Cyclic metal(oid) clusters control platinum-catalysed hydrosilylation reactions: From soluble to zeolite and MOF catalysts

Article abstract of DOI:10.1039/d0sc02391d

The Pt-catalysed addition of silanes to functional groups such as alkenes, alkynes, carbonyls and alcohols, i.e. the hydrosilylation reaction, is a fundamental transformation in industrial and academic chemistry, often claimed as the most important application of Pt catalysts in solution. However, the exact nature of the Pt active species and its mechanism of action is not well understood yet, particularly regarding regioselectivity. Here, experimental and computational studies together with an ad hoc graphical method show that the hydroaddition of alkynes proceeds through Pt-Si-H clusters of 3-5 atoms (metal(oid) association) in parts per million amounts (ppm), which decrease the energy of the transition state and direct the regioselectivity of the reaction. Based on these findings, new extremely-active (ppm) microporous solid catalysts for the hydrosilylation of alkynes, alkenes and alcohols have been developed, paving the way for more environmentally-benign industrial applications. This journal is

Full text of DOI:10.1039/d0sc02391d

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EDGE ARTICLE
Cyclic Metal(oid) Clusters Control Platinum–Catalysed
Hydrosilylation Reactions: From Soluble to Zeolite and MOF
Catalysts
Received 00th January 20xx,
Accepted 00th January 20xx
Miguel Rivero–Crespo,a,b* Judit Oliver–Meseguer,a Klaudia Kapłońska,c Piotr Kuśtrowski,c Emilio
Pardo,d José Pedro Cerón–Carrascoe* and Antonio Leyva–Pérez.a*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The Pt–catalysed addition of silanes to functional groups such alkenes, alkynes, carbonyls and alcohols, i.e. the
hydrosilylation reaction, is a fundamental transformation in industrial and academic chemistry, often claimed as the most
important application of Pt catalysts in solution. However, the exact nature of the Pt active species and its mechanism of
action is not well understood yet, particularly regarding regioselectivity. Here, experimental and computational studies
together with an ad–hoc graphical method, show that the hydroaddition of alkynes proceeds through Pt–Si–H clusters of
3–5 atoms (metal(oid) association) in part–per–million amounts (ppm), which decrease the energy of the transition state
and direct the regioselectivity of the reaction. Based on these findings, new extremely–active (ppm) microporous solid
catalysts for the hydrosilylation of alkynes, alkenes and alcohols have been developed, paving the way for more
environmentally–benign industrial applications.
1. Introduction.
not remain in the final product, is not only a practical and
economical but also
concern.3
a
toxicological and environmental
The Pt–catalysed hydrosilylation of alkenes, alkynes, ketones
and alcohols, among others, is the method of choice to obtain
industrially–useful organosilanes, and unlike hydroborations or
hydrostannylations, only proceeds in the presence of a
catalyst.1–2 After 50 years of intensive research, Pt–based
catalysts are still the catalysts of choice in both Academia and
Industry, due to their inherent and extremely high catalytic
activity for this particular reaction, working in part–per–million
amounts (ppm), at room temperature and with good selectivity
in some cases. The fact that the soluble Pt catalyst is not
removed after the reaction and that the produced
organosilanes are present in industrial silicones in multi–tonnes
per year, has led to a worldwide spread of the inherently toxic
metal.3a Thus, the search for more benign hydrosilylation
catalysts, with similar catalytic activity to soluble Pt but that do
SiR'3
-Isomer
Si
Pt
R
R
R
Pt catalyst
R
Si
SiR'3
R
+
R'3Si
Confined space
Pt
-Isomer
-Isomer
HSiR'3
R
O
(common to carbonyl
R
OSiR'3
Pt
Pt
R
OH
and alcohols, latter with -H2)
R
R
R
R
R'3
R
R'3Si
Si
Pt
Pt Pt
Si
Pt1
vs.
and
H
Ptn
H
m-2
R
Si
R'3
H
R
R'3
H
TS-2
TS-1
Pt3,
SiH-Pt1-SiH,
Chalk-Harrod:
Highly strained
(this work)
Metal(oid) association relaxes the structure
Up to 90%
Up to 99%
SiR'3
R
-Isomer
-Isomer
R
R'3Si
a. Instituto de Tecnología Química. Universitat Politècnica de València–Consejo
Superior de Investigaciones Científicas. Avda. de los Naranjos s/n, 46022,
Valencia, Spain.
Figure 1. Pt–catalysed hydrosilylation of alkenes, alkynes, ketones
and alcohols. The inset shows the postulated Chalk–Harrod
intermediate and the regioselective TS–1 and TS–2 cyclic metal(oid)
intermediates proposed in this work, with the extension to confined
spaces in solid catalysts.
b.Current address: Department of Chemistry and Applied Biosciences. Swiss
Federal Institute of Technology (ETH) Zürich. Vladimir-Prelog-Weg 1-5/10. 8093
Zürich. Switzerland.
c. Faculty of Chemistry, Jagiellonian University. Gronostajowa 2, 30-387 Krakow.
Poland.
d.Instituto de Ciencia Molecular (ICMol), Universidad de Valencia, 46980 Paterna,
Valencia, Spain.
e. Reconocimiento y Encapsulación Molecular (REM), Universidad Católica de
Murcia (UCAM) (Spain).
Heterogeneous catalysis is considered a suitable solution when
soluble catalysts are toxic and not recyclable, and the more
direct and simple way to heterogenise a soluble catalyst is by
supporting it on a simple solid.4 However, examples of truly
heterogeneous Pt–supported solid catalysts for hydrosilylation
reactions, which mimic the extremely high catalytic activity of
* Corresponding author: miguel_angel.rivero_crespo@org.chem.ethz.ch,
jpceron@ucam.edu, anleyva@itq.upv.es. Phone: +34963877800; Fax: +349638
77809.
†Electronic Supplementary Information (ESI) available: General procedures,
additional Tables, Figures and Movies, and compound characterization. See
DOI: 10.1039/x0xx00000x
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Pt in solution, are difficult to find in the literature.5 The main The results in Figure 2 [(see also Figures S1–S3 in the Electronic
View Article Online
reason for this shortage of reported examples is that the exact Supplementary Information (ESI†)] showDthOeI: 1r0a.t1e03e9q/Du0aStCio0n2s39f1oDr
nature of the catalytically active Pt species formed in solution the hydrosilylation of phenylacetylene 1 with triethylsilane 2
during reaction, as well as the structure of key intermediates catalysed by either Pt1 or Pt3, obtained from the initial kinetic
during the different hydrosilylation reactions, remain mainly points, and which are r0=kexp[Pt1][1]-1[2]2 and r0=kexp[Pt3][1]-1[2],
unknown, beyond the postulated Chalk–Harrod intermediate respectively. In other words, the hydrosilylation reaction rate is,
shown in Figure 1.6 The extremely low amount of Pt required to in both cases, linearly proportional to the amount of Pt and
catalyse the hydrosilylation of alkenes and alkynes in solution is inversely proportional to the amount of alkyne but,
well below the detection limits for many experimental unexpectedly, it is quadratically proportional to the amount of
techniques, which has precluded conclusive studies on the Pt silane for Pt1 but not for Pt3. For the latter, saturation kinetics is
active species. If these catalytically active Pt reaction observed at high silane concentration ([2] > 0.50 M). Notice that
intermediates are properly unveiled, the design of new solid the main product for Pt1 is the typical β–(E) alkenylsilane,15 with
catalysts would be easier and more efficient.
>95% selectivity, while for Pt3 is the less common –
alkenylsilane,9,16 with >90% selectivity. However, the formation
of both products is here equally considered to assess the
catalytic activity of the different Pt catalysts.
Here, we show experimental and computational studies,
together with the development of an ad–hoc graphical method,
which support the association of silane molecules with Pt atoms
in solution, prior to or during the rate determining step (rds) of
the reaction, to generate 3–5 atom cyclic metal(oid) clusters
that facilitate the reaction by providing an alternative low–
energy pathway to the classical Chalk–Harrod mechanism. It is
also shown here how these cyclic metal(oid) clusters control the
electronics and sterics during the alkyne hydrosilylation and
governs the final chemo–, regio– and stereoselectivity of the
reaction, and with this information in hand, new and extremely
active soluble and solid–supported Pt catalysts can be designed
not only for the hydrosilylation of alkynes but also for the
hydrosilylation of alkenes and alcohols.
Figure 2. Top: Experimental kinetic equation for the Pt–catalysed
hydrosilylation reaction, notice that [silane] is powered to x. Bottom:
Reaction rate for the hydrosilylation of phenylacetylene 1 vs. [HSiEt3]
and [HSiEt3]X (X = 1 or 2) for either Pt1 (left) and Pt3 (right) at
increasing amounts of triethylsilane 2 maintaining [1] constant, with
Pt Kardstedt (2.5·10-5 M) and TEMPO (only in left graphic, 0.05 M);
toluene (1 mL), and 110 ºC reaction temperature.
2. Results and Discussion.
2.1.
Homogeneous catalysis.
2.1.1 Kinetic experiments and a graphical method unveil
metal(oid) cluster formation during the hydrosilylation of alkynes.
Ligand–free Pt1 and Pt3 were synthesised under typical reaction
conditions for the hydrosilylation of alkynes, and this reaction
was selected since, in contrast to alkenes, alkyne hydrosilylation
involves both regio– and stereoselectivity features.7,8 For the
study, the classical Karstedt’s hydrosilylation catalyst [Pt0–1,3–
divinyl–1,1,3,3–tetramethyldisiloxane] was treated with 10
mol% of 2,2,6,6–tetramethylpiperidine 1–oxyl (TEMPO) or not,
to give ligand–free Pt1 or Pt3, respectively, in >90% yield
according to combined mass spectroscopy and ultraviolet–
visible absorption and emission spectroscopy.9 This
methodology, validated by comparative studies with single–
atom supported Pt1 solids5b and preformed Pt3 Chini’s
clusters10,11 ([Pt3(CO)6]52-) in solution, at different temperatures,
enables to study the catalytic action of both Pt species during
the hydrosilylation reaction without the interference of any
ligand beyond the very same reactants,12 thus avoiding any
modification of the intrinsic catalytic activity associated to the
bare Pt atoms.13 As expected, bigger Pt clusters and
nanoparticles are not the catalytically active species during the
hydrosilylation reaction, since a very clear induction time can be
observed when using Pt NPs supported on carbon as
catalysts.9,14
A
Hammett
plot
for
different
para–substituted
dimethylphenylsilanes prepared by a reported method17 gives a
similar negative slope for both catalysts (Pt1 = -0.50, Pt3 = -
0.60, Figure S4), and the kinetic isotopic effect (KIE) for the
hydrosilylation of 1 with H(D)SiEt3 is 2.0 and 1.7 ± 0.2 for Pt1 and
Pt3 respectively (see ahead for a more precise value through the
graphical method), which confirm that positive charge is being
built and that the Si—H(D) bond is breaking before or during the
rate determining step of the hydrosilylation reaction, for both
Pt1 and Pt3 catalysts respectively. However, this catalytic
similarity between Pt1 and Pt3 is clearly broken when
considering the enthalpy and entropy values in the transition
state (H‡ and S‡). Table 1 shows the H‡ and S‡ values for the
hydrosilylation reaction of 1 with 2, calculated experimentally
with the Eyring–Polanyi equation, and the results show that the
Pt1 catalyst significantly decreases both the enthalpy and
entropy values (~50 KJ·mol-1 and -190 J·K-1·mol-1, respectively,
entry 1) respect to Pt3 (~110 KJ·mol-1 and -25 J·K-1·mol-1, entry
2), which raises the apparent contradiction that the transition
state for Pt1 is more associated than for Pt3. To assess the
influence of the alkyne and the silane in the H‡ and S‡ values
the study was expanded to 1–octyne and 1,1,3,3–
,
3
2 | Chem. Sc., 2020, 00, 1-3
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tetramethyldisiloxane (TMDS) 4, and the results (entries 3–6) in representing product concentration vs. the normalized time–
View Article Online
show that a different alkyne does not modify the energetics of scale f or g(t, T, H‡ or S‡). In this waDy,OtI:h1e0.1t0ra39n/sDit0iSoCn02s3t9a1tDe
the transition state while a different silane indeed does. These enthalpy H‡ or entropy value S‡ is easily obtained by
results fit with the different reaction order respect to Pt iterations with any available user graphic software (Excel®,
atomicity found for the silane but not for the alkyne, and Origin®,…) until all kinetic profiles overlay at different
suggest that not only the nature but also the number of silane temperatures, which can be done either just visually, adjusting
molecules are key in the transition state of the hydrosilylation to a polynomic equation until obtaining the best fit in terms of
reaction, at least with Pt1.
R2 or by minimizing the Euclidean distance between the data
points. The latter approach was used in this work (see ESI). This
methodology gives the activation parameters directly from raw
concentration data, avoiding the propagation of errors
associated to the initial rates, which can be as high as ±8% at
R2=0.997 for the classical method (Figure S6 and Tables S1–S5),
but only up to ±4% at nearly perfect fittings (R2>0.999) for the
new graphical method. Noteworthy, the graphical method
overrides the experimental distortions arising from induction
times, since these induction times are included in the whole
kinetic profile, which was validated for a simulated (Figures S7–
8 ad Tables S6–7) and a reported related reaction23 (Figure S9).
Table 1. Transition state enthalpy and entropy values for the
hydrosilylation of phenylacetylene 1 or 1–octyne 3 (0.5 M) with
triethylsilane 2 (0.6 M) or 1,1,3,3–tetramethyldisiloxane 4
(TMDS, 0.125M) catalysed by Pt1 and Pt3 (2.5·10-5 M). Reactions
carried out at temperatures of 80, 95, 110 and 125 ºC. In
brackets, values obtained by the graphical method.
Entry Alkyne
/ Silane
Pt
H‡ (KJ·mol-1)
S‡ (J·K-1·mol-1)
1
Pt1
Pt3
Pt1
Pt3
Pt1
Pt3
53  4 (52  2)
114  6 (109  2)
52  3 (54  3)
103  12 (123  4)
24  3 (38  2)
28  3 (41  2)
-175  10 (-156  6)
-2  16 (5  6)
1 / 2
2
퐵푇
‡
‡
‡
퐻 푅푇

푘(푇) = 푘 푒 푅푒 ―
풇 풕, 푻, 푯
푆
퐻
푅푇
‡
―

(
)
= 푡푇푒
3
-171  8 (-168  15)
22  33 (40  17)
-252  9 (-210  20)
-242  6 (-188  5)

ℎ
3 / 2
푇
푇
4
푇
‡
‡
‡
푆

푇
푇
푆
―
퐻
푘퐵
푇


푇
푅푇
푅푇
푅푇
푚
푚
(
)
=
푘
푚(푇) =
푚푒푇
푚푒
품 풕, 푻, 푺 ‡
푡푘
퐵푇
ℎ
푚푒푇
(
)

(
)
ℎ
5
1 / 4
6
In order to confirm these energetic differences, a more accurate
graphical method was developed, since the initial rates
approximation encounters some limitations in reactions with
induction times, catalyst deactivation, late steady state reach
and very fast rates,18 which can be the case here. In this context,
Reaction Progress Kinetic Analysis (RPKA)19 and Variable Time
Normalization Analysis (VTNA)20 have emerged in recent years
as powerful tools to measure reaction orders from catalysed
reactions with high precision in few experiments.21 However,
both RPKA and VTNA do not cover other kinetic and
thermodinamic parameters such as transition state enthalpy
and entropy, linear free energy relationships (LFER) and KIE
(Figure S5).22 Despite temperature scanning methodologies
have been recently developed to calculate energetic activation
parameters,22d this methodology is much more complex in term
of instrumentation and data treatment. For all these reasons,
and inspired in the simple graphical methodologies commented
above, a graphical method to calculate the H‡, S‡, Hammett
parameter () and KIE values, using the tools available in our
laboratory, was developed here. This new methodology does
not only allow to calculate with high precision the energetic
values in few experiments, but also propose a way to calculate
the errors associated to the measurements (see ESI for
discussion).
Figure 3. Top: Equations employed in the graphical method to obtain
the enthalpy and entropy transition state values H‡ and S‡.
Bottom: Example of transition state enthalpy (a) and entropy (b)
calculation using time–normalized scales f and g respectively for a
bimolecular reaction carried out at three different temperatures: 60,
80 and 100 ºC.
The results of H‡ and S‡ obtained by the graphical method are
shown in Table 1 (see also Figures S10–S15) and reinforce the
conclusions obtained with the traditional kinetic values, i.e. the
transition state for Pt1 is less energetic (lower H‡) and more
associated (lower S‡) than for Pt3. Figure 4 shows that the
graphical method can also be applied to the Hammett
parameter (, Figure S16 and Tables S8–S10) and KIE (see ESI for
discussion), which gives  = 0.50 for Pt1 catalyst and KIE =
1.91(5) for Pt3 catalyst, exactly the same value than for Pt1 but
with much more precision (see above and Table S11). These
results confirm the energetic and isotopic kinetic data obtained
for the hydrosilylation of alkynes with Pt1 and Pt3, and support
the hypothesis of metal(oid) Pt–silane cluster formation during
the transition state of the reaction.
Figure 3 illustrates the basis of our approach for a simulated
bimolecular reaction A+BP, i.e. the hydrosilylation reaction,
with the calculation of the enthalpy and entropy transition state
values (H‡ and S‡, see ESI for the mathematical model
development). The final methodology is very simple and consists
This journal is © The Royal Society of Chemistry 20xx
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Conversely,
a
rigid disilane molecule such as 1,4–
View Article Online
DOI: 10.1039/D0SC02391D
푘푋
푘퐻
bis(dimethylsilyl)benzene 5, bearing two Si–H moieties far away
from each other and incapable to form a 5–member ring, still
shows a quadratic [silane]2 dependence, as it occurs with
triethylsilane 2. Moreover, Figure 5 also shows that 2–
(dimethylsilyl)pyridine 6 acts as a functional ligand for Pt1,
placing a Si–H bond at the appropriate distance to form a
double five and six–member ring with the incoming silane and
showing a linear [silane] dependent rate, faster at low silane
concentrations than with naked Pt1 (Figure S17). Notice that the
hydrosilylation between the alkyne reagent and the ligand does
not occur, thus the ligand remains unaltered during reaction,
and that the highly coordinating nature of the pyridine moiety
gives a less active homogeneous catalyst than Pt1 at higher
concentrations. These results, together, strongly support that
the negative transition state entropy measured for Pt1
corresponds to a highly associative, cyclic metal(oid) transition
state such as TS–1, which directs to the anti–Markovnikov
hydrosilylation of alkynes and gives the β–(E) alkenylsilane
푋
(
)
= 푡푒
푙표푔
=
ℎ 푡,
,
푋
푋

 
(
)
푘퐻
푘퐷
= 퐾퐼퐸
푖(푡, 퐾퐼퐸) = 푡(퐾퐼퐸) ―1
(
)
a
b
KIE(IR) = 1.7 ± 0.2
KIE(GM) = 1.91 ± 0.05
0.5
0.5
H-Si
D-Si
H-Si
D-Si
[P]
[P]
0
0
i(t, KIE)
time (s)
0
50
0
50
Figure 4. Top: Equations employed in the graphical method to obtain
the Hammett value () and the KIE. Bottom: Kinetic profiles and KIE
calculated by a) initial rates (IR) method and b) graphical method
(GM), for the Pt3 catalyst.
2.1.2 Reactivity experiments supports a cyclic metal(oid) products.
cluster.
The observed change in the silane reaction order from +1 to +2
at low [silane] for Pt1, indicates that two silane molecules are
involved in the mechanism either in a separate, sequential or
cooperative manner, which clearly escapes from the Chalk–
Harrod mechanism, modified or not.24 To shed light on this, the
hydrosilylation of 3 with equimolecular amounts of HSiEt3 and
DSiPr3 was carried out, and the formation of a mixture of
deuterated and non–deuterated scrambled hydrosilylation
products, together with scrambled reactants (DSiEt3 and
HSiPr3), was observed by gas chromatography coupled to mass
spectrometry (GC–MS) in the first reaction minutes, which
discards that two separate Si–H units operate during reaction.
Two consecutive and reversible oxidative additions of the silane
over the Pt(0) atom is very unlikely since it would lead to a
highly energetic Pt(IV) intermediate. Considering that the
oxidative addition of non–polar bonds such as Si–H is accepted
to occur via concerted and not SN2 mechanism,25a a more
reasonable interpretation is the cooperative formation of a
transition state with two silane molecules bound to the Pt1
R
R'3
Si
Pt1
H
H
Si
R
R'3
SiH-Pt1-SiH
TS-1
a
atom, which is represented in Figure 5 (see also Figure 1). In this Figure 5. Top left: Proposed transition state TS–1 during the
cyclic structure, named TS–1, two silane molecules would form hydrosilylation of alkynes with Pt1 catalyst. Top right: Reaction rate
a five–member ring with the Pt1 atom to enable a cooperative for the hydrosilylation of phenylacetylene
1 (0.5 M) with
oxidative addition through a less strained transition state, which triethylsilane 2 using in–situ generated Pt1 from Kardstedt’s catalyst
resembles reported intermediates for the Pd–catalysed Wacker (2.5·10-5 M) without (a) or with silanepyridine 6 (b). Bottom: Reaction
oxidation25b and Au2–catalysed oxidative heteroarylations,25c rate for the hydrosilylation of phenylacetylene 1 (1M in toluene) with
the former with three water molecules forming a chain in–situ generated Pt1 from Kardstedt’s catalyst (2.5·10-5 M) at 110 ºC,
coordinated to Pd in the key oxypalladation step. TS–1 explains with respect to a) silane concentration and b) squared silane
why TMDS 4 decreases the activation energy (see Table 1), since concentration, with silanes 2, 4 and 5.
disilane 4 acts as a bidentate molecule to easily generate the
cyclic intermediate.26 If this is the case, 4 should have a first
The occupancy of the free coordinating sites in TS–1 by alkynes
order dependence in the rate equation, which is indeed
is supported by the negative dependence of the hydrosilylation
observed in Figure 5: the kinetic points for the hydrosilylation
rate with alkyne concentration, for both Pt1 and Pt3 catalysts,
of 1 with disilane 4 bearing two flexible Si–H moieties close to
which indicates that the saturation of the coordinative sites of
each other, adjusts much better to a first order [silane]
Pt with alkyne molecules occurs at the very beginning of the
catalytic cycle, in accordance with the alkynophilic nature of the
dependence, and reacts much faster than monosilane 2.
4 | Chem. Sc., 2020, 00, 1-3
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metal. This hypothesis is supported by the enhanced reaction
rate found for electron–rich alkynes (worse π–acceptors),
following the order: electron–deficient phenylacetylenes <
electron–rich phenylacetylenes < alkyl alkynes, assessed by the
negative slope in the Hammett plot (Figure S18), the decrease
in the reaction rate when phenylacetylene 1 is added during the
hydrosilylation of 1–octyne 3, but not the reverse, in
accordance with the stronger π–accepting ability of 1 (Figure
S19), and the lack of kinetic isotopic effect (KIE) found for
deuterated phenylacetylene (1.1 ± 0.1 for both Pt1 and Pt3),
which indicates that σ–Pt alkynylide species do not participate
during reaction. Thus, the incoming silane must displace
coordinating alkyne molecules in the inner sphere of the Pt
through σ–interaction,27 to give the key cyclic metalloid cluster.
View Article Online
DOI: 10.1039/D0SC02391D
Electronic effects
 
Pt
Pt
3
1 or
R1
R2
Same selectivity
(R1=R2= Aryl, alkyl)
R
R
R
SiR'3
Pt1
R
Pt
 
R1
R2
Pt Pt
R
Si
Pt3
R'3
H
R
SiR'3
(R1= Aryl, alkyl; R2= H, SiMe3)
TS-2
Pt3,
Steric effects (Pt3)
A four–member metal(oid) cyclic transition state may also be
involved for Pt3 clusters, in this case with just one silane
molecule, shown in Figure 6 as TS–2 (see also Figure 1). The
linearity of the rate equation with [2] (see above) supports this
intermediate. Furthermore, taking advantage of the different
regioselectivity found for Pt3 and that the regioselectivity comes
from the electronic effect produced by alkyne polarization, the
reactivity of different internal alkynes where the two carbon
atoms are electronically similar, was tested. Figure 6 also shows
that no α/β differentiation occurs between Pt1 and Pt3 catalysts
while, in contrast, an extremely polarized internal alkyne such
as trimethylsilylphenylacetylene 7, where the charge difference
between the two carbon atoms is comparable to a terminal
alkyne (Table S12), produces a change in the α/β isomer ratio
from 0.37 (Pt1) to 0.60 (Pt3). These results support that the
electronics of the alkyne is determinant for the regioselectivity
during the Pt3–catalysed hydrosilylation reaction, but not with
the Pt1 catalyst.28 Regarding sterics, electronically similar ortho–
and para–substituted phenylacetylenes show that the former
react lower with Pt1 clusters than with Pt3 (Figure 6 and S20–
21), and these results make sense considering the cyclic
metal(oid) transition state around Pt1 is more congested than
for Pt3, thus enhancing steric effects during reaction (Figure
S21).29 The so–called ortho effect, initially described for the
hydrostannylation of alkynes30 and associated with higher
differences in 13C NMR for internal and external carbon atoms,
X
X
Figure 6. Proposed transition state TS–2 during the hydrosilylation of
alkynes with Pt3 catalyst, and electronic and steric factors of the
alkyne affecting reaction regioselectivity for Pt3.
2.1.3 Computational studies support the energetic feasibility of
the cyclic metal(oid) cluster TS–1.
Aiming at further assessing the energetic preference for the
different transition states during the hydrosilylation reaction,
density functional theory (DFT) calculations have been
systematically conducted along the oxidative addition and
hydroplatination steps, for both the proposed metal(oid)
cluster TS–1 and Chalk–Harrod mechanism (TS–3). For the
records, the locations of both hydrogen and silicon atoms have
been monitored during the reaction with propyne, which is
used here as a minimal model system of alkyne moiety. Figure
7 summarizes the predicted energetic changes for both α and β
products, and the nature of the transition states is confirmed by
analysing the vibrational normal modes associated to the
reaction coordinate (movies S1–S4 in ESI).
cannot be invoked here since it is caused not only by electronic According to the calculations, the proposed TS–1 metal(oid)
but also by steric effects.31 Two–dimensional nuclear magnetic cluster decreases the barrier for the hydride addition
resonance, nuclear Overhauser effect spectroscopy (NOESY) of (intermediates [A]) in ca. 10 kcal/mol (black and blue lines)
the α–vinylsilane product, deuterated or not, shows that both respect to the standard Chalk–Harrod intermediate TS–3 (red
the anti– and the Markovnikov hydrosilylation with Pt1 and Pt3 and green lines), regardless the regiochemistry of the product.
proceeds completely in a syn–fashion (Figures S22–S24), which This stabilization comes from the significantly less constrained
is in accordance with the observed products and previous environment around the Pt atom when two silane molecules
studies on silylplatination and reductive elimination steps.32
bind and cooperate, compared to the canonical Chalk–Harrod
configuration. Furthermore, the smallest energy barrier for the
final silylation step (intermediates [B]) corresponds to the
formation of the β–isomer trough the TS–1 intermediate (black
line), thus the DFT calculations predict the right product after a
silane self–assisted hydroplatination process. All these
accumulated theoretical outcomes back up the hypothesis of a
cyclic metal(oid) cluster formation.
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account the extraordinary level of dilution of the ligand–free Pt
View Article Online
-5
DOI: 10.1039/D0SC02391D
catalyst during the hydrosilylation reaction (10 M), which
discards any of these techniques for reliable data. In any case,
the here proposed cyclic metal(oid) clusters are more
elaborated, precise and energetically–favored intermediate
species for the hydrosilylation of alkynes that the postulated,
canonical and highly strained 3–membered–ring transition
state in the Chalk–Harrod mechanism. Notice that ultrasmall Rh
clusters were early reported as active catalyst for the
hydrosilylation of alkynes,33 and that other noble metal clusters
(including Pt) can easily form under similar reaction conditions
and catalyse related reactions such as the hydrogenation of
alkenes,34 carbene–mediated couplings35 and carbon–carbon
cross–coupling reactions.14,36
Me
Me
Me
Me
H
H
H
H
A
Pt1
Pt1
Ptn
H
Ptn H
SiR'3
SiR'3
SiR'3
Me
SiR'3
Me
SiR'3
SiR'3
Me
Me
TS-1
TS-1
,
TS-3
,
,
TS-3
Chalk-Harrod,
hydride insertion
Chalk-Harrod,
hydride insertion
,
hydride insertion
hydride insertion
Me
Me
B
Me
Me
SiR'3
Pt1
Ptn SiR'3
L
SiR'3
Ptn SiR'3
L
Pt1
SiR'3
SiR'3
H
H
Me
Me
Me
Me
TS-1
,
TS-1
TS-3
silylation
,
Chalk-Harrod,
beta
,
TS-3
Chalk-Harrod,
alpha
,
beta
silylation
alpha
silylation
silylation
Chalk-Harrod (alpha)
Chalk-Harrod (beta)
TS-1 (alpha)
10
0
TS-1 (beta)
a) Proposed reaction mechanism
Pt precatalyst
A
1) Alkyne
coordination
-10
-20
-30
-40
m x
m
R
5) Reductive
elimination
SiR'3
-Isomer
R
R
R'3Si
R
2) Silane
coordination
HSiR'3
-Isomer
Ptn
B
R
2 x R
R
R
R
R
R
H
Reaction coordinate
Pt1
and
m-2
Pt
Pt Pt
R'3Si
H
Ptn
R'3Si
R
m-1
R
SiR'3
H
Hydroplatination
(Chalk-Harrod)
Figure 7. Computed energy profile for the Pt–catalysed Markovnikov
(alpha product) and classical anti–Markovnikov (beta product)
hydrosilylation of propyne. [A] refers to the hydride insertion step
and [B] to the silylation step (L refers to a propyne or solvent
molecule, not involved in the calculations). Red and green lines
illustrate the predicted reaction by using the standard Chalk–Harrod
mechanism (TS–3), while blue and black curves stand for the reaction
through two silane molecules bound to a single Pt1 atom (TS–1). In
all cases, both α and β products are simulated.
R
Silylplatination
Pt
Pt Pt
3) Oxidative
addition (rds)
Si
R'3
H
R
R
R'3Si
H
4) Migratory insertion
(regioselectivity)
Pt1
m-2
H
R'3Si
b) Proposed transition states for oxidative addition (step 3)
R
R
R
R
R'3
Si
H
Si
R'3
R
R'3Si
Pt
Pt Pt
Pt1
vs.
and
Ptn
H
R
m-2
H
R
Si
R'3
H
TS-1
Pt1,
TS-2
Pt3,
TS-3
, Chalk-Harrod
2.1.4 Proposed reaction mechanism and complete kinetic
equations for the Pt–catalysed hydrosilylation of alkynes
.
Figure 8. (a) Chalk–Harrod mechanism (n=1) and the alternative
mechanism found in this work (n = 1 and 3). (b) Transition states for
the oxidative addition proposed in this study.
Figure 8 shows a plausible reaction mechanism, on the basis of
the above experiments and compared with the classical Chalk–
Harrod mechanism. The first steps are similar to the accepted
Chalk–Harrod mechanism,1,7,24 i.e. 1) alkyne coordination to Pt,
2) silane coordination and 3) oxidative addition of the silane.
However, at this point, the metal(oid) cluster association
occurs, and the reaction follows a different reaction pathway
depending on the type of Pt–silane cluster formed. If two silane
molecules bind to a single Pt1 atom (TS–1), migratory insertion
through a silane self–assisted hydroplatination process is
favored and the anti–Markovnikov product is formed after
reductive elimination. In contrast, if one silane molecules bind
the Pt3 cluster (TS–2), the Markovnikov regioselectivity is
preferred by the cooperative dissociation of one silane
Taking into consideration the elementary steps of the
mechanism proposed in Figure 8, validated not only by
experimental but also by computational results and the
graphical method, a theoretical kinetic equation for the
Markovnikov hydrosilylation of alkynes catalysed by Pt1 can be
calculated, using the steady state approximation (Figure S25 for
mathematical development). With the appropriate
simplifications, the resulting calculated equation resembles the
obtained experimentally (vide supra), with the quadratic
[silane]2 dependence:
푘1푘2푘3
푟푎푡푒 푃푡1
=
[푆]2[푃푡0]
molecule on the Pt3 cluster, to give
a regioselective
2
(
)(
)
(
)
푘 ―1[퐴] + 푘1[푆] 푘 ―2[퐴] + 푘3 + 푘2 푘3푘5[퐴] + 푘1[푆] [푆]
silylplatination of the polarized terminal alkyne. Of course, a
direct visualization of TS–1 and TS–2 by experimental
techniques such as NMR or X–ray spectroscopy is not possible
and, whereas this visualization would be desirable for III–A and
III–B to confirm the proposed mechanism, it must be taken in
Which can be approximated at low [silane]:
풌ퟏ풌ퟐ풌ퟑ
)(
풓풂풕풆 푷풕ퟏ
=
[푺]ퟐ[푷풕ퟎ]
(
)
풌 ―ퟏ[푨] 풌 ―ퟐ[푨] + 풌ퟑ
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And the same applies for the Pt3 catalyst (Figure S26), which
gives the expected reaction orders after simplifications:
View Article Online
DOI: 10.1039/D0SC02391D
풌ퟏ풌ퟐ풌ퟑ
ퟎ
[
]
풓풂풕풆 푷풕ퟑ
=
[푺] 푷풕
풌ퟐ풌ퟑ + (풌 ―ퟐ + 풌ퟑ)(풌 ―ퟏ[푨] + 풌ퟏ[푺])
Therefore, the mechanism in Figure 8 is supported not only by
kinetic, isotopic, thermodynamic and computational data, but
also by the theoretical rate equations.
2.2.
Heterogeneous catalysis.
2.2.1 Hydrosilylation of alkynes.
Solid catalysts are preferred in industry because can be
easily separated from the reaction media and recycled, or
engineered in continuous processes. The cyclic metal(oid)
transition states of low entropy TS–1 and TS–2, with molecular
sizes of around 1–2 nm, are formed under very diluted reaction
conditions since the formation of >3 atom Pt aggregates is
somehow hampered, and intramolecular reactions are favored
vs. intermolecular ones. Thus, a solid catalyst able to
compartmentalize the active Pt species may mimic this catalytic
behavior, and microporous solids with nanometric channels and
cavities where Pt atoms can be accommodated, stabilized and
electronically tuned, such as zeolites and metal organic
frameworks (MOFs), are thus suitable candidates to design Pt–
supported catalysts for the hydrosilylation of alkynes. The
nanometric confined space of these microporous materials is a
convenient place to maximize the cyclic metal(oid) formation,
not only by the size but also by the intrinsic weak interactions
Figure 9. Top: Hydrosilylation of 1–octyne
3 (0.5 M) with
triethylsilane 2 (1.2 equivalents) in toluene (1 ml) at 110 ºC using
Pt1/NaY (A) and Pt3/NaY (B) as catalysts (2.5·10–4 M). Bottom: High
resolution transmission electron microscopy (HR–TEM) images
of Pt1/NaY catalyst A) fresh and B) reused 5 times.
of the metal and reactants with the framework walls, which Figure 9A also shows that the Pt1/NaY catalyst could be reused
assists to further reduce the entropy of the transition state.37 without any induction time (up to five times), without loss of
Recent protocols describe the precise incorporation of isolated activity and with just a small fraction of the confined Pt3
Pt1 atoms and Pt clusters within the channels of zeolites and aggregates being converted into nanoparticles (< 2 nm),40
MOFs, thus it is not necessary to develop new synthetic according to high resolution transmission electron microscopy
methodologies but just, perhaps, to apply and optimize known (HR–TEM) measurements (Figure 9, bottom). These results
materials.38 Thus, a 1 wt% of Pt single atoms and Pt3 clusters strongly support that the active Pt3 species are formed in–situ
were prepared inside the typical NaY zeolite by reported and stabilized within the zeolite framework, ready to be further
methods (Pt1/NaY38a and Pt3/NaY39 respectively), and the reused without leaching. In accordance with the leaching
kinetic results for the hydrosilylation of 1–octyne 3 with results, the preformed Pt3/NaY sample losses activity and still
triethylsilane 2 catalysed by these solid materials are shown in shows some induction time (Figure 9B).
Figure 9.
Figure 10 shows the results for the hydrosilylation reaction of
Both Pt1/NaY and Pt3/NaY, in ppm, give the –vinylsilane different alkynes and silanes catalysed by Pt1/NaY, and it can be
product, but while Pt1/NaY showed a long induction time (~25 seen that just 1 mg of the zeolite per mmol of alkyne (50 ppm
minutes) and no leaching of Pt into solution, as assessed by the of Pt) is enough to achieve full conversion and good selectivity
hot filtration test (Figure 9A) and ICP analysis (<1% of supported towards the corresponding –vinylsilanes, including aromatic
Pt into solution), Pt3/NaY showed, in contrast, a shorter (products 8a-c–10 and 12–13) and alkyl alkynes (products 11a-
induction and leaching of inactive Pt species to the solution b and 14–15), aromatic (products 8b-c, 11b, 12 and 15) and alkyl
(~10% measured by ICP), which evolve to active species after 50 silanes (products 8a–11a and 14–14), and also an internal
minutes (Figure 9B). In a similar way as Pt3 clusters are formed alkyne (product 14). Monosusbtituted phenylsilane PhSiH3
in–situ in solution from simple homogeneous precursors,9,16 we reacts selectively once with an aromatic alkyne to give the
reasoned here that Pt atoms inside Pt1/NaY could also evolve to corresponding silane 8c in reasonable 51% yield (the rest is 
form Pt3 clusters under reaction conditions, remaining within isomer and not polyalkylated silanes), and it does not react with
the zeolitic framework, while the pre–formed Pt3 clusters inside an alkyl alkyne (product 11c). The silane HSi(OEt)3 did not yield
the zeolite Pt3/NaY are less stabilized by the framework and the desired product neither, in any case, but rather
tend to leach out, which converts the Pt3/NaY just in a simple decomposition. The reaction can also be run at room
reservoir of Pt.
temperature, provided that more solid catalyst (20 mg per
mmol of alkyne) is used, and this result indicates that the active
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and zeolite–stabilized Pt3 species can be formed inside the Figure 11 shows the single crystal X–ray diffractogram of the
View Article Online
DOI: 10.1039/D0SC02391D
zeolite without thermal requirements. To our knowledge, this Pt1/MOF, where the single Pt atoms are marked with an
is the first efficient and recyclable solid catalyst for the arrow.38b As it can be seen, the MOF is constituted by ~1 nm
Markovnikov hydrosilylation of alkynes.41 Previous tests9 channels with homogeneously distributed isolated Pt atoms
showed that Pt/C does not really catalyse to the alpha product stabilized on the framework walls by electronic
but just generate species in solution with low alpha selectivity. interactions.35,38b,38d This structure resembles that expected in
Besides, not only the selectivity but also the activity is the cavities and channels of the zeolite, thus, it is not surprising
considerably higher in Pt/zeolites than in Pt/C. At this stage, and that the Pt1–supported microporous MOF catalyses the
in order to gain more information about the Pt species on the hydrosilylation of alkenes similarly or even better as Pt1/NaY
solid during reaction, a 195Pt solid NMR experiment was does. Figure 11 also shows the scope for the hydrosilylation of
envisaged, however, the acquisition time to have a reliable terminal alkenes catalysed by Pt1/MOF, and not only aromatic
result was estimated to be unacceptably long.
(products 18–23) but also alkyl alkenes (products 24–25)
engage well in the reaction. Different electron poor silanes are
reactive, including PhSiH3 as a selective monosilylation partner,
however, triethylsilane 2 was not suitable in this case.
Pt1/NaY (50 ppm Pt)
R
SiR'3
R1
R2
HSiR'3
+
R
+
R'3Si
-Isomer
Toluene, 110 ºC, 24 h
-Isomer
Table 2. Results for the hydrosilylation of styrene 16 (0.5 M)
with dimethylphenylsilane 17 (1.2 equivalents) to give (2–
phenylethyl)dimethylphenylsilane 18 with different soluble and
solid Pt catalysts (100 ppm); acac = acetylacetonate, COD =
cyclooctadiene. GC yields using n–dodecane as an external
standard. Only the  isomer product was found.
Cl
Me
R=Et, 82%a,b (3.5:1)c
8a
SiEt3
SiEt3
SiR'3
8b
8c
R=Me2Ph, 68% (3:1)
R=PhH2, 51%a,b (1:1)
10
91% (7:1)
9
86% (9:1)
11a
11b
11c
R=Et, 14% (1:6)
R=Me2Ph, 55% (1.5:1)
R=PhH2, <5%
MeO
SiMe2Bn
SiR'3
100 ppm of Pt
SiMe2Ph
+
HSiMe2Ph
12
52% (4:1)
18
16
Toluene, 110 ºC, 19 h
17
Et
O
HO
15
35% (1.4:1)
SiEt3
14
56% (1.5:1)
SiMe2Ph
Entry
Pt catalyst
Yield (%) TOF0 (h-1)
SiEt3
13
77% (5:1)
Karstedt
Pt(acac)2
86
75
14
89
90
6
455
393
73
1
2
3
4
5
6
7
8
9
Figure 10. Hydrosilylation of alkynes (0.5 M) with silanes (1.2
equivalents) in toluene (1 ml) at 110 ºC using Pt1/NaY (0.5 mg, 50
a
ppm in Pt) as a catalyst. 61% at 25 ºC reaction temperature with
Pt(NH3)4Cl2·xH2O
Pt(COD)(Me)2
Pt/C
b
twenty times more catalyst. GC yield of the  isomer after full
466
477
31
conversion, isolated yields can be achieved with the oxidation
method in 5–10% lower amounts. Not all products were isolated.
c  ratio at the maximum conversion measured.
PtCl2/SiO2
Pt1/NaY
96
100
100
514
453
526
2.2.2 Hydrosilylation of alkenes.
Pt1/MOF
The catalyst of choice in the literature and in Industry for the
hydrosilylation of alkenes is the same than for alkynes, i.e. the
Karstedt’s catalyst.42 Indeed, the Chalk–Harrod mechanism for
alkynes has just been accommodated from previous studies
with alkenes,7,24 thus, it may be expected that the solid Pt
catalysts shown here could be of applicability for the
hydrosilylation of alkenes, particularly Pt1/NaY.5b,43 Table 2
shows the results for the hydrosilylation of styrene 16 with
dimethylphenylsilane 17 with 100 ppm of different soluble and
solid Pt catalysts, and it can be seen that not only Pt1/NaY (entry
7) but also a recently reported Pt1/MOF solid catalyst38b (entry
8) gives the best yields of terminal silane product 18 with initial
turnover frequencies (TOF) comparable to the Karstedt´s
catalyst (entry 1). Notice that a solid silica catalyst with
supported PtCl2, without removing the chloride anions, gives a
marginal yield, while Pt/C gives good yields after long induction
times, in accordance with the literature.44
Pt1/NaY (calc. at 200 ºC)
Pt1/MOF (100 ppm of Pt)
HSiR'3
SiR'3
+
R
R
Toluene, 110 ºC, 24 h
SiMe2Ph
SiR'3
1a
21b
2
R= OMe 80%
R= Br 26%
R
18
19
20
;
;
R3 = Me2Ph
85%
29%
O2N
SiMe2OTMS
= Me2Bn
= Me2OTMS 66%
;
22
58%
H2N
SiMe2Bn
Pt1/MOF
SiMe2Ph
25
94%
n-Hex
SiPhH2
24
94%
23
87%
Figure 11. Single crystal X–ray diffractogram of the Pt1/MOF
catalyst (the arrow marks the Pt atoms, grey balls) and scope for
the hydrosilylation of terminal alkenes. NMR yields using
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dibromomethane as an internal standard. Only the  isomer with just 100 ppm of Pt in 1,4–dioxane as the solvent of choice.
View Article Online
DOI: 10.1039/D0SC02391D
product was found. Products were generally not isolated.
With this result in hand, different soluble and solid Pt catalysts
were tested, and the results are shown in Table 3.
Karstedt´s catalyst is the only efficient soluble Pt catalyst from
those tested (entries 1–4), and considering that Karstedt´s
catalyst is the only with Pt(0) atoms, and that Pt/C is also quite
effective (entry 5), it seems that Pt(0) could be the active redox
species for the dehydrogenative silylation of alcohols. This
hypothesis fits well with the studies above on alkynes and alkenes,
and also with the lower activity shown by PtCl2/SiO2 and Pt1/MOF
(entries 6–7) respect to Pt1/NaY (entry 8), which is able to easily
modulate the Pt site by the negatively–charged zeolite
framework. Indeed, Pt1/NaY calcined at 200 ºC, which tends to
reduce further the Pt atoms,38a gives a 99% yield of 27 and the
highest initial TOF (entry 9). Calcination at higher temperature
to provoke metal aggregation (entry 10),52 and decreasing or
increasing the electronic density of the zeolite with a different
charge compensating cation to Na+ (H+ and Cs+, entries 11 and
12 respectively),38a do not improve the catalytic activity and
supports the need of fine tuning the Pt site on truly
heterogeneous microporous solid catalysts for the
hydrosilylation reaction, when leaching does not occur (Figure
S30). These facts point again to an in–situ formation of the Pt
catalyst, and having in mind the H–atom bridges found in the
metal(oid) cluster intermediates and the decisive effect of the
zeolite confined space, it is possible to propose a tentative
mechanism for the dehydrogenative silylation of alcohols based
on metal(oid) cycles (Figure S31).53 Although speculative, this
mechanism should be taking into consideration for future
catalyst design based on Pt for the dehydrogenative
hydrosilylation of alcohols.
A hot filtration test for the hydrosilylation of styrene 16 with the
Pt1/MOF catalyst shows that the ~30% of the catalytic activity
comes from active species in solution (Figure S27, left),
however, most of the catalytic activity lies on the solid under
reaction conditions. Furthermore, when one looks the kinetic
profile closely, a short but clear induction time of 5 min can be
seen (Figure S28, right), an induction time that is not present at
all when the hydrosilylation of 16 is catalysed even by tiny
amounts of Karstedt´s catalyst in solution, according to our
experiments (Figure S29). The induction time with Pt1/MOF may
indicate that the initial Pt1 on the MOF is not the active species
and evolves to active Pt1 during reaction. Coulombic forces are
behind the stabilization of Pt on the negatively–charge walls of
the microporous solids, thus it could occur that subtle electronic
modifications of the Pt1 atom by donation of electron density
from the solid framework to the metal atom could modulate
and ultimately increase the catalytic activity.45 Indeed, by just
heating Pt1/NaY in an oven at 200 ºC for 1 hour before reaction,
the hydrosilylation of 16 proceeds with full conversion and with
the highest TOF measured (entry 9 in Table 2). X–ray
photoelectron spectroscopy (XPS) in combination with CO–
probe low temperature infrared (IR) measurements of the
Pt1/NaY sample calcined at 200 ºC show that the amount of
Pt(0) increases at least twice respect to Pt(II) in non–calcined
Pt1/NaY and Pt1/MOF (Figure S29). These results support the
formation of catalytically active, reduced Pt1 species within the
microporous catalyst. Notice that the Pt1 annotation used here
does not necessarily presuppose hardly reduced species, since
discussion focus on Pt atomicity rather than on the exact
valence state.
Table 3. Results for the dehydrogenative hydrosilylation of 1–
octanol 26 with dimethylphenylsilane 17 to give
octyldimethylphenylsiloxane 27 with different Pt catalysts (100
ppm); acac = acetylacetonate, COD = cyclooctadiene. GC yields
using n–dodecane as an external standard.
2.2.3 Dehydrogenative hydrosilylation of alcohols.
Both the hydrosilylation of carbonyl compounds (aldehydes and
ketones)46 and the dehydrogenative hydrosilylation of
alcohols47 lead to the same O–silylated products (see Figure 1),
which are classical functional groups to protect alcohols, carry
out additional manipulations in the molecule or finally recover
the alcohol functionality after deprotection, or even trigger
unexpected reactions.48 Besides, alcohols are common
precursors for carbonyl compounds (by oxidation) and are
widely available and stable compounds, particularly primary
alcohols. In view of this, we studied here the direct
hydrosilylation of alcohols and not the carbonyl hydrosilylation,
the former usually performed with other soluble catalysts
rather than Pt49 despite very early studies showed that the
Speier´s catalyst H2PtCl6·6H2O was catalytically active in
solution,50 and that simple metal nanoparticles can also catalyse
the reaction.47,51 First, the catalytic performance of the
Karstedt´s catalyst at ppm for the hydrosilylation of 1–octanol
26 with dimethylphenylsilane 17 was studied, and the results
(Table S13) show that, after optimization of reactant, solvent
and catalyst amount, reaction temperature and time,
octyldimethylphenylsiloxane 27 can be produced in 96% yield
17
HSiMe2Ph
, Pt (100 ppm)
OSiMe2Ph
OH
27
1,4-Dioxane (1M), 110 ºC, 22 h
(- H2)
26
Entry
Pt catalyst
Yield (%) TOF0 (h-1)
Karstedt
96
26
33
38
87
37
35
93
99
126
30
1
2
3
4
5
6
7
8
9
Pt(acac)2
Pt(NH3)4Cl2·xH2O
Pt(COD)(Me)2
Pt/C
37
43
99
PtCl2/SiO2
Pt1/MOF
42
42
106
118
Pt1/NaY
Pt1/NaY (calc. at 200 ºC)
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61
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only the hydrosilylation of alkynes but also of alkVeineweAsrtaicnledOntlhinee
dehydrogenative hydrosilylation of alDcoOhI:o1l0s.,103a9n/Dd0ScCa0n239b1De
Pt1/NaY (calc. at 300 ºC)
Pt1/HY
recycled. These results provide
a plausible alternative
Pt1/CsY
mechanism for the long–accepted Chalk–Harrod mechanism
and give tools to design new hydrosilylation reactions based on
ppm of Pt, of relevance in industrial chemistry.
Pt3/NaY
Figure 12 shows that the Pt1/NaY solid calcined at 200 ºC for 1
hour, catalyses the dehydrogenative hydrosilylation of alkyl
alcohols (products 28–32, 34 and 36), benzyl alcohols (product
33) and phenol derivatives (products 34 and 37–38) in good
yields and with just 100 ppm of Pt, and with different silanes.
Interestingly, the disilane H2SiPh2 yield only the monosilylated
product without further reaction, and the silane HSi(OEt)3 was
also reactive in this case, to give products 30 and 37,
respectively, in very good yields. To our knowledge, this is the
first Pt–based efficient solid catalyst for the dehydrogenative
hydrosilylation of alcohols.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
Financial support Financial support by MINECO through the
Severo Ochoa program (SEV–2016–0683), Excellence program
(CTQ 2017–86735–P, CTQ 2017–87974–R) and Retos Col. (RTC–
2017–6331–5) is acknowledged. E.P. acknowledges the financial
support of the European Research Council under the European
Union's Horizon 2020 research and innovation program / ERC
Grant Agreement No 814804, MOF–reactors. M.A.R.–C. thanks
ITQ for the concession of a contract and J.O.–M. thanks to the
Juan de la Cierva program for the concession of a contract. We
thank V.B.dLL. and N.G.G. for performing degree projects in the
ITQ labs. This research was partially supported by the
supercomputing infrastructure of Poznan Supercomputing
Center. The Electron Microscopy Service of the UPV is also
acknowledged.
Pt1/NaY (calc. 200 ºC, 100 ppm of Pt)
ROH
+
HSiR3
ROSiR3
1,4-Dioxane (1M), 110 ºC, 24 h, (- H2)
Me
OSiHPh2
Me
27
28
29
30
31
OSiR3
Me
32
82%
R3 = Me2Ph
= Me2Bn
96%
88%
Me
= Me2OTMS 93%
Pt Pt
Pt
= HPh2
= (OEt)3
87%
53%
OSiMe2OTMS
33
71%
OSiMe2Bn
OSiMe2Ph
35
int
Notes and references
Pt/NaY
O2N
69%
OSi(OEt)3
34
89%
1
a) B. M. Trost and Z. T. Ball, Synthesis, 2005, 6, 853–887. For
a book see b) B. Marciniec, H. Maciejewski, C. Pietrasuk and
P. Pawluc, Hydrosilylation: A Comprehensive review on
Recent Advances., Springer Netherlands, 2009.
OSiEt3
Me OSiMe2Ph
Me
Me
Me
Me
O
O
38
41%
37
90%
36
40%
2
For some recent works see a) P.–W. Long, X.–F. Bai, F. Ye, L.
Li, Z. Xu, K.–F. Yang, Y.–M. Cui, Z.–J. Zheng, L.–W. Xu, Adv.
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Xu, L.–W. Xu. Appl. Organomet. Chem. 2018, 32, e4037; d)
Y. Zhu, T. Cao, C. Cao, J. Luo, W. Chen, L. Zheng, J. Dong, J.
Zhang, Y. Han, Z. Li, C. Chen, Q. Peng, D. Wang, Y. Li, ACS
Catal. 2018, 8, 10004–10011; e) M. A. Guseva, D. A.
Alentiev, E. V. Bermesheva, I. A. Zamilatskovc, M. V.
Bermeshev, RSC Adv. 2019, 9, 33029–33037; f) L. Chen, I. S.
Ali, G. E. Sterbinsky, J. T. L. Gamler, S. E. Skrabalak, S. L. Tait.
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Figure 12. Schematic representation of the Pt1/NaY catalyst and
catalytic activity for the dehydrogenative hydrosilylation of
different alcohols with silanes under the indicated reaction
conditions. GC yields using n–dodecane as an external standard.
Products were generally not isolated.
Conclusions
The regioselectivity of the Pt–catalysed hydrosilylation of
alkynes is controlled by in–situ formed cyclic Pt–silane
metal(oid) clusters, and while the cluster formed by one Pt
atom and two silane molecules direct to the β–(E)–vinylsilane
product, the cluster formed by three Pt atoms and one silane
molecule directs to the α–vinylsilane product. These cyclic
unstrained transition states lower down the activation energies
of the reaction by facilitating the rate determining step of the
reaction, i.e. the oxidative addition of the silane. These Pt
intermediates are, in principle, suitable to be stabilized within
the nanometric confined space of zeolites and MOFs, and
indeed, the Pt–supported solids catalyse very efficiently not
3
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Chemical Science
View Article Online
DOI: 10.1039/D0SC02391D
Cyclic Metal(oid) Clusters Control Platinum–Catalysed Hydrosilylation
Reactions: From Soluble to Zeolite and MOF Catalysts
Miguel Rivero–Crespo,* Judit Oliver–Meseguer, Klaudia Kapłońska, Piotr Kuśtrowski, Emilio Pardo, José
Pedro Cerón–Carrasco* and Antonio Leyva–Pérez.*
TABLE OF CONTENTS
R
R'3
Si
Pt1
Confined
H
space
H
Si
R'3
R
and
Part-per-millions of Pt-supported
R
R
R
Zeolite Y and MOFs catalyse:
Pt
- Markovnikov hydrosilylation of alkynes
- Alkene hydrosilylation
- Dehydrogenative silylation of alcohols
Pt Pt
R
R'3Si
H
Metal(oid) association relaxes hydrosilylation intermediates versus the accepted highly-
strained Chalk-Harrod mechanism and enables solid catalyst design