The article information
 Linyan Wang, Chaotun Cao, Chenzhong Cao
 王琳艳, 曹朝暾, 曹晨忠
 Substituent Effects on Reduction Potentials of Metasubstituted and Parasubstituted Benzylideneanilines
 间位取代与双对位取代氮苄叉苯胺还原电位中取代基效应
 Chinese Journal of Chemical Physics, 2016, 29(2): 260264
 化学物理学报, 2016, 29(2): 260264
 http://dx.doi.org/10.1063/16740068/29/cjcp1508173

Article history
 Dated: Received August 13, 2015
 Accepted on October 30, 2015
b. School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China;
c. Key Laboratory of Theoretical Organic Chemistry and Function Molecule, Ministry of Education, Hunan Provincial University Key Laboratory of QSAR/QSPR, Hunan University of Science and Technology, Xiangtan 411201, China
In the molecule of benzylideneanilines (abbreviated as XBAYs,XPhCH=NPhYs),CH=N is a bridge linking two aromatic rings,in which one ring carries substituent X and another ring carries substituent Y. Changes of X and Y in XBAY can affect its molecular overall electron distribution and its properties. Therefore,the substituent effects play an important role in the studies of this kind of compounds,and attain great interest in recent years [1, 2, 3]. In addition,substituent effects are also the focus of quantitative structure property/activity relationship (QSPR/QSAR) [4, 5, 6].
Based on the mechanism of electrochemical reduction,the reduction progress of title compounds and their derivatives firstly carried out on the carbon atom of CH=N,and the more positive charge the carbon atom has,the easier the XBAY is to be reduced and the more positive its potential value is [7, 8, 9, 10]. So,the electrochemical reduction potentials $E_{\textrm{(Red)}}$ values of XBAYs can be determined by the charge of the carbon atoms which is affected by the substituents of X and Y. In previous reports,the substituent effects on the reduction potentials of XBAYs or the analogous compounds were studied [11, 12, 13]. For example,Celik et al. ever analyzed the effects of parasubstituents on the halfwave potentials of several substituted benzaldehyde oximes and acetophenone oximes,and pointed out that their halfwave potentials could be correlated well with Hammett substituent constant $\sigma$ [12]. Their work promotes the studies of the substituent effects on the analogous compounds in the field of electrochemistry [12]. The previous studies mostly placed emphasis on the substituent effects by changing the kind of substituents. However,there are few quantitative structure property/activity relationship studies about the substituent effects by changing the position of the substituents.
Recently,Cao et al. analyzed the substituent effects on the $E_{\textrm{(Red)}}$ for 52 samples of 4,4$'$disubstituted XBAYs,and obtained a fourparameter equation [14],as shown in Eq.(1),in which $\sigma_\textrm{F}$(X) is the inductive effect of X,$\sigma_\textrm{R}$(X) and $\sigma_\textrm{R}$(Y) are the conjugative effectsof X and Y respectively,$\sigma _{\textrm{CC}}^{\textrm{ex}} \left( X \right)$ is the excitedstate substituent constant of X.
$ \begin{array}{l} {E_{({\rm{Red}})}} = {\rm{  2}}.{\rm{23}} + {\rm{0}}.{\rm{35}}{\sigma _{\rm{F}}}\left( {\rm{X}} \right) + {\rm{0}}.{\rm{52}}{\sigma _{\rm{R}}}({\rm{X}}) + \\ \;\;\;\;\;\;\;\;\;\;\;\;\;{\rm{0}}.{\rm{11}}{\sigma _{\rm{R}}}({\rm{Y}})  0.15\sigma _{{\rm{CC}}}^{{\rm{ex}}}\left( {\rm{X}} \right)\\ \;\;R = 0.9756,{R^2} = 0.9518,\\ \;\;S = 0.052,n = 52,F = 232.07 \end{array} $  (1) 
In Ref.[14],the $E_{\textrm{(Red)}}$ values of the 52 samples of 4,4$'$disubstituted XBAYs were not corrected by ferrocene. In this work,their $E_{\textrm{(Red)}}$ values were corrected by ferrocene [15],and a modified regression equation was obtained (shown as Eq.(2)).
$ \begin{array}{l} {E_{({\rm{Red}})}} = {\rm{  1}}.{\rm{96}} + {\rm{0}}.{\rm{35}}{\sigma _{\rm{F}}}\left( {\rm{X}} \right) + {\rm{0}}.{\rm{52}}{\sigma _{\rm{R}}}({\rm{X}}) + \\ \;\;\;\;\;\;\;\;\;\;\;\;{\rm{0}}.{\rm{11}}{\sigma _{\rm{R}}}({\rm{Y}}){\rm{  0}}.{\rm{15}}\sigma _{{\rm{CC}}}^{{\rm{ex}}}\left( {\rm{X}} \right)\\ \;\;\;R = 0.9756,{R^2} = 0.9518,\\ \;\;\;S = 0.052,n = 52,F = 232.20 \end{array} $  (2) 
Then we want to know if Eq.(2) will still be applicable in the case of X and/or Y changed from paraposition to metaposition. If not,what are the difference and the reasons? To answer above questions,49 samples of 3,4$'$/4,3$'$/3,3$'$substituted benzylideneanilines (as shown in Fig. 1) were synthesized and the substituent effects on their $E_{\textrm{(Red)}}$ values were analyzed,and meaningful results were obtained.
Ⅱ.EXPERIMENTS A.Preparation of XBAYsThe substituted benzylideneanilines were all synthesized with the solventfree method according to Fig. 1 [24, 25]. They were purified with anhydrous alcohol,and were confirmed with $^1$H NMR and $^{13}$C NMR. The NMR spectra were recorded with Bruker AV 500 MHz in CDCl$_3$ at room temperature at an approximate concentration. The detailed data of the synthesized compounds are available in the supplementary materials.
B.Measurment of redox potentialsThe electrochemical experiments were carried out by cyclic voltammetry (CV) and using a CS300 electrochemical apparatus in deaerated acetonitrile under nitrogen atmosphere at 298 K. $n$Bu$_4$NPF$_6$ (0.1 mol/L) in acetonitrile was employed as the supporting electrolyte. A standard threeelectrode cell consists of a glassy carbon disk as work electrode,a platinum wire as a counter electrode,and 0.01 mol/L AgNO$_3$/Ag (in 0.1 mol/L $n$Bu$_4$NPF$_6$/acetonitrile) as reference electrode. The scanning speed was 50 mV/s. Ferrocene was taken as an external reference. For example,Fig. 2 is the cyclic voltammetric curve of $m$FBANMe$_2$p.
Ⅲ.RESULTS AND DISCUSSIONFortynine samples of 3,4$'$/4,3$'$/3,3$'$substituted benzylideneanilines were synthesized and their $E_{\textrm{(Red)}}$ were measured by CV method. Their $E_{\textrm{(Red)}}$ values corrected by ferrocene were summarized in Table Ⅰ.
Firstly,the parameters in Eq.(2) were assumed to be applicable for correlating with the $E_{\textrm{(Red)}}$ of the 3,4$'$/4,3$'$/3,3$'$substituted XBAYs. So the correlation between the experimental $E_{\textrm{(Red)}}$ values of Table Ⅰ with the parameters $\sigma_\textrm{F}$(X),$\sigma_\textrm{R}$(X),$\sigma_\textrm{R}$(Y) and $\sigma _{\textrm{CC}}^{\textrm{ex}} \left( X \right)$ was carried out,and Eq.(3) was obtained.
$ \begin{array}{l} {E_{({\rm{Red}})}} = {\rm{  1}}.{\rm{89}} + {\rm{0}}.{\rm{47}}{\sigma _{\rm{F}}}\left( {\rm{X}} \right) + {\rm{0}}.{\rm{39}}{\sigma _{\rm{R}}}({\rm{X}}) + \\ \;\;\;\;\;\;\;\;\;\;\;{\rm{0}}.{\rm{24}}{\sigma _{\rm{R}}}({\rm{Y}})  0.05\sigma _{{\rm{CC}}}^{{\rm{ex}}}\left( {\rm{X}} \right)\\ \;\;\;\;R = 0.8869,{R^2} = 0.7866,\\ \;\;\;\;S = 0.091,F = 40.54,n = 49 \end{array} $  (3) 
As seen from Eq.(3),its correlation result is not good enough and worse than that of Eq.(2). The coefficients of the parameters in Eq.(2) were different from those of corresponding parameters in Eq.(3),as well as their intercepts. It implies that the factors affecting the $E_{\textrm{(Red)}}$ of 3,4$'$/4,3$'$/3,3$'$substituted XBAYs may be different from those of parasubstituted XBAYs. Therefore,we used the parameters listed in Table Ⅰ to make regression analysis against the $E_{\textrm{(Red)}}$ values of the 49 samples once again,and obtained the optimality equation (Eq.(4)).
$ \begin{array}{l} {E_{({\rm{Red}})}} = {\rm{  1}}.{\rm{96}} + {\rm{0}}.{\rm{46}}\sigma \left( {\rm{X}} \right) + {\rm{0}}.{\rm{29}}\sigma ({\rm{Y}})\\ \;\;\;\;\;\;\;\;\;\;\;{\rm{  }}0.05\sigma _{{\rm{CC}}}^{{\rm{ex}}}\left( {\rm{X}} \right) + 0.03\Delta \sigma _{{\rm{CC}}}^{{\rm{e}}{{\rm{x}}^2}}\\ R = 0.9461,{R^2} = 0.8951,S = 0.064,F = 93.83,n = 49 \end{array} $  (4) 
In Eq.(4),$\sigma$(X)=$\sigma_\textrm{F}$(X)+$\sigma_\textrm{R}$(X),$\sigma$(Y)=$\sigma_\textrm{F}$(Y)+$\sigma_\textrm{R}$(Y),and $\Delta \sigma _{{\rm{CC}}}^{{\rm{e}}{{\rm{x}}^2}}$=[$\sigma _{{\rm{CC}}}^{{\rm{ex}}}\left( {\rm{X}} \right)  \sigma _{{\rm{CC}}}^{{\rm{ex}}}(Y)$]$^2$. Obviously,the correlation of Eq.(4) is much better than that of Eq.(3),and its standard deviation $S$ is down to 0.064 from 0.091 of Eq.(3). And the number of variables employed in Eq.(4) is equal to that in Eq.(3). Furthermore,the intercept of Eq.(4) is equal to that of Eq.(2). It can be explained as follows: in case X and Y groups all are H atom,both of 4,4$'$substituted XBAYs and 3,4$'$/4,3$'$/3,3$'$substituted XBAYs all returned to the parent molecule,benzylideneaniline (HBAH). Thus,the intercepts of Eq.(4) and Eq.(2) should be equal to each other,which express the reduction potential of HBAH.In addition,the plot of the $E_{\textrm{(Red)calcd.}}$ values calculated by Eq.(4) vs. the experimental $E_{\textrm{(Red)expt.}}$ values for 3,4$'$/4,3$'$/3,3$'$substituted XBAYs of Table Ⅰ was made and shown in Fig. 3.
Comparing Eq.(4) and Eq.(2),it can be seen that the factors affecting the $E_{\textrm{(Red)}}$ of 3,4$'$/4,3$'$/3,3$'$substituted XBAYs are different from those of 4,4$'$substituted XBAYs. Here the relative importance of parameters in each equation is investigated from the relative contributions ($\psi_\textrm{r}$) or fraction contributions ($\psi_\textrm{f}$) of the corresponding parameters to $E_{\textrm{(Red)}}$ [20, 21].
$ {\psi _{\rm{r}}}\left( i \right) = {m_i} = {\bar X_i} $  (5) 
$ {\psi _f}\left( i \right) = \frac{{{R^2}\left {{\psi _r}\left( i \right)} \right}}{{\sum\limits_i {\left {{\psi _r}\left( i \right)} \right} }} \times 100% $  (6) 
The $m_i$ and ${\bar X_i}$ are the coefficient and the average value of the $i$th parameter in each equation,and the $R$ is the correlation coefficient of each equation. The sum is over the parameters in the equations. The contribution results for the corresponding parameters of the equations are all shown in Table Ⅱ.
Table Ⅱ shows that for the 4,4$'$substituted XBAYs,the main contribution to the $E_{\textrm{(Red)}}$ is the X group,in which the total contribution of $\sigma_\textrm{F}$(X),$\sigma_\textrm{R}$(X) and $ \sigma _{\textrm{CC}}^{\textrm{ex}} ( {\rm{X}}) $ is 86.63%. The contribution of Y group is only 8.57% and the contribution of $\sigma_\textrm{F}$(Y) can be ignored. For the 3,4$'$/4,3$'$/3,3$'$substituted XBAYs,the contribution of X group to the $E_{\textrm{(Red)}}$ is down to 62.16%,the contribution of Y group to the $E_{\textrm{(Red)}}$ rises to 14.11%,and the contribution of $\Delta \sigma _{{\rm{CC}}}^{{\rm{e}}{{\rm{x}}^2}}$ is also important. In the 4,4$'$substituted XBAYs,the contribution of $\sigma_\textrm{R}$ of X or Y is larger than that of $\sigma_\textrm{F}$,while the contribution of $\sigma_\textrm{R}$ of X or Y is equal to that of its $\sigma_\textrm{F}$ in 3,4$'$/4,3$'$/3,3$'$substituted XBAYs,so the $\sigma$(X) and $\sigma$(Y) were employed in Eq.(4).
It should be paid attention that the effects of X group on the $E_{\textrm{(Red)}}$ are larger than that of Y group in both 4,4$'$substituted XBAYs and 3,4$'$/4,3$'$/3,3$'$substituted XBAYs. The reasons may be as follows: the distance between X and carbon atom of CH=N is closer than that between Y and the carbon atom,and the aniline ring with Y group is twisted out of the CC=NC plane by 41$^{\circ}$55$^{\circ}$ exhibited by crystal structures of XBAY molecules [22, 23],which may hinder the transmission of the conjugative effect some what from Y to the CH=N.
Ⅳ.CONCLUSIONAn optimality equation (Eq.(4)) with four parameters was obtained for the electrochemical reduction potentials $E_{\textrm{(Red)}}$ of 49 samples of 3,4$'$/4,3$'$/3,3$'$substituted XBAYs. The results indicated that the factors affecting the $E_{\textrm{(Red)}}$ of 3,4$'$/4,3$'$/3,3$'$substituted XBAYs are different from those of 4,4$'$substituted XBAYs. As regards the $E_{\textrm{(Red)}}$ of 3,4$'$/4,3$'$/3,3$'$substituted XBAYs,the contributions of $\sigma_\textrm{F}$ and $\sigma_\textrm{R}$ of X (or Y) are equal to each other,so the parameters $\sigma$(X) and $\sigma$(Y) rather than $\sigma_\textrm{F}$ and $\sigma_\textrm{R}$ were employed,and the contribution of $\Delta \sigma _{{\rm{CC}}}^{{\rm{e}}{{\rm{x}}^2}}$ is also important and not negligible. Compared with 4,4$'$substituted XBAYs,X group contributes less to the $E_{\textrm{(Red)}}$ of 3,4$'$/4,3$'$/3,3$'$substituted XBAYs,while Y group contributes more. Finally,X group contributes more than Y group to the $E_{\textrm{(Red)}}$ of substituted XBAYs wherever they are in paraposition or metaposition.
Ⅴ.ACKNOWLEDGMENTSThis work was supported by the National Natural Science Foundation of China (No.21272063),the Scientific Research Fund of Hunan Provincial Education Department (No.14C0466),and the Natural Science Foundation of Hunan (No.14JJ3112).
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b. 湖南科技大学化学化工学院, 湘潭 411201;
c. 湖南科技大学, 理论有机化学与功能分子教育部重点实验室, 分子构效关系湖南省普通高校重点实验室, 湘潭 411201