metal complexation in ionic liquids

Annual Reports on the Progress of Chemistry, Section A

Annual Reports on the Progress of Chemistry, Section A Vol 104

Annual Reports on the Progress of Chem

istrySection A

Vol 104

0260-1818(2008)104:1;1-#

Annual Reports on the Progress of Chemistry provides comprehensive coverage of the most recent developments in the classical fields of chemical science. Published in three sections, this unique service offers critical assessments from expert international authors, on an annual basis. The short, easily accessible articles combined with the depth of information make Annual Reports an invaluable teaching resource.

Annual Reports on the Progress of Chemistry, Section A covers recent developments in inorganic and bioinorganic chemistry. Every year, all the elements are reviewed, as is progress in:

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Top authors from around the world offer their approach to, and views of, the particular subject, written to a very high standard.

In Volume 104, Andrew Abbott, Gero Frisch and Karl Ryder write on metal complexation in ionic liquids and Elaine Moore on the computational modelling of inorganic solids. This is the latest in the continuing series of chapters written by leading authorities, reviewing current perspectives and future developments in specific subject areas within inorganic chemistry.

Inorganic Chem

istry

Inorganic Chemistry

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http://dx.doi.org/10.1039/b716593p

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http://pubs.rsc.org/en/journals/journal/IC?issueid=IC008104

Metal complexation in ionic liquids

Andrew P. Abbott,* Gero Frisch and Karl S. RyderDOI: 10.1039/b716593p

This Chapter reviews the area of inorganic complexes in ionic liquids. Itfocuses primarily on the literature published during the calendar year 2007but more generally it discusses the lack of information and the problemsassociated with collecting data in these solvents. The applications in whichinorganic complexes play a vital role are highlighted in this review togetherwith the importance that speciation plays in controlling the chemistry ofmetal ions.

1. Introduction

There can be few classes of material that have received as much interest over as short

a period of time as ionic liquids. In 2007 alone there were 1634 papers, 419 patents

and 168 reviews published in the area; quite a meteoric rise in activity considering the

total number of publications ten years previously had been just 9. In truth the subject

is considerably older claiming its origin in the synthesis of ethylammonium nitrate in

1914.1 The first systematic studies involved the chloroaluminate systems and evolved

from attempts to electrodeposit aluminium using molten salts. The whole concept of

ionic liquids comes from concerted attempts to make eutectics of aluminium chloride

with accessible melting points. Eutectic mixtures of KCl with AlCl3 (67 mol%) for

example have a melting point of 128 1C.2 These are well known to contain mixtures

of AlCl4� and Al2Cl7

� and it is this charge delocalisation that lowers the lattice

energy and produces the concomitant decrease in the melting point. The first major

studies in ionic liquids occurred when KCl was substituted by firstly pyridinium and

then later imidazolium chloride.3–5

The term ionic liquid was coined to differentiate a class of materials from molten

salts which had primarily inorganic cations and anions. The difference between the

two classes of salts was arbitrarily chosen to be a melting point of 100 1C. The

principle underlying the formation of a low melting point salt is that the ions are

large and non-symmetrical thus decreasing the lattice energy. There are thousands

on ionic liquid systems that have been characterised in the literature and millions

that are claimed in the patent literature. This review is not intended to cover the

whole area of ionic liquids in general and the newcomer to the field is directed to the

book of Welton and Wasserscheid6 which is recognised as the authoritative

summary of the ionic liquids in synthesis. In 2007 comprehensive reviews were

written on general reviews of applications,7–9 analytical chemistry and separation

science,10–15 reaction media,16–20 electrochemistry,21–24 enzyme-mediated reac-

tions25–28 and computer simulation29–36 which are the main areas of academic

research.

Department of Chemistry, University of Leicester, Leicester, UK LE1 7RH

REVIEW www.rsc.org/annrepa | Annual Reports A

Annu. Rep. Prog. Chem., Sect. A, 2008, 104, 21–45 | 21

This journal is �c The Royal Society of Chemistry 2008

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This article, however, covers the largely overlooked aspect of speciation of metals

in ionic liquids. The area of metal complexes in ionic liquids has largely been

overlooked by inorganic chemists, due mostly to the complexity in obtaining

unambiguous data, this review will mostly highlight the areas that need to be

studied to support the extensive field of applications that are currently under

development.

2 Classification of ionic liquids

There are numerous methods by which ionic liquids have been classified including

the anion, the acidity, the physical properties and solvation properties.

2.1 Classification by anion

The most commonly used classification is that of the anion since this is generally the

major influence upon the chemistry of the system. Broadly, ionic materials can be

subdivided into two classes depending upon their anions; those with discrete anions

(second generation ionic liquids) e.g. BF4�, (CF3SO2)2N

� (Tf2N�) and [N(CN)2]

�

and those with complex anions with a number of anionic species (first generation

ionic liquids) e.g. chloroaluminates AlCl4�, Al2Cl7

�.6

Each of these anion systems can be further subdivided depending on the cation

e.g. imidazolium, pyridinium or quaternary ammonium. Some examples of anions

and cations are given in Table 1.

Complex anions form from the interaction between a Lewis or Brønsted acid Y,

and a Lewis base (usually a halide anion e.g. Cl�) and can be classified depending on

the type of Lewis or Brønsted acid Y:

Type 137–40

Y = MClx, M = Zn, Sn, Fe, Al, Ga

Type 241

Y = MClx � yH2O, M = Cr, Co, Cu, Ni, Fe

Type 3 42,43

Y = RZ, Z = CONH2, COCH2OH, OH

The relative proportions of anionic species depend on the ionic liquid composition

and the Lewis acidity of the metal. The ability to vary the composition of Lewis or

Brønsted acid adds an additional dimension to the tuneability of the eutectic-based

ionic liquids. The term Deep Eutectic Solvents42 has been coined to describe these

systems, although their physical properties are indistinguishable from the corre-

sponding system with a discrete anion. Most studies are currently being carried out

with ionic liquids with discrete anions as these tend to be more inert and have lower

viscosities.

Classification of the liquids in terms of chemical composition does not necessarily

provide an insight for the user but this is no different to the situation encountered

with molecular liquids. Methanol, for example, has very different solvent properties

to t-butanol. The same is true for ionic liquids only it is more complicated as both the

anion and cation affect the properties of the fluid. In general, the cation controls the

physical properties of the liquid such as phase behaviour, conductivity and viscosity

22 | Annu. Rep. Prog. Chem., Sect. A, 2008, 104, 21–45


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whereas the anion has a larger role in the chemical properties such as speciation of

metals and reactivity.

2.2 Classification by acidity

Most ionic liquids with discrete anions are neutral or very weakly basic. They have

weak electrostatic interactions with the cation and tend, therefore, to have low

melting points and low viscosities. Examples include BF4�, SCN�, p-toluene

sulfonate and methanesulfonate. An increasing number of Lewis and Brønsted

acids and bases are being incorporated for specific applications. Protonated ammo-

nium, pyridinium and imidazolium salts have been produced44–49 as well as the well

known Lewis acidic metal based eutectics such as Al, Ga and Zn. Basic anions

include acetate, lactate and [N(CN)2]�.50–52 In addition, amphoteric anions such as

hydrogen sulfate and dihydrogen phosphate have also been reported. The area of

Lewis basic ionic liquids has recently been reviewed by MacFarlane et al.53

2.3 Brønsted acidity

Whilst a great deal of the interest in ionic liquids has focussed on Lewis acidity,

recently attention has been directed towards a more fundamental understanding of

Table 1 A selection of cations and anions used to make ionic liquids



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the acid base character of the liquids.54 This interest has been motivated by the

increasing popularity of ionic liquids for synthetic applications6 and the realisation

that conventional approaches based on data acquired for aqueous systems are not

easily transferable to ionic liquid environments. Put simply and from a synthetic

perspective, when a molecule or reagent is added to an ionic liquid, it is desirable to

have some knowledge as to whether that molecule will act as an acid or base in order

to predict the outcome of a particular reaction. Conventionally the concept of

acidity is identified with the quantification of hydrogen ion activity; for aqueous

environments this problem was solved many years ago with the pH scale. Here the

operative acidic and basic species are H3O+ and OH�, respectively (together with

hydrates thereof) and these species are formed in the presence of a solute that is

either a stronger acid than H2O, or a stronger base than OH�. This is known as

solvent levelling. In organic solvents the situation is more complex but the theory for

low ionic strength solutions (I o 0.1) is well developed.54,55 In ionic liquids,

however, the situation is less clear partly because of high ionic strength and because

of the number of different ionic species present. The latter complication comes not

only from the possible number of ionic components but also from the existence of

speciation equilibria for each component (discussed above). Measurement of the

position of acid base equilibria can, in principle, be attempted by potentiometric

methods, however, determination and interpretation of such data would, in many

cases, be impossibly difficult for essentially the same reasons.

In a recent review of Brønsted acidity of ionic liquids in synthesis54 it was

concluded that the position of acid base equilibria in a wide range of liquids

correlates more closely with gas phase proton affinities (of the conjugate base) than

with pKa values determined from aqueous solution. This is presumably because of

the high solvation energies in aqueous environment and the lack of solvent in most

ionic liquids.

There is a considerable body of literature devoted to the behaviour of inorganic

acids e.g. HCl, HBr, in chlorometalate ionic liquids, especially AlCl3/imidazolium

systems. This is because systems derived from Al2Cl7� or Al2Br7

� are superacidic

and hence very good proton donors i.e. the conjugate bases, including AlCl4�, are

very weak (HAlCl4 and HAlBr4 are unknown).56–58 This has led to some very

interesting observations including unexpected proton exchange in the [emim]+

cation (at ring positions 4 and 5),59 also when anthracene is added to acidic

[emim]Cl/AlCl3 a green colour develops attributed to the protonated anthracene.60

Other examples include ligand exchange reactions of ferrocene61 under similar

conditions. Much of the available literature is focussed on classical organic reaction

types exploring the influence of acid base equilibria and liquid based proton donor

speciation on, for example, reaction yields. There have been very few in situ structure

determinations for metal based systems other than as components of a chlorome-

talate liquid.

Acid base equilibria of deep eutectic solvents is an area that has not yet been

represented in the literature. However, for liquids that are composed of eutectics and

binary mixtures of choline chloride with hydrogen bond donors such as glycol, urea,

and carboxylic acids then the scope for acid base behaviour is vast. This subject has

appeared indirectly in some recent work of ours.43 When solid sodium bicarbonate is

added to a dry eutectic mix of choline chloride and oxalic acid no evolution of CO2 is

observed and the insoluble solid remains stable. However, if a small amount of H2O

is titrated in then rapid effervescence is observed. This tells us firstly, that the

hydrogen ion activity in the neat liquid is relatively low and second, suggests that the



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acidity of the liquid can be tuned by addition of water, or other co-solvent, to suit

reaction conditions.

2.4 Classification by physical properties

One characteristic by which ionic liquids can be characterised is clearly the

conductivity of the fluid. Contrary to what might immediately be assumed, ionic

liquids have comparatively poor conductivity. Angell62 has shown that for ionic

liquids with discrete anions the molar conductivity is inversely proportional to the

viscosity of the liquid and this correlation is virtually independent of the cation and

anion. This has been used as a method of defining the properties of an ionic liquid.

We have recently used hole theory to show that Angell’s observation is due to a

different mechanism of charge transfer in ionic liquids from that encountered in ionic

solutions or high temperature molten salts.63,64 We proposed that the main property

that distinguished ionic liquids from molecular liquids was the mechanism by which

charge travelled and proposed that one possible classification of an ionic liquid was a

system in which hole mobility dominated and the Nernst–Einstein equation was

valid.65

2.5 A need for a different classification?

What is clear is that all ionic liquids form due to delocalisation of charge and this can

be described by an equilibrium:

cation + anion + complexing agent " cation + complex anion (1)

or potentially,

cation + anion + complexing agent " complex cation + anion (2)

with the vast majority focussing on the former case. The confusion arises from themagnitude of the equilibrium constant. For discrete anions such as BF4

� and even[N(CN)2]

� the equilibrium lies clearly to the right of eqn (1). For eutectic basedliquids the equilibrium constant depends upon the strength of the Lewis or Brønstedacid such that a variety of complex anions are possible. Hence, all of the followingcould be described as ionic liquids:

Cat+ Cl� + AlCl3 " Cat+ + AlCl4� (3)

Cat+ Cl� + urea " Cat+ + Cl� � urea (4)

LiClO4 + 3.5H2O " Li+ � xH2O + ClO4� � yH2O (5)

R3N + HX " R3NH+ X� (6)

To further complicate66,67 this already complex system we have recently shown thateutectics do not necessarily need a quaternary ammonium, phosphonium orsulfonium cation to form a fluid with the properties of an ionic liquid. Simpleinorganic salts can also disproportionate to form both anionic and cationic specieswhen complexed with simple amides and alcohols.68 For example,

ZnCl2 + urea " ZnCl+ � urea + ZnCl3� (7)

Other new classes of materials are being discovered all the time including polyelec-trolytes and zwitterionic systems. These systems have the physical properties ofionic liquids but cannot be easily classified into any of the above classes of liquids.The key problem also occurs when a solute is dissolved in an ionic liquid. It hasbeen shown by numerous groups that solutes such as water and alcohols can



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significantly affect the physical properties of an ionic liquid, but for otherionic liquids water has little effect upon the physical properties. There must be apoint where the solute (or the Lewis or Brønsted acid in the case of eqns (3)–(6)) arein considerable excess and the system becomes a solution of salt in the soluteor acid. Many ionic liquids with discrete anions are hydrophilic and the absorptionof water is found to sometimes have a significant effect upon the viscosity andconductivity of the liquids.69–71 What is required is a simple method of distinguishingthe properties of ionic liquids from those of a concentrated solution.While the problem has been identified, little has been done to address the definitionof why an ionic liquid is fundamentally different from a molecular liquid.We propose that the difficulty arises due to the grey region between a dilutemolecular solute in an ionic liquid and a concentrated ionic solution. The concen-tration at which the system goes from ionic to molecular character dominating thephysical properties depends upon the activity of the solute and the ionic character ofthe ionic liquid.

2.5.1 Activity coefficients. Deviations from ideal behaviour are

notoriously difficult to characterise in molecular solvents and so it is unsurprising

that little has so far been attempted in ionic media. Some solute–solvent interaction

data has appeared recently in the form of activity coefficients for a range of

polar organic solutes in imidazolium and phosphonium based ionic liquids.

The data have, in the most part, been obtained from gas–liquid chromatography

using the ionic liquid as the stationary phase.72–76 In some cases the partial molar

excess enthalpies of the solutes at infinite dilution in the ionic liquid were also

obtained. Isobaric vapour–liquid equilibria data were also measured for ternary

ionic liquid/alcohol/water systems and used to obtain activity coefficients for the

solutes. The activity coefficients showed that [emim]+ has a stronger interaction with

water than [bmim]+ although the interactions with 1-propanol or 2-propanol were

weaker.77 Significantly more work needs to be carried out in this area. Our group is

currently attempting to use electrochemical measurements to determine

activity coefficients for metal salts in solution and determine how these vary

with concentration.

2.6 The key issue

In principle there is no difference in the way in which metal ions (M) behave in ionic

liquids and any molecular solvent (S). The metal ion interacts with whatever ligands

(L) are available and the equilibrium constant, and hence the species present, are

dependent upon the concentration of species in solution and the relative Lewis or

Brønsted acidity.

Mx+ + yL + zS " [MLySz]x+

For most systems the solvent species is neutral and a relatively weak ligand. Thechemistry of ligands in water naturally dominates the literature and the activity ofthe ligand is controlled by the ligand–solvent interaction. In an ionic liquid thesolvent species that dominates complexation is the anion which generally has asignificant Lewis basicity. The fact that it is generally present in 5 to 10 mol dm�3

concentration means that even generally strong ligands can be displaced. Anotherissue is clearly that if the anion dominates solvation then most d- and p-blockelements will be present as anionic species in solution which will affect issues such asreactivity and double layer properties (see below).

Mx+ + yL + zS� " [MLy Sz]x�z



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Attempts have been made to characterise the solvent properties of ionic liquids in ananalogous manner to that used for molecular solvents. This approach seeks to likenthe predominantly ionic environment to molecular systems. An alternative view is topragmatically ignore the overall solvent properties and characterise the resultantmetal complexes formed in solution. Both methods have advantages and difficultieswhich are discussed below.

3 Solvent properties of ionic liquids

For most molecular liquids classification methods are derived from that developed

by Brønsted who used the parameters of dielectric constant, acidity and basicity to

differentiate between different types of solvent groups. Solvents are classified

depending on whether they are neutral amphiprotic, protogentic, protophilic,

dipolar aprotic or inert. A similar scale is not simple to determine for ionic systems

due to the complexity and applicability of the concept of a dielectric to a charged

medium. A limited number of studies have been performed using high frequency

dielectric response spectroscopy in the GHz region of the spectrum. The results show

that most of the ionic liquids studied to date have dielectric constants in the range 10

to 12. Higher dielectric constants were measured for liquids with more polar anions

such as ethylsulfate. The effects of both anions and cations are contained in a

number of articles.78,79 In general the dielectric constant values are lower than would

be expected from the corresponding polarity parameter and solubility data and this

could be due, in part, to the method by which the dielectric constant data were

obtained. The relative merits of the experimental techniques are discussed by

Bright.80

3.1 Polarity parameters

Some of the discrepancies encountered comparing high and low frequency dielectric

constant data can be overcome by the measurement of polarity parameters.

Numerous semi-empirical scales have been applied to ionic liquids. These are based

upon linear free energy relationships.

In an absorption or emission process it is a radiative electronic transition that

links the initial relaxed and final Franck–Condon states. When considering a

molecule surrounded by a medium, each of its electronic states will be stabilised

or destabilised by an amount known as the solvation energy, which is dependent on

the polarity of the medium. Thus, it is the influence of this solvating medium on the

electronic absorption and emission spectra of the solute that gives rise to the

definition of solvatochromism.

The solvatochromic shift between two solvents is the difference in solvation

energies between the final and initial states. These shifts are important not only

for the description of the relative energies of electronic states of molecules, but also

for the experimental determination of some important physical properties such as

the polarisability and the dipole moment of a molecule. However, they can often also

provide information about specific interactions such as hydrogen bonding and, on a

microscopic level, solvatochromic shift data can provide information on solvation in

the cybotactic region of a solute molecule.

The single parameter ENT scale81 and the multi parameter Kamlet and Taft82 scale

have most commonly been applied to ionic liquids. These provide parameters on a

normalised scale. These parameters have only so far been determined for a limited

number of pyridinium, imidazolium and quaternary ammonium based ionic liquids



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with a small range of discrete anions. ENT values have been obtained for Cl�, SCN�,

NO3�, ClO4

�, BF4�, PF6

� and Tf2N�,83–87 whereas the Kamlet and Taft parameters

have been determined for BF4�, PF6

� and Tf2N�.88–90 Table 2 shows selected values

for a range of ionic liquids and comparators with molecular solvents. It can be seen

that most ionic liquids have a medium polarity as expressed by the ENT (lower than

methanol but higher than acetone) and changes to the cation structure have little

effect on the polarity. Differentiating the polarity contributions into polarisability,

p*, hydrogen bond donor properties, a, and hydrogen bond acceptor properties, b,shows that they have a high polarisability which would be expected given the

aromatic nature of the cation. The hydrogen bond donor properties arise primarily

from the relatively acidic protons on the cation whereas hydrogen bond acceptor

properties result primarily from basic anions such as TfO�. These data are invalu-

able for understanding the solubility, reactivity and activity of dipolar solutes and

Lee has shown that good correlation exists between the Kamlet and Taft parameters

and the activity coefficients of dipolar solutes.91 Kobrak90 demonstrated that the

polarisability, p*, correlated well with the molar volume of the ionic liquid and

devised a qualitative model to predict the solvent polarity with ionic substitution or

derivatisation. The data also show some interesting characteristics for example the

hydrogen bonding ability of these fluids appears to increase as the temperature

increases whereas the polarisability decreases.89 More comprehensive reviews of

ionic liquid polarity have recently been written by Welton92 and Reichardt.93

The difficulty arises with metal ion solubility because the speciation depends

primarily on Lewis acid–base interactions which are not directly comparable with

the parameters described above. What is needed is a Lewis basicity scale that

describes the affinity of the anion for the metal.

3.2 Speciation in ionic liquids

So, many of the applications for which ionic liquids are currently being employed

involve the incorporation of metal ions in solution. This can be for metal deposition,

metal dissolution, metal processing, catalysts for organic reactions etc. One of the

key issues concerning the reactivity of metals in ionic liquids is the species that form

Table 2 Polarity parameters of various ionic and molecular liquids144

Liquid ENT a b p*

[n-Octylpyridinium]+[Tf2N]� 0.588 0.51 0.28 0.99

[2-Methyloctylpyridinium]+[Tf2N]� 0.554 0.48 0.35 0.95

[3-Methyloctylpyridinium]+[Tf2N]� 0.576 0.50 0.33 0.97

[1-Butyl-2,3-dimethylimidazolium]+[BF4]� 0.541 0.381 0.239 1.010

[1-Butyl-3-methylimidazolium]+[BF4]� 0.630 0.627 0.376 1.047

[1-Butyl-3-methylimidazolium]+[PF6]� 0.669 0.634 0.207 1.032

[1-Butyl-3-methylimidazolium]+[TfO]� 0.656 0.625 0.464 1.006

[1-Butyl-3-methylimidazolium]+[Tf2N]� 0.644 0.617 0.243 0.984

[n-Butylmethylpyridinium]+[Tf2N]� 0.544 0.427 0.252 0.954

Water 1.000 1.12 0.14 1.33

Methanol 0.762 1.05 0.61 0.73

Acetone 0.35 0.20 0.54 0.70

Dichloromethane 0.31 0.04 �0.01 0.79

Hexane 0.009 0.07 0.04 �0.12



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upon dissolution. Metal ions being generally Lewis acidic will complex with Lewis or

Brønsted bases to form a variety of complexes. In aqueous based media the

chemistry of H+ and OH� dominate the species formed in solution, the redox

properties and the solubility of metals. Speciation has been characterised for the vast

majority of metals and media as a function of electrochemical potential and pH

through Porbaix diagrams. These are vital for a range of corrosion and hydro-

metallurgical processing techniques.

The study of metal ion speciation in ionic liquids has started only very recently.

The picture is considerably more complex than that in aqueous solutions because of

the differing Lewis basicities of the anions. In eutectic based ionic liquids the

speciation is also governed by composition. Our concepts of metals in solution are

so dominated by hydroxide ions that it is very difficult to forget the behaviour of

metals in aqueous systems and appreciate potential applications of ionic fluids. For

example, aluminium is known to be passive at neutral pH but dissolve in strongly

basic or acidic solutions. In some ionic liquids aluminium will anodically dissolve

whereas it is immune in others depending upon the Lewis basic strength of the anion.

This means that dip coatings of a wide variety of metals can be achieved on

aluminium in some ionic liquids.

One of the reasons that speciation is poorly understood is that due to the difficulty

in acquiring structural data and identifying species. In molecular solvents speciation

is frequently elucidated by crystallising the metal complex and determining a crystal

structure using X-rays. A different mindset is, however, required for ionic solvents.

Most metals form ionic complexes which are difficult or impossible to crystallise

from solution and evaporation of the solvent is naturally not an option in most

cases. What is therefore required is a series of in situ techniques that can be used to

piece together metal ligation in solution. The following sections describe some of the

recent efforts and techniques to determine structure and speciation of metal ions in

ionic liquids.

The speciation of neat second generation ionic liquids can be investigated by single

crystal diffraction in most cases. This also works for some ionic liquids of the first

generation, in cases where they form stoichiometric phases and are not mixtures or

eutectics. Many have been described in a recent review94 for imidazolium based

compounds. Other neat ionic liquids and solutes have to be studied in liquid state. In

this case the advantage is that in situ the species are studied under representative

reaction conditions. Here, we will provide just a few examples to illustrate the

potential of the most common liquid state techniques used to determine speciation

and properties of metal complexes in ionic liquids.

3.3 NMR/EPR

Clearly NMR spectroscopy has become the most powerful method to determine

speciation in the liquid state. In ionic liquids it has been extensively used for those

systems where appropriate nuclei are available. The organic parts of ionic liquids like

imidazolium based cations can be investigated with standard 1H and 13C techniques,

well known from organic chemistry. But spectra based on other elements, mainly27Al for the Al-based first generation ionic liquids, have also been used. There are

also several examples of NMR experiments with specific pulse sequences, that have

been used to probe the physical and chemical properties of the liquids. The vast

majority of NMR experiments on metal complexes have been performed on neat

chloroaluminate ionic liquids or their mixtures with molecular solvents, but there are



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also few other examples e.g. the investigation of acidic [bmim]Cl/SnCl2 mixtures for

hydroformylation reactions by Wasserscheid et al.95

One of the first NMR experiments in ionic liquids was performed by Osteryoung

et al.96 Although they found that the 13C NMR spectra of [epy]Br/AlCl3 mixtures

were identical to those of neat [epy]Br, they observed an upshift of all signals on the

addition of benzene. This indicates that significant ion pairing occurs, when a non-

polar solute is introduced into the ionic liquid. Hydrogen bonding was shown to play

a significant role in basic [emim]Cl/AlCl3 mixtures97 through 1H-NOE techniques.

Dual spin probe experiments with the 13C and 27Al nuclei were used by Carper

et al.98 to probe aggregation in these liquids. Many investigations of chloroalumi-

nate moieties as a function of [cat]Cl/AlCl3-ratio using 27Al NMR have been

reported. The benefits of 27Al NMR are evident, since it directly probes the element

which changes speciation upon composition of the liquid. Due to the quadrupolar

moment of the 27Al nucleus, the line width of the NMR signal can indicate the

symmetry of Al species. High symmetry of the probed species reduces quadruploar

relaxation and thus results in a sharper signal for [AlCl4]� than for [Al2Cl7]

�. Wilkes

et al.99 used this method to show, that in a 1:1 mixture of [emim]Cl and AlCl3,

orthoaluminate [AlCl4]� is the only anionic species, whereas in acidic (i.e. Al-rich)

mixtures the dialuminate species [Al2Cl7]� becomes more significant. A combination

of 17O and 27Al NMR was employed by Osteryoung et al. to determine the

speciation in chloroaluminate based ionic liquids containing oxygen impurities100

and a series of different oxochloroaluminates was found.

The investigation of solutes through NMR spectroscopy is more difficult than that

of neat ionic liquids since it usually requires isotopically substituted solvents for 1H

and 13C NMR. Nevertheless, other nuclei like 31P, which play an important role in

organic catalysis, have been be probed.101,102 There are also some examples where

solid state (MAS) techniques have been employed to investigate, for example ionic

liquids in polymers103 or Zn speciation in [bmim]Cl/ZnCl2 ionic liquids.104

EPR spectroscopy has barely been used to study ionic liquids. Gedanken et al.

used X-band EPR105 to complement their data from several other methods on the

speciation and properties of [Ni(C4H6N2)6](BF4)2. However, EPR has been em-

ployed to probe radical transition states. This can be particularly useful for

electrochemical reactions as shown by Osteryoung et al.106 with transitional radical

cations in polypyrrole films in [emim]Cl/AlCl3 or by Kucherov et al.107 who detected

W/Mo(V) species with EPR during olefin metathesis in ionic liquids. Osteryoung

et al. also used several spin probes to investigate donor/acceptor properties and

hydrogen bonding in chloroaluminate ionic liquids.108 Nevertheless it should be

noted, that EPR is never a routine technique but always requires a tailored setup and

an extensive knowledge of the method. Both EPR and NMR spectroscopic studies in

ionic liquids have been extensively reviewed by Bankmann and Giernoth.109

3.4 UV/Vis

UV-Vis spectroscopy has mainly been employed as an assisting technique to verify

results from other spectroscopic methods or computer simulations. For example,

Visser et al.110 obtained supporting evidence for their conclusions from the EXAFS

analysis of uranium complexes in imidazolium based ionic liquids by UV-Vis

spectra. Nevertheless, UV-Vis spectroscopy could possibly be a very useful technique

especially as a fingerprint method, when the species under investigation is established

in other media, e.g. aqueous solutions.



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3.5 Vibrational spectroscopy

Although vibrational spectroscopy has been employed extensively to determine

speciation and physical properties of organic moieties in ionic liquids, few examples

have been reported for metal complexes. Nevertheless, IR and Raman spectra can

yield important information on the nature of inter- and intra-molecular bonds,111

conformation and conformational changes in ionic liquids. For example, Nanbu

et al.112 used FT-Raman spectroscopy in reflection mode to determine the orienta-

tion of [emim]+ cations on charged gold electrode surfaces in situ. There are a few

examples, where Raman spectroscopy has been used to probe the speciation of metal

complexes, such as that by Osteryoung et al.113 where Raman spectroscopy was used

to determine aluminium speciation in [bpy]Cl/AlCl3 mixtures as a function of

composition, providing further evidence for the [AlCl4]� and [Al2Cl7]

� species

identified in the NMR experiments.99 Freeman et al.114 showed analogous results

for [FeCl4]� and [Fe2Cl7]

� in the [bmim]Cl/FeCl3 system from a combination of

Raman spectroscopy and ab initio calculations. A review on Raman spectroscopy in

ionic liquids has been published by Berg.115

To probe molecular dynamics and investigate possible reaction properties of ionic

liquids RIKES (Raman induced Kerr effect spectroscopy) has proven to be a useful

tool. RIKES uses a pulsed Raman laser to excite intermolecular librational modes in

a liquid. Information on molecular dynamics within the liquid can be obtained from

birefringence effects of the optically anisotropic excited state and its relaxation

dynamics to the isotropic ground state. Although RIKES can be a very useful

technique, especially when combined with computer simulations, to our knowledge

there have been no experiments to investigate metal complexation. Several articles

reviewing molecular dynamics116 and the nano structure117 of ionic liquids with

RIKES can be found in the literature.

3.6 Neutron and X-ray methods

Neutron and X-ray scattering techniques have been used for structure determination

in amorphous solids and melts and can also be used for ionic liquids. Especially the

isotopic (NDIS) or isomorphic (XDIS) substituted variants have been used respec-

tively, since they overcome the phase problem that is a major issue of scattering

techniques. Takahashi et al. combined neutron diffraction and ab initio calcula-

tions118 in [emim]Cl/AlCl3 melts, and found that the speciation of Al was the ortho-

and dichloroaluminate complexes, known from NMR experiments,99 as well as their

associates with [emim]+ ions. Notwithstanding this, these methods are limited to

those systems where isotopes with appropriate neutron scattering length or iso-

morphic substitution with much heavier or lighter atoms is possible.

X-ray absorption fine structure (EXAFS) provides a more generic approach to the

first coordination sphere of metal complexes. The EXAFS arises from a modulation

in intensity of the X-ray absorption edge by interference with the photoelectrons,

which are backscattered from the neighbouring atoms. Fourier transformation

provides a radial distribution function in real space within a radius of approx.

600 pm. EXAFS is element specific and shows the local arrangement around the

excited element also for low concentrations. This makes it a powerful method to

probe solutes like transition metal complexes, which cannot be accessed easily by

other methods. Regardless, there are clear limitations, since too large or complex

coordination shells make the spectrum hard to interpret.



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To-date, EXAFS has mainly been used for high temperature molten salts but the

advantages of the technique are the same for ionic liquids. For example, EXAFS in

melts of Rb2ZnCl4119 showed the coordination of both Zn and Rb in the liquid to be

similar to that in solid Rb2ZnCl4 and not to solid RbCl and ZnCl2, respectively.

Hence, also in the liquid state, Rb2ZnCl4 must be regarded as rubidium–chlorozin-

cate (Rb+ + [ZnCl4]2�) rather than rubidium-zinc-chloride (Rb+ + Zn2+ + Cl�).

Similar results can be found for complexes in aqueous solution and also in ionic

liquids, e.g. Carmichael et al.120 showed that the Ni speciation in [emim]2[NiCl4] and

[C14mim]2[NiCl4] did not change significantly upon melting.

EXAFS can also be employed for speciation experiments of solutes in ionic

liquids. Dent et al.121 studied chloro-manganate(III), -cobaltate(III) and -nickelate(III)

complexes in [emim]Cl/AlCl3 ionic liquids. All three metals were found to be isolated

[MCl4]� species in the chlorine rich mixtures, while they shared three chloride ions

with aluminum inM(AlCl4)3 moieties in the case of acidic mixtures. EXAFS can also

be employed to investigate catalytic species in situ as shown by Hardacre et al.122 in

studies on the Heck reaction in several imidazolium based ionic liquids using Pd-

EXAFS.

Another key feature of EXAFS is the determination of metal oxidation states,

since the energy of the absorption edge depends on the charge of the probed atom.

Pitner et al.123 investigated uranium species from anodic oxidation of uranium metal

in [emim]Cl. The EXAFS spectra showed the uranium to be in oxidation states IV

and V, and not in III as first expected by the authors. A summary of EXAFS

measurements in ionic liquids has been published by Hardacre.124

3.7 Potentiometry

For eutectic based ionic liquids speciation can be determined using FAB-MS and

this shows that simple halometalate complexes are produced with one, two and

sometimes three metal ions. The formation of these complexes can be described by

simple equilibria. Heerman and D’Olieslager125 measured the potential of the cell

Al|[bpy]Cl, AlCl38[bpy]Cl (1�x), AlCl3(x)|Al

and showed that the data could be fitted to the Nernst equation over the regionx = 0.6 to 0.67. They found that the equilibrium constant for reaction (8),

2Al2Cl7� # AlCl4

� + Al3Cl10� (8)

was 2.93 � 10�3 at 60 1C i.e. Al2Cl7� is the most abundant species in solution. We

have used the same technique for the analogous choline chloride (ChCl):zincchloride cell at 60 1C for various mole fraction, x.

Zn|ZnCl2 (0.667), ChCl (0.333)8ZnCl2(x), ChCl (1�x)|Zn

Here we have obtained an equilibrium constant of 2.0 � 10�5 which would beexpected on Lewis acidity arguments.

The above examples illustrate that there is no universal approach to the analysis of

metal species in ionic liquids. Most published experiments on both speciation and

properties in ionic liquids require tailored setups and often a combination of several

analytical techniques. Further examples are given below for specific applications and

it will be shown that complementary techniques can give conflicting evidence about

the species present.



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4 Applications of ionic liquids

The need for metal speciation data can be highlighted through a brief survey of two

common applications. The solubility of metal complexes and their reactivity in

solution are of key importance to metal oxide dissolution and the electrodeposition

of metals.

4.1 Processing of metal oxides

Metal oxides have traditionally been processed using hydrometallurgical techniques

based upon dissolution in mineral acids and bases. Separation has usually been

achieved using solvent extraction with specific chelating agents for given metals.

Over the past 10 years there have been significant developments in the use of ionic

liquids. An increasing number of groups are now using ionic liquids for solvatome-

tallurgical processing.

4.1.1 Elements of the f-block. Here a significant number of studies, primarily

using imidazolium based liquids, have concentrated on f-element chemistry and in

particular actinide ion separations from spent fuel radioactive wastes. Amongst

other attributes, these systems have been selected for their stability under prolonged

radioactive exposure.126 The coordination chemistry of actinides in ionic liquids has

recently been reviewed by Rogers et al.127

Dai et al.128 were amongst the first to study ionic liquids for metal oxide

processing. They studied the dissolution of UO3 in imidazolium chloroaluminate

melts. The solubility was found to be in the range 1.5–2.5 � 10�2 M in Lewis basic

melts and the main metallic species in solution was found to be [UO2Cl4]2�. The

solvation of uranium and europium ions in these liquids has been modelled by

Chaumont and Wipff.129

Issues associated with water sensitivity of chloroaluminate melts can be overcome

using discrete anions such as PF6� and Tf2N

� and this is where most studies have

focussed recently.130 Dai et al.131,132 have used crown ethers and calixarenes to

extract a range of metals from aqueous solutions. Studies by Rogers et al. used

azonaphthols and crown ethers133 in a similar manner. Recently Visser et al.134,135

have also developed a new class of task-specific ionic liquids with monoaza-crown

ether fragments covalently attached to the imidazolium cations and have shown that

they can be used for the biphasic extraction of Cs+ and Sr2+. Nockemann et al.136

have used protonated betaine bistriflamide salts to dissolve metal oxides such as

UO2, ZnO, CdO and HgO. The metal salts can be stripped from the ionic liquids by

treatment with acidic aqueous solutions. The two phases were miscible above 55 1C

but separated below this temperature.

The area of imidazolium based ionic liquids with discrete anions as extraction

solvents has recently been reviewed by Dietz.137 The review covers the use of neutral

and anionic complexing agents such as crown ethers and organophosphorous acids.

The use of task specific ionic liquids has been reviewed separately by Davis.138

As described above the main analytical techniques applied to determine speciation

of f-block elements in ionic solution have been UV-Vis spectroscopy coupled with

EXAFS.139 An interesting variant on this was recent work by Servaes et al.140 who in

addition to EXAFS and UV-Vis spectroscopy used magnetic circular dichroism to

elucidate the structure of [UO2(NO3)3]�. As with many of these investigations



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corroboration of speciation was obtained from comparison of spectra with standard

samples which relies on standards being available.

The majority of studies have used ionic liquids for liquid–liquid extraction for

which the ionic liquid needs to be immiscible with water. Partition coefficients have

been determined for a number of standard compounds, but modelling this depends

upon an understanding of the structure of the species present. The selectivity in

metal separation arises from the ligand type and its selectivity for the metal of

interest together with the partition coefficient for the extracting solvent i.e. classical

solvent–solvent extraction. An alternative approach is to use ionic liquids to

selectively extract the solid from the solid matrix relying on a difference in solute

solubility in the ionic liquid. There are natural limitations with this approach such as

the slow dissolution kinetics and issues concerning inclusion compounds. A more

fundamental issue is that the volume of ionic liquid required to extract from a solid is

larger than that required to concentrate from an aqueous solution, meaning that

some types of ionic liquids may not be suitable.

4.1.2 Elements of the d-block. Type 3 eutectics (R4N+Cl �HBD)42,43 have the

major advantage that they are viable to use on a large scale and the variability of the

hydrogen bond donor (HBD) makes them tuneable. We have shown that these types

of ionic liquids can dissolve a range of metal oxides43,141 and that they can be used to

separate metals from a complex mixture using electrochemistry.142 These liquids are,

however, all totally miscible with water and cannot be used for biphasic extraction.

The solubility of 17 metal oxides in the elemental mass series Ti through Zn has been

reported in three ionic liquids based on choline chloride.143

The judicious choice of hydrogen bond donor allows the application of these

solvents to metal oxide processing. Table 3 shows the solubility of three metal oxides

in similar eutectic mixtures and it can be seen clearly that the solubility can be

strongly influenced by the hydrogen bond donor. A wide range of metal oxides can

be dissolved in deep eutectic solvents as can be seen from Fig. 1. This selectivity of

metal oxide solubility has been used for the study of metal extraction from complex

matrices. The only speciation studies that have been carried out for these systems has

been using FAB-MS. This has shown a variety of chlorometalate species many of

which are not intuitively logical. This is probably an artefact of the technique rather

than unique chemistry occurring in the high chloride environment. EXAFS studies

on these systems are currently on going and should resolve the issue of speciation.

The use of Type 3 eutectics has been applied to the reprocessing of waste material

from the Electric Arc Furnace (EAF).142 A dust forms when volatile metals, such as

zinc and lead, pass into the vapour phase at the high operating temperature of the

EAF. The metals are readily oxidized when they leave the furnace. These metals

form the resulting dust as free oxides and in the form of composite structures with

Table 3 Solubility of ZnO, CuO and Fe3O4 in three deep eutectic solvents at 50 1C43

Solubility/mol dm�3

Ionic liquid CuO Fe3O4 ZnO

1 Malonic acid:1 Choline chloride 0.246 0.071 0.554

1 Oxalic acid:1 Choline chloride 0.071 0.341 0.491

2 Phenylpropionic acid:1 Choline chloride 0.473 0.014 40.491



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iron oxides. The Type 3 eutectics were used to selectively extract metals from a mixed

matrix and allow them to be recovered with high efficiencies and in high purity.142

During this project a protocol was developed to selectively dissolve elements from

the EAF dust. A demonstrator module has been built, Fig. 2, to recover toxic

components such as lead and cadmium while also recovering the major soluble

component, which is zinc oxide. The insoluble portion, which is primarily iron oxide

with a smaller proportion of aluminosilicates can be recycled into the iron industry.

While significant amounts of data have been gathered for these systems it is difficult

to model the solubility without a knowledge of the soluble species identity as solute

size and polarisability are required for most models. We have recently shown that

there is a rough correlation between the solubility of a metal oxide and the charge

density of the metal centre but this can clearly not explain the anomalous behavior

shown in Table 3 which must be due to solute polarisability.

4.2 Electrochemical processing in ionic liquids

Another important technological application area of ionic liquids in which metal ion

speciation plays a key role is that of electrochemical deposition and dissolution.

Here, the metal ion coordination chemistry and structure influence directly the

Fig. 1 Range of metal oxides dissolved in deep eutectic solvents.

Fig. 2 Pilot plant for selective metal recovery room EAF dust using deep eutectic solvents.



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thermodynamic (redox potential) and kinetic parameters that determine the rate of

electrochemical deposition as well as the morphology and stability of the resultant

metal coating. This area has been extensively reviewed recently.144 Once again

progress in the fundamental science underpinning this area is hindered by the lack of

structural data and by the complexity of the coordination equilibria. For example,

the complexity of the speciation issue can clearly be seen from Fig. 3 which shows

CuCl2 � 2H2O in a variety of ionic liquids including choline chloride based ones as

well as [emim]+ systems with a variety of anions. Each anion complexes the Cu

centre in a different way and results in a different colour. In turn, each will naturally

affect the reactivity of the metal and its redox properties.

The electrochemistry of many p- and d-block elements has been studied in

chlorometalate melts and also, recently, by us in deep eutectic solvents based on

binary mixtures of choline chloride with either urea or ethylene glycol.141,145–147

Many others have also been recently reviewed.144 We have found that metals in

groups 3–5 (Sc–V) cannot be deposited within the potential window of the ionic

liquid. Those in groups 6–8 (Cr–Fe) can be reduced but not stripped whereas Co, Ni

and Pd all give a quasi-reversible deposition response. The majority of the remaining

elements (e.g. Cu, Ag, Zn, Hg, In, Sn and Bi) show chemically reversible deposition

and stripping responses. The reason behind this is not clearly understood but is

thought to result from the speciation of the metal complexes in solution or close to

the electrode/solution interface and this link needs to be urgently addressed.

4.2.1 Effect of speciation. Electrochemical deposition processes are a key part of

manufacturing industries as diverse as high density electronics, and high fashion (e.g.

modern furniture). Electrolytic deposition is useful for surface modifications of bulk

materials (including metals and polymers) with applications including wear (friction)

resistance, corrosion resistance, heterogeneous catalysis, nano structured materials,

decorative and speciality coatings, e.g. for H2 storage. Conventional electroplating

processes, from aqueous electrolytes, are dominated by presence of oxide, hydroxide

and aqua complexes and underpinned by more than 150 years of fundamental

studies in the area of inorganic coordination chemistry. Historically, metal deposi-

tion processes have been developed, improved and optimised by fine tuning of the

Fig. 3 Solutions of CuCl2 � 2H2O, 20 mM, in various ionic liquids: (a) [emim]OAc, (b)

[emim]SCN, (c) 2(ethylene glycol):1ChCl, (d) 2(urea):1ChCl, (e) [emim]HSO4, (f) [emim]EtSO4,

(g) 2(ethylene glycol):1ChCl with added excess NH3, (h) 2(ethylene glycol):1ChCl with added

excess en.



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complex chemistry. This has often involved the addition of toxic co-ligands such as

cyanide, thiourea, or the salts of strong acids, NO3�, SO4

2� etc.; these are often

termed brighteners because they produce metal deposits that are smooth and

reflective. As well as this, the mechanistic steps for metal ion reduction, including

electron transfer and ligand displacement, can be modified at the electrode surface

by an array of reagents including low molecular weight polymers (e.g. PEG) that

affect the surface energy of the electrode/electrolyte interface and/or perturb the

double layer structure. Wetting agents and surface adsorbates fall into this category.

Generally their role in coordination chemistry and metal ion reduction is less well

understood because of the kinetic nature of these (often surface specific) interactions.

In fact, to some considerable extent their widespread use, although effective, is

anecdotal!

Ionic liquids and deep eutectic solvents, have no solvent molecules and essentially

no (or at least very little) water. Consequently it is not surprising that the bright-

eners, wetting agents, complexing agents and levellers developed to fine tune

electrolytic deposition processes from aqueous electrolytes do not function in ionic

liquids. It nevertheless remains the case that the complex chemistry of the metal ions

has a critical influence on the deposit formed. This is manifested in the adhesion and

coherence of the deposited metal film, together with the surface morphology, crystal

size, thickness and grain structure.

4.2.1.1 Aluminium. There is a very strong interest in the deposition of reactive

metals such as Al, primarily as anti corrosion coatings in aerospace components.

Aluminium can be electrolytically reduced in ionic liquids because of the large

cathodic window. Several groups have recently reported deposition of Al from

chloroaluminate melts using alkylimidazolium chloride148,149 or trimethylphenyl-

ammonium chloride.150 In each case speciation equilibria are cited, eqns (8) and (9);

it is generally acknowledged that Al deposition is only possible from Lewis acidic

melts (i.e. where the mole fraction of AlCl3 in the mixture 4 0.5).151

2AlCl4� " Al2Cl7

� + Cl� (9)

Hence, it is clear that the position of the speciation equilibria are critical to thedeposition process although it remains the case that there are virtually no in situstructural data on these systems. We have recently studied these systems using 27AlNMR (see earlier), probe microscopy and many other techniques. We have devel-oped a process via a combination of complexing agents and diluents that produceshigh quality, bright adherent Al coatings. Although we cannot give details here(since there are process patents pending) this example illustrates that practical issuescan be addressed with analytical insight into the coordination chemistry supportedby structural data. It is also observed that balance of speciation effects theelectrochemical nucleation mechanism of electrolytic deposits. Whilst Al depositionfrom chloroaluminate melts has been shown to be relatively insensitive to cationeffects148,150 (3D instantaneous model), addition of LiCl to the melt changes thenucleation mechanism and produces powdery black deposits.

4.2.1.2 Chromium. In the case of chromium there is a motivation to develop a

plating process that will replace the current technology that relies on aqueous CrVI.

The use of CrVI will shortly be banned (except for reserved applications) under EU

law because of toxicological concerns. Chromium plating is possible from type 2

deep eutectic solvents using a mixture of CrCl3 � 6H2O and choline chloride.145

Interestingly, cyclic voltammetry of CrCl3 � 6H2O in this liquid reveals no sign of

proton reduction normally associated with labile H2O ligands. From this we deduce



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that the water molecules are strongly bound to the Cr centre and/or that proton

reduction is rendered unfavourable by strong hydrogen bonding between the

protons of the H2O ligands and the Cl� ions of the choline chloride. Cyclic

voltammetry also shows step-wise reduction of the CrIII to CrII followed by electrode

passivation and then, at more cathodic potentials, reduction of CrII to Cr0 is

observed with bulk deposition. The passivation process associated with the Cr3+/2+

couple is not well understood and whilst it is known that CrII coordination chemistry

in aqueous solution is very complex, there is no evidence to suggest that the situation

is any simpler in ionic liquids. This is another example where in situ structural data,

vital to understanding the redox electrochemistry and associated coordination

chemistry, are still lacking.

4.2.2 Brighteners and complexing agents. Studies of other metal electrolytic

deposition processes, including nickel147 and zinc,68,146 have been faced with similar

problems. Whilst there have been several recent studies of, for example, zinc

deposition from chlorozincate systems37c,38 these have not included any attempts

to determine structural parameters of the zinc species or to correlate these with the

morphology and properties of the resultant metal coating. In general very little work

has been carried out into the influence of ligand complexation on the electrode

reactions of deposition processes and, commonly, the metal deposits from electro-

lytic process in a range of ionic liquids are quite rough and dull in appearance. We

have taken a systematic, if empirical, approach to ligand chemistry with Ni2+ and

Zn2+ in type 3 deep eutectic solvents that has yielded useful results. Smooth and

optically reflective (bright) coatings of metallic zinc and nickel have been obtained

from ethylene glycol and urea based type 3 eutectics using ethylene diamine (en) or

acetylacetonate (acac).152 The FAB-MS of NiCl2 in choline chloride:2(ethylene

glycol) shows the presence of NiCl3� and Ni2Cl5

� as dominant species. This solution

has a lMax at 415 nm in the UV-Vis (green) that differs from the spectrum of

NiCl2 � 6H2O in aqueous solution; this is consistent with the absence of water in the

coordination sphere. Successive addition of up to three stoichiometric equivalents of

en to this system produces no change in the intensity ratios of the FAB-MS data but

the colour changes rapidly from green to purple.147 The lMax value of the purple

liquid is similar to that for [Ni(en)3]2+ in an aqueous environment and so it seems

likely that NiCl3�, Ni2Cl5

� and [Ni(en)3]2+ all exist in solution. Certainly the colour

change in this system, as well as the cyclic voltammetry is convincing evidence of a

change of bulk speciation upon addition of en but the corresponding, contrary,

FAB-MS data in this case serves to illustrate the limitations of this (ex situ)

technique that have already been discussed earlier in this article. A similar sequence

of observations has also been described for addition of acac to the same system

where bulk speciation changes were evidenced both by colour change and cyclic

voltammetry but where FAB-MS showed no difference. Additionally we have

studied zinc deposition under similar conditions and found that only en serves as

a brightener. Here, there is no colour change associated with ligand binding and so

comparative structural information is even more elusive.

Our main focus in this area has been to study the effects upon electrochemical

response and physical properties of the Zn deposit of the liquid composition.146 For

example, comparing results of deposition from both choline chloride:2(ethylene

glycol) and choline chloride:2(urea). Here, we observe that the cyclic voltammetry of

ZnCl2 in each liquid is quite different and that the rates potential dependent



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adsorption (upd), nucleation and growth are a strong function of the liquid

composition. These effects do not correlate directly with viscosity of the liquid

suggesting that a difference in Zn2+ speciation is responsible for these observations.

However, in a very recent EXAFS study we have observed only a single zinc

containing bulk species, ZnCl4�, to be present in both liquids. Whilst this result is

intuitively consistent with the high Cl� ion concentration (ca. 4.8 M) in both of these

liquids, it suggests that the coordination chemistry of the zinc ions at the electrode

surface controls the deposition mechanism and further complicates the elucidation

of these processes.

4.2.3 Mass transport and composition. Further to the above discussion, the

position of speciation equilibria extending to oligomeric chlorometalate anions also

has an influence on the rate of mass transport of metal containing ions to the

electrode surface during deposition, since these will have a range of diffusion

coefficients. Consequently, the magnitude of mass transport limited current, during

a deposition process, will be a function of the liquid composition. Similar arguments

also apply to the electrodeposition of metal alloys in ionic liquids. This is especially

attractive and has significant advantages over aqueous processes because of the

compression of redox potentials observed in ionic liquids as a consequence of the

high chloride concentration.153 Several groups have recently reported electrodeposi-

tion of metal alloys from chlorometalate systems154,155 or from type 2 deep eutectic

solvents.156 Here, there are possibilities for the formation of mixed metal species and

the scope for these to influence and control the electrochemical reduction processes

but although FAB MS indicate that this is likely,156 once again there are no reliable

in situ data. Other complications also arise because of concentration profiles that are

necessarily set up during a sustained electrolytic process. The existence of concen-

tration profiles implies that the composition of the liquid is not constant over the

volume of liquid closest to the electrode surface i.e. the diffusion layer. The

consequence of this is that the speciation equilibria in this region of the liquid could

be substantially different to those of the bulk. In the extreme, rapid and complete

depletion of one component of the ionic liquid, e.g. ZnCl2, during electrolysis at the

electrode surface could result in a change of phase, i.e. freezing. This rather radical

notion could help to explain the electrochemical passivation effects observed by

some researchers. Alternatively, diffusion layer speciation effects might explain rapid

and abrupt phase changes in growth morphology observed for some deposits, Fig. 4.

The zinc deposit shown in Fig. 4, was deposited at constant potential over a period

of time. At short time scales the deposit consists of densely packed micro crystals ca.

1 mm in size. However, at longer times growth of this phase ceases abruptly in favour

of large, flat platelets orientated approximately normal to the electrode plane.

4.2.4 Dissolution processes. In addition to electrochemical deposition, electro-

chemical dissolution processes are also dominated by the coordination chemistry of

the complex metal ions as they are formed from the metallic substrate during

oxidation and released into solution. These are important because dissolution

processes are often used as the counter electrode reaction during electrochemical

deposition. For example, use of a sacrificial Zn anode is used during cathodic Zn

electrodeposition in order to maintain the bulk zinc ion concentration during

electrolysis. Consequently, there is an imperative to understand the mechanistic

steps and structural intermediates associated with both anodic and cathodic



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processes.153,157 There are also other commercially important electrochemical pro-

cesses such as electropolishing; here surface roughness is minimised by anodic

electrochemical dissolution of sharp surface features.158,159 In such a process it is

the electrochemical dissolution kinetics and thermodynamics that determine the

surface finish and process parameters. Typically this process is used to prepare high

quality components manufactured from stainless steel (containing Fe, Ni, Cr, as well

as other minor components) or high performance alloys (e.g. Ni/Co) for the

pharmaceutical, aerospace or automotive industries. Issues here include the relative

solubility and speciation of metal ions e.g.Ni, Co, Fe, Cr, as well as the possibility of

mixed metal complexes. We have recently shown that despite the different solubilities

of Fe, Ni and Cr chlorides and glycolates in type 3 liquids, the metals in alloys such

as stainless steel (e.g. standard grade SS316) are removed during anodic dissolution

in the same proportions as those originally present in the alloy (i.e. de-alloying does

not occur).159

4.2.5 Galvanic processes. Finally, as well as electrolytic electrochemical processes

there are many catalytic or galvanic redox reactions of metal based solution species

that are utilised in surface processing. We have recently reported the galvanic

deposition of metallic silver on a copper surface160 that involves reduction of Ag+

by metallic copper and concomitant oxidation of Cu0 to Cu+ in a choline chloride/

ethylene glycol, type 3, deep eutectic solvent. This system is unique to this class of

ionic liquids and distinct from the aqueous speciation for two reasons, (i) AgCl,

probably present as AgCl2�, is moderately soluble and (ii) Cu+ is stable in the liquid.

This is in contrast to the aqueous environment where AgCl is insoluble and Cu+

undergoes rapid disproportionation. These two points are illustrated clearly in

Fig. 5. The deposition potentials of the two metals are evident (Fig. 5(a) and (b))

with the reversible couple corresponding to Cu2+/+. This process is now utilised in

the coating of copper components with silver for the manufacture of high density

printed circuit boards.161

Fig. 4 Scanning electron micrograph of Zn metal deposited from 0.3 M ZnCl2 in type 3 deep

eutectic solvent (2(ethylene glycol):1ChCl) in the presence of 0.3 M NH3.



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Conclusions

This review has highlighted the importance of metal speciation in ionic liquids in

controlling the reactivity and redox properties of metals in solution. Beyond this it

has hopefully posed more questions than it has answered. The question of classifica-

tions of ionic liquid has been raised; at what composition does an ionic liquid stop

and a molecular solution begin? The meaning and more importantly the quantifica-

tion of Lewis and Brønsted acidity has been highlighted and this needs to be urgently

addressed. Numerous methodologies have been highlighted to determine speciation

in ionic liquids but there are problems associated with many of them and little

correlation exists between the species predicted. To some extent, however, the issues

are the same with molecular solvents and again new methodologies are needed to

address this issue. The application of ionic liquids in the field of metal processing has

already led to processes being operated on the 100–1000 kg scale but the key issue of

speciation dominates the dissolution and deposition of metals and hampers optimi-

zation.

The field of ionic liquids has been dominated over the last 10 years by physical

characterization and synthetic application. The necessity for coordination chemists

to become involved in addressing the fundamental issues of metal speciation in ionic

media has hopefully been highlighted in this review.

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metal complexation in ionic liquids

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