A Functional Zn(II) Metallacycle Formed from an N-Heterocyclic

Apr 12, 2017 - School of Chemistry, University of Hyderabad, Central University P.O., Hyderabad 500046, India ...... DFT calculations were performed w...
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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)-1Himidazol-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.

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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 frameworkcontaining 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 NHCbased metal complexes were used as fluorescence-based receptors/probes for the recognition of biologically relevant ions: for example, Fe3+ and periodate (IO4−) ions, the ones reported here.11−13 Neutral iron and its cationic form are some © 2017 American Chemical Society

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 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 Received: January 12, 2017 Published: April 12, 2017 5017

DOI: 10.1021/acs.inorgchem.7b00098 Inorg. Chem. 2017, 56, 5017−5025

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Inorganic Chemistry 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.

positions (Figure S10 in the Supporting Information). Importantly, the singlet of the carbene-H, appearing at δ 10.39 ppm in the 1H NMR spectrum of ligand L, completely 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. 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

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 theory (DFT) calculations to corroborate this host−guest recognition.

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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). The 1H NMR spectrum of 1 clearly shows signals for the carbene-H, aromatic, and methyl protons at their respective 5018

DOI: 10.1021/acs.inorgchem.7b00098 Inorg. Chem. 2017, 56, 5017−5025

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Inorganic Chemistry Table 1. Crystal Data and Structure Refinement Details for Ligand L and Zn(II) Metallacycle 1 empirical formula formula wt temp (K) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcalc (g/cm3) μ (mm−1) F(000) cryst size (mm3) radiation 2θ range for data collection (deg) completeness to θ (%) index ranges no. of rflns collected no. of indep rflns no. of data/restraints/params goodness of fit on F2 final R indexes (I > 2σ(I)) final R indexes (all data) largest diff peak/hole (e Å−3) a

L

1

C34H49Br0.66Cl0.34N4O3 662.708 173(2) orthorhombic Pbca 18.9774(8) 16.3898(4) 23.4059(7) 90 90 90 7280.1(4) 8 1.198 1.635 2767 0.2 × 0.18 × 0.15 Cu Kα (λ = 1.54184) 3.777−71.911 99.9 −22 ≤ h ≤ 23, −10 ≤ k ≤ 19, −24 ≤ l ≤ 28 20154 7002 (Rint = 0.0456) 7002/423/444 1.017 R1 = 0.0714, wR2 = 0.2042 R1 = 0.1153, wR2 = 0.2384 0.99/−0.420

C111H128N12O2Cl11Zn4 2313.68 100(2) triclinic P1̅ 15.443(12) 16.958(13) 16.998(13) 65.686(9) 65.686(9) 73.124(9) 3783(5) 1 1.015 0.861 1199 0.2 × 0.15 × 0.13 Mo Kα (λ = 0.71073) 3.062−42.52 100 −15 ≤ h ≤ 15, −17 ≤ k ≤ 17, −17 ≤ l ≤ 17 54954 8432 (Rint = 0.0988) 8432/991/866 1.531 R1 = 0.1302, wR2 = 0.3708 R1 = 0.1759, wR2 = 0.3992 1.50/−0.72

R1 = Σ∥Fo| − |Fc∥/Σ|Fo|; wR2 = {Σ[w(|Fo|2 − |Fc|2)2]/Σ[wFo4]}1/2.

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 P1̅. In this case, the asymmetric unit is composed of two crystallographically independent coordination complex components, 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 ×

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). Furthermore, to probe the counterion composition (occupational 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 5019

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Figure 2. Calculated molecular orbitals of ligand L (left) and metallacycle 1 (right) in the gas phase (isovalue 0.02).

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−, S2O52−, 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 shown in Figure 3b. Figure 3b shows that when the ratio CFe3+/ 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 from 33/2 to higher CFe3+ 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 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 CIO−4 ions. 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 biphasic quenching is observed (inset of Figure 3d). From the changes in Fe3+- and IO4−-dependent fluorescence intensity,

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). 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. Selective Recognition of Fe3+ and IO4−. The fluorescence-based selective recognition of a number of biologically 5020

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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, 1 H NMR titration and HRMS analyses were carried out. The 5021

DOI: 10.1021/acs.inorgchem.7b00098 Inorg. Chem. 2017, 56, 5017−5025

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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 ion (IO4−) can also behave as a mild reagent for C−H bond activation26−29 and is responsible for the carbene-H activation and clamping of the IO4− ion through a carbene−H···IO4− interaction. Thus, we can also claim that this is an example of shape-specific molecular recognition as well.

Figure 5. Bar diagram representation of relative fluorescence intensity of a 1 mM solution of 1 upon addition of 10 equiv of IO4− in the presence of 10 equiv of background anions (An−) in DMF.

H 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 and the IO4− ions. Notably and interestingly, this carbene-H did not show any further δ shift upon further addition of IO4− ions, thereby suggesting the 1/1 complexation/interaction. However, under similar circumstances, we were not able to record the 1H NMR spectrum in the presence of Fe3+ ion, as 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) 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

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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. Nevertheless, this is a unique example of the utilization of an NHC precursor based metallacycle as a fluorescence-based dual sensor for the biologically relevant Fe3+ and IO4− ions. The 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.

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EXPERIMENTAL SECTION

Materials and Methods. Starting materials such as 2,6diisopropylaniline, nBu4NBr3, glyoxal (40% in water), paraformaldehyde, 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

Figure 6. 1H NMR spectra of metallacycle 1 before (blue trace) and after (red trace) the addition of 1 equiv of KIO4 in DMSO-d6.. 5022

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Inorganic Chemistry 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 D8Advance 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. 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 roundbottom 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. 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 graphitemonochromated 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%

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 SHELXL2014/7.35 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 crystallographic 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, [email protected]. ac.uk). Density Functional Theory (DFT) Calculations. DFT calculations 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.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00098. 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)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for S.K.D.: [email protected]. ORCID

Samar K. Das: 0000-0002-9536-6579 Notes

The authors declare no competing financial interest.

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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. 5023

DOI: 10.1021/acs.inorgchem.7b00098 Inorg. Chem. 2017, 56, 5017−5025

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Inorganic Chemistry

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D. S. Kothari postdoctoral fellowship (Higher) (Ref. No. F.4-2/ 2006(BSR)/CH/13-14/0192). The authors also thank Prof. 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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