D. Tomczyk et al. / Inorganica Chimica Acta 390 (2012) 70–78
77
Table 3
The summary of values calculated from electrochemical studies, T = 293 K.
Ef (V) vs. SCE
a
(1 ꢁ a)
trans-[MnIIIisocyclam(H2O)2]3+/trans-[MnIIisocyclam(H2O)2]2+
ꢁ0.06 0.02
[MnIIIMnIV
trans-[MnIIIisocyclam(H2O)2]3+
[MnIIIMnIV -O)2isocyclam2]3+
(l
-O)2isocyclam2]3+/[MnIIIMnIII
(l
-O)2isocyclam2]2+
0.01 0.03
0.21 0.03
0.52 0.04
(l
0.62 0.03 (0.02 V sꢁ1), 0.43 0.04 (0.3 V sꢁ1
)
0.4 to 1.2 V) and at different scan rates ranging from 0.02 to
0.3 V sꢁ1, high enough to finish the experiment within 2 min. In
this time the concentration of the solution could have changed
and infrared spectroscopy. Magnetic measurements showed high-
spin character of mononuclear complex and antiferromagnetism of
the binuclear one. Cyclic voltammetry of neutral aqueous solutions
maximally by 1% (Fig. 1). Comparable statistic error of
a
and
of binuclear complex and estimated transfer coefficient (1 ꢁ
showed complex mechanism of the dimer [MnIIIMnIV(
-O)2isocy-
clam2]3+ oxidation to [MnIVMnIV( -O)2isocyclam2]4+, whereas in
a)
(1 ꢁ
a
) values calculated both for the lowest and the highest scan
l
rates proves that the concentration decrease does not influence
l
significantly the calculation of transfer coefficients.
acidic media these measurements showed hydrolysis of oxygen
Cathodic transfer coefficients
[MnIIIisocyclam(H2O)2]3+ and [MnIIIMnIV
change significantly with scan rate, so the Table 3 summarises only
a
of reduction processes of trans-
bridges and process characteristic for mononuclear complex.
(l
-O)2isocyclam2]3+ did not
Exhaustive electrolysis of aqueous solution of [MnIIIMnIV(
l-O)2iso-
cyclam2]3+ at its reduction and oxidation potentials, and electronic
spectra of these solutions showed the possibility of both reduction
and oxidation of dimer of Mn(III) and Mn(IV) ions to [MnIIIMnIII
average values. The average cathodic transfer coefficient
a of
reduction process of trans-[MnIIIisocyclam(H2O)2]3+ is of low value
which results from distinct irreversibility of the electrode process.
(l l
-O)2isocyclam2]2+ and [MnIVMnIV( -O)2isocyclam2]4+, respectively.
The average
a
value of reduction process of [MnIIIMnIV(
l
-O)2isocy-
Voltammetric measurements of neutral aqueous trans-[MnIIIisocy-
clam(H2O)2]3+ solution after longer scanning showed oxidation of
this complex analogically to the binuclear one, which was verified
by controlled potential electrolysis (yield 32%) and electron spec-
troscopy. Based on voltammetric measurements one can conclude,
that dimerisation occurs for trans-[MnIIIisocyclam(H2O)2]3+ and is
preceded by trans ? cis isomerisation of Mn(II) ion complexes.
Mononuclear complex trans-[MnIIIisocyclam(H2O)2]3+ in acidic
media undergoes one-electron reduction to ions of the same
symmetry.
clam2]3+ indicates on electron transfer according to E mechanism
[52], while definitely varied anodic transfer coefficient values
(1 ꢁ
a
) dependent on scan rate and different from (1 ꢁ
a
) = 0.5,
show complicated mechanism of [MnIIIMnIV(
oxidation process.
l
-O)2isocyclam2]3+
Formal potential of the couple trans-[MnIIIisocyclam(H2O)2]3+
/
trans-[MnIIisocyclam(H2O)2]2+ in HCl medium, pH 0.99, was calcu-
lated using polarisation curves method [53] from curves recorded
at scan rates 5, 7, 10 and 20 mV sꢁ1. Formal potential of the couple
[MnIIIMnIV(
l
-O)2isocyclam2]3+/[MnIIIMnIII(
l
-O)2isocyclam2]2+ was
The over mentioned investigation proves significant stabilisa-
tion of Mn(III) ion as the result of macrocyclic effect both for mono-
and binuclear complexes. Formal potential values Ef (ꢁ0.06 V
for trans-[MnIIIisocyclam(H2O)2]3+/trans-[MnIIisocyclam(H2O)2]2+cou-
calculated based on curves recorded at higher scan rates, i.e. 10,
20, 30 and 40 mV sꢁ1 because of lower stability of these ions in
the potential range of the Ist redox couple, i.e. from 0.2 to ꢁ0.4 V
in KCl medium, pH 7.0 (then, the electrode process did not take
longer than 2 min and the lowering of concentration was not high-
er than 1%). Comparable statistic errors of Ef values calculated for
different scan rates prove that the low stability of binuclear com-
plex does not influence significantly this way of calculation of for-
mal potential.
ple and 0.01 V for [MnIIIMnIV( -O)2isocyclam2]3+/[MnIIIMnIII(
l l-
O)2isocyclam2]2+ couple) are considerably lower than EoMn3þ=2þ
Furthermore, the possibility of oxidation of mononuclear complex
to dimeric [MnIVMnIV( -O)2isocyclam2]4+ of potentially higher oxi-
.
l
dation capability (Epa of IInd redox couple about 0.9 V) (Fig. 4) gives
a chance to utilise trans-[MnIIIisocyclam(H2O)2]3+ which is stable in
As oxidation of complex ion [MnIIIMnIV
occurs according to complicated mechanism, formal potential of
[MnIVMnIV( -O)isocyclam2]4+/[MnIIIMnIV( -O)2isocyclam2]3+ redox
couple was not calculated.
(l
-O)2isocyclam2]3+
aqueous solutions instead of unstable [MnIIIMnIV(
l-O)2isocy-
clam2]3+ obtaining the same catalytic effect, what is proved by
l
l
our current further investigation.
Average formal potential values, Ef, are presented in Table 3.
Low Ef values suggest efficient thermodynamic stabilisation of high
oxidation states of manganese ions resulting from macrocyclic ef-
fect, which is the reason, why Mn(II) ions tend in neutral aqueous
medium in the presence of isocyclam to form binuclear Mn(III) and
Mn(IV) as well as mononuclear Mn(III) complexes.
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4. Conclusions
Syntheses of mononuclear Mn(III) complex and binuclear
mixed-valence Mn(III)–Mn(IV) di-l-oxo complex were described.
The results of investigation of trans-[MnIIIisocyclam(H2O)2]3+ sug-
gest, analogically to trans-[MnIIIcyclamCl2]Cl [37], an isomerisa-
tion of cis-[MnIIIisocyclam(H2O)2]3+ ? trans-[MnIIIisocyclam(H2O)2]3+
type. The synthesis of [MnIIIMnIV(
l-O)2isocyclam2](ClO4)3 shows
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