nm, excitation and emission slits 3 nm) which is ascribed to the
emission of the central imidazolium moiety. Upon addition of
10 μL of a 5 mM solution of Mn+ ions to 3 mL of host 1
(except Fe3+ ions), the fluorescence emission intensity of 1
does not obviously change, whereas the addition of the same
amount of Fe3+ ions causes a great decrease in emission
intensity of 1 (Figure 3a,b). Notably, the titration of a 3 × 10−3
mM solution of Fe3+ ion with a 2 × 10−3 mM solution of 1
showed a regular decrease in fluorescence intensities from
399.7 nm as we increased the concentration of Fe3+ ions, as
3+
shown in Figure 3b. Figure 3b shows that when the ratio CFe
/
C1 is below 3/2, the emission intensity decreases sharply with
increasing concentration of Fe3+ ions. However, when the ratio
is increased from 3/2 to 33/2, the decrease in emission
intensity becomes gradual. Further, an increase in the ratio
3+
from 33/2 to higher CFe does not lead to nonemissive 1.
The Stern−Volmer graph clearly indicates that the
recognition of Fe3+ ions is an example of diphasic quenching
with Ksv1 and Ksv2 values of ca. 0.23 × 105 M−1 (R2 = 0.967) and
ca. 1.5 × 106 M−1 (R2 = 0.985), respectively, in DMF at 25 °C
(inset of Figure 3b).23 A similar observation is also noticed in
the recognition of various aforementioned (An−) anions. Out of
a number of examined An− ions, a decrease in emission
DFT Calculations. Furthermore, to gain insight into the
electronic structures of ligand L and its Zn(II) metallacycle 1,
DFT calculations were performed using the geometric
parameters obtained from single crystal X-ray diffraction
analyses. The optimized geometries remain similar to those
determined by X-ray crystallography. The calculated molecular
orbital (MO) surfaces are shown in Figure 2. The highest
occupied molecular orbitals (HOMO) of L are mostly centered
on the imidazolium moiety; however, the lowest unoccupied
molecular orbitals (LUMO) are located all over the ligand L.
On the other hand, the HOMO and LUMO of the metallacycle
1 have significant contributions from the imidazolium moiety as
located all over L with almost no contributions from the Zn(II)
ions. The HOMO−LUMO energy gaps for ligand L and
metallacycle 1 are found to be ca. −0.56 eV and −0.36 eV,
respectively.
−
intensity was only noticed with IO4 ions. In addition, the
titration results of a 6 × 10−3 mM solution of IO4− ion with a 3
× 10−3 mM solution of 1 also showed a regular decrease in the
fluorescence intensity (Figure 3d) and fluorescence was almost
−
completely quenched after addition of excess C ions.
IO4
The relevant Stern−Volmer graph displays that the
−
recognition of IO4 ions is an example of monophasic
quenching with a Ksv value of ca. 1.4 × 103 M−1 (R2
=
0.984) in DMF at 25 °C, in contrast to Fe3+ quenching, where
Selective Recognition of Fe3+ and IO4 . The fluores-
biphasic quenching is observed (inset of Figure 3d). From the
−
−
cence-based selective recognition of a number of biologically
changes in Fe3+- and IO4 -dependent fluorescence intensity,
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Figure 3. (a) Fluorescence spectra of metallacycle 1 (1.0 × 10−6 mol L−1) upon addition of different cations (Mn+) (10 μL of a 5 mM solution of all
guests was added to 3 mL of host 1) in DMF (λ 399.7 nm). (b) Fluorescence titration of metallacycle 1 (2 × 10−3 mM) in DMF with increasing Fe3+
concentration (addition of 10 μL of Fe3+ ions per time; λex 320 nm). The inset in (b) gives a Stern−Volmer plot for Fe3+ ion recognition. (c)
Fluorescence spectra of metallacycle 1 (1.0 × 10−6 mol L−1) upon addition of different anions (An−) (10 μL of a 5 mM solution of all guests was
−
added to 3 mL of host 1) in DMF (λ 399.7 nm). (d) Fluorescence titration of metallacycle 1 (3 × 10−3 mM) in DMF with increasing IO4
concentration (addition of 20 μL of IO4 ions per time, λex 320 nm). The inset of (d) gives a Stern−Volmer plot for IO4 ion recognition.
−
−
the detection limits were also calculated and found to be 2.5 ×
10−6 and 6.3 × 10−5 M, respectively (Figures S20 and S21 in
the Supporting Information). In order to further explore the
special selective ability of metallacycle 1 as a receptor for Fe3+
−
and IO4 ions, contest experiments were carried out.
For this, receptor 1 (1 mM) was first mixed with 10 equiv of
Fe3+ or IO4 ions (as the case may be) and then 10 equiv of
−
−
various Mn+ or An− ions (except Fe3+ or IO4 ions) was added
followed by the monitoring of competition actions by
measurement of the fluorescence spectra. Importantly, we did
not observe any significant influence of these competitive ions
on the intensities of receptor 1, as shown in Figures 4 and 5,
thereby proving the recognition ability of receptor 1. Notably, a
closer look at the structure of receptor 1 reveals that the
carbene-H atoms are the most prominent binding sites for Fe3+
Figure 4. Bar diagram representation of the relative fluorescence
intensity of a 1 mM solution of 1 upon addition of 10 equiv of Fe3+ in
the presence of 10 equiv of background cations (Mn+) in DMF.
−
and IO4 ions. Further, to get in-depth information on the
binding mode between metallacycle 1 and Fe3+ or IO4 ions,
−
1H NMR titration and HRMS analyses were carried out. The
E
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similar size; however, they do not display any type of
fluorescence quenching response under identical conditions,
thereby confirming the selectivity of the aforementioned ions in
the cavity. Herewith, we have argued that the combined effects
of (i) the oxidative nature of ions and (ii) the shape of the
concerned ions are responsible for this selective recognition.
In addition, FeCl3 is also well-known for C−H activation24,25
and is responsible for the carbene-H activation during the
−
recognition process and thus capture of the FeCl4 ion via a
carbene−H···FeCl4− interaction. In a similar manner, periodate
Figure 5. Bar diagram representation of relative fluorescence intensity
−
−
ion (IO4 ) can also behave as a mild reagent for C−H bond
of a 1 mM solution of 1 upon addition of 10 equiv of IO4 in the
activation26−29 and is responsible for the carbene-H activation
presence of 10 equiv of background anions (An−) in DMF.
−
−
and clamping of the IO4 ion through a carbene−H···IO4
1H NMR spectral differences for IO4− ion are shown in Figure
6. The most prominent signal for the carbene-H of metallacycle
1 has been shifted upfield by 0.029 ppm by addition of 1 equiv
of IO4− ion. The rest of the signals of the aromatic and aliphatic
hydrogen almost appeared at their respective places (Figure 6).
This change in the chemical shift value could be ascribed to the
possible hydrogen-bonding interactions between the carbene-H
interaction. Thus, we can also claim that this is an example of
shape-specific molecular recognition as well.
SUMMARY AND CONCLUSION
■
In summary, we have successfully demonstrated the synthesis
and characterization of a novel L ligand and its Zn(II)
metallacycle 1, leading to its utilization as a fluorescence-based
−
receptor for the selective recognition for Fe3+ and IO4 ions.
−
and the IO4 ions. Notably and interestingly, this carbene-H
−
did not show any further δ shift upon further addition of IO4
Nevertheless, this is a unique example of the utilization of an
ions, thereby suggesting the 1/1 complexation/interaction.
However, under similar circumstances, we were not able to
NHC precursor based metallacycle as a fluorescence-based dual
sensor for the biologically relevant Fe3+ and IO4 ions. The
−
1
record the H NMR spectrum in the presence of Fe3+ ion, as
clear demonstration of metallacycle 1 as a fluorescence-based
receptor for the selective recognition of Fe3+ and IO4− ions will
unlock access to the design of new N-heterocyclic ligand based
coordination materials as sensors in the near future.
this is a d5 system (paramagnetic). Furthermore, HRMS studies
of 1·FeCl4− and 1·IO4− display M+ peaks at m/z 2511.5413 and
− +
−
2503.6887, which correspond to [M·FeCl4 ] and [M·IO4
−
H]+ peaks, respectively, and also provide additional evidence
for the 1/1 complexation (Figures S22 and S23 in the
Supporting Information). Interestingly, we observed that the
cavity of the receptor 1 showed shape-specific molecular
recognition, as this is only sensitive to FeCl3 and IO4− having a
particular shape in solution. We have argued that the presence
of lattice Cl− ion in the cavity of metallacycle 1 (Figure 1b)
EXPERIMENTAL SECTION
■
Materials and Methods. Starting materials such as 2,6-
diisopropylaniline, nBu4NBr3, glyoxal (40% in water), paraformalde-
hyde, pyridine-4-boronic acid, PdCl2(PPh3)2, potassium tert-butoxide
(KOtBu), and anhydrous DMF were purchased from Sigma-Aldrich.
However, trimethylsilyl chloride (TMSCl), anhydrous Na2CO3, and
ZnCl2 (anhydrous) were purchased from TCI, AVRA, and CDH
chemicals, respectively, and used as received. The solvents were of
analytical grade and purified wherever necessary per the standard
literature.30 The microanalytical data were obtained with a FLASH EA
1112 Series CHNS Analyzer. The NMR spectroscopic measurements
were carried out with a Bruker Avance 400 spectrometer. The FTIR
spectra were recorded with a Perkin-Elmer FTIR 2000 spectrometer.
Thermogravimetric analysis (TGA) measurements were performed on
an SDT Q600 (V20.9 Build 20) instrument (Artisan Technology
−
might be responsible for the conversion of FeCl3 to FeCl4 .
Both FeCl4 and IO4 are tetrahedral in shape. We have
−
−
calculated the cavity size of the zinc metallacyle (20.052 ×
9.066 Å2) as well as the molecular dimensions of FeCl4 (6.02
−
× 6.02 Å2) and IO4− (6.38 × 6.38 Å2) ions and found that the
sizes of these two molecules fit well with the cavity size of the
metallacycle (see Table S2 in the Supporting Information for
details). In addition, we further investigated other ions having
1
Figure 6. H NMR spectra of metallacycle 1 before (blue trace) and after (red trace) the addition of 1 equiv of KIO4 in DMSO-d6..
F
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Group, Champaign, IL) under an N2 atmosphere at a heating rate of
10 °C per min. The UV−visible absorbance spectra were recorded
using a Perkin-Elmer Lambda-25 spectrophotometer. The field
emission scanning electron microscopy (FESEM) imaging with energy
dispersive X-ray spectroscopy (EDXS) was carried out on a Carl Zeiss
Model Ultra 55 microscope. The fluorescence-based sensing studies
were performed with a Cary Eclipse fluorescence spectrophotometer.
The powder X-ray diffraction patterns were recorded on a Bruker D8-
Advance diffractometer using graphite-monochromated Cu Kα1 (λ =
1.5406 Å) and Kα2 (λ = 1.54439 Å) radiation. The samples were
ground and subjected to the range θ = 5−40° with a scan rate of 1°/
min at room temperature.
completeness. Data reduction and unit cell refinement for L were
performed using CryAlisPro v38.43,33 while those for 1 was performed
using SAINT-Plus.34 The structures were solved by direct methods
and a refinement procedure by a full-matrix least-squares method,
based on F2 values against all reflections, as performed by SHELXL-
2014/7.35 The hydrogen atoms were fixed at calculated positions with
isotropic thermal parameters. Non-hydrogen atoms were refined
anisotropically except for OW1 and Cl6 for 1. In the crystal structure
of L, occupational disorder between bromine and chlorine was found
on the halogen position. The position of the halogen site was partially
occupied by bromine (66.3% occupancy) and chlorine (33.7%
occupancy). Furthermore, zinc (Zn2 and Zn3) and chloride (Cl3,
Cl4, Cl5, Cl6, Cl7, and Cl8) atoms of 1 were found to be positionally
disordered and were solved using the PART command. For 1, their
positions were refined anisotropically with site occupancy factors of
0.5390 (Zn2A), 0.4610 (Zn2B), 0.5181 (Cl3A) and 0.4819 (Cl3B),
0.5406 (Cl4A) and 0.4594 (Cl4B), and 0.5406 (Cl5A) and 0.4594
(Cl5B). The lattice occluded chloride ion and water molecule of 1
have relatively large thermal displacement amplitudes, indicating
positional disorder. The hydrogen atoms of the lattice water molecules
for both L and 1 could not be located from the difference Fourier map;
however, their contributions are included in the empirical formulas.
Solvent accessible voids (SAVs) were calculated without removing
lattice water and chloride ion, as obtained by using grid points/probe
radius of 1.2 Å from the nearest Van der Waals surface with the
PLATON software.22 Details of the crystallographic data collection
and structural solution parameter are given in Table 1. CCDC file
numbers 1515034 and 1515035 contain supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif (or from the Cambridge
Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge
CB2 1EZ, U.K.; fax, +44(0)1223-336033; e-mail, deposit@ccdc.cam.
ac.uk).
Synthesis of Ligand {[C37H43N4]·Cl0.337/Br0.663·3H2O} (L). The
ligand L has been synthesized by using a Suzuki−Miyaura coupling
reaction,31,32 where N,N′-1,3-bis(4-bromo-2,6-diisopropylphenyl)-
imidazolium chloride (1.164 g, 2 mmol, 1 equiv) was dissolved in
20 mL of DMF and degassed with N2 for 15 min followed by addition
of Na2CO3 solution (10 mL, 2 M). Pyridine-4-boronic acid (492 mg, 4
mmol, 2 equiv) and PdCl2(PPh3)2 as catalyst (280 mg, 0.399 mmol,
0.2 equiv) were added to the reaction mixture under a N2 atmosphere
(Scheme S1 in the Supporting Information). Finally the reaction
mixture was stirred at 100 °C for 8−9 h and the progress of the
reaction was monitored by TLC. After completion of the reaction the
resultant solution was diluted with H2O (20 mL) and then filtered off.
A white solid of L was obtained which was soluble in excess H2O. A
single crystal, suitable for X-ray diffraction analysis, was obtained from
the water after the aqueous solution of L stood for 5−6 days. Yield:
0.820 g (62%). Anal. Calcd for C37H43Br0.66Cl0.34N4·2H2O: C, 68.94;
H, 7.35; N, 8.69. Found: C, 68.62; H, 7.41; N, 8.57. FTIR spectrum
(KBr disk, selected peaks): 3498 (OH), 1624 (CN) cm−1. 1H NMR
(400 MHz, DMSO-d6): δ 10.39 (s, 1H, Ha), 8.74 (d, 4H, J = 6 Hz,
Hc), 8.70 (s, 2H, Hb) 7.92 (s, 8H, Hd+e), 2.45 (q, 4H, J = 6.8 Hz, Hf),
1.38 (d, 12H, J = 6.8 Hz, Hg), 1.28 (d, 12H, J = 9.2 Hz, Hg′). 13C NMR
(100 MHz, DMSO-d6) δ 150.81 (C1), 146.48 (C2), 146.40 (C9),
141.40 (C3), 139.86 (C8), 131.13 (C7), 126.85 (C6), 123.76 (C4),
122.38 (C5), 29.47 (C10), 24.46 (C11), 23.47 (C11′). HRMS (ESI): [M
+ 2]+ calcd for [C37H43Br0.66Cl0.34N4·3H2O] 666.60, found 666.36.
Synthesis of [{Zn2(L)1.5Cl5}·0.5Cl·H2O] (1). The metalation
reaction was performed in an oven-dried 5 mL two-neck round-
bottom flask. The ligand L (25 mg, 0.04 mmol, 1 equiv) was dissolved
in 5 mL of dry DMF and degassed with N2 followed by the addition of
potassium tert-butoxide (5 mg, 0.048 mmol, 1.2 equiv), and this
mixture was stirred at room temperature for 4 h. To this reaction
mixture was added anhydrous ZnCl2 (15.4 mg, 0.11 mmol, 3 equiv),
and stirring was continued for a further 24 h at room temperature
under an N2 atmosphere. After completion of the reaction, a colorless
solution was obtained which was filtered off and diffused with diethyl
ether. A single crystal, suitable for X-ray diffraction analysis, was
obtained after a period of 10−15 days. Yield (based on L): 30 mg
(65%). Anal. Calcd for C111H128Cl5.5N12Zn2·4H2O: C, 65.74; H, 6.76;
N, 8.29. Found: C, 65.26; H, 6.63; N, 8.15. FTIR spectrum (KBr disk,
selected peaks): 3448 (OH), 1656 (CC) cm−1. 1H NMR (400
MHz, DMSO-d6): δ 10.20 (s, 2H, Ha), 8.70 (d, J = 6 Hz, 8H, He), 8.43
(d, J = 7 Hz, 4H, Hb), 7.76 (d, J = 7 Hz, 8H, Hd), 7.74 (s, 8H, Hc),
2.42 (sextet, J = 7 Hz, 8H, Hf), 1.34 (d, J = 7 Hz, 24H, Hg), 1.23 (d, J
= 7.6 Hz, 24H, Hg′). 13C NMR (100 MHz, DMSO-d6): δ 162.8 (C1),
150.5 (C2), 146.4 (C9), 141.2 (C3), 139.8 (C8), 131.3 (C7), 126.8
(C6), 123.9 (C5), 122.7 (C4), 29.5 (C10), 24.4 (C11), 23.5 (C11′).
HRMS (ESI): [M + 1]+ calcd for 0.5[C111H128N12O2Cl11Zn4] 1167.70,
found 1167.6183.
Density Functional Theory (DFT) Calculations. DFT calcu-
lations were performed with the Gaussian 09 software36 package
employing the B3LYP functional37,38 using the LanL2DZ basis
set39−41 for Zn and 6-31G* basis set42 for C, N, H, and O. Geometric
parameters obtained from X-ray structure analyses were used as a
starting point for geometry optimization in the ground state.
Frequency calculations were performed to confirm the optimized
structures to be true minima on the potential energy surfaces.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b00098.
■
S
Experimental details, NMR spectra, crystal structures,
XRPD patterns, TGA-DTA plots, FESEM with EDX
plots, detection limit plots, HR mass spectra, and
bonding parameters. (PDF)
X-ray crystallographic data for ligand L (CIF)
X-ray crystallographic data for metallacycle 1 (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail for S.K.D.: skdas@uohyd.ac.in.
■
ORCID
Single-Crystal X-ray Diffraction Analysis. Suitable, apparently
single crystals of L and metallacycle 1 were selected. Data collection
for ligand L was carried out on an Oxford XCalibur CCD
diffractometer equipped with graphite-monochromated Cu Kα
radiation (λ = 1.54184 Å). However, data collection for 1 was carried
out on a Bruker SMART APEX II CCD diffractometer using graphite-
monochromated Mo Kα (λ = 0.71073 Å) radiation. The frames were
collected at T = 173 K (for L) and 100 K (for 1). The crystals of 1
were found to be poorly diffracting; several data sets were collected,
and the best set was chosen with a resolution of 0.98 Å with 100%
Samar K. Das: 0000-0002-9536-6579
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the SERB, DST, Government of India (Project No.
SB/S1/IC-34/2013), for financial support. G.K. thanks the
University Grants Commission (UGC) for the award of a Dr.
G
DOI: 10.1021/acs.inorgchem.7b00098
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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Rajeev Gupta and Gulshan Kumar, Department of Chemistry,
Delhi University, for helping in the sensing studies. We also
acknowledge UPE-II, University of Hyderabad.
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