Solid Molecular Frustrated Lewis Pairs in a Polyamine Organic Framework for the Catalytic Metal-free Hydrogenation of Alkenes

Article abstract of DOI:10.1002/cctc.201701783

We report for the first time a metal-free heterogeneously catalyzed hydrogenation using a semi-solid frustrated Lewis pair (FLP). The catalyst consists of a solid polyamine organic framework and molecular tris(pentafluorophenyl)borane (BCF) that form a semi-immobilized FLP in situ in the catalytic hydrogenation of diethyl benzylidenemalonate. ¹¹B NMR spectroscopy proves the successful hydrogen activation by the FLP. Furthermore, the B?N interactions between the polyamine and BCF are investigated by IR and solid state NMR spectroscopy. The FLP 1,4-diazabicyclo[2.2.2]octane (DABCO)/BCF, which combines the features of a FLP and a classical Lewis adduct, functions as molecular reference in both, catalysis and characterization. Furthermore, computational studies enable a better insight into the hydrogen activation through DABCO/BCF and polyamine/BCF.

Full text of DOI:10.1002/cctc.201701783

Accepted Article
Title: Solid Molecular Frustrated Lewis Pairs in a Polyamine Organic
Framework for the Catalytic Metal-free Hydrogenation of Alkenes
Authors: Marcus Rose, Andrea Willms, Hannah Schumacher, Tarnuma
Tabassum, Long Qi, Susannah L. Scott, and Peter J. C.
Hausoul
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Solid Molecular Frustrated Lewis Pairs in a Polyamine Organic
Framework for the Catalytic Metal-free Hydrogenation of Alkenes
Andrea Willms[a], Hannah Schumacher[a], Tarnuma Tabassum[b], Long Qi[b,c], Susannah L. Scott[b,c]
Peter J. C. Hausoul[a] and Marcus Rose*[a,d]
,
Abstract: We report for the first time a metal-free heterogeneously
catalyzed hydrogenation using a semi-solid frustrated Lewis pair
(FLP). The catalyst consists of a solid polyamine organic framework
and molecular tris(pentafluorophenyl)borane (BCF) that form a semi-
immobilized FLP in situ in the catalytic hydrogenation of diethyl
benzylidenemalonate. 11B NMR spectroscopy proves the successful
hydrogen activation by the FLP. Furthermore, the B-N interactions
between the polyamine and BCF are investigated by IR and solid
state NMR spectroscopy. The FLP 1,4-diazabicyclo[2.2.2]octane
(DABCO)/BCF, which combines the features of a FLP and a
classical Lewis adduct, functions as molecular reference in both,
catalysis and characterization. Furthermore, computational studies
examples of FLPs and their application in homogeneous
catalysis have been shown. In 2012, Inés et al. reported a
system of DABCO/B(C6F5)3 (BCF) as FLP which catalyzes the
hydrogenation of electron-poor allenes and alkenes in very good
yields (Scheme 1).[12,13]
In general, due to their convenient processing and
separation from the reaction solution heterogeneous catalysts
are favored for process implementation. Hence, solid FLP
recently emerged as a hot topic in this field. Some examples of
either solid, immobilized or semi-immobilized FLPs have been
published recently. Supported FLPs for heterogeneously
catalyzed hydrogenation reactions are mentioned by Szeto et al.
as well as Xing et al.[4,14] Szeto et al. report a silica-based FLP
for the Z-selective reduction of alkynes. For immobilization they
stirred an ether solution of (4-hydroxyphenyl)biphenyl phosphine
and [(≡SiO)2Al(iBu(Et2O)] at 25°C for 12 h. The resulting silica-
supported surface phosphine is then combined with Piers’
reagent or BCF to give the supported FLP. This FLP was
characterized by DRIFT spectroscopy as well as MAS NMR
spectroscopy. It shows a remarkable activity and selectivity in
the hydrogenation of alkynes.[4] In contrast, Xing et al. describe a
procedure to obtain silica-supported BCF (s-BCF), although
silica-supported BCF was already reported earlier on.[14–16]
However, their procedure differs from the procedures described
before and further they combine this s-BCF with a Lewis base,
tri-tert-butyl phosphine, to yield a solid FLP. In the example
mentioned before they proved the successful heterolytic
cleavage of H2 under mild conditions by the immobilized FLP by
NMR studies.[14] Recently, Trunk et al. reported for the first time
a semi-immobilized FLP in microporous polymer networks. BCF
is immobilized in two different polyphosphines, i.e.,
triarylphosphine polymers.[17] Interactions between P and B and
the “frustrated” character are proven by 31P solid state NMR.
They report a chemical shift in comparison to the pure polymer
due to a “less frustrated” state of the solid and solvent-free FLP.
Furthermore, they proved a successful solid FLP-catalyzed
H2/D2 activation and exchange by 1H NMR experiments.
enable
a better insight into the hydrogen activation through
DABCO/BCF and polyamine/BCF.
Introduction
Hydrogenation reactions represent
a crucial step in the
synthesis of many bulk and fine chemicals.[1] The use of
transition metal catalysts like organometallic Pd and Ni
complexes as well as early transition metals such as Ti and Nb
in hydrogenation reactions is widely known.[2,3] However, even
traces of metal are unfavorable in fine chemicals and
pharmaceuticals.[4] Stephan et al. were the first to report a
reversible metal-free H2 activation using the concept of
frustrated Lewis pairs (FLP) in 2006.[5] FLPs consist of sterically
hindered Lewis acids and bases which do not form adducts but
rather “frustrated complexes”. Due to the unquenched acidity
and basicity they are capable of activating small molecules such
as SO2[6], CO2[7], N2O,[8] H2[5,9,10], in the case of H2 via a
heterolytic cleavage.[3,11] Since the first publication a lot of effort
has been put into the research field of FLPs and numerous
[a]
A. Willms, H. Schumacher, Dr. P. J. C. Hausoul, Prof. Dr. M. Rose
Institut für Technische und Makromolekulare Chemie
RWTH Aachen University
Herein, we report for the first time a semi-solid molecular
FLP consisting of an insoluble polyamine organic framework
exhibiting Lewis base sites in combination with molecular BCF.
This semi-solid FLP successfully activates H2 and enables the
heterogeneously catalyzed hydrogenation of the electron-poor
alkene diethyl benzylidenemalonate as model substrate.
Worringerweg 2, 52074 Aachen (Germany)
T. Tabassum, L. Qi, Prof. Dr. S. L. Scott
Department of Chemistry & Biochemistry
University of California, Santa Barbara, California 93106, USA
L. Qi, Prof. Dr. S. L. Scott
Department of Chemical Engineering
University of California, Santa Barbara, California 93106, USA
Prof. Dr. M. Rose
[b]
[c]
[d]
Ernst-Berl-Institut, Technische Chemie II
Technische Universität Darmstadt
Alarich-Weiss-Straße 8, 64287 Darmstadt (Germany)
* E-mail: rose@tc2.tu-darmstadt.de
Supporting information for this article is given via a link at the end of
the document.
Scheme 1. Catalytic hydrogenation of diethyl benzylidenemalonate.[12,13]
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Results and Discussion
polyamine does not exhibit a permanent porosity in dry state
although it is three-dimensionally crosslinked. Nitrogen
physisorption isotherms exhibit a type II behavior (Figure S1)
and reveal only external surface area with a low specific surface
area of 10 m²g-1. As both monomers exhibit a methylene group
between the functional groups and the aromatic ring,
respectively, the framework is apparently not sufficiently rigid to
exhibit permanent porosity. However, as in the catalytic
experiments toluene is applied as the solvent, the polyamine
swells to a certain extent opening up the pore system and the
nitrogen centers get sufficiently accessible for the molecular
Lewis acid during catalysis to form an in situ FLP. Even small
amounts of toluene result in a slight but observable extension of
the polymer volume due to swelling of the framework structure.
For other solvents this behavior is even more pronounced.
TG/DSC measurements (Figure S3) reveal that the polymer is
thermally stable under the reaction conditions of the catalytic
Materials synthesis and characterization
Our concept of supported FLPs is similar to the approach of
Trunk et al.[17] It is composed of a polymeric organic framework,
which exhibit heteroatoms as connectors, which function as
Lewis acid or base centers, and the corresponding counterpart.
To form a FLP, the polymer requires appropriate electronic and
structural features. On the one hand, the Lewis base sites need
to be accessible for the Lewis acid, but also sterically hindered
enough so that the Lewis acid cannot form the adduct but rather
only a weak “frustrated” interaction. In this work, we obtained a
solid frustrated Lewis Pair, which consists of a polyamine
organic framework, which exhibits tertiary amine groups as the
Lewis
base
centers
in
combination
with
tris(pentafluorophenyl)borane as the Lewis acid part. The novel
polyamine organic framework is synthesized by a N-alkylation of
p-xylylenediamine with 1,4-bis(bromomethyl)benzene inspired
by a synthesis route of Singh et al., who reported aqueous-
mediated N-alkylations of various primary and secondary
amines with halides by using NaHCO3 as a base in aqueous
solution.[18] This procedure was adjusted to our purpose of
obtaining an insoluble, crosslinked polymeric organic framework
by a convenient synthetic approach (Scheme 2).
hydrogenation
of
diethyl
benzylidenemalonate.
The
decomposition of the polyamine starts at around 293 °C in
argon, whereas the catalytic hydrogenation is carried out at
80 °C which is far below the decomposition temperature.
To obtain more information about the interaction of BCF
with the polyamine, the latter was impregnated with BCF and
examined by means of ATR IR spectroscopy (Figure 1). A new
IR band appears at 1276 cm-1 which can be assigned to neither
the pure polyamine nor BCF but rather to the B-N interaction
between the polyamine and BCF, indicating weak LA-LB
interactions as evidenced by comparison with interactions
between DABCO/BCF (Figure 2). To prove this, the literature-
known molecular FLP DABCO/BCF was investigated in
comparison by ATR IR spectroscopy as well as liquid state 11B
NMR studies. DABCO and BCF are considered as a FLP in a
toluene solution being able to activate hydrogen for the catalytic
reduction of electron-poor allenes and alkenes.[12,13] However,
when combining a solution of DABCO in toluene with a BCF
solution at r.t., a white precipitate is obtained which is not
expected for the formation of a molecular FLP. IR investigations
show a new IR absorption band at 1279 cm-1 , which is assigned
to the boron-nitrogen interaction between DABCO and BCF
(Figure 2). The hypothesis that this band indicates a Lewis
adduct formation is underlined by the fact that a precipitate is
formed. Nevertheless, catalytic activity is obtained for both, the
molecular and the solid FLP.
Scheme 2. Synthesis of polyamine organic framework: p-xylylenediamine/1,4-
bis(bromomethyl)benzene/NaHCO3=1/2.2/4.4;
SDS = sodium dodecyl sulfate.
H2O/EtOH/THF
(1/1/1),
The resulting polymer was characterized using CHN elemental
analysis, SEM, EDX, solid state NMR, N2 physisorption,
TG/DSC, XRD and IR measurements and further applied in the
hydrogenation of diethyl benzylidenemalonate in combination
with the Lewis acid BCF. SEM measurements show
polydisperse particles with a flaky structure, which agglomerate
due to electrostatic charge (Figure S2). According to elemental
analysis the content of C, H and N is lower than theoretically
calculated for an ideal structure. This discrepancy results
partially from hydrolyzation as well as unreacted residual
bromine which is also detected by EDX spectroscopy. 13C CP
MAS NMR (Figure S 4) confirms the proposed structure.
However, the small signal at -36.2 ppm indicates residual
bromine at the terminal building blocks of the polyamine. The
signals above 200 ppm and also at 74 ppm and 62 ppm are
spinning sidebands of the main signal at 132.8 ppm and
142.5 ppm. The asymmetry of the spinning sideband at 62 ppm
is due to an overlay with a peak originated from the carbon
atoms directly bonded to the N atoms.
Catalytic performance
The polyamine organic framework in combination with BCF was
applied
in
the
catalytic
hydrogenation
of
diethyl
benzylidenemalonate as model reaction. The procedure and
reaction conditions were based on publications by the Alcarazo
group.[12,13] Therefore, the autoclave with glass inlet was charged
with a magnetic stirrer and evacuated at 120°C in high vacuum
to remove air and moisture. Diethyl benzylidenemalonate, LA
and LB were weighed into a Schlenk tube and dissolved in
toluene inside the glovebox. After transferring the solution into
the autoclave, it was pressurized with H2. The catalytic
experiments were carried out at different temperatures with
varying catalyst amounts and different reaction times (Table 1).
After the reaction, the solid was filtered off and the reaction
solution was analyzed by GC using ethyl heptanoate as an
internal standard to obtain the yield of diethyl benzyl malonate.
For three-dimensionally crosslinked polymers
a high
specific surface area is expected as can be seen for different
examples such as hypercrosslinked polymers[19–21] and
polyphosphines, which are also applied in catalytic
reactions.[22,23] However, the herein synthesized amorphous
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1276 cm-1
1279 cm-1
Polyamine + BCF
BCF + DABCO
DABCO
Polyamine
BCF
BCF
1400
1400
1300
1200
1100
1000
900
1300
1200
1100
1000
900
wavenumber [cm-1]
wavenumber [cm-1]
Figure 2 ATR-IR absorption spectra of BCF in toluene, DABCO in toluene and
BCF and DABCO in toluene at r.t. The dashed line shows the position of the
new absorption band at 1279 cm-1 of the combination of BCF and DABCO in
toluene.
Figure 1. ATR-IR absorption spectra of pure BCF, polyamine and BCF-
impregnated polyamine measured under inert gas atmosphere. The dashed
line shows the position of the new absorption band at 1276 cm-1 of the BCF-
impregnated polyamine due to the weak LA-LB interactions.
For homogeneous systems it is rather challenging to
separate the catalyst from the reaction solution. Because of this,
we replaced one part of the FLP by an appropriate solid, namely
the solid polyamine organic framework. Due to the lack of
porosity, an excess (referred to the nitrogen content) of polymer
is used to ensure the sufficient accessibility of Lewis base
centers. The experimental procedure was modified for the
application of polyamine as the autoclave is charged with the
polyamine first and heated up to 120 °C in high vacuum to
remove the residual moisture and air adsorbed on the polymer.
Subsequently, the FLP was formed in situ in the toluene reaction
mixture splitting hydrogen for the efficient catalytic
hydrogenation of diethyl benzylidenemalonate. Applying
10 mol-% of BCF and 50 mg of polyamine a yield of 100 % was
achieved under the same reaction conditions (500 rpm, 80 °C,
24 h) as used for the DABCO/BCF (10 mol-%) system. Full
conversion was achieved already after six hours (Table 1,
entry 9), rendering the combination of the polyamine with BCF
an even more active catalyst compared to the molecular
counterpart. Hence, the electronic and structural features of the
polyamine are appropriate to form an in situ FLP in combination
with BCF to catalyze the hydrogenation. Furthermore, studies by
liquid phase 11B NMR show proof of the capability of hydrogen
splitting through the polyamine and BCF as the NMR spectrum
In addition to the polyamine/BCF system, also catalytic tests
using only the Lewis acid in combination with DABCO as
molecular base were performed to better compare to the
experiments from earlier reports on that model reaction.[12,13] It
should be noted that only the reduction of the C-C double bond
was observed in our experiments. Applying the pure LA BCF
(10 mol%) in the catalytic reaction already leads to conversion of
diethyl benzylidenemalonate achieving a yield of 61 % after 24 h
of reaction time. This behavior was not described by Nicasio et
al. who reduced electron-poor alkenes and allenes including
diethyl benzylidenemalonate, previously. They describe the
cycloisomerization of an allene catalyzed by solely BCF, but not
the hyrdogenation.[12,13] However, the Lewis acid is not capable
of activating hydrogen on its own as the counterpart to absorb
the proton is missing, which was also proven by 11B NMR
experiments, in which no BCF-hydride could be detected (see
below). It is highly likely that BCF forms a FLP with the substrate
itself which is capable of splitting hydrogen to auto-catalytically
hydrogenate diethyl benzylidenemalonate. A similar behavior is
already reported in literature for BCF in combination with
imines.[24–31] Experimental evidence of the autoinduced catalysis
of imine reduction with BCF was reported by Tussing et al.[30,31]
However, as soon as substrate, H2 and BCF are present during
the reaction, the reduction of diethyl benzylidenemalonate
occurs. As a result, no BCF-hydride is observable in the 11B
NMR spectrum after the reaction as the activated hydrogen
reduces the substrate. However, applying a combination of
LB/LA leads to even higher yields than obtained by the semi-
autocatalytic reduction of the substrate. The combination of BCF
and DABCO (10 mol %) leads to an increase in yield of diethyl
benzylmalonate up to 95 % which is only slightly lower than the
yield reported in literature (98 %) but within the GC error of ~3 %.
This implies that the FLP formed by DABCO and BCF is more
active than the one consisting of the substrate alkene and BCF.
shows
a
characteristic doublet for the BCF-hydride at
around -25 ppm (Figure 3).
As mentioned above, the polymer exhibits a very low
specific surface area in dry state. However, in catalysis it shows
an excellent activity due to swelling in toluene which allows
appropriate accessibility of the borane to the nitrogen centers of
the polymer. Furthermore, the reaction shows no pressure
dependence above 20 bar hydrogen pressure (Table 1).
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catalytic hydrogenation of diethyl benzylidenemalonate.
However, the average yield is around 50 % (Table 2, entry 6)
under these reaction conditions. The yield fluctuates to a certain
degree which is attributed to the sensitivity of the catalytic
system. Even small amounts of moisture lead to deactivation of
the LA due to its high Lewis acidity. Besides, the LA catalyzes
the reaction in combination with the substrate as stated above.
Nonetheless, the yield obtained using a combination of LB and
LA is always higher than when only applying the LA (Table 1,
entries 2, 3, 5-9; Table 2). To lower the moisture sensitivity of
the system in future developments, a feasible method might be
to substitute the BCF with compounds that exhibit a lower Lewis
acidity and, hence, a higher moisture tolerance. In literature,
there are already some examples for moisture-tolerant FLPs, in
which the LA exhibits, e.g., partial chlorine instead of fluorine
substitution.[32,33]
Table 1. Catalytic hydrogenation of diethyl benzylidenemalonate with varying
hydrogen pressure and reaction time using 10 mol% LA and 50 mg
polyamine.
Entry[a]
LA [mol%]
Base
H2 [bar]
t [h]
Yield[d] [%]
1
2
3
4
5
6
7
8
9
-
-
60
60
60
60
60
40
20
40
40
24
24
24
24
24
24
24
18
6
0
10
10
-
-
61
DABCO[b]
Polyamine
Polyamine[c]
Polyamine[c]
Polyamine[c]
Polyamine[c]
Polyamine[c]
95
0
10
10
10
10
10
100
100
100
96
H2 activation
Liquid phase 11B NMR spectra show a new signal at around -3
to -4 ppm (Figure 3) for the combination DABCO/BCF in toluene,
whereas the signal of pure BCF in toluene-d8 is located at
around 60 ppm. A chemical shift of -3 to -4 ppm can be assigned
to signals of either boron complexes, trivalent or tetravalent
boron species and in systems where there is direct bonding of
nitrogen to boron. The distinct shift from 60 ppm to -3/-4 ppm
indicates a major change in the chemical environment of the
boron atom. That implies that DABCO and BCF do not form a
FLP to common comprehension but rather a Lewis adduct with a
more pronounced interaction. This behavior has already been
shown, e.g, for lutidine/BCF by Geier et al.[34] In their study,
lutidine/BCF is explored for H2 activation, despite the fact that a
11B chemical shift is observed at -3.9 ppm for the lutidine/BCF
adduct,[34] which is in the same range as the signal for the
DABCO/BCF adduct. However, they proved H2 activation and
claimed that the adduct and the FLP exist in equilibrium.[34] As
the catalytic reduction of the electron-poor alkene diethyl
benzylidenemalonate is possible using the combination of
DABCO and BCF, a similar behavior regarding the hydrogen
activation is expected.
100
[a] Reaction conditions: 45 mL Hastelloy autoclave, glas inlet, magnetic stirrer,
8 mL toluene solution, 80°C, 500 rpm. [b] 10 mol%. [c] 50 mg. [d] Yield
determined via GC using ethyl heptanoate as standard.
Table 2. Catalytic hydrogenation of diethyl benzylidenemalonate using
5 mol% LA and 25 mg polyamine.
Entry[a]
LA [mol%]
Base
H2 [bar]
60
Yield[d] [%]
1
2
3
4
5
6
7
8
9
10
5
5
DABCO[b]
67
15
11
2
-
60
5
-
40
5
-
20
5
Polyamine[c]
Polyamine[c]
Polyamine[c]
Polyamine[c]
-
60
33
51
24
0
5
40
For experimental proof, the molecular FLP of DABCO and
BCF was used to activate hydrogen and analyzed by means of
11B NMR (Figure 3). The resonance of BCF at 60 ppm as well as
the signal assigned to the DABCO-BCF adduct at -3/-4 ppm was
not visible in the 11B NMR spectrum while a doublet at -24.3 ppm
was observed. This doublet is assigned to the BCF-hydride
which is in accordance with its literature-known chemical shift.[35–
5
20
5[e]
5[e]
-
40
40
0
Polyamine[c]
60
0
37]
Hydrogen activation through BCF and DABCO is not only
[a] Reaction conditions: 45 mL Hastelloy autoclave, glas inlet, magnetic stirrer,
8 mL toluene solution, 80°C, 24 h, 500 rpm. [b] 5 mol%. [c] 25mg. [d] Average
yield determined via GC using ethyl heptanoate as standard. [e] BPh3.
possible at 60 bar H2 and an elevated temperature of 80 °C.
When exposing the DABCO/BCF system in toluene-d8 to a
hydrogen pressure of 0.3 bar (rel.), slightly above atmospheric
pressure, at 80°C, the 11B NMR spectrum shows again the
doublet at -24 ppm indicating the presence of [HB(C6F5)3]- in the
solution. Certainly, the hydrogen splitting under these conditions
is not as efficient as at 60 bar and 80 °C as there is still
precipitated adduct present as proven by the peak at -3/-4 ppm
in the 11B NMR spectrum. A similar behavior is observed when
exposing the DABCO/BCF adduct to 60 bar H2 at 25 °C. Clearly,
hydrogen activation is observable but also there is a distinct
signal that proves the presence of the DABCO/BCF adduct.
When lowering the H2 pressure to 20 bar, still 100 % product is
obtained. Shortening the reaction time shows that the
hydrogenation takes place fast using this catalyst amount. After
6 h and 18 h the yield was 100 % and 96 % (Table 1, entries 7,
8), respectively. The variation in yield is within the GC error
which can be assumed to be ~3% for the applied method.
Reducing the amount of catalyst to half, i.e., 25 mg of
polyamine combined with 5 mol-% of BCF, leads to lower
conversions as expected in most cases. Using fresh BCF and
applying a hydrogen pressure of 40 bar yields up to 86 % in the
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proves the capability of the herein synthesized polyamine
combined with BCF for hydrogen activation.
Solid-state MAS NMR
For a comprehensive investigation of the solid FLP and its
hydrogen activation behavior, solid-state 1H, 11B and 14N MAS
NMR experiments were carried out on the polyamine and the
BCF-loaded polyamine in dry and wetted samples. For the latter,
the materials were wetted with dry and oxygen-free toluene-d8 to
create a paste-like sample.
BCF
The 11B NMR spectra (Figure S 5) of the dry BCF-
impregnated polyamine show no presence of free BCF. Instead,
all signals have shifted to lower fields and we propose that this is
mostly due to interactions somewhere in between the “frustration”
and adduct-formation. The more downfield shifted signals could
also arise from π-π interactions. In the spectra, multiple 11B
species are observable. A greater number of species may
originate from the distribution of different sites, namely due to
interactions of the borane with different (e.g. primary or
secondary) amine species and other species as residual
bromine terminal groups. The signal at approximately 8-14 ppm
may result from the formation of a bromide complex with the
aromatic borane. The rather upfield shifted signals could occur
due to interaction of B with stronger nucleophiles such as the N
sites in the polyamine.
DABCO + BCF, in CDCl3
DABCO + BCF, 60 bar H2, 80 °C
DABCO + BCF, 0.3 bar H2 (rel.), 80 °C
To obtain well-resolved spectra, the materials were wetted
with toluene-d8. Due to the addition of the solvent the polymer
swells, thus rendering higher mobility of the molecular borane
species in the framework pore system and allowing better-
resolved signals. For a more suitable evaluation, the peaks were
assigned by deconvolutions of the spectra of the dry and wetted
material (Figure 4). The sharp signals at -4.9 ppm (dry)
and -4.6 ppm (wetted) can be assigned to direct bonding
between B and N. These chemical shifts are near the chemical
shifts of the DABCO/BCF adduct and the lutidine/BCF.[34]
Therefore, it is likely that the polyamine/BCF and the
DABCO/BCF system both exist in equilibrium as an adduct and
a FLP similar to the scenario reported by Stephan et al.[34] We
propose that the two broad signals in both spectra most likely
arise from subsurface-level incorporation of the borane species
embedded within the polyamine framework. In this regard, the
signals with the most upfield shifts at -8.7 ppm (dry)
and -10.5 ppm (wetted) are probably indicative of borane
species embedded in the polymer framework that are interacting
with amines by a dative bond. A fraction of embedded active
sites is suggested to be less accessible to reactants and
solvents. Upfield chemical shifts have also been attributed to
anions like [HB(C6F5)3]-. However, this is not probable due to the
fact that the sample was not exposed to hydrogen and these
shifts are usually even more upfield-shifted as they normally
appear at around -25 ppm which was also observed for
molecular FLPs (Figure 3) and in previous reports.[35–37]
DABCO + BCF, 60 bar H2, r.t.
Polyamine + BCF, 60 bar H2, 80 °C
Figure 3. Liquid 11B NMR measured in toluene-d8. An autoclave equipped with
magnetic stirrer and glass inlet was charged with Lewis base, Lewis acid and
toluene-d8, pressurized with H2 and left to stir for 24 h. The mixture of DABCO
and BCF was measured in CDCl3 because it dissolves the precipitate better.
The mixture as well as pure BCF were measured after mixing without the
autoclave procedure.
Thus, the DABCO/BCF system is capable of activating hydrogen
at room temperature and also under very low H2 pressures. The
hydrogen activation takes place efficiently under elevated
pressures at elevated temperatures.
Substituting the molecular Lewis base with the solid
polyamine organic framework under the same conditions (60 bar
H2, 80°C, 24 h), 11B NMR investigations of the liquid phase show
only weak signals (Figure 3, bottom) indicating
a low
concentration of the boron species in solution. This implies that
the boron species is adsorbed and immobilized within the pore
system of the organic framework to a large extent, but there is
still a molecular boron species in the liquid phase that would
result in tremendous leaching under continuous processing
conditions. The adsorption of the BCF-hydride is assigned to an
electrostatic interaction between the BCF-hydride anion and the
positively charged polymer due to the heterolytic cleavage of
hydrogen. However, a small doublet at around -25.1 ppm is
observed that indicates the formation of [HB(C6F5)3]-. This
Certainly, to exclude this possibility, the evaluation of the
amount of exchangeable H in primary and secondary amines
would be beneficial. The broad 11B resonances at 12.2 ppm (dry)
and 17.1 ppm (wetted) are also assigned to boranes on
subsurface-level but not with close proximity to the nitrogen sites.
Downfield shifts in 10B NMR spectra have been observed for
sterically hindered, bulky secondary and tertiary amines which
were complexed with boranes. These did not form adducts in
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a) dry
b) wetted
Original
Original
Simulated
Simulated
2.7
17.1
-4.9
-10.5
0.4
12.5
-4.6
-8.7
40.4
150
100
50
0
-50
-100
-150
150
100
50
0
-50
-100
-150
d [ppm]
d [ppm]
Figure 4. Spectral deconvolutions showing 11B chemical shift assignment for the dry (a) and wetted (b) BCF-impregnated polyamine.
contrast to the studied primary amine.[38] We propose that these
signals shed light into the “frustrated interactions” expected for
our system, as also Trunk et al. showed in the case of 31P NMR
spectroscopy for their polyphosphine/BCF FLP.[17] In their study,
they directly used changes in 31P chemical shifts to account for
frustrated type of interactions in their system. Another possibility
is that the chemical shift is indicative for interactions between
the electron-rich aromatic rings of the polyamine and the
electron-poor ones in borane, resulting in π-π stacking. The π-π
stacking has been suggested for such FLPs containing aromatic
rings.[35,39,40] N-benzyl imines and N-phenyl imines complexed
with BCF have shown restricted rotation in the B-N bond due to
sterical hindrance and π-π stacking.[39] Besides, intermolecular
π-π stacking between catalysts containing C6F5 and aromatic
groups has been demonstrated to affect the catalytically active
conformation even in organic solvent and hence, the
enantioselectivity.[40] However, there is no literature example of
π-π stacking with NMR evidence. At 40 ppm in the 11B NMR
spectrum of the wetted BCF-impregnated polyamine there is an
additional small peak. The BCF itself has a chemical shift of
65-59 ppm (in CD2Cl2/C6D6, CD2Cl2, C6D6, CDCl3).[41–44] Because
of this, it is hypothesized that this signal results from the
appearance of a borane species with less interaction with the
polymer when the solvent is added. This would also match with
the fact that the [HB(C6F5)3]- is detectable after H2 activation
using the combination of polyamine and BCF in toluene-d8, as
can be seen in Figure 3. However, it should be noted that in
literature a minor signal at 30-40 ppm has been observed for a
borane-tetrahydroquinoline adduct in its 10B spectrum in addition
to the B-N signal at approximately -4 ppm. This signal has not
been assigned.[44] Table 3 gives an overview of all assigned
signals.
1
The H MAS NMR spectra of the polyamine and the BCF-
impregnated polyamine (Figure 5) both in dry (a) and wetted (b)
state were measured to probe changes in chemical shifts due to
the addition of the BCF. The spectra show broad signals due to
strong dipolar interactions as is often the case in a 3D polymeric
network that contains interacting protons. The most intense
signal of dry polyamine (Figure 5a) is observed at 6.7 ppm and
does not change significantly through the impregnation with BCF.
This signal is assigned to aromatic protons which typically show
chemical shifts at ~ 7 ppm. The signal at 1.3 ppm appears due
to aliphatic protons. However, the latter is well-resolved for the
BCF-impregnated polyamine but not for the polyamine itself. The
spectrum of the wetted polyamine (Figure 5b) also shows
broadening and less-resolved signals. Through impregnation
with BCF the signals become much better resolved, although
also here the chemical shifts do not change. This implies that
the incorporation of the BCF reduces the rigidity of the polymer
network, presumably by decreasing the amount of π-π
interactions between adjacent aromatic pore walls. This effect
appears to be even more pronounced in the wetted material
compared to the dry one.
Table 3. Tentative assignments of the resonances in 11B MAS NMR spectra of
dry and wetted BCF-impregnated polyamine.
Observed δ11B [ppm]
Literature δ11B
Entry
Assignments
[ppm]
Dry
Wetted
-4.6
Computational studies
-3.9[34]
B-N dative bonds, surface-
level
To underline the experimental results, computational
calculations were performed for the molecular DABCO/BCF
system. The calculations were performed for 80 °C and 25 °C,
respectively (Figures S 7-15). In reference to the isolated system
(isolated DABCO and isolated BCF, each in toluene) the Gibbs
free energy for the formation of an adduct species with strong
B-N interaction (to be referred to as adduct) is
ΔGadduct,80 = 7.0 kcal/mol at 80 °C in toluene. The calculation for
the adduct without or with low B-N interaction (to be referred to
as FLP) results in ΔGFLP,80 = 1.7 kcal, meaning that this state is
1
2
3
-4.9
-10.5 (b)
12.2 (b)
-8.7 (b)
17.4 (b)
-
-
B-N dative bonds,
subsurface-level
B-N frustrated
interactions/ π-π stacking,
subsurface-level
4
-
40
65-59
weakly interacting borane
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reduced, meaning that the energetic barrier is lower for both FLP
and adduct, whereas the FLP formation is favored in comparison
to the adduct formation.
a) dry
The calculated IR spectrum of the DABCO/BCF adduct
(Figure S6) with B-N interaction matches the experimental
spectrum of the DABCO/BCF system in toluene indicating the
presence of the adduct although its formation is energetically
less favored. Based on the experimental and computational
evidence the FLP is the favored species, but due to the
precipitation from the solution, the adduct is formed according to
the principle of LeChatelier. However, in solution the FLP is the
dominating species.
*
*
*
*
polyamine
The H2 activation of the DABCO/BCF FLP is energetically
favored in toluene at 80 °C with a calculated Gibbs free energy
of ΔGFLP,H2,80 = -6.3 kcal/mol. For comparison of the molecular
with the polymeric system an extended model molecule is
chosen for the calculations (Figure S12). The FLP (denotation
analogous to DABCO/BCF system) formation has a Gibbs free
energy of ΔGsFLP,80 = 3.8 kcal/mol, meaning that this reaction is
slightly endergonic. However, the energetic barrier is low in
analogy to the results for the DABCO/BCF system. The H2
activation starting from the polyamine/BCF FLP results in a
Gibbs free energy of ΔGsFLP,H2,80 = -2.3 kcal/mol, showing that
the reaction is exergonic and the FLP is capable of
heterolytically cleaving H2. These results are similar for the
polyamine/BCF and the DABCO/BCF system in toluene and
underline the liquid-state 11B NMR experiments in which the
formation of [HB(C6F5)3]- was proven.
*
*
*
*
BCF-impregnated polyamine
60
40
20
0
-20
-40
-60
 [ppm]
a) wetted
Conclusions
polyamine
In conclusion, a novel insoluble, three-dimensionally crosslinked
polyamine organic framework was prepared and successfully
applied
in
the
catalytic
hydrogenation
of
diethyl
benzylidenemalonate in combination with molecular BCF as
strong Lewis acid. It proves that the structural and electronic
features of the polymer are appropriate to form a semi-solid in
situ FLP under the hydrogenation reaction conditions. Hence,
our approach to combine a Lewis basic polymer and its
counterpart for FLP formation is promising to bridge the gap
from molecular to solid FLP catalysis. 11B NMR experiments
proved the capability of heterolytic hydrogen cleavage as well as
the presence of the respective adduct and the FLP. 1H
experiments suggest that the incorporation of BCF increases the
accessibility of the swellable polyamine. Upon impregnation with
BCF the signals become much better-resolved. This implies that
the rigidity of the polymer framework is probably reduced by the
impregnation with BCF in the swollen state. Besides, the BCF-
impregnated polyamine is more accessible and mobile to the
solvents in comparison to the polyamine itself.
Computational studies show a very small energetic barrier
for the FLP formation of the DABCO/BCF system in comparison
to the isolated adduct. For the latter the Gibbs free energy is
higher and hence, its formation thermodynamically less favored.
However, also adduct formation takes place to some extent due
to precipitation. The H2 activation starting from the FLP is
exergonic. This means H2 activation through this system is
possible which supports the experimental data. The
polyamine/BCF FLP shows a similar behavior with a slightly
BCF-impregnated polyamine
60 40 20
0
-20
-40
-60
 [ppm]
Figure 5: 1H MAS of polyamine (blue) and BCF-impregnated polyamine
(black) in dry (a; MAS rate: 8 kHz) and wetted (b; MAS rate: 4 kHz) state. The
spinning sidebands are denoted with asterisks.
apparently thermodynamically favored at 80 °C in comparison to
the adduct. However, due to molecular motion and collisions
interaction, the adduct formation is not completely excluded and
in this case the adduct species precipitates under kinetic control,
which was observed in the experiment. Nevertheless, due to the
lower energetic barrier of the FLP formation, the FLP state of the
system is favored, meaning the H2 activation is feasible based
on the thermodynamic properties of the system.
For the calculations at room temperature the Gibbs free
energy is lower in both cases, namely ΔGFLP,RT = 0.7 kcal for the
FLP and ΔGadduct,RT = 4.5 kcal for the adduct, respectively. This
is expected as with lower temperature the molecule motion is
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better-resolved signals were observed such that changes in chemical
shifts are easier to observe. All 11B spectra were measured by single-
pulse echo MAS at room temperature using 10 μs of pulse length and 1 s
of delay time, at MAS rate of 8 kHz unless otherwise stated. The spectra
were calibrated with cubic boron nitride as an external standard. All 1H
spectra were measured at room temperature using single-pulse MAS
with 10 μs of pulse length and 1 s of delay time, at MAS rate of 8 kHz.
The chemical shifts were calibrated with solid adamantane as external
reference. The wetted samples were spun at a lower rate, 4 kHz, due to
presence of the solvent in the rotor. All spectra are only qualitative for
identification of species rather than for quantification. The number of
scans varies from sample to sample because it was chosen according to
the best signal-to-noise ratio.
endergonic Gibbs free energy for the FLP formation and an
exergonic Gibbs free energy for the H2 activation proving that
hydrogen activation is thermodynamically favored.
Overall, a polyamine was synthesized and characterized
which is capable of H2 cleavage and furthermore of
heterogeneously catalyzing the hydrogenation of the electron-
poor alkene diethyl benzylidenemalonate. The DABCO/BCF
system acts as a molecular model system. The experimental
data is underlined by computational calculations. This proves
that organic framework materials and accessible polymers,
which exhibit molecular Lewis basic or acidic centers, are a
promising class of materials to further develop the
heterogenization of molecular FLP and, thereby, bridge the gap
from homo- to heterogeneous catalysis for these innovative
materials and metal-free catalysts.
Acknowledgements
We gratefully acknowledge financial support by the German
Research Foundation (Grant No. RO 4757/5-1) and the
Excellence Initiative of the German federal and state
governments. We kindly thank Karl-Josef Vaeßen for TG DSC
and XRD measurements, Noah Avraham for CHN analyses,
Elke Biener and Hannelore Eschmann for GC analyses, Dr.
Alexander Schwedt and Claudia Pütz from the Central facility for
Electron Microscopy (RWTH Aachen University) for SEM EDX
measurements. Also, we thank Jens Kothe for DFT calculations.
Financial support of the U.S. Department of Energy, Office of
Science, Division of Basic Energy Sciences, under the Catalysis
Science Initiative (DE-FG-02-03ER15467) is gratefully
acknowledged for funding the solid state NMR measurements
making use of the facilities of the Materials Research Laboratory
(MRL) at UC Santa Barbara, supported by the NSF MRSEC
Program under award DMR 1720256.
Experimental Section
Polyamine preparation
To
a
suspension of p-xylylenediamine, NaHCO3 and SDS in
water/ethanol (v/v=1/1) a solution of 1,4-bis(bromomethyl)benzene in
THF was added dropwise while stirring. The reaction mixture was left to
stir at r.t. overnight. The resulting solid was filtered off and washed
several times with water, methanol, THF and acetone. After that, it is
washed by soxhlett extraction with EtOAc. Finally, it was dried in vacuo
at 60°C. Anal. calcd.(%): C, 84.67; N, 8.23; H, 7.11. Found: C, 70.54; N,
7.42; H, 6.11.
Catalytic hydrogenation
All steps were carried out in Ar inert gas atmosphere using Schlenk-
technique and a Glovebox. Diethyl benzylidenemalonate was dried over
molecular sieve 4 Å and solvents were dried prior to use over
sodium/benzophenone and stored in a Labstar Glovebox by mBraun. For
the hydrogenation reaction MRS-5000 autoclaves (45 mL) by Parr
equipped with a glass inlet and a magnetic stirrer were charged with the
prepared polyamine and evacuated at 120°C using high vacuum.
Tris(pentafluorophenyl)borane and diethyl benzylidenemalonate were
dissolved in toluene and added to the prepared polymer in the autoclave
with a cannula. The autoclave was pressurized with H2 and the reaction
was carried out at 80°C with various reaction times. Yields were
determined by GC analytics using ethyl heptanoate as standard.
Keywords: Polyamine organic framework • Heterogeneous
Catalysis • Frustrated Lewis Pairs • Hydrogenation • Alkenes
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Entry for the Table of Contents (Please choose one layout)
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Frustrated frameworks: A solid
molecular frustrated Lewis pair was
synthesized from a novel polyamine
organic framework and a borane. It
performs excellent in the metal-free
heterogeneously catalysed
Andrea Willms, Hannah Schumacher,
Peter J. C. Hausoul, Tarnuma
Tabassum, Long Qi, Susannah L. Scott,
and Marcus Rose*
Page No. – Page No.
hydrogenation of alkenes. The
frustration and hydrogen activation
were proven spectroscopically.
Solid Molecular Frustrated Lewis
Pairs in a Polyamine Organic
Framework for the Catalytic Metal-
free Hydrogenation of Alkenes
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