Properties of (μ-oxo)di-iron complexes and catalytic activity toward cyclohexane oxidation

Article abstract of DOI:10.1590/S0103-50532010000400004

We report herein the synthesis and characterization of two dinuclear m-oxo iron compounds obtained through the reactions of FeSO₄?7H ₂O and FeCl₃?6H₂O with 1-(bis-pyridin-2-ylmethyl-amino)-3-chloropropan-2-ol (L5), which resulted in the compounds [(SO₄)(L5)Fe(μ-O)Fe(L5) (SO₄)]?6H ₂O, 1, and [Cl(L5)Fe(μ-O)Fe(L5)Cl]Cl₂?2H ₂O, 2. The electronic spectra of both compounds show absorption bands only in the UV range. The electrochemical analysis showed that the dinuclear unit is more stable under reduction in compound 1 than in compound 2, while the Mo?ssbauer spectroscopy revealed that the monodentate ligands (sulfate and chloride) have a significant influence on the Mo?ssbauer parameters determined for 1 and 2, particularly on the quadrupole splitting values. Both compounds were studied as catalysts in reactions of cyclohexane oxidation, using H₂O₂ and t-BuOOH as oxidants, in a substrate:oxidant: catalyst ratio of 1000:1000:1. Cyclohexanol, cyclohexanone, cyclohexyl hydroperoxide, t-butyl cyclohexyl peroxide and adipic acid were formed during the process. The experiments revealed that compound 2 is, in general, more active than compound 1 in promoting cyclohexane oxidation.

Full text of DOI:10.1590/S0103-50532010000400004

J. Braz. Chem. Soc., Vol. 21, No. 4, 603-613, 2010.
Printed in Brazil - ©2010 Sociedade Brasileira de Química
0103 - 5053 $6.00+0.00
Properties of (m-Oxo)di-iron Complexes and Catalytic Activity Toward
Cyclohexane Oxidation
Gabrieli L. Parrilha,a Sarah S. Ferreira, a Christiane Fernandes,a Giselle C. Silva,b
Nakédia M. F. Carvalho,b O. A. C. Antunes,b,† Valderes Drago,c Adailton J. Bortoluzzid
and Adolfo Horn Jr.*,a
aLaboratório de Ciências Químicas, Universidade Estadual do Norte Fluminense,
28013-602 Campos dos Goytacazes- RJ, Brazil
bInstituto de Química, Universidade Federal do Rio de Janeiro, Cidade Universitária,
CT Bloco A-641, 21945-970 Rio de Janeiro-RJ, Brazil
c Departamento de Física and d Laboratório de Bioinorgânica e Cristalografia, Departamento de
Química, Universidade Federal de Santa Catarina, 88040-900 Florianópolis-SC, Brazil
Neste artigo são apresentadas a síntese e a caracterização de dois compostos binucleares de
•
ferro, os quais contêm uma ponte m-oxo. Os compostos [(SO4)(L5)Fe(m-O)Fe(L5)(SO4)] 6H2O, 1,
•
e [Cl(L5)Fe(m-O)Fe(L5)Cl]Cl2 2H2O, 2, foram obtidos nas reações entre 1-(bis-piridin-2-ilmetil-
•
•
amino)-3-cloropropan-2-ol (L5) e os sais FeSO4 7H2O e FeCl3 6H2O, respectivamente. Os espectros
eletrônicos dos complexos apresentam absorções somente na região do ultravioleta, sendo que a
análise eletroquímica revelou que, após a formação da espécie FeIIIFeII, a unidade binuclear do
composto 1 é mais estável do que a do composto 2. Os ligantes monodentados (sulfato e cloreto)
exercem influência sobre os parâmetros Mössbauer determinados para 1 e 2, particularmente
sobre os desdobramentos de quadrupolo. Os compostos foram empregados como catalisadores
em reações de oxidação do cicloexano, usando H2O2 e t-BuOOH como oxidantes em uma razão
substrato:oxidante:catalisador de 1000:1000:1. Os resultados indicam que o composto 2 é um
catalisador mais eficiente que o composto 1.
We report herein the synthesis and characterization of two dinuclear m-oxo iron compounds
•
•
obtained through the reactions of FeSO4 7H2O and FeCl3 6H2O with 1-(bis-pyridin-2-ylmethyl-
amino)-3-chloropropan-2-ol (L5), which resulted in the compounds [(SO4)(L5)Fe(m-O)Fe(L5)
•
•
(SO4)] 6H2O, 1, and [Cl(L5)Fe(m-O)Fe(L5)Cl]Cl2 2H2O, 2. The electronic spectra of both
compounds show absorption bands only in the UV range. The electrochemical analysis showed
that the dinuclear unit is more stable under reduction in compound 1 than in compound 2,
while the Mössbauer spectroscopy revealed that the monodentate ligands (sulfate and chloride)
have a significant influence on the Mössbauer parameters determined for 1 and 2, particularly
on the quadrupole splitting values. Both compounds were studied as catalysts in reactions of
cyclohexane oxidation, using H2O2 and t-BuOOH as oxidants, in a substrate:oxidant:catalyst ratio
of 1000:1000:1. Cyclohexanol, cyclohexanone, cyclohexyl hydroperoxide, t-butyl cyclohexyl
peroxide and adipic acid were formed during the process. The experiments revealed that compound
2 is, in general, more active than compound 1 in promoting cyclohexane oxidation.
Keywords: di-iron complex, m-oxo bridge, methane monooxygenase, cyclohexane oxidation
Introduction
thermodynamic and kinetic stability of their C-C and
C-H bonds, which makes their transformation difficult to
perform.1 The evolution of living systems has provided
many ways to overcome this difficulty, such as developing
different metalloenzymes, like the cytochrome P-450
family and the methane monooxygenases. These are very
Under mild conditions, alkanes are inert compounds
in the presence of most chemical reagents due to the
†Deceased. This paper is dedicated to his memory.
*e-mail: adolfo@uenf.br

604
Properties of (m-Oxo)di-iron Complexes and Catalytic Activity
J. Braz. Chem. Soc.
specialized systems able to promote specific hydrocarbon
oxidation.2, 3
In order to contribute to a better understanding of how
different ligand groups affect the physicochemical and
structural properties of coordination compounds, we have
recently published the synthesis and characterization of a
family of mononuclear iron complexes containing bis(2-
pyridylmethyl)amine derivative ligands (Figure 1: L1- L4).
Results showing the reactivity of these iron compounds with
H2O2 and O2, as well as their use as catalysts in cyclohexane
oxidation, were reported.8,18-20 Following this same line,
we report herein the synthesis and characterization of two
new dinuclear iron complexes [(SO4)(L5)Fe(m-O)Fe(L5)
Along with copper,iron was selected by nature to form
the majority of natural metalloenzymes, which participate
in the oxidation of different organic compounds and have
inspired several research groups in the search for the
so-called biomimetic compounds that have some functional
or structural analogies with the natural systems.4 In this
regard, several heme and non-heme iron complexes have
been employed in studies on hydrocarbon oxidation
including those of our group.3-9
•
•
Soluble methane monooxygenase (sMMO), which is
found in methanotrophic bacteria, is one of the natural
systems that promote hydrocarbon oxidation, carrying out
the conversion of methane to methanol. The active site of
this MMO system contains a di-iron center coordinated
to the amino acid residues glutamate and histidine, and to
water molecules.9 During the sMMO catalytic cycle the
oxidation state of the iron ions changes from FeII-FeII, the
most reduced state, to FeIV-FeIV, the most oxidized one.
The latter is the main reactive species of the cycle, being
responsible for the formation and hydroxylation of alkyl
radicals. Intermediate species such as FeII-FeIII and FeIII-FeIII
have also been characterized in this catalytic cycle.10
Several synthetic compounds have been described in
the literature as structural and/or functional models for
the active site of MMO. For example, the deprotonation
of the compound [Fe2(O)(OH)(6tla)2](ClO4)3 (6tla =
tris(6-methylpyridyl-2methyl)amine) resulted in the
first diamond core bis(m-oxo)di-iron(III) compound
that mimics the sMMO active site containing FeIIIFeIII.11
The reaction of [Fe2O(5-Me-tpa)2(OH)(H2O)](ClO4)3
(5-Me-tpa = tris(5-methyl-2-pyridylmethyl)amine) with
H2O2 resulted in a high valence complex containing the
FeIII(O)2FeIV unit, which has also been characterized by
spectroscopic and structural methods and is claimed to be
analogous to the intermediate Q observed in the catalytic
cycle of MMO.12 Other di-iron complexes have been
synthesized as models for the sMMO peroxo intermediate.13
From the catalytic point of view, a number of mono and
dinuclear iron complexes have been studied in reactions
of hydrocarbon oxidation.7,8,14-16 In general, cyclohexane
is the first substrate chosen as a model, because its
manipulation and transformation is much easier than that
of methane. Furthermore, the products obtained from
cyclohexane oxidation, cyclohexanol and cyclohexanone,
are precursors to adipic acid and caprolactam, which have
extensive industrial applications in Nylon-6 and Nylon-66
manufacturing.Due to the high amount of energy expended
in the actual process, the development of catalytic systems
that operate under mild conditions is desired.17
(SO4)] 6H2O, 1, and [Cl(L5)Fe(m-O)Fe(L5)Cl]Cl2 2H2O,
2, which were synthesized with a new member of the
bis(2-pyridylmethyl)amine family: L5 = 1-(bis-pyridin-
2-ylmethyl-amino)-3-chloropropan-2-ol (Figure 1). An
alcohol function was incorporated into this ligand aiming
to compare its coordination ability with the ligands
that possess amide (L3) and carboxylate (L4) groups.
Both complexes 1 and 2 had their catalytic activity
in cyclohexane oxidation investigated. The synthesis,
molecular structure and nuclease activity of 1 have been
published previously.21
Figure 1. Family of ligands containing the bis(2-pyridylmethyl)amine unit
(L1-L5). The structure for ligand L6 is included because it is discussed
in the text.
Results and Discussion
Syntheses
The complexes were synthesized according to the
scheme in Figure 2. During the preparation of 1, a very
small amount of a brown solid was formed, which was
eliminated by filtration. In the synthesis of 2, immediate
precipitation of a yellow solid was observed after the addition
of the iron salt to the solution containing L5. We tried to
isolate this product, but it is highly deliquescent. An initial
characterization of this yellow solid, performed by infrared
spectroscopy, revealed the absence of Fe-O-Fe stretching,
indicating that it might be a mononuclear iron complex.
This difficulty, concerning the isolation of a stable solid,

Vol. 21, No. 4, 2010
Parrilha et al.
605
was solved by refluxing. However, with this procedure, only
a red solid could be isolated, characterized as a (m-oxo)di-
iron compound. Interestingly, it was observed that the use
of L5 results in dinuclear m-oxo iron compounds, while the
other members of this family (L1-L4) produce mononuclear
iron compounds under similar reaction conditions. We have
previously published the synthesis of copper and manganese
compounds with L5. For these metal ions, only mononuclear
compounds were isolated.22, 23
to the oxo bridge. For example, the Fe-Namine bond lengths
in the compound [(tpa)(OH)FeOFe(H2O)(tpa)]3+ (tpa =
tris(2-pyridylmethyl)amine) are 2.194(8) Å when the N
is cis to the oxo bridge and 2.264(8) Å when it is trans.
In the compound [(tpa)ClFeOFeCl(tpa)]2+, the aliphatic
nitrogen atoms are cis to the oxo bridge and the Fe-N bond
distances are 2.227(6)Å.24 For the complex [(trispicMeen)
ClFeOFeCl(trispicMeen)], where trispicMeen = N,N,N’-
tris-(2-pyridylmethyl)-N´-methylethane-1,2-diamine, the
Both iron complexes are soluble only in polar solvents,
including water, MeOH, CH3CN, dimethylformamide
(DMF) and dimethylsulfoxide (DMSO). Conductivity
analysis revealed that both compounds behave as
electrolytes in water, indicating that, at least for 1 and
under the conditions employed, the sulfate group should be
displaced from the iron coordination environment.
Namine trans to the oxo bridge shows an Fe-N bond length
of 2.270(16)Å, while the Namine atoms cis to m-O the values
are 2.243(15) and 2.261(12) Å.25
In 1 the alcohol groups are located trans to the
monodentate sulfate group and cis to the oxo bridge. The
Fe-Oalcohol distances (average: 2.140 Å) indicate that the
alcohol group is protonated. Also, its hydrogen atoms
were found from the Fourier difference map. The Fe-Oalcohol
(ca. 2.140 Å) and Fe-Npyridine (2.138 Å) bond lengths are
very similar, indicating that, at least in terms of structural
parameters, both groups have similar basicity.
X-ray molecular structure
The ellipsoid plots of the molecular structure of
complexes 1 and 2 are presented in Figure 3, the main
bond distances and angles are shown in Table 1 and the
crystallographic data are given in Table 2.
The molecular structure of 1 has been published
previously. Compound 1 shows a neutral dinuclear unit
with the iron(III) ions connected by a m-oxo bridge
(Fe-O-Fe = 163.2(3)o). Each iron(III) ion is also coordinated
by one L5 ligand molecule and one monodentate sulfate
ion, resulting in a N3O3 coordination environment around
the metal centers.
The bis(2-pyridylmethyl)amine units from the
ligand molecules adopt a meridional coordination mode
(Fe-Namine: average = 2.236 Å, Fe-Npy: average = 2.138 Å),
with the pyridine groups trans to each other. The same
coordination mode was observed in mononuclear copper
complexes synthesized with L5;22 however, it differs from
the mononuclear seven-coordinate manganese compound,
in which the pyridine groups are in cis arrangement.23
Although it is recognized that the oxo bridge exerts a
significant trans effect, it is not very pronounced in 1, since
the Fe-Namine bond lengths (ca. 2.23 Å) are in the range
observed for iron compounds containing tertiary amines cis
The cation 2 lies on a crystallographic inversion center
and, accordingly, it has exact Ci local symmetry, which
gives the chloride ions an anti configuration, similar to
that observed in the compound [(tpa)ClFeOFeCl(tpa)]2+.24
In complex 1, in its turn, the sulfate groups show a syn
configuration. The L5 coordination mode also differs
significantly from that observed in 1. In 1 the Namine atoms
are trans to the oxo bridge, while in 2 the alcohol groups
occupy this position. In contrast, the Namine in 2 is cis to
the oxo bridge, with Fe-Namine distances of 2.191(3) Å.
The pyridyl groups are positioned trans to each other, as
observed in 1.
…
In 2, the Fe-O-Fe unit is linear,which makes the Fe Fe
distance(3.580(5)Å)slightlylongerthanin1(3.5323(13)Å).
The oxo bridge in 2 shows a more pronounced trans effect
when compared with 1, resulting in a Fe-Oalcohol bond length
of 2.241(2)Å.This bond distance was approximately 2.140Å
in 1, with the alcohol coordinated cis to the oxo bridge.
Curiously, although there are different groups trans to the
oxo bridge in 1 (amine) and 2 (alcohol), the bond lengths
are practically the same: 2.236(5) in 1 and 2.241(2) Å in 2.
The Fe-N bond lengths in 2 are slightly shorter than in 1.
Figure 2. Synthesis of compounds 1 and 2.

606
Properties of (m-Oxo)di-iron Complexes and Catalytic Activity
J. Braz. Chem. Soc.
Table 1. Selected bond lengths and angles for complexes 1 and 2
Both isomeric forms of the ligand are observed in 2 (R
around Fe1i and S around Fe1), while only the R isomer
is present in 1.
Complex 1
Fe1-O3
Bond lengths (Å)
1.777(4)
1.961(4)
2.140(5)
2.144(4)
2.145(5)
2.235(5)
Fe2-O3
Fe2-O5S
1.793(4)
1.958(4)
2.120(5)
2.135(4)
2.146(6)
2.237(5)
Wehavepreviouslyreportedthecoordinationbehaviorof
a similar ligand {N-(2-hydroxybenzyl)-N-(2-pyridylmethyl)
[(3-chloro)(2-hydroxy)] propylamine} (L6 in Figure 1)
which differs from L5 by the presence of one pyridine and
one phenol group instead of two pyridines as observed in
L5. L6 forms a mononuclear compound when it reacts with
Fe1-O1S
Fe1-N12
Fe1-O1
Fe2-N32
Fe2-O2
Fe1-N22
Fe1-N1
Fe2-N42
Fe2-N2
8
•
FeCl3 6H2O, while dinuclear iron complexes were obtained
Angles (°)
•
•
by reactions with FeSO4 7H2O and Fe(ClO4)3 9H2O. The
structural characterization of the dinuclear complexes
synthesized with L6, [Fe2(L6)2(H2O)2]2+, [Fe2(L6)2(OAc)]+
and [Fe2(L6)2(SO4)],26 revealed that the iron ions are bridged
by alkoxide groups from the ligand instead of presenting
an oxo bridge, as observed for 1 and 2.
O3-Fe1-O1S
O3-Fe1-N12
O1S-Fe1-N12
O3-Fe1-O1
O1S-Fe1-O1
N12-Fe1-O1
O3-Fe1-N22
O1S-Fe1-N22
N12-Fe1-N22
O1-Fe1-N22
O3-Fe1-N1
O1S-Fe1-N1
N12-Fe1-N1
O1-Fe1-N1
N22-Fe1-N1
Complex 2
Fe1-O1
104.6(2)
103.7(2)
O3-Fe2-O5S
O3-Fe2-N32
O5S-Fe2-N32
O3-Fe2-O2
O5S-Fe2-O2
N32-Fe2-O2
O3-Fe2-N42
O5S-Fe2-N42
N32-Fe2-N42
O2-Fe2-N42
O3-Fe2-N2
O5S-Fe2-N2
N32-Fe2-N2
O2-Fe2-N2
N42-Fe2-N2
Bond lengths (Å)
Fe1-N1
108.71(19)
105.0(2)
87.1(2)
86.96(19)
90.58(18)
163.84(18)
84.06(19)
103.89(19)
93.02(19)
151.5(2)
89.78(18)
161.20(18)
84.72(19)
102.2(2)
88.4(2)
152.4(2)
91.04(19)
165.05(19)
86.11(19)
76.9(2)
88.56(18)
167.0(2)
88.38(19)
76.53(19)
76.48(17)
75.00(19)
75.55(17)
75.7(2)
1.7897(5)
2.121(3)
2.123(3)
2.191(3)
2.241(2)
2.2758(9)
Fe1-N12
Fe1-O2
Fe1-N22
Fe1-Cl2
Angles (°)
O1-Fe1-N12
O1-Fe1-N22
N12-Fe1-N22
O1-Fe1-N1
N12-Fe1-N1
N22-Fe1-N1
O1-Fe1-O2
N12-Fe1-O2
92.69(8)
92.79(8)
N22-Fe1-O2
N1-Fe1-O2
O1-Fe1-Cl2
N12-Fe1-Cl2
N22-Fe1-Cl2
N1-Fe1-Cl2
O2-Fe1-Cl2
Fe1-O1-Fe1i
82.69(11)
76.28(10)
104.59(3)
100.93(8)
102.35(9)
162.74(8)
86.60(7)
153.88(12)
92.60(8)
76.27(11)
77.99(12)
168.63(7)
87.03(11)
180.00(5)
Figure 3. ORTEP diagrams for 1 (top) and 2 (bottom) with thermal
ellipsoids at 40% probability. Hydrogen atoms and solvates are omitted
for clarity. Symmetry code for 2: -x,-y,-z+1.
and of the alcohol group at 3450 cm-1. The asymmetric
stretching of the oxo bridge (nas Fe-O-Fe) gives rise to a
band of medium intensity at 832 for 1 and 825 cm-1 for 2,
presenting higher wavenumber values when compared with
[(tpa)ClFeOFeCl(tpa)]2+ (nas = 816 cm-1).24
The Mössbauer spectra of the complexes were
collected at room temperature (Figure 4). The spectra
for both complexes consist of a single quadrupole
doublet, indicating the presence of only one type of iron
Spectroscopic and electrochemical characterization
The infrared spectra of the complexes are very similar,
with the major difference being related to the presence
of sulfate groups in 1, resulting in bands at 1175, 1124,
1106 and 1036 cm-1. Bands characteristic of pyridine
groups are observed at 1610, 1573, 1487 and 1449 cm-1,

Vol. 21, No. 4, 2010
Parrilha et al.
Complex 1
607
Table 2. Crystal data and structure refinement for complexes 1 and 2
Complex 2
C30H40Cl6Fe2N6O5
889.08
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
C30H48Cl2Fe2N6O17S2
1011.46
293(2)
293(2)
0.71073
0.71073
Monoclinic
P 2/c
Triclinic
–
P 1
19.104(4)
15.071(3)
16.826(3)
8.4405(7)
b (Å)
10.2902(12)
c (Å)
12.4575(16)
a (°)
66.714(3)
b (°)
108.81(3)
84.629(3)
g (°)
72.324(3)
Volume (Å3)
4585.6(16)
946.50(18)
Z
4
1.465
1
1.560
Density (calc.) (Mg/m3)
-1
m (mm
)
0.911
1.236
F(000)
2096
456
Crystal size (mm3)
0.40 × 0.26 × 0.23
1.13 to 25.08
0.24 × 0.18 × 0.10
3.56 to 26.05
Theta range (°)
Index ranges
-22 ≤ h ≤ 0; 0 ≤ k ≤ 17; -18 ≤ l ≤ 20
8380
-10 ≤ h ≤ 10; -12 ≤ k ≤ 12; -15 ≤ l ≤ 15
10816
Reflections collected
Independent reflections
Absorption correction
Transmission factors
Refinement method
Data / parameters
Goodness-of-fit on F2
R indices [I > 2s(I)]
R indices (all data)
8124 [R(int) = 0.0249]
Psi-scan
3709 [R(int) = 0.0227]
Multiscan
0.808 and 0.769
Full-matrix least-squares on F2
8124 / 502
0.886 and 0.756
Full-matrix least-squares on F2
3709 / 233
1.090
1.053
R1 = 0.0681, wR2 = 0.1985
R1 = 0.1387, wR2 = 0.2171
R1 = 0.0488, wR2 = 0.1272
R1 = 0.0594, wR2 = 0.1329
nucleus in the solid state for each complex. The observed
isomer shifts (d 0.34 for 1 and d 0.32 mm s-1 for 2) are
characteristic of high spin iron(III). The quadrupole
splitting values (DEq = 1.67 for 1 and DEq = 0.98 mm s-1
for 2) indicate that the iron atoms possess an anisotropic
electronic environment. Since high spin FeIII centers show
a dependence of DEq on the local symmetry and on the
electronic environment,27 the higher value of DEq obtained
for compound 1 might be related to a less symmetric
coordination environment around the iron ions or it might
reflect the presence of dianionic sulfate ions, which have
a very distinct basicity when compared with the chloride
ions coordinated in 2.
shoulders were observed for complex 1 and two bands and
one shoulder for complex 2, all of them located in the UV
range and presenting e values typical of charge transfer
transitions. The band of highest energy is located near
255 nm (e ca. 21×103 dm3 mol-1 cm-1) for both complexes,
and is attributed to ligand-centered transitions (pyridine).
The shoulders between 300 and 400 nm observed in
the spectra of 1 can be attributed to oxoFeIII LMCT
transitions.28
Complex 2 also shows a band at 339 (e = 6.5×103
dm3 mol-1 cm-1) and a shoulder near 400 nm (e = 3.5×103
dm3 mol-1 cm-1). Similar bands have also been observed
in mononuclear iron compounds containing chloride
coordinated to iron(III) ions, and were assigned to LMCT
ClFeIII.18 Since complex 2 possesses both oxo and
The electronic spectra of the complexes in CH3CN
solution are very similar (Figure 5). One band and two

608
Properties of (m-Oxo)di-iron Complexes and Catalytic Activity
J. Braz. Chem. Soc.
chloride ligands coordinated to iron(III), it is possible that
the absorptions observed between 300 and 450 nm are
composed of LMCT ClFeIII and oxoFeIII.
Both complexes show no distinct absorption features
in the visible range, which is characteristic of complexes
containing linear Fe-O-Fe units. Hence, it is possible to
hypothesize that the Fe-O-Fe core in 1 might show a linear
Fe-O-Fe arrangement in solution, 29 although in the solid
state its angle is 163o.
The cyclic voltammograms of 1 and 2 are rather distinct
(Figure6).TheinternalstandardFc/Fc+isobservedat0.418V
(DEp = 125 mV), and the free L5 base does not show any
electrochemical process between −1.8 to 1.0 V.
Compound 1 shows two quasi-reversible redox processes
at − 0.690 (DEp = 0.151 V) and −1.30 V (DEp = 0.222 V)
associated with the redox couples FeIIIFeIII/FeIIIFeII and
FeIIIFeII/FeIIFeII. Furthermore, it is possible to see another
oxidation wave at −0.322 V versus Fc/Fc+, which means
that the anodic wave for the first redox process, observed at
−0.690Vvs.Fc/Fc+,isassociatedwithtwooxidativeprocesses.
A similar behavior has been observed for the compound
[(trispicMeen)ClFeIIIOFeIIICl(trispicMeen)]2+,25 forwhichthe
anodicprocessforthefirstredoxcouplewascomposedoftwo
oxidation waves attributed to the presence of two species in
equilibrium, a six and a five coordinated FeII. Since a similar
feature is observed in the cyclic voltammogram of 1, the
presenceoftwospeciesinequilibriumcanbeproposed.Thus,
after the first process of reduction, the sulfate anion possibly
leaves the coordination environment of the iron center and
six and five coordinated iron(II) ions coexist on the electrode
surface, resulting in the two anodic processes.
Figure 4. Mössbauer spectra for compounds 1 (top) and 2 (bottom).
The cyclic voltammogram of 2 shows two very well
defined irreversible processes, one cathodic at −0.426 and
another anodic at −0.223 V versus Fc/Fc+. There is a second
irreversible cathodic process with low current intensity at
−0.826Vvs. Fc/Fc+.Theelectrochemicalprofilepresentedby
2 is similar to that reported by Costes and co-workers.30 For
(L)Fe-O-Fe(L) compounds, where L is a Schiff base, Costes
suggested that, after the first step of reduction - which leads to
theformationofthemixedvalenceFeIIIFeII -thedinuclearunit
is broken, resulting in mononuclear species that are oxidized
atamorepositivepotential.Thus, itispossibletohypothesize
that there is a chemical reaction associated with the reduction
process of 2 at −0.426 V, resulting in the breaking of the
dinuclear unit and formation of mononuclear species. This
explains why the electrochemical behavior of 2 is so distinct
fromthatof1. Thepresenceofasecondcathodicprocesswith
low current intensity may suggest that a small amount of the
dinuclear unit remains on the electrode surface.
Figure 5. Electronic spectrum of 1 (top) and 2 (bottom) in acetonitrile
-4
-5
-5
at different concentrations: (____)1.0×10 ; (----) 7.5×10 ; ( ) 5.0×10 and
•
• • •
-5
( -) 2.5×10 mol dm-3. lmax (nm) / e (dm3 mol-1 cm-1): 1 = 252/2.1×104;
326/6.1×103; 384/4.0×103. 2 = 254/2.2×104; 345/6.6×103; 420/3.5×103.
The insets show the plots of A versus [complex], used to calculate the
e values.
•
•
The anodic process observed at −0.223 V is related
to the oxidation of the mononuclear iron(II) species

Vol. 21, No. 4, 2010
Parrilha et al.
609
electrochemically formed on the electrode surface. The
formationofthismononuclearspeciesisalsosupportedbythe
fact that the anodic wave at −0.223V is in the range observed
active site, experiments aiming to evaluate whether the
complexes could have monoxygenase activity were carried
out. Cyclohexane was employed as the substrate and the
reactions were performed employing different solvents
(MeCN/BuOH), oxidants (hydrogen peroxide or tert-
butylperoxide)andtemperatures(25and50oC).Cyclohexanol
(Cy-OH), cyclohexanone(Cy=O), cyclohexylhydroperoxide
(Cy-OOH), t-butyl cyclohexyl peroxide (Cy-OOt-Bu) and
adipic acid (AA) were formed during the oxidation process
(Scheme 1, Table 3). Control reactions were carried out in
the absence of the catalysts and no oxidation products were
observed. It is also important to note that the yields reported
in Table 3 were based on the substrate, since other kinds of
reactions (catalase activity, insertion of oxo groups in the
structure of the complexes etc.) may take place.
for the mononuclear iron compounds containing L1 (E1/2
=
−0.347 V versus Fc/Fc+), L2 (E1/2 = −0.257 V versus Fc/Fc+)
and L3 (E1/2 = −0.222 V versus Fc/Fc+).
Comparingthemolecularstructuresof1and2,itispossible
to explain their distinct electrochemical behaviors. For 1, the
alcoholgroupboundtooneoftheironcentershasanorientation
that allows it to form a hydrogen bond with the sulfate group
boundtotheotherironcenterand/orwiththeoxobridge.Onthe
otherhand,thealcoholgroupspresentin2arecoordinatedtrans
to the oxo bridge, which does not allow any kind of interaction
with the groups coordinated to the second iron center. Thus,
the lack of intramolecular hydrogen bonding in 2 could lead to
the rupture of the dinuclear unit after reduction, while for 1 it is
proposed that the intramolecular hydrogen bond is responsible
for the maintenance of the dinuclear structure.
Scheme 1. Products observed during cyclohexane oxidation promoted
by compounds 1 and 2. Cy-OH: cyclohexanol; Cy=O: cyclohexanone;
Cy-OOH: cyclohexyl hydroperoxide; Cy-OOt-Bu: tert-butylcyclohexyl
peroxide; AA: adipic acid.
At 25 oC, employing acetonitrile as the solvent and H2O2
as the oxidant (Table 3, entry 1), complex 2 was more active
than complex 1. Adipic acid was the main product obtained
for complex 2, followed by Cy-OOH, Cy-OH and Cy=O.
Interestingly, adipic acid is virtually absent when complex
1 is used, indicating that the two complexes have different
behaviors in promoting the cyclohexane oxidation under
these conditions.
o
Interestingly, when the temperature is raised to 50 C
(Table 3, entry 2) the total conversions observed for 1 and 2
are similar. Under this condition, the product adipic acid was
not observed, differing significantly from the results obtained
at lower temperature, mainly for complex 2. For 1, the main
product obtained is Cy-OOH, while, for complex 2, Cy=O
was obtained in larger amounts. This suggests that a different
oxidation pathway may be in place.
When the oxidant was changed from H2O2 to t-BuOOH,
o
practically no oxidation was observed at 25 C (Table 3,
entry 3), but it reaches 19% (1) and 16% (2) at 50 C
Figure 6. Cyclic voltammograms for compounds 1 (top) and 2 (bottom)
under argon atmosphere. Ferrocene was used as internal standart for
complex 1 and as external standart for complex 2. Scan rate: 100 mV s-1.
Support electrolyte: 0.1 mol dm-3 solution of tetrabutylammonium
perchlorate in acetonitrile. Pseudo-referene electrode: Pt wire. (----) base
o
(Table 3, entry 4). Under this condition, results indicate
the same trend concerning the oxidation products for both
complexes, and the major product formed is Cy=O, which
suggests that the catalytic species should be the same for
the two precursors. Furthermore, this result reveals that the
temperature has an important effect on both the yield and
oxidation mechanism. The influence of the temperature was
also observed for compound 1 when H2O2 was employed
line, ( ) ligand, (____) complex
• • • •
Cyclohexane oxidation
Sincethemolecularstructuresof1and2aredinuclearand
adinuclearironcoreispresentatthemethanemonooxygenase

610
Properties of (m-Oxo)di-iron Complexes and Catalytic Activity
J. Braz. Chem. Soc.
as the oxidant (Table 3, entry 1). Entries 3 and 4 indicate
that the interaction between the catalysts and the oxidants
(mainly for t-BuOOH) would not occur so easily at room
temperature, probably due to some steric hindrance, since
coordination mode of the polydentate ligand around the iron
ions; ii) the charges and types of the monodentate ligands
(SO42- and Cl-); iii) the charges presented by the complexes;
iv) the redox potential of the metal center. Since several
variables can be related to each other, at the present time it is
not possible to establish the factors which drive the activity of
the iron complexes synthesized with the ligand L5.
Considering the molecular structure, in compound 2 the
chloroligandsshowananticonfigurationtoeachother,whilein
1,thesulfateionsaresyn.Thisstructuraldifferencemayexplain
the higher activity of compound 2, since the iron centers in this
compound are more accessible to the oxidant than those in 1.
Furthermore, electrochemical data showed that the dinuclear
unit in 2 is not very stable. Since we have observed previously
that the mononuclear iron compounds synthesized with L1-
L4 and L6 are more active in cyclohexane oxidation than the
dinuclearcompoundspresentedhere,itispossiblethattheeasier
conversion of the dinuclear 2 in a mononuclear compound
makes this complex a better catalyst that compound 1.
o
it is facilitated at 50 C. Thus, it is possible to suggest
o
that, at the higher temperature (50 C), the coordination
environment of the iron center changes (ligand exchange
or rupture of the dinuclear unit), facilitating the catalyst-
oxidant interaction, which results in a higher yield for the
oxidation reaction.
The change of solvent from acetonitrile to t-BuOH had a
negative effect on the total conversion for both temperatures
and oxidants. Firstly, we decided to study the cyclohexane
oxidationint-BuOHduetothefactthatbothcomplexes1and
2areinsolubleinthissolvent.Thus, aheterogeneouscatalysis
could take place. However, the addition of the oxidants to the
complexes made them soluble, resulting in a homogeneous
system. When H2O2 was used as the oxidant (entries 5 and 6)
a very low amount of oxidation products were observed at
room temperature. However, at 50 oC, complex 2 showed the
highest selectivity toward cyclohexanol.
Conclusions
The system t-BuOOH/t-BuOH (Table 3, entries 7 and 8)
gives reasonable results only at 50 oC for complex 2. Under
these conditions complex 2 showed the highest production
and selectivity for Cy=O.
This study revealed that the tripodal tetradentate ligand
L5 yields the formation of (m-oxo)di-iron(III) compounds.
This feature is independent of the oxidation state of the iron
salt employed in the synthesis (FeII or FeIII). However, the
behaviorsrelatedtothecoordinationmodeoftheliganddiffer,
andthisaccountsforthedistinctelectrochemicalbehaviorsand
probablytothecatalyticactivities.Ingeneral,compound 2was
more effective than compound 1 in promoting cyclohexane
oxidation, although the results for both complexes showed
a dependence on the reaction conditions (solvent, oxidant,
temperature). The two complexes were more active in the
medium with H2O2 as the oxidant and acetonitrile as the
solvent at 50 °C, furnishing the highest turnover values under
these conditions. Complexes 1 and 2 showed lower activity
thanthemononuclearcomplexessynthesizedwiththeligands
L1-L4andL6.Thepresenceofamorelabileligand(chloride),
the lower steric hindrance, as well as the higher propensity
to form mononuclear compounds observed for compound 2,
can be considered to explain its higher activity as a catalyst
for oxygenation reactions when compared with 1.
The two complexes presented here are less effective in
promoting cyclohexane oxidation than the mononuclear
iron complexes synthesized with the ligands L1-L4 and
L6.7,8 In contrast to this tendency, Li and co-workes31
reported similar results in the cyclohexane oxidation when
the mononuclear [Fe(tpoen)Cl]PF6 and the dinuclear
[{Fe(tpoen)}2(m-O)](ClO4)4 compounds were employed
as catalysts (tpoen = N-(2-pyridylmethoxyethyl)-N,N-
bis(2-pyridylmethyl)amine). On the other hand, the
dinuclear iron compound [Fe2OL2(MeOH)2(NO3)2](NO3)2
(L = 2,6-bis(N-methylbenzimidazol-2-yl)pyridine) was
able to promote the cyclohexane oxidation while the
mononuclear iron compound with the same ligand, [FeLCl3],
was inactive. Compounds 1 and 2 showed lower activity
than the compounds [{Fe(tpoen)}2(m-O)](ClO4)4 and
[Fe2OL2(MeOH)2(NO3)2](NO3)2.
Several effects may account for the oxygenase activity of
the iron compounds: the redox potential, the lability of the
ligands, the presence of ligands able to form hydrogen bonds
andsterichindranceeffects.32-34 Theanalysisofourdatareveals
thatcompounds1and2areverydistinctcatalysts.Theactivity
ofeachcompoundisdependentontheexperimentalconditions
(solvent, temperature, oxidant) employed in the oxidation
reaction. Since both are dinuclear m-oxo compounds and
containthesamepolydentateligand, thedifferencesobserved
in the reactivity may be related to the following points: i) the
Experimental
Materials and methods
Reagents and solvents were used as received from
commercialsources.Onlythesolventsusedforelectrochemical
and electronic spectroscopy analyses were of spectroscopic
grade. L5 was synthesized as described previously.22

Vol. 21, No. 4, 2010
Parrilha et al.
611
Table 3. Results for the cyclohexane oxidation catalyzed by complexes 1 and 2 after 24 h
Entry
T (°C)
Oxidant
Solvent
Catalyst
Yield (%)a
Cy-OH/
Cy=O
TNb
Cy-OH
Cy=O
Cy-OOH Cy-OOt-Bu
AA
Total
6.6
17.3
21.5
17.9
1
1
2
3
4
5
6
7
8
rt
50
rt
H2O2
H2O2
ACN
ACN
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1.0
2.9
5.4
3.9
1
-
1
5.6
5.7
12.6
2.8
-
-
-
7.7
-
72
173
217
185
23
-
-
2.9
1.5
0.35
-
3.5
11.2
-
-
-
-
t-BuOOH
t-BuOOH
H2O2
ACN
-
-
-
-
-
-
1.4
1.1
-
-
-
-
50
rt
ACN
5.9
5.5
-
9.4
8.3
-
2.2
1.0
-
-
18.9
15.9
-
0.63
0.66
-
191
161
-
-
t-BuOH
t-BuOH
-
-
-
2.1
1.2
-
-
1
-
3.1
2.2
9.5
-
-
38
27
92
-
50
rt
H2O2
1
-
-
-
5.6
-
2.9
-
-
1
-
1.9
-
t-BuOOH t-BuOH
t-BuOOH t-BuOH
-
-
-
-
-
-
6.8
-
6.8
4.8
16.5
-
73
55
167
50
1.4
1.5
1.9
-
0.9
2.6
11.6
-
2.3
-
0.22
a Yields were calculated in relation to the substrate, using the equation (1), which relates the yield calculated from chromatographic analysis and the yield
calculated from titration. For the chromatographic yield, the equations (2) and (3) were employed, using the corrected areas of the products (considering
the corresponding response factor). b TN, turnover number; calculated as mol of products per mol of catalyst. Rt = RAA + Rrc (1), where Rt = total yield;
RAA = yield of adipic acid obtained from titration; Rrc = corrected chromatographic yield;
(2), where Rc = chromatographic yield; nc = number
of moles analyzed by GC (nc = nt -naa; naa = mols of adipic acid; nt = number of moles of substrate);
(3), whereAp = corrected area of the products
from GC; At = total corrected area (sum of the areas of products and substrate from GC).
UV-Vis spectra were recorded on a Shimadzu 1601PC
UV-Visspectrophotometerinacetonitrile.Infraredspectrawere
collectedonaFTIRNicoletMagna-IR760spectrophotometer,
with the sample dispersed in KBr. Mössbauer spectra were
obtainedusingaWisselinstrumentintheconstantacceleration
mode with transmission geometry. A 57Co/Rh source was
maintained at room temperature. The resultant spectra were
least-squares fitted to Lorentzian shaped lines using the
NORMOS software (Wissel Company). Metallic iron was
used for energy calibration and also as a reference for the
isomer shift (d) scale. Conductivity measurements were
carried out with solutions containing 1×10-3 mol dm-3 of the
complexes, using a BioCristal NT CVM conductivimeter,
employing a conductivity cell CA150. Cyclic voltammetry
experiments were carried out using an Autolab PGSTAT10
potentiostat/galvanostat and a three electrode system, with
a glassy carbon disk as the working electrode, a platinum
wire as the auxiliary electrode and a platinum wire as the
pseudo-reference electrode. As the supporting electrolyte,
a 0.1 mol dm-3 solution of tetrabutylammonium perchlorate
in acetonitrile was used. The redox couple Fc/Fc+ (0.400 V
versusNHE)wasusedastheinternalstandard.35 Cyclohexane
was purified before handling through successive extractions
with H2SO4, H2O and NaHCO3 solution (1%), respectively,
and distilled over CaH2. The oxidants, H2O2 and tert-butyl
hydroperoxide(t-BuOOH), weretitratedusingtheiodometric
method.TheoxidationproductswereanalyzedinanHP5890
gas chromatographer with FID detector and a DB-5 column.
•
Synthesis of [(SO4)(L5)Fe(m-O)Fe(L5)(SO4)] 6H2O (1)
This complex was obtained through the reaction of a
methanolicsolution(50cm3)ofL5(2mmol, 0.58g)followed
•
by the addition of an equimolar amount of solid FeSO4 7H2O
(0.56g).Thesolutionacquiredimmediatelyagreenish-brown
color. The reaction mixturewas stirred for one hour, filtered
and left to stand. The next day a red microcrystalline material
began to precipitate. After 3 days, this microcrystalline solid
was filtered, washed with cold propan-2-ol and dried under
vacuum (0.48 g, 48%). Found: C, 33.95; H, 4.9; N, 7.6;
•
C30H36Cl2Fe2N6O11S2 9H2OrequiresC, 33.8;H, 5.1;N, 7.9%.
IR(KBr)nmax/cm-1:3463(OH),1610,1573,1487,1449(C=C,
C=N,pyridine),1175,1124,1106,1036(SO4),832(Fe-O-Fe).
-1
LM = 22.7 cm2 W mol-1 (DMF: no electrolyte).36 This solid
was recrystallized in MeOH, rendering red diamond shaped
crystals suitable for X-ray analysis.
•
Synthesis of [Cl(L5)Fe(m-O)Fe(L5)Cl]Cl2 2H2O (2)
2mmol(0.58g)ofL5weredissolvedin20cm3 ofpropan-

612
Properties of (m-Oxo)di-iron Complexes and Catalytic Activity
J. Braz. Chem. Soc.
2-ol, followed by the addition of a solution (propan-2-ol,
dried with anhydrous Na2SO4 and analyzed by GC. Retention
times and mass spectra compared with standards were used
to characterize most of the reaction products. Yields were
calculatedtakingintoaccountthedifferentresponsefactorsto
FIDofthesubstrate(cyclohexane)andproducts(cyclohexanol
and cyclohexanone) through external standardization. For
cyclohexyl hydroperoxide, the response factor to FID was
consideredtobethesameasthatofcyclohexanol.Theaqueous
phase was titrated with NaOH to quantify the total acid
compoundsobtainedinthereaction, expressedasadipicacid.
3
•
20 cm ) containing an equimolar amount of FeCl3 6H2O. A
yellowsolidwasimmediatelyformed.Methanol(50cm3)was
added and the solution was refluxed for 1.5 h, rendering an
orange solution. This solution was kept at room temperature,
and after four days red single crystals suitable for X-ray
analysiswereisolated(0.39g,44%). Found:C=40.5;H=4.4;
•
N, 9.3; C30H36Cl6Fe2N6O3 2H2O requires C = 40.5; H = 4.5;
N = 9.45. IR (KBr ) nmax/cm-1: 3421 (OH), 1607, 1570,
1483, 1445 (C=C, C=N, pyridine), 825 (Fe-O-Fe).
-1
LM = 147.8 cm2 W mol-1 (DMF: 2:1 electrolyte type).36
Supplementary Information
Crystallographic analysis
The crystallographic data (atomic coordinates and
equivalent isotropic displacement parameters, calculated
hydrogen atom parameters, anisotropic thermal parameters
and bond lengths and angles) have been deposited at the
Cambridge Crystallographic Data Center (deposition numbers
CCDC672086andCCDC672087).Copiesofthisinformation
may be obtained free of charge from: CCDC, 12 Union Road,
Cambridge, CB2 1EZ, UK (Fax: +44-1223-336-033; e-mail:
deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).
Crystallographic analysis was carried out on a CAD-4
diffractometerforcomplex1andonanAPEXIIdiffractometer
for complex 2, at room temperature. For complex 1, the
electronic density attributed to highly disordered solvent
molecules was removed using squeeze correction with
PLATON.37 All non-hydrogen atoms were refined with
anisotropic displacement parameters, except for the oxygen
atoms of the six water molecules of crystallization. Hydrogen
atoms attached to carbon atoms were fixed at their idealized
positions, whereas the H atoms of the alcohol groups were
found from the Fourier map and treated with a riding model.
The hydrogen atoms for the water molecules were not
found. For complex 2, all non-hydrogen atoms were refined
anisotropically. The counterion chloride (Cl3) is disordered
over two alternative positions, with refined occupancy of
0.52(5) and 0.48(5). H atoms of the alcohol group and of the
water molecule were found from the Fourier map and also
treatedwitharidingmodel.OtherHatomswereplacedattheir
idealized positions, with C-H distances and Ueq values taken
from the default settings of the refinement program. Further
crystallographic information is given in Table 2.
Acknowledgments
The authors are grateful for the financial support received
from CNPq (Jovens Pesquisadores and INCT-Catálise),
CAPES, FAPERJ(JovemCientistadoNossoEstado, Pronex)
and FUJB. The authors also thank Dr. Manfredo Hörner
and Dr. Robert A. Burrow (Departamento de Química,
UniversidadeFederaldeSantaMaria)forthecrystallographic
facilities use.
References
1. Crabtree, R. H.; Chem. Rev. 1995, 95, 987.
Cyclohexane oxidation
2. Denisov, I. G.; Makris, T. M.; Sligar, S. G.; Schlichting, I.; Chem.
Rev. 2005, 105, 2253.
The reactions were carried out in a 50 cm3 round bottom
flask under stirring for 24 h. Different solvents, temperatures
and oxidants were employed, using an experimental
optimization method. The catalyst:substrate:oxidant ratio
was 1:1000:1000. The reagent amounts were: 0.75 cm3 of
cyclohexane (7×10-3 mol), 0.59 cm3 of H2O2 or 0.93 cm3
of t-BuOOH (7×10-3 mol), 7.4 mg of 1 or 6.0 mg of 2
(7×10-6 mol). Acetonitrile (MeCN) and t-butanol (t-BuOH)
were used as the solvents (10 cm3) and the experiments were
3. Costas, M.; Chen, K.; Que, L. Jr.; Coord. Chem. Rev. 2000, 200,
517;Tshuva, E.Y.;Lippard, S. J.;Chem. Rev. 2004, 104, 987;Bois,
J. D.; Mizoguchi, T. J.; Lippard, S. J.; Coord. Chem. Rev. 2005,
200, 443.
4. Que Jr., L.; Tolman, W. B.; Nature 2008, 455, 333.
5. Tang, J.; Gamez, P.; Reedijk, J.; Dalton Trans. 2007, 4644; Pan,
Z.; Newcomb, M.; Inorg. Chem. 2007, 46, 6767; Nam, W.; Jin, S.
W.; Lim, M. H.; Ryu, J.Y.; Kim, C.; Inorg. Chem. 2002, 41, 3647;
Halma, M.; Bail, A.; Wypych, F.; Nakagaki, S.; J. Mol. Catal. A:
Chem. 2006, 243, 44; Nakagaki, S.; Castro, K.A. D. F.; Machado,
G. S.; Halma, M.; Drechsel, S. M.; Wypych, F.; J. Braz. Chem.
Soc. 2006, 17, 1672;Wada,A.; Ogo, S.; Nagatomo, S.; Kitagawa,
T.; Watanabe,Y.; Jitsukawa, K.; Masuda, H.; Inorg. Chem. 2002,
o
carried out at two temperatures: room temperature (25 C)
and 50 °C. The reactions were quenched by the addition of
an aqueous 0.4 mol dm-3 solution of Na2SO4, followed by
extraction with 10 cm3 of diethyl ether. The ether layer was

Vol. 21, No. 4, 2010
Parrilha et al.
613
41, 616; England, J.; Britovsek, G. J. P.; Rabadia, N.; White,A. J.
Silva, M. S.; Pich, C. T.; Terenzi, H.; Horn, A. Jr.; Inorg. Chem.
Commun. 2008, 11, 643.
P.; Inorg. Chem. 2007, 46, 3752.
6. Olsen,M.H.N.;Salomão,G.C.;Drago,V.;Fernandes,C.;Horn,A.
Jr.;Cardozo-Filho, L.;Antunes, O.A. C.;J. Supercrit. Fluids2005,
34, 119; Salomão, G. C.; Olsen, M. H. N.; Drago, V.; Fernandes,
C.; Cardozo-Filho, L.; Antunes, O. A. C.; Catal. Commun. 2007,
8, 69; Esmelindro, M. C.; Oestreicher, E. G.; Márquez-Alvarez,
H.; Dariva, C.; Egues, S. M. S.; Fernandes, C.; Bortoluzzi, A. J.;
Drago, V.; Antunes, O. A. C.; J. Inorg. Biochem. 2005, 99, 2054.
7. Carvalho, N. M. F.; Horn, A. Jr.; Antunes, O. A. C.; Appl. Catal.,
A 2006, 305, 140.
22. Horn, A. Jr.; Fernandes, C.; Bortoluzzi, A. J.; Vugman, N. V.;
Herbst, M. H.; J. Mol. Struct. 2005, 749, 96; Fernandes, C.;
Parrilha, G. L.; Lessa, J.A.; Santiago, L. J. M.; Kanashiro, M. M.;
Boniolo, F. S.; Bortoluzzi, A. J.; Vugman, N. V.; Herbst, M. H.;
Horn, A. Jr.; Inorg. Chim. Acta 2006, 359, 3167.
23. Lessa, J. A.; Horn, A. Jr.; Pinheiro, C. B.; Farah, L. L.; Eberlin,
M. N.; Benassi, M.; Catharino, R. R.; Fernandes, C.;Inorg. Chem.
Commun. 2007, 10, 863.
24. Hazell, A.; Jensen, K. B.; Mckenzie, C. J.; Toftlund, H.; Inorg.
Chem. 1994, 33, 3127.
8. Silva, G. C.; Parrilha, G. L.; Carvalho, N. M. F.; Drago, V.;
Fernandes, C.; Horn,A. Jr.;Antunes, O.A. C.; Catal. Today 2008,
133, 684.
25. Nivorozhkin, A. L.; Anxolabéhère-Mallart, E.; Mialane, P.;
Davydov, R.;Guilhem, J.;Cesario, M.;Audière, J.-P.;Girerd, J.-J.;
Styring, S.; Schussler, L.; Seris, J.-L.; Inorg. Chem. 1997, 36, 846.
26. Horn, A. Jr.; Neves, A.; Vencato, I.; Drago, V.; Zucco, C.;
Werner, R.; Haase, W.; J. Braz. Chem. Soc. 2000, 11, 7; Horn,
A. Jr.; Vencato, I.; Bortoluzzi, A. J.; Horner, R.; Silva, R. A. N.;
Szpoganicz, B.; Drago, V.; Terenzi, H.; de Oliveira, M. C. B.;
Werner, R.; Haase, W.; Neves, A.; Inorg. Chim. Acta 2005, 358,
339; Horn,A. Jr.;Vencato, I.; Bortoluzzi,A. J.; Drago, V.; Novak,
M. A.; Neves, A.; J. Braz. Chem. Soc. 2006, 17, 1584.
27. Greenwood,N.N.;Gibb,T.C.;MössbauerSpectroscopy,Chapman
and Hall: London, 1971.
9. Rosenzweig, A. C.; Frederick, C. A.; Lippard, S. J.; Nordlund, P.;
Nature 1993, 366, 537; Whittington, D. A.; Lippard, S. J.; J. Am.
Chem. Soc. 2001, 123, 827.
10. Basch, H.;Mogi, K.;Musaev, D. G.;Morokuma, K.;J.Am. Chem.
Soc. 1999, 121, 7249.
11. Zang,Y.;Dong,Y.;Que, L. Jr.;Kauffmann, K.;Muenck, E.;J.Am.
Chem. Soc. 1995, 117, 1169.
12. Dong, Y.; Fujii, H.; Hendrich, M. P.; Leising, R. A.; Pan, G.;
Randall, C. R.; Wilkinson, E. C.; Zang,Y.; Que, L. Jr.; Fox, B. G.;
Kauffmann, K.; Münck, E.; J. Am. Chem. Soc. 1995, 117, 2778;
Hsu, H. F.; Dong,Y.; Shu, L.;Young, V. G. Jr.; Que, L. Jr.; J. Am.
Chem. Soc. 1999, 121, 5230.
28. Marchi-Delapierre, C.; Jorge-Robin, A.; Thibon, A.; Ménage,
S.; Chem. Commun. 2007, 1166; Nagataki, T.; Tachi,Y.; Itoh, S.;
J. Mol. Catal. A: Chem. 2005, 225, 103;Taktak, S.; Kryatov, S.V.;
Rybak-Akimova, E. V.; Inorg. Chem. 2004, 43, 7196.
29. Wilkinson, E. C.; Dong, Y.; Que, L. Jr.; J. Am. Chem. Soc. 1994,
116, 8394; Ito, S.; Okuno, T.; Matsushima, H.; Tokii, T.; Nishida,
Y.;J. Chem. Soc., DaltonTrans. 1996, 4037;Norman, R. E.;Holz,
R. C.;Ménage, S.;O’Connor, C. J.;Zhang, J. H.;Que, L. Jr.; Inorg.
Chem. 1990, 29, 4629.
13. Kryatov, S. V.; Taktak, S.; Korendovych, I. V.; Rybak-Akimova,
E. V.; Kaizer, J.; Torelli, S.; Shan, X.; Mandal, S.; MacMurdo, V.
L.; Payeras, A. M.; Que, L. Jr.; Inorg. Chem. 2005, 44, 85; Kim,
S. O.; Sastri, C. V.; Seo, M. S.; Kim, J.; Nam, W.; J. Am. Chem.
Soc. 2005, 127, 4178; Li, F.; Wang, M.; Ma, C.; Gao, A.; Chen,
H.; Sun, L.; Dalton Trans. 2006, 2427; Zhang, X.; Furutachi, H.;
Fujinami, S.; Nagatomo, S.; Maeda, Y.; Watanabe, Y.; Kitagawa,
T.; Suzuki, M.; J. Am. Chem. Soc. 2005, 127, 826.
30. Costes, J.-P.; Tommasino, J.-B.; Carré, B.; Soulet, F.; Fabre, P.-L.;
Polyhedron 1995, 14, 771.
14. Pardo, E.; Lloret, F.; Carrasco, R.; Muñoz, M. C.; Temporal-
Sánchez, T.; Ruiz-García, R.; Inorg. Chim. Acta 2004, 357, 2713.
15. Britovsek, G. J. P.; England, J.;White,A. J. P.; Inorg. Chem. 2005,
44, 8125.
31. Li, F.;Wang, M.;Ma, C.;Gao,A.;Chen, H.;Sun, L.;DaltonTrans.
2006, 2427.
32. Wang, X.; Wang, S.; Li, L.; Sundberg, E. B.; Gacho; G. P.; Inorg.
Chem. 2003, 42, 7799.
16. Carson, E. C.; Lippard, S. J.; Inorg. Chem. 2006, 45, 837; Britovsek,
G. J. P.; England, J.; Spitzmesser, S. K.;White,A. J. P.;Williams, D.
J.;DaltonTrans.2005,945;Nan,W.;Acc.Chem.Res.2007,40,522.
17. Schuchardt, U.; Cardoso, D.; Sercheli, R.; Pereira, R.; Cruz, R. S.;
Guerreiro, M. C.; Mandelli, D.; Spinacé, E.V.; Pires, E. L.; Appl.
Catal., A 2001, 211, 1.
33. Leising, R. A.; Kim, J.; Pérez, M. A.; Que, L. Jr.; J. Am. Chem.
Soc. 1993, 115, 9524.
34. Ito, S.; Okuno, T.; Matsushima, H.;Tokii, T.; Nishida,Y.; J. Chem.
Soc., Dalton Trans. 1996, 4479.
35. Gagne, R. R.; Koval, C. A.; Lisensky, G. C.; Inorg. Chem. 1980,
19, 2854.
18. Carvalho, N. M. F.; Horn, A. Jr.; Bortoluzzi, A. J.; Drago, V.;
Antunes, O. A. C.; Inorg. Chim. Acta 2006, 359, 90.
19. Carvalho, N. M. F.; Horn, A. Jr.; Faria, R. B.; Bortoluzzi, A. J.;
Drago, V.;Antunes, O. A. C.; Inorg. Chim. Acta 2006, 359, 4250.
20. Carvalho, N. M. F.;Antunes, O.A. C.; Horn,A. Jr.; Dalton Trans.
2007, 1023.
36. Geary, W.; Coord. Chem. Rev. 1971, 7, 81.
37. Spek, A. L.; Acta Crystallogr., Sect. A: Found. Crystallogr. 1990,
46, C34.
Received: March 31, 2009
21. Parrilha, G. L.; Fernandes, C.; Bortoluzzi, A. J.; Szpoganicz, B.;
Web Release Date: December 11, 2009