Dinuclear Zinc(II) Macrocyclic Complex as Receptor for Selective

Article pubs.acs.org/IC

Dinuclear Zinc(II) Macrocyclic Complex as Receptor for Selective Fluorescence Sensing of Pyrophosphate Lígia M. Mesquita,† Vânia André,‡ Catarina V. Esteves,† Tiago Palmeira,§ Mário N. Berberan-Santos,§ Pedro Mateus,*,† and Rita Delgado*,† †

Instituto de Tecnologia Química e Biológica António Xavier, Universidade Nova de Lisboa, Avenida da República, 2780-157 Oeiras, Portugal ‡ Centro de Química Estrutural, Instituto Superior Técnico, and §Centro de Química-Física Molecular, Instituto Superior Técnico, Universidade de Lisboa, Avenida Rovisco Pais, 1049-001 Lisboa, Portugal S Supporting Information *

ABSTRACT: A new diethylenetriamine-derived macrocycle known as L, bearing 2-methylquinoline arms and containing m-xylyl spacers, was prepared in good yield by a one-pot [2 + 2] Schiff base condensation procedure, followed by reduction with sodium borohydride. Up to now this is the first hexaazamacrocycle with appended fluorophore units. Single-crystal X-ray diffraction determination of the dinuclear zinc(II) complex of L showed that metal centers are located at about 7.20(2) Å from one another. This complex exhibits only weak fluorescence in aqueous solution, but the addition of 1 equiv of pyrophosphate (PPi) caused a 21-fold enhancement of the fluorescence intensity. The sensor response is linear up to a value of 10 μM HPPi3− and has a detection limit of 300 nM. The receptor behaves as a highly selective sensor for pyrophosphate as other anions, including phosphate, phenylphosphate (PhP), adenosine monophosphate (AMP), adenosine diphosphate (ADP), and adenosine triphosphate (ATP), failed to induce any fluorescence change and practically do not affect the fluorescence intensity of the sensor in the presence of HPPi3−. Competition titrations carried out in aqueous solution at pH 7.4 [in 20 mM 3-(N-morpholino)propanesulfonic acid (MOPS) buffer] by spectrofluorometry revealed a high association constant value of 6.22 log units for binding of PPi by the dinuclear zinc(II) receptor, one of the highest reported values for colorimetric/fluorometric sensors able to work under real aqueous physiological conditions, while association constant values for binding of the other phosphorylated substrates are in the 5.51−4.03 log unit range.

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INTRODUCTION

adenosine triphosphate (ATP), is still proving very difficult to achieve.6 Chemosensors for PPi able to perform in aqueous solutions have in common a receptor unit that may rely on hydrogen bonding, electrostatic interactions, or coordination bonds (or combinations of these) to selectively bind the anion. These receptors can be covalently attached to a fluorescent or colorimetric reporter7a−n (see representative examples a−c in Chart 1) or can provide a binding site for a fluorescent or colorimetric indicator that should be displaced by the analyte7o−s (example d in Chart 1). Dinuclear metal complexes of ditopic macrocyclic compounds have been shown to display high affinity toward PPi owing to their ability to bind this substrate in a bridging manner, forming cascade complexes.8 However, these complexes lack the ability to report the binding event. With this in mind, and with the knowledge that quinoline-based zinc(II) complexes have interesting fluorescent properties,9 in this work a new

Binding and sensing of phosphorylated substrates by synthetic receptors with high selectivity in aqueous medium is still one of the greatest challenges of supramolecular chemistry.1 Among them, pyrophosphate (PPi) has been one of the most pursued targets because it plays crucial roles in numerous biological processes, such as energy storage, signal transduction, DNA/ RNA polymerization, and muscle contraction, and it controls metabolic processes through participation in various enzymatic reactions.2 Additionally, pyrophosphate has become vital as a biomarker for several diseases like chondrocalcinosis and arthritis and for the clinical diagnosis and therapy of cancer.3,4 Therefore, it is not surprising that researchers have focused their attention on detection and discrimination of pyrophosphate, with many fluorescent and colorimetric chemosensors being described in recent years.5,6 However, to date only a few chemical sensors can detect PPi under real aqueous physiological conditions.7 In addition, its selective sensing over other phosphorylated species, including phosphate, adenosine monophosphate (AMP), adenosine diphosphate (ADP), and © XXXX American Chemical Society

Received: November 10, 2015

A

DOI: 10.1021/acs.inorgchem.5b02596 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Chart 1. Examples of Metal Complex Chemosensorsa

a

(a−c) With fluorescent or colorimetric reporter covalently bound; (d) with an external fluorescent or colorimetric indicator.

Chart 2. Representation of Dinuclear Zinc(II) Complex of L and Phosphorylated Substrates Studied in This Work

diethylenetriamine-derived macrocycle known as L, containing m-xylyl spacers and bearing 2-methylquinoline arms, was synthesized and studied. Chart 2 schematically represents the dinuclear zinc(II) complex of L and the phosphorylated anions studied. The L compound is the first hexaazamacrocycle containing appended fluorophore units described until now, and furthermore, its dinuclear zinc(II) complex selectively detects PPi by fluorescence in 100% aqueous solution.

pH value for the complete complexation (Figure S1 in Supporting Information). Thus, ligand L was titrated with a Zn2+ solution, buffered at pH = 7.4, and the corresponding spectral changes were monitored by both UV and fluorescence spectrometries (Figure 1). The absorption spectra consist of a band at 230 nm and three less intense bands at 292, 304, and 317 nm, as commonly observed in quinoline-based compounds.9 The fluorescence spectra consist of a broad band centered at 380 nm. At pH = 7.4, the ligand exhibits almost no emission [fluorescence quantum yield 1.1 × 10−3, close to that of the model compound fluorophore 2-(chloromethyl)quinoline; see Table S1 in Supporting Information]. Significant spectral changes were observed in the UV spectra upon addition of Zn2+ to L ligand (Figure 1a), with the observation of two clear isosbestic points. Concomitantly, a chelation enhancement of fluorescence (CHEF) was observed upon increasing the Zn/L molar ratio (Figure 1b), caused both by a 5-fold increase of fluorescence quantum yield (to 5.4 × 10−3) and by the increase of the absorption coefficient at the excitation wavelength (Figure 1a). On the other hand, the average fluorescence lifetimes of both ligand and zinc complex are nearly identical, 3.8 and 4.0 ns. The fluorescence quantum yield increase as opposed to the near constancy of the lifetimes, which are dominated by the nonradiative decay constant, can be explained by an increase of the radiative rate constant for fluorescence, in agreement with the observed increase in the absorption coefficient of the

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RESULTS AND DISCUSSION Synthesis of Ligand. Hexaazamacrocycles are typically prepared by reaction of a dialdehyde with a triamine, followed by reduction with sodium borohydride. In the present case, a 2-methylquinoline group was incorporated in the central nitrogen atom of the diethylenetriamine, protected in the form of phthtalimide, followed by deprotection with hydrazine in ethanol/chloroform at room temperature (rt). Then, the L compound was prepared in one pot through [2 + 2] cyclization reaction between isophthaldehyde and 4-(2-quinolinylmethyl)1,4,7-triazaheptane, followed by reduction with sodium borohydride (Scheme 1), as described for related compounds containing hydroxyethyl10 and methylimidazole11 pendants. Study of Zinc(II) Complexes of L. The effect of pH on fluorescence of the zinc(II) complexes (at 2:1 Zn2+/L ratio) was studied in order to choose the appropriate pH value for subsequent studies. Fluorescence starts increasing at pH ≈ 6.5 and reaches a maximum at about pH 7.4, which is the optimum B


Article

Inorganic Chemistry Scheme 1. Synthetic Procedure Used To Obtain L

Figure 1. (a) UV absorption and (b) fluorescence (λexc = 302 nm; excitation slit 20 nm; emission slit 10 nm) spectra of L in the presence of increasing amounts of Zn(ClO4)2 in aqueous solution buffered with 3-(N-morpholino)propanesulfonic acid (MOPS; 20 mM) at pH 7.4 and T = 298.2 K. CL = 50 and 20 μM, respectively.

first absorption band (Figure 1a and Table S1 in Supporting Information). Fitting of the changes in absorbance and fluorescence by means of the HypSpec program12 allowed us to determine the effective stepwise constants (Keff) at pH 7.4 for the formation of mono- and dinuclear zinc(II) complexes of L, with values of 6.23 ± 0.06 and 4.85 ± 0.06 log units, respectively. Single-Crystal X-ray Diffraction Studies. Crystals were grown by slow evaporation of an aqueous solution containing ligand L and 2 equiv of Zn(NO3)2 at pH 7.2. However, the crystal structure determined by single-crystal X-ray diffraction data revealed the presence of carbonate anions formed by atmospheric CO2 fixation, as observed with other polynuclear zinc(II) complexes described in the literature.13 The molecular structure of the complex cation of [Zn4L2(CO3)3]2NO3 is shown in Figure 2, along with the relevant atomic notation adopted. The asymmetric unit contains two crystallographically independent [Zn4L2(CO3)3]2+ cations, each composed of two dinuclear complexes bridged by one carbonate anion (Figure 2).

Several water molecules are also in the structure, but due to their high disorder it was not possible to accurately determine the hydration level. Although four dinuclear complexes are present in the asymmetric unit, they differ only slightly in bond lengths and angles. Thus, henceforth the discussion will be focused on only one of the dinuclear macrocyclic complexes. Selected distances and angles given in Table 1 are ranges of the values found in the four dinuclear complexes. The ligand is bound to each Zn center by three nitrogen atoms of a diethylenetriamine subunit and a quinolyl nitrogen atom, and the metal coordination sphere is completed by the binding of oxygen atoms of carbonate anions. The Zn1 coordination environment can be described as a strongly distorted octahedron due to the binding of a carbonate anion in a bidentate mode. Zn2, on the other hand, is pentacoordinate, adopting a square-pyramidal geometry. The intermacrocycle Zn1···Zn1′ distance of 5.271(1)− 5.291(1) Å is much smaller than the Zn1···Zn2 intramacrocycle separation of 7.132(1)−7.179(1) Å. The macrocycle adopts a conformation not suitable for accommodation of a carbonate anion in a bridging mode. Nonetheless, C


Article

Inorganic Chemistry

Figure 2. (a) X-ray crystal structure of the complex cation of [Zn4L2(CO3)3]2NO3; (b) detail of half unit of the complex cation.

Table 1. Selected Distances and Angles in Coordination Spheres of [Zn4L2(CO3)3]2NO3 Complexa Distances (Å) Zn1−N1 Zn1−N28 Zn1−N4 Zn1−N41 Zn1−O1 Zn1−O2 Zn2−N19 N1−Zn1−O2 N4−Zn1−N41 N28−Zn1−O1 N1−Zn1−N4 N1−Zn1−N28 N1−Zn1−N41 N1−Zn1−O1 N28−Zn1−N41 N28−Zn1−O2 N4−Zn1−N28 N4−Zn1−O2 N4−Zn1−O1 O1−Zn1−O2 a

2.132(8)−2.160(7) 2.039(5)−2.078(5) 2.137(5)−2.163(5) 2.187(6)−2.229(6) 2.455(4)−2.589(5) 2.082(4)−2.123(5) 2.138(4)−2.176(4)

Zn2−N16 Zn2−N13 Zn2−N52 Zn2−O6 Zn2−O5 Zn1−Zn2 Zn2−Zn2 (same complex) Angles (deg)

150.4(3)−154.4(2) 145.0(2)−148.3(3) 167.8(2)−172.2(2) 82.4(2)−83.7(2) 85.1(3)−86.0(3) 77.0(3)−77.4(3) 97.2(3)−99.6(2) 92.0(3)−95.4(2) 117.5(2)−122.8(2) 109.0(2)−111.6(2) 94.2(2)−96.4(2) 76.9(2)−82.7(2) 52.8(2)−55.5(2)

O1−Zn1−N41 O2−Zn1−N41 N16−Zn2−O4 N19−Zn2−N52 N16−Zn2−N19 N16−Zn2−N13 N16−Zn2−N52 N13−Zn2−N19 N19−Zn2−O4 N52−Zn2−O4 N52−Zn2−N13 O4−Zn2−N13

2.118(4)−2.187(5) 2.030(5)−2.075(4) 2.242(5)−2.404(4) 1.951(4)−1.948(3) 2.398(4)−2.655(4) 7.132(1)−7.179(1) 5.271(1)−5.291(1) 75.9(2)−80.1(2) 89.8(2)−91.3(2) 160.7(2)−168.2(2) 143.2(2)−147.1(2) 80.4(2)−81.6(2) 84.9(2)−86.2(2) 72.8(2)−76.1(2) 107.3(2)−109.9(2) 100.8(2)−107.6(2) 91.9(2)−97.6(2) 91.6(2)−96.7(2) 105.6(2)−106.5(2)

Values presented correspond to the minimum and maximum observed.

the fluorescence spectrum of the [Zn2L]4+ receptor was characterized by a very weak emission band centered at 380 nm (Figure 1b). Upon addition of 1 equiv of HPPi3− to the solution of [Zn2L]4+ receptor, a remarkable 21-fold enhancement of fluorescence intensity was observed, as well as a blue shift of 10 nm. The estimated fluorescence quantum yield of the HPPi complex is 0.13. As observed with the zinc complexation effect, the average fluorescence lifetimes of both [Zn2L]4+ and its complex with HPPi3− are again nearly identical, 4.0 and 3.8 ns, respectively, confirming that the emitting species is essentially the same in both cases, notwithstanding the observed 10 nm emission blue shift (see Tables S1 and S2 in Supporting Information). The increase in fluorescence quantum yield can thus be again attributed to a significant increase of the radiative decay constant of the complexed quinoline fluorophore. Further addition of HPPi3− caused a decrease of fluorescence intensity (Figure S5 in Supporting Information), which may be ascribed to removal of zinc(II) from the complex by the HPPi3− anion. The fluorescence intensity follows a linear dependence on HPPi3− concentration (Figure S6 in Supporting Information) up to a value of 10 μM and with a detection limit of

one of the carbonate anions is coordinated to Zn1 and almost encapsulated into the macrocyclic cavity, and it is reasonable to assume that, in solution and in the presence of stoichiometric amounts of anion, the carbonate may bridge the two zinc centers. This is supported by the electrospray ionization mass spectra (ESI-MS), performed by dissolving the crystals in water, which revealed the presence of the [Zn2L(CO3)]2+ complex cation and no trace of the [Zn4L2(CO3)3]2+ one (see Figure S2 in Supporting Information). Another feature supporting encapsulation of the anion in bridging mode was found by crystals grown in aqueous solutions of L but in the presence of copper(II) salt instead of zinc(II). Unfortunately, the data obtained lacked the required characteristics for publication due to poor quality of the crystals, although the overall structural features of the complex can be ascertained; see Figure S3 in Supporting Information. Anion Binding by Dinuclear Zinc(II) Complex of L. The effect of addition of anionic substrates on the fluorescence spectrum of the dinuclear zinc(II) complex of L was examined at pH 7.4 in aqueous solution buffered with 20 mM MOPS and at 298.2 K (Figure S4 in Supporting Information). As previously mentioned, in the absence of an anionic substrate, D


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Inorganic Chemistry

As expected, monophosphorylated anions, such as PhP2−, HPO42−, and HAMP−, have very similar effective association constant values, as they must have similar binding modes. The diphosphorylated HADP2− is bound with an effective association constant of the same order as the monophosphate anions, which suggests that this anion is only coordinated through one of the phosphoryl groups. Surprisingly, the triphosphorylated anion HATP3−, in spite of having the same charge as HPPi3−, has a much smaller effective association constant value, which is most likely related to a steric hindrance effect caused by the bulky quinoline pendant arms. The fact that HPPi3− is the only studied anion causing fluorescence enhancement of the receptor, which act as a sensor for this anion, is certainly related to very strong binding to the receptor, which leads to an important effect on excited-state intramolecular charge transfer in the quinoline ring. The blue shift of the fluorescence band of the sensor upon binding of HPPi3− is also in agreement with a decrease in electron density at the nitrogen in the quinoline ring. Consequently, all the other studied anions do not bind strongly enough to cause a significant change of excited-state charge transfer in the quinoline ring and consequent fluorescence enhancement.

300 nM (S/N = 3), a value of similar magnitude when compared with other PPi sensors in the literature.7 Furthermore, no fluorescence change was observed in the presence of any of the other anions studied, including HPO42−, PhP2−, HAMP−, HADP2−, and HATP3−, indicating that [Zn2L]4+ receptor acts as a selective fluorescent sensor for HPPi3− in aqueous solution buffered at pH 7.4 with MOPS. Moreover, the fluorescence intensity of the sensor in the presence of HPPi3− is practically unaffected by the presence of 1 equiv of competing anions (see Figure 3).

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CONCLUSIONS Two quinoline units were appended to a diethylenetriaminederived hexaazamacrocycle, L, for the first time. Moreover, it was found in this work that the compound L, when converted in a dinuclear zinc complex, acts as a selective fluorescent sensor for pyrophosphate, in aqueous solution and at physiological pH. The thermodynamically stable dinuclear zinc(II) complex, although exhibiting only weak fluorescence in aqueous solution, upon addition of 1 equiv of HPPi3− experiences a 21-fold enhancement of the fluorescence intensity. The fluorescence intensity follows a linear dependence on HPPi3− concentration up to a value of 10 μM, with a detection limit of 300 nM. The sensor response is highly selective, as other anions including HPO42−, PhP2−, HAMP−, HADP2−, and HATP3− failed to induce any fluorescence change and practically do not affect the fluorescence intensity of the sensor in the presence of HPPi3− anion. The very high association constant value of 6.22 log units found for binding of HPPi3− by the dinuclear zinc(II) complex of L is one of the largest reported values among the few colorimetric/fluorometric sensors able to work under real aqueous physiological conditions. This, along with the lower association constant values for binding of the other phosphorylated substrates, greatly contributes to the selectivity observed. The present study shows that the well-established ability of dinuclear metal complexes of ditopic macrocycles to exhibit high affinity toward PPi in aqueous physiological conditions8 can be made more useful when the fluorophore reporter is directly appended to the macrocyclic framework. Unfortunately, the short excitation and emission wavelengths preclude the usefulness of the sensor for in vivo conditions. Ongoing work in our group is focusing on appending fluorophores with higher excitation/emission wavelengths in macrocyclic architectures and will be reported in due time.

Figure 3. Fluorescence intensity change of [Zn2L]4+ (blue) upon addition of each anion (red bars) and upon addition of HPPi3− in the presence of 1 equiv of each anion (hatched bars) in aqueous solution buffered at pH 7.4 (20 mM MOPS) and at 25 °C. CR = CA = 20 μM. λexc = 302 nm; excitation slit 10 nm; emission slit 5 nm.

Competition spectrofluorometric titrations14 carried out at pH 7.4 in buffered aqueous solution with 20 mM MOPS (see Figures S7−S12 in Supporting Information) revealed the high association constant value of 6.22 log units for the binding of HPPi3− by dinuclear zinc(II) receptor, which is one of the highest reported values for colorimetric/fluorometric sensors able to work under real aqueous physiological conditions.7i,n,o Association constant values for the binding of other phosphorylated substrates are in the 5.51−4.03 log unit range (see Table 2). Table 2. Effective Association Constantsa log Keffb

equilibrium reaction 4+

[Zn2L] [Zn2L]4+ [Zn2L]4+ [Zn2L]4+ [Zn2L]4+ [Zn2L]4+

+ + + + + +

PhP ⇌ [Zn2L(PhP)] HPO42− ⇌ [Zn2L(HPO4)]2+ HPPi3− ⇌ [Zn2L(HPPi)]+ HAMP− ⇌ [Zn2L(HAMP)]3+ HADP2− ⇌ [Zn2L(HADP)]2+ HATP3− ⇌ [Zn2L(HATP)]+ 2−

2+

4.70(1) 4.03(4) 6.22(1) 4.18(3) 5.00(1) 5.51(1)

a

Keff values for the indicated equilibria were determined by fluorescence spectroscopy in aqueous solution at pH = 7.4 buffered with 20 mM MOPS and at 298.2 K. bValues in parentheses are standard deviations in the last significant figures.

The strong binding of HPPi3− can be ascribed to its higher negative charge but also to the fact that the size of this anion fits better to the distance of the two zinc centers by coordination of two oxygen atoms from the two phosphoryl groups while causing minimum steric strain on the system. This was also suggested by Martell and co-workers15 when a related macrocyclic copper(II) complex was used as receptor for the mentioned anion.

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EXPERIMENTAL SECTION

General Considerations. All solvents and reagents used in synthesis were commercially purchased reagent-grade quality and were used as supplied without further purification, except 1,5-diphthalimido3-azapentane, which was prepared according to literature methods.16 E


Article

Inorganic Chemistry

concentrated HClO4 (3.4 mL) was added dropwise, under stirring. The precipitate formed was filtered off and washed with cold CH3OH. 1 H NMR (400 MHz, D2O) δ (ppm) 2.91 (8H, t, J = 6.0 Hz, NCH2CH2NH), 3.30 (8H, t, J = 6.0 Hz, NCH2CH2NH), 3.36 (4H, s, NCH2 xyl), 4.35 (8H, s, NCH2QN), 7.09 (4H, d, J = 7.56 Hz, H2 and H4 xyl), 7.15−7.18 (2H, m, H3 xyl), 7.24 (2H, s, H6 xyl), 7.48−7.50 (2H, m, H4 QN), 7.50−7.52 (2H, m, 2 H, H8 QN), 7.63 (2H, t, J = 7.77 Hz, H7 QN), 7.76 (2H, d, J = 8.08 Hz, H9 QN), 7.92 (2H, d, J = 8.48 Hz, H6 QN), 8.06 (2H, d, J = 8.68 Hz, H3 QN). Anal. Calcd for C44H58N8.6ClO4·2CH3OH: C, 40.6; H, 4.9; N, 8.2. Found: C, 41.00; H, 5.18; N, 8.63. The precipitate was then dissolved in water and NaOH was added until pH 13. The solution was extracted with chloroform (3 × 50 mL), and the organic layer was dried with anhydrous Na2SO4 and evaporated to give a yellow oil. Yield 2.25 g (61%). 1H NMR (400 MHz, CDCl3) δ (ppm) 1.89 (4H, s, NH), 2.71 (16H, m, NCH2CH2NH), 3.64 (8H, s, NCH2 xyl), 3.91 (4 H, s, NCH2QN), 7.09 (4H, d, J = 7.56 Hz, H2 and H4 xyl), 7.17 (2H, t, J = 6.0 Hz, H3 xyl), 7.24 (2H, s, H6 xyl), 7.50 (2H, d, J = 8.0 Hz, H3 QN), 7.50 (2H, t, J = 8.0 Hz, H8 QN), 7.63 (2H, t, J = 8.0 Hz, H7 QN), 7.76 (2H, d, J = 8.0 Hz, H9 QN), 7.92 (2H, d, J = 8.0 Hz, H6 QN), 8.06 (2H, d, J = 8.0 Hz, H4 QN). 13C NMR (100 MHz, CDCl3) δ (ppm) 47.27 (NCH2CH2NH), 53.96 (NHCH2 xyl), 54.61 (NCH2CH2NH), 62.37 (NCH2QN), 121.06 (C3 QN), 126.27 (C8 QN), 126.77 (C2, C4 xyl), 127.50 (C5 QN), 127.69 (C6 xyl, C9 QN), 128.40 (C3 xyl), 129.17 (C6 QN), 129.56 (C7 QN), 136.58 (C4 QN), 140.58 (C1 xyl), 147.76 (C10 QN), 160.77 (C2 QN). ESI-MS (CH3OH) m/z 693.3 [M + H]+, 731.2 [M + K]+. Crystals of [Zn4L2(CO3)3]2NO3. Ligand L (2.1 mg, 3 μmol) was dissolved in aqueous solution (3 mL) containing Zn(NO3)2 and KNO3 (0.002 and 0.1 M, respectively). The pH was adjusted to 7.2 and the mixture was allowed to slowly evaporate at rt. Colorless single crystals suitable for X-ray crystallographic determination were obtained within 7 days. ESI-MS (H2O, pH = 7.0) m/z 946.5 [Zn2L(CO3) (NO3)]+, 1010.5 [Zn2L(NO3)3]+ (see Figure S2 in Supporting Information). Zinc(II) Complexes of L: Ultraviolet and Fluorescence Titrations. Solutions of H6L·6ClO4 were prepared in degassed Milli-Q water buffered to pH 7.4 (0.020 M MOPS) at 5.0 × 10−5 M for UV and 2.0 × 10−5 M for fluorescence measurements. Stock solutions of Zn(ClO4)2 were prepared at 2.50 × 10−3 M and 5.0 × 10−4 M, respectively. Titrations were performed at 298.2 ± 0.1 K in 1 cm quartz cells. Volumes of Zn(ClO4)2 stock solution were added by means of Hamilton syringe to a stock solution of H6L·6ClO4 (2.50 mL). After each addition the solutions were kept stirring for 10 min, and then the spectra were recorded. For each titration, 16−27 spectra were acquired. The pH value was verified before and after the titrations. Effective stability constant values (Keff) for formation of mono- and dinuclear zinc(II) complexes were determined by use of the HypSpec program.12 Errors quoted are standard deviations of the overall constants given directly by the program for the input data, which include all experimental points. Anion Binding Studies. Fluorescence Titrations. All stock solutions were prepared in degassed Milli-Q water and buffered to pH 7.4 (0.020 M MOPS). A stock solution of [Zn2L]4ClO4 was prepared at 8.0 × 10−5 M by mixing H6L·6ClO4 and 2 equiv of Zn(ClO4)2 and raising the pH to 7.4. Stock solutions of the anionic substrates (potassium salts of HPPi3−, HPO42−, PhP2−, HATP3−, HADP2−, and HAMP−) were prepared at 5.0 × 10−4 M. Titrations were performed at 298.2 ± 0.1 K in 1 cm quartz cell. The fluorescence emission spectrum of a MOPS solution (0.020 M) was considered the blank reading. Volumes were added by means of Hamilton syringe. After each addition the solutions were kept stirring for 10 min, after which spectra were recorded in the 275−450 nm region (λexc = 302 nm). For each titration, 18−27 spectra were acquired. The pH value was verified before and after each titration. ESI-MS spectra recorded for aqueous solutions at pH = 7.4 of the receptor [Zn2L]4+ with HPPi3− and HATP3− anions, respectively, confirm that the receptor bind the anions (see Figures S24−S27 in Supporting Information).

NMR spectra used for characterization of products were recorded on a Bruker Avance III 400 (1H at 400.13 MHz and 13C at 100.61 MHz). The reference used for 1H NMR measurements in CDCl3 was tetramethylsilane (TMS), and in D2O, 3-(trimethylsilyl)-propanoic acid-d4 sodium salt was used. Peak assignments are based on peak integration and multiplicity for one-dimensional (1D) 1H spectra and in correlation spectroscopy (COSY), nuclear Overhauser enhancement spectroscopy (NOESY), and heteronuclear multiple quantum coherence (HMQC) experiments (Figures S13−S23 in Supporting Information). Microanalyses were carried out by ITQB Microanalytical Service. UV−vis absorption spectra were recorded with a PerkinElmer Lambda 45 spectrophotometer and fluorescence spectra on Cary Eclipse and Horiba Jobin-Yvon Fluorolog 3 spectrofluorometers. For fluorescence quantum yield determinations, anthracene in ethanol (ΦF = 0.27) was used as the reference.17 Time-resolved fluorescence intensity decays were obtained by the single-photon timing method with laser excitation and microchannel plate detection, with the setup described in ref 18. Excitation and emission wavelengths were 302 and 370 nm, respectively. Caution! Although no problems were found during this work with perchlorate salts, these compounds should be considered potentially explosive. Syntheses. 3-(2-Quinolinylmethyl)-1,5-diphthalimido-3-azapentane. A mixture of 1,5-diphthalimido-3-azapentane (6.43 g, 17.7 mmol), quinoline-2-carbaldehyde (2.78 g, 17.7 mmol), and sodium triacetoxyborohydride (5.45 g, 25.7 mmol) in dichloroethene (DCE; 65 mL) was stirred under nitrogen atmosphere and at rt for 12 h. Then a 2 M NaOH solution (65 mL) was added, the organic layer was extracted, and the aqueous phase was washed with dichloromethane (DCM; 3 × 50 mL). The organic layers were dried over with anhydrous Na2SO4, which was filtered off, and the solvent was evaporated to dryness to give the desired product, which was recrystallized from CH3OH. Yield 5.74 g (64%). 1H NMR (400 MHz, CDCl3) δ (ppm) 2.92 (4H, t, J = 6.0 Hz, NCH2CH2NPhth), 3.80 (4H, t, J = 6.0 Hz, NCH2CH2NPhth), 4.00 (2H, s, NCH2QN), 7.20 (1H, d, J = 8.0 Hz, H3 QN), 7.43 (1H, d, J = 8.0 Hz, H9 QN), 7.46 (1H, t, J = 8.0 Hz, H7 QN), 7.58 (1H, d, J = 8.0 Hz, H6 QN), 7.65−7.71 (9H, m, PhthN + H8 QN), 7.97 (1H, d, J = 12 Hz, H4 QN). 4-(2-Quinolinylmethyl)-1,4,7-triazaheptane. 3-(2-Quinolinylmethyl)-1,5-diphthalimido-3-azapentane (5.74 g, 11.4 mmol) was dissolved in a mixture of ethanol (260 mL) and CHCl3 (52 mL). Hydrazine monohydrate (7.2 mL, 148 mmol) was added and the solution was stirred for 24 h at rt. Then the solvents were evaporated to give a yellow oil that was dissolved in water. The pH was raised to about 12 with addition of 6 M KOH, and the mixture was extracted with CHCl3 (3 × 50 mL). The organic layer was dried with anhydrous Na2SO4 and evaporated to give a yellow oil. Yield 2.60 g (94%). 1H NMR (400 MHz, CDCl3) δ (ppm) 1.36 (4H, s, NH), 2.65 (4H, t, J = 6.0 Hz, NCH2CH2NH2), 2.80 (4H, t, J = 6.0 Hz, NCH2CH2NH2), 3.94 (2H, s, NCH2QN), 7.52 (1H, t, J = 8.0 Hz,, H8 QN), 7.62 (1H, d, J = 8.0 Hz, H3 QN), 7.69 (1H, t, J = 8.0 Hz, H7 QN), 7.80 (1H, d, J = 8.0 Hz, H9 QN), 8.04 (1H, d, J = 8.0 Hz, H6 QN), 8.13 (1H, d, J = 8.0 Hz, H4 QN). 13C NMR (100 MHz, CDCl3) δ (ppm) 40.00 (NCH2CH2NH2), 58.22 (NCH2CH2NH2), 62.07 (NCH2QN), 121.03 (C3 QN), 126.34 (C8 QN), 127.50 (C5 QN), 127.71 (C9 QN), 129.11 (C6 QN), 129.63 (C7 QN), 136.38 (C4 QN), 147.70 (C10 QN), 160.89 (C2 QN). 6,20-(Quinolin-2-ylmethyl)-3,6,9,17,20,23-hexaazatricyclo[23.3.1.111,15]triaconta-1(29),11,13,15(30),25,27-hexaene, L. A solution of isophthalaldehyde (1.44 g, 10.7 mmol) in CH3OH (100 mL) was added dropwise to a solution of 4-(2-quinolinylmethyl)-1,4,7triazaheptane (2.60 g, 10.7 mmol) in CH3OH (100 mL). The mixture was allowed to react for 12 h at rt. Solid NaBH4 (8.09 g, 214 mmol) was added, and the mixture was left under stirring at rt until the bubbling ceased and then under reflux for 3 h. The solution was filtered off and evaporated to dryness. Then water (50 mL) was added to the mixture, and the solution was made strongly basic with 6 M KOH and extracted with DCM (3 × 80 mL). The organic layer was dried with anhydrous Na2SO4 and evaporated to give a yellow oil. This oil was dissolved in the least possible amount of CH3OH and then F


Article

Inorganic Chemistry Solutions of the [Zn2L]4+ receptor were first titrated with PhP2− by addition of volumes of stock solution of this anion to solutions of [Zn2L]4ClO4 (2.500 mL) in order to determine the respective association constant. The remaining association constants were determined by competition experiments between PhP2− and each of the other anions by the receptor. The experiments were carried out by addition of PhP2− stock solution to a solution of [Zn2L]4+ together with the competing anion in equimolar amounts (2.0 × 10−5 M). Association constants of the remaining anions were determined by use of the HypSpec program,12 by considering as constant the previously determined Keff value for association of [Zn2L]4+ with PhP2−. Errors quoted are standard deviations of the overall constants given directly by the program for the input data, which include all experimental points from all titration curves. Single-Crystal X-ray Diffraction. Crystals of [Zn4L2(CO3)3]2NO3 suitable for X-ray diffraction study were mounted with Fomblin in a cryoloop. Data were collected on a Bruker AXS-Kappa Apex II diffractometer with graphite-monochromated radiation (Mo Kα, λ = 0.710 73 Å) at 150 K. The X-ray generator was operated at 50 kV and 30 mA and X-ray data collection was monitored by the APEX219 program. All data were corrected for Lorentzian, polarization, and absorption effects by use of SAINT19 and SADABS19 programs. SIR9720 and SHELXS-9721 were used for structure solution and SHELXL-97 for full-matrix least-squares refinement on F2. These three programs are included in the package of programs WINGX-version 1.80.05.22 Non-hydrogen atoms were refined anisotropically. Fullmatrix least-squares refinement was used for non-hydrogen atoms with anisotropic thermal parameters. All the hydrogen atoms were inserted in idealized positions and allowed to refine in the parent carbon atom. Squeeze was used to deal with the high number of disordered water molecules present in the asymmetric unit. Restraints had to be used to refine the nitrate ions. Low crystal quality led to a low observed/ unique ratio and precluded a better refinement. Molecular diagrams presented are drawn with PyMOL.23 PLATON24 was used to calculate bond distances and angles as well as hydrogen-bond interactions. Table 3 summarizes data collection and refinement details. CCDC-1432771 contains supplementary crystallographic data for this Article. These data can be obtained free of charge from

The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac. uk/data_request/cif or as Supporting Information.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02596. Two tables listing fluorescence properties of solutions; 27 figures showing 1D and 2D NMR and ESI mass spectra of the L compound, ESI mass spectra of the complexes, and fluorescence spectra recorded on the course of the titrations (PDF) Crystallographic data for Zn4L2(CO3)3]2NO3 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.M.). *E-mail: [email protected] (R.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Fundaçaõ para a Ciência e a Tecnologia (FCT) for the financial support under Project PTDC/QEQSUP/2718/2012 including the fellowship of L.M. We also acknowledge support by FCT (RECI/BBB-BQB/0230/2012) for the NMR spectrometers as part of the National NMR Facility. The X-ray facilities thank FCT for funding (RECI/ QEQ-QIN/0189/2012). M. C. Almeida from the ITQB Analytical Services Unit is acknowledged for providing elemental analysis and ESI-MS data. P.M., V.A., and C.V.E. acknowledge FCT for fellowships SFRH/BPD/79518/2011, SFRH/BPD/78854/2011, and SFRH/BD/89501/2012, respectively.

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Table 3. Crystallographic Data and Experimental Details for [Zn4L2(CO3)3]2NO3 compound chemical formula formula wt temp, K wavelength, Å crystal form and color crystal size, mm crystal system space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z d, mg·cm−3 μ, mm−1 θ range, deg reflns collected/unique Rint GOF final R indices [I > 2σ(I)]

ASSOCIATED CONTENT

S Supporting Information *

REFERENCES

(1) Hargrove, A. E.; Nieto, S.; Zhang, T.; Sessler, J. L.; Anslyn, E. V. Chem. Rev. 2011, 111, 6603−6782. (2) Heinonen, J. K. Biological Role of Inorganic Pyrophosphate; Kluwer Academic Publishers: Norwell, MA, 2001. (3) Wuthier, R. E.; Bisaz, S.; Russell, R. G. G.; Fleisch, H. Calcif. Tissue Res. 1972, 10, 198−206. (4) Xu, S. Q.; He, M.; Yu, H. P.; Cai, X. K.; Tan, X. L.; Lu, B.; Shu, B. H. Anal. Biochem. 2001, 299, 188−193. (5) Kim, S. K.; Lee, D. H.; Hong, J.-I.; Yoon, J. Acc. Chem. Res. 2009, 42, 23−31. (6) Lee, S.; Yuen, K. K. Y.; Jolliffe, K. A.; Yoon, J. Chem. Soc. Rev. 2015, 44, 1749−1762. (7) (a) Anbu, S.; Kamalraj, S.; Paul, A.; Jayabaskaran, C.; Pombeiro, A. J. L. Dalton Trans. 2015, 44, 3930−3933. (b) Hai, Z.; Bao, Y.; Miao, Q.; Yi, X.; Liang, G. Anal. Chem. 2015, 87, 2678−2684. (c) Wang, J.H.; Xiong, J.-B.; Zhang, X.; Song, S.; Zhu, Z.-H.; Zheng, Y.-S. RSC Adv. 2015, 5, 60096−60100. (d) Bhowmik, S.; Ghosh, B. N.; Marjomäki, V.; Rissanen, K. J. Am. Chem. Soc. 2014, 136, 5543−5546. (e) Huang, F.; Feng, G. RSC Adv. 2014, 4, 484−487. (f) Anbu, S.; Kamalraj, S.; Jayabaskaran, C.; Mukherjee, P. S. Inorg. Chem. 2013, 52, 8294−8296. (g) Xu, Q.-c.; Wang, X.-f.; Xing, G.-w.; Zhang, Y. RSC Adv. 2013, 3, 15834−15841. (h) Zhu, W.; Huang, X.; Guo, Z.; Wu, X.; Yu, H.; Tian, H. Chem. Commun. 2012, 48, 1784−1786. (i) Yang, S.; Feng, G.; Williams, N. H. Org. Biomol. Chem. 2012, 10, 5606−5612. (j) Roy, B.; Rao, A. S.; Ahn, K. H. Org. Biomol. Chem. 2011, 9, 7774−7779. (k) Ravikumar, I.; Ghosh, P. Inorg. Chem. 2011, 50, 4229−4231. (l) Wen, J.; Geng, Z.; Yin, Y.; Zhang, Z.; Wang, Z. Dalton Trans. 2011,

[Zn4L2(CO3)3]2NO3 2[C91H104Zn4N16O9]4[NO3] 3902.98 150(2) 0.710 73 block, blue 0.12 × 0.06 × 0.04 triclinic P1̅ 22.323(2) 24.481(2) 24.687(2) 111.030(5) 91.991(5) 101.809(5) 12 239(2) 2 1.059 0.830 0.890−26.804 50 685/19 567 0.0845 0.859 R1 = 0.0771, wR2 = 0.2231 G


Article

Inorganic Chemistry 40, 1984−1989. (m) Lee, H. N.; Xu, Z.; Kim, S. K.; Swamy, K. M. K.; Kim, Y.; Kim, S.-J.; Yoon, J. J. Am. Chem. Soc. 2007, 129, 3828−3829. (n) Lee, D. H.; Kim, S. Y.; Hong, J.-I. Angew. Chem., Int. Ed. 2004, 43, 4777−4780. (o) Lee, D. H.; Im, J. H.; Son, S. U.; Chung, Y. K.; Hong, J.-I. J. Am. Chem. Soc. 2003, 125, 7752−7753. (p) Lee, J. H.; Jeong, A. R.; Jung, J.-H.; Park, C.-M.; Hong, J.-I. J. Org. Chem. 2011, 76, 417− 423. (q) Yu, W.; Qiang, J.; Yin, J.; Kambam, S.; Wang, F.; Wang, Y.; Chen, X. Org. Lett. 2014, 16, 2220−2223. (r) Svane, S.; Kjeldsen, F.; McKee, V.; McKenzie, C. J. Dalton Trans. 2015, 44, 11877−11886. (s) Zwicker, V. E.; Long, B. M.; Jolliffe, K. A. Org. Biomol. Chem. 2015, 13, 7822−7829. (8) (a) Qin, L.; Reibenspies, J. H.; Carroll, R. I.; Martell, A. E.; Clearfiled, A. Inorg. Chim. Acta 1998, 270, 207−215. (b) Nation, D. A.; Martell, A. E.; Carroll, R. I.; Clearfield, A. Inorg. Chem. 1996, 35, 7246−7252. (9) (a) Mikata, Y.; Ugai, A.; Ohnishi, R.; Konno, H. Inorg. Chem. 2013, 52, 10223−10225. (b) Mikata, Y.; Sato, Y.; Takeuchi, S.; Kuroda, Y.; Konno, H.; Iwatsuki, S. Dalton Trans. 2013, 42, 9688− 9698. (c) Meng, X.; Wang, S.-X; Zhu, M. Quinoline-Based Fluorescence Sensors. In Molecular Photochemistry - Various Aspects; Saha, S., Ed.; InTech: 2012; DOI: 10.5772/2058. (d) Ichimura, C.; Shiraishi, Y.; Hirai, T. Tetrahedron 2010, 66, 5594−5601. (e) Mameli, M.; Aragoni, M. C.; Arca, M.; Atzori, M.; Bencini, A.; Bazzicalupi, C.; Blake, A. J.; Caltagirone, C.; Devillanova, F. A.; Garau, A.; Hursthouse, M. B.; Isaia, F.; Lippolis, V.; Valtancoli, B. Inorg. Chem. 2009, 48, 9236−9249. (f) Nolan, E. M.; Lippard, S. J. Acc. Chem. Res. 2009, 42, 193−203. (g) Williams, N. J.; Gan, W.; Reibenspies, J. H.; Hancock, R. D. Inorg. Chem. 2009, 48, 1407−1415. (h) Mikata, Y.; Yamanaka, A.; Yamashita, A.; Yano, S. Inorg. Chem. 2008, 47, 7295−7301. (i) Aragoni, M. A.; Arca, M.; Bencini, A.; Blake, A. J.; Caltagirone, C.; De Filippo, G.; Devillanova, F. A.; Garau, A.; Gelbrich, T.; Hursthouse, M. B.; Isaia, F.; Lippolis, V.; Mameli, M.; Mariani, P.; Valtancoli, B.; Wilson, C. Inorg. Chem. 2007, 46, 4548−4559. (j) Shiraishi, Y.; Ichimura, C.; Hirai, T. Tetrahedron Lett. 2007, 48, 7769−7773. (k) Mikata, Y.; Wakamatsu, M.; Kawamura, A.; Yamanaka, N.; Yano, S.; Odani, A.; Morihiro, K.; Tamotsu, S. Inorg. Chem. 2006, 45, 9262−9268. (l) Wu, D.-Y.; Xie, L.-X.; Zhang, C.-L.; Duan, C.-Y.; Zhao, Y.-G.; Guo, Z.-J. Dalton Trans. 2006, 3528−3533. (m) Mikata, Y.; Wakamatsu, M.; Yano, S. Dalton Trans. 2005, 545−550. (n) Gan, W.; Jones, S. B.; Reibenspies, J. H.; Hancock, R. D. Inorg. Chim. Acta 2005, 358, 3958− 3966. (o) Kimber, M. C.; Mahadevan, I. B.; Lincoln, S. F.; Ward, A. D.; Tiekink, E. R. T. J. Org. Chem. 2000, 65, 8204−8209. (p) Prodi, L.; Bolletta, F.; Montalti, M.; Zaccheroni, N. Coord. Chem. Rev. 2000, 205, 59−83. (10) Yang, D. X.; Li, S. A.; Li, D.-F.; Tang, W. X. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 2002, 58, o11−o13. (11) Qi, Z.-P.; Cai, K.; Yuan, Q.; Okamura, T.-a.; Bai, Z.-S.; Sun, W.Y.; Ueyama, N. Inorg. Chem. Commun. 2010, 13, 847−851. (12) Gans, P.; Sabatini, A.; Vacca, A. Ann. Chim. 1999, 89, 45−49. (13) (a) Liu, X.; Du, P. W.; Cao, R. Nat. Commun. 2013, 4, 2375. (b) Notni, J.; Schenk, S.; Görls, H.; Breitzke, H.; Anders, E. Inorg. Chem. 2008, 47, 1382−1390. (c) Trösch, A.; Vahrenkamp, H. Inorg. Chem. 2001, 40, 2305−2311. (d) Adams, H.; Bradshaw, D.; Fenton, D. E. J. Chem. Soc., Dalton Trans. 2001, 3407−3409. (e) Mao, Z.-W.; Liehr, G.; van Eldik, R. J. Chem. Soc., Dalton Trans. 2001, 1593−1600. (f) Mao, Z.-W.; Heinemann, F. W.; Liehr, G.; van Eldik, R. J. Chem. Soc., Dalton Trans. 2001, 3652−3662. (g) Dietrich, J.; Heinemann, F. W.; Schrodt, A.; Schindler, S. Inorg. Chim. Acta 1999, 288, 206−209. (h) Schrodt, A.; Neubrand, A.; van Eldik, R. Inorg. Chem. 1997, 36, 4579−4584. (i) Bazzicalupi, C.; Bencini, A.; Bencini, A.; Bianchi, A.; Corana, F.; Fusi, V.; Giorgi, C.; Paoli, P.; Paoletti, P.; Valtancoli, B.; Zanchini, C. Inorg. Chem. 1996, 35, 5540−5548. (j) Ehlers, N.; Mattes, R. Inorg. Chim. Acta 1995, 236, 203−207. (k) Chen, X.-M.; Deng, Q.Y.; Wang, G.; Xu, Y.-J. Polyhedron 1994, 13, 3085−3089. (l) Looney, A.; Han, R.; McNeill, K.; Parkin, G. J. Am. Chem. Soc. 1993, 115, 4690−4697. (m) Murthy, N. N.; Karlin, K. O. J. Chem. Soc., Chem. Commun. 1993, 1236−1238. (n) Kitajima, N.; Hikichi, S.; Tanaka, M.; Moro-oka, Y. J. Am. Chem. Soc. 1993, 115, 5496−5508. (o) Kajiwara, T.; Yamaguchi, T.; Kido, H.; Kawabata, S.; Kuroda, R.; Ito, T. Inorg.

Chem. 1993, 32, 4990−4991. (p) Looney, A.; Parkin, G.; Alsfasser, R.; Ruf, M.; Vahrenkamp, H. Angew. Chem., Int. Ed. Engl. 1992, 31, 92−93. (q) Kato, M.; Ito, T. Inorg. Chem. 1985, 24, 504−508. (14) Due to the fact that an excess of HPPi3− competes with L for Zn2+ and to the expected high value of its association constant, competition titrations were preferred. See the Experimental Section for more details. (15) Nation, D. A.; Martell, A. E.; Carroll, R. I.; Clearfield, A. Inorg. Chem. 1996, 35, 7246−7252. (16) Ng, C. Y.; Motekaitis, R. J.; Martell, A. E. Inorg. Chem. 1979, 18, 2982−2986. (17) (a) Dawson, W. R.; Windsor, M. W. J. Phys. Chem. 1968, 72, 3251−3260. (b) Suzuki, K.; Kobayashi, A.; Kaneko, S.; Takehira, K.; Yoshihara, T.; Ishida, H.; Shiina, Y.; Oishi, S.; Tobita, S. Phys. Chem. Chem. Phys. 2009, 11, 9850−9860. (18) Menezes, F.; Fedorov, A.; Baleizao, C.; Valeur, B.; BerberanSantos, M. N. Methods Appl. Fluoresc. 2013, 1, 015002. (19) Bruker Analytical Systems, Madison,WI, 2005. (20) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115−119. (21) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (22) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837−838. (23) PyMOL Molecular Graphics System, version 1.2r3pre, Schrödinger LLC. (24) Spek, A. L. Acta Crystallogr., Sect. D: Biol. Crystallogr. 2009, 65, 148−155.

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Dinuclear Zinc(II) Macrocyclic Complex as Receptor for Selective

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