B.M. Pires et al. / Inorganica Chimica Acta 407 (2013) 69–81
79
was obtained in buffer, with first-order (1.03 ± 0.06) in relation to
cluster [(L)
2
(Na+)
3
N
2
]+, m/z 447.2879, with units of propylene
ꢁ3 ꢁ1
the complex and a kobs of 3.50 ꢂ 10
s
, 25-fold higher than in
oxide, probably from the plastic bottle of the H
tween 250–400 m/z can be assigned to iron mononuclear
2 2
O . The species be-
water.
II
+
III
The catalytic efficiency of the title complex was also studied in
acetonitrile (Fig. 10) in order to compare with published data, since
most of the available kinetic experiments for synthetic models
complexes with compositions [Fe (L)(H
2
O)N
(CH ] , [Fe (L)(H O)
3 2 4 2
-OH)(CH CN)(H O) N ] of m/z 277, 317, 331, 349 and 389,
2
] , [Fe (L)(l-OH)
+
II
+
II
+
III
3 2
CN)N ] , [Fe (L)(H
2
O)
4
N
2
2
5
N
2
]
and [Fe (L)
+
(l
ꢁ2
were carried out in organic solvent. kcat (5.37 ꢂ 10 ) in CH
3
CN
respectively. The peak of m/z 305 increases with time and does
was 381-fold higher than in water and 15-fold higher than in buf-
fer. The same trend is reported for other complexes [76], some
authors have shown that the catalase-like activity is accelerated
at higher pH values as well as in aprotic solvents as acetonitrile.
Probably, a high concentration of protons is capable of generating
inactive or less active species. The difference between the catalytic
efficiency in aqueous solution and in acetonitrile may also be re-
lated to the different electron transfer rates in both solvents, as evi-
denced by the cyclic voltammetry analyses. Furthermore, when the
experiments were performed in water a precipitate was formed,
indicating a faster decomposition of the initial complex.
not refer to an iron species, it can be attributed to the cluster
+
+
2 2 3
[(L)(Na ) (N ) ] and indicates the complex degradation over time.
A control test where pure water was added to the solution instead
of hydrogen peroxide showed no changes from the original com-
plex spectrum. We conclude that the active species in acetonitrile
is a mononuclear species.
EPR spectra of the complex in presence of H
lected in CH CN at 77 K, from 0 to 20 min of reaction (Fig. S7).
All the measurements furnished EPR silent spectra. This is in agree-
ment with the ESI(+)-MS experiments, once the majority of the
species formed after reaction with peroxide are Fe(II) and EPR si-
lent. Similar results were found for the binuclear complex [Fe2-
2 2
O were also col-
3
3.5.2. Progress of H
O
2 2
disproportionation reactions followed by
4 2 2 4 4
O(bipy) (OH ) ][ClO ] , where the final products of the reaction
UV–Vis, ESI-MS/Q-TOF and EPR
were mononuclear iron(II) and iron(III) species [25]. We believe
that in our case the concentration of the iron(III) species observed
on the ESI(+)-MS spectra are too low to give an EPR signal.
The catalase-like activity of the title complex in water, in buffer
and in acetonitrile was monitored by electronic spectroscopy. The
aqueous and buffered solutions presented similar spectra, as
shown in Fig. 3(a). During the reaction, no significant change was
observed besides the increase of the spectral line in the region of
3.6. Cyclohexane oxidation
5
00–900 nm (Fig. S4), which can be originated from light disper-
Some authors [25,27] have described the competition between
sion caused by suspended particles. The spectral line increased fas-
ter in water than in buffer and it is associated to the orange
precipitate observed during the experiments. These results also
indicate that the generation of the precipitate is pH-dependent.
Different from the aqueous solutions, in acetonitrile an isos-
bestic point was observed around 600 nm, with the decay of the
band at 675 nm and the increasing in the intensity of the band at
hydrogen peroxide dismutation and oxidation chemistry, when a
substrate is present, using ( -oxo)diiron(III) complexes. In order
l
to verify if the title tetranuclear complex is promiscuous or selec-
tive for either catalase- or monooxygenase-like activity, we carried
4 4
out the cyclohexane oxidation with [Fe (l-O)(l-OH)(l-OAc)
(
L) ](ClO and H , in CH CN. The oxidized products (measured
2
4
)
3
2
O
2
3
after 24 h), cyclohexanol and cyclohexanone, were formed in small
yields (yield cyclohexanol/cyclohexanone: 0.8/0.7% (25 °C) and
.9/2.6% (50 °C)). Cyclohexyl hydroperoxide was also formed
10.2 at 25 °C and 28.1 at 50 °C), being the major product in both
temperatures. Control experiments were conducted under the
same conditions with no iron source and no oxidation of cyclohex-
ane was observed in both temperatures. Control experiments using
5
50 nm (Fig. 11). As the band at 675 nm was attributed to the
Fe–O–Fe core, its disappearance indicates that the new species
formed does not possess an oxo bridge.
2
(
Since ESI is a gentle ionization technique and causes little or no
fragmentation of the sample, it has been used extensively to study
reaction mechanisms involving organic or inorganic species [79].
The ESI(+)-MS spectrum of the complex in acetonitrile (Fig. 12)
showed the presence of a few low molecular weight mononuclear
Fe(ClO
hexanol (1.8% at 25 °C; 1.4 at 50 °C) and cyclohexanone (2.2% at
5 °C; 1.1 at 50 °C) were comparable to the values obtained with
the tetramer in both temperatures. Cyclohexyl hydroperoxide
2.9% at 25 °C; 42.4 at 50 °C) was also the major product. These re-
4 3
) as catalyst were also conducted and the yields of cyclo-
II
+
species with the compositions [Fe
2
(
l
-OH)(
l
-O)] (m/z 145),
2
II
II
-OAc)]+ (m/z 187), [Fe (
III
+ +
[
[
Fe
Fe
2
(
l
l
-O)(
l
l
-OAc)
3
Na ] (m/z 256) and
+
+
2
(
-O)
2
(l
-OAc)
2
(H )
3
] (m/z 265), and some tetranuclear spe-
(
II
+
+
4 4
cies with the compositions [Fe (l-O)(l-OAc)(L-H )(L)(ClO )]
sults reinforce the low monooxygenase activity of the tetranuclear
complex, which oxidation of cyclohexane are in the level of iro-
n(III) salts and no influence from the ligand was observed. Cyclo-
hexane oxidation using iron(III) salts has been reported
previously, where high yields were obtained, with cyclohexyl
hydroperoxide as the major product as well [80]. Therefore,
m/z 747), [FeII
l
l
(
l
-OAc)
(L-H )(L)(ClO
+
)] (m/z 849), [Fe
+
III
(
(
(
4
3
4
4
+
+
III
-O)
-OAc)
2
(
l
3
-OAc)
2
(L-H )(L)(ClO
4
)
2
]
4
(m/z 920) and [Fe (l-O)
2
+
(L) (ClO
2
4
)
2
] (m/z 981). This indicates that the solid-state
structure is not well preserved in acetonitrile, but some of the spe-
cies formed in solution still maintain the tetranuclear core.
In order to get some information about the mechanism, the
reaction of the complex with H
in acetonitrile at room temperature. H
plex solution, which was directly infused into the ESI source of a
Q-TOF mass spectrometer. The spectra are presented in Fig. 12
and they were recorded before the addition of the substrate and
after 0.2, 3, 9 and 20 min the reaction has started. The addition
[
4 4 2 4 3
Fe (l-O)(l-OH)(l-OAc) (L) ](ClO ) seems to prefer the pathway
O
2 2
was monitored by ESI(+)-MS
was added to the com-
that leads to the active species involved in the H
2 2
O decomposition
2 2
O
than those toward oxidation chemistry [25,27].
4. Conclusions
2
of H O
2
immediately caused a major shift in the spectrum, only
4 4 2 4 3
In summary, the complex [Fe (l-O)(l-OH)(l-OAc) (L) ](ClO )
peaks in the range of 100–650 m/z were observed and any of these
was synthesized through the reaction between the known ligand
peaks refers to a tetranuclear core. Peaks in the range of 100–
(1,3-bis[(2-aminoethyl)amino]-2-propanol)
and
the
-oxo)(l
salts
-hy-
2
50 m/z may refer to low molecular weight complexes caused by
the cleavage of the complex in solution. The series of ions above
00 m/z differing by m/z 58 repeating units is not related to an
Fe(ClO
droxo)bis(
4
)
3
ꢀxH
2
O and NaOAcꢀ3H
2
O, and possesses a (
-alkoxo)tetra(l-carboxylato)tetrairon core. The com-
l
l
4
plex shows catalase-like activity in water, in TRIS buffer and in
CH CN. The kinetic experiments results revealed a Michaelis–Men-
ten behavior in relation to the H and a first-order kinetic in
iron-containing species since the characteristic first peak from
the iron isotope pattern is absent. They can be attributed to the
3
2 2
O