Hexanuclear and undecanuclear iron(iii) carboxylates as catalyst precursors for cyclohexane oxidation

Article abstract of DOI:10.1039/c3dt50966d

Two multinuclear complexes [Fe₆(μ₃-O) ₂(μ₄-O₂)L₁₀(OAc) ₂(H₂O)₂]·2.625Et₂O·2. 375H₂O (1) and [Fe^III₁₁Cl(μ₄-O) ₃(μ₃-O)₅L₁₆(dmf) _2.5(H₂O)_0.5]·Et₂O·1. 25dmf·3.8H₂O (2), where HL = 3,4,5-trimethoxybenzoic acid and dmf = dimethylformamide, have been prepared from trinuclear iron(iii) carboxylates via their structural rearrangement in dimethylformamide or diethyl ether-dimethylformamide 9:1, respectively, and slow vapor diffusion of diethyl ether into the reaction mixture. Both compounds have been characterized by X-ray diffraction, optical, Moessbauer spectroscopy, and magnetic measurements. Complex 1 possesses a hexanuclear ferric peroxido-dioxido {Fe₆(O ₂)(O)₂}¹²⁺ core unit, which adopts a recliner conformation, while complex 2 contains an unprecedented {Fe₁₁O ₈Cl}¹⁶⁺ core, in which 9 ferric ions are six-coordinate and the remaining two are five-coordinate. Another structural feature of note of the undecanuclear core is the presence of a deformed cubane entity {Fe ₄(μ₃-O)(μ₄-O)₃}⁴⁺. Both complexes act as catalyst precursors for the oxidation of cyclohexane to cyclohexanol and cyclohexanone with aqueous H₂O₂, in the presence of pyrazinecarboxylic acid. Remarkable TONs and TOFs (the latter mainly for 1) with concomitant quite good yields have been achieved under mild conditions. Moreover, 1 exhibits remarkably high activity in an exceptionally short reaction time (45 min), being unprecedented for any metal catalyzed alkane oxidation by H₂O₂. The catalytic reactions proceed via Fenton type chemistry.

Full text of DOI:10.1039/c3dt50966d

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Hexanuclear and undecanuclear iron(III) carboxylates as
catalyst precursors for cyclohexane oxidation†
Cite this: DOI: 10.1039/c3dt50966d
Miljan N. M. Milunovic,a Luísa M. D. R. S. Martins,b,c Elisabete C. B. A. Alegria,b,c
Armando J. L. Pombeiro,*c Regina Krachler,a Günter Trettenhahn,d
Constantin Turta,e Sergiu Shovaf and Vladimir B. Arion*a
Two multinuclear complexes [Fe6(µ3-O)2(µ4-O2)L10(OAc)2(H2O)2]·2.625Et2O·2.375H2O (1) and [FeIII11Cl-
(µ4-O)3(µ3-O)5L16(dmf)2.5(H2O)0.5]·Et2O·1.25dmf·3.8H2O (2), where HL = 3,4,5-trimethoxybenzoic acid
and dmf = dimethylformamide, have been prepared from trinuclear iron(III) carboxylates via their struc-
tural rearrangement in dimethylformamide or diethyl ether–dimethylformamide 9 : 1, respectively, and
slow vapor diffusion of diethyl ether into the reaction mixture. Both compounds have been characterized
by X-ray diffraction, optical, Mössbauer spectroscopy, and magnetic measurements. Complex 1 possesses
a hexanuclear ferric peroxido–dioxido {Fe6(O2)(O)2}12+ core unit, which adopts a recliner conformation,
while complex 2 contains an unprecedented {Fe11O8Cl}16+ core, in which 9 ferric ions are six-coordinate
and the remaining two are five-coordinate. Another structural feature of note of the undecanuclear core
is the presence of a deformed cubane entity {Fe4(µ3-O)(µ4-O)3}4+. Both complexes act as catalyst precur-
sors for the oxidation of cyclohexane to cyclohexanol and cyclohexanone with aqueous H2O2, in the
presence of pyrazinecarboxylic acid. Remarkable TONs and TOFs (the latter mainly for 1) with concomi-
tant quite good yields have been achieved under mild conditions. Moreover, 1 exhibits remarkably high
activity in an exceptionally short reaction time (45 min), being unprecedented for any metal catalyzed
alkane oxidation by H2O2. The catalytic reactions proceed via Fenton type chemistry.
Received 12th April 2013,
Accepted 27th June 2013
DOI: 10.1039/c3dt50966d
www.rsc.org/dalton
Hexanuclear iron(III) clusters, and, in particular, those contain-
ing a central peroxido group bridging four ferric ions in a
Introduction
Low nuclearity iron complexes with structural motifs {Fe2O}4+, µ4–η2:η2 fashion, have been prepared by using basic iron(III)
{Fe3O}7+ and {Fe4O2}8+ are suitable starting materials for the carboxylates containing the {Fe3O}7+ unit as starting materi-
synthesis of higher nuclearity complexes1–3 due to their rela- als.4b,c,6 The range of this type of polynuclear cage has been
tively easy chemical rearrangement in solution.4 The dominant extended by using displacement reactions of initially used carb-
structural unit in polynuclear iron complexes is {Fe3O}7+, oxylates by other carboxylate or phosphonate ligands.4c,7,8
A
although iron clusters with an {Fe4O4}4+ cubane entity or with hexanuclear iron(III) complex with three µ4–η2:η2 peroxido
a defected double-cubane core have also been reported.5 ligands bridging two Fe3O triangles resulted from a similar
reaction of basic iron(III) acetate with H2O2 in the presence of
NaOCH3 as a base.4b The family of undecanuclear iron com-
aUniversity of Vienna, Institute of Inorganic Chemistry, Währinger Strasse 42,
plexes is also small. In the past 25 years just a few compounds
A-1090 Vienna, Austria. E-mail: vladimir.arion@univie.ac.at
have been synthesized.4e,f,9–11 Some of them possess SMM fea-
bÁrea Departamental de Engenharia Química, ISEL, R. Conselheiro Emídio Navarro,
tures4f,9 due to their capacity to retain magnetization after
1959-007 Lisboa, Portugal
cCentro de Química Estrutural, Complexo I, Instituto Superior Técnico,
removal of a magnetic field. Others were investigated as model
compounds for ferritin,4e a physiologically important iron
Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisboa, Portugal.
E-mail: pombeiro@ist.utl.pt
storage protein in mammals. However, their catalytic ability
dUniversity of Vienna, Institute of Physical Chemistry, Währinger Strasse 42,
remains little explored. Hydrocarbons, particularly alkanes, are
A-1090 Vienna, Austria
eInstitute of Chemistry of the Academy of Sciences of Moldova, Academiei str. 3,
interesting compounds as the main constituents of natural oil
and gas and their C–H bonds can be converted to C–OH or
CvO functional groups leading to the formation of more valu-
able products for a fine chemical synthesis. This requires the
use of transition metals as catalysts. Nature uses iron to
oxidize C–H bonds by dioxygen activation mechanisms, in
MD-2028, Chisinau, Moldova
f‘Petru Poni’ Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda 41A,
700487 Iasi, Romania
†Electronic supplementary information (ESI) available. CCDC 889029 and
933668. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3dt50966d
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which high-valent oxido-iron species act as actual oxidizing were acquired using a conventional spectrometer in the
agents.12,13 Therefore, the synthesis of metal-based systems constant-acceleration mode (MS4, Edina, USA) equipped with
with similar catalytic ability is a challenge in catalytic a 57Co source (3.7 GBq) in a rhodium matrix. Isomer shifts are
chemistry.4a,14–16 Variable architecture of their cores and struc- quoted relative to α-Fe at room temperature. The spectra of the
tural similarity with active centers of biologically important samples measured at 300 and 80 K were fitted using the
enzymes such as methane monooxygenase (MMOx),17 toluene NORMOS Mössbauer Fitting Program. The sample measured
monooxygenase (TMOx),18 Δ-9-desaturase (Δ-9-D),19 and ribo- at 4.2 and 2.7 K was inserted inside an Oxford Instruments
nucleotide reductase (RNR)20 make them well suited for assay- Mössbauer-Spectromag 4000 Cryostat, which has a split-pair
ing the catalytic activity in organic reactions.4a,c,21 We have a superconducting magnet system for applied fields up to 5 T,
general interest in the metal catalyzed, mild oxidative with the field of the sample oriented perpendicular to the
functionalization of alkanes and alcohols.4a,22–40 The use of gamma-ray direction. Magnetic data were obtained with a
polynuclear iron(III) carboxylates in alkane oxidation has been Quantum Design MPMS SQUID susceptometer. Magnetic sus-
recently reported by us,4a and peroxido species formation has ceptibility measurements were performed in the 2–300 K
been hypothesized on reaction of the hexanuclear iron(III) temperature range under a 0.1 T applied magnetic field,
complex with hydrogen peroxide. This prompted us to attempt and diamagnetic corrections were applied by using Pascal’s
the synthesis of other polynuclear iron carboxylates, isolation constants.42 Isothermal magnetization measurements were
of peroxido species and elucidation of their role in catalytic performed up to 6 T at 2 K. Gas chromatographic (GC)
alkane oxidation.
measurements were carried out using a FISONS Instruments
Herein, we report on the synthesis, X-ray diffraction struc- GC 8000 series gas chromatograph with a FID detector and a
ture, spectroscopic characterization, magnetism and catalytic capillary column (DB-WAX, column length: 30 m; internal dia-
activity of two multinuclear iron(III) carboxylates, namely a meter: 0.32 mm) and the Jasco-Borwin v.1.50 software. The
hexanuclear µ4–η2:η2-peroxido complex [Fe6(µ3-O)2(µ4-O2)- temperature of injection was 240 °C. The initial temperature
L10(OAc)2(H2O)2]·2.625Et2O·2.375H2O (1) and an undecanuc- was maintained at 100 °C (for peroxidative oxidation of cyclo-
lear complex [FeIII11Cl(µ4-O)3(µ3-O)5L16(dmf)2.5(H2O)0.5]·Et2O· hexane, alkane carboxylation and hydrocarboxylation or 80 °C
1.25dmf·3.8H2O (2) with
a new core topology, where for n-heptane oxidation to corresponding alcohols and
HL = 3,4,5-trimethoxybenzoic acid. The compounds were ketones) for 1 min, then raised to 180 °C (10 °C min−1) and
tested as catalyst precursors for cyclohexane oxidation to held at this temperature for 1 min. Helium was used as the
cyclohexanol and cyclohexanone, the mainstream starting carrier gas. GC-MS analyses were performed using a Perkin
materials in the production of adipic acid, nylon-6,6′, and Elmer Clarus 600C instrument (He as the carrier gas),
polyamide-6.41
equipped with a 30 m × 0.22 mm × 25 μm, BPX5 (SGE) capil-
lary column.
Preparation of complexes
Experimental section
Materials and physical measurements
[Fe3OL6(H2O)3]NO3·HL. 3,4,5-Trimethoxybenzoic acid (HL)
(3.00 g, 14.14 mmol) was neutralized with a solution of
Pyrazinecarboxylic acid (Hpca), trifluoroacetic acid, n-butyric sodium hydrogen carbonate (1.19 g, 14.14 mmol) in water
acid and 3,4,5-trimethoxybenzoic acid were purchased from (20 mL) under stirring at 50–60 °C until effervescence of CO2
Sigma Aldrich, nitric acid (65%) and potassium peroxodi- was completed. To the filtered solution of the sodium salt of
sulfate from Fluka, FeCl3·6H2O from Riedel-de-Haën and 3,4,5-trimethoxybenzoic acid at room temperature was added
Fe(NO3)3·9H2O, FeSO4·7H2O and Fe(CF3SO3)3 from Acros Organ- dropwise a solution of iron(III) nitrate nonahydrate (2.85 g,
ics. Acetonitrile (Riedel-de-Haën), cyclohexane (Aldrich), cyclo- 7.07 mmol) in water (25 mL). The reaction mixture was stirred
pentane (Aldrich), cyclohexanol (Aldrich), cyclohexanone for 15 min and allowed to stand at room temperature. The
(Aldrich), cycloheptanone (Aldrich), n-pentane (Aldrich), orange precipitate was filtered off, dissolved in CHCl3
n-heptane (Aldrich), hydrogen peroxide (30% and 50% in water) (200 mL), and solution filtered. After removing the solvent, the
(Fluka), diethyl ether (Riedel-de-Haën), triphenylphosphine, red solid was dried in vacuo overnight. Yield 2.78 g, 66.5%.
diphenylamine (Fluka), bromotrichloromethane (Fluka), nitro- Found: C, 47.61; H, 4.86; N, 0.43. Calc. for C70H84Fe3NO42
methane (Aldrich), ethane (AlphaGaz), carbon monoxide (Air (Mr 1778.9 g mol−1): C, 47.26; H, 4.76; N 0.79. ESI-MS in MeOH
Products) and dinitrogen (Air Liquid Portugal) were used as (positive): m/z 1450 ([Fe3OL6]+). ESI-MS in MeOH (negative):
received from the suppliers. The solvents were dried over m/z 1183.5 ([Fe2OL5]−).
appropriate drying agents and degassed using standard
[Fe6(µ3-O)2(µ4-O2)L10(OAc)2(H2O)2]·2.625Et2O·2.375H2O (1).
methods. Elemental analysis was carried out by the Micro- To a solution of [Fe3OL6(H2O)3]NO3·HL (0.8 g, 0.45 mmol) in
analytical Service of the Institute of Physical Chemistry of the methyl ethyl ketone (9 mL), a 30% solution of H2O2 (0.2 mL)
University of Vienna. UV-vis spectra were recorded on an was added and the reaction mixture was stirred at room temp-
Agilent 8453 UV-vis spectrophotometer. The MIR spectra were erature for 1 h. The slow diffusion of Et2O into the reaction
measured on
a Bruker Vertex 70 FT-IR spectrometer mixture produced dark-red X-ray diffraction quality crystals
(4000–400 cm−1) by using an ATR unit. The Mössbauer spectra in ca. 10 days, which were filtered off and dried in vacuo.
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A sample for X-ray diffraction study was left in contact with the follows: 0.05–26 μmol of the catalyst precursor (1 or 2) and
mother liquor to prevent the loss of solvent. Yield: 0.45 g, 8.6–130 μmol of pyrazinecarboxylic acid (Hpca) (i.e. a (1 or
75.0%. Found: C, 46.00; H, 4.43. Calc. for C104H124Fe6O62 (Mr 2) : Hpca molar ratio of 1 : 5 to 1 : 150) were dissolved in MeCN
2701.1 g mol−1): C, 46.24; H, 4.63. λmax(MeCN)/nm 265 (ε/dm3 (3.00 mL) with vigorous stirring, whereafter 10.00 mmol of
mol−1 cm−1 130 000), 343 (24 000). ν/cm−1: 3470, 2942, 2837, 30% H2O2 (1.02 mL) and 5.00 mmol of cyclohexane (0.54 mL)
1589, 1544, 1450, 1436, 1389, 1318, 1225, 1182, 1122, 1002, were added and the reaction solution was stirred for 6 h, at
958, 882, 783, 758, 728.
room temperature (r.t.) and normal pressure. In the experi-
[Fe3OL6(H2O)3]Cl·HL. 3,4,5-Trimethoxybenzoic acid (HL) ments with radical traps, CBrCl3 (5.00 mmol) or NHPh2
(4.00 g, 18.85 mmol) was neutralized with a solution of (5.00 mmol) was added to the reaction mixture. The product
sodium hydrogen carbonate (1.58 g, 18.85 mmol) in water analysis was performed as follows: 90 μL of cycloheptanone
(55 mL) under stirring at 60 °C until effervescence of CO2 was (internal standard), 10.00 mL of diethyl ether (to extract the
completed. To the filtered solution of the sodium salt of 3,4,5- substrate and the organic products from the reaction mixture)
trimethoxybenzoic acid was added dropwise a solution of iron(III) and an excess of triphenylphosphine (in order to reduce the
chloride hexahydrate (2.54 g, 9.43 mmol) in water (39 mL). formed cyclohexyl hydroperoxide to the corresponding
The reaction mixture was stirred at 60 °C for 4 h. The orange alcohol, and hydrogen peroxide to water, following a method
precipitate was filtered off, dried in vacuo overnight and stored developed by Shul’pin)46 were added. The obtained mixture
in a desiccator over CaCl2. Yield 3.65 g, 75.7%. Found: C, was stirred for 10 min and then a sample (1 μL) was taken
47.81; H, 4.84; Cl, 2.03. Calc. for C70H84ClFe3O39 (Mr 1752.4 g from the organic phase and analyzed by gas chromatography
mol−1): C, 47.98; H, 4.83; Cl, 2.02. ESI-MS in MeOH (positive): (GC) using the internal standard method. Blank tests indicate
m/z 1450 ([Fe3OL6]+).
[FeIII11Cl(µ4-O)3(µ3-O)5L16(dmf)2.5(H2O)0.5]·Et2O·1.25dmf·3. complex or the oxidant.
that no oxidation takes place in the absence of the iron
8H2O (2). A solution of [Fe3OL6(H2O)3]Cl·HL (0.56 g,
For n-heptane oxidation, 1.3 μmol of 1 or 2, a catalyst
0.36 mmol) in dmf–Et2O = 1 : 9 (50 mL) was stirred at room precursor : Hpca molar ratio of 1 : 50, a 50% aqueous solution
temperature for 1 h. Slow diffusion of Et2O into the reaction of H2O2 (0.2 mL) and n-heptane (100.0 μL) were vigorously
mixture produced X-ray diffraction quality dark-red crystals stirred, in MeCN (up to 2.50 mL), whereafter the reaction solu-
of 2 in two months. Yield: 0.05 g, 11.0%. The sample for tion was stirred for 3 h, at 50 °C and normal pressure. The
elemental analysis was dried in vacuo for 6 h. Anal. Found: product analysis by GC was performed directly on the reaction
C, 45.53; H, 4.21; N, 0.33; Cl, 1.16. Calc. for C166.75H197.75
-
mixture.
Hydrocarboxylation and carboxylation reactions of
ClFe11N2.25O93.25 (Mr 4375.3 g mol−1): C, 45.77; H, 4.56; N,
0.72; Cl, 0.81. λmax(MeCN)/nm 265 (ε/dm3 mol−1 cm−1 alkanes were carried out in a stainless steel autoclave with a
264 000), 350 sh (54 400). ν/cm−1: 3510, 3074, 2939, 2836, 1645, capacity of 13.00 mL. Typical conditions are as follows:
1546, 1385, 1223, 1186, 1120, 998, 776, 760.
0.002 mmol of 1 or 2, 2 mmol of potassium peroxodisulfate
K2S2O8 (1000 : 1 molar ratio of K2S2O8 to catalyst precursor),
MeCN (2.50 mL), H2O (2.50 mL) (total solvent volume
X-ray crystallography
X-ray diffraction measurements were performed on a Bruker 5.00 mL) for hydrocarboxylation or TFA (5.00 mL) for carb-
X8 APEXII CCD diffractometer. Single crystals of 1 and 2 were oxylation, the substrate (1.00 mmol of liquid alkanes and 10
positioned at 40 mm from the detector, and 2491 and 6982 atm (2.66 mmol) of C2H6) and a Teflon-coated magnetic stir-
frames were measured, each for 30 and 20 s over 1° scan ring bar were placed in the autoclave. Then, the autoclave
width. The data were processed using SAINT software.43 Crystal was closed and flushed with CO (three times for removing
data, data collection parameters, and structure refinement the air) and finally pressurized with 20 atm (5.33 mmol) of
details are given in Table S1.† The structures were solved by CO (due to the high toxicity of CO, all the manipulations
direct methods and refined by full-matrix least-squares tech- should be performed with due care!). The autoclave was
niques. The disorder in some carboxylate ligands in 1 was introduced into an oil bath and heated at 60 °C for 4 h, for
resolved with restraints on the displacement parameters and hydrocarboxylation, or at 80 °C for 20 h, for carboxylation,
on bond distances using mainly the EADP and SADI instruc- with magnetic stirring of the reaction solution. After the reac-
tions of SHELX-97, respectively. The following software pro- tion was complete, the autoclave was cooled with an ice bath
grams and computer were used: structure solution SHELXS-97, to room temperature, degassed, opened and the contents
refinement SHELXL-97,44 molecular diagrams ORTEP-3,45 transferred to a flask. Diethyl ether was added to extract the
computer, Intel CoreDuo. CCDC 933668 and CCDC 889029 organic products. The mixture was vigorously stirred and fil-
contain the supplementary crystallographic data for this tered off, and 90 μL of cycloheptanone (GC internal stan-
paper.
dard) were added to the filtered ether solution, whereafter
aliquots (1 μL) of the solution were analyzed by GC using
the internal standard method. Blank tests indicated that
the hydrocarboxylations also proceed in the metal-free
Typical procedures for the catalytic oxidation of alkanes and
product analysis
The peroxidative oxidation reactions were carried out in systems.47–49 The formation of dicarboxylic acids was not
Schlenk tubes and under a dinitrogen or air atmosphere as observed.
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Results and discussion
Crystal structure of 1
The result of the X-ray diffraction study of 1 is shown in Fig. 1.
The hexanuclear complex has Ci point symmetry. The structure
consists of two [Fe3(µ3-O)(H2O)(µ2-O2CR)4(µ2-OAc)]2+ units
bridged by two carboxylate ligands (µ2-O2CR)− and a µ4,η2:η2-
peroxido ion (Fig. 1). The atoms Fe1 and Fe3 are spanned by
an acetate ligand adopting a µ2–η1:η1 binding mode. The
coordination number of the distal atom Fe2 is completed to
six by coordination of water molecules.
Fig. 2 Recliner conformation of the {Fe6(O2)(O)2}12+ core in 1.
As in [Fe6(O2)(O)2(O2CCMe3)12(HO2CCMe3)2], the core unit
[Fe6(O2)(O)2]12+ in 1 adopts a recliner conformation as shown
in Fig. 2.4c This contrasts with the ‘butterfly’-like conformation
discovered in other related complexes.4c The dihedral angle
between the mean plane through Fe1, Fe2 and Fe3 and that
through Fe1, Fe3 and Fe1i, Fe3i in 1 is at 137.6°. Atoms
marked with i were generated by symmetry transformation −x,
−y + 1, −z + 2. The peroxido oxygen atoms O1 and O1i come
Crystal structure of 2
Likewise, complex 2 crystallizes in the triclinic centrosym-
metric space group P1. The result of an X-ray diffraction study
ˉ
is shown in Fig. 3. The structure of 2 consists of an
{FeIII11O8}17+ core, which possesses C1 point symmetry, with
three µ4-O2− and five µ3-O2− bridges. Of the 11 ferric ions, 9
are six-coordinate while the remaining 2 (Fe7 and Fe8) are five-
coordinate. Peripheral coordination is provided by 15 bridging
carboxylate groups adopting a µ2–η1:η1 binding mode and one
µ2–η1:η2-carboxylate (O39 acts as a bridging atom between Fe9
and Fe10), one chlorido ligand and three dmf–H2O molecules.
One interesting feature of the core structure in 2 is the pres-
ence of a distorted cubane {Fe4O4}4+ unit, in which the iron
atoms Fe2, Fe3, Fe4, Fe7 and oxygen atoms O1, O2, O3, O4 are
involved (Fig. 4 and 5). The distortion is manifested by the
lack of bonding interaction between Fe7 and O2 (Fe7–O2
2.925 Å compared to Fe2–O7 at 1.955 Å). Of the four oxygen
atoms building the distorted cubane unit, three (O1, O3 and
O4) are µ4-O2− and O2 acts as a µ3-O2− ligand. The atom Fe7
plays an important role in the core structure sharing its
out from the mean plane through Fe1, Fe3, Fe1i and Fe3 by
i
0.16 and −0.16 Å, respectively. Thus the weakly pyramidally
distorted geometry of peroxido oxygen atoms can be men-
tioned here. The sum of bond angles around O1 is 355.7°.
This value, which indicates the extent of planarity of peroxido
oxygen atoms, is at 357.4 and 359.2° in [Fe6(O2)-
(O)2(O2CPh)12(H2O)2].6 Strong pyramidality of peroxido oxygen
atoms was observed in [Fe6(O2)3(O)2(OAc)9]−,4b where the sum
of bond angles around peroxido oxygen atoms is 322.0° and
331.8°.
Fig. 1 The molecular structure of 1. Hydrogen atoms, trimethoxy and some tri-
methoxyphenyl groups were omitted for clarity. Selected bond distances (Å) and
bond angles (°): Fe1–O1 2.047(3), Fe1–O2 1.889(3), Fe2–O2 1.895(3), Fe3–O2
1.895(3), O1–O1i 1.471(5), Fe1–O1–Fe3i 127.75(14), Fe1–O2–Fe2 122.39(15),
Fe1–O2–Fe3 114.55(15), Fe2–O2–Fe3 122.83(15). Atoms marked with i were
generated by symmetry transformation −x, −y + 1, −z + 2.
Fig. 3 The molecular structure of 2. Hydrogen atoms, trimethoxy and some tri-
methoxyphenyl groups were omitted for clarity.
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served not only as a solvent, but also as a source of acetic acid.
Complex 2 was prepared by dissolution of basic iron(III) carb-
oxylate [Fe3OL6(H2O)3]Cl·HL (HL = 3,4,5-trimethoxybenzoic
acid) in a mixture of dmf–Et2O 1 : 9 and crystallized within two
months by slow diffusion of Et2O into the reaction mixture at
room temperature.
Mössbauer spectroscopy and magnetic susceptibility
measurements
The Mössbauer spectra of 1 show a symmetric quadrupole
doublet with an isomer shift δ = 0.54 mm s−1 and a splitting
ΔEQ = 0.75 mm s−1, and δ = 0.43 mm s−1 and ΔEQ = 0.75 mm
s−1, at 80 and 300 K, respectively (Fig. 6 and Fig. S1†), indicat-
ing a high-spin ferric compound.4a
Fig. 4 The core unit {Fe11O8Cl}16+ of 2. Selected bond distances (Å) and bond
angles (°): Fe1–O1 1.917(4), Fe2–O1 2.089(4), Fe3–O1 2.074(4), Fe4–O1 2.033(4),
Fe2–O2 1.955(4), Fe3–O2 1.946(4), Fe5–O2 1.892(4), Fe2–O3 1.968(4), Fe4–O3
2.077(4), Fe6–O3 1.959(4), Fe7–O3 2.095(4), Fe3–O4 1.997(4), Fe4–O4 2.001(4),
Fe7–O4 2.115(4), Fe8–O4 1.990(4), Fe5–O5 1.939(4), Fe7–O5 1.904(4), Fe9–O5
1.948(4), Fe6–O6 1.965(4), Fe7–O6 1.923(4), Fe10–O6 1.894(4), Fe7–O7 1.931(4),
Fe8–O7 1.874(4), Fe11–O7 1.897(4), Fe9–O8 1.929(4), Fe10–O8 1.939(4),
Fe11–O8 1.895(4), Fe8–Cl1 2.2411(17), O1–Fe2–O2 83.53(15), O1–Fe2–O3
83.40(15), O1–Fe3–O2 84.13(15), O1–Fe4–O3 82.13(15), O1–Fe4–O4 83.79(15),
O2–Fe3–O4 95.45(16), Fe4–O4–Fe7, Fe2–O3–Fe7 116.23(19), Fe4–O3–Fe7
96.16(16).
The Mössbauer spectrum of 2–57Fe (compound 2 prepared
from 57Fe) at 80 K displays a quadrupole doublet (Fig. 7) with
an isomer shift δ = 0.57 mm s−1 and a splitting ΔEQ = 0.88 mm
s−1 very close to those reported for a hexanuclear ferric carb-
oxylate with a [Fe6(µ3-O)3(µ2-OH)]11+ core (δ = 0.53 mm s−1
,
Fig. 5 Distorted cubane unit {Fe4 (μ3-O)(μ4-O)3}4+ of 2.
position with three µ3-oxido centered iron(III) triangles (Fig. 4).
The dihedral angles formed between the plane through Fe7,
Fe5, Fe9 with those through Fe7, Fe8, Fe11 and Fe7, Fe6, Fe10
are of 68.4 and 72.7°, respectively. Note that the coordination
environment of the five-coordinate atom Fe7 consists of two
µ4-O2− and three µ3-O2− ligands exclusively, while that of Fe8
consists of µ4-O2− (O4), µ3-O2− (O7), and two µ-carboxylate
ligands in the basal plane of the square-planar pyramid and
apical chloride. The terminal iron ions Fe9, Fe10 and Fe11 are
joined together by the µ3-oxido bridging ligand O8. The Fe1
atom is connected to O1 of the cubane entity. The other four
octahedral sites are occupied by 4 carboxylate oxygens (of four
different 3,4,5-trimethoxybenzoate ligands), while the sixth
position by dmf and H2O with equal probability.
Fig. 6 Mössbauer spectrum of 1 at 80 K (δ = 0.54 mm s−1, ΔEQ = 0.75 mm s−1).
Syntheses
Complex 1 was obtained by oxidation of [Fe3OL6(H2O)3]-
NO3·HL, where HL = 3,4,5-trimethoxybenzoic acid, with H2O2
in methyl ethyl ketone and crystallized within two weeks by
slow diffusion of diethyl ether into the reaction mixture at
Fig. 7 Mössbauer spectrum of 2–57Fe at 80 K (δ = 0.57 mm s−1, ΔEQ
=
room temperature. On this occasion methyl ethyl ketone 0.88 mm s−1).
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ΔEQ = 0.96 mm s−1).4a The parameters of 2 are also close to observed for [FeIII(gma)(PBu3)] (gmaH2 = glyoxal-bis(2-mercapto-
those of 1.
anil); up to 8%)39b or [Fe{HOCH2C(pz)3}2]2[Fe2OCl6](Cl)2·
The temperature dependence of the magnetic susceptibility 4H2O (pz = pyrazolyl; up to 14%),25,34b,52 comparable with
of 2 is shown in Fig. S2.† The room temperature χMT value is those exhibited by Li[FeCl2{SO3C(pz)3}] (up to 24%),34a by the
11.68 cm3 K mol−1, which is much less than the value Co(III)–Fe(III) Schiff base complex [Co4Fe2OSae8]·4dmf·H2O
48.13 cm3 K mol−1 expected for 11 uncoupled S = 5/2 centers (H2Sae = salicylidene-2-ethanolamine) (up to 26%)53 and by
with g = 2. As the temperature is decreased, χMT decreases stea- the hexanuclear iron(III) p-nitrobenzoate complex [Fe6O3(OH)-
dily to a value of 0.55 cm3 K mol−1 at 2.0 K, indicating strong (p-NO2C6H4COO)11(dmf)4] (20%) which displays a much lower
antiferromagnetic interactions between the FeIII ions. This is TON (100).4a They are lower than those for Fe(III)–Cr(III) hydro-
suggestive of an S = 1/2 ground state with a partially populated xides39a (up to 45%) but the latter system requires the use of a
low lying S = 3/2 excited state as χMT is calculated to be 0.375 strong acid (HNO3) and leads to a much lower TON value of
and 1.875 cm3 K mol−1 for S = 1/2 and S = 3/2 systems with g = 30. The combination of such a high yield and TON (and TOF),
2.0. We have not attempted to simulate the magnetic suscepti- reached in a short reaction time and under mild conditions, is
bility data, as a large number of different exchange parameters very rare and indicates exceptional activity of the catalytic
would be required, taking into account the high nuclearity and system based on complex 1. For example, the above hetero-
low symmetry of 2.
metallic Co4Fe2 Schiff base complex reaches concomitantly high
TON (3.6 × 103) and yield (26%) values only after 5 h reaction
time (TOF = 7.2 × 102 h−1), in the presence of HNO3 (a much
stronger acid than Hpca in our case).53 Moreover, the oxidation
of cyclohexane with hydrogen peroxide in the presence of deca-
methylosmocene, (Me5C5)2Os, and pyridine, also an exception-
ally active system,50 needs 24 h to exhibit higher TON values
(up to ca. 5.1 × 104) although the TOF values are lower (up to
ca. 2.1 × 103 h−1) and the yields remain low (up to ca. 6%).
A high selectivity toward the formation of cyclohexanol and
cyclohexanone is exhibited by our systems, since no traces of
by-products were detected by GC analysis of the final reaction
mixtures.
For oxidation in the presence of 2, in general, cyclohexanol
is the main product after treatment of the final reaction
mixture with triphenylphosphine, and ca. 92% selectivity
(Table 1, entry 34) can thus be reached toward alcohol for-
mation, expected to occur mainly via cyclohexyl hydroperoxide
(ROOH). However, the yield of cyclohexanone increases upon
prolonged reaction times (Table 1, entry 52). For 1, the trend is
not so clear, but cyclohexanone is the main product in most of
the performed experiments.
The presence of Hpca (pyrazinecarboxylic acid) is essential
for these reactions, since without it only traces of products are
formed (Table 1, entries 1 and 26, Fig. 9). The addition of
Hpca (up to an Hpca : catalyst precursor molar ratio of 25 : 1
(1) or 50 : 1 (2)) leads to a remarkable yield growth for both
alcohol and ketone (Fig. 9). A high promoting effect of that
heteroaromatic acid on the peroxidative oxidation of alkanes
catalyzed by homogeneous iron species39b,54 on the oxyfunctio-
nalization of cyclohexane with dioxygen catalyzed by scorpio-
nate vanadium complexes,55 as well as by pyrazole–rhenium
complexes supported on modified silica gel,38 has been pre-
viously recognized. The role of Hpca, via coordination of pca−
to iron, upon displacement of labile ligands in 1 or 2 (namely
H2O and dmf), conceivably consists in assisting proton-trans-
fer from ligated H2O2 to an oxido- or a hydroxido-ligand, thus
promoting the formation of hydroperoxyl (HOO˙) and hydroxyl
(HO˙) radicals, as was proposed46,54,56 for vanadium catalyst–
Hpca–H2O2 systems and supported by theoretical calcu-
lations.54c,57 However, an excessive amount of Hpca leads to a
Catalytic oxidation of alkanes
Complexes 1 and 2 act as catalyst precursors for the peroxida-
tive oxidation of cyclohexane with aqueous H2O2, in the pres-
ence of pyrazinecarboxylic acid.
Peroxidative oxidation of cyclohexane
The system 1 (or 2)–pyrazinecarboxylic acid (Hpca)–H2O2 exhi-
bits remarkable catalytic activity toward the oxidation of cyclo-
hexane to cyclohexanol (OL) and cyclohexanone (ONE), via
cyclohexyl hydroperoxide (CyOOH, primary product), accord-
ing to Scheme 1 (without a defined stoichiometry) and
Table 1. The formation of CyOOH is proved by using the
method proposed by Shul’pin.46 In fact, addition of PPh3 prior
to the GC analysis of the products results in a marked increase
of the amount of cyclohexanol (due to reduction of CyOOH by
PPh3) and a corresponding decrease of cyclohexanone, as
observed in other catalytic systems.23d,25,34,46,49–51
Overall turnover numbers (TON, moles of product per mol
of catalyst precursor) and yields up to ca. 2.2 × 103 (1) or 3.5 ×
103 (2) and 22% (1) or 20% (2) (relative to the alkane), respecti-
vely, have been achieved at room temperature (r.t.), after
45 min (1) or 6 h (2) reaction time (Fig. 8). In fact, 1 is, to our
knowledge, the first metal catalyst precursor able to provide
such combined high yield and TON values in a remarkably
reduced reaction time (corresponding to a TOF value of ca. 3.0
× 103 h−1) and under the mild reaction conditions used.
The achieved TONs (and TOFs) are remarkably high, much
above those reported4a,34,39a,b,52 for iron or other metal
complex catalysts in spite of the mild conditions, in aqueous
acetonitrile at a low temperature with a green oxidant, in the
presence of a weak acid. The yields are also higher than those
Scheme 1 Peroxidative oxidation of cyclohexane.
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Table 1 Peroxidative oxidation of cyclohexane with H2O2 (selected data)a
Yieldb (%)
OL
Entry
Catalyst
n(Hpca)/n(precat.)
n(H2O2)/n(precat.) × 10−3
Reaction time (h)
ONE
Totald
TONc
1
2
3
4
5
6
7
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
—
5
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
1
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.25
0.5
1
0.1
3.7
3.9
9.7
5.1
2.8
1.9
0.9
8.8
5.3
2.3
2.4
5.5
8.4
7.9
6.4
6.2
6.0
6.1
5.0
3.8
7.5
1.7
0.9
6.5
0.4
3.7
1.2
7.5
6.3
0.2
12.4
11.4
14.7
4.2
9.7
10.3
12.2
12.7
2.6
5.7
13.4
5.6
2.5
2.2
3.9
3.6
5.3
8.0
11.3
10.4
6.8
0.2
0.9
13.2
0
0.1
3.8
10.5
22.1
12.9
9.4
8.4
6.1
18.3
11.2
5.7
1
380
0.1
6.6
12.4
7.8
6.6
6.5
5.2
9.5
5.9
3.4
7.3
9.0
8.2
6.0
5.6
5.4
5.1
4.1
1.8
10.6
11.1
0
15
25
35
50
100
150
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
—
5
1.05 × 103
2.21 × 103
1.29 × 103
940
840
610
183
112
570
970
8
9e
10f
11g
12
13
14
15
16
17
18
19
20
21
22
23h
24i
25j
26
27
28k
29
30
31k
32
33
34
35
36
37
38
39
40
41
42
43l
44m
45
46
47
48
49
50
51
52
53h
54i
55j
9.7
14.5
16.6
13.9
12.0
11.6
11.1
10.2
6.8
14.4
18.6
1.7
1.0
16.5
0.4
1.45 × 103
1.66 × 103
1.39 × 103
1.20 × 103
1.16 × 103
1.16 × 103
1.02 × 103
680
3
6
12
24
30
0.75
0.75
0.75
0.75
0.75
0.75
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
0.25
0.5
0.75
1
3
6
10
24
6
6
6
4
1.44 × 103
1.86 × 103
170
40
20
20
20
40
40
40
40
40
40
40
40
40
20
20
20
20
20
1
0.1
10.0
0
0.9
0
1.5
3.4
0
100
1.65× 103
81
4.6
1.2
9.0
9.7
80
289
5
25
35
35
50
100
150
5
1.65 × 103
2.24 × 103
42
0.2
4.7
4.0
1.2
1.0
3.5
7.2
5.9
4.8
1.1
3.7
6.0
0.6
1.2
2.0
2.6
3.4
4.7
6.3
8.5
9.0
7.5
1.0
0.1
6.3
17.1
15.4
15.9
5.2
13.2
17.5
18.0
17.5
3.7
9.4
19.8
6.2
3.7
4.2
3.42 × 103
2.73 × 103
1.59 × 103
520
25
35
100
150
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
1.32 × 103
1.75 × 103
1.80 × 103
1.75 × 103
36
2
90
20
20
20
4
4
4
4
4
4
4
1.98 × 103
623
335
81
129
137
193
255
380
376
6.5
7.0
10.0
14.3
19.8
19.4
14.3
1.2
4
276
118
20
20
20
1.0
19.6
102
1.96 × 103
a Reaction conditions (unless stated otherwise): acetonitrile (3.0 mL), cyclohexane (5.0 mmol), r.t., under dinitrogen; (0.25–10) μmol of 1 or
(0.25–10) μmol of 2, (1 or 2) : Hpca (molar ratio 1 : 5 to 1 : 150), H2O2 (1000 : 1 to 40 000 : 1 molar ratio of oxidant to catalyst precursor). Percentage
of yield, TON determined by GC analysis (upon treatment with PPh3). b Molar yield (%) based on substrate, i.e. moles of products (cyclohexanol
(Ol) and cyclohexanone (One)) per 100 mol of cyclohexane. c Turnover number (moles of product per mol of catalyst precursor). d Moles of
cyclohexanol + cyclohexanone per 100 moles of cyclohexane. e 2nd run of catalyst resulting from 1 used in entry 4 (same reaction conditions). f 3rd
run of catalyst resulting from 1 used in entry 4 (same reaction conditions). g 4th run of catalyst resulting from 1 used in entry 4 (same reaction
conditions). h Reaction in the presence of CBrCl3 (5 mmol). i Reaction in the presence of Ph2NH (5 mmol). j Reaction performed in the presence
of O2. k Reaction in the presence of nitric acid instead of Hpca (same amount). l 2nd run of catalyst resulting from 2 used in entry 41 (same
reaction conditions). m 3rd run of catalyst resulting from 2 used in entry 41 (same reaction conditions).
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Fig. 10 Dependence of the overall yield (mol%, based on substrate) of the pro-
Fig. 8 Dependence of the overall yield (mol%, based on substrate) of the pro-
ducts on the reaction time. Reaction conditions: MeCN (3.0 mL), cyclohexane
(5.0 mmol), 0.5 μmol of 1 or 2.5 μmol of 2, n(H2O2)/n(catalyst precursor)
(2 × 104), n(Hpca)/n(1) (25.0), n(Hpca)/n(2) (50.0), r.t.
■
◆
ducts on the amount of oxidant (H2O2, molar ratio relative to 1 ( ) or 2 ( )) in
the oxidation of cyclohexane. Reaction conditions: n(H2O2)/n(catalyst precursor)
(0–4 × 104), MeCN (3.0 mL), cyclohexane (5.0 mmol), 0.25–10 μmol of 1 or 2,
n(Hpca)/n(1) (25.0), n(Hpca)/n(2) (50.0), 45 min (1) or 6 h (2), r.t.
GC-MS for the conditions of entries 4 and 42, for 1 and 2, in
Table 1.
The oxidation of cyclohexane proceeds even if a very small
relative amount of 1 (or 2) is used leading to rather high TONs
(Table 1, e.g., entry 32 for 2). The effect of the catalyst precur-
sor amount on the overall yield is illustrated in Fig. 11, the
latter increasing with the former up to ca. 22% (1) or 20% (2)
for an n(catalyst precursor)/n(cyclohexane) molar ratio of
ca. 1 × 10−4 and 3 × 10−4, respectively. Higher catalyst amounts
result in a yield drop, possibly due to overoxidation (Fig. S7
and S8†).
Under the same reaction conditions, FeSO4·7H2O, Fe-
(CF3SO3)3 and [Fe3OL6(H2O)3]NO3·HL (HL = 3,4,5-trimethoxy-
benzoic acid) demonstrated markedly lower catalytic activity in
terms either of yields of products or TOFs (see Fig. S9†).
Experiments in the presence of O2 were also performed
(Table 1, entries 25 and 55). For 1, the obtained overall yield
and TON values for 0.75 h in air (Table 1, entry 25) are quite
similar to the ones obtained for 1 h under dinitrogen (Table 1,
entry 14), suggesting only a slightly promoting effect of the air
■
◆
Fig. 9 Effects of the Hpca amount (molar ratios relative to 1 ( ) or 2 ( )) and
□
of the HNO3 amount (molar ratio relatively to 2 ( )) on the overall yield (mol%,
based on substrate) of the products of cyclohexane oxidation. Reaction con-
ditions: n(H2O2)/n(catalyst precursor) (2 × 104), 0.5 μmol of 1 or 2, MeCN
(3.0 mL), cyclohexane (5.0 mmol), n(Hpca)/n(1 or 2) (150–0), n(HNO3)/n(2)
(35–0), 45 min (1) or 6 h (2), r.t.
decrease in the overall yield of cyclohexanol and cyclohexa-
none (Fig. 9, Fig. S3 and S4†), conceivably due to the compe-
tition with H2O2 for the active metal center, eventually leading
to inactive Fe(pca)2 type species.54d
The activity of 2 is also promoted by addition of a mineral
acid such as HNO3 (Table 1, entries 28 and 31; Fig. 9), as recog-
nized by us for other metal catalysts,23d,25,31–34,36,39a,b,51,52,58,59
but to a much lower (almost insignificant) extent, even
showing an inhibiting role for an n(HNO3)/n(catalyst precur-
sor) ratio of 35 (Fig. 9).
The effect of the peroxide-to-catalyst precursor molar ratio
was investigated and is depicted in Fig. 10 (Fig. S5 and S6†).
The increase of the peroxide amount up to an n(H2O2)/n(cata-
lyst precursor) molar ratio of 2 × 104 leads to the maximum
product yield of 22% (1) or 20% (2). Further increase of the
n(H2O2)/n(catalyst precursor) molar ratio results in a slight yield
drop, eventually due to overoxidation reactions at higher H2O2
amounts. In fact, some products of overoxidation such as 1,4-
cyclohexanedione and 1,2-epoxycyclohexane were detected by
Fig. 11 Dependence of the overall yield (mol%, based on substrate) of the pro-
■
◆
ducts on the amount of catalysts 1 ( ) or 2 ( ) in the oxidation of cyclohexane.
Reaction conditions: n(H2O2)/n(catalyst precursor) (2 × 104), MeCN (3.0 mL),
cyclohexane (5.0 mmol), 0.25–10 μmol of 1 or 2, n(Hpca)/n(1) (25.0), n(Hpca)/
n(2) (50.0), 45 min (1) or 6 h (2), r.t.
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atmosphere. In accord with more oxidant availability, the ratio leads to CyOO˙, and CyOOH can then be formed upon
cyclohexanone–cyclohexanol increased. For 2, the obtained H-abstraction from H2O2 by CyOO˙ (Scheme 2, reactions 2 and 3)
results (Table 1, entry 55) are also not significantly different or upon reduction of the latter to CyOO− by FeII followed by
from the ones performed under dinitrogen. Moreover, experi- protonation. Metal-assisted decomposition of CyOOH to CyO˙
ments using O2 as the sole oxidant were performed for both and CyOO˙ (reactions 4 and 5) would then afford the cyclo-
catalyst precursors 1 and 2, but no oxidation product could be hexanol (CyOH) and cyclohexanone (Cy–HvO) products (reactions
detected. This is consistent with the proposed mechanism (see 6 and 7).64 For the metal (iron) assisted steps, a change of the
below) which requires the presence of H2O2 as the source of metal oxidation state (+2/+3) often occurs13d,16g,65 and the iron
the hydroxyl radical.
centers of our catalyst precursors appear to be particularly
On completion of each stage, the products were analyzed favorable.
and the assumed catalyst (which precipitates upon addition of
ether) was isolated by filtration. The subsequent cycles were
initiated upon addition of new standard portions of all the
other reagents; in a second, third and fourth run, the observed
activity was 83%, 61% and 51% (for catalyst resulting from 1,
Table 1, entries 9, 10 and 11, respectively) or 32% and 17%
(for catalyst resulting from 2, Table 1, entries 43 and 44,
respectively) of the initial one, with maintenance of the selecti-
vity. This recycling possibility was not reported34,39a,b,52,53 for
other catalytic iron systems for the peroxidative oxidation of
cyclohexane.
In order to examine the general type of mechanism involved
in the studied oxidation, we also performed it in the presence
of radical traps. The peroxidative oxidation of cyclohexane is
believed to proceed mainly via a radical mechanism which
involves both carbon- and oxygen-centered radicals, in view of
the inhibition effect observed when it is carried out in the
presence of either a carbon-radical trap such as CBrCl3
(Table 1, entry 23 (1) or entry 53 (2)) or an oxygen-radical trap
such as Ph2NH60 (Table 1, entry 24 (1) or entry 54 (2)). In the
Peroxidative oxidation of n-heptane
Compound 2 is also active for the peroxidative oxidation of
linear alkanes, such as n-heptane, to the corresponding alco-
hols and ketones. In fact, after 3 h stirring at 50 °C, n-heptane,
in the presence of 2, H2O2 (50% aqueous solution)–MeCN and
Hpca (1 : 50), yields heptanol and heptanone, with a selectivity
of 1 : 9 : 10 : 10 for C(1) : C(2) : C(3) : C(4) (relative normalized
reactivities of H atoms at carbon atoms C(1), C(2), C(3) and C(4)).
These selectivity parameters are comparable to those achieved
by using the systems [Cu(H3L1)(NCS)]/t-BuOOH (H4L1 = N,N,N′,N′-
tetrakis(2-hydroxyethyl)ethylenediamine) (1 : 10 : 8 : 11),66
FeSO4–H2O2 (1 : 5 : 5 : 4.5)54b or [Fe2(HPTB)(μ-OH)(NO3)2](NO3)2–
Hpca–H2O2 [HPTB = N,N,N′,N′-tetrakis(2-benzimidazolylmethyl)-
2-hydroxo-1,3-diaminopropane] (1 : 6 : 6 : 5),54a which are believed
to operate via the HO˙ radical.
Hydrocarboxylation and carboxylation of alkanes to carboxylic
acids
experiments performed in the presence of CBrCl3, the for- Following the recent system for the mild hydrocarboxylation of
mation of CyBr was detected (13.4 and 14.2% relative to CyH, Cn alkanes into Cn+1 carboxylic acids developed by some of
respectively for 1 and 2) by GC analysis.
us,47–49,67 we tested the catalytic behavior of 1 using C2, C5
Metal-catalyzed (and Hpca-assisted) decomposition of H2O2 linear and C5, C6 cyclic alkanes by reacting the alkane (sub-
can lead to the oxygen-centered radicals HOO˙ and HO˙, upon strate) with carbon monoxide, water and potassium peroxodi-
oxidation of H2O2 by FeIII and reduction by FeII, respectively, sulfate, at 60 °C, for 4 h and in a neutral H2O–MeCN medium.
as suggested for some Fe39a,b,52–54,61 and V30,34,46,52,56,62 cata- In contrast to most of the known methods for the synthesis of
lysts. Water is believed to promote H+-shift steps toward the carboxylic acids,68 including the carboxylation of alkanes with
formation of HO˙ from H2O2.56a,63 The cyclohexyl radical Cy˙ TFA,69,70 the carboxylation of alkanes with CO and H2O pro-
is then formed upon H-abstraction from cyclohexane CyH by ceeds even in the absence of metal-catalyst (metal-free).47
HO˙ (Scheme 2, reaction 1). Reaction of Cy˙ with dioxygen However, the presence of both H2O and MeCN is crucial since
the carboxylation does not proceed in either only water or only
acetonitrile, or in the absence of both.
Under typical conditions, the hydrocarboxylations in the
presence of complex 2 show maximum overall yields (based on
alkane) of the corresponding carboxylic acid of ca. 7.6, 8.2,
16.4 or 9.5% for the conversion of cyclopentane, cyclohexane,
n-pentane or ethane, respectively. However, in these studies, 2
did not exhibit a promoting effect, since metal-free transform-
ations yield47 8.2, 12.3, 17.6 or 9.9% for the conversions of the
corresponding alkanes.
Compound 2 also shows low catalytic activity (TON = 5,
yield = 3.2%), although with high selectivity, for the oxidation
of ethane to acetic acid (only product), in a single-pot process,
under mild conditions, in the presence of potassium peroxodi-
Scheme 2 Mechanistic consideration of catalytic oxidation of cyclohexane.
sulfate and trifluoroacetic acid (TFA).
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Stoichiometric oxidation reactions of cis-stilbene and ada-
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Similarly, it has been shown that 2 reacts also with Hpca.
mantane with peroxido complexes [Fe6(O2)(O)2(O2CCMe3)12- A new band with a maximum between 400 and 500 nm devel-
(HO2CCMe3)2] and [Fe6(O2)(O)2(O2CCMe3)12(py)2]·0.5Me2CO in oping in time immediately after addition of a solution of Hpca
benzene or py/HO2CCMe3 have been studied.4c The reactions to the solution of 2 in MeCN–dmf is seen in Fig. 13.
performed at 70 and 66 °C, respectively, produced benz-
The changes in the UV-vis spectra of the solution of 2 in
aldehyde, trans-stilbene oxide, and oxoproducts of adaman- MeCN–dmf 10 : 1 used as a blank after simultaneous addition
tane, namely 1-ol and 2-one, as well as 2-(1-Ad)py and 2-(2-Ad)- of H2O2 and Hpca in the first 15 s also indicate formation of
py in low yields. In the case of adamantane, the tert/sec selecti- new species (minimum between 350 and 400 nm in Fig. 14).
vity achieved (∼8) was different from that resulting from After 1 h, precipitation of an orange product was observed.
catalytic Gif-type oxygenation of adamantane by [Fe3O-
To establish the role of 2 in catalytic oxidation reactions,
(O2CCMe3)6(py)3]/O2/Zn or H2O2 (∼3–5), suggesting that the 2–57Fe was synthesized and the mixture of 2–57Fe, Hpca and
peroxido species do not directly participate as active oxidants H2O2 in molar ratio 1 : 50 : 40 000 in MeCN was monitored by
in those catalytic reactions.
Mössbauer spectroscopy. The Mössbauer spectrum of the
solution prepared and frozen immediately showed one quadru-
pole doublet with an isomer shift δ = 0.49 mm s−1 and a
splitting ΔEQ = 0.62 mm s−1 (Fig. S12†). Comparison of
these parameters with those of 2–57Fe (δ = 0.57 mm s−1 and
ΔEQ = 0.88 mm s−1) indicates that the initial complex reacts
very quickly with H2O2 and/or Hpca in agreement with UV-vis
UV-vis and Mössbauer spectroscopy kinetic studies involving
complex 2
Although the structure of the catalyst remains elusive, optical
and Mössbauer spectra provide compelling evidence for the
transformation of 2 into a new Fe(III) high spin compound
with a more symmetric ligand environment of Mössbauer
ions.
Kinetic studies of the systems 2–H2O2 (1 : 2000) at 0, 15, 30,
and 45 °C, 2–Hpca (1 : 50) at 45 °C, and 2–Hpca–H2O2
(1 : 50 : 40 000) at 15 °C by UV-vis spectroscopy indicate the for-
mation of new species. The solution of 2 in MeCN–dmf was
stable over at least 3 h (Fig. S10†). Dimethylformamide was
added to prevent dissociation of coordinated dmf to iron in 2.
The solution of 2 in MeCN–dmf (10 : 1) was used as a blank,
and changes in the spectral bands with minima at about
394 nm and maxima at nearly 490 nm (Fig. 12), immediately
after addition of 30% H2O2 aqueous solution, indicate that 2
reacts with H2O2, presumably with formation of peroxido
species. The rate constant of this reaction was examined by an
Arrhenius plot (Fig. S11†), which gave an activation energy
value of EA = 76.17 kJ mol−1 and a frequency factor A = 2.03 ×
1011 s−1. These parameters can be compared with those calcu-
lated for the reaction of [Fe6O3(OH)(p-NO2C6H4COO)11(dmf)4]
with H2O2 (EA = 48 kJ mol−1, A = 1.9 × 107 s−1).4a
Fig. 13 UV-vis spectra of the 2–Hpca system (c2 = 1.25 × 10−4 M; 2 : Hpca =
1 : 50; T = 45 °C; t = 1–70 s; in MeCN–dmf = 10 : 1).
Fig. 12 UV-vis spectra of the 2–H2O2 system (c2 = 1.25 × 10−4 M; 2 : H2O2
1 : 2000; T = 45 °C, t = 0.5–25.5 s; in MeCN–dmf = 10 : 1).
=
Fig. 14 UV-vis spectra of the 2–H2O2–Hpca system (c2 = 1.25 × 10−4 M;
2 : H2O2 : Hpca = 1 : 40 000 : 50; T = 15 °C, t = 1–15 s; in MeCN–dmf = 10 : 1).
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Fig. 15 Mössbauer spectrum of the 2–57Fe/H2O2/Hpca system at 80 K after
1 h.
measurements, leading to a more symmetrical ligand environ-
ment of the Mössbauer ions. The asymmetry of the doublet
obtained for the mixture frozen after 30 min (Fig. S13†) indi-
cates continuous transformation of the initial complex. The
iron ions remain in the same high spin state, but the spectrum
is approximated by one doublet with a low value of quadrupole
splitting at 0.42 mm s−1 and another nearly 3 times larger,
ΔEQ = 1.10 mm s−1. The solution which was stirred at room
temperature for 1 h and then frozen gave a Mössbauer spec-
trum as one doublet with isomer shift δ = 0.45 mm s−1 and
ΔEQ = 0.52 mm s−1. These parameters are comparable with
Fig. 16 Zero-field and magnetic field Mössbauer spectra of the solid isolated
from the 2–57Fe/H2O2/Hpca system after 1 h reaction time at 4.2 and 2.7 K. The
probability distribution of the magnetic hyperfine field is shown on the right
side (δ = 0.48 mm s−1 and ΔEQ = 0.58 mm s−1).
Conclusions
those of the Mössbauer spectrum of the solid finally isolated The family of hexanuclear iron complexes containing a central
from the solution after 1 h reaction time shown in Fig. 15 with peroxido group bridging four ferric ions in an almost planar
δ = 0.40 mm s−1 and ΔEQ = 0.56 mm s−1
.
µ4–η2:η2 fashion and that of undecanuclear iron(III) carboxy-
The Mössbauer spectrum at 4.2 K also represents a doublet lates have been further extended by preparation of [Fe6(O)2(O2)-
(Fig. 16). Even at this very low temperature the rapid motion of L10(OAc)2(H2O)2] with two different carboxylate ligands, 3,4,5-
the iron electronic moments causes the quenching of the mag- trimethoxybenzoate and acetate (the latter is presumably
netic hyperfine structures. The observed values for δ and ΔEQ formed in situ by oxidation of methyl ethyl ketone used as a
(see the caption of Fig. 16) are in good agreement with those solvent), and [Fe11Cl(µ4-O)3(µ3-O)5L16(solvent)3] with an un-
obtained at higher temperatures. The line width is relatively precedented topology of the [FeIII11ClO8]16+ core consisting of
large, which is due to the lowering of fast electronic relaxation nine six-coordinate and two five-coordinate ferric ions. The
processes. At 2.7 K the spectrum is dominated by intermediate present study shows that 1 and 2 can act as efficient catalyst
relaxation processes. It is significantly broadened, with wings precursors for the partial oxygenation of alkanes to alkyl per-
that extend from about −6 to +6 mm s−1. The spectra at 2.7 K oxides, alcohols and ketones. Remarkable TONs and TOFs (the
and applied magnetic fields of 3 and 5 T represent broad latter mainly for 1) with concomitant quite good yields have
sextets with very large line widths. This feature looks typical of been achieved under mild conditions, for the peroxidative oxi-
a broad distribution of hyperfine fields and indicates the pres- dation of cyclohexane, in the presence of pyrazinecarboxylic
ence of a distribution of spin–spin relaxation frequencies, acid as a promoter. Moreover, 1 exhibits remarkably high
which is caused by superposition of spectra arising from activity in an exceptionally short reaction time (45 min), being
chemically nonequivalent iron sites.
unprecedented for any metal catalyzed alkane oxidations by
Although attempts to perform the same reaction at larger H2O2. The low catalytic activity of systems containing simple
scale to further characterize the emerging catalyst failed, the iron(III) salts, µ3-oxido iron(III) carboxylates, as well as inactivity
Mössbauer data clearly indicate that 2 underwent transform- of the Fe(pca)2 complex indicate that most probably the
ations in the studied oxidation reactions. Moreover, the species acting as real catalysts are iron(III) carboxylates with
compound resulting from 2 is different from 1, which was higher nuclearity. Mössbauer spectroscopy and UV-vis data
initially regarded by us as a possible reaction product of 2 with provide evidence that original complex 2 reacts with pyrazine-
H2O2 under the conditions explored in catalytic oxidation carboxylic acid–H2O2 with formation of a new high spin
reactions.
iron(III) species with a more symmetric ligand environment of
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Mössbauer ions. We hypothesize that replacement of mono-
dentate (H2O, dmf) ligands or even in part 3,4,5-trimethoxy-
benzoate may take place. Formation of peroxido species in this
case cannot be ruled out as well.
2067–2069; (c) P. Baran, R. Boca, I. Chakraborty,
J. Giapintzakis, R. Herchel, Q. Huang, J. E. McGrady,
R. G. Raptis, Y. Sanakis and A. Simopoulos, Inorg. Chem.,
2008, 47, 645–655; (d) H. Li, Z. J. Zhong, W. Chen and
X.-Z. You, J. Chem. Soc., Dalton Trans., 1997, 463–464.
6 W. Micklitz, S. G. Bott, J. G. Bentsen and S. J. Lippard,
J. Am. Chem. Soc., 1989, 111, 372–374.
7 E. I. Tolis, M. Helliwell, S. Langley, J. Raftery and
R. E. P. Winpenny, Angew. Chem., Int. Ed., 2003, 42,
3804–3808.
8 S. Khanra, S. Konar, A. Clearfield, M. Helliwell,
E. J. L. McInnes, E. Tolis, F. Tuna and R. E. P. Winpenny,
Inorg. Chem., 2009, 48, 5338–5349.
9 A. M. Ako, V. Mereacre, Y. Lan, W. Wernsdorfer, R. Clerac,
C. E. Anson and A. K. Powell, Inorg. Chem., 2010, 49, 1–3.
Acknowledgements
This work was supported by the Austrian Science Fund (Project
P19629-N19), the Fundação para a Ciência e a Tecnologia
(FCT), Portugal, and projects PTDC/QUI-QUI/102150/2008,
PTDC/EQU-EQU/122025/2010 and PEst-OE/QUI/UI0100/2011
and by the European Regional Development Fund, Sectoral
Operational Programme ‘Increase of Economic Competitive-
ness’, Priority Axis 2 (SOP IEC-A2-O2.1.2-2009-2, ID 570, COD
SMIS-CSNR: 12473, Contract 129/2010-POLISILMET). We 10 C. Boskovic, H. U. Guedel, G. Labat, A. Neels,
thank Alexander Roller for collection of the X-ray data.
W. Wernsdorfer, B. Moubaraki and K. S. Murray, Inorg.
Chem., 2005, 44, 3181–3189.
11 E. C. Sanudo, M. Font-Bardia, X. Solans and R. H. Laye,
New J. Chem., 2011, 35, 842–848.
12 P. C. A. Bruijnincx, K. G. van Koten and R. J. M. K. Gebbink,
Chem. Soc. Rev., 2008, 37, 2716–2744.
References
1 (a) W. H. Armstrong and S. J. Lippard, Inorg. Chem., 1985,
24, 981–982; (b) D. M. Kurtz, Chem. Rev., 1990, 90, 585–606; 13 (a) M. Sono, M. P. Roach, E. D. Coulter and J. H. Dawson,
(c) T. Ookubo, H. Sugimoto, T. Nagayama, H. Masuda,
T. Sato, K. Tanaka, Y. Maeda, H. Okawa, Y. Hayashi, A. Uehara
and M. Suzuki, J. Am. Chem. Soc., 1996, 118, 701–702.
2 (a) A. K. Boudalis, Y. Sanakis, C. P. Raptopoulou, A. Terzis,
J.-P. Tuchagues and S. P. Perlepes, Polyhedron, 2005, 24,
1540–1548; (b) C. E. Anson, J. P. Bourke, R. D. Cannon,
U. A. Jayasooriya, M. Molinier and A. K. Powell, Inorg.
Chem., 1997, 36, 1265–1267.
Chem. Rev., 1996, 96, 2841–2887; (b) B. J. Wallar and
J. D. Lipscomb, Chem. Rev., 1996, 96, 2625–2657;
(c) E. I. Solomon, T. C. Brunold, M. I. Davis, J. N. Kemsley,
S.-K. Lee, N. Lehnert, F. Neese, A. J. Skulan, Y.-S. Yang and
J. Zhou, Chem. Rev., 2000, 100, 235–349; (d) M. Costas,
M. P. Mehn, M. P. Jensen and L. Que, Chem. Rev., 2004,
104, 939–986.
14 M. C. Esmelindro, E. G. Oestreicher, H. Marquez-Alvarez,
C. Dariva, S. M. S. Egues, C. Fernandes, A. J. Bortoluzzi,
V. Drago and O. A. C. Antunes, J. Inorg. Biochem., 2005, 99,
2054–2061.
3 (a) W. H. Armstrong, M. E. Roth and S. J. Lippard, J. Am.
Chem. Soc., 1987, 109, 6318–6326; (b) T. C. Stamatatos,
A. K. Boudalis, Y. Sanakis and C. P. Raptopoulou, Inorg.
Chem., 2006, 45, 7372–7381; (c) J.-L. Tian, W. Gu, S.-P. Yan, 15 A. E. Shilov and G. B. Shul’pin, Chem. Rev., 1997, 97,
D.-Z. Liao and Z.-H. Jiang, Z. Anorg. Allg. Chem., 2007, 633, 2879–2932.
478–481; (d) A. Horn, A. Neves, A. J. Bortoluzzi, V. Drago 16 (a) Y. He and C. R. Goldsmith, Chem. Commun., 2012, 48,
and W. A. Ortiz, Inorg. Chem. Commun., 2001, 4, 173–176.
4 (a) G. Trettenhahn, M. Nagl, N. Neuwirth, V. B. Arion,
W. Jary, P. Poechlauer and W. Schmid, Angew. Chem., Int.
Ed., 2006, 45, 2794–2798; (b) I. Shweky, L. E. Pence,
G. C. Papaefthymiou, R. Sessoli, J. W. Yun, A. Bino and
S. J. Lippard, J. Am. Chem. Soc., 1997, 119, 1037–1042;
(c) R. Celenligil-Cetin, R. J. Staples and P. Stavropoulos,
Inorg. Chem., 2000, 39, 5838–5846; (d) E. J. Seddon,
J. C. Huffman and G. Christou, Dalton, 2000, 4446–4452;
(e) S. M. Gorun and S. J. Lippard, Nature, 1986, 319, 666–
668; (f) L. F. Jones, E. K. Brechin, D. Collison, M. Helliwell,
T. Mallah, S. Piligkos, G. Rajaraman and W. Wernsdorfer,
Inorg. Chem., 2003, 42, 6601–6603; (g) C. Canada-Vilalta,
10532–10534; (b) K. Chen and L. Que, J. Am. Chem. Soc.,
2001, 123, 6327–6337; (c) R. A. Leising, B. A. Brennan,
L. Que, B. G. Fox and E. Münck, J. Am. Chem. Soc., 1991,
113, 3988–3990; (d) T. Okuno, S. Ito, S. Ohba and
Y. Nishida, J. Chem. Soc., Dalton Trans., 1997, 3547–3551;
(e) Y. Mekmouche, C. Duboc-Toia, S. Menage, C. Lambeaux
and M. Fontecave, J. Mol. Catal. A: Chem., 2000, 156, 85–89;
(f) X. Wang, S. Wang, L. Li, E. B. Sundberg and
G. P. Gacho, Inorg. Chem., 2003, 42, 7799–7808;
(g) J. England, G. J. P. Britovsek, N. Rabadia and
A. J. P. White, Inorg. Chem., 2007, 46, 3752–3767;
(h) J. England, C. R. Davies, M. Banaru, A. J. P. White and
G. J. P. Britovsek, Adv. Synth. Catal., 2008, 350, 883–897.
T. A. O’Brien, M. Pink, E. R. Davidson and G. Christou, 17 D. A. Whittington and S. J. Lippard, J. Am. Chem. Soc.,
Inorg. Chem., 2003, 42, 7819–7829. 2001, 123, 827–838.
5 (a) I. A. Gass, C. J. Milios, A. G. Whittaker, F. P. A. Fabiani, 18 M. H. Sazinsky, J. Bard, D. Di and S. J. Lippard, J. Biol.
S. Parsons, M. Murrie, S. P. Perlepes and E. K. Brechin, Chem., 2004, 279, 30600–30610.
Inorg. Chem., 2006, 45, 5281–5283; (b) J.-P. Tong, F. Shao, 19 Y. Lindqvist, W. Huang, G. Schneider and J. Shanklin,
J. Tao, R.-B. Huang and L.-S. Zheng, Inorg. Chem., 2011, 50, EMBO J., 1996, 15, 4081–4092.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
20 H. Eklund, U. Uhlin, M. Farnegardh, D. T. Logan and
P. Nordlund, Prog. Biophys. Mol. Biol., 2001, 77, 177–268.
21 (a) P.-G. Lassahn, V. Lozan, G. A. Timco, P. Christian,
706–716; (b) T. F. S. Silva, M. F. C. Guedes da Silva,
G. S. Mishra, L. M. D. R. S. Martins and A. J. L. Pombeiro,
J. Organomet. Chem., 2011, 696, 1310–1318.
C. Janiak and R. E. P. Winpenny, J. Catal., 2004, 222, 260– 35 G. S. Mishra, J. J. R. Frausto da Silva and A. J. L. Pombeiro,
267; (b) B. Singh, J. R. Long, F. F. de Biani, D. Gatteschi J. Mol. Catal. A: Chem., 2007, 265, 59–69.
and P. Stavropoulos, J. Am. Chem. Soc., 1997, 119, 7030– 36 E. C. B. Alegria, M. V. Kirillova, L. M. D. R. S. Martins and
7047. A. J. L. Pombeiro, Appl. Catal., A, 2007, 317, 43–52.
22 (a) P. J. Figiel, M. N. Kopylovich, J. Lasri, M. F. C. Guedes 37 (a) A. M. Kirillov, M. Haukka, M. F. C. Guedes da Silva and
da Silva, J. J. R. Frausto da Silva and A. J. L. Pombeiro,
Chem. Commun., 2010, 46, 2766–2768; (b) P. J. Figiel,
A. Sibaouih, J. U. Ahmad, M. Nieger, M. T. Raeisaenen,
A. J. L. Pombeiro, Eur. J. Inorg. Chem., 2005, 2071–2080;
(b) G. B. Shul’pin, M. V. Kirillova, T. Sooknoi and
A. J. L. Pombeiro, Catal. Lett., 2008, 123, 135–141.
M. Leskelae and T. Repo, Adv. Synth. Catal., 2009, 351, 38 G. S. Mishra, E. C. B. Alegria, L. M. D. R. S. Martins,
2625–2632; (c) P. J. Figiel, M. Leskela and T. Repo, Adv.
Synth. Catal., 2007, 349, 1173–1179.
J. J. R. Frausto da Silva and A. J. L. Pombeiro, J. Mol. Catal.
A: Chem., 2008, 285, 92–100.
23 (a) K. T. Mahmudov, M. N. Kopylovich, M. F. C. Guedes da 39 (a) M. N. Kopylovich, A. M. Kirillov, A. K. Baev and
Silva, P. J. Figiel, Y. Y. Karabach and A. J. L. Pombeiro, J. Mol.
Catal. A: Chem., 2010, 318, 44–50; (b) M. N. Kopylovich,
K. T. Mahmudov, M. Haukka, P. J. Figiel, A. Mizar,
J. A. L. da Silva and A. J. L. Pombeiro, Eur. J. Inorg. Chem.,
A. J. L. Pombeiro, J. Mol. Catal. A: Chem., 2003, 206,
163–178; (b) R. R. Fernandes, M. V. Kirillova, J. A. L. da
Silva, J. J. R. Frausto da Silva and A. J. L. Pombeiro, Appl.
Catal., A, 2009, 353, 107–112.
2011, 4175–4181; (c) A. M. Kirillov, M. N. Kopylovich, 40 N. V. Gerbeleu, S. S. Palanciuc, Yu. A. Simonov, A. A. Dvorkin,
M. V. Kirillova, M. Haukka, M. F. C. Guedes da Silva and
A. J. L. Pombeiro, Angew. Chem., Int. Ed., 2005, 44, 4345–
P. N. Bourosh, M. T. Reetz, V. B. Arion and K. Toellner, Poly-
hedron, 1995, 14, 521–527.
4349; (d) M. N. Kopylovich, A. C. C. Nunes, K. T. Mahmudov, 41 H. Ulrich, Raw Materials for Industrial Polymers, Hanser
M. Haukka, T. C. O. MacLeod, L. M. D. R. S. Martins, Publ., 1988.
M. L. Kuznetsov and A. J. L. Pombeiro, Dalton Trans., 2011, 42 P. Pascal, Ann. Chim. Phys., 1910, 19, 5–70.
40, 2822–2836.
43 SAINT-Plus, version 7.06a, and APEX2, Bruker-Nonus AXS
Inc., Madison, WI, 2004.
24 A. M. Kirillov, M. N. Kopylovich, M. V. Kirillova,
E. Y. Karabach, M. Haukka, M. F. C. Guedes da Silva and 44 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystal-
A. J. L. Pombeiro, Adv. Synth. Catal., 2006, 348, 159–174. logr., 2008, A64, 112–122.
25 R. R. Fernandes, J. Lasri, M. F. C. Guedes da Silva, 45 M. N. Burnett and J. K. Johnson, ORTEPIII, Report
J. A. L. da Silva, J. J. R. Frausto da Silva and A. J. L. Pombeiro,
J. Mol. Catal. A: Chem., 2011, 351, 100–111.
ORNL-5138, Oak Ridge National Laboratory, Oak Ridge,
TN, 1996.
26 C. Di Nicola, Y. Y. Karabach, A. M. Kirillov, M. Monari, 46 (a) G. B. Shul’pin, J. Mol. Catal. A: Chem., 2002, 189, 39–66;
L. Pandolfo, C. Pettinari and A. J. L. Pombeiro, Inorg.
Chem., 2007, 46, 221–230.
27 Y. Y. Karabach, A. M. Kirillov, M. Haukka, M. N. Kopylovich
and A. J. L. Pombeiro, J. Inorg. Biochem., 2008, 102, 1190–
1194.
28 M. V. Kirillova, A. M. Kirillov, M. F. C. Guedes da Silva and
A. J. L. Pombeiro, Eur. J. Inorg. Chem., 2008, 3423–3427.
29 K. R. Gruenwald, A. M. Kirillov, M. Haukka, J. Sanchiz and
A. J. L. Pombeiro, Dalton Trans., 2009, 2109–2120.
(b) G. B. Shul’pin, C. R. Chim., 2003, 6, 163–178;
(c) G. B. Shul’pin, Y. N. Kozlov, L. S. Shul’pina,
A. R. Kudinov and D. Mandelli, Inorg. Chem., 2009, 48,
10480–10482; (d) G. B. Shul’pin, Y. N. Kozlov, L. S. Shul’pina
and P. V. Petrovskiy, Appl. Organomet. Chem., 2010, 24,
464–472.
47 M. V. Kirillova, A. M. Kirillov, M. L. Kuznetsov, J. A. L. da
Silva, J. J. R. Frausto da Silva and A. J. L. Pombeiro, Chem.
Commun., 2009, 2353–2355.
30 L. S. Shul’pina, M. V. Kirillova, A. J. L. Pombeiro and 48 M. V. Kirillova, A. M. Kirillov and A. J. L. Pombeiro, Appl.
G. B. Shul’pin, Tetrahedron, 2009, 65, 2424–2429. Catal., A, 2011, 401, 106–113.
31 S. Contaldi, C. Di Nicola, F. Garau, Y. Y. Karabach, 49 (a) M. V. Kirillova, A. M. Kirillov and A. J. L. Pombeiro, Adv.
L. M. D. R. S. Martins, M. Monari, L. Pandolfo, C. Pettinari
and A. J. L. Pombeiro, Dalton Trans., 2009, 4928–4941.
32 C. Di Nicola, F. Garau, Y. Y. Karabach, L. M. D. R. S. Martins,
Synth. Catal., 2009, 351, 2936–2948; (b) M. V. Kirillova,
A. M. Kirillov and A. J. L. Pombeiro, Chem.–Eur. J., 2010, 16,
9485–9493.
M. Monari, L. Pandolfo, C. Pettinari and A. J. L. Pombeiro, 50 G. B. Shul’pin, M. V. Kirillova, Y. N. Kozlov, L. S. Shul’pina,
Eur. J. Inorg. Chem., 2009, 666–676.
33 D. S. Nesterov, V. N. Kokozay, V. V. Dyakonenko,
A. R. Kudinov and A. J. L. Pombeiro, J. Catal., 2011, 277,
164–172.
O. V. Shishkin, J. Jezierska, A. Ozarowski, A. M. Kirillov, 51 (a) T. F. S. Silva, K. V. Luzyanin, M. V. Kirillova, M. F. C. Guedes
M. N. Kopylovich and A. J. L. Pombeiro, Chem. Commun.,
2006, 4605–4607.
34 (a) T. F. S. Silva, E. C. B. A. Alegria, L. M. D. R. S. Martins
and A. J. L. Pombeiro, Adv. Synth. Catal., 2008, 350,
da Silva, L. M. D. R. S. Martins and A. J. L. Pombeiro, Adv.
Synth. Catal., 2010, 352, 171–187; (b) T. F. S. Silva, G. S. Mishra,
M. F. C. Guedes da Silva, R. Wanke, L. M. D. R. S. Martins
and A. J. L. Pombeiro, Dalton Trans., 2009, 9207–9215.
This journal is © The Royal Society of Chemistry 2013
Dalton Trans.

View Article Online
Paper
Dalton Transactions
52 R. R. Fernandes, J. Lasri, A. M. Kirillov, M. F. C. Guedes da
Silva, J. A. L. da Silva, J. J. R. Frausto da Silva and
A. J. L. Pombeiro, Eur. J. Inorg. Chem., 2011, 3781–3790.
(b) J. A. Howard, in Free Radicals, ed. J. K. Kochi, Wiley-
Interscience, 1973, vol. 2, p. 3; (c) R. E. Huie and
C. L. Clifton, Int. J. Chem. Kinet., 1989, 21, 611–619.
53 D. S. Nesterov, E. N. Chygorin, V. N. Kokozay, V. V. Bon, 61 (a) R. R. Fernandes, J. Lasri, M. F. C. Guedes da Silva,
R. Boca, Y. N. Kozlov, L. S. Shul’pina, J. Jezierska,
A. Ozarowski, A. J. L. Pombeiro and G. B. Shul’pin, Inorg.
Chem., 2012, 51, 9110–9122.
J. A. L. da Silva, J. J. R. Frausto da Silva and A. J. L. Pombeiro,
Appl. Catal., A, 2011, 402, 110–120; (b) S. Tanase, P. Marques-
Gallego, W. R. Browne, R. Hage, E. Bouwman, B. L. Feringa
and J. Reedijk, Dalton Trans., 2008, 2026–2033; (c) S. Tanase,
C. Foltz, R. de Gelder, R. Hage, E. Bouwman and J. Reedijk,
J. Mol. Catal. A: Chem., 2005, 225, 161–167.
54 (a) G. V. Nizova, B. Krebs, G. Suss-Fink, S. Schindler,
L. Westerheide, C. Gonzalez and G. B. Shul’pin, Tetra-
hedron, 2002, 58, 9231–9237; (b) G. B. Shul’pin,
G. V. Nizova, Y. N. Kozlov, L. G. Cuervo and G. Suess-Fink, 62 G. B. Shul’pin, Y. N. Kozlov, G. V. Nizova, G. Suss-Fink,
Adv. Synth. Catal., 2004, 346, 317–332; (c) R. Z. Khaliullin,
A. T. Bell and M. Head-Gordon, J. Phys. Chem. B, 2005, 109,
S. Stanislas, A. Kitaygorodskiy and V. S. Kulikova, J. Chem.
Soc., Perkin Trans. 2, 2001, 1351–1371.
17984–17992; (d) G. B. Shul’pin, M. V. Kirillova, 63 M. L. Kuznetsov and A. J. L. Pombeiro, Inorg. Chem., 2009,
L. S. Shul’pina, A. J. L. Pombeiro, E. E. Karslyan 48, 307–318.
and Y. N. Kozlov, Catal. Commun., 2013, 31, 32–36; 64 G. B. Shul’pin and G. Suss-Fink, J. Chem. Soc., Perkin Trans.
(e) A. M. Kirillov and G. B. Shul’pin, Coord. Chem. Rev., 2, 1995, 1459–1463.
2013, 257, 732–754; (f) G. B. Shul’pin, C. C. Golfeto, 65 (a) M. Costas, K. Chen and L. Que, Coord. Chem. Rev., 2000,
G. Suess-Fink, L. S. Shul’pina and D. Mandelli, Tetrahedron
Lett., 2005, 46, 4563–4567.
55 G. S. Mishra, T. F. S. Silva, L. M. D. R. S. Martins and
200–202, 517–544; (b) A. Company, L. Gomez, M. Gueell,
X. Ribas, J. M. Luis, L. Que and M. Costas, J. Am. Chem.
Soc., 2007, 129, 15766–15767.
A. J. L. Pombeiro, Pure Appl. Chem., 2009, 81, 1217– 66 A. M. Kirillov, M. V. Kirillova, L. S. Shul’pina, P. J. Figiel,
1227.
K. R. Gruenwald, M. F. C. Guedes da Silva, M. Haukka,
A. J. L. Pombeiro and G. B. Shul’pin, J. Mol. Catal. A: Chem.,
2011, 350, 26–34.
56 (a) G. B. Shul’pin, D. Attanasio and L. Suber, J. Catal.,
1993, 142, 147–152; (b) Y. N. Kozlov, V. B. Romakh,
A. Kitaygorodskiy, P. Buglyo, G. Suess-Fink and 67 (a) A. M. Kirillov, Y. Y. Karabach, M. V. Kirillova,
G. B. Shul’pin, J. Phys. Chem. A, 2007, 111, 7736–7752;
(c) M. V. Kirillova, M. L. Kuznetsov, V. B. Romakh,
L. S. Shul’pina, J. J. R. Frausto da Silva, A. J. L. Pombeiro
and G. B. Shul’pin, J. Catal., 2009, 267, 140–157.
M. Haukka and A. J. L. Pombeiro, Dalton Trans., 2011, 40,
6378–6381; (b) A. M. Kirillov, J. A. S. Coelho, M. V. Kirillova,
M. F. C. Guedes da Silva, D. S. Nesterov, K. R. Gruenwald,
M. Haukka and A. J. L. Pombeiro, Inorg. Chem., 2010, 49,
6390–6392.
57 M. V. Kirillova, M. L. Kuznetsov, Y. N. Kozlov,
L. S. Shul’pina, A. Kitaygorodskiy, A. J. L. Pombeiro and 68 Ulmann’s Encyclopedia of Industrial Chemistry, ed. B. Elvers,
G. B. Shul’pin, ACS Catal., 2011, 1, 1511–1520. Wiley-VCH, Weinheim, 6th edn, 2002.
58 M. N. Kopylovich, T. C. O. MacLeod, M. Haukka, 69 (a) C. Jia, T. Kitamura and Y. Fujiwara, Acc. Chem. Res.,
G. I. Amanullayeva, K. T. Mahmudov and A. J. L. Pombeiro,
J. Inorg. Biochem., 2012, 115, 72–77.
2001, 34, 633–639; (b) Y. Fujiwara, K. Takaki and
Y. Taniguchi, Synlett, 1996, 591–599.
59 A. M. Kirillov, M. N. Kopylovich, M. V. Kirillova, 70 (a) M. V. Kirillova, M. L. Kuznetsov, P. M. Reis, J. A. L. da
M. Haukka, M. F. C. Guedes da Silva and A. J. L. Pombeiro,
Angew. Chem., Int. Ed., 2005, 44, 4345–4349.
60 (a) L. M. Slaughter, J. P. Collman, T. A. Eberspacher and
J. I. Brauman, Inorg. Chem., 2004, 43, 5198–5204;
Silva, J. J. R. Frausto da Silva and A. J. L. Pombeiro, J. Am.
Chem. Soc., 2007, 129, 10531–10545; (b) M. V. Kirillova,
A. M. Kirillov, P. M. Reis, J. A. L. da Silva, J. J. R. Frausto da
Silva and A. J. L. Pombeiro, J. Catal., 2007, 248, 130–136.
Dalton Trans.
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