Stepwise Synthesis of Tetra-imidazolium Macrocycles and Their N-Heterocyclic Carbene Metal Complexes

A modular stepwise synthetic method has been developed for the preperation of tetra-imidazolium macrocycles. Initially a series of three bis(imidazolylmethyl)benzene precursors were alkylated with 1,2-dibromoethane to produce the corresponding bis-bromoethylimidazolium bromide salts. In the second step the bis-bromoethylimidazolium bromide salts were reacted with selected bis(imidazolylmethyl)benzene molecules to produce a series of two symmetrical and three asymmetrical tetra-imidazolium macrocycles. These tetra-imidazolium salts act receptors for anions and 1H-NMR titration studies were used to determine the association constants between two of the macrocycles and the halide anions chloride, bromide and iodide. The tetra-imidazolium salts are precursors for N-heterocyclic carbene (NHC) ligands and the corresponding silver(I), gold(I), and palladium(II) NHC complexes have been prepared. Varied structures were obtained, which depend on the chosen macrocyclic ligand and metal ion and in the case of the coinage metals Ag(I) and Au(I), mono, di, and hexanuclear complexes were formed.


INTRODUCTION
Imidazolium linked macrocycles have attracted significant recent attention because of their capacity to act as anion receptors and to function as pro-ligands for the synthesis of N-heterocyclic carbene metal complexes. Due to the great importance of negatively charged anionic species in biology, the preparation of receptor molecules designed to recognize and sense anions is an area of great research interest (Beer and Gale, 2001;Gale, 2003;Martínez-Máñez and Sancenón, 2003). Imidazolium groups are now well-recognized for their favorable features for the generation of anion receptors, which result both from electrostatic and hydrogen bonding interactions (Alcalde et al., 1999(Alcalde et al., , 2007Chellappan et al., 2005;Wong et al., 2005;Yoon et al., 2006;Xu et al., 2010). For example, a series of tetra-imidazolium linked macrocyclic compounds e.g., I (Figure 1) were shown to bind to biologically relevant anions such as chloride and hydrogen sulfate (Wong et al., 2005;Serpell et al., 2011). Additionally, compounds of this type have been used as selective luminescent sensors for nucleic acids and nucleotide derivatives such as DNA, RNA, ATP, and GTP and to sense and image RNA in the living cells, as a result of strong C-H + · · · A − hydrogen bonding interations (Neelakandan and Ramaiah, 2008;Ahmed et al., 2011Ahmed et al., , 2012Shirinfar et al., 2013).
A range of imidazolium linked macrocyclic compounds have been previously reported and both direct macrocyclization (Bass et al., 2010;Altmann et al., 2015;Toure et al., 2016) and stepwise macrocyclization (Mesquida et al., 2013) processes have been previously described for the synthesis of compounds of this type. In the direct macrocyclization approach, equal quantities of a bis-imidazole precursor, and a dihaloalkane are combined under high dilution conditions to produce the desired macrocyclic compound. For example, the imidazolium linked macrocycles II and III were prepared using the direct macrocyclization approach (Hahn et al., 2008;Schulte to Brinke et al., 2013). This approach can be limited due to unintended reactivity between the precursor compounds, which can produce complex reaction mixtures and poor yields of the desired product. Alternatively, stepwise macrocyclization can be used for the synthesis of polyimidazolium linked macrocyclic compounds (Schulte to Brinke and Hahn, 2015) and the synthesis of the tetra-imidazolium macrocycle I was achieved via sequential alkylation of bis-imidazolium precursors (Wong et al., 2005).
Polyimidazolium linked macrocycles have also been utilized as precursors for N-heterocyclic carbene metal complexes and previously the Au(III) complex IV (Figure 2) was prepared the reaction of KAuCl 4 with a macrocyclic tetra-imidazolium salt II (Mageed et al., 2017). Additionally, dinuclear Au(II) and mixed valence Au(I)/Au(III) complexes of have been prepared by the oxidation of dinuclear Au(I) complexes of macrocyclic NHC ligands (Mageed et al., 2018). In the past decade, a number of Ag(I) complexes of macrocyclic NHC ligands that display wide range of structures have been reported (Mckie et al., 2007;Hahn et al., 2008;Schulte to Brinke et al., 2013;Altmann et al., 2016;Fei et al., 2017;Lu et al., 2017Lu et al., , 2018. For example, a sandwich like tetranuclear Ag(I) complex featuring two tetra-NHC ligands was prepared V (Figure 2) and this complex was utilized as a precursor for the preparation of Au(I), Ni(II), Pd(II), and Pt(II) complexes (Altmann et al., 2016). In addition, macrocyclic tetra-NHC ligands have been shown to act as tetradentate ligand for square-planer metals such as Pd(II), Ni(II), Cu(I), and Pt(II)  and the interesting square-planar Pt(II) complex VI (Figure 2) was prepared by metal-template reaction from the tetrakis(trimethylphosphane)platinum(II) triflate and 2-azidophenylisocyanide (Hahn et al., 2005). The biological properties of metal complexes of NHC-based cyclophane and macrocyclic ligands have also been of significant recent interest and Ag(I) and Au(I) complexes have been shown to possess potent antimicrobial and anticancer activities, respectively (Aweda et al., 2013;Shah et al., 2013;Nomiya et al., 2018;Pöthig et al., 2018). Youngs et al. have been particularly active in this field (Hindi et al., 2009;Johnson et al., 2017) and have described the potent antimicrobrial properties of a Ag(I)-NHC complexes of cyclophane ligands (Melaiye et al., 2005). Additionally, a series of Au(I) complexes of related cyclophane-based NHC ligand systems were shown to be selectively toxic to cancer cells as a result of an antimitochondrial mechanism (Barnard et al., 2004(Barnard et al., , 2006. Meyer and more recently Kühn have also extended the application of macrocyclic tetra-NHC ligands to the synthesis of iron complexes, that provide fascinating models of reactive intermediates that are generated in the catalytic cycles of a range of heme and non-heme iron enzymes (Meyer et al., 2013;Anneser et al., 2015).
As imidazolium linked macrocycles offer the potential for the generations of novel sensors for biologically significant anions and as precursors for NHC metal complexes of metals that are well-known for their biological properties, we became interested in developing new strategies for the synthesis of compounds of this type. In the present paper, we report a novel modular stepwise synthetic approach for the synthesis of tetraimidazolium macrocycles. This approach involves the initial synthesis of bis-bromoethylimidazolium bromide precursors, which can then be utilized for the formation of either symmetrical or asymmetrical tetra-imidazolium macrocycles. These tetra-imidazolium salts bind anions in solution and association constants between two of the macrocycles and the halide anions chloride, bromide, and iodide were determined for two macrocycles. A range of silver(I), gold(I), and palladium(II) NHC complexes have been prepared from these pro-ligands.

Synthesis of Tetra-imidazolium Macrocycles
The ortho-phenylene linked tetra-imidazolium macrocycle 9·Br 4 was prepared via a stepwise macrocyclization procedure. Initially, 1,2-bis(imidazolylmethyl)benzene 1 (Baker et al., 2001) was alkylated with an excess of 1,2-dibromoethane (30 equivalents) to obtain the bis-bromoethylimidazolium bromide salt 5·Br 2 in a moderate yield (Scheme 1). The 1 H-NMR spectrum of 5·Br 2 shows the imidazolium C2-H proton resonates as a singlet signal at 9.50 ppm, while the ethylene group protons resonate as two upfield shifted triplet signals at 4.00 and 4.68 ppm. The macrocycle 9·Br 4 , was prepared by heating an equimolar mixture of 5·Br 2 and 1 in a solvent system consisting of a 1:4 mixture of DMF and acetonitrile under high dilution conditions (Scheme 1) and was isolated in a moderate yield of 29.7%. Compound 9·Br 4 gave a simple 1 H-NMR spectrum consistent with its high symmetry (point group D 2h ) with downfield shifted signals for the imidazolium protons which resonated at 9.45 (C2-H) and 7.98 and 7.88 ppm (C4-H/C5-H). The protons of the ethylene linker groups resonated as a singlet signal at 4.81 ppm. A similar method was adopted for the synthesis of the metaphenylene linked macrocycle 10·Br 4 from a mixture of 2 and 6·Br 2 (Scheme 1).
With the successful synthesis of the symmetrical macrocycles 9·Br 4 and 10·Br 4 , we were interested in further exploring the versatility of our stepwise synthetic methodology with the synthesis of asymmetric tetra-imidazolium salts. In initial studies, reaction of either 3 with 5·Br 2 or 1 with 7·Br 2 (Scheme 1) under high dilution condition did not successfully produce the desired asymmetrical tetra-imidazolium salts. Additionally, attempts to use tetra-n-butylammonium bromide (Bu 4 N·Br) as a templation reagent in the reaction of 3 with 5·Br 4 also did not give the desired product. By contrast, the reaction of compound 1 with  7·Br 2 under high dilution condition in presence of Bu 4 N·Br gave the desired product 11·Br 4 in a low yield. The 1 H-NMR spectrum of 11·Br 4 showed the imidazolium C2-H proton as singlet signal at 9.76 ppm and the methyl group on the imidazolium C2 carbon resonated as a singlet signal at 2.64 ppm. In addition, consistent with the lower symmetry structure (point group C 2h ) the benzylic proton resonates as two singlet signals at 5.47 and 5.52 ppm. The asymmetrical pro-ligands 12·Br 4 and 13·Br 4 were prepared in a similar manner from the reaction of either 2 and 8·Br 2 or 2 and 5·Br 2 , respectively, in presence of Bu 4 N·Br. Again, the 1 H-NMR spectrum for 13·Br 4 is consistent with the lower symmetry structure, with the C2-H protons resonating as two singlet signals at 8.99 and 9.13 ppm.

Synthesis of Ag(I), Au(I), and Pd(II) Complexes
The Ag(I) complex 14·(PF 6 ) 6 was prepared by the reaction of pro-ligand 9·Br 4 with Ag 2 O in DMF with the exclusion of light (Scheme 2) and product was isolated as a white crystalline solid in a yield of 21%. The 1 H-NMR spectrum of 14·(PF 6 ) 6 showed no imidazolium C2-H proton signal, indicating that the C2 carbon is deprotonated and coordinated to the metal center as a carbene. The ortho-substituted phenyl-linker group protons resonate as two set of doublets (5.68 and 6.94 ppm) and triplet (7.17 and 7.52 ppm) signals, consistent with a more complex magnetic environment for the Ag(I) complex when compared to the pro-ligand. The 13 C-NMR spectrum for 14·(PF 6 ) 6 revealed a downfield shifted signal at 182 ppm, for which 107 Ag-13 C (d, 1 J = 182.32 Hz) and 109 Ag-13 C (d, 1 J = 209.99 Hz) couplings were observed, which is also consistent with coordination of the C2 carbon to Ag(I). The high-resolution mass spectrum for 14·(PF 6 ) 6 produced a series of peaks consistent with a hexanuclear structure with the general formula [Ag 6 L 3 ] 6+ (where L is the macrocyclic tetra-carbene ligand). For example, a peak was observed at m/z = 372.0334, which corresponds to the formula [C 96 H 96 N 24 Ag 6 ] 6+ (calculated = 372.0419).
Using a similar approach, the Ag(I) complex 17·(PF 6 ) 3 was prepared from the pro-ligand 10·Br 4 , however for this compound the 1 H-NMR spectrum showed a signal at 8.37 ppm, consistent with the C2-H proton being present in the complex. However, the 13 C-NMR spectrum of 17·(PF 6 ) 3 showed a downfield shifted signal at 180.4 ppm that displays 107 Ag-13 C (d, 1 J = 183.58 Hz) and 109 Ag-13 C (d, 1 J = 211.24 Hz) couplings, indicating coordination of the imidazole C2 carbon to Ag(I). These NMR results are consistent with a mononuclear complex, where two of the imidazole units are coordinated to the metal, while the other two remain uncoordinated imidazolium units (Scheme 3). To further investigate this result, a different synthetic method was undertaken where the proligand 10·(PF 6 ) 4 was reacted with AgNO 3 in present of NH 4 OH, however the same mononuclear Ag(I) complex was obtained (Scheme 3). The hexanuclear Au(I) complex 15·Br 6 was prepared by the reaction of 9·Br 4 with (THT)AuCl in present of the mild base sodium acetate (Scheme 2) and the complex was obtained as an off-white solid in 54.2% yield. The same approach was used in an attempt to prepare the Au(I) complex of proligand 10·Br 4 , however no complex could be isolated from the reaction mixture. In the next set of reactions, the pro-ligand 11·Br 4 (with both normal and C2-blocked imidazolium groups) was reacted with (THT)AuCl in presence of sodium acetate. It was anticipated that this ligand might produce a complex displaying both "normal" and "abnormal" NHC coordination modes, however the dinuclear Au(I) complex 18·(PF 6 ) 2 was obtained, which displayed only the "normal" NHC coordination mode (Scheme 4). Using the same method, a dinuclear Au(I) complex derived from 19·(PF 6 ) 4 was also prepared.
Due to their potential to act as tetradentate ligands with metals that adopt square-planar coordination geometries, the Pd(II) complex of pro-ligand 9·Br 4 was prepared. A range of approached have been previously employed for the preparation of Pd(II)-NHC complexes, including in situ deprotonation and metallation (Baker et al., 2001;Fei et al., 2017) and transmetallation via an intermediate Ag(I) complex (Schulte to Brinke et al., 2013;Andrew et al., 2016). In this work the former in situ deprotonation and metallation approach was initially investigated by reacting 9·(PF 6 ) 4 with Pd(OAc) 2 in DMSO, however the desired Pd(II) complex could not be isolated. As described previously, a hexanuclear Ag(I) complex could be prepared from the pro-ligand 9·(PF 6 ) 4 and in a second attempt to synthesize the Pd(II) complex, pro-ligand 9·Br 4 was first reacted with Ag 2 O to form the Ag(I) complex in situ followed by addition of K 2 PdCl 4 (Scheme 5). 1 H-NMR analysis of Pd(II) complex 16·(PF 6 ) 2 showed a relatively simple spectrum with the C4/5 protons of the NHC groups resonating as two sets of doublets at 7.48 and 7.83 ppm. The benzylic protons resonate as two sets of doublet signals at 5.20 and 6.44 ppm (AX pattern), which is consistent with a rigid molecular structure in solution. Furthermore, the 13 C-NMR spectrum showed a downfield shifted signal at 167.41 ppm, which corresponds to the NHC carbene carbon coordinated to the Pd(II) metal center.
The Ag(I) transmetallation approach did not successfully produced a Pd(II) complex of pro-ligand 11·Br 4 . In this attempt, Ag 2 CO 3 was used instead of Ag 2 O to avoid the undesirable oxidative cleavage of the C2-blocking methyl group which has been observed previously (Chianese et al., 2004). In a second synthetic attempt, the pro-ligand 11·Br 4 was reacted with K 2 PdCl 4 in presence of NaOAc. The 1 H-NMR of the crude product showed no peak for the C2-H proton for the "unblocked" imidazole units indicating that these groups are coordinated to the metal center. The C4/5 protons of the C2 "blocked" imidazolium group at 7.34 and 7.66 ppm suggesting SCHEME 3 | Synthesis of Ag(I) complex 17·(PF 6 ) 3 from the pro-ligand 10·Br 4 . (a) Ag 2 O (4.0 eq.), DMF, 70 • C, 3 d. (b) AgNO 3 (4.0 eq.), NH 4 OH (eq.), CH 3 CN, RT, overnight. SCHEME 4 | Synthesis of dinuclear Au(I) complex derived from 11·Br 4 to 12·Br 4 . that C2 blocked imidazolium unit did not react with the metal. Furthermore, the benzylic proton signals at 5.24 and 5.26 ppm and the ethylene linker signal at 4.61-4.70 ppm also indicated an asymmetrical structure of the title complex. The crystal structure in Figure 5 also suggested the Pd(II) complex 20·(PF 6 ) 2 was successfully synthesized. Unfortunately, it was unable to separate this complex as pure product.
Two representations of the X-ray crystal structure for the Au(I) complex 15·Br 2 (PF 6 ) 4 are shown in Figure 4. This remarkable structure reveals that the Au(I) complex adopts a cyclic hexanuclear supramolecular assembly with the overall formula [Au 6 L 3 ] 6+ . In the cation, each of the Au(I) centers adopt linear two-coordinate geometries, and they can be divided into two distinct groups of three atoms each. For the first group, each Au(I) atoms (Au1, Au4, and Au5) is bound to two NHC units on opposite sides of each of the three ligand molecules. While each of the Au(I) atoms in the second group (Au2, Au3, and Au6) are coordinated to NHC donors from adjacent ligand molecules, resulting in the formation of a metallo-macrocycle. In each case, the ligand molecules are bowl shaped to accommodate the two Au(I) coordination modes. Interestingly, the metallo-macrocycle is arrayed around an encapsulated central bromide counterion. This bromide ion displays interactions with Au(I) atoms Au1, Au4, and Au5 with the distances being 3.19488(5), 3.24731(5), and 3.23064(4) Å, respectively. Previously, a hexanuclear Ag(I) complex was reported for a tetra-carbene ligand linked by aliphatic butyl chains .
A representation of the X-ray crystal structure for the Pd(II) 20·(PF 6 ) 2 is shown in Figure 5. The molecular structure shows that the ligand is coordinated to the metal center through the "normal" NHC groups, with the C2 "blocked" groups present as cationic imidazolium units. The Pd(II) center is fourcoordinate, with the two-remaining sited being occupied by bromide ions.

Anion Binding Studies
Due to the tetracationic charge of the imidazolium linked macrocycles prepared in this work, the propensity of 9·(PF 6 ) 4 and 10·(PF 6 ) 4 to bind to the halide anions F − Cl − , Br − , and I − (as their tetra-n-butylammonium halide salts) was evaluated using 1 H-NMR titration experiments. In initial studies, it was  found that the addition of Bu 4 N·F to 10·(PF 6 ) 4 in d 6 -DMSO caused an immediate color change to pale yellow. The 1 H-NMR analysis showed that the color change occurred concurrently with a significant broadening and downfield shift of the imidazolium C2-H signal. In addition, the appearance of new unidentified 1 H-NMR signals were observed which were consistent with decomposition of the macrocyclic receptor. As such this anion was not studied further.
Addition of increasing equivalents of Bu 4 N·Cl (0.25-14.0 eq.) to a solution of 9·(PF 6 ) 4 in d 6 -DMSO caused a significant downfield shift of the resonance corresponding to the imidazolium C2-H signal from 8.96 to 9.64 ppm. In addition, the benzylic proton signal was also shifted downfield from 5.34 to 5.76 ppm. Unfortunately, this study was hampered by the gradual precipitation of the imidazolium salt at higher Cl − concentrations. In a similar manner, addition of increasing  equivalents of Bu 4 N·Br and Bu 4 N·I to 9·(PF 6 ) 4 caused downfield shifts in the imidazolium C2-H signal, although to a lesser extent than that seen for Bu 4 N·Cl, however for these anions no precipitation of the macrocycle was seen. Figure 6 shows the 1 H-NMR titration between 9·(PF 6 ) 4 and Bu 4 N·Br, while Figure 7 shows the change in the imidazolium C2-H chemical shift for 9·(PF 6 ) 4 in response to increasing equivalents of the added halide anion (solid black lines). Similar results were seen for the macrocyclic receptor 10·(PF 6 ) 4 and the plot of experimental titration data is shown in Figure S2. Job plot analysis was then used to evaluate the stoichiometry of the interactions between the macrocyclic receptors and the anions Cl − , Br − , and I − . The results of these studies Figures S3, S4 showed that maxima occurred at close to χ = 0.5 indicating 1:1 stoichiometry for both receptor 9 4+ and 10 4+ with these anions.
Binding (association) constants K a (M −1 ) for the tetraimidazolium macrocycles 9·(PF 6 ) 4 and 10·(PF 6 ) 4 were determined by analysis of the 1 H-NMR titration data using the computer program HypNMR 2018 (Frassineti et al., 1995(Frassineti et al., , 2003. The fitted binding isotherms are shown in Figure 6 and Figure S2 (red dashed lines) and the calculated binding constants are given in Table 1. The values obtained for the compounds studied in this work are similar in magnitude to those reported previously for related compounds.

CONCLUSION
In conclusion, a novel stepwise synthetic strategy is reported that allows for the synthesis of both symmetrical and asymmetrical tetra-imidazolium linked macrocyclic compounds. The synthetic strategy is modular as initially a range of bis-bromoethylimidazolium bromide precursors were synthesized, which when combined with chosen bis(imidazolylmethyl)benzene molecules produced a range of tetra-imidazolium linked macrocycles. These tetra-imidazolium salts are of significant interest as they bind anions in solution and offer the potential for the development of sensors for biologically relevant anions. Using 1 H-NMR titration studies, the association constants between 9·(PF 6 ) 4 and 10·(PF 6 ) 4 and the halide anions Cl − , Br − , and I − were determined and these ranged between 501 and 20 M −1 . Imidazolium salts are precursors for N-heterocyclic carbene (NHC) ligands and given that Ag(I) and Au(I) complexes of NHC ligands have been shown to display antimicrobial (Aweda et al., 2013;Shah et al., 2013) and anticancer (Barnard et al., 2004) properties, respectively, the Ag(I), Au(I), and Pd(II) NHC complexes derived from these tetra-imidazolium linked macrocycles were prepared. The Ag(I) complexes 14 6+ and the analogous Au(I) complex 15 6+ adopted intriguing hexanuclear structures with the general formula [M 6 L 3 ] 6+ . Currently we are exploring the development of silver complexes of NHC ligands as potential antimicrobial agents and supramolecular assemblies such as complex 14 6+ are a particular focus for the slow release of silver ions and the results of these studies will be reported in due course.

EXPERIMENTAL General Details
All solvents and chemicals were purchased from Sigma-Aldrich, Chem Supply, Alfa Aesar, and were used as received unless otherwise stated. Where necessary, solvents were further purified using an Innovative Technology Pure Solv solvent purification system. All experiments were performed under an atmosphere of N 2 unless otherwise stated. 1 H-NMR and 13 C-NMR spectra were recorded on Bruker Advance 500 (500.023 MHz for 1 H and 125.74 MHz for 13 C) and spectra were referenced to solvent resonances. Where required, COZY, HSQC, HMBC, and NOESY 2-dimensional experiments were used to assist assignments. Mass spectra were obtained using an Agilent 6530 Q-TOF LC/MS mass spectrometer fitted with an Agilent electrospray ion (ESI) source.

X-ray Crystallography
Single crystals suitable for X-ray diffraction studies were grown as follows: 5·Br 2 and 9·Br 4 diffusion of diethyl ether into an methanol solution of the title compound; 10·Br 3 PF 6 diffusion of diethyl ether into an acetonitrile solution of the title compound containing two drops of a solution of tetra-n-butylammonium bromide in acetonitrile; 11·Br 4 slow evaporation of a methanol solution of the title compound; 15·Br 2 (PF 6 ) 4 diffusion of ethyl acetate into an acetonitrile solution of the title compound; 20·(PF 6 ) 2 was grown by slow diffusion of diethyl ether into an acetonitrile solution of the titled compound. Crystallographic data for all structures determined are given in Table S1. For all samples, crystals were removed from the crystallization vial and immediately coated with paratone oil on a glass slide. A suitable crystal was mounted in Paratone oil on a glass fiber and cooled rapidly to 173 K in a stream of cold N 2 using an Oxford low temperature device. Diffraction data were measured using an Rigaku Oxford Diffraction SuperNova X-ray Diffraction System mounted with Mo-Kα λ = 0.71073 Å and Cu-Kα λ = 1.54184. Data were reduced and corrected for absorption using the CrysAlis Pro program. The SHELXL2013-2 program was used to solve the structures with Direct Methods, with refinement by the Full-Matrix Least-Squares refinement techniques on F2. The non-hydrogen atoms were refined anisotropically and hydrogen atoms were placed geometrically and refined using the riding model. Coordinates and anisotropic thermal parameters of all non-hydrogen atoms were refined. All calculations were carried out using the program Olex2. Further XRD details are provided in the Supporting Information. CCDC 1885400-1885405 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

H-NMR Titration Studies
A solution of the title compound (10 mg) in d 6 -DMSO (600 µL) and a 1.5 M solution of Bu 4 N·X (X = Cl, Br and I) in d 6 -DMSO were prepared, respectively. To the solution of title compound, increasing equivalents (0.25-14.0 eq.) of 1.5 M Bu 4 N·X solution was added and the resultant solution was thoroughly mixed. The 1 H-NMR spectrum was recorded ∼2 min after each addition at 302 K.

Jobs Plot Analysis
A solution of the title compound (10 mg/mL) in d 6 -DMSO and a 0.050 M solution of Bu 4 N·X (X=Cl, Br and I) in d 6 -DMSO were prepared, respectively. A varied fraction of title compound solution and Bu 4 N·X solution was added and diluted with d 6 -DMSO to 600 µL to maintain the total concentration of substance at 10 mM. The resultant solution was thoroughly mixed and 1 H-NMR spectrum was recorded at 302 K.

AUTHOR CONTRIBUTIONS
PB conceived of the presented idea. ZL synthesized compounds and recorded NMR spectra. NW recorded and analyzed high resolution mass spectra. PB and ZL collected X-ray diffraction data. All authors discussed the results and contributed to the final manuscript.

FUNDING
This research was financially supported by The Australia Research Council, DP150102741 and ZL acknowledges receipt of a La Trobe University postgraduate award.

SUPPLEMENTARY MATERIAL
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.