Triangular Triplatinum Complex with Four Bridging Si Ligands: Dynamic Behavior of the Molecule and Catalysis

Article abstract of DOI:10.1021/acs.organomet.7b00048

A triangular triplatinum(0) complex with bridging diphenylsilylene ligands, [{(Pt(PMe₃)}₃(μ-SiPh₂)₃] (1a), reacts with H₂SiPh₂ to produce the 1:1 adduct, [{Pt(PMe₃)}₃(

Full text of DOI:10.1021/acs.organomet.7b00048

Article
pubs.acs.org/Organometallics
Triangular Triplatinum Complex with Four Bridging Si Ligands:
Dynamic Behavior of the Molecule and Catalysis
Makoto Tanabe, Megumi Kamono, Kimiya Tanaka, and Kohtaro Osakada*
Laboratory for Chemistry and Life Science, Tokyo Institute of Technology, 4259-R1-3 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
S
* Supporting Information
ABSTRACT: A triangular triplatinum(0) complex with bridging diphenylsilylene ligands, [{(Pt(PMe3)}3(μ-SiPh2)3] (1a),
reacts with H2SiPh2 to produce the 1:1 adduct, [{Pt(PMe3)}3(H)2(μ-SiPh2)4] (2a), which was isolated and characterized by
X-ray crystallography. Two Pt−Pt bonds of the triangular Pt3 core are bridged by a diphenylsilylene ligand, while the remaining
Pt−Pt bond has two unsymmetrical bridging Si ligands. Dissolution of 1a and H2SiPh2 at a 1:3 molar ratio forms a mixture of
complex 2a and unreacted 1a. NMR measurement of the solution at −90 °C revealed the structure of 2a as having two hydride
ligands and four bridging silylene ligands. Two 31P{1H} NMR signals of 2a at −90 °C coalesce on warming to −50 °C
owing to facile exchange of the four Si ligands. Reversible addition of H2SiPh2 to 1a yielded 2a with ΔG° = −8.0 kJ mol−1,
ΔH° = −51.7 kJ mol−1, and ΔS° = −146 J mol−1 K−1. Addition of bis(4-fluorophenyl)silane and bis(4-methylphenyl)silane to
the complexes, which have three bis(4-fluorophenyl)silylene and bis(4-methylphenyl)silylene ligands, respectively, also occurs
reversibly in the solution, and the diarylsilane with an electron-withdrawing substituent is favored for the formation of Pt3Si4
complexes. Complex 1a catalyzes hydrosilyation of benzaldehyde with H2SiPh2 to produce diphenyl(benzyloxy)silane along with
concurrent hydrosilyation and dehydrosilyation of phenyl(methyl)ketone. Dehydrogenative coupling of H2SiPh2 and phenol is
also catalyzed to yield diphenyl(phenoxy)silane. The 31P{1H} NMR spectra of the mixtures during the catalytic reaction show 2a
as the major Pt-containing species.
Chart 1. Structures of [{Pt(PMe3)}3(μ-EPh2)3] (E = Si and Ge)
INTRODUCTION
■
Triangular trinuclear complexes of late transition metals are
regarded as minimal metal clusters. These complexes are
normally supported by bridging ligands such as CO, CNR, and
PR2.1,2 The two former ligands have a π-acceptor characteristic
and stabilize a molecule composed of d-electron-rich metal
centers. Recently, triplatinum(0) complexes with three bridging
diphenylsilylene and diphenylgermylene ligands, formulated
as [{Pt(PMe3)}3(μ-EPh2)3] (E = Si (1a) and Ge), have been
reported to have high thermal stability in spite of the
mismatched combination of electron-releasing ligands and d10
metal centers (Chart 1).3,4
We have investigated structures and electronic states of these
complexes and have determined their chemical properties.
Reaction of dimethyl acetylenedicarboxylate with the Pt3Si3
complex, for example, causes expansion of the Pt3 ring to pro-
duce new complexes. A linear Pt3 complex having μ3-silylene
and phenyl ligands and the complex whose Pt2 and Pt1
fragments are separated by an acetylene ligand are obtained
depending on the aryl group.3d In this paper, we report the
addition of H2SiPh2 to complex 1a with three bridging silylene
ligands to form a new Pt3Si4 type complex. The complex
undergoes unique fluxional behavior and catalyzes the hydro-
silyation of benzaldehydes as well as the dehydrocoupling of
diarylsilane with phenol at room temperature.
Received: January 20, 2017
© XXXX American Chemical Society
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DOI: 10.1021/acs.organomet.7b00048
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the metal center and the H and Si atoms.5 Another isomeric
form without interaction between the hydride and the
coordinating Si atom is also possible (Scheme 1c). Nonclassical
interactions among Si, H, and metal atoms were involved in the
two former structures, which have been examined on the basis
of the geometry of the atoms and spectroscopic results of the
complexes reported thus far.5,6
RESULTS AND DISCUSSION
■
Triplatinum Complexes with Four Bridging Si Ligands.
A triplatinum(0) complex with three bridging diphenylsilylene
ligands, [{Pt(PMe3)}3(μ-SiPh2)3] (1a), reacted with an excess
amount of H2SiPh2 to yield the 1:1 adduct, [{Pt(PMe3)}3-
(H)2(μ-SiPh2)4] (2a), as orange crystals (eq 1).
Diplatinum(I) complexes having two bridging Si ligands as
well as their Pd(I) analogues were reported to have structures
that depended on the substituents of the Si ligand and auxiliary
phosphine ligands. The common structure has a rectangular
Pt2Si2 unit composed of four alternatingly long and short Pt−Si
bonds.7 Two unsymmetrically bridging silyl (e.g., μ-η2-HSiR2
and μ-η2-H2SiR) ligands, which are bonded to one metal by
σ-bonding and to the other by Si−H−M bonding (Scheme 1a,b),
favor coordination in opposite directions. A typical complex,
[{Pt(PCy3)}2(μ-η2-HSiMe2)2], has a crystallographic C2 center
of symmetry at the midpoint of the Pt−Pt bond, and its Pt−Si
bond distances are 2.420(2) and 2.324(2) Å.7a
X-ray crystallography of 2a, which was obtained from the
solution of 1a and H2SiPh2 at a 1:5 molar ratio, clarified the
molecular structure as having a triangular Pt3 framework, as shown
in Figure 1. The Pt−Pt bond distances, 2.747(1), 2.795(1), and
A mirror-image symmetrical isomer with μ-silane and
μ-silylene ligands (Scheme 2b) has also been reported,
Scheme 2
although it is rare.8 Braddock-Wilking reported on dinuclear
Pt complexes having bridging μ-η2-SiHAr2 and μ-SiAr2 ligands in
addition to a hydride ligand attached to a Pt center, as shown in
Scheme 2c.4b,9 The hydride and bridging silylene ligands are
bonded on the same side of the Pt−Pt bond, while the μ-silyl
ligand bridges the other side. The Pt−Si bond lengths of the
Pt2Si2 unit of [{PtH(PPh3)2}{Pt(PPh3)}(μ-SiAr2)(μ-η2-SiHAr2)]
(Ar2 = C13H9Br2N) are 2.393(1), 2.341(1), 2.498(1), and
2.315(1) Å,4b and the relationship with their magnitudes is sim-
ilar to that in 2a (2.408(5), 2.378(5), 2.449(5), and 2.397(5) Å).
Thus, the Pt2Si2 unit of 2a resembles that of the unsymmetrical
Pt2 complex with bridging silylene and silyl ligands. X-ray
crystallographic results, however, did not provide the positions of
the hydrogens of the complex in the solid state.
Figure 1. (a) Thermal ellipsoids (50% probability) of 2a and
(b) selected bond distances (Å). The positions of hydrogen atoms
close to Pt2 and Pt3 were not determined. Selected bond angles (deg):
Pt2−Pt1−Pt3 58.72(3), Pt1−Pt2−Pt3 60.87(3), Pt1−Pt3−Pt2
60.41(2), P1−Pt1−Pt2 150.4(1), P1−Pt1−Pt3 150.1(1), P2−Pt2−
Pt1 155.2(1), P2−Pt2−Pt3 143.9(1), P3−Pt3−Pt1 151.4(1),
P3−Pt3−Pt2 143.2(1), Si3−Pt1−Si4 166.4(2), Si1−Pt2−Si2
105.0(2), Si1−Pt2−Si3 116.9(2), Si2−Pt2−Si3 115.3(2), Si1−Pt3−
Si2 104.3(2), Si1−Pt3−Si4 113.7(2), Si2−Pt3−Si4 117.7(2).
2.808(1) Å, are longer than those of 1a (2.697(1)−2.716(1) Å).
The two latter Pt−Pt bonds of 2a are single-bridged by a SiPh2
ligand with Pt−Si bonds of 2.371(4)−2.392(4) Å, while the
corresponding bonds are shorter for those of 1a (2.337(5)−
2.364(5) Å). The other Pt−Pt bond of 2a is bridged by two
Si ligands, and the two Pt and two Si atoms form a concave
quadrilateral (the dihedral angle of two PtSi2 planes: 148.9°)
composed of four Pt−Si bonds with different lengths: Pt2−Si1,
2.408(5) Å; Pt3−Si1, 2.378(5) Å; Pt3−Si2, 2.449(5) Å;
Pt2−Si2, 2.397(5) Å.
In Scheme 3, possible structures of 2a are summarized on the
basis of X-ray crystallographic results. Scheme 3a is composed
of two Pt atoms double-bridged by two SiHPh2 ligands, similar
Scheme 3
Bridging coordination of a secondary silyl group (−SiHR2)
in dinuclear transition metal complexes is classified as the
μ-η2-silyl form (Scheme 1a) or the hydride and μ-silylene forms
(Scheme 1b), depending on the degree of interaction between
Scheme 1
B
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to the dinuclear complex having C2 symmetry in Scheme 2a.
Scheme 3b contains bridging silylene and silyl ligands as well as
a hydride bonded to a Pt atom. Scheme 3c has two bridging
silylene ligands between the Pt centers and two hydride ligands
bonded to each of the Pt centers. Unsymmetrical coordination
of the silylene ligands that bridges a Pt−Pt bond is explained by
interaction of Si atom and the hydride ligand.
DFT calculations were applied to 2a with Gaussian 09
quantum chemistry program package. The optimized structure
of the complex contains two hydrogen atoms positions close to
the elongated Pt−Si bond, and the Si−H and Pt−H distances
are 1.776 and 1.757 Å and 1.738 and 1.769 Å, respectively
(Figure S2). The Wiberg bond indices are obtained as
0.382 and 0.432 (Si−H) and 0.429 and 0.404 (Pt−H). These
results as well as their comparison with a model diplatinum
complex with bridging silyl ligand [{Pt(PMe3)}2(μ-η2-
HSiPh2)2] (Si−H: 0.451 and 0.455, Pt−H: 0.371 and 0.374)
suggest that the calculated structure of complex 2a is similar to
that in Scheme 3b.
1
Figure 2. (a) 31P{1H} NMR (r.t. and −90 °C) and (b) H NMR
(−30 and −90 °C) spectra of 2a in toluene-d8. Peaks with asterisks in
(a) and (b) are due to 195Pt satellites, while H NMR peaks due to
1
29Si−1H coupling are shown with arrows in (b). These spectra were
obtained by dissolving 1a and H2SiPh2 at a 1:3 molar ratio.
Thus, both X-ray crystallography and DFT calculations are
consistent with the structure shown in Scheme 3b. The NMR
results mentioned below, however, indicate that the complex
exists with the structure in Scheme 3c in solution. Since change
of the coordination bonds between Scheme 1a,b is considered
to be facile, isomerization of the molecules among the struc-
tures in Scheme 3a−c may occur smoothly in solution.
Dynamic Behavior of Complex in Solution. Dissolution
of isolated 2a in toluene-d8 at room temperature resulted in
NMR spectra that contain signals corresponding to 1a and
H2SiPh2 at a 1:1 ratio, which indicates high reversibility in the
formation of 2a from a mixture of 1a and H2SiPh2 in the
J
PtSi = 945 Hz) than of the latter, they are assigned to the singly
bridged silylene ligands.
Thus, complex 2a shows two magnetically equivalent hydride
ligands bonded to each of the two Pt centers, which are double-
bridged by the silylene ligands, as shown in Scheme 3c. Raising
the temperature to −30 °C gave rise to a single 31P{1H} NMR
signal attributable to the coalescence of the doublet and triplet
1
observed at −90 °C. The H NMR spectrum at this tempera-
ture shows a single signal for the ortho-hydrogen of the SiPh2
groups and a hydride signal with satellite peaks (apparent
coupling constant, 187 Hz). The fluxional behavior that renders
three phosphorus nuclei equivalent at this high temperature
also renders the apparent coupling constant to be 1/3 that of the
signal at −90 °C.
solution. A solution of 1a and H2SiPh2 at a 1:3 molar ratio shows
2
a new 31P{1H} NMR signal at δ −22.1 (JPtP = 3893 Hz, JPtP
=
3
144 Hz, JPP = 53 Hz at −8 °C). The signal is assigned to
complex 2a, which exists as the major species at this temperature.
Warming to room temperature, however, causes a decrease in
the intensity of this signal of 2a and an increase in that of the
signal of 1a. The spectrum of a mixture of 1a and H2SiPh2 at a
1:10 molar ratio contains 2a as the dominant Pt-containing
species even at room temperature. 2a was characterized by multi-
nuclear NMR spectroscopy in solution using the 1:3 mixture of
1a and H2SiPh2 in deuterated toluene and THF. Although
the three phosphorus nuclei are magnetically equivalent at
−50 °C, cooling the solution to −90 °C causes a doublet at
δ −20.0 and a triplet at δ−22.3 at a 2:1 ratio (3JPP = 50 Hz),
as shown in Figure 2a. The former signal is ascribed to two
PMe3 ligands bonded to the Pt atoms, each of which is bonded
to three Si ligands, while the triplet is due to the ligand bonded
1
The H and 31P{1H} NMR spectra at −90 °C and the
29Si{1H} NMR spectrum at −10 °C contain a minor signal
that may be due to uncharacterized products of the reaction,
such as an isomer of 2a or an adduct of the diarylsilane to 1a
with different stoichiometries (1H: δ −6.4, 31P{1H}: δ −15.4,
29Si{1H}:δ 214). Raising temperature caused coalescence or
changes in the peaks of the H and 31P{1H} NMR spectra,
1
which rendered the minor signals invisible. The minor species
observed at the low temperature are involved in the reversible
reaction on the NMR time scale.
The fluxional process of the molecules is considered to
involve formation of a silyl ligand from a silylene and hydride
ligand, as shown in Scheme 4a. The reaction corresponds to
the isomerization of the molecule with structure in Scheme 3c
1
to the other Pt center. The H NMR spectrum of complex 2a
at −90 °C contains a signal corresponding to two hydrogens
Scheme 4
at δ −6.21, flanked by satellite peaks (JPtH = 520 Hz, Figure 2b).
2
The corresponding signal is also observed in the H NMR
spectrum of the mixture of 1a and excess D2SiPh2 at −10 °C
(δ −6.06). The signals are assigned to the hydride ligands on
the basis of the position of the peak at a high magnetic field,
and coupling with 29Si nuclei suggests interaction between the
H and Si atoms (Scheme 1b). ortho-Hydrogen signals appear
as four peaks with equal intensity in the range of δ 7.4−8.1 at
−90 °C, suggesting that the phenyl groups bonded to Si
atoms are not magnetically equivalent at this temperature. The
29Si{1H} NMR spectrum of 2a contains two signals at δ 291.7
(JPtSi = 981 Hz) and 253.6 (JPtSi = 583 Hz) at −10 °C. Since
data of the former are more similar to those of 1a (δ 279.4,
C
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2JPtP = 145 Hz, 3JPP = 47 Hz) and [{Pt(PMe3)3}(H)2{μ-Si(C6H4-4-
into Scheme 3b. The resultant silyl ligand will cause further
change of the structure of the complex.
2
3
Me)2}4] (2c, δ −22.8, JPtP = 3998 Hz, JPtP = 150 Hz, JPP
49 Hz). The reaction is summarized in eq 2.
=
We reported that Pt−Pd dinuclear complex with bridging Si
ligands exists as the molecule having silyl, silylene, and hydride
ligands at −60 °C, and is converted to the molecule having two
silyl ligands at 25 °C.10 The reaction involves a change in the
coordination mode of Si and H atoms to a Pt center from that
with two σ-bonds (Si−Pt and H−Pt) to the bonding between
Si, H, and Pt atoms, similar to the structural change of the
triplatinum complex shown in Scheme 4b.
Addition of H2Si(C6H4-4-F)2 to complex 1a forms a
complicated mixture of the Pt complexes, which suggests
reversibility of the reactions. Equilibrium constants for the
addition of diarylsilanes to the Pt3Si3 complex were determined
by comparing of the 1H NMR signals assigned to ortho-
hydrogens of 1a−c, diarylsilanes, and 2a−c, which have the
three bridging silylene ligands with the same aromatic groups.
Figure 3 shows van ’t Hoff plots of the reactions.
Scheme 5 summarizes an intramolecular mechanism for
exchange of the Si ligand. The complex with two hydride ligands
Scheme 5
Figure 3. van ’t Hoff plots for the addition of diarylsilanes to
complexes (i) 1a, (ii) 1b, and (iii) 1c. Hammett plots of the
thermodynamic parameters are shown as an inset.
is equilibrated to an intermediate, with a bridging SiHAr2 ligand
via formation of a Si−Ha bond and activation of Pt−Si and
Pt−Ha bonds (A). Rotation of the remaining Pt−Si bond to
form a new bond between the Si−Ha group and the remaining
Pt center produces intermediate (A′). The facile and reversible
intra- and intermolecular switching of the Pt−Si bonds occurs
over all the Si ligands. The NMR spectra of the solution
prepared by dissolving Pt3Si3 complex 1a and H2SiPh2 in 1:3
molar ratio suggested that the mixture is composed of Pt3Si4
complex 2a and unreacted H2SiPh2 below −30 °C.
Thermodynamic parameters are determined as ΔG° =
−8.0 kJ mol−1, ΔH° = −51.7 kJ mol−1, and ΔS° =
−146 J mol−1 K−1 for 2a formation; ΔG° = −10.0 kJ mol−1,
ΔH° = −47.5 kJ mol−1, and ΔS° = −126 J mol−1 K−1 for 2b
formation; and ΔG° = −5.9 kJ mol−1, ΔH° = −48.2 kJ mol−1,
and ΔS° = −142 J mol−1 K−1 for 2c formation. The reaction
enthalpy and entropy are comparable to those of oxidative
addition of H2SiPh2 to a mononuclear Ni(0) complex
with a diphosphinoborane ligand (ΔH° = −50 kJ mol−1,
ΔS° = −113 J mol−1 K−1) and of HSiMe3 with RuH(SiMe3)
(PMe3)4 (ΔH° = −46.0 kJ mol−1, ΔS° = −167 J mol−1 K−1).11
Hammett plots of the equilibrium constants of the reaction versus
σp suggest that the Pt3Si4 adduct is stabilized more significantly
by less electron-donating substituents on the aromatic group.
Catalysis. Generation of the Pt3Si4 complex having hydride
ligands from the reaction of H2SiPh2 with complex 1a prompted
us to conduct the study on the reactions using the triplatinum
complex as the catalyst. Hydrosilyation of aldehydes and ketones
was reported to be catalyzed by the complexes of transition
metals such as Rh,12 Cu,13 and Fe.14 Pt is known as the most
common and effective transition metal for catalysts of olefin
hydrosilyation.15 Although molecular Pt complexes and Pt metal
formed by reduction of them are often employed as the catalyst,
there have been only a few reports on the hydrosilyation of
carbonyl compounds using Pt catalyst. H2PtCl6 promotes
The 31P{1H} and 1H NMR spectra of the mixtures of
1a and excess H2SiPh2 in different concentration show similar
coalescence behavior at −30 and −90 °C (Figures S10 and S11).
This suggests that the pathway in Scheme 5 is operative to the
observed dynamic behavior of the NMR signals and that the
intermolecular exchange of the Si ligand occurs more slowly.
Concentration of the complex and diarylsilane influences both
equilibrium between Pt3Si3 and Pt3Si4 complexes and rate of
their mutual conversion, and detailed NMR studies did not
provide further information on this issue. Thus, we do not
have conclusion on whether the intramolecular process in
Scheme 5 is the sole one of the complex on the NMR time scale
or not.
The 31P{1H} NMR spectra of mixtures of [{Pt(PMe3)}3
{μ-Si(C6H4-4-F)2}3] (1b) and H2Si(C6H4-4-F)2 as well as that
of [{Pt(PMe3)}3{μ-Si(C6H4-4-Me)2}3] (1c) and H2Si(C6H4-
4-Me)2 also contain signals that can be assigned to [{Pt-
(PMe3)3}(H)2{μ-Si(C6H4-4-F)2}4] (2b, δ −23.3, JPtP = 3883 Hz,
D
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hydrosilyation of α,β-unsaturated aldehydes and ketones,
accompanied by isomerization, to produce the silyl enol ether.16
Complex 1a catalyzes hydrosilyation of benzaldehyde
derivatives with H2SiPh2 as the substrates. Benzyloxy(diaryl)-
silane compounds (3a) were obtained as the product in 98%
NMR yield (R = Me), as determined from eq 3.
complex composed of a trianglular Pt3Si4 core and two hydride
and three PMe3 ligands. The resulting complex has four bridging
silylene ligands in solution at a low temperature. Changes in
NMR signals as a function of the temperature are assigned to
the inter- and intramolecular switching of the Pt−Si bonds over
all the Si ligands. Hydride and silylene ligands are converted to
silyl (−SiHAr2) ligands easily and reversibly, which contributes
to the facilitation of fluxional behavior. The addition of H2SiAr2
to the Pt3Si3 complexes occurs reversibly, and the thermody-
namic parameters suggest that electron-withdrawing substitu-
ents on the aryl groups favor the Pt3Si4 adduct and thereby favor
its formation. [{Pt(PMe3)}3(μ-SiPh2)3] catalyzes hydrosilyation
of benzaldehyde and hydrosilyation/dehydrosilyation of methyl-
(phenyl)ketone to yield the expected products, although
previous reports on the Pt-catalyzed hydrosilyation of a CO
group are quite rare. The complex also promotes dehydrogen-
ative condensation of H2SiPh2 with phenol to form diphenyl-
(phenoxy)silane at room temperature. NMR analysis of the
reaction mixtures revealed that the catalyst preserves the cyclic
triplatinum structure during the reaction.
The reaction rate is affected significantly by the substituent
on the aryl group of the diarylsilylene ligand and aldehyde.
A similar hydrosilyation with H2Si(C6H4-4-F)2 catalyzed by 1b
proceeds slowly at room temperature to give the product (3b)
in a lower yield (32%). 4-(Trifluoromethyl)phenylaldehyde is
converted to the corresponding hydrosilyation product (4)
in 43% yield under the same conditions. The 31P{1H} NMR
spectrum of the reaction mixture after 8 h contains the signal
of 2 only, but after completion of the reaction (after 24 h),
the NMR signals corresponding to complex 1 and other
uncharacterized complex (δ 18.4) were observed.
EXPERIMENTAL SECTION
■
General Procedures. All manipulations were carried out using
standard Schlenk line techniques under an atmosphere of argon or
nitrogen or in a nitrogen-filled glovebox (Miwa MFG). Hexane,
toluene, and tetrahydrofuran (THF) were purified by using a Grubbs-
Complex 1a catalyzes hydrosilyation of methyl(phenyl)-
ketone and produces a mixture of the hydrosilyation product
(5, 20%) and that of dehydrosilyation (6, 45%), as shown in
eq 4. A longer reaction time (24 h) produced the hydrosilylated
type solvent purification system (Glass Contour).20 The H, H{1H},
13C{1H}, 29Si{1H}, and 31P{1H} NMR spectra were recorded on Bruker
Biospin Avance III 400 MHz and Avance III HD 500 MHz NMR
1
2
1
spectrometers. The chemical shifts in H and 13C{1H} NMR spectra
were referenced to the residual peaks of the solvents used.21 The peak
positions of the 29Si{1H} and 31P{1H} NMR spectra were referenced to
external SiMe4 (δ 0) and 85% H3PO4 (δ 0) in deuterated solvents.
Elemental analysis was performed using a J-science JM10 or a Yanaco
HSU-20 autorecorder. H2SiPh2 (Aldrich), D2SiPh2 (Aldrich), benzal-
dehyde (TCI), methyl(phenyl)ketone (TCI), and p-cresol (Kanto
Chemical) were purchased and used without further purification.
Complexes [{Pt(PMe3)}3(μ-SiAr2)3] (1a: Ar = Ph, 1b: Ar = C6H4-4-F)
were prepared according to the literature.3d H2Si(C6H4-4-F)2 was
synthesized from LiAlH4 reduction of HClSi(C6H4-4-F)2, which was
obtained by the coupling of HCl3Si (TCI) and the corresponding
Grignard reagent in 1:2 ratio. The H2Si(C6H4-4-Me)2 was obtained via
LiAlH4 reduction of Cl2Si(C6H4-4-Me)2 (Wako Chemical).
dimer (7) composed of a mixture of 5 and 6. The products are
similar to those obtained using other metal catalysis.17 Plausible
mechanisms of the hydrosilyation of ketones catalyzed by
mononuclear Rh complexes have been examined by several
research groups.18 The reactions mostly involve complexes with
silyl ligands as intermediates, although other intermediates having
silylene ligands were also proposed to explain the reaction
mechanisms.
Recently, dehydrogenative couplings of organosilane with
OH-containing compounds using several late transition metal
complexes have been reported.19 This is a thermodynamically
favored reaction, supported by formation of Si−O bonds with
high bond energy and enhanced by the catalysts that forms
silylmetal intermediates. Addition of phenol and H2SiPh2 to a
solution of a catalytic amount of 1a forms diphenyl(phenoxy)-
silane (8, 79%) in a high yield (eq 5).
Preparation of [{Pt(PMe3)}3{μ-Si(C6H4-4-Me)2}3] (1c). To a
toluene solution (6 mL) of [Pt(PMe3)4] (406 mg, 0.81 mmol) was
added H2Si(C6H4-4-Me)2 (190 mg, 0.89 mmol), and the reaction
mixture was stirred at 100 °C for 15 h. The solvent was removed under
reduced pressure to give a residue, which was washed with hexane
(3 mL × 2) and MeCN (2 mL × 3) and dried in vacuo to give 1c
(212 mg, 54%) as a red powder. The single crystals suitable for X-ray
crystallography were obtained by slow diffusion of the hexane/toluene
(5:1) solution at 0 °C. 1H NMR (400 MHz, C6D6, room temperature):
δ 8.22 (d, 12H, C6H4 ortho, 3JHH = 7.6 Hz), 7.14 (d, 12H, C6H4 meta,
3JHH = 7.6 Hz), 2.09 (s, 18H, C6H4CH3), 1.24 (br, 27H, PCH3).
13C{1H} NMR (126 MHz, C6D6, room temperature): δ 150.2 (C6H4
ipso), 137.7 (C6H4 para), 136.6 (C6H4 ortho), 129.2 (C6H4 meta),
22.0 (m, PCH3), 21.7 (C6H4CH3). 31P{1H} NMR (202 MHz, C6D6,
2
3
room temperature): δ 26.9 (JPtP = 2978 Hz, JPtP = 425 Hz, JPP
=
85 Hz). 29Si{1H} NMR (99 MHz, C6D6, room temperature): δ 280
(JPtSi = 938 Hz).
Preparation of [{Pt(PMe3)}3(H)2(μ-SiPh2)4] (2a). To a toluene
solution (2 mL) of 1a (102 mg, 75 μmol) was added excess H2SiPh2
(68 mg, 369 μmol), and the reaction mixture was stirred at room
temperature for 15 h. The solvent was removed under reduced pre-
ssure to give a red residue, which was washed with hexane (3 mL × 3)
and dried in a vacuo to give 2a (58 mg, 38 μmol, 51%). Dissolving
isolated 2a in toluene-d8 at room temperature caused an immediate
CONCLUSIONS
A triplatinum complex with bridging silylene ligands, [{Pt-
(PMe3)}3(μ-SiPh2)3], reacts with H2SiPh2 to produce the
■
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DOI: 10.1021/acs.organomet.7b00048
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
dissociation of H2SiPh2 from the Pt3 core, resulting in formation of 1a
and H2SiPh2 in 1:1 ratio. Therefore, recrystallization of 2a was carried
out by slow diffusion of hexane into the toluene solution containing
excess H2SiPh2, forming the red crystals in a few days at 0 °C. Anal.
Calcd for C70H69P3Pt3Si4 + toluene: C, 46.96; H, 4.47. Found: C,
46.79; H, 4.74.
1H NMR (400 MHz, C6D6, room temperature): δ 7.6 (m, 4H, C6H4),
7.29 (d, 2H, C6H4, JHH = 8.0 Hz), 7.2−6.9 (m, 8H, C6H4), 5.64
(s, 1H, SiH), 4.55 (s, 2H, CH2).
3
Hydrosilyation of Methyl(phenyl)ketone Catalyzed by 1a.
In a NMR tube with a septum cap to a C6D6 solution (0.6 mL) of 1a
(10 mg, 7.4 μmol) were added H2SiPh2 (31 mg, 0.17 mmol) and
CH3COC6H5 (19 mg, 0.16 mmol). The reaction was performed at
60 °C for 54 h to yield a mixture of hydrosilylated product 5 (20%),
dehydrosilylated product 6 (45%), and their coupling dimer, 7 (14%),
in NMR yields. The 1H NMR spectroscopic data of 5−7 were
NMR Characterization of 2a with H2SiPh2. In a J. Young NMR
tube to a toluene-d8 solution (0.6 mL) of 1a (5 mg, 4 μmol) was
added 3-fold H2SiPh2 (2 mg, 11 μmol). The multinuclear NMR
measurements were performed at 293 and 183 K. 1H NMR
(500 MHz, toluene-d8, 265 K): δ 7.66 (br, 16H, C6H5 ortho),
7.3−6.9 (24H, C6H5 meta and para), 1.22 (br, 27H, PCH3), −6.14
consistent with the literature.17 Data for 5: 1H NMR (400 MHz, C6D6,
3
room temperature): δ 5.67 (s, 1H, SiH), 4.95 (q, 1H, CH, JHH
=
2
3
1
(PtH, 2H, JPtH = 187 Hz, JPH = 4.5 Hz). 13C{1H} NMR (126 MHz,
6.4 Hz), 1.42 (d, 3H, CH3, JHH = 6.4 Hz). Data for 6: H NMR
(400 MHz, C6D6, room temperature): δ 5.89 (s, 1H, SiH), 4.89
(d, 1H, = CH2, JHH = 2.4 Hz), 4.65 (d, 1H, = CH2, JHH = 2.4 Hz).
THF-d8, 263 K): δ 151.5 (br, C6H5 ipso), 137.1 (C6H5 ortho), 128.2
(C6H5 para), 127.5 (C6H5 meta), 24.3 (PCH3). 31P{1H} NMR
2
2
2
1
(202 MHz, toluene-d8, 265 K): δ −22.1 (JPtP = 3893 Hz, JPtP
=
Data for 7: H NMR (400 MHz, C6D6, room temperature): δ 5.67
144 Hz, JPP = 53 Hz). 29Si{1H} NMR (99 MHz, THF-d8, 263 K): δ
291.7 (JPtSi = 981 Hz), 253.6 (JPtSi = 583 Hz). H NMR (400 MHz,
(s, 1H, SiH).
3
1
Dehydrocoupling of Phenol and H2SiPh2 Catalyzed by 2a.
In a NMR tube with a septum cap to a C6D6 solution (0.6 mL) of 1a
(11 mg, 8.1 μmol) were added H2SiPh2 (54 mg, 0.29 mmol) and
PhOH (14 mg, 0.15 mmol). The reaction was performed at room
temperature for 96 h to yield diphenyl(phenoxy)silane (8) in 79%
toluene-d8, 183 K):δ 8.11, 7.87, 7.60, 7.36 (br, 16H, C6H5 ortho), 7.3−
6.9 (m, 24H, C6H5 meta and para), 1.3−0.9 (br, 27H, PCH3), −6.21
(s, 2H, PtH, JPtH = 520 Hz). 31P{1H} NMR (161 MHz, toluene-d8,
3
183 K) δ −20.0 (d, JPtP = 3707 Hz, JPP = 50 Hz), −22.3 (t, JPtP
=
1
3
NMR yield. The H NMR spectroscopic data of 8 were consistent
4159 Hz, JPP = 50 Hz).
with the literature.19c 1H NMR (400 MHz, C6D6, room temperature):
δ 7.65 (m, 4H, C6H5), 7.2−7.1 (m, 7H, C6H5), 6.9−6.8 (m, 4H,
C6H5), 5.93 (s, 1H, SiH, JSiH = 218 Hz).
NMR Characterization of [{Pt(PMe3)}3(D)2(μ-SiPh2)4] (2a-d2).
In a J. Young NMR tube to a toluene-d8 solution (0.6 mL) of 1a
(10 mg, 8 μmol) was added D2SiPh2 (4 mg, 21 μmol) to form the
deuterated Pt3Si4 adduct 2a-d2 in a few minutes. 2H{1H} NMR
(77 MHz, toluene, 263 K): δ −6.06 (br, PtD).
X-ray Crystal Structure Analyses. Single crystals of 1c and 2a
suitable for X-ray diffraction were mounted on MicroMountsTM
(MiTeGen). The crystallographic data were collected on a Bruker
SMART APEX II ULTRA/CCD diffractometer equipped with mono-
chromated Mo Kα radiation (λ = 0.71073 Å) at 90 K. Calculations
were carried out using the program packages APEX II and OLEX2 for
Windows. The positional and thermal parameters of non-hydrogen
atoms were refined anisotropically on F2 by the full-matrix least-
squares method using SHELXL-2014. Hydrogen atoms were placed
at calculated positions and refined with a riding mode on their
corresponding carbon atoms. CCDC numbers 1526079 (1c) and
1526710 (2a) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
NMR Characterization of [{Pt(PMe3)}3(H)2{μ-Si(C6H4-4-F)2}4]
(2b). In a J. Young NMR tube to a toluene-d8 solution (0.6 mL) of 1b
(6 mg, 4 μmol) was added H2Si(C6H4-4-F)2 (3 mg, 14 μmol) at room
temperature to form [{Pt(PMe3)}3(H)2{μ-Si(C6H4-4-F)2}4] (2b).
1H NMR (500 MHz, toluene-d8, 293 K): δ 7.38 (br, 16H, C6H4 ortho),
δ 7.3−6.9 (16H, C6H4 meta), 1.09 (br, 27H, PCH3), −6.51 (PtH, 2H,
2
JPtH = 186 Hz, JPH = 5 Hz). 31P{1H} NMR (202 MHz, toluene-d8,
2
3
293 K): δ −23.3 (JPtP = 3883 Hz, JPtP = 145 Hz, JPP = 47 Hz).
NMR Characterization of [{Pt(PMe3)}3(H)2{μ-Si(C6H4-4-Me)2}4]
(2c). In a J. Young NMR tube to a toluene-d8 solution (0.6 mL) of 1c
(6 mg, 4 μmol) was added H2Si(C6H4-4-Me)2 (3 mg, 14 μmol).
1H NMR (500 MHz, toluene-d8, 293 K): δ 7.65 (d, 16H, C6H4 ortho),
7.2−6.9 (16H, C6H4 meta), 2.23 (s, 24H, C6H4CH3), 1.32 (br, 27H,
2
PCH3), −6.11 (PtH, 2H, JPtH = 182 Hz, JPH = 5 Hz). 31P{1H} NMR
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.organomet.7b00048.
■
(202 MHz, toluene-d8, 285 K): δ −22.8 (JPtP = 3998 Hz, 2JPtP = 150 Hz,
S
3JPP = 49 Hz).
Hydrosilyation of p-Tolualdehyde Catalyzed by 1a. In a
NMR tube with a septum cap to a C6D6 solution (0.6 mL) of 1a
(10 mg, 7.4 μmol) were added H2SiPh2 (33 mg, 0.18 mmol) and
p-CH3C6H4CHO (16 mg, 0.13 mmol). The reaction was performed
at room temperature for 24 h to form hydrosilylated products 3a in
98% NMR yield. The 31P NMR signal of 2a was observed during the
NMR spectra (1−8), X-ray structures and the data (1c,
2a), and optimized structure (2a) (PDF)
Cartesian coordinates of 2a and the model (PDF)
1
Crystallographic data of 1c and 2a (CIF)
catalytic reactions. The H NMR spectroscopic data of the product
was consistent with the literature.14c 1H NMR (400 MHz, C6D6, room
temperature): δ 7.67 (d, 4H, C6H5, 3JHH = 7.6 Hz), 7.21 (d, 2H, C6H4,
AUTHOR INFORMATION
Corresponding Author
*E-mail: kosakada@res.titech.ac.jp.
■
JHH = 7.6 Hz), 7.15−7.06 (m, 6H, C6H5), 6.98 (d, 2H, C6H4, JHH
7.6 Hz), 5.71 (s, 1H, SiH), 4.75 (s, 2H, CH2), 2.08 (s, 3H, CH3).
=
Hydrosilyation of p-Tolualdehyde Catalyzed by 1b. In a
NMR tube with a septum cap to a C6D6 solution (0.6 mL) of 1b
(10 mg, 7.1 μmol) were added H2Si(C6H4F-4)2 (36 mg, 0.16 mmol)
and p-CH3C6H4CHO (16 mg, 0.13 mmol). The reaction was
performed at room temperature for 24 h to form hydrosilylated
ORCID
Kohtaro Osakada: 0000-0003-0538-9978
Notes
The authors declare no competing financial interest.
1
products 3b in 32% NMR yield. H NMR (400 MHz, C6D6, room
temperature): δ 7.41 (dd, 4H, C6H4, J = 7.6, 8.4 Hz), 7.14 (app t, 4H,
C6H4, J = 7.6 Hz), 6.8−6.7 (m, 4H, C6H4), 5.55 (s, 1H, SiH), 4.68
(s, 2H, CH2), 2.12 (s, 3H, CH3).
Hydrosilyation of p-CF3C6H4CHO Catalyzed by 1a. In a NMR
tube with a septum cap to a C6D6 solution (0.6 mL) of 1a (10 mg,
7.4 μmol) were added H2SiPh2 (33 mg, 0.18 mmol) and p-CF3C6H4CHO
(16 mg, 0.13 mmol). The reaction was performed at room temper-
ature for 24 h to form hydrosilylated product 4 in 43% NMR yield.
The 31P NMR signal of 2a was observed during the catalytic reactions.
ACKNOWLEDGMENTS
■
This work was financially supported by Grants-in-Aid for
Scientific Research (B) (No. 24350027) and (C) (No.
25410061), for Challenging Exploratory Research (26620040),
and for Scientific Research on Innovative Areas, “New
Polymeric Materials Based on Element Blocks” (15H00725)
from Japan Society for Promotion of Science and from the
F
DOI: 10.1021/acs.organomet.7b00048
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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DOI: 10.1021/acs.organomet.7b00048
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