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AZERBAIJAN CHEMICAL JOURNAL № 4 2022 ISSN 2522-1841 (Onlme)
ISSN 0005-2531 (Print)
UDC 541.13+54-386
ELECTROCEMICALSTUDY OF SYM. POLYMETHYLFERROCENE/SYM. POLYMETHYLFERRICINIUM (MenFc/MenFc+) (n = 6, 8, 10) REDOX SYSTEMS IN ORGANIC SOLUTIONS
N.Z.Ibrahimova, A.Sh.Aliyev, G.M.Jafarov, I.U.Lyatifov, D.B.Tagiyev
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
Received 10.05.2022 Accepted 29.06.2022
Redox systems MenFc/MenFc+ (n=6, 8, 10) consisting of symmetric polymethylferrocene and the corresponding polymethylferricinium cation have been studied by cyclic voltammetry in organic solvents - ac-etonitrile, dichloromethane and dimethylsulfoxide. The values of the diffusion coefficient of the reagents (MenFc and MenFc+) of redox systems have been calculated by the Randles-Sevcik equation, which are consistent with the conclusion that "a one-electron reversible electrochemical reaction is determined by the diffusion of the reagents". The features of the change in the values of the diffusion coefficient, both in the sequence of Fc/Fc+^Me6Fc/Me6Fc+^ Me8Fc/Me8Fc+ ^ Mei0Fc/Mei0Fc+ redox systems and in dependence on the presence or absence of the charge in the reagents, have been interpreted by the change in the volume (dimensions) of the reagents with increasing number of methyl groups and the interaction of polymethylferricinium cations with electrolyte anions in solution.The fact that the diffusion coefficient of the cation in each MenFc/MenFc+ redox system is very close to the diffusion coefficient of the molecule, which allows us to replace the standard electrode potential (Eo) of the system with the half-wave potential (E1/2) according to the Nernst equation. The effect of the nature of the solvent on the value of the half-wave potential of these redox systems has been studied. It has been shown that the appearance of methyl groups in cyclopentadienyl rings significantly weakens the interaction of reagents (especially the iron atom) with solvent molecules, in comparison with the interaction in the Fc/Fc+ system and, as a result, their half-wave potential is less affected by the surrounding medium -solvent molecules and electrolyte ions.
Keywords: reference electrode, redox systems, voltammetry, sym. polymethylferrocene, sym. polymethylferricinium; half-wave potential, diffusion coefficient.
doi.org/10.32737/0005-2531-2022-4-8-14 Introduction
At present, there are no stable reference electrodes in non-aqueous media (organic solvents, ionic liquids), and therefore the development of such electrodes is one of the actual problems of electrochemistry. In this aspect, redox systems (MenFc/MenFc+, n=6, 8, 10) consisting of methyl homologues of ferrocene and ferricinium cations are widely studied [1-4].
An advantage of MenFc/MenFc+ (n=6, 8, 10) systems in comparison with the Fc/Fc+-sys-tem partially applied as a reference electrode arises from the fact that the redoxs potential of MenFc/MenFc+ (n=6, 8, 10) systems depends to a lesser extent on the nature of the solvent than the potential of Fc/Fc+ system. This feature of MenFc/MenFc+ (n=6, 8, 10) redox systems is due to electron-donor properties and spatial dimen-
sions of methyl groups. On the one hand, electron-donor properties of methyl groups partially compensate for a positive charge on the iron atom. On the other hand, emergence of several Me groups in cyclopentadienyl rings noticeably increases spatial dimensions (volume) of the polymethylcyclopentadienyl ring. Both factors will weaken the interaction of solvent molecules both with the central iron atom and aromatic ring of the reagents (MenFc and MenFc+). Therefore, the nature of the solvent should have less effect on the potential of the redox systems MenFc/MenFc+ (n=6, 8, 10) than on the potential of the system Fc/Fc+.
The results of the electrochemical studies presented in the previous article [5] showed that the electrochemical reaction in the polymethylfer-rocene/polymethylferricinium (MenFc/MenFc+, n=6, 8, 10) redox systems is a one-electron re-
versible process limited by diffusion. The results of our research in homogeneous and heterogeneous systems [5-7] have shown that polymet-hylferrocene/polymethylferricinium redox systems, which are quite stable in solutions prepared in an air medium, fully meet the 7 criteria set by the IUPAC for the reference electrode [8].
In view of the above, the purpose of this paper is to study the effect of organic solvents of different nature on the oxidation-reduction potential of MenFc/MenFc+ (n=6, 8, 10) systems and calculate the values of the diffusion coefficients to prove that the electrochemical reaction that takes place in redox systems is a one-
electron reversible process limited by diffusion.
+
+ e
octamethylferrocene
cation of octamethyl-ferricinium
Fig. 1. Scheme of electron transfer reaction in the example of Me8Fc/Me8Fc+ system
Experimental part
Electrochemical study of redox systems of (MenFc/MenFc+, n=6, 8, 10) were conducted in a three electrode cell using the potentiostat of IVIUMSTAT Electrochemical Interface. As the working electrode, a platinum wire was used, platinum plate served as the auxiliary electrode, while silver/silver chloride electrode (Ag/AgCl in acetonitrile, 0.1 M [NBu4][PF6]) was used as the reference electrode. The reference electrode was isolated from the cell solution with the Luggin's capillary, filled with the buffer solution (hexafluorophosphate tetrabutylammo-nium ([NBu4][PF6]), 0.1 M). The concentration of the reagents (MenFc, n=6, 8, 10) in the solution was 1 mM.
As an electrolyte, hexafluorophosphate tetrabutylammonium ([NBu4][PF6]) (Sigma-Al-
drich) was used. Before using, the salt was dried during an hour at the temperature of 1050C by a vacuum pump [9]. The cyclic voltammograms were recorded in the presence of 0.1 M [NBu4][PF6] in acetonitrile (dichloromethane, dimethylsulfoxide) purified from oxygen and traces of water, in the argon atmosphere. Temperature of the system was kept constant (±0.1 K) by a thermostat.
Neutral complexes MenFc (n=6, 8, 10) were sublimated at temperature of 40, 60 and 800C under the pressure of 0.01 MPa, and re-crystallized twice from hexane solution. The salts MenFc+PF6- (n=6, 8, 10) in the form of acetone solution passed through a column with an alumi num oxide with h eight of 3 -4 cm and re-crystallized twice from a mixture of acetone with hexane.
Results and discussion
a. Diffusion coefficients of reagents in MenFc/MenFc+ (n = 0, 6, 8, 10) redox systems
Previous research has shown that [5] the selection of experimental conditions in MenFc/MenFc+ (n=6, 8, 10) systems allowed us to connect the mass transfer between solution-electrode with the diffusion process, because in this case migration and convection processes do not participate in mass transport. According to the theoretical bases of electrochemistry, in such redox systems, the anodic and cathodic peak
currents (ipa, ipc) change from the square root of
1/2
the scan rate of the potential (u ) to the Randles-
Sevcik equation [10-12]. The study [5] proved
1/2
that the graphical dependence ipa (ipc) - v is linear according to the Randles-Sevcik equation. However, the values of diffusion coefficients have not been calculated, which characterize the diffusion process. This did not allow to unequivocally confirm that the diffusion process played a key role in the electron transfer reaction.
Figure 2 shows the cyclic voltammograms of the MenFc/MenFc+ (n=0, 6, 8, 10) systems in acetonitrile solution at different scan rate (v) of the potential, and Table 1 shows the parameters obtained from the voltammograms.
Fc/Fc+
Me6Fc/Me6Fc+
Me»Fc/ Me»Fc
-0.2 -0.1 0.0 Potential, V
Me10Fc/ Me10Fc+
Fig. 2. Cyclic voltammograms of MenFc/MenFc+ (n=0, 6, 8, 10) systems in acetonitrile solution at different scan rates (v) (v: 1 - 0.2; 2 - 0.1; 3 - 0.05; 4 - 0.02 Vs-1).
Table 1. The characteristics of cyclic voltammograms of the redox systems of MenFc/MenFc+ (n = 0, 6, 8, 10)
v (V-s-1) *Epa (V) Epc (V) AEp (V) E1/2 (V)
Fc/Fc+
0.02 0.457 0.390 0.067 0.424
0.05 0.460 0.392 0.068 0.426
0.10 0.467 0.394 0.073 0.431
0.20 0.472 0.398 0.074 0.435
MesFc/MesFc +
0.02 0.136 0.067 0.069 0.102
0.05 0.142 0.071 0.071 0.107
0.10 0.147 0.074 0.073 0.111
0.20 0.151 0.076 0.075 0.113
Me8Fc/Me8Fc +
0.02 0.054 - 0.009 0.063 0.022
0.05 0.055 - 0.010 0.065 0.023
0.10 0.057 - 0.010 0.067 0.023
0.20 0.058 - 0.011 0.069 0.024
Me10Fc/Me10Fc+
0.02 - 0.046 - 0.109 0.063 -0.077
0.05 - 0.044 - 0.112 0.069 -0.078
0.10 - 0.043 - 0.113 0.070 -0.078
0.20 - 0.042 - 0.116 0.074 -0.079
* - Potentials were calculated relatively to acetonitrile solution of silver chloride (Ag/AgCl) reference electrode (buffer solution - 0.1 M [NBuJ [PF6])
Considering the values of cathodic (and anodic) peak currents (zpc, ipa) presented in the Table 1, characterizing MenFc/MenFc+ (n=0, 6, 8, 10) redox systems, the values of diffusion coefficient of each reagent [D(MenFc) and [D(MenFc+)] redox system has been calculated according to the Randles-Sevcik equation and shown in Table 2.
ip = 2.686 •10V/2-A-c-D1/V/2 (1)
n - the number of electrons transferred during the redox process; A - surface area of the electrode (cm ); c - the concentration of the reagent (mol//); D - diffusion coefficient of the reagent (cm /sec); v - scan rate (V/sec).
Based on the data in the Table 2, it is necessary to note two features observed in the change of the diffusion coefficient.
Table 2. Values of diffusion coefficient of reagents in MenFc/MenFc+ (n=0, 6, 8, 10) systems (cm2/sec, acetonitrile, t = 250C)_
Reagents Diffusion coefficient (D; cm /sec)
n=0 n=6 n=8 n=10
MenFc 6.4-10-5 1.68-10-5 0.84-10-5 0.48-10-5
MenFc+ 3.6-10-5 1.23-10-5 0.58-10-5 0.36-10-5
Firstly, within each MenFc/MenFc+ redox system, although the value of the diffusion coefficient of the cation is close to the value of the diffusion coefficient of the molecule, the inequality D(MenFc+)<D(MenFc) is always observed.
In our opinion, this inequality can be explained by the interaction of positively charged ferricinium cations (MenFc+) with negatively charged PF6- anions of 0.1 M electrolyte [(C4H9)4N+(PF6-)], since as a result of this effect, the diffusion of cations in solution slows down. Therefore, the values of the diffusion coefficient of cations (MenFc+) are less than the diffusion coefficient of the corresponding neutral ferrocenes (MenFc), but smaller in each redox pair (Table 2).
The second feature is that the value of the diffusion coefficient decreases in the sequence n = 0 ^ 6 ^ 8 ^ 10 both for the molecule and for the cation. In our opinion, it would be correct to connect this feature of the diffusion coefficient with the increase in the volume (dimensions) of reagents in this sequence, because the diffusion coefficient varies inversely proportional to the
radius of the reagent according to Stokes-Einstein equation (2) [10]:
D = kT/6rcr|a (2)
a - the radius of the reagent, ^ - the viscosity of the solvent, k - the Boltzmann's constant.
Therefore, as the number of methyl groups increases, the value of the diffusion coefficient logically decreases.
It should be noted that the diffusion coefficient of the cation in each MenFc/MenFc+ redox system is very close to the diffusion coefficient of the molecule, which allows the standard electrode potential (E) to be equal to the half-wave potential (£1/2) according to Nernst's equation [7] (3): E ~ £1/2.
E0 = E/2-(RT/nF) ln (DMe Fc/DMe Fc+)1/2 . (3)
n n
For this reason, we characterize the oxidation-reduction potential of the redox systems studied by us with a half-wave potential that can be easily found on a voltammograms.
Thus, the numerical values of the diffusion
_5 2
coefficients («-10 cm/sec) unequivocally confirm that the electrochemical reaction in redox systems MenFc/MenFc+ (n=0, 6, 8, 10) is a reversible process limited by diffusion.
b. Influence of different solvents on the half-wave potential of MenFc/MenFc+ (n=6, 8, 10) redox systems
The independence of half-wave potential of the redox system from the nature of the solvent characterizes the possibility of using this system as a reference electrode. The disadvantage of the Fc/Fc+ redox system, currently proposed as a reference electrode for non-aqueous media, is that the half-wave potential does not remain constant in a number of solvents [13-15]. Therefore, the study of this problem is very important from the practical point of view.
We have tested the effect of solvents of different nature on their half-wave potential to check whether the these studied redox systems meet with this criterion or not. In order to determine the effect of solvents of different nature, we used acetonitrile (CH3CN), dimethyl-sulfoxide ((CH3)2SO) and dichloromethane (CH2Cl2) - solvents that differ significantly in donor number (DN) and viscosity (n) (Table 3) [16].
The cyclic voltammograms of the Me8Fc/Me8Fc+ system recorded in each of listed solvents and the values of the half-wave potential obtained according to them are shown in Figure 3 and in Table 3, respectively. Table 3 shows that the value of the half-wave potential in the system Me8Fc/Me8Fc+ varies by 70 mV (AE1/2=70 mV) towards positive potentials in the sequence CHsCN^CHCh ^ (CHs^SO. The
change in E1/2 value of the other two redox systems (n = 6, 10) is approximately the same in these solvents (AE1/2=65 and 69 mV). The change in the value of the half-wave potential by the action of the solvent («70 mV) in polymethyl-ferrocene/polymethylferricinium systems is significantly less than the change observed in the currently partially applied ferrocene/ferricinium system [13].
Potential v Potential v Potential v
Figure 3. Cyclic voltammograms of Me8Fc/Me8Fc+ system in acetonitrile (CH3CN) (a), dimethylsulfoxide ((CH3)2SO) (b) and dichloromethane (C^Cb) (c) solution at different scan rates (v) (v: 1- 0.2; 2 - 0.1; 3 - 0.05; 4 - 0.02 Vs-1).
Table 3. Comparison of values of half-wave potential of Me8Fc/Me8Fc+ system in different solvents (at 0.1 V-s-1 value of scan rate)_
Solvent CH3CN CH2Q2 (CH3)2SO
DN(kC-mol-1) 59.0 4.2 124.7
П (mPa-s) 0.341 0.410 1.989
Em (mV) 23 48 93
The potential values are given for the silver chloride (Ag/AgCl) reference electrode at a temperature of 298.15 K.
In our opinion, this is due to both electronic and steric interactions of methyl groups in the systems studied by us. Thus, the electrodonor property of the methyl group, on the one hand, leads to a decrease in the value of the positive charge of the iron atom, on the other hand, the fact that the methyl group has a larger volume than the hydrogen atom replaced in the ring increases the distance between the iron atom and the solvent molecules. As a result of the action of both factors, the interaction between the cation and the solvent molecules is significantly weakened, and the half-wave potential of the redox
system is less affected by the solvents than the Fc/Fc+-system.
Conclusion
The calculated values of the diffusion coefficient of the reagents of the MenFc/MenFc+ (n=6, 8, 10) redox systems allow us to unambiguously conclude that a one-electron reversible electrochemical reaction observed in these systems is determined by the diffusion of the reagents.
The studied MenFc/MenFc+ (n=6, 8, 10) re-dox systems are more promising systems than the currently used ferrocene/ferricinium (Fc/Fc+) system as a reference electrode in organic solvents.
References
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SiM. POLiMETiLFERROSEN/SiM. POLiMETiLFERRiSiNiUM (MenFc/MenFc+) (n=6, 8, 10) REDOKS SiSTEMLORiN ÜZVi HOLLEDiCiLORDO ELEKTROKiMYOVi TODQiQi
N.Z. ibrahimova, A.§.Oliyev, Q.M.Cafarov, LU.Latifov, D.B.Tagiyev
Simmetrik qurulu§lu polimetilferrosen va müvafiq polimetilferrisinium kationundan ibarat MenFc/Me,Fc+ (n=6, 8, 10) redoks sistemlar üzvi halledicilarda - asetonitril, dixlormetan va dimetilsulfoksidda tsiklik voltampermetriya üsulu ila tadqiq edilmi§dir. Redoks sistemlari ta§kil edan reagentlarin (MenFc va MenFc+) diffuziya amsalinin qiymatlari Randles-Sevcik tanliyi üzra hesablanmi§ va göstarilmi§dir ki, sistemda ba§ veran reaksiya "diffuziya ila mahdudla§mi§ birelektronlu dönan elektrokimyavi reaksiyadif'. Diffuziya amsalinin qiymatlarinin ham redoks sistemlarin Fc/Fc+ ^ Me^c/Me^^ Me8Fc/Me8Fc+ MeioFc/MeioFc+ ardicilligi üzra, ham da reagentda yükün olub-olmamasindan asili olaraq dayi§masinin xüsusiyyatlari metil qruplannin sayi artdiqca reagentlarin hacminin (ölgülarinin) artmasi va mahluldaki elektrolit anionlari ila polimetilferrisinium kationlarinin qar§iliqh tasiri ila §arh edilmi§dir. Har bir MenFc/MenFc+ redoks sisteminda kationun diffuziya amsalinin molekulun diffuziya amsalina qiymatca gox yaxin olmasi Nernst tanliyina uygun olaraq sistemin standart elektrod potensialini (Eo) yarimdalga potensiali (E1/2) ila avazetmaya imkan vermi§dir. Redoks sistemlarin yarimdalga potensialinin qiymatina halledicinin tabiatinin tasiri öyranilmi^dir. Göstarilmi§dir ki, tsiklopentadienil halqalarinda metil
qmplanmn olmasl reagentlэrin (xйsusilэ dэmir айшипип) hэlledici то1еки11ап Иэ qar§lllqll tэsirim Fc/Fc+ sistemindэki qar§lllqll tэsirlэ mйqayisэdэ эhэmiyyэtli dэrэcэdэ zэiflэdir vэ nэticэdэ эtraf mйhit - hэlledici токкиЛап vэ elektrolit ionlan Ьи sistemlэrin yaпmdalga potensiallna daha az tэsir edir.
Адаг sбzlэr: muqayisэ е1екЬгойи; ге^кя я1я1ет1эг; уо^атрегтеМуа; я1т. роИтеШ/еггояеп; ят. роНтеи^етяШит; yarmdalga potensiah; diffuziya этваЬ.
ЭЛЕКТРОХИМИЧЕСКОЕ ИССЛЕДОВАНИЕ СИМ. ПОЛИМЕТИЛФЕРРОЦЕН/СИМ. ПОЛИМЕТИЛ-ФЕРРИЦИНИЕВЫХ РЕДОКС-СИСТЕМ (MenFc/MenFc+) (П=6, 8, 10) В ОРГАНИЧЕСКИХ
РАСТВОРИТЕЛЯХ
Н.З.Ибрагимова, А.Ш.Алиев, Г.М.Джафаров, И.У.Лятифов, Д.Б.Тагиев
Методом циклической вольтамперометрии в органических растворителях (ацетонитриле, дихлорметане и диметилсульфоксиде) изучены MenFc/MenFc+ (п = 6, 8, 10) редокс-системы, состоящие из симметрично постренных полиметилферроцена и соответствующего катиона полиметилферрициния. В соответствии с уравнением Рандл-Севчика вычислены значения коэффициента диффузии реагентов (MenFc vэ MenFc+) редокс-систем, которые согласуются с выводом, что «одноэлектронная обратимая электрохимическая реакция определяется диффузией реагентов». Особенности изменения значений коэффициента диффузии как в последовательности редокс-систем Fc/Fc+ ^ Me6Fc/Me6Fc+ ^ Me8Fc/Me8Fc+ ^ MeloFc/MeloFc+, так и в зависимости от наличия или отсутствия заряда интерпретированы изменением объема (размеров) реагентов с ростом числа метильных групп и взаимодействием катионов полиметилферрициния с анионами электролита в растворе. Близость значения коэффициента диффузии катиона к значению коэффициента диффузии молекулы в каждой MenFc/MenFc+ редокс-системе позволило, в соответствии с уравнением Нернста, заменить стандартный электродный потенциал системы (Ео) его потенциалом полуволны (Е1/2). Изучено влияние природы растворителя на значение потенциала полуволны этих редокс-систем. Показано, что появление метильных групп в циклопентадиенильных кольцах значительно ослабляет взаимодействие реагентов (в особенности атома железа) с молекулами растворителя, по сравнению с тем же в системе Fc/Fc+ и как следствие, их потенциал полуволны меньше подвергается влиянию окружающей среды - молекул растворителя и ионов электролита.
Ключевые слова: электрод сравнения; редокс-система; вольтамперометрия; сим. полиметилферроцен; сим. полиметилферрициний; потенциал полуволны; коэффициент диффузии.
