Ferrocene-containing macrocyclic triazoles for the electrochemical sensing of dihydrogen phosphate anion

Inorganica Chimica Acta 449 (2016) 31–37

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Ferrocene-containing macrocyclic triazoles for the electrochemical sensing of dihydrogen phosphate anion Chun-Tao Li a, Qian-Yong Cao a,⇑, Jia-Jin Li a, Zhong-Wei Wang b,⇑, Bo-Na Dai a a b

Department of Chemistry, Nanchang University, Nanchang 330031, PR China College of Material Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, PR China

a r t i c l e

i n f o

Article history: Received 19 February 2016 Received in revised form 24 April 2016 Accepted 25 April 2016 Available online 30 April 2016 Keywords: Macrocyclic receptor Triazole Ferrocene Dihydrogen phosphate Electrochemical sensing

a b s t r a c t Two novel ferrocene-containing macrocyclic triazoles (L1 and L2) and their acyclic analogs (L3 and L4) were easily prepared by ‘‘click” reaction. The anions binding abilities of L1–L4 were evaluated by cyclic voltammetry and differential pulse voltammetry methods. The results revealed that the receptors L1–L4 have exclusive electrochemical sensing of H2PO
4 , with the shift in redox potentials of L1–L4 towards the 1 cathodic values. The binding mechanisms between the receptors and H2PO
4 were examined by H NMR titrations and density functional theory (DFT) calculations. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction There is a growing interest in anions recognition due their ubiquitous roles in many chemical and biochemical processes, and some of them are also of great environmental and medical concerns [1,2]. Among them, phosphates such as dihydrogen phosphate, phosphate or pyrophosphate, are of particular interest due to their key roles in fields of information processes, energy storage, and signal transduction [3,4]. Therefore, many man-made receptors that incorporate different anion binding sites (neutral N–H donors, cationic (N–H)+ or (C–H)+ donors) and different signaling subunits (optical or electrochemical units) have been developed to achieve a sensitive and selective detection of phosphates [5–17]. The so called ‘‘click” reaction, a Cu(I)-catalyzed cycloaddition reaction involving azides and alkynes, is widely used in anions supramolecular chemistry due to its high efficiency and mild reaction conditions [18,19]. The resulting 1,2,3-triazole ring has a substantial 5D dipole along the C–H bond that creates an effective C– H  anions hydrogen bonding, which could further be enhanced by converting the triazole unit into a triazolium cation [20,21]. Some triazole-based receptors such as macrocycles [22–24], foldamers

⇑ Corresponding authors. Tel.: +86 791 83969514; fax: +86 791 83969386. E-mail addresses: [email protected] (Q.-Y. Cao), wangzhongwei@fusilinchem. com (Z.-W. Wang). http://dx.doi.org/10.1016/j.ica.2016.04.047 0020-1693/Ó 2016 Elsevier B.V. All rights reserved.

[25–27] and short flexible oligomers [28–30], have particularly higher affinities and selectivities towards chloride anions. Other 1,2,3-triazole-linked dendrimers and polymers for various phosphates recognition properties have also been reported [31–33]. Ferrocene is one of the most useful electrochemically active signaling subunits due to its strong p-donating ability and good reversibility when it displays a one-electron oxidation at a desirable range. Many ferrocene-based receptors for anions recognition have been reported, which reveal a large shift in the redox potential of the ferrocene/ferrocenium redox couple toward cathodic potential when target anions are added to the media [34–40]. In addition, triazole functionalized ferrocenes for anions recognition, especially phosphate ions, have also been reported [41–44]. However, most of them are limited to acyclic systems. Macrocyclic triazoles for electrochemical sensing of anions are rarely investigated. The macrocyclic receptors often show elevated affinities and selectivities in anion complexation due to their high degree of preorganization and rigidity. This study investigates two ferrocene-functionalized macrocycles (L1 and L2, see Scheme 1) and their acyclic analogs (L3 and L4) through the click reaction. The anionic recognition abilities of the receptors L1–L4 are examined in detail by electrochemical methods (cyclic voltammetry and differential pulse voltammetry) and 1H NMR spectrum. The binding mechanisms of the receptors toward H2PO
4 are studied by density functional theory (DFT) calculations.

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C.-T. Li et al. / Inorganica Chimica Acta 449 (2016) 31–37

O

O

N3

Toluene

+

Fe

1

O

2

L2

CuI/DBU

O

4

O N3

O

N N N

N3

+

Fe

O

Toluene Fe

+

3

L1

N N N

N N N

O

N3

Fe CuI/DBU

N3

O

1

N N N

N N N

Fe

DMF

L3

Fe CuSO4·5H2O/NaVc O

5

N N N

O

3

N N N

O

Fe

Fe

DMF

+

Fe

L4

CuSO4·5H2O/NaVc N N N

6

O

Fe

Scheme 1. The synthetic route of the receptors L1–L4.

2. Experimental

2.3. X-ray crystallographic analysis

2.1. Materials and instrumentation

Single crystals of L1 were obtained by direct diffusion of hexane into the CH2Cl2 solution containing L1. A suitable single crystal was mounted on a glass fibber, and the diffraction measurements are taken with a Bruker Smart APEX CCD-based diffractometer using a Mo Ka graphite monochromated radiation. The structure was solved by direct methodology using a SHELXL-97 program [49]. The data refinement and all further calculations are performed using

All the starting materials for synthesis were commercially available and used as received. The precursors 1,8-bis[(2-propyn-1yloxy)methyl]naphthalene (1), 1,10 -bis(azidomethyl)ferrocene (2), 1,8-bis(azidomethyl)naphthalene (3), 1,10 -bis[(2-propyn-1-yloxy) methyl]ferrocene (4), azidomethylferrocene (5) and (2-propyn-1yloxy)methylferrocene (6) were prepared following the methods reported in the literature [45–48]. The electrochemical measurements were performed with a CHI 624C instruments. NMR spectra were recorded using a Varian instrument (400 MHz). The anions ions are perchlorate salts, and were dissolved in CH2Cl2 solution. 2.2. Preparation of the electrochemical titrations The electrochemical measurements were carried out in a onecompartment cell under a nitrogen atmosphere at 25 °C, equipped with a Pt disk working electrode, a platinum wire counter electrode, and an Ag/AgNO3 (0.1 M in CH3CN solution) reference electrode. The platinum working electrode surface was carefully polished with an Al2O3–water slurry, washed with MeOH then sonicated in a H2O–MeOH–CH3CN 1:1:1 mixture at 40 °C for 15 min prior to use. The redox potentials were recorded in CH2Cl2 containing [n-Bu4N]ClO4 (0.10 M) as the supporting electrolyte and are quoted relative to Ag/AgNO3 (0.01 M in CH3CN solution). The utilized scan rate for cyclic voltammetry is 100 mV s
1, and the differential pulse voltammetry (DPV) measurements were performed with pulse width of 50 ms. The concentration of the receptors in the CV/DPV titration was 0.2 mM dissolved in the CH2Cl2 solution.

SHELXL-97

[50]. The non-H atoms were refined anisotropically using weighted full matrix least-squares on F2. Crystallographic data for L1: C30H28FeN6O2, M = 506.43, monoclinic, space group P2(1)/c, a = 6.059(1) Å, b = 28.391(5) Å, c = 15.291(3) Å, b = 100.285(2)°, V = 2588.0(8) Å3, room temperature, Z = 4, l = 0.624 mm
1, q = 1.438 g cm
3, crystal size 0.30  0.25  0.20 mm3, 6464 reflections collected with 4186 being independent; the final R1 and wR (F2) values were 0.0634 and 0.1777, respectively; goodness-of-fit on F = 1.052. The CIF deposition number of L1 is CCDC 1468829. 2.4. DFT calculations The optimized structures are obtained using the GAUSSIAN 09 program [51]. The calculations are performed using a hybrid exchange functional B3LYP [52,53] equipped with a CPCM treatment of the solvent, and the structures were characterized by computation of the vibrational frequencies. A 6-31G (d) basis set was used for both C and H, and 6-31+G (d, p) was used for N, O, S, and P. For the Fe atom, a Lanl2dz basis set was employed [54]. 2.5. Synthesis of L1 Under a nitrogen atmosphere, 1.522 mL (10.0 mmol) of 1,8-diaza[5.4.0]bicycloundec-7-ene (DBU) and 28.5 mg (0.015 mmol)

C.-T. Li et al. / Inorganica Chimica Acta 449 (2016) 31–37

of CuI were added to 200 mL dry toluene. The solution was heated to 70 °C then a mixture of 1 (132 mg, 0.5 mmol) and 2 (148 mg, 0.5 mmol) in dry toluene (100 mL) was added dropwise over 10 h. The mixture was stirred for another 4 h under nitrogen then was cooled to room temperature. The filtrate was concentrated in vacuum to get the crude product, which was processed through a column chromatography (SiO2, CH2Cl2/MeOH, 99:1, v/v) to obtain a pure yellow solid compound L1 (115 mg, 41% yield). 1H NMR (CDCl3): d (ppm) = 7.85 (d, J = 7.9 Hz, 2H), 7.68 (d, J = 6.9 Hz, 2H), 7.61 (s, 2H), 7.45 (t, J = 7.6 Hz, 2H), 5.11 (s, 4H), 5.01 (s, 4H), 4.72 (s, 4H), 4.16 (s, 4H), 3.98 (s, 4H). 13C NMR (100 MHz, CDCl3): d (ppm) = d 145.2, 136.1, 133.6, 131.4, 131.1, 130.7, 125.2, 123.6, 84.0, 73.1, 69.4, 69.1, 63.5, 49.1. ESI-TOF-MS: 560.1627 (M+1), calcd for C30H28FeN6O2 = 560.1623. M.p.: 158–160 °C. IR (KBr, cm
1): 3130, 3080, 2930, 2868, 1949, 1637, 1368, 1112, 1068, 987, 818. 2.6. Synthesis of L2 The compound L2 was synthesized with a 40% yield using a similar method as L1. Yield: 40%. 1H NMR (DMSO-d6): d (ppm) = 8.07 (d, J = 7.9 Hz, 2H), 7.96 (s, 2H), 7.56 (t, J = 7.9 Hz, 2H), 7.25 (d, J = 6.9 Hz, 2H), 6.11 (s, 4H), 4.50 (s, 4H), 4.10 (d, J = 8.5 Hz, 8H), 4.00 (s, 4H). ESI-TOF-MS: 560.1638 (M+1), calcd for C30H28FeN6O2 = 560.1623. M.p.: 202–204 °C. IR (KBr, cm
1): 3103, 3061, 2932, 2851, 2090, 1646, 1372, 1126, 1051, 998, 853. 2.7. Synthesis of L3 Precise amounts of 1 (132 mg, 0.5 mmol), 3 (290 mg, 1.2 mmol), CuSO45H2O (0.1 mmol, 25 mg) and sodium ascorbate (0.15 mmol, 29.5 mg) were added into 5 mL DMF under a nitrogen atmosphere. The mixture was stirred at room temperature for 2 h, and then water (20 ml) was added to form a precipitate. The crude product was filtered off and purified by column chromatography (SiO2, ethyl acetate) to get a yellow solid compound L3 (Yield 86%). 1H NMR (CDCl3): d (ppm) = 7.88 (d, J = 8.2 Hz, 2H), 7.60 (d, J = 7.0 Hz, 2H), 7.46 (dd, J = 9.4, 5.6 Hz, 4H), 5.30 (s, 4H), 5.14 (s, 4H), 4.67 (s, 4H), 4.31 (s, 4H), 4.24 (s, 4H), 4.23 (d, J = 6.6 Hz, 10H). 13C NMR (100 MHz, CDCl3): d (ppm) = 145.0, 135.8, 133.2, 131.3, 130.6, 125.0, 122.1, 80.7, 73.2, 69.2, 69.0, 68.8, 63.1, 50.0. ESITOF-MS: 747.0060 (M+1), calcd for C40H38Fe2N6O2 = 746.1755. M. p.: 172–174 °C. IR (KBr, cm
1): 3142, 3093, 2924, 2861, 1955, 1649, 1379, 1117, 1042, 924, 799.

33

2.8. Synthesis of L4 The compound L4 was obtained with 85% yield following a similar synthetic procedure as L3. 1H NMR (CDCl3): d 8.01 (d, J = 8.2 Hz, 2H), 7.59–7.51 (m, 2H), 7.44 (d, J = 6.9 Hz, 2H), 7.27 (s, 2H), 6.07 (s, 4H), 4.60 (s, 4H), 4.37 (s, 4H), 4.22 (s, 4H), 4.16 (s, 4H), 4.14 (s, 10H). 13C NMR (100 MHz, CDCl3): d (ppm) = 145.7, 137.3, 132.0, 125.8, 77.4, 77.0, 76.7, 70.5, 68.7, 68.8, 68.7, 68.5, 63.2, 55.1. ESITOF-MS: 747.0085 (M+1), calcd for C40H38Fe2N6O2 = 746.1755. M. p.: 69–71 °C. IR (KBr, cm
1): 3136, 3093, 2930, 2869, 1963, 1779, 1317, 1111, 1036, 936, 811. 3. Results and discussion 3.1. Synthesis and characterization The synthetic route of the receptors L1–L4 was shown in Scheme 1. The cyclic receptors L1 and L2 were prepared by the click reaction diazoles (1 and 4) with equal amounts of di-alkynes (2 and 3) under highly diluted toluene solution, using CuI/DBU (1,8-diaza[5.4.0]bicycloundec-7ene) as catalysts. The yields of L1 and L2 were about 40%. In contrast, acyclic receptors L3 and L4 were synthesized by the typical ‘click’ conditions in the presence of CuSO45H2O/sodium ascorbate as the catalysts. The L3 and L4 were obtained with higher yields (85%). It is found that the receptors L1 and L3 are soluble in most organic solvents, such as CHCl3, CH2Cl2, MeOH, DMF and DMSO, but the receptors L2 and L4 show poor solubilities in organic solution, even in DMSO solution. Their chemical structures were fully characterized by 1H NMR, 13C NMR and ESI-MS spectra. The receptor L1 was further characterized by X-ray crystallography. The X-ray quality single crystals of L1 were obtained by slow diffusion hexane into its CH2Cl2 solution, and its ortep structures are shown in Figs. 1 and S1. It can be seen that structure of L1 is not flat, and both the triazole rings adopt a trans conformation. The distance between the two triazole rings is about 6.32 Å, and that of Fe1–C10 is 6.28 Å, respectively. The large cavity present in the structure of L1 may match the size of an H2PO
4 anion when a strong binding is established (see below). 3.2. Anions sensing properties The sensing abilities of L1–L4 toward various anions of were examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) [55] methods in a 0.2 mM CH2Cl2 solution

Fig. 1. The ortep structure of L1 with top view. The hydrogen atoms except in triazole CH were omitted for clarity.

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C.-T. Li et al. / Inorganica Chimica Acta 449 (2016) 31–37

containing 0.1 M [n-Bu4N]ClO4 as a supporting electrolyte. As shown in Fig. 2 and Table 1, the free receptors L1–L4 depict a ferrocene-based one-electron reversible wave with the half-wave potentials (E1/2) of 419 mV, 246 mV, 340 mV and 244 mV for respectively L1, L2, L3, and L4. The relatively high potential obtained with L1 and L3 in comparison with those of L2 and L4 could be attributed to effect of the substituting species. In L1 and L3, the appended substituent of ferrocene in L1 and L3 is an electron withdrawing triazole unit, while that of L2 and L4 is an electron donor alkane ether unit. The electron withdrawing substituent renders the oxidation process of the appended ferrocene more difficult. The addition of 2.0 equivalents H2PO
4 anions in the form of a tetrabutylammonium salt, shifts the Fe(II)/Fe(III) half-potential (DE1/2) towards the cathodic potentials by 220 mV, 80 mV, 240 mV and 200 mV for L1, L2, L3 and L4, respectively (Table 1, Figs. 3 and S2). However, the addition of the anions F
, Cl
and AcO
shows insignificant shifting in potentials. The large shift in potential indicates a strong interaction between the receptors L1–L4 and the H2PO
4 anion through the triazole C–H  anions hydrogen bonding. As a result, the electron density on the anion is transferred to the ferrocenyl center of the hosts, inducing a facile oxidation of the ferrocene moiety of the receptor into ferrocenium

L4

L3

2

L1

L2

ion. The triazole-based receptors often show good binding abilities toward phosphate anions [41–44,56]. The CV and DPV titrations of the receptors L1–L4 in presence of the H2PO
4 anion were investigated (Figs. 4, S3 and S4). The addition of H2PO
4 results in a ‘‘two wave behavior” for L1 with a decrease in the initial half-redox potential (E1/2) at 420 mV and an increase in intensity of the new redox peak located at 200 mV. The appearance of a second redox peak is attributed to the formation of a complex L1H2PO
4 . The CV and DPV titrations experiments also revealed that the shift in potential reached saturation at about 1.0 equiv of H2PO
4 , indicating the formation a 1:1 stoichiometry of the host:guest adduct. The same ‘‘two wave behavior” with a 1:1 complexation stoichiometries is also observed for L3 and L4 with H2PO
4 present. By contrast, the addition of H2PO
4 to L2 induces a ‘‘shifting behavior” phenomenon, where the original redox wave undergoes a shift in potential. The different redox processes observed with each receptor in presence of H2PO
4 , ‘‘shifting behavior” for L2 with a smaller potential shift (DE1/2 =
80 mV) and a ‘‘two wave behavior” for L1, L3 and L4 with larger potential shifts, could be attributed to different binding mechanisms between the hosts and H2PO
4 (details are shown in DFT analyzes). The binding abilities of L1–L4 toward the anions in other solvents such as CH3CN are also investigated and the results are shown in Figs. S5 and S6. Upon addition of H2PO
4 , a similar electrochemical response with a shift in potentials towards the cathodic values is observed in CH3CN. However, the redox peaks of the

I/µA

1

(a)

0

2 1

-1

0

I/µA

-2 -3 0.0

0.2

0.4

0.6

-1 -2

E/V

-3

Fig. 2. The CV profile of L1–L4 in CH2Cl2 solution at the concentration of 0.2 mM.

-4 -0.1

Table 1 The redox potential of L1–L4 before and after the addition of various anions (2.0 equiv). Epa–Epc (mV)

DE1/2 (mV)

0.419 0.199 0.407 0.387 0.414

87 262 107 137 115

220 12 32 5

L2 L2H2PO
4 L2Ac
L2F
L2Cl


0.246 0.167 0.237 0.254 0.231

87 130 135 122 120

79 9 8 15

L3 L3H2PO
4 L3Ac

L3F L3Cl


0.34 0.1 0.348 0.331 0.329

105 238 103 95 85

240 8 9 11

L4 L4H2PO
4 L4Ac

L4F L4Cl


0.244 0.044 0.233 0.231 0.240

85 204 85 81 77

200 11 13 4

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

E/V

(b)

2 1

I/µA

E1/2 (V) L1 L1H2PO
4 L1Ac

L1F L1Cl


L1 L1+2eq AcO L1+2eq CI L1+2eq F L1+2eq Pi

0 -1 -2

-0.2

L2 L2+2eq AcO L2+2eq Cl L2+2eq F L2+2eq Pi

0.0

0.2

0.4

E/V Fig. 3. The CVs of L1 (a) and L2 (b) before and after the addition of various anions (2.0 equiv) in CH2Cl2 solution containing 0.2 mM L1 or L2. Pi denotes H2PO
4.

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C.-T. Li et al. / Inorganica Chimica Acta 449 (2016) 31–37

(a)

2 1

-0.5 -1.0

I/µA

I/µA

0

0.0

(b)

0eq Pi 0.5eq Pi 1eq Pi 1.5eq Pi 2eq Pi

-1

-1.5 -2.0

-2 -3 -0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

-2.5 0.0

0.7

0 eq Pi 0.2 eq Pi 0.4 eq Pi 0.6 eq Pi 0.8 eq Pi 1.0 eq Pi 1.5 eq Pi 2.0 eq Pi 0.1

0.2

0.4

0.5

0.6

E/V

E/V

(c)

0.3

(d)

2

0

1

I/µA

I/µA

-1 0 0 eq Pi 0.5 eq Pi 1.0 eq Pi 1.5 eq Pi 2.0 eq Pi

-1

-2 -0.3

-0.2

-0.1

0.0

-2

0.1

E/V

0.2

0.3

0.4

0.5

-3 -0.2

0 eq Pi 0.2 eq Pi 0.4 eq Pi 0.6 eq Pi 0.8 eq Pi 1.0 eq Pi 1.5 eq Pi 2.0 eq Pi -0.1

0.0

0.1

0.2

0.3

0.4

0.5

E/V

Fig. 4. The CVs and DPVs of L1 (a and b) and L2 (c and d) upon addition of the various amount of H2PO
4 (Pi) in CH2Cl2 solution containing 0.2 mM L1 or L2.



Fig. 5. The 1H NMR spectra of L1 (8.0 mM) upon addition H2PO
4 in CDCl3 solution. The marked N peak after addition of H2PO4 can be attributed to OH protons of H2PO4 .

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Fig. 6. The optimized geometries of L1H2PO
4 (a), L2H2PO4 (b), L3H2PO4 (c) and L4H2PO4 (d). The hydrogen bonds between the host and guest were listed in figure.

receptors become more irreversible upon interaction. The latter could be attributed to the deposition of thick layers of the receptor species on the working electrode surface through adsorption. To gain a better understanding of the binding properties of the receptors L1 and L3 with H2PO
4 anions, further evaluation was performed by 1H NMR spectrum in CDCl3 solution. Fig. 5 shows the in 1H NMR spectra of L1 upon the progressive addition of H2PO
4 . Remarkably, the protons (Ha) within the triazole ring of L1 depict a significant downfield shift by 0.30 ppm in the presence of H2PO
4 . This demonstrates a C–H  anion hydrogen bonding between L1 and H2PO
4 . A similar downfield shift of triazole protons in L3 is observed upon interaction with H2PO
4 (Fig. S7). It should be noted that the shifting magnitude of the macrocyclic receptor L1 is larger than that of the acyclic receptor L3 in presence of 2.0 equivalents H2PO
4 (
0.30 ppm versus
0.05 ppm). The latter indicates that L1 is more sensitive to H2PO
4 than L3 owing to its high degree of preorganization and rigidity. The 1H NMR titrations of L1 with the anions F
and AcO
, often shown as interference in most H2PO
4 chemosensors, were performed in CDCl3 and the results are presented in Fig. S8. In presence of the anions, the changes in 1H NMR of L1 look insignificant. This indicates weaker complexation abilities between L1 and F
or AcO
, which agrees well with the electrochemical titration results. The binding modes of the receptors L1–L4 with H2PO
4 are theoretically studied using DFT calculations at the B3LYP/6-31G level, and most stable geometries are depicted in Fig. 6. The resulting calculated structures suggest the formation of stable 1:1 complexes

between L1–L4 and H2PO
4 . In L1H2PO4 and L4H2PO4 complexes, the H2PO
anion is not bound within the center of the macrocycles 4 L1 and L2 but locates outside the cavity. For the acyclic receptors L3 and L4, the H2PO
4 anion is embedded within the central cleft form cavity. Further observations illustrate that the triazole and the cyclopentadienyl a-position protons of L1, L3 and L4 are

involved in the hydrogen bonding with H2PO
4 . By contrast, only the triazole protons are involved in the interaction between L2 and H2PO
4 , possibly assigned to the weak potential shift observed for L2 in comparison with those of the other receptors L1, L3, and L4. 4. Conclusions In conclusion, two novel ferrocene-containing macrocycles (L1 and L2) and their acyclic analogs (L3 and L4) were synthesized by click reaction. The anionic binding abilities of L1–L4 were evaluated by cyclic voltammetry and differential pulse voltammetry methods. In CH2Cl2 solution, the receptors L1–L4 showed exclusive electrochemical sensing properties of H2PO
4 , with shifts in the ferrocene half-redox peak (DE1/2) by 220 mV, 80 mV, 240 mV, and 200 mV for L1, L2, L3, and L4, respectively. On the other hand, the shifts in potential were insignificant in presence of other anions such as F
, Cl
, and AcO
. The 1H NMR titrations and DFT calculations revealed that the triazole C–H protons in the receptors L1–L4 play a key role in the interaction with H2PO
4 . The cyclopentadienyl a-position C–H protons in L1, L3 and L4 are involved in the hydrogen bonding with H2PO
4 , hence inducing larger potential shifts when compared with L2. Thus, the ferrocene-containing triazoles for electrochemical sensing of H2PO
4 are quite interesting, and the design based in this study can be further modified to construct more developed systems for other anions. Acknowledgments This work was supported by the National Nature Science Foundation of China (Nos. 21162017 and 21462027). Dr. C.-T. Li acknowledged support from the Innovation Experiment Project for Postgraduate Student of Nanchang University.

C.-T. Li et al. / Inorganica Chimica Acta 449 (2016) 31–37

Appendix A. Supplementary material CCDC 1468829 contains the supplementary crystallographic data for L1. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac. uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10. 1016/j.ica.2016.04.047.

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