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Russ.Chem.Bull., Int.Ed., Vol. 56, No. 4, April, 2007
Lomova et al.
catalysts). The electronic absorption spectra of the comꢀ
plexes in DMF remain virtually unchanged upon the adꢀ
dition of KOH or H2O2. Sharp changes in the spectra of
these complexes recorded before gas evolution are obꢀ
served only when KOH and H2O2 are added simultaꢀ
neously. For the CuDPOEPꢀcatalyzed reaction, the specꢀ
trum changes similarly to those described above for the
heterogeneous systems (see Fig. 5, curves 5 and 6). The
EAS of the reaction mixture containing CuTPP cannot
be recorded because of vigorous oxygen evolution. Howꢀ
ever, after CuTPP was removed from the reaction mixꢀ
ture by extraction with chloroform, the EAS exhibit the
catalytic reactions confirms the nature of rateꢀdeterminꢀ
ing step (7) with the transition state in which the reactant
HO2– is axially coordinated on the catalyst.
Thus, the catalytic activity of the studied metal porꢀ
phyrins in the decomposition of H2O2 is due to the ability
of the copper atom in the complex to additionally coordiꢀ
nate the ligands (coordination number 5), as in functionꢀ
ing of the natural catalase1, and low ionization potentials
of the coordinated porphyrin macrocycle. As supposed
above, based on the spectrophotometric data, the inertꢀ
ness of CuTPP and CuDPOEP in the reaction with H2O2
(in the absence of KOH) is actually related to their low
ability to axial coordination. Weak retention of the O2–
ligand in the (O2–)CuP+• πꢀradical complex eliminated
during reduction in the slow step (8) decreases the activaꢀ
tion energy in the case of the complexes CuTPP and
CuDPOEP (see Table 5) compared to other metal porꢀ
phyrins.
absorption of the initial CuTPP (λI
= 538.5 nm) and
max
its πꢀradical cation form (λI
= 700 nm and λII
=
max
max
664 nm). It can be seen from the spectral data that CuTPP
and CuDPOEP do not react with H2O2 in the absence
of KOH, unlike CuOEP and CuTPOEP and similar
manganese(III) complexes studied earlier.10 As will be
shown below, this difference in reactivity can be explained
by variable stability of the axial CuP complexes with an
H2O2 molecule.
For the systems studied, the kinetics of the process
can be interpreted taking into account elementary reacꢀ
tions of coordination of H2O2 on the central atom of the
catalyst molecule and then its decomposition with twoꢀ
electron oxidation ((4), (5)), equilibrium of acidic dissoꢀ
ciation of hydrogen peroxide under the action of alkali (6),
and catalyst reduction to the initial state by the reaction
with HO2– (7)
Among the studied metal porphyrins, CuTPP and
CuDPOEP exhibit substantial catalytic activity in H2O2
decomposition (see Table 2). They increase the reaction
rates and decrease the activation energy 3—4ꢀfold comꢀ
pared to that in the noncatalytic process. The activity of
the complexes CuOEP and CuTPOEP is comparable with
that of the copper salts. Under experimental conditions
(see Tables 2—5), the conversion of H2O2 is low. After
the end of the reaction (after the time interval for which
the kinetic curve remains linear), ∼90% of H2O2 remain
in the system, which is confirmed by iodometric titration.
The study of the influence of addition of copper porphyꢀ
rin to the mixture after the end of the reaction (in the
region of kinetic curve saturation) for the heterogeneous
process showed the absence of a change in the rate of
H2O2 decomposition. Similarly addition of H2O2 to the
reaction mixture resumes gas evolution with a rate someꢀ
what lower than the initial rate of the process. This indiꢀ
cates that the catalyst undergoes no destruction during
the catalytic process, unlike the catalyst (Cl)MnTPP.10
In the latter case, the reaction proceeds under homogeꢀ
neous conditions, which allows one to compare the reꢀ
sults for CuTPP and (Cl)MnTPP. The reaction of peroxꢀ
ide decomposition catalyzed by the copper complex ocꢀ
curs twice as fast. Under the same conditions of the reacꢀ
CuP + H2O2
(H2O2)CuP
(H2O2)CuP,
(4)
(5)
(O2–)CuP+• + H2O,
H2O2 + OH–
HO2 + H2O,
(6)
(7)
–
(O2–)CuP+• + HO2
CuP + OH– + O2.
–
i. Fast, ii. slow.
A kinetic equation (8) for the rateꢀdetermining step (7)
can be written in the form
tion (T = 298 K, СMP = (10.6—11.6)•10–5, CKOH
(1.8—1.9)•10–2, and CH O = 3.98 mol L–1), the value
=
2
2
–dCHO –/dτ = dCO /dτ = k4C(O
–
+•CHO – =
)CuP
2
2
2
2
of W and the activation energy E for the Cu and Mn comꢀ
plexes are equal to (5.4 0.8), (2.6 0.1) mL of O2 min–1
and (25 3), (47 1) kJ mol–1, respectively. The enꢀ
hanced catalytic activity of CuTPP compared to that of
(Cl)MnTPP is probably due to the absence of steric hinꢀ
drance for coordination of an H2O2 molecule at both
sides of the macrocycle plane.
Thus, the results of studies on EAS and kinetics of the
reactions in the CuP—KOH—H2O2 system indicate the
ionꢀmolecular mechanism of H2O2 decomposition cataꢀ
= k4KCCuPCH O COH–.
(8)
2
2
In the case of the heterogeneous reaction, the term
C(O does not enter into kinetic equation (8).
–
+
•
)CuP
2
A comparison of Eqs (2) and (8) and the data in Table 5
show that the proposed scheme of reactions (4)—(7) agrees
well with the experimental kinetic equation (2) of H2O2
decomposition and the activation parameters; in this case,
the rate W = k4K. The negative activation entropy for the
Lomova
