This is the published version:   Dean,PamelaM.,Pringle,JenniferM.andMacFarlane,DouglasR.2010,Structuralanalysisoflowmeltingorganicsalts:perspectivesonionicliquids,Physicalchemistrychemicalphysics,vol.12,no.32,pp.9144‐9153.

Available from Deakin Research Online:  http://hdl.handle.net/10536/DRO/DU:30062935ReproducedwiththekindpermissionofthecopyrightownerCopyright:2010,RoyalSocietyofChemistry

Structural analysis of low melting organic salts: perspectives

on ionic liquids

Pamela M. Dean,* Jennifer M. Pringle and Douglas R. MacFarlane

Received 23rd February 2010, Accepted 3rd June 2010

DOI: 10.1039/c003519j

Ionic liquid-forming salts often display low melting points (a lack of crystallisation at ambient

temperature and pressure) as a result of decreased lattice energies in the crystalline state.

Intermolecular interactions between the anion and cation, and the conformational states of each

component of the salt, are of significant interest as many of the distinctive properties ascribed to

ionic liquids are determined to a large extent by these interactions. Crystallographic analysis

provides a direct insight into the spatial relationship between the cations and anions and provides

a basis for an enhanced understanding of the physico-chemical relationship of the ionic liquids.

This perspective article examines the crystallographic studies of relevance to ionic liquid-forming

organic salts as a basis for the rational design and synthesis of novel ionic liquids.

1. Ionic liquids

1.1 Introduction

Ionic liquids are, simply put, liquids comprised entirely of

ions. The currently favoured, yet arbitrary, definition is that

an ionic liquid is a salt with a melting temperature or glass

transition below the boiling point of water.1 Chemists first

discovered the earliest generation of ILs, then termed ‘molten

salts’, in the mid-19th century when, during a Friedel–Crafts

reaction involving benzene and chloromethane and using an

AlCl3 Lewis acid catalyst to form toluene, a ‘red oil’ formed.

This ‘red oil’, a tetrachloroaluminate anion and a cation, was

one of the earliest documented ionic liquids.1,2 These first

generation ILs, however, were prone to hydrolysis and it was

not until the 1990’s that air and water stable ILs were

generated,1 leading the way for the explosion in IL synthesis

and application. The currently favoured ionic liquids involve

imidazolium, pyrrolidinium, pyridinium, tetraalkylammonium

and tetraalkylphosphonium cations with, for example,

bis(trifluoromethanesulfonyl)amide, trifluoromethanesulfonate,

dicyanamide, p-toluenesulfonate, tetrafluoroborate and hexa-

fluorophosphate anions (Fig. 1).3

ILs exhibit various properties, some of which are uniform to

all ILs and others that are ascribed to particular members of

the IL family.3,4 The observation has been made that over 1018

ionic liquids could be prepared by varying the cation and/or

the anion,5 thus ILs have been termed ‘designer solvents’. This

variability provides excellent opportunities to design new ionic

liquids with different chemical and physical properties,6 but it

also presents a complex challenge in which a rational design

strategy is required in order to produce the liquid in favour of

the crystalline form. Understanding the structural factors that

underpin the formation and stability of low melting salts is key

to the design and development of novel members of this

immense family of liquid salts.

1.2 Towards the design of ionic liquids

The departure from molten salts to the currently used ionic

liquids, which involve organic heterocyclic cations and

inorganic anions, traces back to the 1960s. The aim for the

originator of this research, L. A. King, was to design and

School of Chemistry, Monash University, Wellington Road, Clayton,Victoria, 3800, Australia. E-mail: pamela.dean@sci.monash.edu.au;Fax: +61 3 99054597; Tel: +61 3 99054599

Pamela M. Dean

Dr Pamela Dean received herMSc degree, within supra-molecular chemistry, at theUniversity of Cape Town in2005. Thereafter she wasawarded two university scholar-ships to complete her PhDdegree, involving the structuralanalysis of ionic liquids, atMonash University, Australia,in 2009. Currently her researchfocuses on the structural andenergetic analysis of lowmelting organic ionic salts.

Jennifer M. Pringle

Dr Jenny Pringle received herdegree and PhD, on ionicliquids, at The University ofEdinburgh in Scotland beforemoving to Monash Universityin 2002. In 2008 she started anARC QEII Fellowship, inassociation with the ARCCentre of Excellence forElectromaterials Science(ACES), which focuses herresearch on ionic liquids andplastic crystals into the area ofdye-sensitized solar cells.

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prepare molten salt electrolytes with low melting temperatures

using an alkali chloride–aluminium chloride system to reduce

the temperature-related problems within thermal batteries.1

Until the mid 1990’s the drive towards ionic liquid design was

for electrochemical applications,2,4,7 the choice of ions being

determined by the sensitivity of the ions towards hydrolysis,

their thermal stability, electrochemical window and conductive

ability. It was recognized that the resultant properties of the IL

depended on the structure of the cation and anion, thus

making ILs tunable and task-specific. Consequently, the ions

chosen were of low symmetry and exhibited strong charge

delocalisation, thus reducing the possibility of hydrogen

bonding and subsequent higher viscosity.7 Fluorination

of the anion also effectively reduces the hydrogen bonding

ability, decreases the viscosity and increases the thermal and

electrochemical stability, all of which are favourable for

electrochemical applications.4,7

Subsequent IL design has been founded on these early

studies and directed towards the physicochemical properties

that the ILs exhibit. A lucid example of IL design are the

pyrrolidinium-based ILs. These were initially developed for

electrochemical devices and thus required high conductivities,

liquidity at ambient conditions and chemical and electro-

chemical stability.8 At that time, imidazolium-based ILs were

favoured, but it was unclear as to whether or not the 1-alkyl-3-

alkylimidazolium cations were unreactive at the C(2) position;

the imidazolium C2 atom shows an electron deficit due to the

CQN bond and thus has, relative to other carbon atoms

within the ring, higher acidity. Low symmetry quaternary

ammonium cations were also known, but these typically

display melting points above ambient temperature. Hence, in

an effort to produce sub-ambient melting ILs (known as room

temperature ionic liquids, RTILs) pyrrolidinium salts, a family

of heterocyclic compounds containing an ammonium cation,

were prepared. These display lower symmetry than the easily

accessible ammonium salts, in addition to structurally lying

between the three dimensional quaternary ammoniums and

the essentially planar imidazoliums.8

However, despite attempts to correlate structure with

melting point,9 it is presently difficult to absolutely predict

the melting point of an IL formed from a new cation/anion

combination. Even prediction of which combination will yield

an RTIL is challenging, and it has been reported that there

may be 1 trillion possible cation and anion combinations.5 It is

thus essential to develop a systematic method for selecting new

ion pairs, and to use this as a predictive tool in the rational

design of new ionic liquids. This topic has been the focus of

ongoing research in the field and the subject of numerous

books, articles and reviews.1,3,6,10,11

A novel approach recently developed in our group that

combines the fundamental knowledge acquired from structural

ionic liquid studies and the crystal engineering12,13 method is

the so-called anti crystal-engineering approach.14 Crystal en-

gineering is defined as the ability to assemble molecular or

ionic components into the desired architecture by engineering

a target network of supramolecular interactions.12 Simply put,

crystal engineering is making crystals by design. Supra-

molecular interactions, in this context, include coulombic

attractions and/or repulsions and non-covalent interactions

(van der Waals, hydrogen bonding, etc.). In anti-crystal

engineering this concept is reversed in the pursuit of liquids.

Thus, instead of selecting functional groups that result in

intermolecular interactions that encourage crystallisation, the

opposite is pursued. Accordingly, by using prior knowledge of

the intermolecular interactions and ionic conformations that

occur in ‘traditional’ ionic liquids, combined with the knowledge

of which interactions result in crystallinity, appropriate starting

materials are selected to yield RTILs.

1.3 Recent developments in the application of ionic liquids

The application of an IL is clearly dependent on its physical

properties. As many ILs possess negligible vapour pressure

and high thermal stability they can be favourably used in place

Fig. 1 Select examples of cations and anions used for ionic liquids.

Douglas R. MacFarlane

Professor Doug MacFarlaneleads the Monash IonicLiquids Group at MonashUniversity. He is also theprogram leader of the EnergyProgram in ACES. He was aPhD graduate of PurdueUniversity in 1982 and afterpostdoctoral work at VictoriaUniversity, Wellington tookup a faculty position atMonash University. ProfessorMacFarlane was recentlyawarded an AustralianResearch Council FederationFellowship to extend his work

on Ionic Liquids. He was elected to the Australian Academy ofSciences in 2007. His research interests include the chemistryand properties of ionic liquids and solids and their application ina wide range of technologies from electrochemical (batteries,fuel cells, solar cells and corrosion prevention), to biotechnology(drug ionic liquids and protein stabilisation) and biofuelprocessing.

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of molecular solvents (volatile organic compounds, VOCs),

thus making them desirable in ‘green’ synthesis, separations

and electrolytes.15,16 Additionally, these IL solvents provide

an excellent solvent media for homogeneous catalysis—the

so-called ‘clean catalysis’.6 The high ionic conductivity of ILs

enables their use in a wide variety of electrolyte applications

including batteries, capacitors and solar cells.17 The wide

liquid range and thermal stability of some ILs allows for their

use as lubricants, as non-volatile solvents, as extractants in

chemical synthesis and in electrochemical processes at elevated

temperatures.4,18,19

Recently, biocompatible uses have also emerged, such

as media for enzymatic reactions,20 DNA21 and protein

solubilisation.22 Building on this, applications in the pharma-

ceutical industry include utilising ionic liquids for solubilising

drugs,23 determination of drug enantiomers using optically

active ILs,24 and the use of novel ionic liquids comprising

active pharmaceutical ingredients (APIs).14,25–27 Active ionic

liquids (AILs) represent a new liquid salt phase that may be of

interest to the pharmaceutical industry as an addition to

the repertoire of known phases of active compounds. AILs

complement the various crystalline and amorphous solid salt

phases that are already recognised as different forms of a drug

compound (and patentable as such) and yet mitigate issues of

polymorphism or crystallisation. These active ionic liquids can

be formed from the active ions in combination with simple,

generally recognised as safe (GRAS),28 counter ions. Further,

the possibility exists for a combination of both cationic and

anionic active pharmaceutical ingredients, thereby providing

dual therapeutic action.

1.4 The melting point: structural considerations

Ionic liquids display low melting points (a lack of crystal-

lisation at ambient temperature and pressure) as a result of

lower lattice energies compared to more traditional organic

salts.29,30 Low lattice energies result from the use of large

asymmetric ions that have a shielded or delocalised charge,

thus frustrating coordination and minimising anion–cation

interaction.31,32 Additionally, in some cases the materials can

possess dynamic disorder, such as the rotational disorder/

mobility seen in organic ionic plastic crystals.33,34

Interesting melting point trends are also observed with the

slight modification of specific ions. For example, changes in

the ring substitutions within imidazolium and pyrrolidinium

salts can have a significant effect on the melting point. A well

known trend in the 1-alkyl-3-methylimidazolium cation family

is the reduction of melting point with increasing alkyl chain

length, up to n o 8. With n > 8 the compounds exhibit an

increase in melting point, which is attributed to inter-chain

hydrophobic packing and the subsequent formation of bilayer-

type structures.35 For most pyrrolidinium salts the minimum

melting point occurs for alkyl chain lengths of either n = 3

or 4.36 Furthermore, an increased melting point is observed

with branching of the alkyl chain substituent and/or the

addition of particular functional groups, as discussed later.29

Further to the structural considerations of the respective

ions, the degree of anion–cation contact with respect to the

type, strength and number of interactions is a major factor

determining the lattice energies, melting point, conductivity,

viscosity and general behaviour of ionic liquids.37–40 Thus, a

thorough understanding of the intermolecular interactions in

known ionic liquids is of significant importance for ionic liquid

design. This perspective serves to present a brief overview

of the literature detailing the crystallographic analysis of

low-melting organic salts, focussing on intermolecular inter-

actions and packing and the effects of modification of the ionic

constituents.

2. Crystallographic analysis of low-melting

organic salts

2.1 Introduction

Detailed investigations of the molecular structures and inter-

molecular interactions of ionic liquids have thus far utilised

simulations and theoretical approaches, spectroscopic and

chromatographic methods and X-ray crystallographic

studies.1,3,10,41,42 In the majority of these previous studies, a

correlation has been found between the structure of the solid

and the liquid.10,43 Thus, solid state analysis allows direct

insight into the spatial relationship between the cation and

anion through the elucidation of crystal structures and this

provides a strong basis from which structural features of the

ionic liquid can be further understood. A number of crystallo-

graphic studies of crystallised ILs have been pursued in an

effort to understand both short range and coulombic interactions.

In this vein, we have recently used the technique of Hirshfeld

analysis to investigate the intermolecular interactions,

conformations and packing of a series of bisamide salts

(Fig. 2).44 Hirshfeld analysis is performed using the software

program crystalexplorer.45–47 This software explores packing

modes and intermolecular interactions by generating a unique

Hirshfeld surface and associated fingerprint plot from the

crystal structure of each salt. In this manner all interaction

types are readily identifiable. For example, within the bisamide

salts the existence of anion–anion interactions such as C–F� � �pand C–H� � �p contacts were identified.44 Additionally, the

C–H� � �O interaction was found to be the dominant interaction

in all of the salts studied. As the alkyl chain length of the

cation increased, Hirshfeld analysis was able to identify ‘voids’

in the structure that correspond to non-optimum packing

and a correspondingly lower melting point. These results

demonstrate the applicability of the Hirshfeld surface approach

Fig. 2 Hirshfeld surface for the anion of 1-ethyl-3-methylimidazolium

bis(methanesulfonyl)amide.44

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in investigating the molecular origins of the physical properties

of ILs.

2.2 Phase behaviour

Ionic liquids display complex crystallization behaviour.

Firstly, crystals of ILs that are liquid at room temperature

are technically challenging to grow and select for crystallo-

graphic studies as standard crystal growth techniques are more

difficult to perform because of the low temperatures involved.

In addition, the deliquescent nature of many hydrophilic ILs

make them difficult to prepare for diffraction purposes. Aside

from the practical aspects of crystallographic analysis,

ILs have been found to display very complicated thermal

behaviour, such as supercooling and glass formation, which

are due to kinetic factors, and pre-melting over a wide

temperature range and solid–solid phase transitions which

are thermodynamically controlled.29,40 Several factors affect

the energy barrier that influences whether a kinetic or thermo-

dynamic crystallisation product results, such as molecular

mobility, molecular assembly, packing and the creation of

an interface. Consequently, different crystallisation methods

need to be used to address these issues. For example,

Choudhury et al. developed an in situ crystallization method

(within the diffractometer) involving a combination of zone-

melting and cryo-crystallographic techniques, thus successfully

forming diffraction quality single crystals of several ILs with

melting points between +6.7 to �25.7 1C.41,42 Another group

has successfully used a modification of this theme to produce

crystalline samples of RTILs.48 In addition other novel methods,

such as the use of multiwalled carbon nanotubes, have been

used to crystallize ILs.49

Further studies relating the thermal behaviour of ILs to

their crystal structures have been performed in order to

understand the structural origins. For example, Holbrey et al.

identified polymorphs of [C4mim][Cl], which result from variances

in the alkyl chain conformation, thus concluding that it is the

inefficient packing of these polymorphs, and the subsequent

creation of eutectic mixtures, that results in the inhibition of

crystallization.30 In a later study, it was also suggested that

[C4mim][Cl] and [C4mim][Br] display supercooling and wide

pre-melting as a result of the dynamics of the conformational

change between the gauche–trans and trans–trans conformations

of the butyl group on the cation, thus confirming the effects of

alkyl chain conformation.50

The structural analysis of salts with disordered phases, such

as plastic crystals, has been performed by ourselves and

others.51–55 These salts display several solid–solid transitions

prior to melting, which represent the onset of rotational

disorder and, in some cases free rotation, of the ions within

the lattice.51,53 Plastic crystals are useful as solid state electro-

lytes as the rotational mobility of the ions can result in high

ionic conductivity.34 Henderson et al. have pursued solid-state

structural studies of NTf2-based plastic crystals in order to

elucidate any correlation between the disorder and the melting

point and found that the NTf2 anion displays a C2 2 C2

disordering mechanism, which may contribute to the salts’

low melting behaviour.55,56 Similarly, we have performed

comparative studies on a series of salts containing monohalide

or trihalide anions.53 Crystallographic analysis revealed

significant cation disorder within the plastic crystal pyrrolidinium

salts (Fig. 3). Finally, certain imidazolium-based ionic liquids

have been found to display thermotropic smectic liquid crystalline

mesomorphism and, thus, the so called ‘‘ionic liquid crystal’’

was introduced.57 For example, 1-alkyl-3-methylimidazolium

salts were found to display liquid crystalline behaviour with

longer alkyl chains (n = 12–18), due to the interdigitation of

the long alkyl chains.58–60

2.3 Ionic substitution and/or modification

In the drive towards the target-specific design of ILs two

approaches may be followed; the use of entirely new ions or

modification of an existing ion that has previously been

proven to provide low melting ILs (the so-called task specific

ionic liquids).40 Much research has been carried out within

these two areas but, while the subsequent crystallographic

analysis of the new ILs has provided valuable structural

information, it is still difficult to make broad generalisations.

In the cases discussed below, which include novel ions and/or

significant structural modification, the use of a new functional

group resulted in interactions that yielded a crystalline solid

rather than a RTIL. Thus, these provide important information

for IL design via anti-crystal engineering.

Anion substitution/modification. First generation ILs

included halides and perfluorinated anions such as tetra-

fluoroborate (BF4) and hexafluorophosphate (PF6). However

these anions displayed unfavourable properties, such as

sensitivity to hydrolysis, which encouraged the pursuit of more

‘user friendly’ anions.1 Complex halogen-containing anions

such as bis(trifluoromethanesulfonyl)amide (NTf2), trifluoro-

methanesulfonate (OTf) and halogen-free anions such as

dicyanamide and tosylate were subsequently designed and

produced.7,40,61 The NTf2 anion is particularly popular as it

forms ILs with low viscosity and high thermal and electro-

chemical stability.7 Attempted modifications of this anion

have been pursued,62,63 such as synthesis of the non-fluorinated

bis(methanesulfonyl)amide anion, but in this case the resultant

ILs possessed an increased viscosity and decreased thermal

and electrochemical stability.64

Research into novel anions initially focused on those that

reduce the level of inter-ionic interactions, such as ILs containing

cyano groups, fluorocomplex anions and interhalogens.36,65–67

Thereafter, the use of more complicated anions such as

fluorinated acetylacetones and various derivatives of borate

Fig. 3 Packing diagram of 1-ethyl-1-methylpyrrolidinium tribromide

showing the rotational disorder of the cation.53

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salts, including non-coordinating boron clusters, were synthe-

sised and structurally characterised.68–71 These novel salts

offer relatively low melting points and provide important

insights into the physico-chemical relationship. For example,

crystal structure analysis of several N,N-dialkylimidazolium

carborane salts suggested that it is the packing inefficiency, as

indicated by the positional disorder of the cation, that is the

main determinant of the low melting points of these salts.72

Similarly, crystallographic studies on dimethylpyrrolidinium

salts, including those utilising thiocyanate, iodide and NTf2anions, indicated that the melting point has a strong dependence

on packing efficiency which, in turn, was dependent on the size

of the anion and the number and type of interactions within

the crystal.31

Generally, it can be concluded that the melting point is

dependent on a number of factors:

� the size of the anion—mp decreases with increasing

size.65,71

� number and strength of interactions and/or co-

ordination,66,67,69 which is dependent on the level of

delocalisation.68

� the conformational flexibility—a higher flexibility yields a

lower melting point.71

� packing efficiency—less efficient packing results in a lower

melting point.70

Cation substitution/modification. The most widely utilised

cations are the imidazolium, pyrrolidinium, pyridinium, tetra-

alkylammonium and tetraalkylphosphoniums. IL chemists

have generally remained faithful to these basic structural

foundations, often preferring structural modifications over

the development of entirely novel cations.40 However, some

new cations have been reported, such as the N,N,N0,N0-tetra-

methylguanidinium cation73 and those with biological func-

tion, such as choline and its derivatives.22,74,75 Crystal

structures of these novel cations have been reported, as have

those for the previously discussed pharmaceutically active

ionic liquids (APIs).14,26,76 The crystallization of these APIs

is generally facile given the typical number of functional

groups on the cation and this must be overcome, via anti-crystal

engineering,14 to yield the liquid form of these salts. Crystal

structures for several modified imidazolium-based ILs have

been reported, including imidazolium cations modified with

salicylaldoxime or salen functional groups, which provide a

metal complexation function to the IL.77 Two IL-bis(oxime)-

and IL-salen–copper complexes have been crystallised and

characterised, proving the chelating properties of these novel

task-specific ILs.77 Another example of task specific design is

cation functionalisation with a carboxyl group to produce

metal oxide-solubilizing ILs.78 Crystallographic analysis

of four ionic liquids and five metal complexes involving

functionalised pyridinium, pyrrolidinium, imidazolium and

morpholinium cations and the NTf2 anion clearly illustrate

the strong coordinating ability of the carboxyl group.78

Other examples of the crystallographic study of the effect of

cation functionalisation include the use of alkoxy side

chains,79 amino side chains,80 substitution at the C2 position

of the imidazolium ring,39,81 the addition of a benzyl group

(Fig. 4),39,81 addition of electron withdrawing groups,82 and

the synthesis of an organometallic IL with a chromium

tricarbonyl fragment.83

Thus, in terms of cation variations, the melting point is

generally dependant on:

� the symmetry of the ions; a high cation symmetry tends to

produce a higher melting point.73

� the length and conformation of the alkyl chain and its

ability to crystallise.

� the type and extent of hydrogen bonding.75,78,80

� the addition of functional groups; replacing a methylene

group with an ether group results in lower melting due to lone

electron pair repulsion,79 the addition of electron withdrawing

groups within the pyridinium ring results in higher melting

points,82 the addition of organometallic groups can result in

lower melting points.83

� charge delocalisation; a greater charge delocalisation

results in lower melting points.39

� the number of possible conformations of flexible substituents

and chiral centres.84

2.4 Intermolecular forces within the ionic lattice

Traditional ionic compounds result in highly stable crystalline

lattices as a result of the attraction of unlike charges. The level

of this coulombic attraction is reflected by its lattice energy;

for example, NaCl has a high lattice energy of 788 kJ mol�1.

The magnitude of the lattice energy depends on a number of

factors; the charges of the ions, their size and their arrangement

in the lattice. Thus, if strongly charged ions were smaller

and closer together a higher lattice energy would eventuate.

However, ILs differ from traditional salts as the ions are

usually bulky organics with asymmetric charge distribution.

These features result in inefficient packing and a weaker

coulombic force, which lowers the lattice energy. However,

the same features facilitate the occurrence of short range

interactions, such as ion-dipole and van der Waals forces.

All of these characteristics have come under crystallographic

scrutiny over the past two decades in order to understand

which intermolecular forces prevail in ionic liquids.

Short range interactions. Over the past 20 years, a greater

understanding of hydrogen bonding within crystallised ILs has

been gained through crystallographic analysis. In early work

any evidence of a hydrogen atom on the cation that lay within

van der Waals proximity to a hydrogen bond acceptor would

be granted the definition of a hydrogen bond. However,

much needed criteria have since been established in order to

differentiate between a van der Waals close contact and a

Fig. 4 The asymmetric unit of 1-methyl-2-methyl-3-benzylimidazolium

iodide.81

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hydrogen bond.85,86 Hydrogen bonds are classified according

to their electrostatic and directional nature; these bonds, being

more electrostatic, are thermodynamically stronger than van

der Waals contacts. Furthermore, these bonds are directional

in nature and thus have a distance/angle criterion.86 Hydrogen

bonds are directional because they result from the near linear

alignment of the available electron density of the donor

atom’s orbitals and the vacant orbitals of the acceptor atom.

However, an exactly linear arrangement is usually not observed

as a result of competing intermolecular forces and packing

arrangements.86 The Seddon group have developed a statistical

model to identify hydrogen bonds in the solid state as determined

by their directional nature, and have used it to analyse the

hydrogen bonding within IL uranium halide complexes.87

The imidazolium cation was originally chosen for study

because of the interesting properties that the ring exhibits; a

strong charge delocalisation around the N1–C2–N3 moiety, a

double bond at C4–C5 and a weak delocalisation in the central

area.88 The hydrogen atoms of the ring, i.e. C2–H, C4–H and

C5–H, are reported to exhibit the same charge but the C2–H

displays a stronger Lewis acidity as a result of the delocalised

positive charge.88 Thus, the aim of several of these investigations

was to determine whether or not the C2–H favours hydrogen

bond formation as a result of its acidity, and the results at that

time confirmed this hypothesis.89–91 However, following the

publishing of new distance/angle criterion,13 assessment of

hydrogen bonding in the crystallographic analysis of

[C4dmim][HSO4], [C4dmim][Cl] and [C4dmim]4[FeIICl4]-

[FeIIICl4]2,92 [C4mim][PF6],

93 [C2mim][NTf2],42 [C2mim][NbF6]

and [C2mim][TaF6]48 and [C1mim][NTf2]

94 has determined

that the C2–H hydrogen bond is not the strongest interaction.

Many other pre-existing IL structures may benefit from such

reassessment, including [C2mim][I],89 [C2mim][Cl],95 [C2mim][NO2],37

[C2mim][NO3],37 [C2mim]2[CoCl4],

90 [C2mim]2[NiCl4],90 and

[C2mim][Br].91

Other interactions, apart from hydrogen bonding, can also

play an important role within crystallised ILs. For example,

C–H� � �p contacts between the hydrogen atoms of the cation

and the benzyl groups of the anion within [C4mim][BPh4],96

weak C–H� � �I and C–H� � �F contacts within [C1mpyr][I],

[C3mpyr][PF6] and [C7mpyr][PF6]97 and weak C–H� � �O

contacts in [C1mpyr][NTf2] and [N3,3,3,3][NTf2].33 Crystallo-

graphic analysis clearly suggests that weak close contacts are

the predominant interaction occurring in these low melting

salts.

Long range interactions. Low melting organic salts such as

ionic liquids often display low lattice energies as a result of

inefficient packing of irregularly shaped ions, a diffuseness of

charge and weak interactions.30 These interactions, which may

encompass long-range coulombic and short-range weakly

attractive forces, are dependent on the salt’s geometry and

charge distribution. Organic ions tend to be bulky and have

an asymmetric charge distribution, which can increase the

likelihood of directional short-range contacts and slightly

decrease the strength of the coulombic forces. Most of the

crystallographic studies of low melting salts have focused on a

qualitative investigation of short-range contacts. However,

one cannot fully understand the lattice energy of a crystalline

system by scrutinising only an isolated part of that system. The

overall lattice energy can be considered to be the sum of two

main contributions, as outlined in the following simplified

expression:

UL = Ees + Esr (1)

where UL is the lattice energy, Ees is the electrostatic energy

(the coulombic interaction) and Esr is the short range inter-

action energy (including the atom–atom repulsion interactions);

Ees can be obtained from the following classic equation from

Madelung:

Ees ¼M � qcation � qanion

4pe0dminð2Þ

where dmin is the distance to the nearest counter-ion, qcationand qanion are the ion charges andM is the Madelung constant,

which is dependent on how the ions are arranged in relation to

one another.29 Thus, bulkier ions, with a larger separation

between the ions, should theoretically have lower Ees. In

addition, the equation also suggests that a lower lattice energy

can be expected for salts consisting of singly charged ions and

those exhibiting lower packing efficiency.

We have recently developed a generalised method of

expanding unit cells to calculate the Madelung constant of

organic salts with a known crystal structure.98 This has

allowed the Madelung constant of a range of crystallised ionic

liquids and plastic crystals to be calculated for the first time.

This revealed that their Madelung constants are lower than

those of the inorganic salts studied, which indicates that the

nature of ion packing and the extent of ion pairing in the

organic salt can have a major influence on the electrostatic

energy component of the lattice energy.

Finally, it is important to consider that while the coulombic

interactions within an organic salt will not change significantly

upon melting if the positions of the ions is not greatly altered,

the short range interactions are likely to change significantly as

these interactions are often highly directional and upon melting

the orientation may be significantly affected. Several groups

have successfully pursued lattice energy calculations of ionic

liquids using ionic volumes obtained via theoretical calculations,

or experimental values derived from the unit cell volume in the

crystal structure, in order to explain the melting point.9,99,100

For example, Krossing et al.9 used the Born–Haber–Fajans

cycle to calculate the lattice Gibbs energy of sublimation and

coupled this with the Gibbs energy of solvation at DfusGm = 0

(the melting temperature) using quantum mechanical methods,

thermodynamic approximations and dielectric constants, in

order to understand the processes controlling melting. They

found that the energetically favoured salts (where DG is

negative), which are liquid at ambient temperatures, had large

and conformationally flexible ions that result in small lattice

enthalpies and large entropy changes (a more disordered

system) that favor the liquid state. A large entropy change

on melting created by the onset of hindered or free rotations

about an alkyl chain, for example, tends to produce a low

melting point. In contrast, the appearance of conformational

disorder in the crystal tends to have the opposite effect as the

entropy of the crystal is increased. An example of where

free rotations are possible in the crystalline state is when a

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mesophase, termed the plastic crystalline phase, can appear.

These crystals have an entropy close to that of the liquid state

(hence low entropy of fusion) and hence an elevated melting

point. These plastic crystalline phases have interesting and

useful properties as solid electrolytes.34

Crystal packing and ionic conformation. The predominant

crystal packing type, or a slight variation thereof, appears to

be a loose lattice comprising anion–cation layering

(Fig. 5).101,102 However, as discussed previously, the specific

type of crystal packing and the ionic conformation of low

melting organic salts can have a marked influence on their

thermal behaviour; inefficient packing and pronounced ionic

mobility can significantly decrease the melting point.33,56 For

example, a comparative study of N,N-dimethylpyrrolidinium,

piperidinium and morpholinium iodide salts based on crystal

packing, ring conformation and dynamic disorder revealed

that all of these structural variations have a influence on the

salts’ thermal properties, although no direct correlations could

be drawn.101 Another comparative study of the influence of

ionic mobility and crystal packing on the thermal behaviour

of [C1mpyr][NTf2], [C2mpyr][NTf2], [C1mpy][NTf2] and

[C2mpy][NTf2] (where py = piperidinium) salts revealed that

although the dynamic disorder increases with temperature for

[C1mpyr][NTf2] and [C1mpy][NTf2] the crystal packing does

not change. In contrast, the crystal packing in the low and

high temperature phases for both [C2mpyr][NTf2] and

[C2mpy][NTf2] does change, but the pre-melting phases (phase

I) packing of these two salts are very alike, thus explaining the

similarly low melting points.101

However, packing inefficiency as a result of crystallographic

disorder is clearly not the only cause of low melting points.

For example, cation–cation repulsive interactions and the

existence of polar and non-polar domains have also been

reported to influence thermal behaviour.92 Many instances

of conformational flexibility have been reported, including

crystallographic analysis of cases where the less energetically

preferred conformation crystallises, such as the cis-conformation

for the NTf2 anion;94 the twisted/half-chair ring disorder with

[Cxmpyr] salts;103 and the cis-conformation within [Cxmim]

alkyl chains.30 These effects may contribute to lowered lattice

energies and subsequent low melting points.

2.5 Ionic liquid clathrates, complexes and inclusion compounds

Liquid clathrates, first discovered in the 1970’s,104 are formed

between salt ions and aromatic molecules and it is the distance

between the ions, which are separated by the aromatic molecules,

that determines the formation of localized cage structures.

If the interactions are too weak then the salts are either

completely miscible or immiscible with the aromatics, and if

they are too strong then the salt crystallises out of solution.105

The Roger’s group explored the potential of ILs to form liquid

clathrates when mixed with aromatic hydrocarbons in order to

understand IL-biphasic systems.106 They successfully synthesized

several 1-alkyl-3-methylimidazolium based clathrates, in addition

to the inclusion compound [C1mim][PF6]�0.5C6H6 (Fig. 6).105

They discovered that the same design principles used for novel

ILs, such as low symmetry, organic cations, suppression of

lattice energy, etc., could be used for liquid clathrate formation,

thus illuminating the early work completed by Atwood. In a

similar vein, solid state structural analysis of imidazolium-

based salts containing water have shown that the water

changes the alkyl chain conformation significantly and forms

a hydrogen bonded network with the anion.107,108

ILs provide rich and varied solvent environments and this

has been successfully exploited by co-ordination chemists

to produce numerous crystalline metal–IL coordination

complexes.109–113 The use of metals in ILs can result in novel

properties such as magnetic, photophysical/optical and catalytic

properties.109 Crystal structure analysis of these complexes can

elucidate the effect of the ionic structure on the crystal packing,

new coordination environments and the solvation effects

of the ILs (Fig. 7).110,114,115 In several complexes, such as

[C3mim][Eu(NTf2)4], [C4mim][Eu(NTf2)4], [C4mpyr]2[Eu(NTf2)4],

[mppyr]3[NdI6] and [bmpyr]4[NdI6][NTf2], it is the cation that

has the predominant effect on the overall structure and

composition of the salt.109,111 Structural studies of lanthanide

and alkaline earth metal salts involving the NTf2 anion as a

ligand have revealed that the anion is weakly coordinating,

rather than non-coordinating.113,116 Furthermore, crystal

structure analysis of a range of complexes containing both

NTf2 and iodide anions demonstrates the conformational

flexibility, and thus anionic crystallographic disorder, of the

NTf2 anion.112 Other groups have taken advantage of the

tunable solvent properties, such as solvation strength, of ILs by

Fig. 5 Packing diagram, as viewed down the c-axis, of polymorph IV

of [C2mpyr][NTf2] at 153 K revealing the dynamic disorder and

layering of the ions.101Fig. 6 Packing diagram down the c-axis displaying the channels for

benzene inclusion within the clathrate [C1mim][PF6]�0.5C6H6.105

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using them as novel crystallization solvents for metal–organic

crystal engineering purposes.117–120

The use of ILs for co-crystallization as inclusion compounds

has also been reported.121,122 Calixarene derivatives and

18-crown-6 molecules have been tested as selective ion pair

receptors for ionic liquids. Inclusion compounds containing

pyridinium and imidazolium ionic liquids have been isolated

and crystallographically analysed for insight into the stacking,

intermolecular interactions and ionic conformation in each

compound (Fig. 8).121–123

3. Conclusion

Fundamentally, the design principles of crystal engineering

rely upon an understanding of the type, strength, directionality

and distance dependence of intermolecular or ion–ion interactions

within a crystallised material. Thus, a thorough understanding

of all of the interactions within low melting organic salts,

through detailed crystallographic analysis, allows identification

of the dominant features and, thus, assists in the target-specific

design of new ionic liquids with desired physical properties

such as low melting points. As discussed in this perspective,

the crystallographic analysis of a variety of low melting

organic salts, and the development of techniques such as

Hirshfeld analysis and calculation of the Madelung constant,

has already provided some valuable insight into the physico-

chemical relationships of these unique materials. However, for

the target-specific design of ionic liquids to become a reality, a

deeper understanding of the influence of the structural features

of these salts is key, and this relies on further detailed

structural analysis of additional salt families. This information

will be of considerable interest to the rapidly expanding

numbers of researchers in the ionic liquid field, including those

developing ionic liquids that utilise active pharmaceutical

ingredients, and future work in this area promises to be very

rewarding.

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