Ionic Liquids IIIB - American Chemical Society

Chapter 17

Understanding Reactions in Ionic Liquids

Downloaded by PENNSYLVANIA STATE UNIV on September 7, 2012 | http://pubs.acs.org Publication Date: March 15, 2005 | doi: 10.1021/bk-2005-0902.ch017

Lorna Crowhurst, N. Llewellyn Lancaster, Juan M . Perez-Arlandis, and Tom Welton* 1Department of Chemistry, Imperial College of London, Exhibition Road, London SW7 2AY, United Kingdom *Corresponding author: [email protected]

Abstract Since the discovery of organic salts that are liquid at room temperature and stable to both air and water; ambient-temperature ionic liquids have attracted growing interest as solvents for organic synthesis, using both stoichiometric and catalytic methodologies. Rather than taking a random approach to the selection of reactions for study in ionic liquids, we have been attempting to quantify ionic liquid effects, so that a rational approach to the selection of reactions can be used. In order to do this, we have needed to characterise the ionic liquids' abilities to interact with solute species and to investigate a number of reactions. We have chosen to select simple reactions, principally S 2 processes, and to investigate the effect of the ionic liquids on their rates and to compare these results to those in molecular solvents. In this paper we summarise our findings. N

Introduction Ionic liquids are being advanced as a class of "green" solvents that have the potential to be used in place of volatile organic solvents in synthesis [1]. Although estimates vary, there is no doubt that the number of combinations of anions and cations that will give rise to ionic liquids is vast. This possibility for synthetic variation has lead to ionic liquids being described as "Designer Solvents" [2]. However, in order to be able to realise this possibility it is necessary to know exactly what is being designed. Here we are interested in designing ionic liquids for increased rates of reaction. There is a vast range of 218

© 2005 American Chemical Society

In Ionic Liquids IIIB: Fundamentals, Progress, Challenges, and Opportunities; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.


219 potential ionic liquids; we have restricted ourselves to examples that are liquid at room temperature and give a variety of combinations of cation and anion. The ionic liquids used, together with abbreviations, are shown in Figure 1. A number of organic reactions have been studied in ionic liquids, with varying degrees of success. This has most often involved taking a well-known reaction and performing it in an ionic liquid to see if it "goes". However, this form of study does little to explain how the use of ionic liquids can affect the reactions conducted in them, or how the ionic liquids might be more generally applied. This is in spite of the fact that the study of solvent effects on organic reactions is a well-established area of research [3]. Such studies are required to extrapolate the results to other solvents and to predict reaction outcomes (both rates and selectivities). That is,.to select the right reactions to try in ionic liquids and the right ionic liquids to use for a given reaction. It is just as important to be able to identify reactions for which ionic liquids are likely to be disadvantageous and should be avoided.

[BF y, [PF ]\ [SbF ]\ [CF S0 r ([TfOD, [(CF S0 ) N]-([Tf N]-) 4

6

6

3

3

2 2

3

2

+

[bmim]

Me +

[bm im] 2

Me

Bu

[bmpyf Figure 1: Ionic liquids used



220 It is well known that the outcome of a reaction is affected by the local microenvironment generated around a solute species by a solvent; changing the solvent can impact both on equilibria and rates [3]. Since ionic liquids have the potential to provide reaction media that are quite unlike any other available at room temperature, it is possible that they will have dramatic effects on reactions in them and there have been many claims of great improvements in reaction yields and rates when using ionic liquids [4]. In order to understand how such effects may arise we have chosen to study the rates of a range of relatively simple and well-understood reactions in a variety of ionic liquids and molecular solvents. However, we must also consider how we are going to describe the ionic liquids. Unless we find some set of properties that can be measured, or predicted, against which the rates of the various reactions can be compared, we will generate a set of numbers that will ultimately fail us in our aims to achieve true design. These parameters must also be able to link the ionic liquids to the molecular solvents we are comparing them to.

A

Β

C

Figure 2: Reichardt's dye (A), N, iV-diethyl-4-nitroaniline (B) and 4-nitroaniline (C) The most important characteristics of a solvent are those that determine how it will interact with potential solutes. For molecular solvents, this is most commonly recorded as the polarity of the pure liquid, as expressed through its dielectric constant. A direct measurement of dielectric constant is not possible for conducting liquids and so is not available for the ionic liquids.


221


The Kamlet-Taft [4] system, based on the comparison of effects on the uv-vis spectra of sets of closely related dyes, was selected to probe particular solvent properties. These complimentary scales of hydrogen bond acidity (a), hydrogen bond basicity (β) and dipolarity/polarizability effects (π*) are the ones that we use here. It should be noted that these are empirically derived parameters and not fundamental molecular properties. The values can change according to the selection of dye sets or by the precise method used to calculate the values. We have chosen to use Reichardt's dye, 4-nitroaniline and N, N-diethyl-4nitroaniline, which are shown in Figure 2. The parameters π*, α and β were calculated using the following equations: π* = 0.314(27.52-V

N t

α = -0.186(10.91-v

Re

AMie%M-nitroaniline)

ichardt

2

2

free ion Γ161

71.8 (1.6) 71.8 (1.6) 68.5 (1.8) 79.5 (2.9) 54.4 (2.5)

-42.2 (5.4) -37.3 (5.3) -43.9 (6.0) 7.9 (10) -58.6 (8.4)

84.4 (3.2) 82.9 (3.2) 81.6 (3.6) 77.2 (5.9) 71.9 (5.0)

Kamlett-Taft parameters π* α 0.984 0.617

β 0.243

1.010

0.381

0.239

0.954

0.427

0.252

0.791

0.042

-0.014

0.791

0.042

-0.014

1

Values of Δ// in the ionic liquids are similar to those for the ion-pair in dichloromethane, but very different to that for the free ion in dichloromethane. Therefore, it is not the change of solvent that is leading to the change in the activation enthalpy in goingfromthe free ion in dichloromethane to the other systems, but rather the association of the chloride with a cation (ion-pair in dichloromethane) or cations (ionic liquids). This suggests that the same process is occurring in the rate-limiting step in both cases, i.e., as the chlorine-carbon bond is being formed in the activated complex, a cationchloride association is being broken with an associated enthalpic cost. This is opposed to the "free ion" case where the chlorine-carbon bond is formed without the concomitant breaking of another interaction, giving a lower activation enthalpy. In an ionic liquid, where there is no molecular solvent present to separate the ionic species, a free (molecular solvent solvated) ion cannot occur. An ionic species would always be expected to be coordinated by counter ions of the ionic liquid and we would expect that the chloride will always have its first coordination sphere dominated by cations. In fact recent structural studies in a related system confirm this expectation, and this is discussed later [17]. It is



229 noteworthy that our data suggest AH* more similar to that for the ion-pair in dichloromethane than for the free ion in dichloromethane. An ion-pair is a cation and anion that are coordinated to some extent and contained within a solvent shell; in our system that solvent shell is ionic, but is still more analogous to an ion-pair than afreeanion. The AS* values, however, are more similar to those for the reaction by the free ion in dichloromethane, than the ion-pair in dichloromethane [16]. Since the activation enthalpies are so similar to the ion-pair situation in dichloromethane, this needs to be explained. This reaction follows an SN2 reaction pathway and will form an activated complex as shown in Figure 3. In molecular solvents, this complex is coordinated by neutral solvent molecules. In ionic liquids it is likely that the cations will interact with the chloride and developing p-nbs.

Figure 6; The activated complex for the reaction of me-/?-nbs with chloride. The activation step of an S 2 reaction is an associative process and would therefore be expected to have a negative entropy. In dichloromethane the reaction by the ion-pair has a small positive activation entropy because, as the activated complex is formed, the cation of the ion-pair is liberated as a free solvated cation. This process counterbalances the loss of entropy associated with the formation of the activated complex. When performed in dichloromethane, the anionic leaving group p-nbs does not form an ion-pair in dichloromethane but becomes a free solvated anion. In the ionic liquid /?-nbs does associate with the cations of the ionic liquid. It is proposed that the entropy gained by liberating a cation from its association with the chloride ion is cancelled out by the association of another cation with p-nbs. Hence, the activation entropy for the reaction was what might be expected for an SN2 process, with a value similar to that for reaction by the free ion in dichloromethane. Finally, the AG* κ values have been compared, and a slight trend emerges. It appears that the highest AG value is observed in the [bmim] ionic liquid, and that it falls a little for [bm im] , and still more for [brnpy]*. This trend is to be expected, given that these values should follow the same relationship as the A values. These AG values are higher than those observed for reaction by either the free ion or the ion-pair in dichloromethane, N

298

X

+

+

2

X

2



230 suggesting, as one would expect, that in fact this reaction (with its charged reagent and its transition state with a delocalised charge) is slightly less favoured in ionic liquids than in dichloromethane. However, the activation parameters alone are not sufficient to explain the observed differences in the nucleophilicities of the halides in the different ionic liquids. These parameters reveal the size of the entropy and enthalpy barriers on going from the ground state of the available reactive forms of the reagents and the activated complex. The ground state for the reaction has first to be achieved to give such an available chloride ion before the reaction can occur. The evidence shows that the chloride is not always available to react with the substrate when dissolved in the ionic liquids, although there is still a linear relationship between concentration of chloride and k . So, it seems that there is an equilibrium (Figure 7) that must be accounted for. obs

cat cat>

,cat

caK

cat _^cat

+ me-p-nbs cat

*obs

+ me-p-nbs

cat

products

cat

cat

cat

Figure 7: The formation of the "available" form of CI in an ionic liquid. The left hand side of the equilibrium represents a fully coordinated, "unavailable" chloride, whereas on the right hand side one face of the chloride ion is exposed to the substrate following the dissociation of one [bmim] cation, giving an "available" chloride. This loose association of available chloride with the substrate represents the ground state for the reaction in this system. It should be noted that UV/vis spectroscopy showed that there is no significant interaction between available chloride and the substrate to give rise to a formal intermediate. The extent to which the nucleophile is coordinated by the cation will affect ΛΓ, and thus &„bs and k . This is consistent with our results. Although the nature of the complexes proposed above, and of the equilibrium constant, has not been determined in this work some initial proposals can be made. Hardacre et α/. [17], have demonstrated by neutron diffraction that a CI" ion is fully coordinated by six cations within 6.5 À of the halide in the related [mmim]Cl ([mmim] = 1,3-dimethylimidazolium cation). For the reaction to occur the CI" ion must first come into close proximity with the substrate me-p-nbs. To do this, the CI" ion must dissociate from at least one cation. At first glance, this appears to be simply a matter of breaking the cation-chloride hydrogen bond and so would be expected to correlate with a. However, our data show that the order of availability of chloride to react is [bmim][N(Tf)2] < [bm im][N(Tf) ] < [bmpy][N(Tf) ]. Closer inspection of the +

2

+

2

2

2


231 process reveals that, as well as breaking the cation-chloride hydrogen bond, the formation of the "available" chloride also requires the separation of the cation from the anion and the insertion of the neutral substrate. The contribution of this charge separation to AG is best modelled by considering π*. [bm im][N(Tf) ] has the highest value of π* of all of the ionic liquids used here and we propose that the energetic cost of the separation of charges in this ionic liquid is sufficient to cause an inversion of the expected reactivities of CI" in [bm im][N(Tf) ] and [bmpy][N(Tf) ], when only hydrogen bonding is considered. Downloaded by PENNSYLVANIA STATE UNIV on September 7, 2012 | http://pubs.acs.org Publication Date: March 15, 2005 | doi: 10.1021/bk-2005-0902.ch017

t

2

2

2

2

2

Conclusions This work shows that all ionic liquids are not the same. Therefore one cannot simply take a conventional organic reaction and replace the solvent with a single ionic liquid, then expect the result to be the same as would be achieved in all other ionic liquids. However, by the same token this work reveals that the claims made that ionic liquids can be tailor-made for a given reaction, are true [2]. It is possible to imagine that ionic liquids can be made to have the ideal combination of cation and anion for a given reaction. We can generalise to say that, the Hughes-Ingold rules [14, 15] do seem to apply to ionic liquids and can be used to predict whether the use of ionic liquids is likely to be helpful (or unhelpful) for any given process. However, which ionic liquid should be used requires a deeper understanding of the solvent-solute interactions that are possible between all of the reacting species, any intermediate species and the activated complex for the reaction and the ionic liquids.

Acknowledgements We would like to thank the Leverhulme Trust for a fellowship (NLL) and the Kodak Foundation (JMPA) and GSK (LC) for studentships.

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Ionic Liquids IIIB - American Chemical Society

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