Iron catalyzed oxidation of benzylic alcohols to benzoic acids

Article abstract of DOI:10.1039/c8dt00819a

The bidentate N,O-ligands phenol-pyrazole (HL1), naphthol-pyrazole (HL2) and the commercially available ligand 5-methylphenol-benzotriazole (HL3) were used for the synthesis of novel iron(iii) complexes. The mononuclear iron complexes [FeCl(L1)₂] (1), [FeCl(L2)₂] (2) and [FeCl(L3)₂] (3) are stable to air and moisture, both in the solid state as well as in solution, while the dinuclear, μ-oxido bridged complex [{Fe(L1)₂}₂(μ-O)] (1a) is air sensitive. All four complexes 1, 2, 3 and 1a were investigated for their catalytic activity in the direct one-pot oxidation of primary alcohols to carbonic acids with 30% aq. hydrogen peroxide (H₂O₂) as the oxidation agent. The activity in oxidation reactions of the isolated, mononuclear complexes 1-3 was further compared to their in situ prepared analogues IS1-3. Experimentally obtained results indicate a tendency of higher activity for the oxidation of primary alcohols for the in situ prepared complexes. In conclusion, the oxidation of aromatic primary alcohols to carboxylic acids using isolated iron(iii) complexes and in situ generated complexes in the presence of H₂O₂ results in good to high yields. The reaction is straight-forward, clean and generates water as the only by-product.

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Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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Iron catalyzed oxidation of benzylic alcohols to benzoic acids
B. Stanje,a P. Traar,a J. A. Schachner,a* F. Belaj,a N. C. Mösch-Zanettia
Received 00th January 20xx,
Accepted 00th January 20xx
The bidentate N,O-ligands phenol-pyrazole (HL1), naphthol-pyrazole (HL2) and the commercially available ligand 5-
methylphenol-benzotriazole (HL3) were used for the synthesis of novel iron(III) complexes. The mononuclear iron
complexes [FeCl(L1)2] (1), [FeCl(L2)2] (2) and [FeCl(L3)2] (3) are stable to air and moisture, both in the solid state as well as
in solution, while the dinuclear, µ-oxido bridged complex [{Fe(L1)2}2(ꢀ-O)] (1a) is air sensitive. All four complexes 1, 2, 3
and 1a were investigated for their catalytic activity in the direct one-pot oxidation of primary alcohols to carbonic acids
with 30% aq. hydrogen peroxide (H2O2) as the oxidation agent. The activity in oxidation reactions of the isolated,
mononuclear complexes 1-3 was further compared to their in situ prepared analogues IS1-3. Experimentally obtained
results indicate a tendency of higher activity for the oxidation of primary alcohols for the in situ prepared complexes. In
conclusion, the oxidation of aromatic primary alcohols to carboxylic acids using isolated iron(III) complexes and in situ
generated complexes in the presence of H2O2 results in good to high yields. The reaction is straight-forward, clean and
generates water as the only by-product.
DOI: 10.1039/x0xx00000x
www.rsc.org/
ox.
ox.
O
O
Introduction
R
OH
red.
red.
R
H
R
OH
alcohol
aldehyde
carbonic acid
Iron is one of the most versatile metal centers in the arsenal of
transition metals, possesses several stable oxidation states and
allows various coordination geometries by ligands that make it
an interesting target for chemical applications. An excellent
review article summarizing the development of iron catalysis
written by Bolm et al. in 2004 triggered an enormous surge in
research.1 Today, iron catalysis has reached applications in
virtual all fields of homogeneous catalysis. Such catalysts are
particularly interesting considering that iron is cheap, nontoxic,
readily available and rich in oxidation states.2 Since 2004
significant improvements were made in the field of iron
catalysis, which is evident by the recent publication of a 218
page review article in Chemical Reviews.3 One important
reaction in chemistry is the oxidation of chemical compounds
that is widely used in the laboratory as well as on industrial
scale.4,5 Especially the selective oxidation of alcohols into their
carbonyl derivatives (Scheme 1) is a fundamental reaction in
the pool of oxidation reactions.6 Specifically the selective
cascade oxidation of alcohols to carbonyl compounds is
attractive because the desired molecules can be directly
Scheme 1. Cascade oxidation of alcohols
The direct conversion of primary alcohols to carboxylic acid is
still a challenging task to achieve.8 Traditionally, alcohol
oxidation methods require stoichiometric amounts of
chromium- or manganese-based compounds and chlorinated
solvents, that are hazardous materials requiring special
disposal procedures.9 Efficient methods to achieve this
transformation are the Jones oxidation (CrO3/H2SO4), the
RuCl3/H5IO6 protocol, TEMPO-catalyzed reactions based
oxidation conditions (e.g. with sodium hypochlorite NaOCl, or
bleach) or other chromium- or ruthenium-based methods like
CrO3/H5IO6.10 Using the Jones reagent is the least favorable
protocol to catalyze this transformation, because CrO3 is
generally known to be very toxic and carcinogenic. Compared
to the Jones oxidation, the Swern oxidation is a good
alternative, although it involves a two-step process followed
by the oxidation of the resulting aldehyde to the carboxylic
acid.
Several metal catalysts including palladium, copper,
ruthenium, and cobalt have been developed for the oxidation
of alcohols using environmental friendly oxidants like H2O2 and
O2.9,11 While alcohol oxidations catalyzed by iron complexes
have also been investigated in detail, there is no generally
applicable protocol available and therefore research in this
field is still ongoing. Nevertheless, the one-pot cascade
transformation of alcohols directly to carboxylic acids is still a
relatively uninvestigated field of chemistry.5,12,13,14 In 1969
Welch et al. published the oxidation of primary alcohols to
synthesized in one-pot sequences (e.g. primary alcohols
aldehydes carboxylic acids; secondary alcohols
ketones esters; cycloalkanols cycloalkanones
or α,β-enones, etc).7,8
→
→
→
→
→
→ lactones
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carboxylic acids (based on the previous developed procedure reasoned that the coordination of two bidentate ligands
of Pappo and Becker)15 by using a catalytic amount of RuO4 in instead of one rigid tetradentate ligand like salen could lead to
presence of NaIO4 as secondary oxidant during the preparation a more flexible coordination sphere resulting in different,
of the mold metabolite culmorin.16 Sharpless et al. reported in advantageous catalytic properties.
1981 a single example of the oxidation of a primary alcohol to
N
N
OH
N
OH
N
OH
N
N
the corresponding carboxylic acid. The so called “Sharpless
modification” of the method by Pappo and Becker very quickly
found widespread use in the transformation of primary
alcohols to carboxylic acids.17 A more recent example for an
improvement of the “Sharpless modification” is found in the
work of Prashad et al. A simple substitution of CCl4 with EtOAc
allows the isolation of the carboxylic acid in higher yield (80%
to 93% yield), with the advantage of using less toxic solvent.18
The group of Grützmacher published several examples of
coupling reactions of primary alcohols with water giving
carboxylic acids.19 In 2013 the group of Milstein reported a
Ru(III) complex that was able to catalytically oxidize alcohols to
carboxylic acids in water under basic conditions without the
need of an oxidant (Scheme 2).
N
HL1
HL2
HL3
Figure 1. Bidenate N,O-ligands used in this study.
Results and discussion
Synthetic procedures
Aryloxide ligands with a pyrazol-3-yl-bonded moiety (HL1 and
HL2) were prepared according to literature procedures.24 The
triazole-1-yl-bonded ligand HL3 is commercially available and
was used as received (Figure 1). HL1 and HL2 were successfully
used in our group for the synthesis of oxido-rhenium(V) and
dioxido-molybdenum(VI) complexes and their application as
catalysts for olefin epoxidation reactions.25 Both metal
complexes showed moderate to high yields with excellent
selectivities towards to the epoxidation of cis-cyclooctene. The
Scheme 2. Milstein’s one-pot oxidation of alcohols.14
iron complexes [FeCl(L1-3)2] (1, 2 and 3) were synthesized by
The oxygen atom being transferred originated from the
solvent H2O itself, under liberation of H2 gas.14,20 Modified
versions of Milstein’s catalyst have since appeared.21 Very
recently, a catalytic metal free oxidation of a homoserin
residue to aspartaic acid was reported in a glycopeptide
synthesis.22
the addition of the respective ligands HL1-HL3 to a solution of
FeCl3·6H2O under the reaction condition shown in Scheme 4.
Upon cooling complexes 1-3 precipitated from the reaction
solution and were isolated by filtration. After washing with
cold H2O, EtOH and Et2O they were isolated as dark blue to
green powders in 58-71% yield. In the solid state the
complexes can be stored under ambient conditions without
any sign of decomposition for several months, whereas in
CH3CN solution after several days decomposition occurs. All
complexes are nearly insoluble in apolar solvents (e.g.
pentane, hexanes), poorly soluble in medium polar solvents
(e.g. toluene) and sparingly soluble in polar solvents (e.g.
MeOH, CH3CN and CHCl3). The elemental analyses of all three
complexes were in good agreement with their predicted
A selective iron-catalyzed one-pot oxidation of an alcohol to
the respective carboxylic acid under mild conditions with H2O2
has not been published so far. However, other metals have
shown to be able to convert benzyl alcohol to benzoic acid
under mild, green conditions. In 2003 Punniyamurthy et al.
reported on a cobalt(II)-catalyzed system for the oxidation of
alcohols into carboxylic acids and ketones with hydrogen
peroxide. After 6-9 hours of reaction time a yield of about 70
to 75% of carboxylic acid was obtained for the transformation
of various primary alcohol substrates. To achieve these yields a
relatively high amount of H2O2 (up to 20 equiv.) was needed.
This complex was able to oxidize benzyl alcohol to benzoic acid
with a yield of 76% as shown in Scheme 3.13
values. The molecular structure of
diffraction analysis (vide infra).
1 was confirmed by X-ray
Scheme 3. Cobalt(II) catalyzed oxidation of alcohols.13
The ligands HL1-3 were chosen based on the structure of an
[Fe(III)-salen]Cl complex, that is a widely used iron complex in
various fields of chemistry (Grignard-catalyzed cross-coupling,
oxidation, enzyme models, magnetic materials etc.) (Figure
1).23 We intended to synthesize new iron complexes with a
similar ONNO-coordination sphere as in the [Fe(III)-salen]Cl
complex, but using two bidentate ligands instead. We
Scheme 4. Preparation of mononuclear iron complexes 1-3.
2 | J. Name., 2012, 00, 1-3
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In an attempt to obtain iron(II) complexes, the reaction of from a concentrated solution in DMSO at room temperature.
FeCl2 with these ligands in absolute ethanol under inert Molecular views for and 1a are displayed in Figure 2 and
1
atmosphere was investigated. With HL1 a light red precipitate Figure 3, selected bond angles and lengths are given in Table 2,
could be isolated in 74% yield that analyzed in good agreement crystallographic data and refinement parameters can be found
with the dinuclear complex [{Fe(L1)2}2(ꢀ-O)] (1a) (Scheme 5). in the SI.
Recrystallization from DMSO led to single crystals suitable for
X-ray diffraction analysis confirming the dinuclear nature of
the complex 1a as displayed in Scheme 5 and Figure 3. Thus,
under the employed reaction conditions oxidation occurred to
an iron(III) complex. In the solid state 1a is far less stable
compared to
1 as it decomposes after approx. 2 h under
ambient conditions to unidentified products. Complex 1a is
well soluble in strong polar solvents (e.g. MeOH, CH3CN and
CHCl3) and can be stored in dry DMSO solution for several
weeks if kept under nitrogen. The source of the oxygen atom is
as yet unclear. The high yield hints towards residual water in
the pre-dried ethanol. When ligands HL2 and HL3 were
reacted with FeCl2 under the same conditions, no identifiable
Figure 2. Molecular view of the mononuclear complex
1 showing the numbering
scheme of the non-carbon atoms. Hydrogen atoms have been omitted for clarity.
products formed.
Ar, abs. EtOH,
2 equiv NEt3
reflux, 1 h
N
N
N
O
O
N
O
N
O
FeCl2
+
2 HL1
0.5
Fe
N
O
Fe
N
=
O
O
- 2 equiv [HNEt3]Cl
1a
, 74%
Scheme 5. Synthesis of dinuclear iron complex 1a.
Magnetic susceptibility
All four complexes 1, 1a, 2 and 3 contain a paramagnetic high-
spin iron center. The magnetic moments were experimentally Figure 3. Molecular view of the dinuclear complex 1a showing the numbering scheme
of the non-carbon atoms. Hydrogen atoms have been omitted for clarity.
determined with the help of a magnetic susceptibility balance
(see Experimental Section). The number of unpaired electrons
Table 2. Selected bond lengths (Å) and angles (°) of compounds 1 and 1a
was calculated by applying the spin-only formula.26 Data is
summarized in Table 1. In case of the mononuclear complexes
1
1a
1a
1-3 the determined numbers of unpaired electrons agree quite
well with the expected 5 unpaired electrons for a high-spin d5
Fe1 fragment
Fe2 fragment
metal center and definitively rules out
a
low-spin
Fe1-Cl1/O1
Fe1-O21
2.2703(3)
1.8630(9)
1.8583(8)
2.1201(9)
2.1309(9)
-
1.7898(11)
1.9098(11)
1.8986(11)
2.1531(13)
2.1246(13)
179.86(9)
172.76(5)
120.59(5)
Fe2-O1
1.7893(11)
1.8993(11)
1.9094(11)
2.1273(13)
2.1580(13)
-
configuration. For the dinuclear complex 1a a value of 3.89
unpaired electrons for both iron centers was experimentally
determined indicating antiferromagnetic coupling via the
oxygen bridge.27
Fe2-O61
Fe2-O81
Fe2-N52
Fe2-N72
-
Fe1-O41
Fe1-N12
Fe1-N32
Fe1-O1-Fe2
N12-Fe1-N32
O21-Fe1-O41
170.68(4)
125.49(4)
N52-Fe2-N72
O61-Fe2-O81
172.41(5)
119.45(5)
Table 1. Effective magnetic moment ꢀeff and number of unpaired electron
Complex
ꢀeff
experimental
no. of unpaired electrons
Complex
1 shows a trigonal-bipyramidal coordination with
1
1a
2
6.09
4.79a
6.10
5.27
5
3.89a
5
small deviations from ideal geometry of the iron atom with the
N atoms in axial positions (N12–Fe1–N32 170.68(4)°, O41–
Fe1–O21 125.49(4)°, O21–Fe1–Cl1 119.59(3)°, O41–Fe1–Cl1
114.91(3)°). All the other bond lengths and angels compare
well with previously published, fivefold coordinated iron(III)
3
5
a antiferromagnetic coupling in the dinuclear species
complexes equipped with bidentate N,O-
ligands.27,28
Molecular structures of complexes 1 and 1a
Complex 1a shows approx. D2 symmetry, with the Fe atoms
having trigonal-bipyramidal surroundings (N12–Fe1–N32
172.76(5)Å, N52–Fe2–N72 172.41(5)Å) and are connected by a
linear O bridge (O1–Fe1 1.7898(11)Å, O1–Fe2 1.7893(11)Å;
Fe1–O1–Fe2 179.86(9)°). Dinuclear µ-oxido iron(III) complexes
The molecular structures of complex
1 and 1a were
determined by single crystal X-ray diffraction analysis. Single
crystals of were obtained by slow evaporation of an acetone
1
solution at room temperature and for 1a by slow evaporation
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of the type [L2Fe-O-FeL2] with four bidentate ligands are rather The oxidation of S1 is catalyzed by isolated complexes
scarce in literature.27,29,30 The almost ideal linear arrangement and
(Table 4, entries 1, 2, 5 and 7) as well as by using in situ
of the Fe-O-Fe core (179.86(9)°) in 1a indicates the low steric generated systems IS1 IS3 (Table 3, entries 4, 6 and 8) with
1, 1a, 2
3
-
bulk of ligand moiety L1, which otherwise has been shown to various yields of acid (31-94%) and high selectivities (>96%).
cause significant distortion from linearity (135-180°).30 Also the Complete overview of the data is given in Table 4. No side-
lack of a sixth ligand in the solid state structure of 1, where products were observed during the catalysis reaction, only
often an H2O molecule is coordinated, points to an excellent remaining alcohol, the intermediate aldehyde and the
Lewis basicity of the pyrazole moiety in ligand L1. The small resulting acid were detected.
steric bulk in combination with the excellent donor capabilities
Table 3. Iron-catalyzed oxidation of S1 with H2O2
of ligand HL1, as deduced from the solid state structures,
might explain the unusual catalytic activity in oxidations
Entry
Cat.
Conv.a
[%]
Acida
[%]
81
Aldehyda
[%]
Select.b
[%]
reactions of complexes 1-3
.
1
2
3c
1
1a
1a
IS1
2
91
10
89
Oxidation of alcohols
56
79
97
93
96
95
97
0
31
56
93
86
94
89
93
0
25
23
4
55
71
96
92
98
94
96
0
Complexes 1, 2 and
IS1 IS3 as well as the dinuclear complex 1a were screened in
the catalytic direct one-pot synthesis of carboxylic acids from
3, their in situ generated analogues
−
4
5
7
three different benzylic alcohol substrates (S1-S3) using H2O2
6
IS2
3
2
as oxidizing agent (Scheme 6).
7
6
8
IS3
-
4
9
0
10d
1
0
0
0
0
11
Lit.13
FeCl3 ∙ 6H2O
0
-
0
0
-
0
-
[Co]
76
a
Conversion and yield were determined by HPLC analysis. b Selectivity refers to
benzoic acid. c 4 mol% of 1a was used. d no hydrogen peroxide was added.
Scheme 6. General scheme of the direct one-pot synthesis of carboxylic acids
The highest yield of benzoic acid (94%) with an excellent
selectivity (98%) for the oxidation of S1 was obtained by using
the in situ system IS2 (Table 3, entry 6). The oxidation with the
The oxidation of benzyl alcohol (PhCH2OH) to benzaldehyde
(PhCHO) and benzoic acid (PhCOOH) was chosen as benchmark
reaction for the oxidation of alcohols. Because ligand HL3 is
analogous isolated complex
2 (Table 3, entry 5) yielded 86% of
commercially available complex
3 was used for the
benzoic acid with
mononuclear complex
a
92% selectivity. Comparison of
optimization of the reaction conditions with respect to the
addition time of H2O2, loading of H2O2, solvent, total volume of
the solvent and catalyst loading. The best activity was
observed with 6 equiv. of H2O2 (with respect to substrate) with
an addition time of 80 min via syringe pump and a catalyst
loading of 4 mol% in a total of 2 mL CH3CN. Thus, standard
conditions for all catalytic experiments were 2 mL of CH3CN
1
to dinuclear 1a (Table 3, entries 1, 2
and 3) indicates that only one Fe center in 1a becomes an
active catalyst. At 2 mol% catalyst loading of 1a the yield of
benzoic acid is only 31%, at 4 mol% loading the yield increases
to 56%. Nevertheless this is still far below the yield with
1
(81%). The in situ generated analogue IS1 showed a better
catalytic activity with a yield of acid of 93% with a high
selectivity (Table 3, entry 4). In general, in situ systems (Table
3, entries 4, 6 and 8) showed higher catalytic activities than
their isolated analogues in the oxidation of S1 (Table 3, entries
1, 3, 5 and 7). In order to obtain more mechanistic information
the kinetic conversion profile of substrate S1 was investigated
with 4 mol% of catalyst 1, 2 and
3 and 6 equiv. H2O2 added
over a period of 80 min at 70 °C, unless stated otherwise. For
the dinuclear complex 1a both 2 and 4 mol% of catalyst
loading were used, in order to test if only one or both Fe
centers become active catalysts. Oxidation reactions with in
situ generated complexes IS1
the same conditions by simply mixing the required amounts of
FeCl3·6H2O and ligands HL1 HL3 (Scheme 6). For further
−IS3 were also performed under
with catalyst 3. As seen in Figure 4 the oxidation of S1 to
aldehyde and acid occurs at essentially the same rate within
the first ten minutes, without an induction time. The detected
amount of the intermediate aldehyde confirms the initial
oxidation of S1 to benzaldehyde. From 10 to 25 minutes, the
amount of PhCHO remains roughly constant while the
concentration of product PhCOOH steadily increases. Because
the second oxidation from PhCHO to PhCOOH occurs at the
same rate as the oxidation of S1 to PhCHO, this first oxidation
step could therefore represent the rate determining step.
−
investigations of the reaction kinetics benzaldehyde (PhCHO,
S4) was also screened as substrate (vide infra). Apart from the
desired product (carboxylic acid) and the corresponding
aldehyde (in some cases up to 25%, e.g. in Table 4, entry 13)
no other side products were detected in oxidation reactions of
substrates S1-S3. A summary of all yields can be found in Table
6.
4 | J. Name., 2012, 00, 1-3
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Table 5. Iron-catalyzed oxidation of S3 with H2O2
100
80
60
40
20
Entry
Cat.
Conv.a
[%]
Acida
[%]
Aldehydea
[%]
Select.b
[%]
19
20c
21
1
90
83
93
76
61
86
14
22
7
84
75
92
S1
PhCOOH
PhCHO
1a
IS1
22
23
2
IS2
84
94
70
86
16
8
83
91
24
25
Lit.13
3
93
92
-
84
80
74
9
12
-
90
87
-
0
0
20
40
60
80
100
120
140
160
180
200
IS3
[Co]
t [min]
a
Conversion and yield were determined by HPCL analysis. b Selectivity refers to
Figure 4. Oxidation of S1 showing the concomitant formation of acid and aldehyde; 1
mmol substrate, 4 mol% 3, 6 equiv. H2O2, 70 °C, 2 mL CH3CN.
benzoic acid. c 0.02 mmol of 1a was used.
Again the highest yield of the corresponding acid (86%) with
the best selectivity (92%) was obtained by using the in situ
systems IS1 and IS2 (Table 5, entries 21 and 23). Similar to the
oxidation of substrates S1 and S2 the oxidation of S3 with the
The oxidation of S2 is catalyzed by both the isolated complexes
1, 1a, 2 and 3 (Table 5, entries 14, 15, 17 and 19) as well as by
using in situ generated systems IS1-IS3 (Table 5, entries 16, 18
and 20) in good to high yields (58-96%) with good selectivities
for the mononuclear catalysts (83-98%).
isolated mononuclear complex
1 (Table 5, entry 19) lower
yields of the acid (76%) in contrast to the in situ system IS1
were obtained. Again the dinuclear iron complex 1a (Table 5,
entry 20) resulted in lowest yield of the corresponding
carboxylic acid (61%). In contrast to previous results of S1 and
Table 4. Iron-catalyzed oxidation of S2 with H2O2
Entry
Cat.
Conv.a
[%]
Acida
[%]
77
Aldehydea
[%]
Select.b
[%]
S2 the isolated analogue
3 showed a more less similar catalytic
activity and selectivity than the in situ generated system IS3
(Table 5, entries 24 and 25). The other two in situ generated
systems IS1 and IS2 (Table 5, entries 21 and 23) were showing
12
13c
14
1
93
16
83
1a
83
98
58
96
25
2
70
98
IS1
higher catalytic activities than their isolated analogues
(Table 5, entries 19, 20 and 22).
1 and 2
15
16
2
93
98
80
95
13
3
86
97
IS2
It is known that iron complexes are capable of decompose
H2O2 forming OH uch radical
radicals (Fenton chemistry).31
17
3
96
98
-
90
95
74
4
3
-
94
97
-
˙
S
18
Lit.13
IS3
[Co]
based reactions lead to low selectivities and result in
undefined polymeric materials. In our case the mononuclear
complexes 1-3 as well as their in situ generated analogues IS1-
a
Conversion and yield were determined by HPCL analysis. b Selectivity refers to
3
showed high selectivities for the two oxidation products,
benzoic acid. c 0.02 mmol of 1a was used.
without any other side products being detected. Therefore a
Compared to the oxidation of S1
,
S2 behaves quite similar. The radical pathway seems unlikely during the oxidation of the
highest yield of 4-chlorobenzoic acid (96%) with a high substrates S1-S3.
selectivity (98%) was obtained by using the in situ system IS1 The results with in situ generated complexes as well as with
(Table 4, entry 14). The oxidation with the isolated the isolated complexes are better than those with the best
mononuclear analogue 1 (Table 4, entry 12) yielded 77% of the previously described cobalt-based system of Punniyamurthy et
acid with slightly reduced selectivity (83%). The dinuclear iron al. with H2O2 as the terminal oxidant, as less oxidant and
complex 1a (Table 4, entry 13) again showed the lowest shorter reaction times were required.13 For the cobalt-based
activity and selectivity (58% yield acid, 70% selectivity). In situ system 6-9 hours reaction time and high amounts of the
generated catalysts (Table 4, entries 14, 16 and 18) showed oxidant H2O2 (20 equiv.) were required. In the benchmark
higher activities as well as higher selectivities towards the reaction of benzyl alcohol S1 the catalytic system of
formation of the acid than their isolated analogues (Table 4, Punniyamurthy et al. was able to oxidize S1 to benzoic acid in
entries 12, 13, 15 and 17).
The oxidation of S3 is again catalyzed by all four isolated system has the added value of using non-toxic iron instead of
complexes 1a and (Table 5, entries 19, 20, 22 and 24) as cobalt. It was also demonstrated that the oxygen bridged
well as by using in situ generated systems IS1 IS3 (Table 5, diiron complex 1a is able to catalyze to formation of the acid
entries 21, 23 and 25) in good yields to high yields (61-86%) from primary alcohols, albeit with lowest activities and
81-95% yield vs. 76% (Table 6). Furthermore, our catalytic
1,
,
2
3
-
with high selectivities (83-92%).
selectivities of all tested complexes (Table 6). In case of HL1
based systems the catalytic activity decreases from in situ
prepared complex IS1 to isolated iron complex
1 for all three
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substrates (Table 6). Interestingly the pyrazole based complexes 1-3. An advantage of using well-characterized
complexes and showed significantly lower catalytic isolated complexes as catalysts is the defined structure of the
1
2
activities compared to their in situ generated analogues IS1 compound that allows for mechanistic investigations. In situ
and IS2, whereas the difference in activity is less pronounced generated catalysts may be easier to prepare and show a
for the triazole based systems
the activities for all substrates S1
1a and , complex equipped with a triazole based ligand catalytically active compounds remain unknown, providing less
3
and IS3 (Table 6). Comparing higher conversion of the substrate S1-S3 with excellent
-S3 of all isolated complexes activities and selectivities, but the structures of the
1,
,
2
3
3
is the most effective in the oxidation of alcohol substrates mechanistic and no structural information for further catalyst
(Table 6). The investigation of S2, that contains an electron improvement. Future work based on these results involves the
withdrawing chloro substituent in para-position, showed a investigation of other alcoholic substrates (e.g. primary
slightly higher yield of the corresponding acid compared to the aliphatic alcohols, secondary alcohols etc.).
oxidation of S1 (Table 6). However, for substrate S3, that
contains an even more electron withdrawing -NO2 group in
Experimental
para-position, a slight decrease in conversion of alcohol
compared to the overall conversion of S1 and S2, respectively
General
(Table 6) is observed. Therefore the functional group located
The iron precursors FeCl3, FeCl3·6H2O, FeCl2, H2O2 (30% aq.)
in para-position to the alcohol in the phenyl ring of the
and HL3 were purchased from Sigma Aldrich and used as
substrate does have an effect on the catalytically activity of the
received. Ligands HL1 and HL2 were prepared according to
systems used. However more substrates with different
literature procedures.24 Chemicals were purchased from
functional groups would have to be tested to establish a trend.
commercial sources and were used without further
The better catalytic activities of the in situ generated
purification. All reactions were carried out under air unless
complexes IS1 and IS2 compared to 1 and 2 could be explained
stated otherwise. NMR spectra were recorded with a Bruker
(300 MHz) instrument. Chemical shifts are given in ppm and
are referenced to residual protons in the solvent. Signals are
described as s (singlet), bs (broad singlet), d (doublet), dd
(doublet of doublet), t (triplet), m (multiplet) and coupling
constants (J) are given in Hertz (Hz). Mass spectra were
recorded with an Agilent 5973 MSD-Direct Probe using the EI
ionization technique. Samples for infrared spectroscopy were
measured on a Bruker Optics ALPHA FT-IR Spectrometer
equipped with an ATR diamond probe head. A J-Kem Scientific
dual syringe pump (model 2250) in combination with a JKem
Infinity Controller was used for the slow addition of the
oxidation agent H2O2. HPLC measurements were performed on
an Agilent 1200 Series Rapid Resolution LC System equipped
with a ZORBAX Eclipse XDB-C18 column coupled to a diode
array detector (DAD) Agilent G1315D. Susceptibility
measurements were performed on a MSB Auto Magnetic
Susceptibility Balance of Johnson Matthey. Elemental analyses
were carried out using a Heraeus Vario Elementar automatic
analyzer at the University of Technology Graz.
by the possible formation of different, more active iron species
present in solution. The catalytically active species is in all
systems as yet unclear as complexes 1, 1a, 2 and 3 are only
pre-catalysts. However, it seems likely that the in situ formed
catalysts of the systems IS1 and IS2 are not only comprised of
the isolated complexes
1 and 1a or 2, but also consist of other
catalytically active complexes that lead to higher overall
activities and thus selectivity towards the acid. It is feasible
that the presence of these other additional species result in
higher yields if these complexes are catalytic active. Finally we
also tested the aliphatic substrate 1-octene with complex 3,
but did not observe any conversion to the respective aldehyde
or acid.
Table 6. Summary of yields of benzoic acid
1
2
3
IS1
93
96
86
IS2
94
95
86
IS3
93
95
80
[Co]13
76
1a
56
58
61
S1
S2
S3
81
77
76
86
80
70
89
90
84
74
74
Catalytic oxidation reactions
Conclusions
Conversion and yields were determined by HPLC; the mobile
phase contained a mixture of 25% v/v CH3CN with 75% v/v H2O
acidified with 0.1% v/v formic acid to achieve a separation of
substrates and products. The yield of products was determined
via an internal standard method, where all peak areas were
referenced to the area of the internal standard. The obtained
products were identified by comparison of the retention times
to authentic samples; the retention times as well the peak
forms were in excellent agreement. The mass balance during
the analysis with HPLC of the products and the substrate
added up to ~100% indicating a highly selective process. The
chosen internal standard was nitrobenzene, which proved to
be inert under reaction conditions. No oxidation reaction took
The oxidation of substrates S1
carboxylic acids is achieved by using well-characterized,
isolated iron(III) complexes 1a and as well as by using
their analogous in situ prepared complexes IS1 IS3. It can be
concluded that in principle the synthesized iron(III) complexes
1a and and the in situ systems IS1 IS3 showed better
catalytic activities toward the oxidation of S1 S3 than the best
-S3 to their corresponding
1,
,
2
3
-
1,
,
2
3
-
-
previously described cobalt-based system with hydrogen
peroxide as the terminal oxidant with the added value of
exhibiting more environmentally friendly reaction conditions.13
The oxygen-bridged dinuclear complex 1a is the least
catalytically active catalyst compared to the mononuclear iron
6 | J. Name., 2012, 00, 1-3
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place in control experiments without added catalyst or oxidant Synthesis of complexes
H2O2. Also FeCl3 alone proved to be inactive as a catalyst.
Synthesis of [FeCl(L1)2] (1). To a solution of iron(III) chloride
(0.485 g; 2.99 mmol) in H2O (10 mL) a solution of HL1 (1.052 g;
6.04 mmol) in MeOH (20 mL) was added. The resulting dark
solution was stirred overnight at rt. The formed precipitate
was separated by filtration, washed with H2O and Et2O and
Magnetic susceptibility measurements
The values of mass magnetic susceptibilities χm for the solid
samples were measured under ambient conditions, corrected
for diamagnetic contributions and the number of unpaired
electrons calculated by the spin-only formula.26 The
diamagnetic correction susceptibilities of the ligands were
determined to be -1.45·10-9 m3/mol for HL1, -1.90·10-9 m3/mol
dried to give a dark powder of the iron complex 1. Blue square
shaped crystals, suitable for X-ray crystallography, were
obtained after two days by slow evaporation of an acetone
solution. Yield: 0.930 g (71%). EI-MS: m/z 437.2 [M+]. IR (ATR):
ν
1596, 1517, 1303, 1248, 861, 778, 751, 449 cm-1. Anal. Calc.
for HL2 and -1.80·10-9 m3/mol for HL3
.
for C20H18ClFeN4O2: C, 54.88; H, 4.15; N, 12.80%. Found: C,
54.97; H, 3.99; N, 12.78%.
X-ray crystallography
Synthesis of [{Fe(L1)2}2(ꢀ-O)] (1a). This reaction was carried
X-ray crystal structure analyses were performed on a Bruker out under nitrogen using standard Schlenk techniques. To a
APEX2 area detector diffractometer. All measurements were solution of anhydrous iron(II) chloride (0.109 g; 0.86 mmol) in
performed using graphite-monochromatized Mo Kα radiation dry EtOH (3 mL) a solution of HL1 (0.301 g; 1.72 mmol) in dry
at 100K. The structure was solved by direct methods (SHELXS- EtOH (3 mL) was added, followed by dry NEt3 (3.61 mmol, 503
97) and refined by full-matrix least-squares techniques against µl). The resulting dark red solution was stirred under refluxed
F2 (SHELXL-97).32 The non-hydrogen atoms were refined with for 1h, and after cooling to rt centrifuged. A light red
anisotropic displacement parameters without any constraints. precipitate was separated, washed twice with hot EtOH (7 mL)
The H atoms of the pyrazole and of the phenyl rings were put and THF and dried under vacuum to give 1a. Dark red crystals,
at the external bisectors of the C-C-X angles at C–H distance of suitable for X-ray crystallography, were obtained by slow
0.95Å and one common isotropic displacement parameter was evaporation of a concentrated DMSO solution. Yield: 0.263 g
refined for the H atoms of the same ring. The H atoms of the (74%). EI-MS: m/z 402.2 [M+ - OFe(L1)]. IR (ATR):
ν 1595, 1441,
methyl groups were refined with common isotropic 1306, 834, 747 cm-1. Anal. Calc. for C40H36ClFe2N8O5: C, 58.56;
displacement parameters for the H atoms of the same group H, 4.52; N, 13.66%. Found: C, 57.81; H, 4.26; N, 13.52%.
and idealized geometries with tetrahedral angles, enabling Synthesis of [FeCl(L2)2] (2). To a solution of iron(III) chloride
rotation around the N-C bond, and C-H distances of 0.98 Å. (0.362 g; 2.23 mmol) in EtOH (10 mL) a solution of HL2 (1.001
Crystallographic data for
1 and 1a have been deposited with mg; 4.47 mmol) in EtOH (20 mL) was added. The resulting dark
the Cambridge Crystallographic Data Centre, CCDC 1048055 solution was stirred under reflux overnight. The formed black
and 1048056.
precipitate was filtered off, washed with H2O and EtOH and
dried to give a black powder of the iron complex . Yield: 0.705
g (60%). EI-MS: m/z 537.2 [M+]. IR (ATR):
1560, 1522, 1387,
1323, 790, 760, 738, 447 cm-1. Anal. Calc. for C28H22ClFeN4O2:
C, 62.53; H, 4.12; N, 10.42%. Found: C, 62.44; H, 4.07; N,
10.28%.
2
General procedures for catalytic one-pot oxidation
ν
In a standard experiment the alcohol substrate (1 mmol), the
internal standard nitrobenzene (1 mmol) and the respective
catalyst (1, 2, 3: 4 mol%, S/C ratio = 25:1; 1a: 2 mol%, S/C ratio
Synthesis of [FeCl(L3)2] (3). Ligand HL3 (2.048 g; 9.09 mmol)
was dissolved in acetonitrile (20 mL) with gentle heating. A
solution of iron(III) chloride (0.738 g; 4.55 mmol) and NEt3
(0.835 g; 8.25 mmol) in acetonitrile (20 mL) was prepared and
heated to 50 °C. To this iron precursor solution the hot ligand
solution was quickly added and the resulting mixture was
stirred overnight at 50 °C. The formed precipitate was filtered
off, washed with H2O and Et2O and dried to give a greenish-
= 50:1) were mixed with CH3CN (2 mL) in a Mininert® glass vial
closed with a screw cap containing a septum. The temperature
of the reaction was kept at 70 °C using a sand bath. A
hydrogen peroxide solution (6 mmol, 30% w/w in H2O) was
added to initiate the reaction over a period of 80 min via a
syringe pump. Samples were directly taken out of the reaction
mixture, diluted, quenched with MnO2 and analyzed via HPLC.
The identity of the oxidation products was verified by
comparison to commercially available chemicals. Oxidation
reactions with in situ generated complexes were prepared
according to the procedure above by mixing the respective
black powder of iron complex
3. Yield: 1.433 g (58%). EI-MS:
m/z 539.2 [M+]. IR (ATR): 1560, 1523, 1489, 1387, 1322,
ν
1236, 1081, 891, 790, 760, 447 cm-1. IR: 1492, 1302, 806, 739,
531, 434 cm-1. Anal. Calc. for C27H20ClFeN6O2: C, 57.85; H, 3.73;
N, 15.57%. Found: C, 57.61; H, 3.63; N, 15.61%.
amounts of FeCl3·6H2O and ligands HL1, HL2 or HL3. The
reaction mixture was stirred for 10min at 70 °C before the
addition of H2O2 was started. Conversions, yields and
selectivities were calculated via peak integration of the
chromatograms.
Conflicts of interest
There are no conflicts to declare.
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Ed., 2006, 45, 4776; f) D. R. Jensen, M. J. Schultz, J. A.
Acknowledgements
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The authors gratefully acknowledge support from NAWI Graz.
Platforms in Homogenous Catalytic Reactions with
Metals, John Wiley & Sons, Inc, Hoboken, NJ, 2014;
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Direct, one-pot oxidation of benzyl alcohols to benzoic acid with H2O2 catalyzed by Fe(III) complexes.