Weakly polar aprotic ionic liquids acting as strong dissociating solvent: A typical ionic liquid effect revealed by accurate measurement of absolute pKa of ylide precursor salts

Article abstract of DOI:10.1021/jacs.6b02607

Absolute pK_as of selected salts with different counter-anions were measured with high precision in four aprotic ionic liquids (AILs), which enables a detailed examination of solvation effect of ILs on salts. Interestingly, the counter-anions of the ylide precursor salts, protic amine, and phenol salts of this study, though differing dramatically in size and electron dispersion, were found to have no effect on the respective pK_as of the substrates. This indicates that the ionic species generated upon acidic dissociation of the salts in weakly polar AILs of low dielectric constant (?: 10-15) are not ion-paired, or in other words, behave like free ions as if in strongly dissociating molecular solvents of high polarity (e.g., DMSO). This suggests that the widely assumed ionpairing phenomenon, an issue of much debate, is not important in the AILs under our experimental conditions, presenting a typical ionic-liquid effect on the solvation of charged species in AILs.

Full text of DOI:10.1021/jacs.6b02607

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Communication
Weakly Polar Aprotic Ionic Liquids Acting as Strong Dissociating
Solvent: A Typical “Ionic Liquid Effect” Revealed by Accurate
Measurement of Absolute pKa of Ylide Precursor Salts
Chong Mao, Zedong Wang, Zhen Wang, Pengju Ji, and Jin-pei Cheng
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b02607 • Publication Date (Web): 14 Apr 2016
Downloaded from http://pubs.acs.org on April 16, 2016
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Weakly Polar Aprotic Ionic Liquids Acting as Strong Dissoci-
ating Solvent: A Typical “Ionic Liquid Effect” Revealed by Ac-
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curate Measurement of Absolute pK of Ylide Precursor Salts
a
†
,+
‡,+
†
†,*
†,‡,*
Chong Mao, Zedong Wang, Zhen Wang, Pengju Ji and Jin-Pei Cheng
†
Center of Basic Molecular Science (CBMS), Department of Chemistry, Tsinghua University, Beijing, 100084, China.
‡
State Key Laboratory of Elemento-organic Chemistry, Collaborative Innovation Centre of Chemical Science and Engineer-
ing, Nankai University, Tianjin, 300071, China.
+
These authors contributed equally to this work.
Supporting Information Placeholder
two conceptually diverse media under comparison. It is
known that acid-base equilibria are closely associated with
the polarity of solvent. Thus, perturbation of solvent polar-
ity may cause significant shift to the equilibrium, which
can be directly “sensed” by measuring the equilibrium con-
a
stant (pK ). Therefore, acidity study has often been viewed
ABSTRACT: Absolute pK
ent counter-anions were measured with high precision in
aprotic ionic liquids (AILs), which enables a detailed
examination of solvation effect of ILs on salts. Interest-
ingly, the counter-anions of the ylide precursor salts,
protic amine and phenol salts of this study, though dif-
fering dramatically in size and electron dispersion, were
a
s of selected salts with differ-
3
4
as a very useful probe to reveal valuable information on the
4
microscopic properties of solvent systems. However, the
a
found to have no effect on the respective pK s of the
research in this aspect had focused almost entirely on the
conventional molecular solvent systems in the past, and
hence, chances to disclose the governing factors in ILs with
respect to their unique roles in solvation are actually much
depressed. Although we have recently carried out a system-
substrates. This indicates that the ionic species generated
upon acidic dissociation of the salts in weakly-polar
AILs of low dielectric constant (ε: 10~15) are not ion-
paired, or in other words, behave like “free ions” as if in
strongly dissociating molecular solvents of high polarity
a
atic study on the absolute pK s of several series of C-H
5
6
2c
(
e.g., DMSO). This suggests that the widely assumed
acids, O-H acids, and S-H acids in RTILs (room temper-
ature ILs) which indeed revealed important information
regarding the solvation of the cation and anion of ILs, the
previous works only covered neutral molecules, and the
knowledge on the solvation effect of ILs on ionic substrates
is yet much limited. Compared with the acidic dissociation
processes of neutral substrates, ionic species are more
complicated in that different degree of solvation of initial
ionic substrates may result in various solvent solvated enti-
ties, such as free ions and ion pairs, depending on the polar-
ion-pairing phenomenon, an issue of much debate, is not
important in the AILs under our experimental conditions,
presenting a typical “ionic-liquid effect” on the solvation
of charged species in AILs.
As a rising mainstream solvent, ionic liquids (ILs), the so-
called “designer’s solvent”, have sparked tremendous re-
1
3
search interests during the past decade. Compared with the
ity of the solvent. The present work intends to find if there
rapid development of synthetic applications of ILs, the fun-
damental research on chemical transformations in ILs are
severely lagged behind, however. This is in sharp contrast
to the good understandings in molecular solution chemistry.
As a consequence, some crucial information, such as de-
tailed solvent properties, solvation behaviors of ILs, is not
yet clear. Unlike molecular solvents, ILs are comprised
entirely of cations and anions, and thus, their solvation be-
haviors cannot be expected to be exactly the same as those
found in molecular media. Indeed, ILs may show unique
solvation effects under certain circumstances that are not
likely to occur in molecular solvent. Though this so-called
“ionic liquid effect” was observed in only a few cases till
is anything different or interesting regarding the solvation
in ILs.
Phosphonium ylides, derived from α-substituted tri-
7
phenylphosphonium cations by removal of the α-proton,
are important reactive intermediates for C=C bond for-
8
mation (Wittig reaction ). Due to their synthetic im-
portance, the pK
a
scales of ylide precursor salts have been
9
established in DMSO and in a number of molecular sol-
1
0
vent mixtures. Similarly, the α-proton of N-substituted
pyridinium cations can be removed by relatively strong
11
bases to form nitrogenium ylides. On the other hand, ILs
were also found to be attractive solvent systems for Wittig
reactions, owing to easier separation of products and effi-
2
present, it does provide a distinct chance to gain insight
into the fundamental differences of solvation between these
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Page 2 of 5
1
2
cient reuse of ILs. Notably, the compositions of the ylide
precursor salts are similar to these of ILs formed from ani-
ons and cations, and in this regard, the acidity study on
these “salts dissolved in liquid salts” may offer an ideal
opportunity to understand how and to what extent the cati-
like the acidities of carbon acids in DMSO which corre-
late well with these in the gas phase, reflect the intrin-
sic order of C-H bond ionization.
1
2
3
4
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7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
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2
2
2
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3
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3
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4
4
4
5
5
5
5
5
5
5
5
5
5
6
16
28
1
3
ons and anions of ILs interact with the ionic solutes. In
this work, we wish to firstly report the absolute pK values
of some frequently used ylide precursor salts in 4 common
AILs, [BMIM][NTf ], [BMPY][NTf ], [BM IM][NTf ] and
[BMIM][OTf] (Scheme 1), and secondly address the inter-
esting chemistry disclosed for IL solvation based on de-
2
2
1
1
4
0
6
2
8
Slope = 1.188
R² = 0.997
a
2
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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8
9
0
1
2
3
4
5
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7
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
tailed analysis of the pK
a
data.
Cation
4
6
8
1 0
1 2
1 4
pK in DMSO
a
N
N
N
N
R = n-butyl
R
R
N
R
[BM2IM]+ [BMPY]+
[
BMIM]+
Figure 1. Correlation between pK
um salts in [BMPY][NTf
s of triphenylphosphoni-
a
2
] with these in DMSO.
Anion
O O
O
O
O
O
S
S
S
Table 1 also shows that the pK
salt in various ILs follow an order of [BM
BMPY][NTf ] ≈ [BMIM][NTf ] > [BMIM][OTf],
values of an individual
a
N
O
F3C
CF3
F3C
2
IM][NTf ] >
2
[NTf2]
[OTf]
[
2
2
which is consistent with our previous observation for the
Scheme 1. Cations and anions of the ILs used in this study.
The absolute pK values of the ylide precursor salts in
5
neutral C-H acids in these ILs. Similar to previously
2c,5,6
a
observed,
the anion of ILs plays a dominant role on
Scheme 2 (1a-8) were measured by UV-vis spectroscopic
the acidity of the substrate, which can be rationalized by
comparing the size and electron density of oxygen and
nitrogen in [OTf] and [NTf
sequence, a stronger solvation towards proton by [OTf]
2
than [NTf ] should be responsible for the considerable
acidity difference of ~2.4 pK units, which corresponds a
differential solvation energy of 3.3 kcal mol towards
proton. On the other hand, the cation moiety of the ILs
has only limited influence on the pK values of the sub-
a
5-6
indicator overlapping method (SI). The conjugated bases
−
−
(
ylides) of P-substituted triphenylphosphonium and N-
substituted pyridinium salts were stable under the meas-
urement conditions. Table 1 summarizes the measured pK
2
] , respectively. As a con-
−
−
a
values of 1a-8 in ILs. It is worth mentioning that the AILs
used in this work were scrupulously purified with water
-1
content less than 10 ppm (SI), and these pK
measured in high precision with standard deviation (SD) of
±0.05 pK units, superior to these obtained from electro-
a
values were
≤
14
strates.
chemical approaches.
Interestingly, Table 1 also shows that, albeit bromide
and chloride anion are quite different in size and basicity
and they are much smaller than the anions of ILs, the
halogen anion has practically no effect on the acidities
of P-substituted triphenylphosphonium salts (compare
entries 1a/1b, 4a/4b, and 5a/5b) and N-substituted pyri-
dinium salts (8) in ILs. We also measured the acidities of
several triphenylphosphonium salts with nitrate (NO
as the counter-anion (such as 1 and 4, when X = NO
The measured acidity of the salt with nitrate is the same
as that with chloride or bromide ion as counter-anion.
This suggests that the counter-anions must not be in the
immediate vicinity of the phosphonium/pyridinium cati-
on to affect the positive charge of the latter in these ILs;
in other words, there is no specific ion pair formation
under our experimental conditions. This may be contra-
dictory to one’s intuitive thought. The far enough sepa-
ration of the solute cation and anion should most likely
be caused by strong Coulombic interactions between the
solute and IL ions in terms of strong ionic solvation
Ph P CH COPh X
Ph
3
P CH
2
CN Cl
Ph
3
P CH
2
COMe Cl
3
2
1
a, X = Cl; 1b, X = Br
2
3
Ph P CH CO Et X
Ph
3
P CH
2
NO
2
X
Ph
3
P CH
2
CN Br
3
2
2
4
a, X = Cl; 4b, X = Br
5a, X = Cl; 5b, X = Br
6
N
CH COPh Br
N
CH
2
COMe Cl
−
2
3
)
−
7
8
3
).
Scheme 2. P-substituted triphenylphosphonium and N-
substituted pyridinium salts used in this work, the acidic
hydrogens are indicated by boldface type.
As can be seen from Table 1, the acidifying effect of
4
triphenylphosphonium group is about 3.2×10 times (4.5
pK units) stronger than that of pyridinium group (1a and
7
, 3 and 8) in all four ILs, probably due to a (p-d) π in-
teraction between triphenylphosphonium group and the
1
5
neighboring carbanion. Although the salts are more
acidic in DMSO than in ILs (Table 1), the correlation
(
a
Eqn. 1). Logically, this also says that the pK in this
work represents “free-ion” acidity as usually seen in
strongly polar solvent like water and DMSO, rather than
between the pK
shows an excellent linearity with R of 0.997 (Figure 1).
This indicates that the absolute pK s measured in the ILs,
a
s of salts (1a-6) in DMSO and ILs
2
a
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“
ion-pair” acidity as in non- or weakly polar molecular
and para-ammonium phenol salts (12) in several neat
aprotic ILs were measured with high precision. Detailed
examinations of the data show that the acidities of these
organic salts are all independent of their counter anions.
This implies that the solvated cations and anions of the
substrate salts in the weakly polar aprotic ILs applied in
this work behave like “free ions” as if in strongly disso-
ciating polar molecular media of high dielectric constant
like DMSO, demonstrating a typical “ionic liquid effect”
on the solvation of charged species. The present obser-
vation is clearly not in line with the ion association as-
sumption in neat ILs proposed for solute salts in some
1
media, a crucial criterion for thermodynamic parameter
to be used as a standard quantity.
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Ka
Ph3P-CH2-G(solv.)
+
X (solv.)
Ph3P-CH-G(solv.) + H (solv.) + X (solv.)
G = CN, COPh, COMe, Ph-p-NO2; X = Cl, Br.
Eqn 1. Dissociation equilibrium for ylide precursor salts in
the ILs.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
X
X
X
N
H
N
N
H
H
H
21
previous literature.
9
10
11
X = Cl, NTf2 and OTf
ASSOCIATED CONTENT
R2
Supporting Information.
The Supporting Information is available free of charge on
X
N
R1
OH
R1
12
the ACS Publications website. Detailed pK
procedures, indicators used in this work and their corre-
sponding pK s in the ILs; UV-vis spectra for representative
a
measurement
R = n-C H , R = CH ; X = I, BAr , where ArF
F
1
8
17
2
3
4
stands for 3,5-bis(trifluoromethyl)phenyl
a
measurements; synthesis and characterization of salts.
Scheme 3. The structure of protic amine and phenol salts,
the acidic hydrogens are indicated by boldface type.
AUTHOR INFORMATION
Corresponding Author
To further test the validity of the above deduction, we
next synthesized and measured the acidities of several
protic amine salts (9-11) and para-substituted phenol
salts (12) (Scheme 3) in IL [BMIM][NTf ] (Table 2). As
2
can be seen very clearly, the acidity of these salts is also
*jipengju@mail.tsinghua.edu.cn (P. Ji)
*
jinpei_cheng@mail.tsinghua.edu.cn (J.-P. Cheng)
Notes
−
The authors declare no competing financial interest.
independent of the counter-anion X , regardless of the
dramatic change of counter-anions in their size (cf. Cl
−
ACKNOWLEDGMENT
−
vs. NTf
2
), charge delocalization and binding ability (cf.
−
−
−
We are grateful for the financial grants from National Natu-
ral Science Foundation of China (No. 21390401), the par-
tial of this work was also supported by Tsinghua University
Initiative Scientific Research Program (Nos. 20131080083,
4
I vs. BArF ). For instance, when the X anion of 12 is
changed from the relatively small and localized iodide to
a much bulky and extensively delocalized borate anion,
its acidity in [BMIM][NTf ] remains basically the same
2
(cf. Table 2, entry 4). This is in sharp contrast to the re-
cent finding that the acidity of the phenol salt 12 is much
more acidic with BArF
a non- (or weakly) polar molecular solvent CCl
ion-pairing is presumably dominant. In other words,
the present observation added a strong support to the
“
salt in our IL must be far apart to each other rather than
being ion-paired. It is worth noting that similar phenom-
enon (i.e., a “free ion” status) was also found in the pK
measurement in DMSO,
molecular solvent of high polarity (ε = 46.5 ). The
2
0141081295).
−
−
4
as counter-anion than with I in
, where
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2
015, 137, 4678.
(18) Bordwell, F. G.; Branca, J. C.; Hughes, D. L.; Olmstead, W. N.
J. Org. Chem. 1980, 45, 3305.
(
(
19) Casteel, J. F.; Sears, P. G. J. Chem. Eng. Data 1974, 19, 196.
20) (a) Wakai, C.; Oleinikova, A.; Ott, M.; Weingärtner, H. J. Phys.
Chem. B 2005, 109, 17028. (b) Huang, M.; Jiang, Y.; Sasisanker, P.;
Driver, G. W.; Weingärtner, H. J. Chem. Eng. Data 2011, 56, 1494.
(21) For the ion pair issues in neat ILs, see a recent review: Kirchner,
B.; Malberg, F.; Firaha, D. S.; Hollóczki, O. J. Phys.: Condens. Mat-
ter 2015, 27, 463002, and the references cited therein.
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1
2
3
Table 1. pK
a
Values of P-substituted triphenylphosphonium and N-substituted pyridinium salts (Scheme 2) in ILs and a
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
molecular solvent.
a
b
a
pK in ILs and DMSO
salts
[BMPY][NTf
2
]
[BMIM][NTf
13.3
2
]
[BM
2
IM][NTf
14.2
2
]
[BMIM][OTf]
11.2
DMSO
6.0
1
1
a
13.7
13.7
15.0
15.3
16.9
16.9
b
13.3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
14.8
15.4
15.8
17.5
12.5
5
6.9
7.1
8.5
3
15.0
12.7
14.2
4
a
16.9
4
b
16.9
5a
20.1
20.1
20.1
20.1
21.9
17.7
19.4
20.7
17.4
11.0
5
b
6
22.0
22.6
18.6
19.3
15.5
17.2
13.0
10.7
7
8
18.1
c
19.7 (19.6
5
)
20.2
5
11.8
+
a
b
c
In pK units; standard deviation (SD): ≤ ±0.05 pK unit; ref. 9 and 11; the value in parentheses is for PyN
−
2
CH COMe Br .
−
Table 2. pK
a
Values for the protic amine and phenol salts (Scheme 3) with various counter anion (X ) in
[
BMIM][NTf ](SD ≤ ±0.05).
2
amine salts
X = Cl
12.9
X = NTf
13.0
2
X = OTf
12.9
9
10
16.4
16.4
16.3
1
1
16.4
16.5
16.4
_____
_____
F
phenol salts
X = I
20.5
X = BAr
20.4
4
1
2
TOC:
5
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