A remarkably active iron catecholate catalyst immobilized in a porous organic Polymer

Article abstract of DOI:10.1021/cs400043a

A single-site, iron catecholate-containing porous organic polymer was prepared and utilized as a stable and remarkably active catalyst for the hydrosilylation of ketones and aldehydes. In some instances, catalyst loadings of 0.043-2.1 mol % [Fe] were sufficient for complete hydrosilylation of aldehydes and ketones within 15 min at room temperature. The catalyst can be recycled at least three times without a drop in catalytic activity. This system is an example of an immobilized homogeneous catalyst with no homogeneous analogue.

Full text of DOI:10.1021/cs400043a

Research Article
pubs.acs.org/acscatalysis
A Remarkably Active Iron Catecholate Catalyst Immobilized in a
Porous Organic Polymer
Steven J. Kraft,† Raul Hernandez Sanchez,‡ and Adam S. Hock*,†,§
́
́
́
†Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States
‡Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02139, United States
§Department of Biological and Chemical Sciences, Illinois Institute of Technology, Chicago, Illinois 60616, United States
S
* Supporting Information
ABSTRACT: A single-site, iron catecholate-containing po-
rous organic polymer was prepared and utilized as a stable and
remarkably active catalyst for the hydrosilylation of ketones
and aldehydes. In some instances, catalyst loadings of 0.043−
2.1 mol % [Fe] were sufficient for complete hydrosilylation of
aldehydes and ketones within 15 min at room temperature.
The catalyst can be recycled at least three times without a drop
in catalytic activity. This system is an example of an
immobilized homogeneous catalyst with no homogeneous
analogue.
KEYWORDS: catalysis, hydrosilylation, iron catalysis, porous organic polymer, catechol
ligation is of interest as the redox noninnocent capabilities of
catecholate iron complexes have made the isolation of
complexes that do not participate in either intra/extradiol
cleavage catalysis5a,7a,b,e or metal transport in natural and
biomimetic siderophores difficult.8
INTRODUCTION
■
Metal containing porous organic polymers (POPs)1 have
gained recent interest for applications in catalysis,1b,2 gas
storage,3 and gas separations2d,4 because of their high surface
area three-dimensional networks related to metal organic
frameworks (MOFs) and zeolites. Metalated POPs have several
potential advantages over MOFs and zeolites because of their
greater chemical and thermal stability than MOFs combined
with a more tunable ligand binding environment than zeolites.
Additionally, the potential for rational synthetic variation1b of
POP-based ligand binding sites presents an opportunity to
prepare stable and tunable catalysts that are not accessible to
traditional homogeneous systems. We hypothesized that unique
coordination environment of the POP structure will stabilize
catalysts capable of reactivity patterns that are not possible in
homogeneous, supramolecular, or zeolitic environments.1b
For example, typical homogeneous metal catecholate
complexes frequently form bis- or tris-chelated complexes to
stabilize the metal center. Such complexes lack reactive sites
and are often catalytically inactive or unstable to ligand loss or
other decomposition routes.5 The recent report of a catechol
containing POP, POP A2B1, represents the opportunity for the
isolation of a single catechol bound to a metal.6 POP A2B1 has
been shown to coordinate divalent Mn and Mg complexes
made from Mn(OAc)2·4H2O and Me2Mg as neutral species
without reducing pore volume below a size compatible with
many organic reagents.6 The physical separation of catechol
groups in a porous network combined with the rigid support
offered by POP A2B1 offers a unique opportunity to attempt
catalysis with low-coordinate, divalent metal centers in the
absence of bulky, stabilizing ligands.7 Single catechol site
RESULTS AND DISCUSSION
■
The convenient Fe (II) source, Fe[N(SiMe3)3]2,9 rapidly reacts
with proton sources, and the addition of 1 equiv of
Fe[N(SiMe3)3]2 to a suspension of POP A2B1 in diethyl
ether resulted in complete uptake of the iron complex within 24
h. Attempts at generating a single site catechol containing
complex with addition of 1 equiv of 3,6-di-tert-butylcatechol to
Fe[N(SiMe3)3]2 results in complete consumption of the latter
concomitant with the precipitation of insoluble and intractable
products (Scheme 1).
ATR-FT-IR spectroscopy (Supporting Information, Figure
S1) confirms full metalation of all catechol sites as OH
stretches at 3542 cm−1 of POP A2B1 are no longer visible.
Subsequent washing with diethyl ether afforded Et2OFe(CAT-
POP) without any leeching of unreacted Fe[N(SiMe3)3]2.
1
Analysis of the washings by H NMR spectroscopy reveals
∼85% of the expected HN(SiMe3)2 upon Fe ligation. Upon
further washing with diethyl ether, 13% of the remaining 15%
of HN(SiMe3)2 bound within the material is released. The
remaining 2% of expected amine remains bound in an unknown
fashion within the material and is reflected in the slight
Received: January 21, 2013
Revised: March 5, 2013
© XXXX American Chemical Society
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splitting shifts to 0.69 and 1.15 mm/s. No significant change in
isomer shifts between the inert and the air exposed samples
indicates that oxidation may not occur at the iron center, but
rather within the catechol POP framework itself. This is also
consistent with changes in the quadrupole splitting as
coordination environment around the iron center would result
from catechol oxidation to semiquinone ligands. The deep
color change from brown to black for the air-free POP to O2-
exposed POP without any change in the formal oxidation state
of the iron center is characteristic of oxidized redox-active
semiquinone metal complexes previously reported and
corroborates these results.10 Regardless of the Fe or catechol
oxidation state, the air-exposed Et2OFe(CAT-POP) is inactive
for hydrosilylation catalysis under conditions where the initial
material is operative.
Scheme 1. (Top) Preparation of Porous Organic
Catecholate Ligand CAT-POP A2B1,6 and (Bottom)
Preparation of Et2OFe(CAT-POP)
In the absence of O2 Et2OFe(CAT-POP) is stable whether
suspended in solution or stored as a solid at room temperature
for longer than one week. Solid samples can be heated to 175
°C in vacuo and also retain identical catalytic properties to
unheated material. In contrast, our Fe(II) source Fe[N-
(SiMe3)3]2 is a catalyst for the hydrosilylation of ketones and
aldehydes,11 but it is unstable in solution at room temperature
for extended periods resulting in precipitation of insoluble iron-
containing material.11
Et2OFe(CAT-POP) was screened as a catalyst for hydro-
silylation reactions, important for fine chemical synthesis and
industrially relevant silicon containing materials.12 Rapid
conversion of aldehydes and ketones to the corresponding
silyl ether was observed with catalyst loadings of 2.1 mol %
[Fe] in C6D6 when monitored via H NMR spectroscopy
(Scheme 2). The results for a variety of substrates are collected
in Table 1.
percentage of N found in elemental analysis (1.09 wt %).
Importantly, the addition of excess Fe[N(SiMe3)3]2 to
Et2OFe(CAT-POP) does not result in any appreciable weight
change after washing the solid to remove entrained iron
precursor, corroborating full metalation of catechol binding
sites.
1
Et2OFe(CAT-POP) is extremely O2 sensitive and turns from
its brown color to a deep black upon contact with air. Such
sensitivity might be expected as iron intra/extradiol cleavage
catalysts operate via oxidation in the presence of O2 to cleave
the catechol moiety.5a,7a,b,e Although this black product has not
been successfully characterized, Brunauer−Emmett−Teller
(BET) surface area measurements of as-prepared Et2OFe-
(CAT-POP) (855 m2/g, and pore width distribution ∼12 Å as
the dominant pore size) (Supporting Information, Figure S2)
do not significantly differ from the black product. This indicates
that oxygen exposure does not significantly damage the POP
framework. For comparison, the free POP ligand has a
measured surface area of 1050 m2/g, and the previously
reported Mn and Mg complexes display a similar decrease in
surface area upon metalation (BET surface areas of 600 and
610 m2/g respectively).6
Scheme 2. Catalytic Hydrosilylation by Et2OFe(CAT-POP)
Within 20 min benzaldehyde was fully converted to a mixture
of PhCH2OSiHPh2 and (PhCH2O)2SiPh2 (28:72) upon
reaction with 1 equiv of diphenylsilane (Table 1, entry 1).
The full catalytic potential was observed upon addition of
phenylsilane to a mixture of Et2OFe(CAT-POP) and
benzaldehyde in benzene-d6, resulting in a sufficiently
exothermic reaction to boil the mixture within 10 s (Table 1,
entry 2). Within 2 min the reaction mixture cooled, and
analysis by 1H NMR spectroscopy revealed complete
conversion to (PhCH2O)2SiHPh and (PhCH2O)3SiPh
(48:52). Despite the thermal output of this reaction, Et2OFe-
(CAT-POP) retains its catalytic ability, and further addition of
benzaldehyde to these mixtures resulted in full conversion of
the silane to the highest order alkoxysilane products. Decreased
catalyst loadings to as low as 0.043% with an average turnover
frequency (TOF) of 1.11 s−1 is possible. Complete conversion
of benzaldehyde with Et2OFe(CAT-POP) and phenylsilane
was achieved rapidly for catalyst loadings of 0.43 and 0.043 mol
% (Table 1, entries 3 and 4). Both these reactions are
noticeably exothermic and are complete upon cooling after 10
and 15 min respectively yielding (PhCH2O)2SiHPh and
(PhCH2O)3SiPh (75:25 and 80:20 for 0.43 and 0.043 mol %
respectively). The TOF for the hydrosilylation of aldehydes is
remarkable for iron-based catalysts, and is considerably more
To gain insight into the electronic and coordination
environment at the Fe centers, Mossbauer spectra were
̈
obtained at 90 K for both the air free and the O2 exposed
Et2OFe(CAT-POP) materials (Supporting Information, Figure
S3). The spectrum of Et2OFe(CAT-POP) was fitted to two
symmetric quadrupole doublets with isomer shifts of 0.32 and
0.34 mm/s and quadrupole splittings of 1.47 and 0.89 mm/s
respectively. Although two different iron environments could
be fit, the small difference in isomer shift indicates their
oxidation state and coordination environment are practically
the same. Thus, while there is a degree of irregularity in the
POP polymer structure, the local environment of the Fe sites is
virtually identical according to the Mossbauer spectrum.
̈
Exposure of the material to air results in a very small change
in the isomer shifts to 0.35 mm/s, but significant quadrupole
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a
Table 1. Hydrosilylation of Aldehydes and Ketones with Et2OFe(CAT-POP)
a
b
1
Reaction conditions: substrate (0.356−3.56 mmol), silane (1 equiv), 23 °C, C6D6. Reaction times were determined via H NMR spectroscopy
c
d
after complete consumption of substrate. Product distribution listed in order of products column. All three stereoisomers were observed.
active than our precursor, Fe[N(SiMe3)2]2, which exhibits a
TOF of ∼0.007 s−1 at similar reaction conditions.11 Et2OFe-
(CAT-POP) can be recovered as a solid, and qualitatively
identical catalytic reactivity was observed upon catalyst
recycling by filtration, drying, and reuse (3 times). Also, the
notable stability of Et2OFe(CAT-POP) can be demonstrated
by heating solid samples to 175 °C in the absence of O2 or
storing suspensions for 1 week with no loss of catalytic activity.
Furthermore, the catalyst can be separated from the reaction
mixture, and the filtrate is not active for hydrosilylation.
Although no molecular analogue to Et2OFe(CAT-POP) is
available we treated Fe[N(SiMe3)2]2 with 3,6-di-tert-butyl
catechol in C6D6, diethyl ether, or toluene, and the resulting
separable but intractable mixture of brown solid (36%) and
brown solution (64%) was tested for catalytic hydrosilylation.
Addition of a mixture of benzaldehyde and phenylsilane to the
two materials provided no hydrosilylation products after 30
min. However, full conversion of benzaldehyde to
(PhCH2O)2SiHPh and (PhCH2O)3SiPh was observed after a
24 h period for both materials at room temperature. Unlike
Et2OFe(CAT-POP), however, these mixtures are no longer
active for catalysis after isolation and addition of fresh substrate,
having transformed further to catalytically inactive material.
The slow reaction time of these mixtures and their inability to
be recycled validates the catalytic role of Et2OFe(CAT-POP),
which is far faster kinetically and stable to decomposition under
reaction conditions. Such a rapid hydrosilylation of benzalde-
hyde is atypical for iron-based hydrosilylation catalysts, which
generally catalyze aldehydes with modest rates at ambient
temperature11 and require higher catalyst loading (5 mol %) to
achieve similar speeds13 or elevated temperatures.14
furaldehyde to furfuryl alcohol is an important industrial
process as it is used to produce resins, rubbers, and alternative
chemical feedstocks.16 Hydrosilylation is a model reaction for
selective reduction to the alcohol as direct hydrogenation can
result in coking or reduction of the olefin to furan derivatives.17
Alternatively, the hydrosilylation product can be hydrolyzed
easily to form the desired alcohol.15a 2-Furaldehyde was treated
with Et2OFe(CAT-POP) and phenylsilane; an exothermic
reaction resulting in full conversion to [(C4H3O)CH2O]3SiPh
was again observed within 2 min (Table 1, entry 5). A decrease
in catalyst loading to 0.43 mol % yielded complete conversion
within 240 min generating [(C4H3O)CH2O]3SiPh and
[(C4H3O)CH2O]2SiHPh in a ratio 73:27 (Table 1, entry 6).
However, when the catalyst loading is decreased to 0.043 mol
% the reaction does not progress, most likely because of
commercial impurities in 2-furaldehyde (Table 1, entry 7). This
activity for 2-furaldehyde is considerably higher than typical
Ni18 and Fe19 catalysts for this process that exhibit slower
TOFs. Moreover, this activity is on the order of the capability
of recently reported lanthanide hydride complexes which are
some of the fastest reported for this reaction.15a
Finally, we studied more sterically encumbered ketone
substrates. Acetophenone reacts with diphenylsilane (average
TOF ∼ 0.04 s−1) at a comparable rate to that of benzaldehyde
(average TOF ∼ 0.04 s−1) with diphenylsilane and generates a
sole product, PhMeCHOSiHPh2 (Table 1, entry 8). The
formation of a sole product is consistent with that of
Fe[N(SiMe3)3]2; however, a much faster reaction occurs with
Et2OFe(CAT-POP) than with this iron precursor (average TOF
∼0.015 s−1).11 In contrast, phenylsilane generates the purely
bis-1,2-addition products of (PhMeCHO)2SiHPh and its
diastereomers when reacted with acetophenone which differs
from the 1:6.5 ratio of PhMeCHOSiH2Ph to (PhMe-
CHO)2SiHPh that is observed with Fe[N(SiMe3)3]2 (Table
1, entry 9).11 Ketone hydrosilylation is exothermic like the
We also pursued the less common hydrosilylation substrate,
2-furaldehyde. It is produced by the treatment of pentoses
derived from hemicellulose biomass, which itself is an
undesirable fuel source.15 However, selective reduction of 2-
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corresponding reaction of benzaldehyde and was complete
within 2 min. Unlike hydrosilylation of benzaldehyde and 2-
furaldehyde, the addition of a subsequent equivalent of
acetophenone in either reaction does not result in complete
consumption of Si−H, and is most likely due to the large steric
bulk imparted by the methyl groups in the resulting
alkoxysilanes.
of reaction. Notably, material used in hydrosilylation reactions
(any listed in Table 1) can be reused to perform the
isomerization of 1-hexene and vice versa without activity loss
as circumstantial support for a common catalytic intermediate.
Although recyclability is commonplace among heterogeneous
catalysts, the observed stability of our low-coordinate single site
metal catecholate is extremely rare, and such results reported
herein highlight their stability imparted by unique porous
environments within a POP framework.
A further increase in substrate size to benzophenone resulted
in longer reaction times (1.5 h) with diphenylsilane generating
Ph2CHOSiHPh2 and (Ph2CHO)2SiPh2 (88:12) (Table 1, entry
10). However, decreasing the silane size by using smaller
phenylsilane resulted in complete conversion of benzophenone
within 5 min, producing (Ph2CHO)2SiHPh. The rate of this
reaction is surprising (average TOF ∼ 0.009 s−1; Table 1, entry
11) since this is typically a slow reaction.20 Hydrosilylation of
cyclohexanone with diphenylsilane shows a similar trend, with
longer reaction times (1.5 h) needed with bulkier diphenylsi-
lane leading to (Cyclohexyl)OSiHPh2 and [(Cyclohexyl)-
O]2SiPh2 (94:6) ((Table 1, entry 12), but phenylsilane was
complete within 2 min producing [(Cyclohexyl)O]2SiHPh
(Table 1, entry 13). Although few homogeneous iron
hydrosilylation catalysts perform ketone hydrosilylation with
similar catalyst loadings within 1−3 h at room temperature,11,21
many require elevated temperatures and/or much longer
reaction times than the observed reactivity with this
system.13,14,22 We speculate that the slower qualitative rates
of bulky substrates are consistent with the catalysis occurring
within the pore structure of the POP, rather than exclusively on
the surface; however, more detailed experiments are required to
explicitly address the impact of mass-transport effects and count
active catalytic sites. In addition, the exothermicity of
hydrosilylation severely complicates quantitative kinetic experi-
ments.
CONCLUSION
■
Initial studies have proven site-isolated iron catecholates in
Et2OFe(CAT-POP) are stable, extremely active catalysts for
the hydrosilylation of carbonyl groups. Hydrosilylation of
aldehydes and ketones with phenylsilane are rapid with this
system resulting in exothermic reactions completed within
minutes with obtainable catalyst loadings of as low as 0.043 mol
% [Fe] and an average TOF of 1.11 s−1. Although
hydrosilylation of these substrates with diphenylsilane are
relatively slower, full conversion of substrates are achieved with
catalyst loadings of 2.1 mol % [Fe] on the order of 20 to 90
min. The catalyst is reusable, recyclable for at least three
reaction cycles, and thermally robust allowing catalytic activity
after treatment to 175 °C. Currently, the synthesis of other
catalytic POP materials, exploration of Et2OFe(CAT-POP) as
a catalyst for other transformations, and elucidation of the
origin of stability are all ongoing.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental conditions and synthetic methods for this Article.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Thus, we explored the ability of Et2OFe(CAT-POP) to
hydrosilylate less polar bonds. The hydrosilylation of phenyl-
acetylene was attempted with diphenyl-, phenyl-, and
triisopropyl- silanes. Mixtures of silane, Et2OFe(CAT-POP),
and phenylacetylene resulted in the formation of an
unidentified polymer most likely assigned as polyphenylacety-
lene. Since a relatively small amount of the polyacetylene could
potentially block the POP pores, we also screened diphenyla-
cetylene and benzonitrile as substrates, but no reaction
occurred after 24 h. Attempts to hydrosilylate 1-hexene were
also unsuccessful, but upon stirring for 3 h 2-hexene was
observed. Stirring the suspension of catalyst with silanes for 48
h yields the complete isomerization of 1-hexene to the
thermodynamic equilibrium ratio of 2-hexene and 3-hexene
(2:1) (Scheme 3) at far slower rates than Fe(CO)5.23 The slow
AUTHOR INFORMATION
Corresponding Author
*E-mail: ahock@iit.edu.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge funding by the U.S. Department of
Energy, under Contract No. DE-AC02-06CH11357. R.H.S.
acknowledges CONACYT (Consejo Nacional de Ciencia y
́
́ ́
Tecnologia) and Fundacion Mexico for doctoral fellowship. We
would like to thank Ted Betley, SonBinh Nguyen, and Marc
Johnson for helpful discussions.
Scheme 3. Isomerization Reactions of 1-Hexene
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