Hydrogenolysis of Polysilanes Catalyzed by Low-Valent Nickel Complexes

Article abstract of DOI:10.1002/anie.201907525

The dehydrogenation of organosilanes (R_xSiH_4?x) under the formation of Si?Si bonds is an intensively investigated process leading to oligo- or polysilanes. The reverse reaction is little studied. To date, the hydrogenolysis of Si?Si bonds requires very harsh conditions and is very unselective, leading to multiple side products. Herein, we describe a new catalytic hydrogenation of oligo- and polysilanes that is highly selective and proceeds under mild conditions. New low-valent nickel hydride complexes are used as catalysts and secondary silanes, RR′SiH₂, are obtained as products in high purity.

Full text of DOI:10.1002/anie.201907525

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Accepted Article
Title: Hydrogenolysis of Polysilanes Catalyzed by Low Valent Nickel
Complexes
Authors: Bruno Pribanic, Monica Trincado, Frederik Eiler, Matthias
Vogt, Aleix Comas-Vives, and Hansjörg Grützmacher
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10.1002/anie.201907525
Angewandte Chemie International Edition
Hydrogenolysis of Polysilanes Catalyzed by Low Valent Nickel Complexes
Bruno Pribanic,1 Monica Trincado,1* Frederik Eiler,1 Matthias Vogt,2 Aleix Comas-Vives,3
Hansjörg Grützmacher1*
1 Department of Chemistry and Applied Biosciences, ETH Zürich 8093, Switzerland
2
Universität Bremen, Fachbereich 2 Biologie/Chemie, Institut für Anorganische Chemie und
Kristallographie, Leobenerstr. 7, 28359 Bremen, Germany
3 Chemistry Department, Autonomous University of Barcelona, Cerdanyola del Vallès 08193 Bellaterra,
Catalonia, Spain
Dedicated to Prof. Dr. José Manuel González Díaz on the occasion of his 60th birthday
Abstract
The dehydrogenation of organosilanes (RxSiH4-x) under formation of Si–Si bonds is an intensively
investigated process leading to oligo- or polysilanes. The reverse reaction is little studied. Up to date,
the hydrogenolysis of Si–Si bonds requires very harsh conditions and is very unselective leading to
multiple side products. Here we describe a new catalytic hydrogenation of oligo- and polysilanes, which
is highly selective and proceeds under mild conditions. New low valent nickel hydride complexes are
used as catalysts and secondary silanes, RR’SiH2, are obtained as products in high purity.
Introduction
Hydrosilanes are widely used reagents in the hydrosilylation of unsaturated molecules to give
organosilanes1,2 and are crucial components in CVD (chemical vapor deposition) processes.3,4 Elegant
methods for the use of surrogates for flammable hydrosilanes were developed recently,5 but these
are not atom efficient and require additional synthetic steps. A clean simple in situ method for
especially the preparation of the very reactive hydrosilanes remains therefore desirable.
Organohydrosilanes are commonly produced by halide/hydride exchange reactions from
organohalosilanes, RxSiX4-x. The latter are produced in the Müller6-Rochow7 process (Figure 1a, eq. 1),
the Barry process,8 hydrosilylation reactions,9 and Grignard-type reactions.10 The cleavage of Si–Si
bonds is used to break down the “high-boiling” residues from the Müller-Rochow process (mostly
chlorodisilanes R6-xSi2Clx) to monomers RxSiHCl3-x, either under harsh conditions (HCl gas, up to
900°C)11 or using dihydrogen and expensive precious metals as catalysts (eq. 2).12 Subsequently, the
partially hydrogenated compounds RxSiHCl3-x are reacted again with costly metal hydrides (LiAlH4) in
classical salt metathesis reactions to form the corresponding hydrosilanes (eq. 3).13 The products are
1
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10.1002/anie.201907525
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often absorbed to the complex mixture of metal hydride chloride salts as side products and
consequently difficult to purify. Polysilanes, on the other hand, are easily prepared via a Wurtz-type
reductive coupling of chlorosilanes in high yield (eq. 4).14
Figure 1. a) Industrial Müller-Rochow process for the synthesis of organochlorosilanes (eq. 1).
Hydrogenation of the “high-boiling residue” (eq. 2). Synthesis of hydrosilanes by a classical hydride
transfer reaction (eq. 3). Wurtz-type dehalogenation of chlorosilanes to polysilanes (eq. 4).
Hydrogenation/dehydrogenation equilibrium proposed syntheses of hydrosilanes (eq. 5). b) Selected
examples of low valent nickel olefin and nickel hydride complexes relevant to this work.
The dehydrogenation of silanes, R2SiH2, to oligo- and polysilanes has been intensively investigated
(Figure 1a, eq. 5 left to right)15,16 and both -bond metathesis according to 2 Si–H  Si–Si + H–H
(metallocene catalysts) or a sequence of oxidative/reductive eliminations (catalysts with late
transition metals)17, 18 are accepted mechanisms. Silane dehydrogenation is also promoted by low-
valent nickel catalysts, 19 where Ni hydride species are proposed as intermediates. Notably, the
dinuclear nickel(I) hydride A (Figure 1b) dehydrogenates primary silanes to give polysilanes.20 Also the
related zerovalent [Ni(dmpe)2] [dmpe = 1,2-bis(dimethylphosphino)ethane] produces linear
polysilanes selectively.21 The principle of microscopic reversibility suggests that the opposite reaction,
namely the catalytic hydrogenation of Si–Si bonds, may be possible (Figure 1a, eq. 5 right to left).
Based on experimentally determined bond energies, we estimate that the hydrogenation of SiSi
2
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bonds [Hdiss (SiSi) = 73.1 - 88.0 kcal mol1] is a thermodynamically allowed process [Hdiss (SiH) >
90 kcal mol1].22 Indeed, Rosenberg et al.23 determined an equilibrium constant of Kc = 104 for Ph2HSi–
SiHPh2 + H2  2 Ph2SiH2 at room temperature, which bolsters our expectation that the hydrogenation
of Si–Si bonds is exergonic.24 Yet, the hydrogenation of Si–Si bonds is very rare and only disilanes with
one Si‒Si bond were successfully converted as was reported independently by Braun25,26 and Radius27
using either a Pt complex or the binuclear Ni(0) complex B (Figure 1b) as catalysts achieving turn over
numbers (TON’s) of up to 20. The hydrogenation of higher oligo- or polysilanes is unknown.
Nickel hydride complexes28 have been frequently proposed as intermediates in reactions where
hydrogen is used to cleave chemical bonds (hydrogenolysis)29and have demonstrated high efficiency
in the electrocatalytic production of H2.30 Moreover, in the recent years the interests in this class of
compounds have been primarily determined by the development of synthetic models of the active
sites of nickel-containing hydrogenases and methyl-coenzyme M reductase.31 Inclusion of cooperative
ligands and reactivity studies of well-defined hydride nickel complexes have provide important
advances in these catalytic processes. In contrast to divalent and monovalent nickel hydride species,
very few zero-valent Ni hydrides were described such as: (i) the binuclear olefin Ni(0) complex C which
is only stable at temperatures below -20 C;32 (ii) the anionic carbonyl complex [(Ph3P)2N][Ni2(CO)6H],33
and (iii) complex D containing a Ga(III) center which stabilizes the electron rich Ni center by a NiGa
interaction.34
Results and Discussion
We reported that the monovalent nickel complex [Ni(TFA)(trop2NH)] 1a (trop2NH =bis(5H-
dibenzo[a,d]cyclohepten-5-yl)amine; TFA = trifluoroacetate) is a particularly active catalyst in the
dehydrogenation of amino boranes under mild conditions.35 Spectroscopic studies suggested that two
diamagnetic hydrido Ni complexes 5a and 6a participate in the catalytic cycle. Both contain the
diolefine-amine trop2NH which has remarkable properties as stabilizing ligand for low-valent metal
complexes. 36 , 37 We could now isolate such low-valent nickel hydrides and characterize them
structurally.
3
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Scheme 1. a, b) Synthesis of terminal anionic Ni(0) hydride complexes 5a,b and bridging dinuclear Ni(0)
complexes 6b and 6b. c) Synthesis of potassium salts with terminal anionic Ni(0) silyl complexes 7a,b
and neutral Ni(0) silane complexes 8b and 9b.
When [Ni(TFA)(trop2NH)] 1a reacts with dimethylaminoborane and KOtBu in THF for 30 minutes at
room temperature (r.t.), a 8 : 1 ratio of the mononuclear Ni(0) hydride complex K[NiH(trop2NH)] 5a
and the dinuclear [Ni(0)2(-H)(trop2NH)2] 6a is obtained (Scheme 1a) together with a mixture of the
linear amido borane 3 and cyclic aminoborane (Me2NBH2)2 (4).35 When the amidoborane salt
K[Me2NBH3] (2) was reacted with 1a or 1b, the dinuclear bridging hydride complexes 6a or 6b were
formed selectively. The N-methylated derivative 5b could be obtained as stable Li salt in 56% yield
(Scheme 1b) using Li[AlH4] as reductant and hydride transfer reagent. When this reaction is carried
out rapidly with an excess of Li[AlH4], hydrogen evolution is observed and beside hydride 5b, pale red
crystals of Li[Ni(AlH4)(trop2NMe)] [5b(AlH3)] were isolated at -30 °C. In THF solution, [5b(AlH3)]
completely converts to hydride 5b (r. t., 24 h). The terminal hydride ligand in 5a, b shows a 1H NMR
resonance at  = ‒4.75 and ‒4.59 ppm while the chemical shifts for the bridging hydride in 6a and 6b
are observed at lower frequencies at  = ‒8.12 and ‒6.06 ppm, respectively.28, 38 In order to probe the
dehydrogenation activity vis-a-vis silanes, the dinuclear complex K[Ni2(-H)(trop2NMe)2] 6b was
reacted with three equivalents of Ph2SiH2. Within five minutes, the silyl nickelate 7b and the Ni(0)
silane complex [Ni(trop2NMe)(2-HSiHPh2)] (8b) are formed as 1 : 1 mixture (Scheme 1c). Addition of
4
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K[Me2NBH3] (2) immediately converts the silane complex 8b to the silyl complex 7b under concomitant
liberation of H2 and formation of the cyclic amino borane 4. If the reaction mixture containing 7b, 8b,
and Ph2SiH2 is kept for longer time in a closed reaction vessel at room temperature, the slow formation
of the disilane complex [Ni(trop2NMe)(2-HSiPh2-SiPh2H)] (9b) is observed (for alternative selective
methods to prepare salts 7a,b with Ni(0) silyl anions in high yields directly from 1a,b, see Supp.
Information). The silyl complexes 7a,b and silane complexes 8b and 9b were characterized by 1H-, 29Si-
NMR and two-dimensional 29Si-1H HMQC NMR spectroscopy (See Supp. Information for details).
Single crystals of the terminal hydrido complex [Li(thf)3][NiH(trop2NMe)] 5b and its AlH3 adduct
[Li(THF)3][Ni(AlH4)(trop2NMe)] 5b(AlH3), the bridging hydrido complexes 6a,b, the silyl complexes 7a,b,
the silane complex [Ni(trop2NMe)(2-HSiHPh2)] 8b, and the disilane complex [Ni(trop2NMe)(2-
HSiPh2-SiPh2H)] 9b were isolated and investigated via X-ray diffraction analysis. Plots of the structures
of 5b, 6a, 7b and 8b are shown in Figure 2 (for a complete set of data on all structures see Supp.
Information). The structures show two remarkable features: (i) With exception of 7b, the coordination
sphere around the Ni(0) center in all complexes is trigonal pyramidal. The sum of angles defined by
the hydride ligand and the centroids (ct) of both coordinated C=Ctrop units is close to 360 ° and the Ni
center deviates only little from the best plane through the centroids (ct’s) of the coordinated C=Ctrop
units and the hydride (0.1 – 0.2 Å). In the sterically more encumbered silyl complex 7b with the
trop2NMe ligand, the deviation of the Ni center from the Si, ct1, ct2 plane is larger (0.31 Å). (ii) The Ni-
N bonds are remarkably long exceeding by far 2.2 Å. For comparison, the Ni–N bond in the Ni(I)
complex 1a is much shorter (2.061 Å). The coordinated olefinic bonds trans to the hydride are long (>
1.42 Å) indicating efficient electron donation from the d10-Ni(0) center to the (pp)* orbitals of the
C=Ctrop units. In the bridging hydride complexes 6a,b, the Ni–H–Ni angles vary significantly [6a: 151(3)°;
6b: 121(3)°] which goes hand in hand with a variation of the Ni–Ni distance: 6a: 3.041(1) Å; 6b: 2.760(1)
Å. Both are longer than in [Ni2(-H)(C2H4)4]‒ (C) - the only other known anionic dinuclear Ni(0) hydride
complex – showing a short Ni–Ni distance at 2.596(1) (Ni–H–N 125°) which was attributed to a weak
non-covalent interaction between the d10 valence electron configured Ni(0) centers.32, 39
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a
b
c
d
Figure 2. Molecular structures of 5b, 6a, 7b and 8b.
The Ni–Si bond lengths in 6a,b [2.247(1) Å, 2.229(1) Å] are comparable to other nickel silyl
complexes.40 , 41 The isolation of the rare d10 2-(Si–H)Ni(0) complexes 7b and 8b is noteworthy.
Obviously the back donation of electron density from Ni(0) into the C=Ctrop units is sufficiently strong
to prevent oxidative cleavage under insertion of the Ni center into the Si-H bond.42
6
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Figure 3. a) Electron density contour map of complex 5b in the N–Ni–H plane. b) Electron density
contour map of complex 5b in the trigonal plane with the two C=Ctrop units, the Ni, and the H center.
c) Energy densities and NPA charges (black) at the bond, BCPs (blue), and ring critical points, RCPs
(orange). d) HOMO of 5b at a surface isovalue = 0.055.  = bond critical point,  = ring critical point.
In order to gain some insight into the electronic structure of the anionic low-valent Ni hydride and silyl
complexes, DFT calculations were performed for 5b [B3LYP with mixed basis sets 6-31g(d) for C and N
atoms and 6 31g(d,p) for H atoms, as well as effective core potential LANL2DZ for the nickel metal
center]. Specifically, the Atoms-in-Molecule (AIM) theory of Bader43 allows to identify bond critical
points (bcp = ) and ring critical points (rcp = ).44 As Figure 3a and 3b clearly show, there are bcp’s
between Ni and N, Ni and the C centers of the C=Ctrop units, and Ni and H. Furthermore, rcp’s indicate
metallacyclopropane character of the Ni and C=Ctrop interaction as a result of significant ligand-to-
metal donation, (C=C)Ni, and metal-to-ligand back-donation, Ni*(C=C). The Ni atom carries a
positive partial NPA charge of +0.47 e while all surrounding atoms are negatively charged. Figure 3c
gives the energy densities at these critical points.45 The negative values at the bcp’s between Ni and
all C atoms indicate polar but covalent interactions. The positive value at the bcp between Ni and H
indicates a predominantly ionic interaction Ni+, H‒. The bcp between Ni and N has a small positive
value, which is indicative of a very weak interaction as also seen for van-der-Waals contacts or
hydrogen-bridges. The HOMO of complex 5b is displayed in Figure 3d and shows the anti-bonding
interaction between the occupied dz2-orbital at Ni(0) and the lone pair at N. Based on these data, the
extraordinarily long Ni–N bond in 5b and all other complexes listed here are best described as a
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structurally and electrostatically enforced anti-bonding interaction. This is in stark contrast to the Ni–
Ga interaction in complex D (Figure 1b) which corresponds to a dz2(Ni)p(Ga) two electron donor-
acceptor-interaction.34
The isolation of the disilane complex 9b suggests that the low-valent Ni hydride or silyl complexes may
be catalysts for the dehydrogenation of silanes. This is indeed the case. Either the hydride complexes
5b, 6a,b, the silyl complexes 7a,b, or the precursor complexes 1a,b in combination with amidoborane
2, catalyze the dehydrogenation of Ph2SiH2 at room temperature. With 1 – 5 mol% of the nickel
complexes, a mixture of oligosilanes H(Ph2Si)nH (n ≤ 4) was obtained (see Table S1 in Supp. Information
for details). The principle of micro reversibility implies that the low valent Ni complexes should also
catalyze the hydrogenation of oligo- and polysilanes. Attempts to convert tertiary disilanes such as
Me3Si-SiMe3, Ph3Si-SiPh3, Me2PhSi-SiPhMe2, or Me3Si-SiPh3 to the corresponding tertiary silanes using
6b as catalyst, KOtBu as activator, and 1 bar H2 were little encouraging. The first two silanes did not
react because of low solubility (Ph3Si-SiPh3) and lack of interaction with the activator while Me2PhSi-
SiPhMe2 was converted in 53% yield, with significant scrambling to Me2SiH2 and Me2SiPh2 after
extended reaction times (see Table S3). The asymmetric disilane Me3Si-SiPh3 was hydrogenated to the
expected products Ph3SiH and Me3SiH but in low yield (< 20%). However, the hydrogenolysis of the
oligosilanes H(SiPh2)nH using isolated pure Ni(0) hydrides 5b or 6a,b as catalysts at room temperature
under 1 bar of hydrogen was highly successful (entry 1, Table 1). This reaction is the direct reverse of
the above reported dehydrogenation of Ph2SiH2. The optimal conditions for this and all other oligo-
and polysilane hydrogenation reactions are listed in Table 1 (for an extended set of data on catalyst
screening and optimization, see Tables S2 – S7 in Supp. Information). The reaction can be effectively
extended to cyclic silanes, as Si5Ph10 in which case the easily accessible Ni(I) complex 1b was employed
as catalyst precursor. Note that Ph2SiH2 is slowly reconverted to the initial mixture of oligosilanes
H(SiPh2)nH (n = 2 – 4) when the Ni catalyst is not separated from the reaction mixture and hydrogen
release is permitted.
In addition to the experiments shown in Scheme 1a, we have verified that the trifluoroacetate complex
1b is converted to the hydride complex 6b using KOtBu in the presence of H2. This nucleophilic additive
is also needed to activate the cyclic oligosilanes (entries 2 and 3) or polymeric silanes (entry 4). This is
especially important in reactions with the highly insoluble dimethylpolysilane which upon addition of
small amounts of KOtBu is first converted to soluble (Me2Si)6 and silyl anions before the hydrogenation
proceeds.46 The THF solutions of the methylated silanes and KOtBu as activator must be saturated with
H2 before the nickel catalyst is added, otherwise no conversion is observed. In all cases, good to
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excellent yields of Me2SiH2 were obtained in NMR experiments (up to 92%). The reaction can be easily
scaled up and Me2SiH2 was isolated by distillation from the reaction mixture in 80% yield.
Table 1. Hydrogenation of oligo- and polysilanes catalyzed by nickel complexes.[a]
Additive
(equiv)
H2
(bar)
Reaction Monomer yield
Entry
1[b]
Substrate
cat (mol%)
time (h)
(%)
6a (1)
KAB (0.03)
1
5
48
>99
2
3
1b (0.6)
5b (0.8)
KOtBu (0.03)
48
24
79
92
KOtBu (0.05)
2
4[c]
5b (0.2)
1b (1)
KOtBu (0.01)
LiAlH4 (0.1)
2
5
24
18
90
79
5[d]
[a] Reaction conditions: Silane (0.34 -1.72 mmol Si units) in THF, additive (0.01 – 0.1 equiv, respect
to Si units), [Ni] catalyst (0.2 - 1 mol%) was added and reacted with H2 (1 - 5 bar) at ambient
temperature. [b] Substrate is a mixture of trisilane and tetrasilane in a 2:1 ratio. [c]
Polydimethylsilane Mw = 1100 - 1700. [d] Polymethylphenylsilane Mn (g mol-1) = 28’500, Đ = 1.22.
In additional experiments, the homogeneity of the reaction mixture was investigated. When the
hydrogenolysis of Si6Me12 was performed in the presence of an excess of Hg, no inhibition of the
catalytic reaction was observed. High resolution transmission electron microscopy (HRTEM) gives no
evidence for crystalline metallic nickel particles. A second recharge of substrate (for example Si5Ph10)
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to the reaction mixture results in little loss of activity (75% yield Ph2SiH2 vs. 79% in the first run, see
Table 1, entry 2). However, addition of an excess of triphenylphosphine (3 equivalents with respect to
1b) added to a reaction mixture which had reached about 30% conversion of Si5Ph10 (after ca. 6h)
provokes a considerable decrease in the catalytic activity (45% NMR yield Ph2SiH2 after 48h). These
experiments indicate that very likely nickel complexes in homogeneous solution are the active species.
The direct conversion of the in situ generated Me2SiH2 - in pure form a highly flammable gas at room
temperature (b.p. = ‒20 °C) with a flash point below ‒40 °C - to siloxanes by reaction with water or
anhydrous ethanol according to Me2SiH2 + 2 ROH  Me2Si(OR)2 + 2 H2 was tested. The addition of
water to a cooled (-40 °C) reaction mixture containing the silane leads to the quantitative formation
of a mixture of cyclic and linear silicone oligomers. The corresponding reaction with EtOH affords
cleanly Me2Si(OEt)2 and hydrogen gas as by product (see Supp. Information for details).
In order to gain some insight into a possible mechanism of the hydrogenolysis of Si–Si bonds, DFT
calculations [B3LYP with 6-31g(d) for C and N atoms, 6 31g(d,p) for H atoms, LANL2DZ for the nickel
center] were carried out for the hydrogenolysis of a disilane. We chose HPh2Si–SiPh2H as substrate
and [Ni(H)(trop2NH)]‒ (5a‒) as Ni species (for calculations with complex 5b-, see Supp. Information).
The calculated reaction energy for HPh2Si–SiPh2H + H2  2 Ph2SiH2 is GR = ‒9.3 kcal mol‒1. The free
Gibbs energies (kcal mol‒1) for the individual reaction steps are listed in Figure 4 with respect to the
catalytically active complex I (0.0 kcal mol‒1). The calculations indicate a very similar Minimum Energy
Reaction Path (MERP) for [Ni(H)(trop2NMe)]‒ (5b‒) with the only difference that intermediates which
show a Ni···HN interactions have a Ni···HC interaction between Ni and the N-methyl group instead.
Inclusion of solvent effects do not significantly alter the energy profile (see Figure S18-S21 in Supp.
Information). The calculations indicate that the hydrido nickelate 5a‒ is only a pre-catalyst, which leads
to the active catalyst I. A possible MERP from 5a‒ to I shows a rather shallow energy profile and
involves first a weakly endergonic cleavage of the Ni–N interaction (3.0 kcal mol‒1) proceeding via a
low barrier (2.8 kcal mol‒1). In the next step, the C=Ctrop unit is de-coordinated, which proceeds via a
sizable barrier of 19.5 kcal mol‒1, to give the complex I in an overall endergonic reaction G(5a  I)
=
16.4 kcal mol‒1 (Figure S17 in Supp. Information). This anionic d10-Ni complex I serves as entry point
into the catalytic cycle (Figure 4a) and can be viewed as almost linear 14 electron Ni(0) complex (H-
Ni-ct = 158.4 °; ct = centroid of the C=Ctrop bond) where the electron rich basic Ni center is internally
protonated by the NH group (Ni···H = 2.184 Å; see Figure 4b for a plot of the calculated structure).
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a
b
c
Figure 4. a) Proposed mechanism (DFT; B3LYP, 6-31g(d) for C ,N atoms; 6 31g(d,p) for H, LANL2DZ for
Ni) of the hydrogenolysis of oligosilanes catalyzed by 5a-. Activated complexes at transition states are
shown in blue. Ni···N distances (Å): I: 3.142; TS(I,II): 3.140; II: 3.004; TS(II,III):·2.692; III: 2.322; IV: 2.263;
V: 2.587; TS(V,VI): 2.677; VI: 2.636; TS(VI,VII): 2.510; VII: 2.278; VIII: 2.363; TS(VII,I): 2.974. b) Calculated
structure of I: Ni-H 1.569 Å, Ni-H 1.569 Å, Ni-ct: 1.965 Å, Ni···H 2.184 Å, C=Ctrop 1.439 Å. c) Energy
profile for the reaction Ph2HSi-SiHPh2 + H2  2 Ph2SiH2 catalyzed by I; free Gibbs energies, G, are
given in kcal mol‒1.
In the first step of the catalytic cycle, complex I reacts with disilane via a barrier of 7.6 kcal mol‒1 to
give the 2-Si-H disilane complex II (G1 = ‒0.2 kcal mol‒1) which rearranges to the activated complex
TS(II,III) (7.2 kcal mol‒1) in which the Si–Si bond is co-planar with the Ni–H bond. A -bond metathesis
occurs in which a new Ni–Si and Si–H bond are formed at the expense of cleaving the Si–Si and Ni–H
bond. The Ni silyl complex III formed in that second reaction step loses immediately in step 3 the 2-
coordinated H2SiPh2 ligand as first product molecule to give the 16 electron Ni(0) silyl amino olefin
complex IV. Overall, the reaction II  IV is exergonic by G2+3 = ‒20.0 kcal mol‒1. In the next step 4,
H2 is added in a weakly endergonic reaction (G(IV,V) = 3.5 kcal mol‒1) to yield the H2 complex V. The
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2-bonded H2 ligand is then oxidatively cleaved in step 5 to give the Ni(II) silyl dihydride VI in a further
mildly endergonic reaction, (G5 = 7.2 kcal mol‒1). In step 6, complex VI decomposes via a very small
barrier (1.2 kcal mol‒1) to give the 2-H2SiPh2 complex VII in an exergonic reaction (G6 = -6.9 kcal mol‒
1). Finally, the second equivalent Ph2SiH2 is released in an endergonic reaction step 7 (G7 = 10.6 kcal
mol‒1) to give the nickel hydrido amine olefin complex VIII. This complex immediately rearranges to
give I, which is 3.5 kcal mol‒1 more stable and shows a diminished anti-bonding interaction between
the lone-pair at N and the occupied dz2 orbital at Ni. This final step 8 turns the catalytic cycle over.
Overall, this reaction mechanism involves classical intermediates which undergo -bond metatheses.
Only modest activation barriers below 8 kcal mol‒1 are encountered along the MERP. In agreement
with the proposed mechanism, no significant kinetic isotope effect was observed when D2 was used
instead of H2. A notable feature of the intermediates and activated complexes involved in the catalytic
cycle is the wide range of Ni–N distances. Only the Ni silyl intermediates III and IV, the 2-H2SiPh2
complex VII, and the hydride VIII show Ni–N bond lengths below 2.4 Å. All other species have very
long Ni–N distances which exceed in some cases even 3 Å (see Supp. Information for details).
Conclusion
A new class of low-valent d10-Ni(0) hydride and silyl complexes was synthesized which contain
stabilizing amino bis(olefin) ligands. These complexes are remarkably effective in the hydrogenation
of all Si–Si bonds in oligo- and polysilanes. Some reactions require small amounts of a nucleophilic
additive which is needed to break-up a Si–Si bond according to: (R2Si)x + Nuc‒  (Nuc)R2Si-(SiR2)x-2-
‒
SiR2 . This activation is not necessary when the substrate contains already one Si–H bond which allows
binding to the metal center to give a 2-Si-H complex. The almost quantitative formation of Me2SiH2
from (Me2Si)6 and (Me2Si)x is especially noteworthy. These experimental findings and DFT calculations
give some first insights for further catalyst design and improvement. Key-features of the d10-
[Ni(X)(trop2NR)]‒ (R=H, Me; X = H, SiR3) complexes are: (i) a low-valent electron rich metal center which
promotes both, -bond metathesis and oxidative additions; (ii) a double hemi-labile tripodal ligand
which is achieved by an electrostatically enforced anti-bonding interaction between the Ni and N
center and the intrinsically low binding energy of olefins to metal centers. These special features allow
accessing low-coordinated 14 and 16 electron d10-Ni hydrides and silyl complexes, which readily
interact with Si–H and H–H bonds. At the same time, the rigidity of the central seven-membered
cycloheptatriene-ring of the trop ligand platform ensures sufficient kinetic stabilization of all
intermediates within the catalytic cycle. Future work will seek to enhance the catalytic activity of the
complexes by structural modification of the ligand in such a way that the liberation of one
coordination site is even more facilitated. Additionally, catalytic hydrogenation of Si–Si bonds and
transfer of the transiently formed Si–H to an organic molecule, according to: (SiR2)x + x H2 + 2x A=B 
12
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10.1002/anie.201907525
Angewandte Chemie International Edition
x R2Si(A-BH)2 may be realized which is a highly atom efficient process that employs only relatively
harmless chemicals.
Acknowledgements
B. P., M. T., F. E. and H. G. acknowledge funding from the Swiss National Science Foundation (SNF
200021_162436 / 1) and Eidgenössische Technische Hochschule Zürich (ETH). A.C-V. acknowledges
financial support from the Holcim Foundation, the Spanish MEC and the European Social Fund (Ramon
y Cajal program, RyC-2016-19930). The authors thank A. Rosenthal for assistance with X-ray
crystallographic analysis and R. Huber and I. Kalkman for assistance in dehydrogenation experiments
of diphenylsilane.
Conflict of interest
The authors declare no conflict of interest.
Keywords: Nickel hydride – polysilanes – hydrogenolysis – hydrosilanes – olefin ligands – silyl
complexes – silane complexes
13
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Table of Contents
The Si–Sibonds in polysilanes are converted to Si–H bonds under mild
conditions using low-valent anionic Ni(0) hydride catalysts. The reaction is
reversible and dehydrogenative coupling of hydrosilanes is likewise possible.
Experiments and DFT calculations indicate that low-coordinated 14 and 16
electron d10-Ni hydrides, silyl and silane intermediates are involved in the
catalytic cycle which likely consists of sequential -metathesis and OA/RE
steps.
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