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Bis-Heteroleptic Ruthenium(II) Complex of Pendant Urea Functionalized Pyridyl Triazole and Phenathroline for Recognition, Sensing, and Extraction of Oxyanions Tamal Kanti Ghosh, Sourav Chakraborty, Bijit Chowdhury, and Pradyut Ghosh* Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Kolkata 700032, India S Supporting Information *

ABSTRACT: A new bis-heteroleptic RuII complex based ditopic receptor, 1[PF6]2, having an anion binding triazole −CH unit and appended 4-fluorophenyl urea arm has been developed. 1H NMR and isothermal titration calorimetry (ITC) experiments showed binding of 1[PF6]2 toward oxyanions such as phosphates (e.g., H2PO4− and HP2O73−) and carboxylates (e.g., CH3CO2− and C6H5CO2−) anions selectively. 1H NMR studies showed that highly basic phosphate anions such as HP2O73−/H2PO4− are bound by both −CH and −NH units of complex 1[PF6]2. However, comparatively less basic CH 3CO 2−, C6H5CO2− anions interacted with the urea −NH protons only. Thermodynamic parameters obtained from ITC experiments suggested that binding of all the interacting anions with complex 1[PF6]2 are highly enthalpy and entropy driven processes. Importantly, complex 1[PF6]2 showed extraction of H2PO4−, CH3CO2−, and C6H5CO2− anions from aqueous solution via liquid−liquid extraction with efficiencies of 28%, 74%, and 80%, respectively. The influential role of the urea moiety in the course of extraction is demonstrated by comparison with a model complex, 2[PF6]2. Additionally, complex 1[PF6]2 is capable of selective sensing of phosphate anions among all investigated anions.

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this context, RuII polypyridyl complexes have largely been utilized for photochemical and electrochemical detection of anions.8 Recently, our group has reported RuII−polypyridyl complex based monopodal and tripodal receptors for recognition and sensing of phosphate anions via solely a −CH···anion interaction.9 Extraction of phosphate anions from water is a challenging task due to the high hydration energy associated with solvation of these anions (ΔHhyd = 58.8 kJ/mol for H2PO4− anion). There are very few receptors in the literature on liquid−liquid extraction of phosphate anions,5p and to the best of our knowledge, no of RuII−polypyridyl receptors have been explored/shown for the extraction of anions. In this context, herein we report a new bis-heteroleptic RuII based ditopic receptor, 1[PF6]2, composed of two types of anion binding units, viz. −CH protons of a RuII-bound triazole unit and appended −NH protons of the urea moiety, for selective recognition, sensing, and extraction of oxyanions such as phosphates (HP 2 O 7 3− /H 2 PO 4 − ) and carboxylates (CH3CO2−/C6H5CO2−). The role of the appended urea moiety in the extraction of oxyanions is also established with

INTRODUCTION The development of synthetic anion receptors for recognition, sensing, and extraction of biologically and environmentally important anions is one of the prime goals in supramolecular anion recognition chemistry.1 Recognition, sensing, and extraction of oxyanions such as phosphates are very important because of their vital roles in essential biological processes.2 Excess phosphate in water can lead to significant eutrophication, which subsequently results in the decline of aquatic life. On the other hand, carboxylate anions play an important role in numerous metabolic processes3 and food preservations. The presence of carboxylate anions during wastewater treatment is a great problem in the steel and synthesis industries.4 Recognition and sensing of anions generally occur due to several types of noncovalent interactions of anions with receptor molecules via either neutral/cationic −NH donors (e.g., pyrrole, urea, amide, ammonium, guanidinium, etc.)3,5 or neutral or cationic −CH hydrogen bond donor motifs (e.g., phenyl, triazole, triazolium, and imidazolium).6 For the last two decades, anion binding through urea −NH protons has been extensively studied, where good to excellent selectivity for specific anions has been observed.5n On the other hand, receptor molecules decorated with both −CH and −NH donor units have recently been utilized for recognition and sensing of anions via simultaneous −CH/−NH···anion interactions.7 In © 2017 American Chemical Society

Received: February 21, 2017 Published: April 17, 2017 5371

DOI: 10.1021/acs.inorgchem.7b00473 Inorg. Chem. 2017, 56, 5371−5382

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Inorganic Chemistry the model complex 2[PF6]2, having a solitary triazole −CH unit as an anion binding unit.

spectrometry (Figures S1−S4 in the Supporting Information). Thus, 1[PF6]2 contains an anion binding triazole −CH unit and an appended p-fluorophenyl urea unit to serve a dual role of sensing and extraction of anions. The detailed procedure of preparation of complex 1[PF 6 ] 2 is discussed in the Experimental Section. Characterization details of Complex 1[PF6]2 are provided in Figures S5−S13 in the Supporting Information. Two characteristic peaks observed in an ESI-MS experiment at m/z 393.94 (calcd 394.08) and m/z 933.02 (calcd 933.14) can be assigned to [C40H31FN10ORu]2+ and [C40H31F7N10OPRu]+ species, respectively (Figure S12 in the Supporting Information) with good matches between simulated and experimental isotopic distribution patterns (Figure S13 in the Supporting Information). Single crystals of 1[PF6]2 suitable for X-ray diffraction analysis were obtained via diffusion of diisopropyl ether into a solution of 1[PF6]2 in an acetonitrile/ tetrahydrofuran (3/1, v/v) solvent mixture. Crystal structure analysis of complex 1[PF6]2 showed that the central RuII is placed in a distorted-octahedral geometry where four coordination sites are occupied by four N atoms of two coordinated phenanthroline rings and the other two sites are occupied by two N atoms of the pyridine triazole unit (Figure 1). Crystallographic details and all bond angles and bond lengths are given in Tables S1 and S2 in the Supporting Information. Anion Recognition Studies of 1[PF6]2. 1H NMR Titration Experiment. As the urea −NH and triazole −CH protons showed sharp and distinguishable resonance peaks in DMSO-d6 in comparison to other solvents (e.g., CD3CN), anion binding studies for 1[PF6]2 were carried out with various oxyanions and halides having a tetrabutylammonium (TBA) countercation by 1 H NMR spectroscopy in DMSO-d6. Addition of various anions (e.g., Cl−, Br−, I−, ClO4−, NO3−, HSO4−, etc.) except for phosphates and carboxylates showed insignificant shifts of the resonance positions of both triazole −CH and urea −NH protons even with high amounts (3 equiv) of anions (Figure S14 in the Supporting Information). Downfield shifts of either triazole −CH and urea −NH protons or only urea −NH protons are observed in the presence of phosphates (i.e., H2PO4− and HP2O73−) and carboxylates (e.g., CH3CO2− and C6H5CO2−), respectively (Figure S14). During the course of the 1H NMR titration of complex 1[PF6]2 with TBAH2PO4, the

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RESULTS AND DISCUSSION Designing Aspects and Synthesis of Complex 1[PF6]2. Anion receptors based on a RuII polypyridyl system have emerged as a potential candidate for recognition and sensing of anions because of their unique combination of photophysical, photochemical, and electrochemical properties which can be tuned by strategic modifications of attached ligands.10 RuII polypyridyl complex based anion receptors have largely been used in recognition and sensing of anions,8 with minimum attention being given toward the extraction of recognized anions. Our previously published results with complex 2[PF6]2 suggested that a RuII complex of a triazole-pyridine and phenanthroline hybrid ligand can act as an efficient sensor for H2PO4− and HP2O73− via a CH···anion hydrogen bonding interaction.9 The urea moiety is known as an extractor of anions.11 In order to introduce dual properties such as anion sensing as well as extraction, we have integrated a 4fluorophenyl urea functionality in the architecture of complex 2[PF6]2 to prepare a new bis-heteroleptic ditopic RuII complex based receptor, i.e. complex 1[PF6]2 (Chart 1), from the newly Chart 1. Chemical Structures of Complexes 1[PF6]2 and 2[PF6]2

synthesized ligand L with 84% yield (Scheme 1). L was synthesized in four steps with an overall yield of 76% (Scheme 1) and characterized by NMR spectroscopy and mass Scheme 1. Preparation Scheme of L and Complex 1[PF6]2

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the presence of dicationic receptor molecules, as reported by Molina et al. and others.7a−c Titration data from the shift of the −CHa proton was fitted to a 1:2 binding model,13b which yielded binding constant values of 4.18 and 3.07 for log K1 and log K2, respectively. Similarly, during titration of the complex 1[PF6]2 with HP2O73−, the −NHb peak gradually vanished due to deprotonation and the resonance position of the triazole −CHa proton shifted from 9.11 to 9.68 ppm (Δδ = 0.57 ppm), which saturated after addition of nearly 1 equiv of HP2O73− (Figure S18 in the Supporting Information). Molar ratio analysis showed that complex 1[PF 6 ] 2 formed a 1:1 (host:guest) complex with HP2O73− (Figure S19 in the Supporting Information). Fitting of this data to a 1:1 binding model using WINEQNMR 2 software12 yielded log Ka = 3.91 (Figure S20 in the Supporting Information). 1H-DOSY NMR of both phosphate complexes showed the existence of −NHc and −CH a protons in the ligand backbone without deprotonation, which diffused at the same rate with the other protons of the ligand backbone (Figures S21 and S22 in the Supporting Information). On the other hand, the resonance position of −NHb was shifted from 8.59 to 10.9 ppm (Δδ = 2.31 ppm) and from 8.59 to 11.1 ppm (Δδ = 2.51 ppm) during titration of complex 1[PF 6 ] 2 with TBAO 2 CCH 3 and TBAO2CC6H5, respectively (Figure 2b and Figure S23 in the Supporting Information). In both cases, downfield shift of urea protons ceased after addition of nearly 1 equiv of anion (Figures S24 and S25 in the Supporting Information). Molar ratio plot analysis showed that complex 1[PF6]2 formed a 1:1 (host:guest) complex with both CH3CO2− and C6H5CO2− (Figure S26 in the Supporting Information). Titration data in both cases were fitted to a 1:1 binding model using WINEQNMR 2 software, which yielded log Ka values of 3.80 and 3.63 for CH3CO2− and C6H5CO2−, respectively (Figures S27 and S28 in the Supporting Information). The binding constant value for C6H5CO2− was lower in comparison to the binding constant value of CH3CO2− with complex 1[PF6]2, which was further supported by other techniques discussed herein. It is worthy mentioning here that 2[PF6]2 showed log Ka values of 4.75 and 3.45 for H2PO4− and HP2O73−, respectively.

Figure 1. Single-crystal X-ray structure of complex 1[PF6]2 with thermal ellipsoids drawn at the 30% probability level. Two PF6− anions, tetrahydrofuran (THF) and diisppropyl ether solvent molecules, and all hydrogen atoms except those of urea and the triazole unit are omitted for clarity. Color code: carbon, gray; nitrogen, blue; oxygen, red; hydrogen, sky blue; fluorine, yellowish green; ruthenium, magenta.

resonance position of the triazole −CHa shifted from 9.32 to 9.98 ppm (Δδ = 0.66 ppm), whereas that for the urea −NHb proton shifted from 8.62 to 10.52 ppm (Δδ = 1.9 ppm) with gradual broadening up to an addition of nearly 2 equiv of H2PO4− (Figure 2a and Figures S15 and S16 in the Supporting Information). Although from the above figure it looks like the −NHb proton has disappeared, a closer look showed that the peak was still there even after addition of 2 equiv of anion (Figure S16a). Broadening of the −NHb peak could be due to a strong hydrogen-bonding interaction without deprotonation (in the case of deprotonation the peak would have completely vanished). Molar ratio analysis with the shifts of both −CHa and −NHb resonances showed the formation of a 1:2 host:guest complex between 1[PF6]2 and TBAH2PO4 (Figure S17 in the Supporting Information). A probable explanation for the formation of 1:2 (host:guest) stoichiometry in the case of 1[PF6]2 and TBAH2PO4 could be due to the ability of monomeric H2PO4− anions to form stable dimers in solution in

Figure 2. 1H NMR titration profiles in DMSO-d6 of (a) 1[PF6]2 (2.3 × 10−4 M) with TBAH2PO4 (3.3 × 10−3 M) and (b) 1[PF6]2 (10.2 mM) with TBAO2CCH3 (50.08 mM). 5373


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Figure 3. 31P NMR of (a) free TBAH2PO4 in DMSO-d6 (lower) and complex 1[PF6]2 with 2 equiv of TBAH2PO4 in DMSO-d6 (upper) and (b) free (TBA)3HP2O7 in DMSO-d6 (lower) and complex 1[PF6]2 with 1 equiv of (TBA)3HP2O7 in DMSO-d6 (upper).

Table 1. Comparison of Association Constants and Sensing Properties of Complexes 1[PF6]2 and 2[PF6]2 Obtained from Different Techniques in CH3CN at 298 Ka PL

ITC

host

guest

UV−vis log Ka

log Ka

enhancement factor

1[PF6]2 1[PF6]2 1[PF6]2 1[PF6]2 2[PF6]2 2[PF6]2

TBAH2PO4 (TBA)3HP2O7 TBAO2CCH3 TBAO2CC6H5 TBAH2PO4 (TBA)3HP2O7

5.00, 4.40 4.43 n.d.c n.d. 4.47 4.39

5.18, 4.08 4.62 n.d. n.d. 4.72 4.67

5.1 2.1 n.d. n.d. 6 3

b

sensitivity (μM)

log Ka

ΔH (kJ/mol)

ΔS (J/(mol deg))

0.36 0.54 n.d. n.d. 0.25 0.45

5.19, 4.07 4.75 5.50 5.30 n.d. n.d.

−6.9, −5.4 −30.2 −15.9 −14.9 n.d. n.d.

76.9, 60.1 9.7 52.5 52.1 n.d. n.d.

a

All photophysical data of 2[PF6]2 were incorporated from ref 9a. bIndicates the enhancement in emission intensity of phosphate adducts in comparison to free complex. cNot determined.

Figure 4. Isothermal titration calorimetric plot at 298 K for the addition of a solution of (a) TBAH2PO4 (5.2 mM) to a solution of 1[PF6]2 (0.35 mM) in CH3CN and (b) TBAO2CCH3 (1.285 mM) to a solution of 1[PF6]2 (0.1087 mM) in CH3CN. The upper panel shows the heat pulses experimentally observed in each titration. The lower panel reports the respective time integrals translating as the heat evolved for each aliquot and its coherence to (a) a sequential (n = 2) binding model and (b) a one-site binding model.

anion binding urea unit the binding affinity of −CHa protons toward HP2O73− anion was decreased, which could be due to competitive binding of two strong anion binding units with an anion, which effectively minimizes the interaction of an anion with each recognition unit in comparison to single −CH recognition unit in 2[PF6]2. However, in the case of H2PO4−, due to different binding stoichiometries in the case of 1[PF6]2

Thus, from the binding constant values, it can be clearly seen that the presence of an additional anion binding urea unit in complex 1[PF6]2 has a significant effect on the phosphate binding mode (for H2PO4−) and binding constant values (for HP2O73− and H2PO4−) in comparison to the model complex 2[PF6]2, having the triazole −CH as the solitary anion binding unit in the same solvent. Thus, in the presence of an external 5374


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Inorganic Chemistry Table 2. Excited-State Lifetimes of 1[PF6]2 and 2[PF6]2 in the Absence/Presence of Several Anionsa

a

entry

lifetime (ns)

entry

lifetime (ns)

1[PF6]2 2[PF6]2 1[PF6]2 + H2PO4− 1[PF6]2 + HP2O73−

3.21 5.45 16.9 11.6

2[PF6]2 + H2PO4− 2[PF6]2 + HP2O73− 1[PF6]2 + CH3CO2− 1[PF6]2+C6H5CO2−

28.60 12.01 3.39 3.42

entry 1[PF6]2 1[PF6]2 1[PF6]2 1[PF6]2

+ + + +

lifetime (ns)

HSO4− HCO3− Cl− Br−

3.23 3.15 3.36 3.28

All lifetime values of free 2[PF6]2 and its phosphate adducts were incorporated from ref 9a.

Figure 5. (a) UV−vis spectra of complex 1[PF6]2 (30 μM) in the presence of 2 equiv of various anions (e.g., F−, Cl−, Br−, I−, ClO4−, NO3−, CH3CO2−, C6H5CO2−, CO32−, and HSO4−; [H2PO4−] = 11.9 mM, [HP2O73−] = 9.8 mM, concentration of other anions ∼10 mM) in CH3CN at 298 K. (b) PL spectra of complex 1[PF6]2 (4 μM) in the presence of 2 equiv of various anions (F−, Cl−, Br−, I−, ClO4−, NO3−, CH3CO2−, C6H5CO2−, CO32−, and HSO4−; [H2PO4−] = 2.7 mM, [HP2O73−] = 1.8 mM, concentration of other anions ∼2 mM) in CH3CN at 298 K. (c) Selectivity studies of complex 1[PF6]2 with H2PO4− in the presence of competing anions in CH3CN. Red bars correspond to the emission intensity of complex 1[PF6]2 in the presence of other competitive anions and H2PO4−, and blue bars refer to the emission intensity of complex 1[PF6]2 (10 μM) in the presence of all anions. Legend: (1) complex 1[PF6]2; (2) Cl− (6.2 mM); (3) Br− (7.1 mM); (4) I− (7.9 mM); (5) F− (6.8 mM); (6) NO3− (9.2 mM); (7) HCO3− (8.5 mM); (8) OH− (7.2 mM); (9) CH3CO2− (5.8 mM); (10) C6H5CO2− (9.3 mM); (11) ClO4− (10.1 mM); (12) HSO4− (8.1 mM); (13) HP2O73− (6.3 mM); (14) H2PO4− (6.9 mM).

Table 1). Similarly, when complex 1[PF6]2 was titrated with HP2O73− in CH3CN, the titration data was fitted to a one-site fitting model which provided a binding constant value of log Ka = 4.75 (Figure S29 in the Supporting Information and Table 1). Similarly, when complex 1[PF 6 ] 2 was titrated with TBAO2CCH3 and TBAO2CC6H5 anions in CH3CN, smooth and clear exothermic titration profiles were obtained and the data fit very well to a one-site binding model (Figure 4b and Figure S30 in the Supporting Information). The fitting model provided binding constant values (log Ka) of 5.50 and 5.30 for TBAO2CCH3 and TBAO2CC6H5, respectively (Table 1). Binding constant values obtained for anions from ITC studies were higher than the values obtained from NMR titrations, which could be due to the use of a different solvent in the case of these two studies, as reported previously.5q Thermodynamic values in Table 1 clearly suggested that binding of phosphate and carboxylate anions with 1[PF6]2 was both an enthalpy- and entropy-driven process, which resulted in significantly high binding constant values in all of the anion binding processes. Photophysical Studies of 1[PF6]2 with Anions. Details of photophysical outcome of 1[PF6]2 and previously reported monopodal complex 2[PF6]2 in phosphate sensing are enlisted in Tables 1 and 2. All photophysical studies were carried out in CH3CN. Free 1[PF6]2 showed absorption and emission spectra similar to those of free 2[PF6]2 (Figure S31 in the Supporting Information).9b Quantitative UV−vis and photoluminescence (PL) studies of complex 1[PF6]2 with several anions as a TBA salt showed that only H2PO4− and HP2O73− could be sensed among all of the investigated anions (e.g., F−, Cl−, Br−, I−, ClO4−, NO3−, CH3CO2−, C6H5CO2−, CO32−, and HSO4−) in CH3CN (Figure 5). UV−vis titration data of complex 1[PF6]2

and 2[PF6]2, we could not compare the binding constant values. It is worthy mentioning here that the binding constant values obtained in the case of 1[PF6]2 are comparable with the values reported with similar receptors and anions.3a,5d,6a,5o 31 P NMR Experiment. We have utilized 31P NMR as an additional tool to establish the interaction of H2PO4− and HP2O73− with complex 1[PF6]2 in DMSO-d6 by using triphenylphosphine (PPh3) as an external standard. The 31P signal of free TBAH2PO4 at 2.98 ppm underwent a 0.3 ppm downfield shift in the presence of 0.5 equiv of complex 1[PF6]2 (Figure 3a). Similarly, the 31P resonance of free (TBA)3HP2O7 at −3.52 ppm showed a downfield shift of 0.5 ppm upon addition of 1 equiv of complex 1[PF6]2 (Figure 3b). The downfield shift of parent 31P spectral peaks in both of the above cases clearly suggests the interaction of neighboring oxygen atoms of the phosphorus centers in both phosphate anions with complex 1[PF6]2. Isothermal Titration Calorimetric (ITC) Experiment. In order to gain thermodynamic insight into the anion binding properties of complex 1[PF6]2, we carried out several ITC experiments. Details of the ITC experiments are given in the Experimental Section. Binding of anions (e.g., Cl−, Br−, I−, ClO4−, NO3−, CO32−, and HSO4− as TBA salt) except for phosphates and carboxylates with 1[PF6]2 were found to be too weak to be reliably quantified by ITC studies. All of the thermodynamic parameters obtained from ITC studies are given in Table 1. When the H2PO4− anion was titrated with complex 1[PF6]2 in CH3CN, a smooth and clear exothermic titration profile was obtained, which was fitted to a sequential two-site binding model that yielded binding constant values of 5.19 and 4.07 for log K1 and log K2, respectively (Figure 4a and 5375


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Figure 6. 1H NMR spectra of (a) complex 1[PF6]2, (b) the extracted complex from liquid−liquid extraction between complex 1[PF6]2 and TBAH2PO4, and (c) a 1:2 mixture of complex 1[PF6]2 and TBAH2PO4. (d) 31P NMR spectrum of extracted mass obtained from liquid−liquid extraction between complex 1[PF6]2 and TBAH2PO4.

in the presence of other investigated anions (e.g., Cl−, Br−, I−, F−, NO3−, HCO3−, OH−, CH3CO2−, C6H5CO2−, ClO4−, and HSO4−) (Figure S41 and Table 2). The increase in excitedstate lifetime values of the phosphate adducts could be attributed to the hydrogen-bonding interactions of phosphate anions with 1[PF6]2. Although 1[PF6]2 showed a photophysical response quite similar to that of 2[PF6]2 in the presence of various anions, careful investigation helped us to point out differences between these two receptors toward sensitivity and emission enhancement factor in phosphate detection. As is evident from Table 1, association constant values (log Ka) of 4.67 and 4.72 were obtained for HP2O73− and H2PO4−, respectively, with 2[PF6]2.9a On the other hand, 2[PF6]2 showed detection limit values of 0.25 and 0.45 μM for H2PO4− and HP2O73−, respectively, which were lower in comparison to the values of 1[PF6]2 (Table 1).9a Thus, from these data it is clear that, though the basic mechanism of luminescence “OFF-ON” sensing via phosphate binding is same in both complexes, attachment of a pendant urea unit decreses the sensitivity of the sensor 1[PF6]2 in comparison to the non-urea sensor 2[PF6]2 and similar RuII polypyridyl based model receptors9c because of the decreased binding affinity of the analyte with the triazole −CHa proton, as discussed previously in the section on 1H NMR titration. From all the above experimental techniques, we have seen that the binding constant values of H2PO4− with 1[PF6]2 are higher in comparison to those of HP2O73−. The lower binding affinity of 1[PF6]2 toward HP2O73− in comparison to H2PO4− can be explained as follows. Deprotonation of the −NHb proton upon addition of HP2O73− may lead to a decrease in the charge density on the oxygen atom of HP2O73− probably via formation of H2P2O72− species in solution along with the generation of a negative charge on the ligand backbone which could result in some repulsion between host and guest species. Extraction Studies with Complex 1[PF6]2. Inspired by the fact that complex 1[PF6]2 showed a high degree of selectivity and binding affinity toward oxyanions such as phosphates and carboxylates and the role of the urea unit in anion extraction, we employed a liquid−liquid extraction technique to extract phosphates and carboxylates from aqueous solution into water-immiscible organic solvents. Details of the extraction procedure are given in the Experimental Section.

with HP2O73− (Figure S32 in the Supporting Information) were fitted to a 1:1 (host:guest) binding model, which yielded a binding constant (log Ka) value of 4.43 (Figure S33 in the Supporting Information).13 On the other hand, 1[PF6]2 and H2PO4− showed the formation of a 1:2 (host:guest) complex in solution (Figure S34 in the Supporting Information), having binding constant values of 5.0 and 4.4 for log K1 and log K2, respectively (Figure S35 in the Supporting Information).13 PL titration of complex 1[PF6]2 with HP2O73− and H2PO4− anions resulted in a steady increase in the luminescence intensity of the parent emission band at λmax ∼590 nm up to addition of 1 and 2 equiv of anion, respectively (Figures S36a and S37a in the Supporting Information). PL titration data of 1[PF6]2 with HP2O73− showed formation of a 1:1 (host:guest) complex in solution having a binding constant value (log Ka) of 4.62 (Figure S38 in the Supporting Information and Table 1). On the other hand, H2PO4− formed a 1:2 (host:guest) complex with 1[PF6]2 with binding constant values of 5.18 and 4.08 for log K1 and log K2, respectively, in CH3CN (Figure S39 in the Supporting Information and Table 1). The binding constant values obtained from ITC, UV−vis, and PL measurements in our study are fairly close to each other (Table 1). Slightly different binding constant values for different techniques could be due to differences in principles and instrumentation. However, all techniques show similar trends of binding constants. However, these binding constants are different from those obtained from 1H NMR studies, which could be due to the use of a different solvent (DMSO-d6) in the case of NMR titrations. Our control experiment showed that 1[PF6]2 could sense H2PO4− even in the presence of 5 equiv of other competing anions (e.g., F−, Cl−, Br−, I−, NO3−, HCO3−, OH−, AcO−, C6H5CO2−, ClO4−, and HSO4−) in CH3CN (Figure 5c). From the calibration curve plot we obtained the detection limit of the receptor 1[PF6]2 for phosphate sensing using a detection limit formula (given in the Experimental Section), which yielded a detection limit of ∼0.36 μM for H2PO4−and ∼0.54 μM for HP2O73− (Figure S40 in the Supporting Information and Table 1). On the other hand, time-correlated single photon counting (TCSPC) studies showed that 1[PF6]2 had an excited-state lifetime value of 3.21 ns, which increasd to 16.9 ns (∼5-fold) and 11.6 ns (∼3.5-fold) in the presence of H2PO4− and HP2O73−, respectively (Figure S41 in the Supporting Information and Table 2) with negligible alteration 5376


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Figure 7. 1H NMR of (a) complex 1[PF6]2, (b) 1:1 mixture of complex 1[PF6]2 and TBAO2CCH3, and (c) extracted complex from liquid−liquid extraction between complex 1[PF6]2 and TBAO2CCH3. (d) 13C NMR of extracted mass from liquid−liquid extraction between complex 1[PF6]2 and TBAO2CCH3. In this figure peak a corresponds to the carboxylate carbon atom of acetate and peak b corresponds to the methyl carbon atom of acetate.

and Figure S44 in the Supporting Information). This confirmed the extraction of acetate and benzoate from the aqueous phase into the organic phase with 74% and 80% extraction efficiencies, respectively (Figures S45 and S46 in the Supporting Information). Extraction of CH 3CO2 − and C6H5CO2− into the organic phase was further confirmed by 13 C NMR studies. 13C NMR of extracted mass of carboxylate anions showed resonance positions at δ 174.4 ppm and δ 170.5 ppm for acetate (Figure 7d) and benzoate anions (Figure S47 in the Supporting Information), respectively, which correspond to carboxylate carbon atoms in each case. Additionally in the case of extraction of acetate, a peak at δ 24.2 ppm (Figure 7d) corresponding to the methyl carbon atom of the acetate anion was also observed. Therefore, 1H NMR in combination with 13 C NMR and 31P NMR studies confirmed the extraction of H2PO4−, CH3CO2−, and C6H5CO2− into the organic phase. However, when complex 2[PF6]2 was employed as an extractant of phosphate and carboxylate anions from aqueous solution via liquid−liquid extraction, it failed to extract any such anions from water (Figure S48 in the Supporting Information). During liquid−liquid extraction studies of 2[PF6]2 and TBAH2PO4, the extracted organic phase contained only the methyl-substituted pyridyl triazole precursor ligand. Most probably, in the presence of alkaline phosphate solution, 2[PF6]2 decomposed and generated the precursor triazole ligand. As 2[PF6]2 showed negligible interactions with carboxylate anions, its extraction inefficiency toward these anions is quite reasonable. This result clearly depicted the necessity of the appended urea moiety in the extraction process. In order to determine the efficiency of the receptor to selectively extract the aforementioned anions (i.e. H2PO4−, CH3CO2−, and C6H5CO2−) in the presence of each other and other competing anions from aqueous solution, we have performed liquid−liquid extraction experiments with complex 1[PF6]2 and a mixture of anions (e.g., H2PO4−, CH3CO2−, C6H5CO2−, HSO4−, Cl−, Br−, I−, NO3−, and ClO4−) in a water/ CHCl3 binary solvent mixture. Complex 1[PF6]2 could selectively extract CH3CO2− from aqueous solution of a mixture of these anions (Figure S49 in the Supporting Information). On the other hand, in the absence of

A 5 mL portion of a 10 mM CHCl3 solution of complex 1[PF6]2 was treated with 1 mL of a 4.5 M aqueous solution of TBAH2PO4, and the mixture of these two immiscible solvents was stirred vigorously for 5−6 h. The organic layer was separated, dried, and evaporated to obtain the extracted product. 1H NMR of the extracted product is slightly different from that of the 1:2 mixture complex 1[PF6]2 and TBAH2PO4 (Figure 6). However, both the triazole −CH protons and urea −NH protons showed a significant downfield shift of peak resonances from free 1[PF6]2. This difference in 1H NMR spectra could be due to recognition of highly hydrated H2PO4− during the course of extraction from water. 31P NMR of the extracted complex showed a peak at δ 1.68 ppm (Figure 6d), which clearly suggested extraction of H2PO4− into the organic phase. Calculation of extraction efficiency by using 1,3,5trimethoxybenzene (TMB) as an internal standard14 via 1H NMR showed that complex 1[PF6]2 can extract H2PO4− from water with 28% efficiency (Figure S42 in the Supporting Information). Additionally, UV−vis studies also showed extraction of the H2PO4− anion from aqueous solution into the organic phase with ∼32% efficiency (Figure S43 in the Supporting Information). We have calculated the extraction efficiency by comparing the extinction coefficients of the pure phosphate adduct and the extracted complex (see the Experimental Section for calculations). However, under similar experimental conditions, complex 1[PF6]2 failed to extract (TBA)3HP2O7 into the CHCl3 phase. The inefficiency of complex 1[PF6]2 for pyrophosphate extraction could be due to the high hydration energy associated with the solvation of the highly charged HP2O73− anion in water, which could not be compensated by an H-bonding interaction of the anion with complex 1[PF6]2. Further, complex 1[PF6]2 was utilized to extract carboxylate salts having a TBA counterion from aqueous medium. A 5 mL portion of a 10 mM CHCl3 solution of complex 1[PF6]2 was employed to extract acetate and benzoate anion from 1 mL of an ∼5 M aqueous solution. After liquid−liquid extraction between CHCl3 and H2O the organic layer was separated and 1 H NMR of the extracted mass was similar to that of the acetate/benzoate adduct of complex 1 in DMSO-d6 (Figure 7 5377


Article

Inorganic Chemistry CH3CO2−, complex 1[PF6]2 could extract C6H5CO2− in the presence of several competing anions (e.g., H 2 PO 4 − , C6H5CO2−, HSO4−, Cl−, Br−, I−, NO3−, and ClO4−) via liquid−liquid extraction (Figure S50 in the Supporting Information).

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ΔX =

2 ⎛ 1⎞ G0 + H0 + ⎟ ⎝ K⎠

2 ⎜

⎫ ⎪ ⎬ + 4G0H0⎪ ⎭

(1)

where K is the binding constant, G0 and H0 are the initial concentrations of guest and host, respectively, and ΔX is the change in absorbance or emission intensity for each addition of guest species. For 1:2 (host:guest) binding stoichiometry we have used the fitting equation13

CONCLUSION II

In conclusion, we have synthesized the new Ru complex based receptor 1[PF6]2 by amalgamation of pendant urea functionalized pyridyl triazole and phenanthroline units for selective sensing and extraction of oxyanions such as phosphates and carboxylates. Solution-state 1H NMR and ITC studies showed that the receptor could recognize oxyanions such as phosphates (H2PO4−, HP2O73−) and carboxylates (CH3CO2−, C6H5CO2−). Photophysical studies with 1[PF 6 ]2 showed that only phosphate anions could be selectively sensed in the presence of other competitive anions via enhancement of the RuIIcentered MLCT emission band. On the other hand, 1[PF6]2 showed extraction of H2PO4−, CH3CO2−, and C6H5CO2− anions from aqueous solution via liquid−liquid extraction with extraction efficiencies of 28%, 74%, and 80%, respectively.

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⎧ ⎛ X ⎞⎪⎛ 1 ⎟⎞ ⎜ ⎟⎨⎜G + H + − 0 ⎝ 2H ⎠⎪⎝ 0 K⎠ ⎩

ΔX =

XΔHG1K1[H ]0 [G] + XΔHG2K1K 2[H ]0 [G]2 1 + K1[G] + K1K 2[G]2

(2)

where K1 and K2 are the stepwise association constants, [H]0 is the initial concentration of the host receptor, and ΔX is the change in emission intensity or absorption value. Calculation of Detection Limit. The detection limit (DL) was calculated using the equation DL =

(3 × standard deviation) slope

(3)

SD corresponds to the standard deviation of the value of luminescence intensity of the blank sample, which was measured for 15 consecutive blank samples. The slope was obtained from the linear fit plot of change in PL intensity vs concentration of the guest anion added. Calculation of Excited-State Lifetime. The following equation was used to analyze the experimental time-resolved luminescence decays:

EXPERIMENTAL SECTION

Materials. All reactions were carried out under an argon gas atmosphere followed by workup under ambient conditions. Dichloromethane and acetonitrile were dried over CaH2 and were collected before use. HPLC grade DMSO was purchased from Spectrochem Pvt. Ltd. India and used for ITC studies. RuCl3·xH2O, 1,10-phenanthroline, deuterated solvents, and tetrabutylammonium salts of F−, Cl−, Br−, I−, CH3COO−, C6H5COO−, ClO4−, NO3−, HCO3−, HO−, HSO4−, H2PO4−, and HP2O73− were purchased from Sigma-Aldrich and were used as received. Methods. High-resolution ESI-MS experiments were carried out with a Waters QtoF Model YA 263 mass spectrometer in positive/ negative ESI mode. A sample for mass spectrometry of complex 1[PF6]2 was prepared by dissolving the compound in acetonitrile having concentration 2.1 × 10−6 M. NMR experiments were carried out on an FT-NMR Bruker DPX 500/300 MHz NMR spectrometer. Elemental analysis was performed on PerkinElmer 2500 Series II elemental analyzer, PerkinElmer USA. The absorption and emission studies were performed with a PerkinElmer Lambda 900 UV−vis− NIR spectrometer (NIR = near-infrared) (with a quartz cuvette of path length 1 cm) and a FluoroMax-3 spectrophotometer from Horiba Jobin Yvon, respectively. For the time-correlated single photon counting (TCSPC) measurements, samples were excited at 403 nm using a picosecond diode laser (IBH Nanoled-07) in an IBH Fluorocube apparatus. The luminescence decay data were recorded on a Hamamatsu MCP photomultiplier (R3809) and were analyzed using IBH DAS6 software. Chemical shifts for 1H and 13C NMR spectra were reported in parts per million (ppm), calibrated to the residual solvent peak set, with coupling constants reported in hertz (Hz). Calculation of Binding Constant. Upon addition of anions into a solution of complex 1[PF6]2, several chemical properties were changed, which were utilized to calculate the binding constants. Binding constant values between complex 1[PF6]2 and H2PO4− and HP2O73− were calculated from 1H NMR, UV−vis, and PL titration experiments. The host:guest binding constants were calculated from 1 H NMR titration data using WINEQNMR2 software for both 1:1 and 1:2 (host:guest) binding stoichiometry. Binding constant from UV−vis and PL experiments were calculated using the following equations. For a 1:1 (host:guest) binding stoichiometry we used the fitting equation13

i=1

P(t ) = b +

∑ ai exp(−t / τi) i=n

(4)

Here, P(t) is the decay, n is the number of discrete emissive species, b is a baseline correction factor, αi is a pre-exponential factor, and τi is the excited-state luminescence lifetime associated with the ith component. The average lifetime ⟨τ⟩ was calculated in the case of multiexponential decays from the equation15

τi =

∑ aiτi

(5)

where αi is the contribution from the ith component. Isothermal Titration Calorimetric (ITC) Studies. The isothermal titration calorimetric (ITC) experiments were performed with a Micro-Cal VP-ITC instrument. The titrations were carried out at 298 K in HPLC grade DMSO solvent. A solution of the host in DMSO was placed in the measuring cell. This solution was then titrated with 30 injections of the respective guest solution (10 μL) that was prepared in DMSO. An interval of 220 s was allowed between each injection, and the stirring speed was set at 329 rpm. The obtained data were processed by using Origin 7.0 software that was supplied with the instrument and were fitted to a one-site binding model. A blank titration of plain solvent was also performed and subtracted from the corresponding titration to remove any effect from the heats of dilution from the titrant. X-ray Crystallographic Refinement Details. Crystals of complex 1[PF6]2 suitable for single-crystal X-ray diffraction studies were selected from the mother liquor and immersed in Paratone oil and then mounted on the tip of a glass fiber and cemented using epoxy resin. Intensity data for the all the crystals were collected using Mo Kα (λ = 0.7107 Å) radiation on a Bruker SMART APEX II diffractometer equipped with a CCD area detector at 150 K. The data integration and reduction were processed with SAINT software16 provided with the software package of the SMART APEX II instrument. An empirical absorption correction was applied to the collected reflections with SADABS.17 The structures were solved by direct methods using SHELXL18 and were refined on F2 by the full-matrix least-squares 5378


Article

Inorganic Chemistry technique using the SHELXL-201419 program package. Graphics were generated using PLATON20 and MERCURY 3.2.21 All of the nonhydrogen atoms except for three atoms of the diisopropyl ether solvent molecule (C1, C2, and C3) were refined anisotropically until convergence was reached. Extraction Details. In a typical liquid−liquid extraction experiment, complex 1[PF6]2 was taken up in CHCl3 and aqueous solutions of respective anions were added to it. Two immiscible layers were formed, which were stirred vigorously for about 5−6 h. After that, the organic layer was separated and dried by passing through Na2SO4. Finally, the organic layer was concentrated and diethyl ether was added to obtain the extracted product. The purity and nature of the extracted product were determined by NMR spectroscopy, and the extraction efficiency of complex 1[PF6]2 was calculated by using 1,3,5trimethoxybenzene as an internal standard via calculation of the integration ratio of the peaks from 1H NMR spectroscopy. Extraction efficiency was calculated from the following equations. From 1H NMR:

mixture was stirred for 3−4 h, and then all the volatiles were removed under vacuum. This reaction mixture was dissolved in dry DCM, and 1.4 mL of dry Et3N (10.136 mmol) was added to this mixture; this mixture was stirred for 10 min at room temperature. A 114 μL portion of p-fluorophenyl isocyanate (1.002 mmol) was dissolved in 10 mL of dry DCM and added dropwise to the reaction mixture via a pressureequalizing funnel at 0 °C. After 1 h or so a precipitate appeared and the reaction mixture was stirred for 6 h. After that the solution was filtered and the precipitate was collected. The precipitate was washed with water and diethyl ether and dried overnight to yield the desired product L as a light brown solid (0.290 g, 90% yield). Anal. Calcd for C16H15FN6O (MW = 326.33): C, 58.89; H, 4.63; N, 25.75. Found: C, 56.74; H, 4.29; N, 23.78. FTIR in KBr disk (ν/cm−1): 3346, 3299, 1631, 1579, 1506, 1213, 786, 644. ESI-MS [C16H15FN6NaO]+: calcd, m/z 349.12; found, m/z 349.24. 1H NMR (300 MHz, DMSO-d6): δ 8.593 (m, 3H), 8.044−8.017 (d, 1H, 8.1 Hz), 7.917−7.859 (m, 1H), 7.386−7.316 (m, 3H), 7.062−7.003 (t, 2H, 9 Hz), 6.265−6.227 (t, 1H, 5.7), 4.553−4.515 (t, 2H, 11.4), 3.649−3.92 (m, 2H) ppm. 13C NMR (100 MHz, DMSO-d6): δ 158.5, 155.4, 155.2, 150.0, 149.6, 147.3, 137.2−136.6 (3C), 123.5, 122.9, 119.5, 119.4, 147.7, 115.2, 114.9, 49.9 ppm. Synthesis of Complex 1[PF6]2. cis-[Ru(phen)2Cl2] was synthesized following the reported literature procedure23 and used for the synthesis of the complex. L (0.119 g, 0.364 mmol) and cis[Ru(phen)2Cl2] (0.195 g, 0.367 mmol) were dissolved in 30 mL of a well-degassed ethanol/water binary solvent mixture (2/1; v/v). The mixture was heated to reflux under an argon atmosphere for 48 h until the solution became dark red. After that the reaction mixture was cooled to room temperature and ethanol was evaporated. The solution of the crude complex in water was treated with excess KPF6 salt dissolved in water. An orange-red precipitate was formed after addition, which was filtered, washed with water, and dried under vacuum to give the desired complex 1[PF6]2 as an orange-red crystalline solid (0.226 g, 68% yield). Anal. Calcd for C40H31F13N10OP2Ru (MW = 1177.74): C, 44.58; H, 2.9; N, 13.0. Found: C, 42.60; H, 2.49; N, 12.68. FTIR in KBr disk (ν/cm−1): 3425, 1697, 1521, 1429, 1006, 842, 557. ESI-MS [C40H31FN10ORu]2+: calcd, m/z 394.08; found, m/z 393.94. ESI-MS [C40H31F7N10OPRu]+: calcd, m/z 933.14; found, m/z 933.02. 1H NMR (500 MHz, DMSO-d6): δ 9.396 (s, 1H, −CHa), 8.850 (s, 1H, −NHb), 8.828−8.793 (t, 2H), 8.744−8.727 (d, 1H), 8.662−8.645 (d, 1H), 8.561−8.551 (d, 1H), 8.407−8.299 (m, 6H), 8.070−8.034 (m, 2H), 7.957−7.930 (m, 2H), 7.866−7.839 (m, 1H), 7.743−7.716 (m, 1H), 7.617−7.590 (m, 1H), 7.527−7.516 (d, 1H), 7.285−7.229 (m, 3H), 6.970−6.934 (t, 2H), 6.543 (t, 1H, −NHc), 4.458−4.339 (m, 2H), 3.574−3.279 (m, 2H) ppm. 13C NMR (125 MHz, DMSO-d6): δ 159.1, 156.1, 155.9, 153.6, 153.3, 153.2, 152.2, 151.2, 148.2, 147.9, 147.8, 147.7, 138.8, 137.4, 136.9, 136.8, 130.9, 130.8, 130.4, 128.5, 127.3, 126.9, 126.7, 126.1, 122.8, 120.6, 120.5, 115.7, 115.4, 55.4, 53.3 ppm.

efficiency (%) ⎤ ⎡ area under urea −NH proton peak =⎢ ⎥ ⎣ (area under aromatic −CH proton peak of TMB)/3 ⎦ × 100

(6)

From UV−vis: ⎡ absorbance of extracted product ⎤ concn of extracted product = ⎢ ⎥⎦ ⎣ εXl where ε is the extinction coefficient of the H2PO4− adduct of 1[PF6]2 and l = path length.

extraction efficiency (%) ⎛ ⎞ concn of extracted product =⎜ ⎟ × 100 ⎝ concn of free complex before extraction ⎠ Syntheses. Synthesis of tert-Butyl (2-Azidoethyl)carbamate. The compound was prepared as reported previously.22 Synthesis of tert-Butyl (2-(4-(Pyridin-2-yl)-1H-1,2,3-triazol-1-yl)ethyl)carbamate. In a RB flux, 0.580 g (3.114 mmol) of the compound tert-butyl (2-azidoethyl)carbamate was dissolved in 15 mL of THF. A 350 μL portion of pyridyl alkyne (3.465 mmol) was added to the THF solution, and the mixture was stirred for 1 min. After that 0.181 g of sodium ascorbate (0.913 mmol) was added to it and then 0.078 g of CuSO4·5H2O (0.312 mmol) dissolved in 5 mL of water was added to the mixture. The reaction mixture was stirred at room temperature under an argon atmosphere for 12 h. After the stirring was over, the solvent was evaporated and an aqueous solution of Na2EDTA was added to the crude mixture, and this mixture was stirred for 2−3 h. This solution was extracted two to three times with DCM, and the combined organic solution was collected, washed with brine solution, and finally dried with Na2SO4. This DCM part was evaporated and washed with hexane several times to give 0.242 g of the desired product in 84% yield. Anal. Calcd for C14H19N5O2 (MW = 289.33): C, 58.12; H, 6.62; N, 24.21. Found: C, 56.14; H, 6.19; N, 22.98. FTIR in KBr disk (ν/cm−1): 3222, 2979, 1710, 1602, 1542, 1421, 1251, 1174, 784. ESI-MS [C14H19N5NaO2]+: calcd, m/z 312.14; found, m/z 312.24. 1H NMR (300 MHz, DMSO-d6): δ 8.601−8.577 (m, 1H), 8.519 (s, 1H), 8.013−8.00 (d, 1H, 7.8 Hz), 7.910−7.853 (m, 1H), 7.352−7.306 (m, 1H), 7.031−6.92 (t, 1H, 6.0 Hz), 4.485−4.445 (t, 2H, 6 Hz), 3.449−3.3.390 (m, 2H) ppm. 13C NMR (100 MHz, DMSO-d6): δ 156.1, 150.6, 150.0, 147.6, 137.6, 124.0, 123.3, 119.8, 78.5, 49.8, 28.6 ppm. Synthesis of 1-(4-Fluorophenyl)-3-(2-(4-(pyridin-2-yl)-1H-1,2,3triazol-1-yl)ethyl)urea (L). In an RB flux, 0.290 g (1.003 mmol) of tert-butyl (2-(4-(pyridin-2-yl)-1H-1,2,3-triazol-1-yl)ethyl)carbamate was dissolved in 20 mL of DCM. An excess amount of CF3COOH (2 mL) was dissolved in 10 mL of DCM and added dropwise via a pressure-equalizing funnel in the RB, kept at ∼0 °C. The reaction

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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.7b00473. Spectral characterization of each compound, solutionstate 1H NMR studies, ITC experiments, Pphotophysical studies with complex 1[PF6]2 and anions, extraction studies with anions, crystallographic details of complex 1[PF6]2, and selected bond distances and angles for complex 1[PF6]2 (PDF) Crystallographic data for complex 1[PF6]2 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for P.G.: [email protected]. 5379


Article

Inorganic Chemistry ORCID

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Pradyut Ghosh: 0000-0002-5503-6428 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.G. gratefully acknowledges the Science and Engineering Research Board (SERB; EMR/2016/000900) of India for financial support and the Alexander von Humboldt Foundation for donating a Fluorimeter. T.K.G. and B.C. acknowledge the CSIR for SRF.

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