Comment on “Lewis acidic ionic liquids of crown ether complex cations: preparation and applications in organic reactions” by Y. Liang, J. Wang, C. Cheng and H. Jing, RSC Adv., 2016, 6, 93546

Introduction

A paper published recently in RSC Advances reports on a new group of solvate ionic liquids,¹ generated by the combination of a crown ether (18-crown-6), an alkali metal chloride (NaCl or KCl) and a halide-abstracting agent, which was also a metal chloride (AlCl₃, FeCl₃, or ZnCl₂).² The structures of synthesised solvate ionic liquids were postulated (Fig. 1, left). The anion structure was confirmed using Raman spectroscopy, with bands characteristic of M–Cl stretching frequencies reported, as shown in Fig. 1, right.

Fig. 1 Left: postulated structures of Lewis acidic ionic liquids; right: fragments of Raman spectra, corresponding to M–Cl stretching frequencies. Adapted from ref. 2.

The assignment of bands in spectrum A to [AlCl₄]⁻ and in spectra B and D to [FeCl₄]⁻ is consistent with all literature reports,³ and supported with appropriate citations. However, assignment of the band at 287 cm⁻¹ (spectra C and E) to [ZnCl₃]⁻ is in stark contrast with the current knowledge.

Anionic speciation in chlorometallate ionic liquids has vital impact on their properties: viscosity of ionic liquids with doubly-charged anions is dramatically higher, conductivity is lower, and chemical behaviour differs, therefore it is of utmost importance to have them correctly understood.

Speciation of chlorozincate anions in ionic liquids – Raman spectroscopy and other techniques vs. mass spectrometry

The most common chlorozincate anion is [ZnCl₄]²⁻.⁴ In the Cambridge Structural Database, there is a plethora of deposited single crystal structures with discrete tetrachlorozincate anions – the search for {ZnCl₄} motif returns 849 hits, most of these featuring [ZnCl₄]²⁻.⁵ In contrast, not a single structure with (presumably) trigonal planar [ZnCl₃]⁻ has been deposited to date. In reference books on vibrational spectroscopy, there is no mention of the [ZnCl₃]⁻ anion, but vibrational frequencies for [ZnCl₄]²⁻ are always reported – even in Nakamoto's book from 1963 (ν₁ frequency at 282 cm⁻¹).⁶

Nevertheless, in publications on ionic liquids two opposing speciations are proposed: one assuming the existence of [ZnCl₃]⁻ and [Zn₂Cl₅]⁻ anions,^7,8 and another suggesting the formation of [ZnCl₄]²⁻ and [Zn₂Cl₆]²⁻.⁹ In 2011 we encountered this discrepancy in scientific literature, and decided to address it, as a part of our research on speciation studies of halometallate ionic liquids.^3,10–14

A multi-technique, multi-phase study on chlorozincate(II) ionic liquids with 1-octyl-3-methylimidazolium, [C₈mim]⁺ and 1-ethyl-3-methylimidazolium, [C₂mim]⁺, cations was carried out.¹⁵

In order to study the crystalline phase, single crystals were grown from the [C₂mim]Cl–ZnCl₂ system at varying stoichiometries (defined in terms of molar fractions of ZnCl₂, χ_ZnCl2). The resulting structures were [C₂mim]₂[ZnCl₄] for χ_ZnCl2 = 0.33, and [C₂mim]₂[Zn₂Cl₆] for χ_ZnCl2 = 0.50. Tricoordinate chlorozincate anions were not observed – neither in our study, nor in any other crystallographic work (as per CSD search).⁵ Raman vibrations assigned to the Zn–Cl vibrations in these solids were 280 cm⁻¹ for [ZnCl₄]²⁻, and 264 and 317 cm⁻¹ for [Zn₂Cl₆]²⁻.

In Raman spectroscopic study of the liquid [C₈mim]Cl–ZnCl₂ system, corresponding species were detected, with the 275 cm⁻¹ band for [ZnCl₄]²⁻ at χ_ZnCl2 = 0.33, as well as 269 and 312 cm⁻¹ bands for [Zn₂Cl₆]²⁻ at χ_ZnCl2 = 0.50. Thus, the presence of [ZnCl₄]²⁻ and [Zn₂Cl₆]²⁻ anions was confirmed in both liquid and solid chlorozincate(II) ionic liquids.

Physico-chemical properties of the [C₈mim]Cl–ZnCl₂ system were also studied. Particularly telling was the study of Lewis acidity (Gutmann acceptor number approach, using triethylphosphine oxide as the ³¹P NMR spectroscopic probe). The most significant increase in Lewis acidity was noted at χ_ZnCl2 = 0.33, that is when [ZnCl₄]²⁻ ions start being replaced by [Zn₂Cl₆]²⁻ ions (Fig. 2). Simply based on stoichiometry, such pattern does not agree with the [ZnCl₃]⁻/[Zn₂Cl₅]⁻ speciation, for which there would be no discontinuity in properties at χ_ZnCl2 = 0.33.

Fig. 2 Plot of the ³¹P NMR shifts of triethylphosphine oxide (spectroscopic probe of Lewis acidity) dissolved in various compositions of the [C₈mim]Cl–ZnCl₂ system, and extrapolated to infinite dilution, δ_inf, as a function of composition (χ_ZnCl2). The dotted lines are only visual guides. Adapted from ref. 15.

Doubly-charged speciation was also inferred from the pseudo-phase diagram, generated from glass transition in the [C₈mim]Cl–ZnCl₂ system

Finally, ESI-MS spectra of several [C₈mim]Cl–ZnCl₂ samples were recorded – and, in contrast to all other multiple techniques, suggested the presence of [ZnCl₃]⁻ and [Zn₂Cl₅]⁻.

In the literature, although ESI-MS was successfully used to study speciation in a wide variety of compounds, including some complex ionic liquids, e.g. polyiodides,¹⁶ numerous pitfalls are also reported. Notable are in-depth studies on perturbations in MS by Di Marco and co-workers,^17,18 and a recent work by Clark and co-workers, entitled “Failure of ESI Spectra to Represent Metal-Complex Solution Composition: A Study of Lanthanide–Carboxylate Complexes”.¹⁹ The likely source of these discrepancies lays the fact that ions detected in MS are in fact in the gas phase, not in the liquid. The mechanisms leading to the formation of gas-phase ions from liquid-phase droplets are discussed expertly by Kebarle,^20–22 with a recent tutorial review by Awad et al. offering a simple summary on the introductory level.²³

In our chlorozincate study, under the ESI-MS conditions, the electrostatic factors most likely encouraged the separation of the doubly-negative charge in [Zn₂Cl₆]²⁻ into two singly-charged [ZnCl₃]⁻ anions.¹⁵ Identical conclusions were reached in earlier work by Alves et al., published in Journal of Raman Spectroscopy, who stated “there is no experimental result that supports the formation of [ZnCl₃]⁻ or [Zn₂Cl₅]⁻ species, but there are enough experimental data on the formation of [ZnCl₄]²⁻ and [Zn₂Cl₆]²⁻ species in ZnCl₂/LiCl melts (…). Therefore, we interpret the detection of [ZnCl₃]⁻ species by EMS as the result of the reaction [ZnCl₄]²⁻ → [ZnCl₃]⁻ + Cl⁻”.⁹ They also report the [ZnCl₄]²⁻ anion vibration at 276 cm⁻¹, and list older studies on [ZnCl₄]²⁻ in various environments, with analogous vibrations at 275, 286 and 288 cm⁻¹.

Subsequent investigations were carried out by the licence group using XPS²⁴ and by the Abbott group using EXAFS.²⁵ Both were consistent with the presence of [ZnCl₄]²⁻ and [Zn₂Cl₆]²⁻ species.

Despite this breadth of work, Liang et al. assigned the Raman band at 287 cm⁻¹ to [ZnCl₃]⁻, citing neither of the above-mentioned articles, but a 1999 paper on aqueous chemistry of zinc chloride, published in Desalination, where the existence of such anion is indeed suggested, but with no further literature ref. 26.

Conclusion

Independent work carried out by several groups, using a very wide range of direct and indirect techniques, demonstrates explicitly two facts: (1) vibrational frequency at 287 cm⁻¹ is in good agreement with a band corresponding to [ZnCl₄]²⁻, and should be reported as such, and (2) the presence of [ZnCl₃]⁻ in chlorozincate(II) ionic liquids, albeit often cited, may be traced back to mass spectrometry studies, which return results contradictive to: Raman spectroscopy, XPS, EXAFS, and physico-chemical properties of chlorozincate(II) ionic liquids.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

I would like to thank Prof. Michael D. Ward for his very constructive remarks that contributed to the clarity of this manuscript.

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