Solvent-Free Microwave-Assisted Peroxidative Oxidation of Alcohols Catalyzed by Iron(III)-TEMPO Catalytic Systems

Article abstract of DOI:10.1007/s10562-015-1616-2

The iron(III) complexes [H(EtOH)][FeCl₂(L)₂] (1), [H₂bipy]_1/2[FeCl₂(L)₂].DMF (2) and [FeCl₂(L)(2,2′-bipy)] (3) (L = 3-amino-2-pyrazinecarboxylate; H₂bipy = doubly protonated 4,4′-bipyridine; 2,2′-bipy = 2,2′-bipyridine, DMF = dimethylformamide) have been synthesized and fully characterized by IR, elemental and single-crystal X-ray diffraction analyses, as well as by electrochemical methods. Complexes 1 and 2 have similar mononuclear structures containing different guest molecules (protonated ethanol for 1 and doubly protonated 4,4′-bipyridine for 2) in their lattices, whereas the complex 3 has one 3-amino-2-pyrazinecarboxylate and a 2,2′-bipyridine ligand. They show a high catalytic activity for the low power (10 W) solvent-free microwave assisted peroxidative oxidation of 1-phenylethanol, leading, in the presence of TEMPO, to quantitative yields of acetophenone [TOFs up to 8.1 × 10³ h^-1, (3)] after 1 h. Moreover, the catalysts are of easy recovery and reused, at least for four consecutive cycles, maintaining 83 % of the initial activity and concomitant rather high selectivity. Graphical Abstract: 3-Amino-2-pyrazinecarboxylic acid is used to synthesize three new iron(III) complexes which act as heterogeneous catalysts for the solvent-free microwave-assisted peroxidative oxidation of 1-phenylethanol. [InlineMediaObject not available: see fulltext.]

Full text of DOI:10.1007/s10562-015-1616-2

Catal Lett
DOI 10.1007/s10562-015-1616-2
Solvent-Free Microwave-Assisted Peroxidative Oxidation
of Alcohols Catalyzed by Iron(III)-TEMPO Catalytic Systems
1
1,2
1
•
Anirban Karmakar
1
•
Lu ´ı sa M. D. R. S. Martins
•
M. F a´ tima C. Guedes da Silva
1
Susanta Hazra
•
Armando J. L. Pombeiro
Received: 25 July 2015 / Accepted: 26 August 2015
Ó Springer Science+Business Media New York 2015
Abstract The iron(III) complexes [H(EtOH)][FeCl (L) ]
2
recovery and reused, at least for four consecutive cycles,
maintaining 83 % of the initial activity and concomitant
rather high selectivity.
2
0
(
1), [H bipy] [FeCl (L) ].DMF (2) and [FeCl (L)(2,2 -
2
2
1/2
2
2
bipy)] (3) (L = 3-amino-2-pyrazinecarboxylate; H bipy =
2
0
0
0
doubly protonated 4,4 -bipyridine; 2,2 -bipy = 2,2 -bipyr-
idine, DMF = dimethylformamide) have been synthesized
and fully characterized by IR, elemental and single-crystal
X-ray diffraction analyses, as well as by electrochemical
methods. Complexes 1 and 2 have similar mononuclear
structures containing different guest molecules (protonated
Graphical Abstract 3-Amino-2-pyrazinecarboxylic acid
is used to synthesize three new iron(III) complexes which
act as heterogeneous catalysts for the solvent-free micro-
wave-assisted peroxidative oxidation of 1-phenylethanol.
0
ethanol for 1 and doubly protonated 4,4 -bipyridine for 2)
in their lattices, whereas the complex 3 has one 3-amino-2-
0
pyrazinecarboxylate and a 2,2 -bipyridine ligand. They
show a high catalytic activity for the low power (10 W)
solvent-free microwave assisted peroxidative oxidation of
1
-phenylethanol, leading, in the presence of TEMPO, to
3
quantitative yields of acetophenone [TOFs up to 8.1 9 10
-
h
1
, (3)] after 1 h. Moreover, the catalysts are of easy
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-015-1616-2) contains supplementary
material, which is available to authorized users.
Keywords Alcohol oxidation Á Mononuclear iron(III)
complexes Á TEMPO Á Microwave-assisted catalysis
&
&
&
&
Anirban Karmakar
anirbanchem@gmail.com
Lu ´ı sa M. D. R. S. Martins
lmartins@deq.isel.ipl.pt
1
Introduction
M. F a´ tima C. Guedes da Silva
fatima.guedes@tecnico.ulisboa.pt
The oxidation of primary and secondary alcohols to car-
bonyl compounds is one of the most important reactions in
synthetic organic chemistry, as well as in chemical industry
Armando J. L. Pombeiro
pombeiro@tecnico.ulisboa.pt
[
1–3]. Many catalytic methods have been developed for the
1
Centro de Qu ´ı mica Estrutural, Instituto Superior T e´ cnico,
Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisbon,
Portugal
oxidation of alcohols, the most commonly utilizing
homogeneous catalysts based on different transition metals,
such as Co [3], Mn [4, 5], V [6], Cu [7–10], Fe [11–14], Mo
2
Chemical Engineering Department, ISEL, R. Conselheiro
Em ´ı dio Navarro, 1959-007 Lisbon, Portugal
[
14], Ru [16, 17], and Pd [18]. Unfortunately, most of the
123

A. Karmakar et al.
systems suffer from harsh reaction conditions, high cost,
toxicity of the metal, high reagent load, or they form
unwanted waste. Moreover, they suffer from the usual
separation and recycling difficulties inherent to homoge-
nous catalytic system. To eliminate the harmful wastes, the
development of catalytic oxidation procedures that involve
green oxidants, in particular dioxygen or peroxides [H O ,
aldehydes. Hence, it would be interesting to check its
cooperative action with an iron(III) complex towards the
peroxidative oxidation of 1-phenylethanol to acetophenone.
In this context, we have chosen, as ligand precursor,
3-aminopyrazine-2-carboxylic acid (HL) which, upon
-
ready deprotonation, produces L which can act as a
multidentate ligand with versatile metal-binding and
hydrogen-bonding capabilities. Recently, it has drawn
extensive attention for the construction of various com-
plexes with s-block [31], d-block [32] and f-block [33]
metal ions. However, to our knowledge, no study on the
Fe(III) coordination chemistry with 3-aminopyrazine-2-
carboxylic acid (HL) has been reported.
2
2
t
tert-butylhydroperoxide ( BuOOH or TBHP)], is a matter
of current interest [4–19].
Solvent-free processes [20] eventually assisted by micro-
wave (MW) irradiation (an alternative to the traditional
heating, with advantages [21] of reducing the reaction time
and promoting the yield, selectivity and purity of products)
and using environmentally acceptable and efficient catalyst
and oxidant, under mild conditions are of particular interest.
Very recently, some of us reported very efficient catalytic
systems based on copper(II) [7–10] and Fe(III) [11–14]
complexes for the peroxidative (using TBHP) oxidation of
different primary and secondary alcohols, including a sol-
vent-free MW assisted method which accelerates dramati-
callythereactionrate usingaverylow MWirradiationpower.
The selective oxidation, with a Fe complex, of a secondary
alcohol when a primary one is present in the same molecule
was also reported [14]. However, the catalyst recycling and
reuse was not achieved in those systems.
Hence, the two main objectives of the current work are
as follows: (i) to synthesize new 3-aminopyrazine-2-car-
boxylate Fe(III)-complexes with low solubility; (ii) to
apply them as heterogeneous catalysts for solvent-free MW
assisted peroxidative oxidation of 1-phenylethanol in the
presence of TEMPO.
Herein we report three mononuclear Fe(III) complexes
with ligands derived from 3-amino-2-pyrazinecarboxylic
acid. The structural features of the obtained complexes
[H(EtOH)][FeCl (L) ] (1), [H bipy] [FeCl (L) ].DMF (2)
2
2
2
1/2
2
2
0
and [FeCl (L)(2,2 -bipy)] (3) (L = 3-amino-2-pyrazinecar-
2
0
0
boxylate; H bipy = doubly protonated 4,4 -bipyridine; 2,2 -
2
0
The development of new, efficient, cheaper and easily
recoverable catalysts has been a challenging task, and the
introduction of a cheap heterogeneous catalyst would be a
significant addition in this field. The low cost and non-
toxicity of iron render it a particularly attractive metal in
this regard. Iron has drawn particular attention in catalyst
design also on account of its essential role in fundamental
biological processes such as oxygen transport and sensing,
small-molecule metabolism and electron transfer [22]. Iron
has become largely used in many oxidation reactions, such
as epoxidation [23], sulfoxidation [24], hydroxylation [25],
etc. and in the last two decades iron complexes have also
been developed to catalyze alcohols oxidations [13, 26].
Recently, some iron complexes have been reported by our
group [27, 28], which effectively catalyze the oxidation
reactions of secondary alcohols and cycloalkanes. Besides,
the hydrotris(pyrazol-1-yl)methane iron(II) complex,
heterogenized on carbon nanotubes, is an active, selective
and recyclable catalyst for oxidation of cyclohexane [28].
Thus, catalytic systems with heterogenized (or heteroge-
neous) iron catalysts are promising but no general method
has been developed for an efficient, clean and safe alcohol
oxidation procedure.
bipy = 2,2 -bipyridine, DMF = dimethylformamide) were
established by FT-IR, elemental and single crystal X-ray
diffraction analyses. They are found to be active and highly
efficient and selective heterogeneous catalysts for the sol-
vent-free peroxidative oxidation of 1-phenylethanol to ace-
tophenone (under focused MW irradiation).
2 Experimental
All the chemicals were obtained from commercial sources
and used as received. The infrared spectra
-
1
(4000–400 cm ) were recorded on a Bruker Vertex 70
instrument in KBr pellets; abbreviations: strong (s), med-
ium (m), weak (w), broad and strong (bs), medium and
broad (mb). Carbon, hydrogen and nitrogen elemental
analyses were carried out by the Microanalytical Service of
1
the Instituto Superior T e´ cnico. The H spectra were
recorded at ambient temperature on a Bruker Avance
TM
II ? 300 (UltraShield Magnet) spectrometer operating at
300.130 MHz for proton. The chemical shifts are reported
in ppm using tetramethylsilane as the internal reference;
abbreviations: singlet (s), doublet (d), triplet (t), quartet (q).
From another perspective, the TEMPO (2,2,6,6-tetram-
ethylpiperidine-1-oxyl) radical has been recognized as a
mediator or promoter in several organic reactions [29, 30],
and has been used in the selective oxidation, with a variety of
oxidants, of primary alcohols to the corresponding
2.1 Synthesis of 1
To a hot and stirred ethanol suspension (10 mL) of
3-amino-2-pyrazinecarboxylic acid (0.28 g, 2 mmol) was
1
23

Solvent-Free Microwave-Assisted Peroxidative Oxidation of Alcohols Catalyzed by Iron(III)…
added dropwise an ethanol solution (5 mL) of FeCl .6H O
3
personal computer through a GPIB interface. Cyclic
2
(
0.27 g, 1 mmol) to produce a dark red solution which was
voltammetry (CV) studies were undertaken in 0.2 M
n
[ Bu N][BF ]/MeCN, at a platinum disc working electrode
kept for slow evaporation. After 2 days, the red crystalline
compound containing diffractable crystals that deposited
was collected by filtration and washed with ethanol. Yield:
4
4
(d = 0.5 mm) and at room temperature. Controlled-po-
tential electrolyses (CPE) were carried out in electrolyte
solutions with the above-mentioned composition, in a
three-electrode H-type cell. The compartments were sepa-
rated by a sintered glass frit and equipped with platinum
gauze working and counter electrodes. For both CV and
CPE experiments, a Luggin capillary connected to a silver
wire pseudo-reference electrode was used to control the
working electrode potential. A Pt wire was employed as the
counter-electrode for the CV cell. The CPE experiments
were monitored regularly by CV, thus assuring no signifi-
cant potential drift occurred along the electrolysis. The
solutions were saturated with N2 by bubbling this gas
before each run, the redox potentials of the complexes were
measured by CV in the presence of ferrocene as the internal
standard, and their values are quoted relative to the SCE by
7
8 % (0.35 g) based on Fe. Anal. Calcd. for C H Cl
12 15 2
FeN O (M = 450.05): C, 32.03; H, 3.36; N, 18.67; Found:
6
5
-
1
C, 32.13; H, 3.21; N, 18.18. FT-IR (KBr, cm ): 3421(bs),
301 (bs), 1631 (s), 1542 (m), 1501 (m), 1432 (m), 1381
s), 1330 (m), 1234 (s), 1208 (s), 1109 (m), 925 (m), 841
3
(
(
(
1
6
m), 816 (m), 724 (s), 625 (m); H-NMR (DMSO-d ): 8.24
1H, s, Ar–H), 7.88 (1H, s, Ar–H), 7.37 (2H, bs, –NH2).
2
.2 Synthesis of 2
The mixture of FeCl .6H O (0.27 g, 1 mmol), 3-amino-2-
3
2
0
pyrazinecarboxylic acid (0.28 g, 2 mmol) and 4, 4 -bipyr-
idine (0.15 g. 1 mmol) was dissolved in 4 mL of DMF and
MeOH (1:1) mixture. The resulting solution was sealed in a
vessel and heated at 75 °C for 48 h. It was subsequently
cooled to room temperature (0.2 °C min ), affording
block-type reddish crystals. Yield: 74 % (0.41 g) based on
Fe. Anal. Calcd. for C H Cl FeN O (M = 555.15): C,
5
0/?
using the [Fe(g -C H ) ] redox couple (E = 0.42 V vs.
5
5 2
-
1
SCE) [34–36].
2.5 General Procedure for the Peroxidative
Oxidation of 1-Phenylethanol
1
8
20
2
8 5
3
2
1
8.94; H, 3.63; N, 20.18; Found: C, 38.65; H, 3.41; N,
1
-
0.08. FT-IR (KBr, cm ): 3411 (bs), 3304 (bs), 1621 (s),
554 (m), 1491 (m), 1442 (m), 1383 (s), 1327 (m), 1232
The catalytic tests under MW irradiation (MW) were per-
formed in a focused Anton Paar Monowave 300 MW
reactor using a 5 mL capacity reaction tube with a 10 mm
internal diameter, fitted with a rotational system and an IR
temperature detector.
(
s), 1204 (s), 923 (m), 840 (s), 821 (s), 556 (m), 451 (m);
6
1
H-NMR (DMSO-d ): 8.78 (2H, s, Ar–H), 8.23 (1H, s, Ar–
H), 7.94 (2H, s, Ar–H), 7.87 (1H, s, Ar–H), 7.36 (2H, bs,
–
NH2).
Chromatographic analyses were undertaken by using a
Fisons Instruments GC 8000 series gas chromatograph with
a DB-624 (J&W) capillary column (DB-WAX, column
length: 30 m; internal diameter: 0.32 mm), FID detector,
and the Jasco-Borwin v.1.50 software. The temperature of
injection was 240 °C. The initial temperature was main-
tained at 140 °C for 1 min, then raised 10 °C min to
220 °C and held at this temperature for 1 min. Helium was
used as the carrier gas. The internal standard method was
used to quantify the organic products.
2
.3 Synthesis of 3
The mixture of FeCl .6H O (0.27 g, 1 mmol), 3-amino-2-
3
2
0
pyrazinecarboxylic acid (0.14 g, 1 mmol) and 2,2 -bipyr-
idine (0.15 g. 1 mmol) was dissolved in 4 mL of methanol.
The resulting solution was sealed in a vessel and heated at
-
1
7
5 °C for 48 h. It was subsequently cooled to room tem-
-1
perature (0.2 °C min ), affording reddish crystals. Yield:
1 % (0.34 g) based on Fe. Anal. Calcd. For C H Cl
2
8
1
5
12
FeN O (M = 421.05): C, 42.79; H, 2.87; N, 16.63; Found:
5
Oxidation reactions of the alcohols were carried out in
sealed cylindrical Pyrex tubes under focused MW irradia-
tion as follows: the alcohol (2.5 mmol), TBHP (70 %
aqueous solution, 5.0 mmol) and catalyst precursor 1–3
(1–10 lmol, 0.04–0.4 mol% vs. substrate) were introduced
in the tube which was then placed in the MW reactor. In the
experiments with radical traps, CBrCl3 (2.5 mmol) or
2
-
C, 42.83; H, 2.21; N, 16.18. FT-IR (KBr, cm ): 3422 (s),
1
3
1
8
8
7
7
308 (s), 1639 (s), 1603 (s), 1555 (m), 1497 (w), 1443 (m),
368 (s), 1328 (m), 1235 (m), 1150 (m), 1026 (w), 918 (m),
1
6
38 (m), 821 (m), 772 (s), 731 (m); H-NMR (DMSO-d ):
.63 (1H, s, Ar–H), 8.33 (1H, s, Ar–H), 8.22 (1H, s, Ar–H),
.89 (1H, s, Ar–H), 7.88 (1H, s, Ar–H), 7.40 (1H, s, Ar–H),
.35 (2H, bs, –NH2).
NHPh (2.5 mmol) was added to the reaction mixture. In
2
the experiments with other additives (TEMPO, nitric acid
1 M solution, or potassium carbonate 1 M solution), a
2
.4 Electrochemical Studies
2.5 % additive/substrate molar ratio was used. The system
The electrochemical experiments were performed on an
EG&G PAR 273A potentiostat/galvanostat connected to a
was stirred and irradiated (10 W) for 0.5–2 h at 50–150 °C.
After the reaction, the mixture was allowed to cool down to
123

A. Karmakar et al.
-
1
-1
room temperature. Typically, 150 lL of benzaldehyde
internal standard) and 2.5 mL of MeCN (to extract the
1639 cm ) and symmetric (1368–1383 cm ) stretchings of
carboxylic groups. Asymmetrical and symmetrical N–H
stretching frequencies of the amine groups appear in the
(
substrate and the organic products from the reaction mix-
ture) were added. The obtained mixture was stirred for
-1
-1
3411–3422 cm region. The bands at 1327–1330 cm are
attributed to C–N stretch (aromatic amines) [37].
1
0 min, filtered and then a sample (1 lL) was taken from
the organic phase and analysed by GC using the internal
standard method. Blank tests indicate that only traces
3.2 Crystal Structure Analyses
(
\1 %) of ketone or aldehyde are generated in a metal-free
system.
In order to perform the recycling experiments, first we
The asymmetric unit of the compound 1 contains an
3
?
anionic complex with a Fe ion coordinated by two N,O-
chelated pyrazine carboxylate ligands, two chloride
ligands, and a crystallization protonated ethanol molecule
(Fig. 1). The anionic charge of 1 is balanced by the singly
protonated ethanol which has also been found in other
washed the used catalyst (separated by centrifugation and
filtration of the supernatant solution) with several portions
of methanol and dried it in air at room temperature. It was
then reused for the oxidation reaction as described above.
3
?
cases [38]. The asymmetric unit of 2 comprises a Fe ion
with two N,O-chelated pyrazine carboxylate ligands and
two chloride ligands, together with half a molecule of
3
Results and Discussion
0
doubly protonated 4,4 -bipyridine and one dimethyl for-
3
.1 Syntheses and Characterization
mamide molecule; thus, the anionic charge of 2 is balanced
with the bipyridinium cation, in the ratio one molecule of
the former to half of the latter (Fig. 2). Compound 3 pre-
sents one N,O-chelated pyrazine carboxylate, one N,
The reaction of 3-amino-2-pyrazinecarboxylic acid with
iron(III) chloride hexahydrate in ethanol leads to the for-
-
mation of [H(EtOH)][FeCl (L) ] (1) (L = 3-amino-2-
0
N-chelated 2,2 -bipyridine and two chloride ligands
2
2
pyrazinecarboxylate) (Scheme 1). The syntheses of [H2
0
(Fig. 3). The Fe atoms in 1–3 are in slightly distorted
Cl N O (in 1 and 2) or Cl N O (in 3) octahedral envi-
ronments presenting quadratic elongations of 1.028, 1.030
bipy]1/2[FeCl (L) ].DMF (2) and [FeCl (L)(2,2 -bipy)] (3)
2
2
2
2
2
2
2 3 1
were carried out by solvothermal conditions, by reacting
2
3
-amino-2-pyrazinecarboxylic acid with iron(III) chloride
and 1.025, and angle variances of 69.80, 77.45 and 71.93° ,
in this order.
0
0
hexahydrate in the presence of 4,4 -bipyridine or 2,2 -bipyr-
idine in DMF:MeOH or MeOH, respectively (Scheme 1).
The FT-IR spectra of the complexes show the expected
bands, revealing the characteristic asymmetric (1621–
The Fe–O bond lengths in 1–3 range from 1.9599(15) to
˚
2.0075(15) A, whereas the Fe–N distance achieve
˚
2.231(2)–2.2711(18) A and the Fe–Cl distance 2.2504(6)–
Scheme 1 Synthesis of 1–3 from HL
1
23

Solvent-Free Microwave-Assisted Peroxidative Oxidation of Alcohols Catalyzed by Iron(III)…
Fig. 1 Crystal structures of complexes 1 and 2
˚
.3094(6) A (Table S1). The difference between each type
2
bipyridine, were investigated by cyclic voltammetry (CV)
of these distances, as measured by D(Fe–X) (X = O, N or
and controlled potential electrolysis (CPE), at a platinum
n
working electrode at room temperature in a 0.2 M [ Bu
˚
Cl), varies in the order 3 (nil, 0.0949 or 0.059 A) [ 2
4
˚
(
0.0476, 0.066 or 0.0409 A) [ 1 (0.0077, 0.025 or 0.0021
N][BF ]/MeCN solution. Complexes 1–3 exhibit a first
4
˚
A) indicating the effect of the charge of the complex
single-electron (as measured by CPE) reversible cathodic
wave at ca. 0.06 V versus SCE (Table 1), assigned to the
red
(
compare the results for 1 and 2) and of the presence of a
0
different type of ligand (2,2 -bipyridine in 3).
Fe(III) ? Fe(II) reduction (wave I ). A second irre-
red
versible cathodic wave (II ) in the range -0.62 to
-1.12 V versus. SCE (Table 1) is also observed, which
A number of non-covalent interactions are present in 1
and 2 (Table S2, supplementary information), stabilizing
their crystal packings. One of the amine groups of 1
donates intermolecularly to one of the N-pyridyl atoms of a
vicinal entity while the other donates to a chloride ligand,
thus generating 1D chains which are interconnected by
ethanol molecules giving rise to a 2D network (Fig. 2a).
The medium intense contacts of the methylene groups of
ethanol with O-carboxylate atoms, are worth to be men-
-
can involve the L ligand. A typical cyclic voltammogram
is exemplified for 3 in Fig. 4.
In fact, 3-amino-2-pyrazinecarboxylic acid (HL) is
redox active (Table 1) exhibiting an irreversible reduction
wave at -0.97 V versus SCE. Upon scan reversal, after the
cathodic process, an irreversible anodic wave at 1.16 V
0
versus SCE is also observed. In contrast, 2,2 -bipy does not
˚
tioned (C12-H12AÁÁÁO3, DÁÁÁA 2.787(3) A, \DHA 159°;
exhibit any redox activity in the -2.0 to 1.5 V versus SCE
?
range. For (H bipy)Cl , liberation of H occurs in the
˚
C12-H12BÁÁÁO1, DÁÁÁA 2.822(2) A, \DHA 133°). In 2 the
2
2
?
anionic complex units generate, by themselves, a 3D net-
work (base vectors [100], [001] and [020]) by means of H
contacts connecting the amine substituents as donors and
the N-pyrazine or O-carboxylate atoms as acceptors. This
network is interpenetrated by 1D chains (base vector [021])
linking the protonated bipyridine groups which donate to a
carboxylate group (Fig. 2b). In 3 the most intense contacts
found are those connecting the N3 amine group with the
electrolytic solution, and H reduction is detected as an
irreversible cathodic wave at -0.68 V versus SCE (Pt
electrode) which, as expected [34], shifts to a much more
cathodic value (-1.18 V vs. SCE) when using a vitreous C
2
?
electrode. In the case of compound 2, with the (H bipy)
2
cation, the free proton reduction is not detected conceiv-
-
ably due to its capture by the basic L ligand. The irre-
-
versible oxidation of the Cl anion in (H bipy)Cl is
2
2
˚
Cl ligand (DÁÁÁA 3.428(2) A, \DHA 160°) and which
observed at 1.19 V versus SCE, within the typical [34, 35]
potential range.
2
associate the molecules into pairs. Despite this, the shortest
˚
metal–metal distance was found in this structure (6.597 A),
˚
followed by 1 (7.160 A) and then by 2 (7.762 A).
˚
3.4 Microwave-Assisted Catalytic Peroxidative
Oxidation of 1-Phenylethanol
3
.3 Electrochemical Behaviour
Complexes 1–3 were tested as catalysts for the oxidation of
1-phenylethanol (model substrate), using aqueous tert-
t
butylhydroperoxide ( BuOOH or TBHP, aq. 70 %) as
The electrochemical behaviours of complexes 1–3, as well
as of 3-amino-2-pyrazinecarboxylic acid (HL), doubly
0
0
protonated 4,4 -bipyridine [(H bipy)Cl ] and 2,2 -
2
oxidizing agent, at 50–150 °C, low power (10 W) MW
2
123

A. Karmakar et al.
Fig. 2 Hydrogen bonded networks of 1 (top) and 2 (bottom; hydrogen bonding interactions drawn in black dotted lines)
irradiation, 0.5–2 h reaction time and in a solvent-free
medium (Scheme 2). They display high and similar activ-
ities (for 1 and 2 they are even identical).
towards the formation of the ketone was found, since no
traces of by-products were detected by GC and GC–MS
analyses of the final reaction mixtures (only unreacted
alcohol, apart from the ketone product).
In fact, under typical solvent- and additive-free condi-
tions (Table 2, entries 1–3), 1–3 lead to the production of
acetophenone in 81.3 (1), 80.9 (2) or 83.9 % (3) yield and
turnover frequencies (TOF, moles of product per mol of
Complexes 1–3 were also tested towards the oxidation
of C secondary alcohols, namely cyclohexanol and the
6
linear 2- and 3-hexanol, and the primary alcohols benzyl
alcohol and 1-hexanol. The ketones are the only oxidation
products obtained (Table 3). As expected [7, 9], the ali-
cyclic cyclohexanol is less reactive than 1-phenylethanol:
3
-1
catalyst per hour) up to 8.1 9 10 h (3, for a low catalyst
loading: catalyst/substrate molar ratio of 0.04 %) after
1
.5 h under MW irradiation at 90 °C. A high selectivity
1
23

Solvent-Free Microwave-Assisted Peroxidative Oxidation of Alcohols Catalyzed by Iron(III)…
Scheme 2 MW-assisted oxidation of 1-phenylethanol to acetophe-
none catalysed by 1–3
1
50 °C/10 W MW irradiation) (Table 3, entries 1–3). The
linear aliphatic alcohols, 2-hexanol and 3-hexanol, lead to
even lower yields in the 45–56 % range (Table 3, entries
4–9), under similar reaction conditions, as reported in other
cases [2, 3, 6, 7]. The activity of 1–3 in the oxidation of
benzyl alcohol or 1-hexanol is quite modest since only
rather moderated yields of benzaldehyde (30–34 % range)
or hexanal (22–24 % range) are obtained under the same
reaction conditions (Table 3, entries 10–12 and 13–15,
respectively).
Fig. 3 Crystal structure of complex 3 with atomic labelling scheme
Table 1 Cyclic voltammetric data for compounds 1–3, HL and
2 2
(H bipy)Cl
The very similar catalytic activities of 1–3 are in accord
with their very similar reduction potentials as detected by
cyclic voltammetry (see above), which conceivably deter-
mine the iron promoted decomposition of TBHP (see
mechanistic considerations below) [39, 40].
red
red
ox
(I )
Compound
E
‘
(E
p
) (I
)
E
p
(II
)
E
p
1
0.06
0.05
0.06
-1.12
-1.05
-0.62
–
–
–
–
2
3
a
The catalytic systems of the present study seem to
operate under heterogeneous conditions and, although
yielding ketone products in the range reported for homo-
geneous ones [2, 3, 6, 7], e.g., the tris{2,4-bis(tri-
HL
(-0.97)
b
-0.68
1.16
1.19
c
(
H
2
bipy)Cl
2
–
Potential values in Volt ± 0.02 versus SCE, in a 0.2 M [Bu
MeCN solution, at a Pt disc working electrode determined by using
4 4
N][BF ]/
5
0/?
ox
redox couple (E1/2 = 0.42 V vs. SCE) [34–36]
the [Fe(g -C
as internal standard at a scan rate of 200 mVs
5 5
H )
2
]
chloromethyl)-1,3,5-triazapentadienato}-Fe(III)
com-
-1
plexes/MW/TBHP systems [11], they are advantageous
due to the possibility of recycle and reuse of the catalysts
up to three consecutive catalytic cycles without a marked
loss of activity (see below).
a
Anodic wave generated upon scan reversal following the reduction
wave
b
Proton reduction (shifts to -1.18 V at a vitreous C electrode)
-
Oxidation of Cl
c
MW irradiation (10 W) provides a more efficient syn-
thetic method than conventional heating, thus allowing the
attainment of similar yields in shorter times, as observed in
other cases [2, 3]. For example, in the presence of 3, a
99.3 % yield of acetophenone was obtained after 1 h
reaction under MW irradiation at 150 °C, while the reac-
tion under conventional heating (oil bath) gives only 9 %
yield. It takes 46 h with conventional heating to achieve
the same acetophenone yield as that obtained with MW
after 1 h.
The influences of various reaction parameters, such as
the amount and type of oxidant, time, temperature and
presence of additives, were investigated for the most active
substrate (1-phenylethanol), and the results are summarized
in Table 2 and Figs. 5 and 6.
Fig. 4 Cyclic voltammogram, initiated by the cathodic sweep, of 3 in
n
a 0.2 M [ Bu
4 4
N][BF ]/MeCN solution, at a Pt disc working electrode
-1
5
0/?
The MW-assisted alcohol oxidation depends on the
temperature (Fig. 5). For example, an almost quantitative
yield (99.3 %) of acetophenone is achieved after 1 h at
150 °C (10 W) for 3, in the presence of TEMPO (Table 2,
entry 9), which is much higher than the 67 % yield
(
5 5 2
d = 0.5 mm), run at a scan rate of 200 mVs . * [Fe(g -C H ) ]
in the presence of a catalytic amount of 1–3 (0.4 mol% vs.
substrate) and in the solvent-free medium, the yields of
cyclohexanone are in the 61–66 % range (after 1.5 h at
123

A. Karmakar et al.
Table 2 Selected results for the solvent-free MW-assisted oxidation of 1-phenylethanol using 1–3 as catalyst precursors
b
Yield (%)
-1 c
TON [TOF (h )]
Entry
Catalyst (amount,
mol% vs. substrate)
Reaction
time (h)
Temperature (°C)
Additive or
radical trap
a
1
2
3
4
5
6
7
8
9
1 (0.3)
2 (0.3)
3 (0.3)
–
–
–
81.3
80.9
83.9
68.2
86.0
97.1
71.0
91.1
99.3
99.2
44.0
67.0
94.2
42.7
8.0
271 (181)
270 (180)
280 (187)
227 (454)
287 (287)
324 (216)
237 (473)
304 (304)
331 (331)
331 (220)
147 (147)
223 (223)
314 (314)
142 (71)
27 (27)
1.5
90
0.5
1
90
1 (0.3)
3 (0.3)
90
TEMPO
TEMPO
1.5
0.5
1
90
90
90
1
150
90
10
11
12
13
14
15
16
17
18
19
20
21
1.5
1
50
3 (0.3)
3 (0.3)
3 (0.4)
1
70
TEMPO
1
120
70
2
d
d
1
150
–
TEMPO
21.8
39.2
47.1
6.9
73 (73)
K
2
CO
HNO
Ph NH
3
98 (98)
1
1
150
150
3
118 (118)
17 (17)
2
CBrCl
3
13.4
46.3
34 (34)
e
FeCl
.6H O (0.3)
3 2
–
154 (54)
Reaction conditions unless stated otherwise: 2.5 mmol of 1-phenylethanol, 1–10 lmol (0.04–0.4 mol% vs. substrate) of 1–3, 5 mmol of TBHP
(
a
2 eq., 70 % in H
2
O), 50–150 °C, MW irradiation (10 W power)
Additive: TEMPO, K CO or HNO (2.5 mol% vs. substrate); radical trap: Ph NH or CBrCl (2.5 mmol)
3
2
3
2 3
b
c
d
e
Molar yield (%) based on substrate, i.e., moles of acetophenone per 100 mol of 1-phenylethanol determined by GC
Turnover number = number of moles of product per mol of catalyst precursor; TOF = TON per hour (values in brackets)
5
mmol of H
2 2
O (30 % aqueous solution) instead of TBHP
Included for comparative purposes
obtained for the same reaction time but at 70 °C (Table 2,
entry 12). Furthermore, at this temperature, only 42.7 %
hydrogen peroxide (instead of TBHP) in the presence of
TEMPO (entries 15 and 16, Table 2).
(
3) of acetophenone is obtained for the extended reaction
time of 2 h (Table 2, entry 14).
Moreover, the previously recognised promoting effect of
basic additives [2, 3, 40, 42] is not observed for the present
catalytic systems; in contrast, addition of 1 M solution of
K CO hampers the reaction (Table 2, entry 17; Fig. 6).
Experiments with the cheaper and environmentally
friendly hydrogen peroxide (30 % aqueous solution) as
oxidant are not effective, as attested by the marked yield
lowering e.g., in the case of 3, from 83.9 to 8.0 % (entry
2
3
The presence of HNO also exhibited an inhibitory effect
3
on the acetophenone yield (Table 2, entry 18, Fig. 6), as
found e.g., for the MW-assisted oxidation of 1-pheny-
lethanol with TBHP catalysed by the Cu(II) complex
1
5, Table 2), in accord with the expected decomposition of
H O under the used reaction conditions (150 °C).
2
2
The influence of the presence of 2,2,6,6-tetram-
[Cu(H R)(HL)]ÁH O
[H L = (E)-2-(((1-hydroxynaph-
2
2
2
ethylpiperidyl-1-oxyl (TEMPO), a nitroxyl radical that is a
known [2–4, 7, 38, 41] promoter in oxidation catalysis of
alcohols, was also investigated. The TEMPO additive
provided a significant increase in acetophenone yield from
thalen-2-yl)methylene)amino)benzenesulfonic acid] [7] or
0
Cu(II) complexes bearing the 1,6-bis(2 -pyriyl)-2,5-dithia-
hexane ligand [27, 42]. In fact, the mechanism of alcohol
oxidation with TBHP does not seem to be promoted by the
presence of protons as also verified [43] in the oxidation of
1-phenylethanol with TBHP catalysed by bi- or tetranu-
clear cage-like copper(II) silsesquioxanes.
8
3
3.9 to 99.2 % under the same reaction conditions (entries
and 10, Table 2; Fig. 6). A similar yield enhancement
was observed when the reaction was carried out with
1
23

Solvent-Free Microwave-Assisted Peroxidative Oxidation of Alcohols Catalyzed by Iron(III)…
Table 3 Selected results for the
a
-1 b
TON [TOF(h ]
Entry
Catalyst
Substrate
Product
Yield (%)
solvent-free MW-assisted
oxidation of selected C
6
1
2
3
4
5
6
7
8
9
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
Cyclohexanol
Cyclohexanone
63.9
65.5
61.3
50.3
52.4
55.5
45.2
47.1
48.3
34.0
31.9
29.7
22.3
24.1
22.8
160 (107)
164 (109)
153 (102)
126 (84)
131 (87)
139 (93)
113 (75)
118 (79)
121 (81)
85 (57)
secondary alcohols and of the
primary alcohols benzyl alcohol
and 1-hexanol, using 1–3 as
catalyst precursors
2-Hexanol
2-Hexanone
3-Hexanone
Benzaldehyde
Hexanal
3-Hexanol
10
11
12
13
14
15
Benzyl alcohol
1-Hexanol
80 (53)
74 (50)
56 (37)
60 (40)
57 (38)
t
Reaction conditions (unless stated otherwise: 2.5 mmol of substrate, 5 mmol of BuOOH (aq. 70 %),
10 lmol (0.4 mol% vs. substrate) of 1–3, 62.5 lmol (2.5 mol% vs. substrate) of TEMPO, 150 °C, 1.5 h of
microwave irradiation (10 W)
a
Molar yield (%) based on substrate, i.e., moles of product per 100 mol of substrate determined by GC
b
Turnover number = number of moles of product per mol of metal catalyst; TOF = TON per hour
values in brackets)
(
1
00
8
6
4
2
0
0
0
0
0
5
0
100
150
Fig. 6 Influence of different additives (K
Ph NH or CBrCl ) on the yield of acetophenone from MW-assisted
2 3 3
CO , HNO , TEMPO,
Temperature (°C)
2
3
peroxidative oxidation of 1-phenylethanol catalysed by 3
Fig. 5 Influence of the temperature on the yield of acetophenone
from MW-assisted peroxidative oxidation of 1-phenylethanol catal-
ysed by 3 (1 h reaction time, 10 W)
t
˙
BuOO radicals produced in the iron promoted decompo-
sition of TBHP [23] according to the following equations.
A very strong inhibition effect is observed (Fig. 6) when
the peroxidative oxidation of 1-phenylethanol is carried out
in the presence of either an oxygen-radical trap such as
3
þ
t
2þ
t
Á
þ
Fe þ BuOOH ! Fe þ BuOO þ H
ð1Þ
ð2Þ
ð3Þ
ð4Þ
ð5Þ
2þ
t
3þ
t
Á
Fe þ BuOOH ! Fe ÀOH þ BuO
Ph NH (Table 2, entry 19) or a carbon-radical trap such as
2
3
þ
t
3þ
t
Fe ÀOH þ BuOOH ! Fe ÀOOÀ Bu þ H O
CBrCl (Table 2, entry 20). This suggests that the oxida-
3
2
tion reaction proceeds mainly via a radical mechanism
t
0
t
0
Á
BuO þ RR CHOH ! BuOH þ RR C À OH
BuOO þ RR CHOH ! BuOOH þ RR C ÀOH
which involves both oxygen- and carbon-centred radicals
t
44, 45]. The mechanism may involve e.g., BuO and
t
Á
0
t
0
Á
˙
[
123

A. Karmakar et al.
1
00
ligands, and different organo-cations (protonated ethanol
0
for 1 and half of a doubly protonated 4,4 -bipyridine for 2)
involved in H-bonds with the carboxylate ligand. Complex
8
6
4
2
0
0
0
0
0
0
3 contains both a pyrazine carboxylate and a 2,2 -bipyr-
idine as ligands.
Complexes 1–3 act as efficient and selective catalysts for
the mild MW-assisted oxidation of 1-phenylethanol and C6
secondary alcohols in solvent-free systems, thus widening
the scope of peroxidative catalytic systems suitable for MW
assisted oxidative transformations of alcohols. The activity
follows the order of 1-phenylethanol[ cyclohexanol[
1st
2nd
3rd
4th
Catalyꢀc cycle
Fig. 7 Effect of the catalyst recycling on the yield of acetophenone
from MW-assisted peroxidative oxidation of 1-phenylethanol catal-
ysed by 3
2-hexanol[ 3-hexanol [ benzyl
alcohol [ 1-hexanol.
Moreover, these heterogeneous catalytic systems allowed
their easy recovery and reuse, at least for four consecutive
cycles, maintaining 83 % of the initial activity and a con-
comitant rather high selectivity.
3
þ
t
0
Á
0
Fe ÀOOÀ Bu þ RR C ÀOH ! RR C
t
2þ
¼
O þ BuOOH þ Fe
ð6Þ
Acknowledgments This work has been supported by the Funda c¸ a˜ o
para a Ci eˆ ncia e a Tecnologia (FCT), Portugal (Project UID/QUI/
00100/2013). Authors A. Karmakar and S. Hazra express their grat-
itude to the FCT for post-doctoral fellowships (Ref. Nos. SFRH/BPD/
It may also proceed via the coordination of the alcohol
substrate to an active site of the catalyst, and its deproto-
nation to form the alkoxide ligand, followed by a metal-
centred (and TEMPO assisted) dehydrogenation.
7
6192/2011 and SFRH/BPD/78264/2011).
Catalyst recyclability was investigated for up to four
consecutive cycles for the catalyst precursor 3. On com-
pletion of each stage, the product was analyzed as usually
and the catalyst was recovered by filtration from the
reaction mixture, thoroughly washed and dried (see
experimental). The subsequent cycle was initiated upon
addition of new standard portions of all other reagents. The
filtrate was tested in a new reaction (by addition of fresh
reagents), and no oxidation products were detected.
Moreover, the iron content of the filtrate was determined as
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