Oxidation of cobalt(II) tetraphenylporphyrinate with molecular oxygen Текст научной статьи по специальности «Химические науки»

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DOI: 10.6060/mhc140926z

Oxidation of Cobalt(II) Tetraphenylporphyrinate with Molecular Oxygen

Tatyana M. Ziyadova,a@ Vladimir A. Burmistrov,ab Alexandr S. Semeikin,a and Oskar I. Koifmanab

aResearch Institute of Macroheterocycles, Ivanovo State University of Chemistry and Technology, 153000 Ivanovo, Russian Federation

bG.A. Krestov Institute of Solution Chemistry, Russian Academy of Sciences, 153045 Ivanovo, Russian Federation @Corresponding author E-mail: [email protected]

The state of cobalt complexes with tetraphenylporphyrin in alcoholic solutions has been studied by means of conductometry and electronic absorption spectroscopy. In the presence of oxygen, the Co'1 complex is oxidized into the H-peroxodimer and chloro(tetraphenylporphyrinato)cobalt(III), the latter exists in the form of the solvent-separated ion pair. Kinetic experiments have revealed conditions of the products formation in the course of the CoII complex oxidation. It has been demonstrated that only chloro(tetraphenylporphyrinato)cobalt(III) oxidizes thiols in the presence of oxygen. The elucidated kinetic and activation parameters of the Cd1 + 02 ^ Cd" reaction have demonstrated that its enthalpy is low and positive, whereas its entropy is high and negative.

Keywords: Metalloporphyrin, u-peroxodimer, oxidation, ion pair.

Окисление тетрафенилпорфирината кобальта(П) молекулярным кислородом

Т. М. Зиядова,а@ В. А. Бурмистров,'а,ь А. С. Семейкин,' О. И. Койфмана,ь

НИИ макрогетероциклических соединений, Ивановский государственный химико-технологический университет, 153000 Иваново, Россия

ьИнститут химии растворов им. Г. А. Крестова РАН, 153045 Иваново, Россия @Е-таИ: [email protected]

Методами кондуктометрии и электронной спектроскопии исследовано состояние кобальтовых комплексов тетрафенилпорфина в спиртовых растворах. Установлено, что при доступе кислорода комплекс двухвалентного кобальта окисляется до ¡-пероксодимера и хлоридтетрафенилпорфирината Со111, существующего в растворе в виде сольватно разделенной ионной пары. В ходе кинетического эксперимента определены условия образования ¡-пероксодимера и хлоридтетрафенилпорфирината Со111 при окислении комплекса Со11. Показано, что только тетрафенилпорфиринат кобальта(Ш) способен к окислению меркаптанов в отсутствии кислорода. Рассчитаны кинетические и активационные параметры реакции Со11 + О2 ^ Со111, свидетельствующие о низкой положительной энтальпии и высокой отрицательной энтропии активации этого процесса.

Ключевые слова: Металлопорфирин, д-пероксодимер, окисление, ионная пара.

Introduction

Metal complexes with macroheterocyclic compounds (for instance, porphyrins and phthalocyanines) are of considerable interest for researchers working in the fields of chemistry, biochemistry, and chemical engineering. The interest to such compounds is due to their important part in the biochemical processes. Furthermore, the macroheterocyclic complexes have been widely used in many fields of science and technology, including development of cancer diagnostics and therapy, antiviral blood treatment as well as preparation of dyes, pigments, and various catalysts. Diversity of applications of porphyrins metal complexes is based on their peculiar properties, such as ability to axially coordinate certain small molecules (H2O, O2, etc), chemical and thermal stability, chromophoric properties, and so further. [1-3] The complexes catalytic activity is closely related to their redox properties. In view of the above, this work was devoted to study of oxidation of cobalt(II) tetraphenylpor-phyrinate (ConTPP) with oxygen, extending the previously reported results.[4]

other activation parameters (the enthalpy, the entropy, and the Gibbs energy) were determined as described elsewhere.[6] The rate constant of the reaction can be expressed as:

k =

K)kBT h

(1)

where K* is the concentration equilibrium constant of activated complex formation; kB is Boltzmann's constant; h is Planck's constant.

In transition state theory, the activation Gibbs energy, AG*, is defined by:

AG*=-R T ln(K*),

(2)

where R is gas constant.

The enthalpy of activation AH* and entropy of activation AS* were calculated as:

AH* = Ea - RT AG* = AH* - TAS*

(3)

(4)

The equilibrium constant Keq was calculated using the equation:[7]

Experimental

Cobalt(II) 5,10,15,20-tetraphenylporphyrinate (Co"TPP) was used as model macrocycle. Ethanol (99.9%) was used for the solutions preparation.

Cd'TPPpreparation. 2.0 g (3.25 mmol) of 5,10,15,20-tetra-phenylporphyrinate was extracted with the Soxhlet apparatus into a boiling solution of 3.0 g (12.04 mmol) of cobalt(II) acetate tetrahydrate in 150 ml of acetic acid. Then the mixture was cooled down; the precipitate was filtered off, washed with acetic acid and methanol, and dried in air at 70 °C. Yield: 1.8 g (81 %). UV-Vis (CHCl3) X nm (lge): 529 (4.18), 411 (5.40).

Co"TPP oxidation with oxygen. Oxidation of cobalt(II) tetraphenylporphyrinate was performed as described elsewhere.[4] Concentration of dissolved oxygen was determined using the HQ portable device equipped with the LDO IntelliCALTM detector. Electronic absorption spectra were recorded using the UV-VIS Lambda 20 scanning photometer (Perkin-Elmer), the wavelength setting accuracy being of ±0.1 nm.

Calculation of rate constants, equilibrium constants, activation and thermodynamic parameters ofCo"TPP oxidation. The rate constants were defined by two methods. I. The rate constants were calculated taking advantage of the KinCalc software. The rate and equilibrium constants were computed via the least squares method using the following experimentally determined input parameters: initial concentrations of O2, HCl, and H2O; concentrations of ConTPP, chloro(tetraphenylporphyrinato)cobalt(III) (ClComTPP), and ^-peroxodimer (D) as functions of the reaction time. The concentrations of ConTPP, ClComTPP, and D were determined from the absorbance data. Molar absorptivities of the molecular forms of the cobalt complexes were previously elucidated via the absorption spectra deconvolution;[4] UV-Vis (C2H5OH) X nm (lge): ConTPP 411.8 (4.43); ClComTPP 427.4 (4.83); and D 386 (4.88). II. To calculate the rate constant, we used the program developed in[5] that including a method of calculation based on the use of optical density measurements over the maximum number of points in the series of spectra obtained by the spectrophotometer, which was capable of automatic digital recording. The error in determing the reaction rate constants does not exceed 5 %. The difference between the rate constants determined by these methods does not exceed the error. The activation energy Ea and the pre-exponential factor were determined graphically from the lnk = f(T_1) plot;

K

A -A t o

eq A -A „ „ A -A œ r C -C .^L^L A -A

oo t

(5)

I MC

where A A A^ are optical density of the metallocomplexes solutions at time 0, t and after the reaction, respectively; C° is initial concentration of the ligand, mol/l; CMCo is initial concentration of the metallocomplex, mol/l.

When the reaction is at equilibrium (AG = 0) and according to isobar equation:

AG0 = -RTln(Ke).

(6)

In thermodynamics, the change in Gibbs free energy, AG, is defined as:

AG0 = AH0 - TAS0. Summary

AH° 1 AS"

lnK,

eq

R T R

(7)

(8)

The thermodynamic parameters of the process (the entropy and the enthalpy) were determined graphically from the ln^e =f(T-1) plot: tga = -AH°/R; A = AS°/R.

Conductometry of the cobalt porphyrinates in ethanolic solutions. Conductometric measurements were performed in the cell equipped with platinum electrodes at 298.15±0.15 K. The cell was calibrated using the 0.1 mol/l KCl solution.® The cell constant was of k = 0.2118 cm-1. The specific conductivity (D-1-cm-1) was calculated as

X = k/R

(9)

with R being a measured electrical resistance.

In order to precisely calculate the conductance, the conductivity of pure solvent was accounted for.[9] The true value of the solute conductivity was found as the difference between those of the solution and the solvent:

1

X ^solution ^solvent"

(10)

The excess molar conductivity X was calculated from the true solute conductivity x and the solute concentration C (mol/l):

X = x/C.

(11)

The uncertainty of true solute conductivity and excess molar conductivity determination did not exceed 5 %.

molar conductivity due to formation of the charged complex. However, the excess conductivity of the QCofflTPP solution was twice lower than that of the CTAB solution; hence, the cobalt(III) complex in the presence of Cl- ions in ethanol has likely existed in the form of the solvent-separated ion pairs Cl5-(solv)Com5+TPP

It was previously demonstrated141 that the following equality was held at any reaction time t in the course of Co"TPP interaction with oxygen and hydrochloric acid:

Results and Discussion

As it was shown previously,[4] the dissolved cobalt(II) complex Co"TPP was oxidized into the ^-peroxodimer ConiTPP-0-0-ConiTPP (D) and chloro(tetra-phenylpor-phyrinato)cobalt(III) (ClCoIIITPP) in the presence of air and Cl- ions (KCl or HCl). The electronic absorption studies of CoIITPP oxidation[4] led to reliable estimation of the oxidation state of the complex forming metal (cobalt) in all the complexes forms; however, the complexes dissociation or ion pairs formation could not be elucidated. As the complexes concentration was low (10-5-10-6 mol/l), that could be achieved taking advantage of conductometric measurements.

In view of the above, we have measured the conductivity x and calculated the excess molar conductivity X of the following ethanolic solutions: CoIITPP (under argon), CoIIITPP + the ^-peroxodimer, and CoIIITPP (in the presence of equimolar concentration of HCl). For the sake of comparison we have measured the conductivity of cetyltrimethylam-monium bromide CTAB solution, the organic electrolyte is known to exist in the form of free solvated ions in ethanol at concentration of about 10-5 mol/l.[10]

The data collected in Table 1 have evidenced that the excess conductivity was close to zero in the case of CoIITPP, the background value of 0.09 Q-1-cm2-mol-1 had apparently caused by the presence of admixtures.

CoIITPP oxidation with air oxygen was not accompanied with the conductivity increase (Table 1); hence, the charged particles concentration in the oxidized system was insignificant. Taking into account the presence of the uncharged ^-peroxodimer, it was reasonable to assume that the cobalt(III) complex was not charged as well, and the acidic ligand was either directly bound to the complex forming ion (X-CoIIITPP) or formed the contact ion pair (X8-Co8+TPP). Hydroxyl ion formed via the autoprotolysis of water traces could act as the ligand X.

Introduction of the equimolar amount of HCl into the CoIIITPP solution has led to significant increase of the excess

[ConTPP]0 = [ConTPP]t + [ClCoinTPP]T + + 2[CoinTPP-0-0-Co'nTPP] .

(a)

Hence, the oxidation could be represented by reactions (b) and (c).

2CoIITPP + 02 ^ CoIIITPP-0-0-CoIIITPP (b)

CoIITPP + 02 + Cl- ^ ClCoIIITPP + O2- (c)

In order to compare these stages significance in the course of catalytic oxidation of thiols, we have studied the effect of three forms of cobalt complexes on the oxidation of 1-propanethiol in ethanol. The experiments have revealed that the peroxodimer D, as well as CoIITPP complex,[13] did not oxidize 1-propanethiol, what was confirmed by conclusions made earlier[1112]). Only CoIIITPP is able to oxidize 1-propanethiol rapidly in the oxygen-free solution. Therefore, the reaction (c) is seemed to be important in view of mechanism of catalytic oxidation of thiols. Further kinetic studies of CoIITPP oxidation were performed under conditions of reaction (c) domination. Taking advantage of the specially developed software, we have processed the [CoIITPP] =fz) and [ClCoIIITPP] =fr) kinetic curves and calculated the rate constant of reaction (c) as function of HCl concentration (Table 2).

Table 2. Rate constants of cobalt(II) tetraphenylporphyrinate

oxidation with molecular oxygen in ethanol

(C(CoIITPP) = 1.3410-5 mol/l, C(O2)=6.5910-5 mol/l, at 298.15 K).

C(HCl), mol/l kefP s-1 k, l2mol-2s-1

(1.16±0.07)10-5 (1.96±0.11)10-6 (2.57±0.17)-103

(2.08±0.12)-10-5 (3.13±0.19)10-6 (2.28±0.14)-103

(2.73±0.17)-10-5 (4.47±0.29)-10-6 (2.49±0.15)-103

(5.45±0.32)-10-5 (1.02±0.06)10-5 (2.85±0.21)-103

where k ff and k are the effective and veritable constants of reaction (c).

Table 1. Conductivity x and excess molar conductivity X of the studied compounds (ethanol at 298.15 K).

Solute C, mol/l X, n-1'cm-1 X, fi-1cm2mol-1

Ethanol (pure solvent) - 1.610-7 -

ClCoIIITPP 9.0810-6 3.0310-4 33.37

CoIIITPP 8.8310-6 10.5310-7 0.10

CoIITPP (under Ar) 9.1310-6 9.6310-7 0.09

CTAB 1.6410-4 - 86.90

Kinetic data shown in Table 2 and Figure 1 have evidenced the first order of reaction (c) with respect to HCl. Processing of the kinetic data obtained as varied temperature has allowed calculating the rate and equilibrium constants Keq (Table 3); furthermore, from the Arrhenius-type plots (Figure 2) we have succeeded in elucidation of the activation and thermodynamic parameters of reaction (c) (Table 4). We have supposed that the linear shape of the lnkv =f(T-1) and lnKeq =f(Tt) plots supports the validity of the suggested scheme of reaction (c). In order to correctly determine the equilibrium constant, we have expressed the reactants concentrations as the molar fractions.

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МакрогетероцикJlbl /Macroheterocycles 2015 8(3) 274-278

Table 3. Rate constants and equilibrium constants of cobalt(II) tetraphenylporphyrinate oxidation by oxygen in ethanol (C(ConTPP) = 1.3410-5 mol/l, C(02) = 6.5910-5 mol/l, C(HCl) = 2.7310-5 mol/l).

Table 4. Activation and thermodynamic parameters of reaction (c) at 298.15 K

T, K

kefP S-

k, l2mol-2s-1

K

298.15 (4.47±0.29)-10-6

303.15 (5.07±0.32>10-6

308.15 (5.38±0.34>10-6

313.15 (5.70±0.35>10-6

318.15 (6.19±0.38)10-6

(2.49±0.15)-103 (2.82±0.16)103 (2.99±0.18)103 (3.17±0.20)-103 (3.44±0.23)-103

(3.92±0.33)-104 (4.18±0.34)-104 (4.38±0.36)-104 (4.69±0.39)104 (4.76±0.40)-104

-11,5

lnc0(HCI) [mol/L] -11 -10,5 -10

-9,5

-11 -11,5 -12 -12,5 -13 -13,5

Figure 1. Effective rate constant of cobalt(II) tetraphenylporphyrinate oxidation by oxygen as function of HCl concentration in logarithmic coordinates(C(ConTPP) = 1.34 10-5 mol/l; C(02) = 6.5910-5 mol/l, at 298.15 K).

The low value of activation entropy of the studied reaction (Table 2) was likely due to the nature of ConTPP interaction with oxygen: the both reactants were molecules bearing the unpaired electrons at the reactive site.[1416] The high negative activation entropy was also expected taking into consideration binding of oxygen at the metal ion in the

k x10-3, l2mol-2s-1 v AG", kJ/mol AH", kJ/mol AS", Jmol-1K-1

2.49±0.15 53.6±2.5 9.7±0.7 -147±19

E , kJ/mol K xio-4 a eq AG°, kJ/mol АН°, kJ/mol AS°, Jmol-1K-1

12.2±0.7 3.92±0.33 -26.2±4.2 8.0±0.6 115±16

transition state. The thermodynamic parameters of reaction (c) (Table 2) could not be easily interpreted, possibly, due to the different conditions of the starting reagents and the products solvation.

Acknowledgements. This work was financially supported by Russian Foundation for Basic Research (project no. 1423-00204). The authors are grateful to Dr. Zhurko G.A. for his help.

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3,1 3,15

3,2 3,25 1000/T, К"1

3,3 3,35 3,4

10,8 10,75 10,7 10,65 10,6 10,55

3,1 3,15

3,2 3,25 1000/Т, К"1

3,3 3,35 3,4

Figure 2. Logarithms of the rate constant (a) and equilibrium constant (b) of cobalt(II) tetraphenylporphyrinate oxidation by oxygen as functions of reciprocal temperature.

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Received 19.09.2014 Received revised 13.05.2015 Accepted 19.05.2015