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Ionic Liquids in Green Chemistry
Dr. Nie Wanli
Chemistry Department of NWU, Xi’an
Ionic Liquids in Green Chemistry
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What are Ionic liquids (ILs)?
Why consider of ILs?
The characteristic properties of ionic
liquids
The synthetic methods
Research with ILs
Outlook
What are ionic liquids?
Definition:
------ Quite simply, they are liquids that are composed
entirely of ions.
In the broad sense, this term includes all the molten
salts, for instance, sodium chloride at temperatures
higher than 800 oC.
What are ionic liquids?
------ Ionic liquids are salts that are liquid at low
temperature (<100 oC) which represent a new class
of solvents with nonmolecular, ionic character.
Room temperature Ionic liquids
• Room temperature ionic liquids (RTIL) are salts that
are liquid over a wide temperature range, including
room temperature.
• Variations in cations and anions can produce literally
millions of ionic liquids, including chiral, fluorinated,
and antibacterial IL.
• Large number of possibilities allows for fine-tuning
the ionic liquid properties for specific applications
The driving forces
The problems in the chemical industry with the
volatile organic compounds (VOCs) :
• toxic and/or hazardous
• serious environmental issues, such as
atmospheric emissions and contamination
of aqueous effluents
The driving force in the quest for novel reaction media:
• greener processes
• recycling homogeneous catalysts
What is “green chemistry” ?
Recently ionic liquids have often been
discussed as promising solvents for “clean
processes” and “green chemistry”.
These two catchwords means to reduce drastically
the amounts of side and coupling products and
the solvent and catalyst consumption in chemical
processes.
Why consider Ionic liquids ?
• ILs are environmentally-friendly alternatives
to organic solvents for liquid/liquid extractions.
Catalysis, separations, and electrochemistry.
• ILs will reduce or eliminate the related costs,
disposal requirements, and hazards associated
with volatile organic compounds (VOCs).
• The ability to fine-tune the properties of the IL
medium will allow selection of IL to replace
specific solvents in a variety of different
processes.
Important IL Properties
•
•
•
•
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High ionic conductivity
Non-flammable
Non-volatile
High thermal stability
Wide temperature range for liquid phase (- 40
to + 200°C)
• Highly solvating, yet non-coordinating
• Good solvents for many organic and inorganic
materials
Great promise
• Designability. By combining different anions with
cations, it is possible to generate a huge number of
different ionic liquids, each with their own specific
solvent properties. Some ionic liquids are water
soluble, others are not. Some dissolve typical organic
solvents, other are not.
• They can be functionalized to act as acids, bases or
ligands and have the potential to catalyze certain
reactions in certain systems.
• Ionic liquids are non-volatile, hence they may be used
in high vacuum systems and high temperature
reactions without the requirement of a pressure vessel
to contain the vapors.



They are good solvents for a wide range of both
inorganic, organic and polymeric materials and
unusual combinations of reagents can be
brought into same phase. However they do not
dissolve glass, polyethylene, or Teflon. High
solubility usually implies small reactor volumes
in the final process.
They are immiscible with a number of organic
solvents and provide a non-aqueous, polar
alternative for two phase systems, this has been
used to effect total catalyst recovery in a
number of transition metal catalyzed reactions.
Hydrophobic ionic liquids can also be used as
immiscible polar phase with water.
They are often composed of poorly coordinating
ions, so they have the potential to be highly
polar non-coordinating solvents, this is
particularly important when using transitionmetal based catalysts.
Characteristics of RTIL
• Choice of cation and anion determine physical
properties (e.g. melting point, viscosity, density, water
solubility, etc.)
• Cations are typically big, bulky, and asymmetric
accounting for the low melting points
• The anion contributes more to the overall
characteristics of the IL and determines the air and
water stability
• Melting point can be easily changed by structural
variation of one of the ions or combining different
ions
Typical RTIL Cations
• Room temperature ionic liquids consist of bulky
and asymmetric organic cations such as :
Imidazolium ion
Pyridium ion
Ammonium ion
Scheme 1. Important types of cation
Phosphonium ion
Anions for RTIL
•A wide range of anions is employed, from simple
halides which inflect high melting points, to inorganic
anions such as:
Anions:
• [PF6]- for moisture stable, water immiscible IL
• [BF4]- for moisture stable, but water miscible IL
depending on the ratio of ionic liquid: water, system
temperature, and alkyl chain length in the cation.
• Less common anions include:
Triflate [TfO]
Nonaflate [NfO]
CF3SO2CF3(CF2)3SO2Bis(triflyl)amide [Tf2N] Trifluoroacetate [TA]
(CF3SO2)2NCF3CO2Heptafluorobutanoate [HB]
CF3(CF2)3CO2-
Historical Development
• Ethylammonium nitrate, which is liquids at RT was
first described in 1914.
• In the later 1940s, n-alkylpyridinium
chloroaluminates were studied as electrolytes for
electroplating aluminum.
• The first examples of ionic liquids based on
dialkylimidazolium cations were reported in the early
1980s. They contain chloroaluminate anions and
proved to be useful catalysts/solvents for FriedelCrafts acylations.
• The first example of the new ionic liquids, that
currently are receiving so much attention as novel
media for homogeneous catalysis,
ethylmethylimidazolium tetrafluoroborate was
reported in 1992.
Ionic liquid synthesis

Direct quaternization to form cation
------Alkylation reagents

Indirect quaternization to form cation
Ionic liquid synthesis
General procedures:
NR3
Step I
Step IIa
+ R'X
[R'R3N]+X-
+ Lewis acid MXy
Step IIb
1. + Metal salt M+[A]- MX (precipition)
2. + Bronsted acid H+[A]- HX (evaporation)
3. Ion exchange resin
[R'R3N]+[MXy+1]-
[R'R3N]+[A]-
Scheme 2. synthesis paths for the preparation of ionic liquids
examplified for an ammonium salt.
The types of RTILS
•
organoaluminates
•
air- and water-stable ionic liquids
Organoaluminates
• Since the organoaluminate ionic liquids have donor and
acceptor patterns, The Lewise acidity can be modulated by
the relative amount of the aluminum compound. Acidic or
basic IL attainable through varying the concentration of
the following species:
Al2ClAlCl
Cl- = 2 AlCl47 +
AlCl3
3
ClAlCl4Al2Cl7Acidic
basic
neutral
N
N
N
N
N
N
Et
Me
basic
Et
Me
neutral
Et
Me
acidic
• Basic haloaluminates preclude solvation and solvolysis of
metal ion species
Large
electrochemical windows for both
chloro and bromo ionic liquids.
The
advantage of this controlled Lewis acid
ionic liquids is their use in Ziegler-Natta Type
catalytic reactions
BUT:
moisture sensitive
Table 1. Melting Point (Mp) and Viscosity (n ) of 1-Ethyl-3methylimidazolium Chloride/Aluminum Chloride Ionic liquid at
different Molar Fractions (x) of the Aluminum Compound
x
Mp (oC)
n (p) Table 1.
PH
Table 1. Table
1.
- 60
basic
0.36
1.59
0.50
0.20
2
neutral
0.66
0.16
- 80
acidic
Ambient-Temperature, Air- and Waterstable Ionic liquids
• Can be obtained by the substitution of the halide
anion of the 1,3- dialkylimidazolium cation by other
weekly coordinating anions.
• In order to be liquid at room temperature, the cation
should preferably be unsymmetrical. The melting
point is also influenced by the nature of anion.
• Can be used for the immobilization of transitionmetal catalyst precursors in biphase catalysis.
• Due to their inherent ionic nature, ionic liquids can
effectively stabilize cationic transition-metal special
that are known to be more attractive than their
neutral analogues.
“The melting point is influenced by”
the nature of cation and anion
Applications
•Because of their properties, ionic liquids attract
great attention in many fields, including organic
chemistry, electrochemistry, physical chemistry,
and engineering.
1. as reaction media for synthesis and catalysts
2. in electrochemistry
3. in separation processes
4. as electrolytes in solar cells
5. as lubricants
6. as propellants in small satellites
7. matrixes in MALDI mass spectrometry
8. Applications in other areas
Catalysis in ionic liquids
------general considerations
potentially attractive media for homogeneous catalysis:
•
They have essentially no vapour pressure which
facilitates product separation by distillation.
•
They are able to dissolve a wide range of
organic, inorganic and organometallic
compounds.
•
The solubility of gases, e.g. H2, CO and O2, is
generally good which makes them attractive
solvents for catalytic hydrogenations,
carbonylations, hydroformylations, and aerobic
oxidations.
• They are immiscible with some organic solvents,
e.g. alkanes, and, hence, can be used in two-phase
systems. This gives rise to the possibility of a
multiphase reaction procedure with easy isolation
and recovery of homogeneous catalysts.
• Polarity and hydrophilicity / lipophilicity can be
readily adjusted by a suitable choice of
cation/anion and ionic liquids have been referred
to as ‘designer solvents’.
•
They are often composed of weakly
coordinating anions, e.g. BF4- and PF6- and,
hence, have the potential to be highly polar
yet non-coordinating solvents. They can be
expected, therefore, to have a strong rateenhancing effect on reactions involving
cationic intermediates.
•
Ionic liquids containing chloroaluminate ions
are strong Lewis, Franklin and Brønsted
acids. Protons present in emimAlCl4 have
been shown to be superacidic. Such highly
acidic ionic liquids are, nonetheless, easily
handled and offer potential as non-volatile
replacements for hazardous acids such as HF
in several acid-catalysed reactions.
•Publications to date show that replacing an
organic solvent by an ionic liquid can lead to
remarkable improvements in well-known
processes.
•There are also indications that switching from
a normal organic solvent to an ionic liquid can
lead to novel and unusual chemical reactivity.
This opens up a wide field for future
investigations into this new class of solvents in
catalytic application.
Applications
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Solvent Properties
Transition Metal Catalysed Reaction
Carbocation Chemistry
Separations
Electrochemistry
Photochemistry
Solvent Properties
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Diels-Alder reaction
Aldol condensation
Others
Diels-Alder reaction
O
+
OMe
cyclopentadiene
ionic liquid
r.t
H
+
CO2M
e
H
CO2Me
methyl acrylate ester
exo-form
endo-from
Ionic liquids
Composition
(% AlCl3)
Time (h)
emimCl/(AlCl3)x
48 (basic)
22
4.88
32.3
emimCl/(AlCl3)x
48 (basic)
72
5.25
95
emimCl/(AlCl3)x
51 (acidic)
22
19
53
emimCl/(AlCl3)x
51 (acidic)
72
19
79.4
-
72
4.3
91
bmimBF4
Endo/exo Y. (%)
ratio
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Endo selectivity ----highly polar solvents
Increases in the reaction rate
Allows water sensitive reagents to be used
Simple workup
Ionic liquid can be reused
Aldol Condensation
Aldol I
O
O
hydrogenation
NaOH aq.
2
H
H
solvent,reflux, 3h
1
Aldol II
O
O
O
+
H
NaOH aq.
H
H
solvent,reflux, 3h
3
entry

solvent
reaction
type
conv.(%)
selectivity (%)
1
2*
3
4**
1
bmimBF4
Aldol I
99
64
2
-
33
2
H2 O
Aldol I
100
82
0
-
18
3
emimBF4
Aldol II
100
4
6
69
21
4
bmimBF4
Aldol II
100
3
3
80
14
5
H2 O
Aldol II
100
36
0
59
5
Solubility
Recent activity with RTIL as solvent
• sc-CO2 Stripping after Extraction (J. Brennecke)
• Conductive RTIL (P. Bonhote)
• Ionic liquid-polymer gel electrolytes (R. Carlin)
• Catalytic hydrogenation reaction (J. Dupont)
• Electrochemistry in RTIL (C. Hussey)
• Butene dimerization (H. Olivier)
• Benzene polymerization (B. Osteryong)
• Two-phase separations (R. D. Rogers)
• Friedel-Crafts; regioselectivie alkyl. (K. seddon)
• Organometallic synthesis (T. welton)
… This list is not exhaustive
Transition Metal Catalyzed Reaction
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Hydrogenation
Heck reaction
Stille reaction
Other reactions
Hydrogenation reaction
OH
H2, [RuCl2-(S)-BINAP]2.NEt3 (cat.)
i
PrOH-bmimBF4, r.t., 20 h
O
O
Dupont et al.
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OH
*
P (atm)
conv.(%)
% e.e
1st use
50
100
78 (S)
2nd use
75
100
84 (S)
3rd use
25
90
79 (S)
4th use
100
95
67 (S)
Two phase system
Simple workup -------decantation
Ionic liquid/catalyst phase can be reused
IL in Two-Phase Catalytic Reactions
Heck Reaction (1)
X
Pd(OAc)2
+
bmimBr
NaOAc, 100 ~ 110 oC, 24h
styrene
R
entry
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

stilbenes
R
X
R
conv.(%)
1
I
H
100
2
Br
CHO
100
3
Br
MeCO
79
Polar solvent
Expensive
Phosphine ligand
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Less expensive
High yields
Without phosphine
Heck reaction (2)
OBu
OBu
Br
OBu
+
Pd(OAc)2 (2.5 mol%),
DPPP(2.75 mol%)
+
solvent, Et3N, 100 oC, 18h
enol ethers
Dppp = 1,3-bis(diphenylphosphino)propane
entry
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
solvent
-form
conv.(%)
/
E/Z
1
toluene
23
46/54
68/32
2
DMSO
100
75/25
79/21
3
bmimBF4
50
>99/1
4*
bmimBF4
100
>99/1
High regioselectivity
Simple workup -------distillation
-form
Y.(%)
95
Stille reaction
O
O
I
+
PdCl2(PhCN)2 (cat.)
SnBu3
vinyltributyltin
iodocyclohexenone
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

bmimBF4
Ph3As, CuI, 80 oC, 2h
Y. (%)
1st use
82
2nd use
78
3rd use
72*
Simple workup -------extraction
Ionic liquid/catalyst phase can be reused
Air and moisture stable
Other reactions
• Suzuki-Miyaura coupling reaction
• Trost-Tsuji coupling
• Hydroformylation (biphase)
Stabilize
catalysts
Simple workup
Atom economy
Carbocation Chemistry
• IL containing chloroaluminate anions are
strong Lewis acids and if protons are present
they are superacidic.
•The ionic liquids acts as both a solvent and
catalyst for a acid catalysed processes involve
cationic intermediates, e,g. carbenium and
acylium ions
• Friedel-Crafts alkylations and acylations
• Arene exchange reactions
Friedel-Crafts reaction--acylation
O
O
OMe
Cl
+
anisole
• Y 64% in acetonitrile
•p-/o- = ratio of 93/7
Cu(OTf)2
OMe
bmimBF4
methoxybenzophenone
Y. quant. ( p-/o-ratio = 94/4 )
• quantitatively
• regioselective
Friedel-Crafts reaction--akylations
The Friedel-Crafts alkylation of benzene with
long chain –olefin catalyzed by
chloroaluminate ionic liquids modified by HCl
which was attributed to the superacidities of
these media, were shown to give higher rates
and more favorable product distributions.
Arene exchange reactions
• IL can function as both catalyst and solvent
• In a series of arene exchange reactions on ferrocene, an
acidic [bmim]+ chloroaluminate IL was used where
[Al2Cl7]- is the active Lewis acid.
Reactant
Fe
Arene
Product
Yield (%)
Benzene
Fe(C5H5)(C6H6)+
53
toluene
Fe(C5H5)(C6H5Me)+
64
napthalene
Fe(C5H5)(C10H8)+
53
• Conventional problems with these reactions (e.g., lower
yields with solid arenes) are eliminated.
Separations
• Witting reaction
• Others
Witting reaction
O
PPh3
O
bmimBF4
C C Me + PhCHO
60 oC, 2.5 h
H
Ph HC C C Me + Ph3PO
H
Y. (%)
E/Z (%)
1st use
82
97/3
2nd use
83
6th use
91
• The separation of the product
and triphenylphosphine oxide
• Extractions
• Reuse IL
Reduction
CHO
CH2OH
Bu3B (1 eq.)
ionic liquid
ionic liquid
temp. (oC)
time (h)
Y. (%)
bmimBF4
100
16
93
emimBF4
100
16
90
emimPF6
100
16
96
emimPF6
r.t.
48
94
• Lower temperature
Fluorination
CH2Cl
Me
F
N+
N+
.
H
Me
2 BF4-
F
O
o
N
H
bmimBF4 / MeOH (1 / 1), 20 C, 3 h
solvent
cosolvent
O
+
N
H
N
H
N-fluoro-N’-(chloromethyl)triethylenediamine bis(tetrafluoroborate)
entry
Me
1 3-fluorinated 2-oxoindoles
2
Temp.(oC)
Time (h)
1 (%)
2 (%)
1
MeCN
H2 O
r.t.
over
night
71
small
amount
2
bmimBF4
MeOH
20
3
99
-
• Short reaction time
• High yield
Ring opening reaction
NHR3
O
+
R2
R1
R3-NH2
bmimBF4, r.t.
NHR3
OH +
R1
R1
epoxide
R2
R1
entry
1
OH
Ph
R2
H
R3
R2
product
Ph
Ph
Time (h)
NHPh
OH
Y. (%)
5.0
85
6.0
83
NHPh 6.0
89
6.0
85
OH
2
-(CH2)4-
Ph
NHPh
3
PhOCH2
OH
H
Ph
PhO
OH
4
Bu
H
p-tol
Bu
NH-p-tol
• room temperature, economic
•This reactions require a large excess of the amines
at elevated temperatures. The high temperature
reaction conditions are not only detrimental to
certain functional groups but also to the control of
regioselectivity.
•Subsequently, a variety of activators or promoters
such as metal amides, metal triflates and transition
metal halides have been developed. However, many
of these are often expensive or are needed in
stoichiometric amounts, thus limiting their
practicality.
•In the system using ionic liquids, the reaction
proceeds at room temperature to give aminoalcohols in high yield. After the reaction, the
product was extracted with ether.The ionic liquid was
reused in five runs without any loss of activity.
Enzymatic reaction
OH
oAc
O
lipase
+
Ph
O
bmimBF4, r. t., 3.5 h
Ph
Y. 44% (>99 % e.e.)
• similar yields to those of organic
solvent systems
Others
Electrochemistry
• Unique features of chloroaluminate ionic
liquids include a large electrochemical
window, although these anions are moisture
sensitive
• Possible applications include low cost and
recyclable electrolytes for batteries,
photoelectrochemical cells, and
electroplating
• BF4- and PF6- ionic liquids have been
developed as moisture stable electrolytes
Other types of ionic liquids
As the range of application for ionic liquids
increase, the need for ionic liquids with special
chemical and physical properties also
increases. With this in mind, the term “tastspecific ionic liquid” has been introduced to
described ‘designer’ligands prepared for
special applications. Other types of ionic
liquids:
Concluding remarks
Future IL research Needs:
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Comprehensive toxicity data
Combinatorial approach to IL development
Database of physical properties, chemistries, etc.
Comparators for direct comparison of IL and
traditional solvents
Industrial input into a research Agenda
Economic synthetic pathways
Wider availability
REFERENCES
Further information regarding physical properties, chemistry, and uses of
ionic liquids:
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[2] Wasserscheid P, Keim W. Angew Chem. .Int. Ed. Engl., 2000, 39: 3722.
[3] Freemantle M. (a) Chem . Eng . News, 2000, 78 (May)15: 37-39; (b) Chem .
Eng . News, 2001, 79 (Jan)1: 21-25.
[4] Earle M J, Seddon K R. Pure Appl, Chem., 2000, 72 (7): 1391-1398.
[5] Chum H L, Koch V N et al. J. Am. Chem, Soc., 1975, 97: 3264 .
[6] Wilkes JS et al . Inorg . Chem., 1982, 21: 1236.
[7] a) Blanchard L A et al. Nature, 1999, 399: 28; b) Blanchard L A et al. Ind. Egn.
Chem. Res., 2001, 40: 287.
[8] Chauvin Y, Mumann L, Olivier H. Angew. Chem. Int. Engl., 1995, 34: 2698.
[9] Monteiro A L et al. Tetrahedron Asymmetry, 1997, 2: 177-179.
[10] Song C E, Roh E J. Chem. Commun., 2000: 837-838.
[11] Dullins J E L et al. Organometallics, 1998, 17: 815.
[12] Kakfman D E et al. Synlett., 1996: 1091.
[13] Mathews C J, Smith P J, Welton T. Chem. Commun., 2000: 1249-1250.
[14] Bellefon C de et al . J. Mol . Catal., 1999, 145: 121.
[15] Adam C J et al. Chem. Commun., 1998: 2097-2098.
[16] Boon J A et al. J. Org. Chem., 1986, 51: 48.
[17] Kun Qian, Yonquan Deng. J. Mol. Catal. A: Chem., 2001, 171: 81-84.
[18] Surretle J K D, Green L, Singer R D. Chem. Commun., 1996: 2753-2754.
[19] Wheeler C et al . Chem. Commun., 2001: 887.
[20] Earle M J, McCormac P B, Seddon K R. Chem. Commun, 1998: 2245.
[21] Hagiwara R, Ito Y J. Fluorine Chem., 2000, 105: 221.
[22] Boularre V L, Gree R. Chem. Commun., 2000: 2195-2196.
[23] Gordone L M, McClusky A. Chem. Commun., 1999: 1431-1432.
[24] Kanalka G W, Maladi R R. Chem. Commun., 2000: 2191.
[25] Fischer F, Sethi A , Welton T et al. Tetranedron letters, 1999, 40: 793-796.
[26] Earle M J, McCormac P B, Seddon K R. Gree. Chem., 1999, 1:23-25.
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Thanks