Room-Temperature Activation of Hydrogen by Semi-immobilized Frustrated Lewis Pairs in Microporous Polymer Networks

Article abstract of DOI:10.1021/jacs.6b13147

Porous polymer networks based on sterically encumbered triphenylphosphine motifs, mimicking the basic sites employed in frustrated Lewis pair (FLP) chemistry, were synthesized via Yamamoto polymerization and their interactions with the strong Lewis acid B(C₆F₅)₃ probed. The combinations yield semi-immobilized FLPs, which are able to cleave dihydrogen heterolytically at ambient temperature and low hydrogen pressure.

Full text of DOI:10.1021/jacs.6b13147

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Communication
Room-Temperature Activation of Hydrogen by Semi-Immobilized
Frustrated Lewis Pairs in Microporous Polymer Networks
Matthias Trunk, Johannes F Teichert, and Arne Thomas
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b13147 • Publication Date (Web): 01 Mar 2017
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Room-Temperature Activation of Hydrogen by Semi-Immobilized
Frustrated Lewis Pairs in Microporous Polymer Networks
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Matthias Trunk,1 Johannes F. Teichert,2 Arne Thomas1,*
1Technische Universität Berlin, Department of Chemistry, Functional Materials, Hardenbergstr. 40, 10623 Berlin
2Technische Universität Berlin, Department of Chemistry, Organic Chemistry / Sustainable Synthetic Methods, Straße des
17. Juni 115, 10623 Berlin
Supporting Information Placeholder
Such types of functional groups can be found as catalytic sites in
frustrated Lewis pairs (FLPs).7 The members of this emerging
class of molecular catalysts are solely based on main group ele-
ments and in most cases consist of a boron-based Lewis acid (LA)
and a Lewis base (LB), usually comprising phosphorus or nitro-
gen as basic sites.8 Both active sites are sterically encumbered by
bulky moieties in order to prevent them from forming a strong
covalent bond and consequently quench their opposing reactivi-
ties. Small molecules (H2, CO2 etc.) fitting in between these bulky
groups can be activated through double polarization by the acidic
and basic sites, leading to (zwitter)ionic products (see Scheme
1).6,7,9 For this activation to take place, the steric hindrance must
not be sufficient to prohibit any interaction; in solution a weak but
reversible coordination must be possible lest the polarization be
precluded entirely.10 Subsequently, the activated intermediate(s)
can be used for reactions with a number of substrates. The most
intensely researched application for this type of catalyst is the
heterolytic splitting of dihydrogen and its subsequent use in the
reduction of C=C11, C=N12, and C=O13 double bonds as well as
cleavage of silylenolethers14, among others. Lewis acid and base
can be part of the same molecule, i.e. both intra- and intermolecu-
lar FLPs are capable of activating small molecules.9 In many
cases though the mechanistic properties of an activation process
are deceptively simple. Even in cases of both active sites within
the same molecule theoretical studies rather suggest intermolecu-
lar activation via a twofold head-to-tail interaction between two
intramolecular FLP molecules.15
ABSTRACT: Porous polymer networks based on sterically
encumbered triphenylphosphine motifs, mimicking the
basic sites employed in frustrated Lewis pair chemistry,
were synthesized via Yamamoto polymerization and their
interactions with the strong Lewis acid B(C6F5)3 probed.
The combinations yield semi-immobilized frustrated Lewis
pairs, one of which is able to cleave dihydrogen heterolyti-
cally at ambient temperature and low hydrogen pressure.
Over the last decade, microporous polymer networks have gar-
nered attention as materials for a wide variety of applications.1,2
Emanating from rigid organic building blocks, materials with very
small pores between 0.5 and 2 nm, accessible surface areas of up
to 6400 m2g-1 as well as high thermal and chemical stability3 have
been synthesized by a number of polymerization methods such as
metal-mediated homocoupling reactions, metal-catalyzed cross-
coupling reactions, condensation reactions, and many others.2,4 In
contrast to MOFs and COFs these materials do not exhibit crystal-
linity but, emerging from C–C bond forming reactions, they can
forgo the use of heteroatoms entirely, allowing for a pure hydro-
carbon skeleton. This feature is especially advantageous when
designing catalysts with highly sensitive active sites as it enables
the exclusion of non-orthogonal functional groups.
Whereas a great number of Lewis basic polymers has been pub-
lished, since these are easy to obtain and often display interesting
CO2 adsorption properties, only a handful of Lewis acidic moie-
ties into porous polymers have been reported16,17, none of which
are used for catalytic applications. Due to the use of very strong
Lewis acids such as tris(pentafluorophenyl)borane (commonly
abbreviated to BCF) or related compounds, these catalysts are
often highly sensitive to external factors such as humidity or
oxygen, which makes their handling challenging. To overcome
this issue, extensive studies towards electronic fine-tuning have
produced robust Lewis acids which can be handled without the
use of inert gas techniques.18 Recent advances have furthermore
afforded so-called “inverse” FLPs based on a weak and more
robust Lewis acid but which are used in conjunction with a very
strong and sensitive Lewis base on the other hand.19
In order to be able to enhance stability and recyclability and to
further increase the appeal of FLPs, the incorporation of frustrated
Lewis pairs into solid materials, e.g. a microporous polymer
network, seems an advantageous endeavor. In this work we fo-
cused on the generation of frustrated Lewis pairs emerging from
Scheme 1: Equilibrium state of a generic frustrated Lewis pair
(LA: Lewis acid, LB: Lewis base) and activation of carbon diox-
ide and hydrogen (top); (non-)reversible activation of hydrogen
by reported FLPs5,6 (bottom)
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porous polymers comprising sterically encumbered phosphine
Organic solvents such as toluene, bromobenzene, dichloro-
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2
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7
8
moieties, which are impregnated with a readily available strong
Lewis acid. We demonstrate the ability of these semi-immobilized
FLP systems to activate dihydrogen at ambient temperature and
low hydrogen pressure.
methane and THF are readily absorbed by the polymers, which
exhibit strong swelling properties and form gel-like mixtures with
these solvents. To probe the properties of the synthesized materi-
als as Lewis bases, their behavior towards Lewis acids such as
commercially available tris(pentafluorophenyl)borane (commonly
abbreviated as BCF) has to be examined. When a substance is
pre-dissolved in the absorbed solvent it can be introduced into the
pores of the polymer. Choosing a volatile solvent facilitates dry-
ing of the obtained material. BCF is well soluble in aromatic and
chlorinated solvents. To gain information on the interaction be-
tween BCF as a Lewis acid and the bulky Lewis base, no other
Lewis acidic or basic functions may be present. Therefore, inside
the glovebox, BCF and porous phosphine polymer were suspend-
ed in DCM, which is volatile, non-coordinating, neither acidic nor
basic, readily dissolves BCF, and leads to rapid swelling of the
polymer. When impregnated with BCF solutions in non-
coordinating solvents, e.g. DCM, the Lewis acid is readily ab-
sorbed into the polymer. The impregnation of the yellow polymer
network P2 was accompanied by darkening of the color while
colorless P1 remained colorless. Washing with coordinating
solvents such as diethyl ether or THF restores the original net-
work. Even weakly coordinating solvents such as methanol or
acetone are coordinated preferably over the bulky phosphine
moieties.
The syntheses of the phosphine polymers were carried out accord-
ing to the synthesis procedure for porous polymer networks via
Yamamoto
polymerization20
starting
from
tris(p-
bromophenyl)phosphine derivatives (see Scheme 2) with increas-
ing steric demand around the phosphorus atoms. The structure of
unmethylated triphenylphosphine polymer has been described
previously.21,22 Due to the use of less reactive aryl chloride start-
ing material harsher synthesis conditions were required, whereas
in this work a milder synthesis protocol23 was applied due to the
exclusive use of the more reactive brominated species.
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Both polymers (see Figure 2) as well as their composites with
BCF exhibit strong fluorescence under UV irradiation (bottom
row) at 254 (weak) and 366 nm (strong). BCF is readily absorbed
by the polymer and quenches the fluorescence (see supporting
information) especially for polymer P1 pointing to a stronger
interaction with the Lewis acid. (Figure S1) Furthermore, after
addition of BCF the finely dispersed polymer particles contract
and sediment within a matter of minutes whereas the empty pol-
ymer particles tend to collect at the surface of DCM.
Scheme 2: Syntheses of triarylphosphine polymers; a) i-PrMgCl,
THF, 0 °C, 1 h, then PCl3, THF, -78 °C to r.t., 12 h; b) Ni(COD)2,
DMF/THF, r.t., 15 h c) n-BuLi, Et2O, -78 °C, 2 h, then PCl3,
Et2O, -78 °C to r.t., 12 h d) Ni(COD)2, DMF/THF, r.t., 22 h
P1
P2
without BCF
with BCF
without BCF
with BCF
Purification via Soxhlet extraction with methanol and drying in
vacuo at 80 °C yields the product networks (Figure S1). Tris(2-
methylphenyl)phosphine polymer P1 was collected as white and
light solid whereas tris(2,6-dimethylphenyl)phosphine polymer
P2 has an intense yellow color and fluffy texture. P1 and P2
feature surface areas of 1045 and 947 m2 g-1, respectively, with P2
featuring a remarkable hysteresis (see Figure 1). These results are
in line with the reported surface area of unmethylated tri-
phenylphosphine polymer of 1284 m2 g-1.21 The decrease in sur-
face area can thus be interpreted as a result of the increased mon-
omer weight and pore blocking by increasing amounts of methyl
groups.
600
400
200
Figure 2: P1 (left block) and P2 (right block) in DCM suspension
before and after addition of BCF. Top row: UV lamp off, bottom
row: under irradiation of UV light (366 nm)
P1
P2
0
Solid state magic angle spinning NMR spectroscopy of the dried
solids confirms the assumed structures (see Figure 3 and Fig-
ure S 2 in the supporting information). The 31P NMR spectra each
exhibit one main peak in the region of -34 and -36 ppm alongside
a small impurity at approximately +32 ppm, indicative of small
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure
Figure 1: Nitrogen sorption isotherms of P1 (blue) and P2 (red)
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amounts of phosphine oxide present in each network. These traces
lecular counterpart of P2, tris(2,6-dimethylphenyl)phosphine, and
of phosphine oxide have been reported previously21 and seem to
be the result of a side reaction of the Yamamoto polymerization
since the polymers were synthesized under rigorously inert condi-
tions. Additionally, P1 and P2 are prone to phosphine oxide for-
mation in air at elevated temperatures over elongated periods of
time and should therefore be stored under inert gas.
BCF to the solid state NMR spectrum of the phosphine molecule
on its own. As expected, in the “dry” FLP, the phosphorus signal
is shifted downfield by ca. 10 ppm compared to the phosphine
(see Figure S 3 in the supporting information).
1
2
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According to the molecular compounds5,6, the combinations
P1/BCF and P2/BCF should form frustrated Lewis pairs and be
active towards the cleavage of dihydrogen. When used in combi-
nation with BCF, the molecular equivalent of P1, tris(o-
tolyl)phosphine has been reported to activate hydrogen reversibly
at ambient conditions to form [(o-C6H4Me)3PH][HB(C6F5)3]
whereas the molecular counterpart of P2, trimesitylphosphine,
forms the hydrogen adduct [(1,3,5-C6H2Me3)3PH][HB(C6F5)3]
which remains stable even when exposed to temperatures of up to
150 °C. To prove the hydrogen activation capability of our poly-
meric materials, we performed H/D isotope scrambling experi-
ments typically employed to demonstrate FLP behavior in a new
material24 for composites of BCF with P1, P2, and their molecular
Upon impregnation with BCF and subsequent solvent removal,
the 31P solid state NMR signals are shifted to lower field as ex-
pected when coordinating to an acidic species. It is noteworthy
that nearly the entire signal of the phosphine within he polymers
is shifted downfield, showing that almost all phosphine groups in
the polymer networks P1 and P2 are accessible to the molecular
Lewis acid. Notably, the extent of the shift strongly correlates
with the degree of methylation of the triphenylphosphine deriva-
tive. For P1 a shift of 14.7 ppm is observed whereas for P2 a shift
of only 6.5 ppm is observed (see Figure 3). This shift difference
indicates that the bond strength decreases with increasing steric
hindrance around the central phosphorus atom, which means that
the higher the degree of frustration is, the weaker the emerging
bond is rendered. This observation is quite remarkable. According
to Heine et al.,10 the formation of a FLP is usually not observed at
ambient temperature in solution, i.e. the solution NMR spectra of
the individual components do not differ from the spectra of stoi-
chiometric mixtures. Due to the only weakly coordinating nature
of the FLP, the favorable interactions gained from association of
Lewis acid and base are dominated by the entropic contribution of
the dissociation process. In the solid state on the other hand, after
solvent removal, Lewis acid and base are forced into and kept in a
“not-so-frustrated” state, so that the interaction can actually be
observed, and its strength determined by the difference in the
chemical shifts, .
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counterparts,
tris(o-tolyl)phosphine
and
tris(2,6-
dimethylphenyl)phosphine. To that end, inside the glovebox a
Young-type NMR tube was charged with a stoichiometric mix-
ture of P1/BCF or P2/BCF, cyclohexane-d12, and the suspension
subjected to ultrasonication for 15 minutes to ensure sufficient
intermixing of the poorly soluble BCF and the polymer. Subse-
quently, the tube was pressurized with an H2/D2 mixture (ca.
1
1:1 v/v) to 6 bar, and analyzed via H NMR spectroscopy begin-
ning several hours after pressurization of the tube. In both cases
an emerging triplet can be observed at 4.54 ppm shortly after
pressurization, indicating the formation of HD (see Figure 4). The
control reaction comprising only porous phosphine polymer P2
and solvent under H2/D2 pressure did not result in any conversion
to HD. Although the control reaction with BCF as the sole reac-
tant gave the scrambling product albeit at much lower rate than
the BCF/P2 composite, it must be pointed out that it has been
proven theoretically25 and experimentally under low hydrogen
pressure5 that BCF alone is not active in the heterolytic cleavage
of H2. In our case, residues of protic solvents attached to BCF or
trace water on the glass surface might be responsible for provid-
ing a proton shuttle in the activation of hydrogen. Furthermore, if
a composite of BCF and another highly microporous polymer
network devoid of any functional groups (PPN-6, 4172 m2 g-1,
also prepared by Yamamoto polymerization) is used, no hydrogen
activation product can be detected. It seems therefore that the
impregnation procedure of a polymer has a negating effect on the
activity of BCF itself. This could be caused by the highly hydro-
phobic and non-polarizable surface of the networks. It can there-
fore be assumed that the observed reactivity of the composites is
not only independent of but inherently much higher than the
*
*
P2 + BCF
Δδ = 6.5 ppm
*
P2
activity
of
“pure”
BCF.
The
composite
tris(o-
*
*
tolyl)phosphine/BCF gives the expected statistical H2/HD ratio
after merely a few hours. This shows that the incorporation of the
active sites into the polymer imposes a strong kinetic restriction
on the H/D exchange process.
P1 + BCF
Δδ = 14.7 ppm
P1
40
20
0
-20
 / ppm
-40
-60
-80
Figure 3: Impregnation procedure of triphenylphosphine deriva-
tive polymers (exemplified for P2) followed by 31P NMR spec-
troscopy; P1 (blue) and P2 (red) before (upper spectra) and after
(lower spectra) impregnation with BCF. Asterisks denote spinning
sidebands.
To verify this hypothesis we compared the chemical shifts of the
solid state NMR spectrum of a “dry” FLP, consisting of the mo-
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ACKNOWLEDGMENT
A)
261 h
189 h
93 h
69 h
45 h
21 h
3 h
1
2
3
4
5
6
7
8
The authors thank Christina Eichenauer for surface area and TGA
measurements. We also thank Douglas Stephan for his insights
into the subject of FLP materials. This work was funded by the
ERC Project ORGZEO (Grant-Nr.: 278593) and the DFG (Cluster
of Excellence UniCat).
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 / ppm
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(10)
(11)
(12)
(13)
(14)
(15)
BCF + P1
BCF + P2
P2 only
0
50
100
150
200
250
300
Reaction time / h
Figure 4: A) Kinetic measurement of H/D exchange determined
via H NMR and B) integral ratio of HD/H2 plotted as a function
1
(16)
(17)
(18)
(19)
(20)
of time
We hereby report the first synthesis of two semi-immobilized
frustrated Lewis pairs with the basic components embedded into
the backbones of microporous polymer networks. Addition of the
commercially
available
Lewis
acid,
tris(pentafluorophenyl)borane, to polymers structurally akin to
trimesitylphosphine yielded two functional heterogeneous hydro-
gen activation catalysts. Their ability to cleave dihydrogen at
ambient temperature and low pressure was demonstrated via
isotope scrambling experiments.
(21)
(22)
Further combinations of Lewis acidic and basic species need to be
created to obtain heterogeneous catalyst systems capable of split-
ting dihydrogen at low pressures and to enable catalytic hydro-
genation. Furthermore, full heterogenization of frustrated Lewis
pairs could even further enhance the reactivity and applicability of
such systems.
(23)
(24)
(25)
ASSOCIATED CONTENT
Supporting Information
Experimental details on synthesis of monomers and polymers
and hydrogen activation. 13C-NMR of polymers and molecu-
lar model compounds, fluorescence spectra and TGA of the
polymers
The Supporting Information is available free of charge on the
ACS Publications website.
AUTHOR INFORMATION
Corresponding Author
eMail: arne.thomas@tu-berlin.de
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