Reactivity of an NHC-stabilized pyramidal hydrosilylene with electrophilic boron sources

Article abstract of DOI:10.1039/c9dt00608g

Silylenes have become an indispensable tool for molecular bond activation. Their use for the construction of silicon-boron bonds is uncommon in comparison to the numerous studies on silylene-derived silicon-element bond formations. Herein we investigate t

Full text of DOI:10.1039/c9dt00608g

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DOI: 10.1039/C9DT00608G
ARTICLE
Reactivity of an NHC-stabilized pyramidal hydrosilylene with
electrophilic boron sources
a
a
b
a
a
Gizem Dübek, Daniel Franz, Carsten Eisenhut, Philipp J. Altmann, Shigeyoshi Inoue*
Received 00th January 20xx,
Accepted 00th January 20xx
Silylenes have become an indispensable tool for molecular bond activation. Their use for the construction of silicon-boron
bonds is uncommon in comparison to the numerous studies on silylene-derived silicon-element bond formations. Herein we
DOI: 10.1039/x0xx00000x
investigate the reactivity of the pyramidal NHC-coordinated hydrosilylene tBu
LMe4 = 1,3,4,5-tetramethylimidazolin-2-ylidene) with various boron-centered electrophiles. The reaction of 1 with THF·BH
or H N→BH afforded the silylene complex 1→BH or the product of insertion of the silicon(II) atom into an N–H bond with
concomitant dehydrogenation along the HN–BH moiety (2). The respective conversion of 1 with BPh yields 1→BPh which
readily reacts with excess LMe4 to form the more stable complex LMe4→BPh
with release of 1. Treatment of 1 with the
haloboranes Et O→BF , BCl , BBr and Me S→BBr resulted in the formation of the Lewis acid-base adducts 1→BX (X = F,
Cl, Br) and an equilibrium with their auto-ionization products [1 BX significantly
]+[BX ]− slowly develops. The ratio of 1→BX
increases with rising atomic number of the halide, thus 1→BF majorly transforms within hours while 1→BBr is near-
quantitatively retained over time. Accordingly, the complex 1→BPhBr was isolated after conversion of 1 with PhBBr
3
SiSi(H)LMe4 (1; NHC = N-heterocyclic carbene,
3
3
3
3
3
3
3
2
3
3
3
2
3
3
2
2
4
3
3
3
2
2
.
insertion pathways. Taking into account their ylidenic character
it does not come as a surprise that there has been made
frequent use of silylenes as electron-pair donors toward boron-
centered electrophiles. An early report of Metzler and Denk
from 1996 described the formation of the adduct between a
Introduction
There has always been a strong link between the chemistry of
silicon and boron as implied by the diagonal relationship of
these metalloids in the Periodic Table of Elements. Boron-
doped silicon semiconductors are a prominent example from
material science in which the combination of these elements
five-membered ring N-heterocyclic silylene (NHSi) and B(C
6 5 3
F )
which slowly transforms to the product of Si-insertion into a B–C
1
6
bond. However, neither the adduct nor the insertion product
were structurally characterized in the solid state (i.e. XRD study,
XRD = X-ray diffraction). In fact, more than 20 years
1
-3
fostered tremendous innovation.
Molecular chemistry
benefits from the particular properties of the silicon-boron
bond. It is sufficiently stable to craft durable compounds but
also susceptible to mild methods of chemoselective cleavage to
enable metallyl group transfer, thus, enriching the ever-growing
library of organometallic synthesis. The ylidenic compound
class of silylenes are a subtype of molecular silicon complexes
with one lone pair majorly located at the metalloid center. As a
result, the formal oxidation number +II is assigned to the silicon
atom. Silylenes have gained outstanding attention as key
4
-6
compounds to bring forward new ways for bond activation and
catalysis.4
,7-15
In particular, silylenes may act as ligands to
enhance the catalytic activity of transition metal
complexes.9
,10,14
Moreover, the potentially ambiphilic silicon(II)
atom itself may engage in bond activations via addition and
a. Department of Chemistry, WACKER-Institute of Silicon Chemistry and Catalysis
Research Centre, Technische Universität München, Lichtenbergstr. 4, 85748
Garching bei München.
b.
Institut für Chemie, Technische Universität Berlin, Straße des 17. Juni 135, Sekr.
Fig. 1 Selected examples for outcomes of conversions of silylenes with
C2, 10623 Berlin, Germany.
various boron sources.
†
Electronic Supplementary Information (ESI) available: Depiction of analysis
spectra and crystallographic details. CCDC 1896328-1896331. See
DOI: 10.1039/x0xx00000x
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Journal Name
As a distinct characteristic 1 marks three sites thVaietwmArtaicylerOenalincet
5
2
later, one can easily survey the stock of compounds in which
conversions of a silylene with an electrophilic borane derivative with boron-centered electrophiles: (i) tDhOeI:y1l0id.1e0n39ic/Cs9iDteT0a0t60t8hGe
resulted in the structural characterization of a species that silicon center, (ii) the dative bond between the NHC and the
1
7-39
contained a silicon-boron bond.
Often a silicon-coordinated silicon atom which may be cleaved by electrophilic attack, and
borane moiety affords isolation of an otherwise elusive kind of (iii) the Si–H functionality which may readily transfer a hydrogen
silylene (i.e. “acceptor stabilization”). A prominent example for atom (Scheme 1). Considering the importance of silicon-boron
1
this concept is the silylene dihydride A described by Rivard and compounds to the community it was apparent to systematically
coworkers (Fig. 1).30 Structurally related compounds have also study the reactivity of 1 towards various types of electrophilic
2
3 29,33
been reported (A , A ).
Bis(guanidato)-, as well as boron-sources commonly encountered in molecular chemistry
bis(amidinato)silylenes tend to switch between isomers with a (e.g. hydro-, organo-, haloboranes). We took the marked
three- or with a four-coordinate silicon atom. The high- scarceness of reports on simple Lewis acid base adducts of
coordinate species may be stabilized in the form of borane silylenes with haloboranes as a particular motivation for our
adducts of type B1 (Fig. 1).
-3
23,26
The amidinato ligand was also investigation.
implemented in four-coordinate adducts between silylene and
1
,2
21,31
the trihydroborane group (C , Fig. 1).
Similar to type B the
silylene adducts D1 are observed in solution whereas for the
-3
Results and discussion
Conversions with trihydroborane complexes.
respective “free” silylenes (with no borane moiety attached) the
coordination number of the silicon centers rapidly changes (Fig.
A plethora of chemical transformations has derived from
complexes of NHCs with the parent borane (i.e. NHC→BH ).
3
In sharp contrast, only few silylene adducts with the
trihydroborane group are known as outlined above. In fact,
two-coordinate silylenes do not commonly form simple and
stable Lewis acid base adducts upon reaction with borane
1
7
1
).
5
3-56
Despite these various examples for “acceptor stabilization”
silylenes were often found to be prone to insertion of the silicon
atom into boron-heteroatom bonds as implied by the
1
6
pioneering study of Metzler and Denk (vide supra). Jutzi and
coworkers described the insertion of the high-coordinate
3 2 3 3 3
complexes (e.g. THF·BH , Me S→BH , H N→BH ; here stable
silylene center of Cp*
conversion with Cp*BCl
2
Si into boron-chloride bonds upon its
means to be isolatable at room temperature in the condensed
phase). This will be majorly due to the enhanced Lewis acidity
of low-coordinate silylenes which causes side-reactions (i.e.
hydride shift from B to Si) that follow after the initial
coordination between the metalloid centers. It is apparent, that
the prospect for forming stable Lewis acid-base adducts is
higher for three-coordinate silylenes as the silicon center is less
electrophilic because of the stabilizing effect of an additional
electron-pair donor. Accordingly, the conversion of 1 with a
slight excess (1.3 equiv.) of borane tetrahydrofurane complex
3
8
2
to afford E (Fig. 1). In agreement with
the high reactivity of bonds between boron and the heavier
halides (e.g. Cl, Br, I) this type of insertion was also observed for
low-coordinate silylenes (i.e. two-coordinate NHSi) as
demonstrated by Braunschweig and coworkers with the
isolation of F1 and by the group of Iwamoto (G , Chart 1).
,2
1,2
24
1
,2
As verified by the synthesis of H the ylidenic center in silylenes
may also insert into boron-hydrogen bonds, as well as
3
7
unpolarized boron-boron bonds (Fig. 1). Interestingly, the
group of Chiu reported the conversion of a two-coordinate
bulky NHSi with borabicyclo[3.3.1]nonyl triflate (9-(OTf)BBN) to
3 3
(THF·BH ) furnished the chiral adduct 1→BH (isolated: 93%) as
suggested by multinuclear NMR spectroscopy, high-resolution
mass spectrometry (HRMS), as well as XRD structural study
3
9
furnish the product of boron-oxygen insertion. Obviously, the
triflate group complies to its pseudohalide character and, thus,
the reactivity of the ambiphilic NHSi with the boron-triflate
functionality is reminiscent of Braunschweig’s study on treating
NHSi with organoborohalides. In addition to these synthetic
examples the reader is also referred to theoretical studies on
1
1
(
−
Scheme 1). The B NMR spectrum in C
40.8 ppm (J = 95 Hz) that collapses into a singlet in the proton
decoupled experiment. This confirms a BH group with a four-
coordinate boron nucleus (cf. A : δ( B) = −46.2 ppm, J = 93 Hz,
6 6
D reveals a quartet at
3
1
11
4
0
the insertion of silylene into boron-element bonds. In
consideration of the hitherto outlined scope of compounds we
were surprised that systematic investigations of one particular
silylene’s reactivity towards different types of boron sources is
rather uncommon with a respective report of Cui being a rare
example.25 In the context of discussing silylene-borane adducts
one should note the small number of boryl-substituted
silylenes, as well as the exceptional reaction of a disilicon(0)
complex with THF·BH
Moreover, silicon-silicon multiple bonded
systems with boryl functionalities have been reported.
3
to outstanding silylene-borane
complexes.3
4,41,42
4
3-47
Recently, we have isolated the NHC-stabilized hydrosilylene
and studied its reactivity towards transition metal complexes Scheme 1 Reactive sites at the NHC-stabilized hydrosilylene 1 and its
e.g. Ni(COD) , Fe(CO) , W(CO)
; COD = 1,5-cyclooctadiene) and conversions with trihydroboranes.
functional organic groups (e.g. carbonyls, alkynes; Scheme 1).
1
(
2
5
5
4
8-
2
| J. Name., 2012, 00, 1-3
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four-coordinate metalloid centers. In support of this reaction
View Article Online
DOI: 10.1039/C9DT00608G
3 3
pathway the formation of tBu SiSiH , as well as dihydrogenated
Me4
NHC (i.e. L
2
H , dihydrogenated at the formerly ylidenic carbon
atom) was suggested by NMR study and verified by HRMS
analysis. Accordingly, half an equivalent of 1 is consumed to
produce one equivalent of H
reacts with the remaining silylene to yield 2 (Scheme 1).
Reminiscent of this reactivity treatment of 1,3-
diketiminosilylene with gaseous NH has been described to
2 2
N–BH which in the following
a
3
5
7
afford the product of N–H insertion. On the other hand, the
conversion of
complex with H
a
1,3-diketiminosilylene tricarbonylnickel
3
N→BH resulted in dihydrogenation of the
3
silylene with one hydrogen atom bonded to the silicon center
5
8
and one additional hydrogen atom in the ligand backbone. In
1
1
the B NMR analysis of 2 in C
6 6
D the compound gives rise to a
Fig. 2 Ellipsoid plot (30% level) of the molecular structure of 1→BH
3
in signal at −18 ppm which is deshielded with respect to 1→BH in
3
the single crystal. H-atoms omitted (except on B, Si). Wireframe model accordance with replacement of a hydride at a four-coordinate
for tert-butyl groups. Selected interatomic distances [Å] and angles [°]: boron-center for a more σ electron-withdrawing nitrogen atom.
Si2-B1 = 2.009(5), Si2-C2 = 1.942(3); Si1-Si2-B1 = 123.3(2), C2-Si2- The Si–H proton is observed at 5.2 ppm (d, J = 4 Hz) in the
H
1
HH
B1 = 108.2(2), Si1-Si2-C2 = 111.3(2). Note: The Si2 and B1 atoms are NMR analysis. The 29Si NMR spectrum of the complex exhibits a
disordered over two sites (occupancy levels: 0.85/0.15 each) and only signal at −46.3 ppm for SiH which is downfield shifted from the
2
the higher occupied sites are considered.
precursor (1: δ(29Si) = −137.8 ppm). The single crystal structure
of 2 was elucidated by XRD methods (Fig. 3). As expected the B–
6 6
in C D
). In the 1H NMR spectrum the prominent Si–H N distance of 1.542(4) Å in this bulky silylaminoborane is shorter
functionality gives rise to a quartet at 4.31 ppm with the than the length of the dative bond in typical trihydroborane
3
coupling to the boron-bonded hydrogen atoms resolved ( JHH
5
=
2 3
complexes of bulky primary amines (e.g. H (Dip)N→BH , B–N =
Hz, 29Si satellites with JSiH = 150 Hz). Notably, this is markedly 1.620(2) Å; H (Ar*)N→BH , B–N = 1.641(3) Å; Ar* = 2,6-
2 3
1
59
shifted to lower field in comparison to the precursor (1: δ( H)SiH (Ph CH) -4-Me-C H ). In agreement with the proposed
2
2
6 2
4
8
=
6 6
3.17 ppm in C D ). Despite the quadrupolar momentum of dehydrogenation this distance relates to the B–N single bond
the B nucleus the NMR signal of the Si atom is observed as a length in the bulky organylaminoborane adduct L ·BH N(H)Dip
broad peak at −77 ppm in the INEPT experiment. As a prominent (B–N (mean) = 1.54 Å).29
1
1
29
Dip
2
structural parameter the Si–B distance in the single crystal XRD
1
study of 1→BH
3
is found at 2.009(5) Å which resembles A
Conversions with triarylboranes.
2
,3
2
(
1.992(2) Å) and is longer than in A (1.976(2) Å (A ), 1.965(2)
If a mixture of a strong Lewis acid (typically a triarylborane) and
a Lewis base does not recombine to the respective adduct
3
1,2
1
2
21,30
Å (A )), as well as C (1.962(1) Å (C ), 1.972(2) Å (C ), Fig. 2).
The bond length between the silicon center and the ipso-carbon
atom of the NHC group seems to be unaffected by coordination
of the silylene to the BH
→BH , as well as for 1). In contrast, the band of the Si–H
stretching mode in 1→BH
3
fragment (Si–CNHC: 1.942(3) Å for
1
3
3
is observed at higher wavenumber
2083 cm ) in comparison to the “free” silylene 1 (1984 cm )
−1
−1
(
which suggests strengthening of the Si–H interaction upon
bonding to the Lewis acid.
As an alternate source of the trihydroborane group we
converted
Me N→BH
temperature which agrees with the pronounced stability of this
adduct as compared to THF·BH . On the contrary, the treatment
of 1 with one equivalent borane ammonia complex (H N→BH
furnishes the silylaminoborane complex 2 (46% yield isolated)
Scheme 1). Its formation can be rationalized by
dehydrogenation of H N→BH to produce H N–BH (note: this
1
with borane trimethylamine complex
(
3
3
). However, no reaction occurred at room
3
3
3
)
(
3
3
2
2
aminoborane is prone to aggregation) and formal insertion of Fig. 3 Ellipsoid plot (30% level) of the molecular structure of 2 in the
the silylene into the N–H bond. In the course of this reaction the single crystal. H-atoms omitted (except on B, N, Si). Wireframe model
NHC ligand ends up attached to the boron center which is for tert-butyl groups. Selected interatomic distances [Å] and angles [°]:
reasonable as otherwise a five-coordinate silicon complex and Si2-N3 = 1.703(3), B1-N3 = 1.542(4), B1-C1 = 1.635(4); Si1-Si2-
a three-coordinate boron atom would coexist instead of two N3 = 114.4(1), Si2-N3-B1 = 123.2(2), N3-B1-C1 = 110.2 (2).
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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3
after exposing 1 to the
Considering the isolation of 1→BH
parent borane source THF·BH we dec
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D
i
O
deI:
d
10.
t
1
o
039
p
/
r
C
o
9
b
D
e
T0
bo n
60ro8G
0
3
halide reagents going from lower to higher atomic number.
When the three-coordinate silylene 1 in toluene solution was
brought into contact with Et O→BF a colorless precipitate
2 3
rapidly formed (Scheme 3). The solid was obtained in 87% yield
after a period of 2 h and redissolved in deuterated
fluorobenzene. The 11B NMR analysis reveals a quartet at 4.8
ppm which is produced by a BF
3
group and deviates from the
Scheme 2 Conversions of the hydrosilylene 1 with triarylboranes.
precursor (note: Et O→BF is the external standard for the 0
2
3
ppm value of the 11B nucleus). Moreover, weaker signals were
observed at 10.5 ppm (triplet) and 0.4 ppm (sharp singlet),
respectively. The shifts agree with four-coordinate boron
(
usually due to steric hindrance) the chemical potential of the
system may be exploited for the activation of bonds in
unhindered Small Molecule substrates (i.e. Frustrated Lewis
Pair (FLP) chemistry).60-65 In order to probe the bulky silylene 1
for FLP characteristics it was converted with the archetypical
boron-centered
centers and the resonances are diagnostic for a BF
BF
]−. In fact, the intensity of the weaker signals significantly
increased upon storage of the sample tube for a period of 20 h
while the ratio of the BF species decreased in the mixture.
2
group and
[
4
Lewis
acids
triphenylborane
and
3
tris(pentafluorophenyl)borane (Scheme 2). The conversion of 1
SiSi(H)LMe4
was expected to afford the complex LMe4→BPh
3 3
with BPh which
Accordingly, a signal pattern assigned to one tBu
3
1
may easily form via abstraction of the NHC from the silylene.66
moiety had been observed in the H NMR spectrum after 2 h
reaction time and two additional signals in a 1:1 ratio rose after
storage of the sample for 20 h. From the NMR study we
The cleavage of the dative type bond between silicon and NHC
by virtue of triarylborane has previously been reported to afford
such type of NHC-borane adducts.25,66,67 However, when
repeating the conversion in C
monitoring the course of the reaction we recognized the
formation of the proposed complex 1→BPh
ppm is assigned to the SiH hydrogen in the H NMR analysis
3
which is reminiscent of the respective chemical shift in 1→BH .
The B NMR spectrum reveals a signal at −3.2 ppm. Hence, the
conclude that the initial product 1→BF
3
slowly transforms into
[
1 BF
2 2
]+[BF ]−. Notably, the transformation equilibrates over
4
6 6
D in an NMR sample tube and
time and full reaction to the auto-ionization product was not
observed.
Moving our systematic investigation to the next heavier
halide we treated a yellow colored solution of 1 in toluene with
3
. A signal at 4.38
1
1
1
3
one equivalent of BCl (as a 1 M solution in heptane, Scheme 3).
1
1
As expected a colorless solid was isolated in 71% yield and
combustion elemental analysis confirmed the stoichiometric
signal of the three-coordinate borane precursor (cf. BPh
O)68 is shifted to a value for the chemical shift
67 ppm in Et
typical of four-coordinate boron nuclei. Another persuasive hint
3
: δ( B)
=
2
composition of the trichloroborane compound 1·BCl
NMR analysis revealed the product to contain one major type
of the tBu
SiSi(H)LMe4 moiety and additional species are hardly
found. The 11B NMR analysis (in CD
Cl ) showed a resonance at
.5 ppm which is in accordance with a four-coordinate boron
3
. The 1H
1
13
toward the putative 1→BPh
correlation experiment in which a cross-peak is given rise to by
coupling between the ipso-carbon atoms of the BPh group and
the SiH hydrogen atom. We verified the stability of the complex
in C solution at 50 °C for a period of two days. Nevertheless,
the "free" silylene 1 is readily released upon conversion of
→BPh is
with LMe4 and the overtly more stable LMe4→BPh
furnished. Interestingly, when bringing 1 into contact with the
more potent Lewis acid B(C we were not able to assign any
3
is given by the H C HMBC
3
2
2
3
5
6 6
D
1
3
3
6 5 3
F )
silicon-containing species neither in the product mixture nor as
a temporary intermediate.
Conversions with haloboranes.
In contrast to the boron sources mentioned in the previous
paragraphs (e.g. trihydroboranes, triarylboranes) the
conversion of silylenes with haloboranes is majorly limited to
examples of the groups of Jutzi, of Braunschweig, and of
Iwamoto as pointed out in the introduction. In addition, a
respective study of Tokitoh and coworkers is to be highlighted
3
6
in particular. In these cases insertion of the silicon atom into a
boron-halide bond occurs.
Scheme 3 Reactions of the hydrosilylene 1 with different haloboranes.
4
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in CDCl did not
solution in heptane) to an NMR sample of 1·BCl
3
3
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1
1
DOI: 10.1039/C9DT00608G
change the B NMR spectrum except for the rising of a signal at
6 ppm produced by “free” BCl . This suggests that the auto-
ionization proceeds very slowly.
4
3
As expected, a colorless precipitate also formed upon
treatment of a yellow solution of the pyramidal silylene 1 in
1
toluene solution with BBr
the isolated solid (in CD Cl
the elemental combustion analysis agrees with the proposed
stoichiometric composition 1·BBr . A proton resonance at 4.76
3
(Scheme 3). The H NMR spectrum of
2
2
) diagnoses one product species and
3
ppm is assigned to the SiH hydrogen atom (Si,H-satellites:
1J(Si,H) = 168 Hz, 1 H, Si-H) which proves that the conceivable
hydride-abstraction from the hydrosilylene by the Lewis acid is
no relevant side reaction. Interestingly, while we isolated the
product in moderate yield upon conversion of 1 with BBr
the use of the milder boron tribromide source Me S→BBr
in resulted in higher amounts of obtained product (74%). This also
3
(50%)
2
3
Fig. 4 Ellipsoid plot (30% level) of the molecular structure of 1→BBr
3
the single crystal. H-atoms omitted (except on Si). Wireframe model for means that 1 possesses a higher affinity to BBr than
3
tert-butyl groups. Selected interatomic distances [Å] and angles [°]:Si1- dimethylsulphide. The 11B NMR analysis of 1·BBr (in CDCl )
3
3
B1 = 2.045(3),
Si1-C1 = 1.922(3);
Si2-Si1-B1 = 130.1(1),
C1-Si1-
shows the presence of a four-coordinate boron center as
implied by a signal at −11.8 ppm (95 Hz) which is significantly
B1 = 104.9(1), Si2-Si1-C1 = 113.6(1).
shifted to higher field with respect to BBr
and also with regard to the value of 4.5 ppm for the presumed
→BF , as well as 5.5 ppm for 1→BCl . The upfield shift is easily
3 2 2
(39 ppm in CD Cl )
nucleus and suggests the formation of the Lewis acid base
adduct 1→BCl (note: uncomplexed BCl produces a resonance
3
3
1
3
3
at about 40–45 ppm depending on the analytic setup).
Nevertheless, a very weak signal at 6.9 ppm hints towards the
explained by the "heavy atom effect" that is imposed by
bromine to inflict an upfield shift on the NMR signals of
attached nuclei.73 For comparison the 11B nucleus of the NHC
presence of [BCl
intensity ratio (a strong and a weak signal) it is precluded that
the 5.5 ppm signal is produced by [1 BCl
]+ the formation of
which via auto-ionization of 1→BCl should coincide with the
BCl
]− content. Most likey, the observation of the minor
amounts of [1 BCl
]+ is hampered by signal broadening. Thus,
we surmise that 1→BCl is prone to a similar auto-ionization
process as presumed for 1→BF but the equilibrium is shifted
more to the 1→BCl side. Notably, from the literature a few
examples can be retrieved for bidentate nitrogen-based ligands
to exert auto-ionization on BCl to furnish [BCl
]− salts of chelate
4
]− in solution. Due to the discrepancy in the
iPr
3
adduct L →BBr had been reported to resonate at −15 ppm (in
2
2
, LiPr = 1,3-diisopropyl-4,5-dimethyl-imidazolin-2-ylidene). A
signal produced by [BBr
6 6
C D
3
]− (expected at about −24 ppm) is not
4
[
4
observed. The structural formulation of 1·BBr
base complex 1→BBr is supported by the XRD study conducted
on single crystals grown from a concentrated solution of 1·BBr
3
as the Lewis acid
2
2
3
3
3
3
in a 1:1 mixture of dichloromethane with hexane (Fig. 4). The
Si–B distance amounts to 2.045(3) Å and, thus, is very similar to
the respective distance in 1→BH . The Si–CNHC bond length is
3
determined to 1.922(3) Å which is only marginally shorter than
observed in the trihydroborane complex, hence, the bonding
situation within the SiLMe4 fragment seems to be affected only
to a small degree by the Lewis acidity of the attached borane
group. At this point it is to emphasize that we are not aware of
any structural report on a simple Lewis acid base adduct
between a silylene and a haloborane group as related examples
commonly involve transfer of a halide from the boron- to the
silicon atom.
We conclude that the susceptibility of the system consisting
of 1 and a boron trihalide to auto-ionization decreases with a
rise in atomic number of the halide. This agrees with the general
trend reported for complexes between boron and diorganyl
3
3
4
fashioned boronium dichloride cation complexes.69-72 For
carbene or silylene ligands, however, salts of the type
[
2 2 4
(ligand) BX ][BX ] (with X = halogen) are to the best of our
knowledge scarcely reported in the literature. The formation of
chloroborane species bearing one or two groups of 1 was
corroborated by our ESI mass spectrometric analysis (positive
+
mode) in which signals were assigned to [1·BCl
2
] ,
(LMe4)]+, and the auto-ionization product [1
] .
+
[
1·BCl
2
2
BCl
2
Furthermore, the spectrum includes two peaks correlated with
the
(1)BCl
of 1·BCl
(1→BCl
stoichiometries
[(1)BHCl(SiH(SitBu
3
)1]+
and
+
[
2
(SiH(SitBu
3
)1] . In fact, we obtained crystalline batches
3
and from these single crystals of 1→BCl
3
, as well as
picked.
+
]−
[
2
←SiH(SitBu
3
)←1] [BCl
4
have
been
2 3
compounds of the heavier chalcogens (i.e. R E→BX with E = S,
Se, Te and X = halogen). The complex stabilities of these
increase in the order F < Cl < Br for the respective chalcogen.74-
Unfortunately, the insufficient quality of these data prohibits
discussion of the respective structural parameters (see the ESI
for details on mass spectrometry and structure depiction). It is 76
Moreover, the pronounced stability of the widely employed
3
also worth noting that incremental addition of BCl (as a 1 M
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
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Page 6 of 10
ARTICLE
Journal Name
the tribromoborane and phenyldibromoborane fragments.
View Article Online
2
DOI: 10.1039/C9DT00608G
Notably, the silicon-boron bond length of 2.024(3) Å in F is
shorter by about 0.05 Å in comparison to that in 1→BPhBr
2
. The
2
respective bond length in G , however, amounts to 2.077(2) Å
which is very similar to our new complex. Consequently, it can
hardly be concluded on the type of silicon-boron interaction
(
e.g. dative bond, single bond) from this structural parameter
on the scarce basis of reported examples. We envisaged that
→BPhBr constitutes a promising precursor for reductive
1
2
debromination, as well as bromide abstraction experiments to
produce novel types of silylene-stabilized organoborylene
systems and borenium cation species. In fact, Lin, Xie and
coworkers have reported a borylene complexe stabilized by a
bis(silylene) ligand with four-coordinate silicon centers bearing
amidino groups. Unfortunately, our attempts of exposing
1
BPhBr
naphthalenide, NaSitBu
Similarly, treating 1→BPhBr
Ag[Al(OC(CF ], K[B(C ]) yielded an untraceable product.
2
to common reducing agents (e.g. KC
) afforded ill-defined product mixtures.
with bromide scavengers (e.g.
8
, sodium
2
Fig. 5 Ellipsoid plot (30% level) of the molecular structure of 1→BPhBr in the
3
single crystal. H-atoms omitted (except on Si). Wireframe model for tert-butyl
groups. Selected interatomic distances [Å] and angles [°]: Si1-B1 = 2.074(3),
Si1-C1 = 1.931(3); Si2-Si1-B1 = 131.9(1), Si2-Si1-C1 = 113.1(1), C1-Si1-
B1 = 103.0(1).
2
) )
3 3 4
6 5 4
F )
Conclusions
[
BF
4
]− anion explains why the boron fluoride system is
The build-up of silicon-boron bonds by reaction of a three-
coordinate silylene with electrophilic boron sources was
systematically investigated. We treated the NHC-stabilized
particularly prone to auto-ionization.
Because we had studied the triarylborane complex 1→BPh and
3
the trihaloborane adducts 1→BX it was obvious that the study
3
of a mixed system, that is an arylhaloborane, needed to be
included in our systematic investigation. Further reason was
given by the recent reports of Braunschweig and coworkers
who have disclosed various insertions of two-coordinate NHSi
into boron-halide bonds of arylhaloboranes and we sought to
contrast these reactivities by employing a three-coordinate
silylene in organohaloborane chemistry.22,24 Thus, we brought
Me4
Me4
pyramidal hydrosilylene tBu
tetramethyl-imidazolin-2-ylidene)
organoboranes and haloboranes. The reaction of 1 with
THF·BH or H N→BH afforded the silylene complex 1→BH or
3
SiSi(H)L
(1, L
= 1,3,4,5-
with
trihydroboranes,
3
3
3
3
the product (2) of ammoniaborane dehydrogenation with
concomittant insertion of the silicon(II) atom into an N–H bond.
Conversion of 1 with BPh
intermediate 1→BPh complex in solution which readily
converts with additional L
. Treatment of 1 with the haloboranes Et
discoloration and formation of a precipitate (Scheme 3). A and Me S→BBr resulted in formation of the Lewis acid-base
3
leads to the formation of the
3
PhBBr
2
into contact with 1 in toluene solution and, reminiscent
Me4
Me4
to L →BPh
3
and "free" silylene
O→BF , BCl , BBr
3
of the related conversions described above, we observed
1
2
3
3
2
3
sample of the isolated product (70% yield isolated) in CDCl
solution exhibited a resonance at −1 ppm in the 11B NMR auto-ionization products [1
analysis which is deshielded in comparison to 1→BBr
and significantly increased with rising atomic number of the halide.
corresponds to the typical chemical shift expected for four- Accordingly, the complex 1→BPhBr was isolated after
conversion of 1 with PhBBr . The relative stability of 1→BE (E =
H, F, Cl, Br), as well as 1→BPhBr strongly correlates with the
3
3
adducts 1–BX (X = F, Cl, Br) which slowly equilibrated to the
2 2 4 3
BX ][BX ]. The ratio of 1→BX
3
2
coordinate boron nuclei. Additionally, the observation of the
prominent SiH signal at 4.50 ppm in the H NMR spectrum and
the 13C NMR resonance assigned to the silicon-bonded carbene
center provides evidence that ligand exchange reactions
between the metalloid centers are inhibited. We surmise that
2
3
1
2
relative stability of respective borane dimethylsulphide
adducts.
We envisage that the use of complexes between silylenes
and boranes will complement the toolkit of organometallic
synthesis similar to the ubiquitous compound class of carbene-
borane complexes. The silylene-haloborane compounds in
particular provide high prospect for access to hitherto unknown
low-coordinate silicon-boron complexes via dehalogenation
methods.
the silylene Lewis acid base adduct 1→BPhBr
fashion as found in the related reaction between 1 and BBr
This is confirmed by the structural characterization of single
crystals of 1→BPhBr (Fig. 5). Geometric parameters concerning
metalloid coordination differ only by increments from the
tribromoborane congener 1→BBr , that is the Si–B bond and
the Si–CNHC are slightly elongated (2.074(3) Å and 1.931(3) Å)
2
forms in similar
3
.
2
3
with respect to the higher brominated derivative. This can be Conflicts of interest
attributed to the small increase of steric repulsion caused by the
phenyl group rather than differences in the Lewis acidities of
The authors affirm that there are no conflicts to declare.
6
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
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Page 7 of 10
Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
Journal Name
ARTICLE
removed in vacuum to afford a colorless solid. Yield: 40 mg,
Acknowledgements
We gratefully acknowledge financial support from WACKER
Chemie AG and the European Research Council (SILION
View Article Online
DOI: 10.1039/C9DT00608G
4
6%. Colorless crystals suitable for X-ray diffraction analysis
1
were obtained from a toluene:pentane (1:1) mixture at 5 °C. H
3
NMR (400.1 MHz, C
H, Si-H ), 3.30 (s, 6H, N-CH
6 6
C-CH D
). 11B NMR (128.4 MHz, C
6 6
D
, 298 K): δ [ppm] = 5.24 (d, J(H,H) = 4 Hz,
6
37394). We thank Lorenz J. Schiegerl (TUM) and Dr. Maria
2
2
3
), 1.37 (s, 27H, C(CH
, 298 K): δ [ppm] = −17.8 (t,
, J(B,H) = 94 Hz). C{ H} NMR (100.6 MHz, C , 298 K): δ
ppm] = 122.6 (C-CH ), 31.8 (C(CH ), 31.1 (N-CH ), 23.7
C(CH ), 7.8 (C-CH ), n.a (:CN ). Si{ H} NMR (79.5 MHz, C
98 K): δ [ppm] = −46.3 (SiH ), 2.4 ppm (tBu Si). APCI-HRMS m/z
380.3093 [M − H] , calc: 380.3083.
3 3
) ), 1.21 (s, 6H,
Schlangen-Ahl (TU Berlin) for mass spectrometric analyses.
3
1
13
1
BH
[
(
2
=
2
6 6
D
3
)
3 3
3
Experimental section
2
9
1
)
3 3
3
2
6 6,
D
General Considerations. All experiments and manipulations
were carried out under an atmosphere of dry argon using
standard Schlenk techniques or an MBraun glovebox
workstation. Glassware was heat dried under vacuum prior to
use. Solvents were dried by standard methods. NMR spectra at
ambient temperature (298 K) were recorded on a Bruker
2
3
+
3 3
Synthesis of 1→BPh : Freshly sublimed triphenylborane (BPh )
(
34.4 mg, 0.14 mmol) and silylene 1 (45 mg, 0.13 mmol) were
added to a NMR sample tube and dissolved in C (0.5 mL).
After sealing the NMR-tube the spectroscopic investigation was
6 6
D
1
AV400US, DRX400, AVHD300, or AV500C device. δ( H) and
1
3
δ( C) were referenced internally to the relevant residual
1
6 6
processed. H NMR (400.1 MHz, C D , 298 K): δ [ppm] =7.81 (d,
1
1
solvent resonances. δ( B) was referenced to Et
2
O→BF
external standard. δ( Si) was referenced to tetramethylsilane
TMS) (δ = 0 ppm) as an external standard. Abbreviations: s =
singlet, q = quartet, n.o. = not observed, Elemental analyses
EA) were conducted with a EURO EA (HEKA tech) instrument
3
as an
2,6
3,5
4
6
C
3
3
H, C -H, C
), 4.38 (s, Si,H-Satellites: J(Si,H) = 149 Hz, 1H, Si-H), 2.90 (s,
H, N-CH ), 2.62 (s, 3H, N-CH ), 1.17 (s, 27H, C(CH ), 1.14 (s,
). 11B NMR (160.5 MHz, C
H, C-CH ), 1.07 (s, 3H, C-CH , 298
, 298 K):
), 156.6 (PhC), 136.8 (PhCH), 127.5 (C-CH ),
26.5 (PhCH), 126.3 (C-CH ), 123.6 (PhCH), 36.0 (N-CH ), 33.8
N-CH ), 32.4 (C(CH ), 24.6 (C(CH ), 8.4 (C-CH ), 7.7 (C-CH ).
was distilled and stored 29
6 5 6 5
H ), 7.22 (t, 6H, C -H, C H ), 7.12 (m, 3H, C -H,
2
9
1
6 5
H
(
3
3
3 3
)
3
3
1
6 6
D
(
13
K): δ [ppm] = −3.2 (BPh
3 6 6
). C{ H} NMR (125.8 MHz, C D
4
8
equipped with CHNS combustion analyzer. The silylene 1,
δ [ppm] = 161.8 (:CN
1
2
3
7
6
77
Me
procedures. THF·BH
BBr were used as received. Et
in a fridge under argon. BPh was sublimed at 80 °C prior to use.
2
S·BBr
3
and PhBBr
2
were prepared according to literature
N→BH , BCl (1.0 M in heptane) and
O→BF
3
3
3
, H
3
3
3
(
)
3 3
)
3 3
3
3
3
3
2
3
Si{ H} NMR (99.4 MHz, C , 298 K): δ [ppm] = −76.6 (Si-H),
1
6 6
D
3
2
4.4 (tBu Si).
3
3 3
Synthesis of 1→BH : A solution of THF·BH (1.0 M, 0.3 mL, 0.30
3 2 3
Synthesis of 1·BF : Et O→BF (0.05 mL, 0.36 mmol) was added
mmol) in THF was added to a solution of silylene 1 (80 mg, 0.23
mmol) in THF (5 mL) at ambient temperature dropwise. The
color of the solution changed from yellow to colorless
immediately. The reaction solution was stirred for additional 30
dropwise to a yellow solution of 1 (86 mg, 0.25 mmol) in 10 mL
toluene. Immediate decolorization followed by formation of a
colorless precipitate occured. The suspension was stirred for 2
hours at room temperature and the phases were separated. The
colorless solid was washed with pentane (10 mL) and dried in
3
min. All volatiles were removed in vacuum to give 1→BH as
colorless solid (77 mg, 93%). Colorless crystals suitable for single
crystal X-ray diffraction analysis were obtained at ambient
1
vacuum to give 1·BF
300.1 MHz, C F, 298 K): δ [ppm] = 4.39 (s, 1H, Si-H), 4.01 (s,
H, N-CH ), 3.58 (s, 3H, N-CH ), 1.79 (s, 6H, C-CH ), 1.50 (s, 27H,
)). B NMR (96.3 MHz, C F, 298 K): δ [ppm] = 4.46 (q,
J = 85.3 Hz). F NMR (376.5 MHz, C , 298K)): δ [ppm] =
138.12 (q, J(B,F) = 36 Hz). Si{ H} NMR (99.4 MHz, C F, 298
K): δ [ppm] = −84.0 (Si-H), 19.4 (tBu Si). Elemental analysis (%):
Calcd. for C19 Si : C, 54.27; H, 9.59; N, 6.66. Found: C,
2.79; H, 9.61; N, 6.22 (the low value for C is reasoned by the
formation of incombustible boron- and silicon carbides).
3
(90 mg, 87%). Mono-adduct: H NMR
(
6 5
D
1
temperature from a benzene solution. H NMR (400.1 MHz,
3
3
3
3
3
C
6
D
6
, 298 K): δ [ppm] = 4.31 (q, J(B,H) = 4.8 Hz, Si,H-satellites:
11
(C(CH
)
3 3
6 5
D
1
J(Si,H) = 150 Hz, 1H, Si-H), 3.71 (s, 3H, N-CH
3
), 3.09 (s, 3H, N-
), 1.19 (s, 3H, C-
, 298 K): δ [ppm] =
). C{ H} NMR (100.6 MHz, C
), 127.0 (C-CH ), 126.9 (C-CH ), 35.0
), 32.1 (C(CH ), 24.5 (C(CH ), 8.5 (C-CH ),
). Si{ H} INEPT NMR (79.5 MHz, C
19
6 6
D
CH
CH
3
), 1.36 (s, 27H, C(CH
)
3 3
), 1.29 (s, 3H, C-CH
3
1
29
1
−
6 5
D
1
1
3
), n.o. (BH). B NMR (128.4 MHz, C
6 6
D
3
1
13
1
−40.8 (q, J(B,H) = 93 Hz, BH
3
6 6
D ,
H40BF N
3 2
2
2
98 K): δ [ppm] = 161.8 (:CN
2
3
3
5
(N-CH
3
), 34.1 (N-CH
3
3
)
3
3
)
3
3
2
9
1
8
=
.1 (C-CH
3
D
6 6
, 298 K): δ [ppm]
+
-77.0 (Si-H), 13.1 (tBu
3
Si). APCI-MS m/z = 365.2977 [M − H] ,
calc: 365.2974. IR (KBr) ṽ [cm ] = 2974 (w), 2950 (w), 2885 (w),
Synthesis of 1·BCl
3
: To a solution of 1 (100 mg, 0.28 mmol) in 10
-1
mL toluene, BCl (1M in heptane, 0.3 mL, 0.3 mmol) was added
3
2
1
852 (s), 2311 (br, B-H), 2238 (w, B-H), 2083 (m, Si-H), 1914 (w),
648 (w), 1468 (m), 1437 (m), 1385 (s), 1364 (m), 1131 (w), 1021
dropwise at ambient temperature. The yellow solution turned
colorless immediately and a colorless precipitate formed. The
resulting suspension was stirred overnight. After filtration the
colorless powder was dried in vacuum for 2 hours to give
(s), 931 (w), 888 (s), 814 (s), 775 (m), 591 (s).
Synthesis of 2: To a solution of silylene 1 (250 mg, 0.71 mmol)
in 10 mL toluene, NH BH (22 mg, 0.71 mmol) in 5 mL toluene
1
analytically pure 1·BCl
CDCl , 298 K): δ [ppm] = 4.49 (s, Si,H-satellites: J(Si,H) = 165 Hz,
H, Si-H), 3.98 (s, 3H, N-CH
3
. Yield: 95 mg, 71%. H NMR (400.1 MHz,
3
3
1
3
was added dropwise at ambient temperature. The yellow
solution turned colorless immediately. The reaction mixture
stirred additional 3 hours; toluene was removed under vacuum,
hexane was added (2 x 15 mL) and it was filtered. Hexane was
1
3
), 3.81 (s, 3H, N-CH
3
), 2.23 (d, J = 6.5
11
Hz, 6H, C-CH
CD Cl
MHz, CD
3
), 1.21 (s, 27H, (C(CH
, 298 K): δ [ppm] = 5.45 (h1/2 = 90 Hz). C{ H} NMR (100.6
Cl , 298 K): δ [ppm] = 155.5 (:CN ), 129.6 (C-CH ), 128.5
3
)
3
)). B NMR (128.4 MHz
13
1
2
2
2
2
2
3
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
Page 8 of 10
ARTICLE
Journal Name
(
C-CH
3
), 37.6 (N-CH
3
), 35.5 (N-CH
). 29Si{ H} NMR (79.5 MHz,
), 9.4 (C-CH
Si), n.o. (Si-H). Elemental
Si : C, 48.57; H, 8.58; N,
.96. Found: C, 48.28; H, 8.31; N, 5.61.
3
), 31.9 (C(CH
3
)
3
), 24.4 4.
M. Oestreich, E. Hartmann and M. MewaldV,iewChAertmicle. ORnelivn.e,
2013, 113, 402-441.
C. Kleeberg and C. Borner, Eur. J. Inorg. Chem., 2013, 2013,
799-2806.
E. Yamamoto, R. Shishido, T. Seki and H. Ito,
Organometallics, 2017, 36, 3019-3022.
1
DOI: 10.1039/C9DT00608G
(C(CH
3
3
) ), 10.0 (C-CH
3
3
5
6
7
8
.
.
.
.
CD
2 2 3
Cl , 298 K): δ [ppm] = 19.3 (tBu
2
analysis (%): Calcd. for C19
5
H40BCl
3
N
2
2
Y. Mizuhata, T. Sasamori and N. Tokitoh, Chem. Rev., 2009,
Synthesis of 1→BBr
1
3
: To a solution of 1 (176 mg, 0.5 mmol) in
(0.05 mL, 0.5 mmol, d: 2.65 g/mL) was
added dropwise. The yellow solution turned colorless after 15
1
09, 3479-3511.
M. Asay, C. Jones and M. Driess, Chem. Rev., 2011, 111,
54-396.
0 mL toluene, BBr
3
3
min and a precipitate formed. The reaction mixture was stirred 9.
overnight. The suspension was filtered and the obtained off-
white powder dried in vacuum for 2 hours to give analytically 10.
B. Blom, M. Stoelzel and M. Driess, Chem.–Eur. J., 2013, 19,
40-62.
Z. Benedek and T. Szilvási, RSC Advances, 2015, 5, 5077-
5
086.
3
pure compound 1→BBr (152 mg, 50%). Colorless crystals were
grown from dichloromethane:hexane (1:1) at room
temperature. (The same product could also be obtained by
1
1
1.
2.
C. Marschner, Eur. J. Inorg. Chem., 2015, 2015, 3805-3820.
T. Troadec, A. Prades, R. Rodriguez, R. Mirgalet, A.
Baceiredo, N. Saffon-Merceron, V. Branchadell and T. Kato,
Inorg. Chem., 2016, 55, 8234-8240.
using the more convenient Me
2
S→BBr
3
adduct instead of BBr
Cl , 298 K): δ [ppm] =
3
1
with 74% yield). H NMR (400.1 MHz, CD
2
2
1
1
3.
4.
J. A. Cabeza, P. García-Álvarez and D. Polo, Eur. J. Inorg.
Chem., 2016, 2016, 10-22.
S. Raoufmoghaddam, Y.-P. Zhou, Y. Wang and M. Driess, J.
Organomet. Chem., 2017, 829, 2-10.
C. Weetman and S. Inoue, ChemCatChem, 2018, 10, 4213-
4228.
N. Metzler and M. Denk, Chem. Commun., 1996, 1996,
2657-2658.
1
4
.76 (s, Si,H-satellites: J(Si,H) = 168 Hz, 1 H, Si-H)), 4.00 (s, 3H,
N-CH
3
), 3.80 (s, 3H, N-CH
3
), 2.23 (d, J = 3.2 Hz, 6H, C-CH
)). B NMR (128.4 MHz, CD Cl , 298 K): δ [ppm]
−11.8 (h1/2: 95 Hz). C{ H} NMR (100.6 MHz, CDCl
), 128.9 (C-CH ), 127.8 (C-CH ), 37.9 (N-CH
), 31.9 (C(CH ), 24.3 (C(CH ), 9.9 (C-CH
), 9.3 (C- 16.
). Si{ H} NMR (79.5 MHz, CDCl , 298 K): δ [ppm] = 22.0
Si), n.o. (Si-H). Elemental analysis (%): Calcd. for
Si : C, 37.83; H, 6.68; N, 4.64. Found: C, 38.8; H,
.66; N, 4.12.
3
), 1.22
1
1
(s, 27H, (C(CH
)
3 3
2
2
1
3
1
=
3
, 298 K): δ 15.
),
[
ppm] = 156.3 (:CN
2
3
3
3
3
5.3 (N-CH
3
3
)
3
)
3 3
3
2
9
1
CH
3
3
1
7.
D. Wendel, A. Porzelt, F. A. D. Herz, D. Sarkar, C. Jandl, S.
Inoue and B. Rieger, J. Am. Chem. Soc., 2017, 139, 8134-
(tBu
3
19
C H40BBr
6
N
3 2
2
8
137.
H. Wang, L. Wu, Z. Lin and Z. Xie, J. Am. Chem. Soc., 2017,
39, 13680-13683.
1
1
2
8.
9.
0.
1
2 2
Synthesis 1→BPhBr : PhBBr (105 mg, 0.43 mmol) in 3 mL
Y. Suzuki, S. Ishida, S. Sato, H. Isobe and T. Iwamoto,
Angew. Chem. Int. Ed., 2017, 56, 4593-4597.
Y. Li, R. K. Siwatch, T. Mondal, Y. Li, R. Ganguly, D. Koley and
C.-W. So, Inorg. Chem., 2017, 56, 4112-4120.
toluene was added dropwise to a solution of 1 (150 mg, 0.43
mmol) in 7 mL toluene. The yellow solution gradually
decolorized, approximately 10 minutes later a white precipitate
formed. For a complete conversion, the suspension was stirred 21.
S. Kaufmann, S. Schäfer, M. T. Gamer and P. W. Roesky,
Dalton Trans., 2017, 46, 8861-8867.
H. Braunschweig, T. Brückner, A. Deißenberger, R. D.
Dewhurst, A. Gackstatter, A. Gärtner, A. Hofmann, T.
Kupfer, D. Prieschl, T. Thiess and S. R. Wang, Chem.–Eur. J.,
overnight. It was filtered and the colorless solid was washed
2
2.
with 10 mL hexane and dried in vacuum to give analytically pure
1
1
→BBr
2
Ph. Yield: 180 mg, 70%. H NMR (400.1 MHz, CDCl
3
, 298
K): δ [ppm] = 7.86-7.64 (m, 2H, Ph-H), 7.18-6.89 (m, 3H, Ph-H),
2
017, 23, 9491-9494.
1
4
.50 (s, Si,H-satellites: J(Si,H) = 161 Hz, 1H, Si-H), 3.92 (s, 3H, N-
2
2
3.
4.
F. M. Mück, J. A. Baus, R. Bertermann, C. Burschka and R.
Tacke, Organometallics, 2016, 35, 2583-2588.
A. Gackstatter, H. Braunschweig, T. Kupfer, C. Voigt and N.
Arnold, Chem.–Eur. J., 2016, 22, 16415-16419.
H. Cui, M. Wu and P. Teng, Eur. J. Inorg. Chem., 2016, 2016,
4123-4127.
CH
C(CH
1/2: 550 Hz). C{ H} NMR (75.5 MHz, CDCl
57.5 (:CN ), 133.8 (PhCH), 128.4 (C-CH ), 127.3 (C-CH
PhCH), 125.5 (PhCH), 37.7 (N-CH ), 35.0 (N-CH ), 31.7 (C(CH
4.0 (C(CH ), 9.8 (C-CH ), 9.1 (C-CH
). Si{ H} NMR (99.4 MHz, 26.
CDCl , 298 K): δ [ppm] = 20.2 (tBu Si), n.o. (Si-H). Elemental
analysis (%): Calcd. for C25 Si : C, 50.01; H, 7.55; N,
.67. Found: C, 49.05; H, 7.27; N, 4.47 (the low value for C is
3
), 3.65 (s, 3H, N-CH
3
), 2.17 (s, 6H, C-CH
). B NMR (128.4 MHz, CDCl , 298 K): δ [ppm] = −0.83
, 298 K): δ [ppm] =
3
), 1.06 (s, 27H,
1
1
)
3 3
3
1
3
1
(
1
(
2
h
3
2
3
3
), 126.6 25.
),
3
3
3 3
)
2
9
1
)
3 3
3
3
K. Junold, J. A. Baus, C. Burschka, C. FonsecaꢀGuerra, F. M.
Bickelhaupt and R. Tacke, Chem.–Eur. J., 2014, 20, 12411-
3
3
1
2415.
H45BBr N
2 2
2
2
7.
R. Rodriguez, T. Troadec, T. Kato, N. Saffon-Merceron, J.-
4
M. Sotiropoulos and A. Baceiredo, Angew. Chem. Int. Ed.,
reasoned by the formation of incombustible boron- and silicon
carbides).
2
012, 51, 7158-7161.
S. Inoue and K. Leszczyńska, Angew. Chem. Int. Ed., 2012,
1, 8589-8593.
2
2
8.
9.
5
S. M. I. Al-Rafia, R. McDonald, M. J. Ferguson and E. Rivard,
Chem.–Eur. J., 2012, 18, 13810-13820.
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Dalton Transactions
Page 10 of 10
View Article Online
DOI: 10.1039/C9DT00608G
An NHC-stabilized three-coordinate hydrosilylene dehydrogenates ammonia borane and forms more
stable complexes with BH , BPh , BBr and BPhBr but less stable ones with BF , and BCl for which
3
3
3
2
3
3
ligand scrambling occurs.