Tuning Cyclohexane Oxidation: Combination of Microwave Irradiation and Ionic Liquid with the C-Scorpionate [FeCl2(Tpm)] Catalyst

Article abstract of DOI:10.1021/acs.organomet.6b00620

For the first time, microwave (MW) irradiation was successfully applied to peroxidative cyclohexane oxidation catalyzed by a C-scorpionate complex, [FeCl2(Tpm)] (1; Tpm = hydrotris(pyrazol-1-yl)methane), providing a highly selective and fast eco-friendly

Full text of DOI:10.1021/acs.organomet.6b00620

Article
pubs.acs.org/Organometallics
Tuning Cyclohexane Oxidation: Combination of Microwave
Irradiation and Ionic Liquid with the C‑Scorpionate [FeCl2(Tpm)]
Catalyst
Ana P. C. Ribeiro,† Luísa M. D. R. S. Martins,*,†,‡ Maxim L. Kuznetsov,† and Armando J. L. Pombeiro†
†Centro de Química Estrutural, Instituto Superior Tec
́
nico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
‡Chemical Engineering Department, Instituto Superior de Engenharia da Lisboa, Instituto Politec
́
nico de Lisboa, R. Conselheiro
Emídio Navarro, 1959-007 Lisboa, Portugal
S
* Supporting Information
ABSTRACT: For the first time, microwave (MW) irradiation was
successfully applied to peroxidative cyclohexane oxidation catalyzed
by a C-scorpionate complex, [FeCl2(Tpm)] (1; Tpm = hydrotris-
(pyrazol-1-yl)methane), providing a highly selective and fast eco-
friendly procedure to produce a KA (cyclohexanol + cyclohexanone)
oil mixture (up to ca. 100% selectivity) with 28% yield in 1 h at 50 °C
in MeCN. Water, organics, and ionic liquids (ILs, [bmim][N(CN)2]
and [bmim][BF4]) were used as solvents, and their interactions with
1 were determined by DFT calculations. The possibility to recycle and
reuse 1 (up to nine consecutive cycles) without loss of activity is also
observed for the IL [bmim][BF4]. Moreover, higher catalytic activities
under additive-free conditions are obtained with the ILs in
comparison to those with the other solvents. Tuning the alcohol/
ketone selectivity is also possible by choosing the appropriate IL
solvent.
Within our interest in the oxidation of alkanes, in particular
of cyclohexane due to its industrial relevance,21,22 namely by C-
scorpionate complexes (bearing tripodal ligands comparable to
cyclopentadienyl on electronic and coordination grounds),23,24
the main objectives of the present study consisted of
developing a selective, efficient, and sustainable protocol for
the oxidation of cyclohexane to KA oil (cyclohexanone (K) and
cyclohexanol (A) mixture) catalyzed by the C-scorpionate
Fe(II) catalyst [FeCl2(Tpm)] (Tpm = hydrotris(pyrazol-1-
yl)methane),25 by (i) replacing common organic solvents by
suitable ILs in order to recycle and reuse the catalyst and (ii)
using MW irradiation as a reaction accelerating agent,
improving the yield and selectivity of the desired products.
Two ILs (Figure 1), 1-butyl-3-methylimidazolium dicyana-
mide ([bmim][N(CN)2], hydrophilic) and 1-butyl-3-methyl-
imidazolium tetrafluoroborate ([bmim][BF4], hydrophobic),
were tested as solvents under MW irradiation (up to 6 h) in a
temperature range of 30−80 °C, and their interaction with the
catalyst molecule was rationalized by DFT calculations.
INTRODUCTION
■
Direct selective oxidation of C−H bonds remains a challenging
goal in organic synthesis.1−3 Although several approaches have
been developed1−10 to overcome safety and ecological
drawbacks, namely those using catalysts based on cheap and
abundant metals such as iron, the search for iron-based catalytic
systems operating under more sustainable conditions still
continues.
Among several requirements for a suitable catalyst, its
recyclability with possibility to be used in consecutive cycles is
often mandatory for an economically feasible process. More-
over, high selectivity, short reaction times, and practical, easy
workups are also relevant. Thus, the use of microwave (MW)
radiation as an alternative energy source (that generates heat
evenly throughout the reactor) is appealing, eventually
enhancing product purity, selectivity, and yield as well as
being more energy efficient and economical in comparison to
conventional heating methods.11,12 However, the application of
MW irradiation in alkane functionalization is essentially an
unexplored field.13,14
To our knowledge, this is the first time that the successful use
of ionic liquids as solvents for the oxidation of alkanes with an
iron catalyst under MW irradiation has been reported.
Another important topic to consider in sustainable catalytic
processes concerns the use of solvents. The application of
room-temperature ionic liquids (ILs) in organic synthesis
provides a good alternative to volatile organic solvents,15−17 but
Special Issue: Hydrocarbon Chemistry: Activation and Beyond
Received: August 4, 2016
their use in alkane oxidative functionalization remains virtually
unexplored.18−20
© XXXX American Chemical Society
A
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Under the optimized conditions, cyclohexanol (A) and
cyclohexanone (K) were the only products detected by GC-MS
analysis, indicating a high selectivity of the oxidation system
under these conditions. The high selectivity toward such
products (typically above 95%, reaching 100%) is confirmed by
the mole balance between the converted substrate and those
products.
A maximum total (KA) yield of 33.0% was obtained after 6 h
of MW irradiation at 50 °C using 0.2 mol % vs CyH of 1 in
MeCN and in the presence of Hpca (entry 5 of Table 1; see
optimization assays in Table S1 in the Supporting Information).
However, as depicted in Figure 2, after 1 h of MW irradiation
(28% total yield, entry 3 of Table 1) under the above
conditions, the yield enhancement is not considerable.
The yield values now obtained are significantly higher than
that reported (total yield of 17%)30,31 without MW assistance.
Moreover, they are achieved in a much shorter reaction time (1
h vs 6 h).30,31
Hpca, known as an effective cocatalyst for alkane oxidation
with H2O2 (involving the formation of the hydroxyl radical) at
room temperature or under conventional heating,10,30−33 also
acts efficiently under MW irradiation (compare entries 4 and 6
of Table 1). Its effect is also much more pronounced than those
observed with inorganic acids such as nitric and sulfuric acids
(compare entries 3, 7, and 8 of Table 1), as reported for the
non-MW-assisted catalytic system.25
The efficacy of the 1/MW/MeCN/Hpca catalytic system is
dependent on the catalyst amount (Table S1 in the Supporting
Information and Figure 2) and type and amount of oxidant
(compare entries 10, 20, and 21 of Table S1), as well as on the
reaction temperature (compare entries 8, 18, and 19 of Table
S1). Decomposition of H2O2 hampered its use at higher
temperatures (above 50 °C), which led to a marked decrease in
the yield (entry 19, Table S1). “Overoxidation” products, such
as 1,4-cyclohexanediol, detected by GC-MS for higher
temperatures (80 °C), can also account for the observed
lower yields of A and K.
Figure 1. Schematic structure representation of (a) 1-butyl-3-
methylimidazolium dicyanamide ([bmim][N(CN)2]) and (b) 1-
butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]).
RESULTS AND DISCUSSION
■
The knowledge that the complex [FeCl2(Tpm)] (1; Tpm =
hydrotris(pyrazol-1-yl)methane) is one of the best C-
scorpionate transition-metal catalysts for the peroxidative
(with aqueous H2O2) oxidation of cyclohexane (CyH) in
acetonitrile and in the presence of pyrazinecarboxylic acid
(Hpca)23,25 prompted us to try to further increase the efficiency
of the homogeneous catalytic system by using unconventional
conditions.
Thus, the catalytic activity of 1 was tested under microwave
(MW) irradiation, in different solvents: water, acetonitrile, and
two room-temperature ionic liquids (RTILs), 1-butyl-3-
methylimidazolium tetrafluoroborate ([bmim][BF4]) and 1-
butyl-3-methylimidazolium dicyanamide ([bmim][N(CN)2]),
in a temperature range of 30−80 °C over 0.25−6 h, according
to Scheme 1 and Tables 1 and 2.
Scheme 1. MW-Assisted Oxidation of Cyclohexane with
Aqueous H2O2 Catalyzed by [FeCl2(Tpm)] in Different
Solvents
a
Table 1. Selected Data for the MW-Assisted Oxidation of Cyclohexane (CyH) with H2O2 Catalyzed by [FeCl2(Tpm)], in
MeCN and in the Presence of Hpca
b
yield/%
c
d
e
entry
time/h
additive
Hpca
cyclohexanone (K)
cyclohexanol (A)
total
A/K
total TON
total TOF/h−1
1
0.25
0.5
1
0.4
3.7
8.7
13.3
20.3
20.3
20.6
2.1
9.1
17.0
27.9
31.5
33.0
4.8
21.8
3.6
2.7
1.5
0.6
0.8
0.7
1.2
2.9
1.5
1.5
0.7
46
85
184
170
141
53
28
8
2
Hpca
3
Hpca
7.6
141
159
165
24
4
3
Hpca
11.2
12.4
2.7
5
6
Hpca
6
3
no additive
HNO3
H2SO4
K2CO3
CBrCl3
NHPh2
Hpca
7
1
4.3
2.9
7.2
36
36
30
22
8
8
1
2.7
3.2
5.9
30
9
1
1.1
3.2
4.3
22
10
11
3
1.9
2.9
4.7
24
3
1.3
1.9
3.2
16
5
f
12
3
14.9
0.0
10.3
0.0
25.2
0.0
128
43
g
13
h
3
Hpca
14
3
no additive
2.3
1.9
4.2
0.8
21
7
a
Reaction conditions (unless stated otherwise): MeCN (3.0 mL), CyH (5.0 mmol), H2O2 (10.0 mmol), 10.0 μmol of 1 (0.2 mol % vs CyH),
n(additive)/n(1) = 40, 50 °C, 0.25−6 h of MW irradiation. Yields and TONs were determined by GC analysis (upon treatment with PPh3).9,26−29
b
c
d
Percentage molar yield ((mol of product)/(mol of cyclohexane)). Ratio between the molar concentrations of A and K. Total turnover number,
e
f
g
h
defined as (mol of A and K)/(mol of 1). TOF = TON/h. Without PPh3 treatment. Without 1. Under an N2 atmosphere.
B
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a
Table 2. Selected Data for MW-Assisted Additive-Free Oxidation of Cyclohexane (CyH) with H2O2 Catalyzed by
[FeCl2(Tpm)] in ILs
b
yield/%
c
d
e
entry
solvent
time/h
additive
cyclohexanone (K)
cyclohexanol (A)
total
A/K
total TON
total TOF/h−1
1
2
3
4
5
6
7
8
0.25
0.5
1
0.8
1.1
1.8
1.7
1.7
0.9
0.6
0.5
0.2
1.6
2.9
2.5
6.3
3.3
7.4
3.1
5.7
17
37
63
94
97
36
6
67
74
63
31
16
36
6
10.6
16.9
17.7
6.3
12.4
18.6
19.4
7.2
5.9
3
9.9
6
10.4
7.0
[bmim][N(CN)2]
1
Hpca
1
HNO3
K2CO3
0.5
1.1
0.8
1
0.9
1.4
1.8
7
7
f
9
6
0.5
0.7
2.5
4
0.6
16
16
g
10
6
17.3
16.5
18.9
19.4
10.8
8.2
95
97
h
11
6
12
13
14
15
16
17
18
19
0.25
0.5
1
1.6
4.3
0.5
1.1
2.1
4.0
4.2
0.1
0.5
1.9
0.1
3.3
3.8
2.1
5.4
0.3
0.3
0.3
0.3
0.3
0.3
0.2
0.3
0.3
0.3
0.3
11
27
47
81
84
3
43
55
47
27
14
3
7.2
9.3
3
12.1
12.3
0.4
16.1
16.5
0.5
6
[bmim][BF4]
1
Hpca
1
HNO3
K2CO3
2.1
2.6
13
39
3
13
39
0.4
13
14
1
5.9
7.8
f
20
6
0.4
0.5
g
21
h
6
12.0
12.3
15.3
16.5
77
84
22
6
a
Reaction conditions (unless stated otherwise): IL (3.0 mL), CyH (5.0 mmol), H2O2 (10.0 mmol), 10.0 μmol of 1 (0.2 mol % vs. CyH),
n(additive)/n(1) = 40, 50 °C, 0.25−6 h of MW irradiation. Yields and TONs were determined by GC analysis (upon treatment with PPh3).9,26−29
b
c
d
Percentage molar yield ((mol of product)/(mol of cyclohexane)). Ratio between the molar concentrations of A and K. Total turnover number,
e
f
g
h
defined as (mol of A and K)/(mol of 1). TOF = TON/h. Without 1. Under N2 atmosphere. Without PPh3 treatment.
Scheme 2
Figure 2. Effect of the reaction time and catalyst amount on the yields
of cyclohexanol (A) and cyclohexanone (K) obtained from the MW-
assisted oxidation of CyH in MeCN (values from Table S1 in the
severe (160 °C, 100 W MW power) than ours. Cyclohexanol,
Supporting Information).
cyclohexanone, cyclohexyl hydroperoxide, and adipic acid were
obtained with yields up to 17.0% and selectivities up to 35%.14
Evidence for a radical mechanism, involving the formation of
cyclohexyl hydroperoxide (Scheme 1), as proposed for the non-
MW-assisted system,25 arises from the application of Shul’pin’s
method:7−9,26−29 the addition of PPh3 prior to the GC analysis
of the products resulted in a marked increase in the amount of
A (due to the reduction of the hydroperoxide by PPh3, with
concomitant formation of phosphane oxide) with a decrease of
K (compare entries 4 and 11 of Table 1).
Moreover, addition of a radical trap (e.g., Ph2NH or CBrCl3)
to the reaction mixture results in the near-suppression of the
catalytic activity (see entries 16 and 17 of Table S1 in the
Supporting Information). This behavior also supports the
hypothesis of a free radical mechanism for the cyclohexane
oxidation carried out in this study (Scheme 2).8−10,23,24,32−35
Experiments in the absence of O2 (dinitrogen atmosphere)
were also performed (Table 1, entry 14). The obtained overall
yield and TON values under the same reaction conditions, but
in air (Table 1, entry 6), are quite similar to those obtained
under dinitrogen (Table 1, entry 14), indicating only a slightly
promoting effect of the air. It is noteworthy that O2 is formed
even when the reaction is initiated under an inert gas
atmosphere, on account of dismutation of H2O2 and of
CyOO• (Scheme 2, eq 9).
Our catalytic system is much more selective than the Fe(III)
complexes bearing bis-2-pyridylmethylamine (BMPA) ligands
and BPMA derivatives reported14 to catalyze the MW-assisted
oxidation of CyH with H2O2 in acetonitrile (to our knowledge,
the only ones until now) but under reaction conditions more
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The isolation of 1 from the reaction mixture in MeCN was
not possible, thus disabling the performance of consecutive
catalytic cycles.
to pure cyclohexanone (at high pressure and 250 °C, in the
presence of copper and chromium oxides), which is then used
to make caprolactam, the monomer for producing PA 6.21,36
A
With the aim of being able to recycle and reuse the catalyst,
we addressed room-temperature ionic liquids (ILs) as solvent
media and studied their effect on the catalytic activity of this
iron C-scorpionate complex for the above reaction. [bmim]-
[BF4] and [bmim][N(CN)2] were found to be suitable IL
solvents for [FeCl2(Tpm)] and were selected as the reaction
media. To our knowledge, this is the first time that ILs have
been used as solvents for the oxidation of alkanes with an iron
catalyst under MW irradiation. Selected results for the catalytic
systems 1/MW/H2O2/[bmim][N(CN)2] and 1/MW/H2O2/
[bmim][BF4] are presented in Table 2.
selective oxidation of the cyclohexane to cyclohexanone process
would therefore be an improvement in the PA 6 production.
The recyclability of 1 in the used IL media (see the
Experimental Section) is depicted in Figure 4. The activity and
A maximum total (KA) yield of 19.4 or 16.5% was obtained
after 6 h of MW irradiation at 50 °C using 0.2 mol % of 1 vs
CyH in [bmim][N(CN)2] or [bmim][BF4], respectively (entry
5 or 16, respectively, Table 2). Cyclohexanol (A) and
cyclohexanone (K) were the only products detected by GC-
MS analysis under the optimized conditions, as observed for
the 1/MW/MeCN/Hpca catalytic system. The oxidation of
CyH with H2O2 in IL media led to total yields considerably
lower in comparison to reactions run under the same
conditions in MeCN and in the presence of Hpca (33%,
Table 1). Interestingly, no yield enhancement due to the
presence of an additive such as Hpca, HNO3, or K2CO3
(entries 6−8 and 17−19 for [bmim][N(CN)2] or [bmim]-
[BF4], respectively, Table 2) was observed in both ILs. On the
contrary, such additives had a marked inhibitory effect.
Although a higher total (KA) yield is obtained for the
reaction performed in MeCN and in the presence of Hpca,
important advantages are observed when ILs are used as
solvents, such as (i) an enhanced total (KA) yield under
additive-free conditions (4.8% in MeCN vs. 18.6% in
[bmim][N(CN)2] or 16.1% in [bmim][BF4]; entry 6 of
Table 1 vs entry 4 or 15 of Table 2), (ii) the recycling and reuse
of 1 in a considerable number of consecutive catalytic cycles
(see below), and (iii) the alcohol/ketone selectivity tuning
dependent on the IL. As depicted in Figure 3, high selectivity
Figure 4. Effect of the catalyst recycling on the total (A + K) yield for
the MW-assisted oxidation of cyclohexane with H2O2 catalyzed by 1 in
[bmim][N(CN)2] or [bmim][BF4]. Reaction conditions: IL (3.0 mL),
CyH (5.0 mmol), H2O2 (10.0 mmol),10.0 μmol of 1, 3 h of MW
irradiation at 50 °C. Each bar represents a run, and the respective yield
value is from Table S2 in the Supporting Information.
selectivity (Table S2 in the Supporting Information) of 1 in
[bmim][BF4] are maintained through a considerably high
number of consecutive cycles: e.g., after the ninth cycle, 1 still
retains 92% of its initial activity. Moreover, the ICP analysis of
the corresponding supernatant phase revealed a leaching of 1
inferior to 1.7%. However, the reuse of 1 in [bmim][N(CN)2]
was not as successful (Figure 4).
In view of the hydrosolubility of [FeCl2(Tpm)] (1), the
oxidation of CyH catalyzed by this complex was also tested
under the above conditions but in water. Although active in this
solvent (Figure 5, Table S3 in the Supporting Information), the
performance of 1 in the presence of Hpca is much lower than
that in acetonitrile with the same additive. In fact, the obtained
Figure 3. Effect of the reaction time and IL (solvent) on the yield of
cyclohexanol (A) and cyclohexanone (K) obtained from the MW-
■
assisted oxidation of CyH in [bmim][N(CN)2] ( ) and [bmim][BF4]
●
( ).
toward cyclohexanol formation is obtained in [bmim][N-
(CN)2] (entries 1−5, Table 2), whereas in [bmim][BF4] the
formation of cyclohexanone is clearly favored (entries 12−16,
Table 2). Such an uncommon feature of our 1/MW/H2O2/IL
catalytic systems can be of applied significance. For example, in
the current production process of polyamide 6 (PA 6), the KA
mixture obtained from cyclohexane oxidation is first converted
Figure 5. Effect of the solvent on the total (KA) yield for the MW-
assisted oxidation of cyclohexane with H2O2 catalyzed by 1. Yield
values were obtained after 3 h MW irradiation under the conditions of
Table S3 in the Supporting Information.
D
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KA yield (5.6%, entry 2, Table S3) in water (with Hpca) after 3
h of MW irradiation is comparable with that obtained for the
same reaction time in MeCN but in the absence of Hpca (4.8%,
entry 6, Table 1). When the reaction was run in water under
additive-free conditions, 4.3% KA yield was achieved.
Solvent-free conditions were also attempted, but the pressure
generated hampered performing the catalytic assay.
The interaction of the catalyst [FeCl2(Tpm)] (1) with the IL
1-butyl-3-methylimidazolium dicyanamide ([bmim][N(CN)2])
was investigated using theoretical DFT calculations at the
M06/6-31+G* (Fe-ECP) level of theory. The coordination
polyhedron of the equilibrium structure of complex 1
corresponds to a tetragonal pyramid with one of the donor
N atoms being at the axial position (Figure 6). Thus, the iron
Formation of the bmim·2 associate from 1 and [bmim][N-
(CN)2] (optimized as an ion pair) is highly exergonic (by
−26.7 kcal mol−1), indicating that the interaction of the catalyst
with IL is very efficient.
The concurrent interaction of 1 with IL, water, and
acetonitrile was also investigated. With this aim, the structures
of two complexes, i.e. [bmim][FeCl2{N(CN)2}(Tpm)](H2O)
(bmim·2·H2O) (with bmim and water molecules in the second
coordination sphere) and [bmim][FeCl2(H2O)(Tpm)][N-
(CN)2] (bmim·3·N(CN)2) (with bmim and N(CN)2 in the
second coordination sphere), have been optimized. In both
complexes, the outer-sphere H2O or N(CN)2 species are
situated near the methine group of the tris-pyrazolyl ligand,
forming several C−H···O or C−H···N hydrogen bonds. The
calculated Gibbs free energy change of N(CN)2 for H2O
substitution (that is, the energy of reaction bmim·2·H2O →
bmim·3·N(CN)2) is slightly negative (−2.2 kcal mol−1, Table
3). However, considering that the concentration of the IL
(solvent) is significantly higher than that of H2O (introduced to
the system with aqueous H2O2), the predominant form in
solution should be complex bmim·2.
The coordination of acetonitrile (another solvent used in this
work) to the pentacoordinated complex 1 is also energetically
favorable (by −9.9 kcal mol−1). However, the substitution of
MeCN for H2O in [FeCl2(NCMe)(Tpm)] (4) is noticeably
exergonic in the gas phase (by −5.2 kcal mol−1); however, it is
slightly disfavored when solvent effects are taken into account
(by 1.1 kcal/mol).
CONCLUSIONS
■
This work demonstrated that a combination of a C-scorpionate
iron complex catalyst and well-adjusted reaction parameters (in
particular, the use of a IL as solvent and of MW irradiation) can
lead to highly selective, efficient, fast, and reusable catalytic
systems for the mild oxidation of cyclohexane with hydrogen
peroxide.
Figure 6. Calculated equilibrium structures.
atom has one empty coordination place in complex 1. Such a
structure indicates that a solvent molecule (e.g., an anion of IL)
can be easily coordinated to the metal. Indeed, the equilibrium
structure of the coordinatively saturated complex [FeCl2{N-
(CN)2}(Tpm)]− (2) was found as a result of geometry
optimization (Figure 6). The formation of this complex from 1
In fact, the combination of MW irradiation with the use of
[FeCl2(Tpm)] (1) as catalyst in IL solvents led to (i) a high
selectivity with an enhanced total (KA) yield under additive-
free conditions and short reaction times, (ii) higher catalytic
activities under additive-free conditions, (iii) the possibility of
recycling and reuse of 1 in a considerable number of
consecutive catalytic cycles with retention of its activity and
selectivity, and (iv) alcohol/ketone selectivity tuning by choice
of the appropriate IL. These sustainable features of our 1/MW/
H2O2/IL catalytic systems are of synthetic significance, in
particular for the production of polyamide 6.
−
and N(CN)2 is energetically favorable in terms of both gas-
phase Gibbs free energy and SMD energy in solution (by −8.7
and −12.3 kcal mol−1, respectively, Table 3).
The interaction of 1 with both the cation and anion of the
ionic liquid was also considered. As a result, the equilibrium
structure of the neutral associate [bmim][FeCl2{N(CN)2}-
(Tpm)] (bmim·2) was obtained. The bmim cation in bmim·2
is situated between the Cl− and N(CN)2− ligands, forming with
them several C−H···Cl and C−H···N hydrogen bonds (Figure
6). Such a position of the bmim cation in the second
coordination sphere corresponds to the most extended regions
of the negative electrostatic potential in complex 2 (Figure 7).
To our knowledge, this is the first report, for the oxidation of
alkanes, of the successful use of ionic liquids with an iron
catalyst under MW irradiation, providing a more sustainable
approach to KA oil synthesis.
Table 3. Calculated Free Energy Changes (in kcal mol−1) of Theoretically Considered Reactions
reaction
ΔG (ΔESMD)
[FeCl2(Tpm)] (1) + N(CN)2 → [FeCl2{N(CN)2}(Tpm)]− (2)
−8.7 (−12.3)
−9.9 (−18.7)
−15.1 (−15.5)
−26.7 (−25.6)
−2.2 (−0.1)
−5.2 (1.1)
−
[FeCl2(Tpm)] (1) + MeCN → [FeCl2(NCMe)(Tpm)] (4)
[FeCl2(Tpm)] (1) + H2O → [FeCl2(H2O)(Tpm)] (3)
[FeCl2(Tpm)] (1) + [bmim][N(CN)2] → [bmim][FeCl2{N(CN)2}(Tpm)] (bmim•2)
[bmim][FeCl2{N(CN)2}(Tpm)](H2O) (bmim·2·H2O) → [bmim][FeCl2(H2O)(Tpm)][N(CN)2] (bmim·3·N(CN)2)
[FeCl2(NCMe)(Tpm)] (4) + H2O → [FeCl2(H2O)(Tpm)] (3) + MeCN
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Figure 7. Electrostatic potential distribution in complex 1.
of both analyses allows us to estimate the amount of cyclohexyl
hydroperoxide. In the experiments with radical traps, CBrCl3 (5.00
mmol) or NHPh2 (5.00 mmol) was added to the reaction mixture.
Blank experiments were performed in the different solvent media
and confirmed that no CyH oxidation products (or only traces, below
1%) were obtained in the absence of 1.
Catalyst recyclability in IL media was investigated for several
consecutive cycles. After completion of each run, the products were
analyzed as detailed previously and the IL with the solubilized catalyst
was recovered by drying it under vacuum overnight at 70 °C. Each
cycle was initiated after the preceding one upon the addition of new
typical portions of all other reagents.
Computational Details. The full geometry optimization of the
molecular structures was carried out at the DFT level of theory using
M06 functional38 with the help of the Gaussian-09 program package.39
No symmetry operations were applied for any of the structures
calculated. The geometry optimization was carried out using a
relativistic Stuttgart pseudopotential which describes 10 core electrons
and the appropriate contracted basis set (8s7p6d1f)/[6s5p3d1f] for
the iron atom40 and the 6-31+G* basis set for other atoms. The
Hessian matrix was calculated analytically for all optimized structures
to prove the location of correct minima (no imaginary frequencies)
and to estimate the thermodynamic parameters, the latter being
calculated at 25 °C.
The solvent effects were estimated by single-point calculations on
the basis of gas-phase geometries. For the approximation of solvent
effects in the solution of IL, the SMD-GIL method41 was used with the
parameters ε = 11.5, n = 1.43, γ = 61.24, ∑α2H = 0.229, ∑β2H = 0.265,
φ = 0.2, and ψ = 0. For the approximation of the solvent effects in
water and acetonitrile solutions, the standard SMD method42 was
applied.
A better understanding of how the IL properties (e.g., charge,
bulkiness, ionic mobility, viscosity, and density) can affect the
performance of the catalytic system seems to be worthy of
further development, a topic that will be addressed in future
studies. The generality of the above advantages in the field of
alkane oxidation should also be tested by extending the
investigation to other alkanes and catalysts.
EXPERIMENTAL SECTION
■
Materials and Physical Methods. All the reagents and solvents
were purchased from commercial sources and used as received. Water
used in all the syntheses and catalytic studies was double-distilled. The
scorpionate iron(II) complex 1 has been synthesized according to a
reported procedure and characterized accordingly.25 1-Butyl-3-
methylimidazolium tetrafluoroborate ([bmim][BF4]) and 1-butyl-3-
methylimidazolium dicyanamide ([bmim][N(CN)2]) were prepared
by anion exchange of 1-butyl-3-methylimidazolium bromide, upon
reaction with Na[BF4] or Na[N(CN)2], respectively,37 and used after
drying for 24 h at 70 °C under high vacuum with stirring.
1H and 13C NMR spectra were recorded at ambient temperature on
a Bruker Avance II + 300 (UltraShield Magnet) spectrometer
operating at 300.130 and 75.468 MHz for proton and carbon-13,
respectively, at ambient temperature. Far-infrared spectra FIR (400−
200 cm−1) were recorded on a Vertex 70 spectrophotometer in CsI
pellets.
Catalytic reactions were performed in borosilicate tubes (5 mL
capacity with a 10 mm internal diameter) under focused microwave
irradiation (MW) in an Anton Paar Monowave 300 reactor fitted with
a rotational system and an IR temperature detector.
Chromatographic analyses were undertaken by using a Fisons
Instruments GC 8000 series gas chromatograph with a DB-624 (J&W)
capillary column (flame ionization detector) and the Jasco-Borwin
v.1.50 software. The internal standard method was used to quantify the
organic products.
MW-Assisted Peroxidative Oxidation of Cyclohexane. Cyclo-
hexane oxidations were carried out in air in a cylindrical Pyrex tube
placed in the microwave reactor. Typically, the catalyst 1 (0.2−10.0
μmol) was added to 3 mL of solvent (CH3CN, H2O or IL), and an
additive (optional) was added. Cyclohexane (0.54 mL, 5 mmol) and
30% aqueous H2O2 solution (10 mmol) were then added in that order.
The reaction vessel was closed and the contents were stirred and
irradiated (up to 20 W) for 0.25−6 h, at 30−80 °C. After the reaction,
the mixture was cooled to room temperature and the organics were
extracted with diethyl ether (5 mL).
All products formed were identified by GC and retention times
compared with those of commercially available products. Cyclo-
heptanone (0.09 mL) was used as the GC internal standard. The
reaction mixtures were analyzed twice by GC: with and without
addition of an excess of solid triphenylphosphine, following a method
developed by Shul’pin.7,26−29 The addition of PPh3 to the final organic
phase reduces cyclohexyl hydroperoxide, if formed, to the correspond-
ing alcohol and hydrogen peroxide to water. Comparison of the results
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00620.
■
S
Additional selected data on catalytic oxidation of
cyclohexane and DFT results (PDF)
Cartesian coordinates for the calculated structures (XYZ)
AUTHOR INFORMATION
Corresponding Author
*E-mail for L.M.D.R.S.M.: lmartins@deq isel.ipl.pt.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work has been partially supported by the Foundation for
Science and Technology (FCT) of Portugal (UID/QUI/
■
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