BINOL-linked 1,2,3-triazoles: an unexpected fluorescent sensor with anion–π interaction for iodide ions

Abstract

A new family of cyclic and acyclic BINOL-derived triazoles has been prepared for the organocatalytic silylation and subsequent use of fluorescent sensors, in which this type of receptor can unexpectedly recognize I⁻ with good selectivity. The spectral analysis, including UV and NMR titrations, demonstrated that the anion–π interaction of I⁻ to the triazole ring was responsible for the formation of a weak charge-transfer complex.

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Since 2001/2002, the copper(I)-catalyzed Huisgen [3 + 2] cycloaddition¹ of terminal alkyne and azide reactants has been widely known as the “click”-reaction or CuAAC-reaction.^2,3 In particular, it has recently become one of the most important methods for heterocyclic ring formation because of its high atom-economic transformation and mild conditions; it has found widespread application in chemistry and advanced materials.⁴ For example, many molecules with triazole motifs are becoming increasingly important building blocks and linkers in drug and medicinal chemistry, as well as in catalysis.⁵ More interestingly, a broad variety of triazole-based compounds have been introduced as ligands in transition metal catalysis because they are thermally stable and can be easily achieved.⁶ In addition, the direct use of triazoles as organocatalysts was also disclosed in several transformations that were promoted by the progress of organocatalysis.⁷

We had previously reported an interesting example of divergent catalysis with a copper(I) catalyst, which was controlled by amine-functional macromolecular polysiloxanes (Scheme 1).⁸ In this work, we successfully demonstrated the interesting ability of the secondary amine-functional polysiloxane (AFP-L2) that led to oxidative coupling in the copper-mediated Huisgen reactions of general azides and alkynes with good yields and selectivities. Moreover, it is also surprising that the use of diamine-functional polysiloxane (AFP-L1) as a polymeric ligand led to classic Huisgen [3 + 2]-cycloadditions in excellent yields. However, the subsequent investigation of the catalytic application of these triazoles or bis-triazoles in homogeneous catalysis was not carried out. Inspired by the previous reports on the organocatalyzed kinetic resolution/silylation of alcohols,⁹ we herein carried out a study on the catalytic activity of various chiral triazoles in the asymmetric silylation of alcohol.

Scheme 1 Previous work: divergent synthesis of triazoles.¹²

The novel cyclic triazole dimer derivatives (Scheme 2) could be prepared from binaphthol (BINOL) according to previous reports,^8,10 in which the key step was the copper-promoted click reaction or the oxidative coupling from the dipropargyl and the substituted azide (see ESI†). The classical copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) of the monopropargyl and dipropargyl compounds with azide provided the desired mono- and di-click adducts (3, 6 and 2, 5, respectively) in good yields. In addition, the click reactions mediated by the polysiloxane-supported secondary amine allowed the preparation of novel heterocyclic compounds, such as bis-triazoles 1 and 4. All of these compounds 1–6 were characterized by NMR, HRMS and FTIR (also see ESI†).

Scheme 2 The cyclic and acyclic BINOL-based triazole derivatives.

Initially, we selected BINOL-linked triazoles 1 and 2 as model organocatalysts in the catalytic asymmetric kinetic resolution/silylation of 1-phenylethanol (8). As shown in Scheme 3, the promising results from the kinetic resolution/silylation of 1-phenylethanol with hexamethyldisilazane (HMDS) promoted by the BINOL-linked triazole (1) suggested that the Lewis basic triazole would prove to be a viable organocatalyst in this reaction. Notably, an enantiomeric excess of 1-phenylethanol was detected in the presence of a catalytic amount of BINOL-linked triazole (Entry 1 of Scheme 3, 5% ee of 8a by triazole 1), whereas the combination of TBAI with BINOL-linked triazole 1 or 2 led to poor conversion and no enantioselectivity (Entries 2, 4, and 6 of Scheme 3). Thus, the low reactivity of the BINOL-linked triazole and TBAI raised concerns about the possibility of quenching of the catalytic activity of BINOL-linked triazoles, in which the combined catalyst system might result in the formation of anion–π interactions of BINOL-linked triazole and TBAI. Inspired by the previous work on anion–π interactions¹¹ and the aforementioned experimental results,¹³ we hypothesized that the possible anion–π interactions of BINOL-linked triazoles with TBAI would lead to the establishment of BINOL-linked triazoles as an iodide ion sensor.

Scheme 3 Kinetic resolution of 1-phenylethanol (8) with HMDS (7) by (S)-BINOL-linked triazole-promoted catalytic asymmetric silylation.

Notably, the substituted triazoles have been used as chemosensors for various cations and anions in the past.^14–16 For example, the triazole-based sensors are selective for Cu²⁺, Hg²⁺, Ag⁺, Zn²⁺, Fe³⁺ and Cd²⁺,¹⁴ and few reports are known about the recognition of anions such as Br⁻, HP₂O₇³⁻ and SO₄²⁻.¹⁵ However, although this electron-poor heterocycle can be readily synthesized, the triazole unit has not been given sufficient attention in the research of fluorescent sensors, and accordingly, the interaction between an anion and the electron-deficient triazole system have been neglected as a foothold for the construction of efficient anion receptors. In the past decades, the development of fluorogenic chemosensors for selectively sensing anions has attracted considerable attention because of the fundamental roles played by anions in a wide range of biological, chemical, and environmental processes.¹⁷ Among the anions, the detection of iodide anions is of particular interest because of its biological activities, such as neurological and thyroid functions.¹⁸ However, only a few iodide anion sensors have been developed to date because of the lack of efficient receptors,¹⁹ mainly ascribed to the reason that the iodide anion possesses the characteristics of large size, low-charge density, and low hydrogen-bonding ability, thus the binding capacities of them with receptors are relatively weak. Therefore, it remains a challenge to design chemosensors that can selectively bind iodides.

Notably, almost all the reported triazole anion receptors were based on hydrogen bonding and size selectivity.²⁰ Simultaneously, there is a growing interest in anion–π interactions, which are the non-covalent contacts between an anion and an electron-deficient aromatic ring.²¹ Until now, only Caballero and Molina reported that the bis(triazolium)-based receptor can recognize the hydrogen pyrophosphate anion based on anion–π interaction and hydrogen bonding.²² Interestingly, to date, there has been no report on the selective detection of I⁻ based on click addition-generated triazoles.

Initially, it was found that all of the compounds in Scheme 2 gave strong fluorescent emissions at 355–370 nm. The fluorescent titration of compound 1 with I⁻ anion (TBAI) is shown in Fig. 1, where a notable fluorescence quenching at 355–367 nm was observed. The fluorescence intensity of 1 could be quenched by almost 94% with the addition of 150 equiv. of I⁻. The efficiency of quenching was further studied by plotting a modified Stern–Volmer plot, as shown in the inset of Fig. 1, and the fluorescent quenching coefficient K_sv is further calculated from the following equation:²³

lg(F₀ − F)/I = n lg K_a − n lg 1/{[Q₀] − [P₀] (F₀ − F)/F₀},

where F₀ and F represent the fluorescence intensity in the absence and presence of I⁻, respectively; [Q₀] is the molar concentration of the quencher I⁻; and [P₀] is the molar concentration of the ligand. K_sv is the quenching constant, and n is the ratio between iodide and the ligand. From the Stern–Volmer plot, the K_sv and n are calculated to be 8.6 × 10³ and 1.22, respectively, for Ligand 1, whereas for compound 4, the K_sv and n are 9.4 × 10³ and 1.06 (Table 1), respectively, which indicate that the promising anion-binding affinities exist possibly due to the anion–π interaction between I⁻ and the electron-deficient triazole ring system. From Table 1, it is clear that the K_sv of CH₂COOEt-substituted Ligands 4 and 6 are higher than that of the corresponding CH₂Ph-substituted 2 or 3. This is possibly because of the electron withdrawing ability of the CH₂COOEt-substituted Ligands 4 or 6, which can enhance the electron-withdrawing character of the triazole ring. This can then strengthen the binding affinities between I⁻ and the triazole unit. The quenching mechanism can be explained by the heavy-atom effect, e.g. from the formation of a charge-transfer complex as previously discussed. The “heavy-atom” interaction between the ground state of the triazole-containing complex and the inorganic anion (iodide) leads to an enhancement of the spin–orbit coupling and thus the associated fluorescence quenching.²⁴

Fig. 1 Fluorescence spectra of Ligand 1 (5 μM) in CH₃CN with the increasing concentration of I⁻, inset shows a modified Stern–Volmer plot of Ligand 1 upon the addition of I⁻ in CH₃CN.

Table 1 Quenching constants K_sv and binding sites n from the titration of Ligand 1–6 with TBAI^a

Ligand	Ksv (mol^−1)	n	R
a r is the regression coefficient.
1	8.6 × 10^3	1.22	−0.995
2	5.5 × 10^3	1.30	−0.996
3	7.1 × 10^3	1.22	−0.995
4	9.4 × 10^3	1.06	−0.972
5	7.5 × 10^3	1.09	−0.978
6	8.3 × 10^3	1.04	−0.980

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As shown in Fig. 2, the UV-vis spectra showed a broad absorption band centered at 234 nm. For compounds 2, 3, 4, 5 and 6, the absorption maxima were all observed at 231 nm. The titration of all ligands with I⁻ were also monitored by UV-vis spectroscopy. Upon the addition of I⁻, all the compounds showed a slight redshift of 13–15 nm. The redshift in the absorption can be explained by the formation of the charge-transfer complex between the donor (I⁻) and the π acceptor (the triazole ring). Simultaneously, a new peak was observed, improving at 212 nm upon the continuous addition of TBAI, which may be ascribed to the interaction of the iodide with the triazole ring.

Fig. 2 UV-vis titration spectra of Ligand 1 (5 μM) in CH₃CN with increasing concentration of I⁻ in CH₃CN.

The ¹H-NMR spectrum of compound 6 (Fig. 3) mainly displays four sets of signals. The first characteristic set of signals is ascribed to the naphthalene protons, which appeared at δ = 7.94, 7.88, 7.46, 7.34 and 7.22 ppm, respectively. The second set appeared as multiplets at δ = 7.13 ppm and as doublets at δ = 6.87 ppm due to the phenyl protons. The –CH proton of the triazole ring appears as a singlet at δ = 6.77 ppm. Finally, the three different signals of AB-quartet pattern protons from ArO–CH₂– and –CH₂–COOEt appear at δ = 5.26, 5.04 and 4.65 ppm, respectively. With the addition of 10 equiv. of TBAI to the compound 1–6 in CDCl₃, the solution color gradually turns from colorless to pale green, which indicates the formation of an anion–π interaction complex.

Fig. 3 ¹HNMR spectrum (500 MHz, CDCl₃) of Ligand 6 before (a and c) and after (b and d) addition of 10 equiv. of I⁻.

Fig. 3 further depicts the ¹HNMR spectra of compound 6 after the addition of TBAI. Obviously, all the signals were shifted up-field. These results agree well with the formation of 6–I complex: (1) the complexation with I⁻ will weaken the electron-withdrawing character of the triazole ring, and as a result, the ¹HNMR signals of triazole-CH₂ (three AB quartet) are shifted up-field (△δ = −0.094, −0.065 and −0.050 ppm). (2) The interaction with I⁻ will enhance the π electron density of the naphthalene and phenyl ring, which caused the up-field shift of the corresponding protons (△δ = −0.067, −0.069, and −0.069 for naphthalene, △δ = −0.076 and −0.076 ppm for phenyl). (3) In contrast, the triazole C–H only shows a small up-field shift (△δ = −0.040), apparently the C–H bonds play little role in recognition, i.e., almost no hydrogen bonding exists in the sensing process. Thus, the spectral analysis, including UV and NMR titration, demonstrated that anion–π interaction of I⁻ to the triazole ring was an indirect evidence for the deactivation in the catalytic silylation of alcohol with HMDS.

To obtain a better understanding of the observed florescent properties and molecule recognition with iodide, theoretical calculations were performed using a Gaussian 09 software package.²⁵ The electron density diagrams for HOMO and LUMO orbitals of BINOL-derived triazoles 1–3, as well as the BINOL-derived ether 9 (Φ_F = 0.15) showed that they exhibited poor selectivity for iodide.²⁶ Notably, the Φ_F values of 1, 2, 3, 4, 5, and 6 are 0.40, 0.47, 0.66, 0.60, 0.55 and 0.49, respectively. It is noteworthy that the electron density of both the HOMO and LUMO of these compounds is predominantly distributed along the binaphthyl unit. However, for the BINOL-derived triazole 3 containing a benzyl group and a triazole, the HOMO level has the charge localized on the benzyl ring, while the LUMO level has charge distributed along the naphthyl ring that linked with benzyl ether. This feature indicates that the energies of the orbitals could be affected by the triazole and benzyl units. These electronic effects and fluorescent analysis/molecule recognition of the BINOL-derived triazoles would be beneficial for the molecular design of task-directed fluorescent sensors in the near future (Table 2).

Table 2 Frontier orbital shape and energy (in eV) calculated at B3LYP/6-31G(d,p) level of theory¹⁵

	LUMO (eV)	HOMO (eV)
(9) ΦF = 0.15
	−0.939	−5.289
BINOL-derived triazole 3 (see Scheme 2) ΦF = 0.66
	−1.180	−5.610
BINOL-derived triazole 2 (see Scheme 2) ΦF = 0.47
	−1.310	−5.581
BINOL-derived triazole 1 (see Scheme 2) ΦF = 0.40
	−1.091	−5.547

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Conclusions

In summary, compelling experimental evidence for the existence of anion–π interactions in this molecular recognition system is directly provided by the model kinetic resolution/silylation of 1-phenylethanol with hexamethyldisilazane. With or without the addition of TBAI, the catalytic activity of BINOL-linked triazoles dramatically varied because of the contributions from anion–π interactions. On the basis of the catalytic activity of BINOL-linked triazoles in the asymmetric silylation of alcohol, we firstly discovered that a new type of fluorescent sensor with a cyclic or acyclic BINOL skeleton containing triazole groups showed high fluorescence emission and high selective recognition for iodide. The spectral analysis, including UV and NMR titration, demonstrated that anion–π interaction between I⁻ and the triazole ring was responsible for the formation of a weak charge-transfer complex. Therefore, we have demonstrated the ability of BINOL-derived triazoles as sensors to detect iodide ions in solution, and the fluorescence is almost completely quenched by the heavy-atom effect. This fluorescence recognition of I⁻ possesses the following features: (1) the sensing processes are constructed on the basis of anion–π interaction between the I⁻ donor and the electron-deficient triazole ring. (2) All of the BINOL-derived triazole sensors 1–6 exhibit good selectivity toward I⁻, which supported the triazole unit to be a promising backbone in the molecular design of the fluorescent sensor for selective interaction with iodide. Thus, the novel investigation in this manuscript was catalysis-initiated fluorescent analysis, which would be an attractive concept in the analytical chemistry. Undoubtedly, the evidence that triazole-containing BINOL-derived sensors have the functionality of anion–π recognition may contribute to metal-free organocatalysis in the near future, which opens new perspectives for the construction of supramolecular complexes with anions.

Acknowledgements

Financial support by the National Natural Science Foundation of China (NSFC Grant no. 21173064, 51303043 and 21472031), Zhejiang Provincial Natural Science Foundation of China (LR14B030001), and Hangzhou Science and Technology Program (20130432B06) is appreciated.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra09589h

‡ J. F. Zou and C. Y. Wang contributed equally to this work.

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