Size-Selective Detection of Picric Acid by Fluorescent Palladium

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Size-Selective Detection of Picric Acid by Fluorescent Palladium Macrocycles Sushil Kumar,† Ram Kishan,† Pramod Kumar,† Sanya Pachisia, and Rajeev Gupta* Department of Chemistry, University of Delhi, Delhi 110 007, India S Supporting Information *

of P atoms in coordination to PdII ion(s).16 Mass spectrometry (MS) spectra of both 1 and 2 display peaks for [1-Cl]+ and [2Cl]+ or [2-2Cl]2+ in chloroform and ethanol (EtOH), respectively (Figures S15−S20). Thus, both NMR and MS spectra assert a stable nature of 1 and 2 in the solution state. Diffraction studies for 1 and 2 illustrate palladium macrocycles wherein a square-planar PdII ion is coordinated via two P atoms from the ligand(s) in addition to two Cl atoms (Figure 1). In two

ABSTRACT: This work presents the synthesis and characterization of two palladium-based fluorescent macrocycles offering hydrogen-bonding cavities of contrasting dimensions. Both palladium macrocycles function as chemosensors for the detection of nitroaromatics, whereas the larger macrocycle not only illustrates nanomolar detection of picric acid but also transports its significant amount from an aqueous to an organic phase.

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itroaromatic compounds (NACs) are extensively used explosives1 and known ingredients of industrially and commercially relevant products.2 NACs enter the environment through multiple pathways, and their higher than permissible exposure limit significantly affects our health.3 For example, NACs can cause liver malfunction and damage to blood cells and kidney and can lead to neurological disorders.3,4 Therefore, not only restricted use but also constant monitoring of such pollutants is required to protect the environment.5 Among the different NACs, picric acid (PA) has been identified as one of the most commonly used industrial compounds. PA is an integral ingredient of several medicines, germicides, pesticides, industrial liquids, dyes, and tanning agents.6 Thus, detection of various NACs in general7 and PA in particular has been a topical issue.8 As a result, there has been an upsurge in the development of multiresponsive sensors for the detection of NACs and identification of their trace level for both security and environmental reasons.9 NACs are electron-deficient and thus can interact with electron-rich molecules.9 Using this feature, a variety of π-electron-rich sensors9 including organic−inorganic hybrid materials,10 covalent organic polymers,11 metal−organic frameworks,12 and dendrimers13 have been developed. Such sensors often detect various NACs with limited success to discriminate between the related NACs. 14 Such a fact necessitates the importance of a cavity-based sensor that can discriminate between related substrates of varying dimensions. This work presents two palladium macrocycles of varying dimensions furnished with hydrogen-bonding cavities as fluorescent sensors for the size-selective-encapsulation-led discriminative detection of PA. The reaction of ligands L1 and L215 with [Pd(CH3CN)2Cl2] afforded palladium macrocycles 1 and 2, respectively (Scheme S1). 1 and 2 exhibit N−H stretches at 3294−3302 cm−1 due to the presence of amidic NH groups (Figures S1 and S2). Such a fact is concurred by the appearance of N−H signals at 10.98 ppm (for 1) and 11.07 ppm (for 2) in their 1H NMR spectra (Figures S3−S14). 31P NMR spectra for 1 and 2 suggest the involvement © XXXX American Chemical Society

Figure 1. Crystal structures of 1 and 2. Thermal ellipsoids are drawn at the 30% probability level, while lattice solvent molecules are omitted for clarity. Selected bond distances (Å) and bond angles (deg) for 1: Pd− P1, 2.3456(13); Pd−P2, 2.3647(13); Pd−Cl1, 2.2740(16); Pd−Cl2, 2.2958(13); P1−Pd−P2, 175.84(6); Cl1−Pd−Cl2, 175.05(7). Selected bond distances (Å) and bond angles (deg) for 2: Pd−P1, 2.3326(19); Pd−P2, 2.3206(19); Pd−Cl1, 2.289(2); Pd−Cl2, 2.298(2); P1−Pd− P2, 172.33(7); Cl1−Pd−Cl2, 177.94(8).

macrocycles, both the P atoms and Cl− anions are arranged trans to each other. Importantly, while palladium macrocycle 1 displays a 1 + 1 self-assembly of a ligand and a PdII ion, 2 is an example of 2 + 2 self-assembly. As a result, 1 offers a smaller macrocyclic cavity of 6.492 × 4.902 Å2 dimensions, while 2 exhibits a much larger cavity of 14.922 × 12.788 Å2. The diphenylphosphine groups create a sterically hindered environment that inducts nonplanarity in both macrocycles. Such a nonplanarity results in the distortion of the pyridine-2,6dicarboxamide unit, which, in turn, has placed NH groups in nonplanar orientations. As a consequence, both macrocycles (particularly 2) are better suited to accommodate a guest with the propensity to accept multiple hydrogen bonds. To test this hypothesis, we attempted sensing of NACs as the potential Received: November 4, 2017

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DOI: 10.1021/acs.inorgchem.7b02813 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

was determined by the quantitative titration of two macrocycles (1 μM) with PA (0−100 μM) in EtOH (Figures S26 and S27). The binding constants (Kb) for 1 (2.47 × 104 M−1) and 2 (1.03 × 105 M−1) were calculated from the Benesi−Hildebrand plots (Figure 2b).19 Stern−Volmer constants (KSV) of 6.73 × 104 M−1 for 1 and 2.61 × 105 M−1 for 2 were determined from the Stern− Volmer plots (Figure 2c).19 High values of both binding as well as Stern−Volmer constants for 2 suggest its high affinity toward PA compared to 1. Such a notable difference is further reflected in the observed detection limits of 287 and 71 nM for 1 and 2, respectively (Figures S28 and S29).19 These values advocate the high sensitivity of 2 toward PA even when present at the trace level in a sample (Table S1). Competitive binding studies for 1 and 2, in the presence of other NACs, illustrate that no other substrate has effectively interfered with PA detection (Figure S30). Job’s experiment was performed to determine the stoichiometry of the binding event by titrating an EtOH solution of 1 and 2 against PA and monitoring the change in the emission intensity.19 Job’s plot analysis endorses a 1:1 stoichiometry for both palladium macrocycles toward PA (Figure 2d). Benesi− Hildebrand fitting plots further confirmed a 1:1 stoichiometry for both macrocycles toward PA (Figure 2b).19 The detection of PA by photoresponsive 1 and 2 was also investigated by UV−visible spectral titrations (Figure S31). λmax at 342 nm for 1 (20 μM) was found to be red-shifted (ca. 10 nm) after the addition of PA (20 equiv). A more prominent spectral change was noted in the case of 2. Because 2 displayed “switch− OFF” behavior after recognition of PA, the same quenching nature was noted under UV light (Figure S32). Our attempts to grow single crystals of 1⊂PA and 2⊂PA were unsuccessful. Therefore, we resorted to docking studies to interpret the difference in the binding of PA with 1 versus 2 (Figure 3). Such a study became essential considering the fact

guests because of their ability to create hydrogen bonds with suitable hydrogen-bond donors/acceptors and their electrondeficient nature.9 Excitation of palladium macrocycles 1 and 2 at 350 nm resulted in emissions at 432 and 428 nm, respectively. For NAC sensing, 1 and 2 were titrated with a variety of substrates in EtOH or tetrahydrofuran (THF; Figures 2 and S21−S23). In several

Figure 2. (a) Bar diagram displaying the relative change in the emission intensity for a 1 μM EtOH solution of 1 (green) and 2 (blue) at λmax = 432 and 428 nm, respectively, upon the addition of different analytes (20 μM), (b) Benesi−Hildebrand plots, (c) Stern−Volmer plots, and (d) Job’s plots for 1 and 2, respectively.

cases, fluorescence quenching was noticed as soon as NACs were introduced into a solution of 1 and 2. Notably, the extent of quenching was higher for 2 compared to 1. Among the different NACs, maximum quenching was observed for PA, followed by 2,4-dinitrophenol (2,4-DNP), whereas 2-, 3-, and 4-nitrophenols (2-NP, 3-NP, and 4-NP) as well as 2-, 3-, and 4-nitroanilines (2NA, 3-NA, and 4-NA) resulted in lower quenching.17 On the other hand, related substrates without −OH groups, such as nitrobenzene (NB), 1,3-dinitrobenzene (1,3-DNB), 1,4-dinitrobenzene (1,4-DNB), 4-nitrotoluene (4-NT), 2,4-dinitrotoluene (2,4-DNT), and 2,4,6-trinitrotoluene (TNT), were not effective. Similarly, substrates with an acidic group such phenol and benzoic acid (BA) as well as aliphatic substrates containing −NO2 group(s), e.g., nitroethane (NE) and cyclotrimethylenetrinitramine (RDX), did not cause a significant change to the emission intensity of either 1 or 2. To evaluate and understand the role of the −NO2 groups in PA, a number of related substituted phenols were screened.18 In all cases, an imperceptible change in the emission intensity infers negligible interaction of such substituted phenols to that of any of the macrocycles (Figure S24). These experiments infer the requirement of −NO2 group(s) that most probably make the −OH group acidic. In order to delineate the role of the −OH group in PA, we studied the effect of sodium picrate and methyl picrate on the emission intensities of 1 and 2. Both of these substrates only resulted in negligible quenching, thus confirming that a phenolic −OH group is an essential requirement for the detection of PA (Figure S25). Collectively, these experiments therefore conclude that a substrate must have an −OH group in addition to strong electron-withdrawing −NO2 group(s) for its effective sensing by the present macrocycles.17 Subsequently, all studies were performed with PA using two palladium macrocycles. In successive studies, change of the fluorescence intensity

Figure 3. Molecular docking structures of the palladium macrocycles (a) 1 and (b) 2 with PA. See Figures S33 and S34 for different views.

that the cavity dimensions of 1 (6.492 × 4.902 Å2) were actually smaller than the molecular dimensions of PA (6.813 × 6.457 Å2). In contrast, 2 offered a much larger cavity (14.922 × 12.788 Å2).20 Therefore, while 2 should not have any difficulty in accommodating PA within its cavity, 1 may have to adopt a different conformation to accommodate or at least interact with a molecule of PA. Interestingly, docking of 1 illustrates a molecule of PA sitting above the pyridine-2,6-dicarboxamide fragment of the macrocycle. As a result, the arene ring of PA prominently interacts with the NH groups of the palladium macrocycle in the form of N−H···π interactions whereas the −OH group of PA B

DOI: 10.1021/acs.inorgchem.7b02813 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Lopes26 have recognized the importance of ketones, while Kudo et al.28 have illustrated a remarkable effect of the functional group in the extraction abilities. Uslu27 applied an innovative reactive technique combined with an organic phase for the efficient extraction of PA. These examples illustrate that the extraction of PA from an aqueous solution has attracted significant interest; however, it still remains a notable challenge. We therefore attempted possible extraction of PA from an aqueous solution, realizing the outstanding detection ability of the palladium macrocycle 2. For such studies, PA present in an aqueous layer was extracted using dichloromethane (CH2Cl2) containing 2. Satisfyingly, 2 was able to extract 15.2 equiv of PA within 1 min (Figure S41).29 These results are noteworthy and demonstrate significant extraction of PA from an aqueous layer to a CH2Cl2 layer mediated by 2. Importantly, 1 could only extract a negligible amount (2.6 equiv) under identical conditions (Figure S42). This experiment not only illustrates the practical significance of PA extraction from a contaminated sample but also demonstrates a notable difference in the extraction abilities of two macrocycles that can be related to the cavity size being offered by them. In conclusion, this work illustrated two palladium macrocycles offering hydrogen-bonding cavities of varying dimensions and their role in the encapsulation-led remarkable detection of PA.30 The larger macrocycle was able to transport a significant amount of PA from an aqueous to an organic phase, thus creating practical extraction possibilities.

creates hydrogen bonds with the Npyridine, Oamide, and Namide atoms (Figure 3a). On the other hand, the interaction of PA with 2 was completely different. Here, a molecule of PA was found to comfortably fit within the macrocyclic cavity of 2. The inclusion of PA was made possible considering a large cavity being offered by 2. Such an inclusion was manifested by an array of hydrogen bonding as well as π···π interactions between 2 and PA. In particular, the NH groups of the pyridine-2,6-dicarboxamide fragment formed hydrogen bonds with the O atoms of the nitro groups of PA (Figure 3b).21,22 In addition, π···π interactions (3.6−4.8 Å) between the arene ring of PA and the arene/phenyl rings of 2 helped to sandwich a molecule of PA between two panes of the macrocycle. Notably, the presence of Pd ions at both ends of the macrocycle enhanced its rigidity, thus creating a cavity of fixed dimensions that assisted in the selective recognition of PA. We therefore infer that both size-selective inclusion of PA within the cavity of 2 and its ability to form hydrogen bonds resulted in its significant sensing, as reflected by its nanomolar detection limit.20 Importantly, the MS spectrum of a mixture of 2 and PA in EtOH displays a peak at m/z 1901.1623 corresponding to [(2-2Cl)(PA-H)(H2O)]+ that matches well with the simulated pattern and confirms 2⊂PA (Figure S37). On the other hand, 1 did not display the 1⊂PA peak in the MS spectrum, thus further concurring with docking studies (Figure S38). Finally, fluorescence lifetime measurements for both 1 and 2 in the presence of PA exhibited notable differences, therefore suggesting the binding of PA with macrocycles (Figure S39).23 Notably, both docking and MS studies illustrated the inclusion of PA within the cavity of 2, while 1 was only partially able to interact with PA. However, both macrocycles were only able to detect an aromatic substrate having an −OH group while containing electron-withdrawing −NO2 group(s).17 Such a fact also suggests the involvement of electronic characteristics of both a host (palladium macrocycles) and a guest (NACs). In this context, the electronic properties of both the host and guest can be used to delineate a sensing event.24 Generally, the conduction band (CB) of an electron-rich host is situated above the lowest unoccupied molecular orbital (LUMO) of an electron-deficient guest (e.g., NACs).25 Upon photoexcitation, an electron is moved from the CB of the host to the LUMO of the guest, resulting in emission quenching. However, such an electron-transfer mechanism is not the only reason for fluorescence quenching, and an effective overlap between the absorption band of the guest to that of the emission band of the fluorescent host can also result in sufficient resonance energy transfer.24,25 The extent of resonance energy transfer depends on the spectral overlap between the guest’s absorption band and the host’s emission band.24,25 Figure S40 illustrates an ample spectral overlap between the absorption spectrum of PA and the emission spectra of both 1 and 2, particularly 2. On the other hand, assorted NPs (2-NP, 3-NP, 4-NP, and 2,4-DNP) as well as NAs (2-NA, 3-NA, and 4-NA) exhibited limited spectral overlap and therefore moderate sensing, whereas the remaining substrates display insignificant spectral overlap. In essence, these observations not only support maximum quenching efficiency compared to other NACs but also illustrate the importance of a hydrogen-bonding-based cavity as well as size match in the recognition of PA by 2. Being an explosive and highly toxic compound, PA has to be removed from aqueous streams and wastewaters.26 Therefore, extensive attempts have been made for its removal, and solvent extraction methods are the most preferred ones.27 Ferreira and

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02813. Experimental data including figures and tables (PDF) Accession Codes

CCDC 1582152−1582153 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Web page: http://people. du.ac.in/~rgupta/. ORCID

Rajeev Gupta: 0000-0003-2454-6705 Author Contributions †

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

R.G. acknowledges SERB, New Delhi (EMR/2016/000888), and the University of Delhi for financial support. S.K., R.K., and P.K. thank CSIR and UGC for RA and Dr. D. S. Kothari Postdoctoral Fellowship, respectively. C

DOI: 10.1021/acs.inorgchem.7b02813 Inorg. Chem. XXXX, XXX, XXX−XXX

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Addicoat, M. A.; Petkov, P. S.; Heine, T.; Banerjee, R. Solid State Organic Amine Detection in a Photochromic Porous Metal Organic Framework. Chem. Sci. 2015, 6, 1420−1425. (d) Joarder, B.; Desai, A. V.; Samanta, P.; Mukherjee, S.; Ghosh, S. K. Selective and Sensitive Aqueous-Phase Detection of 2,4,6-Trinitrophenol (TNP) by an AmineFunctionalized Metal−Organic Framework. Chem. - Eur. J. 2015, 21, 965−969. (e) Tu, M.; Wannapaiboon, S.; Khaletskaya, K.; Fischer, R. A. Engineering Zeolitic-Imidazolate Framework (ZIF) Thin Film Devices for Selective Detection of Volatile Organic Compounds. Adv. Funct. Mater. 2015, 25, 4470−4479. (13) (a) Rose, A.; Zhu, Z.; Madigan, C. F.; Swager, T. M.; Bulovic, V. Sensitivity Gains in Chemosensing by Lasing Action in Organic Polymers. Nature 2005, 434, 876−879. (b) Geng, Y.; Ali, M. A.; Clulow, A. J.; Fan, S.; Burn, P. L.; Gentle, I. R.; Meredith, P.; Shaw, P. E. Unambiguous Detection of Nitrated Explosive Vapours by Fluorescence Quenching of Dendrimer Films. Nat. Commun. 2015, 6, 8240−8248. (14) (a) Sun, X.; He, J.; Meng, Y.; Zhang, L.; Zhang, S.; Ma, X.; Dey, S.; Zhao, J.; Lei, Y. Microwave-Assisted Ultrafast and Facile Synthesis of Fluorescent Carbon Nanoparticles from a Single Precursor: Preparation, Characterization and Their Application for the Highly Selective Detection of Explosive Picric Acid. J. Mater. Chem. A 2016, 4, 4161− 4171. (b) Wang, K.; Tian, X.; Jin, Y.; Sun, J.; Zhang, Q. Heterometallic Hybrid Open Frameworks: Synthesis and Application for Selective Detection of Nitro Explosives. Cryst. Growth Des. 2017, 17, 1836−1842. (15) Kumar, S.; Kishan, R.; Gupta, R. Convenient Route for the Synthesis of Ortho-, Meta-, and Para-Substituted Diphenylphosphinoamine. Patent File No. 201611008186. (16) The 31P{1H} NMR spectra of 1 and 2 exhibit resonances at 25.87 and 32.66 ppm, respectively, which are 30−40 ppm downfield-shifted from their respective ligands (L1, −4.16 ppm; L2, −7.34 ppm). (17) The use of 2-, 3-, and 4-NA resulted in moderate quenching in the fluorescence emission of both palladium macrocycles. These results suggest that such substrates closely mimic the critical features of nitrophenols, i.e., the −OH group (as the hydrogen-bond donor) and the −NO2 group(s) (as the hydrogen-bond acceptor) in addition to their electron-deficient nature. See: Jiang, X.; Liu, Y.; Wu, P.; Wang, L.; Wang, Q.; Zhu, G.; Li, X.-L.; Wang, J. A metal−organic framework with a 9-phenylcarbazole moiety as a fluorescent tag for picric acid explosive detection: collaboration of electron transfer, hydrogen bonding and size matching. RSC Adv. 2014, 4, 47357−47360. (18) Substituted phenols screened: 4-methoxyphenol, 4-chlorophenol, 2,4-di-tert-butylphenol, 2,6-di-tert-butylphenol, 2,4,6-tri-tert-butylphenol, 2,4,6-trichlorophenol, 2,4,6-tribromophenol, and 2,4,6-trimethylphenol. See Figure S24. (19) (a) Srivastava, S.; Gupta, B. K.; Gupta, R. Lanthanide-Based Coordination Polymers for the Size-Selective Detection of Nitroaromatics. Cryst. Growth Des. 2017, 17, 3907−3916. (b) Kumar, P.; Kumar, V.; Gupta, R. Detection of the Anticoagulant Drug Warfarin by Palladium Complexes. Dalton Trans. 2017, 46, 10205−10209. (20) Docking studies, performed for other substrates, exhibited either ineffective encapsulation or insufficient interaction of a substrate to that of the palladium macrocycle 2. See Figure S35 for the molecular dimensions of various substrates. (21) The hydrogen bonding of NH groups of the pyridine-2,6dicarboxamide fragment to that of NO2 group(s) of trinitrobenzene has been crystallographically noted. See: Kim, S. K.; Lim, J. M.; Pradhan, T.; Jung, H. S.; Lynch, V. M.; Kim, J. S.; Kim, D.; Sessler, J. L. SelfAssociation and Nitroaromatic-Induced Deaggregation of Pyrene Substituted Pyridine Amides. J. Am. Chem. Soc. 2014, 136, 495−505. (22) Importantly, both ligands L1 and L2 also exhibited quenching in their emission intensities upon reaction with PA in both EtOH and THF (Figure S36 and Table S2). Although the extent of quenching was not as significant as that noted for palladium macrocycles 1 and 2, these experiments support that the ligands plausibly interacted with PA via amidic NH groups. (23) While 1 and 2 display fluorescence lifetimes of 0.34 and 0.89−1.01 ns, respectively, the excited-state decay profiles are described by relatively fast decays of 0.30 and 0.77−0.87 ns for 1 + PA and 2 + PA, respectively. See Figure S39 and Table S3.

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DOI: 10.1021/acs.inorgchem.7b02813 Inorg. Chem. XXXX, XXX, XXX−XXX