Synthesis of mixed silylene-carbene chelate ligands from nheterocyclic silylcarbenes mediated by nickel

Article abstract of DOI:10.1002/anie.201409739

The Ni^II -mediated tautomerization of the N-heterocyclic hydrosilylcarbene L²Si(H)(CH₂)NHC₁, where L² = CH(C=CH₂)(CMe)(NAr)₂, Ar = 2,6-iPr₂C₆H₃ ; NHC = 3,4,5-trimethylimidazol-2-yliden-6-yl, leads to the first N-heterocyclic silylene (NHSi)-carbene (NHC) chelate ligand in the dibromo nickel(II) complex [L¹SiD(CH₂)(NHC)NiBr₂]₂ (L¹ = CH(MeC=NAr)₂). Reduction of 2 with KC₈ in the presence of PMe₃ as an auxiliary ligand afforded, depending on the reaction time, the N-heterocyclic silyl-NHC bromo NiII complex [L²Si(CH₂)NHCNiBr(PMe₃)] ₃ and the unique Ni⁰ complex [h²(Si-H){L²Si(H)(CH₂)NHC}Ni(PMe₃)₂] ,inf>4 featuring an agostic Si-H→!Ni bonding interaction. When 1,2-bis(dimethylphosphino)ethane (DMPE) was employed as an exogenous ligand, the first NHSi-NHC chelate-ligand-stabilized Ni0 complex [L¹SiD(CH₂)NHCNi(dmpe)] 5 could be isolated. Moreover, the dicarbonyl Ni⁰ complex 6, [L¹SiD-(CH₂)NHCNi(CO)₂], is easily accessible by the reduction of 2 with K(BHEt₃) under a CO atmosphere. The complexes were spectroscopically and structurally characterized. Furthermore, complex 2 can serve as an efficient precatalyst for Kumada-Corriu-type cross-coupling reactions.

Full text of DOI:10.1002/anie.201409739

.
Angewandte
Communications
DOI: 10.1002/anie.201409739
Carbene Complexes
Synthesis of Mixed Silylene–Carbene Chelate Ligands from N-
Heterocyclic Silylcarbenes Mediated by Nickel**
Gengwen Tan, Stephan Enthaler, Shigeyoshi Inoue, Burgert Blom, and Matthias Driess*
Abstract: The NiII-mediated tautomerization of the N-hetero-
cyclic hydrosilylcarbene L2Si(H)(CH2)NHC 1, where L2 =
=
CH(C CH2)(CMe)(NAr)2, Ar = 2,6-iPr2C6H3; NHC = 3,4,5-
trimethylimidazol-2-yliden-6-yl, leads to the first N-heterocy-
clic silylene (NHSi)–carbene (NHC) chelate ligand in the
dibromo nickel(II) complex [L1SiD(CH2)(NHC)NiBr2] 2 (L1 =
=
CH(MeC NAr)2). Reduction of 2 with KC8 in the presence of
PMe3 as an auxiliary ligand afforded, depending on the
reaction time, the N-heterocyclic silyl–NHC bromo NiII com-
plex [L2Si(CH2)NHCNiBr(PMe3)] 3 and the unique Ni0
complex [h2(Si-H){L2Si(H)(CH2)NHC}Ni(PMe3)2] 4 featur-
Scheme 1. Selected NHSi–transition-metal complexes that can serve
as precatalysts for organic transformations.
ꢀ
ing an agostic Si H!Ni bonding interaction. When 1,2-
bis(dimethylphosphino)ethane (DMPE) was employed as an
exogenous ligand, the first NHSi–NHC chelate-ligand-stabi-
lized Ni0 complex [L1SiD(CH2)NHCNi(dmpe)] 5 could be
isolated. Moreover, the dicarbonyl Ni0 complex 6, [L1SiD-
(CH2)NHCNi(CO)2], is easily accessible by the reduction of 2
with K(BHEt3) under a CO atmosphere. The complexes were
spectroscopically and structurally characterized. Furthermore,
complex 2 can serve as an efficient precatalyst for Kumada–
Corriu-type cross-coupling reactions.
N-Heterocyclic carbenes (NHCs) have been one of the
most widely utilized supporting ligands in TM chemistry in
the last decades.[7] These complexes exhibit numerous supe-
rior activities in comparison to phosphine complexes owing to
the stronger s-donor ability of NHC ligands.[8] According to
previous studies, both NHCs and NHSis can drastically
exceed the s-donor ability of phosphines and feature
a strong trans effect. Combining these two ligand types in
one chelate molecule could enable new coordination features
at TMs. Moreover, the presence of both strong s-donating
moieties might facilitate the coordination and dissociation of
other ligands, thereby improving the reactivity or catalytic
performance of respective TM complexes. Until now, no such
mixed silylene–carbene ligand system or a respective TM
complex has been reported. The difficulty to synthesize
a mixed NHSi–NHC chelate ligand is most likely due to the
reactive nature of NHSi groups, which can react with NHCs
via CD!SiII coordination or insertion of the SiII atom of the
S
ilylenes are emerging as a novel class of versatile steering
ligands in the coordination chemistry of the transition metals
(TMs), and their complexes have demonstrated remarkable
features in small-molecule activation and as precatalysts for
various types of organic transformations.[1] For instance,
several N-heterocyclic silylene (NHSi) TM complexes have
[2]
ꢀ
shown to be active precatalysts for C C bond formations,
[2+2+2] cyclotrimerizations,[3] borylations of arenes,[4] ketone
hydrosilylations,[5]
and
organic
amide
reductions
NHSi into a C H bond of the NHC, precluding the formation
ꢀ
(Scheme 1).[6] These investigations demonstrate that NHSis
are not simply spectator ligands, but can also tune the
electronic properties of the TM centers and ultimately change
the reactivity and selectivity of the emerging TM complexes.[4]
of a NHSi–NHC chelate ligand.[9]
In 2010, we reported the silicon(II)-based (metal-free) sp3
ꢀ
C H activation of the 1,3,4,5-tetramethylimidazol-2-ylidene
by the zwitterionic NHSi L2SiD,[10] affording the N-heterocyclic
2
2
=
hydrosilylcarbene 1, L Si(H)(CH2)NHC (L = CH(C CH2)-
(CMe)(NAr)2, Ar= 2,6-iPr2C6H3; NHC = 3,4,5-trimethylimi-
dazol-2-yliden-6-yl).[11] Herein we describe the unexpectedly
facile formation of the first mixed NHSi–NHC chelate
[*] G. Tan, Dr. S. Enthaler, Prof. Dr. S. Inoue, Dr. B. Blom,
Prof. Dr. M. Driess
1
1
=
complex [L SiD(CH2)(NHC)NiBr2] 2 (L = CH(MeC NAr)2)
through the hydrogen-atom migration (tautomerization)
from the silicon atom to the exocyclic methylene group in
1 mediated by NiBr2. Remarkably, the reduction of 2 with
KC8 in the presence of PMe3 does not lead to the expected
NHSi–NHC (Me3P)2Ni0 complex but to the silyl-NHC bromo
NiII complex 3 and the hydrosilyl–NHC(Ni0) complex 4 with
Technische Universitꢀt Berlin, Department of Chemistry:
Metalorganics and Inorganic Materials, Sekr. C2
Strasse des 17. Juni 135, 10623 Berlin (Germany)
E-mail: matthias.driess@tu-berlin.de
[**] We are grateful to the Cluster of Excellence UniCat for financial
support (sponsored by the Deutsche Forschungsgemeinschaft and
administered by the TU Berlin). We thank Dr. Jan Dirk Epping and
Samantha Voges for NMR measurements. S.I. wishes to thank the
Alexander von Humboldt foundation (Sofja Kovalevskaja Program)
for financial support.
ꢀ
an agostic Si H!Ni bonding interaction, respectively
(Scheme 2). However, the analogous reduction of 2 in the
presence of 1,2-bis(dimethylphosphino)ethane (DMPE) fur-
nishes the first mixed NHSi–NHC(DMPE)Ni0 complex 5
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201409739.
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appears at d = 8.3 ppm in the 29Si{1H} NMR spectrum, which
is shifted upfield when compared to that of the pincer-type
bis(NHSi) NiII complex (d = 20.2 ppm) depicted in
Scheme 1.[2d]
Single crystals of 2 suitable for X-ray diffraction analysis
were obtained from concentrated toluene solutions at ꢀ208C
(Figure 1).[12] The complex crystallizes in the orthorhombic
Scheme 2. Synthesis of the mixed NHSi–NHC NiII complex 2, and its
stepwise reduction with KC8 in the presence of PMe3 to form 3 and 4,
respectively.
Figure 1. Molecular structure of compound 2. Ellipsoids are set at
50% probability; all hydrogen atoms (except H15 and H24) and
solvent molecules (toluene) are omitted for clarity.
Scheme 3. Reductions of 2 in the presence of DMPE and CO to afford
the Ni0 complexes 5 and 6, respectively.
space group P212121. Noteworthy is that the nickel center
exhibits a somewhat distorted square-planar geometry, with
a sum of the angles around nickel of 365.038. The short
distances of Br1 with the H15 and H24 atoms (2.884 and
2.990 ꢀ) from the iPr groups indicate strong intramolecular
hydrogen bonding, which can be attributed to the drastically
downfield-shifted resonance signal for the methine protons in
the 1H NMR spectrum (see above). This also rationalizes the
(Scheme 3). Moreover, we further report the remarkably high
catalytic activity of 2 for Kumada–Corriu-type cross-coupling
reactions.
Treatment of 1 with [NiBr2(dme)] (dme = 1,2-dimethoxy-
ethane) in toluene at 508C afforded the mixed NHSi–NHC-
stabilized NiII complex [L1SiD(CH2)(NHC)NiBr2] 2 which was
isolated as dark red crystals in 82% yield (Scheme 2). We
propose that the formation of 2 proceeds first via the
coordination of the NHC moiety in 1 to NiBr2, which then
ꢀ
slight distortion of the nickel configuration. The Ni1 Si1
distance of 2.1553(8) ꢀ is similar to those in a pincer-type
bis(NHSi) NiII complex (2.1737(7) ꢀ; Scheme 1),[2d] whereas
it is longer than those in NHSi-supported Ni0(h6-arene)
complexes (2.0369(6)–2.0936(10) ꢀ), which have a somewhat
stronger Ni!SiII p back-donation.[13] The substantially longer
ꢀ
triggers the hydride migration from the Si H moiety to the
terminal CH2 group. This affords the silylene species that
spontaneously coordinates to the nickel(II) center yielding
complex 2, bearing the chelate NHSi–NHC ligand [L1SiD-
(CH2)(NHC)] 1a. The proposed process is further supported
by density functional theory (DFT) calculations, which reveal
that the coordination of the NHC to NiBr2 and the subsequent
formation of complex 2 are energetically favorable (ꢀ4.2 kcal
molꢀ1 and ꢀ29.1 kcalmolꢀ1, respectively; see the Supporting
Information for details).
ꢀ
Ni1 Br2 distance of 2.4437(6) ꢀ in comparison to that of
ꢀ
Ni1 Br1 (2.3538(5) ꢀ) might be attributed to a stronger trans
effect of the NHSi compared to that of the NHC moiety.
Compound 2 is the first TM complex stabilized by
a chelate NHSi–NHC ligand, which encouraged us to inves-
tigate its reduction to isolable Ni0 complexes. The desired
debromination of 2 with two molar equivalents of KC8 in the
presence of PMe3 as an exogenous ligand was carried out in
THF. Unexpectedly, after a reaction time of 3 h at room
temperature, only the new silyl-NHC monobromo NiII com-
plex 3, [L2Si(CH2)NHCNi(Br)(PMe3)], could be isolated in
76% yield (Scheme 2). Complex 3 was formed through
dehydrobromination of 2; a similar dehydrobromination
occurred in the preparation of L2SiBr2, which is the dibromo
precursor for synthesizing L2SiD.[10] After a prolonged reaction
time at room temperature (4 h), the reduction resulted in the
formation of the unprecedented hydrosilyl-NHC Ni0 complex
[h2(Si-H)][L2Si(H)(CH2)NHCNi(PMe3)2] 4 with an agostic
Compound 2 is highly soluble in CH2Cl2 and THF, and
moderately soluble in toluene. The protons at the methyl
groups of the b-diketiminato ligand are observed at d = 1.57
and 1.66 ppm as singlet resonance signals in the 1H NMR
spectrum in CD2Cl2 solutions. Interestingly, the resonance
signals of the methine protons of the iPr groups exhibit two
septet signals at d = 2.95 and 5.03 ppm with one being shifted
significantly downfield. This is likely a consequence of
intramolecular hydrogen bonding between the methine pro-
tons with one of the bromide atoms attached to the nickel
center, which is consistent with respective metric parameters
in the molecular structure determined by single-crystal X-ray
diffraction analysis (see below). The 29Si resonance signal
ꢀ
Si H!Ni interaction, which could be isolated in 68% yield
(Scheme 2). The mechanism for the formation of 4 is
unknown but might proceed via the NiI species [L2Si-
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(CH2)NHCNiI(PMe3)], generated by one-electron reduction
of 3. However, the latter intermediate is expected to be highly
reactive, and can likely abstract a hydrogen atom from the
8.8 Hz) for 4a and a broad signal around d = 2.5 ppm for 4.
The variable temperature NMR spectroscopic studies show
that the two broad 31P resonance signals for 4 are converted
into two doublet signals with a coupling constant of 35.4 Hz
when the sample is cooled down to ꢀ208C. The broad 29Si
resonance signal of 4 becomes a triplet signal with a coupling
constant of 71.5 Hz at ꢀ798C owing to the coupling with two
31P nuclei of two chemically equivalent PMe3 moieties.
The hydride signal is observed at d = ꢀ7.8 ppm in the
1H NMR spectrum with 29Si satellites (JSi,H = 87.5 Hz), which
is shifted substantially upfield compared to that of the [h2(Si-
H)Ni0(PPh3)] complex (d = ꢀ2.9 ppm).[17] This might be due
to the stronger s-donating property of the ligand 1. The
environment (THF or other sources).[14] Subsequent reductive
0
ꢀ
elimination of the Si H moiety, would yield the Ni complex 4
as the final product.
Compound 3 was isolated as red crystals from concen-
trated n-hexane solutions; it exhibits good solubility in n-
hexane, benzene, and toluene. Akin to complex 2, owing to
the hydrogen bonding between the bromide atom and the
methine hydrogen atoms, two downfield-shifted proton
resonance signals (d = 4.52 and 5.44 ppm) could be observed
in the 1H NMR spectrum in [D6]benzene solutions. The
protons of PMe3 resonate at d = 1.64 ppm as a doublet (2JH,P
=
coupling constant of JSi,H = 87.5 Hz is much higher than the
1
8.0 Hz), while the corresponding 31P resonance signal is
observed at d = ꢀ14.9 ppm in the 31P{1H} NMR spectrum.
Owing to the coupling with the 31P nucleus, the 29Si signal
results in a doublet at d = 12.2 ppm (2JSi,P = 45.9 Hz) in the
29Si{1H} NMR spectrum, which is shifted downfield in
comparison to that of complex 2 (d = 8.3 ppm). Complex 3
is the first example of any nickel complex containing a chelate
silyl-NHC ligand. It is noteworthy that silyl-functionalized
NHC cobalt and iron complexes have recently been reported
by Deng and co-workers.[15]
typical values for Si M H complexes (< 20 Hz) with oxida-
ꢀ ꢀ
ꢀ
tive addition of Si H bonds to TM centers. The large value of
the minimum T1 (1319 ms) determined by T1 measurement
for Si H hydride at 298 K and 400 MHz clearly indicates
a h (Si H) coordination to the nickel center in 4.
ꢀ
2
[18]
ꢀ
The
ꢀ
stretching frequency for the Si H bond is observed at n =
1746 cmꢀ1 in the IR spectrum, which is in the range of the
frequency for an s-silane TM complex (n = 1650–
1800 cmꢀ1).[19] The Si1 H1 bond length (1.738(19) ꢀ) of 4
ꢀ
2
ꢀ
(Figure 2) is also in the range of those reported for h (Si H)
The molecular structure of complex 3 is portrayed in
Figure 2. It crystallizes in the monoclinic space group P21/c.
TM complexes (1.70–1.90 ꢀ). Therefore, complex 4 can best
2
0
ꢀ
ꢀ
be described as a h (Si H)Ni complex with an agostic Si
H!Ni bonding interaction.
ꢀ
The C1 C2 bond (1.360(2) ꢀ) is significantly shorter than the
ꢀ
C4 C5 distance (1.496(2) ꢀ), indicating its double-bond
In an attempt to isolate the chelate NHSi–NHC stabilized
Ni0 complex, we further probed to utilize the chelate
diphosphine ligand DMPE and CO as additional supporting
ligands. Accordingly, the reduction of 2 with KC8 was carried
out in THF in the presence of one molar equivalent of DMPE.
In contrast to the case of PMe3 as an exogenous ligand, the
reaction afforded the desired Ni0 complex 5, [L1SiD-
(CH2)NHCNi0(dmpe)], in 43% yield as dark-green crystals
(Scheme 3). This result shows that the ligand 1a is also
capable of stabilizing Ni0 species. The 31P{1H} resonance of 5 is
revealed as a singlet signal at d = 15.8 ppm with 29Si satellites
(2JSi,P = 30.9 Hz) in [D6]benzene solutions. Correspondingly,
2
a triplet signal (d = 60.0 ppm, JSi,P = 30.9 Hz) is observed in
Figure 2. Molecular structures of 3 (left) and 4 (right). Ellipsoids are
set at 50% probability; all hydrogen atoms (except those at C1 and
Si1) are omitted for clarity.
the 29Si{1H} NMR spectrum owing to the coupling with two
chemically equivalent 31P nuclei from the DMPE ligand.
The nickel center of 5 (Figure 3) exhibits a tetrahedral
ꢀ
geometry, and the Ni Si bond (2.1740(18) ꢀ) is slightly
character. The Ni1 Si1 bond (2.2273(4) ꢀ) is slightly longer
shorter than those in [L3SiD(O)DSiL3Ni(cod)] (L3 = PhC-
ꢀ
ꢀ
than the Si Ni dative bond (2.1553(8) ꢀ) in 2, whereas the
value is comparable to those in the silyl nickel complexes
[(2,4,6-Me3C6H2)B(o-Ph2PC6H4)2)(m-H)NiE]
(E = SiH2Ph,
2.2379(4) ꢀ; SiHPh2, 2.2479(7) ꢀ).[16]
The NMR spectra of complex 4 in [D6]benzene or
[D8]toluene solutions at room temperature are rather com-
plex and are consistent with the coexistence of the two species
4 and 4a in solutions owing to the coordination and
dissociation of one of the PMe3 ligands (Scheme 2). This is
exemplified by one sharp (d = ꢀ18.8 ppm) and two broad
resonances (d = ꢀ27.9 and ꢀ31.3 ppm) for PMe3 groups in the
31P{1H} NMR spectrum in [D8]toluene solutions, of which
belong to 4a and 4, respectively. In analogy, the 29Si{1H} NMR
Figure 3. Molecular structures of 5 (left) and 6 (right). Ellipsoids are
set at 50% probability; all hydrogen atoms are omitted for clarity.
2
spectrum reveals one doublet signal (d = ꢀ19.2 ppm, JSi, P
=
2216
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(NtBu)2;
cod = cyclooctadiene)
(2.1908(7)
and
bearing Me, tBu, CF3, and Me2N groups at para positions,
2.1969(7) ꢀ).[20] DFT calculations revealed a significant
p back-donation from the nickel center to both silicon and
carbon centers, as indicated by the highest-occupied molec-
ular orbital (HOMO). Furthermore, a p-type bonding orbital
(HOMOꢀ1) is present and this orbital appears to be
respectively. Moreover, the effect of the halide leaving group
was studied, showing excellent yields with I, Br, Cl (7a–e,
> 99%). With 2,4,6-Me3C6H2Br as the substrate, a significant
decrease in the yield is observed which might be a result of the
steric hindrance from the mesityl group (7g, 29%).
ꢀ
ꢀ
delocalized over the Si Ni C framework of complex 5
(Supporting Information, Figure S28). To understand the
electron-donating property of the ligand 1a, we further
synthesized the carbonyl complex [L1SiD(CH2)NHCNi(CO)2]
6.
Complex 2 is also capable of catalyzing the cross-coupling
reactions of the heterocyclic bromides with 4-MeC6H4MgCl
with yields of more than 99% (7h) and 58% (7i). Strikingly,
3
2
ꢀ
a 86% yield for the sp –sp C C bond formation with 1-octyl
bromide and 4-MeC6H4MgCl was obtained (7j), whereas no
cross-coupling product was detected when the chelate bis-
(NHSi) Ni complex [L3SiD(O)DSiL3Ni(cod)] was used as the
precatalyst.[2c] When tBuMgCl was employed, the coupling
reaction with 4-MeOC6H4Br gave 31% yield along with 69%
yield of the hydrodehalogenation product anisole (7k).
Overall, complex 2 shows very good performance for
Kumada–Corriu-type cross-coupling reaction and moreover
highlights the promising potential of the NHSi–NHC ligand
supported TM complexes for other organic transformations.
In summary, with the assistance of NiII, the reaction of
ligand 1 with NiBr2 afforded the first mixed NHSi–NHC
supported NiII complex 2. Its reduction by KC8 in the
presence of PMe3 afforded the silyl-NHC bromo NiII complex
Reduction of 2 with two molar equivalents of K(BHEt3)
under an atmosphere of CO afforded complex 6 as brown
crystals in 60% yield (Scheme 3).[21] The 13C resonances of
CO and carbene ligands are observed at d = 198.8 and
206.2 ppm, respectively, which are comparable to those of
[(NHCCy)2Ni(CO)2] (d = 198.3 and 205.2 ppm; NHCCy
=
(CHNCy)2CD, Cy = cyclohexyl).[23] The 29Si nucleus resonates
at d = 66.8 ppm in the 29Si{1H} NMR spectrum, which is
shifted downfield when compared to that of 5 (d = 60.0 ppm).
This is likely due to the stronger p-acidity of CO than
phosphine ligands, which also results in decreasing the p back-
donation from the Ni center to the silicon and NHC carbon
ꢀ
centers. This consequently leads to longer Ni1 Si1
2
0
ꢀ
ꢀ
(2.2131(13) ꢀ) and Ni1 C33 (1.947(4) ꢀ) bond lengths of 6
3 and the h (Si H)Ni complex 4 depending on the reaction
time. Using DMPE as the auxiliary ligand, the analogous
reduction smoothly furnishes Ni0 complex 5. The substantially
low IR stretching frequencies of 6 suggest that 1a is a better s-
donor than two phosphine or NHC ligands. Moreover,
complex 2 is a highly efficient precatalysts for Kumada–
Corriu-type cross-coupling reaction. Exploring new com-
plexes for other catalytic transformations of organic sub-
strates and the coordination features of 1 and 1a toward other
TMs are currently under investigation in our laboratory.
(Figure 3) in comparison to those in 5 (2.1740(18) and
1.918(7) ꢀ, respectively). The IR stretching vibrations for
the CO ligands are observed at n = 1952 and 1887 cmꢀ1, which
are blue-shifted in comparison to those observed in
[{(dmpm)Ni(CO)2}2] (n = 1991 and 1927 cmꢀ1; dmpm = bis(-
dimethylphosphino)methane)[22] and [(NHCMes)2Ni(CO)2]
(n = 2050 and 1877 cmꢀ1, NHCMes = (CHNMes)2CD, Mes =
2,4,6-Me3C6H2).[23] This suggests that the NHSi–NHC ligand
1a is a stronger s-donor than two phosphine or NHC ligands.
To highlight the influences of the chelate NHSi–NHC
ligand 1a on the reactivity of the nickel center, we further
tested the catalytic performance of 2 in Kumada–Corriu-type
cross-coupling reactions (Scheme 4).[24] It was found that
complex 2 can efficiently catalyze the cross-coupling reactions
of 4-MeC6H4MgCl with aromatic halides (yields: > 99%)
Received: October 3, 2014
Published online: December 30, 2014
Keywords: cross-coupling · homogeneous catalysis ·
.
main-group chemistry · N-heterocyclic carbenes · silicon
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