A New Mode of Chemical Reactivity for Metal-Free Hydrogen Activation by Lewis Acidic Boranes

Article abstract of DOI:10.1002/anie.201900861

We herein explore whether tris(aryl)borane Lewis acids are capable of cleaving H₂ outside of the usual Lewis acid/base chemistry described by the concept of frustrated Lewis pairs (FLPs). Instead of a Lewis base we use a chemical reductant to generate stable radical anions of two highly hindered boranes: tris(3,5-dinitromesityl)borane and tris(mesityl)borane. NMR spectroscopic characterization reveals that the corresponding borane radical anions activate (cleave) dihydrogen, whilst EPR spectroscopic characterization, supported by computational analysis, reveals the intermediates along the hydrogen activation pathway. This radical-based, redox pathway involves the homolytic cleavage of H₂, in contrast to conventional models of FLP chemistry, which invoke a heterolytic cleavage pathway. This represents a new mode of chemical reactivity for hydrogen activation by borane Lewis acids.

Full text of DOI:10.1002/anie.201900861

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: A New Mode of Chemical Reactivity for Metal–Free Hydrogen
Activation by Lewis Acidic Boranes
Authors: Chris Slootweg, Elliot Bennett, Elliot Lawrence, Robin
Blagg, Anna Mullen, Fraser MacMillan, Andreas Ehlers,
Andrew Ashley, Daniel Scott, Joshua Sapsford, and Gregory
Wildgoose
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201900861
Angew. Chem. 10.1002/ange.201900861
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10.1002/anie.201900861
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A New Mode of Chemical Reactivity for Metal–Free Hydrogen
Activation by Lewis Acidic Boranes
Elliot L. Bennett,[a] Elliot J. Lawrence,[a] Robin J. Blagg,[a] Anna S. Mullen,[a] Fraser MacMillan,[a]
Andreas W. Ehlers,[b,c] Daniel J. Scott,[d] Joshua S. Sapsford,[d] Andrew E. Ashley,*[d] Gregory G.
Wildgoose,*[a] and J. Chris Slootweg*[b]
Abstract: We herein explore whether tris(aryl)borane Lewis acids are
capable of cleaving H2 outside of the usual Lewis acid/base chemistry
described by the concept of “frustrated Lewis pairs” (FLPs). Instead
of a Lewis base we use a chemical reductant to generate stable
radical anions of two highly-hindered boranes: tris(3,5-
dinitromesityl)borane and tris(mesityl)borane. NMR spectroscopic
characterization reveals that the corresponding borane radical anions
by the relative strengths of the Lewis acidic/Lewis basic
components and the degree of steric encumbrance between
5
]
them.[1b-e,
This contrasts with the transition-metal–based
complexes and biological systems that have dominated
hydrogenation catalysis for the previous 150 years.[6] In these
complexes, the metal center provides both vacant and filled
acceptor/donor orbitals at a single reactive site; the chemistry is,
to a large extent, operating under redox control of the metal center,
and homolytic H2 bond cleavage is common.
activate
(cleave)
dihydrogen,
whilst
EPR spectroscopic
characterization, supported by computational analysis, reveals the
intermediates along the hydrogen activation pathway for the first time.
This radical–based, redox pathway involves homolytic cleavage of H2,
in contrast to conventional models of FLP chemistry which invoke a
heterolytic cleavage pathway. This represents a new mode of
chemical reactivity for hydrogen activation by borane Lewis acids.
The heterolytic mechanism proposed for FLP activation of
H2 is found generally to be in good agreement with observed
trends in reactivity, and it has been supported by a number of
computational studies.[4] Nevertheless, definitive experimental
proof has remained elusive (perhaps unavoidably so). As such it
is interesting to consider that observed patterns of FLP reactivity
could also be consistent with alternative H2 activation pathways.
In particular, these trends could also be consistent with plausible
radical mechanisms, in which initial single-electron transfer (SET)
from the Lewis base to the Lewis acid would transiently generate
highly reactive radical pairs capable of activating H2. For example,
while the thermodynamic and kinetic ability of an FLP to activate
H2 is well known to correlate with the hydride ion affinity of the
Lewis acid (consistent with heterolytic bond cleavage), these
parameters both also correlate well with the one-electron
reduction potential of the Lewis acid (consistent with SET). Indeed,
recent studies have implied that for some families of borane Lewis
acids, reduction potentials may even be a better indicator of
reactivity towards H2 than hydride ion affinities.[7]
The chemistry of Lewis acidic boranes reacting with H2 is now
almost exclusively described by the Lewis acid/base conceptual
framework of “frustrated Lewis pairs” (FLPs),[1 ] introduced by
Douglas Stephan in 2006.[ 2 ] While some precise mechanistic
details are still debated,[3] in general the ability of FLPs to cleave
H2 relies on the cooperative action of the two reactive centers that
are sterically encumbered (“frustrated”) within an encounter
complex of the Lewis acid–base pair. The Lewis acid, which is
most often an organoborane, provides a vacant acceptor orbital,
and the Lewis base, typically a phosphine or amine, provides a
donor orbital with which to cleave the strong H–H bond.[ 4 ]
Activation of H2 by borane–based FLPs is therefore widely
thought to involve heterolytic bond cleavage, and to be controlled
There is also a growing body of evidence for the occurrence
of radical mechanisms when small molecules such as NO,
Ph3SnH, and peroxides are used as the substrates of FLP
reactions.[ 8 ] To date, however, these “frustrated radical pair”
(FRP) mechanisms have not been observed with H2. Indeed, no
FLP is known to cleave H2 via a radical mechanism. Our previous
work studying the electrochemistry of FLP components, together
with the recent evidence for radical pathways in FLPs and FRPs
reported by others, raises an obvious question that this article sets
out to answer: can boranes react with H2 outside of an FLP
chemical framework, if they are allowed to operate via a hitherto
unknown redox controlled, radical reaction pathway instead?
To test our hypothesis we carefully selected two boranes as
models: tris(3,5-dinitromesityl)borane, 1, and tris(mesityl)borane,
2 (Scheme 1). Both boranes have essentially identical steric
shielding of the central boron atom by the six ortho methyl groups
on the mesityl rings, leading to the formation of long–lived borane
radical anions upon reduction.[9,10] Neither borane is currently
known to be active for H2 activation within an FLP. The addition of
six electron-withdrawing nitro groups in 1 shifts the reduction
[a]
[b]
Dr. E. L. Bennett, Dr. E. J. Lawrence, Dr. R. J. Blagg, A. S. Mullen,
Dr. F. MacMillan, Prof. Dr. G. G. Wildgoose
School of Chemistry, University of East Anglia, Norwich Research
Park, Norwich, NR4 7TJ (United Kingdom)
E-mail: g.wildgoose@uea.ac.uk
Dr. A. W. Ehlers, Assoc. Prof. Dr. J. C. Slootweg
Van 't Hoff Institute for Molecular Sciences
University of Amsterdam, Science Park 904, PO Box 94157,
1090 GD Amsterdam (The Netherlands)
E-mail: j.c.slootweg@uva.nl
Dr. A. W. Ehlers
Department of Chemistry, Science Faculty
University of Johannesburg
[c]
[d]
PO Box 254, Auckland Park, Johannesburg (South Africa)
Dr. D. J. Scott, J. S. Sapsford, Dr. A. E. Ashley
Molecular Sciences Research Hub, Imperial College White City
Campus, 80 Wood Lane, London W12 0BZ (United Kingdom)
E-mail: a.ashley@imperial.ac.uk
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201900861
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potential in a positive direction to –1.57 V vs. Cp2Fe0/+ (see
reduction at the nitro groups is observed by NMR, EPR, nor IR
spectroscopic characterization of the reaction products.
Supporting Information), making
1
as electrophilic and
comparably facile to reduce as the archetypal electron deficient
The very negative redox potential of 2 necessitates the use
of a stronger reducing agent. When a solution of 2 in THF-d8 is
reduced over sodium metal[10] and heated in the presence of H2
the appearance of a doublet in the 11B NMR spectrum at –14.5
ppm (1JB,H = 78 Hz), and a corresponding 1:1:1:1 quartet in the 1H
NMR spectrum at 3.75 ppm (1JH,B = 77 Hz) is observed,
characteristic of the formation of [Na][2–H] (Figure 1b).
borane B(C6F5)3 used in FLP chemistry (–1.52 V vs. Cp2Fe0/+),[7b,e-
g]
and much easier to reduce than 2 (ca. –2.8 V vs. Cp2Fe0/+).[11]
The NO2 groups in 1 also provide useful electron paramagnetic
resonance spectroscopic markers for the characterization of
reaction intermediates.
Scheme
1.
Reduction
of
tris(3,5-dinitromesityl)borane,
1,
and
tris(mesityl)borane, 2, and subsequent reaction with H2.
To examine whether the radical anions of Lewis acidic
boranes are capable of cleaving hydrogen, a solution of 1 in either
CD2Cl2
or
THF-d8 was chemically reduced using
decamethylcobaltocene (Cp*2Co, E0 = –1.94 V vs. Cp2Fe0/+),[12]
heated in the presence of H2, and the reaction periodically
monitored using multinuclear NMR spectroscopy (see Supporting
Information for details). Figure 1a shows the resulting 11B NMR
spectra. The formation of the borohydride product [Cp*2Co][1–H]
is clearly evident by the observation of a characteristic doublet at
–13.6 ppm (1JB,H = 82 Hz) in the 11B NMR spectrum and the
corresponding 1:1:1:1 quartet at +3.8 ppm (1JH,B = 82 Hz) in the
1H NMR spectrum. The spectral assignment was further
confirmed by comparison to an authentic sample of [Na][1–H]
(see Figure S10). Control experiments using D2 in protio–CH2Cl2
or protio–THF produced the analogous result, the generation of
[Cp*2Co][1–D] (Figures S11–S12), observed as a partially
resolved triplet at −13.6 ppm in the 11B NMR spectrum.
In these reactions, the cleavage of H2/D2 must be homolytic
as there is no apparent plausible mechanism to allow for the
formation of H+ (no counter anion) which must be produced via
heterolytic scission of H2. Whilst very strong acids are known to
protonate Cp*2Co,[13] there is no evidence for the formation of this
observable in these reactions. To examine the proposed radical
homolytic dihydrogen cleavage mechanism, 1 was again reduced
with Cp*2Co under H2 but this time in the presence of 1 equiv of
the radical spin–trap TEMPO ((2,2,6,6-tetramethylpiperidin-1-
yl)oxyl). TEMPO was selected because it does not coordinate to
the bulky borane 1 and has a more negative reduction potential
than Cp*2Co,[14] thus precluding any possible redox inhibition to
form [TEMPO]– and 1, which together could subsequently
participate in “normal” FLP H2 activation. In the presence of
TEMPO no H2 cleavage was observed, consistent with inhibition
of a radical reaction by the TEMPO spin–trap. Additional control
experiments confirm that Cp*2Co alone does not activate H2 under
these conditions and that THF/1 mixtures do not result in the
observable formation of [1–H]– via a “solvent–FLP” mechanism[15]
in the absence of a reducing agent. Crucially, no evidence of
Figure 1. Overlaid 11B NMR spectra expanded over the B–H bond region of
interest, showing the progression of H2 cleavage by chemical reduction of 1 in
CD2Cl2 (a) and 2 in THF (b). Inset: The corresponding 11B NMR spectra
recorded at the start and end of the experiments showing the conversion of the
parent borane starting material to the borohydride product upon reduction and
exposure to H2.
The experiments described above clearly indicate that the
borane radical anions 1•– and 2•– can cleave H2 in the absence of
any exogenous Lewis base. These reactions are, however, slow
in comparison to typical FLP H2 activation reactions. In the case
of the model borane 1 this is advantageous, since it enables the
reaction to be monitored in real time and observe reaction
intermediates along the H2 cleavage pathway using EPR
spectroscopy.
Solutions of 1 dissolved in either CD2Cl2 or THF-d8 were
chemically reduced using Cp*2Co (see Supporting Information)
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and the EPR spectra resulting from exposure to H2 were recorded
(Figures 2a-d). Simulation of the EPR spectra yields the isotropic
produces a radical species consistent with that observed in
Figures 2b–c. Computation reveals the structure of this
hyperfine coupling constants for the various 1H, 14N, and11B nuclei, intermediate to be [(Ar2B(H)–Ar(H)]•– with hydride attached at a
given in Table 1. These data, supported by DFT calculations
(performed for the identifiable intermediates of both 1 and 2 and
detailed in the Supplementary Information), enable us to observe
and characterize the structures of the intermediates and gain
valuable insights into the reaction mechanism (given
schematically in Figure 3) and the corresponding energetic profile
by which organoborane radicals cleave H2 homolytically (Figure
4).
four-coordinate boron centre, and H• carried on one of the
aromatic rings (denoted as [1–{H2}]•– with specific reference to
borane 1). DFT models indicate that there is little energetic
discrimination for the H• to be attached to one or other carbon
positions around the aromatic ring. Spin density calculations (see
Data S1) confirm, however, that the isomer with the H•
predominantly located at a meta carbon on the ring, ipso to one
of the nitro groups, is consistent with the observed EPR spectra
(Figures 2b–c). Here the unpaired electron is coupled only to one
of the nitrogen nuclei in the nitro groups of the aryl ring system
and is not coupled to the boron nucleus at all (Table 1).
After 48 hours of heating, the EPR spectrum changes once
again (Figure 2d) to reveal a 1:2:2:2:1 five–line hyperfine coupling
pattern of a new persistent paramagnetic species. This does not
fit the expected coupling pattern from two nitro groups which
would give rise to a 1:2:3:2:1 splitting pattern. Instead, it arises
from near coincident hyperfine coupling with both an additional
single hydrogen atom and the boron nucleus (similar to DFT
calculations of a hydrogen–boron adduct).[ 17 ] This then, is a
neutral [1–H]• intermediate resulting from cleavage of the H2
molecule.
insert Table 1 here (see below)
Figure 2. EPR spectra of 1•- formed via chemical reduction of 1, recorded under
an atmosphere of N2 (a), upon first exposure to H2 but prior to heating (b), after
heating under H2 for 10 minutes (c), and after heating under H2 for 48 hours (d).
The structures of the paramagnetic species are shown with ring substituents
removed for clarity.
Upon reduction of 1 under N2, the EPR spectrum shown in
Figure 2a is observed, which is characteristic of 1•– with hyperfine
coupling of the unpaired electron spin density to the boron
nucleus as well as the methyl and nitro substituents on the
aromatic rings (Table 1).[16] The initiation step is calculated to be
exothermic for both compounds (−56.7 and −11.7 kcal·mol-1 for 1
and 2, respectively) and reflects the relative LUMO energy and
reduction potential of each borane.
Figure 2b shows the resulting spectrum recorded upon first
exposing the reaction to H2 and before heating. An immediate
change is evident with the appearance of a sharp 1:1:1 three–line
signal superimposed on the original signal of the 1•– parent. After
heating the reaction for a further 10 minutes this three–line signal
dominates the EPR spectral response (Figure 2c) for the next 48
hours. The only change to the system is the addition of H2 and
computational modelling of the possible interactions between 1•–
and H2 reveal two propagation pathways. Propagation (1a)
produces the diamagnetic borohydride product, and is
endothermic (+30.8 and +28.1 kcal·mol-1 for 1 and 2) albeit to a
lesser extent than homolytic H2 splitting itself (+107.1 kcal·mol-1
at this level of theory). The alternative pathway, Propagation (1b)
avoids the release of free H–atom radicals and is slightly
exothermic (−6.1 and −1.4 kcal·mol-1 for 1 and 2). This reaction
Figure 3. The proposed radical chain–propagation mechanism for the homolytic
cleavage of H2 upon reduction of organoborane Lewis acids. Inset: the chemical
structures corresponding to the [Ar2B(H)–Ar(H)]•– and [Ar2B–Ar(H)]•
intermediates (substituents on the aryl rings have been omitted for clarity).
Once again there are two possible pathways that result in the
formation of the [1–H]• intermediate: Propagation (2a) and
Propagation (2b). Propagation (2a) is exothermic by −37.1
kcal·mol-1 and −30.8 kcal·mol-1 for 1 and 2, respectively.
Interestingly, computation suggests that if [1–H]• is formed with
the hydrogen atom at boron, as one might expect, the hydrogen
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atom immediately hops from the boron atom onto the aromatic
ring system, until it arrives at the para carbon atom which is the
most stable isomer in the case of 1 (whereas the meta position is
most stable in 2, see Table S1). This is supported by what is
observed experimentally during the EPR monitoring of hydrogen
splitting by 1 where the magnitude of the resulting H• atom
hyperfine coupling fits well with coupling to spin density on the
ring system in the para position located between the two nitro
groups (Figure 2d).
If the parent borane is present in excess of the radical anion
(Propagation (2b)), the hydrogen atom produced in step (1a)
(considered as [Ar2B(H)–Ar(H)]•–) may be transferred, and the
borohydride product and the neutral [Ar3B–H]• radical
intermediate is formed. Using the values calculated for
propagation steps 1a and 2a, step 2b is energetically neutral. In
the system reported herein, it is unlikely that the parent borane is
present in excess of the radical anion initially, but as the reaction
proceeds through step 3b and the consumption of the [Ar2B(H)–
Ar(H)]•– progresses, this stabilization may become more relevant
towards the end of the reaction. This situation may also have
relevance to potential radical–FLP hydrogen cleavage
mechanisms, where the parent borane is most likely present in
excess of any potential radical anion intermediates throughout.
The final step in the reaction, which cannot be observed by
EPR, is the formation of the diamagnetic [1–H]– product, which is
Figure 4. Postulated reaction profile showing the relevant reaction
intermediates involved in each step (ring substituents removed for clarity, steps
labeled as in Figure 3) together with the associated change in energy values
along each reaction step obtained from DFT calculations.
In summary, using two model boranes which produce stable
radical anions upon one-electron reduction, we have successfully
demonstrated homolytic dihydrogen cleavage in the absence of a
Lewis base. This represents a new mode of chemical reactivity by
Lewis acidic boranes towards H2 that opens up new borane and
potentially other main group chemistries beyond the framework of
conventional FLPs. The reaction between the model borane
radical anions and H2 is slow, and the intermediates are
sufficiently stabilized so that, for the first time, we can observe
several distinct intermediates along the homolytic dihydrogen
cleavage pathway using EPR spectroscopy and can model the
energetics of the reaction pathway computationally. We are
currently exploring the application of boryl radical H2 activation as
a convenient route to more active borane hydride species, which
may have application in catalysis and energy materials.
1
detected by 11B and H NMR analysis of the reaction mixture at
the end of the experiment. Aside from the obvious recombination
of 2H• to form H2 (the reverse of step 1), there are two termination
pathways: Termination (3a) (−39.5 and −49.6 kcal·mol-1 for 1 and
2, respectively), and Termination–Propagation (3b) (−39.3 and
−48.2 kcal·mol-1 for 1 and 2, respectively). Step 3a may also be
written as [Ar3B]•– + H•  [Ar3B–H]– for consistency with the rest
of the scheme, or as a termolecular reaction: 2[Ar3B]•– + H2 
2[Ar3B–H]–. Step 3b yields both the terminal borohydride product
and regenerates the parent neutral borane for further reaction in
propagation step 2a. Note that whilst it would appear from Figures
2c–d that the EPR spectra are dominated by the [1–{H2}]•– and [1–
H]• species, respectively, simulation of the spectral data reveals
that these spectra are each superimposed over the parent 1•–
radical anion species. As the reaction proceeds with heating the
weighting between the systems changes (1•– : [1–{H2}]•– = 98.5 :
1.5 in Figure 2b; 73.0 : 27.0 in Figure 2c, and 1•– : [1–H]• = 63.0 :
37.0 in Figure 2d. The rate of consumption of 1•– as measured by
EPR (Figures 2a–d) correlates with the rate of conversion to
borohydride as measured by NMR spectroscopy (Figure 1a).
Acknowledgements
The research leading to these results has received funding from
the European Research Council under the ERC Grant Agreement
no. 307061 (PiHOMER). GGW and AEA thank the Royal Society
for financial support via University Research Fellowships
(UF/130336 and UF/160395 respectively). FM thanks the Royal
Society for support via a Wolfson Research Merit Award. AM
acknowledges the faculties of Science and Medicine at the UEA
for funding a PhD studentship. JCS acknowledges the Council for
Chemical Sciences of The Netherlands Organization for Scientific
Research (NWO/CW) for a VIDI grant (723.012.101). We
acknowledge the use of the EPSRC funded National Chemical
Database Service hosted by the Royal Society of Chemistry, and
the EPSRC UK National Mass Spectrometry Facility (NMSF) at
the University of Swansea. We thank the EPSRC UK National
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Crystallography Service at the University of Southampton for the
collection of the crystallographic data.
Keywords: Lewis acids • boranes • radicals • EPR • dihydrogen
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10.1002/anie.201900861
Angewandte Chemie International Edition
COMMUNICATION
Table 1. EPR spectral parameters obtained by simulation of the experimental spectra recorded in Figures 2A–D.
Simulated Spectra
Parameter
Figure 2a
1•–
Figure 2b
1•–
Figure 2c
1•–
Figure 2d
1•–
[1–{H2}]•–
2.00619
–––
[1–{H2}]•–
2.00640
–––
[1–H]•
2.00404
35.4
0.7
g-value
2.00475
23.2
2.00473
23.3
2.00473
23.3
2.00473
23.3
A (11B) / MHz
A (14N, meta–NO2) / MHz
A (1H, ortho–CH3) / MHz
A (1H, para–CH3) / MHz
A (1H) / MHz
3.6
3.4
36.5
3.4
37.3
3.5
4.8
4.2
–––
4.2
–––
4.2
0.5
7.9
7.8
–––
7.8
–––
7.9
1.8
–––
–––
–––
–––
–––
–––
32.2
0.26
37.0%
Linewidth (Gaussian) / mT
Weighting
0.15
0.25
0.22
0.25
0.22
0.20
–––
98.5 %
0.038452
1.5 %
73.0 %
0.034853
27.0 %
63.0%
0.060845
RMSD
0.022532
Entry for the Table of Contents
COMMUNICATION
We find that if Lewis acidic boranes
similar to those used in FLP chemistry
are exposed to H2 in the presence of
common reducing agents, instead of
the Lewis bases required in FLP
chemistry, then homolytic H2 cleavage
occurs. For the first time, we are able
to experimentally observe and
Elliot L. Bennett, Elliot J. Lawrence,
Robin J. Blagg, Anna S. Mullen, Fraser
MacMillan, Andreas W. Ehlers, Daniel J.
Scott, Joshua S. Sapsford, Andrew E.
Ashley,* Gregory G. Wildgoose,* and
J. Chris Slootweg*
Page No. – Page No.
structurally characterize a series of
intermediates formed during the H2
cleavage process at a main group
borane complex.
A New Mode of Chemical Reactivity
for Metal–Free Hydrogen Activation
by Lewis Acidic Boranes
This article is protected by copyright. All rights reserved.