B. Xie et al. / Journal of Molecular Liquids 268 (2018) 610–616
613
Table 1
Alkalinity, Mulliken atomic charges, and Knoevenagel reaction performance of six hydroxypyridine anion-based PILs.
PILs
Ve (mL)
pKb
Mulliken atomic charges of anion
Yields of Knoevenagel reactiona (%)
N
O
[DBUH][2 Op]
[DBUH][3 Op]
[DBUH][4 Op]
[TMGH][2 Op]
[TMGH][3 Op]
[TMGH][4 Op]
18.9
18.5
18.2
19.7
20.3
19.3
2.6
5.7
3.4
2.6
5.5
3.1
−0.485
−0.376
−0.389
−0.438
−0.373
−0.409
−0.636
−0.690
−0.680
−0.658
−0.665
−0.650
96
83
90
94
84
89
a
Reaction conditions: benzaaldehyde (20 mmol) and ethyl cyanoacetate (20 mmol) stirred in ethanol (25 mL) with 2 mmol hydroxypyridine anion-based PILs at room temperature for
20 min; Isolated yields of product.
3. Results and discussion
However, for [3 Op]− anion, the substituent of oxygen atom is located
in the meta-position of the pyridine ring, which will weaken the ability
of the nitrogen atom to share the negative charge of the oxygen atom
and lead to weaker alkalinity of PILs with [3 Op]− anion compared
with PILs with [2 Op]− or [4 Op]−. Therefore, we believe there must
be lie in a positive cooperative interaction between electronegative ni-
trogen and oxygen atoms of anions of hydroxypyridine anion-based
PILs [3], resulting in the significant increase of the alkalinity.
To confirm the above analysis, we calculated the Mulliken atomic
charge of the nitrogen and oxygen atoms in the anion of six studied
PILs using the Gaussian 03 program [30], which is listed in Table 1. As
can be seen in Table 1, the order of the Mulliken atomic charge of the ni-
trogen atom in the anion of the studied PILs is as follows:
3.1. Comparison of the alkalinity of hydroxypyridine anion-based PILs
According to the titration curves and their first derivative curves of
six hydroxypyridine anion-based PILs illustrated in Fig. 1, the pKb values
for the studied PILs calculated from Eq. (2) are obtained and listed in
Table 1. As can be seen in Table 1, the pKb values of six hydroxypyridine
anion-based PILs are b6, which is similar to that of the azole-based PILs
[4], whereas much smaller than that of traditional alkylimidazolium-
based aprotic ILs and 1,1,3,3 tetramethylguanidinium-based PILs [21],
indicating that the studied hydroxypyridine anion-based PILs has strong
alkalinity. For example, the pKb of 1,1,3,3 tetramethylguanidinium ace-
tate PIL, which is usually used as basic solvent for the synthesis of po-
rous metal-organic frameworks nanospheres [34, 35], is 9.61 and
larger than that of the studied hydroxypyridine anion-based PILs.
Clearly, the studied hydroxypyridine anion-based PILs exhibit much
stronger alkalinity compared with the traditional ILs. Therefore, these
hydroxypyridine anion-based PILs may be a suitable candidate for
basic solvent and catalyst.
−
½2 Opꢀ− N½4 Opꢀ− N½3 Opꢀ
which is much larger than that of the nitrogen atom in pyridine (pKb =
−0.161 [3]). Interestingly, the order of the Mulliken atomic charge of
the oxygen atom in the anion of the studied PILs is exactly opposite to
that of the nitrogen atom, which is as follows: [2 Op]− b [4 Op]−
b
As shown in Table 1, one can see that the order of pKb values of six
hydroxypyridine anion-based PILs is as follows:
[3 Op]−. For example, the Mulliken atomic charges of the nitrogen
atom in [DBUH][2 Op], [DBUH][4 Op], and [DBUH][3 Op] are −0.485,
−0.389, and − 0.376, respectively, while those of the oxygen atom is
−0.636, −0.680, and −0.690, respectively. Therefore, it is certain that
there is a positive cooperative interaction between electronegative ni-
trogen and oxygen atoms of anions of hydroxypyridine anion-based
PILs by formation the π-electron delocalization, which effectively im-
proves the alkalinity of PILs. Moreover, the position of the nitrogen
atom of anion has a significant influence on the cooperative interaction.
When the substituent of oxygen atom is located in the ortho-position or
para-position of the pyridine ring of anion, the cooperative interaction
between electronegative nitrogen and oxygen atoms is significant,
transferring the negative charge of oxygen atom to the nitrogen atom
by the conjugate effect to form the π-electron delocalization. Further-
more, in PILs with [2 Op]−, the electronegativity of the nitrogen atom
will also attract the negative charge of oxygen atom, leading to the co-
operative interaction and the alkalinity of PILs with [2 Op]− slightly
greater than those of PILs with [4 Op]−, respectively. When the substit-
uent of oxygen atom is located in the meta-position of the pyridine ring
of anion, the cooperative interaction between electronegative nitrogen
and oxygen atoms is relatively weak because of the lack of the conjugate
effect between the electronegative oxygen and nitrogen atoms.
½TMGHꢀ½2 Opꢀ ¼ ½DBUHꢀ½2 Opꢀb½TMGHꢀ½4 Opꢀb½DBUHꢀ½4 Opꢀb½TMGHꢀ
ꢁ ½3 Opꢀb½DBUHꢀ½3 Opꢀ
indicating that the structure of anion has a significant influence on the
alkalinity of the studied PILs, and the effect of the type of cation on the
alkalinity can be neglected, which is consistent with the results obtained
from the solvatochromic method for [Cxmim]-type ILs [36, 37] and the
results obtained from the potentiometric titration method for azole-
based PILs [4]. Why does the structure of anion affect the alkalinity of
the studied PILs? By comparing with the structure of three anions of
the studied PILs, it is clear that the position of nitrogen atom or the po-
sition of the substituent of oxygen atom on the pyridine ring (Scheme 1)
is different, which will not only affect the change distribution of the
anion, but also affect the alkalinity of the studied PILs. For [2 Op]− and
[4 Op]− anions, the substituent of oxygen atom is located in the ortho-
position and para-position of the pyridine ring, respectively. Conse-
quently, the nitrogen atom may share the negative charge of the oxygen
atom by formation the π-electron delocalization, thereby leading to the
increase the stability of anions and the enhanced the alkalinity of PILs.
CN
CHO
O
O
ILs
NC
+
H2O
O
O
Scheme 2. Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate catalyzed by hydroxypyridine anion-based PILs.