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CHEMICAL PROBLEMS 2019 no. 2 (17) ISSN 2221-8688
UDC 54 - 386
RESEARCH INTO KINETICS OF ELECTRON EXCHANGE REACTIONS IN THE SYSTEM SYM. OCTAMETHYLFERROCENE/SYM. OCTAMETHYLFERRICINIUM
HEXAFLUOROPHOSPHATE
N.Z. Ibrahimova, G.M. Jafarov, D.B. Taghiyev, I.U. Lyatifov
Acad. M. Nagiyev Institute of Catalysis and Inorganic Chemistry National Academy of Sciences of Azerbaijan AZ1143 Baku, H. JavidAve., 113; e-mail: [email protected]
Received 12.05.2019
Abstract: The constant of electron exchange rate (kex.) in the system of sym. octamethylferrocene/sym. octamethylferricinium hexafluorophosphate
[(CH3)4C5H]2Fe/(CH3)4C5H]2Fe PF—] (hereinafter are denoted as MeFc+/0) was determined by method of 1H NMR line broadening. The appearance of eight methyl groups in each reagent of the redox-system of Me8Fc+m increases the constant of exchange rate approximately 4.8 times, in comparison with the unsubstituted ferrocene-ferricinium system [(C5H5)2Fe/(C5H5)2Fe+PF6-] (Fc+/0). The increase of exchange rate in series of Fc+l0^Me6Fc+l0^Me8Fc+l0^Me10Fc+l0, though it corresponds to the electron donor property of the methyl group, is interpreted by us as the result of reduction of reorganization energy of solvent molecule (acetone) by increasing the reagent volumes of the redox couples of 'Me„Fc+l0 (n = 6, 8, 10).
Keywords: sym. octamethylferrocene, cation of sym. octamethylferricinium, chemical shift, electron exchange
DOI: 10.32737/2221-8688-2019-2-310-315
Introduction
Over the past 10-15 years an emphasis of a great number of research works has been laid on the study into kinetics and mechanism of electron exchange in the system of ferrocene-ferricinium and their alkyl derivatives [1-3]. The main reason of an emphasis on these objects is due to the necessity of replacement of unstable in a number of solvents ferrocene-ferricinium reference electrode (Fc+/0) by more promising, from the point of IUPAC standards, systems -highly substituted alkyl derivatives of ferrocene [2, 3]. In spite of numerous experimental and theoretical studies of electron exchange in the homogeneous and electrochemical heterogeneous systems [1-8],
no mechanism of its proceeding has completely been clarified [3, 7]. This work is a continuation of our previous researches [7, 8] revealing that the electron exchange process in the system of Me6Fc+/0 is determined not by the electron conjugation of the reactants in the transition state and the dynamics of the solvent molecules but by the energy of the reorganization of the solvent molecules around reactants. We consider that the study of electron exchange in redox couples whose components are representatives of two complete homologous series (ferrocene and cation of ferricinium) can provide us with even greater clarity about the mechanism of this process.
Results and discussion
1H NMR study into sym. octamethylferrocene and paramagnetic cation of sym. octamethylferricinium.
NMR spectra of diamagnetic sym. are obtained in deuteroacetone at 25°C. The octamethylferrocene and paramagnetic sym. values of chemical shifts (v) and half-widths octamethylferricinium hexafluorophosphate of resonance lines (W) are given in Table 1.
Table 1. The observed chemical shifts in 1H NMR spectra of sym. Me8Fc and sym. Me8Fc+PF6" in deuteroacetone (v in Hz, on tetramethylsilane; in square brackets are presented the half-widths of resonance lines (W) in Hz)*
sym. Me8Fc (c iamagnetic complex) sym. Me8Fc+PF6 (paramagnetic complex)
Vd; H(Cp) [Wd] vd; H(C^) [Wd] vp; Н(Cр) [Wp] vp; H(CH3) [Wp]
957 [«1 Hz] 516 [«1 Hz] 498 [«1 Hz] 8430 [2060 Hz] -8640 [405 Hz] -11310 [405 Hz]
: - spectrometer Bruker-300 MHz, t = 25 °C,
The spectrum of cation of sym. Me8Fc+ differs from the spectrum of appropriate diamagnetic complex - sym. Me8Fc mainly in two aspects: a) large range of proton chemical Me8Fc+/0. Since, in the 1H NMR spectrum of shifts and b) great width of resonance lines (Table 1).
We have calculated isotropic shifts (Av) of protons of methyl groups [Av(HMe)] and cyclopentadienyl ring [Av(Hring)] necessary for
vD(HMe)ave. = (516 + 498) : 2 = 507 Hz
calculation of constant of electron exchange rate on the basis of 1H NMR chemical shifts of a component of the redox-system of sym.
the diamagnetic and paramagnetic reagents, two resonance signals of protons of the Me groups are observed, we calculated average values of isotropic shifts (Av):
Vp(HMe)ave. = [-8640 + (-11310)] : 2 = - 9975 Hz Av(HMe)ave. = |vd - vp| = |[507 -(-9975)]| = 10482 Hz
VD(HMe)aVe. - average value of the chemical shift of protons of methyl groups of diamagnetic reagent in Hz;
vP(HMe)ave. - average value of the chemical shift of protons of methyl groups of paramagnetic reagent in Hz;
Av(HMe)ave. - average value of the isotropic shift of protons of methyl groups in Hz.
1H NMR study of electron exchange in the system of sym. Me8Fc
+/0
NMR research of the redox-system of sym. Me8Fc+/0 in deuteroacetone, first of all, made it possible to establish that this system is chemically stable in the solution: shift (vDP-vD) of resonance line frequency of protons of diamagnetic component in an invariable molar fraction of the cation salt (fp) and various total concentrations (Ctotal) remained constant.
Further, on the basis of calculated isotropic shifts (Av), half-widths of the resonance lines (WD and WP) and molar fraction of paramagnetic salt in the mixture (fp), according to the formula (1), the constant of exchange rate (kex) in the system of sym.
Me8Fc+/0 [7] was calculated:
kex — ■
4 n fD fr (Av)2
(WDP - fDWd - fpWp)Ctotal
(1)
where Wdp - half-widths of the resonance fP - molar fraction of paramagnetic salt
line of the diamagnetic component in the in the mixture calculated from expression
presence of paramagnetic salt (220 Hz);
CoMFc + CoMFc +
/D =-= —--(OMFc = Me8Fc)
^OMFc
P Comfc + + Co— ctotal
The study of concentration dependence of the rate constant made it possible to establish that the kinetic equation of electron
exchange rate = k[Me8Fc] •
exchange reaction is expressed by Equation 2, and the reaction order is equal to two:
[Me8Fc+] (mol//-sec.) (2)
The constant value of electron exchange approximately 4.8 times more than exchange
rate in the system of sym. octamethylferro- rate constant in the system Fc+/0 [9], previously
cene/cation of sym. octamethylferricinium offered as a reference electrode: [/U(Me8Fc+/0) = 2.2-107 //mol-sec] is
kex(Me8Fc+/0) = 2.2-107 « kex(Fc+/0) • 4.8 « (4.6-106) • 4.8 (//mol-sec.) (3)
on this basis we can conclude that the system of sym. Me8Fc+/0 offered as the reversible reference electrode satisfies the condition of reversibility - the most important requirement presented for the reference electrodes by IUPAC [6].
The constant values of electron exchange rate in the system of sym. octamethyl-ferrocene/cation sym. octamethylferricinium, along with the reference data on the systems of ferrocene/cation of ferricinium [9],
1,2,4,1',2',4'-hexamethylferrocene/cation of 1,2,4,1',2',4'- hexamethylferricinium [7] and decamethylferrocene/cation of
decamethylferricinium [9] are given in Table 2. The value of rate constant in the system of sym. Me8Fc+/0 confirms the results obtained in the work [7] - the exchange rate increases together with the rise in the methyl groups number in the composition of reagents of the redox couple (Table 2).
Table 2. The constants of electron exchange rate in the systems of Fc+/0, Me6Fc+/0, sym. Me8Fc+/0 and Me10Fc+/0 in deuteroacetone at 25°C
System Fc+/0 Me6Fc+/0 Me8Fc+/0 Me10Fc+/0
kex (/-mol-1-sec-1) 4.6-106 [9]* 1.5107** 2.2-107 2.4-107 [9]
* - in square brackets [ ], the references are presented
** - corrected values kex in system of sym. Me6Fc0/+ (see Experimental part)
The observed increase in the electron exchange rate in a series of the redox couple of Fc+/0^Me6Fc+/0^Me8Fc+/0^Me10Fc+/0 (Table 2) basically corresponds to the electron-donor property of methyl substituent. Really, by taking into consideration this property, it can be assumed that under the impact of the increasing number of methyl groups, the effective positive charge on the iron atom must consistently decrease by causing spatial expansion of e2g orbitals. The expansion of e2g
orbitals, in its turn, should lead to an increase in the degree of e2g-e2g overlapping in collision complex (if, in collision complex, the reagents are "side by side"), and consequently to an increase in the rate of electron exchange.
However, if we take into consideration that in the methyl homologues of ferrocene and the cation of ferricinium, methyl groups are inplane of the cyclopentadienyl ring, it is obvious that methylation of ring will be accompanied by a significant (not less than 3-4 A) increase in
the distance between atoms of iron molecules and cations in the collision complex leading to noticeable reduction of overlap of e2g orbitals in comparison with the overlapping in the unsubstituted ferrocene-ferricinium complex of collision. As a consequence, electron exchange rate in the redox couple of methyl homologues of ferrocene and ferricinium, for steric reasons must be decreased in comparison with the system of Fc+/0. But, in the experiment [7, 9 and Table 2 of the work], the opposite is observed.
Therefore, the increase of electron exchange rate in the reviewed systems should be retraced in changes of other factors that may affect the exchange rate.
The role of the factors capable of affecting the electron exchange rate has been reviewed in the theory of electron transfer of R.A. Marcus
[10]. In previous work [7], it has been noted that, from four factors affecting the value of free activation energy, in the systems of MenFc+/0 and in the exchange conditions selected by us, the impact of three factors (by ionic strength of solution, Coulombic forces of interaction between reagents and energy expended on changes in the collision complex) can be disregarded. Therefore, free activation
energy ( AGex ) is defined only by the reorganization energy value of solvents ( AG*r ), so AGeX « AG 1 should be
SR
accepted.
The values of reorganization energy of acetone for the systems of Fc+/0 and sym. Me8Fc+/0 calculated by the improved Marcus formula (Xiang-Yuan Li's formula [11] (4)) are respectively 11.22 and 9.35 (kJ/mol):
AG
SR
NAe2
r 1 Ï r 1 Ï r 1Ï +
2r
2r.
a
v y
i 1 - 1 ^
^op (Ss —1)
(4)
where a- the sum of effective radii of reagents r and r2, e - electron charge;
s0 - vacuum permittivity (it is used in the International System of Units); sop - optical dielectric permittivity of solvent permeability; ss - static dielectric solvent permeability.
In the presented calculations of reorganization energy, the radius for each of components of couple of sym. MegFc+/0 has been taken equal to the radius accepted for reactants of the couples of Me10Fc+/0. Probably, if it was possible to take into account some asymmetry of van der Waals surface for the couples of Me8Fc+/0 in the reorganization energy formula (4) (i.e., actually to take into consideration the difference of their radius from the radius of the couple Me10Fc+/0), then the reorganization energy (9.35 kJ/mol) would have some what
increased value and more appropriate to the real physical picture.
However, despite this degree of approximation, the general conclusion arising from the observed tendency of change AGSR
is that the exchange rate constant consistently rises with the increase in substitution degree of hydrogen atoms of C5-ring by methyl groups and the reason is the decrease of acetone molecule reorganization energy with an increase in volumes of reactants in the couples of MenFc+/0 (n = 6, 8, 10), in comparison with the couple of Fc+/0.
Experimental part
The reactants containing the redox couple of sym. Me8Fc+/0 were synthesized by methods indicated in the works [12, 13]. For NMR research, methylferrocenes were twice sublimated, and hexafluorophosphate salts
were twice recrystallized from acetone solution.
1H NMR spectra were obtained in deuteroacetone by means of spectrometer Bruker-300 MHz with the internal standard
(tetramethylsilane) at a temperature of 25°C. The total concentration of reagents in the systems studied by us varied within 0.0300.45 M, but molar fraction of paramagnetic salt fp was 0.01-0.350.
The errors in values of exchange rate constants computed on the basis of uncertainties of half-widths of resonance lines are about 10%. The difficulties of defining half-widths of resonance lines of spectra of the electron exchange process in the systems of
sym. Me6Fc+/0 and sym. Me8Fc are caused
by overlapping absorption bands belonging to protons of Me groups with the different numbers of vicinal Me groups. The repeated, more thorough analysis of the overlapping bands in the spectrum of the system of sym. Me6Fc+/0 led to a small correction of initial value kex=1.67-107 [7]. The corrected value kex for the system of sym. Me6Fc+/0 is equal to 1.5-107.
Molar fractions were also calculated on the basis of the values of vDP relatively to vD and vP, according to the formula:
fp =
v
DP
Av
where vDP - average values of chemical shifts of protons of the diamagnetic component
in the mixture with appropriate paramagnetic salt in Hz (-743.1 Hz), wherein fp was 0.119.
References
1. Matsumoto M. and Swaddle T.W. Volumes of activation for electrode processes of various charge-types in nonaqueous solvents. Can. J. Chem., 2001, vol. 79, pp. 1864-1869.
2. Matsumoto M. and Swaddle T.W. The decamethylferrocene(+/0) electrode reaction in organic solvents at variable pressure and temperature. Inorg. Chem., 2004, vol. 43, pp. 2724-2735.
3. Torriero A.A.J. Characterization of decamethylferrocene and ferrocene in ionic liquids: argon and vacuum effect on their electrochemical properties. E/ectrochimica Acta, 2014, vol. 137, pp. 235-244.
4. Kirchner K., Dang S.Q., Stebler M., Dodgen H., Wherland S., Hunt J. P. Temperature, pressure, and electrolyte dependence of the ferrocene/ferrocenium electron self-exchange in acetonitrile-d3. Inorg. Chem., 1989, vol. 28, pp. 3604-3606.
5. Nielson R. M., McManis G. E., Safford L. K., Weaver M. J. Solvent and electrolyte effects on the kinetics of ferrocenium-ferrocene self-exchange. A reevaluation. J. Phys. Chem., 1989, vol. 93, pp. 2152-2157.
6. Gritzner G., Kuta J. Recommendations on reporting electrode potentials in nonaqueous solvents. J. Pure App/. Chem., 1984, vol. 56, pp. 461-466.
7. Ibrahimova N.Z., MaMMadov I.G., Jafarov G.M., Salimov R.M., Lyatifov I.U. NMR investigation of diamagnetic 1,2,4,1 ',2',4'-hexamethylferrocene, paramagnetic 1,2,4,1',2',4'-hexamethylferricinium hexafluorophosphate and electron exchange between them. Chemical Problems. 2017, no. 1, pp. 51-58.
8. Ibrahimova N.Z., MaMMadov I.G., Jafarov G.M., Lyatifov I.U. Kinetics of electron-exchange reaction in systems consisting of sandwich type complexes of iron. News of Baku University. 2018, no. 1, pp. 26-31.
9. Yang E.S., Chan M.S., Wahl A.C. Electron exchange between ferrocene and ferricenium ion. Effects of solvent and of ring substitution on the rate. J. Phys. Chem., 1980, vol. 84, pp. 3094-3099.
10. Marcus R.A., Sutin N. Electron transfers in chemistry and biology. Biochim. Biophys. Acta, 1985, vol. 811, pp. 265-322.
11. Xiang-Yuan Li. An overview of continuum models for nonequilibrium solvation: Popular theories and new challenge. International Journal of Quantum Chemistry. 2015, vol. 115, pp. 700-721.
12. Ibrahimova N.Z., MaMMadov I.G., Jafarov G.M., Salimov R.M., Lyatifov I.U. Polymethylferrocenes and relevant
v
D
polymethylferricinium cations. News of Baku University. 2016, no. 1, pp. 27-32. 13. Nesmeyanov A.N., Materikova R.B., Lyatifov I.R., Kurbanov T.Kh., Kochetkova
N.S. Sym.-polymethylferricinium Hexa-fluorphosphates. J. Organometal. Chem., 1978, vol. 145, pp. 241-243.
SiM. OKTAMETiLFERROSEN/SiM. OKTAMETiLFERRiSiNiUM-HEKSAFL ÜORFOS-FAT SiSTEMiNDd ELEKTRON MÜBADiLd REAKSiYASININ KiNETiKASININ
TdDQiQi
N.Z. ibrahimova, Q.M. Сэ/эгог, D.B. Tagiyev, i.U. L9tifov
AMEA-nin akad. M.Nagiyev adina Kataliz vs Qeyri-üzvi Kimya institutu AZ1143, Baki, H.Cavidpr., 113; e-mail: [email protected]
Sim. oktametilferrosen/sim. oktametilferrisinium-heksaflüorfosfat [(CH3)4CH]2Fe/(CH3)4C5H]2Fe+ PF6-] (vs ya Ме8рс+/0) sisteminds ba§ versn elektron mübadils reaksiyasinin sürst sabiti (kex.) 1Н NMR rezonans zolaqlarinin geni§lsnmssi üsulu ils müsyysn edilmi§dir. Me8Fc+/0 sisteminin hsr bir reagentinds 8 sdsd metil qrupunun yaranmasi bu sistemin sürst sabitinin ^ferrosen/ferrisinium [(C5H5)2Fe/(C5H5)2Fe+PF6-] (Fc+/0) sistemins nszsrsn tsxminsn 4.8 dsfs artmasina ssbsb olur. Fc+/0^Me6Fc+/0^Me8Fc+/0^Me10Fc+/0 sirasi üzrs mübadils reaksiyasinin sürstinin artmasi metil qrupunun elektronodonor xassssins uygun olsa da, bu hadiss Me„Fc+/0 (n = 6, 8, 10) redoks-cütlsrinds reagentlsrin hscminin artmasi nsticssinds aseton molekullarinin reorqanizasiya enerjisinin azalmasi ils izah olunmu^dur.
Agar sözbr: sim. oktametilferrosen, oktametilferrisinium kationu, kimysvi sürü^ms, elektron mübadils
ИЗУЧЕНИЕ КИНЕТИКИ РЕАКЦИИ ЭЛЕКТРОННОГО ОБМЕНА В СИСТЕМЕ СИМ. ОКТАМЕТИЛФЕРРОЦЕН/ГЕКСАФТОРФОСФАТСИМ. ОКТАМЕТИЛ-
ФЕРРИЦИНИЯ
Н.З. Ибрагимова, Г.М. Джафаров, Д.Б. Тагиев, И.У. Лятифов
Институт катализа и неорганической химии им. акад. М.Нагиева Национальной АН Азербайджана AZ1143 Баку, пр.Г.Джавида, 113; e-mail: [email protected]
Константа скорости электронного обмена (kex.) в системе сим. октаметилферроцен/гекса-фторфосфат сим. октаметилферрициния [(СН3)4С5Н]2Fe/(CH3)4C5H]2Fe+PF6-] (далее обозначаются как Ме8Фс+/0) определена методом уширения 1НЯМР линий. Появление восьми метильных групп в каждом реагенте редокс-системы Ме8Фс+/0 увеличивает константу скорости обмена примерно в 4.8 раза, по сравнению с незамещенной ферроцен-феррициниевой системой [(С5Н5)2Fe/(C5H5)2Fe+PFi] (Fс+/0). Увеличение скорости обмена в ряду Fo^'0-Ме^с+/0—Ме^с+/0 -^Ме1сРс+/0 хотя и соответствует электронодонорному свойству метильной группы, однако интерпретируется нами как результат уменьшения энергии реорганизации молекул растворителя (ацетона) с увеличением объемов реагентов редокс-пар Ме^с+/0 (n = 6, 8, 10). Ключевые слова: сим. октаметилферроцен, катион сим. октаметилферрициния, химический сдвиг, электронный обмен.
