Use of chemical kinetics for the description of metal porphyrin reactivity

Journal of Porphyrins and Phthalocyanines

J. Porphyrins Phthalocyanines 2012; 16: 1040–1054

DOI: 10.1142/S1088424612500769

Published at http://www.worldscinet.com/jpp/

Use of chemical kinetics for the description of metal

porphyrin reactivity

Tatyana N. Lomova^a◊, Mariya E. Klyueva^a,b, ElenaYu.Tyulyaeva*^aand

Nataliya G. Bichan^a

^aG. A. Krestov Institute of the Solution Chemistry of Russian Academy of Sciences, Akademicheskaya str. 1, Ivanovo

153045, Russia

^bIvanovo State University of Chemistry and Technology, Engel’s str. 7, Ivanovo 153000, Russia

Received 20 January 2012

Accepted 16 March 2012

ABSTRACT: The results of use of chemical kinetics receptions, approaches and methods for the study

of porphyrins and their metal complexes reactivity are discussed on an example of oxidation, acid-basic,

and catalytic reactions of rhodium, palladium, and rhenium complexes of porphyrin in liquid solutions.

The peculiarity of the porphyrin reaction rates is analyzed in a brief context of general provisions of the

chemical kinetics. The opportunity to use the quasistationarity principle at the deﬁnition of the kinetic

equation of the reactions with participation of metal porphyrins is shown. The transition from the process

kinetic description to consideration of its mechanism is explored.

KEYWORDS: porphyrins, rhodium, palladium, rhenium complexes, synthesis, reactions, kinetics,

mechanism.

chemical reactions in total process in the closed reactor

INTRODUCTION

of periodic action [2].

In spite of the fact that chemical kinetics methods are

A rate represents a barrier of course of chemical

known rather for a long time they obviously are used in

reaction. Spontaneous, from the point of view of

the chemistry of porphyrin compounds not enough. To

certain extend this position connected with industrial-

thermodynamic criterion Gibbs free energy, reaction

can proceed with too low speed to be appreciable in

scale applications of porphyrins is developed poorly.

real conditions. This phenomenon concerns to the full

Hence there are not necessary to make technological

calculations of reactions using a kinetics equation.

However chemical kinetics surely represents one of the

most promising and exploited method for the study of

of the reactions with participation of metal porphyrins

because of their high stability and low reactivity in such

reactions, as formation, oxidation, and dissociation.

A chemical kinetics method gets special value for

mechanism of reactions. That allows writing a lot of

the study of such reactions. It is possible to describe

equilibriums and completing stages independent from

reactivity quantitatively, having deﬁned a rate constant

each other for a real not simple process and obtaining

with use of the method and to obtain the rate law —

a limiting reaction rate [1]. Within the limits of

phenomenological kinetics the change of concentration

connected with physical processes in reacting system

the kinetic equation, allowing to calculate the reaction

rate at any concentration of reagents. Due to this

possibility the method is a base for an optimization of

is not considered in the rate equation. It means that the

synthesis and designing of porphyrin structures. Kinetic

equation describes only system consisting of elementary

description and knowledge about elementary limiting

reaction of chemical process are of great interest

because of the possibility to regulate the rate of the

^SPP full member in good standing

reactions of metal porphyrins which are the potential

components of the novel materials in catalyses and

*Correspondence to: Elena Yu. Tyulyaeva, email: teu@isc-

ras.ru

USE OF CHEMICAL KINETICS FOR THE DESCRIPTION OF METAL PORPHYRIN REACTIVITY 1041

electronics. In particular, the kinetic study shows a role

reactions, intermediate compounds, and kinetically

signiﬁcant equilibriums were determined.

of the chemically generated cation radicals of metal

porphyrins as intermediates in the reactions catalyzed

by these compounds.

EXPERIMENTAL

Rare kinetics investigations in the field of porphyrin

reactivity are reported every year [3–7]. However

supramolecular assemblies [6], dendrite nanoprobes

[4] or compounds with more complex structures

[7–10] such as J-aggregates, Langmuir–Blodgett films

and covalently linked tetrapyrrole macrocycles were

studied and a rate of a photoinduced electron transfer

or electron transport and electrocatalytic activity [4–7,

11] was determined without a full kinetic description

of processes in most cases. In other works [12, 13] the

progressofareaction,asmeasuredbythedisappearance

of the reagent, was monitored by UV-vis spectroscopy

and by thin layer chromatography without a chemical

kinetics method application. Our recent works

have showed an ample opportunities of kinetic method

for the study of structure-properties relationship

[14–16] and for determining of elementary stages

in a complex chemical process with a fractional,

zero or negative reaction order with respect to a

reagent [17].

Complexes 1–7 were prepared [18, 19] by complexing

of PdCl₂solvated complex with the porphyrin ligand in

dimethylformamide (DMFA). Pd^IIP 3–7 was synthesized

by A. S. Semeikin at the Ivanovo State University of

Chemical Technology. The spectral characteristics of the

compounds were studied in this work or were obtained

earlier [20].

(5,10,15,20-Tetraphenylporphinato)palladium(II)

PdTPP, 1. The complex was prepared as described in

[18]: H₂TPP and PdCl₂in the molar ratio 1:10 were boiled

in DMF until H₂TPP bands disappeared from the UV-vis

spectrum of the reaction mixture. Then, the solvent was

distilled in vacuo, the residue was dissolved in CHCl₃

and puriﬁed by repeated preparative chromatography on

Al₂O₃using CHCl₃. UV-vis (CHCl₃): l_max, nm (log e)

541 (3.92), 511 (4.11), 402 (4.84). IR (KBr): n, cm^-1

benzene ring vibrations, 696, 744 (g C–H); 1071, 1075 (d

C–H); 1485, 1562, 1598 (n C=C); 3040, 3054 (n C–H);

pyrrol ring vibrations, 791 (g C–H); 1016 (C₃–C₄, n C–N,

d C–H); 1312 (n C–N); 1438 (n C=N); 1540 (skeletal

vibrations in pyrrol ring), 2853, 2973 (n C–H); Pd–N

-440, 470. ¹H NMR (CDCl₃): d, ppm 8.81 (t, 8H, C₄H₂N,

J = 3 Hz), 8.16 (t, 8H, o-Ph, J = 3 Hz), 7.75 (m, 8H,

m-Ph, 4H, p-Ph).

The results of the kinetic description of acid-basic and

oxidation reactions of palladium, rhodium, and rhenium

complexes both with 5,10,15,20-tetraphenyl21H,23H-

porphine and of 2,3,7,8,12,13,17,18-octaethyl21H,23H-

porphin substituted at meso-positions with monophenyl,

5,15-diphenyl, 5,10-diphenyl, triphenyl, and tetraphenyl

groups (complexes 1–9) are presented in this work. The

kinetics of the heterogenic catalytic reactions of H₂O₂

(2,3,7,8,12,13,17,18-octaethylporphinato)-

palladium(II) PdOEP, 2. The complex was prepared as

described in [18]. UV-vis (CHCl₃): l_max, nm (log e) 545

(4.75), 511 (4.35), 392 (5.31). IR (KBr): n, cm^-1pyrrol

ring vibrations, 796 (g C–H); 1317, 1305 (n C-N); 1465,

1451 (n C=N); 1554, 1602, 1632 (skeletal vibrations in

decomposition in

a water-dimethylformamide-KOH

system was studied as well. The role of palladium(II)

porphyrin catalysts in a formation of elementary

[O=ReTPP]₂O, 9

M = Pd, n = 2, PdTPP 1

R₁= R₂= R₃= R₄= H, PdOEP, 2

M = Rh, X = Cl, n = 3, (Cl)RhTPP, 8

R₁= Ph, R₂= R₃= R₄= H, PdMPOEP, 3

R₁= R₃= Ph, R₂= R₄= H, Pd^5,15DPOEP, 4

R₁= R₂= Ph, R₃= R₄= H, Pd^5,10DPOEP, 5

R₁= R₂= R₃= Ph, R₄= H, PdTriPOEP, 6

R₁= R₂= R₃= R₄= Ph, PdTetPOEP, 7

J. Porphyrins Phthalocyanines 2012; 16: 1041–1054

1042 T.N. LOMOVA ET AL.

pyrrol ring); ethyl vibrations, 837, 835 (g C–H methine

group); 1273 (d C–H methine group), 2869, 2963 (n

vibrations in pyrrol ring); ethyl vibrations, 833, 844, 874 (g

C–H methine group); 1263, 1274 (d C–H methine group),

2870, 2966 (n CH₃); 2929 (n CH₂); Pd-N -486. ¹H NMR

(CDCl₃): d, ppm 9.62 (d, H, meso); 8.21 (d, 8H, o-Ph),

7.71, 7.62 (d, t, 9H, m-Ph, 4H, p-Ph), 3.68, 2.52, 2.26 (q, q,

q, 16 H, CH₂), 1.60, 0.65, 0.46 (t, t, t, 16 H, CH₃).

1

CH₃); 2929 (n CH₂); Pd-N -484. H NMR (CDCl₃): d,

ppm 10.12 (s, 4H, meso), 4.08 (q, 16 H, CH₂), 1.92 (t,

24 H, CH₃).

(2,3,7,8,12,13,17,18-octaethyl-5-phenylporphinato)-

palladium(II) PdMPOEP, 3. The synthesis was

described in [19]. UV-vis (CHCl₃): l_max, nm (log e) 550

(4.59), 516 (4.34), 400 (5.50). IR (KBr): n, cm^-1benzene

ring vibrations, 710, 763 (g C–H); 1088, 1167 (d C–H);

1450, 1599 (n C=C); 3020, 3057 (n C–H); pyrrol ring

vibrations, 781 (g C–H); 1003 (C₃–C₄, n C–N, d C–H);

1317 (n C-N); 1465 (n C=N); 1500, 1631 (skeletal

vibrations in pyrrol ring); ethyl vibrations, 840 (g C–H

methine group); 1259, 1278 (d C–H methine group),

2870, 2963 (n CH₃); 2929 (n CH₂); Pd-N -486. ¹H NMR

(CDCl₃): d, ppm 10.10, 10.00 (d, d, 3H, meso), 8.17 (d,

2H, o-Ph), 7.63, 7.79 (t, m, 2H, m-Ph, H, p-Ph), 3.97,

2.65 (m, m, 16 H, CH₂), 1.91, 1.81, 1.12, 1.00 (m, m, t,

t, 24 H, CH₃).

(2,3,7,8,12,13,17,18-octaethyl-5,15-diphenylpor-

phinato)palladium(II) Pd^5,15DPOEP, 4. The synthesis

was described in [19]. UV-vis (CHCl₃): l_max, nm (log e)

549 (4.47), 517 (4.28), 403 (5.25). IR (KBr): n, cm^-1

benzene ring vibrations, 705, 764 (g C–H); 1091, 1173 (d

C–H); 1444, 1600 (n C=C); 3020, 3056 (n C–H); pyrrol

ring vibrations, 784 (g C–H); 1003 (C₃–C₄, n C–N, d

C–H); 1317 (n C-N); 1450, 1465 (n C=N); 1498, 1632

(skeletal vibrations in pyrrol ring); ethyl vibrations, 842

(g C–H methine group); 1261, 1279 (d C–H methine

group), 2870, 2963 (n CH₃); 2929 (n CH₂); Pd-N -486.

¹H NMR (CDCl₃): d, ppm 9.79 (s, 2H, meso), 8.20 (d,

4H, o-Ph), 7.63, 7.79 (d, t, 4H, m-Ph, 2H, p-Ph), 3.90,

3.79, 2.54, 2.35 (q, q, q, q, 16 H, CH₂), 1.79, 1.64, 0.68,

0.60 (t, t, t, t, 24 H, CH₃).

(2,3,7,8,12,13,17,18-octaethyl-5,10-diphenylpor-

phinato)palladium(II) Pd^5,10DPOEP, 5. The synthesis

was described in [19]. UV-vis (CHCl₃): l_max, nm (log e)

561 (4.23), 527 (4.30), 412 (5.25). IR (KBr): n, cm^-1

benzene ring vibrations, 705, 758 (g C–H); 1174 (d

C–H); 1443, 1599 (n C=C); 3020, 3056 (n C–H); pyrrol

ring vibrations, 780, 794 (g C–H); 1002 (C₃–C₄, n C–N, d

C–H); 1314 (n C-N); 1464 (n C=N); 1501, 1632 (skeletal

vibrations in pyrrol ring); ethyl vibrations, 839, 856, 880

(g C–H methine group); 1261, 1286 (d C–H methine

group), 2870, 2964 (n CH₃); 2929 (n CH₂); Pd-N -484.

¹H NMR (CDCl₃): d, ppm 9.98 (s, 2H, meso), 8.14 (d,

4H, o-Ph), 7.69, 7.62 (d, t, 4H, m-Ph, 2H, p-Ph), 3.88,

2.61 (q, q, 16 H, CH₂), 1.75, 0.98 (t, t, 24 H, CH₃).

(2,3,7,8,12,13.17,18-octaethyl-5,10,15-triphenylpor-

phinato)palladium(II) PdTriPOEP, 6. The synthesis

was described in [19]. UV-vis (CHCl₃): l_max, nm (log e)

570 (4.38), 536 (4.38), 422 (5.32). IR (KBr): n, cm^-1

benzene ring vibrations, 704, 761 (g C–H); 1089, 1173 (d

C–H); 1443, 1598 (n C=C); 3020, 3057 (n C–H); pyrrol

ring vibrations, 794 (g C–H); 1003 (C₃–C₄, n C–N, d

C–H); 1314 (n C-N); 1465 (n C=N); 1501, 1631 (skeletal

(2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenyl

porphinato)palladium(II) PdTetPOEP, 7. The

synthesis was described in [19]. UV-vis (CHCl₃): l_max

,

nm (log e) 580 (3.92), 544 (4.21), 433 (5.27). IR (KBr):

n, cm^-1benzene ring vibrations, 702, 760 (g C–H); 1085

(d C–H); 1443, 1598 (n C=C); 3021, 3057 (n C–H);

pyrrol ring vibrations, 802 (g C–H); 1003 (C₃–C₄, n

C–N, d C–H); 1317, 1329 (n C-N); 1462 (n C=N); 1501,

1632 (skeletal vibrations in pyrrol ring); ethyl vibrations,

860, 879 (g C–H methine group); 1261 (d C–H methine

group), 2870, 2969 (n CH₃); 2929 (n CH₂); Pd-N -486.

¹H NMR (CDCl₃): d, ppm 8.22 (d, 8H, o-Ph, J = 4 Hz),

7.68 (m, 8H, m-Ph, 4H, p-Ph), 4.2 (d, 16 H, CH₂, J = 24

Hz), 2.4 (d, 16 H, CH₂, J = 23 Hz), 1.95 (d, 24 H, CH₃,

J = 7 Hz), 0.5 (d, 24 H, CH₃, J = 7 Hz).

(5,10,15,20-tetraphenylporphinato)chlororho-

dium(III) (Cl)RhTPP, 8. H₂TPP was boiled with RhCl₃

in a mole ratio of 1:5 in PhCN until the bands of H₂TPP

disappeared from the electronic absorption spectra of

the reaction mixture. After the end of the reaction, the

solvent was distilled in vacuum the reaction mixture was

dissolved in chloroform and chromatographed twice on

a column packed with Al₂O₃(Brockmann activity grade

II) using chloroform. UV-vis (CHCl₃): l_max, nm (log e)

569 (3.79), 535 (4.33), 421 (5.31). IR (KBr): n, cm^-1

phenyl substituents, 700, 749 (g C–H); 1077, 1079 (d

C–H); 1489, 1533, 1575 (n C=C); 3026, 3047 (n C–H);

pyrrole fragments, 788 (g C–H); 1014 (C₃–C₄, n C–N,

d C–H); 1349 (n C–N); 1436 (n C=N); 1540 (skeletal

vibrations of pyrrole ring), 2827, 2907 (n C–H); Rh–N

-457, Rh–Cl –518, 549. ¹H NMR (CDCl₃): d, ppm 8.93

(s, 8H, C₄H₂N, J = 3 Hz), 8.19 (d, 4H, o-Ph, J = 3 Hz),

8.34 (d, 4H, o′-Ph, J = 3 Hz), 7.76 (M, 8H, m-Ph, 4H,

p-Ph).

µ-oxo-bis[(oxo)(5,10,15,20-tetraphenyl21H,23H-

porphinato)]rhenium(V) [O=ReTPP]₂O, 9. The

starting compound H₂ReCl₆was prepared by reaction of

ReO₂with HCl under the heating. H₂TPP was boiled with

H₂ReCl₆in a mole ratio of 1:2 in PhOH until the bands

of H₂TPP disappeared from the electronic absorption

spectrum of the reaction mixture. After the end of the

reaction, the solvent was cooled and the reaction mixture

was extracted in chloroform, multiply washed with warm

watertoremovephenolandchromatographedonacolumn

packed with Al₂O₃(Brockmann activity grade II) using

chloroform. Two zones were obtained: diluted green and

concentrated green. Both zones were rechromatographed.

Green compound was chromatographed on a column

packed with silicagel using ﬁrstly benzene then mixed

solution CHCl₃–C₂H₅OH as eluents. Three individual

zones were separated: (1) orange (Cl)ReTPP (Fig. 1,

J. Porphyrins Phthalocyanines 2012; 16: 1042–1054

USE OF CHEMICAL KINETICS FOR THE DESCRIPTION OF METAL PORPHYRIN REACTIVITY 1043

continuous stirring conditions were determined

volumetrically, by measuring the volume of

oxygen released. An aqueous solution of H₂O₂

(18.1 0.2 M) was added to a solution of a Pd

complex and KOH in DMFA. The concentrations

of complexes, KOH, and H₂O₂were varied

over the ranges 10^-6–10^-4, 0.0018–0.036, and

0.9–7.22 M, respectively. The W values were

determined from the slope of the linear portion

of the V_O2–t dependence optimized by the

method of least squares (Microsoft Excel) with

respect to the positive direction of the abscissa

axis. The activation parameters of the reaction E

and DS^#were determined from the temperature

dependents of W.

Fig. 1. UV-vis spectra in chloroform: (1) (Cl)ReTPP, (2) [O=ReTPP]₂O

and (3) (PhO)ReTPP

The reaction complex formation between

5,10,15,20-tetraphenyl21H,23H-porphin and

H₂ReCl₆in boiling phenol was studied by means of

investigating of intermediates and products structure

with using of the spectral methods. The m-oxo-dimer

[O=ReTPP]₂O dissociation reaction was studied

by spectrophotometrical titration. The equilibrium

constants were calculated using the equation (1), derived

in accordance with the laws of mass action and the

Bouguer–Lambert–Beer for a mixture of colored start

compounds and products [1], where A₀, A_∞, and A_pare the

absorbances of solutions of the starting metal porphyrin,

reaction products, and their equilibrium mixture at the

operating wavelength.

line 1), (2) H₂TPP, and (3) green [O=ReTPP]₂O (Fig. 1,

line 2). Greenish brown zone was chromatographed on

a column packed with silicagel using ﬁrstly benzene

then mixed solution CHCl₃–C₂H₅OH as eluents. Two

individual zones were separated: (1) pink (PhO)ReTPP

(Fig. 1, line 3) and (2) green [O=ReTPP]₂O.

[O=ReTPP]₂O yield — 75%. UV-vis (CHCl₃): l_max

,

nm (log e) 620 (sh), 582 (4.51), 461 (5.34), 333 (4.88).

IR (solid chaotic layer): n, cm^-1phenyl substituents, 702,

754 (g C–H); 1072, 1179 (d C–H); 1485, 1575, 1597 (n

C=C); 2954, 3056 (n C–H); pyrrole fragments, 801 (g

C–H); 1018 (C₃–C₄, n C–N, d C–H); 1341 (n C–N); 1440

(n C=N); coordinating centre, 418 (Re–N); 568, 630, 854

(Re–O–Re); 961, 946 (Re=O). ¹H NMR (CDCl₃): d, ppm

9.07 (d, 8H o-Ph, J = 7.6 Hz), 8.79 (s, 16H, C₄H₂N, J =

3 Hz), 8.01 (t, 8H m-Ph, J = 7.6 Hz), 7.60 (t, 8H p-Ph,

J = 7.6 Hz), 7.47 (d, 8H o′-Ph, J = 7.6 Hz), 7.04, 6.87 (t,

t, 8H m′-Ph, J = 7.6 Hz). ¹H NMR (D₂SO₄): d, ppm 9.13

(m, 8H, C₄H₂N), 8.52, 8.23 (t, t, 8H o-Ph), 8.06 (t, 8H

m-Ph, 4H p-Ph).

A_p- A_o

A_∞- A_o

A_p- A_o

A_∞- A_o

1

K =

⋅

(1)

A_p- A_o

A_∞- A_o







 ⁿ





C_AcOH-C_M^o_P

⋅

1-

RESULTS AND DISCUSSIONS

UV-vis spectra were recorded on an Agilent 8453

UV-vis, Specord M40, and SF-26 spectrophotometers. IR

and ¹H NMR spectra were recorded on Specord M80 and

AV A NCE-500 (Bruker, tetramethylsilan as the internal

standard), Bruker (200 MHz, hexamethyldisiloxane as

the internal standard) spectrometers, respectively.

The kinetics of acid-basic and oxidation reactions

of complexes 1–8 were studied spectrophotometrically.

Solutions were thermostated in a cell installed in the

spectrophotometer and heated with a U4 standard water

thermostat. The error of temperature determination

was 0.1 K. The strength of aqueous sulfuric acid was

determined by acid-base titration with an error within

0.15%.

Kinetics of oxidation reactions of rhodium(III) and

palladium(II) porphyrins by aerated sulfuric acid

The porphyrin complexes 1–8 do not change at the

time and at high temperatures in organic solvents and in

AcOH and show the complex stability in liquid solutions.

In medium containing sulfuric acid rhodium(III)

and palladium(II) porphyrins undergo to irreversible

transformation, which is apparent in evolution of UV-vis

spectra (Fig. 2). The state and reactions of complexes 1–4,

7 in concentrated aqueous sulfuric acid and of complexes

2–4, 7 in mixed solutions AcOH–H₂SO₄was studied

quantitatively in [20]. The interaction between complex

8 and concentrated sulfuric acid was studied in this work.

The UV-vis spectra of complexes sharply change in

strong acid. The spectra of compounds 1, 7 and 8 (Fig. 2,

the bottom line on the right part of ﬁgure) in concentrated

H₂SO₄contain two broad bands in the visible region with

The rates W of the decomposition of H₂O₂of reagent

grade in the presence of complexes 2–7 in the DMFA

(chemically pure)–KOH (chemically pure)–H₂O

(doubly distilled) system at 343–363 K and atmospheric

pressure in a temperature-controlled reactor under

J. Porphyrins Phthalocyanines 2012; 16: 1043–1054

1044 T.N. LOMOVA ET AL.

(Equations 3 and 4) is observed for complexes 1, 7, and

8 in the course of the transformation indicating by the

existence of an isosbestic point between bands at 408

and 545 nm in the set of spectral curves (Fig. 2). For

the palladium complex with H₂TetPOEP its oxidized

-

species was isolated in an unchanged state (with HSO₄as

opposite ion) by reprecipitation from sulfuric acid.

PdP…H⁺…R + O₂+ H⁺→ PdP^+·+ HO₂^·+ H⁺...R (3)

(Cl)RhTPP…H⁺…R

+ O₂+ H⁺→ (Cl)RhTPP^+·+ HO₂^·+ H⁺...R (4)

Analogous process (Equation 3) takes place in the

case of complexes 2–4 in mixed solutions AcOH–H₂SO₄

with C_H2SO= 0.6–8.0 M at 333–363 K and of complex

Fig. 2. The (Cl)RhTPP UV-vis spectrum changes in 16.52 M

H₂SO₄for 22000 at 323 K

4

7 in AcOH – 0.1–1.0 M H₂SO₄at 298 K. However,

the complex 4 react in molecular form and complex 7

undergo oxidation in solvating process.

l

_maxof 538, 690 nm; 562, 710 nm, and 593 (sh), 637 (sh)

Kinetic data presented on Tables 1 and 2, and

Figs 2–4 are described by the third-order or second-

order rate equations (5–8) obtained within the limits

of phenomenological kinetics. The left part of these

equationsisanexperimentalrateofacomplexprocessthat

is proportional with a reacted compounds concentration

in a corresponding degrees (n) determined experimentaly.

At the ﬁrst the order with respect to metal porphyrin with

controlled concentrations was examined. One is equal 1

according to data of Tables 1 and 2. At the second the

orders with respect to acid particles of the solvent were

obtained by the extra concentrations method — due to

accordingly because of the ion-molecule association

of metalloporphyrin with the solvent proton (Equation (2),

R is an acid anion or H₂O). Compounds 2 and 3

have analogous spectra in AcOH-H₂SO₄with C_H2SO

=

4

0.6–8.0 M.

K

PdP + H⁺…R

PdP…H⁺…R.

(2)

 →

← 

Partial proton transfer from the solvent to the complex

occurs at the C_mesoatom of metalloporphyrins [21]. This

interaction is not full proton transfer but the hydrogen

bonding without aromatic system decomposition. High

speciﬁc electron spectra of aromatic metal porphyrin

H⁺-associates were obtained for a lot of porphyrin

complexes [21]. The rhodium(III) and palladium(II)

complexes studied in this work, accordingly with their

chloro-ligand and 4d⁸electronic conﬁguration, satisfy

the conditions required for association with proton

in strong acid media containing sulfuric acid — the

presence an electron donor ligand or an Pd–N dative p

component in the coordination interaction and in both

cases — the absence electron–acceptor substituents in

the macrocycle and phenyl rings [21]. In mixed solutions

AcOH–H₂SO₄weak acid media two electron–acceptor

phenyl substituents (complex 4) are enough in order to

H⁺-associate formation was not probably.

Spectral study of the complexes 2–4 at 298 K and 1,

7 and 8 at high temperatures showed that the reaction

product in concentrated H₂SO₄for both group of

complexes is the precursor complex oxidized in the

aromatic macrocycle (the p-cation radical PdP^+·). This

complex form is identiﬁed by the characteristic wide-

band electronic spectrum between 500 and 700 nm

and the band with a peak at ≈ 750 nm (Fig. 2, the top

line on the right part of ﬁgure). This UV-vis spectrum

was obtained for the product of one electron chemical

oxidation of PdTPP in CH₂Cl₂[22]. The transition of

the precursor complex into its p-cation radical PdP^+··

a relation logk_obsvs. logC⁰

(in AcOH or in H₂O)

H SO

2

with the slope ratio equal to the⁴reaction order n (Figs 3

and 4). One allowed to determine the rate constants for

the reactions (3 and 4), named a kinetic coefﬁcient in a

case of the complex reactions (Table 3), and to discuss

the elementary reactions in the transformations of metal

porphyrin complexs after obtaining the additional data

about reactions (no reversibility, a product nature, oxygen

absence inﬂuence).

The rate low for complexes 1 and 7 in concentrating

sulfuric acid is:

–dC_PdP/dt = k × C_PdP× [H₂SO₄]²,

(5)

(6)

(7)

for complexes 2 and 3 in AcOH–H₂SO₄

2

–dC_PdP/dt = k × C_PdP× (C⁰_H2SO

)

4

for complex 4 in AcOH–H₂SO₄

–dC_Pd5,15_DPOEP/dt = k × C_Pd5,15_DPOEP× C⁰_H2SO

4

The rate low for complex 8 in concentrating sulfuric

acid analogues to Equation (5) is:

dC_(Cl)RhTPP/dt = k × C_(Cl)RhTPP^.× [H₂SO₄]²

Here [H₂SO₄] and C⁰

are equilibrium and

(8)

H SO

2

4

initial sulfuric acid concentration. Table 3 lists the rate

constants and activation parameters for the reactions of

PdTPP and PdTetPOEP with H₂SO₄found in the course

J. Porphyrins Phthalocyanines 2012; 16: 1044–1054

USE OF CHEMICAL KINETICS FOR THE DESCRIPTION OF METAL PORPHYRIN REACTIVITY 1045

Table 1. Effective kinetic parameters^aof the oxidation reaction of PdMPOEP or Pd^5,15DPOEP in

mixed solutions AcOH–H₂SO₄

C_H^O,M

T, K

k_obs× 10⁵, s^-1

E_obs, kJ.mol^-1

-DS^#_obs, J.mol^-1.K^-1

₂SO₄

PdMPOEP

348

353

358

348

353

358

1.60 0.15

2.3 0.2

3.5 0.3

2.0 0.2

2.8 0.2

4.28 0.40

2.79

81

79

4

6

111 13

115 20

3.22

3.65

4.07

348

353

358

2.7 0.1

3.8 0.3

5.6 0.7

75

4

2

124 13

348

353

358

3.3 0.2

4.7 0.3

75

68

124

141

7

6.85 0.60

348

353

3.9 0.3

5.4 0.3

4.50

Pd^5,15DPOEP

2.40

333

338

343

333

338

343

1.63 0.10

1.93 0.15

2.18 0.20

2.12 0.20

2.5 0.15

2.8 0.2

28

27

4

3

260 13

260 10

3.08

3.43

4.11

333

338

343

2.54 0.24

2.95 0.20

3.4 0.3

27

23

1

2

259

269

3

7

333

338

343

3.15 0.20

3.60 0.2

4.03 0.35

^aObserved rate constant (k_obs), activation energy (E_obs) and activation entropy (-DS^#_obs) calculated

from log k–1/T plot.

of the least-squares ﬁt (logk^T_obsvs. log [H₂SO₄]). The

in H₂SO₄(Equations 9, 10); rate Equation (5) can only be

interpreted on the assumption that the associated proton

(Equation 2) is involved in the reaction and R = HSO₄^-:

presentation in log k^T_obs–log[H₂SO₄] coordinates uses

the H₂SO₄equilibrium concentration in strong aqueous

sulfuric acid; therefore, H₂SO₄ionization preequilibrium

has already been included into Equation (5).

PdP…H⁺…R + O₂→ (O₂)PdP + H⁺…R, fast (9)

(O₂)PdP + H₂SO₄→ PdP^+·+ HO₂^·+ HSO₄^-, slow (10)

The rate lows (5–8) are analogous to dissociation

reactions of some metal porphyrin coordination centre.

However the transformations of rhodium and palladium

complexes in H₂SO₄are not the reaction leading up

to complex dissociation. The proton of the H⁺-associated

complex species is not involved in the dissociation

of porphyrin complexes [24, 25]. Other mechanisms

operate in the transformations of investigating complexes

Writing rate equation for slow reaction (10) with

+

C_{(O )PdP}equal to C_{PdP…H …R}, so K × C_PdP× C_H

(K is

…R

2

equilibrium constant of H⁺-associate formation (2))

give the third-order rate Equation (5) in which k = k_slow

K. Thus, PdTPP and PdTetPOEP in strong sulfuric acid

experience multistage oxidation. Kinetically signiﬁcant

J. Porphyrins Phthalocyanines 2012; 16: 1045–1054

1046 T.N. LOMOVA ET AL.

Table 2. Effective kinetic parameters^aof the oxidation reaction of PdTetPOEP in concentrated

H₂SO₄

[H₂SO₄]^b, M

T, K

k_obs× 10⁴, s^-1

E_obs, kJ.mol^-1

-DS^#_obs, J.mol^-1.K^-1

C_H^O,M

₂SO₄

17.48

17.74

17.86

18.03

18.23

11.096

11.646

11.630

11.606

11.573

298

343

353

358

363

(1.4 0.5)·10^-3

0.53 0.02

1.40 0.03

2.35 0.10

4.0 0.1

104

103

104

102

4

5

3

2

32 12

12.406

12.875

12.836

12.803

12.760

298

343

353

358

363

(2.1 0.4)·10^-3

0.64 0.02

1.64 0.05

2.8 0.1

33 15

4.7 0.2

12.504

13.462

13.414

13.376

13.330

298

343

353

358

363

(2.8 0.6)·10^-3

0.68 0.01

1.89 0.06

3.1 0.1

30

35

33

9

6

5.1 0.1

13.753

14.084

14.028

13.987

13.938

298

343

353

358

363

(4 1)·10^-3

0.77 0.01

2.17 0.06

3.4 0.2

5.4 0.2

14.551

15.208

15.151

15.106

15.053

298

343

353

358

363

(4.4 0.9)·10^-3

0.92 0.02

2.45 0.10

4.10 0.15

6.6 0.2

^aObserved rate constant (k_obs), activation energy (E_obs) and activation entropy (-DS^#_obs) calculated

from logk–1/T plot; ^bequilibrium sulfuric acid concentration from [23].

are the rate-controlling stage of one-electron oxidation of

pK_a= 4.3 or 4.5 according to [26] and [27]. The

equilibrium (13) is displaced probably to the right because

of the big surplus of oxygen in comparison with PdP. It is

obvious, that coordination of molecule O₂on the Equation

(13) is accompanied by destruction H-associate PdP…

H⁺…R in reaction of complexes 2 and 3. Slow reaction

(14) passes at action of H₂SO₄molecules, as acid particle

with the highest concentration in the reaction mixture.

The rate equation can be written down in view of

equality C_H⁰_SO= [H₂SO₄] + [AcOH₂⁺] and [H₂SO₄]

metalloporphyrin by coordinated dioxygen with the help

of H⁺and partial protonation of the macrocyclic complex

(Equation 2).

The oxidation reaction, for complexes 2 and 3 in

AcOH–H₂SO₄analogous with reaction (3), for complex

4 is:

PdP + O₂+ H⁺→ PdP^+·+ HO₂^·

(11)

The scheme of the plain reactions for this case is:

two preequilibriums and slow reaction of one-electronic

oxidation

2

4

~ C_H⁰_SO(because of the small value of pK_a) by the

2

4

Equations (15) and (16) for complexes 2, 3 and for

complex 4 correspondently. The ﬁrst equality have

been written in view of equilibrium (13) — how many

molecules of (O₂)PdP have reacted in (14) during dt,

so much these have obtained (so much PdP molecules

have reacted) according to (13) [1]. The second equality

is a mathematical expression of the law of mass action

H₂SO₄+ AcOH ↔ AcOH₂⁺+ HSO₄^-;

K_a= [AcOH₂⁺] × [HSO₄^-]/[H₂SO₄]

PdP + O₂↔ (O₂)PdP

(12)

(13)

(14)

(O₂)PdP + H₂SO₄→ PdP^+·+ HO₂^·+ HSO₄^-, slow

J. Porphyrins Phthalocyanines 2012; 16: 1046–1054

USE OF CHEMICAL KINETICS FOR THE DESCRIPTION OF METAL PORPHYRIN REACTIVITY 1047

toward coordination center dissociation because of easy

p-cation-radical formation. The modiﬁcation of meso-

positions of coordinating ligand can be considered as

a way of change of electron donating properties of the

complexes important in oxidation reactions and partial

carry of a proton from the medium.

Alkaline decomposition of hydrogen peroxide in a

water-dimethylformamide medium catalyzed by

palladium(II) porphyrins

The kinetic and spectral description of alkaline

decomposition of H₂O₂in a water-dimethylformamide

medium catalyzed by Pd^IIP (complexes 2–7) is an

example of successful research of the chemical reaction

route (stoicheometric mechanism).

Table 3. Kinetic parameters of the oxidation reaction of PdOEP,

Fig. 3. Dependence of observed rate constant (log k^T_obs) on the

PdMPOEP, Pd^5,15DPOEP in mixed solutions AcOH–H₂SO₄and

PdTPP, PdTetPOEP and (Cl)RhTPP in concentrated H₂SO₄

initial concentration of H₂SO₄(logC⁰_{H SO}) for PdOEP inAcOH–

2

4

H₂SO₄with logC⁰_{H SO}= 0.9 ÷ 1.4 M at temperatures (1) 353,

4

(2) 358, (3) 363 K.²R²= 0.98 ÷ 0.99

T, K

k^a× 10⁶, M^-2.s^-1

E^a,b, kJ.mol^-1

-DS^{# a,b}

,

J.mol^-1.K^-1

PdTPP

343

348

353

298

1.05 0.16

1.5 0.2

1.9 0.2

4.6 ^.10^-2

60

5

195 20

PdTetPOEP

343

353

0.39 0.01

1.05 0.03

1.74 0.03

2.87 0.04

1.83 ^.10^-3

104

3

70 10

358

363

298^b

PdOEP

298^b

353

358

363

1.15

100

138

191

70

1

7

5

131

3

Fig. 4. Dependence of observed rate constant (logk_obs) on

equilibrium concentration of H₂SO₄(log C_{[H SO ]}) for the reaction

2

4

of (Cl)RhTTP in H₂SO₄at temperatures (1) 328, (2) 323, and

(3) 318 K. R²are equal 0.98, 0.99, and 0.98, respectively

PdMPOEP

298^b

348

353

358

0.008

2.2

3.3

91

102 23

254 16

-17^d

[1] for limiting elementary reaction (14). In next the

equalities C_{(O )PdP}and [H₂SO₄] are expressed according to

5.3

2

equilibrium transformations (12) and (13).

PdD^5,15POEP

298^b

1.2^c

5.5^c

6.8^c

7.8^c

33

–dC_PdP/dt = –dC_{(O )PdP}/dt

2

333

338

343

= k_slow× C_{(O )PdP}× [H₂SO₄]

2

= k_slow× K^–1× K × C_PdP× C_{H …R}× [H₂SO₄]

+

a

= k_slow× K^–1× K × C_PdP× (C⁰_{H SO}

)

4

(15)

(16)

2

a

2

(Cl)RhTPP

298

318

323

328

0.4^d

2.0

2.5

8.0

120^d

–dC_Pd5,15_DPOEP/dt

= –dC_{(O )Pd}5,15_DPOEP/dt

= k_slow×²C_{(O ) Pd}5,15_DPOEP× [H₂SO₄]

2

= k_slow× C_Pd5,15_DPOEP× C⁰_H2SO

4

^aKinetic coefﬁcient (k) in Equations (5–8), activation energy

(E) and activation entropy (-DS^#); ^bfound by extrapolation of the

The high stability of (Cl)RhTPP and PdP towards

acids and oxidation of their aromatic part are caused by

the strong sp-bonding of the central atom. According

to obtained data these aromatic complexes are stable

logk–1/T plot; M^-1.s^-1; are represented rough values because

of appreciable differences of the reaction order at various

temperatures from exact value n = 2 (Equation 8).

c

d

J. Porphyrins Phthalocyanines 2012; 16: 1047–1054

1048 T.N. LOMOVA ET AL.

Table 4. Rate of H₂O₂decomposition (W) catalyzed by palladium(II) porphyrins at various

alkaline (KOH) and hydrogen peroxide (H₂O₂) concentrations at T = 343 K

Complex

PdOEP

C_KOH, M

W^a, mL_O2.min^-1

W^b, mL_d2.min^-1

C_H^O,M

₂SO₄

0.0018

0.018

0.036

0.37 0.02

2.20 0.17

3.58 0.16

0.902

1.805

3.61

5.415

7.22

0.30 0.02

0.60 0.03

2.20 0.17

4.62 0.17

7.78 0.25

PdMPOEP

0.902

1.805

3.61

5.415

7.22

0.30 0.03

0.60 0.02

1.59 0.09

3.19 0.08

6.99 0.23

Pd^5,15DPOEP

Pd^5,10DPOEP

0.018

1.53 0.14

1.56 0.11

1.82 0.14

0.902

1.805

3.61

5.415

7.22

0.40 0.05

0.75 0.06

1.51 0.06

3.28 0.13

6.43 0.15

PdTriPOEP

PdTetPOEP

0.0018

0.018

0.036

0.42 0.01

1.40 0.09

2.76 0.07

0.902

1.805

3.61

5.415

7.22

0.50 0.04

0.80 0.05

1.63 0.07

3.66 0.12

6.96 0.25

^aC_H₂_O= 3.61 M, C_PdP^.10⁵, M: 2.15, 2.80, and 0.34, 3.40, 13.6 for PdOEP, PdTriPOEP, and

2

Pd^5,10DPOEP, ^bC_PdP^.10⁵, M: 2.15, 2.82, 2.90, and 2.10 for PdOEP, PdMPOEP, Pd^5,10DPOEP,

and PdTetPOEP.

dC_O/dτ = k¢ × (C_PdP)⁰× (C_{H O})²×C_KOH× (C_O)⁰

Decomposition process was studied over a wide

2

range of reagent concentrations. The results of the

study of PdP catalytic activity are presented in Table

4 and Figs 5 and 6. These data show that W increases

with growing of the concentrations of KOH and H₂O₂.

In all cases, there is a satisfactory linear correlation

between the logarithm of W and the concentration of

KOH with a slope close to one. It is found that logW

(18)

= k¢ × (C_{H O})²×C_KOH

2

linearly depends from logC_{H O}in two regions of H₂O₂

2

concentrations, 0.9–3.6 and ²3.6–7.2 M H₂O₂(Fig. 6).

The order of the reaction in C_{H O}is close to 1 and 2,

2

respectively. At last the reaction order with respect

to the concentration of the catalysts is close to zero

because the rate W is almost independent of C_PdP. Zero

reaction order with respect to PdP is evidence of a

heterogeneous catalytic process.

Named results are in line with the total second-order

and third-order kinetic equations for the ranges C_{H O}

=

2

0.9–3.6 and 3.6–7.2 M:

Fig. 5. Dependences of the rate of H₂O₂decomposition (W)

on the concentration of catalyst (C_cat) (1) PdTetPOEP, (2)

Pd^5,15DPOEP, (3) Pd^5,10DPOEP, (4) PdMPOEP, (5) PdTriPOEP

at H₂O₂concentration 3.61 M and KOH concentration 0.018 M

at temperature 343 K

dC_O/dτ = k × (C_PdP)⁰×C_{H O}×C_KOH× (C_O)⁰

2

(17)

= k ×C_{H O}×C_KOH

2

J. Porphyrins Phthalocyanines 2012; 16: 1048–1054

USE OF CHEMICAL KINETICS FOR THE DESCRIPTION OF METAL PORPHYRIN REACTIVITY 1049

by optimizing of the same dependence and the logW-

logC_KOHdependence (complex 2) (Table 5).

In the electronic absorption spectra of freshly

prepared mixtures PdP–DMFA–H₂O–KOH, when gas

release is insigniﬁcant and there is no precipitate, the

Q(0, 0) and Q(0, 1) absorption bands of the initial

PdP complexes in DMFA with maxima at 508–545

and 540–581 nm, respectively (Fig. 7a), experience

broadening and bathochromic shifts by 3–23 nm (Fig.

7b). An increase in overall absorption background

and the appearance of absorption in the longwave

region and visible spectrum region adjacent to the

UV range are observed. Such electronic absorption

spectra are characteristic of p radical cation forms

of metalloporphyrins obtained by chemical and

electrochemical oxidation [27, 28]. After gas release

terminates, complexes from a solution of 2, 3 and 4

heterogeneous systems were isolated into CHCl₃, and

these solutions were thoroughly washed from DMFA,

KOH, and H₂O₂. The electronic absorption spectra of

solutions in chloroform contained bands of initial PdP.

Changes in the spectra of PdP in the PdP–DMFA–

H₂O–KOH–H₂O₂system compared with its solution in

DMFA can therefore be explained by the formation of

p radical cation forms PdP^+•.

Fig. 6. Dependences of the rate of H₂O₂decomposition

(lgW) on the H₂O₂concentration (lgC_{H O}) in the presence

2

of catalyst PdMPOEP with concentration ²2.82 × 10^-5M and

KOH concentration 0.018 M at temperature 343 K. R²

=

0.995 and 0.999 to left and to right broken lines, R²= 0.992

to solid line

Table 5. Rate constants of H₂O₂decomposition^acatalyzed by

palladium(II) porphyrins at 343 K

Complex

k′, mL_O.l². mol^-1.

k′′, mL_O.l³.min^-1.

2

min^-1

14.5 1.8

18.1

mol^-3

4.5 0.9

5.07

The elementary reactions in the complex

transformations of H₂O₂(Equation 19) showed on the

Scheme 1 explain the kinetic Equations (17) and (18).

PdOEP

PdMPOEP

Pd^5,10DPOEP

PdTetPOEP

24.3

5.7

2H₂O₂→ H₂O + O₂

(19)

29.2

6.2

^aKinetic coefﬁcients in Equation (17) (k) and (18) (k′), respectively.

In order to maintain the nature of k and k′ we can use

the relations like in the case of kinetics of rhodium(III)

and palladium(II) porphyrins oxidation reactions. A

rate low for limiting stages we will write in view of

fast irreversible reactions and previous equilibrium

(Scheme 1). The kinetic equations of slow stages (20, 21)

in the scheme are written as Equations (22) and (23).

The main rate constants k and k′ for PdP at T = 343 K

were found by optimizing of the logW–C_{H O}dependence

2

(complexes 3, 5, 7) or from two valu²es for each

constant determined in two independent experiments

Fig. 7. UV-vis spectra of (1) PdMPOEP and (2) Pd^5,15DPOEP in (a) DMFA and (b) DMFA–H₂O–KOH–H₂O₂mixture with H₂O₂

concentration 3.61 M and KOH concentration 0.018 M at temperature 298 K

J. Porphyrins Phthalocyanines 2012; 16: 1049–1054

1050 T.N. LOMOVA ET AL.

Scheme 1. The transformations in PdP–DMFA–H₂O–KOH–H₂O₂system for the intervals (a) C_{H O}= 0.9–3.6 M and (b) C_{H O}=

2 2

2

3.6–7.2 M

 ^k⁴→



^k



^'^s

→

(30)

-

(O^2-)PdP^+·+ HO₂^-

(HO₂^-)(O^2-)PdP^+·

PdP + OH + O₂, slow (20)

k′ = k₅× K₁× K₃× K₄

PdP + OH^-+ O₂, slow (21)

Thus catalysis with PdP follows a more complex

scheme than simple decomposition of H₂O₂in an

alkaline medium. At the initial stages (before the radical-

ion mechanism begins to operate), we have a quasi-

equilibrium system with acid-base catalysis [29],

-dC_HO_-/dτ = dC /dτ

O₂

2

= k₄×C_(O_2-_)PdP_+·×C_HO_-

(22)

(23)

2

= k₄× K ×C_PdP×C_{H O}×C_OH_-

2

H₂O₂+ OH^-↔ HO₂^-+ H⁺

2HO₂^-→ 2OH^-+ O₂

(31)

(32)

-dC_(HO_-

/dτ = dC /dτ

)(O^2-)PdP^+·

O₂

2

= k₅^¢×C

(HO₂^-)(O^2-)PdP^+·

Equation (22) for limiting stage on Scheme 1a is

identical to experimental Equation (17). It is possible to

write down following equality

According to [29], the rate of such a reaction increases

up to the complete shift of equilibrium (31) to the

right and then decreases. In experiments with PdP, the

concentration of OH^-is much lower than C_{H O}(Table 4),

2

and the rate of the decomposition of H₂O₂²increases

for all PdP as grows; that is, all the experiments were

performed in the region preceding maximum rate rise.

Within the ﬁrst range of concentrations, the PdP–

H₂O₂–KOH system (Scheme 1a) does indeed work as

a quasi-equilibrium system. The coordination of the

second peroxide molecule (in the form of HO₂^-), which

is a slow and irreversible reaction at low concentrations

(Scheme 1a), occurs rapidly to equilibrium state with

K₄(Scheme 1b), and only then the product slowly

decomposes with the regeneration of PdP and oxidation

in a two-electron redox process, which ends by releasing

gaseous O₂.

C

= K₄×C_(O_2-_)PdP_+·×C

= K₃×C_{H O}×C_OH_-

(HO₂^-)(O^2-)PdP^+·

HO₂^-

(24)

(25)

C

HO₂^-

2

C_(O_2-

= C

,C

(H₂O₂)PdP (H₂O₂)PdP

)PdP^+·

= K₁×C_PdP×C_{H O}

(26)

(27)

2

C_(HO

= K₄× K₁× K₃×C_PdP× (C_{H O})²×C_OH_-

₂)(O^2-)PdP^+·

2

dC_O/dt = k₅× K₄× K₁× K₃×C_PdP× (C_{H O})²×C_OH_-

(28)

2

Equation (23) for limiting stage on Scheme 1b is

identical to experimental Equation (18). Thus kinetic

coefﬁcients k and k’ include the thermodynamic constants

of kinetic signiﬁcant equilibriums:

Thus the ion-molecular mechanism of hydrogen

peroxidedecompositionwassubstantiatedusingchemical

kinetics method. According to this mechanism, the only

stable radicals involved in the reaction are the p radical

cation forms of palladium(II) porphyhrin catalysts.

k = k₄× K

(29)

J. Porphyrins Phthalocyanines 2012; 16: 1050–1054

USE OF CHEMICAL KINETICS FOR THE DESCRIPTION OF METAL PORPHYRIN REACTIVITY 1051

Fig. 8. Absorption spectra of [O=ReTPP]₂O on addition of AcOH in benzene and curves of the spectrophotometric titration at

l

_max= 461 nm; C_AcOH= 2.92 × 10^-4–1.30 × 10^-2M (the broken lines and (a); C_AcOH= 1.30 × 10^-2–2.92 M (the solid lines and (b), 298 K

The reversible dissociation of rhenium m-oxo-dimer

The reaction of [O=ReTPP]₂O in benzene-AcOH

was studied at 298 K in wide AcOH concentration range

(2.92 × 10^-4–2.92 M). It is shown that the presence of

protons allows to reversible µ-oxo bridge destruction.

The electronic absorption spectra of [O=ReTPP]₂O in

benzene in the presence of various amounts of AcOH

and according spectrophotometric titration curves are

shown in Fig. 8. Two spectral series changes are as the

content of AcOH increases. These series with C_AcOH

=

2.92 × 10^-4–1.30 × 10^-2M and C_AcOH= 1.30 × 10^-2–2.92

M include:

— bands of increasing intensity with l_max= 333 nm

and bands of decreasing intensity with l_max= 461 nm

(Fig. 8,a);

— bands of increasing intensity with l_max= 333 nm

and batochromic displacement of band from 461 to 498

nm (Fig. 8,b).

A_p- A_O

A_∞- A_p

Fig. 9. Dependence of indicator ratio (

) on AcOH concen-

log

tration (log C_AcOH) for the reaction between [O=ReTPP]₂O

and AcOH in benzene at the (1) ﬁrst and the (2) second stages.

R²= 0.99

These spectral results and the shape of the

spectrophotometric titration curves show that the reaction

occurs in two stages. The reversibility of these stages was

substantiated by the study of changes in the electronic

absorption spectrum as the concentration of AcOH

decreased. The stoichiometry for the stepwise reactions

was established using the curves of Equation (1) in a

logarithmical form (Fig. 9).

At low AcOH concentrations the destruction of

m-oxo-bridge takes place and the equilibrium constant is

K₁= (5.4 1.0) × 10²M^-1. It was found experimentally

that the equilibrium at the ﬁrst stage was established

in approximately 5min and characterized by the

stoichiometry 1:1 (Fig. 9, line 1 (tan a = 1.2)). These data

allow to the equation for the ﬁrst reaction step in the form;

The second stage (34) of the reaction is observed at

AcOH concentrations from 5.83 × 10^-2to 2.92 M and

characterized by slow batochromic displacement of

band from 461 to 498 nm. The equilibrium constant is

K₂= (35 7) M^-2. Two AcOH molecules take part in the

reaction (Fig. 9, line 2 (tan b = 1.9)).

O=Re(AcO)TPP + O=Re(OH)TPP

K₂

←→

+ 2AcOH

O=Re(AcO)(AcOH)TPP

+O=Re(AcO)(H₂O)TPP (34)

It is obvious that constant K₂represents a passing

of the reactions in the ﬁrst coordination sphere —

coordination of AcOH by O=Re(AcO)TPP and

O=Re(OH)TPP in the molecular and anion form

K₁

←→

[O=ReTPP]₂O + AcOH

O=Re(AcO)TPP

+ O=Re(OH)TPP

(33)

J. Porphyrins Phthalocyanines 2012; 16: 1051–1054

1052 T.N. LOMOVA ET AL.

Fig. 10. Selected electronic spectra obtained during the titration of [O=ReTPP]₂O with AcOH at 293 K. C_AcOH, M: (a) 2.92 × 10^-4,

(b) 0.29, (c) 14.5. The broken lines show the calculated absorption based on the best ﬁt

--------- [O=ReTPP]₂O; –·–·–·–· O=Re(OH)TPP; –··–··–··–· O=Re(AcO)TPP. Absorption maxima are calculated using a Gaussian

peak deconvolution

correspondently. The protonation of OH^-ligand takes

place as well (34).

ﬁnal compound (CDCl₃+ 5 M AcOH) conﬁrming OH^-

ions are absent in the coordination sphere of complex.

From EAS data (Fig. 10) it is important that the further

increasing of AcOH concentration (3 M and above) does

not change the compound EAS. The ﬁnal spectrum by

titration and the spectrum of [O=ReTPP]₂O in 100%

AcOH are analogies.

The composition of the coordination sphere of

stepwise reaction products with AcOH was conﬁrmed

1

by the EAS (Fig. 10) and H NMR methods. There

1

is proton signal of OH^–at d = -2.5 [30] in H NMR

spectrum of [O=ReTPP]₂O in intermediate area of

acetic acid concentration (spectrum in CDCl₃+ 0.38 M

Only the ratio of kinetic coefﬁcients of direct

1

AcOH). Those disappears in H NMR spectrum of the

and return reactions can be obtained for the Re^v

J. Porphyrins Phthalocyanines 2012; 16: 1052–1054

USE OF CHEMICAL KINETICS FOR THE DESCRIPTION OF METAL PORPHYRIN REACTIVITY 1053

complex transformations because of a fast equilibrium

scientiﬁc-pedagogical Staff of innovative Russia” (2009–

2013, Action 1.1 - II turn).

establishment. The study of the mechanism is not

obviously possible without the kinetic description of

reactions.

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concentrations the destruction of m-oxo-bridge takes

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J. Porphyrins Phthalocyanines 2012; 16: 1054–1054

Article Doi

Use of chemical kinetics for the description of metal porphyrin reactivity

DOI: 10.1142/S1088424612500769

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Article abstract of DOI:10.1142/S1088424612500769

Full text of DOI:10.1142/S1088424612500769

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