Solid state frustrated Lewis pair chemistry

Article abstract of DOI:10.1039/c8sc01089g

In solution the PCy₃/B(C₆F₅)₃ pair is rapidly deactivated by nucleophilic aromatic substitution. In the solid state deactivation is effectively suppressed and the active frustrated phosphane/borane Lewis pair sp

Full text of DOI:10.1039/c8sc01089g

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Solid State Frustrated Lewis Pair Chemistry
Long Wang,a Gerald Kehr,a Constantin G. Daniliuc,a Melanie Brinkkötter,b Thomas Wiegand,c Anna-
Lena Wübker,b Hellmut Eckert,b,d,* Lei Liu,f Jan Gerit Brandenburg,e,f Stefan Grimme,f,* Gerhard
Erkera,*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
In solution the PCy3/B(C6F5)3 pair is rapidly deactivated by nucleophilic aromatic substitution. In the solid state deactivation
is effectively suppressed and the active frustrated phosphane/borane Lewis pair splits dihydrogen or adds to sulfur
dioxide. A variety of phosphane/B(C6F5)3 pairs have been used to carry out active FLP reactions in the solid state. The
reactions were analyzed by DFT calculations and by solid state NMR spectroscopy. The solid state dihydrogen splitting
reaction was also carried out under near to ambient conditions with suspensions of the non-quenched phosphane/borane
mixtures in the fluorous liquid perfluoromethylcyclohexane.
www.rsc.org/
characterized and their reactions reported.12-23 The vast
majority of them relies on avoiding the effective neutralizing
Lewis acid/Lewis base adduct formation by steric hindrance,
Introduction
Lewis acids and bases when brought together in solution
typically undergo rapid formation of strong adducts. Similar to
the neutralization reaction of Brønsted acids and bases this
leads to an annihilation of the typical Lewis acid and Lewis
base properties.1,2 This situation can be changed if one
effectively hinders the Lewis pair from the neutralizing adduct
formation, e.g. by electronic means3 or, more commonly, by
attaching very bulky substituents at the core atoms of the
pair.4-6 This invariably leads to situations where active Lewis
acids and active Lewis bases are present in a solution at the
same time, opening possibilities for cooperative reactions with
added substrates. Such “frustrated Lewis pairs” (FLPs)7 have
very successfully been used within the last decade for small
molecule binding and activation, most notably among this the
metal-free splitting and activation of dihydrogen.8,9 A number
of new reaction types have been found in this way10,11 and a
great variety of different FLPs have been devised,
and this has led to the discovery and development of a great
variety of interesting reactions in solution.24-26
We thought that we should not confine ourselves to searching
for FLP reactions in the liquid phase. One may safely assume
that large molecules in the solid state are more or less rigidly
confined to their positions in the crystal lattice. Therefore,
Lewis acids and Lewis bases should effectively be hindered
from adduct formation or other deactivating reaction
pathways even in the absence of efficient steric hindrance by
their substituents as long as we keep them in the solid state.
This may actually set the scene for possibly finding new
frustrated Lewis pairs and, consequently, new FLP reactions,
by exposing such Lewis pairs to suitable reagents in the solid
state. We have tried this principle and found that FLP
chemistry can be done in this way.
There had been some reports about frustrated Lewis pair
related behavior at certain heterogeneous catalysts. In these
cases the active sites were part of the catalytic solid. Activation
of small molecules at such systems had been achieved either
thermally or by photolysis.27-29 There have also been a few
reports about heterogenized Lewis acids, bonded to suitable
supports, that have been employed in FLP type reactions.30
Our here reported case is distinctly different: we have
employed solid physical mixtures of phosphane Lewis bases
with the strong B(C6F5)3 boron Lewis acid and reacted them
under suitable conditions with selected small molecules.
Activation occurred and the phosphane/borane pair became
consumed with selective formation of the FLP reaction
products. First examples of this FLP development will be
described below in this account.
a.Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster,
Corrensstraße 40, 48149 Münster, Germany. Email: erker@uni-muenster.de
b.Institut für Physikalische Chemie and Graduate School of Chemistry, Westfälische
Wilhelms-Universität Münster, Corrensstraße 30, 48149 Münster, Germany,
Email: eckerth@uni-muenster.de
c. Laboratorium für Physikalische Chemie, ETH Zürich, Vladimir-Prelog-Weg 2, 8049
Zürich, Switzerland
d.Institute of Physics in Sao Carlos, University of Sao Paulo, CEP 369, Sao Carlos SP
13566-590, Brazil
e. London Centre for Nanotechnology, University College London, 17-19 Gordon
Street London, WC1H 0AH, United Kingdom
f. Mulliken Center for Theoretical Chemistry, Institut für Physikalische und
Theoretische Chemie, Universität Bonn, Beringstraße 4, 53115 Bonn,Germany,
Email: grimme@thch.uni-bonn.de
† Electronic Supplementary Information (ESI) available: additional experimental
details, further spectral and crystallographic data, additional data from the solid
state NMR and theoretical studies. CCDC 1515080–1515082. See DOI: xxxxxxxxxxxx
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We carried out an ample characterization of the solid product
material directly (i.e. without ever dissolving it) by solid state
NMR spectroscopy. Evidence for the solid state hydrogenation
of the PCy3/B(C6F5)3 mixture comes from the 31P and 11B MAS
NMR data (Figure 1). In the 31P NMR spectrum we recognize
the starting material at 7.1 ppm, whereas the phosphonium
ion gives a broad signal at 30.1 ppm. In the 11B MAS NMR
spectrum the signal of the free B(C6F5)3 gives rise to the
previously documented second-order quadrupolar lineshape34
whereas after the hydrogenation a much narrower signal
appears at -24.9 ppm. In addition, the spectrum reveals the
presence of a minor amount of the substitution product at -2.4
ppm. The latter is the only product formed when the reaction
is carried out in solution. The complete NMR characterization
of the substitution product both in solution and the solid state
is given in the ESI. In a control experiment, the formation of
only a very minor amount of the substitution product 3a was
observed in the solid state NMR spectra of a PCy3/B(C6F5)3
mixture subjected to identical reaction conditions in the
absence of H2.
Results and discussion
Tricyclohexylphosphane
(
1a
)
and tris(pentafluorophenyl)-
borane (2) represent a Lewis base/Lewis acid pair that is
known to react rapidly in solution. It undergoes a nucleophilic
aromatic substitution by the phosphane at the para-position of
a C6F5 group of the borane4,31 to eventually yield the reaction
product 3a 7,32
reactivity of the 1a
.
This favorable SNAr reaction eliminates any FLP
pair very effectively in solution. Even in
/
2
the presence of 50 bar H2 no significant hydrogen splitting is
observed, the formation of 3a even prevails under these
conditions. We isolated this compound in 86 % yield and
confirmed its formation by X-ray diffraction and by NMR
spectroscopy [Scheme 1 (a) (H2); for details see the ESI].32
The situation is drastically different in the solid state: we
mixed equimolar quantities of PCy3 (1a) and B(C6F5)3 (2) and
exposed it in a glass vial inside a steel autoclave to 50 bar of
dihydrogen for a total of 10 days with constant agitation by a
Teflon coated magnetic stirring bar. After this time a sample
was taken and dissolved in D2-dichloromethane. The NMR
analysis revealed the hydrogen splitting product
+
-
[HPCy3 ][HB(C6F5)3 ] (4a) had been formed as the by far major
product [Scheme 1 (b) (H2)]. Only negligible if any amounts of
the SNAr product 3a, which would have been formed readily
from any residual PCy3/B(C6F5)3 upon dissolving in
dichloromethane, were present in the in situ samples. The
phosphonium/ hydridoborate product 4a was identified by its
1
1
typical 31P (δ 33.2, JPH ~ 443 Hz) and 11B (δ -25.3, JBH ~ 92 Hz)
NMR signals with correlated 1H NMR features at δ 5.15 (d, JPH
1
= 444.0 Hz) and δ 3.59 (br 1:1:1:1 q, [B]H), respectively.
Workup of a representative sample eventually furnished the
salt 4a isolated in 81% yield on a 100 mg scale. Crystallization
from dichloromethane/pentane gave single crystals which
were used to confirm the formation of the FLP H2 splitting
product under these special conditions by X-ray diffraction (for
details see the ESI).33 The salt 4a is an active reducing agent. Its
reaction with the bulky N-phenyl-4-methylacetophenon-imine
gave the respective secondary amine reduction product (24h
at 70°C, 86% conversion, for details see the ESI).
Figure 1. 11B{1H} MAS (left) and 31P{1H} CPMAS NMR spectra (right) of the B(C6F5)3/PCy3
control mixture sample (grey trace) and the hydrogenation sample (black trace) from
the solid-state reaction with H2. Minor side products are labelled by +. The product 3a
of the SNAr reaction is indicated by an asterisk.
Figure 2 shows the 1H MAS NMR spectrum of the FLP-H2
adduct 4a acquired at 20.0 T and a MAS spinning frequency of
60.0 kHz with the EASY scheme for suppression of background
signals from the MAS probe and the MAS rotor cap.35 Under
these conditions, the strong 1H-1H dipolar couplings are
sufficiently suppressed, even though residual line broadening
is still detected owing to higher order terms in the Hamiltonian
which are not fully eliminated even at 60.0 kHz.36 A distinct
doublet (J(1H-31P) ~ 430 Hz) can be identified at 5.5 ppm which
is assigned to P-bound hydrogen, whereas the singlet at 4.0
ppm is assigned to the B-bound hydrogen (the expected
multiplet is not resolved in this case).
Scheme 1. Reactions of the PR1R2R3/B(C6F5)3 FLP systems in a dihydrogen and SO2
atmosphere, respectively, in solution (a) and in the solid state (b).[Cy: cyclohexyl]
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was carried out analogously as the one described above (r.t.,
50 bar H2, 3 days). Our analysis of a product sample dissolved
in CD2Cl2 revealed almost exclusive formation of the
dihydrogen FLP splitting product 4b [Scheme 1b (H2)]. It shows
1
1
a characteristic H NMR [P]H doublet at δ 6.13, JPH = 459.3 Hz
(31P : δ 30.6) and a broad 1:1:1:1 intensity [B]H quartet at δ
3.64 (11B: δ -25.3, d, 1JBH ~ 94 Hz ).
Keeping the PPhCy2/B(C6F5)3 mixture (1b/2) in CD2Cl2 solution
for 12 hours under H2 (50 bar) gave a different result. The NMR
analysis showed the formation of a ca. 1:6 mixture of the salt
4b with the substitution product 3b. [Scheme 1a (H2)] The
major zwitterionic phosphonium/fluoroborate product 3b was
eventually isolated as a white solid in 82% yield. It shows a 11B
Figure 2. 1H MAS NMR spectrum of a B(C6F5)3/PCy3 mixture after the solid-state
reaction with H2, acquired using the EASY scheme for probe and rotor cap background
suppression measured at 20.0
T and a
MAS frequency of 60.0 kHz (a) and NMR doublet (1JBF ~ 70 Hz) at δ -0.6 and a sharp 31P NMR signal
at δ 33.6. The 19F NMR spectrum shows the [B]F resonance at -
192.9, two signals of the bridging C6F4 group and three
resonances (o, p, m) of the remaining B(C6F5)2 unit with a
Δδ19Fm,p shift difference of 4.9 ppm, which is typical for a
borate type structure (for details and the depicted NMR
spectra see the ESI).
corresponding line shape simulation (b). + marks a small signal remaining from the
rotor cap.
This assignment is supported by 11B{1H} and 31P{1H}
heteronuclear correlation experiments (Figure 3) which show
intense cross-peaks linking these resonances to the
corresponding 31P and 11B NMR signals of the solid-state
hydrogenation product. Further support for this assignment
In this case the nucleophilic aromatic substitution by the
markedly less nucleophilic phosphane PPhCy2 (1b) relative to
1
comes from H{11B} REAPDOR experiments. The obtained peak
PCy3 (1a) seems to allow the FLP reaction to compete as a
assignments are in agreement with solution-state NMR data
1
(vide supra), as well as DFT computations of H NMR chemical
minor pathway in solution. This becomes continued in the
reaction of the much less nucleophilic phosphane PPh2(tBu)
+
shifts for the isolated cationic H-PCy3 and anionic H-B(C6F5)3
-
(
1c) which in solution together with the B(C6F5)3 Lewis acid
species (5.4 and 4.5 ppm on a B3-LYP/def2-TZVP level of
theory, respectively). The absence of an encounter complex is
proven by 31P{11B} REAPDOR and 11B{31P} REDOR experiments
(see the ESI). As previously discussed, such experiments can
probe the B———P distance by measuring the strength of the
heteronuclear 11B-31P dipole-dipole interactions in both FLPs
and their reaction products.37 In the present material, no
dephasing was observed over a dipolar mixing time of ~ 5 ms.
Comparing these experimental data with corresponding two-
spin simulations we can conclude that the boron-phosphorus
distance must be larger than 600 pm. Thus, all the
experimental data are consistent with well-separated
phosphonium and borate ions.
gave a ratio of the SNAr andFLP products of 3c : 4c ~ 2:1 under
our typical reaction conditions (r.t., CH2Cl2 solution, 12 hours,
50 bar H2).[Scheme 1a (H2)] Without H2 only the substitution
product (3c) was formed in solution (isolated in 76% yield, see
the ESI for its characterization). On the contrary, the reaction
of the PPh2(tBu) / B(C6F5)3 Lewis base / Lewis acid mixture in
the solid state (r.t., 50 bar H2, 3 days) gave almost pure H2-
splitting product 4c, [Scheme 1b (H2)] [31P NMR: δ 31.4 (1JPH
~
474 Hz), 11B: δ -25,2 (1JBH ~ 91 Hz), 19F: Δδ19Fm,p = 3.0 ppm]
which we isolated from the workup procedure involving
recrystallization from CH2Cl2/pentane in 60% yield (details of
the characterization of the compounds 3c and 4c see the ESI).
Quantum chemical simulations were used to investigate the
mechanistic details of the different FLP reactivities in all three
states of matter, i.e. gas, liquid, and solid phase. The FLP/H2
thermochemistry is for the first time investigated here in the
solid state using relatively high-level periodic quantum
chemistry methods. We employed a hierarchy of theoretical
methods, ranging from semi-empirical tight-binding
Hamiltonians to accurate London dispersion corrected hybrid
density functionals.38-46 More discussion of methodological
points and computational details can be found in our previous
benchmark study47 and in the ESI. The main representative
results of the PCy3/B(C6F5)3 FLP are shown in Figure 4.
Figure 3. 1H/31P (left) and 1H/11B (right) HETCOR spectra of a HB(C6F5)3 /HPCy3 mixture
4a after the solid-state reaction with H2. Cross-peaks denote the B-H and P-H
correlations. The additional 1H/11B crosspeak seen in the right part of the figure at
(1.3/-5.2) ppm arises from the substitution product 3a.
-
+
The reaction of the phosphane PPhCy2 (1b) with the Lewis acid
B(C6F5)3 (2) in the solid state proceeds similarly. The reaction
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experimentally in a mixture of the solid particles. According to
these results which are shown in Figure 5, at the interface the
components (in particular the Lewis base) are spatially not
constrained, possibly due to a mismatch of the molecular
surfaces (a kind of interface strain). This leads partially to a
“liquid phase” behavior of the FLPs in the solid state. It is seen
that the Lewis acid and base components could move rather
freely at the contact surface and adopt molecular FLP
conformations enabling hydrogen activation as in solution. (for
details see the ESI)
Figure 4. Calculated Gibbs free energies for the reaction of PCy3/B(C6F5)3 with H2 in the
solution (toluene) and in the solid state (see see the supplementary material for
computational details). All energy values are given in kcal mol−1. Inserted figures are
the overlays of the HF-3c calculated crystal structures of PCy3/B(C6F5)3 (green) and
[HPCy3+][HB(C6F5)3 ] (blue) (a), and crystal structures of PCy3/B(C6F5)3 (green) and the
-
Figure 5. Snapshots of the periodic BO-MD simulation at the DFTB-D3 level of theory
for the PCy3 + B(C6F5)3 FLP (1a/2). Color legend: P yellow, B pink, C black, F green and H
white.
SN2Ar product 3a (orange) (b). Hydrogen atoms except P-H and B-H are omitted for
clarity .
All shown reactions are exergonic, and both the solvent and
the crystal phase stabilize the products 4a and 3a by 10 to 15
kcal/mol compared to the gas phase. In this regard the FLP
reaction to 4a is in fact thermodynamically feasible. However,
the competing product 3a is in solution significantly preferred
over 4a (-50.7 kcal mol−1 vs. -15.5 kcal mol−1) and due to the
expected high mobility it can readily react and prevent the
desired FLP reaction. In contrast, the crystal field provides
much more pronounced energy barriers, which kinetically
stabilizes the reactant and enables the targeted reaction to
product 4a. These higher energy barriers can be rationalized
The new solid state phosphane / borane FLP reactions are not
limited to the splitting of dihydrogen. We exposed the 1:1
mixture of PCy3 (1a) and B(C6F5)3 (2) for 4 hours at r.t. in the
solid state to SO2 gas (1.5 bar).48 A sample was dissolved in
CD2Cl2 and its NMR spectra revealed the almost quantitative
formation of the P/B FLP SO2 addition product 5. [Scheme 1 (b)
(SO2)] The product was also directly identified from the solid
obtained by solid state NMR spectroscopy (see the ESI),
indicating essentially quantitative conversion. The distorted
four-coordinate boron environment in
= -0.6 ppm, and a nearly axially symmetric electric field
gradient, with CQ = 1.54 MHz and
Q = 0.15. The 31P MAS-NMR
5 is characterized by δiso
by
a simple geometric comparison (inset of Figure 4).
η
Apparently, the solid state reaction to 4a requires substantially
less rearrangements of the crystal compared to 3a. Thus, we
can identify two key roles that drive the reaction in the solid
phase: (1) The crystal environment can adopt the solvent role
in enhancing the FLP reactivity, typically explained by both the
electrostatic screening and undirectional London dispersion
stabilization of the FLP products. (2) The static crystal field can
selectively suppress certain undesired reaction routes, which is
not possible in a liquid or gas environment with high molecular
mobility.
spectrum shows a single sharp signal at 51.5 ppm. In this case,
11B{31P} REDOR and 31P{11B} REAPDOR experiments
consistently point towards a B———P internuclear distance of 450
pm, which is in good agreement with the distance of 434 pm
from the crystal structure.
We performed the reaction on a preparative scale and isolated
the product
CH2Cl2 / pentane in 84 % yield. The product shows the typical
11B (δ -0.3) and 19F NMR features (three resonances, Δδ19Fm,p
5 as a white solid after recrystallization from
=
6.4 ppm) of the borate section of the molecule and a
phosphonium 31P NMR signal at δ 50.0 (for further details see
the ESI).
While point (1) makes the heterogeneous formulation of
typical FLP reactions possible, (2) goes beyond it and opens
possibilities for new FLP systems as compellingly
demonstrated for the here discussed compound. An additional
important prerequisite for the discussed reaction is the
possible diffusion of H2 gas through the reactant crystal. Our
molecular dynamics (MD) simulation (for details see the ESI)
confirm that H2 can actually move more or less freely through
the channels of the crystal thereby generating the correct
conditions for the H2 activation to take place. Moreover, we
have conducted MD simulations for a model of the interface
between the Lewis acid and base as it may occur
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essentially identical to those obtained from the solid-state
reaction.
Table 1.
A
Comparison of the Dihydrogen Splitting Reaction by the
Phosphane/Borane FLPs 1/B(C6F5)3 in Solution, in the Dry Solid State and in the
Fluorous Liquid Perfluoromethylcyclohexane.
b
No.
1a
1b
1c
solutionc
dry solid
in F11C6-CF3
PCy3
only 3a
> 90% conv. to 4aa
60% conv. to 4a
PPhCy2
3b : 4b ~ 6 :1 ca. 95% conv. to 4bd ca. 95% conv.to 4b
3c : 4c ~ 2 : 1 ca. 95% conv. to 4cd ca. 98% conv.to 4c
t
PPh2 Bu
a
b
c
10 days, r.t., 50 bar H2; 10 hours, r.t. 1.5 bar H2; d2-dichloromethane, 12
hours, r.t.; d 3 days, r.t., 50 bar H2
Figure 6. Molecular structure of the solid state P/B FLP SO2 addition product 5. Selected
bond lengths (Å): S1-O1 1.445(2), S1-O2 1.564(2), P1-S1 2.268(1), B1-O2 1.545(3) and
bond angles (°): O1-S1-O2 110.8(1), O1-S1-P1 106.3(1), O2-S1-P1 93.4(1), ΣB1CCC 334.0,
ΣP1CCC 334.1.
The reaction of the PPhCy2 (1b)/B(C6F5)3 pair with dihydrogen
proceeded at least equally well in this fluorous liquid. Under
analogous conditions a ca. 95% conversion to the H2-splitting
product 4b was achieved within the 10 h reaction time. The
Compound 5 was characterized by X-ray diffraction (Figure 6).
The X-ray crystal structure analysis shows the newly formed
P1-S1 and O1-B1 bonds. Both the phosphorus and the boron
atoms show pseudo-tetrahedral coordination geometries. The
sulfur coordination geometry is distorted trigonal-pyramidal.
We also exposed the PCy3/B(C6F5)3 pair to SO2 in solution but
only observed the formation of the substitution product 3a
[Scheme 1 (a) (SO2); see the ESI for details].
t
PPh2 Bu (1c)/B(C6F5)3 system even slightly surpassed this result.
We obtained
a near to quantitative conversion to the
HPPh2tBu+/HB(C6F5)3 salt within 10 h at near to ambient
conditions in the inert perfluoromethylcyclohexane liquid (see
the ESI for details).
-
Conclusions
Our study has shown so far that an agitated mixture of
Our study has shown that frustrated Lewis pair behavior can
be achieved by other means than the usual electronic3,19 or
steric modifications4-9 of Lewis acids and bases. In the cases
reported here we have deviated from the ubiquitous method
of using sterically bulky substituents at the phosphorus Lewis
base in order to prevent neutralizing adduct formation or
other deactivating reactions with the strong B(C6F5)3 boron
Lewis acid. On the contrary, the phosphanes used in this study
are rather nucleophilic and in solution they undergo rapid
nucleophilic aromatic substitution at one C6F5 ring of the Lewis
acid replacing fluoride which consequently then becomes
bonded to the boron atom31 with annihilation of its Lewis
acidic features. In all the cases looked at in our study this FLP
deactivating reaction is very efficiently suppressed by localizing
the individual Lewis acidic and Lewis basic molecules inside
their solid state lattices. This prevents them effectively from
undergoing the bimolecular deactivation reaction.
particles of the phosphanes 1a
-c with particles of the B(C6F5)3
Lewis acid 2 did very effectively evade the deactivating SNAr
reaction that these pairs rapidly undergo in solution. Instead,
they retained their frustrated Lewis pair character and,
consequently, showed the ability to split dihydrogen
heterolytically. While this observation is probably of a far-
reaching principal interest, the rather harsh conditions of the
solid state FLP H2-splitting (50 bars of dihydrogen, 3 to 10 d
reaction time) made this far from a conveniently applicable
procedure.
Fluorous liquids show some extraordinary properties.49-56 They
do not mix with a variety of common organic solvents; they
show an enhanced solubility of many gases in them, among
,
them dihydrogen.49 56-58 Moreover, many organic and element-
organic compounds, among them the phosphanes 1a
B(C6F5)3 ) are insoluble in them. Therefore, we decided to
carry out the solid state FLP dihydrogen splitting reaction in
perfluoromethylcyclohexane (F11C6-CF3). In typical
experiment (see the ESI for details) we suspended an
equimolar mixture of PCy3 1a and B(C6F5)3 in
-c and
(
2
From the solid state NMR and the DFT analysis in conjunction
with general principles about molecular diffusion in the solid
state, we assume that the individual Lewis acid and base
components do not easily mix on a molecular level in our
experiments, but that initially we are dealing with separate
solid state phosphane and borane particles. This lets us
assume that the FLP dihydrogen splitting reaction must take
place at the surface, respectively the interface between
phosphane and borane solid state entities; the ensuing
reaction is, however, probably facilitated by the easy
permeability of the respective crystal lattices by dihydrogen
(and other gases). The dihydrogen splitting reaction may then
have formed the phosphonium/hydridoborate salt initially at
the surface, but it may be assumed that accumulation of that
a
(
)
perfluoromethylcyclohexane and stirred the suspension for 10
h in a dihydrogen atmosphere at near to ambient conditions
(r.t., 1.5 bar H2). Workup was simply done by evaporation of
the volatiles. A sample of the obtained white powdery solid
was then subjected to NMR analysis in D2-dichloromethane
solution. It showed that a ca. 60% conversion to the hydrogen
+
-
splitting product HPCy3 /HB(C6F5)3
(4a) had been achieved.
The remaining starting material had become converted to the
SNAr reaction product 3a under the conditions of the NMR
analysis in solution. The solid state NMR spectra of the
products obtained after the suspension reaction were
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G. C. Welch and D. W. Stephan, J. Am. Chem. Soc., 2007, 129,
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P. Spies, G. Erker, G. Kehr, K. Bergander, R. Frohlich, S.
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5074.
species might create a local situation resembling an ionic
liquid, which might facilitate diffusion and mixing since
eventually we obtained homogeneous solid samples of the
respective dihydrogen splitting products. The MD simulations
which support this view are currently due to the large system
size too approximate (short) to draw any quantitative
conclusions nor are we able to simulate further parts of the
solid state reaction dynamically. Nevertheless, the MD and
static theoretical results clearly support the above described
picture of partially “molten” material at the interface with
sufficient molecular flexibility for activation of almost freely
diffusing molecular hydrogen.
7
8
9
G. C. Welch, L. Cabrera, P. A. Chase, E. Hollink, J. D. Masuda,
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Although the mechanistic aspects of our solid state FLP
reactions must remain somewhat speculative at this time, we
have greatly improved its practical applicability by using the
fluorous liquid effect.55-57 This has made dihydrogen splitting
reactions readily available from frustrated Lewis pair
combinations which cannot be kept active in other ways.33 We
have further found first indications that the resulting
[P]H+/[B]H- product 4a can be usefully employed in imine
reduction and we have shown that the solid state FLP
reactions are not confined to the dihydrogen splitting
reactions but can be developed beyond. The solid state
approach can make FLPs available for small molecule
activation beyond using the conventional methods leading to
Lewis pair “frustration”. It needs to be explored if this will
open new pathways of extending FLP chemistry beyond its
existing scope, for example by opening FLP routes to the large
field of heterogeneous catalysis, here to be performed without
the aid of metals.27-30
19 M.-A. Courtemanche, M.-A. Légaré, L. Maron and F.-G.
Fontaine, J. Am. Chem. Soc., 2013, 135, 9326-9329.
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21 E. Theuergarten, J. Schlosser, D. Schluns, M. Freytag, C. G.
Conflicts of Interest
There are no conflicts to declare.
Daniliuc, P. G. Jones and M. Tamm, Dalton Trans., 2012, 41
9101-9110.
,
22 E. L. Kolychev, T. Bannenberg, M. Freytag, C. G. Daniliuc, P.
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23 B. Inés, D. Palomas, S. Holle, S. Steinberg, J. A. Nicasio and M.
Alcarazo, Angew. Chem. Int. Ed., 2012, 51, 12367-12369.
Acknowledgements
Financial support from the European Research Council (G.E.)
and the Deutsche Forschungsgemeinschaft (Leibniz award
(S.G.)) is gratefully acknowledged. We thank Prof. Beat Meier
and the ETH Zürich for supporting the high field solid-state
NMR measurements.
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,
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26 D. J. Scott, M. J. Fuchter and A. E. Ashley, J. Am. Chem. Soc.,
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theoretical study.
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