Reductive Dehydrogenation of a Stannane via Multiple Sn?H Activation by Frustrated Lewis Pairs

Article abstract of DOI:10.1002/anie.201610254

A bulky substituted stannane Ar*SnH₃(Ar=2,6-(2′,4′,6′-triisopropylphenyl)phenyl) was treated with the well-known frustrated Lewis pair (FLP) PtBu₃/B(C₆F₅)₃in varying stoichiometries. To some degree, h

Full text of DOI:10.1002/anie.201610254

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International Edition: DOI: 10.1002/anie.201610254
German Edition: DOI: 10.1002/ange.201610254
Frustrated Lewis Pair
À
Reductive Dehydrogenation of a Stannane via Multiple Sn H
Activation by Frustrated Lewis Pairs
Christian P. Sindlinger,* Frederik S. W. Aicher, Hartmut Schubert, and Lars Wesemann*
Abstract: A bulky substituted stannane Ar*SnH3 (Ar* = 2,6-
(2’,4’,6’-triisopropylphenyl)phenyl) was treated with the well-
known frustrated Lewis pair (FLP) PtBu3/B(C6F5)3 in varying
stoichiometries. To some degree, hydride abstraction and
adduct formation is observed, leading to [Ar*SnH2(PtBu3)]+
which is rather unreactive toward further dehydrogenation. In
a competing process, the FLP proved to be capable of
completely striping-off hydrogen and hydrides to generate the
first cationic phosphonio-stannylene [Ar*Sn(PtBu3)]+. This
behavior provides insight into the activation/abstraction mech-
anism processes involved in these Group 14 hydride deriva-
tives.
dehydrogenation of organotin(IV) trihydrides we anticipated
the abstraction of hydrogen in its ionic, charge separated form
of H+/HÀ from the rather non-polar stannane Ar*SnH3 by an
FLP. Successful dehydrogenation using N-heterocyclic car-
benes (NHC),[7] which can be seen as single-atom-based
“frustrated” Lewis pairs, encouraged us to use the iconic P/B
FLP PtBu3/B(C6F5)3.[1c,8] Reductive dehydrogenation of
a dihydrogermane with a NHC/B(C6F5)3 FLP was previously
described by Roesky.[9]
We now exposed the sterically protected stannane
Ar*SnH3 to the FLP mixture and predicted the generation
of dimeric Ar*SnH along with [HPtBu3]+ and [HB(C6F5)3]À
T
he activation of small, non-polar molecules, such as hydro-
gen or silanes, via strong polarization stemming from the
combination of sterically hindered Lewis acids and bases (so
called “frustrated” Lewis pairs, FLP) has been a successful
concept for a variety of synthetic chemical applications for the
last decade.[1]
It was strongly suggested, that the key step toward the
À
activation of E H moieties is the initial interaction with the
strong Lewis acids (LA) leading to labile, hydride-bridged
À
[RnE H····LA] complexes and generating a positively polar-
ized E center activated for further nucleophilic attacks.[2] It
was only recently, that such elusive complexes have been
isolated and studied in the solid state.[3]
Since reports of borane-promoted carbonyl hydrosilyla-
tions by Piers and co-workers,[4] the B(C6F5)3 activation of
Scheme 1. Reductive dehydrogenations of Ar*SnH3 using a frustrated
Lewis pair under formation of mixtures including a phosphine-stabi-
lized SnIV cation [Ar*SnH2(PtBu3)]+ (1) and a phosphonio-stannylene
[Ar*Sn(PtBu3)]+ (2).
[5]
À
R3E H (E = Si, Ge, Sn) toward additions to unsaturated
functional groups has been intensely studied with a strong
focus on silanes. For the reaction of B(C6F5)3 with R3SnH
À
(R = alkyl), owing to the weaker Sn H bond, complete
hydride abstraction was reported.[5d,6] We are particularly
interested in the chemistry of organotin trihydrides RSnH3
(R = bulky substituents) which may provide up to three
hydrogen atoms. In the course of our interest in the reductive
(Scheme 1, top). However, if one equivalent of borane is
added to an equimolar mixture of Ar*SnH3 and PtBu3 in
benzene, instead of forming a blue solution of [Ar*SnH]2,[10]
the colorless mixture immediately turned deep red-orange
and slowly a deep red oily phase separated. On addition of the
polar solvent (o-C6H4F2) a homogeneous mixture is obtained.
Neither phosphine nor borane individually revealed obvious
reactions with Ar*SnH3, also marking the distinguished less-
hydridic nature of Ar*SnH3 compared to R3SnH.[5d,6,11]
The resulting solution was revealed to be a defined
mixture of compounds including some residual Ar*SnH3
(Scheme 1, below). All nuclei are NMR-active and (except
for 11B) feature spins of I = 1/2 allowing an almost unambig-
uous assignment of the molecular constitutions by means of
heteronuclear NMR spectroscopy. Along with some free
phosphine, the mixture contains the expected phosphonium
cation [HPtBu3]+ and the hydridoborate [HB(C6F5)3]À. The
[*] Dr. C. P. Sindlinger, F. S. W. Aicher, Dr. H. Schubert,
Prof. Dr. L. Wesemann
Institut fꢀr Anorganische Chemie
Eberhard Karls Universitꢁt Tꢀbingen
Auf der Morgenstelle 18, 72076 Tꢀbingen (Germany)
E-mail: lars.wesemann@uni-tuebingen.de
christian.sindlinger@gmx.de
Dr. C. P. Sindlinger
Present address:
Department of Chemistry, University of Oxford
12 Mansfield Road, OX13TA, Oxford (UK)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201610254.
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sharp shape of 1H- and 19F-NMR-signal sets for [HB(C6F5)3]À
are indicative of complete consumption of B(C6F5)3 in these
mixtures.
First, a phosphine adduct to the SnIV dihydrostannyl
cation [Ar*SnH2(PtBu3)]+ (1) is formed. Only a few phos-
phine adducts to stannylium ions are known.[12] The NMR
spectra characteristically feature a doublet signal at dH =
5.90 ppm for the two Sn-bonded hydrides. These split up to
2
a doublet arising from a JP-H coupling of 40 Hz and further
1
reveal 119/117Sn satellites with a JSn-H coupling constant of
2100/2008 Hz. This coupling constant is almost 180 Hz greater
than that observed for the parent neutral Ar*SnH3; an
Figure 1. ORTEP of [Ar*Sn(PtBu3)]+ (2). Thermal ellipsoids are set at
50% probability and protons, the counteranion, and lattice benzene
are omitted for clarity. Canonical frontier molecular orbitals at an
isodensity value of. 0.05 are shown. Selected distances [ꢂ] and
angles [8]: Sn–P 2.7071(3), Sn–C1 2.1876(12); closest Sn-ortho-CTrip of
the two Trip groups 3.27/3.35; P-Sn-C1 113.38(3), C4-C1-Sn 158.0(1).
HOMO–LUMO gap: 5.610 eV.
increase that may be rationalized by a higher Sn s-orbital
1
share in the Sn H bonding. The H-coupled 119Sn NMR signal
À
appears at dSn = À340 ppm as a triplet, corroborating the
constitution of a Sn atom with two hydrogen atoms attached
(further splitting from a small 1JSn-P coupling is not resolved).
The shift is in the range of tetra-substituted stannanes and the
predicted 119Sn chemical shift for a computationally deter-
mined structure of this cation is in reasonable agreement
(dSn,calcd = À382 ppm). The 31P NMR signal for [Ar*SnH2-
(PtBu3)]+ (1) is observed at dP = 72.1 ppm with tin satellites
revealing a very small 1JSn-P coupling of only 32 Hz which is an
order of magnitude smaller than in [Me3PSnMe3]+.[12c] The
formation process of 1 resembles Burfordꢀs approach to
stannylphosphonium cations.[12c]
phenomenon for ArSnX and usually rationalized by steric
effects.[16] The P-Sn-C angle amounts to 113.38(3)8. From the
combination of a large R-E-(L) angle for a cationic [RE(L)]+
species we anticipate further interesting reactivity.[14a,17] The
flanking Trip groups mildly tilt over towards the cationic tin
center indicating weak arene interactions. The Sn–arene
distances are comparable to those computationally predicted
for [Ar’Sn(MeNHC)]+ (Ar’ = 2,6-Mes2C6H3-, Mes = 2,4,6-
Me3C6H2-).[18] However, computational probing reveals a sig-
nificant impact on the 119Sn NMR shift (Figure 2) and there-
The second compound formed in the reaction is assigned
to the phosphine adduct to the SnII cation [Ar*Sn(PtBu3)]+
(2). It reveals a 119Sn NMR resonance at dSn = 1420 ppm as
a doublet due to strong 1JSn-P coupling of about 1990 Hz, in the
range known for P–SnII compounds.[13] The downfield shift is
indicative of dicoordinate SnII compounds and the computa-
tionally predicted 119Sn NMR shift (dSn,calcd = 1387 ppm) is in
excellent agreement with the experimental data. No further
splitting of the signal is observed in 1H-coupled spectra,
confirming that all hydrogen atoms have been removed from
the Sn. 31P NMR spectroscopy reveals a signal at dP =
95.0 ppm with the respective tin satellites. The drastic increase
1
of JSn-P coupling constants upon reductive dehydrogenation
from [Ar*SnH2(PtBu3)]+ (1) to [Ar*Sn(PtBu3)]+ (2) by almost
two orders of magnitude may mark the differing nature of the
Figure 2. Computational examination of the arene interaction on the
119Sn NMR shift on hypothetical [ArSn(PtBu3)]+ derivatives. Structural
modifications have been made to the optimized [Ar*Sn(PtBu3)]+
structure and the shifts were directly computed on the basis of these
structures without further re-optimization. Shifts dSn are given in parts
per million, relative to tetramethylstannane.
À
P Sn bond in these species. The Sn–P coupling behaves invers
1
À
to couplings of the Sn H moiety where JSnIV-H is often one
order of magnitude bigger than 1JSnII-H. For neutral phosphinyl
1
stannylenes a JSn-P coupling of around 1450 Hz was repor-
ted.[13a]
A
31P-EXSY-NMR experiment only revealed chem-
ical exchange of the free phosphine with [Ar*Sn(PtBu3)]+ (2),
but not with [Ar*SnH2(PtBu3)]+ (1) on the examined NMR
time scale.
fore strongly indicates an electronic interaction.[18] Flanking
arene interactions in Group 14 cations have been observed
previously.[19] Attempts to obtain crystals of [Ar*SnH2-
(PtBu3)]+ (1) from the reaction mixtures only yielded color-
less needles of phosphonium hydridoborate [HP(tBu)3][HB-
(C6F5)3].[1c]
X-ray diffraction studies revealed the molecular structure
of 2 with the phosphine being arranged almost symmetrically
between the two flanking Trip-groups (Trip = 2,4,6-iPr3C6H2-;
Figure 1). Compound 2 represents an example of the rare
structural motif of cationic, dicoordinate tetrylenes.[14] An
example of phosphine adducts to cationic SnII transition metal
According to 31P NMR spectroscopy, the initial integra-
tion ratio of 2/1/PtBu3/[HPtBu3]+ is (roughly) 1.3:2:1:2.[20]
These mixtures were formed within a couple of minutes and
only slowly showed some further conversions and partial
decomposition over several days. Small amounts of dehydro-
coupling product Ar*H2SnSnH2Ar* were formed under
complexes is known.[15] The Sn P bond of 2.7071(3) ꢁ is fairly
À
long compared to a neutral phosphino stannylene Ar*SnP-
(SiMe3)2 (2.527(1) ꢁ).[13a] The tin atom strongly deviates (ca.
228) from the central phenyl plane, which is an established
2
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consumption of residual Ar*SnH3. After two weeks, the 31P-
NMR spectrum revealed integration ratios of approximately
1.7:1.3:1:4 indicating further consumption of the PtBu3 and
[Ar*SnH2(PtBu3)]+ (1) in favor of [HPtBu3]+ and distannane
Ar*H2SnSnH2Ar*.
However, we found no indication for the formation of the
phosphine adduct to the neutral hydrostannylene Ar*SnH-
(PtBu3) which is considered to be the intermediate after
deprotonation of [Ar*SnH2(PtBu3)]+ (1) by the phosphine
(Scheme 2). Ar*SnH(PtBu3) is not stable and expected to
readily dissociate to Ar*SnH. The formation of
Ar*H2SnSnH2Ar* is traced back to Ar*SnH reacting with
Ar*SnH3.[22] The dimeric hydrostannylene [Ar*SnH]2 could
not be detected during the reaction.[10]
Table 1: Composition of the reaction mixture from the reaction of
Ar*SnH3 + 2 equiv FLP in C6D6/o-C6H4F2 over time according to 31P NMR
spectra integrals (in %).[20]
Time
[Ar*Sn(PtBu3)]+ [Ar*SnH2(PtBu3)]+ PtBu3 [HPtBu3]+
25 min
7 days
20
30
28
13.6
6.4
22
9.1
4.3
30
47.3
57.3
14 days[a] 32.1
[a] 1H NMR spectroscopy also reveals formation of a minor amount of
new unassigned products without a corresponding 31P signal.
fact, that the expected product [Ar*Sn(PtBu3)]+ (2) is not
formed quantitatively in these reaction mixtures that still
contain both unreacted B(C6F5)3 and PtBu3, was surprising.
Over two weeks, along with some minor decomposition
Considering that [Ar*SnH2(PtBu3)]+ (1) is the formal
product of the reaction with one equivalent of FLP and
[Ar*Sn(PtBu3)]+ (2) the respective product of the reaction
with two equivalents of FLP, we reasoned that selective
formation of [Ar*Sn(PtBu3)]+ (2) should occur if Ar*SnH3
was treated with two equivalents of the FLP. Under these
1
(only apparent in the H NMR spectrum), [Ar*Sn(PtBu3)]+
(2) slowly becomes the predominant Sn-containing species
while [Ar*SnH2(PtBu3)]+ (1) and FLP are consumed. The
NMR signals of the hydridoborate in the late-stage mixture
are sharp, indicating that B(C6F5)3 has fully been converted
into the hydridoborate.
1
conditions, the initial H NMR spectrum confirms complete
consumption of Ar*SnH3 and formation of a mixture exclu-
sively containing [Ar*SnH2(PtBu3)]+ (1), [Ar*Sn(PtBu3)]+
(2), PtBu3, and [HPtBu3]+ along with [HB(C6F5)3]À. Unlike
in the 1:1 mixture, the hydride signal for [HB(C6F5)3]À appears
as a broad singlet rather than a broad quartet, which indicates
the presence of residual free B(C6F5)3 interacting with the
hydridoborate. This assumption is further supported by
19F NMR spectroscopy which reveals broad resonances,
rather than sharp signal sets for pure [HB(C6F5)3]À.
31P NMR spectroscopy provides a rough approximation of
the relative ratios of phosphine derivatives (Table 1), differ-
ing from the reaction with only one equivalent of FLP. The
The initially fast, then strongly decelerated formation rate
of [Ar*Sn(PtBu3)]+ (2) as the product of the reaction of
Ar*SnH3 with two equivalents of the FLP, is puzzling. To
rationalize this, we propose the initial step to be a reversible
formation of a weakly associated borane–stannane complex
(I, Scheme 2) of Ar*SnH3 and B(C6F5)3. Similar structurally
characterized silane-borole/silane-alane complexes have been
isolated by the groups of Piers and Chen.[3] Although no
obvious change in NMR spectroscopic features becomes
apparent on mixing Ar*SnH3 and B(C6F5)3 in benzene
solutions at room temperature, we observed a significantly
increased H–D scrambling rate in mixtures of Ar*SnH3 and
Scheme 2. Suggested rationalization of the hydride/proton/hydride abstraction sequences for the formation of [Ar*Sn(PtBu3)]+ (2).[21]
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Ar*SnD3 when the borane is present. This is an indication for
activation involving the putative borane stannane complex
(Scheme 2).[23] The stannane–borane complex can now react
with the phosphine in two distinct pathways (Scheme 2, IIa/
IIb). Since the borane does apparently not react with
Ar*SnH3 to fully abstract a hydride forming [Ar*SnH2][HB-
(C6F5)], the involvement of the phosphine is crucial to
Forschungsgemeinschaft (DFG). C.P.S. is indebted to Prof.
Simon Aldridge (Oxford) as a supportive host and for the
provision of access to computational resources.
Conflict of interest
À
successful heterolytic cleavage of Sn H bonds. We propose
the target of the nucleophilic phosphine attack to be criterion
for determining the final composition of products.
The authors declare no conflict of interest.
À
Keywords: cations · dehydrogenation · E H activation ·
According to step IIa (Scheme 2) the phosphine reacts, as
originally anticipated, as a Brønsted base attacking the
frustrated Lewis pair · stannylenes
À
positively polarized, protic Sn H moiety and abstracting
a proton. The elusive Ar*SnH then further reacts with
a second equivalent of FLP forming the neutral base-
stabilized stannylene (step IIIa, Scheme 2) and hydride
abstraction (step IV).[24] In the second pathway, IIb, the
phosphine attacks at the Sn atom forming the dihydrostannyl
cation [Ar*SnH2(PtBu3)]+ (1) in a substitution reaction
similar to the formation of Burfordꢀs [Me3SnPMe3]+.[12c]
According to 31P EXSY-NMR spectra, this adduct formation
is irreversible. For steric reasons, a proton abstraction from
[Ar*SnH2(PtBu3)]+ (1) by PtBu3 is not favorable and thus
slow, preventing rapid further conversion. To explain the
experimentally observed formation of product mixtures,
step II is considered to be unselective with proton abstraction
IIa and adduct formation IIb being competing mechanistic
routes of similar reaction rates. On the way to [Ar*Sn-
(PtBu3)]+ (2), substantial amounts of tin hydride species are
therefore trapped by formation of the rather unreactive SnIV
species [Ar*SnH2(PtBu3)]+ (1). Even after prolonged reaction
times we have not observed substantial (i.e. greater than 3–
5%) decomposition of the cationic phosphonium compounds
via the deprotonation/isobutene elimination sequence that
would either lead to phosphinyl stannane Ar*SnH2PtBu2 or
phosphinyl stannylene Ar*SnPtBu2.
Herein we showed that stannanes can be reductively
dehydrogenated by FLP and that the FLP is able to
completely strip off all the hydrogen atoms from a mono-
organo stannane to form a cationic phosphonio-stannylene
via reductive dehydrogenation. Observation of a dihydros-
tannyl phosphonium cation provided valuable and unprece-
dented insight into the competing processes involved in the
stannane–FLP interaction. This may be fruitful for further
studies of FLP-activated Group 14 hydrides RnEH4Àn—espe-
cially with n ꢀ 2. We are currently exploring further reactiv-
ities of the cationic species created and trying to shed light on
the influence of the organic substituent at Sn.
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4
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[20] Please note, that these integrals do not necessarily correlate with
the molar distribution, since the 31P experiment was not
specifically set up for the integrability (d1 = 1s).
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[23] J. Koller, R. G. Bergman, Organometallics 2012, 31, 2530 – 2533.
[24] The order of these two steps can also be reversed.
[18] C. P. Sindlinger, F. Aicher, L. Wesemann, Inorg. Chem. 2016,
2017, 56, 548—560.
Manuscript received: October 19, 2016
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Frustrated Lewis Pair
C. P. Sindlinger,* F. S. W. Aicher,
H. Schubert,
L. Wesemann*
&&& — &&&
Reductive Dehydrogenation of
À
a Stannane via Multiple Sn H Activation
by Frustrated Lewis Pairs
You can’t leave your H on! Frustrated
Lewis pairs are shown to completely
abstract hydrogen from a monoorgano
stannane.
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!