Additional effect of para-hydroxyl on the reduction potentials of the N-benzylidenebenzenamines

Article abstract of DOI:10.1002/poc.4028

The reduction potential E_Red of disubstituted N-benzylidenebenzenamines XArCH NArY (abbreviated as XBAY) is a measure of the electron acceptance ability of the Lowest-energy Unoccupied Molecular Orbital (LUMO) in molecule, which is closely related to the electronic effects of substituents X and Y in molecule. Previous studies on the E_Red of XBAY reported by Luo, Wang, and Yuan all did not involve these molecules containing hydroxyl. In this work, 28 samples of disubstituted N-benzylidenebenzenamines containing para-hydroxyl, (abbreviated as 4-OHBAY or XBAOH-4′) were synthesized, and their E_Red values were measured. Then, 127 compounds were taken as a complicated E_Red data set, which involved 28 compounds (4-OHBAY and XBAOH-4′) of this work and 99 compounds XBAY reported by Luo. Based on these data E_Red set, through a quantitative regression analysis method and comparison of the factors affecting the E_Red, the following results are obtained: (a) Because the phenolic hydroxyl OH can dissociate H⁺ to form phenolic oxygen anion, the change regularity of E_Red of compounds 4-OHBAY and XBAOH-4′ is somewhat different from that of E_Red of XBAY compounds without hydroxyl OH. That is, hydroxyl OH has an additional effect on the E_Red, which decreases the E_Red value. (b) The additional effects of 4-OH on the E_Red is different from that of 4′-OH. The effect of hydroxyl OH attached to the aldehyde aromatic ring on the E_Red is more than that of OH attached to the amine aromatic ring.

Full text of DOI:10.1002/poc.4028

Received: 29 June 2019
DOI: 10.1002/poc.4028
Revised: 25 September 2019
Accepted: 29 September 2019
R E S E A R C H A R T I C L E
Additional effect of para-hydroxyl on the reduction
potentials of the N-benzylidenebenzenamines
Chao-Tun Cao | Wei Zhou | Chenzhong Cao
Key Laboratory of Theoretical Organic
Abstract
Chemistry and Function Molecule,
Ministry of Education, Hunan Provincial
University Key Laboratory of
The reduction potential ERed of disubstituted N-benzylidenebenzenamines
XArCH NArY (abbreviated as XBAY) is a measure of the electron acceptance
ability of the Lowest-energy Unoccupied Molecular Orbital (LUMO) in mole-
cule, which is closely related to the electronic effects of substituents X and Y in
molecule. Previous studies on the ERed of XBAY reported by Luo, Wang, and
Yuan all did not involve these molecules containing hydroxyl. In this work,
28 samples of disubstituted N-benzylidenebenzenamines containing para-
hydroxyl, (abbreviated as 4-OHBAY or XBAOH-40) were synthesized, and their
ERed values were measured. Then, 127 compounds were taken as a compli-
cated ERed data set, which involved 28 compounds (4-OHBAY and XBAOH-40)
of this work and 99 compounds XBAY reported by Luo. Based on these data
ERed set, through a quantitative regression analysis method and comparison of
the factors affecting the ERed, the following results are obtained: (a) Because
the phenolic hydroxyl OH can dissociate H+ to form phenolic oxygen anion,
the change regularity of ERed of compounds 4-OHBAY and XBAOH-40 is some-
what different from that of ERed of XBAY compounds without hydroxyl
OH. That is, hydroxyl OH has an additional effect on the ERed, which decreases
the ERed value. (b) The additional effects of 4-OH on the ERed is different
from that of 40-OH. The effect of hydroxyl OH attached to the aldehyde aro-
matic ring on the ERed is more than that of OH attached to the amine
aromatic ring.
QSAR/QSPR, School of Chemistry and
Chemical Engineering, Hunan University
of Science and Technology, Xiangtan,
China
Correspondence
Chenzhong Cao, Key Laboratory of
Theoretical Organic Chemistry and
Function Molecule, Ministry of
Education, Hunan Provincial University
Key Laboratory of QSAR/QSPR, School of
Chemistry and Chemical Engineering,
Hunan University of Science and
Technology, Xiangtan 411201, China.
Email: czcao@hnust.edu.cn
Funding information
National Nature Science Foundation of
China, Grant/Award Number: 21672058
K E Y W O R D S
additional effect, para-hydroxyl N-benzylidenebenzenamine, reduction potential, substituent
electronic effect
1 | INTRODUCTION
their molecular C N double bond to CH–NH single
bond.[4–8] For examples, Jia et al[4] employed sodium
Schiff bases are a kind of important organic compounds
containing C N double bond. They are not only widely
used as ligand in coordination chemistry[1–3] but also are
able to be converted into various amines by reducing
tetrahydroborate (NaBH4) to reduce the imine C N dou-
ble bond of ruthenium (II) p-cymene complexes con-
taining Schiff-base ligands [Ru(p-cymene)LCl] and
prepared half-sandwich ruthenium complexes, which
J Phys Org Chem. 2019;e4028.
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https://doi.org/10.1002/poc.4028

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CAO ET AL.
have active catalysts. Tan et al[5–10] used NaBH4 to reduce
the imine C N double bond in the synthesis of novel
3-(1,3,4-thiadiazolyl)-1,3-bezoxazines, which have fungi-
cidal activity. Liu et al[11] synthesized 4-[(5-aryl-1,-
3,4-thiadiazol-2-ylamino)methyl]phenol compounds via
the nucleophilic addition reaction of 5-aryl-1,-
3,4-thiadiazol-2-amine with 4-hydroxybenzaldehyde and
the reduction of C N double bond in the presence of
NaBH4 reducing agent.
Previous studies have shown that the reducibility of
C N bond of Schiff bases is closely related to the sub-
stituents, which attach to the ring of aromatic alde-
hyde or aromatic amine. Not only the reducibility of
Schiff bases but also the reducibility of other aromatic
compounds are greatly affected by the substituents in
their molecules. As regards the effect of substituents
on the reducibility of organic compounds, it has been
studied in-depth by many chemical researchers.
Celik[12] and Zhu[13] have studied the reduction of
molecular ground state. Therefore, it is better to employ
both the substituent ground state constant (Hammett
constant σ) and the excited state substituent constant
[20]
ex
σ
P
to correlate the ERed, which can give good
CC
ex
results. The method of employing the σ and the σ has
CC
been applied well to quantify the ERed of disubstituted N-
phenyl-α-phenylnitrones XArCH N(O)ArY (abbreviated
as XPNY), disubstituted N-(phenyl-ethylene)-anilines
XArC (Me) NArY (abbreviated as XPEAY),[21] and N-
benzylidenebenzenamines containing meta-substituents
(abbreviated as p/m-XBAY-p/m).[22] Yuan et al[23] further
extended this method to quantify the ERed of other kinds
of compounds, such as substituted naphthalene, and also
obtained good results. The above mentioned results indi-
cate that it is a new and effective method by employing
the substituent ground state constant (Hammett constant
[20]
ex
CC
σ) and the excited state substituent constant σ
quantify the ERed of organic compounds.
to
It should be noted that aromatic compounds con-
taining hydroxyl groups are also a sort of important
organic compounds and are widely used. For examples,
they were used in plasmon-assisted reactions on a metal
surface,[24] exploring and elaborating the novel excited
state dynamical behavior.[25] We analyzed carefully the
ERed values of more than 160 compounds of three series
of compounds (involving XBAY, XPNY, and XPEAY)
reported by Luo[21] and noticed that none of these
compounds contains para-hydroxyl OH. We want to
know if the equation for calculating the ERed of XBAY
reported in Luo[21] can be applied to the compounds
4-OHArCH NArY (abbreviated as 4-OHBAY) and
XArCH NArOH-40 (abbreviated as XBAOH-40) con-
taining hydroxyl OH or not. Thus, we synthesized two
compounds, 4-OHArCH NC6H5 and C6H5CH NArOH-
40, and measured their ERed, which is −2.55 and − 2.38 V,
respectively. Then, the ERed values of the two compounds
were predicted by using Equation (1) reported by Luo.[21]
The predicted values are −2.17 and − 2.07 V, respectively.
It can be seen that the experimental ERed values of them
decrease more than 0.3 V compared with the predicted
ERed value, and the error of the experimental ERed versus
the predicted ERed is much higher than the experimental
error. This experimental phenomenon reminds us that
the ERed of bi-aryl Schiff bases containing para-hydroxyl
OH may have additional behavior. To explore this topic,
we designed and synthesized a series of bi-aryl Schiff
bases containing para-hydroxyl OH, namely, 4-OHBAY
and XBAOH-40, and determined their ERed values. We,
employing the compounds 4-OHBAY and XBAOH-40, try
to investigate whether the ERed of para-OH bi-aryl Schiff
bases has additional behavior or not and expect to pro-
vide a theoretical reference for further understanding the
change regularity of the ERed of bi-aryl Schiff bases.
these
GCH NPhY structure, and Sauro[14] has studied the
reduction potential (ERed of aryl-substituted
compounds
containing
XPhCH NG
or
)
acetophenone azines (XPhC (Me) N–N C (Me)PhY.
Celik, Zhu, and Sauro all have observed that the ERed
values of C N bonds in the investigated compounds
are linearly correlated to the Hammett parameter σ of
substituents X and Y. Heyes[15] studied the reductions
of the pyrylium and thiopyrylium cations. Yamataka[16]
studied the reductions of para-substituted benzophe-
nones. The results reported by Heyes and Yamataka
also show that the ERed values of the investigated com-
pounds are greatly affected by the Hammett parameter
σ of the substituents in molecules. Dosa et al[17] stud-
ied the relationship between the reduction rate and
the ERed of substituted azobenzene. Liu et al[18] studied
the relationship between the reduction activity and the
ERed of para-substituted bromobenzene. The works of
Dosa and Liu show that there is a linear relationship
between the reduction activity and the ERed. It can be
seen from above mentioned study results that the sub-
stituent electronic effect, the ERed, and the reduction
activity of organic compounds are closely related.
Therefore, exploring the relationship between the ERed
of organic compounds and the substituent electronic
effect is of great significance for understanding the
reduction activity of organic compounds.
Recently, our team[19] studied the effect of
substituents on ERed of para-disubstituted N-
benzylidenebenzenamines XArCH NArY (abbreviated
as XBAY) and pointed out that when XBAY molecule
receives an electron to be reduced to radical anion
(XBAY−), the electron distribution in the molecule is sim-
ilar to that of molecular excited state rather than

CAO ET AL.
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2 | RESULTS AND DISCUSSION
this work (Table 1). Then, based on these data ERed set, a
regression analysis was carried out, and Equation (2) was
generated by employing the same parameters used in
Equation (1).
2.1 | Determination results of reduction
potential
In order to investigate systematically the effect of para-
hydroxyl OH on the ERed of bi-aryl Schiff bases, 28 sam-
ples of model compounds, 4-OHBAY and XBAOH-40
(Figure 1), were synthesized according to the method
reported by Cao et al,[26–28] and the ERed values of the
obtained model compounds were determined. The ERed
data were listed in Table 1.
ERed = −2:17 + 0:53σðXÞ + 0:14σðYÞ−0:18σeCxCðXÞ−0:085Im
ð2Þ
R = 0:8537,R2 = 0:7285,S = 0:14,n = 127,F = 81:96:
The result of Equation (2) shows that the correlation of
Equation (2) is worse than that of Equation (1) when the
ERed values involve these of XBAY, 4-OHBAY, and
XBAOH-40. The standard deviation S is up to 0.14 V,
which is much higher than the experimental error. It
means that the ERed values of 4-OHBAY and XBAOH-40
may be affected by other factors.
2.2 | Quantitative correlation of ERed for
the model compounds
In our previous work,[19,21,22] we reported the relation-
ship between ERed and the electronic effects of substitu-
ents X and Y for the compounds XBAY, who are without
hydroxyl OH, and proposed Equation (1) to express the
[21]
change regularity of their ERed
.
2.3 | Discussion
ERed = −2:02 + 0:46σðXÞ + 0:13σðYÞ−0:11σeCxCðXÞ−0:20Im
As regards the molecules 4-OHBAY and XBAOH-40, they
may occur ionization equilibrium in solution because of
the presence of phenolic hydroxyl OH, as shown in
Figure 2.
ð1Þ
R = 0:9573,S = 0:06,n = 99,F = 257:70,
After dissociation of H+ from molecule 4-OHBAY or
XBAOH-40, phenol-oxygen anion is formed, which can
prevent the molecule from accepting electrons from the
electrode and reduces the ERed. In order to measure the
effect of Figure 2A,B on the ERed of 4-OHBAY and
XBAOH-40, we used the equalized electronegativity me-
thod proposed by Bratsch[35] (Pauling electronegativity)
to calculate the partial charge QO of O atoms on the
OH group in 4-OHBAY and XBAOH-40, respectively.
The O atom charges of 4-OH and 40-OH are represented
where the σ is Hammett electronic effect constant.
ex
The σ
is excited-state substituent constant. The X
CC
and Y in brackets indicate the substituents in XBAY
molecules. The Im is the indicator variable of group 3/30-
X/Y in the molecule XBAY, if there is a group at 3− or 30
−
position in the molecule XBAY, which are
without hydroxyl OH, Im = 1, otherwise, Im = 0. The R,
S, n, and F are the correlation coefficient, standard devia-
tion, data points used in equation, and Fisher ratio,
respectively.
0
by the symbols QO4 and QO4 , respectively, whose values
0
can be seen in Table 1. Then, the QO4 and QO4 were
Firstly, we took 127 compounds as a complicated ERed
data set, which involved 99 samples of XBAY reported by
Luo[21] and 28 samples of 4-OHBAY and XBAOH-40 in
employed as parameters to express the additional contri-
butions of 4-OH and 40-OH to the ERed, and the contri-
ex
butions of σ and σ
of 4-OH and 40-OH were all
CC
incorporated into the contributions of QO. Next, the QO4
0
and QO4 were added into Equation (2), and the 127 ERed
values were regressed once again. Then Equation (3) was
obtained.
ERed = −2:02 + 0:46σðXÞ + 0:13σðYÞ−0:11σCexCðXÞ
ð3Þ
0
−0:20Im + 1:79 QO4 + 1:26 QO4
FIGURE 1 The synthetic route of 4-OHBAY and XBAOH-40
molecules
R = 0:9750,R2 = 0:9419,S = 0:06,n = 127,F = 324:09:

4 of 7
CAO ET AL.
TABLE 1 The values of ERed/V of the molecules 4-OHBAY and XBAOH-40 and the σF, σR, and σCexC of substituents X and Y
c
d
σF(X)a
0.33
0.33
0.33
0.33
0.33
0.33
0.33
0.33
0.33
0.33
0.33
0.33
0.33
0.33
0.29
0.01
0.00
0.45
0.42
0.45
0.51
0.65
0.12
–0.07
0.34
0.37
0.39
0.56
σF(Y)a
0.29
0.01
0.00
0.45
0.42
0.45
0.38
0.12
–0.07
0.34
0.37
0.39
0.43
0.56
0.33
0.33
0.33
0.33
0.33
0.33
0.33
0.33
0.33
0.33
0.33
0.33
0.33
0.33
σR(X)a
–0.70
–0.70
–0.70
–0.70
–0.70
–0.70
–0.70
–0.70
–0.70
–0.70
–0.70
–0.70
–0.70
–0.70
–0.56
–0.18
0.00
σR(Y)a
–0.56
–0.18
0.00
σ
(X)b
σ
(Y)b
QO
ERed,exp.
ex
ex
Compd.
1
X
Y
CC
CC
4-OH
4-OH
4-OH
4-OH
4-OH
4-OH
4-OH
4-OH
4-OH
4-OH
4-OH
4-OH
4-OH
4-OH
4-OMe
4-Me
H
40-OMe
40-Me
H
–0.19
–0.19
–0.19
–0.19
–0.19
–0.19
–0.19
–0.19
–0.19
–0.19
–0.19
–0.19
–0.19
–0.19
–0.50
–0.17
0.00
–0.50
–0.17
0.00
–0.29
–0.30
–0.29
–0.28
–0.29
–0.29
–0.26
–0.29
–0.30
–0.28
–0.29
–0.29
–0.26
–0.29
–0.29
–0.30
–0.29
–0.28
–0.28
–0.29
–0.28
–0.27
–0.29
–0.30
–0.28
–0.28
–0.29
–0.28
–2.68
–2.64
–2.55
–2.62
–2.51
–2.50
–2.58
–2.48
–2.50
–2.41
–2.40
–2.42
–2.47
–2.40
–2.50
–2.44
–2.38
–2.41
–2.17
–2.35
–1.91
–1.87
–2.35
–2.40
–2.22
–2.23
–2.27
–2.21
2
3
4
40-F
–0.39
–0.19
–0.22
0.16
0.06
5
40-Cl
–0.22
–0.33
–0.12
0.10
6
40-Br
7
40-CF3
30-OMe
30-Me
30-F
8
0.00
9
0.00
–0.03
0.02
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
0.00
30-Cl
0.00
0.02
30-Br
0.00
–0.03
0.09
30-CF3
30-CN
40-OH
40-OH
40-OH
40-OH
40-OH
40-OH
40-OH
40-OH
40-OH
40-OH
40-OH
40-OH
40-OH
40-OH
0.00
0.00
0.56
–0.70
–0.70
–0.70
–0.70
–0.70
–0.70
–0.70
–0.70
–0.70
–0.70
–0.70
–0.70
–0.70
–0.70
–0.19
–0.19
–0.19
–0.19
–0.19
–0.19
–0.19
–0.19
–0.19
–0.19
–0.19
–0.19
–0.19
–0.19
4-F
–0.39
–0.19
–0.22
0.15
0.06
4-Cl
–0.22
–0.33
–0.70
–1.17
0.10
4-Br
4-CN
4-NO2
3-OMe
3-Me
3-F
0.13
0.00
0.00
–0.03
0.02
0.00
3-Cl
0.00
0.02
3-Br
0.00
–0.03
0.56
3-CN
0.00
aValues were from Hansch et al.[29]
bValues were from author's previous studies.[20,30–34]
cFor 4-OH, QO indicates QO4; For 40-OH, QO indicates QO4
dValues determined by this work.
0
.
Compared with Equation (2), the correlation coefficient
R of Equation (3) is significantly increased, and the
standard deviation S is reduced from 0.14 to 0.06 V,
which is less than half of the S in Equation (2) and is
within the experimental error. The intercept of
Equation (3), the coefficients in front of the variables
additional effect of 4-OH on the ERed is different from
that of 40-OH, and the former is greater than the latter. It
should be pointed out that because the charge QO of O
ex
σ(X), σ(Y), σ ðXÞ, and Im all are exactly the same as
CC
those in Equation (1). Equation (3) indicates that
(a) 4-OH and 40-OH groups have indeed additional effects
on the ERed, and the more negative the charge of O atom
is, the more the ERed will be reduced. (b) The effects of
4-OH and 40-OH on the ERed are different from those of
other groups X and Y of XBAY molecules; (c) the
FIGURE 2 The ionization equilibrium for 4-OHBAY A, and
XBAOH-40 B

CAO ET AL.
5 of 7
atom is usually negative, the additional effect of OH
makes the ERed decrease. For example, in molecule,
4-OHArCH NC6H5, the QO4 of OH is −0.29, the addi-
tional effect of 4-OH is 1.79 × (−0.29) = −0.52 V; in mole-
cule C6H5CH NArOH-40, the additional effect of 40-OH
is 1.26 × (−0.29) = −0.37 V.
range is also wide, which is in going from −1.43 (com-
pound 4-NO2BACN-40) to −2.68
V
(compound
4-OHBAOMe-40). The obtained Equation (3) has good
correlation; its standard error is only 0.06 V. The average
absolute error
of
the calculated
value by
Equation (3) versus the experimental value is only
0.05 V, which is within the experimental error. Figure 4
is the plot of the calculated value ERed,cal. Versus the
experimental value ERed,exp. From the calculation results
and Figure 4, it can be seen that Equation (3) expresses
well the additional effect of 4-OH and 40-OH on the
change regularity of ERed of bi-aryl Schiff bases.
Also the results of this paper are consistent with the
experimental facts reported. For example, the reduction
of compounds containing 4-OH-ArCH N structure[11]
and other X-ArCH N structure[5–10] by NaBH4, the for-
mer takes longer than the latter under the same reduc-
tion conditions, that is, the former is more difficult to be
reduced.
The additional effect of para-hydroxyl OH on the ERed
can be attributed to the ionization shown in Figure 2. In
order to illustrate the effect of ionization on the ERed, we
briefly discuss it as follows. Taking substituents NMe2,
OH, and OMe for examples, their Hammett constant σ
values are −0.83, −0.37, and − 0.27,[29] respectively, while
the ERed values of 4-NMe2BAH, 4-OHBAH, and
4-OMeBAH are −2.11, −2.55, and − 1.98 V, respectively.
The ERed values of HBANMe2-40, HBAOH-40, and
HBAOMe-40 are −2.01, −2.38, and − 2.06 V, respectively.
Among the three substituents mentioned above, the σ
value of OH is in the middle position, while the ERed
value of the compounds containing OH is the smallest in
above two series of compounds, respectively. This phe-
nomenon cannot be reasonably explained by the contri-
bution of σ value of substituents but can only be
explained by the additional effect of OH, that is, the ERed
decrease originates from the ionization of OH. In order to
compare intuitively the difference of effect of 4-OH ver-
sus 40-OH on the ERed, we plot the ERed with 4-OHBAX
and XBAOH-40 isomers containing the same group X and
got Figure 3. It can be seen from Figure 3 that 4-OH
reduces ERed more than 40-OH does.
There are 127 compounds employed in Equation (3).
The substituents in these compounds are distributed in
the para- and meta-positions of aldehyde and amine aro-
matic rings, and the substituents involve a wide elec-
tronic effect range, which is in going from strong
electron-donating group[19] NMe2 (σ = −0.83) to strong
electron-withdrawing group NO2 (σ = 0.78). The ERed
FIGURE 4 Plot of the calculated ERed,cal with
Equation (3) versus the experimental ERed,exp for the 127 model
compounds
FIGURE 3 Comparison of the ERed for the
isomers 4-OHBAX and XBAOH-40 containing
the same group X

6 of 7
CAO ET AL.
3 | CONCLUSION
reference electrode and a platinum wire as counter
electrode. Then the ERed values were obtained by using
cyclic voltammetry (CV), scanning with CS300 electro-
chemical instrument (Ferrocene as an internal
standard).
By systematically and quantitatively analyzing the factors
affecting the ERed of 4-OHBAY, XBAOH-40, and XBAY
without hydroxyl OH, we can obtain the conclusions:
(a) Because phenolic hydroxyl OH can dissociate H+ to
form phenolic oxygen anion, the change regularity of
ERed of compounds 4-OHBAY and XBAOH-40 is some-
what different from that of ERed of compounds XBAY
without hydroxyl OH. That is to say, OH group has a
additional effect on the ERed, which decreases ERed value.
(b) The additional effect of 4-OH on the ERed is different
from that of 40-OH. The effect of OH group attached to
the aldehyde aromatic ring on the ERed is more than that
of OH attached to the amine aromatic ring.
ACKNOWLEDGEMENT
Authors are very grateful to the National Nature Science
Foundation of China (21672058).
ORCID
Chenzhong Cao https://orcid.org/0000-0001-5224-7716
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1
terized by H NMR and 13C NMR (AV500 MHZ) after
vacuum drying. The detailed data and NMR spectra of
model compounds in Table 1 were reported in the
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Additional supporting information may be found online
in the Supporting Information section at the end of this
article.
How to cite this article: Cao C-T, Zhou W,
Cao C. Additional effect of para-hydroxyl on the
reduction potentials of the N-
benzylidenebenzenamines. J Phys Org Chem. 2019;
e4028. https://doi.org/10.1002/poc.4028
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