Spectral and kinetic investigation on oxidation and reduction of water soluble porphyrin-manganese(III) complex

Article abstract of DOI:10.1016/j.ica.2009.12.015

In this work the oxidation and reduction reactions of Mn^III-Coproporphyrin-I (Mn^III-CPI) have been studied and four forms of manganese-CPI complexes have been characterized. This complex was observed to be highly reactive (at basic pH) towards Mn(II), hypochlorite, hydrogen peroxide and oxone, forming [Mn^IV(O)CPI(OH)]^- that was unstable and, after a short time, formed again [Mn^IIICPI(OH)₂]^-. With an excess of NaClO, a further oxidation of the complex [Mn^IV(O)CPI(OH)]^-, provoked a significant spectral change for the [Mn^V(O)CPI(OH)] formation that showed, in the time, a partial polymerization. [Mn^IIICPI(OH)₂]^- was reduced by sodium dithionite to form the very unstable complex of [Mn^IICPI(OH)]^- that successively degraded with Mn(II) release.

Full text of DOI:10.1016/j.ica.2009.12.015

Inorganica Chimica Acta 363 (2010) 1561–1567
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Spectral and kinetic investigation on oxidation and reduction of water
soluble porphyrin–manganese(III) complex
*
Rita Giovannetti , Leila Alibabaei, Filippo Pucciarelli
Chemistry Section of Environmental Sciences Department, Camerino University, Via S. Agostino, 1, 62032 Camerino, Italy
a r t i c l e i n f o
a b s t r a c t
In this work the oxidation and reduction reactions of MnIII–Coproporphyrin-I (MnIII-CPI) have been stud-
ied and four forms of manganese–CPI complexes have been characterized. This complex was observed to
be highly reactive (at basic pH) towards Mn(II), hypochlorite, hydrogen peroxide and oxone, forming
[MnIV(O)CPI(OH)]ꢀ that was unstable and, after a short time, formed again [MnIIICPI(OH)2]ꢀ. With an
excess of NaClO, a further oxidation of the complex [MnIV(O)CPI(OH)]ꢀ, provoked a significant spectral
Article history:
Received 3 June 2009
Received in revised form 17 November 2009
Accepted 7 December 2009
Available online 13 January 2010
change for the [MnV(O)CPI(OH)] formation that showed, in the time,
a partial polymerization.
Keywords:
[MnIIICPI(OH)2]ꢀ was reduced by sodium dithionite to form the very unstable complex of [MnIICPI(OH)]ꢀ
that successively degraded with Mn(II) release.
Coproporphyrin-I
Manganese-complexes
Kinetic
Published by Elsevier B.V.
UV–Vis spectroscopy
1. Introduction
Mn(IV) and Mn(V) for the oxidation of Mn(III)-complexes. All these
compounds can be easily spectrophotometrically distinguished
The ability of porphyrins to form complexes with numerous
metal ions has been the object of many studies [1–5]. Special
attention was devoted to manganese, iron and chromium, that
are essential metal ions in some biological systems involved in
the reactions of electrons transfer. Because the manganese–por-
phyrin complexes were found to be similar to the biologically ac-
tive compounds, they have more extensively studied [6–9]. These
compounds were also used as catalysts for the oxygenation of al-
kanes, alkenes and compounds containing nitrogen and sulfur
[10–12]. The reactivity of the Mn(III)-porphyrin concerns the
changes in the oxidation states of Mn in the complexes for its high
among them because they have different absorption spectra [15]
from which is possible to know the oxidation number of Mn. High
valent manganese species with porphyrins, in aqueous solutions,
form metal-oxo complexes that are very reactive and several of
these species are employed catalytically in synthesis and in the
oxygen production [16]. Several oxo-manganese(V) porphyrin
complexes have been prepared and characterized by different
authors [17–21].
Interest in the reactions of the acid 2,7,12,17 tetrapropionic of
3,8,13,18 tetramethyl-21H, 23H-porphyrin called Coproporphy-
rin-I (CPI) with manganese in the various oxidation states is moti-
vated by its biological importance because CPI is a product of the
metabolism of glutamate together with protoporphyrin IX, Copro-
porphyrin, uroporphyrin III [22]. Numerous works have been made
in our laboratory on the reactions of CPI with different metal ions
with the aim to determine the kinetic and equilibrium constants,
and the reaction mechanisms [23–26].
In the present work we have studied the oxidation and reduc-
tion reactions of manganese(III)–CPI by spectrophotometric mea-
surements in different experimental conditions and in the
presence of oxidant and reducing agents, in order to characterize
the Mn–CPI complexes in the different oxidation states of the cen-
tral metal ion; the kinetics of all reactions have been studied and
the constant rates of these have been calculated. For better inves-
tigate on the reactivity of MnIIICPI in neutral and basic pH, the reac-
tion with Mn(II) excess and with several oxidant and reducing
agents have been performed.
Åꢀ
reactivity with O2 [13]; it is known that this reaction and those
with reactive nitrogen, particularly involved in many pathological
processes including inflammation, ischemia, hemorrhagic shock,
neurological disorders, carcinogenesis and age. The most impor-
tant properties of manganese in complex biological systems is
the flexibility, resulting from highly variable oxidation states of
the metal, that can be easily obtained in aqueous solutions. The
complexes of manganese – porphyrins are known to have in fact
the central metal with formal oxidation number +2, +3, +4, +5
[14]. Manganese, in the complexes obtained by the reaction of
Mn(II) with the porphyrins, has oxidation number +3, so the com-
plex of Mn(II) can be obtained only by reduction, while those of
* Corresponding author. Fax: +39 737 402202.
E-mail address: rita.giovannetti@unicam.it (R. Giovannetti).
0020-1693/$ - see front matter Published by Elsevier B.V.
doi:10.1016/j.ica.2009.12.015

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plexes takes usually the oxidation state of Mn(III) with high-spin
2. Experimental
2.1. Materials
d4 configuration, as demonstrated by other authors with measures
of magnetic susceptibility [29]. Manganese in the complex with the
CPI, at neutral pH, was coordinated with two water molecules and
is in the form [MnIIICPI(H2O)2]+, therefore the reaction between the
CPI and Mn(II) in aqueous solution could be represented with the
equilibrium:
All chemicals were of analytical-reagent grade and used with-
out further purification. All the solutions were prepared with
Milli-Q water. Stock solution of free ligand CPI 4.18 ꢁ 10ꢀ4
mol dmꢀ3 was prepared by dissolving 76.1 mg of CPI-dihydrochlo-
ride in 500 ml of 0.61 ꢁ 10ꢀ4 mol dmꢀ3 sodium hydroxide solution.
Stock solutions containing 1.06 ꢁ 10ꢀ2 mol dmꢀ3 of Mn(II), were
prepared by solving weighted suitable quantities of MnCl2 in water
and standardized by Inductively Coupled Plasma (JY24R).
k1
CPI þ MnðIIÞ þ 2H2O !½MnIIICPIðH2OÞ ꢂþ
2
The inset of Fig. 1 shows a typical ‘‘d-type-hyper” spectrum [28] of
[MnIIICPI(H2O)2]+ obtained. In order to clarify the mechanism of the
reaction, kinetic studies of the complexation reaction have been
carried out measuring the absorbances as a function of time
at various wavelengths (Fig. 1). Plotting ln([Mn(II)]t[CPI]0)–
ln([Mn(II)]0[CPI]t) versus time t, a good straight line has been ob-
tained, confirming a second order kinetic with respect to reagent
concentrations, as previously observed for other CPI-metal com-
plexes [23–26]. The k1 value obtained from the slope of this straight
line was 10.60 dm3 molꢀ1 sꢀ1 slightly lower than that previously
obtained at basic pH [23]. In this complex, the two water molecules
in the axial position could be deprotonated by pH increase with a
corresponding spectral change. The pH change involved only the
coordinated water molecules in the complex because the spectrum
of porphyrin free base did not change by pH increase; the equilibria
are:
2.2. Methods
For absorbance measurements, all the reactions have been car-
ried out in the dark because of the photosensitivity of CPI [26] and
were studied and monitored, for the required time and at 20 °C, by
a Hewlett–Packard 8452A diode array spectrophotometer with a
1-cm quartz cell well-stoppered and connected to Lauda K2R
thermostat. A Metrohm 655 pH-meter with a combined electrode
(Inlab 413) was used for pH determination.
3. Results and discussion
Aqueous solutions of MnIIICPI have been obtained by the reac-
tion of CPI with Mn(II) at neutral pH. The color of the solution
changed slowly from pink to deep orange and a great transforma-
tion of the CPI spectrum [23] with the formation of five bands has
been observed: three more intense, one of which in the UV at
242 nm, two in the Soret band at 366 and 458 nm and two other
less intense in the Q zone at 542 and 572 nm. Three other small
bands at 300, 414 and 800 nm have been observed (Fig. 1). The
spectrum obtained showed a d-type hyperporphyrin character
and the reason of this remarkable spectral difference [27,28], was
attributable to a greater interaction between Mn(III) and porphy-
rin. Spectra of metal-porphyrin complexes with these characteris-
ka1
½MnIIICPIðOH2Þ ꢂþ $½MnIIICPIðOH2ÞðOHÞꢂ þ Hþ
ð1Þ
2
ka2
½MnIIICPIðOH2ÞðOHÞꢂ $½MnIIICPIðOHÞ ꢂꢀ þ Hþ
ð2Þ
2
Therefore, at pH 7 [MnIIICPI(H2O)2]+ was prevalent, while at pH 13,
[MnIIICPI(OH)2]ꢀ predominated. The loss of one or two protons from
[MnIIICPI(H2O)2]+ in the reaction with NaOH, as already observed by
other authors for other complexes of this type [30,31], allowed to
carry out a spectrophotometric acid – base titration for determining
the dissociation constants. The titration was performed in the pH
range from 6.77 to 13.36 by adding NaOH to a solution of [MnIIICPI
(H2O)2]+ and measuring the absorbance values at 348 and 366 nm
after each addition. The gradual decrease, during the titration, of
the bands at 366, 458, 542 and 572 nm with the increase of two
new bands at 348 and 556 nm has been observed (Fig. 2); simulta-
tics are not common and are generated by the charge transfer of
p
orbitals of porphyrin into those empty of manganese. In this metal,
the electrons 3d are all of fairly high energy and are therefore all
used in the bond; for this reason, manganese in this type of com-
Fig. 1. Time resolved UV–Vis spectrum for the reaction between Mn(II) 4.56 ꢁ 10ꢀ5 mol dmꢀ3 with CPI 1.48 ꢁ 10ꢀ5 mol dmꢀ3 at 20 °C. The spectra was recorded at 20 min
intervals; in inset the final spectrum of the complex [MnIIICPI(H2O)2].

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Fig. 2. Spectral change of [MnIIICPI(H2O)2]+ 0.75 ꢁ 10ꢀ5 mol dmꢀ3 in the pH range 6.7–13.4.
MnIVðOÞðOHÞ2 þ ½MnIIICPIðOHÞꢂꢀ
neously the solution showed a color change from orange to yellow
always more deep. During this titration a well-defined isosbestic
point at 353 nm has been observed that was a proof of the presence
of only two absorbent species, or that, if in the presence of three
species, two of these had similar spectral characteristics; in the in-
set of Fig. 2 the titration plot is showed.
k2
!½MnIVðOÞCPIðOHÞꢂꢀ þ Mn3þ þ 2OHꢀ
The complexes of Mn(IV) with several porphyrins are known to
have a short lifetime and decayed back to reform [MnIIICPI(OH)2]ꢀ
[33]. In order to clarify the mechanism of this reaction, kinetic
studies of this reaction have been carried out measuring the
absorbance changes as a function of time at various wavelengths
(Fig. 3). Plotting ln([MnIIICPI(OH)2]t[MnIVCPI(O)(OH)]0)–ln([MnIII
CPI(OH)2]0 [MnIVCPI(O)(OH)]t) versus t, a good straight line has
been obtained, confirming a second order kinetic with respect to
reagent concentrations. The k2 value obtained from the slope of
For the calculation of pKa’s, the adsorbance values at pH > 10
have been used. Knowing that A348
= e
348[MnIIICPI(OH)2]+ and
A366
=
e
366[MnIIICPI(H2O)2], in the point corresponding to the inter-
section of two curves of Fig. 2, A348 = A366
, and therefore
e
348[MnIIICPI(OH)2]+ =
e
366[MnIIICPI(H2O)2]+, that replaced in the
Eqs. (1) and (2) gave:
this straight line was 15.28 dm3 molꢀ1 sꢀ1
.
2
2
½Hþꢂ ½MnIIICPIðOHÞ2ꢂꢀ ½Hþꢂ e366
Ka1 ꢁ Ka2
¼
¼
½MnIIICPIðH2OÞ ꢂþ
e348
3.2. Reactions of MnIIICPI with oxone
2
From the values of e366 = (7.90 0.03) ꢁ 104 Mꢀ1 cmꢀ1 and e348
=
(5.46 0.02) ꢁ 104 Mꢀ1 cmꢀ1 calculated at pH 7 and 13, respec-
tively, an estimated value of (25.60 0.02) for pKa1 + pKa2 has been
obtained. This value was comparable with those of Mn complexes
with other porphyrins [32] and depended by the force with which
the water molecules was linked to the axial ion central.
Oxone (potassium peroxy mono persulfate) is one of the most
powerful oxidants available, is stable in the pH range from 1 to 7
showing only a very slow hydrolysis to form hydrogen peroxide,
while, in basic solution, the product of decomposition is O2. At neu-
tral pH, [MnIIICPI(H2O)2]+ reacted only in the excess of oxone with
a fast disappearance of the spectrum for likely decomposition of
the complex. At basic pH, the same reaction showed an initial spec-
tral variation, involving a slow decrease of the absorbance at 366,
458 542 and 572 of the complex with the simultaneous formation
of three new bands at 348, 400, 502 and 610 nm (Fig. 4) corre-
sponding to the formation of [MnIV(O)CPI(OH)]ꢀ as in the reaction
with Mn(II) above described.
3.1. Reactions of MnIIICPI with Mn(II)
When an excess of Mn(II) was added to a basic solution
(pH > 10.5) of [MnIIICPI(OH)2]ꢀ, it has been observed in few min-
utes that the solution color changed from deep yellow to yellow–
green with a corresponding decrease of the absorbance at 366,
458 542 and 572 nm and with the simultaneous formation of
new bands at 348, 400, 562 and 610 nm (Fig. 3). After few minutes,
the band at 400 nm disappeared and the spectrum of
[MnIIICPI(OH)2]ꢀ was restored. In anaerobic solutions, obtained un-
der Argon atmosphere, this reaction did not occur and for this rea-
son the band at 400 nm could be attributed to the probable
formation of [MnIV(O)CPI(OH)]ꢀ derived from the reaction of [MnI-
VO(OH)2] with the complex. In fact it is well-known that Mn(II)
ions, in basic aqueous solution and in the presence of O2 give rise
to the formation of Mn(IV) ions according to the following
reaction:
In this case the reaction occurred because oxone in the basic
solutions released molecular O2 removing two electrons from the
metal porphyrin. Under these conditions, these two reactions
occurred:
2½MnIIICPIðOHÞ2ꢂꢀ þ OHꢀ ! ½MnIVðOÞCPIðOHÞꢂꢀ þ eꢀ þ H2O
1=2O2 þ 2eꢀ þ H2O ! 2OHꢀ
That summed give:
K3
½MnIIICPIðOHÞ2ꢂꢀ þ 1=2O2 ! 2½MnIVðOÞCPIðOHÞꢂꢀ þ H2O
2Mn2þ þ 4OHꢀ þ O2 () 2MnIVOðOHÞ
Successively the decrease of the spectrum obtained for likely oxida-
tive degradation of [MnIV(O)CPI(OH)]ꢀ has been observed. In order
to explain the kinetic mechanism of the formation of various spe-
cies during the reaction, the absorbance changes as a function of
2
The excess of MnIVO(OH)2 promoted the formation of the complex
[MnIV(O)CPI(OH)]ꢀ through the reaction:

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Fig. 3. Time resolved UV–Vis spectrum for the reaction at basic pH between Mn(II) 4.44 ꢁ 10ꢀ5 mol dmꢀ3 and [MnIIICPI(OH)2]ꢀ 1.48 ꢁ 10ꢀ5 mol dmꢀ3 at 20 °C; the spectra
was recorded at 30 s intervals.
Fig. 4. Time resolved UV–Vis spectrum for the reaction between oxone and [MnIIICPI(OH)2]ꢀ 1.48 ꢁ 10ꢀ5 mol dmꢀ3 at 20 °C. The spectra was recorded at 30 s intervals; in
inset the final spectrum of the complex [MnIV(O)CPI(OH)]ꢀ.
time at various wavelengths, were measured. Plotting ln([MnIII
ular O2.Kinetic studies of this reaction have been carried out
CPI(OH)2]t[O2]0)–ln([MnIIICPI(OH)2]0[O2]t) as a function of time t, a
good straight line has been obtained, confirming a second order ki-
measuring the absorbance changes as a function of time at var-
ious wavelengths. Plotting ln([MnIIICPI(OH)2]t[O2]0)–ln([MnIIICPI
netic with respect to reagent concentrations. The k3 value obtained
(OH)2]0[O2]t) as a function of time t, a good straight line has been
obtained, confirming a second order kinetic with respect to reagent
concentrations. The k4 value obtained from the slope of this
straight line was 155.34 dm3 molꢀ1 sꢀ1. The same reaction con-
ducted with an excess of H2O2 led, after the formation of
[MnIV(O)CPI(OH)]ꢀ, to the final disappearance of all bands for likely
oxidation of porphyrin ring.
from the slope of this straight line was 229.36 dm3 molꢀ1 sꢀ1
.
3.3. Reactions of MnIIICPI with H2O2
In the reaction of [MnIIICPI(H2O)2]+ with H2O2, at neutral pH, no
spectral change was observed. The same reaction conducted at ba-
sic pH in the stoichiometric ratio, initially caused the decrease of
the absorbances at 366, 458, 542 and 572 of [MnIIICPI(OH)2]ꢀ with
the simultaneous formation of the characteristic bands of
[MnIV(O)CPI(OH)]ꢀ (as previously observed in the reactions with
Mn(II) and oxone) that was instable and decayed back to
[MnIIICPI(OH)2]ꢀ (Fig. 5). The formation of [MnIV(O)CPI(OH)]ꢀ oc-
curred because H2O2 at basic pH decomposes into H2O and molec-
3.4. Reactions of MnIIICPI with hypochlorite
In the reaction of [MnIIICPI(H2O)2]+ with NaClO at neutral pH,
the decrease of all bands of the spectrum, for likely oxidative trans-
formation of porphyrin ring, has been observed. The same reaction
conducted at basic pH, with NaClO in stoichiometric ratio, caused a

R. Giovannetti et al. / Inorganica Chimica Acta 363 (2010) 1561–1567
1565
Fig. 5. Time resolved UV–Vis spectrum for the reaction at basic pH between H2O2 and [MnIIICPI(OH)2]ꢀ 1.48 ꢁ 10ꢀ5 mol dmꢀ3 at 20 °C. The spectra was recorded at 30 s
intervals.
spectral change corresponding to the [MnIV(O)CPI(OH)]ꢀ formation
spectral change (inset of Fig. 6), forming a band at 384 nm with
that slowly reverted to [MnIIICPI(OH)2]ꢀ (Fig. 6). The semireactions
lower absorbance and two weaker bands in the area Q, respectively,
at 526 (b band) and 562 nm (a band) [28]. The obtained compound
in this case were:
was very stable and its spectrum is typical of a porphyrin metal
complex. The low and large absorption band in the Soret region is
characteristic of an aggregate form, while the bands in the Q zone
½MnIIICPIðOHÞ2ꢂꢀ þ OHꢀ ! ½MnIVðOÞCPIðOHÞꢂꢀ þ eꢀ þ H2O
CIOꢀ þ H2O þ 2eꢀ ! Clꢀ þ 2OHꢀ
confirmed that the complex obtained is very stable because
a band
is greater than b [28]. For these reasons it has been hypothesized
that this behavior corresponded to a complex obtained for the fur-
ther oxidation of Mn(IV) into Mn(V) through the reactions:
The overall reaction was:
2½MnIIICPIðOHÞ2ꢂꢀ þ CIOꢀ ! 2½MnIVðOÞCPIðOHÞꢂꢀ þ Clꢀ þ H2O
Also in this case this reaction showed a second order kinetic with
respect to the reagent concentrations: in fact, plotting
ln([MnIIICPI(OH)2]t[ClOꢀ]0)–ln([MnIIICPI(OH)2]0[ClOꢀ]t), as a func-
tion of time t, a good straight line has been obtained the slope of
which represented the k5 value of 148.29 dm3 molꢀ1 sꢀ1. The same
reaction conducted in the excess of reagent, initially caused the for-
mation of [MnIV(O)CPI(OH)]ꢀ, that after at least 12 h evolved, with a
½MnIVðOÞCPIðOHÞꢂꢀ ! ½MnVðOÞCPIðOHÞꢂ þ eꢀ
CIOꢀ þ H2O þ 2eꢀ ! Clꢀ þ 2OHꢀ
that summed:
Fig. 6. Time resolved UV–Vis spectrum for the reaction at basic pH between NaClO and [MnIIICPI(OH)2]ꢀ 1.48 ꢁ 10ꢀ5 mol dmꢀ3 at 20 °C. The spectra was recorded at 30 s
intervals; in inset the final spectrum of the complex [MnV(O)CPI(OH)].

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2½MnIVðOÞCPIðOHÞꢂꢀ þ ClOꢀ þ H2O
! 2½MnVðOÞCPIðOHÞꢂ þ Clꢀ þ 2OHꢀ
The complex [MnV(O)CPI(OH)], having neutral character, showed
easily an arrangement into aggregate form. In fact, slowly, part of
the complex polymerized with formation of a precipitate that was
instable in strongly acidic solution likewise to other MnV–porphyrin
complexes [17–21]. The kinetic of this reaction was of second order
with respect to reagent concentrations, the k6 value obtained was
6.88 dm3 molꢀ1 sꢀ1
.
3.5. Reactions of MnIIICPI with dithionite
In the reaction of [MnIIICPI(H2O)2], at neutral pH, with an excess
of Na2S2O4, the decrease of the complex bands due to the demetal-
lation with the formation the dimeric form of CPI has been ob-
served. It is probable that Mn(III) in the complex was reduced to
Mn(II) that leaving the porphyrin core, restored CPI, that, in the
presence of Na2S2O4, dimerized for the salt effect. The same reac-
tion conducted at basic pH, caused the formation of [MnIICPIOH]ꢀ
with a spectral variation involving a slow decrease of the absor-
bance at 348 nm, the simultaneous formation of a new band at
414 nm and two bands of lower intensity at 544 (b band) and
Fig. 8. Summary of all studied Mn–CPI complexes.
Table 1
Summary of the spectral characteristics of Mn–CPI complexes.
574 nm (
a band) (Fig. 7). The obtained spectrum successively de-
Complex
Soret band k (nm)
Q band (b, a) k (nm)
creased up to the complete disappearance, for probable decompo-
[MnIICPIOH]ꢀ
414
544, 574
542, 572
556
502, 610
526, 562
sition of [MnIICPIOH]ꢀ without release of CPI free ligand.
[MnIIICPI(H2O)2]+
[MnIIICPI(OH)2]ꢀ
[MnIVOCPI(OH)]ꢀ
[MnVOCPI(OH)]
366, 458
348, 458
400
2½MnIIICPIðOHÞ2ꢂꢀ þ S2O2ꢀ ! 2½MnIICPIOHꢂꢀ þ 2HSOꢀ3
4
384
½MnIICPIOHꢂꢀ ! decomposition
Also in this case the reaction of [MnIICPIOH]ꢀ formation was of the
second order respect to reagent concentrations, in fact, plotting
ln([MnIIICPI(OH)2]t[S2O4ꢀ]0)–ln([MnIIICPI(OH)2]0[S2O4ꢀ]t) as a func-
tion of time t, a good straight line has been obtained the slope of
4. Conclusions
The studies reported in this work have highlighted that manga-
nese in the CPI complexes can assume four oxidation degrees by
oxidation or reduction of the central metal ion in the complex
[MnIIICPI(OH)2]ꢀ. All the results indicated that this complex re-
acted with oxone, hydrogen peroxide and hypochlorite forming
[MnIVOCPI(OH)]ꢀ with kinetic constants of the second order re-
spect to reagent concentrations and with comparable values
which represented the k7 value of 55.23 dm3 molꢀ1 sꢀ1
.
In the Fig. 8 is showed a summary of all the complexes obtained
in the reaction of [MnIIICPI(OH)2]– with several reagents in differ-
ent conditions while in Tables 1 and 2 are reported all spectral
characteristics and kinetic constants.
Fig. 7. Time resolved UV–Vis spectrum for the reaction at basic pH between Na2S2O4 with [MnIIICPI(OH)2]ꢀ 1.48 ꢁ 10ꢀ5 mol dmꢀ3 at 20 °C. The spectra was recorded at 30 s
intervals; in inset the final spectrum of the complex [MnIICPIOH]ꢀ.

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1567
Table 2
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Second order rate constants for Mn–CPI complexes.
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Reagents
Product
k (dm3 molꢀ1 sꢀ1
)
Mn(II)
CPI
Mn(II)
oxone
H2O2
NaClO
[MnIIICPI(H2O)2]+
[MnIVOCPI(OH)]ꢀ
[MnIVOCPI(OH)]ꢀ
[MnIVOCPI(OH)]ꢀ
[MnIVOCPI(OH)]ꢀ
[MnVOCPI(OH)]
[MnIICPIOH]ꢀ
10.60 0.04
15.28 0.08
229.36 0.13
155.34 0.11
148.29 0.11
6.88 0.02
[MnIIICPI(OH)2]ꢀ
[MnIIICPI(OH)2]ꢀ
[MnIIICPI(OH)2]ꢀ
[MnIIICPI(OH)2]ꢀ
[MnIIICPI(OH)2]ꢀ
[MnIIICPI(OH)2]ꢀ
NaClO excess
Na2S2O4
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55.23 0.05
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The first reaction has been carried out at neutral pH, while all others at pH 10.5.
decreasing in the order: oxone, hydrogen peroxide and hypochlo-
rite relatively to their oxidation powers. With Mn(II), the same
reaction was about 10 times slower because this was a displace-
ment reaction of Mn(III) with Mn(IV) formed from Mn(II) in alka-
line solution. Further oxidation has been obtained only with an
excess of hypochlorite for the probable formation of the complex
[MnVOCPI(OH)] and presented a kinetic constant value of about
20–30 times lower respect to oxidation reaction. This complex
was very stable, uncharged and for this reason, in the time slowly
polymerized forming a precipitate.
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6052.
The reduction reaction of [MnIIICPI(OH)2]– with dithionite gave
the formation [MnIICPIOH]ꢀ that showed a kinetic constant of the
second order respect to reagent concentrations, lower respect to
the oxidation reactions; this complex was very instable and suc-
cessively degraded with Mn(II) release. The spectral characteristics
have confirmed the different stabilities of the obtained complexes
decreasing in the order: [MnV(O)CPI(OH)] > [MnIIICPI(H2O)2]+ ꢃ
[MnIIICPI(OH)2]ꢀ > [MnIV(O)CPI(OH)]ꢀ > [MnIICPI(OH)]ꢀ.
Acknowledgments
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5766.
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The authors would like to express their gratitude to Professor
Vito Bartocci for his very important scientific suggestions.