Synthesis of Imidazolium Salts Linked to a t-Butylcalix[4]arene Framework and the Isolation of Interesting By-Products

Abstract

A series of functionalized calix[4]arenes were prepared that contain mono- and bis-(alkoxy)imidazolium groups that are linked to the lower rim of a t-butylcalix[4]arene framework. These molecules have potential as anion-complexation reagents and as precursors to N-heterocyclic carbene complexes that are attached to a calixarene framework. They were prepared by the preliminary reaction α,ω-dibromoalkanes with the parent t-butylcalix[4]arene to give bis-ω-bromoalkoxy groups that are connected to the calix[4]arene framework in the 25- and 27-positions. The reaction of the bis-substituted calixarenes with TiCl₄ led to the removal of one bromoalkoxy group to give mono-substituted derivatives. Both the mono- and bis-functionalized calixarenes were reacted with N-substituted imidazoles to give a series of mono- or bis-imidazolium salts with the imidazolium group tethered to the calix[4]arene via O–(CH₂)_n– linkages (n = 2, 4, or 6). Unexpected bis-calix[4]arene products, in which the calixarenes are linked together via bridging organic groups, were obtained in some of these reactions. One bridge consists of two calixarenes linked together via two –C₂H₄– groups while the other had a –O–C₄H₈–imidazolium-C₄H₈–O– linker tethering the two calix[4]arenes together. Both these species were characterized by single crystal X-ray diffraction studies. The structures both had significant disorder but, nevertheless, the data do establish their structures. That the imidazolium-substituted calix[4]arene cations are precursors to N-heterocyclic carbene complexes of nickel was demonstrated by the reaction of a mono-imidazolium-substituted calix[4]arene with nickelocene to give the fully characterized N-heterocyclic carbene nickel complex linked to the calix[4]arene group.

1. Introduction

A lot of our recent work has focused on the chemistry and the catalytic activity of N-heterocyclic carbene complexes of nickel, of generic formula [Ni(h⁵-C₅H₅)(NHC)(X)] where NHC = a N-heterocyclic carbene and X = Cl, Br or I [1,2,3,4]. A subset of this research has involved the catalytic potential of these species when they are linked to a calixarene framework [5]. The nickel complexes are prepared in a well-known reaction by treating nickelocene with imidazolium salts [6]. The research reported here describes the synthesis of such salts prior to their conversion to organometallic compounds and the isolation of interesting side products during such syntheses.

Imidazolium groups linked to calix[4]arenes are known, though these groups are usually attached to the upper rim of the calixarene after elimination of the t-Bu groups. Some recent examples include the preparation and use in CuAAC catalysis of carboxytriazolyl amphiphilic derivatives of calix[4]arenes that are prepared via upper rim imidazolium-substituted calix[4]arenes [7], their use in aqueous solution for Suzuki coupling [8], and the synthesis of NHC polymeric particles linked to calix[4]arenes as supports for active palladium nanoparticle catalysts by the same Russian group [9]. Upper rim-substituted calixarenes with imidazolium salts have been used to prepare metal NHC complexes, as the imidazolium salts are precursors to NHC complexes, and these species have found use as catalysts [10,11]. Indeed, the nearby calixarene cavity has been likened to a nanoreactor [12] for these catalysts.

Bis- and tetrakis-imidazolium salts that are linked to the lower rim of a calix[4]arene have been prepared by us [13], and others [14,15], and catalytic studies have appeared on the use of p-tert-butylthiacalix[4]arene-linked Pd-NHC complexes that are derived from imidazolium groups linked to a p-tert-butylthiacalix[4]arene [16], which are related to what we report in this paper. One such catalytic system, formed in situ, consisted of imidazolium salts, combined with Cs₂CO₃ as the base and Pd(OAc)₂ as the palladium source [17]. These species were used as catalysts for the Suzuki–Miyaura coupling reaction [18,19]. Another example includes the use of a compound that contains bis-(methylimidazolium)groups linked to t-butylcalix[4]arene that finds use as both a surfactant and a catalyst for Ullmann coupling in aqueous media [20].

It is possible to make some generalizations that compare and contrast the two types of calixarene substitution. In general, upper rim substitution, i.e., on the para-position relative to the phenolic OH group, does not lead to major conformational changes in the calixarene [21], while lower rim substitution can lead to more conformational changes that are probably due to different modes of H-bonding that may be possible on substitution. Nevertheless, we have not seen such conformational changes in our research so far, as all our lower rim-substituted calix[4]arenes retain the cone configuration of their parent. In general, it is easier to prepare lower rim derivatives as the OH or the methylene groups are more reactive [22]. As the phenolic OH groups are more chemical reactive, it is also, in principle, easier to prepare substituted derivatives via O–H bond activation.

Imidazolium containing cationic species that are part of a calixarene have been probed as complexing agents for inorganic anions such as H₂PO₄− and HSO₄−, towards which they exhibit some selectivity [23]. Recently tetrakis-imidazolium salts have shown antibacterial activity on par with some antibiotics [24].

This manuscript delineates the synthesis and characterization of a series of mainly mono-substituted calixarenes with imidazolium ligands and the isolation of small quantities of unexpected products with interesting structures. The conversion of one of the mono-substituted species into a nickel-N-heterocyclic carbene (NHC) complex demonstrates that, in principle, all the new imidazolium salts can be converted in good yields to nickel NHC derivatives. The question of whether functionalizing the lower rim of a calix[4]arene through monosubstitution and subsequently transforming these species into organometallic derivatives enhances or diminishes their catalytic performance remains open. Equally interesting is the question of how the spatial relationship between the calixarene scaffold and the organometallic fragment influences activity. By adjusting the length of the methylene linker, this distance can be systematically varied, yet the precise effect on catalytic potential remains unclear. Subsequent syntheses of nickel NHC complexes that are grafted onto calix[4]arene frameworks, that in principle can be prepared from the imidazolium salts described in this manuscript, should shed light on some of these questions.

2. Results

2.1. Synthesis of 25,27-Bis(alkoxy)imidazolium t-Butylcalix[4]arenes

t-Buylcalix[4]arene can be functionalized by reaction with α,ω-dibromoalkanes in the presence of a base, as shown in Scheme 1, to give bis(alkoxybromo) compounds 1a–1c with varying length methylene spacers that we have previously reported for 1a and 1b [13]. The bis(hexoxybromo) compound, 1c, is new and its characterization data are given in the Experimental section. Its ¹H NMR spectra agree with its proposed geometry, and the data are consistent with a molecular geometry that is similar to what is observed for 1a and 1b.

2.2. Synthesis and Structure of Compound 2

When the synthesis of 1a was modified (different solvents and conditions were employed compared to its previous synthesis)—see the Experimental section—the yields obtained from 1a were significantly lower, but a few small crystals (<5 mg, that was not weighed) of another compound (2) were isolated as shown in Scheme 2. Under these conditions, a lot of the initial t-butylcalix[4]arene did not react. We were unable to obtain a ¹³C NMR spectrum of this compound, but its ¹H NMR displays a spectrum of a symmetric molecule. All –OCH₂CH₂O– protons are equivalent and appear as a singlet. A mass spectrum of the compound was unfortunately not obtained as the ¹H NMR tube broke, and the sample was lost.

The bridging methylene groups of the rim of the calixarene appear as a characteristic AB multiplet with a coupling constant of 13 Hz, indicative of a cone conformation for the calix[4]arene. Two different ^tBu peaks are observed and the two different kinds of phenoxy protons display two singlets in the ¹H NMR of 2. Its structure is shown in Scheme 2.

The structure of 2 was established by a single crystal X-ray diffraction study and is shown in Figure 1. There is significant disorder in the unit cell, as is frequently the case with calixarenes, which are relatively large and conformationally non-rigid molecules. Two different crystals were subjected to the X-ray diffraction study, and both gave relatively poor data due to twinning problems and did not give high θ reflections. Compound 2 crystallizes with four molecules of dichloromethane (two of which also exhibit disorder which could not be properly modeled) in the asymmetric unit. Furthermore, many of the t-butyl groups are disordered over two positions. One of the two bridging O–C₂H₄–O units is also disordered in a zig-zag arrangement, leaving the oxygen atoms in both disordered chains in the same position. Despite the unsatisfactory data, we believe the structure still establishes the molecule’s structure shown in Figure 1.

There are a number of H-bonded interactions in this molecule. One calixarene exhibits 2 O–H···O bonds of 2.220 and 2.247 Å between the O–H hydrogen atoms and the oxygen atoms of the two ethoxy groups. The other calixarene shows the same kind of hydrogen bond (O–H···O = 2.200 Å) to one of the ethoxy groups, but the other O–H hydrogen interacts with an O–H oxygen atom in the other calixarene (O–H···O–H) = 2.188. These interactions may persist in solution as the OH protons resonate at d = 7.23 ppm in the ¹H NMR spectrum, which is ≈2–3 ppm upfield compared to their usual positions in di-substituted calix[4]arenes that we have reported earlier [13]. The hydrogen atoms of one of the disordered CH₂Cl₂ molecules also probably exhibits a H-bonding interaction with one of the hydroxy group oxygen atoms. These interactions are highlighted in Figure 1.

2.3. Synthesis of Mono-Substituted Calix[4]arenes

Mono-substituted (ω-bromoalkoxy)t-butylcalix[4]arene compounds were then targeted. A variety of literature methods exist for the preparation of such species, but some methods we tried did not give good results in our hands. A direct mono-alkylation method consists of reacting the calix[4]arene with an alkylating agent in the presence of a weak base such as CsF [25], Na₂CO₃ [26], and K₂CO₃ (in solvents such as DMF or acetonitrile). Stronger alkalis such as Ba(OH)₂ in DMF, or NaH suspended in toluene [27], and NaOMe in CH₃CN [28] have been proposed. However, attempts to synthesize mono-substituted calix[4]arenes by using the base route led to mixtures of mono- and di-substituted calixarenes as well as unreacted starting material and were unproductive in our hands. Mixtures were obtained that were difficult to separate into their constituents via column chromatography.

Other approaches to mono-substitution involved dealkylation from di- or tetra-substituted calix[4]arenes: protecting groups such as iodotrimethylsilane were used on the phenolic OH functionalities [29,30] followed by deprotection of the trimethylsilyl ether. We attempted an alternative approach that avoided protection and de-protection of the OH groups by attempting the dealkylation from a di-substituted derivative using a strong Lewis acid. Both AlCl₃ [31,32] and TiCl₄ [33] have been employed in the literature reports. The proposed mechanism for dealkylation using oxophilic TiCl₄ followed by treatment with aqueous HCl is shown in Scheme 3 [29]. This method worked well in our hands and allowed us to prepare mono-substituted calix[4]arenes.

The structures and syntheses of mono(w-bromoalkoxy)t-butylcalix[4]arenes 3a, 3b, and 3c are shown in Scheme 4. Compounds 1 were refluxed in toluene with 1.5 equivalents of TiCl₄ for two days, during which the reaction mixture turned red, suggesting the formation of TiCl₄ calixarene complexes, possibly one, or a mixture of both, of the two molecules depicted on the bottom of Scheme 3. The addition of 1 M HCl turned the mixture orange, and colorless compounds 3a–3c were obtained pure via column chromatography. During the syntheses of compound 3 it was observed that the ease of dealkylation to give compounds 3 from 1 increases as the length of the alkyl chain in compound 1 increases. This is also reflected in the increasing yields of the reaction: 3a (37%) < 3b (47%) < 3c (86%).

The lower symmetries of compounds 3 compared to 1 result in more complex ¹H NMR spectra. The t-Bu protons in 3 exhibit three signals, in a 1:2:1 ratio in their respective spectra. Two signals are seen, in a 2:1 ratio, for the OH groups. Compounds 3 have effective C_s symmetry in solution and there are three symmetrically distinct aromatic rings. The phenoxy-ring linked to the bromoalkoxy group is bisected by an effective mirror plane (on the ¹H NMR time scale) and its two protons are chemically equivalent. The same situation holds for the distal phenolic ring, and the two aromatic protons of this ring also appear as a singlet. However while the other two phenolic rings are related by an effective mirror plane, one proton in each ring is closer to the bromoalkoxy subsituent than the other, so the two protons of each ring appear as an AB doublet. Furthermore, based on the coupling constants seen for the two distinct Ar–CH₂–Ar groups, the calix[4]arenes in all compounds 3 (as seen for compounds 1) adopt a cone conformation.

2.4. Synthesis of Mono- and Bis-ω-alkoxyimidazolium-Substituted Calix[4]arenes

The dibromo-compounds 1a and 1b were previously reported by us to react with N-substituted imidazoles to afford imidazolium salts 4a and 4b (Scheme 5) that are tethered via an alkoxy-chain to opposite ends of the calix[4]arene at the 25 and 27-positions [13]. We extended this synthesis to form 4c and 4d from 1c, which contain –O(CH₂)₆– linkages between the imidazolium group and the calixarene.

Mono(w-alkoxybromo)t-butylcalix[4]arenes 3 are transformed into mono(w-alkoxy-N-mesityl-imidazolium)t-butylcalix[4]arene species 5a–5c by refluxing compounds 3 for 2 days in acetonitile with N-(2,4,6-trimethylphenyl)imidazole (Scheme 6). Compounds 5 are analogs of 4 but have just one imidazolium group tethered to the calix[4]arene skeleton. They were all fully characterized by ¹H, ¹³C NMR, IR, mass spectroscopy, and elemental analysis, and their melting points were also determined. All NMR data again suggest that the cone-conformation is adopted for these compounds in solution. The compounds are all quite hydroscopic and it is hard to remove all traces of water.

The ¹H NMR spectra of these salts exhibit an effective mirror plane in solution as is seen for compounds 3, and their aromatic protons appear as a two singlets and an AB doublet. Signals seen for the CH₂Br signals in 3 disappear, and new downfield peaks are seen that correspond to the CH₂N peaks for the methylene group attached to the imidazole nitrogen atom in 5, as well as new aromatic signals attributed to the imidazolium and mesitylene protons. While the chemical shifts in most of the hydrogen atoms in these imidazolium salts appear in their normal chemical shift ranges, the chemical shifts in all the OH protons and frequently the imidazolium protons are somewhat variable, and shifts up to ±0.5 ppm are occasionally observed. Concentration-dependent H-bonding in solution may be responsible for this. In addition, the presence of small quantities of water, which is hard to entirely eliminate, may also form H-bonds in solution with these protons. The melting points for these salts (and other similar salts to be discussed shortly) lie in the 160–240 °C range: longer alkoxyalkyl chain imidazolium compounds 5 have lower melting points.

The corresponding PF₆− or BF₄− salts of 5b (5b_P or 5b_B) have been prepared by treating 5b with either KPF₆ or NaBF₄, respectively, in anion substitution reactions, in MeOH. The ¹H NMR spectra of these three compounds (Figure 2) show significant differences in the chemical shifts for the CH₂N and the acidic imidazolium C–H protons in the spectra of 5b_P or 5b_B as compared to 5b, clearly indicating different interactions of the cations of 5b, 5b_P, or 5b_B with their respective anions, and thus, anion recognition effects. Further studies of these interactions will be reported later.

The reactions of other imidazoles with 3b were attempted. The expected analogs of compounds 5b were indeed formed and detected by ¹H NMR in reactions with N-methyl-, N-phenyl-, and N-(2,6-diisopropylphenyl)imidazoles. However, in most cases, poor yields combined with significant purification problems led to them not being isolated pure. Only 5d, formed by reacting N-(2,6-diisopropylphenyl)imidazole with 3b via microwave synthesis, was obtained pure, in a comparatively low 11% yield as shown in Scheme 7.

Similar problems were observed in reactions of 3b with imidazole and benzimidazole, but the imidazole reaction led to small quantities of 6a, a (bis)t-butylcalix[4]arene species linked together by a bridging O–C₄H₈–imidazole–C₄H₈–O group, as shown in Scheme 8. A similar product 6b was also detected by mass spectroscopy for the benzimidazole reaction with 3b.

2.5. Single Crystal X-Ray Diffraction of Compound 6a

The structure of 6a was determined by a single crystal X-ray diffraction study which established the structure shown in Scheme 8. The molecule consists of two t-butylcalix[4]arenes bridged by a O–C₄H₈–imidazolium-C₄H₈–O- unit. Unfortunately, and as is often the case with calixarenes, this structure exhibits quite significant disorder. Seven of the eight t-Bu groups have each of their Me-groups sitting in two sites. There are large voids in the structure where the bromide anion is presumably sitting, but it could not be accurately located or modeled, and the SQUEEZE program [34] was used to handle this. The molecule crystallizes with two molecules of methanol in the asymmetric unit, which were located. Nevertheless, despite the small crystal size, quality, and the serious disorder problem, we believe the data still allows the molecular geometry to be established as indicated in Scheme 8. Figure 3 depicts the crystallographically determined molecular structure of 6a.

As observed with the other compounds described here, calix[4]arenes in 6a adopt cone-conformations. Many of their oxygen atoms are involved in hydrogen-bonding interactions. The 2 OH groups in one calix[4]arene, that flank the oxygen atom that is linked to the C₄H₈ chain, have their hydrogen atoms directed towards it and have short O–H --O distances of 1.84 and 2.31 Å, respectively. The other calixarene shows more H-bonding interactions: one of the solvent methanol oxygen atoms shows a O_MeOH -- H–O_calix distance of 2.04 Å. There are other short O–H–O distances as well as a short imidazolium CH -- OH interaction of 2.12 Å. These are shown in Figure 4.

2.6. Reaction of the Mono-Imidazolium Compound 5a with Nickelocene

The longer-term goals of the syntheses described in this manuscript are the conversion of the calix[4]arene bound imidazolium salts into nickel N-heterocyclic carbene complexes and a subsequent investigation of the catalytic properties of these molecules. To demonstrate that the synthesis of such species is feasible, we reacted compounds 5a with nickelocene, as shown in Scheme 9.

The reaction proceeded in a similar manner to previously described work [6] with non-calixarene bound imidazolium salts with nickelocene. A mixture of 5a and nickelocene (carried out under argon as solutions of nickelocene are air-sensitive) led to the green reaction mixture turning red over the 18 h spent in refluxing dioxane, and to the eventual isolation of compound 7 in excellent yield. Unfortunately, good X-ray quality crystals of 7 could not be obtained. However, all other characterization data fully agree with its formulation and structure as shown in Scheme 9.

A signal for the η⁵-C₅H₅ protons is seen at δ = 4.78 ppm in the ¹H NMR. Nickelocene itself is paramagnetic, with two unpaired electrons, and has a reported ¹H NMR signal at d = −252.9 ppm in 1,2,3,4-tetrafluorobenzene [35,36]. It also shows broad signals in the range of −300 to −200 ppm in the solid-state MAS ¹H NMR spectrum (these chemical shifts are strongly temperature-dependent [37]).

A corresponding peak for the cyclopentadienyl carbon atoms is observed at d = 92 ppm in the ¹³C NMR. There is also a peak that corresponds to the carbenoid carbon at 163 ppm, in the region characteristic of such Ni-NHC carbon atoms which are found at d = 160 ± 10 ppm typically [38]. None of the other compounds described in this manuscript exhibit a ¹³C NMR chemical shift greater than 150 ppm. The MS and microanalysis data also fully support the proposed formulation: the MS shows a peak with m/e = {M − Br]⁺ with the correct isotopic envelope for a mono-nickel species.

3. Materials and Methods

3.1. General

All reactions were carried out using Schlenk techniques under an atmosphere of Ar. All chemicals were purchased from the Sigma-Aldrich chemical company (Saint Quentin, France) or Fisher (Illkirch, France). Commercial compounds were used as received. Solvents were purified and dried from traces of water by distillation over Na-benzophenone ketyl (THF, ether, dioxane), Na (toluene) or CaH₂ (CH₃CN, CH₂Cl₂). Chromatography was carried out on silica gel 60 (63–200 µM). ¹H and ¹³C NMR spectra were recorded at 298 K either on a FT-Bruker Ultra Shield 300 or on a Bruker Spectrospin 400 spectrometer, operating at 300.13 or 400.14 MHz, respectively, for the ¹H spectra and at 75.47 or 100.62 MHz for the ¹³C spectra, using CDCl₃ as the solvent. Chemical shifts (∂) and coupling constants (J) are in ppm and Hz, respectively. Peaks are singlets unless otherwise stated (d = doublet, t = triplet, qn = apparent quintet, sep = septet, m = multiplet). The NMR spectra are shown in the Supplementary Information Tables. IR spectra were collected on a Nicolet 380 FT IR (Thermofisher, Schwerte, Germany) equipped with a diamond SMART ORBIT ATR accessory; the frequencies are recorded in cm⁻¹. Microanalytic data and mass spectra were measured by the Analytical and Inorganic Chemistry platform of the Hubert Curien Pluiridisciplinary institute (UMR 7188, CNRS), Strasbourg, France. Mass spectra were obtained on MicroTOF-Q instrument. Melting (or sometimes decomposition) points were registered on fusion capillary Bucchi Melting point M-560 instrument. Chemical products were purchased as received from standard chemical companies (Aldrich). t-Butylcalix[4]arene [39], N,N-bis-2,4,6-trimethylphenyl)imidazolium, and N,N-bis-2,6-diisopropylphenyl)imidazolium chlorides were prepared via literature methods [40,41,42].

3.2. Syntheses and Spectroscopic Data of 5,11,17,23-Tetra-t-butyl-26,28-dihydroxy-25,27-bis(2-bromoethoxy)calix[4]arene 1a, and 2

para-t-Butylcalix[4]arene (3.50 g, 5.39 mmol), K₂CO₃ (1.90 g, 13.75 mmol), and acetonitrile (50 mL) were placed in a 500 mL round-bottomed flask and stirred for 2 h. Subsequently, 1,2-dibromoethane (4.65 mL, 11.35 mmol) in acetonitrile (150 mL) was added and the mixture was refluxed for 4 d. Heating was then stopped and when the mixture cooled down to ambient temperature, the solvent was removed under vacuum, and the residue was taken up in CH₂Cl₂ (150 mL). Excess base was neutralized by addition of 1 M HCl (aq). The mixture was washed with water (2 × 150 mL), and the organic phase was separated and then dried using anhydrous MgSO₄. Addition of ethyl acetate precipitated excess unreacted p-tert-butylcalix[4]arene, which was removed by filtration. When some CH₂Cl₂ was added, a few white crystals of a compound, which was subsequently shown to be 2, precipitated. The remaining solution was concentrated and then purified by column chromatography on a silica gel column and was eluted using a CH₂Cl₂/cyclohexane 50:50 mixture to give 1a (C₄₈H₆₂O₄Br₂, M = 862.81 g mol⁻¹, 1.20 g, 1.39 mmol) as a white powder in 26% yield. ¹³C NMR of 1a (not reported earlier) [7]: 150.4, 149.2, 147.0, 141.4, 132.3, 127.6, 125.5, 125.0, 75.9, 29.4, 34.0, 33.9, 31.81, 31.08, 31.81, 29.41. ¹H NMR of 2: 7.20 (4H, OH), 7.07 (8H, Ar-H), 6.76 (8H, Ar-H), 4.33, and 3.38 (AB doublet, 8H and 8H, Ar-CH₂-Ar, J = 13.2), 1.35 (18H, ^tBu), 0.98 (18H, ^tBu).

3.3. Synthesis and Spectroscopic Data of 5,11,17,23-Tetra-t-butyl-26,28-dihydroxy-25,27-bis(6-bromohexoxy)calix[4]-arene, 1c

t-Butylcalix[4]arene (3.244 g 5.00 mmol) and K₂CO₃ (1.54 g, 11.14 mmol) and acetone (50 mL) were placed in a 500 mL round-bottomed flask equipped with an argon inlet and a reflux column. The mixture was stirred for 3 h at ambient temperature. 1,6-dibromohexane (7.58 mL, 19.6 mmol) in acetone (100 mL) was then added, and the mixture was refluxed for 2 d, and then allowed to cool to room temperature. The solvent was removed in vacuo and the residue was dissolved in CH₂Cl₂; excess base was neutralized by adding 1 M HCl and the organic layer was separated using a separating funnel, washed with water (2 × 150 mL), and dried over anhydrous MgSO₄. The filtrate was then concentrated and purified by column chromatography using silica gel and eluted with a mixture of CH₂Cl₂–petroleum ether (3:7) and then concentrated. Compound 1c (C₅₆H₇₈Br₂O₄, M = 975.02 g mol⁻¹, 1.80 g. 1.85 mmol) was obtained as a white powder in 37% yield. ¹H NMR: d 7.60 (2H, OH), 7.04 (4H, Ar-H), 6.81 (4H, Ar-H), 4.28, and 3.31 (8H, AB doublet, Ar-CH₂-Ar, J_AB = 11.2), 3.98 (t, 4H, OCH₂, J = 6.5), 3.46 (t, 4H, CH₂-Br, J = 6.8), 2.03 (qn, 4H, CH₂-CH₂-CH₂-Br, J = 5.8), 1.96 (qn, 4H, CH₂-CH₂-CH₂-Br, J = 3.2), 1.72 (qn, 4H, CH₂-CH₂-CH₂-O, J = 6.5), 1.62 (qn, 4H, CH₂-CH₂-CH₂-O, J = 3.5), 1.29 (18H, t-Bu), 0.98 (18H, t-Bu). ¹³C NMR: 150.9, 150.1, 147.0, 141.6, 132.9, 128.0, 125.7, 125.3, 76.4, 53.3, 34.2, 34.1, 34.0, 33.0, 32.0, 31.9, 31.3, 30.0, 28.2, 25.4.

3.4. Synthesis and Spectroscopic Data of 5,11,17,23-Tetra-t-butyl-26,27,28-trihydroxy-25(2-bromoethoxy)calix[4]arene, 3a

1a (1.00 g, 1.16 mmol) was placed in a 250 mL two-necked round-bottomed flask and TiCl₄(THF)₂ (434 mg, 1.30 mmol) was added together with toluene (40 mL). The mixture turned red and was refluxed for 2 d. After cooling, HCl(aq) (100 mL of a 1.0 M solution) were added and the mixture was stirred for 10 h. The orange organic phase was separated and washed with H₂O (100 mL) and then dried over anhydrous MgSO₄. The solution was concentrated and then subjected to silica gel column chromatography and eluted with a 50:50 mixture of CH₂Cl₂ and petroleum ether. Removal of the solvent led to 3a (C₄₆H₅₉BrO₄, M = 755.86 g mol⁻¹, 310 mg, 0.410 mmol, 35%) as a white powder. ¹H NMR: 10.12 (1H, OH), 9.25 (2H, OH), 7.06 (2H, Ar-H), 7.04 (d, 2H, Ar-H, J = 2.5), 7.03 (2H, Ar-H), 6.97 (d, 2H, Ar-H, J = 2.3), 4.47 (t, 2H, OCH₂, J = 6,), 4.36 (d, 2H, Ar-CH₂-Ar, J = 13.2), 4.25 (d, 2H, Ar-CH₂-Ar, J = 13.4), 3.98 (t, 2H, CH₂-Br, J = 5.9), 3.42 (d, 4H, Ar-CH₂-Ar, J = 13.5), 1.20 (9H, ^tBu), 1.19 (18H, ^tBu), 1.16 (9H, ^tBu). ¹³C NMR: 148.6, 148.3, 148.2, 147.5, 143.4, 143.1; 133.2, 128.2, 127.6, 127.4, 126.5, 125.8, 125.6, 125.5, 76.0, 34.3, 34.14, 34.08, 33.1, 32.4, 31.6; 31.3, 29.5.

3.5. Synthesis and Spectroscopic Data of 5,11,17,23-Tetra-t-butyl-26,27,28-trihydroxy-25(4-bromobutoxy)calix[4]arene, 3b

The dibromo compound 1b (2.00 g, 2.17 mmol) was placed in a 250 mL round-bottomed flask equipped with an argon inlet and a reflux. TiCl₄(THF)₂ (1.50 eq) and toluene (80 mL) were added and the mixture refluxed for 2 d and then cooled to room temperature. HCl aq (100 mL of a 1 M solution) were added, and the mixture was stirred for 10h. The organic phase was then separated, washed twice with water (2 × 100 mL), dried over MgSO₄, and then filtered and concentrated. The solution was then subjected to column chromatography on silica gel and eluted with a 40:60 mixture of dichloromethane and petroleum ether and was concentrated to give 3b (C₄₈H₆₃BrO₄, M = 783.91 g·mol⁻¹, 1.47 g, 1.10 mmol) as a white powder in 86% yield. ¹H NMR: 10.13 (1H, OH), 9.52 (2H, OH), 7.10 (2H, Ar-H), 7.07 (d, 2H, Ar-H, J = 2.5), 7.00 (2H, Ar-H, J = 2.3), 4.33 (d, 2H, Ar-CH₂-Ar, J = 13.0), 4.28 (d, 2H, Ar-CH₂-Ar, J = 13.8), 4.17 (t, 2H, CH₂-O, J = 6.1), 3.65 (t, 2H, CH₂-Br, J = 6.1,), 3.46 (d, 2H, Ar-CH₂-Ar, J = 13.8), 3.44 (d, 2H, Ar-CH₂-Ar, J = 13.0), 2.38 (qn, CH₂-CH₂-O, 2H), 2.27 (qn, CH₂-CH₂-Br, 2H), 1.24 (9H, ^tBu), 1.22 (18H, ^tBu), 1.20 (9H, ^tBu). ¹³C NMR: 149.3, 148.7, 148.4. 147.9, 143.7, 143.3, 133.6, 128.4, 128.1, 127.8, 126.6, 126.0, 125.8, 76.2, 34.40, 34.25, 34.0, 33.6, 33.1, 32.4, 31.63, 31.62, 31.4, 29.3, 28.6.

3.6. Synthesis and Spectroscopic Data of 5,11,17,23-Tetra-tert-butyl-26,27,28-trihydroxy-25(6-bromohexoxy)calix[4]arene, 3c

1c (1.50 g, 1.54 mmol), TiCl₄(THF)₂ (1.5 eq), and toluene (40 mL) were placed in a 250 mL round-bottomed flask equipped with a reflux condenser and an argon inlet, and the mixture was refluxed for 2d. After being allowed to cool to room temperature, HCl aq (100 mL of a 1.0 M solution) were added and the mixture was stirred rapidly for 10 h. The organic phase was separated and washed with water (2 × 100 mL), dried over anhydrous MgSO₄, and the solution was then concentrated and subjected to column chromatography on silica gel. Elution with a dichloromethane–petroleum ether (40:60), followed by removal of most of the solvent, led to precipitation of 3c as a white powder (C₅₀H₆₇BrO₄, M = 811.97 g mol⁻¹, 0.90 g, 1.1 mmol). ¹H NMR: 10.16 (1H, OH), 9.57 (2H, OH), 7.07 (2H, Ar–H), 7.04 (d, 2H, J = 2.4, Ar-H), 7.03 (2H, Ar-H), 6.97 (d, 2H, Ar-H, J = 2.4), 4.33 (d, 2H, Ar-CH₂-Ar, J = 12.9), 4.26 (d, 2H, Ar-CH₂-Ar, J = 13.4), 4.12 (t, 2H, CH₂-O, J = 7), 3.49 (t, 2H, CH₂-Br, J = 6.7), 3.44 (d, 2H, Ar-CH₂-Ar, J = 5.7), 3.41 (d, 2H, Ar-CH₂-Ar, J = 7.2), 2.16 (qn, 2H, CH₂-CH₂-CH₂-O, J = 7.3), 1.99 (qn, 2H, CH₂-CH₂-CH₂-Br, J = 7.1), 1.85 (qn, 2H, CH₂-CH₂-CH₂-O, J = 6.8), 1.69 (qn, 2H, CH₂-CH₂-CH₂-Br, J = 8.3), 1.20 (9H, tBu), 1.19 (18H, tBu), 1.17 (9H, tBu). ¹³C NMR: 149.5, 148.6, 148.3, 143.8, 143.3, 133.7, 128.5, 128.3, 127.8, 126.6, 126.0, 125.9, 125.8, 76.9, 34.5, 34.2, 34.14, 34.09, 33.2, 32.9, 32.5, 31.7, 31.1, 29.9, 28.2, 25.3. IR: 3295, 3049, 2953, 2866, 1600, 1483, 1455, 1361, 1297, 1201, 1183, 982, 871, 781. ESI-HRMS: m/z for C₅₀H₆₇BrO₄K⁺, 849.3870 (calc. 849.3859). CHN Microanalysis for C₅₀H₆₇BrO₄.-0.5H₂O: Calc.(expt.), C, 73.15 (73.06); H, 8.35 (8.38); N, 0.00 (<0.1).

3.7. Synthesis and Spectroscopic Data of 5,11,17,23-Tetra-t-butyl-26,28-dihydroxy-25,27-bis(6-N-(2, 4, 6-trimethylphenyl)imidazoliumhexoxy)calix[4]arene Dibromide, 4c

1c (0.30 g, 0.3 mmol) were placed in a 25 mL round-bottomed flask equipped with a reflux condenser, and 0.34 g (1.82 mmol) of 2,4,6-trimethylphenyl-imidazole (340 mg, 1.82 mmol) was added followed by dioxane (5 mL) The mixture was heated at 80 °C for 2 days, and then was allowed to cool to room temperature and evaporated to dryness. The residue was dissolved in CH₂Cl₂ (1 mL), and Et₂O (10 mL) and pentane (10 mL) were added, and the mixture was stirred for 1 h. The insoluble precipitate that formed was filtered and wash with Et₂O (2 × 30 mL) to yield 4c (C₈₀H₁₀₆Br₂N₄O₄, M = 1347.53 g·mol⁻¹, 250 mg, 0.18 mmol, 62%) as a beige-colored powder. Melting point: 225 °C. ¹H NMR: 10.26 (2H, N-CH-N), 8.37 (2H, CH-N), 7.30 (2H, CH-N), 7.11 (2H, OH), 7.01 (4H, Ar-H), 6.94 (4H, Ar-H), 6.72 (4H, Ar-H), 4.87 (t, 4H, CH₂-N, J = 7.3), 4.16 (d, 4H, Ar-CH₂-Ar, J = 13.1), 3.89 (t, 4H, OCH₂, J = 5.9), 3.26 (d, 4H, Ar-CH₂-Ar, J = 13.0), 2.30 (6H, Ar-CH₃), 2.17 (qn, 4H, CH₂-CH₂-CH₂-N, J = 7.3), 2.04 (12H, Ar-CH₃), 1.89 (qn, 4H, CH₂-CH₂-CH₂-N, J = 5.1) 1.78 (qn, 4H, CH₂-CH₂-CH₂-O, J = 7.5 Hz), 1.56 (qn, 4H, CH₂-CH₂-CH₂-O, J = 6.9 Hz), 1.26 (18H, ^tBu), 0.91 (18H, ^tBu). ¹³C NMR: 150.3, 149.8, 147.0, 141.9, 141.2, 137.7, 134.1, 132.3, 130.8, 129.9, 128.0, 125.5, 125.2, 124.2, 122.9, 76.3, 49.8, 33.93, 33.86, 31.7, 31.6, 31.0, 30.4, 29.9, 25.7, 25.2, 21.1. 17.7. IR: 3381; 3131; 3038; 2951; 2863; 1607; 1564; 1545; 1484; 1461; 1361; 1299; 1200; 1124; 1000; 870; 756; 670; 635. ESI-HRMS: m/z for C₈₀H₁₀₆Br₂N₄O₄ ([M − Br]⁺: calc. 593.4101, expt., 593.408. CHN Microanalysis for C₈₀H₁₀₆Br₂N₄O₄.0.5 H₂O: Calc.(expt.), C, 70.83 (70.80); H, 7.95 (7.90); N, 4.13 (4.46).

3.8. Synthesis and Spectroscopic Data of, 5,11,17,23-Tetra-t-butyl-26,28-dihydroxy-25,27-bis (6-N-(1-methylimidazolium)hexoxy)calix[4]arene Dibromide, 4d

3c (200 mg, 0.20 mmol), 1-methylimidazole (0.15 mL, 1.84 mmol), and dioxane (4 mL) were placed in a 25 mL round-bottomed flask equipped with a reflux condenser, and the mixture was heated to 80° for 2 d. After cooling to ambient temperature, the solvent was removed in vacuo. The residue was dissolved in CH₂Cl₂ (1 mL) and diethylether (15 mL) was added, and the mixture was stirred for 1 h. The off-white precipitate that formed was filtered, washed with diethylether (2 × 30 mL), and dried to yield 4d (C₆₄H₉₀Br₂N₄O₄, M = 1139.23 g mol⁻¹, 127 mg, 0.111 mmol, 54%) as a light-yellow powder. Melting point: 214 °C. ¹H NMR: 0.24 (s, 2H, N-CH-N), 7.67 (s, 2H, CH-N), 7.40 (s, 2H, CH-N), 7.33 (s, 2H, OH) 7.04 (s, 4H, H-Ar), 6.73 (s, 4H, H-Ar) 4.50 (t, 4H, J = 7 Hz, CH₂-N) 4.18 (d, 4H, J = 13.1, Ar-CH₂-Ar), 4.04 (6H, N-CH₃); 3.89 (t, 4H, OCH₂, J = 6.3,) 3.29 (d, 4H, Ar-CH₂-Ar, J = 13.1), 2.04 (qn, 4H, CH₂-CH₂-CH₂-N, J = 7.2), 1.86 (qn, 4H, J = 6.5, CH₂-CH₂-CH₂-N) 1.72 (qn, 4H, CH₂-CH₂-CH₂-O, J = 7.7) 1.50 (qn, 4H, CH₂-CH₂-CH₂-O, J = 6.2), 1.27 (18H, ^tBu), 0.91 (18H,^tBu). ¹³C NMR: 150.4, 149.8, 147.0, 142.0; 137.4, 132.0, 125.5, 125.3, 123.3, 122.7, 76.1, 53.4, 49.7, 36.7, 33.94, 33.89, 33.89, 31.7, 31.0, 30.1, 29.7, 25.7, 25.1. IR: 3369, 3145, 3048, 2952, 2863, 1570, 1484, 1462, 1361, 1297, 1196, 1166, 1124, 998, 871, 733, 621. ESI-HRMS: m/e for C₆₄H₉₀Br₂N₄O₄: [M − Br]⁺ 1057.6150 (calc. 1057.6140).

3.9. Synthesis and Spectroscopic Data of 5,11,17,23-Tetra-t-butyl-26,27,28-trihydroxy-25-2-(N-2,4,6-trimethyl-phenyl)imidazolium)ethoxycalix[4]arene Bromide, 5a

3a (300 mg, 0.40 mmol), 2,4,6-trimethylphenylimidazole (220 mg, 1.18 mmol), and acetonitrile (4 mL) were added to a 25 mL round-bottomed flask equipped with an argon inlet and a reflux condenser. The mixture was refluxed for 2 d, and then the solvent was removed. Pentane (15 mL) and diethylether (3 mL) were added and the mixture was stirred for 1 h. The white precipitate was filtered and washed with pentane (2 × 30 mL) to give 5a (C₅₈H₇₃BrN₂O₄, M = 942.11 g mol⁻¹, 210 mg, 0.220 mmol, 56%) as a light-beige powder. Melting point: 243 °C. ¹H NMR: 10.73 (H, N-CH-N), 9.87 (H, OH), 9.04 (2H, OH), 8.49 (H, CH-N), 7.26 (H, CH-N), 7.06 (d, 2H, Ar-H, J = 2.3), 7.02 (2H, Ar-H), 6.99 (2H, Ar-H), 6.97 (d, 2H, Ar-H, J = 2.2), 5.48 (t, 2H, CH₂-N, J = 3.6), 4.70 (t, J = 3.6, OCH₂, 2H), 4.11 (d, 2H, Ar-CH₂-Ar, J = 2.4), 4.07 (d, 2H, Ar-CH₂-Ar, J = 1.4), 3.44 (d, 2H, Ar-CH₂-Ar, J = 6.6), 3.40 (d, 2H, Ar-CH₂-Ar, J = 7.4), 2.32 (3H, Ar-CH₃), 2.10 (6H, Ar-CH₃), 1.19 (18H, ^tBu), 1.18 (9H, ^tBu), 1.13 (9H, ^tBu). ¹³C NMR: 149.1, 148.5, 148.0, 146.8, 144.3, 143.9, 141.5, 138.3, 134.3, 132.5, 130.8, 130.0, 128.2, 127.7, 126.9, 126.8, 125.9, 125.0, 122.9, 76.2, 51.1, 34.3, 34.1, 34.0, 32.9, 32.2, 31.5, 31.4, 31.1, 21.1, 17.7. IR: 3261, 3049, 2954, 2867, 1598, 1557, 1543, 1483, 1454, 1361, 1297, 1255, 1201, 1163, 1126, 1048, 910, 873, 781, 673. ESI-HRMS: m/z for C₅₈H₇₃BrN₂O₄: (M – Br)⁺; calc. 861.5570, expt. 861.5600. Microanalysis for C₅₀H₆₇BrO₄.0.5H₂O: Calc. (expt.) C, 72.56 (72.35); H, 7.87 (7.78); N, 2.92 (3.03).

3.10. Synthesis and Spectroscopic Data of 5,11,17,23-Tetra-t-butyl-26,27,28-trihydroxy-25-4-N-(2,4,6-trimethylphenyl)imidazolium-butoxycalix[4]arene, 5b

3b (400 mg, 0.510 mmol), 2,4,6-trimethylphenylimidazole (285 mg, 1.53 mmol), and acetonitrile (5 mL) were placed in a 25 mL round-bottomed flask equipped with an argon inlet and a reflux condenser, and the mixture was refluxed for 2 d, cooled, and evaporated to dryness. Pentane (30 mL) was added, and the suspension was stirred for 2 h and then filtered and washed with pentane (2 × 30 mL), and dried to give 5b (C₆₀H₇₇BrN₂O₄, M = 970.17 g mol⁻¹, 390 mg 0.40 mmol, 79%) as a light-yellow powder. Melting point: 228 °C. ¹H NMR: 10.68 (1H, N-CH-N), 10.32 (1H, OH), 9.58 (2H, OH), 8.06 (1H, CH-N), 7.17 (1H, CH-N), 7.07 (d, 2H, Ar-H, J = 2.0), 7.06 (2H, Ar-H), 7.04 (2H, Ar-H), 6.98 (2H, Ar-H), 6.97 (d, 2H, Ar-H, J = 2.2), 5.11 (t, 2H, CH₂-N, J = 7.5), 4.23 (d, 2H, Ar-CH₂-Ar, J = 13.0), 4.15 (d, 2H, Ar-CH₂-Ar, J = 13.2), 4.14 (t, 2H, OCH₂, J = 6.1), 3.43 (d, 4H, Ar-CH₂-Ar, J = 13.0), 2.71 (qn, 2H, CH₂-CH₂-N, J = 7.4), 2.32 (3H, Ar-CH₃), 2.31 (qn, 2H, CH₂-CH₂-O, J = 7.1), 2.09 (6H, Ar-CH₃), 1.20 (9H, ^tBu), 1.19 (18H, ^tBu), 1.16 (9H, ^tBu). ¹³C NMR: 148.8, 148.6, 148.0, 147.3, 144.3, 143.7, 141.4, 138.4, 134.3, 133.2, 130.7, 129.9, 128.3, 128.2, 127.3, 126.6, 125.9, 125.8, 123.3, 122.8, 49.7, 34.3, 34.1, 34.0, 33.1, 32.1, 31.49, 31.46, 31.2, 27.3, 26.5, 21.1, 17.7.

IR: 3241, 3150, 3019, 2957, 2869, 1597, 1563, 1549, 1483, 1461, 1392, 1361, 1288, 1202, 1068, 1005, 871, 781, 670. ESI-HRMS: m/z for C₆₀H₇₇BrN₂O₄: (M − Br]⁺ calc. 889.5877; expt. 889.5851. CHN Microanalysis for C₆₀H₇₇BrN₂O₄.0.5MeCN: Calc.(expt.), C, 73.95 (73.90); H, 7.99 (8.02); N, 3.53 (3.39).

3.11. Synthesis of the Hexafluorophosphate and Tetrafluoroborate Salts of 5b, 5,11,17,23-Tetra-t-butyl-26,27,28-trihydroxy-25-(4-N-(2,4,6-trimethylphenyl)imidazoliumbutoxy)-calix[4]arene Hexafluorophosphate (5b_P) and Tetrafluoroborate (5b_B)

5b, (100 mg, 0.100 mmol) and methanol (30 mL) were placed in a 50 mL round-bottomed flask. KPF₆ (230 mg, 1.20 mmol) or NaBF₄ (136 mg, 1.20 mmol) were added together with water (3 mL) and the mixture stirred for 18 h at ambient temperature. The solvent was then removed, and the residue was washed with water (15 mL). Dichloromethane (25 mL) was then added, and the mixture was vigorously shaken. The organic phase was separated, dried over MgSO₄, filtered, and the solvent was then concentrated and placed on a silica gel column. Elution using a 10:1 mixture of CH₂Cl₂/EtOH afforded 5b_P (C₆₀H₇₇F₆N₂O₄P, M = 1035.55 g mol ⁻¹, 60 mg, 0.058 mmol, 58%) or 5b_B (C₆₀H₇₇BF₄N₂O₄, M = 977.09 g mol⁻¹, 48 mg, 0.049 mmol, 49%) as pale-yellow powders.

Data for 5b_P: Melting point, 174 °C. ¹H NMR: δ 10.31 (1H, OH), 9.58 (2H, OH), 8.97 (1H, N-CH-N), 7.98 (1H, CH-N), 7.22 (1H, CH-N), 7.08 (d, 2H, Ar-H, J = 2.1), 7.07 (2H, Ar-H), 7.03 (2H, Ar–H), 6.99 (2H, Ar-H), 6.97 (d, 2H, Ar-H, J = 2.2), 4.74 (t, 2H, CH₂-N, J = 7.9), 4.23 (d, 2H, Ar-CH₂-Ar, J = 13.1), 4.14 (t, 2H, OCH₂, J = 6.3), 4.12 (d, 2H, Ar-CH₂-Ar, J = 13.6), 3.45 (d, 2H, Ar-CH₂-Ar, J = 13.8), 3.41 (d, 2H, Ar-CH₂-Ar, J = 13.4), 2.67 (qn, 2H, CH₂-CH₂-N, J = 7.5), 2.32 (3H, Ar-CH₃), 2.23 (qn, 2H, CH₂-CH₂-O, J = 6.4), 2.05 (6H, Ar-CH₃), 1.20 (9H, ^tBu), 1.19 (18H, ^tBu), 1.16 (9H, ^tBu). ¹³C NMR: 148.7, 148.0, 147.3, 144.3, 143.8, 141.5, 136.8, 134.4, 133.2, 129.9, 128.2, 127.4, 126.7, 125.9, 125.8, 123.9, 123.2, 75.5, 50.0, 34.3, 34.0, 33.1, 32.1, 31.5, 31.2, 27.0, 26.5, 21.1, 17.3. ¹⁹F NMR (376 MHz): −72.36 (d, J = 712.4, PF₆). ³¹P NMR (161 MHz): −144.32 (septet, J = 712.2, PF₆).

Data for 5b_B: Melting point: 213 °C. ¹H NMR: 10.31 (1H, OH), 9.58 (2H, OH), 9.39 (1H, N-CH-N), 7.98 (1H, CH-N), 7.22 (1H, CH-N), 7.08 (d, Ar-H, 2H, J = 2.1), 7.07 (2H, Ar-H), 7.03 (2H, Ar-H), 6.99 (2H, Ar-H), 6.97 (d, 2H, Ar-H, J = 2.2), 4.88 (t, 2H, CH₂-N, J = 7.9), 4.23 (AB d, 2H, Ar-CH₂-Ar, J = 13.1), 4.14 (t, 2H, OCH₂, J = 6.3), 4.12 (AB d, 2H, Ar-CH₂-Ar, J = 13.6), 3.45 (AB d, 2H, Ar-CH₂-Ar J = 13.8), 3.41 (AB d, 2H, Ar-CH₂-Ar J = 13.4), 2.67 (qn, 2H, CH₂-CH₂-N, J = 7.5), 2.32 (3H, Ar-CH₃), 2.23 (qn, 2H, CH₂-CH₂-O, J = 6.4), 2.05 (6H, Ar-CH₃), 1.20 (9H, ^tBu), 1.19 (18H, ^tBu), 1.16 (9H, ^tBu). ¹³C NMR: 148.7, 148.0, 147.3, 144.3, 143.8, 141.5, 136.8, 134.4, 133.2, 129.9, 128.2, 127.4, 126.7, 125.9, 125.8, 123.9, 123.2, 75.5, 50.0, 34.3, 34.0, 33.1, 32.1, 31.5, 31.2, 27.0, 26.5, 21.1, 17.3. ¹⁹F NMR (376 MHz) −151.8 (BF₄). ¹¹B NMR (128 MHz) −0.88 (BF₄).

3.12. Synthesis and Spectroscopic Data of 5,11,17,23-Tetra-t-butyl-26,27,28-trihydroxy-25(6-(2,4,6-trimethylphenyl)imidazolium-hexoxy)calix[4]arene Bromide, 5c

The mono-substituted calixarene 3c (300 mg, 0.37 mmol) was placed in a 25 mL round-bottomed flask equipped with a reflux condenser and 2,4,6-trimethylphenylimidazole (410 mg, 2.2 mmol) and dichloromethane (5 mL). The mixture was refluxed for 2 days, and after cooling the solution was subjected to column chromatography. Elution with a CH₂Cl₂/EtOH mixture (10:1) and concentration of the eluted solution yielded 5c (C₆₂H₈₁BrN₂O₄, M = 998.22 g·mol⁻¹, 110 mg, 0.11 mmol, 30%) as a pale-yellow powder. Melting point, 150 °C. ¹H NMR: 10.62 (1H, N-CH-N), 10.23 (1H, OH), 9.56 (2H, OH); 7.09 (1H, CH-N), 7.06 (2H, Ar-H), 7.04 (d, 2H, J = 2.5, Ar-H), 7.02 (2H, Ar-H), 6.97 (2H, Ar-H), 6.96 (d, 2H, J = 2.5, Ar-H), 4.84 (t, 2H, J = 7.3, CH₂-N), 4.28 (d, 2H, J = 13.0, Ar-CH₂-Ar), 4.19 (d, 2H, J = 13.6, Ar-CH₂-Ar), 4.09 (t, 2H, J = 6.3, OCH₂), 3.42 (d, 2H, J = 2.2, Ar-CH₂-Ar), 3.38 (d, 2H, J = 2.2, Ar-CH₂-Ar), 2.31 (3H, Ar-CH₃), 2.19 (qn, 2H, J = 7.5, CH₂-CH₂-N), 2.12 (qn, 2H, J = 7.1, CH₂-CH₂-O), 2.06 (6H, Ar-CH₃) 1.86 (qn, 2H, J = 7.3, CH₂-CH₂-CH₂-N), 1.64 (qn, 2H, J = 7.7, CH₂-CH₂-CH₂-O), 1.20 (9H, ^tBu); 1.18 (18H, ^tBu); 1.16 (9H, ^tBu). ¹³C NMR: 148.9, 148.8, 148.61, 147.3, 143.9, 141.3, 138.3, 134.2, 133.4, 130.7, 129.90, 128.2, 127.5, 126.5, 125.8, 123.2, 122.9, 49.8, 34.4; 34.2, 34.1, 33.2, 32.2, 31.61, 31.57, 31.3, 27.5, 26.8, 21.2, 17.8. IR: 3338, 3047, 2959, 2868, 1596, 1562, 1543, 1483, 1390, 1362, 1298, 1242, 1193, 1122, 1060, 870, 804, 758, 672. ESI-HRMS: m/z for C₆₃H₈₁₃BrN₂O₄, (M − Br)⁺: calc. 931.635; expt. 931.630.

3.13. Synthesis and Spectroscopic Data of 5,11,17,23-Tetra-t-butyl-26,27,28-trihydroxy-25-(4-(2,6-diisopropylphenyl)imidazolium-butoxy)calix[4]arene Bromide, 5d

3b (150 mg, 0.191 mmol), 2,6-diisopropylphenylimidazole (131 mg, 0.57 mmol), and acetonitrile (4 mL) were placed in a 5 mL vial, sealed, and heated to 150 °C for 1 h. After cooling, the solvent was removed, the residue was dissolved in a minimum of CH₂Cl₂, and the product was precipitated by addition of pentane to give 5d (C₆₃H₈₃BrN₂O₄, M = 1012.25 g mol⁻¹, 20 mg, 0.021 mmol, 11%) as a light-yellow powder. ¹H NMR: 10.80 (1H, N-CH-N), 10.37 (1H, OH), 9.61 (2H, OH), 8.07 (1H, CH-N), 7.52 (t, 1H, J = 7.8, Ar-H), 7.28 (d, 2H, J = 7.8, H-Ar), 7.20 (1H, CH-N), 7.07 (d, 2H, J = 2.1,Ar-H), 7.04 (2H, Ar-H), 7.02 (2H, Ar-H), 6.96 (d, 2H, J = 2.2, Ar-H), 5.20 (t, 2H, J = 7.1, CH₂-N), 4.23 (d, 2H, J = 13.2, Ar-CH₂-Ar), 4.17 (d, 2H, J = 13.6, Ar-CH₂-Ar), 4.15 (t, 2H, J = 6.0, OCH₂), 3.46 (d, 2H, J = 2.3, Ar-CH₂-Ar), 3.42 (d, 2H, J = 2.9, Ar-CH₂-Ar), 2.73 (qn, 2H, J = 7.3, CH₂-CH₂-N), 2.34 (m, 2H, J = 6.8, CH(CH₃)₂), 2.34 (qn, 2H, J = 6.8, CH₂-CH₂-O), 1.23 (d, 6H, J = 6.1, CH(CH₃)₂), 1.20 (9H, ^tBu), 1.19 (18H, ^tBu), 1.16 (9H, ^tBu), 1.06 (d, 6H, J = 6.5, CH(CH₃)₂. ¹³C NMR: 148.8, 148.7, 148.0, 147.3, 145.5, 144.4, 143.8, 139.0, 133.2, 131.9, 130.2, 128.3, 128.2, 127.3, 126.6, 125.9, 125.8, 124.7, 124.6, 124.3, 122.7, 75.8, 49.8, 34.3, 34.1, 34.0, 33.1, 32.1, 31.5, 31.2, 28.8, 28.0, 27.4, 26.6, 26.0, 24.5, 24.0. IR: 3338, 3047, 2959, 2868, 1596, 1562, 1543, 1483, 1390, 1362, 1298, 1242, 1193, 1122, 1060, 870, 804, 758, 672. ESI-HRMS: m/z for C₆₃H₈₃N₂O₄ (M − Br⁺): 931.635, calc. 931.63.

3.14. Synthesis and Spectroscopic Data of 7, Bromo-(η⁵-cyclopentadienyl) {5,11,17,23-Terta-tert-butyl-25-(2-(2,4,6-trimethylphenyl)imidazole-2-ylidiene-ethyloxy)-26,27,28-trihydroxycalix[4]-arene)nickel

5a (100.0 mg, 0.106 mmol) and nickelocene (24 mg, 0.127 mmol) were placed in a 25 mL Schlenk tube under argon, and 1,4-dioxane (5 mL) was added. The mixture was refluxed under argon for 18 h and the green solution slowly turned reddish brown. After being allowed to cool to ambient temperature, the solution was filtered through a Celite pad to give a deep red filtrate. The dioxane was concentrated under vacuum, pentane (15 mL) was added, and after cooling in a −20 °C freezer, 7 (C₆₃H₇₈BrN₂NiO₄, M = 1065.92 g·mol⁻¹, 110 mg, 0.103 mmol, 97%) was obtained as deep red powder. ¹H NMR: 9.93 (1H, CHN), 9.20 (2H, OH), 7.98 (1H, CHN); 7.07 (2H, Ar-H), 7.06 (d, 2H, Ar-H, J = 2.7), 7.02 (2H, Ar-H), 6.99 (1H, OH), 6.97 (d, 2H, Ar-H, J = 2.3), 5.64 (t, 2H, OCH₂ J = 7.6), 4.99 (t, 2H, CH₂-N, J = 8.8), 4.80 (5H, C₅H₅), 4.30 and 4.17 (AB d, 4H, Ar-CH₂-Ar, J = 12.9), 3.47 and 3.43 (d, 2H, Ar-CH₂-Ar, J = 13.5), 2.40 (3H, Ar-CH₃), 1.20 (18H, ^tBu), 1.20 (6H, Ar-CH₃), 1.19 (9H, ^tBu), 1.16 (9H, ^tBu). ¹³C NMR: 163.6, 149.4, 148.6, 148.4, 147.3, 144.0, 143.4, 139.2, 137.0, 133.0, 129.3, 129.2, 128.3, 127.7, 127.0, 126.8, 125.9, 125.8, 125.7, 125.4, 124.0, 92.0, 77.2, 76.7, 52.8, 34.3, 34.1, 34.0, 33.0, 32.3, 31.5, 31.4, 31.2, 21.2. IR: 3263, 3159, 3029, 3012, 2951, 2869, 1597, 1561, 1549, 1538, 1469, 1482, 1366, 1328, 1219, 1168, 1011, 898, 781, 693. ESI-HRMS: m/z for C₆₃H₇₈BrN₂NiO₄ ([M − Br]⁺) 984.5321, calc. 984.5315. CHN Microanalysis for C₆₃H₇₈BrN₂NiO₄: Calc.(expt.), C, 70.99 (70.75); H, 7.38 (7.31); N, 2.63 (2.51).

3.15. Single Crystal X-Ray Diffraction Studies

X-ray diffraction data for 2 and 6a were all collected on a Bruker APEX II DUOKappa-CCD diffractometer (Karlsruhe, Germany) equipped with an Oxford Cryosystem liquid N₂ device, using Mo-Kα radiation (λ = 0.71073 Å) at the University of Strasbourg structural facility. All structures were solved using SHELXS-97 [43,44]. Single crystals of 2 and 6a were obtained by crystallization at 4 °C from dichloromethane (2) or as small crystals from acetonitrile. The crystal–detector distance was 38 mm. The cell parameters were determined (APEX2 software, [45]) from reflections taken from three sets of six frames, each at 10 s exposure. The refinement and all further calculations were carried out using SHELXL-2019 [46]. The H-atoms were included in calculated positions and treated as riding atoms using SHELXL default parameters. The non-H atoms were refined anisotropically, using weighted full-matrix least-squares on F2. A semi-empirical absorption correction was applied using SADABS in APEX2 [39]. For compounds 2 and 6a, the SQUEEZE instruction in PLATON [32] was applied, and for 6 the residual electron density corresponded to a bromide and 3.5 molecules of methanol.

4. Conclusions

New mono- and di-distal imidazolium cations 4 and 5 that are linked to para-t-butylcalix[4]arenes along the outer rim have been synthesized and fully characterized. In addition, the interesting quasi-dimeric molecules 2 and 6 were isolated in small quantities. The formation of nickel N-heterocyclic carbene complexes from the mono-substituted calix[4]arene imidazolium salts is envisaged, and this is shown to be feasible with the reaction depicted in Scheme 9. Such complexes have the potential to act as Suzuki-Miyaura catalysts and is the subject of current research. There are also indications that some of the imidazolium salts have potential in cation recognition. Future work is planned to investigate this further.