Iron-based pre-catalyst supported on polyformamidine for C–C bond
formation
Stephan Enthaler,*
a
Sebastian Krackl,
ab
Jan Dirk Epping,
b
Bj€
orn Eckhardt,
ac
Steffen M. Weidner
d
and Anna Fischer
a
Received 10th November 2011, Accepted 30th December 2011
DOI: 10.1039/c2py00540a
In the present study the incorporation of iron into an organic polymer, composed of formamidine
subunits [R–N]C(H)–NH–R], has been examined. The catalytic ability of the recyclable material was
investigated in the iron-catalyzed formation of C–C bonds. After optimization of the reaction
conditions, excellent yields and chemoselectivities were feasible.
Introduction
The direct functionalization of aromatic C–H bonds is one of the
key transformations for industry as well as academia to access
organic compounds with higher values starting from low cost
materials.
1
In this regard, one of the most frequently applied
methodologies for C–C bond formations is the Friedel–Crafts
reaction. The impact of this method is outlined by several
applications in the synthesis of bulk-, fine- or agrochemicals.
Classically, aluminium chloride is used to force the reaction
towards the C–C bond formation.
2
However, several difficulties
arise with the use of AlCl
3
,e.g., problematic separation and
recovery, corrosion, toxicity, and moisture sensitivity.
3
Recently,
manifold alternatives have been accounted, because of modern
requirements for higher efficiency, better selectivity and a greener
chemistry.
3,4
Within those improvements the application of iron
as a catalyst seems to be a promising tool to accomplish several
of those requests. For instance, the low price, the availability, the
low toxicity and the biocompatibility of iron sources are
favourable aspects.
5
Indeed, during the past years a number of
Friedel–Crafts reactions has been established based on iron
catalysts.
6,7
However, those methods apply homogeneous
systems, which do not allow an easy recovery or reuse of the
catalyst. Therefore, the incorporation of the catalyst on sup-
porting materials can be a choice to solve this problem. Manifold
approaches have been reported for the heterogenization,
recycling and catalytic application of iron-based materials, e.g.,
metal–organic frameworks, mesoporous materials, montmoril-
lonite, and ionic liquids.
8,9
On the other hand, the embedding of
ligands in a heterogeneous matrix allows the coordination/fixa-
tion of metals, tuning of the catalyst abilities and, as a conse-
quence, recycling of the catalyst. However, this interesting access
has not been accounted for those reactions. Based on this
heterogeneous ligand concept, novel materials for immobiliza-
tion of metal catalysts are highly desired. More recently, some of
us studied the application of well-defined formamidine ligands in
the iron-catalyzed oxidation of carbon–carbon double bonds and
their use as synthons for the construction of molybdenum based
dimensional networks.
10,11
Due to the straightforward synthesis
and great availability of the starting materials, an attractive
ligand class can be provided. In addition, they can be easily
incorporated into polymers containing the formamidine unit as
the key motif (Fig. 1).
12
Herein, we report on our initial studies on the application of
iron supported on polyformamidine based polymers in the iron-
catalyzed C–C bond formation.
Results and discussion
For the synthesis of polyformamidine a procedure was per-
formed in accordance with the protocol reported by B€
ohme and
co-workers with slight modifications.
12
A dimethylsulfoxide
Fig. 1 Catalyst strategy based on homogeneous and heterogeneous
formamidine ligands.
a
Technische Universit€
at Berlin, Department of Chemistry, Cluster of
Excellence ‘‘Unifying Concepts in Catalysis’’, Straße des 17. Juni
135/C2, D-10623 Berlin, Germany. E-mail: stephan.enthaler@tu-berlin.
de; Fax: +493031429732; Tel: +493031422039
b
Technische Universit€
at Berlin, Department of Chemistry, Metalorganics
and Inorganic Materials, Technische Universit€
at Berlin, Straße des 17.
Juni 135/C2, D-10623 Berlin, Germany
c
Technische Universit€
at Berlin, Department of Chemistry, Straße des 17.
Juni 124/TC03, D-10623 Berlin, Germany
d
Bundesanstalt f€
ur Materialforschung und -pr€
ufung (BAM), Federal
Institute for Materials Research and Testing, Richard-Willst€
atter-Straße
11, D-12489 Berlin, Germany
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solution of para-phenylene diamine (1) and triethyl orthoformate
was stirred at 140 C for 24 hours under non-inert conditions
(Scheme 1). After work-up a yellow-orange powder was
obtained, which is highly stable up to a temperature of 300 C.
Noteworthy, the compound is highly insoluble in various organic
solvents (e.g., DMSO, THF, toluene, CH
2
Cl
2
). Applying the
13
C
NMR chemical shift of the [N]C(H)–N] group as a probe, we
found for the polyformamidine 2a chemical shift of 146.4 ppm,
which is in agreement with well-defined formamidine ligands
(e.g.,N,N0-bisphenylformamidine, d¼149.5 ppm).
13
For 2,
13
C
{1H} CP/MAS NMR was applied, due to the poor solubility,
resulting in broad overlapping signals. In case of the FT-IR for
the N,N0-bisphenylformamidine (1679 cm
1
) and 2(1665 cm
1
),
a shift of 19 cm
1
for the band of the C]N functionality was
detected. Furthermore, attempts to elucidate the molecular
weight (MALDI TOF MS) of the polymeric compound failed
due to the poor solubility.
To support iron on the polyformamidine, 2was refluxed for 24
hours in THF with different loadings of FeCl
3
(Scheme 1). After
work-up a color change from yellow to brown was observed
correlating with the amount of iron. The obtained FeCl
3
@PF
composites were investigated with several analytical tools. In
case of the FT-IR, the band for the C]N functionality in
FeCl
3
@PF2 was shifted to 1674 cm
1
, while the unmodified
polyformamidine resulted in a band at 1665 cm
1
. This increase
can probably be attributed to the coordination of iron to the C]
N of the formamidine subunit. In case of the FeCl
3
@PF1
material a small shoulder (1665 cm
1
) of the band at 1674 cm
1
was observed, which can be assigned to the uncoordinated for-
mamidine units. Whereas for FeCl
3
@PF2 the band at 1674 cm
1
is broad and probably overlaps the band for the free for-
mamidine. Moreover, the existence of iron in the polymer was
determined by energy dispersive X-ray (EDX) spectroscopy.
Applying the nitrogen content as a probe, we clearly see an
increased incorporation at higher loadings, which was addi-
tionally approved by elemental analysis. Electron microscopy of
the doped and undoped polymeric materials showed large
agglomerates of differently sized particles in the range of
approximately 100 nm to 5 mm. Comparing the SEM image of
the undoped polymer (Fig. 2a) to the FeCl
3
@PF2 composite
(Fig. 2b) demonstrates that the loading process does not have
a significant influence on the morphology of the polymer and
thus underlines its intrinsic stability during the loading process.
A SEM-element-mapping shows that iron is dispersed almost
homogeneously in the material (Fig. 2c). The intrinsic stability of
the polymer was furthermore approved by X-ray diffraction
measurements (Fig. 3) of the doped and undoped polymeric
material. The received values are in agreement with the WAXS
pattern of the homopolymer reported in the literature.
12a
Apparently, the crystallinity and crystallite size remained
unchanged during the FeCl
3
loading procedure. As no diffrac-
tion peaks of FeCl
3
could be identified, one can assume that all
the iron in the sample is in a coordination state. The shoulder of
the reflex at 25.9, at 28.3for FeCl
3
@PF1 and 27.1for
Scheme 1 Synthesis of polyformamidine 2and iron supported on pol-
yformamidine 2.
Fig. 2 SEM: (a) 2, (b) FeCl
3
@PF2, and (c) iron mapping.
Fig. 3 XRD: 2(solid curve), FeCl
3
@PF1 (dash-dotted curve), and
FeCl
3
@PF2 (dotted curve); curves are normalized to the reflex at 25.9.
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FeCl
3
@PF2, respectively, has an increasing intensity and a shift
to smaller angles with higher FeCl
3
content was observed. This
effect can be probably attributed to coordinated FeCl
3
and its
influence on the crystallization of the polymer. This could be
further evidenced by TEM, as no iron hydroxide or iron oxide
nanoparticle formation was observed. To evaluate the surface
area of the catalysts, nitrogen adsorption/desorption measure-
ments were performed both on unloaded and loaded polymers.
From BET analysis the obtained polymers have surface areas in
the range of 20–50 m
2
g
1
.
With the material in hand we became interested in the catalytic
abilities. As a model reaction the Friedel–Crafts alkylation was
chosen. Initial studies on the influence of the reaction conditions
were carried out with an excess of anisole (3) and 1-bromoada-
mantane (4) under solvent free conditions for one hour at 100 C
(Table 1). First, unmodified FeCl
3
was applied as a pre-catalyst
resulting in an excellent yield (>99%) with a regioselectivity of
85% to the formation of the para-substituted isomer 5(Table 1,
entry 1).
In addition, a combination of catalytic amounts of FeCl
3
and
N,N0-bisphenylformamidine was tested in a ‘‘homogeneous
approach’’ to investigate the influence of the formamidine unit
on the reaction outcome. A significantly lower yield (19%) was
obtained compared to unmodified FeCl
3
(Table 1, entries 1 and
2). In contrast, FeCl
3
@PF2 was tested with a catalyst loading of
1.0 mol%, resulting in a good yield (84%) and an excellent
regioselectivity (97%) (Table 1, entry 4). This result shows clearly
the advantages of the heterogeneous approach in comparison to
unmodified FeCl
3
and the homogeneous approach. For
FeCl
3
@PF1 with only 1.0 mol% Fe lower yields were obtained;
hence, further studies were carried out with FeCl
3
@PF2. The
metal-free polymer was tested as catalyst for the C–C bond
formation (Table 1, entry 3), and showed no activity as expected.
However, with increasing the catalyst loading towards 2.5 and
5.0 mol% a slight improvement of the reaction outcome was seen
(Table 1, entries 5 and 6). Performing the reaction for 24 hours
led to full conversion (>99%) and an excellent selectivity (97%)
(Table 1, entry 7). Subsequently, the effect of the reaction
temperature was studied. Decreasing the temperature (80 C)
resulted in no product formation, while for an increased
temperature (120 C) nearly full conversion was observed within
30 minutes with comparable regioselectivity (Table 1, entries 8
and 9).
Since an excess of anisole (4)(3.9 equiv) was applied the
effect of the amount of anisole was studied under solvent free
conditions. Reducing the amount of anisole to a ratio of 1 : 1
(anisole : 1-bromoadamantane) the yield of the product 5
decreased significantly to 28%, while the selectivity for the para-
isomer is excellent with >99% (Table 1, entry 10). Changing the
ratio to 2 : 1 (3:4) the yield increased to 53% accompanied by an
excellent selectivity (Table 1, entry 11).
Additionally, the outstanding Lewis acid zinc(II) triflate was
supported on polyformamidine and the corresponding material
[Zn(OTf)
2
loading 14.0 mol%] was tested as a catalyst (5.0 mol%)
under conditions in accordance with Table 1, entry 6.
14
However,
the obtained yields (35%) and regioselectivities (76%) are
significantly lower than the results achieved with iron, demon-
strating the excellent abilities of FeCl
3
@PF2.
Noteworthy, FeCl
3
@PF2 is insoluble in the starting materials
(3and 4) and in the reaction mixture after the reaction is
completed and cooled to room temperature, due to an excess of
4. This fact allows the easy separation of FeCl
3
@PF2 from the
reaction mixture and a potential reuse of the catalyst compared
to the homogeneous approach. Indeed, the material was recycled
four times and subjected to catalysis with a small loss of activity,
but with constant selectivity (Fig. 4). In more detail, FeCl
3
@PF2
was filtered after catalysis, washed with acetone and dried in
vacuum before resubmission. In contrast, Zn(OTf)
2
@PF showed
full depletion of the activity after the first run. To push this
concept further, a stationary catalyst phase was built up. The
polymer was placed on the top of a filter and catalytic amounts
of FeCl
3
were added. After addition of anisole (3) and
Table 1 FeCl
3
@PF-catalyzed alkylation of anisole (4)
a
Entry Catalyst (mol%) T/C Conv. (%)
Select. (5)
(%)
Yield (5)
(%)
1 FeCl
3
(1.0) 100 >99 85 85
2
b
FeCl
3
(1.0) 100 19 >99 19
32100 <1 <1 <1
4 FeCl
3
@PF2 (1.0) 100 87 97 84
5 FeCl
3
@PF2 (2.5) 100 92 97 89
6 FeCl
3
@PF2 (5.0) 100 88 97 85
7
c
FeCl
3
@PF2 (5.0) 100 >99 97 97
8 FeCl
3
@PF2 (5.0) 80 <1 <1 <1
9
d
FeCl
3
@PF2 (5.0) 120 95 97 92
10
e
FeCl
3
@PF2 (2.5) 100 28 >99 28
11
f
FeCl
3
@PF2 (2.5) 100 53 99 53
12
g
FeCl
3
@PF2 (5.0) 100 26 85 22
a
Reaction conditions: 0.48 mmol 1-bromoadamantane, 1.85 mmol
anisole, 20 mg FeCl
3
@PF2, 1 h. The conversion and yield were
determined by GC (30 m Rxi-5ms column, 40–300 C) using dodecane
as internal standard.
b
0.41 mmol N,N0-bisphenylformamidine.
c
24 h.
d
Reaction time: 30 min.
e
3:41:1.
f
3:41:2.
g
1-Chloroadamantane
instead of 3.Fig. 4 FeCl
3
@PF2-catalyzed alkylation of anisole-recycling
experiments.
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1-bromoadamantane (4) the mixture was heated to 100 C with
a flow of nitrogen from underneath. Once the reaction was
finished the flow was switched off and the residue was washed
with dichloromethane and filtered. The yield and selectivity were
measured by GC-MS. The remaining material was subjected to
catalysis again. After several runs a depletion in activity was
noticed (vide supra). The material was washed with an excess of
triethylamine to remove HBr (Fig. 5). Finally, fresh FeCl
3
was
added and the recharged material was applied in the Friedel–
Crafts alkylation of anisole (3) and 1-bromoadamantane (4).
Interestingly, mapping experiments carried out with the used
catalyst material (after 1 cycle) pointed out that the iron is still
present and furthermore homogeneously dispersed in the for-
mamidine polymer (Fig. 6). Moreover, the measurements indi-
cated the presence of significant amounts of bromide in the
material, which originated from the side product hydrobromic
acid bonded to the formamidine unit to obtain formamidinium
hydrobromide functionalities.
Conclusions
In summary, we have demonstrated the usefulness of a material
composed of iron supported on a polyformamidine ligand in the
iron-catalyzed Friedel–Crafts alkylation. After investigation of
the reaction conditions excellent yields and selectivities were
feasible. Moreover, the catalyst material can be easily separated
from the reaction mixture and can be subjected again to catalysis.
Experimental section
General
1
H and
13
C NMR spectra were recorded with a Bruker Avance III
200 spectrometer (
1
H: 200.13 MHz;
13
C: 50.29 MHz) using the
proton signals of the deuterated solvents as reference.
13
C{1H}
CP/MAS NMR spectra were recorded on a Bruker Avance 400
spectrometer (
13
C: 100.57 MHz) using a 4 mm double resonance
HX MAS probe. The CP spectra were recorded with a cross-
polarization time of 2 ms and composite pulse
1
H decoupling was
applied during the acquisition. IR spectra were recorded either
on a Nicolet Series II Magna-IR-System 750 FTR-IR or on
a Perkin Elmer Spectrum 100 FT-IR. Melting points (mp) were
determined on a BSGT Apotec II capillary-tube apparatus and
are uncorrected. GC-MS measurements were carried out on
a Shimadzu GC-2010 gas chromatograph (30 m Rxi-5ms
column) linked with a Shimadzu GCMA-QP 2010 Plus mass
spectrometer. Elemental analyses were performed on a Perkin-
Elmer Series II CHNS/O Analyzer 2400. Powder X-ray diffrac-
tion (XRD) measurements were recorded on a Bruker D8
Advance with CuK
a
-radiation (l¼0.1546 nm) and scintillator
detector. Nitrogen adsorptions were carried out with an Auto-
sorb-1-C from Quantachrome. Prior to measurement, the
samples were degassed overnight at 100 C. The surface area was
determined using the Brunauer–Emmett–Teller (BET) method.
SEM measurements were performed on a JEOL 7401 F equipped
with an EDX Bruker Quantax XFlash 4010 Detector.
Synthesis of polyformamidine (2)
To a solution of 1,4-benzenediamine (0.43 mol) in DMSO (500
mL) was added triethyl orthoformate (0.87 mol) at room
temperature. The solution was stirred at 140 C for 24 hours.
During that time a precipitate was formed. After cooling to room
temperature the mixture was treated with acetone (500 mL) and
the precipitate was filtered and washed with acetone. The solid
was purified in a Soxhlet extractor using acetone as solvent for 24
hours. The yellow-orange powder was dried in vacuum at 80 C
for 8 hours. Yield ¼82%; mp ¼>300 C;
13
C{1H} CP/MAS
NMR (50 MHz) d¼146.4 (br), 133.1 (br), 140.2 (br), 124.1 (br),
120.4 (br), 117.8 (br), 115.4 (br) ppm; IR (KBr): ~
n¼2917, 2857,
1665, 1637, 1498, 1308, 1205, 982, 820, 526 cm
1
; elemental
Fig. 5 Matrix reloaded—reactivation and re-charge of the PF.
Fig. 6 Mapping experiments with the used catalyst material.
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analysis: anal. calcd for C
7
H
6
N
2
(based on repeating unit) (%): C:
71.17, H: 5.12, N: 23.71. Found: C: 67.21, H: 5.25, N: 22.89.
Synthesis of iron supported on polyformamidine
A suspension of FeCl
3
(Fe@PF1: 0.09 mmol; Fe@PF2: 1.2
mmol) and polyformamidine 5(8.5 mmol) in THF (50 mL) were
refluxed for 24 hours. The volatiles were removed in vacuum and
the residue was purified in a Soxhlet extractor using acetone as
solvent for 24 hours. The red powder was dried in vacuum at 80
C for 8 hours. Fe@PF1: IR (KBr): ~n¼2917 w, 2857 w, 1674 s,
1665 (shoulder of 1674), 1637 s, 1498 s, 1308 m, 1205 m, 982 w,
820 m, 526 m, cm
1
; elemental analysis (%): found: C: 65.98, H:
5.15, N: 22.16. Fe@PF2: IR (KBr): ~
n¼2924 w, 2848 w, 1674 s,
1508 s, 1311 m, 1203 m, 979 w, 823 m, cm
1
; elemental analysis
(%): found: C: 61.59, H: 4.54, N: 17.23.
Catalytic reactions
In a typical reaction, a mixture of 1-bromoadamantane (0.48
mmol), Fe@PF2 (20 mg), an excess of anisole (1.85 mmol) and n-
dodecane (internal standard) was stirred at 100 C for one hour.
The catalyst was removed by filtration via a short plug of silica
gel applying dichloromethane as the eluent. The filtrate was
analyzed by GC-MS and the products were quantified by
comparison with a calibration curve of the authentic compound.
Catalyst recycling experiments were carried out by filtration of
the reaction mixture and subsequent washing of the catalyst three
times with acetone.
Characterization of 5and 6was performed by comparison of
the NMR data with previously reported data.
15,16
Acknowledgements
This work was supported by the Cluster of Excellence ‘‘Unifying
Concepts in Catalysis’’ (sponsored by the Deutsche For-
schungsgemeinschaft and administered by the Technische Uni-
versit€
at Berlin). The authors thank Peter Frenzel and Dr.
Stephan Heitz for technical support.
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