Iron phthalocyanine as new efficient catalyst for catalytic transfer hydrogenation of simple aldehydes and ketones

Article abstract of DOI:10.1002/aoc.3247

Catalytic transfer hydrogenation (CTH) of various aldehydes and ketones was studied using iron phthalocyanine catalyst, in order to substitute the typically used rare transition metals (Ir, Rh, Ru) with an easily available and less expensive metal. Iron phthalocyanine was found to be an efficient hydrogenation catalyst and its immobilized version was successfully prepared. The immobilized iron phthalocyanine was also active in the CTH reaction of various carbonyl compounds, and it was easy to handle and possible to recycle.

Full text of DOI:10.1002/aoc.3247

Full Paper
Received: 1 August 2014
Revised: 2 October 2014
Accepted: 15 October 2014
Published online in Wiley Online Library: 17 November 2014
(wileyonlinelibrary.com) DOI 10.1002/aoc.3247
Iron phthalocyanine as new efficient catalyst
for catalytic transfer hydrogenation of simple
aldehydes and ketones
Péter Bataa, Ferenc Notheisza, Peter Klusonb and Ágnes Zsigmonda*
Catalytic transfer hydrogenation (CTH) of various aldehydes and ketones was studied using iron phthalocyanine catalyst, in order
to substitute the typically used rare transition metals (Ir, Rh, Ru) with an easily available and less expensive metal. Iron phthalo-
cyanine was found to be an efficient hydrogenation catalyst and its immobilized version was successfully prepared. The
immobilized iron phthalocyanine was also active in the CTH reaction of various carbonyl compounds, and it was easy to handle
and possible to recycle. Copyright © 2014 John Wiley & Sons, Ltd.
Keywords: heterogenized complexes; catalytic transfer hydrogenation; reusable catalyst
characteristic regions of the visible part of the spectrum, phthalocy-
anines have been used as pigments and dyes. Recently other appli-
Introduction
Reduction of carbonyl functionality of aldehydes and ketones by
metal-catalysed transfer hydrogenation (CTH) using a suitable
hydrogen donor is a valuable synthetic tool and it has proven to
be an elegant alternative to hydrogenation with molecular
hydrogen.[1] Since this latter method is often associated with risks
and it usually requires high-pressure apparatus, the alternative
method has been applied in several laboratories. Organic mole-
cules such as unsaturated hydrocarbons, primary and secondary
alcohols, and formic acid and its salts have been used as hydrogen
sources in CTH reactions.[2,3] Transition metals such as Rh, Ir, Ni and
Pd and for carbonyl reduction Ru complexes and propan-2-ol in the
presence of base can catalyse the reduction.[4]
The substitution of the above mentioned rare transition metals
(Ir, Rh, Ru) with easily available and less expensive elements is a
major challenge in the study of CTH reactions. Consequently, the
application of iron-based catalysts would be desirable. Homoge-
neous iron catalysts have already been used for carbon–carbon
bond formation, such as olefin polymerizations, cross-couplings
and reduction of nitro compounds.[5] However, much less attention
has been paid to iron-catalysed transfer hydrogenation. Only a few
groups have reported the utilization of iron salts and iron
complexes, containing tetradentate phosphine or aminophosphine
ligands, in the reduction of ketones and α,β-unsaturated carbonyl
compounds to yield alcohols and α,β-saturated carbonyl
compounds.[6–10]
cations based on their distinctive property of generating an active
species upon light absorption have appeared. Phthalocyanines,
which are inactive in photocatalysis, can also be used as oxidation
catalysts in the dark, together with common oxidation agents: hy-
drogen peroxide, peroxyacetic acid and tert-butylhydroperoxide.[15]
Referring to the environmental application of phthalocyanines, it
is important to note that most of them are virtually non-toxic
and they have relatively simple preparation methods, even on
a large scale.
With a final goal of developing a cheap, efficient, heterogeneous
catalyst for CTH reactions, we prepared an iron phthalocyanine
complex, Fe(Pc), and applied it in the CTH of various aldehydes
and ketones. Since for practical purposes the preference for hetero-
geneous catalysts over homogeneous ones is well known, we
also prepared a heterogenized version of Fe(Pc), and applied it
in CTH reactions.
Experimental
Catalyst
Fe(Pc) was prepared using solvent-free conditions, and applying
microwave irradiation. Here, only brief information is given; details
are described elsewhere.[16] Iron(II) chloride (2.5 g, 0.02 mol) and
10.25g (0.08 mol) of phthalonitrile were used for the preparation.
The homogeneous mixture of the two components was heated in
An effective transfer hydrogenation method was reported
recently for the reduction of ketones applying propan-2-ol as the
hydrogen source and a combination of iron salts, tridentate amines
and phosphines as catalysts. Successful hydrogenation of alkoxy-
and aryloxy-substituted acetophenones was also reported in the
presence of iron–porphyrin complexes.[11,12]
Macrocyclic compounds known as phthalocyanines were first
described in the 1930s.[13] Since then, many interesting applications
have appeared due to their unique physical and chemical
properties.[14] Due to their stability and light absorption in
*
Correspondence to: Ágnes Zsigmond, University of Szeged, Department of Organic
Chemistry, Dóm tér 8, 6720 Szeged, Hungary. E-mail: azsig@chem.u-szeged.hu
a University of Szeged, Department of Organic Chemistry, Dóm tér 8, 6720, Szeged,
Hungary
b ICPF, Academy of Sciences of the Czech Republic, Rozvojova 135/2, 165 02,
Prague, Czech Republic
Appl. Organometal. Chem. 2015, 29, 45–49
Copyright © 2014 John Wiley & Sons, Ltd.

P. Bata et al.
a beaker for 6 min in a microwave oven (an intense blue colour was
developed). After cooling the reaction mixture to room tempera-
ture, the complex was crushed to a fine powder and dissolved in
20 ml of acetone. The mixture was refluxed for 30 min and the
remaining powder was filtered and dried. With this treatment the
non-complexed ligand was removed. In order to get a higher and
constant activity, another washing with concentrated H2SO4 was
applied and the final catalyst (3.6g) was a deep blue fine power.
Anchoring of the complex was carried out in two steps, using a
method developed by Augustine and co-workers.[17–19] Al2O3
(1.5 g) was suspended in 30 ml of methanol. Phosphotungstic acid
hydrate (386.7mg, 0.15 mmol) was dissolved in 25 ml of methanol
and this solution was added dropwise into the alumina suspension
with efficient stirring. The stirring was continued for one day at
room temperature under argon atmosphere. The solid residue
was filtered off and suspended in 30 ml of methanol. Fe(Pc) com-
plex (56.8mg, 0.1 mmol) was dissolved in 40 ml of deoxygenated
dimethylformamide and this solution was slowly added dropwise
with stirring to the suspension. The stirring was continued for
another day. The solution was removed and the solid residue was
washed with methanol, until a colourless solution was obtained.
Finally the light blue solid was dried at 30 °C for 2 h in vacuum and
for one day under argon. The amount of catalyst obtained was 1.5 g.
Results and Discussion
With a final goal of developing a new and efficient heterogeneous
catalyst for CTH of carbonyl functionality, the Fe(Pc) complex and its
heterogenized version were prepared. Iron complexes under
homogeneous conditions are usually applied in oxidation reac-
tions; their applicability in reductions has been rather neglected
in the past. We also wished to examine the possibility of replac-
ing the generally applied rare transition metal catalysts with the
Fe(Pc) complex.
Catalyst Preparation and Characterization
As already detailed, the Fe(Pc) complex was prepared using a
solvent-free method, applying microwave irradiation. Each of
the catalytic samples experienced the same activating routine:
dissolution in concentrated H2SO4 followed by precipitation.
The complex obtained was characterized using FT-IR spectros-
copy with the spectra revealing characteristic bands for (Pc).
The surface anchoring of the Fe complex was done using the
Augustine method, with heteropolyacid as the anchoring agent.
The heterogenized catalyst was also characterized using FT-IR
spectroscopy.[22,23] The spectra of the support, the ‘free’ complex
and the heterogenized catalyst were all recorded (Fig. 1).
A comparison of these spectra shows that the Fe(Pc) complex is
anchored to the surface of alumina. The characteristic bands for Fe
(Pc) (1515, 1494, 1333, 1164, 1118 and 752 cmꢀ1) appear clearly in
the spectrum of the immobilized sample as well. As a matter of
fact a slight shift (a couple of cmꢀ1) is observed in the spectrum
of the immobilized complex. This proves that the Fe(Pc) complex
is really anchored onto the surface. The slight shift is caused by
small changes in the structure of the complex during its grafting
to the surface.
Characterization of Catalyst
The homogeneous complex and the heterogenized catalyst were char-
acterized using spectroscopic methods. The presence of immobilized
complex was demonstrated using FT-IR spectroscopy.[20,21] The FT-IR
spectra of the support, the neat complex and the heterogenized
samples were also obtained. The spectra were recorded with samples
in KBr pellets, using a Bio-Rad FTS-65A spectrophotometer, in the range
400–4000 cmꢀ1
.
Since during the preparation of immobilized catalyst the
uncomplexed ligand was removed by extraction, the metal content
of the sample gives the complex content. The metal content was
determined using inductively coupled plasma atomic emission
spectroscopy (ICP-AES), and for determination the sample was dis-
solved in concentrated HNO3. The metal content of this solution
was determined with a Jobin Yvon 24-type ICP-AES instrument.
In order to determine the complex concentration of the
immobilized samples, the samples were dissolved in concentrated
HNO3 and the metal content of these solutions was determined
using the ICP-AES method. Since the extraction procedure removes
all the uncomplexed ligand from the support surface, the residual
iron can be found in the immobilized complex. The anchored
catalyst is found to contain 35.8μmol Fe gꢀ1 catalyst. This value is
in a good agreement with that determined for the immobilized
samples (prepared using the same method).[24]
General Procedure for Catalytic Transfer Hydrogenation
In a 5 ml Schlenk tube, 0.0038 mmol of Fe(Pc) catalyst was added
into 1 ml of propan-2-ol and the tube was purged with argon.
Sodium hydroxide (0.19 mmol) in 0.5ml of propan-2-ol and
0.38 mmol of starting material (aldehyde or ketone) in 0.5 ml of
propan-2-ol were injected into the reaction vessel. The reaction
mixture was heated for 6 or 12 h at 80 °C with efficient stirring.
The solution was cooled to room temperature and filtered over a
plug of silica. The conversion was measured using GC without
further manipulations and the products were identified using
chromatographic standards or the GC-MS technique.
Catalyst Recycling
The immobilized catalyst was applied in three subsequent runs.
After the reaction the stirring was interrupted and the immobilized
catalyst sunk to the bottom of the Schlenk tube. The reaction
mixture was removed via a Pasteur pipette and the settled catalyst
was carefully purged with argon and a new run was started.
Figure 1. FT-IR spectra of Fe(Pc), of the Al2O3 support and of the
heterogenized complex.
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Appl. Organometal. Chem. 2015, 29, 45–49

Catalytic transfer hydrogenation of carbonyl functionality
Catalytic Transfer Hydrogenations of Aldehydes
Table 3. Role of carbonyl compound molecular structure
Entry Starting material
Product
Conversion
(%)
To check the ability of Fe(Pc) to catalyse CTH reactions, various
aldehydes with different chemical environments for the C¼O
functional group were chosen. Since the type and the amount
of base are important in the CTH reaction, we investigated the
role of several frequently used bases, namely NaOH, KOH and
KtBuO, using benzaldehyde as the hydrogenation substrate
(Table 1).
It is evident that the type of base is a much less critical parameter
than the amount of base. Applying 0.095 mmol of the base, the
conversion reaches more than 90%, while half of this amount
decreases the conversion significantly (55%). However, changing
the base produces only a moderate effect on conversion (NaOH,
95.0%; KOH, 91.5%; KtBuO, 93.0%). We also investigated the influ-
ence of the reaction temperature (Table 2, entries 1–3) and the
amount of the Fe complex (Table 2, entries 4–6). The conversion
increases from 40 to 80 °C. The optimum amount of the Fe(Pc)
complex is 0.0038 mmol.
Essential knowledge was also obtained from the reactions with
differently substituted benzaldehydes under optimized reaction
conditions (Table 3). All substituted benzaldehyde derivatives are
active in this reaction except for 4-hydroxybenzaldehyde. It is very
likely the OH group coordinated to the Fe(Pc) complex causes the
observed low catalyst activity. In contrast, excellent conversion is
obtained for benzaldehydes substituted with electron-donating
groups – MeO and di-MeO (Table 3, entries 2 and 7); even 4-
chlorobenzaldehyde yields almost total conversion. Substitution
with electron-withdrawing substituents, such as NO2, decreases
the conversion. The smallest decrease is observed in the para posi-
tion and the highest in the ortho position (Table 3, entries 3–5). We
can attempt to explain this phenomenon in terms of the ionic char-
acter of the hydrogenation mechanism. An electron-withdrawing
1
2
3
4
5
6
7
Benzaldehyde
Benzyl alcohol
> 99
> 99
91
2-MeO-benzaldehyde
4-NO2-benzaldehyde
3-NO2-benzaldehyde
2-NO2-benzaldehyde
4-Cl-benzaldehyde
2,4-di-MeO-
2-MeO-benzyl alcohol
4-NO2-benzyl alcohol
3-NO2-benzyl alcohol
2-NO2-benzyl alcohol
4-Cl-benzyl alcohol
2,4-di-MeO-benzyl
alcohol
57
25
> 99
98
benzaldehyde
8
4-OH-benzaldehyde
4-OH-benzyl alcohol
0
Reaction conditions:4 h, 80 °C, 2 ml of propan-2-ol, 0.0038 mmol of Fe
(Pc), 0.38 mmol of aldehyde, 0.095 mmol of NaOH.
substituent affects the reaction rate electronically, the same as it
does in aromatic electrophilic substitution, destabilizing the
carbocation intermediate and causing its slow formation.
It is well known that in practical catalytic processes heteroge-
neous catalysts are preferred over homogeneous ones. With this
in mind, heterogenized versions of the Fe(Pc) complex were also
prepared. The anchoring of the complex using Augustine’s
method, with heteropolyacid as an anchoring agent, is described
in the experimental section.
When using the heterogenized catalyst, the same reaction
conditions were applied as in the homogeneous set-up. Suppos-
ing all the measured Fe(Pc) complex is anchored onto the Al2O3
support, we have applied 0.074 g (0.0038 mmol) of Fe(Pc)/Al2O3
catalyst. However, this amount provides only moderate conver-
sions. Increasing the amount of the catalyst does not improve
the conversion, but using half of the original amount leads to
nearly total benzaldehyde conversion. We also had to modify
the reaction time (6h) compared to the homogeneous condi-
tions in order to increase the conversion. Finally, the amount
of base was doubled (0.19mmol of NaOH) in the heterogeneous
system. Then moderate to excellent yields are observed for var-
ious aldehydes (Table 4).
Table 1. Effect of base in the Fe(Pc)-catalysed transfer hydrogenation
reaction of benzaldehyde
Entry Base
Amount (mmol) Temperature (°C) Conversion (%)
1
2
3
4
5
6
NaOH
NaOH
KOH
0.095
60
60
60
60
60
60
95.0
79.5
91.5
55.0
93.0
62.0
0.0475
0.095
As is clearly evident from Table 4, all the studied aldehydes are
reduced to the corresponding alcohols with moderate to excellent
yields under optimized conditions. In other words, the immobilized
Fe(Pc) complex is found to be feasibly active for the CTH reaction of
KOH
0.0475
0.095
KtBuO
KtBuO
0.0475
Reaction conditions: 4 h, 2 ml of propan-2-ol, 0.0038 mmol of Fe(Pc),
0.38 mmol of benzaldehyde.
Table 4. Fe(Pc)/Al2O3 in CTH of differently substituted aldehydes
Entry Starting material
Product
Conversion
(%)
Table 2. Effect of temperature and amount of catalyst
1
2
3
4
5
6
7
Benzaldehyde
Benzyl alcohol
> 99
> 99
85.5
67
2-MeO-benzaldehyde
4-NO2-benzaldehyde
3-NO2-benzaldehyde
2-NO2-benzaldehyde
4-Cl-benzaldehyde
2-MeO-benzyl alcohol
4-NO2-benzyl alcohol
3-NO2-benzyl alcohol
2-NO2-benzyl alcohol
4-Cl-benzyl alcohol
Entry Temperature (°C) Amount of catalyst (mmol) Conversion (%)
1
2
3
4
5
6
40
60
80
60
60
60
0.0038
0.0038
0.0038
0.0019
0.0038
0.0057
50
80
92
72
80
85
33
70
2,4-di-MeO-benzaldehyde 2,4-di-MeO-benzyl
alcohol
97
Reaction conditions: 6 h, 80 °C, 2 ml of propan-2-ol, 0.04 g
(0.0019 mmol) of FePc/Al2O3, 0.19 mmol of NaOH, 0.38 mmol of
starting material.
Reaction conditions:4 h, 2 ml of propan-2-ol, 0.0038 mmol of Fe(Pc),
0.38 mmol of benzaldehyde, 0.0475 mmol of NaOH.
Appl. Organometal. Chem. 2015, 29, 45–49
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P. Bata et al.
various aldehydes. Obviously the most important advantages of the
immobilized complex are its easy separation and the possibility to
recycle it.
Table 6. Fe(Pc)-catalysed transfer hydrogenation reaction of differently
substituted ketones
Entry
Starting material
Product
Conversion
(%)
1
2
3
4
Acetophenone
1-Phenylethanol
96
86
63
42
Catalytic Transfer Hydrogenations of Ketones
4-Me-acetophenone
4-Me-1-phenylethanol
4-MeO-acetophenone 4-MeO-1-phenylethanol
Having obtained reasonable results for the Fe(Pc)-catalysed transfer
hydrogenation reactions of aldehydes, we extended the study to
the reaction of ketones. It is well known that the C¼O functionality
is much less reactive in ketones than in aldehydes. First we deter-
mined the optimum reaction conditions using acetophenone as
model substrate for hydrogenation. In the first step we increased
the reaction time to 12 h, due to the expected lower reactivity of
ketones. Since the type and the amount of base are again impor-
tant factors, we investigated several frequently used bases (NaOH,
KOH and KtBuO) under homogeneous conditions (Table 5).
As evident from Table 5, unlike aldehydes, in reactions with
ketones both the type and the amount of base matter, and they af-
fect the conversion of acetophenone. However, we can also see that
the type of base is a more influential parameter than its amount.
Best results are achieved with NaOH: application of 0.38 mmol of
NaOH can give a conversion of more than 90%; 0.19 mmol of NaOH
yields only 82% conversion. The same amount of KOH and KtBuO re-
sults in 33 and 49% conversion. The optimum amount of the catalyst
was also investigated with the application of 0.19 mmol of NaOH.
Using 0.0019 mmol of Fe(Pc) the conversion of acetophenone is
64%, while 0.0057 mmol results in 77% conversion. Thus the opti-
mum amount of catalyst is 0.0038 mmol producing 82% conversion.
The optimum in reaction time was also determined using
0.19 mmol of NaOH: during 8 h the conversion is 58%; increasing
to 16 h the conversion also increases to 86%. The standard reaction
time (used in most experiments) of 12 h provides 82% conversion.
Under the optimum reaction conditions, we carried out the reac-
tion using several differently substituted aromatic ketones. The re-
sults obtained are given in Table 6. It is evident that the substituted
ketones are less reactive than the original acetophenone, in spite
of the substituents being of electron-donating character. These
results are not in agreement with the results obtained for
aldehydes, where the electron-donating substituents enhance
the reactions.
3,5-di-MeO-
3,5-di-MeO-1-
phenylethanol
acetophenone
Reaction conditions: 12 h, 80 °C, 2 ml of propan-2-ol, 0.0038 mmol of Fe
(Pc), 0.38 mmol of NaOH, 0.38 mmol of starting material.
Table 7. Fe(Pc)/Al2O3-catalysed transfer hydrogenation reaction of dif-
ferently substituted ketones
Entry
Starting material
Product
Conversion
(%)
1
2
3
4
Acetophenone
1-Phenylethanol
90
71
30
22
4-Me-acetophenone
4-Me-1-phenylethanol
4-MeO-acetophenone 4-MeO-1-phenylethanol
3,5-di-MeO-
3,5-di-MeO-1-
phenylethanol
acetophenone
Reaction conditions: 12 h, 80 °C, 2 ml of propan-2-ol, 0.04 g of Fe(Pc)/
Al2O3, 0.38 mmol of NaOH, 0.38 mmol of starting material.
Recycling the Fe(Pc)/Al2O3 Catalyst in CTH Reactions
The most important advantage of immobilized complexes is their
recyclability. This is a crucial point if the complex is either expensive
or difficult to prepare (or both). Fe(Pc) is not expensive and it is
readily available, but also in such a case it is worth investigating
its reusability. Figure 2 shows the results obtained in the hydroge-
nation of benzaldehyde and acetophenone with Fe(Pc)/Al2O3. As
Fig. 2 shows, the heterogenized Fe(Pc) complex is well recyclable:
in three sequential experiments it reveals only a slight decrease of
its activity. This effect is more pronounced for the ketone than for
the aldehyde reactant.
As before, we also used the heterogenized complex for the CTH
reaction of ketones. The results are presented in Table 7. Conse-
quently we used 0.04 g of Fe(Pc)/Al2O3 catalyst. All other conditions
were same as in the homogeneous reaction set-up. The activities of
the homogeneous and heterogeneous catalysts are found to be
similar, conversions are rather moderate and the trends in the effect
of substrate molecular structure are same (see Table 6).
Table 5. Effect of base in the Fe(Pc)-catalysed transfer hydrogenation
of acetophenone
Entry
Base
Amount (mmol)
Conversion (%)
1
2
3
4
5
NaOH
NaOH
NaOH
KOH
0.095
0.19
0.38
0.19
0.19
52
82
95
33
49
Figure 2. Reusability of heterogenized Fe(Pc) complex in the
hydrogenation of an aldehyde and a ketone. (Reaction conditions: 6 h,
80 °C, 2 ml of propan-2-ol, 0.04 g of Fe(Pc)/Al2O3, 0.19 mmol of NaOH,
0.38 mmol of benzaldehyde; 12 h, 80 °C, 2 ml of propan-2-ol, 0.04 g of
FePc/Al2O3, 0.38 mmol of NaOH, 0.38 mmol of acetophenone.).
KtBuO
Reaction conditions: 12 h, 80 °C, 2 ml of propan-2-ol, 0.0038 mmol of Fe
(Pc), 0.38 mmol of acetophenone.
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Appl. Organometal. Chem. 2015, 29, 45–49

Catalytic transfer hydrogenation of carbonyl functionality
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Considering the reported and discussed results, we can conclude that
the Fe(Pc) complex is well suited for the CTH reactions of various
aldehydes and ketones. The application of this complex in oxidation
reactions is well known; however, in hydrogenations only very few
data have been reported. Most of the known hydrogenation catalyst
systems are based on rare metals such as Ru, Rh, Ir and Pd. As a result
of their restricted availability and high price, the search for more eco-
nomical and environmentally friendly catalysts based on more readily
available and cheaper analogues (Fe, Cu, Zn, Mn) is a real challenge.
To summarize, CTH of differently substituted aldehydes and
ketones was studied, using Fe(Pc) catalyst and propan-2-ol as the
hydrogen source. This catalytic system produces moderate to excel-
lent conversions for the various simple aldehydes and reasonable
conversions for ketones. The heterogenized version of Fe(Pc) catalyst
was also studied in both cases and it provided similar conversions as
under homogenous conditions. It was shown that the catalyst was
easy to separate, and it was then successfully used in repeated runs.
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Acknowledgements
The authors express their thanks and gratitude for a research grant
(103191 OTKA NN) supporting this research.
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Appl. Organometal. Chem. 2015, 29, 45–49
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