Arene platform based hexa-amide receptors for anion recognition: single crystal X-ray structural and thermodynamic studies

Abstract

Five newly synthesized hexa-amide receptors (L1–L5) and previously reported three such receptors (L6–L8) have been explored to investigate binding propensity of anions of various shapes and sizes. Single crystal X-ray structures of five new anion complexes of receptors (L1–L5), complexes (1–5) and previously reported seven such complexes of (L6–L8), complexes (6–12) fall under four categories of conformations such as aaabbb (A), ababab (B), aabaab (C) and aaaaaa (D) depending upon basicity of the anions as well as substituents on the receptor backbones. Moderately basic guest; chloride exhibits two different conformers with A and B patterns with L1 (R = –oNO₂C₆H₄, complex 1) and L4 (R = –pFC₆H₄, complex 2) respectively in 1 : 2 (host : guest) stoichiometry. On the other hand, strongly basic acetate complexes of L2 (R = –pNO₂C₆H₄), L3 (R = –mCF₃C₆H₄), L5 (R = 4-pyridyl), L6 (R = –C₆F₅), L7 (R = –mNO₂C₆H₄) and L8 (R –oCF₃C₆H₄) i.e. complexes 3–8 respectively, exhibit a chair like conformer (A) with 1 : 2 (host : guest) stoichiometry. Our previous studies showed recognition of nitrate–water cluster (9) by L6 and nitrate (10) by L8 with (B) and (C) conformations respectively and an unusual conformation (D) was isolated in cases of L7 and L8 with hydrated fluoride clusters [(F)₄(H₂O)₁₀]⁴⁻ (11) and [(F)₄(H₂O)₆]⁴⁻ (12) respectively. Solution state ITC and NMR studies have also shown 1 : 2 stoichiometry of host : guest binding of acetate and fluoride with L1–L8.

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Introduction

For several years, anion coordination chemistry has grown in different dimensions based on receptor molecules, platforms, and anionic guests.^1–9 Various neutral tripodal receptors and polyammonium macrocycles are reported in the literature which are capable of encapsulating or isolating numerous anions like sulfate,^10–14 arsenate,^15–21 fluoride,^22–32 thiosulfate,³³ perrhenate^34,35 etc.^36–48 Interestingly, isolation of the fluoride–water^49–57 and chloride–water^58,59 clusters have attracted a great deal of attention recently. Chloride is an integral part of chloride channels which is responsible for transport in human body. Oxyanions such as acetate and nitrate are of immense importance as they play crucial roles in various biological systems, catalysis and environmental issues. For example, carboxylates have specific biochemical behaviors and act as crucial components in metabolic pathways.^60–66 Self-assembled supramolecular architectures have shown encapsulation of guests in its cavity via hydrogen bonding interactions. Compared to conventional platforms which are known in anion coordination chemistry, hexapodal receptors based on arene platforms are less explored. Hence these class of receptors are promising new candidates in anion coordination chemistry.^67–71 Recently, we have shown recognition of [(F)₄(H₂O)₁₀]⁴⁻ and [(F)₄(H₂O)₆]⁴⁻ cluster in the dimeric capsular assembly and compartmental recognition of nitrate and acetate by hexa-amide receptors.^51,52,57 Herein we report a series of neutral hexa-amide receptors that have shown structural diversities upon guest binding depending on basicity of the guests and substitutions on the receptor backbone. Solid state structural analysis and solution state anion binding^72–74 with this new generation neutral receptors are discussed in detail.

Results and discussion

The hexa-amide receptors L1–L5 are synthesized with various electron withdrawing groups like –NO₂ (L1, L2 and L7) –F (L4, L6), –CF₃ (L3 and L8) which are attached in different positions to assess their role in anion complexation via electronic and steric effects (Scheme 1).^51,52,57 Crystallization with various anions resulted acetate, nitrate, chloride and fluoride guest bound structures with this series of receptors. Crystals suitable for single crystal X-ray diffraction studies are obtained for chloride complexes, complex 1 [(L1)·(Cl)₂·(TBA)₂] and 2 [(L4)·(Cl)₂·(H₂O)₂·(TBA)₂]. Single crystals of three new acetate complexes i.e. 3 [(L2)·(CH₃COO)₂·(TBA)₂], 4 [(L3)·(CH₃COO)₂·(TBA)₂], 5 [(L5)·(CH₃COO)₂·(TBA)₂] are isolated in moderate yields and compared with previously reported acetate complexes of L6, L7 and L8, i.e. 6 [(L6)·(CH₃COO)₂·(TBA)₂] and 7 [(L7)·(CH₃COO)₂·(TBA)₂] and 8 [(L8)·(CH₃COO)₂·(TBA)₂] respectively (Fig. 1). Apart from these new complexes, previously reported nitrate and fluoride complexes of L6, L7 and L8 i.e. 9 [(L6)·(NO₃)·(TBA)], 10 [(L8)·(NO₃)·(TBA)], 11 [(L7)₂·(F)₄·(H₂O)₁₀·(TBA)₄] and 12 [(L8)₂·(F)₄·(H₂O)₆·(TBA)₄] are also accounted and compared to provide a comprehensive overview on binding phenomenon (Fig. 1). So the present work will focus on the various anion complexes and how the structural information can be related to the conformational variation of the hexapodal receptors depending upon basicity of encapsulated anionic guests. Thus our discussions on anion recognition by a series of new generation hexapodal receptors will gradually progress as solid state binding morphology studies, solution state ¹H-NMR titration studies with both fluoride, and acetate, solution state ITC studies with acetate. Crystallographic tables and hydrogen bonding parameters are included in the ESI.†

Scheme 1 Synthesis of hexa-amide receptors L1–L8.

Fig. 1 Schematic diagram showing binding of guest to the receptors in complexes 1–12.

Single crystal X-ray crystallographic studies

Structural description of chloride complexes (1 & 2). Fig. 2 show single crystal X-ray structures of chloride complexes of L1 and L4, namely 1 and 2 respectively. Both the chloride complexes show 1 : 2 (host : guest) stoichiometry in solid state. Chloride recognition pattern in complex 1 follows the trend as in acetate complexes (3–8) where L1 is in the chair like aaabbb conformation (Table 1S, Fig. 26S, ESI†). However, a change in conformation of hexa-amide receptor is observed upon recognition of hydrated chloride in complex 2 when the substitution on the side arms is modified from ortho nitro to para fluoro in L4. Thus L4 with para fluoro substitution on the side arms encapsulate chloride–water cluster [Cl₂(H₂O)₂]⁻ in both the compartments where each compartment is composed of tripodal clefts with aaabbb conformation (the most stable conformer, Tables 1S and 2S, ESI†). Fig. 2b shows the coordination pattern of chloride in the tripodal cleft where Cl⁻ is hexacoordinated to L4, TBA and H₂O via two N–H⋯Cl, two C–H⋯Cl and two O–H⋯Cl interactions respectively (Fig. 26S, ESI†). The [Cl₂(H₂O)₂]²⁻ cluster looks like a deformed quadrilateral where both Cl1 and Cl2 are hydrogen bonded to O5 and O6 by O–H⋯Cl interactions. The distances between amide nitrogen centres of the tripodal cleft in complex 1 (A conformer) is found to be much higher with respect to complex 2 (B conformer). Thus, the change in conformation from A to B creates more cleft volume for the incoming guest that is clearly evident from the Cl⁻ vs. [Cl₂(H₂O)₂]²⁻ recognition in L1 vs. L4 receptor. Hence from these two structures we can conclude that moderately strong guest chloride is unable to dictate structural conformation of the hexa-amide receptors which is unlikely in case of strong anions like acetate and fluoride.

Fig. 2 Single crystal X-ray structures of complexes 1 and 2 which showed compartmental recognition of Cl⁻ and [Cl₂(H₂O)₄]²⁻ in A and B conformation respectively.‡

Structural description of acetate complexes (3–8). Fig. 51S† represents the structure of acetate complexes of L2, L3 and L5 i.e. complexes 3–5 in solid state where a 1 : 2, host : acetate recognition is clearly evident. Complexes 3–5 maintain aaabbb conformation which is observed previously in the cases of L6, L7 and L8 (complex 6, 7 & 8 respectively).^51,52 In all these six complexes we observe that the amide protons are directed towards cavity of the tripodal cleft that bind AcO⁻. The one oxygen atom of AcO⁻ has strong hydrogen bonding interactions with two amide –NH centres whereas the other oxygen centre is hydrogen bonded to the remaining amide centre (Tables 4S–6S, ESI†). In all six acetate complexes, the distances between the adjacent amide nitrogen centers are found to be around ∼3 Å and distance between distant nitrogen centers ranges around ∼5.5 Å. It is evident that aaabbb conformation is complementary towards AcO⁻ recognition. Thus, irrespective of the ligand substitution, all the acetate complexes (3–8) attain a preferred aaabbb conformation and 1 : 2 host/guest stoichiometry. It dictates the fact that basicity of the guest anion plays a pivotal role to choose the conformation of binding for the receptor. This statement becomes even more firm when we focus on the fluoride–water clusters (complexes 11 & 12) where fluoride is recognized in the cavity of a dimeric capsular assembly of L7 and L8 as fluoride–water clusters i.e. [(F)₄(H₂O)₁₀]⁴⁻ (11)⁵² and of [(F)₄(H₂O)₆]⁴⁻ (12)⁵⁷ in aaaaaa conformation. Thus basic anions like fluoride and acetate can arrest the receptors in a particular conformation every time irrespective of substitution on the platform whereas borderline basic anions like chloride and nitrate fail to do so. Thus conformational diversity is observed in cases of border line basic anions.

Solution state anion binding

Isothermal Titration Calorimetry (ITC) and ¹H-NMR titration studies are employed to enlighten the solution state binding propensity of the hexa-amide receptors. Qualitative ¹H-NMR studies of anions (F⁻, Cl⁻, Br⁻, I⁻, NO₃⁻, CH₃COO⁻) with L1–L8 (except L2) are carried out in DMSO-d₆ which reveals only binding of F⁻ and CH₃COO⁻ in solution and ¹H-NMR titrations reveal 1 : 2 stoichiometry of binding which is evident from the anion equivalent plot analysis which matched perfectly with the ITC data. CH₃COO⁻ shows the lowest stability constant with L6 (β = 1.02 × 10⁵) and the highest with L5 (β = 1.07 × 10⁸) followed by the stability constant with L4 (β = 6.53 × 10⁷) amongst all the studied receptors. We have monitored the downfield shift of amide –NH peak in ¹H-NMR titrations in order to calculate the respective stability constants of CH₃COO⁻ to the hexa-amide receptors. L7 and L1, both with –NO₂ substitution on the platform show comparable stability constant with acetate in the order of ∼10⁶ (Table 1, Fig. 3, ESI†). In the case of, L7 the –NH proton is shifted to around 0.966 ppm whereas L1 shows a shift of 0.644 ppm (Fig. 3 and 48S, ESI†). Between –CF₃ substituted receptors, L8 (β = 5.97 × 10⁵) and L3 (β = 1.03 × 10⁷), the former shows a higher stability constant value with CH₃COO⁻ with shift of –NH proton around 0.934 ppm.⁵⁷ The representative Job's plot analysis of L3 with both CH₃COO⁻ and F⁻ revealed 1 : 2 binding stoichiometry in DMSO-d₆ at 298 K (ESI, Fig. 64S and 65S†). The selectivity order of CH₃COO⁻ towards hexa-amide receptors estimated by both ITC and ¹H-NMR titrations is found to be, L5 > L4 ≈ L3 > L1 ≈ L7 > L8 ≈ L6.

Table 1 Stability constants (K₁, K₂) and ΔG values obtained from ¹H-NMR & ITC titration experiments with TBAF and TBAAcO with the receptors in DMSO-d₆ & DMSO at 298 K^a

Ligands	TBAF		TBAAcO		TBAAcO (ITC)
	Stability constant (K1 & K2) (NMR)	ΔG (kJ mol^−1) (NMR)	Stability constant (K1 & K2) (NMR)	ΔG (kJ mol^−1) (NMR)	Stability constants (K1 & K2) (ITC)	ΔH (kJ mol^−1)	TΔS (kJ mol^−1)	ΔG (kJ mol^−1)
a β represents the overall stability constant.
L1	K1 = 6.78 × 10^4 ± 9.7 × 10^2	ΔG1 = −27.6	K1 = 7.33 × 10^3 ± 1.71 × 10^2	ΔG1 = −22.0	K1 = 9.96 × 10^3 ± 4.7 × 10^2	ΔH1 = −8.0	TΔS1 = 14.8	ΔG1 = −22.8
	K2 = 4.01 × 10^3 ± 3.12 × 10^2	ΔG2 = −20.6	K2 = 1.00 × 10^3 ± 42	ΔG2 = −17.1	K2 = 383 ± 15	ΔH2 = −24.8	TΔS2 = −10.0	ΔG2 = −14.8
	β = 2.72 × 10^8 ± 3.03 × 10^5		β = 7.33 × 10^6 ± 7.18 × 10^3		β = (3.81 ± 0.07) × 10^6
L3	K1 = 1.81 × 10^3 ± 26.4	ΔG1 = −18.6	K1 = 2.55 × 10^4 ± 2.65 × 10^2	ΔG1 = −25.1	K1 = 2.82 × 10^4 ± 8.1 × 10^2	ΔH1 = −7.4	TΔS1 = 18.0	ΔG1 = −25.4
	K2 = 1.62 × 10^2 ± 4.69	ΔG2 = −12.6	K2 = 4.04 × 10^2 ± 9.71	ΔG2 = −14.9	K2 = 1.32 × 10^3 ± 40	ΔH2 = −8.7	TΔS2 = 9.1	ΔG2 = −17.8
	β = 2.93 × 10^5 ± 1.23 × 10^2		β = 1.03 × 10^7 ± 2.57 × 10^3		β = (3.72 ± 0.10) × 10^7
L4	K1 = 2.46 × 10^3 ± 4.78 × 10^2	ΔG1 = −19.3	K1 = 4.70 × 10^4 ± 9.72 × 10^2	ΔG1 = −26.7	K1 = 6.56 × 10^3 ± 40	ΔH1 = −27.9	TΔS1 = −6.1	ΔG1 = −21.8
	K2 = 2.27 × 10^2 ± 20	ΔG2 = −13.4	K2 = 1.39 × 10^3 ± 4.23 × 10^2	ΔG2 = −17.9	K2 = 5.90 × 10^3 ± 60	ΔH2 = 14.1	TΔS2 = 35.7	ΔG2 = −21.6
	β = 5.58 × 10^5 ± 9.56 × 10^3		β = 6.53 × 10^7 ± 4.11 × 10^5		β = (3.87 ± 0.14) × 10^7
L5	K1 = 6.76 × 10^3 ± 1.97 × 10^2	ΔG1 = −21.8	K1 = 2.21 × 10^4 ± 7.68 × 10^2	ΔG1 = −24.8	K1 = 2.32 × 10^4 ± 5.0 × 10^2	ΔH1 = −82.5	TΔS1 = −57.4	ΔG1 = −25.0
	K2 = 5.90 × 10^3 ± 2.70 × 10^2	ΔG2 = −21.5	K2 = 4.85 × 10^3 ± 1.08 × 10^2	ΔG2 = −21.0	K2 = 1.98 × 10^4 ± 5.3 × 10^2	ΔH2 = 32.6	TΔS2 = 57.1	ΔG2 = −24.5
	β = 3.98 × 10^7 ± 5.32 × 10^4		β = 1.07 × 10^8 ± 8.29 × 10^4		β = (4.59 ± 0.28) × 10^8
L6	K1 = 8.52 × 10^4 ± 5.06 × 10^3	ΔG1 = −28.1	K1 = 8.98 × 10^2 ± 67	ΔG1 = −16.8	K1 = 8.85 × 10^2 ± 52	ΔH1 = −15.2	TΔS1 = 1.6	ΔG1 = −16.8
	K2 = 9.60 × 10^3 ± 5.81 × 10^2	ΔG2 = −22.7	K2 = 1.14 × 10^2± 11.6	ΔG2 = −11.7	K2 = 1.71 × 10^2 ± 12	ΔH2 = 53.2	TΔS2 = 65.9	ΔG2 = −12.7
	β = 8.18 × 10^8 ± 2.94 × 10^6		β = 1.02 × 10^5 ± 7.77 × 10^2		β = (1.51 ± 0.07) × 10^5
L7	K1 = 4.24 × 10^4 ± 7.14 × 10^3	ΔG1 = −26.4	K1 = 4.63 × 10^3 ± 26.7	ΔG1 = −20.9	K1 = 1.83 × 10^4 ± 8.5 × 10^2	ΔH1 = −11.7	TΔS1 = 12.6	ΔG1 = −24.3
	K2 = 1.76 × 10^3 ± 3.08 × 10^2	ΔG2 = −18.5	K2 = 3.55 × 10^2 ± 60.8	ΔG2 = −16.1	K2 = 601 ± 29	ΔH2 = −16.5	TΔS2 = −0.6	ΔG2 = −15.9
	β = 7.46 × 10^7 ± 2.20 × 10^6		β = 1.64 × 10^6 ± 1.62 × 10^3		β = (1.61 ± 0.10) × 10^6
L8	K1 = 7.40 × 10^3 ± 1.8 × 10^2	ΔG1 = −22.1	K1 = 1.81 × 10^3 ± 43.8	ΔG1 = −18.6	K1 = 4.17 × 10^3 ± 1.1 × 10^2	ΔH1 = −7.5	TΔS1 = 13.1	ΔG1 = −20.6
	K2 = 5.60 × 10^3 ± 2.39 × 10^2	ΔG2 = −21.4	K2 = 3.30 × 10^2 ± 6.98	ΔG2 = −14.4	K2 = 234 ± 8.3	ΔH2 = −9.9	TΔS2 = 3.6	ΔG2 = −13.5
	β = 4.14 × 10^7 ± 4.30 × 10^4		β = 5.97 × 10^5 ± 3.05 × 10^2		β = (9.76 ± 0.68) × 10^5

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Fig. 3 (A) Partial ¹H-NMR (300 MHz) spectral changes of L1 in DMSO-d₆ with gradual addition of AcO⁻ (298 K), [L1] = 11.80 mM. The respective ratio of concentrations are [AcO⁻]/[L1]: (i) 0, (ii) 0.17, (iii) 0.51, (iv) 0.85, (v) 1.19, (vi) 1.53, (vii) 1.87, (viii) 2.21, (ix) 2.55, (x) 2.89. (B) Anion equivalent plot of titration for L1 with AcO⁻ in DMSO-d₆. (C) Partial ¹H-NMR (300 MHz) spectral changes of L3 in DMSO-d₆ with gradual addition of AcO⁻ (298 K), [L3] = 9.06 mM. The respective ratio of concentrations are [AcO⁻]/[L3]: (i) 0, (ii) 0.15, (iii) 0.45, (iv) 0.74, (v) 1.04, (vi) 1.34, (vii) 1.64, (viii) 1.94, (ix) 2.24, (x) 2.54, (xi) 2.83. (D) Anion equivalent plot of titration for L3 with AcO⁻ in DMSO-d₆.

The stability constants of F⁻ with the receptors are estimated by ¹H-NMR titration only (Table 1). During the stability constant estimation of these receptors with F⁻, we have monitored the shift of aromatic –CH protons. Among all the receptors, L6, L7 and L1 bind with F⁻ with comparatively higher stability constants. L6, shows highest overall stability constant (β = 8.18 × 10⁸) with F⁻ where K₁ and K₂ are estimated as 8.52 × 10⁴ and 9.60 × 10³ respectively with an upfield shift of –CH proton around 0.161 ppm (Fig. 47S, ESI†). Among the –CF₃ substituted receptors, L8 binds F⁻ with higher stability constants (K₁ = 7.40 × 10³, K₂ = 5.60 × 10³) compared to that of L3 (K₁ = 1.81 × 10³, K₂ = 1.62 × 10²). L4 has a β value of 5.58 × 10⁵ with a downfield shift of –CH protons by 0.160 ppm (Fig. 41S, ESI†). L5 showed 0.134 ppm downfield shift of pyridyl –CH protons with β value 3.98 × 10⁷ (Fig. 44S, ESI†). Fig. 4 shows the partial titration spectra of L1 and L3 to F⁻ and anion equivalent plot analysis of the same respectively. All the other molar ratio plots, anion equivalent plots and stack plots of ¹H-NMR titrations are depicted in the ESI (Fig. 33S–50S, ESI).† The binding affinity order of hexa-amide receptors towards F⁻ is found to be L6 ≈ L1 > L7 ≈ L5 ≈ L8 > L4 ≈ L3. The hexapodal receptors differ only in substitutions on the side arms and thus the overall binding process should be similar in all the cases. A representative ¹H-NMR titration in presence of excess TBAI (10 equivalents) between L3 and CH₃COO⁻ is also carried out in DMSO-d₆. TBAI is used to maintain constant polarity and ionic strength of the media. By this way also we found the stability constants are matching well with the above mentioned data. Furthermore, we have included the residual plots of titration in the ESI (Fig. 52S–63S).†

Fig. 4 Partial ¹H-NMR (300 MHz) spectral changes of L1 in DMSO-d₆ with gradual addition of F⁻ (298 K), [L1] = 8.98 mM. The respective ratio of concentrations are [F⁻]/[L1]: (i) 0, (ii) 0.15, (iii) 0.46, (iv) 0.76, (v) 1.07, (vi) 1.38, (vii) 1.69, (viii) 1.99, (ix) 2.30, (x) 2.61. (B) Anion equivalent plot of titration for L1 with F⁻ in DMSO-d₆. (C) Partial ¹H-NMR (300 MHz) spectral changes of L3 in DMSO-d₆ with gradual addition of F⁻ (298 K), [L3] = 12.32 mM. The respective ratio of concentrations are [F⁻]/[L3]: (i) 0, (ii) 0.11, (iii) 0.32, (iv) 0.53, (v) 0.74, (vi) 1.16, (vii) 1.48, (viii) 1.69, (ix) 1.90, (x) 2.23, (xi) 2.55. (D) Anion equivalent plot of titration for L3 with F⁻ in DMSO-d₆.

However, measureable data in ITC study are only obtained for CH₃COO⁻ with different hexa-amide receptors (L1–L8, except L2) in DMSO. Unfortunately, binding of TBACl and TBANO₃ with hexa-amides in DMSO can't be reliably quantified by ITC measurements. Though, ITC profiles of F⁻ binding to the receptors have shown fitting in a 1 : 2 (sequential sites) model still the Chi^{^2}/DoF values and the standard deviations are very large and thus are not included. The thermodynamic and kinetic parameters associated with CH₃COO⁻ to different hexa-amide receptors binding obtained from ITC titration are tabulated in Table 1. All the receptors have shown exothermic profiles towards CH₃COO⁻ binding (Fig. 5 and 27S–32S, ESI†). These titration data fit well to a sequential binding model where binding sites are taken as two. During titration with CH₃COO⁻, we have not observed any heat pulse beyond 2 equivalents of anions which confirms the choice of model. Instrument accuracy check experiment is also performed which is mentioned in Fig. 66S.† In the case of L1, first CH₃COO⁻ binding is entropy driven and the second one is enthalpy driven. Acetate shows the lowest stability constant with L6 (β = 1.51 × 10⁵) and the highest with L5 (β = 4.59 × 10⁸) followed by the stability constant of L4 (β = 3.87 × 10⁷) amongst all the studied receptors. The thermodynamic parameters associated for binding of L6 to CH₃COO⁻ binding is quite different from other receptors. Here ΔH₁ is found to be −15.2 kJ mol⁻¹ which is much lower than that of ΔH₂ (53.2 kJ mol⁻¹) in terms of magnitude. L5 show lower ΔH₁ (−82.5 kJ mol⁻¹) value compared to ΔH₂ (32.6 kJ mol⁻¹).

Fig. 5 Representative ITC profiles of L3 (A) and L4 (B). Here concentrations maintained during titration were L3 (0.2654 mM), L4 (0.2548 mM) and TBAAcO (4.704 mM). Details of the titration parameters are mentioned in the inset of each picture.

Thermodynamic parameters of anion binding can be described from the substitution nature of the hexa-amides. In case of nitro-substituted receptors L1 and L7, ΔS₁ value is higher compared to ΔS₂. This means that binding of first guest is associated with release of more solvent molecules. This phenomenon is more prominent in case of L7. In both the cases the binding of second acetate is found to be enthalpy driven. The –CF₃ functionalised receptors namely L3 and L8 have also shown higher ΔS₁ value compared to ΔS₂ and the second acetate binding is also enthalpy driven likewise the case of L1 and L7. In case of –F functionalised receptors namely L4 and L6 the trend is quite different from that of L1, L3, L7 and L8. Both L4 and L6 have shown enthalpy driven binding of first acetate and entropy driven binding of the second one which means the binding of second acetate is associated with release of more solvent molecules from the solvation sphere. Thus substitutions on the hexapodal scaffold lead to generate different thermodynamic behaviour in solution.

Further, the ITC data are substantiated with the ¹H-NMR titration data of hexa-amide receptors. So both ITC and NMR titration studies have shown similarity in binding stoichiometry like that of observed in solid state structures. However solution state studies do not provide us the scope to comment on the conformation of the receptors during binding processes. But, both these studies reflect perfect matching in terms of stoichiometry with the solid state structures.

Experimental section

General synthetic procedure for L1–L8

L6, L7 and L8 were synthesized according to our previously reported procedures.^51,52,57 Hexakis(aminomethyl)benzene (0.303 g, 1.2 mmol) and 1 mL triethylamine were dissolved in 100 mL dry tetrahydrofuran and stirred at 0 °C for 20 minutes under N₂ atmosphere. Substituted benzoyl chlorides (R-COCl, 7.80 mmol, 6.5 equivalents) were added from a dropping funnel to the reaction mass under N₂ atmosphere with constant stirring. Formation of off-white precipitate was observed immediately. The temperature of the reaction was gradually brought to room temperature, and the reaction was stirred continuously for 24 h. Subsequently, the reaction mixture was filtered and the solid was washed with THF and water. The precipitate was further washed with diethyl ether and dried in air.

Characterization data of L1

¹H NMR, 300 MHz (DMSO-d₆) δ ppm: 4.83 (s, 12H, –CH₂), 7.48–7.49 (d, 6H, –CH), 7.64–7.66 (m, 12H, –CH), 8.00–8.03 (d, 6H, –CH), 8.95 (b, 6H, –NH); ¹³C NMR, 75 MHz (DMSO-d₆) δ ppm: 124.02, 129.24, 130.77, 132.11, 133.33, 137.31, 146.91, 165.09. Yield: 82%. HRMS (ESI): m/z 1170.1260 [M + Na]⁺, 1148.1459 [M + H]⁺. Elemental analysis: calcd for L1: C, 56.55; H, 3.69; N, 14.65. Found: C, 56.20; H, 3.58; N, 14.25.

Characterization data of L2

¹H NMR, 300 MHz (DMSO-d₆) δ ppm: 4.80 (s, 12H, –CH₂), 7.45–7.47 (d, 6H, –CH), 7.61–7.63 (m, 12H, –CH), 7.98–8.00 (d, 6H, –CH) 8.93 (s, 6H, –NH); ¹³C NMR of L2 could not be recorded due to its low solubility in DMSO-d₆. Elemental analysis: calcd for L2: C, 56.55; H, 3.69; N, 14.65. Found: C, 56.01; H, 3.83; N, 14.08.

Characterization data of L3

¹H NMR, 300 MHz (DMSO-d₆) δ ppm: 4.82 (d, 12H, –CH₂), 7.47–7.49 (m, 6H, –CH), 8.09–8.13 (m, 12H, –CH), 8.38 (s, 6H, –CH), 8.97 (b, 6H, –NH); ¹³C NMR, 75 MHz (DMSO-d₆) δ ppm: 121.94, 123.49, 125.55, 129.06, 131.15, 134.24, 137.39, 164.26. Yield: 79%. HRMS (ESI): m/z 1308.1714 [M + Na]⁺, 1286.2754 [M + H]⁺. Elemental analysis: calcd for L3: C, 56.08; H, 3.29; N, 6.54. Found: C, 56.17; H, 3.20; N, 6.58.

Characterization data of L4

¹H NMR, 300 MHz (DMSO-d₆) δ ppm: 4.73–4.74 (b, 12H, –CH₂), 7.10–7.13 (d, 12H, –CH), 7.73–7.78 (d, 12H, –CH), 8.64 (b, 6H, –NH); ¹³C NMR, 75 MHz (DMSO-d₆) δ ppm: 114.68, 114.97, 129.88, 137.35, 162.12, 164.80, 165.41. Yield: 67%. HRMS (ESI): m/z 1008.1010 [M + Na]⁺. Elemental analysis: calcd for L4: C, 65.85; H, 4.30; N, 8.53. Found: C, 65.31; H, 4.16; N, 8.43.

Characterization data of L5

¹H NMR, 300 MHz (DMSO-d₆) δ ppm: 4.79–4.80 (b, 12H, –CH₂), 7.56–7.58 (d, 12H, –CH), 8.52–8.54 (d, 12H, –CH), 8.90 (s, 6H, –NH); ¹³C NMR, 75 MHz (DMSO-d₆) δ ppm: 122.09, 137.74, 141.97, 150.67, 165.34. Yield: 90%. HRMS (ESI) could not be recorded as it is not soluble in common organic solvents except DMSO. Elemental analysis: calcd for L5: C, 65.30; H, 4.79; N, 19.04. Found: C, 64.96; H, 4.87; N, 18.88.

The ¹³C peak for –CH₂ of the benzene plane is found to fall in the region of ¹³C peak of DMSO carbon. Hence this peak is untraceable in ¹³C NMR.

Synthesis and characterization data of complexes 1–5

The complexes (1–5) were crystallized in a glass beaker by adding 4 equivalents of tetrabutylammonium salt at room temperature. The details of the crystallization processes and characterization data are enlisted below.

Complex 1. Empirical formula: [L1(Cl)₂(TBA)₂], solvent of crystallization: acetonitrile. ¹H NMR, 300 MHz (DMSO-d₆) δ ppm: 0.93 (t, 24H, –NCH₂CH₂CH₂CH₃), 1.29–1.32 (m, 16H, –NCH₂CH₂CH₂CH₃), 1.57 (m, 16H, –NCH₂CH₂CH₂CH₃), 3.14–3.20 (t, 16H, –NCH₂CH₂CH₂CH₃), 4.81 (s, 12H, –CH₂), 7.47–7.50 (m, 6H, –CH), 7.63–7.65 (m, 12H, –CH), 7.99–8.02 (m, 6H, –CH), 9.026 (s, 6H, –NH); ¹³C NMR, 75 MHz (DMSO-d₆) δ ppm: 13.49, 19.20, 23.05, 57.50, 123.97, 129.28, 130.75, 132.05, 133.28, 137.22, 146.96, 165.03. Yield: 40%. Elemental analysis: for complex 1: C, 59.40; H, 6.84; N, 11.28. Found: C, 60.16; H, 6.87; N, 11.06.†

Complex 2. Empirical formula: [L4(Cl)₂(H₂O)₄(TBA)₂], solvent of crystallization: acetonitrile. ¹H NMR, 300 MHz (DMSO-d₆) δ ppm: 0.90–0.95 (t, 24H, –NCH₂CH₂CH₂CH₃), 1.29–1.32 (m, 16H, –NCH₂CH₂CH₂CH₃), 1.57 (m, 16H, –NCH₂CH₂CH₂CH₃), 3.14–3.19 (t, 16H, –NCH₂CH₂CH₂CH₃), 4.73 (s, 12H, –CH₂), 7.08–7.14 (t, 12H, –CH), 7.81–7.86 (t, 12H, –CH), 8.86 (b, 6H, –NH); ¹³C NMR, 75 MHz (DMSO-d₆) δ ppm: 13.50, 19.21, 23.06, 57.51, 114.98, 130.14, 137.22, 162.13, 164.76. Yield: 30%. Elemental analysis: calcd for complex 2: C, 64.84; H, 8.58; N, 6.41. Found: C, 65.16; H, 8.67; N, 6.71.†

Complex 3. Empirical formula: [L2(CH₃COO)₂(TBA)₂], solvent of crystallization: acetone. ¹H NMR, 300 MHz (DMSO-d₆) δ ppm: 0.91–0.94 (t, 24H, –NCH₂CH₂CH₂CH₃), 1.27–1.30 (m, 16H, –NCH₂CH₂CH₂CH₃), 1.55 (m, 22H, NCH₂CH₂CH₂CH₃ & –OOCCH₃), 3.12–3.15 (t, 16H, –NCH₂CH₂CH₂CH₃), 4.64 (s, 12H, –CH₂), 7.93–7.95 (d, 24H, –CH), 9.79 (b, 6H, –NH); ¹³C NMR, 75 MHz (DMSO-d₆) δ ppm: 13.46, 19.18, 23.03, 57.50, 122.76, 128.61, 136.62, 138.96, 148.44, 163.40. Yield: 35%. Elemental analysis: calcd for complex 3: C, 59.59; H, 6.67; N, 10.81. Found: C, 60.08; H, 6.78; N, 11.04.†

Complex 4. Empirical formula: [L3(CH₃COO)₂(TBA)₂], solvent of crystallization: acetone–dioxane (1 : 1, v/v) binary solvent mixture. ¹H NMR, 300 MHz (DMSO-d₆) δ ppm: 0.91–0.94 (t, 24H, –NCH₂CH₂CH₂CH₃), 1.28–1.30 (m, 16H, –NCH₂CH₂CH₂CH₃), 1.45 (s, 6H, –OOCCH₃), 1.56 (m, 16H, –NCH₂CH₂CH₂CH₃), 3.13–3.19 (t, 16H, –NCH₂CH₂CH₂CH₃), 4.68 (s, 12H, –CH₂), 7.31 (m, 6H, –CH), 7.58–7.61 (d, 6H, –CH), 8.02–8.05 (d, 6H, –CH), 8.12 (s, 6H, –CH), 9.72 (b, 6H, –NH); ¹³C NMR, 75 MHz (DMSO-d₆) δ ppm: 13.44, 19.19, 23.04, 23.92, 57.49, 122.00, 123.78, 125.61, 127.19, 128.75, 130.98, 134.22, 136.69, 163.73, 174.39. Yield: 40%. Elemental analysis: calcd for complex 4: C, 59.07; H, 6.20; N, 5.74. Found: C, 59.38; H, 6.18; N, 6.09.†

Complex 5. Empirical formula: [L5(CH₃COO)₂(TBA)₂], solvent of crystallization: acetone. ¹H NMR, 300 MHz (DMSO-d₆) δ ppm: 0.92–0.94 (t, 24H, –NCH₂CH₂CH₂CH₃), 1.28–1.31 (m, 16H, –NCH₂CH₂CH₂CH₃), 1.39 (s, 6H, –OOCCH₃), 1.53–1.58 (m, 16H, –NCH₂CH₂CH₂CH₃), 3.13–3.18 (t, 16H, –NCH₂CH₂CH₂CH₃), 4.61–4.62 (s, 12H, –CH₂), 7.72–7.74 (d, 12H, –CH), 8.42–8.44 (d, 12H, –CH), 9.95 (b, 6H, –NH); ¹³C NMR, 75 MHz (DMSO-d₆) δ ppm: 13.48, 19.20, 23.04, 24.35, 57.49, 121.11, 136.49, 140.30, 149.72, 163.64, 174.67. Yield: 60%. Elemental analysis: calcd for complex 5: C, 67.47; H, 8.30; N, 12.24. Found: C, 68.06; H, 8.12; N, 12.61.†

Details of complexes 6–12

These complexes were previously reported by our group and are included here to get a detailed overview of the binding phenomenon both experimentally and theoretically.^51,52,57

Single crystal X-ray crystallographic details

The crystallographic details of complexes 1–5 are given in Tables 1S and 2S.† In each case, a crystal of suitable size is dipped in paratone oil after collecting from mother liquor. Then it is mounted on the tip of a glass fibre and cemented using epoxy resin. Intensity data for all crystals are collected using MoKα (λ = 0.7107 Å) radiation on a Bruker SMART APEX diffractometer equipped with a CCD area detector at 120 K. The data integration and reduction are processed with SAINT^75a software. An empirical absorption correction is applied to the collected reflections with SADABS.^75b The structures are solved by direct methods using SHELXTL⁷⁶ and are refined on F² by the full-matrix least-squares technique using the SHELXL-97 (ref. 77) program package. Graphics are generated using PLATON-97 (ref. 78) and MERCURY 3.1.⁷⁹

Isothermal titration calorimetric studies

The solution-state binding affinity of the receptors (L1 and L3–L8) with acetate are performed by ITC experiments. L2 was sparingly soluble in DMSO and hence solution state studies with this receptor can't be carried out. In a typical ITC experiment, a solution of the acetate as its tetrabutylammonium salt in DMSO is titrated into a solution of receptor at 298 K. Exothermic titration profiles are obtained for all the receptors upon titration with CH₃COO⁻ and subsequent fitting to a 1 : 2 binding profile provide access to the stability constants (K₁, K₂), enthalpy change (ΔH₁, ΔH₂), entropy change (TΔS₁, TΔS₂), and free energy change (ΔG₁, ΔG₂) of the binding processes. The titration data are fitted in a sequential site model (ligand in cell) with number of binding sites two. Saturation beyond 2 equivalents of guests is observed in each case. Though ITC profiles of the fluoride showed fitting in 1 : 2 (sequential sites) model but with very high Chi^{^2}/DoF values. Hence it is not included in the current manuscript. Blank titration data is subtracted from the titration data in order to obtain accurate thermodynamic parameters of binding. Origin 7.0 is used as software for analysis. The upper panel of the VP-ITC output figure shows the heat pulses which are observed experimentally in each titration step with respect to time. The lower panel reports the respective time integrals translating as the heat absorbed or evolved for each aliquot and its coherence to a 1 : 2, sequential binding model. During each titration, a solution of concentration 0.25 mM (approximately) is placed in the cell at 298 K temperature. This solution is then titrated with 28 injections of 10 μL each of a 4.704 mM TBAAcO solution prepared in DMSO. An initial delay of 240 s is allowed before each titration. Interval of 220 s is allowed between each injection and the stirring speed is set at 329 rpm.

¹H-NMR titration studies

¹H NMR titrations with all the receptors (L1–L8, excluding L2) with both tetrabutylammonium acetate and fluoride are carried out in a 300 MHz NMR instrument. The receptors are soluble in 0.45 mL of DMSO-d₆ at room temperature. The guest anion concentration is maintained almost 10 times more concentrated with respect to receptor concentration. 10 μL aliquot of anion prepared in DMSO-d₆ is added and shaken well before recording the NMR data during titration. Amide –NH peak is monitored in case of acetate and used to calculate stability constant values from WINEQNMR2 software.⁸⁰ The anion equivalent plot and molar ratio plot of NMR titrations with both acetate and fluoride show 1 : 2 (host : guest) binding pattern and thus 1 : 2 fitting model is chosen in WINEQNMR2 to calculate the stability constants (K₁, K₂) values. Similarly for fluoride, the same fitting model is chosen as it also showed 1 : 2 binding pattern in anion equivalent plot and molar ratio plot analysis. Aromatic –CH protons are monitored in case of fluoride titrations to calculate the stepwise biding constants. The stability constant values calculated from NMR titrations which are in good resemblance with the value calculated from ITC studies.

Conclusions

In conclusion, we have shown binding of anions and hydrated anions of different dimensionalities to the hexa-amide receptors with different structural arrangements (conformers A, B, C, D) in single crystal X-ray structural studies. Strongly basic anions (Hofmeister series) like, fluoride and acetate have directed the conformation of hexa-amide receptors towards highly disfavored orientations (conformation A and D) despite the diverse substitutions on the receptor backbone where the rest of the anions failed to do so. Hence, in the case of chloride and nitrate, structural diversities are found that are further directed by steric effects on the receptor center. Compartmental recognition of [Cl₂(H₂O)₂]²⁻ cluster is evident in case of L4. The solution state ¹H-NMR titration and ITC binding studies showed 1 : 2 binding stoichiometry with fluoride and acetate which is found to be in good resemblance with solid state studies. Among the studied ligands, L5 showed selectivity towards acetate with stability constant value [4.59 × 10⁸ (ITC) and 1.07 × 10⁸ (NMR)] whereas L6 showed selectivity towards fluoride with an stability constant value of 8.18 × 10⁸ (NMR) among the studied receptors. L1, L6, L7 & L8 have shown selectivity towards fluoride over acetate and the rest (L3–L5) have shown selectivity towards acetate.

Acknowledgements

P. G. gratefully acknowledges the Department of Science and Technology (DST), New Delhi, India for financial support through Swarnajayanti Fellowship. S. C. and R. D. would like to acknowledge IACS, Kolkata, India for fellowship.

References

1. A. Bianchi, K. Bowman-James and E. García-España, Supramolecular Chemistry of Anions, Wiley-VCH, New York, 1997.
2. J. L. Sessler, P. A. Gale and W.-S. Cho, Anion Receptor Chemistry: Monographs in Supramolecular Chemistry, RSC Publishing, Cambridge, UK, 2006.
3. E. Garcia-España, P. Díaz, J. M. Llinares and A. Bianchi, Coord. Chem. Rev., 2006, 250, 2952–2986.
4. C. A. Schalley, Analytical Methods in Supramolecular Chemistry, Wiley-VCH, Weinheim, 2007.
5. (a) E. V. Anslyn, J. Org. Chem., 2007, 72, 687–699; (b) S. Kubik, Chem. Soc. Rev., 2009, 38, 585–605.
6. P. Ballester, Chem. Soc. Rev., 2010, 39, 3810–3830.
7. M. Arunachalam and P. Ghosh, Chem. Commun., 2011, 47, 8477–8492.
8. A. E. Hargrove, S. Nieto, T. Zhang, J. L. Sessler and E. V. Anslyn, Chem. Rev., 2011, 111, 6603–6782.
9. Anion Coordination Chemistry, ed. K. Bowman-James, A. Bianchi and E. Garcia-España, Wiley-VCH, New York, 2012.
10. D. A. Jose, D. K. Kumar, B. Ganguly and A. Das, Inorg. Chem., 2007, 46, 5817–5819.
11. Y. Li, K. M. Mullen, T. D. W. Claridge, P. J. Costa, V. Felix and P. D. Beer, Chem. Commun., 2009, 7134–7136.
12. R. Custelcean, A. Bock and B. A. Moyer, J. Am. Chem. Soc., 2010, 132, 7177–7185.
13. C. Jia, B. Wu, S. Li, X. Huang, Q. Zhao, Q.-S. Li and X.-J. Yang, Angew. Chem., Int. Ed., 2011, 50, 486–490.
14. (a) I. Ravikumar and P. Ghosh, Chem. Soc. Rev., 2012, 41, 3077–3098; (b) R. Custelcean, Chem. Commun., 2013, 49, 2173–2182.
15. R. Nickson, J. McArthur, W. Burgess, K. M. Ahmed, P. Ravenscroft and M. Rahman, Nature, 1998, 395, 338–339.
16. S. K. Acharrya, P. Chakraborty, S. Lahiri, B. C. Raymahashay, S. Guha and A. Bhowmik, Nature, 1999, 401, 545–546.
17. D. K. Nordstrom, Science, 2002, 296, 2143–2145.
18. A. H. Smith, P. A. Lopipero, M. N. Bates and C. M. Steinmaus, Science, 2002, 296, 2145–2146.
19. M. L. Polizzotto, B. D. Kocar, S. G. Benner, M. Sampson and S. Fendorf, Nature, 2008, 454, 505–509.
20. M. Elias, A. Wellner, K. Goldin-Azulay, E. Chabriere, J. A. Vorholt, T. J. Erb and D. S. Tawfik, Nature, 2012, 491, 134–137.
21. R. Dutta, P. Bose and P. Ghosh, Dalton Trans., 2013, 11371–11374.
22. B. Dietrich, J.-M. Lehn, J. Guilhem and C. Pascard, Tetrahedron Lett., 1989, 30, 4125–4128.
23. M. Shionoya, H. Furuta, V. Lynch, A. Hamiman and J. L. Sessler, J. Am. Chem. Soc., 1992, 114, 5714–5722.
24. S. Mason, J. M. Llinares, M. Morton, T. Clifford and K. Bowman-James, J. Am. Chem. Soc., 2000, 122, 1814–1815.
25. C. J. Woods, S. Camiolo, M. E. Light, S. J. Coles, M. B. Hursthouse, M. A. King, P. A. Gale and J. W. Essex, J. Am. Chem. Soc., 2002, 124, 8644–8652.
26. M. A. Hossain, J. M. Llinares, S. Mason, P. Morehouse, D. Powell and K. Bowman-James, Angew. Chem., Int. Ed., 2002, 41, 2335–2338.
27. C. A. Ilioudis, D. A. Tocher and J. W. Steed, J. Am. Chem. Soc., 2004, 126, 12395–12402.
28. M. A. Hossain, P. Morehouse, D. Powell and K. Bowman-James, Inorg. Chem., 2005, 44, 2143–2149.
29. G. W. Bates, P. A. Gale and M. E. Light, Chem. Commun., 2007, 2121–2123.
30. M. Cametti and K. Rissanen, Chem. Commun., 2009, 2809–2829.
31. Q. Q. Wang, V. W. Day and K. Bowman-James, J. Am. Chem. Soc., 2013, 135, 392–399.
32. S. Chakraborty, R. Dutta, M. Arunachalam and P. Ghosh, Dalton Trans., 2014, 2061–2068.
33. A. Basu and G. Das, Dalton Trans., 2012, 10792–10802.
34. P. D. Beer, P. K. Hopkins and J. D. Mc- Kinney, Chem. Commun., 1999, 1253–1254.
35. V. Amendola, G. Alberti, G. Bergamaschi, R. Biesuz, M. Boiocchi, S. Ferrito and F. P. Schmidtchen, Eur. J. Inorg. Chem., 2012, 21, 3410–3417.
36. R. Custelcean and M. G. Gorbunova, J. Am. Chem. Soc., 2005, 127, 16362–16363.
37. J. R. Butchard, O. J. Curnow, D. J. Garett and R. G. A. R. Maclagan, Angew. Chem., Int. Ed., 2006, 45, 7550–7553.
38. P. S. Lakshminarayanan, E. Suresh and P. Ghosh, Angew. Chem., Int. Ed., 2006, 45, 3807–3811.
39. S. S. Zhu, H. Staats, K. Brandhorst, J. Grunenberg, F. Gruppi, E. Dalcanale, A. Lutzen, K. Rissanen and C. A. Schalley, Angew. Chem., Int. Ed., 2008, 47, 788–792.
40. M. A. Hossain, M. A. Saeed, F. R. Fronczek, B. M. Wong, K. R. Dey, J. S. Mendy and D. Gibson, Cryst. Growth Des., 2010, 10, 1478–1781.
41. A. L. Cresswell, M. M. Piepenbrock and J. W. Steed, Chem. Commun., 2010, 46, 2787–2789.
42. M. C. Das, S. K. Ghosh, S. Sen and P. K. Bharadwaj, CrystEngComm, 2010, 12, 2967–2974.
43. M. R. Krause, R. Goddard and S. Kubik, J. Org. Chem., 2011, 76, 7084–7095.
44. M. N. Hoque, A. Basu and G. Das, Cryst. Growth Des., 2012, 12, 2153–2157.
45. R. Custelcean, P. V. Bonnesen, N. C. Duncan, X. Zhang, L. A. Watson, G. V. Berkel, W. B. Parson and B. P. Hay, J. Am. Chem. Soc., 2012, 134, 8525–8534.
46. A. Basu and G. Das, Chem. Commun., 2013, 49, 3997–3999.
47. S. Chakraborty, M. Arunachalam, P. Bose and P. Ghosh, Cryst. Growth Des., 2013, 13, 3208–3215.
48. S. Saha, B. Akhuli, S. Chakraborty and P. Ghosh, J. Org. Chem., 2013, 78, 8759–8765.
49. M. Cametti and K. Rissanen, Chem. Commun., 2009, 2809–2829.
50. M. Arunachalam and P. Ghosh, Chem. Commun., 2009, 5389–5391.
51. M. Arunachalam and P. Ghosh, Org. Lett., 2010, 12, 328–331.
52. M. Arunachalam and P. Ghosh, Chem. Commun., 2011, 47, 6269–6271.
53. Q. Q. Wang, V. W. Day and K. Bowman-James, Angew. Chem., Int. Ed., 2012, 51, 2119–2123.
54. M. A. Hossain, M. A. Saeed, A. Pramanik, B. M. Wong, S. A. Haque and D. R. Powell, J. Am. Chem. Soc., 2012, 134, 11892–11895.
55. Q. Q. Wang, V. W. Day and K. Bowman-James, J. Am. Chem. Soc., 2013, 135, 392–399.
56. S. Chakraborty, R. Dutta, M. Arunachalam and P. Ghosh, Dalton Trans., 2014, 2061–2068.
57. S. Chakraborty, R. Dutta, B. M. Wong and P. Ghosh, RSC Adv., 2014, 4, 62689–62693.
58. R. Custelcean and M. G. Gorbunova, J. Am. Chem. Soc., 2005, 127, 16362–16363.
59. M. Mascal, L. Infantes and J. Chisholm, Angew. Chem., Int. Ed., 2006, 45, 32–36.
60. C. Schmuck and J. Lex, Org. Lett., 1999, 1779–1781.
61. S. J. Brooks, P. A. Gale and M. E. Light, Chem. Commun., 2005, 4696–4698.
62. R. P. Schwarzenbach, B. I. Escher, K. Fenner, T. B. Hofstetter, C. A. Johnson, U. Von Gunten and B. Wehrli, Science, 2006, 313, 1072–1077.
63. R. G. Foulkes, Fluoride, 2007, 40, 229–237.
64. M. Zhang, A. G. Wang, T. Xia and P. He, Toxicol. Lett., 2008, 179, 1–5.
65. T. Wurtz, S. Houari, N. Mauro, M. MacDougall, H. Peters and A. Berdal, Toxicology, 2008, 249, 26–34.
66. J. R. Hiscock, C. Caltagirone, M. E. Light, M. B. Hursthouse and P. A. Gale, Org. Biomol. Chem., 2009, 7, 1781–1783.
67. F. Vögtle and E. Weber, Angew. Chem., Int. Ed. Engl., 1974, 13, 814–816.
68. J. V. Gavette, A. L. Sargent and W. E. Allen, J. Org. Chem., 2008, 73, 3582–3584.
69. D. Das and L. J. Barbour, J. Am. Chem. Soc., 2008, 130, 14032–14033.
70. D. Das and L. J. Barbour, Chem. Commun., 2008, 5110–5112.
71. D. Das and L. J. Barbour, Cryst. Growth Des., 2009, 9, 1599–1604.
72. M. A. Saeed, A. Pramanik, B. M. Wong, S. A. Haque, D. R. Powell, D. K. Chand and M. A. Hossain, Chem. Commun., 2012, 48, 8631–8633.
73. M. A. Hossain, M. A. Saeed, A. Pramanik, B. M. Wong, S. A. Haque and D. R. Powell, J. Am. Chem. Soc., 2012, 134, 11892–11895.
74. A. Pramanik, D. R. Powell, B. M. Wong and M. A. Hossain, Inorg. Chem., 2012, 51, 4274–4284.
75. (a) SAINT and XPREP, version 5.1, Siemens Industrial Automation Inc., Madison, WI, 1995; (b) G. M. Sheldrick, SADABS, Empirical Absorption Correction Program, University of Göttingen, Göttingen, Germany, 1997.
76. G. M. Sheldrick, SHELXTL Reference Manual, Version 5.1, Bruker AXS, Madison, WI, 1997.
77. G. M. Sheldrick, SHELXL-97: Program for Crystal Structure Refinement, University of Göttingen, Göttingen, Germany, 1997.
78. A. L. Spek, PLATON-97, University of Utrecht, Utrecht, The Netherlands, 1997.
79. Mercury 3.1, Supplied with Cambridge Structural Database, CCDC: Cambridge, UK, 2009.
80. M. J. Hynes, J. Chem. Soc., Dalton Trans., 1993, 311–312.

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Footnotes

† Electronic supplementary information (ESI) available: Characterisation data of L1–L8 & complex 1–5, H-bonding table of complexes, ITC titration profile, qualitative ¹H-NMR spectra, ¹H-NMR titration profiles, Job's plots and anion equivalents plot of receptors with CH₃COO⁻ & F⁻. CCDC 944133, 944134, 944137, 996872 and 996873. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra08747c

‡ Color codes: carbon: yellow, oxygen: red, hydrogen: gray, nitrogen: cyan, fluorine: dark green, chlorine: light green.

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