Divalent Silicon-Assisted Activation of Dihydrogen in a Bis(N-heterocyclic silylene)xanthene Nickel(0) Complex for Efficient Catalytic Hydrogenation of Olefins

Article abstract of DOI:10.1021/jacs.7b07167

The first chelating bis(N-heterocyclic silylene)xanthene ligand [Si^II(Xant)Si^II] as well as its Ni complexes [Si^II(Xant)Si^II]Ni(η²-1,3-cod) and [Si^II(Xant)Si^II]Ni(PMe₃)₂ were synthesized and fully characterized. Exposing [Si^II(Xant)Si^II]Ni(η²-1,3-cod) to 1 bar H₂ at room temperature quantitatively generated an unexpected dinuclear hydrido Ni complex with a four-membered planar Ni₂Si₂ core. Exchange of the 1,3-COD ligand by PMe₃ led to [Si^II(Xant)Si^II]Ni(PMe₃)₂, which could activate H₂ reversibly to afford the first Si^II-stabilized mononuclear dihydrido Ni complex characterized by multinuclear NMR and single-crystal X-ray diffraction analysis. [Si^II(Xant)Si^II]Ni(η²-1,3-cod) is a strikingly efficient precatalyst for homogeneous hydrogenation of olefins with a wide substrate scope under 1 bar H₂ pressure at room temperature. DFT calculations reveal a novel mode of H₂ activation, in which the Si^II atoms of the [Si^II(Xant)Si^II] ligand are involved in the key step of H₂ cleavage and hydrogen transfer to the olefin.

Full text of DOI:10.1021/jacs.7b07167

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Article
Divalent Silicon-Assisted Activation of Dihydrogen
in a Bis(N-heterocyclic silylene)xanthene Nickel(0)
Complex for Efficient Catalytic Hydrogenation of Olefins
Arseni Kostenko, Shenglai Yao, Matthias Driess, and Yuwen Wang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b07167 • Publication Date (Web): 31 Aug 2017
Downloaded from http://pubs.acs.org on August 31, 2017
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Divalent Silicon-Assisted Activation of Dihydrogen in a Bis(N-
heterocyclic silylene)xanthene Nickel(0) Complex for Efficient
Catalytic Hydrogenation of Olefins
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Yuwen Wang, Arseni Kostenko, Shenglai Yao and Matthias Driess*
Metalorganics and Inorganic Materials, Department of Chemistry, Technische Universität Berlin, Straße des 17, Juni
135, Sekr. C2, 10623 Berlin, Germany
ABSTRACT: The first chelating bis(N-heterocyclic silylene)xanthene ligand [SiII(Xant)SiII] as well as its Ni
complexes [SiII(Xant)SiII]Ni(η2-1,3-cod) and [SiII(Xant)SiII]Ni(PMe3)2 were synthesized and fully characterized.
Exposing [SiII(Xant)SiII]Ni(η2-1,3-cod) to 1 bar H2 at room temperature quantitatively generated an unexpected
dinuclear hydrido Ni complex with a four-membered planar Ni2Si2 core. Exchange of the 1,3-COD ligand by PMe3
led to [SiII(Xant)SiII]Ni(PMe3)2 which could activate H2 reversibly to afford the first SiII stablized mononuclear
dihydrido Ni complex characterized by multinuclear NMR and single-crystal X-ray diffraction analysis.
[SiII(Xant)SiII]Ni(η2-1,3-cod) is a strikingly efficient precatalyst for homogeneous hydrogenation of olefins with a
wide substrate scope under 1 bar H2 pressure at room temperature. DFT calculations reveal a novel mode of H2
activation, in which the SiII atoms of the [SiII(Xant)SiII] ligand are involved in the key step of H2 cleavage and
hydrogen transfer to the olefin.
metal complexes is emerging as an efficient alternative
INTRODUCTION
way due to their environment-friendly and cost-effective
properties.11,13 While heterogeneous nickel catalysts have
Success in homogeneous transition metal catalysis greatly
depends upon the development of well-designed ligand
systems.1 Among the diverse ligand systems, chelating
ligands are prominent owing to their significant ad-
vantages in controlling the electronic and geometric
properties of metal complexes. Since the first isolation of
the N-heterocyclic silylene (NHSi) iron complex by Welz
and Schmid in 1977,2 NHSi ligands with strong σ-donating
nature3 have fueled tremendous interest in organometal-
lic chemistry.4 Recently, we have introduced a NHSi moi-
ety5 into chelating ligand scaffolds, leading to the discov-
ery of various and versatile bis(NHSi) ligands (Chart 1),6
whose transition metal complexes have shown excellent
catalytic performance in borylation, hydrosilylation and
other homogenous catalytic transformations.4 Nonethe-
less, thorough catalytic and mechanistic studies upon the
organometallic systems bearing diverse bis(NHSi) ligands
are still in their infancy compared with their N-
heterocyclic carbene (NHC) counterparts.
been widely used in hydrogenation of olefins, e.g. Raney
nickel,12 studies of homogeneous hydrogenation catalyzed
by nickel complexes are still rare.13 Bouwman et al. pio-
neered studies on the nickel catalyzed homogeneous
hydrogenation of 1-octene under high pressure (50 bar).13a-
c In 2012, Hanson et al. reported a nickel hydride complex
Bidentate Bis(NHSi) Ligands
Ph
Ph
Ph
tBu
N
N
tBu
tBu
N
tBu
tBu
tBu
tBu
N
N
tBu
tBu
tBu
N
N
Si
Si
Si
Si
Si
C
O
Fe
C
Si
N
N
tBu
N
tBu
N
N
Ph
Ph
Ph
2012
2010
2016
Tridentate Bis(NHSi) Ligands
tBu
tBu
tBu
tBu
N
N
Ph
N
N
Ph
Ph
Ph
Si
Si
Si
Si
N
N
N
N
tBu
Homogeneous hydrogenation of olefins, a field mainly
dominated by precious metal (Rh,8 Ir,9 Ru10) -based cata-
lysts, is one of the most powerful and atom economical
methods in organic synthesis.7 More recently, hydrogena-
tion catalyzed by earth-abundant first-row transition
O
O
tBu
tBu
N
N
N
tBu
tBu
tBu
2012
2014
Chart 1. Examples of Chelating Bis(NHSi) Ligands
1
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Figure 1. Molecular structures of 1, 2 and 3. Thermal ellipsoids are drawn at 50% probability level. H atoms and solvent
molecules are omitted for clarity.
[(CyPNHPCy)NiH](BPh4) {CyPNHPCy = HN[CH2CH2P(Cy)2]2}
catalyzed alkene hydrogenation under 4 bar H2 pressure
at 80 oC.13d Two nickel–borane species, (MesDPBPh)Ni-
SiCl afforded the chelating ligand [SiII(Xant)SiII] 1 as yel-
low
Scheme 1. Synthesis of Bis(NHSi)xanthene 1 and
Nickel Complexes [Si(Xant)Si]Ni(η2-1,3-cod) 2 and
[Si(Xant)Si]Ni(PMe3)2 3
[
MesDPBPh = MesB(o-PPh2C6H4)2] and pincer-type (tBuPBPt-
Bu)NiH, were synthesized by Peters and coworkers, both
of which were successfully applied as active catalysts for
olefin hydrogenation under 1 bar H2 pressure at room
temperature.13e-f In 2015, Lu et al. conducted the hydro-
genation of unhindered olefins with a gallium supported
bimetallic Ni(0) complex under mild conditions.13g De-
spite all these significant contributions, the substrate
scope is still largely limited to less sterically hindered
terminal and internal alkenes. Therefore, the develop-
ment of new ligands for nickel-catalyzed homogeneous
hydrogenation of olefins is highly desirable.
Ph
tBu
N
1)
N
N
2.0 equiv. s-BuLi
Et2O, -78 oC
RT, 7 h
tBu
Si
Br
Br
tBu
tBu
O
O
N
Si
Si
Ph
2) 2.0 equiv.
Cl
N
N
tBu
Ph
tBu
RT, 4 h
1
Et2O, -78 oC
1,5-COD
isomerized to
1,3-COD
1.0 equiv.
Ni(cod)2
Et2O, RT
Herein, we report the synthesis of a novel type of che-
lating bis(NHSi) ligand bearing a xanthene scaffold as
well as its Ni complexes [SiII(Xant)SiII]Ni(η2-1,3-cod) 2 and
[SiII(Xant)SiII]Ni(PMe3)2 3 (Scheme 1). Remarkably, H2 can
be activated by 2 and 3 under very mild reaction condi-
tions to generate the unexpected dinuclear Ni complex 4
and the first structurally characterized dihydrido Ni com-
plex 5 stabilized by aforementioned bis(NHSi) ligand,
respectively. Complex 2 is a strikingly efficient catalyst in
hydrogenation of olefins with a broad substrate scope
under smooth reaction conditions (1 bar H2, RT). DFT
calculations suggest an unprecedented hydrogenation
mechanism where the SiII atoms in [SiII(Xant)SiII] ligand
play a significant role in assisting the H2 cleavage, stabiliz-
ing the dihydrido Ni intermediate and hydrogen transfer-
ring to the olefin.
Ph
N
Ph
tBu
tBu
N
N
tBu
N
tBu
tBu
Si
Si
2.5 equiv. PMe3
PMe3
PMe3
Ni
Ni
O
O
Et2O, RT
-1,3-COD
Si
Si
N
tBu
N
N
N
tBu
tBu
Ph
Ph
3
2
crystals in 70% isolated yields. Its molecular structure was
unambiguously confirmed by NMR spectroscopy and X-
ray diffraction analysis (Figure 1). The 29Si NMR spectrum
of 1 shows a singlet at δ 17.3 ppm comparable to that re-
ported for a related carborane-bridged bis(NHSi) (δ 18.9
ppm).6c The Si−Si distance of 4.316 Å reveals a suitable
space for coordination of transition-metals.
Treatment of 1 with Ni(cod)2 (cod = 1,5-cyclooctadiene )
in Et2O at RT led to the formation and isolation of un-
precedented [SiII(Xant)SiII]Ni(η2-1,3-cod) (1,3-cod = 1,3-
cyclooctadiene) complex 2 as dark red crystals with a
coordinatively unsaturated 16 valence-electron Ni(0)
center, where isomerization of 1,5-cyclooctdiene occurred
(Scheme 1). While monitoring the coordination process
RESULTS AND DISCUSSION
Synthesis of the Bis(NHSi)xanthene 1 and its
Ni Complexes 2 and 3. The bis(NHSi) chelating ligand
1 is readily accessible in a one-pot synthesis (Scheme 1).
Dilithiation of 4,5-dibromo-9,9-dimethylxanthene with 2
molar equiv. of sec-BuLi in Et2O, followed by salt-
metathesis reaction with the chlorosilylene [PhC(NtBu)2]-
1
by H NMR spectroscopy, we could observe the rapid
generation of the symmetrical species [SiII(Xant)SiII]Ni(η4-
1,5-cod) and ‘free’ 1,5-cyclooctadiene after 15 min. Four
2
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hours later, the 1H NMR spectrum of the reaction mixture
exhibited a new set of unsymmetrical signals which could
1
2
3
4
5
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be assigned to the final products [SiII(Xant)SiII]Ni(η2-1,3-
cod) 2 and ‘free’ 1,3-cyclooctadiene (Figure S8); such Ni-
mediated olefin isomerizations are well known.14 The 29Si
NMR spectrum of 2 reveals a singlet at δ 61.4 ppm, which
is downfield shifted compared with the precursor 1 (δ 17.3
ppm). Single crystals of 2 suitable for X-ray diffraction
analysis were obtained from concentrated Et2O solution
at RT (Figure 1). The nickel center adopts a trigonal-
planar geometry and is disordered due to the rapid coor-
dination exchange between the two C=C bonds (C51=C52,
C46=C53), which also accounts for the two broad reso-
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1
nances of the olefin protons in the H NMR spectrum
(Figure S5). The C51=C52 distance (1.360(7) Å) in 2 is
shorter than C46=C53 distance (1.415(6) Å) because of the
back-donation from the Ni center to the π* orbital of the
C46=C53 bond.
The reaction of [SiII(Xant)SiII]Ni(η2-1,3-cod) 2 with PMe3
in Et2O at RT furnished red crystals of [SiII(Xant)SiII]-
Ni(PMe3)2 3 in 62% isolated yields (Scheme 1). NMR spec-
troscopy and X-ray diffraction analysis of the latter dis-
play a symmetric tetrahedral geometry around the nickel
center (Figure 1).
Figure 2. Molecular structure of 4. Thermal ellipsoids
are drawn at 50% probability level. H atoms (except H5
and H7) and solvent molecules are omitted for clarity.
Analogous dinuclear Ni complexes containing two
Ni−H−Si 3c-2e bonds are rare and resulted from Si−H
bond activation by Ni sites in close proximity.17 To the
best of our knowledge, complex 4 is the first dinuclear Ni
complex with a Ni2Si2 core obtained from activation of H2.
Scheme 2. H2 Activation by 2 to Give the Dinuclear
Ni2H2 Complex 4.
1
tBu
The H NMR spectrum features four signals at δ 2.21,
N
Ph
N
Ph
Ph
N
1.45, 1.26 and 0.37 ppm in a 1:1:1:1 intensity ratio corre-
sponding to the inequivalent tBu groups. In addition, the
resonance associated with the Ni−H−Si fragments is ob-
served at δ −4.59 ppm with silicon satellites (1JSiH = 75.0
Hz) indicating the presence of η2-(Si−H) interaction.15d
The 29Si{1H} NMR spectrum displays two signals at δ 114.0
and 54.9 ppm, one at lower magnetic field (δ 114.0 ppm)
for Ni−H−Si moiety akin to other dinuclear η2-(Si−H)ꢀNi
complexes,17 and the signal at higher field (δ 54.9 ppm)
stems from the silylene group. The 29Si DEPT NMR spec-
trum shows one resonance at δ 113.9 ppm further confirm-
tBu
N
tBu
N
tBu
tBu
N
Si
tBu
tBu
Si
Ni
H2 (1 bar)
Et2O, RT
H
Ni
O
O
O
H
Ni
Si
Si
N
N
Si
N
tBu
N
tBu
tBu
N
tBu
Ph
N
Ph
Ph
tBu
2
4
Activation of H2 by the Ni Complexes 2 and 3.
With all these encouraging results in hand, we proceeded
to examine if H2 could be activated by 2 and 3. Treatment
of 2 with 1 bar H2 at room temperature afforded the unex-
pected diamagnetic dinuclear Ni complex 4 as yellow-
brown crystals in 67% isolated yields (Scheme 2), with its
molecular structure shown in Figure 2. The central struc-
ture moiety of 4 features a four-membered planar Ni2Si2
core with two bridging hydride atoms lying approximately
in the same plane. The hydrogen atoms connected to Ni
atoms were located in the electron density map and re-
fined isotropically. The Si−H distances of 1.66(5) and
1.67(4) Å (1.66 Å in the M06L/S DD-def2-SVP optimized
structure) imply the formation of a η2-(Si−H) σ complex.
These Si−H bond distances are significantly longer than
that of an uncoordinated Si−H moiety (1.40-1.50 Å), but
shorter than the distances (1.90-2.40 Å) corresponding to
secondary interactions of SiIV to element-hydrogen
bonds.15 The Ni−Ni distance of 2.5159(10) Å lies in the
typical range of Ni−Ni single bonds (2.35-2.56 Å).16,17,22a
1
ing the existence of Si−H bonds. The in situ H NMR
study of H2 activation process clearly indicates that
[SiII(Xant)SiII]Ni(coe) (coe = cyclooctene) was initially
formed via the monohydrogenation of cod, and the fol-
lowing hydrogenation of the coe ligand led to the release
of ‘free’ cyclooctane and 4.
Scheme 3. Reversible H2 Activation by Complex 3 to
Give Dihydrido Ni Complex 5
Ph
Ph
N
tBu
tBu
O
H2 (1 bar)
C6D6, RT
- PMe3
N
N
N
tBu
tBu
Si
Si
PMe3
PMe3
PMe3
Ni
Ni
O
H
H2
H
+ PMe3
Si
Si
N
tBu
N
tBu
N
N
tBu
tBu
Ph
Ph
3
5
3
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Exposing [SiII(Xant)SiII]Ni(PMe3)2 3 to 1 bar H2 at room
pressure, the appearance of the resonance of H2 (δ 4.43
ppm) in the H NMR spectrum suggests a reversible H2
activation process and HD scrambling (Figure S21).
1
1
2
temperature in C6D6 resulted in an immediate color
1
change from red to orange (Scheme 3). The resulting H,
31P, 29Si NMR spectra revealed the formation of the new
diamagnetic dihydrido Ni complex 5 together with 10%
residual complex 3, free ‘PMe3’ and trace amount of
Ni(PMe3)4. Complex 5 regenerates 3 in 29% NMR yield
upon removal of H2 by three freeze-pump-thaw cycles,
consistent with a reversible H2 activation process (Figure
S20). The low conversion could attribute to the loss of the
3
4
5
6
7
8
9
To our delight, single-crystals of 5 could be obtained
from Et2O under H2 atmosphere after several attempts
(Figure 3). It crystallizes in the space group P21/c with the
nickel(II) site adopting a distorted square pyramidal co-
ordination geometry. Two H atoms at the Ni center had
been located in the electron density map and refined
isotropically with a tilt towards Si2. The Ni−H distances
(1.57(8) and 1.62(6) Å) lie in the typical hydrido nickel
bond range (1.30-1.70 Å),21 while Ni−Si distances (2.1469(8),
2.1666(8) Å) are slightly shorter than those observed in
[SiII(Xant)SiII]Ni(η2-1,3-cod) (2.1998(16), 2.2108(18) Å) and
[SiII(Xant)SiII]Ni(PMe3)2 (2.2134(4), 2.2254(4) Å) probably
due to the higher oxidation state of Ni in 5. Based on
these evidences, 5 can be described as a SiII stabilized
dihydrido Ni complex. DFT calculations at M06L/SDD-
def2/SVP level of theory show that the reaction of 3 with
H2 to form complex 5 and ‘free’ PMe3 is energy favorable
by 4.0 kcal mol-1. According to previous reports, mononu-
clear dihydrido Ni complexes, usually considered as the
initial intermediate of homolytic H2 cleavage, are highly
unstable which can only be observed at low temperature
or in solutions with the assistance of a boron atom.13e,22 As
far as we know, complex 5 is the first structurally charac-
terized SiII stabilized dihydrido Ni complex.
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volatile PMe3 during the degassing process. The H NMR
spectrum of 5 displays a doublet at δ −1.51 ppm (2 H, 2JPH
=
15.4 Hz) with 29Si satellite signals (1JSiH = 44.2 Hz) attribut-
1
ed to the two Ni−H bonds. The JSiH coupling constant
indicates a significantly bonding interaction between the
silicon and hydrogen atom. The doublet at δ 1.78 ppm
(2JPH = 4.3 Hz) is assigned to the coordinated PMe3, while
the corresponding 31P signal appears at δ −28.3 ppm. In
the 29Si{1H} NMR spectrum, 5 features a doublet (δ 9.7
2
ppm, JPSi = 19.4 Hz) owing to the coupling with one 31P
nucleus, which shows a high field shift relative to complex
3 (δ 70.1 ppm). A doublet at δ 9.7 ppm is also observed in
the 29Si DEPT NMR spectrum further confirming a strong
1
interaction between the Si and H atoms. In the H-
coupled 29Si NMR spectrum, the resonance (δ 9.7 ppm)
1
splits into a triplet of doublets (2JPSi = 19.1 Hz, JHSi = 44.1
Hz) because of coupling with two H atoms and one P
atom. Since only one signal was observed for these two
seemingly distinct SiII atoms, we propose fast exchange of
the two SiII atoms with respect to dihydrido ligand inter-
actions. Although the T1 (min) value was not determined
in the temperature range of 183-273 K due to the freezing
point of toluene-d8 (178 K), the T1 value (766 ms, 500 MHz)
at 183 K was much larger than the typical values (<35 ms)
for dihydrogen metal complexes.18 In addition, the small
H−D coupling (2JDH = 3.8 Hz)19 when exposing complex 3
to 1 bar HD further ruled out the existence of the corre-
sponding η2-H2 Ni adduct (Figure S21).20 Under 1 bar HD
Catalysts Screening for Hydrogenation. Given
the high reactivity of 2 and 3 towards dihydrogen, we next
investigated their catalytic behavior in hydrogenation of
olefins. Choosing norbornene as the standard substrate,
we compared the catalytic hydrogenation competencies
of 2-4 as well as with that of the related Xantphos (4,5-
bis(diphenylphosphino)-9,9-dimethylxanthene) support-
ed Ni complex (Table 1). When 2 was mixed with nor-
bornene (50 equiv.) in C6D6 at RT, a new set of resonances
Table 1. Catalysts Screening for the Hydrogenation of
Norbornenea
Cat. (2 mol %)
H
2 (1 bar), RT
C6D6, 24 h
Entry Catalyst
Conv.b
>99%
32%
1
[SiII(Xant)SiII]Ni(η2-1,3-cod) 2
[SiII(Xant)SiII]Ni(PMe3)2 3
2
3
4
5
6
Dinuclear Ni complex 4
65%
Ni(cod)2 + Xantphos (ratio 1:1)
Ni(cod)2 + [SiII(Xant)SiII] 1 (ratio 1:1)
[Si(Xant)Si]Ni(η2-1,3-cod) 2 + Hgc
6%
>99%
>99%
aReaction conditions: norbornene (0.054 mmol), ferrocene
(0.022 mmol, internal standard) and 2 mol % Ni catalyst in
0.45 mL C6D6 under 1 bar H2 at RT for 24 h. bConversion was
1
Figure 3. Molecular structure of 5. Thermal ellipsoids
are drawn at 50% probability level. H atoms (except H1
and H5) are omitted for clarity.
monitored by H NMR with ferrocene as an internal stand-
ard. cHg to catalyst ratio 250:1.
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Scheme 4. Substrate Scope of Complex 2 Catalyzed Olefins Hydrogenationa
1
2
3
4
5
6
7
8
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aReaction conditions: olefin (0.054 mmol), ferrocene (0.022 mmol, internal standard) and 2 mol % Ni catalyst in 0.45 mL C6D6
under 1 bar H2 at RT. The catalytic reactions were performed in a sealed J. Young NMR tube without stirring. Conversion was
monitored by H NMR with ferrocene as the internal standard. Reaction condition: olefin (1.07 mmol), ferrocene (0.43 mmol,
internal standard) and 0.1 mol % Ni catalyst in 3 mL C6D6 under 1 bar H2 at RT. The catalytic reactions were performed in a
Schlenk tube with stirring. TON = turnover number, TOF = turnover frequency. cIsolated yields (6s 0.27 mmol).
1
b
1
appeared in the H NMR spectrum, corresponding to the
immediate formation of [SiII(Xant)SiII]Ni(η2-norbornene)
and ‘free’ 1,3-COD ligand (Table 1, entry 1). Then exposing
of this mixture to 1 bar H2 resulted in the complete con-
version of norbornene to norbornane after 24 h. After
completion, the color of the solution changed from red to
yellow brown and 4 could be observed as the only nickel
same reaction conditions (Table 1, entry 5). Furthermore,
the hydrogenation of norbornene catalyzed by 2 was un-
affected in the presence of excess Hg in accordance with a
homogenously catalyzed process (Table 1, entry 6).23
Olefins Hydrogenation Catalyzed by the Ni
Complex 2. Inspired by the efficient hydrogenation of
norbornene catalyzed by 2, we expanded the substrate
scope to a variety of olefins (Scheme 4). Unless otherwise
noted, all of the hydrogenation reactions were conducted
in a sealed J. Young NMR tube. Under the optimized reac-
tion conditions (2 mol % catalyst, 1 bar H2 in C6D6 at RT),
terminal alkenes were readily reduced to the correspond-
ing alkanes in quantitative yield (7a-7f), and isomeriza-
tions of these alkenes were observed during the hydro-
genation process. Complete hydrogenation of internal
alkenes such as 2-hexene 6g and cyclic substrates 6h-6k
could be achieved, albeit longer time was required for
more sterically hindered olefins (6j). For the bulky tetra-
substituted olefin tetramethylethylene 6l, only 11.5% con-
version could be reached after 12 h at RT. The olefins
containing one (6m-6r) or two (6s, 6t) phenyl groups
were quantitatively hydrogenated under the same condi-
tions. The hydrogenation of 1,1-diphenylethylene 6s was
conducted on a larger scale (0.27 mmol) under the stand-
ard conditions affording pure 7s in 91% isolated yields
only after filtration. Functional groups such as methoxy
(6n), trifluoromethyl (6o) and cyano (6p) were well toler-
ated, though the cyano group may interacts with the Ni
active site of the catalyst stronger, resulting in
1
species according to the resulting H NMR spectrum.
However, 4 could not be detected during the catalytic
reaction process. To find out whether 4 can be trans-
formed into [SiII(Xant)SiII]Ni(η2-norbornene), the com-
plex 4 was treated with 3 molar equiv. of norbornene in
C6D6. No reaction was observed after 12 h at RT or at 50 oC,
o
and higher temperature (75 C) caused decomposition of
4
instead of the formation of [SiII(Xant)SiII]Ni(η2-
norbornene). In addition, under the same catalytic condi-
tions, the hydrogenation catalyzed by complex 4 only
gave 65% conversion after 24 h (Table 1, entry 3). Taken
all together, it is reasonable to suggest that 4 is not an
active intermediate in the [SiII(Xant)SiII]Ni(η2-1,3-cod)
catalyzed hydrogenation. The usage of complex 3 as cata-
lyst afforded low conversion (32%) presumably due to the
stronger coordination of PMe3 to the Ni center compared
with that of the olefin (Table 1, entry 2). When the com-
mercially available Xantphos ligand supported Ni com-
plex was applied as catalyst for the hydrogenation reac-
tion, only 6% conversion was achieved after 24 h (Table 1,
entry 4). For comparison, the 1:1 mixture of Ni(cod)2 and
[SiII(Xant)SiII] 1 was tested as catalyst and no decrease in
catalytic ability was observed (>99% conv.) under the
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)
)
1
1
-
-
TS(F-G)
1
2
3
4
5
6
7
8
l
l
o
o
m
m
TS(D-E)
l
l
24.8
13.7
a
a
c
c
k
k
TS(C-D)
TS(E-F)
20.9
(
(
G
H
9.8
∆
∆
∆
∆
∆
∆
∆
∆
15.4
5.8
8.6
-0.7
15.7
6.6
9.9
-1.7
0.0
TS(B-C)
3.8
-2.7
6.2
7.6
-3.4
3.7
-4.5
-3.8
7.9
TS(H-A)
H
H
H
H
-0.9
14.1
-7.5
H
H
Si
Si
Si
Si
Si
Si
17.1
-4.8
Si
Si
Ni
Ni
Ni
H
Ni
H
Si
Si
Si
Ni
Si
Si
Si
H2C CH2
A
Si
Si
H2C CH2
C
H2C CH2
D
Ni
H2C CH2
Ni
Ni
H2C CH2
H
H
H2C
9
Si
Si
A
+
H2C
H2C CH2
B
H2C
CH2H
Ni
C
CH2H
H2
+
H2
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13
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60
G
H
F
H2C
HH2C CH2H
E
CH2H
-26.4
-35.5
H
R.C.
Figure 4. Calculated Potential Energy Surface (PES) of the Proposed Mechanism of Hydrogenation of Ethylene by 2b.
The proposed mechanism of ethylene hydrogenation is
a much larger reaction time (110 h). Worthy to note, 4-
vinylpyridine (6u) could also be hydrogenated quantita-
tively after 66 h. Unsaturated carbonyl complexes (6v-6x)
could also be fully converted to ketones (7v-7w) and ester
(7x) with excellent chemoselectivity. Furthermore, non-
conjugated and conjugated dienes (6y, 6ab-6ad), serving
as active substrates, yielded the corresponding alkanes
quantitatively. Lower conversion (68%) was afforded for
the hydrogenation of 2,3-dimethyl-1,3-butadiene (6z) due
to its isomerization to tetramethylethylene after mono-
hydrogenation. Alkynes such as phenylacetylene and 2-
methyl-1-hexen-3-yne, turned out to be unreactive likely
due to the strong coordination ability of a C≡C bond to-
wards the Ni active site. In order to determine the per-
formance of the catalyst with respect to turnover number
(TON) and turnover frequency (TOF), we have chosen 1-
dodecene and styrene as substrates and carried out the
catalytic reactions in a Schlenk tube. The TON and TOF
values could reach up to 1000 and 250 h-1, respectively,
with styrene (Scheme 4), which is an excellent result
among preceding Ni catalyzed homogeneous hydrogena-
tion of olefins.13
presented in Figure 4. The calculations of the mechanism
were performed at the B3LYP-D3/SDD-def2SVP level of
theory, on a model system (2b) in which the Ph and tBu
substituents of the silylene ligand were replaced by me-
thyls and the methyls of the xanthene backbone were
replaced by hydrogens.
In the first step of the proposed mechanism, complex A
[SiII(Xant)SiII]Ni(η2-CH2CH2) coordinates to a molecule of
dihydrogen without a barrier to form complex B. Com-
plex B converts, through a low-barrier TS(B-C), to the
dihydrogen complex C at 8.6 kcal mol-1, which then un-
dergoes a dihydrogen splitting via a TS(C-D) found at 15.4
kcal mol-1 generating complex D (at 9.9 kcal mol-1) in an
endergonic step of 1.3 kcal mol-1. The next step is a migra-
tory insertion of the coordinated ethylene to the Ni−H
bond via TS(D-E) to afford E (at 7.6 kcal mol-1). In com-
plex E the hydride binds directly to the Si atom of the
silylene ligand causing the ligand N-heterocyclic ring
opening (see details below). Complex E isomerizes to
complex F (at 3.7 kcal mol-1) via TS(E-F), bringing the
hydride back to Ni center. Further isomerization from
intermediate F to G brings the hydride to a position that
will allow reductive elimination of the hydrogenated
product in the following step. This isomerization is en-
dergonic by 2.5 kcal mol-1 and proceeds via the highest
transition state TS(F-G) found at 24.8 kcal mol-1 on the
PES, making this transformation the rate determining
Mechanism. H2 cleavage on transition metal centers
usually occurs via homolytic or heterolytic mechanism.24
For the homolytic process, oxidative addition of H2 to an
electron-rich metal center affords a dihydrido metal com-
plex. Heterolytic process generally occurs in a Lewis base-
transition metal system, in which the mental center ac-
cepts a hydride and the assisted Lewis base ligand traps a
proton. Recently, H2 cleaved by a Lewis acid assisted tran-
sition metal system has been presented as a new mode of
H2 activation.25 For example, the Peters group found that
(PhDPBiPr)Ni [PhDPBiPr = PhB(o-iPr2PC6H4)2] can activate
H2 to generate a bridging hydrido-borohydrido Ni com-
plex, where the B and Ni atoms cooperatively cleave H2 in
a concerted mechanism with a lower activation energy
compared with homolytic mechanism.20d,26 Since the
mechanistic studies of H2 activation by Ni complexes are
rare, here we use DFT calculations to gain insight into the
mechanism of complex 2 catalyzed hydrogenation of
olefins.
Figure 5. B3LYP-D3/SDD-def2SVP Optimized Geometries
of the Intermediates C and D.
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Figure 6. NBOs of the TS(C-D). σ*(H−H), occ. 0.55 el.,
Table 2. Calculated Parameters of C, D and 5.
1
2
3dxy(Ni), occ. 1.72 el., σ (Ni−Si), occ. 1.65 el..
r(Ni-H) (Å) r(Si-H) (Å) WBI(Ni-H) WBI(Si-H) q(H)
larly to proposed intermediate D, contains strong interac-
tion between one of the Si atoms and the bound hydrides.
This is supported by short Si−H bond distances (1.646,
1.657 Å) and high WBI(Si−H) (0.67, 0.67) (Table 2). Addi-
tionally, the WBI(Ni−H) of 0.11, 0.12 and q(H) = -0.22, -
0.23 are similar to those in D. The interaction of the Ni
center with the hydrides is achieved via donation of elec-
trons from Ni d orbitals to σ*(Si−H).
3
4
5
6
7
8
9
C
D
5
1.710,
1.711
2.932,
2.944
1.798,
1.726
0.10,
0.10
0.19,
0.14
0.11,
0.12
0.04,
0.04
0.50,
0.58
0.67,
0.67
-0.02,
-0.02
-0.22,
-0.24
-0.22,
-0.23
1.598,
1.668
1.614,
1.652
1.646,
1.657
step in the proposed mechanism. G coordinates to an
additional molecule of olefin yielding complex H which
reductively eliminates to release ethane and regenerates
the catalytic species A. This step is exergonic by 40.5 kcal
mol-1.
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Complex D transfers a hydride to the coordinated ole-
fin to form complex E in which the second hydride mi-
grates to the silicon atom of the silylene resulting in
opening of the N-heterocyclic ring by dissociation of the
N−Si bond, with Si−N distance of 2.83 Å compared with
r(Si−N) of 1.89, 1.90 Å in the closed ring (Figure 7). This
type of ring opening is also observed in complex 4 (Figure
2), which is generated by exposing 2 to dihydrogen
(Scheme 2).
One of the most interesting features about the pro-
posed mechanism is the activation of the dihydrogen
through formation of the intermediate complex D from C
(Figure 5). This activation step of the dihydrogen is pre-
sumably assisted by the SiII atoms of the bis(NHSi) ligand.
In the dihydrogen complex C, the Ni−H bond distances
are 1.710, 1.711 Å with Ni−H Wiberg Bond Indexes (WBI) of
0.10, 0.10 and -0.02,-0.02 el. charges on the H atoms, sug-
gesting only a weak interaction between the Ni center and
the coordinated dihydrogen molecule (Table 2). The Si−H
distances of 2.932, 2.944 Å and WBI(Si−H) = 0.04, 0.04
imply almost no interaction between the silylene Si cen-
ters and the H atoms of the dihydrogen. In complex D, as
expected for a metal dihydrido complex, significant short-
ening of the Ni−H [r(Ni−H) = 1.598, 1.668 Å] bonds are
observed, with WBI of 0.19, 0.14 and NBO charges on the
hydrides of -0.22, -0.24. Additionally, D also exhibits a
strong interaction between the silylene ligand and the
hydrides, based on r(Si−H) = 1.798, 1.726 Å and WBI of
0.50, 0.58. NBO analysis of the transition state TS(C-D)
suggests that the dihydrogen activation is achieved by
interaction of the Ni 3dxy orbital and the σ(Ni−Si) orbital
with the σ*(H−H) (Figure 6). Seminal work by Sabo-
Etienne et al. revealed that SiIV could assist hydrogen
exchange in which (η2-H−SiIV)Ru complexes converted to
(η2-H−H)Ru complexes via a σ–complex assisted metathe-
sis (σ-CAM) process.27 However, cooperative SiII atom
assisted H2 cleavage at a Ni center has never been report-
ed as yet.
Figure 7. B3LYP-D3/SDD-def2SVP Optimized Geometry
of the Intermediate E.
CONCLUSIONS
We could synthesize and fully characterize a new
strong σ-donating bis(NHSi)xanthene ligand whose coor-
dination behavior towards Ni was explored. Treatment of
the bis(NHSi)xanthene ligand 1 with Ni(cod)2 led to
[SiII(Xant)SiII]Ni(η2-1,3-cod) 2, and its subsequent reaction
with PMe3 resulted in [SiII(Xant)SiII]Ni(PMe3)2 3. Unprec-
edentedly, the dinuclear Ni2H2 complex 4 with two bridg-
ing Si−HꢀNi was furnished through the reaction of 2
with 1 bar H2 at room temperature. Mixing complex 3 and
H2 led to a reversible H2 activation and afforded the first
silylene-assisted dihydrido Ni complex confirmed by in
situ NMR spectroscopy and single-crystal X-ray diffrac-
tion analysis. Complex 2 can act as a strikingly efficient
hydrogenation catalyst for olefins: 27 examples involving
terminal and bulky internal olefins are reported which are
hydrogenated quantitatively under very mild condition (1
bar H2, RT). Based on DFT calculations, the bis(NHSi)
ligand 1 can assist the Ni center to cleave dihydrogen
during the hydrogenation process, which is a new mode
for H2 activation. Studies aimed at applying these
bis(NHSi) Ni complexes in other catalytic transformations
are ongoing.
The experimental support for this type of cooperative
activation is observed in complex 5 (Figure 3) which is
formed upon reaction of [SiII(Xant)SiII]Ni(PMe3)2 with di-
hydrogen (Scheme 3). DFT calculations show that 5, simi-
ASSOCIATED CONTENT
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Experimental procedures, characterizations, crystallo-
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available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*matthias.driess@tu-berlin.de
9
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENT
We are grateful to the Cluster of Excellence UniCat (EXC 314-
2, sponsored by the Deutsche Forschungsgemeinschaft and
administered by the TU Berlin) for financial support. Y.W.
gratefully acknowledges financial support by the China
Scholarship Council. A.K. is grateful to the Alexander von
Humboldt Foundation for a postdoctoral fellowship. We
thank Dr. Zhenbo Mo and Yu-Peng Zhou for helpful discus-
sion, Dr. Somenath Garai for XRD structural determinations
and Paula Nixdorf for assistance in the XRD measurements.
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