Kinetic analysis and sequencing of Si-H and C-H bond activation reactions: Direct silylation of arenes catalyzed by an iridium-polyhydride

Article abstract of DOI:10.1021/jacs.0c07578

The saturated trihydride IrH3{κ3-P,O,P-[xant(PiPr2)2]} (1; xant(PiPr2)2 = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) coordinates the Si-H bond of triethylsilane, 1,1,1,3,5,5,5-heptamethyltrisiloxane, and triphenylsilane to give the σ-complexes Ir

Full text of DOI:10.1021/jacs.0c07578

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Article
Kinetic Analysis and Sequencing of Si−H and C−H Bond Activation
Reactions: Direct Silylation of Arenes Catalyzed by an Iridium-
Polyhydride
́
Miguel A. Esteruelas,* Antonio Martínez, Montserrat Olivan, and Enrique Onate
̃
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ABSTRACT: The saturated trihydride IrH3{κ3-P,O,P-[xant(PiPr2)2]} (1; xant(PiPr2)2 = 9,9-
dimethyl-4,5-bis(diisopropylphosphino)xanthene) coordinates the Si−H bond of triethylsilane,
1,1,1,3,5,5,5-heptamethyltrisiloxane, and triphenylsilane to give the σ-complexes IrH3(η2-H-
SiR3){κ2-cis-P,P-[xant(PiPr2)2]}, which evolve to the dihydride-silyl derivatives IrH2(SiR3){κ3-
P,O,P-[xant(PiPr2)2]} (SiR3 = SiEt3 (2), SiMe(OSiMe3)2 (3), SiPh3 (4)) by means of the
oxidative addition of the coordinated bond and the subsequent reductive elimination of H2.
Complexes 2−4 activate a C−H bond of symmetrically and asymmetrically substituted arenes to
form silylated arenes and to regenerate 1. This sequence of reactions defines a cycle for the
catalytic direct C−H silylation of arenes. Stoichiometric isotopic experiments and the kinetic
analysis of the transformations demonstrate that the C−H bond rupture is the rate-determining
step of the catalysis. As a consequence, the selectivity of the silylation of substituted arenes is
generally governed by ligand−substrate steric interactions.
coordination of the σ-bond to the metal center of an
unsaturated compound. The interaction involves σ-donation
from the σ-orbital of the bond to empty orbitals of the metal
and back bonding from the metal to the σ*-orbital of the
bond.1
INTRODUCTION
■
Transition metal-mediated cross-coupling reactions involving
elemental steps of σ-bond activation in both substrates
(Scheme 1) are challenging from a conceptual point of view.
The intermolecular C−H silylation of arenes without the use
of directing groups is a particular type of this class of cross-
coupling reactions of great interest. The silylated products are
useful precursors to commercial polymers and copolymers and
can be also used as intermediates for organic synthesis. The
reactions are environmentally friendlier than the classical
procedures of synthesis of arylsilanes, minimizing waste
formation, and offer the possibility of reaching alternative
regioselectivities.2 The catalysis involves the activation of the
Si−H bond of the silane3 and a C−H bond of the arene,4
before the coupling of the substrates. Although transition
metal−silyl derivatives are a prominent group of compounds,
comparable in relevance to the aryl derivatives, there is
significantly less information on silane Si−H bond splitting
than on the arene C−H bond activation. Two features
distinguish silicon from carbon and hydrogen: its higher
electropositive character and the hypervalent ability. As a
consequence, a greater variety of interactions M−HSi than M−
Scheme 1. General Scheme for Cross-Coupling Reactions
Involving Two σ-Bond Activations
The reason is the need of sequencing the splitting of the σ-
bonds in the metal coordination sphere, which requires an
adequate difference between the activation energies of the
bond rupture reactions. Thus, the success of the cross-coupling
demands the control of the sequencing process, for which it is
necessary to know and to govern such parameters. The latter is
achieved by having a deep knowledge of what is happening just
before the σ-bond cleavage step. It is generally assumed that
the first step for a σ-bond activation reaction is the
Received: July 17, 2020
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© XXXX American Chemical Society
A

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HC have been proposed, while a consensus as to how to
distinguish between them has not yet been reached.3b,5 Much
effort has been centered on the stabilization and character-
ization of these interactions, whereas the H−Si cleavage
process has received less attention. Often, M−HSi compounds
are treated as still pictures of a situation more than as
transitory species on the way to the Si−H activation.6
Catalysts for the arene C−H silylation include complexes of
ruthenium,7 rhodium,8 iridium,9 nickel,10 platinum,11 and
some rare-earth metals.12 Knowledge of the identity of species
by which the catalysts functionalize the C−H bond is scarce.
Recently, it has been proposed that the iridium systems
generated from [Ir(μ-OMe)(η4-COD)]2 (COD = 1,5-cyclo-
octadiene) and phenanthroline ligands work through only one
cycle involving both dihydride-silyl and hydride-disilyl
complexes (Scheme 2). In a sequential manner the former
activates the Si−H bond of the silane, whereas the second adds
a C−H bond of the arene to subsequently promote the silyl−
aryl coupling.9n
selective head-to-head (Z)-dimerization of terminal alkynes,21
whereas the osmium-tetrahydride OsH4{κ3-P,O,P-[xant-
(PiPr2)2]} also catalyzes the latter22 and the hydroxo-
osmium(IV) derivative OsH3(OH){κ3-P,O,P-[xant(PiPr2)2]}
dehydrogenates formic acid to H2 and CO2.23 Reactions
catalyzed by rhodium complexes include dehydropolymeriza-
tion of H3B·NMeH2,24 dehydrogenation of ammonia borane25
and alkanes,26 monoalcoholysis of diphenylsilane,27 borylation
of arenes,28 decyanative borylation of nitriles,29 borylation of
alkynes,30 dehalogenation of chloroalkanes, and homocoupling
of benzyl chloride.31 Recently, we have observed that the
iridium-trihydride IrH3{κ3-P,O,P-[xant(PiPr2)2]} catalyzes the
direct borylation of arenes with the help of dihydride-boryl and
hydride-diboryl derivatives. The three compounds are involved
in two catalytic cycles, which have the dihydride-boryl complex
as a common intermediate because of its ability to activate
both B−H and C−H bonds (Scheme 3).32
Scheme 3. Mechanism for the Borylation of Arenes
Catalyzed by the Trihydride IrH3{κ3-P,O,P-[xant(PiPr2)2]}
Scheme 2. Dihydride-Silyl- and Hydride-Disilyl-Mediated
Silylation of Arenes
The diagonal relationship between the elements of rows 2
and 3 is well-known, being particularly marked for boron and
silicon.33 With that in mind, we reasoned that complex
IrH3{κ3-P,O,P-[xant(PiPr2)2]} should also be an efficient
catalyst for the silylation of arenes. Thus, in order to build a
catalytic cycle for the arene C−H silylation, related to those
shown in Scheme 3, we decided to study the activation of the
Si−H bond of silanes promoted by this trihydride and the C−
H bond activation of arenes promoted by the resulting silyl
products, paying particular attention to the kinetics of the
processes and to the reaction intermediates. This paper
demonstrates that the iridium-promoted arene C−H silylation
can also take place through trihydride and dihydride-silyl
complexes without the help of hydride-disilyl species.
The ligands used for stabilizing C−H silylation catalysts are
usually monodentate and bidentate. Pincer ligands are having a
tremendous impact in current catalysis because of their ability
for stabilizing uncommon species, which open novel
approaches.13 However, they have been scarcely used for
supporting these catalysts. As far as we know, only one catalyst
bearing a ligand of this class has been previously employed. In
2007, Tilley and co-workers reported that complex Ir(κ3-
NSiN)H(OTf)(COE) (NSiN = bis(8-quinolyl)methylsilyl,
OTf = triflate, COE = cyclooctene) is active for arylsilane
redistribution and for the dehydrogenative silylation of
arenes.14
9,9-Dimethyl-4,5-bis(diisopropylphosphino)xanthene (xant-
(PiPr2)2) is an ether-diphosphine that has demonstrated to
have a higher capacity than other POP-diphosphines to act as
pincer.15 However, its flexibility along with the hemilabile
character of the ether function allows it to adapt to the
requirements of the participating intermediates of the catalytic
cycles.16 As a proof-of-concept, species bearing the diphos-
phine coordinated in κ3-mer,17 κ3-fac,18 κ2-cis,19 and κ2-trans20
fashions have been isolated, whereas notable catalysts of
platinum group metals stabilized by this ligand have proved to
be efficient for a wide range of reactions. For the elements of
the iron triad, it should be mentioned that the ruthenium
complex RuH(η2-H2BH2){κ3-P,O,P-[xant(PiPr2)2]} is an
efficient catalyst precursor for the hydrogen transfer from 2-
propanol to ketones, the α-alkylations of phenylacetonitrile
and acetophenone with alcohols, and the regio- and stereo-
RESULTS AND DISCUSSION
■
Si−H Bond Activation. As expected for the diagonal
relationship between boron and silicon, trihydride complex
IrH3{κ3-P,O,P-[xant(PiPr2)2]} (1) activates the Si−H bond of
silanes such as triethylsilane, 1,1,1,3,5,5,5-heptamethyltrisilox-
ane, and triphenylsilane (Scheme 4). Treatment of toluene
solutions of 1 with 1.0 equiv of the silanes, at room
temperature, for 18 h leads to the corresponding dihydride-
silyl derivatives IrH2(SiR3){κ3-P,O,P-[xant(PiPr2)2]} (SiR3 =
SiEt3 (2), SiMe(OSiMe3)2 (3), SiPh3 (4)) and molecular
hydrogen. Complexes 2−4 were isolated as white solids in 49−
55% yield.
Complex 3 was characterized by X-ray diffraction analysis.
The structure, which has two molecules chemically equivalent
B
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Scheme 4. Reactions of 1 with Silanes
but crystallographically independent in the asymmetric unit
(Figure 1 shows one of them), reveals a disposition trans for
The monitoring of the activations by 1H and 31P{1H} NMR
spectroscopy in toluene-d8 reveals that the reactions are
quantitative and take place via the intermediates ASiR3 shown
in Scheme 4. At room temperature, characteristic features of
these transitory species are a broad hydride resonance centered
1
between −11.2 and −12.2 ppm in the H NMR spectra and a
broad signal centered between −1 and −5 ppm in the 31P{1H}
NMR spectra. The half-life of these intermediates depends
upon the substituents attached to the silicon atom, increasing
in the sequence SiPh3 < SiMe(OSiMe3)2 < SiEt3. The half-life
of intermediate ASiEt3 is long enough to allow its spectroscopic
study at 183 K. Figure 2 collects the most informative NMR
spectra at this temperature. The 31P{1H} NMR spectrum (a)
shows two doublets (2JP−P = 18 Hz) at 8.7 (a1) and −5.9 (a2)
ppm and a broad singlet at −12.8 (b1) ppm. The COSY
31P−31P spectrum (b) confirms that the doublets correspond
to the same molecule (A), which bears a κ2-P,P-cis-
diphosphine with inequivalent PiPr2 groups, whereas the
broad singlet represents other κ2-P,P-cis-diphosphine species
with chemically equivalent PiPr2 moieties (B). The molar ratio
between them is approximately 2:1. The HMBC 31P−1H
spectrum (c) reveals hydride resonances for A at −9.7 (a1),
−11.2 (a2), −12.6 (a3), and −14.4 (a4) ppm, whereas those of
B appear at −9.7 (b1), −10.0 (b2), and −14.2 (b3+b4) ppm.
According to an A:B molar ratio of 2:1, the 1H{31P} spectrum
in the high-field region (Figure S19) displays an
a1+b1:b2:a2:a3:b3+b4:a4 intensity ratio of 3:1:2:2:2:2. Further-
Figure 1. Molecular diagram of complex 3 (ellipsoids shown at 50%
probability). All hydrogen atoms (except the hydrides) are omitted
for clarity. Selected bond distances (Å) and angles (deg): Ir(1)−P(1)
= 2.252(4), 2.245(4), Ir(1)−P(2) = 2.263(4), 2.262(4), Ir(1)−O(3)
= 2.339(9), 2.324(9), Ir(1)−Si(1) = 2.262(4), 2.271(4); P(1)−
Ir(1)−P(2) = 156.83(15), 161.33(14), P(1)−Ir(1)−O(3) = 82.0(3),
81.1(3), P(2)−Ir(1)−O(3) = 81.5(3), 81.4(3), P(1)−Ir(1)−Si(1) =
98.91(15), 98.00(15), P(2)−Ir(1)−Si(1) = 100.05(15), 100.18(16),
H(01)−Ir(1)−H(02) = 173(7), 169(7), Si(1)−Ir(1)−O(3) =
170.4(3), 172.8(3).
1
more, on the basis of this spectrum and the H one, a trans
disposition of the hydrides corresponding to resonances a3 and
a4 and the PiPr2 groups of A (2JH−P = 104 and 113 Hz,
respectively), and between the hydrides assigned to signal
b3+b4 and the PiPr2 moieties of B, can be deduced. The signal
the hydride ligands (H(01)−Ir(1)−H(02) = 173(7)° and
169(7)°). The coordination polyhedron around the iridium
center can be rationalized as a distorted octahedron with the
ether-diphosphine coordinated in a mer fashion (P(1)−Ir(1)−
P(2) = 156.83(15)° and 161.33(14)°, P(1)−Ir(1)−O(3) =
82.0(3)° and 81.1(3)°, and P(2)−Ir(1)−O(3) = 81.5(3)° and
81.4(3)°) and the silyl group located trans to the diphosphine
oxygen atom (Si(1)−Ir(1)−O(3) = 170.4(3)° and 172.8(3)°).
The Ir(1)−Si(1) bond lengths of 2.262(4) and 2.271(4) Å
compare well with those reported for other iridium(III)-silyl
2
b3+b4 is the AA′ part of an AA′XX′ spin system with JA‑X
=
2JA′‑X′ = 116 Hz. The decoupling of the 31P nucleus brings to
light hidden 29Si satellites on the signal a2. At first glance, the
1JH−Si value of 43 Hz suggests some degree of Si−H interaction
and the formation of a “symmetric oxidative addition
product”.35 The Si−H interaction in A and the value of the
1JH−Si are also supported by the HMQC 29Si−1H spectrum (d),
which contains the cross-spot between the resonance a2 and
the Ir−Si resonance, which is observed at 2.8 ppm in the
29Si{1H} NMR spectrum. This bidimensional spectrum also is
consistent with the Si−H interaction in B, showing a cross-spot
complexes.34 The H, 31P{1H}, and 29Si{1H} NMR spectra of
1
2−4, in benzene-d6, at room temperature are consistent with
1
1
the structure shown in Figure 1. In the H NMR spectra, the
between Ir−Si and b1 resonances, although the value of JH−Si
most noticeable feature is the resonance corresponding to the
equivalent hydrides, which appears as a triplet (2JH−P = 17 Hz)
between −5.2 and −6.2 ppm. The 31P{1H} NMR spectra show
a singlet between 44 and 54 ppm, in agreement with equivalent
PiPr2 moieties. The 29Si{1H} NMR spectra contain an Ir−Si
resonance, which is observed as a triplet (2JSi−P = 10 Hz)
between −9 and −59 ppm.
cannot be measured in this case. All these spectroscopic
features together suggest that ASiEt3 exists in solution as a
mixture of two species, A and B, which have structures
resembling that calculated by Schley and co-workers for the
compound IrH4(SiEt3)(PPh3)2;36 i.e., an octahedron formed
by the diphosphine κ2-cis-P,P coordinated, three hydrides fac-
disposed, and the Si−H bond situated trans to a hydride.
C
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Figure 2. (a) 31P{1H} NMR spectrum (202.46 MHz, C7D8, 183 K); (b) 31P−31P COSY spectrum (202.46 MHz, C7D8, 183 K); (c) HMBC
31P−1H NMR spectrum (202.46/500.13 MHz, C7D8, 183 K); and (d) HMQC 29Si−1H NMR spectrum (99.36/500.13 MHz, C7D8, 183 K; *
denotes excess of HSiEt3) of ASiEt3
.
Optimization of this structure by DFT calculations (see
Supporting Information) shows the silicon atom in position
anti with regard to the oxygen of the diphosphine. Keeping this
disposition there are several rotamers involving the alkyl
substituents of the diphosphine and silane. The isopropyl
substituents of the PiPr2 groups can be disposed in positions
eclipsed or alternating, which gives rise to structures bearing
equivalent or inequivalent PiPr2 groups, in agreement with the
spectroscopic observations. The disposition of the isopropyl
groups slightly modifies the position of the silicon atom, which
changes the P−Ir−Si angles.
The criterion of the value of 1JH−Si is ambiguous to establish
the nature of the Si−H interaction. So, we decided to prepare a
monodeuterated ASiEt3 species, by reaction of 1 with Et3SiD,
after reasoning that the rupture of the Si−D bond should give
rise to deuterium distribution between all hydride positions,
since at room temperature the six signals observed at 183 K
coalesce to give only one, while if the Si−D bond was
maintained, only the intensities of resonances b1 and a2 should
Figure 3. High-field region of the 1H NMR spectra at 183 K of ASiEt3
(500 MHz, toluene-d8) (a) and ASiEt3‑d (b) and of the 2H NMR
spectrum (76.77 MHz, toluene) of ASiEt3‑d (c).
1
2
be modified. The high-field region of the H and H NMR
spectra of the monodeuterated ASiEt3 intermediate at 183 K
(Figure 3) clearly supports the second possibility; that is, ASiR3
are species on the way to the rupture of the Si−H bond.
Having clarified the nature of the ASiR3 intermediates, we
subsequently analyzed the deuterium present in the final
product from the reaction of 1 with DSiEt3, finding about 12%
of deuterium in the hydride positions (Figure S20). It is a
75:25 mixture of isotopomers 2 and 2-d1 (Scheme 5). The
presence of 2 in the mixture indicates that the creation of a
coordination vacancy in 1 by reductive elimination of
molecular hydrogen does not occur, in spite of its saturated
character. Such reductive elimination takes place after the Si−
Scheme 5. Reaction of 1 with DSiEt3
D rupture. Furthermore, the lower percentage of 2-d1 in the
mixture is consistent with a rapid reductive elimination, which
is only governed by the H−D bond energy. The rate-
D
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obst
[1] = [1]0e−k
determining step of the overall Si−H bond activation process is
the Si−H bond rupture. According to this, a significant
slowdown of the reaction is also observed by changing HSiEt3
to DSiEt3.
1
(1)
[1]0k1obs
obst
[e−k − e−k t
]
1
2
[ASiMe(OSiMe3)2] =
k2 − k1obs
(2)
(3)
(4)
The isotope labeling experiments previously mentioned
provide only a qualitative picture of the Si−H bond activation
promoted by 1. In order to gain quantitative insight of the
process, the kinetic study of the reaction sequence shown in
Scheme 4 was carried out. 1,1,1,3,5,5,5-Heptamethyltrisiloxane
was selected as a model of silane because the half-life of the
ASiMe(OSiMe3)2 species is intermediate between those of ASiPh3
and ASiEt3. The study was performed by 31P{1H} NMR
spectroscopy, in toluene, under pseudo-first-order conditions
using silane concentrations between 0.76 and 1.52 M, for an
initial concentration of 1 ([1]0) of 0.037 M and a temperature
range of 278−298 K. Figure 4 shows the 31P{1H} spectrum of
the reaction mixture, as a function of time, at 288 K, for a
concentration of silane of 1.12 M.
obst
[1]0
k1obs − k2
[k2e−k − k1obse−k t
]
1
2
[3] = [1]0 +
where
obs
k1 = k1[HSiMe(OSiMe3)2]
Values of k1obs, k1, and k2, for each temperature, obtained
from these expressions, are collected in Table 1. A plot k1obs vs
[HSiMe(SiOMe3)2] (Figure 6) provides a value of (1.89
0.1) × 10−4 M−1·s−1 for k1 at 288 K, whereas the value of k2 at
this temperature is (2.38
0.7) × 10−5 s−1. The activation
parameters for the coordination of the silane, calculated from
the corresponding Eyring analysis (Figure 7), are ΔH1⧧ = 19.5
2.1 kcal·mol−1 and ΔS1⧧ = −7.5 3.4 cal·K−1·mol−1. These
⧧
values yield an activation energy ΔG1 at 298 K of 21.7 3.1
kcal·mol−1. The Eyring analysis for the transformation from
⧧
ASiMe(OSiMe3)2 to 3 (Figure 8) gives values of ΔH2 = 23.1
2.4 kcal·mol−1 and ΔS2 = 0.5 3.1 cal·K−1, which afford an
⧧
⧧
activation energy ΔG2 at 298 K of 23.0
3.3 kcal·mol−1·
mol−1.
The negative value of the activation entropy for the
formation of ASiMe(OSiMe3)2 is consistent with the intermolec-
ular character of the process and suggests an ordered transition
state involving the participation of the silane. Because complex
1 is saturated and the coordination of the Si−H bond requires
the κ3-to-κ2 change in the coordination of the diphosphine, by
dissociation of the hemilabile oxygen, that entropy value points
out in favor of a hypervalent hydride−silicon interaction
previous to the oxygen dissociation. Such an interaction, which
is broken in the coordination, should decrease the activation
energy of the dissociation. The value of the activation entropy
for the transformation from ASiMe(OSiMe3)2 to 3, close to zero, is
in agreement with the fast nature of the elimination of
molecular hydrogen and points out the Si−H rupture as the
rate-determining step. Table 1 also contains values of the rate
Figure 4. Stacked 31P{1H} NMR spectra (121.4 MHz, toluene, 288
K) showing the course of the reaction of 1 with 1.12 M
HSiMe(OSiMe3)2 (PPh3 used as internal standard).
obs‑d
d
constants k1
and k2 obtained for reactions with DSiMe-
(OSiMe3)2. The ratio k1obs/k1obs‑d of 1.16 0.01 confirms that
the silane does not have a direct participation in the rate-
determining transition state for the transformation of 1 into
The dependence of the amounts of 1, ASiMe(OSiMe3)2, and 3
with time (Figure 5) is as expected for two consecutive
irreversible reactions and fits to eqs 1−3, respectively.37
A
SiMe(OSiMe3)2, although it is present. In contrast, the ratio k2/
d
k2 gives a primary isotope effect of 2.40
0.11, which
corroborates the rupture of the Si−H bond as the rate-
determining step for the formation of 3.38
Scheme 6 summarizes the Si−H bond activation of silanes
promoted by the trihydride 1, on the basis of the previously
mentioned results. The silane-assisted dissociation of the
hemilabile ether of the diphosphine affords the unsaturated
intermediates I, which subsequently coordinate the silane to
give ASiR3. The oxidative addition of the coordinated Si−H
bond, in the rate-determining step, leads to II. The latter
rapidly eliminates molecular hydrogen to give III. Finally, the
recoordination of the diphosphine ether group yields 2−4.
C−H Bond Activation. Dihydride-silyl complexes 2−4
react in benzene solution to give R3Si-Ph and the trihydride
derivative 1 (Scheme 7).
Figure 5. Composition of the mixture as a function of time for the
reaction of 0.037 M 1 with 1.12 M HSiMe(OSiMe3)2 at 298 K: 1
(blue •), ASiMe(OSiMe3)2 (orange •), 3 (gray •).
The transformation from 3 to 1 was followed by 31P{1H}
NMR spectroscopy as a function of time between 333 and 353
E
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obs
Table 1. Rate Constants for the Transformations of 1 into ASiMe(SiOMe3)2 (k1 and k1, Calculated According to Eqs 1 and 4,
Respectively) and of ASiMe(SiOMe3)2 into 3 (k2, Calculated According to Eq 3)
T (K)
[1]o (M)
[HSi] (M)
[DSi] (M)
k1 × 10−4 (s−1
obs
)
k1 × 10−4 (M−1·s−1
)
k2 × 10−5 (s−1
)
278
283
288
293
298
288
293
298
288
288
288
288
0.037
0.037
0.037
0.037
0.037
0.037
0.037
0.037
0.037
0.037
0.037
0.037
1.12
1.12
1.12
1.12
1.12
(0.63 0.07)
(1.36 0.06)
(2.34 0.04)
(4.12 0.10)
(7.53 0.08)
(2.02 0.05)
(3.52 0.08)
(6.49 0.07)
(1.41 0.10)
(1.81 0.06)
(2.51 0.08)
(2.81 0.07)
(0.56 0.07)
(1.21 0.06)
(2.09 0.04)
(3.68 0.10)
(6.72 0.08)
(1.80 0.05)
(3.14 0.08)
(5.79 0.07)
(1.86 0.10)
(1.91 0.06)
(1.88 0.08)
(1.85 0.07)
(0.49 0.04)
(1.14 0.10)
(2.36 0.11)
(4.36 0.04)
(9.07 0.07)
(1.03 0.10)
(1.77 0.09)
(3.78 0.12)
(2.32 0.07)
(2.43 0.10)
(2.45 0.09)
(2.33 0.07)
1.12
1.12
1.12
0.76
0.95
1.33
1.52
Scheme 6. Si−H Bond Activation of Silanes Promoted by 1
obs
Figure 6. Plot of k1 versus [HSiMe(OSiMe3)2].
Scheme 7. Reactions of Complexes 2−4 with Benzene
Figure 7. Eyring plot for the transformation of 1 into ASiMe(SiOMe3)2
.
where [3]0 and [3] are the concentrations of 3 at the initial
time and time t, respectively. The values obtained for k3 are
collected in Table 2. The activation parameters obtained from
⧧
the Eyring analysis (Figure 9) are ΔH3 = 23.2
2.2 kcal·
mol−1 and ΔS3 = −12.2 2.9 cal·K−1·mol−1. These values
yield an activation energy ΔG3 at 298 K of 26.8 3.0 kcal·
⧧
⧧
Table 2. Rate Constants for the Transformation of 3 into 1
(k3)
Figure 8. Eyring plot for the transformation of ASiMe(SiOMe3)2 into 3.
T (K)
[3]o (M)
arene
k3 × 10−5 (s−1
)
333
338
343
348
353
343
0.037
0.037
0.037
0.037
0.037
0.037
C6H6
C6H6
C6H6
C6H6
C6H6
C6D6
0.81 0.08
1.49 0.07
2.71 0.10
3.92 0.07
6.41 0.05
0.88 0.08
K. The decrease of 3 with the corresponding increase of 1
(Figure S1) is an exponential function of time, fitting to an
expression of first-order:
[3]
ln
= −k3t
[3]0
(5)
F
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Scheme 9. Mechanism for the Silylation of Arenes
Figure 9. Eyring plot for the transformation of 3 into 1.
mol−1. The rate of the reaction of 3 with benzene-d6 is
d
significantly slower than that with benzene. The ratio k3/k3
gives a primary isotope effect of 3.1 0.1, which supports the
rupture of the aromatic C−H bond as the rate-determining
step of the coupling.
These results are consistent with a classical mechanism for
the C−H bond activation of the arene, which can be
summarized according to Scheme 8. The coordination of the
Scheme 8. Proposed Mechanism for the C−H Bond
Activation of Arenes
hydrogen atmosphere and complex 1 also catalyzes the
hydrogenation of olefins faster than the arene silylation,32
cyclohexene in a silane:olefin 1:1 molar ratio was employed as
hydrogen acceptor. Under these conditions, complex 1
promotes the silylation of benzene and mono- and
disubstituted benzenes.
The behavior of the trihydride 1 during the silylation of
benzene was followed by 31P{1H} NMR, to gain insight into
the one-pot functionalization. Figure 10 shows the 31P{1H}
spectrum of the catalytic mixture as a function of time. In
agreement with the cycle shown in Scheme 9, including the
activation energies of the stoichiometric reactions, trihydride 1
is initially transformed into the dihydride-silyl complex 3,
which is the only detected species, while the silane is present in
the solution. Once the silane is consumed, complex 3
organic substrate to the metal center of the unsaturated
intermediates III shown in Scheme 6 should give the σ-
derivatives IV, which would evolve to V by oxidative addition
of the coordinated C−H bond. The rapid reductive
elimination of the functionalized product should lead to I,
which could regenerate 1 by coordination of the diphosphine
ether group.
Catalytic Cross-Coupling: Silylation of Arenes. The
sequencing of reactions summarized in Schemes 4 and 7 gives
rise to a cycle like that shown in Scheme 1, which is different
from those summarized in Schemes 2 and 3. The sequencing is
possible because there is a suitable difference between the
activation energies of the Si−H and C−H bond activation
processes of 1,1,1,3,5,5,5-heptamethyltrisiloxane and benzene,
⧧
⧧
respectively (ΔG2 < ΔG3 ). According to this, complex 1
catalyzes the cross-coupling between the silane and arenes to
afford functionalized arenes and molecular hydrogen (Scheme
9).
The catalysis was carried out at 110 °C, using the arene as
solvent, since the rate-determining step for the cross-coupling
is the rupture of a C−H bond of the arene, according to the
activation energy values collected in Scheme 9. Furthermore,
because complexes 2−4 regenerate 1 and the silane under a
Figure 10. Stacked 31P{1H} NMR spectra (121.4 MHz, benzene, 358
K) showing the course of the catalytic silylation as a function of time.
G
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quantitatively regenerates the trihydride 1. Any hydride-disilyl
derivative related to those shown in Scheme 2 or analogous to
the hydride-diboryl compound collected in Scheme 3 is not
observed. The presence of 3 during the catalysis, as main metal
species, strongly supports that the C−H bond cleavage of the
arene is the rate-determining step of the functionalization, as
expected, as the stoichiometric reaction of 3 with benzene has
the highest activation energy from the stoichiometric reactions
involved in the cycle shown in Scheme 9. In order to have
additional evidence that the cycle of Scheme 9 operates under
catalytic conditions, we determined the activation parameters
for the benzene silylation under one-pot conditions. The values
of kcatobs collected in Table 3 were calculated by measuring the
Trihydride 1 tolerates one of the widest varieties of
functionalities, which includes CH3, OCH3, CF3, F, Cl, and
Br (Scheme 10). The selectivity of the functionalization is
Scheme 10. Silylation of Arenes (% Isolated Yields in
Parentheses)
Table 3. Rate Constants for the Catalytic Silylation of
a
obs
Benzene (kcat
)
T (K)
kcat × 10−5 (s−1
)
obs
338
343
348
353
0.41 0.08
0.76 0.09
1.35 0.10
1.94 0.09
a
[1]0 = 0.028 M, [HSi] = 0.28 M, [cyclohexene] = 0.28 M.
1
decrease of the amount of silane in the H NMR spectrum of
the catalytic solution as a function of time, starting from an
initial silane concentration of 0.28 M and a concentration of 1
of 0.028 M, in the temperature range 338−353 K. The
corresponding Eyring analysis (Figure 11) yields values of
governed by steric factors, as expected for reactions controlled
by the activation energy of the C−H bond cleavage of the
arene. In this context, it should be mentioned that the
activation energy for the rupture of a C−H bond depends
upon two factors: the dissociation energy of the C−H bond
and the stability of the σ-intermediate. So, because within an
arene the strength of the different C−H bonds is generally
similar, the difference in activation energy (ΔΔG⧧) between
the distinct C−H bonds mainly depends on the stability of the
respective σ-intermediates, which is governed by the steric
hindrance experienced by the coordinated C−H bond. As a
consequence, the C−H bond activation is kinetically
controlled by steric factors; that is, the less sterically hindered
C−H bonds are generally the first activated ones. Thus, the
functionalization takes place at the least sterically hindered C−
H bonds of the aromatic ring. With the exception of
fluorobenzene, monosubstituted benzenes give meta- and
para-substituted products. The molar ratio between the
isomers is modulated by the electronic nature of the
substituent, increasing the meta:para ratio according to the
sequence MeO < Me < Cl < CF3 ≈ Br; that is, electron-
withdrawing groups disfavor the para isomer with regard to the
donating substituents. 1,3-Disubstituted benzenes undergo
Figure 11. Eyring plot of the catalytic reaction of the silylation of
benzene.
ΔHcat⧧ = 24.1 2.6 kcal·mol−1, ΔScat⧧ = −11.6 3.2 cal·K−1·
⧧
mol−1, and ΔGcat = 27.5 3.3 kcal·mol−1 at 298 K. To our
delight, these values compare well with those obtained for the
reaction of 3 with benzene.
The cycle shown in Scheme 9 significantly differs from that
proposed for the iridium-phenanthroline catalysts. In contrast
to the cycle shown in Scheme 2, hydride-disilyl species are not
necessary for the catalysis, in our case. In both cycles,
dihydride-silyl species participate, but their functions are
different. While in the cycle shown in Scheme 2 it activates
the Si−H bond of the silane, in our cycle it activates the C−H
bond of the arene. The mechanism proposed for the iridium-
phenanthroline catalysts and our mechanism are also different
from that summarized in Scheme 3, for the borylation of
arenes, but the three are consistent. In this context, it should be
noted that the combination of the cycles shown in Schemes 2
and 9 formally give rise to the mechanism depicted in Scheme
3.
H
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ν(C−O−C) 1095 (m). 1H NMR (300.13 MHz, C6D6, 298 K): δ 7.00
exclusively silylation at the meta position with respect to both
substituents. The fluorine atom shows a marked ability to
approach the silyl group, in agreement with its well-known
capacity to direct to the ortho position the C−H bond
activation of arenes, mediated by transition complexes.28,39
Thus, fluorobenzene and 1,3-difluorobenzene give mixtures of
the three possible isomers; the silylation of the latter merits
particular mention, which affords the silylated product bearing
the silyl group situated ortho to one of the fluorine substituents
and para to the other with high selectivity (70%). The reason
for this fact appears to be related to an increase of the Si−C
bond energy due to the ortho-fluorine substitution. The effect
has been explained in terms of a rise of the ionic component of
the bond by the inductive effect of the fluorine atom.40
3
(m, 2H, CH-arom POP), 6.72 (d, JH−H = 7.5, 2H, CH-arom POP),
6.61 (t, 3JH−3 H = 7.5, 2H, CH-arom POP), 2.35 (m, 4H, PCH(CH3)2),
1.17 (dvt, JH−H = 7.2, N = 18.0, 24H, PCH(CH3)2), 1.04 (m, 6H,
Si(CH2CH3)3), 0.95 (m, 9H, Si(CH2CH3)3, 0.90 (s, 6H, CH3), −5.91
2
(t, JH−P = 17.4, 2H, Ir−H). 13C{1H} NMR (75.47 MHz, C6D6, 298
K): δ 156.8 (vt, N = 10.3, C-arom), 132.5 (vt, N = 4.9, C-arom),
129.7 (s, CH-arom), 125.9 (s, CH-arom), 125.0 (vt, N = 31.4, C-
arom), 124.3 (vt, N = 5.2, CH-arom), 34.4 (s, C(CH3)2), 28.9 (s,
C(CH3)2), 26.0 (vt, N = 29.1, PCH(CH3)2), 19.5 (vt, N = 4.9,
3
PCH(CH3)2), 18.4 (s, PCH(CH3)2), 14.9 (t, JC−P = 2.4,
Si(CH2CH3)3), 10.6 (s, Si(CH2CH3)3). 31P{1H} NMR (121.49
MHz, C6D6, 298 K): δ 47.6 (s). 29Si{1H} NMR (59.63 MHz,
2
C6D6, 298 K): δ −9.3 (t, JSi−P = 8.8).
Preparation of IrH2[SiMe(OSiMe3)2]{κ3-P,O,P-[xant(PiPr2)2]}
(3). A solution of IrH3{κ3-P,O,P-[xant(PiPr2)2]} (100 mg, 0.16
mmol) in toluene (3 mL) was treated with 1,1,1,3,5,5,5-heptamethyl-
trisiloxane (45 μL, 0.16 mmol), and the resulting mixture was stirred
at room temperature for 18 h. After this time, the yellowish solution
was evaporated to dryness to afford a yellow residue. Pentane was
added to afford a white solid, which was washed with cold pentane (2
× 1 mL) and dried in vacuo. Yield: 70 mg (49%). Anal. Calcd for
C34H63IrO3P2Si3: C, 47.58; H, 7.40. Found: C, 47.58; H, 7.41. HRMS
(electrospray, m/z): calcd. for C34H62Si3IrO3P2 [M − H]+ 857.3104;
found 857.3135. IR (cm−1): ν(Ir−H) 1758 (w), δs(Si−CH3) 1246
(m), ν(C−O−C) 1033 (m). 1H NMR (300.13 MHz, C6D6, 298 K): δ
7.24 (m, 2H, CH-arom), 6.93 (d, 3JH−H = 7.5, 2H, CH-arom), 6.84 (t,
3JH−H = 7.5, 2H, CH-arom), 2.67 (m, 4H, PCH(CH3)2), 1.44 (dvt,
CONCLUDING REMARKS
■
This study reveals that the Si−H bond activation of silanes
promoted by the trihydride IrH3{κ3-P,O,P-[xant(PiPr2)2]} and
the C−H bond activation of arenes mediated by dihydride-silyl
derivatives of formulas IrH2(SiR3){κ3-P,O,P-[xant(PiPr2)2]}
can be sequenced in order to build catalytic reactions involving
direct silylation of arenes mediated by the trihydride IrH3{κ3-
P,O,P-[xant(PiPr2)2]}.
Stoichiometric isotopic labeling experiments as well as the
results of a detailed kinetic study have demonstrated that the
Si−H bond activation takes place through the σ-complexes
IrH3(η2-H-SiR3){κ2-cis-P,P-[xant(PiPr2)2]} and that the oxida-
tive addition of the coordinated bond to the metal center is the
determining step of the activation.
Isotopic labeling experiments and kinetic results of the C−H
bond activation indicate that it occurs through a classical
mechanism where the C−H bond cleavage is the rate-
determining step. Its activation energy is higher than that of
the Si−H bond activation. Thus, the C−H bond rupture is the
rate-determining step of the catalysis, and, as a consequence,
the selectivity of the silylation of monosubstituted and 1,3-
disubstituted arenes is generally governed by ligand−substrate
steric interactions.
In summary, the catalytic cycle for the direct silylation of
arenes catalyzed by a saturated polyhydride bearing a pincer
ligand has been built on the basis of stoichiometric isotopic
labeling experiments, the kinetic analysis of the involved σ-
bond activation reactions, and the full characterization of the
key σ-intermediate for the Si−H bond activation.
3JH−H = 7.5, N = 16.2, 12H, PCH(CH3)2), 1.21 (dvt, 3JH−H = 6.9, N =
13.8, 12H, PCH(CH3)2), 1.12 (s, 6H, CH3), 0.79 (s, 3H,
2
SiMe(OSiMe3)2), 0.43 (s, 18H, SiMe(OSiMe3)2), −6.11 (t, JH−P
=
17.1, 2H, Ir−H). 13C{1H} NMR (75.48 MHz, C6D6, 298 K): δ 156.6
(vt, N = 10.7, C-arom), 133.2 (vt, N = 4.8, C-arom), 130.2 (s, CH-
arom), 126.4 (s, CH-arom), 125.6 (vt, N = 30.9, C-arom), 124.4 (vt,
N = 5.1, CH-arom), 34.4 (s, C(CH3)2), 29.8 (s, C(CH3)2), 25.8 (vt,
N = 30.6, PCH(CH3)2), 19.4 (vt, N = 5.3, PCH(CH3)2), 18.3 (s,
PCH(CH3)2), 15.5 (s, SiMe(OSiMe3)2), 3.2 (s, SiMe(OSiMe3)2).
31P{1H} NMR (161.99 MHz, C6D6, 298 K): δ 53.0 (s, triplet under
off-resonance decoupling conditions). 29Si{1H} NMR (59.63 MHz,
2
C6D6, 298 K): δ −7.9 (s, SiMe(OSiMe3)2), −58.2 (t, JSi−P = 10.9,
SiMe(OSiMe3)2).
Preparation of IrH2(SiPh3){κ3-P,O,P-[xant(PiPr2)2]} (4). A
solution of IrH3{κ3-P,O,P-[xant(PiPr2)2]} (100 mg, 0.16 mmol) in
toluene (3 mL) was treated with HSiPh3 (41 mg, 0.16 mmol), and the
resulting mixture was stirred at room temperature for 18 h. After this
time, the yellowish solution was evaporated to dryness to afford a
yellow residue. Pentane was added to afford a white solid, which was
washed with pentane (2 × 1 mL) and dried in vacuo. Yield: 72 mg
(51%). Anal. Calcd for C45H57IrOP2Si: C, 60.31; H, 6.41. Found: C,
60.08; H, 6.56. HRMS (electrospray, m/z): calcd for C45H56SiIrOP2
[M − H]+ 895.3201; found 895.3177. IR (cm−1): ν(Ir−H) 1772 (w),
ν(C−O−C) 1087 (m). 1H NMR (300.13 MHz, C6D6, 298 K): δ 8.39
(d, 3JH−H = 7.2, 6H, SiPh3), 7.29 (t, 3JH−H = 7.2, 6H, SiPh3), 7.18 (m,
EXPERIMENTAL SECTION
■
General Information. All reactions were carried out with
exclusion of air using Schlenk-tube techniques or in a drybox.
Instrumental methods and X-ray details are given in the Supporting
Information. In the NMR spectra the chemical shifts (in ppm) are
referenced to residual solvent peaks (1H, 13C{1H}) or external 85%
H3PO4 (31P{1H}), SiMe4 (29Si), or CFCl3 (19F). Coupling constants J
and N (N = JP−H + JP′−H for 1H and N = JP−C + JP′−C for 13C{1H}) are
given in hertz.
3
3H, SiPh3), 7.01 (m, 2H, CH-arom POP), 6.94 (d, JH−H = 7.5, 2H,
3
CH-arom), 6.81 (t, JH−H = 7.5, 2H, CH-arom POP), 1.59 (m, 4H,
3
PCH(CH3)2), 1.16 (s, 6H, CH3), 1.02 (dvt, JH−H = 7.2, N = 13.8,
2
24H, PCH(CH3)2), −5.28 (t, JH−P = 16.8, 2H, Ir−H). 13C{1H}
NMR (75.47 MHz, C6D6, 298 K): δ 156.8 (vt, N = 10.6, C-arom),
144.4 (s, C SiPh3), 138.5 (s, CH SiPh3), 132.4 (vt, N = 4.8, C-arom),
129.8 (s, CH-arom), 127.1 (s, CH SiPh3), 125.9 (s, CH SiPh3), 125.7
(s, CH-arom), 125.0 (vt, N = 31.0, C-arom), 124.2 (vt, N = 5.6, CH-
arom), 34.2 (s, C(CH3)2), 28.6 (br s, C(CH3)2), 23.6 (vt, N = 30.1,
PCH(CH3)2), 18.7 (vt, N = 5.1, PCH(CH3)2), 17.3 (s, PCH(CH3)2).
31P{1H} NMR (121.49 MHz, C6D6, 298 K): δ 44.9 (s). 29Si{1H}
Preparation of IrH2(SiEt3){κ3-P,O,P-[xant(PiPr2)2]} (2). A
solution of IrH3{κ3-P,O,P-[xant(PiPr2)2]} (100 mg, 0.16 mmol) in
toluene (3 mL) was treated with HSiEt3 (25 μL, 0.16 mmol), and the
resulting mixture was stirred at room temperature for 18 h. After this
time, the yellowish solution was evaporated to dryness to afford a
yellow residue. Pentane was added to afford a white solid, which was
washed with pentane (2 × 1 mL) and dried in vacuo. Yield: 65 mg
(55%). Anal. Calcd for C33H57IrOP2Si: C, 52.70; H, 7.64. Found: C,
52.81; H, 7.59. HRMS (electrospray, m/z): calcd for C33H56SiIrOP2
[M − H]+ 751.3200; found 751.3203. IR (cm−1): ν(Ir−H) 1757 (w),
2
NMR (59.63 MHz, C6D6, 298 K): δ −25.5 (t, JSi−P = 10.4).
Spectroscopic Characterization of IrH3(η2-H-SiEt3){κ2-cis-
P,P-[xant(PiPr2)2]} (ASiEt3). In the glovebox, an NMR tube was
charged with a solution of 1 (10 mg, 0.016 mmol) and HSiEt3 (3 μL,
0.02 mmol) in toluene-d8 (0.42 mL), and the NMR spectra of the
resulting solution were recorded immediately. 1H NMR (400.13
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MHz, C7D8, 298 K): δ 7.01 (m, 4H, CH-arom POP), 6.91 (t, JH−H
7.6, 2H, CH-arom POP), 2.38 (m, 4H, PCH(CH3)2), 1.39 (s, 6H,
=
experiments, which were run in duplicate. In the glovebox, an NMR
tube was charged with a solution of 1 (7.7 mg, 0.012 mmol),
1,1,1,3,5,5,5-heptamethyltrisiloxane (33 μL, 0.12 mmol), and cyclo-
hexene (12 μL, 0.12 mmol) in benzene (0.42 mL), and a capillary
tube filled with a solution of 1,4-dioxane (used as internal standard) in
benzene-d6 was placed in the NMR tube. The tube was immediately
introduced into an NMR probe preheated at the desired temperature,
and the reaction was monitored by H NMR at different intervals of
time (a d1 = 10 s was used in order to ensure accurate integration of
the signals).
3
3
CH3), 1.28 (dd, JH−H = 7.6, JH−P = 15.2, 12H, PCH(CH3)2), 1.13
3
3
(dd, JH−H = 7.2, JH−P = 12.8, 12H, PCH(CH3)2), 0.96 (m, 6H,
Si(CH2CH3)3), 0.79 (m, 9H, Si(CH2CH3)3, −12.09 (br, 4H, Ir−H).
1H NMR (500.13 MHz, C7D8, 183 K, high-field region, relative
intensities): δ −9.73 (m, 3, Ir−H), −9.99 (m, 1, Ir−H), −11.19 (t,
2JH−P = 15.5, 2, Ir−H), −12.49 (dd, 2JH−P = 15.5, 2JH−P = 104, 2, Ir−
H), −14.04 (m, 2, Ir−H), −14.33 (dd, 2JH−P = 21, 2JH−P = 111, 2, Ir−
1
H). H{31P} NMR (500.13 MHz, C7D8, 183 K, high-field region,
1
General Procedure for the Silylation Reactions. In an argon-
filled glovebox an Ace pressure tube was charged with 1 (12.7 mg,
0.02 mmol), HSiMe(OSiMe3)2 (100 μL, 0.36 mmol), cyclohexene
(33 μL, 0.36 mmol), pentadecane (10 μL, 0.036 mmol), as internal
standard, and 1.5 mL of the arene. The resulting mixture was stirred at
110 °C for 18 h. After this time the yield of the silylation reaction was
determined by GC on an Agilent Technologies 6890N gas
chromatograph with a flame ionization detector, using an HP-
Innowax column (30 m × 0.25 mm; film thickness 0.25 μm). The
injector temperature was 250 °C, and the FID temperature was 300
°C. The oven temperature began at 60 °C for 5 min, then 15 °C per
minute to 200 °C, and finally 200 °C for 13 min. Then the arene was
evaporated under reduced pressure to afford a crude reaction mixture.
The identity of the silylation product was confirmed by 1H, 13C{1H},
and 29Si{1H} NMR spectroscopies, as well as by GC-MS analyses.
The isolated yields were calculated after purification of the crude
reaction mixture by flash chromatography over silica gel using diethyl
ether as eluent and by evaporation to dryness.
relative intensities): δ −9.73 (br s, 3, Ir−H), −9.99 (br s, 1, Ir−H),
−11.19 (br s, 2, Ir−H), −12.60 (br s, 2, Ir−H), −14.12 (br s, 2, Ir−
H), −14.40 (br s, 2, Ir−H). 31P{1H} NMR (202.46 MHz, C7D8, 298
K): δ −1.2 (br s). 31P{1H} NMR (202.46 MHz, C7D8, 183 K): δ 8.7
(d, 2JP−P = 18.1), −5.9 (d, 2JP−P = 18.1), −12.8 (br s). 29Si{1H} NMR
(99.36 MHz, C7D8, 183 K): δ 2.8 (br).
Reaction of IrH3{κ3-P,O,P-[xant(PiPr2)2]} (1) with DSiEt3. Two
Wilmad screw-cap NMR tubes were charged with 1 (10 mg, 0.016
mmol). To the first NMR tube was added 0.42 mL of toluene and to
the second was added 0.42 mL of toluene-d8. DSiEt3 (3 μL, 0.02
mmol) was added to both samples, and they were periodically
1
2
checked by NMR spectroscopy. After 28 h, the H and H NMR
1
spectra showed the presence of 2 and 2-d1. The H NMR (300.13
MHz, C7D8, 298 K) data were identical to those reported for 2 with
the exception of the decrease of the intensity of the triplet at −5.76
ppm (2JH−P = 17.4 Hz) corresponding to IrH2 and the appearance of a
new triplet at −5.61 ppm (2JH−P = 17.4 Hz) corresponding to the
IrHD isotopomer, with the deuterium incorporation at the hydride
positions being 25%. 2H NMR (46.07 MHz, toluene, 298 K): δ −5.63
(s, IrD).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c07578.
■
NMR Spectroscopic Study of the Transformation of IrH3{κ3-
P,O,P-xant(PiPr2)2} (1) into ASiMe(OSiMe3)2 and IrH2[SiMe-
(OSiMe3)2]{κ3-P,O,P-[xant(PiPr2)2]} (3). The experimental proce-
dure is described for a particular case, but the same method was used
in all experiments, which were run in duplicate. In the glovebox, an
NMR tube was charged with a solution of 1 (10 mg, 0.016 mmol) and
1,1,1,3,5,5,5-heptamethyltrisiloxane (129 μL, 0.47 mmol) in toluene
(0.42 mL), and a capillary tube filled with a solution of the internal
standard (PPh3) in benzene-d6 was placed in the NMR tube. The tube
was immediately introduced into an NMR probe at the desired
temperature, and the reaction was monitored by 31P{1H} NMR at
different intervals of time.
sı
Experimental details, NMR data of the silylation
products, crystallographic data, computational details
and energies of calculated complexes, and NMR spectra
(PDF)
Cartesian coordinates of the optimized structures (XYZ)
X-ray crystal data (CIF)
Accession Codes
Determination of the Reaction Order for 1,1,1,3,5,5,5-
Heptamethyltrisiloxane in the Transformation of 1 into
CCDC 2014866 contains the crystallographic data for this
paper. These data can be obtained free of charge via www.ccdc.
cam.ac.uk/data_request/cif, or by e-mailing data_request@
ccdc.cam.ac.uk, or by contacting the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: + 44 1223 336033.
A
SiMe(OSiMe3)2. The experimental procedure is analogous to that
described for the transformation of 1 into ASiMe(OSiMe3)2 and 3,
starting form 1 (10 mg, 0.016 mmol, 0.0373 M) and variable
concentrations of 1,1,1,3,5,5,5-heptamethyltrisiloxane (from 0.747 to
1.495 M) in toluene (0.42 mL). The experiments were carried out at
288 K.
NMR Spectroscopic Study of the Transformation of
IrH2[SiMe(OSiMe3)2]{κ3-P,O,P-[xant(PiPr2)2]} (3) into IrH3{κ3-
P,O,P-xant(PiPr2)2} (1). The experimental procedure is described
for a particular case, but the same method was used in all experiments,
which were run in duplicate. In the glovebox, an NMR tube was
charged with a solution of 3 (14 mg, 0.016 mmol) in benzene (0.42
mL), and a capillary tube filled with a solution of the internal standard
(PPh3) in benzene-d6 was placed in the NMR tube. The tube was
immediately introduced into an NMR probe preheated at the desired
temperature, and the reaction was monitored by 31P{1H} NMR at
different intervals of time.
NMR Spectroscopic Study of the Catalysis. In the glovebox, an
NMR tube was charged with a solution of 1 (15 mg, 0.023 mmol),
1,1,1,3,5,5,5-heptamethyltrisiloxane (64 μL, 0.23 mmol), and cyclo-
hexene (24 μL, 0.23 mmol) in benzene (0.42 mL). The tube was
introduced into an NMR probe preheated at 85 °C, and the reaction
was monitored by 31P{1H} NMR at different intervals of time.
Determination of the Activation Parameters of the
Catalytic Silylation of Benzene. The experimental procedure is
described for a particular case, but the same method was used in all
AUTHOR INFORMATION
Corresponding Author
■
́
́
Miguel A. Esteruelas − Departamento de Quımica Inorganica,
́
́
́
́
Instituto de Sıntesis Quımica y Catalisis Homogenea (ISQCH),
́
́
Centro de Innovacion en Quımica Avanzada
(ORFEO−CINQA), Universidad de Zaragoza−CSIC, 50009
Zaragoza, Spain; orcid.org/0000-0002-4829-7590;
Email: maester@unizar.es
Authors
Antonio Martínez − Departamento de Quımica Inorganica,
́
́
́
́
́
́
Instituto de Sıntesis Quımica y Catalisis Homogenea (ISQCH),
́
́
Centro de Innovacion en Quımica Avanzada
(ORFEO−CINQA), Universidad de Zaragoza−CSIC, 50009
Zaragoza, Spain; orcid.org/0000-0002-7292-8591
́
́
́
Montserrat Olivan − Departamento de Quımica Inorganica,
́
́
́
́
Instituto de Sıntesis Quımica y Catalisis Homogenea (ISQCH),
́
́
Centro de Innovacion en Quımica Avanzada
J
https://dx.doi.org/10.1021/jacs.0c07578
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
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F. L.; Bergman, R. G.; Brookhart, M. Silicon-Hydrogen Bond
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(c) Matthews, S. L.; Pons, V.; Heinekey, D. M. Silane Complexes
of Electrophilic Metal Centers. Inorg. Chem. 2006, 45, 6453−6459.
(d) Vyboishchikov, S. F.; Nikonov, G. I. Rhodium Silyl Hydrides in
Oxidation State + 5: Classical or Nonclassical? Organometallics 2007,
26, 4160−4169. (e) McGrady, G. S.; Sirsch, P.; Chatterton, N. P.;
Ostermann, A.; Gatti, C.; Altmannshofer, S.; Herz, V.; Eickerling, G.;
Scherer, W. Nature of the Bonding in Metal-Silane σ-Complexes.
Inorg. Chem. 2009, 48, 1588−1598. (f) Gutsulyak, D. V.;
Vyboishchikov, S. F.; Nikonov, G. I. Cationic Silane σ-Complexes
of Ruthenium with Relevance to Catalysis. J. Am. Chem. Soc. 2010,
132, 5950−5951. (g) Scherer, W.; Meixner, P.; Barquera-Lozada, J.
(ORFEO−CINQA), Universidad de Zaragoza−CSIC, 50009
Zaragoza, Spain; orcid.org/0000-0003-0381-0917
́ ́
Enrique Onate − Departamento de Quımica Inorganica,
̃
́
́
́
́
Instituto de Sıntesis Quımica y Catalisis Homogenea (ISQCH),
́
́
Centro de Innovacion en Quımica Avanzada
(ORFEO−CINQA), Universidad de Zaragoza−CSIC, 50009
Zaragoza, Spain; orcid.org/0000-0003-2094-719X
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c07578
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the MINECO of Spain (Projects
CTQ2017-82935-P and RED2018-102387-T (AEI/FEDER,
E.; Hauf, C.; Obenhuber, A.; Bruck, A.; Wolstenholme, D. J.;
̈
́
UE)), Gobierno de Aragon (Group E06_20R, project
Ruhland, K.; Leusser, D.; Stalke, D. A Unifying Bonding Concept for
Metal Hydrosilane Complexes. Angew. Chem., Int. Ed. 2013, 52,
6092−6096. (h) Komuro, T.; Arai, T.; Kikuchi, K.; Tobita, H.
Synthesis of Ruthenium Complexes with a Nonspectator Si,O,P
Chelate Ligand: Interconversion between a Hydrido(η2-silane)
Complex and a Silyl Complex Leading to Catalytic Alkene
Hydrogenation. Organometallics 2015, 34, 1211−1217. (i) Mai, V.
H.; Korobkov, I.; Nikonov, G. I. Half-Sandwich Silane σ-Complexes
of Ruthenium Supported by NHC Carbene. Organometallics 2016, 35,
936−942. (j) Price, J. S.; Emslie, D. J. H.; Berno, B. Manganese Silyl
Dihydride Complexes: A Spectroscopic, Crystallographic, and
Computational Study of Nonclassical Silicate and Hydrosilane
Hydride Isomers. Organometallics 2019, 38, 2347−2362.
LMP148_18, and predoctoral contract to AM.), FEDER, and
the European Social Fund is acknowledged.
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