Towards a catalytic hydrogenolysis of silicon-silicon bonds: Formation of Si-H bonds from disilanes and H2 at platinum

Article abstract of DOI:10.1039/b815586k

Reactions of the disilanes Cl₂MeSiSiMeCl₂, ClMe ₂SiSiMe₂Cl and Me₃SiSiMe₃ led to the products of oxidative addition cis-[Pt(SiMeCl₂)₂(PEt ₃)₂] (2

Full text of DOI:10.1039/b815586k

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www.rsc.org/dalton | Dalton Transactions
Towards a catalytic hydrogenolysis of silicon-silicon bonds: Formation of
Si–H bonds from disilanes and H2 at platinum
Anja Roscher,a Andreas Bockholtb and Thomas Braun*a
Received 8th September 2008, Accepted 7th November 2008
First published as an Advance Article on the web 12th January 2009
DOI: 10.1039/b815586k
Reactions of the disilanes Cl2MeSiSiMeCl2, ClMe2SiSiMe2Cl and Me3SiSiMe3 led to the products of
oxidative addition cis-[Pt(SiMeCl2)2(PEt3)2] (2), cis-[Pt(SiMe2Cl)2(PEt3)2] (3) and cis-[Pt(SiMe3)2(PEt3)2]
(4). Complex cis-[Pt(SiMe2Cl)2(PEt3)2] (3) converts in the presence of dihydrogen into two equivalents
HSiMe2Cl and complex trans-[PtH2(PEt3)2] (10). NMR investigations reveal the initial formation of
trans-[PtH(SiMe2Cl)(PEt3)2] (5) and one equivalent HSiMe2Cl.
Introduction
Results
Silicon–hydrogen bond cleavage can be considered as a key step in
a variety of transition-metal mediated reactions including hydrosi-
lylations, dehydrogenative silylations and polysilane formation.1
Important elementary steps of catalytic cycles are the oxidative
addition of a Si–H bond, s-bond metathesis reactions or the
addition of a Si–H bond at a silylene ligand.1–5
We became interested in a transition-metal catalyzed “hy-
drogenolysis” of a Si–Si bond of disilanes using dihydrogen as
a second substrate. Such a reaction route is of considerable
importance, because it opens up new opportunities to access
higher-value silanes which can be applied for further synthesis.1
It should also be thermodynamically conceivable because a
Si–H bond with a bond dissociation energy of 380–420 kJ mol-1
is more stable than a Si–Si bond (310–330 kJ mol-1), whereas
the bond dissociation energy of H2 is 436 kJ mol-1.6 The reverse
reaction, a dihydrogenative coupling of silanes, can be catalyzed
by early transition metal complexes.1,6,7 Late transition metal
complexes typically exhibit competing catalytic activities for
both dehydrocoupling and substituent redistribution reactions of
hydrosilanes.1,5,8
Si–Si activation of disilanes at Pt(0)
Treatment of a solution of [Pt(PEt3)3] (1) in THF with
Cl2MeSiSiMeCl2 affords the silyl complex cis-[Pt(SiMeCl2)2-
(PEt3)2] (2) as well as of [PtCl(PEt3)3]Cl und MeSi(SiMeCl2)3.
The latter compounds have been identified by comparison of
their NMR data with the literature.10 Their formation can be
explained by redistribution reactions via◦metal silylene species
starting from 2.1,11 However, a reaction at 0 C leads spontaneously
to a selective generation of 2 (Scheme 1). A conversion of 1
with ClMe2SiSiMe2Cl to give cis-[Pt(SiMe2Cl)2(PEt3)2] (3) has
been described before and proceeds at room temperature within
30 min.12 We found that a reaction of 1 with Me3SiSiMe3 is
very slow and the generation of cis-[Pt(SiMe3)2(PEt3)2] (4) with
approximately 50% conversion within 3 weeks was observed at
room temperature (Scheme 1). The facilitated oxidative addition
with electronegative substituents at silicon is consistent with
literature data.12,13
Catalytic processes, which involve a hydrogenolysis of disilanes
are rare and have been covered by patent specifications. Chlori-
nated disilanes can be converted into silanes in the presence of
dihydrogen and metal salts such as NiCl2, CuCl2, AlCl3 or FeCl2.
The transformations can be heterogeneous or homogeneous
and proceed with low selectivity usually under drastic reaction
conditions such as high pressure and higher temperatures.9 No
mechanistic investigations on elementary steps of a putative
catalytic cycle have been carried out.
In this paper, we report on a selective transformation of Si–Si
bonds with H2 into Si–H bonds in a homogeneous phase. Studies
of elementary steps led to the development of a cyclic process
for the hydrogenolysis of disilanes, which proceeds under mild
conditions. The reaction cascade can be repeated several times,
but the “catalyst” has to be regained after each cycle.
Scheme 1 Si–Si oxidative addition of disilanes at 1.
aHumboldt-Universita¨t zu Berlin, Institut fu¨r Chemie, Brook-Taylor-Str. 2,
12489 Berlin, Germany
1
Selected NMR data of 2–4 are given in Table 1. The 29Si{ H}
bWacker AG, Consortium fu¨r elektrochemische Industrie, Zielstattstr. 20,
81379 Mu¨nchen, Germany
NMR spectra show virtual triplets for the silicon atoms with
couplings to the phosphorus atoms. The 31P{ H} NMR spectra
1
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Table 1 Selected NMR data of 2–4
1
1
1H NMRa
31P{ H} NMR
29Si{ H} NMR
Complex
d/ppm
3JH,Pt/Hz
d/ppm
1JP,Pt /Hz
2JP,Si/Hz
d/ppm
1JSi,Pt/Hz
2
3
4
1.51
1.02
0.35
15.6
21.0
24.5
13.1
14.2
16.5
1872
1751
1586
98
76
60
46
45
3.3
1622
1438
1175
a Signals for the methyl groups bound at Si.
display a singlet for the phosphines in the mutually cis position.
The platinum–phosphorus coupling constants are characteristic
for Pt(II) compounds with a phosphine in the trans position to
synthesis of the silyl complex 3, the latter subsequently converts
in the presence of dihydrogen into two equivalents HSiMe2Cl
and complex trans-[PtH2(PEt3)2] (10). NMR investigations reveal
the initial formation of trans-[PtH(SiMe2Cl)(PEt3)2] (5) and one
equivalent HSiMe2Cl (Scheme 3). The conversion is very selective
and no platinum chloro complexes are formed. The silyl complex 5
was also prepared via an alternative reaction pathway by treatment
of 1 with HSiMe2Cl (Scheme 3).
1
2
a silyl group.12,14 Note that the increasing size of JP,Pt , JP,Si , and
1JSi,Pt coupling constants mirror the decreasing trans-influence of
the silyl group with an increasing number of chlorine substituents.
Reactivity of the silyl complexes
Treatment of complex cis-[Pt(SiMeCl2)2(PEt3)2] (2) with H2 at
1 atm leads to the formation of small amounts of HSiMeCl2.
In contrast, the reaction of cis-[Pt(SiMe2Cl)2(PEt3)2] (3) with
H2 at 1 atm gave the silane HSiMe2Cl and complex trans-
[PtH(SiMe2Cl)(PEt3)2] (5) as well as considerable amounts of
the platinum compounds trans-[PtCl(SiMe2Cl)(PEt3)2] (6), trans-
[PtHCl(PEt3)2] (7) and trans-[PtCl2(PEt3)2] (8) in a ratio of 63 :
1
18 : 10 : 9 according to the 31P NMR spectrum. The H NMR
spectrum reveals the presence of complex 6, HSiMe2Cl and
HMe2SiOSiMe2H in a ratio of approximately 10 : 4 : 1. The
generation of siloxane and the chloro complexes indicates the
presence of adventitious water which converts HSiMe2Cl into
HCl and the siloxane. Independent experiments on the reactivities
of [Pt(PEt3)3] (1) and cis-[Pt(SiMe2Cl)2(PEt3)2] (3) support this
assumption (Scheme 2). Thus, treatment of 3 with HCl results in
the formation of the chloro complex trans-[PtCl(SiMe2Cl)(PEt3)2]
(6) and HSiMe2Cl, whereas 1 reacts with one equivalent HCl to
furnish trans-[PtHCl(PEt3)2] (7). More HCl leads to the generation
of trans-[PtCl2(PEt3)2] (8). For the formation of 7 and siloxane
a hydrolysis of the Pt–Si in 6 is also conceivable.15 The chloro
complexes 6–8 have been described before.16
Scheme 3 Reactivity of the disilyl complex 3.
Complex 5 was characterized by its NMR spectroscopic data.
1
The H NMR spectrum displays a signal at d -3.32 ppm with a
platinum–hydrogen coupling of 910 Hz, which can be assigned to
the metal bound hydrogen nucleus. A resonance at d 1.01 ppm with
a 3JH,Pt coupling constant of 28.0 Hz reveals the SiMe2Cl ligand.
The data are consistent with those of other Pt(II) silyl hydrido
species.3,18 A singlet in the 31P NMR spectrum at d 21.9 ppm
with platinum satellites (1JP,Pt = 2836 Hz) indicates the presence
of phosphines in a mutual trans position.19
A
1H,29Si-HMBC
NMR spectrum shows a signal at d 50.1 ppm in the 29Si domain
which correlates with the resonances for the hydrido ligand and
the SiMe2Cl groups. The silicon–platinum coupling constant is
1314 Hz.
Scheme 2 Formation of platinum chloro complexes.
Note that for the synthesis of the starting compound 1 water
has been used in the work-up process.17 We therefore recrystallised
1 in the presence of PEt3 at 193 K yielding colourless crystals
of [Pt(PEt3)4] (9), which are free of water. Complex 9 is only
stable at low temperature and can be transformed into 1 at
333 K and 2 Torr within 2 h.18 If this batch of 1 is used for the
Development of a cyclic process for the formation of HSiMe2Cl
After the successful formation of HSiMe2Cl starting from
the disilane ClMe2SiSiMe2Cl, we studied the reactivity of the
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dihydride trans-[PtH2(PEt3)2] (10) towards further activation re-
actions. Indeed, an NMR experiment reveals that treatment of
complex 10 with ClMe2SiSiMe2Cl furnishes the activation product
cis-[Pt(SiMe2Cl)2(PEt3)2] (3) within one day, but the starting
compound 10 is still present. Encouraged by these results, we
turned our attention to a possible catalytic hydrogenolysis of
ClMe2SiSiMe2Cl. The formation of HSiMe2Cl with H2 in the
presence of ClMe2SiSiMe2Cl and cis-[Pt(SiMe2Cl)2(PEt3)2] (3) was
investigated. Small amounts of Et3N have been added to trap
adventitious HCl. After several hours reaction time at atmospheric
pressure the stoichiometric generation of HSiMe2Cl and only
compound 10 was observed. Apparently, there is no catalytic
conversion of the Si–Si bond in the presence of free hydrogen. If the
oxidative addition of ClMe2SiSiMe2Cl proceeds at {Pt(PEt3)2},
the generation of the latter, and therefore also of 3, might be
hampered by free H2, which is with {Pt(PEt3)2} in equilibrium
with trans-[PtH2(PEt3)2] (10). Additional phosphine also does not
lead to catalytic turnovers. However, addition of PEt3 to a solution
of trans-[PtH2(PEt3)2] (10) gives the trisphosphine complex trans-
[PtH2(PEt3)3] (11) (Scheme 2). After one hour reaction time, the
generation of [Pt(PEt3)3] (1) and [Pt(PEt3)4] (9) is observed by
reductive elimination of H2. Complex 9 does also react with
ClMe2SiSiMe2Cl to yield 3, but the reaction is slower than with 1
as starting compound and the conversion is not complete.
To facilitate the reductive elimination of H2 from 10 or 11 we
treated a solution consisting of 3, PEt3, NEt3 and an excess of
ClMe2SiSiMe2Cl by turns with argon and H2. However, the gen-
eration of HSiMe2Cl was still stoichiometric. Yet, a regeneration
of 1 is possible by application of a vacuum at 11. Thus, HSiMe2Cl
can be separated from the platinum compounds by trap-to-trap
distillation. Subsequent addition of PEt3 and the application of
vacuum yields 1. The latter can be used for the activation of the
next batch of ClMe2SiSiMe2Cl and its conversion into HSiMe2Cl.
This reaction cascade can be performed several times to yield two
equivalents of HSiMe2Cl from ClMe2SiSiMe2Cl and H2 in each
cycle.
Discussion
Oxidative addition reactions of disilanes are depicted in Scheme 1.
The reactivity of the disilanes increases in the row Me3SiSiMe3 <<
Me2ClSiSiClMe2 < MeCl2SiSiCl2Me. Comparable trends have
been observed in palladium or platinum catalysed bissilylation
reactions with disilanes.12,13 Note that electronegative substituents
at silicon lead to a Si–Si bond with a s*-orbital, which is
lower in energy. This facilitates a concerted oxidative addition
at electron rich metal centres.12,20 Ito et al. showed that hexam-
ethyldisilane reacts with [Pt3(CNAd)6] (Ad = Adamantyl) to yield
[Pt(SiMe3)2(CNAd)2].21 Si–Si bond cleavage reactions at disilanes
with Si–H functionalities are thought to be initiated by a Si–H
activation.22–24
Based on these observations, which are described above it
is possible to draft a cyclic process for the hydrogenolysis of
ClMe2SiSiMe2Cl (Scheme 4). After activation of ClMe2SiSiMe2Cl
at [Pt(PEt3)3] (1) the disilyl complex cis-[Pt(SiMe2Cl)2(PEt3)2] (3)
is furnished. Treatment of 3 with H2 gives the dihydride trans-
[PtH2(PEt3)2] (10) and HSiMe2Cl. The addition of phosphine
gives trans-[PtH2(PEt3)3] (11), which is in equilibrium with 1. The
reductive elimination of H2 is then facilitated and by Si–Si bond
cleavage a cyclic process is completed.
The disilyl complex cis-[Pt(SiMe2Cl)2(PEt3)2] (3) reacts with
dihydrogen to give trans-[PtH2(PEt3)2] (10) and two equivalents of
HSiMe2Cl. The hydrido silyl species trans-[PtH(SiMe2Cl)(PEt3)2]
(5) has been identified as intermediate. This reaction sequence
is unique. It has been reported that cis-[Pt(SiMePh2)2(PMe2Ph)2]
reacts with H2 at 1.5 bar in a reversible reaction to yield trans-
[PtH(SiMePh2)(PMe2Ph)2] and HSiMePh2, but the bissilyl com-
plex was synthesised from cis-[PtCl2(PMe2Ph)2] and MePh2SiLi.25
No formation of a platinum dihydride was observed. In con-
trast, the triphenylphosphine complex cis-[PtH(SiHPh2)(PPh3)2]
is not susceptible to a reductive elimination step. Instead the
2
dimer [Pt(m-h (Si,H)-SiHPh2)(PPh3)]2 is generated upon heating.26
Mechanistically, the reactions of 3 or 5 with H2 might proceed via
a Pt(IV) species, which give after reductive elimination 5 or 10,
respectively.27 As an alternative s-bond metathesis pathways are
also conceivable.4 Note that trans-[PtH3(SiH2Cl)(PCy3)3] reacts
above 193 K to give H2 and trans-[PtH(SiH2Cl)(PCy3)2], but no
silane formation has been observed.18 Complex 5 can be synthe-
sised via an alternative reaction pathway by oxidative addition of
HSiMe2Cl starting from the platinum(0) compound 1. Perutz et al.
demonstrated that [Pt(PCy3)2] reacts with HSiMe2Cl or HSiMeCl2
at 200 K to give initially the cis oxidative addition products which
rearrange to the trans isomers at room temperature.3
Complex trans-[PtH2(PEt3)2] (10) reacts with Me2ClSiSiClMe2
to give the activation product cis-[Pt(SiMe2Cl)2(PEt3)2] (3). Mech-
anistically, the reaction might proceed via Pt(IV) or Pt(0) species.16
We favour the latter reaction pathway, because the presence
of free H2 inhibits the oxidative addition of the disilane. This
observation is in accordance with our studies on the stoichiometric
conversion of 10 into 3, which proceeds only in the absence of
dihydrogen.
Scheme 4 Cyclic process for the transformation of ClMe2SiSiMe2Cl into
HSiMe2Cl.
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dried in vacuo to obtain 5 as yellow oil. Found: C 32.40, H 7.21.
Conclusions
C14H37ClPtP2Si requires C 31.97, H 7.09). Yield 90 mg (73%).
-1
1
A cyclic process for the hydrogenolysis of ClMe2SiSiMe2Cl has
been developed (Scheme 4). All of the elementary steps of a
putative catalytic cycle have been realised including the oxidative
addition of a Si–Si- bond and the hydrogenolysis of the new metal–
silicon bonds by H2 (Scheme 4). The reaction cascade is close to
being catalytic, but a regeneration step to remove excess of H2
hampers a smooth catalysis.
˜
n(ATR)/cm 1971 (PtH). H-NMR (500 MHz, C6D6) d/ppm:
-3.32 (br, s + satellites, Dn 1 = 27 Hz, 1H, JH,Pt = 910 Hz, PtH),
2
0.90–0.93 (m, 18 H, PCH2CH3), 1.01 (br, s + satellites, 6 H, JH,Pt
=
1
28.0 Hz, SiCH3), 1.56–1.60 (m, 12 H, PCH2CH3). 31P{ H}-NMR
(202.4 MHz, C6D6) d/ppm: 21.9 (s + satellites, JP,Pt = 2836 Hz).
1
29Si{ H}-NMR (C6D6, 99.4 MHz) d/ppm: 50.1 (s + satellites,
JSi,Pt = 1314 Hz).
Reaction of cis-[Pt(SiMe2Cl)2(PEt3)2] (3) with HCl
Experimental
A solution of cis-[Pt(SiMe2Cl)2(PEt3)2] (3) (115 mg, 0.19 mmol)
in benzene (5 mL) was treated with a solution of HCl (95 mL,
0.19 mmol) in ether. The reactions mixture was stirred for 10 min
at room temperature and the volatiles were removed in vacuo.
The resulting yellow oil was washed with hexane (1 mL) and
dried in vacuo. The NMR data confirm the presence of trans-
[D6]benzene was dried by stirring over potassium and then distilled
under vacuum. The silanes were obtained from Aldrich or the
Wacker AG and distilled before use. [Pt(PEt3)3] (1) was prepared
according to the literature.17
The NMR spectra were recorded on a Bruker DRX 500 or
Bruker Avance 600 spectrometer at 300 K. The 1H NMR chemical
shifts were referenced to residual C6D5H at d = 7.15 ppm. The
1
[PtCl(SiMe2Cl)(PEt3)2] (6).16 Yield 86 mg (81%).29Si{ H}-NMR
1
31P{ H} NMR spectra were referenced externally to H3PO4 at d
(C6D6, 99.4 MHz) d/ppm: 22.4 (s + satellites, JSi,Pt = 1564 Hz).
0.0 ppm. Infrared spectra were recorded on a Bruker Vector 22
2
spectrometer. Abbreviation: N = JPSi,cis + 2JPSi,trans
.
Reaction of [Pt(PEt3)3] (1) with HCl
A solution of [Pt(PEt3)3] (1) (82 mg, 0.15 mmol) in benzene (5 mL)
was treated with solution of HCl (75 mL, 0.15 mmol) in ether.
The volatiles were removed in vacuo and the colourless residue
was washed twice with hexane (2 mL). The colourless residue
was dried under vacuum. The NMR data confirm the presence of
trans-[PtHCl(PEt3)2] (7).16 Yield 66 mg (97%).
Synthesis of cis-[Pt(SiMeCl2)2(PEt3)2] (2)
A solution of 1 (97 mg, 0.18 mmol) in 5 mL THF was treated with
MeCl2SiSiCl2Me (162 mL, 0.90 mmol) at 0 ◦C. The solution turned
from red to yellow within a few minutes. After 10 min, the reaction
mixture was warmed to room temperature and the volatiles were
removed in vacuo. The residue was dissolved in 10 mL hexane and
the solution was filtered. The filtrate was concentrated to 3 mL
and stored at 243 K. Yellow crystals precipitated after one day.
The crystals were filtered off and dried in vacuo. Yield 110 mg
Reaction of cis-[Pt(SiMe2Cl)2(PEt3)2] (3) with H2
H2 gas was bubbled for 20 min into a yellow solution of 3
(124 mg, 0.20 mmol) in THF (5 mL) until the solution turned
colourless. The 1H, 29Si and 31P NMR spectroscopic data reveal the
initial formation of trans-[PtH(SiMe2Cl)(PEt3)2] (5) after 10 min.
Complex trans-[PtH2(PEt3)2] (10) and HSiMe2Cl were furnished
after 30 min reaction time.10
(93%). 1H-NMR (500 MHz, C6D6) d/ppm: 0.83 (dt, 18 H, JH,P
15.8 Hz, JH,H = 7.5 Hz, PCH2CH3), 1.51 (s + satellites, 6 H, JH,Pt
=
=
15.6 Hz, SiCH3), 1.74 (apparent quint, 12 H, JH,H = JP,H = 7.5 Hz,
1
PCH2CH3). 31P{ H}-NMR (202.4 MHz, C6D6) d/ppm: 13.1 (s +
satellites, JP,Pt = 1872 Hz, JP,Si = 98 Hz). 29Si{ H}-NMR (C6D6,
1
99.4 MHz) d/ppm: 45.8 (vt + satellites, JSi,Pt = 1622 Hz, N =
Reaction of cis-[Pt(SiMeCl2)2(PEt3)2] (2) with H2
196 Hz).
H2 gas was bubbled for 1 h into a yellow solution of 2 (90 mg,
Formation of cis-[Pt(SiMe3)2(PEt3)2] (4)
0.14 mmol) in 5 mL THF (5 mL) until the solution turned
1
colourless. The H and 31P NMR spectroscopic data reveal the
A solution of 1 (38 mg, 0.07 mmol) and Me3SiSiMe3 (0.6 mL,
2.88 mmol) in 0.5 mL C6D6 was transferred into an NMR tube.
According to the NMR data, the solution contained 1 and 4 in a
ratio of 1 : 1 after 3 weeks. Analytical data for 4: 1H-NMR (C6D6,
300 MHz) d/ppm: 0.35 (s + satellites, 18 H, JH,Pt = 24.5 Hz,
SiCH3), 1.09 (dt, 18 H, JH,P = 15.8, JH,H = 7.5 Hz, PCH2CH3),
1.75 (apparent quint, 12 H, JH,H = JP,H = 7.5 Hz, PCH2CH3).
presence of 2 and HSiMeCl2 in a ratio of 10 : 2.
Reaction of trans-[PtH(SiMe2Cl)(PEt3)2] (5) with H2
H2 gas was bubbled for a few seconds into a yellow solution of 5
(74 mg, 0.14 mmol) in benzene (5 mL) until the solution turned
1
colourless. The H and 31P NMR spectroscopic data reveal the
1
31P{ H}-NMR (C6D6, 121.5 MHz) d/ppm: 16.5 (s + satellites,
formation of trans-[PtH2(PEt3)2] (10) and HSiMe2Cl.10
1
JP,Pt = 1586 Hz, JP,Si = 60 Hz). 29Si{ H}-NMR (C6D6, 99.4 MHz)
d/ppm: 3.3 (vt + satellites, JSi,Pt = 1175 Hz, N = 120 Hz).
Synthesis of trans-[PtH(SiMe2Cl)(PEt3)2] (5)
Stepwise treatment of cis-[Pt(SiMe2Cl)2(PEt3)2] (3) with H2 and
ClMe2SiSiMe2Cl under recovery of [Pt(PEt3)3] (1)
Hydrogen was bubbled into a solution of cis-[Pt(SiMe2Cl)2(PEt3)2]
(3) (193 mg, 0.31 mmol) and Et3N (132 mL, 0.94 mmol) in THF
(5 mL) for 20 min. The reaction mixture was distilled by trap-
to-trap distillation. The NMR spectroscopic data of the distillate
confirmed the presence of two equivalents HSiMe2Cl (integration
An orange solution of 1 (132 mg, 0.24 mmol) in 5 mL THF was
treated with HSiMe2Cl (26 mL, 0.24 mmol). The solution turned
light yellow within several minutes. The volatiles were removed in
vacuo and the residue was washed with hexane (1 mL) and then
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on an external standard). The residue was treated with PEt3
(44 mL, 0.31 mmol) to give, after application of vacuum, complex
[Pt(PEt3)3] (1). Yield 163 mg (96%).
Complex 1 can again be used for a Si–Si activation of
ClMe2SiSiMe2Cl. The reaction cycle was performed three times
and 2.7 equivalents of ClMe2SiSiMe2Cl (based on the amount of
3) were transformed into HSiMe2Cl.
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Acknowledgements
We are grateful to S. Hinze and J. Voigt (both in Berlin) for
experimental assistance.
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1382 | Dalton Trans., 2009, 1378–1382
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