Hydrosilylation of Terminal Alkynes Catalyzed by a ONO-Pincer Iridium(III) Hydride Compound: Mechanistic Insights into the Hydrosilylation and Dehydrogenative Silylation Catalysis

Article abstract of DOI:10.1021/acs.organomet.6b00471

The catalytic activity in the hydrosilylation of terminal alkynes by the unsaturated hydrido iridium(III) compound [IrH(κ³-hqca)(coe)] (1), which contains the rigid asymmetrical dianionic ONO pincer ligand 8-oxidoquinoline-2-carboxylate, has been studied. A range of aliphatic and aromatic 1-alkynes has been efficiently reduced using various hydrosilanes. Hydrosilylation of the linear 1-alkynes hex-1-yne and oct-1-yne gives a good selectivity toward the β-(Z)-vinylsilane product, while for the bulkier t-Bu-C≡CH a reverse selectivity toward the β-(E)-vinylsilane and significant amounts of alkene, from a competitive dehydrogenative silylation, has been observed. Compound 1, unreactive toward silanes, reacts with a range of terminal alkynes RC≡CH, affording the unsaturated η¹-alkenyl complexes [Ir(κ³-hqca)(E-CH=CHR)(coe)] in good yield. These species are able to coordinate monodentate neutral ligands such as PPh₃ and pyridine, or CO in a reversible way, to yield octahedral derivatives. Further mechanistic aspects of the hydrosilylation process have been studied by DFT calculations. The catalytic cycle passes through Ir(III) species with an iridacyclopropene (η²-vinylsilane) complex as the key intermediate. It has been found that this species may lead both to the dehydrogenative silylation products, via a β-elimination process, and to a hydrosilylation cycle. The β-elimination path has a higher activation energy than hydrosilylation. On the other hand, the selectivity to the vinylsilane hydrosilylation products can be accounted for by the different activation energies involved in the attack of a silane molecule at two different faces of the iridacyclopropene ring to give η¹-vinylsilane complexes with either an E or Z configuration. Finally, proton transfer from a η²-silane to a η¹-vinylsilane ligand results in the formation of the corresponding β-(Z)- and β-(E)-vinylsilane isomers, respectively.

Full text of DOI:10.1021/acs.organomet.6b00471

Article
pubs.acs.org/Organometallics
Hydrosilylation of Terminal Alkynes Catalyzed by a ONO-Pincer
Iridium(III) Hydride Compound: Mechanistic Insights into the
Hydrosilylation and Dehydrogenative Silylation Catalysis
Jesus J. Perez-Torrente,*,† Duc Hanh Nguyen,‡ M. Victoria Jimenez,† F. Javier Modrego,*,†
́
́
́
Raquel Puerta-Oteo,† Daniel Gomez-Bautista,† Manuel Iglesias,† and Luis A. Oro†
́
†Departamento de Química Inorgan
ea-ISQCH, Universidad de
Zaragoza-CSIC, Facultad de Ciencias, C/Pedro Cerbuna, 12, 50009 Zaragoza, Spain
‡Universite
́
Lille, CNRS, Centrale Lille, ENSCL, Universite Artois, UMR 8181, Unite de Catalyse et Chimie du Solide (UCCS),
́
ica, Instituto de Síntesis Química y Catal
́
isis Homogen
́
́
́
F-59000 Lille, France
S
* Supporting Information
ABSTRACT: The catalytic activity in the hydrosilylation of
terminal alkynes by the unsaturated hydrido iridium(III)
compound [IrH(κ3-hqca)(coe)] (1), which contains the rigid
asymmetrical dianionic ONO pincer ligand 8-oxidoquinoline-2-
carboxylate, has been studied. A range of aliphatic and aromatic 1-
alkynes has been efficiently reduced using various hydrosilanes.
Hydrosilylation of the linear 1-alkynes hex-1-yne and oct-1-yne
gives a good selectivity toward the β-(Z)-vinylsilane product,
while for the bulkier t-Bu-CCH a reverse selectivity toward the
β-(E)-vinylsilane and significant amounts of alkene, from a
competitive dehydrogenative silylation, has been observed.
Compound 1, unreactive toward silanes, reacts with a range of terminal alkynes RCCH, affording the unsaturated η1-
alkenyl complexes [Ir(κ3-hqca)(E-CHCHR)(coe)] in good yield. These species are able to coordinate monodentate neutral
ligands such as PPh3 and pyridine, or CO in a reversible way, to yield octahedral derivatives. Further mechanistic aspects of the
hydrosilylation process have been studied by DFT calculations. The catalytic cycle passes through Ir(III) species with an
iridacyclopropene (η2-vinylsilane) complex as the key intermediate. It has been found that this species may lead both to the
dehydrogenative silylation products, via a β-elimination process, and to a hydrosilylation cycle. The β-elimination path has a
higher activation energy than hydrosilylation. On the other hand, the selectivity to the vinylsilane hydrosilylation products can be
accounted for by the different activation energies involved in the attack of a silane molecule at two different faces of the
iridacyclopropene ring to give η1-vinylsilane complexes with either an E or Z configuration. Finally, proton transfer from a η2-
silane to a η1-vinylsilane ligand results in the formation of the corresponding β-(Z)- and β-(E)-vinylsilane isomers, respectively.
by the classic Chalk−Harrod mechanism.3 This inner-sphere
INTRODUCTION
■
mechanism involves the oxidative addition of the hydrosilane,
followed by sequential coordination and hydrometalation of the
alkyne as the regiodetermining step, and results in the
predominant formation of the most thermodynamically stable
β-(E)-vinylsilane after reductive elimination. This mechanism
seems to be operative with Pt-based catalysts4 but is unable to
explain the formation of β-(Z)-vinylsilane, the trans-addition
product, or silylalkyne derivatives resulting from the com-
petitive dehydrogenative silylation process observed with Rh-,5
Ir-,6,7 and Ru-based8 catalysts.
Transition-metal-catalyzed hydrosilylation of terminal alkynes is
the most straightforward and atom-economical methodology
for the preparation of vinylsilanes.1 Unsaturated hydrosilylation
products are valuable synthetic intermediates due to their
versatility, ease of handling, low toxicity, and stability relative to
other vinyl-metal species.2 The hydrosilylation of terminal
alkynes can afford several isomeric vinylsilanes, and therefore,
control of the regio- and stereoselectivity of the reaction is a
key issue. In general, the selectivity depends upon several
factors such as the catalyst, the substituents on the hydrosilane
and the alkyne, and the reaction conditions. In this context, a
deeper understanding of the reaction mechanisms is pivotal for
the development of more active and selective catalytic
systems.1b
The formation of both types of products can be explained by
a modified Chalk−Harrod mechanism that entails the
silylmetalation of the alkyne by migratory insertion into the
M−Si bond to give a (Z)-silylvinylene metal complex.1 The
The catalytic hydrosilylation of terminal alkynes normally
proceeds with anti-Markovnikov regiochemistry and predom-
inantly syn-addition stereochemistry, which can be rationalized
Received: June 10, 2016
© XXXX American Chemical Society
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formation of the β-(Z)-vinylsilane requires the metal-assisted
isomerization of the (Z)-silylvinylene intermediate to the
thermodynamically more favorable (E)-silylvinylene complex
with the relief of steric strain between the metal and the
adjacent silane as the driving force of the process. This
isomerization proceeds through a η2-vinylsilane or metal-
lacyclopropene intermediate in the mechanism proposed by
Crabtree et al.9 for Ir-catalyzed reactions (Scheme 1i) or a
Scheme 2. Oxidative Hydrometalation Step in the Wu−Trost
Hydrosilylation Mechanism Leading to the Formation of the
α-Vinylsilane Isomer
Scheme 1. Crabtree−Ojima Mechanism for Alkyne
Hydrosilylation
an intermediate silylcarboxonium ion that transfers the silyl
moiety to the alkyne, resulting in outstanding β-Z selectivity.12
The well established modified Chalk−Harrod mechanism
relies on the detection of metal intermediates by spectroscopic
techniques, kinetic analysis, deuterium labeling, or reactivity
studies on well-defined models of reaction intermediates.13
Strong support for this mechanism also comes from the
detection of key reaction intermediates by electrospray
ionization mass spectrometry (ESI-MS).14 Interestingly, a
number of computational studies on the mechanism of
ruthenium-catalyzed alkyne hydrosilylation have been recently
reported,11,15 in contrast with those based on rhodium and
iridium catalysts, which have been mainly focused on the
hydrosilylation of alkenes or carbonyl compounds.16
The modified Chalk−Harrod mechanism perfectly fits to a
catalytic cycle passing through Pt0/PtII or MI/MIII (M = Rh, Ir)
intermediates. However, in the case of iridium(III) precatalysts
the possible involvement of IrIII/IrV species as catalytic
intermediates cannot be excluded. In fact, experimental and
theoretical studies have provided evidence for the intermediacy
of IrV species in iridium-catalyzed C−H activation/functional-
ization processes,17 alkene hydrogenation reactions,18 or the
outer-sphere hydrosilylation of carbonyl compounds.19 In this
context, it is worth mentioning that computational studies on
the mechanism of the stereoselective hydrosilylation of terminal
alkynes catalyzed by [Cp*IrCl2]2 support an Ir(III/V) catalytic
cycle assisted by the ring slippage of the Cp* ligand.20
Pincer ligands are valuable structural motifs for the design of
transition-metal complexes for selective stoichiometric and
catalytic transformations.21 O-based anionic pincer ligands have
attracted significant attention for supporting high-oxidation-
state metal complexes with vacant coordination sites.22 The
robustness and stability of the ligand framework in combination
with strongly electron donating hard oxygen donor atoms
provide access to higher oxidation states of the metal centers by
modulating the electron density at the metal via a hard/hard
interaction or π-donating effects.23 We have recently shown the
potential of tridentate dianionic ONO pincer ligands for the
design of unsaturated iridium(III) compounds which have
shown catalytic activity in C−H activation/functionalization
processes.24,25
zwitterionic carbene species in the mechanism proposed by
Ojima et al.10 for Rh-catalyzed processes (Scheme 1ii). Thus,
because the reductive elimination is the rate-determining step,
the less thermodynamically stable β-(Z)-vinylsilane isomer is
formed as the kinetic product. In addition, β-H elimination
from (E)-silylvinylene intermediates affords the silylalkyne
derivative and dihydrido complex, which can be reduced by the
alkyne to give the corresponding alkene (Scheme 1iii).
A third mechanistic proposal, the Wu−Trost mechanism, has
been put forward to explain the unusual Markovnikov
regioselectivity and exclusive trans-addition stereochemistry
found in the hydrosilylation of terminal and internal alkynes by
cationic cyclopentadienyl RuII catalysts.11 The reaction
proceeds through an oxidative hydrometalation step, which
involves the oxidative addition of the Si−H bond concerted
with hydrometalation and results in the formation of a
ruthenacyclopropene intermediate, which undergoes reductive
silyl migration to the carbene to afford the α-vinylsilane isomer
(Scheme 2). On the other hand, we have recently proposed an
outer-sphere ionic mechanism for the hydrosilylation of
terminal alkynes catalyzed by mononuclear RhIII and IrIII
catalyst precursors featuring a functionalized bis-N-heterocyclic
carbene ligand. The mechanism involves the heterolytic
splitting of the hydrosilane assisted by the electrophilic metal
center and the solvent (acetone), resulting in the formation of
We report herein on the catalytic activity of the unsaturated
hydrido iridium(III) compound [IrH(κ3-hqca)(coe)] (hqca =
8-oxidoquinoline-2-carboxylate) in the hydrosilylation of
terminal alkynes. In principle, the presence of a rigid
asymmetrical dianionic ONO pincer ligand could facilitate
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the stabilization of IrV intermediates, thereby driving the
catalytic reaction through a Ir(III/V) mechanism. Thus, the
hydrosilylation and dehydrogenative silylation mechanisms
have been investigated by theoretical calculations at the DFT
level. In addition, a reactivity study leading to the preparation of
a series of iridium(III) η1-alkenyl complexes is also described.
of a η1-(E)-alkenyl ligand resulting from the anti-Markovnikov
syn addition of the Ir−H bond to the alkynes. The CH
protons of the η2-cyclooctene ligand were observed as a
complex multiplet centered around δ 5.12 ppm. The 13C{1H}
NMR spectra are in full agreement with the unsymmetrical
structure of the complexes and show a set of eight resonances
for the coe ligand, two of them at around δ 88−86 ppm for the
CH carbons.
RESULTS AND DISCUSSION
■
The labeled complex [IrD(κ3-hqca)(coe)] (1-d1) was
prepared in situ by H/D exchange in a THF/MeOH-d4
solution in a few minutes. Reaction of 1-d1 with 1-hexyne
gave the deuterium-labeled complex [Ir(κ3-hqca)(E-CH
CD(CH2)3CH3)(coe)] (3-d1), which was isolated as a red
solid in 96% yield. The full characterization of 3-d1 supports the
regio- and stereoselectivity of the insertion process in 1 that
proceeds in a syn fashion.26 Thus, the lack of a resonance at δ
4.30 ppm in the 1H NMR spectrum of 3-d1 and the deuterium-
coupled triplet resonance at δ 133.46 ppm (1JC‑D = 26.6 Hz)
confirm the deuterium incorporation at the β carbon of the
alkenyl ligand.
The square-pyramidal iridium(III) complex [IrH(κ3-hqca)-
(coe)] (1) was straightforwardly prepared by reaction of
[Ir(μ-OH)(coe)2]2 with H2hqca (8-hydroxyquinoline-2-carbox-
ylic acid). Interestingly, compound 1 exists as dinuclear
assemblies [IrH(κ3-hqca)(coe)]2 in noncoordinating solvents
and as labile mononuclear solvates under polar solvent
solutions. The presence of a vacant coordination site trans to
the hydrido ligand in 1 allowed the preparation of several
octahedral [IrH(κ3-hqca)(coe)L] (L = py, PPh3) complexes.25
The presence of a hydrido ligand in 1 prompted us to study its
reactivity with alkynes, in spite of the fact that its stereo-
chemistry is unsuitable for the insertion into the Ir−H bond.
Reaction of [IrH(κ3-hqca)(coe)] (1) with Terminal
Alkynes. Treatment of a tetrahydrofuran solution of 1 with a
range of terminal alkynes RCCH (1:5 molar ratio) for 48 h
at 298 K gave dark red solutions, from which the η1-alkenyl
complexes [Ir(κ3-hqca)(E-CHCHR)(coe)] were isolated as
red solids in good yields (Scheme 3). The reactions with the
The alkyne insertion reaction in 1 has been studied by DFT
calculations using CH3CCH as an alkyne model. Coordina-
tion of propyne to give the octahedral complex 1-alky, which
has an alkyne molecule in a cis position relative to the hydrido
ligand, is −7.0 kcal/mol downhill. The insertion of propyne
into the Ir−H bond to give the η1-alkenyl complex [Ir(κ3-
hqca)(E-CHCHCH3)(coe)] (6) is calculated to be further
downhill by −7.6 kcal/mol and requires an activation energy of
21 kcal/mol, which is affordable at room temperature (Scheme
4).
Scheme 3. Synthesis of η1-Alkenyl [Ir(κ3-hqca)(E-CH
CHR)(coe)] (2−5) Complexes
Scheme 4. (Bottom) Computed Energies (ΔH, kcal/mol) for
the Insertion of Propyne into the Ir−H Bond of 1 (ONO′ =
8-Oxidoquinoline-2-carboxylato) and (Top) Structure of
TS1‑alky‑6
linear alkynes 1-hexyne and 1-octyne proceeded cleanly, and
the η1-alkenyl complexes 3 and 4 were obtained in yields higher
than 95%. However, complexes 2 and 5, obtained from the
reaction with the functionalized alkyne 3-methoxy-1-propyne
and the bulky 3,3-dimethyl-1-butyne, respectively, required
chromatographic purification and consequently were obtained
in lower yield (60−70%). The complexes were fully
characterized by elemental analysis, mass spectrometry, and
NMR spectroscopy. The MS (MALDI-TOF) spectra of the
compounds showed the molecular ion with the correct isotopic
distribution and also the peaks corresponding to the loss of the
alkenyl ligand. The complexes have a limited solubility in most
organic solvents, including dichloromethane and methanol,
although they have an acceptable solubility in a mixture of both
solvents. Thus, the NMR spectra were routinely recorded in a
MeOH-d4/CD2Cl2 mixture. The 1H NMR spectra of the
complexes showed the expected set of five resonances in the
range δ 8.3−6.8 ppm for the tridentate 8-oxidoquinoline-2-
carboxylato ligand, which were assigned with the help of two-
dimensional NMR techniques (see the Experimental Section).
The spectra also showed two resonances at δ 6.8−6.2 ppm (d)
and 4.6−4.3 ppm (dt, 3JH−H ≈ 14 Hz, 2−4; d, 5) for the α and
β protons of the η1-alkenyl ligand, respectively. The large 1JH−H
coupling constant of ∼14 Hz is in agreement with the presence
In compound 1-alky the propyne ligand lies parallel to the
ONO plane. Due to the asymmetry of this ligand, two
orientations of the alkyne ligand are possible but DFT
calculations show that they differ by just 0.1 kcal/mol; thus,
only one of the insertion paths has been studied. In the
transition structure from 1 to the insertion derivative 6 the
propyne ligand rotates and is positioned perpendicular to the
ONO plane and parallel to the Ir−H bond (Scheme 4). The
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Ir−H elongates to 1.65 Å in the TS from 1.54 Å in compound
1-alky. The propyne ligand loses linearity in the process of
changing its coordination mode from π-alkyne to σ-alkenyl, and
consequently, the C−C−C bond angle changes from 163.3° in
1-alky to 154.1° in the TS, and the C−C−H angle changes to
139.1° from 167.0°.
The insertion of internal alkynes into the Ir−H bond of 1
proceeds with the same selectivity as has been demonstrated in
the reaction with but-2-yne. However, the formation of the
expected compound [Ir(κ3-hqca)(E-(CH3)CCHCH3)(coe)]
is accompanied by a second species (20%) that could not be
separated by chromatography. It is likely that this minor species
results from the isomerization of the alkenyl ligand, although
we have not yet been able to fully characterize this species by
NMR.
of the carbonyl complex [Ir(κ3-hqca)(E-CHCHCH2OMe)-
(coe)(CO)] (2-CO), which is in equilibrium with 2 (40:60
ratio, NMR evidence). The formation of 2-CO is supported by
the IR spectrum, which features a strong absorption at 2035
cm−1 corresponding to the terminal carbonyl stretching ν(CO)
band. The carbonylation of 2 is reversible, and in fact, all
attempts to isolate 2-CO in the solid state resulted in the
crystallization of 2.
Hydrosilylation of 1-Alkynes. Complex [IrH(κ3-hqca)-
(coe)] (1) was found to be an efficient catalyst precursor for
the hydrosilylation of terminal alkynes. The catalytic reactions
were performed in CDCl3 at 60 °C, using a 2 mol % catalyst
loading and a slight excess of hydrosilane, and routinely
1
monitored by H NMR spectroscopy. A range of aliphatic and
aromatic 1-alkynes was efficiently reduced in approximately 1 h
using HSiMePh2, HSiMe2Ph, and HSiEt3 as hydrosilanes
(Table 1). Hydrosilylation of terminal alkynes can result in
three isomeric vinylsilane derivatives: (Z)- or (E)-1-silyl-1-
alkenes, products from the anti-Markovnikov addition, namely
β-(Z)- and β-(E)-vinylsilane isomers, respectively, and 2-silyl-1-
alkene from the Markovnikov addition (α isomer). In addition,
the formation of the dehydrogenative silylation products,
alkynylsilane and the corresponding alkene, has been observed
to some extent, particularly in the case of sterically demanding
substituents on the alkyne and/or the hydrosilane (Scheme 6).
The hydrosilylation of hex-1-yne with HSiMePh2 catalyzed
by 1 was complete in 1 h, giving 88% selectivity to β-(Z)-
vinylsilane (Table 1, entry 1). The reactions with HSiMe2Ph
and HSiEt3 are even faster, with β-(Z) selectivities of 83% and
68%, respectively (entries 3 and 4). The reaction profile of
conversion and selectivity versus time for the hydrosilylation of
hex-1-yne with HSiMePh2 catalyzed by 1 is shown in Figure 1.
A steady increase in the amount of β-(E)-vinylsilane and alkene
can be seen from the beginning of the reaction, reaching 8%
and 4%, respectively, after the complete conversion of the
alkyne. Interestingly, the η1-alkenyl complex [Ir(κ3-hqca)(E-
CHCH(CH2)3CH3)(coe)] (3) exhibited a similar catalytic
performance, in terms of both activity and selectivity, in
comparison to that of hydrido complex 1 (entries 2 and 5), and
then, catalyst 1 was used throughout the catalytic study.
Hydrosilylation of oct-1-yne showed comparable results, as
moderate to good selectivity to β-(Z)-vinylsilane was attained
albeit with a slight increase in the amount of the alkene product
(Table 1, entries 6−8). Interestingly, the reaction with HSiEt3
was completed in only 30 min with a β-(Z) selectivity of 70%
(entry 8). However, a reverse selectivity toward the β-(E)-
vinylsilane and a significant amount of alkene (up to 30%) were
observed when using the bulky t-Bu-CCH as substrate,
which follows the trend observed with rhodium(I) catalysts
having alkylamino-functionalized NHC ligands.5c In the
particular case of HSiMePh2, the reaction is slower than with
linear long alkyl chain alkynes, giving 56% conversion after 1 h
(entry 9). The hydrosilylation of t-Bu-CCH with HSiEt3 was
performed at room temperature, giving 60% conversion in 2 h
with the same selectivity (entries 10 and 11). In sharp contrast,
excellent regio- and stereoselectivity toward the β-(Z)-vinyl-
silane was attained with the substrate Et3SiCCH. Although
the reaction with HSiMePh2 is much slower with only 17%
conversion in 1 h, HSiEt3 provided the best catalytic
performance with 94% selectivity and 87% conversion in only
1.3 h (entries 12 and 13).
Reactivity of [Ir(κ3-hqca)(E-CHCHCH2OMe)(coe)] (2).
The unsaturated complexes [Ir(κ3-hqca)(E-CHCHR)(coe)]
feature a vacant site trans to the alkenyl ligand that makes
possible the preparation of an octahedral complex by reaction
with monodentate neutral ligands. Reaction of 2 with
triphenylphosphine at room temperature cleanly gave the
octahedral adduct [Ir(κ3-hqca)(E-CHCHCH2OMe)(coe)-
(PPh3)] (2-PPh3), which was isolated as a yellow solid in
69% yield (Scheme 5). The coordination of the PPh3 ligand
Scheme 5. Reactivity of [Ir(κ3-hqca)(E-CH
CHCH2OMe)(coe)] (2)
became evident in the 31P{1H} NMR, which features a singlet
at δ −11.25. The α carbon of the η1-alkenyl ligand appears as a
doublet at δ 138.13 ppm with a large 2JC−P = 113.6 Hz coupling
constant in the 13C{1H} NMR spectrum, which supports the
coordination of the PPh3 ligand trans to the alkenyl ligand. In
addition, the α and β vinyl protons were observed at δ 6.79 and
4.74 ppm in the 1H NMR as dd and ddt due to the long-range
coupling to the phosphorus atom with 3JH−P and 4JH−P coupling
constants of 6.8 and 8.0 Hz, respectively. The presence of the
coe ligand in 2-PPh3 was confirmed in both the 1H and
13C{1H} NMR spectra, which showed two resonances for the
CH protons and carbons at δ 5.40 and 5.20 ppm and at δ
85.85 and 85.40 ppm, respectively.
The adduct [Ir(κ3-hqca)(E-CHCHCH2OMe)(coe)(py)]
(2-py) was prepared by reaction of 2 with an excess of pyridine
and isolated as an orange solid in excellent yield. The
stereochemistry of 2-py was unequivocally established by
1
means of a H−1H NOESY spectrum, which showed cross
peaks between the ortho pyridine protons and the nearest
>CH2 protons of the coe ligand and between the α proton of
the η1-alkenyl ligand and the CH coe protons. Thus, the
alkenyl and pyridine ligands in 2-py are disposed mutually
trans, thereby confirming that py occupies the vacant position
in 2 (Scheme 5).
The carbonylation of a solution of 2 in dichloromethane/
methanol for 1 h at room temperature resulted in the formation
The hydrosilylation of phenylacetylene proceeds with
moderate selectivity toward the β-(Z)-vinylsilane isomer
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a b
,
Table 1. Hydrosilylation of Terminal Alkynes with 8-Oxidoquinoline-2-carboxylato Iridium(III) Complexes
entry
catalyst
RCCH (R)
n-Bu
silane
time (h)
conversn (%)
β-(Z) (%)
β-(E) (%)
α (%)
alkene (%)
1
1
3
1
1
3
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
HSiMePh2
HSiMePh2
HSiMe2Ph
HSiEt3
1
100
100
100
100
100
100
100
100
56
88
91
83
68
62
83
72
70
13
9
8
7
0
0
0
8
8
3
0
7
5
3
4
0
0
9
6
6
2
5
5
2
9
16
4
2
2
1
3
0.75
0.75
0.75
1
14
21
28
8
3
4
3
5
HSiEt3
2
6
n-Hex
t-Bu
HSiMePh2
HSiMe2Ph
HSiEt3
6
7
0.75
0.5
1
20
13
51
58
57
10
3
8
8
10
31
30
30
8
9
HSiMePh2
HSiEt3
10
11
12
13
14
15
16
17
18
19
20
21
22
0.5
100
59
c
HSiEt3
2
9
SiEt3
Ph
HSiMePh2
HSiEt3
1
17
82
94
45
71
63
29
80
43
87
40
44
1.3
1
87
3
HSiMePh2
HSiEt3
100
100
100
100
100
100
100
90
37
18
25
66
11
50
9
9
0.2
0.5
1
5
HSiEt3
6
4-MeO-C6H4
4-CF3-C6H4
HSiMePh2
HSiEt3
3
0.15
1.15
4
HSiEt3
2
c
HSiEt3
1.5
2
HSiMePh2
2.5
0.5
40
27
11
13
HSiEt3
98
a
b
1
Conversion and selectivities were calculated by H NMR. Experiments were carried out in CDCl3 at 60 °C using a HSiR3/RCCH/Ir ratio of
c
110/100/2, with [Ir] = 3.08 mM. Room temperature.
Scheme 6. Hydrosilylation and Dehydrogenative Silylation
of Terminal Alkynes
Figure 2. Reaction profile of conversion versus time for the
■
●
hydrosilylation of PhCCH (purple ), n-BuCCH (red ), and
◆
▲
Et3SiCCH (green ) with HSiEt3 and of n-BuCCH (blue
)
with HSiMePh2, catalyzed by 1 in CDCl3 at 60 °C.
tions with HSiEt3 are faster than those with HSiMePh2, as is
exemplified in the case of hex-1-yne in Figure 2.
The selectivity in the hydrosilylation of alkynes is also
influenced by the catalyst ability to promote isomerization
reactions once the alkyne substrate has been completely
consumed. Both the isomerization of β-(Z)-vinylsilane to the
more thermodynamically stable β-(E)-vinylsilane isomer or to
the corresponding allyl-silyl derivatives, in the case of linear
Figure 1. Reaction profile of conversion and selectivity versus time for
the hydrosilylation of n-BuCCH with HSiMePh2 catalyzed by 1 in
◆
CDCl3 at 60 °C: β-(Z)-vinylsilane (blue ); β-(E)-vinylsilane (red
■
▲
●
); 1-hexene (green ); n-BuCCH (purple ).
alkyl chain alkynes, have been frequently observed.5b,c,6
e
Although catalyst precursor 1 has shown no substantial
isomerization activity for aliphatic alkynes at reasonable
reaction times after the consumption of the substrate, the β-
(Z) → β-(E) vinylsilane isomerization process strongly
determines the observed selectivity in the case of aromatic
alkynes. The monitoring of the hydrosilylation of phenyl-
acetylene with HSiEt3 showed that the reaction was complete in
12 min with 71% selectivity to β-(Z)-vinylsilane. However, after
30 min the selectivity dropped to 63% as a consequence of the
(Table 1, entries 14 and 15). Remarkably, the formation of
poly(phenylacetylene) was not observed, which is in contrast
with some rhodium(I) hydrosilylation catalysts that also
promote the polymerization of substituted aromatic alky-
nes.5c,27 As can be observed in the selected reaction profiles of
Figure 2, the hydrosilylation of phenylacetylene with HSiEt3 is
faster than that of the aliphatic alkynes hex-1-yne and
(triethylsilyl)acetylene. In addition, the hydrosilylation reac-
E
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β-(Z) → β-(E) vinylsilane isomerization (entries 15 and 16). In
agreement with these results, we have found that compound 1
efficiently promotes the isomerization of (Z)-PhCH
CHSiMe2Ph into the (E)-vinylsilane isomer under the same
experimental conditions, giving 90% conversion in 3 h (see the
Supporting Information).
conversion was attained in only 1.5 h with 87% of β-(Z)-
vinylsilane and a β-(Z)/β-(E) ratio as high as 9.6 (entry 20).
The reactivity trend observed with catalyst 1 contrasts with
that of some ruthenium catalysts in which electron-withdrawing
substituents increased phenylacetylene reactivity toward hydro-
silylation.28 On the other hand, no major effects arising from
the electronic effects of para substitution of phenylacetylenes,
neither in the activity nor in the selectivity, were found in
catalytic systems based on RhIII and IrIII catalyst precursors
featuring a functionalized bis-N-heterocyclic carbene ligand.6,12
Mechanism of the Hydrosilylation of 1-Alkynes
Catalyzed by [IrH(κ3-hqca)(coe)] (1). Compound 1 does
not react with hydrosilanes such as HSiMePh2 and HSiEt3 in
CHCl3 at 60 °C. Thus, the η1-alkenyl complexes [Ir(κ3-
hqca)(E-CHCHR)(coe)] formed in the reaction of 1 with
terminal alkynes should play a key role in the mechanism of the
hydrosilylation of terminal alkynes. According to these
experimental observations, the catalytic reaction should start
by reaction of the alkenyl species with hydrosilane.
The influence of the electronic effects on the hydrosilylation
of alkynes has been studied with the alkynes (4′-methox-
yphenyl)- and (4′-trifluoromethylphenyl)acetylene. As can be
seen in Table 1, the hydrosilylation of the phenylacetylene
derivative with a −CF3 electron-withdrawing substituent at the
para position is slower than that of phenylacetylene, giving also
poor selectivity (entries 21 and 22). Although a preference for
the β-(Z)-vinylsilane isomer was also observed (40−44%),
significant amounts of α-vinylsilane (up to 16%) and the
corresponding alkene (11−13%) were produced. In order to
confirm the formation of the corresponding silylalkyne
derivative resulting from the competitive dehydrogenative
silylation, which is difficult to identify in the 1H NMR
spectrum of the reaction mixture, the outcome of the
hydrosilylation of (4′-trifluoromethylphenyl)acetylene with
HSiEt3 was studied by 29Si{1H} NMR spectroscopy. The
spectrum showed three resonances at δ 5.84, 3.29, and 1.02
ppm which were assigned to the α-, β-(E)-, and β-(Z)-
In order to shed light on the reaction mechanism, a
computational study at the DFT level has been carried out
using HSiMe3 as a model for silanes and propyne as an alkyne
model. A double catalytic cycle based on Crabtree’s proposal
has been studied.6 The proposed mechanism for the
e
1
vinylsilane isomers, respectively, with the help of a H−29Si
hydrosilylation and dehydrogenative silylation of terminal
alkynes catalyzed by 1 is shown in Scheme 7. In the following
discussion all energies are reported in terms of ΔH. One of the
key steps in the process is the competitive reaction of
intermediate 10 in an unimolecular β-H elimination process,
to follow a reductive silylation path through intermediate 13, or
an alternative bimolecular reaction with a silane molecule to a
silylation process through 11 (Scheme 7). A bimolecular
process suffers a penalty because of the overestimation of
entropy in the Gibbs energy calculations favoring the
unimolecular process. While some approximate corrections
have been proposed, we think that ΔH reproduces sufficiently
the observed experimental trends and will be used throughout
the whole discussion.29 Full data (electronic energy, enthalpy,
and free energies in the gas phase) for every intervening
molecule or transition state are included in the Supporting
Information.
The reaction of 6 with HSiMe3 displaces the coe ligand from
the metal coordination sphere in an exothermic step (ΔH =
−8.3 kcal/mol) to give 7, which then starts the catalytic cycle
(Scheme 7). This is a η2-silane compound which leads to the
formation of propene by σ-bond Si−H metathesis via TS7‑8′
(ΔH⧧ = +4.8 kcal/mol) (Figure 4). An intrinsic reaction
coordinate (IRC) calculation from the transition state produces
a η2-alkene agostic complex 8′ which eventually rearranges to a
more stable π-coordinated alkene complex 8. This step is
exothermic from 7 releasing ΔH = −30.3 kcal mol−1.
Interestingly, no intermediate corresponding to an oxidative
addition of silane to yield an Ir(V) species has been found. As
shown in Figure 4, the Ir−H distance in TS7−8′ is 1.60 Å, while
the Si−H (2.05 Å) and the C−H (1.99 Å) bond distances are
both long. These data suggest that the breaking Si−H bond is
quite advanced while the C−H bond is not completely formed
in the transition state. In addition, the Ir−H bond has a
considerable hydridic character, which is compatible with a
hydrogen transfer step occurring via an oxidative hydrogen
migration mechanism.11a,15a
HMBC NMR correlation spectrum. The resonance at δ −3.57
ppm, which showed correlation exclusively with the ethyl
resonances of the silane moiety, was assigned to the silylalkyne
derivative. Furthermore, both the silylalkyne derivative and the
corresponding alkene, but not the dehydrodimerization Et3Si-
SiEt3 product, were detected by GC/MS (see the Supporting
Information).
The presence of a −OMe electron-donating substituent at
the para position resulted in an increase in the activity. The
hydrosilylation of (4′-methoxyphenyl)acetylene with HSi-
MePh2 gave 66% of β-(E)-vinylsilane isomer with a β-(Z)/β-
(E) ratio of 0.44, likely as a result of the β-(Z) → β-(E)
vinylsilane isomerization after the end of the hydrosilylation
reaction (entry 17). The monitoring of the reaction of (4′-
methoxyphenyl)acetylene with HSiEt3 evidenced complete
conversion in 0.15 h, giving 80% of β-(Z)-vinylsilane with a
β-(Z)/β-(E) ratio of 7.3 (entry 18). However, this ratio steadily
dropped to 0.9 after 1.15 h due to β-(Z) → β-(E) vinylsilane
isomerization (entry 19 and Figure 3). Interestingly, when the
hydrosilylation was performed at room temperature, complete
Figure 3. β-(Z)/β-(E) ratio for the hydrosilylation of (4-MeO-
C6H4)CCH with HSiEt3 catalyzed by 1 in CDCl3 at 60 °C. Total
consumption of (4-MeO-C6H4)CCH occurred at 9 min.
Substitution of the alkene ligand in 8 by propyne is favored
(ΔH = −3.9 kcal mol−1) and gives complex 9 with a square
F
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Scheme 7. Proposed Mechanism for the Hydrosilylation (Left) and Dehydrogenative Silylation (Right) of Terminal Alkynes
Catalyzed by 1
activation energy for this process is ΔH⧧ = +21.6 kcal/mol
from 9a to 10a. The compounds 10a,b, which feature a η2-
vinylsilane ligand perpendicular to the ONO plane, are
endothermic relative to 9a,b by ca. 6.0 kcal/mol. Interestingly,
η2-vinylsilane metal species, also called metallacyclopropene
complexes, have been proposed as key intermediates in several
hydrosilylation mechanisms.11b,30,31a
In the next step, an HSiMe3 molecule enters in the
coordination sphere of the iridium center by bonding in a η2
fashion while the η2-vinylsilane ring opens by breaking the Ir−
Cα bond and rotates around the Ir−Cβ bond, producing an
ensuing η1-vinylsilane ligand. This step is the origin of the
stereoselectivity of the global reaction, as the approach of the
HSiMe3 can take two pathways relative to the substituents on
the Cα atom (Scheme 7). This leads to two different TSs for
each isomer of 10, which eventually produce two square-
Figure 4. Structure of TS7−8′ with some relevant bond distances.
pyramidal complexes, 11a,b, where the final η1-vinylsilane
ligand has either a Z or E configuration, respectively. A total of
four transition structures are then possible. As stated above, the
different orientations relative to the ONO ligand are, in
practice, irrelevant. Those two leading to a (Z)-silylvinylene
ligand are the most feasible with an activation energy of ΔH⧧ =
+5.6 kcal/mol, the difference between both of them being 0.4
kcal/mol. Those which will yield a (E)-silylvinylene compound
are ca. 3 kcal/mol higher in energy with an activation energy of
ΔH⧧ ≈ +8 kcal/mol. During this process the C−C distance
varies form 1.39 Å in the η2-vinyl complex to 1.35 Å in the TSs
and reaches a value of 1.34 Å in the final η1-vinyl complexes 11,
with small differences in the third decimal place for both
isomers. More significant is the change in the Ir−C distances,
which starts at a value of 1.86 Å in both isomers of 10 and ends
at a value of 2.04 Å in 11, reflecting a significant change in the
metal−carbon bond order, with an intermediate value of 1.94 Å
in the TS.
pyramidal geometry. In this complex the ONO pincer ligand
and the alkyne are coplanar while the silyl group is located at
the apical vertex of the pyramid. Due to the asymmetry of the
ONO ligand, two possible orientations of the alkyne molecule
are possible although the difference in stability between them
(9a and 9b) is quite small, just 0.3 kcal mol−1. After this step
the reaction pathway splits into the two separated hydro-
silylation and dehydrogenative silylation cycles (Scheme 7).
This common intermediate to both cycles is shown as origin
and reference in the energy profile for the hydrosilylation cycle
shown in Figure 5.
The silyl group in 9 migrates to the α carbon of the alkyne
ligand. Due the asymmetry of the ONO pincer ligand two TSs
are possible although, as it has been shown above, the energy
difference between both TSs is negligible (ΔH = 0.25 kcal/
mol) with an energy difference of ΔH = 0.2 kcal/mol between
both possible isomer products, 10a and 10b, resulting from the
location of the silyl substituent relative to the two different
functional groups of the ONO pincer ligand (Figure 6). The
Formation of 11a is exothermic relative to 10a (ΔH = −9.3
kcal/mol), while 11b is exothermic by a slightly lesser amount
(ΔH = −6.5 kcal/mol). A more favorable transition state and a
G
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Figure 5. Energy profile for the hydrosilylation cycle (Scheme 7, left). TS10a‑13 and 13 lead to the dehydrogenative silylation pathway that has been
omitted for clarity (both energy profiles are shown together in the Supporting Information). 9 has been chosen as starting compound for the
diagram as it is the common point between both cyclic processes.
Scheme 8. Reaction of 10a with HSiMe3 To Give β-(E)
(Left) or β-(Z) (Right) Vinylsilane Isomers
Figure 6. Structure of one of the isomers of the η2-vinylsilane
compound 10.
slightly greater stability of the final intermediate complex drive
the process toward the formation of the (Z)-silylvinylene
iridium derivative. The steric interaction between the entering
silane molecule and the silyl substituent is at the origin of this
difference. The entrance of the HSiMe3 molecule in the iridium
coordination sphere, by any of the two planes of the
metallacyclopropene moiety, moves away from it the closest
substituent on the η2-vinylsilane moiety, while the rotation
around the C−C bond approaches the second substituent to
the entering silane. In this way, attack from the H side of the η2-
vinylsilane rotates the H atom away from the entering silane
but then approaches the silyl substituent to it. This approach
would yield ultimately the β-(E)-vinylsilane isomer (Scheme 8,
left). An attack from the opposite side relieves any steric
repulsion between the entering silane molecule and the silyl
substituent, making the formation of the β-(Z)-vinylsilane
isomer much easier than that of the β-(E) counterpart (Scheme
8, right).
The η2-HSiMe3 ligand transfers the H atom to the η1-
vinylsilane ligand in an easy σ-bond Si−H metathesis process,
resulting in the formation of the η2-coordinated vinylsilane
H
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Figure 7. Energy profile for the dehydrogenative silylation cycle (Scheme 7, right). TS10a‑11b and TS10a‑11a lead to the hydrosilylation pathway that
has been omitted for clarity (both energy profiles are shown together in the Supporting Information). 9 has been chosen as the starting compound
for the diagram, as it is the common point between both cyclic processes.
products 12a,b (Scheme 7). Attempts to optimize Ir(V)
structures that would correspond to an hypothetical oxidative
addition of HSiMe3 have been unsuccessful. In a fashion similar
to that described above for TS7‑8′, the process occurs via an
oxidative hydrogen migration mechanism. The activation
energy from 11a to 12a is ΔH⧧ = +2.7 kcal/mol and from
11b to 12b is ΔH⧧ = +4.7 kcal/mol. The final η2-vinylsilane
products are both of similar energy (there is a difference of just
0.2 kcal/mol). The replacement of the formed η2-vinylsilane
ligand by a new molecule of alkyne starts a new catalytic cycle.
The silylpropyne product resulting from the competitive
dehydrogenative silylation process can be formed by β-H
elimination from the η2-vinylsilane ligand in compounds 10a,b
(Scheme 7). Passing through TS10a‑13 involves a relatively low
activation energy of ΔH⧧ = +16.6 kcal/mol (Figure 7), but this
energy is larger than those of the hydrosilylation steps (around
5−8 kcal/mol, depending on isomer). The process is slightly
exothermic (ΔH = −3.6 kcal/mol) and generates the hydrido
silylalkyne complex 13 (only one isomer has been considered),
which on replacement of the silylpropyne moiety for propyne
generates 14. Then, insertion of the alkyne into the Ir−H bond
(ΔH⧧ = 22.7 kcal/mol) yields 15, a η2-vinyl complex, by a
slightly endothermic process (ΔH = +3.4 kcal/mol). The
opening of the iridacyclopropene moiety in 15 by coordination
of a silane molecule to give 7, which restarts the catalytic cycle,
is an exothermic process (ΔH = −22.4 kcal/mol).
plane defined by the ONO pincer ligand, also avoiding the
steric clash with the entering hydrosilane. Thus, increasing the
size of the silane makes this pathway, leading to the β-(Z)-
vinylsilane product, more competitive. However, the steric
influence of the alkyne substituent must also be taken into
consideration because the substituent of the alkyne is directed
toward the plane of the ONO ligand after the opening of the
metallacyclopropene intermediate. Then, increasing the bulki-
ness of the alkyne substituent introduces additional steric
effects that strongly influence the β-(Z)/β-(E) ratio. As a larger
substituent is subject to a greater steric repulsion with the
ONO ligand, the hydrosilylation process becomes less favorable
with respect to the β-H elimination, which results in an
increasing amount of the dehydrogenative silylation products
(R = t-Bu, Table 1).
CONCLUSIONS
■
The compound [IrH(κ3-hqca)(coe)] (1), which contains the
rigid asymmetrical dianionic ONO pincer-type ligand 8-
oxidoquinoline-2-carboxylate, reacts with terminal alkynes to
yield the series of η1-alkenyl complexes [Ir(κ3-hqca)(E-CH
CHR)(coe)]. Alkyne insertion into the Ir−H bond proceeds in
a syn fashion, as shown by the deuterium incorporation at the β
carbon of the alkenyl ligand from the deuterium-labeled
compound [IrD(κ3-hqca)(coe)]. The vacant coordination site
at the unsaturated complex [Ir(κ3-hqca)(E-CH
CHCH2OMe)(coe)] can be occupied by monodentate neutral
ligands such as PPh3 and pyridine, while CO binds reversibly.
The compound [IrH(κ3-hqca)(coe)] (1) is an efficient
catalyst precursor for the hydrosilylation of a range of aliphatic
and aromatic 1-alkynes. Hydrosilylation of linear 1-alkynes
(hex-1-yne and oct-1-yne) gives a good selectivity toward the β-
(Z)-vinylsilane product, while for the bulkier t-Bu-CCH a
reverse selectivity toward the β-(E)-vinylsilane and significant
amounts of alkene, from the competitive dehydrogenative
The mechanism described above helps to explain some of the
trends observed in the catalytic performance of 1. In general,
good selectivity to the β-(Z)-vinylsilane product is achieved in
the hydrosilylation of linear 1-alkynes such as hex-1-yne and
oct-1-yne. However, the attained selectivity drops when the size
of the hydrosilane is decreased: HSiMePh2 > HSiMe2Ph >
HSiEt3 (Table 1). As can be seen in the transition state leading
to the intermediate 11b from 10a (Scheme 7, right), the silyl
fragment of the η1-vinylsilane ligand is directed away from the
I
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mL) was treated with 5 equiv of the appropriate alkyne, and the
mixture was stirred for 48 h at room temperature. The resulting dark
red solution was evaporated under vacuum to give the corresponding
compounds [Ir(κ3-hqca)(alkenyl)(coe)] as red solids. In some cases a
chromatographic purification step was required. Thus, the crude
compound was dissolved in CH2Cl2/MeOH and then transferred to
an alumina column (10 × 1.5 cm). Elution with CH2Cl2/MeOH (10/
1) gave red solutions of the complexes, from which the compounds
were obtained as red solids by removing the solvent under vacuum.
The numbering scheme for the 8-oxidoquinoline-2-carboxylate ligand
is provided in Figure 8.
silylation, have been observed. The selectivity in the hydro-
silylation of alkynes is also influenced by the catalyst’s ability to
promote the vinylsilane isomerization. Although no substantial
isomerization activity has been found for aliphatic alkynes at
reasonable reaction times after the consumption of the
substrate, the β-(Z) → β-(E) vinylsilane isomerization strongly
determines the observed selectivity in the case of aromatic
alkynes.
The mechanism of the hydrosilylation process has been
studied by DFT calculations. The catalytic cycle, on the basis of
Crabtree’s proposal, passes through Ir(III) intermediates and
splits into two different cycles, leading to hydrosilylation or
dehydrogenative silylation having two common intermediates.
The key intermediate is a η2-vinylsilane complex resulting from
the silyl migration to the alkyne in a Ir(III)-silyl species having a
η2-alkyne ligand. This intermediate, via a high-energy β-H
elimination process, may yield the dehydrogenative silylation
products. Alternatively, reaction of the η2-vinylsilane complex
with silanes is an easier pathway. Coordination of a silane
molecule causes the opening of the metallacyclopropene and is
the origin of the selectivity between the (Z)- or (E)-1-silyl-1-
alkene products, β-(Z) and β-(E) isomers, respectively. The
attack of silane onto the two different faces of the
iridacyclopropene ring has a slightly different activation energy
which gives rise to different η1-vinylsilane ligands with either an
E or Z configuration. σ-bond Si−H metathesis results in H
transfer from the silane to the η1-vinylsilane ligand and the
formation of the corresponding β-(Z)- and β-(E)-vinylsilane
isomers, respectively. As a consequence, the hydrogen and silyl
fragments on the hydrosilylated products arise from two
different silane molecules.
Figure 8. Numbering scheme for NMR data.
[Ir(κ3-hqca)(E-CHCHCH2OMe)(coe)] (2). [IrH(κ3-hqca)(coe)]
(1) (0.083 g, 0.170 mmol) and 3-methoxy-1-propyne (0.68 mmol,
60 μL), chromatographic purification. Yield: 67% (0.064 g). Anal.
Calcd for C22H26IrNO4: C, 47.13; H, 4.67; N, 2.50. Found: C, 47.05;
H, 4.51; N 2.39. MS (MALDI, CH2Cl2/MeOH, m/z): 560.1 [M]+,
1
490.0 [M − alkenyl]+. H NMR (400.162 MHz, 298 K, CD2Cl2/
MeOH-d4): δ 8.24 (d, H, 3JH−H = 8.8, H-4), 7.75 (d, 1H, 3JH−H = 8.8,
3
3
H-3), 7.45 (t, 1H, JH−H = 8.0, H-6), 7.01 (d, 1H, JH−H = 8.0, H-5),
6.92 (d, 1H, 3JH−H = 8.0, H-7) (hqca), 6.83 (d, 1H, JH−H = 14.7, Ir−
3
3
CHCH−), 5.12 (m, 2H, CH, coe), 4.56 (dt, 1H, JH−H = 14.6,
3
7.3, Ir−CHCH−), 3.61 (d, 2H, JH−H = 6.6, > CH2), 2.85 (s, 3H,
−OCH3), 2.25−2.12 (bm, 2H), 2.02−2.11 (bm, 2H), 1.85−1.95 (bm,
2H), 1.59−1.73 (bm, 2H), 1.37−1.57 (bm, 2H), (>CH2, coe).
13C{1H} NMR (75.468 MHz, 298 K, CD2Cl2/MeOH-d4): δ 168.61
(C-8), 141.31 (C-2), 139.42 (C-9), 134.81 (C-6), 131.41 (C-4),
129.95 (C-10) (hqca), 126.91 (Ir−CHCH−), 119.15 (C-3), 113.42
(C-5), 111.36 (C-7) (hqca), 106.81 (Ir−CHCH−), 86.67, 86.31
(CH, coe), 71.53 (>CH2), 53.54 (−OCH3), 28.45, 28.39, 24.47
(2C), 22.82, 22.77 (>CH2, coe). IR (ATR, cm−1): ν(CO), 1650, 1614
(s).
EXPERIMENTAL SECTION
■
Synthesis. All experiments were carried out under an atmosphere
of argon using Schlenk techniques or a glovebox. Solvents were
obtained from a Solvent Purification System (Innovative Technolo-
gies). CD2Cl2, benzene-d6, and toluene-d8 (Euriso-top) were dried
using activated molecular sieves. MeOH-d4 (<0.02% D2O, Euriso-top)
was used as received. [IrH(κ3-hqca)(coe)] (1) was prepared from
[Ir(μ-OH)(coe)2]2 and 8-hydroxyquinoline-2-carboxylic acid
(H2hqca) following the procedure recently described.25 Alkynes and
hydrosilanes were generally obtained from Aldrich, except for
Et3SiCCH and HSiMe2Ph (Lancaster) and t-Bu-SiCCH (Acros
Organics). All reagents were used as received without further
purification.
[Ir(κ3-hqca)(E-CHCH(CH2)3CH3)(coe)] (3). [IrH(κ3-hqca)(coe)]
(1) (0.196 g, 0.400 mmol) and 1-hexyne (2.0 mmol, 240 μL). Yield:
98% (0.225 g). Anal. Calcd for C24H30IrNO3: C, 50.33; H, 5.28; N,
2.45. Found: C, 50.40; H, 5.12; N, 2.37. MS (MALDI, CH2Cl2/
MeOH, m/z): 605.9 [M + MeOH]+, 572.0 [M]+, 490.1 [M −
alkenyl]+, 380.8 [M − alkenyl − coe]+. 1H NMR (300.13 MHz, 298 K,
3
CD2Cl2/MeOH-d4): δ 8.22 (d, 1H, JH−H = 8.6, H-4), 7.75 (d, 1H,
3JH−H = 8.7, H-3), 7.45 (t, 1H, 3JH−H = 8.0, H-6), 7.01 (d, 1H, 3JH−H
8.1, H-5), 6.91 (d, 1H, 3JH−H = 7.9, H-7), (hqca), 6.17 (d, 1H, 3JH−H
14.5, Ir−CHCH−), 5.11 (m, 2H, CH, coe), 4.30 (dt, 1H, 3JH−H
=
=
=
Scientific Equipment. Elemental analyses were carried out in a
PerkinElmer 2400 CHNS/O analyzer. NMR spectra were recorded on
Bruker AV-400 and AV-300 spectrometers. Chemical shifts are
reported in ppm relative to tetramethylsilane and referenced to
partially deuterated solvent resonances. Coupling constants (J) are
given in hertz. Spectral assignments were achieved by combination of
14.5, 6.9, Ir−CHCH−), 2.21 (2H), 2.05 (2H), 1.96−1.76 (6H),
1.69 (1H), 1.61−1.37 (3H), 0.98−0.76 (4H), (>CH2), 0.57 (t, 3H,
3JH−H = 6.9, −CH3). 13C{1H} NMR (75.468 MHz, 298 K, CD2Cl2/
MeOH-d4): δ 171.01 (C-8), 143.81 (C-2), 141.93 (C-9), 137.12 (C-
6), 133.79 (C-4) (hqca), 133.50 (Ir−CHCH−), 132.54 (C-10),
121.59 (C-3), 115.85 (C-7), 113.90 (C-5), (hqca), 100.21 (Ir−CH
CH−), 88.92, 88.31, (CH, coe), 33.88 (2C), 32.82 (>CH2), 30.98,
30.91, 26.98, 25.33, 25.23, 21.61 (>CH2, coe), 13.50 (s, −CH3). IR
(ATR, cm−1): ν(CO), 1628, 1613 (s).
1
1H−1H COSY, NOESY, 13C DEPT and APT, H−13C HSQC, and
1H−13C HMBC experiments. Electrospray mass spectra (ESI-MS)
were recorded on a Bruker MicroTof-Q instrument using sodium
formiate as reference. MALDI-TOF mass spectra were obtained on a
Bruker Miocroflex mass spectrometer using DCTB (trans-2-[3-(4-tert-
butylphenyl)-2-methyl-2-propenylidene]malononitrile) or dithranol as
matrix. FT-IR spectra were collected on a Nicolet Nexus 5700 FT
spectrophotometer equipped with a Nicolet Smart Collector diffuse
reflectance accessory. Gas chromatography/mass spectrometry (GC/
MS) analyses were performed on an Agilent 7673 GC autosampler
system with an Agilent 5973 MS detector operating in EI ionization
method at 70 eV, equipped with an Phenomenex ZB-5HT apolar
capillary column (0.25 um film thickness, 30 m × 0.25 mm i.d.).
Reaction of [IrH(κ3-hqca)(coe)] (1) with Alkynes. General
Procedure. A solution of [IrH(κ3-hqca)(coe)] (1) in THF (30−40
[Ir(κ3-hqca)(E-CHCD(CH2)3CH3)(coe)] (3-d1). [IrH(κ3-hqca)-
(coe)] (1; 0.050 g, 0.102 mmol) was dissolved in a mixture of THF
(30 mL) and MeOH-d4 (0.5 mL) and the solution stirred at room
temperature for 10 min. 1-Hexyne (0.5 mmol, 60 μL) was added, and
then the mixture was stirred for 48 h at room temperature. Removal of
solvents under vacuum afforded the compound as a red solid. Yield:
96% (0.056 g). 1H NMR (300.13 MHz, CD2Cl2/MeOH-d4): δ 6.16 (s,
1H, Ir−CHCD−). 13C{1H} NMR (75.468 MHz, CD2Cl2/MeOH-
1
d4): δ 133.46 (t, JC‑D = 26.6, Ir−CHCD−), 100.06 (Ir−CH
CD−) (selected resonances).
J
DOI: 10.1021/acs.organomet.6b00471
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
[Ir(κ3-hqca)(E-CHCH(CH2)5CH3)(coe)] (4). [IrH(κ3-hqca)(coe)]
(1; 0.100 g, 0.204 mmol) and 1-octyne (1.0 mmol, 150 μL). Yield:
95% (0.117 g). Anal. Calcd for C26H34IrNO3: C, 51.98; H, 5.70; N,
2.33. Found: C, 51.80; H, 5.61; N, 2.45. MS (MALDI, CH2Cl2/
Workup as described above afforded the compound as a yellow solid.
Yield: 69%: (0.114 g). Anal. Calcd for C40H41IrNO4P: C, 50.38; H,
5.02; N, 1.70. Found: C, 50.26; H, 4.75; N, 1.65. MS (MALDI,
1
CH2Cl2, m/z): 715.2 [M − coe]+, 487.1 [M − PPh3 − alkenyl]+. H
NMR (400.162 MHz, 298 K, CD2Cl2): δ 7.72 (d, 1H, 3JH−H = 8.6, H-
4, hqca), 7.30−7.10 (m, 17H, 2H of hqca and 15H of PPh3), 6.79 (dd,
1
MeOH, m/z): 600 [M]+, 489.90 [M − alkenyl]+. H NMR (300.13
3
MHz, 298 K, CD2Cl2/MeOH-d4): δ 8.22 (d, 1H, JH−H = 8.6, H-4),
3
3
3
7.76 (d, 1H, 3JH−H = 8.5, H-3), 7.46 (t, 1H, 3JH−H = 7.8, H-6), 7.01 (d,
1H, 3JH−H = 8.1, H-5), 6.92 (d, 1H, 3JH−H = 8.0, H-7) (hqca), 6.19 (d,
1H, 3JH−H = 14.4, Ir−CHCH−), 5.13 (m, 2H, CH, coe), 4.32 (dt,
1H, JH−H = 15.4, JH−P = 6.8, Ir−CHCH−), 6.76 (d, 1H, JH−H
=
3
7.8, H-5), 6.66 (d, 1H, JH−H = 7.8, H-7) (hqca), 5.40 (m, 1H), 5.20
3
4
(m, 1H) (CH, coe), 4.74 (ddt, JH−H = 15.4 and 6.5, JH−P = 8.0,
3
5
3
Ir−CHCH−), 3.50 (dd, 2H, JH−H = 6.5, JH−P = 2.0, > CH2,
alkenyl), 2.64 (s, 3H, −CH3), 2.12 (br m, 2H), 1.98 (br m, 1H), 1.90
(br m, 1H), 1.77 (br m, 2H), 1.13−1.05 (br m, 6H) (>CH2, coe).
1H, JH−H = 14.4, 6.5, Ir−CHCH−), 2.23 (m, 2H), 2.07 (m, 2H),
2.01−1.77 (m, 4H), 1.69 (2H), 1.61−1.37 (m, 4H), 1.11−0.75 (m,
3
8H) (>CH2, coe and alkenyl), 0.71 (t, 3H, JH−H = 7.0 −CH3).
13C{1H} NMR (75.468 MHz, 298 K, THF-d8): δ 174.09 (CO),
13C{1H} NMR (75.468 MHz, 298 K, CD2Cl2/MeOH-d4): δ 170.98
(C-8), 143.78 (C-2), 137.08 (C-6), 133.79 (C-4) (hqca), 133.46 (Ir−
CHCH−), 132.50 (C-10), 121.60 (C-3), 115.83 (C-7), 113.88 (C-
5) (hqca), 100.29 (Ir−CHCH−), 88.89, 88.30 (CH, coe), 34.08
(>CH2, coe), 31.83 (>CH2, alkenyl), 30.97, 30.90 (>CH2, coe), 30.44,
28.17 (>CH2, alkenyl), 26.97 (2C), 25.31, 25.23 (>CH2, coe), 22.87
(>CH2, alkenyl), 13.87 (−CH3). IR (ATR, cm−1): ν(CO), 1612 (s).
[Ir(κ3-hqca)(E-CHCH(C)(CH3)3)(coe)] (5). [IrH(κ3-hqca)(coe)]
(1; 0.100 g, 0.204 mmol) and 3,3-dimethyl-1-butyne (1.0 mmol, 124
μL), chromatographic purification. Yield: 62% (0.072 g). Anal. Calcd
for C24H30IrNO3: C, 50.33; H, 5.28; N, 2.45. Found: C, 50.41; H,
5.17; N, 2.33. MS (MALDI, CH2Cl2/MeOH, m/z): 606.2 [M +
MeOH] +, 574.3 [M]+, 490.2 [M − alkenyl]+. 1H NMR (400.16 MHz,
298 K, CD2Cl2/MeOH-d4): δ 8.21 (d, 1H, 3JH−H = 8.8, H-4), 7.75 (d,
2
172.45 (C-8), 143.53 (C-2), 143.37 (C-9) (hqca), 138.13 (d, JC−P
113.6, Ir−CHCHCH−), 136.64 (C-6, hqca), 135.34, (d, 2C, 2JC−P
10, PPh3), 133.77 (C-4, hqca), 133.44 (C-10, hqca), 131.49 (4JC−P
=
=
=
1
3
2.2, PPh3), 130.61 (d, JC−P = 35.6, PPh3), 129.57 (d, 2C, JC−P = 9,
PPh3), 127.81 (Ir−CHCH−), 122.78 (C-3), 115.86 (C-5), 113.32
(C-7) (hqca), 85.85, 84.80, (CH, coe), 76.12 (d, JC−P = 12.7,
>CH2), 56.39 (−CH3) (alkenyl), 32.25, 31.96, 27.85, 27.74, 27.21,
26.27 (>CH2, coe). 31P{1H} NMR (161.99 MHz, 298 K, CD2Cl2): δ
−11.25 (s). IR (ATR, cm−1): ν(CO), 1676 (s).
[Ir(κ3-hqca)(E-CHCHCH2OMe)(coe)(CO)] (2-CO). Carbon
monoxide was bubbled through a solution of [Ir(κ3-hqca)(E-CH
CHCH2OMe)(coe)] (2; 0.040 g) in MeOH-d4/CD2Cl2 (1 mL) at
room temperature for 1 h and the resulting solution transferred to an
NMR tube. 1H NMR (300.13 MHz, 298 K, CD2Cl2): selected
3
3
1H, JH−H = 8.8, H-3), 7.45 (t, 1H, JH−H = 8.0, H-6), 7.01 (d, 1H,
3JH−H = 8.0, H-5), 6.92 (d, 1H, 3JH−H = 8.1, H-7) (hqca), 6.19 (d, 1H,
3JH−H = 14.7, Ir−CHCH−), 5.08 (m, 2H, CH, coe), 4.33 (d, 1H,
3JH−H = 14.6, Ir−CHCH−), 2.19 (m, 2H), 2.05 (m, 2H), 1.89 (m,
2H), 1.68 (m, 2H), 1.58−1.40 (m, 4H), (>CH2, coe), 0.59 (s, 9H,
−CH3). 13C{1H} NMR (75.468 MHz, 298 K, CD2Cl2/MeOH-d4): δ
170.96 (C-8, hqca), 144.78 (Ir−CHCH−), 143.74 (C-2), 141.90
(C-9), 137.06 (C-6), 133.75 (C-4), 132.53 (C-10), 121.54 (C-3),
115.85 (C-7), 113.87 (C-5) (hqca), 93.82 (Ir−CHCH−), 88.97,
88.27 (CH, coe), 33.24 (-C(CH3)3), 30.97, 30.92 (>CH2, coe),
30.10 (s, 3C, −CH3), 30.05, 27.00, 25.35, 25.25 (>CH2, coe). IR
(ATR, cm−1): ν(CO), 1629, 1613.
3
resonances for 2-CO, δ 8.35 (d, 1H, JH−H = 8.7, H-4), 7.84 (d, 1H,
3JH−H = 8.7, H-3), 7.59 (t, 1H, 3JH−H = 8.2, H-6), 7.15 (d, 1H, 3JH−H
8.2, H-5), 7.08 (d, 1H, 3JH−H = 8.2, H-7) (hqca), 6.45 (d, 1H, 3JH−H
=
=
15.4, Ir−CHCH−), 5.35 (m, 3H, CH coe and Ir−CHCH−).
IR (CH2Cl2, cm−1): ν(CO), 2035 (s).
General Procedure for Hydrosilylation of 1-Alkynes. An
NMR tube was charged under argon with the catalyst precursor (1.54
× 10−3 mmol, 2 mol %), CDCl3 (0.5 mL), the corresponding alkyne
(0.077 mmol), and silane (0.085 mmol). The solution was kept in a
thermostated bath at 60 °C and monitored by 1H NMR spectroscopy.
The vinylsilane reaction products were unambiguously characterized
on the basis of the coupling patterns and constants of vinylic protons
in the 1H NMR spectra and subsequent comparison to literature
values.31 Values for J ranged from 17 to 19 Hz for β-(E), 13 to 16 Hz
for β-(Z), and 1 to 3 Hz for α vinylsilanes.
Calculation Details. DFT calculations have been carried out with
Gaussian 0932 using the B3LYP functional. For Ir atoms the lanl2dz
and its associated basis set supplemented33 with an f function was
used, and the 6-31G** basis set was used for the rest of the atoms.
Prop-1-yne was used as a model of the alkyne substrates, and
trimethylsilane was used as a silane model. Energies are reported in the
discussion in terms of enthalpy. All stationary structures have been
characterized by frequency calculations. For transition structures a
single imaginary frequency was found and additional IRC calculations
in both directions of the transition vector were performed to ensure
the connection to the related end points. When the IRC calculations
were finished before reaching the minima, additional optimizations
were performed from the end point reached so far.
[Ir(κ3-hqca)(E-CHCHCH2OCH3)(coe)(py)] (2-py). Pyridine
(0.5 mL) was added to a solution of [Ir(κ3-hqca)(E-CH
CHCH2OMe)(coe)] (2; 0.112 g, 0.200 mmol) in THF (20 mL) to
give an orange solution after stirring at room temperature for 14 h.
The volatiles were removed under reduced pressure, and the residue
was dissolved in the minimum amount of CH2Cl2 (5 mL). Addition of
n-pentane (20 mL) led to the precipitation of an orange solid, which
was washed with n-pentane (3 × 5 mL) and dried under vacuum.
Yield: 97% (0.124 g). Anal. Calcd for C27H31IrN2O4: C, 50.69; H,
4.88; N, 4.38. Found: C, 50.52; H, 5.01; N, 4.42. MS (ESI, CH3CN,
1
m/z): 639.4 [M − H]+, 561.4 [M − py]+. H NMR (400.162 MHz,
3
4
298 K, CD2Cl2): δ 8.55 (dt, 2H, JH−H = 4.8, JH−H = 1.8, o-H, py),
8.01 (d, 1H, 3JH−H = JH−H = 8.6, H-4, hqca), 7.68 (tt, 1H, 3JH−H = JH−H
= 7.8, JH−H = 1.8, p-H, py), 7.63 (d, 1H, JH−H = 8.8, H-3), 7.46 (t,
1H, 3JH−H = 7.8, H-6) (hqca), 7.28 (td, 2H, 3JH−H = 6.3, 4.5, m-H, py),
4
3
3
3
7.01 (d, 1H, JH−H = 7.8, H-5, hqca), 7.00 (d, 1H, JH−H = 14.4, Ir−
3
CHCH−), 6.90 (d, 1H, JH−H = 8.1, H-7, hqca), 5.16 (m, 2H, 
3
CH, coe), 4.82 (dt, 1H, JH−H = 15.2, 6.6, Ir−CHCH−), 3.66 (m,
ASSOCIATED CONTENT
2H, > CH2, alkenyl), 2.90 (s, 3H, −CH3), 2.13 (m, 4H), 1.90−1.77
(m, 2H), 1.75−1.44 (m, 4H), 1.39−1.26 (m, 2H) (>CH2, coe).
13C{1H} NMR (75.468 MHz, 298 K, CD2Cl2): δ 175.04 (CO),
171.12 (C-8) (hqca), 149.11 (2C, py), 142.78 (C-2), 141.71 (C-9)
(hqca), 138.26 (py), 136.27 (C-6), 133.40 (C-4), 132.14 (C-10)
(hqca), 130.02 (Ir−CHCH−), 125.59 (2C, py), 121.58 (C-3, hqca),
118.65 (Ir−CHCHCH−), 115.68 (C-7), 113.07 (C-5) (hqca),
87.70, 87.66 (CH, coe), 74.94 (>CH2), 56.17 (−OCH3), 30.90,
30.84, 26.74, 26.72, 26.17, 26.11 (>CH2, coe). IR (ATR, cm−1):
ν(CO), 1672.
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00471.
Electronic energy, enthalpy, and free energy in the gas
phase for intermediates and transition states. Multi-
nuclear NMR and GC/MS data of the hydrosilylation of
1-ethynyl-4-(trifluoromethyl)benzene with triethylsilane,
and reaction profile of the isomerization of β-(Z)-
PhCHCHSiMe2Ph catalyzed by 1 (PDF)
[Ir(κ3-hqca)(E-CHCHCH2OCH3)(coe)(PPh3)] (2-PPh3). PPh3
(0.110 g, 0.420 mmol) and [Ir(κ3-hqca)(E-CHCHCH2OMe)(coe)]
(2; 0.112 g, 0.200 mmol) were reacted in THF (30 mL) for 3 h.
K
DOI: 10.1021/acs.organomet.6b00471
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Optimized coordinates for catalytic intermediates and
Article
Commun. 2012, 48, 9480−9482. (b) Zanardi, A.; Peris, E.; Mata, J. A.
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Tetrahedron Lett. 2000, 41, 907−910.
transition states (XYZ)
AUTHOR INFORMATION
Corresponding Authors
*E-mail for J.J.P.-T.: perez@unizar.es.
*E-mail for F.J.M.: modrego@unizar.es.
(7) (a) Viciano, M.; Mas-Marza, E.; Sanau, M.; Peris, E.
Organometallics 2006, 25, 3063−3069. (b) Mas-Marza, E.; Sanau,
M.; Peris, E. Inorg. Chem. 2005, 44, 9961−9967. (c) Mas-Marza, E.;
Poyatos, M.; Sanau, M.; Peris, E. Inorg. Chem. 2004, 43, 2213−2219.
(8) (a) Dragutan, V.; Dragutan, I.; Delaude, L.; Demonceau, A.
Coord. Chem. Rev. 2007, 251, 765−794. (b) Menozzi, C.; Dalko, P. I.;
Cossy, J. J. Org. Chem. 2005, 70, 10717−10719. (c) Maifeld, S. V.;
Tran, M. N.; Lee, D. Tetrahedron Lett. 2005, 46, 105−108. (d) Nagao,
M.; Asano, K.; Umeda, K.; Katayama, H.; Ozawa, F. J. Org. Chem.
2005, 70, 10511−10514. (e) Arico, C. S.; Cox, L. R. Org. Biomol.
Chem. 2004, 2, 2558−2562. (f) Katayama, H.; Taniguchi, K.;
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2000, 2, 1887−1889.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support from the Ministerio de Economia y
Competitividad (MINECO/FEDER) of Spain (Project
■
́
́ ́
CTQ2013-42532-P) and Diputacion General de Aragon
(DGA/FSE E07) is gratefully acknowledged. R.P.-O. acknowl-
edges her fellowship from the MINECO (BES-2011-045364).
The authors gratefully acknowledge the resources from the
supercomputer “Caesaraugusta” (node of the Spanish Super-
computer Network) and the technical expertise and assistance
provided by BIFI-Universidad de Zaragoza.
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M
DOI: 10.1021/acs.organomet.6b00471
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