Iridium Pincer Catalysts for Silane Dehydrocoupling: Ligand Effects on Selectivity and Activity

Article abstract of DOI:10.1021/acs.organomet.5b00486

Catalytic reactions of bisphosphinite pincer-ligated iridium compounds p-X^R(POCOP)IrHCl (POCOP) [2,6-(R₂PO)₂C₆H₃, R = ⁱPr, X = H (1); R = ^tBu, X = COOMe (2); = H (3); = NMe₂ (4)] with primary and secondary silanes have been performed. Complex 1 is primarily a silane redistribution precatalyst, but dehydrocoupling catalysis is observed for sterically demanding silane substrates or with aggressive removal of H₂. The bulkier compounds (2-4) are silane dehydrocoupling precatalysts that also undergo competitive redistribution with less hindered substrates. Products generated from reactions utilizing 2-4 include low molecular weight oligosilanes with varying degrees of redistribution present or disilanes when employing more sterically demanding silane substrates. Selectivity for redistribution versus dehydrocoupling depends on the steric and electronic environment of the metal but can also be affected by reaction conditions. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.organomet.5b00486

Article
pubs.acs.org/Organometallics
Iridium Pincer Catalysts for Silane Dehydrocoupling: Ligand Effects
on Selectivity and Activity
Neil T. Mucha and Rory Waterman*
Department of Chemistry, University of Vermont, Burlington, Vermont 05405-0125, United States
S
* Supporting Information
ABSTRACT: Catalytic reactions of bisphosphinite pincer-
ligated iridium compounds p-XR(POCOP)IrHCl (POCOP)
i
t
[2,6-(R2PO)2C6H3, R = Pr, X = H (1); R = Bu, X =
COOMe (2); = H (3); = NMe2 (4)] with primary and
secondary silanes have been performed. Complex 1 is primarily a
silane redistribution precatalyst, but dehydrocoupling catalysis is
observed for sterically demanding silane substrates or with
aggressive removal of H2. The bulkier compounds (2−4) are
silane dehydrocoupling precatalysts that also undergo compet-
itive redistribution with less hindered substrates. Products
generated from reactions utilizing 2−4 include low molecular
weight oligosilanes with varying degrees of redistribution present or disilanes when employing more sterically demanding silane
substrates. Selectivity for redistribution versus dehydrocoupling depends on the steric and electronic environment of the metal
but can also be affected by reaction conditions.
metallocenes, although these methods of polymerization still
generally suffer from lower than desired molecular weights and
high polydispersity.
INTRODUCTION
■
Polysilanes are not primarily prepared by metal catalysis,
despite how essential metal catalysis is for the preparation of
specialty polyolefins.1 This is a surprising dichotomy given the
premium on monodisperse, linear polysilanes of high molecular
weight to promote optimal σ-electron delocalization.2,3 Reasons
for the lack of industrial adoption of metal-catalyzed silane
dehydrocoupling depend on the general category of catalyst
employed. For example, well-studied and highly active d0 metal
catalysts tend to suffer from competitive backbiting and
formation of cyclosilanes, which can limit molecular weight
and increase polydispersity.4
Despite the fact that the first report on silane dehydrocou-
pling used Wilkinson’s catalyst,5 initial observations of low
dehydrocoupling activity with late metals stunted their
development relative to d0 metal catalysts.6 Additionally, the
use of late metal catalysts in dehydrocoupling is potentially
problematic due to two metal-catalyzed side reactions, the
oxidation of Si−Si bonds7 and redistribution.8 Thus, the
potential advances that metal catalysis can offer to polysilane
preparation cannot be realized until the factors that impact
metal catalysis are better understood. Efforts to minimize these
side reactions have included promoting hydrogen loss,9
employing designer substrates,10,11 inhibiting silyl coordina-
tion,12 and rigorous exclusion of water and oxygen.
Iridium pincer complexes bearing the POCOP ligand
−
(POCOP = 2,6-(tBu2PO)2C6H3 ) are thermally robust
molecules that have shown high activity as dehydrogenation
catalysts using both alkanes and amine-boranes as substrates.20
Although iridium-catalyzed silane redistribution is a known
reaction,8,21−23 the high activity of POCOP−iridium com-
pounds for dehydrogenation catalysis and the ability of the
ligand to be easily tuned to modulate reactivity warranted
investigation of these systems as potential silane dehydrocou-
pling catalysts. Herein, we report the synthesis of a series of p-
XR(POCOP)IrHCl [p-XR(POCOP) = 2,6-(R2PO)2C6H3, R =
t
iPr, X = H (1); R = Bu, X = COOMe (2); = H (3); = NMe2
(4)] compounds and their ability to facilitate either
dehydrocoupling or redistribution depending upon the
substrate and, more importantly, reaction conditions.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of p-XR(POCOP)IrHCl
Compounds. Efforts to prepare iPr(POCOP)IrHCl (1)24 by
reaction of [(COE)2IrCl]2 with iPr(POCOP)-H (iPr(POCOP)-
H = 2,6-(iPr2PO)2C6H3) yielded a mixture of products
including 1 in low yield (ca. 10%, 31P δ = 173.0). A closely
related chloro-bridging compound, iPr(POCOP)IrH(μ-Cl)2Ir-
(COE)2,25 has diagnostic NMR resonances (31P δ = 153.4; Ir−
H δ = −24.0) that mirror those of the major product of the
While conditions for promoting linear polysilanes over
cyclosilanes are known for d0 metallocene complexes, general
conditions to promote dehydrocoupling over redistribution are
not known.4 However, late metal catalysts using nickel,13−16
rhodium,17,18 and platinum19 have shown dehydrocoupling
activity and selectivity comparable to that of the group 4
Received: June 7, 2015
© XXXX American Chemical Society
A
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reaction (31P δ = 153.0; Ir−H δ = −25.1), which is tentatively
proposed as the chloro-bridged dimer [iPr(POCOP)IrH(μ-
Cl)]2. Successful preparation of analytically pure 1 was
accomplished by refluxing a toluene solution of [(COE)2IrCl]2
and iPr(POCOP)-H ligand for 16 h under a H2 atmosphere.
from the less than linear P(1)−Ir−P(2) and C(1)−Ir−Cl
angles, which suggest the presence of an apical ligand with
small steric requirements (Table 2).29 There is no apparent
structural difference between compounds 2, 3, and 4 to suggest
a significant trend based on the para substitution. Compound 1
is isostructural with the tert-butyl analogues, although it has a
substantially longer Ir−Cl bond length (Table 2).
By analogy to 3,26 the p-XtBu(POCOP)IrHCl (tBu(POCOP)
−
=
2, 6-(t Bu2 PO)2 C6 H3
)
compounds (2
=
p-
COOMetBu(POCOP)IrHCl; 4 = p-NMe2tBu(POCOP)IrHCl)
were prepared by refluxing a toluene solution of the appropriate
ligand precursor and [(COD)IrCl]2, then isolated by washing
with cold pentane and subsequent filtration. The yield of 4
(81%) was slightly lower than that for 2 (88%), which is
attributed to greater solubility of 4 in pentane. Compounds 1,
2, and 4 were characterized by multinuclear NMR and infrared
spectroscopy, combustion analysis, and single-crystal X-ray
diffraction.
Catalytic Reactions with Primary Silanes: Reactions of
PhSiH3 in a Sealed NMR Tube. Benzene-d6 solutions of
PhSiH3 were treated with 2 mol % of each of 1−4 in J-Young
type, PTFE-valved NMR tubes. These solutions were degassed
via two freeze−pump−thaw cycles, heated for 16 h at 120 °C,
and then analyzed by 1H NMR spectroscopy. Trace amounts of
PhSiH2Cl (δ 5.05) were observed by 1H NMR spectroscopy. It
is therefore presumed that 1−4 undergo ligand exchange to the
respective iridium dihydride species, (POCOP)Ir(H)2, of which
31
1
p-HtBu(POCOP)Ir(H)2 is known. Analysis of the H NMR
spectra of all reactions reveals products from two competing
reactions: dehydrocoupling and redistribution.
Like related POCOP−nickel compounds with substituted
pincer aryl backbones,27 the values of 31P NMR resonances for
1−4 fall into a narrow range. There is no apparent correlation
between the identity of the substituent para to iridium and the
31P NMR chemical shift values observed for 1−4. The hydride
chemical shifts of compounds 2−4 are in a narrow range, with 2
displaying the most downfield shift and 4 the most upfield shift,
indicating a modest effect from the para substituent (Table 1).
t
In reactions with Bu-substituted 2−4, silane dehydrocou-
pling is favored and evident by the presence of 1,2-
diphenyldisilane (PhSiH2)2, H2, linear oligosilanes with 1H
NMR resonances in the range δ 4.3−4.8, and cyclic oligomers
with resonances in the range δ 5.2−5.5. In addition to
dehydrocoupling products, signals due to Ph2SiH2 and SiH4 are
also present in the 1H NMR spectra, demonstrating competitive
silane redistribution.
Table 1. Diagnostic NMR Data (δ) for Compounds 1−4 in
a
Benzene-d6 Solution
In contrast, 1 favors redistribution of PhSiH3 over
dehydrocoupling and yields Ph2SiH2 (73%) as the main
product of the reaction with Ph3SiH (4%) present as well
(Table 3). It is worthwhile to note that the product percentages
herein are based on normalized integration of Si−H resonances
postreaction and reflect only the postreaction product
distributions of these reactions. Trace amounts of dehydrocou-
pling products, including (PhSiH2)2, (Ph2SiH)2, and PhSiH2−
SiPh3, were detected and suggest that 1 may engage in
dehydrocoupling catalysis under alternative reaction conditions.
Comparing compounds 1−4, the phosphine substituent
impacts the catalytic activity and product distribution. The less
bulky derivative, 1, demonstrates higher activity toward silanes
with nearly complete consumption of substrate in 0.5 h but
dramatically favors redistribution at silicon over dehydrocou-
pling. In contrast, the bulkier tBu derivatives are less active and
require >12 h of reaction time to reach completion, but these
compounds favor dehydrocoupling products over redistribution
(Table 3).
1
compound
ipr[Ir], 1
Ir−H H NMR
31P NMR
−36.7
173.0
176.5
175.3
176.3
2
p-COOMetBu[Ir], 2
−40.1, JPH = 12.9
2
p-HtBu[Ir], 3
−40.7, JPH = 13.1
2
p-NMe2tBu[Ir], 4
−41.1, JPH = 13.3
a
[Ir] = (POCOP)IrHCl.
For compound 1, the hydride resonance is broadened to a
singlet and shifted downfield to δ −36.7. This difference in
chemical shift is consistent with the ligand environment of 1
i
that possesses the less electron-rich Pr phosphine ligand
t
relative to the Bu phosphine substituents employed in 2−4.
These diagnostic resonances compare favorably to p-
HtBu(POCOP)IrHCl (3), reported by Brookhart and co-
workers (Table 1).26
Suitable crystals were grown for compounds 1, 2, and 4 and
subjected to single-crystal X-ray diffraction studies (Figure 1).
The compounds are square-based pyramidal at iridium, as
demonstrated by calculated τ values of 0.10, 0.02, and 0.08 for
1, 2, and 4, respectively.28 The hydride ligand was located for
all three compounds from the Fourier difference map.
Additional evidence for the location of this ligand comes
Among the series of compounds screened, the relative
amount of dehydrocoupling versus redistribution in the product
distribution was affected by the substituents para to iridium
t
more than the conversion. Each of the three Bu phosphine
Figure 1. Molecular structure of 1, 2, and 4 (left to right) with thermal ellipsoids rendered at the 30% probability level. Hydrogen atoms, except
H(100) located on iridium, are omitted for clarity.
B
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a
Table 2. Select Bond Lengths (Å) and angles (deg) for Compounds 1−4
iPr[Ir], 1
p-COOMetBu[Ir], 2
p-HtBu[Ir],30
3
p-NMe2tBu[Ir], 4
Ir−C(1)
2.033(5)
1.992(4)
2.000(3)
1.996(3)
Ir−P(1)
Ir−P(2)
Ir−Cl
P(1)−Ir−P(2)
C(1)−Ir−Cl(1)
2.2994(14)
2.2977(14)
2.5501(13)
156.79(5)
176.26(13)
2.2871(13)
2.2864(13)
2.3933(13)
160.37(4)
177.69(14)
2.2897(10)
2.2925(11)
2.3947(12)
159.90(4)
179.50(11)
2.2890(8)
2.2940(8)
2.3954(8)
159.79(3)
176.33(9)
[Ir] = (POCOP)IrHCl. Data for 3 are reproduced from Wendt and co-workers.30
a
The observed product distributions for 1−4 can best be
explained by considering their relative steric environments.
Iridium-catalyzed silane redistribution is proposed to occur via
silylene intermediates.8 Both a vacant coordination site on the
metal and subsequent Si−H/R activation are necessary to form
silylene complexes from organosilanes.32 Thus, the heightened
reactivity of 1 for redistribution is a likely consequence of the
more open metal coordination sphere, which can accommodate
both the silylene ligand and an incoming silane substrate. For
compounds 2−4, reductive elimination to form Si−Si bonds
must be accelerated under steric pressure from the tert-butyl
substituents. The more subtle electronic effect in reactions with
2−4 may be a function of the less electron-rich system, favoring
reductive elimination to the lower iridium oxidation state. An
alternative possibility is that the more electron-rich system
stabilizes the higher iridium oxidation state, allowing for silylene
intermediates to form. The former hypothesis is more attractive
because the reactivity of 2 is lower and the selectivity for
dehydrocoupling is greater than those of 3 and 4, which
suggests that Si−H bond activation may be less favorable for
the more electron-deficient compound.
Table 3. Product Distributions of NMR Tube-Scale
Reactions of PhSiH3 with Catalytic 1−4
a
products
b
cmpd
conversion (%) redistribution dehydrocoupling
c
ipr[Ir], 1
97
75
83
86
83
5
13
70
65
49
p-COOMetBu[Ir], 2
p-HtBu[Ir], 3
18
37
p-NMe2tBu[Ir], 4
a
b
1
Reaction conditions: 2 mol % [Ir], 120 °C, 16 h. Measured by H
NMR spectroscopy with respect to an internal standard of C6Me6.
c
Trace (Ph2SiH)O (≤1%) was observed in this reaction.
complexes (2−4) converted 70−80% of the starting material to
products after heating for 16 h. Compound 2 afforded the
lowest conversion, but it displayed the highest selectivity for
dehydrocoupling products over redistribution products. Con-
versely, 4 had the highest conversion of starting material and
also gave the greatest amount of secondary silane and SiH4
relative to dehydrocoupling products (Table 3).
1
Figure 2. H NMR spectra in benzene-d6 solution of reactions of 1 with PhSiH3 both sealed and open to a N2 atmosphere and reactions of 2−4
open to a N2 atmosphere after 16 h. Redistribution products are seen with compound 1, while dehydrocoupling products are primarily seen with
compounds 2, 3, and 4.
C
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a
Table 4. Product Distributions for Reaction of PhSiH3 with Catalytic 1−4 under N2
products (%)
SiH2Ph−SiPh3
14
bc
TOF (h−1
,
cmpd
)
Ph2SiH2
Ph3SiH
12
(Ph2SiH)2
20
oligosilanes
d
iPr[Ir],
1
219
19
54
3
p-COOMetBu[Ir], 2
97
89
79
p-HtBu[Ir], 3
39
11
p-NMe2tBu[Ir], 4
53
15
6
a
b
Reaction conditions: 2 mol % of [Ir], 120 °C, 16 h. Substrate completely consumed in all reactions. Measured by 1H NMR spectroscopy (Si−H
c
d
region). Measured at 0.25 h reaction time with respect to an internal standard of C6Me6. Trace (Ph2SiH)O (≤3%) was observed in this reaction.
Figure 3. 29Si{1H} DEPT (θ = 90°, 135°) NMR spectra in benzene-d6 solution of reactions of 2 with PhSiH3 open to a N2 atmosphere after 16 h.
Three distinct regions are visible and correspond to the number of Si and Ph groups present.
Reactions of Primary Silanes Open to N2 Atmosphere.
Reactions were run open to a N2 atmosphere and rapidly
stirred to facilitate dissipation of evolved gases. Under these
conditions, the reaction of PhSiH3 spiked with an internal
standard of C6Me6 with 2 mol % of 1 at reflux in toluene
solution for 16 h resulted in the complete conversion of PhSiH3
to primarily silane redistribution products. Compared to the
same reaction run in an NMR tube (vide supra), reaction of 1
and PhSiH3 open to N2 showed increased activity (TOF = 219
h−1, sampled at 15 min) based on the extent of redistribution
products present (Figure 2).
at reflux for 16 h completely consumed the substrate and
yielded a mixture of dehydrocoupling and redistribution
products. When reactions are stopped after 2 h, the disilane
(PhSiH2)2 is the dominant product of these reactions with the
conversions ranging from 30% to 75%. Extending the reaction
time to 16 h yields a mixture of oligosilanes with terminal
−SiH2 groups having apparently undergone redistribution. This
observation demonstrates that redistribution occurs slower than
dehydrocoupling for these compounds, which is consistent with
work using Wilkinson’s catalyst.9 Compared to the sealed NMR
tube reactions, reactions open to N2 yielded a higher
percentage of oligosilanes relative to redistribution products
as measured by 1H and 29Si NMR spectroscopy (Figure 2). The
product distributions of 2, 3, and 4 were consistent with the
NMR-scale reactions, although greater activity was observed
under N2 with TOFs = 19, 39, and 53 h−1, respectively, based
on sampling after 15 min reaction time.
In addition to Ph2SiH2, which is the dominant product under
both sets of reaction conditions, Ph3SiH, (Ph2SiH)2, and
1
PhSiH2−SiPh3 are also present as determined by H and 29Si
NMR spectroscopy. These products were minimal (<4%) in
the sealed variant of this same reaction (Table 4). Under these
conditions, H2 and SiH4 cannot be detected although it is
presumed that the open manifold facilitates dissipation of these
gases and drives conversion of the substrate. The presence of
disilane products continues to demonstrate that 1 can act as a
dehydrocoupling catalyst. However, redistribution is still
favored under these conditions, and observation of (Ph2SiH)2
and PhSiH2−SiPh3 suggests that dehydrocoupling may occur
after redistribution. Trace quantities of the hydrolysis or
oxidation product (Ph2SiH)2O were observed in the product
mixture of this reaction, most likely due to oxidation from O2
or the Florisil used to remove the catalysts from these
reactions.11
Although the gaseous products H2 and SiH4 cannot be
detected under these conditions, a key redistribution product,
Ph2SiH2, is still present in considerable amounts (3−15%). The
relative selectivity of 2, 3, or 4 for dehydrocoupling over
redistribution is unchanged from the NMR scale, where 2
demonstrated the highest selectivity for dehydrocoupling and 4
produced the greatest amount of secondary silane (Table 4).
1
Further analysis of the products by H NMR spectroscopy
reveals complex, overlapping signals in the region δ 3.0−6.0. To
gain more information about the structure of these oligosilane
products, DEPT 29Si NMR (θ = 90°, 135°) spectroscopy was
utilized to determine the number of hydrogen atoms per silicon
atom.33,34 The extent of oligomerization in these reactions is
Treatment of PhSiH3 with 2 mol % of compounds 2−4
spiked with an internal standard of C6Me6 in toluene solution
D
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low based on the number of 29Si resonances present, with
signals seen in three distinct regions of the DEPT 29Si NMR
spectra (Figure 3).
Table 5. Product Distributions of Preparative-Scale
Reactions of (o-tol)SiH3 with 1−4 under N2
a
products (%)
The most downfield signals of the silane products are located
b
conversion
(%)
[(o-tol)
SiH2]2
(o-
tol)2SiH2
redistributed
oligomers
from δ = −25 to −35 and correlate to proton resonances in the
cmpd
iPr[Ir] 1
1
range δ = 5.2−5.5 as determined by H−29Si HSQC NMR
spectroscopy. Other than the 29Si resonance for Ph2SiH2, all of
the signals present in this region correspond to Ph2SiH−
groups that most likely formed from redistribution of a terminal
−PhSiH2 unit. The broad range of overlapping resonances from
δ = −30 to −32 likely corresponds to products containing more
than one −PhSiH− unit in a cyclosilane that consists of mostly
Ph2Si− units (e.g., Ph8Si5H2).35,36
100
100
100
99
100
7
p-COOMetBu[Ir] 2
66
79
26
27
17
66
p-HtBu[Ir] 3
4
p-NMe2tBu[Ir] 4
8
a
b
Reaction conditions: 2 mol % of [Ir], 120 °C, 16 h. Measured by 1H
NMR spectroscopy with respect to an internal standard of C6Me6.
The second region of silane products in the DEPT 29Si NMR
spectra is in the range δ = −55 to −70 and corresponds to
proton resonances in the range δ 4.4−4.8 as determined by
1H−29Si HSQC NMR spectroscopy. The signals from δ = −56
to −60 are due to terminal PhSiH2− groups. Resonances in the
δ = −60 to −70 range, with the exception of that of (PhSiH2)2,
correspond to internal −PhSiH− groups of linear oligosi-
lanes.37 From these spectra no information regarding the
polymer microstructure is evident, other than to suggest an
atactic structure based on the number of signals present.38 In
addition to the linear di- through tetrasilanes −(PhSiH)n− (n =
2−4) present, the products of these reactions are disilanes such
as Ph2SiH−SiH2Ph or oligosilanes that differ in the number of
phenyl substituents due to redistribution.
was decreased versus reactions with PhSiH3, and [(o-tol)SiH2]2
was the major product observed upon reaction for all Bu
t
compounds. The electron-rich compound 4 again was the most
active toward further dehydrocoupling/redistribution of the
dominant disilane product, producing the linear trisilane and
other redistributed oligosilane products such as (o-tol)SiH2−
SiH2−(o-tol)2SiH. These products were present in only trace
1
amounts in reactions using 2 and 3 as detected by H and 29Si
NMR spectroscopy and GC-MS.
Reactions of 2−4 with naphthylsilane and mesitylsilane did
not reach completion and reinforce the notion that product
selectivity for silane dehydrocoupling is governed by steric
factors. As with the other substrates, reaction of NapSiH3 (Nap
= naphthyl) with 2 mol % of 1 exclusively gave the
redistribution product Nap2SiH2. In comparison, treatment of
NapSiH3 with 2 mol % of 2−4 in toluene solution at reflux
overnight yielded partial conversion to the disilane (NapSiH2)2
as the major product. The secondary silane Nap2SiH2 (≤6%)
was present in all reactions, and trace amounts of trisilane
(≤4%) were also observed in the 29Si NMR spectra for the
reactions with 3 and 4.42 Compound 2 converted only 25% of
the starting material to products under these conditions, while
reaction of this substrate with 3 and 4 converted 72% and 62%
of NapSiH3 to the disilane, respectively.
The most upfield 29Si NMR resonances occurred in the
region δ = −100 to −130 and correspond to proton resonances
in the range δ 3.3−3.7 as determined by 1H−29Si HSQC NMR
spectroscopy. Resonances in this range are typical of internal
silane atoms lacking organic substituents of the type H2Si-
(SiH2Ph)2 (δ = −110.6) or HSi(SiH3)3 (δ = −131.5),39 which
could have resulted from the incorporation of SiH4 into the
backbone40 or the redistribution of a substituted oligosilane.
Products of this type that can be conclusively identified from
these reaction mixtures include Ph2SiH−SiH3 and Ph2SiH−
SiH2−SiH2Ph, although other unidentified products of this type
are also present in this region.41
For all of the previous substrates discussed, the iPr phosphine
t
compound 1 yielded redistribution products, while the Bu
Due to the complexity and overlapping of the peaks seen in
complexes 2−4 primarily yielded dehydrocoupling products.
This pattern of reactivity breaks upon treatment of MesSiH3
(Mes = mesityl) with 2 mol % of 1 at reflux for 16 h in toluene
solution, which yields oligosilane dehydrocoupling products with
minimal redistribution (Table 6). On the basis of 29Si NMR
spectroscopy and GPC data, linear products up to the
tetrasilane H(MesSiH)4H were present. The few remaining
1
the Si−H region in the H and 29Si NMR spectra for these
reactions, complete assignment of Si−H signals to specific
products cannot be accomplished. Therefore, gel permeation
chromatography (GPC) was used to obtain approximate
molecular weight information for silane products produced by
compounds 2, 3, and 4. Relative to polystyrene standards (Mw
489−2780), GPC traces for the oligosilanes analyzed were
broad, corresponding to multiple low molecular weight
products (Mw = 190−720), in agreement with the 29Si NMR
data. To probe steric effects of the silane substrate on catalytic
activity and product distributions, primary arylsilanes including,
o-tol- (o-tol = o-tolyl), naphthyl-, and mesitylsilane were used in
reactions with 1−4. In these reactions, more sterically
encumbered primary arylsilanes tempered the extent of both
dehydrocoupling and redistribution for 1−4, but the relative
activity and type of products (dehydrocoupling vs redistrib-
ution) generated were not affected. With (o-tol)SiH3 as the
substrate, reaction with 1 exclusively yielded the redistribution
product (o-tol)2SiH2 (Table 5).
1
unassigned peaks (ca. 5% by H NMR) are presumably due to
cyclic oligomers (Table 6).
Rosenberg has shown that the selectivity of Wilkinson’s
catalyst for dehydrocoupling over redistribution of secondary
silanes could be enhanced through the removal of hydrogen
that is produced.9 In this iridium system, the chemoselectivity
of 1−4 for dehydrocoupling over redistribution is steric in
nature. Therefore, it was hypothesized that rigorous removal of
hydrogen could further influence the product selectivity of
these catalysts for nonvolatile substrates.
Ambient-temperature reactions of neat MesSiH3 and 0.2 mol
% of each of catalyst 1−4 were conducted under reduced
pressure. Catalysts were used at a lower loading in these
reactions to ensure solubility in neat silane. After 4 h under
these conditions, 1 completely consumed the starting material
and yielded only the dehydrocoupling product, (MesSiH2)2. No
redistribution to Mes2SiH2 or further dehydrocoupling of the
Reactions of (o-tol)SiH3 using 2−4 produced the same types
of redistributed oligosilanes by comparison to reactions with
PhSiH3, although each catalyst yielded a different distribution
of products. Oligosilane product formation with (o-tol)SiH3
E
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a
Table 6. Product Distributions of Preparative-Scale Reactions of MesSiH3 with 2, 3, and 4 under N2
products (%)
b
cmpd
iPr[Ir] 1
conversion (%)
(MesSiH2)2
Mes2SiH2
oligomers
97
20
25
37
18
24
56
8
1
1
2
52
p-COOMetBu[Ir] 2
p-HtBu[Ir] 3
1
p-NMe2tBu[Ir] 4
58
a
b
1
Reaction conditions: 2 mol % of [Ir], 120 °C, 16 h. Measured via H NMR spectroscopy with respect to an internal standard of C6Me6
Figure 4. Reactions of 1 with (n-oct)SiH3 under both N2 and reduced pressure conditions and reaction of 2−4 under N2. Redistribution occurs upon
reaction with compound 1, while dehydrocoupling products are seen with 2−4 and with 1 under reduced pressure.
disilane to the trisilane or higher oligomers was evident under
these conditions. Thus, not only selectivity for dehydrocoupling
was achieved, but also greater product selectivity was seen.
Under these conditions, no reaction occurred with 2, 3, or 4.
Given the greater preference for aryl redistribution relative to
alkyl groups,18 n-octylsilane (n-oct = n-octyl) was also screened.
Treatment of (n-oct)SiH3 with 2 mol % of 1 at reflux for 16 h
in toluene solution gave complete consumption of the substrate
and yielded the redistribution product (n-oct)2SiH2 as
determined by 1H and 29Si NMR spectroscopy. No
redistribution to the tertiary silane product (n-oct)3SiH was
observed. Under the same conditions, reactions of (n-oct)SiH3
with 2, 3, or 4 yielded oligosilane dehydrocoupling products
and a small amount (≤11%) of (n-oct)2SiH2. Analysis of the
internal silane centers of the type H2Si(SiH2R)2 or HSi-
(SiH3)3.39 Products of catalysis with 3 were analyzed by GPC
to obtain molecular weight information. Traces for the samples
analyzed contained broad peaks that corresponded to a
distribution of low molecular weight products (Mw = 300−
930).4 In these systems, the degree of oligomerization increases
as substrate conversion increases, which suggests a step growth
mechanism.
For (n-oct)SiH3, product selectivity for dehydrocoupling
versus redistribution using 1 can be enhanced by controlling
the concentration of H2 gas produced. Under dynamic vacuum,
0.2 mol % of 1 completely consumed the n-octSiH3 within 3 h
and yielded oligooctylsilane and <1% (n-oct)2SiH2 (Figure 4).
Product analysis via GPC shows two broad overlapping peaks
corresponding to a Mw of 2910 and 1100, respectively. Again, a
combination of steric factors and H2 management led to
substantial improvement in the selectivity of this catalysis.
Reactions of Secondary Silanes. The secondary silanes
PhMeSiH2 and Ph2SiH2 were used as substrates to probe
dehydrocoupling activity in a system potentially less prone to
redistribution. However, treatment of neat PhMeSiH2 with 2
mol % of 1 for 16 h at reflux facilitated extensive redistribution
of the substrate to the tertiary silanes Ph2MeSiH, PhMe2SiH,
1
product mixtures of each reaction via H NMR spectroscopy
showed that the disilane [(n-oct)SiH2]2 is the predominant
single product for each catalyst in this series. Reactions of para-
substituted 2 and 4 with n-octSiH3 generated trace amounts of
oligosilanes, while the reaction of 3 with (n-oct)SiH3 showed a
majority of the [(n-oct)SiH2]2 was converted to higher order
oligosilane products (Figure 4). Similar to reactions with
PhSiH3, products with upfield 29Si NMR resonances in the
region δ −110 to −130 were produced and are indicative of
F
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Organometallics
Article
a
Table 7. Product Distributions of Preparative-Scale Reactions of 1−4 with PhMeSiH2 Open to a N2 Atmosphere
products (%)
b
cmpd
iPr[Ir] 1
conversion (%)
unknown
Ph3SiH
2
Ph2MeSiH
Ph2SiH2
PhMe2SiH
27
Me2SiH2
dehydrocoupling
100
24
71
3
p-COOMetBu[Ir] 2
4
6
9
7
38
8
3
5
3
7
20
20
p-HtBu[Ir] 3
74
5
p-NMe2tBu[Ir] 4
65
9
16
a
b
1
Reaction conditions: 2 mol % of [Ir], 120 °C, 16 h. Measured via H NMR spectroscopy with respect to an internal standard of C6Me6
1
and trace amounts of Ph3SiH as seen by H and 29Si NMR
spectroscopy. To gain insight into the complete set of
redistribution products produced, a sealed NMR-scale reaction
of PhMeSiH2 and 2 mol % of 1 was heated for 50 min at 120
°C, and the products were analyzed by 1H NMR spectroscopy.
In addition to the tertiary silane products seen in the 16 h
reaction, Ph2SiH2, Me2SiH2, MeSiH3, and trace amounts of
PhSiH3 were present in the 50 min reaction mixture (Table 7).
None of these primary or secondary silane products were
observed after 16 h of reaction with 1, which implies these
volatile intermediate products (e.g., Me2SiH2 and MeSiH3)
escape through the open manifold.
reactions catalyzed by 1 also yield dehydrocoupling products.
The more sterically encumbered compounds 2−4 are active
primary silane dehydrocoupling catalysts that undergo com-
petitive redistribution with less hindered substrates to yield
oligosilane products as determined by GPC. Effects of a
functional group para to iridium in the POCOP ligand were
also considered as a source of selectivity and activity in these
reactions. It was determined that the para electron-withdrawing
compound 2 showed the lowest activity, but the highest
selectivity for dehydrocoupling products, which is consistent
with decreased Si−H bond activation. The electron-rich
compound 4 had the highest catalytic activity, but also showed
the greatest propensity to yield redistribution products. These
results provide initial design parameters for dehydrocoupling
catalysts to avoid competitive redistribution. Additionally, this
work demonstrates that control of reaction conditions remains
a pivotal factor for governing late-metal reactivity toward
organosilanes.
t
Reactions of the Bu compounds 2−4 with PhMeSiH2
suffered from both incomplete conversion (24−74%) and
poor selectivity for dehydrocoupling relative to redistribution.
For this substrate, 3 and 4 were the most active (74% and 65%
conversion, respectively), although their dehydrocoupling
products accounted for a maximum of only 20% of the product
distribution and were limited to the simple disilane
(PhMeSiH)2. The major products of the tBu suite of
compounds are Ph2SiH2 in reactions with 2 and 3 and
PhMe2SiH in reactions with 4 (Table 7).
ASSOCIATED CONTENT
* Supporting Information
■
S
Supporting Information including experimental details, charac-
terization data for all compounds, and data from catalytic runs
is available. The Supporting Information is available free of
charge on the ACS Publications website at DOI: 10.1021/
acs.organomet.5b00486.
Refluxing a toluene solution of Ph2SiH2 for 16 h with 2, 3, or
4 yielded minimal conversion (≤5%) to the redistribution
product Ph3SiH along with trace amounts (≤1%) of the
dehydrocoupling products (Ph2SiH)2 and PhSiH2−SiPh3.
Treatment of Ph2SiH2 with 2 mol % of 1 at reflux for 16 h
in toluene solution gave complete consumption of the substrate
and yielded the major products Ph3SiH (15%), Ph3Si−SiH2−
SiPh3 (18%), and Ph3Si−PhSiH−SiHPh2 (50%) as determined
by 1H−29Si HSQC and 29Si DEPT NMR spectroscopy.41 In the
reaction of PhSiH3 with 1, traces of trisilane Ph3Si−PhSiH−
SiHPh2 were detected via 1H NMR spectroscopy, although this
resonance integrated to <1% of the products in that reaction
(vide supra). Compound 1 initially redistributes PhSiH3 to
Ph2SiH2 and SiH4, and the use of Ph2SiH2 as a substrate
bypasses this initial redistribution, resulting in greater amounts
of dehydrocoupling products capped with trisubstituted silanes.
Other products include (Ph2SiH)2 and Ph3Si−SiH2Ph
(combined 5%), while the remaining products are likely
disilane redistribution products that were produced in only
trace amounts.
AUTHOR INFORMATION
Corresponding Author
*E-mail: rory.waterman@uvm.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the U.S. National Science
Foundation (CHE-1265608 to R.W.). N.T.M. thanks the
Vermont Space Grant Consortium (VSGC) for support
through a NASA Experimental Program to Stimulate
Competitive Research (EPSCoR) VSGC Graduate Research
Fellowship. We thank Dr. Anthony Wetherby for the initial
screening of PhSiH3 with 3 and valuable discussions.
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A series of iridium pincer compounds were shown to be active
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