Efficient and selective alkene hydrosilation promoted by weak, double Si-H activation at an iron center

Article abstract of DOI:10.1039/d0sc01749c

Cationic iron complexes [Cp?(iPr2MeP)FeH2SiHR]+, generated and characterized in solution, are very efficient catalysts for the hydrosilation of terminal alkenes and internal alkynes by primary silanes at low catalyst loading (0.1 mol%) and ambient tempera

Full text of DOI:10.1039/d0sc01749c

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DOI: 10.1039/D0SC01749C
ARTICLE
Efficient and Selective Alkene Hydrosilation Promoted by
Weak, Double Si–H Activation at an Iron Center
Patrick W. Smith,a† Yuyang Dong,a† and T. Don Tilleya*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Cationic iron complexes [Cp*(iPr2MeP)FeH2SiHR]+, generated and characterized in solution, are very efficient catalysts for
the hydrosilation of terminal alkenes and internal alkynes by primary silanes at low catalyst loading (0.1 mol%) and ambient
temperature. These reactions yield only the corresponding secondary silane product, even with SiH4 as the substrate.
Mechanistic experiments and DFT calculations indicate that the high rate of hydrosilation is associated with an inherently
low barrier for dissociative silane exchange (product release).
Harrod mechanism, which can result in a mixture of linear and
branched products due to its reliance on reversible olefin
insertion into a metal hydride.
Introduction
Olefin hydrosilation is one of the most commercially important
reactions, used to produce various silicon-containing materials
and fine chemicals.1,2 As practiced, this reaction mainly employs
Pt-based catalysts3,4 which, given the limited supply and high
Another aspect of a silylene-based mechanism is the double Si–
H bond activation that occurs at the metal center.25,30 Double
Si–H activations of this type occur to varying degrees, from
moderate activation to give η3-silane complexes (A, Scheme
1),30–32 to more strongly activated cases that have undergone a
double oxidative addition to form a silylene dihydride (B,
C).26,33–35 Structures along this continuum exhibit high
electrophilicity at silicon and greater reactivity for the terminal
Si–H bond.
Since the silylene-based mechanism is well established for
ruthenium complexes of the type [Cp*(iPr3P)Ru(H)2SiHR]+,
catalytic transformations with analogous iron complexes could
provide increased activity due to the enhanced lability of metal-
ligand bonds in 3d complexes. This report describes the utility
of Cp*(iPr2MeP)FeH(N2)36 (1) as a precatalyst for alkene
hydrosilation via generation of a cationic iron intermediate. This
Fe system is associated with higher rates of conversion for many
substrates, and a marked selectivity toward terminal olefins
demand for platinum, creates
a strong motivation for
development of alternative catalysts based on more earth-
abundant metals such as Fe, Co, and Ni.5-22 There are profound
differences between the chemical properties of Pt and those of
the base metals; most notably the lighter 3d metals have a
higher propensity for one-electron reaction steps, which may
preclude the oxidative addition/reductive elimination cycles
associated with noble metal catalysis. For this reason, the
search for base metal hydrosilation catalysts must include
discovery of alternative mechanistic pathways suitable for the
3d metals.23
Previous work in this laboratory identified a new mechanism for
hydrosilation24,25 associated with cationic silylene complexes of
Ru24,26 and Ir.27 Unique to this mechanism is the Si–C bond
forming step, in which a Si–H bond in a silylene ligand directly
adds across the olefin.28,2 This process is attractive for the
design of new hydrosilation catalysts based on 3d metals such
as iron, as this critical bond formation does not rely on a metal-
centered redox process. Hydrosilation reactions catalyzed by
cationic silylene complexes form secondary silane products with
linear regioselectivity for the Si–H addition. This selectivity
represents an advantage over the more conventional Chalk-
a. Department of Chemistry, University of California, Berkeley, Berkeley, California
94720-1460, USA. E-mail: tdtilley@berkeley.edu
† Current addresses: P. W. S.: Department of Chemistry, Massachusetts Institute of
Technology, Cambridge, Massachusetts 02139, USA. Y.D.: Department of Chemistry
and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge,
Massachusetts 02138, USA.
Scheme 1. Representation of the cationic silylene mechanism for
hydrosilation with three selected resonance structures for the
cationic complexes.
Electronic Supplementary Information (ESI) available: Experimental details,
characterization
data,
and
computational
details
(PDF).
See
DOI: 10.1039/x0xx00000x
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ambient temperature.
that underscores fundamental mechanistic differences with the
related Ru catalyst.
View Article Online
DOI: 10.1039/D0SC01749C
Yielda
Silane
Product
Time (h)
> 98
6
Results and Discussion
Si
SiH3
a
b
c
H2
Precursors to the catalytic species of interest are neutral iron
complexes of the type Cp*(iPr2MeP)Fe(H)2SiH2R. Compound 1
reacts with primary silanes (RSiH3, R = Trip,37 Ph, p-Tol, Mes; Trip
= 2,4,6-triisopropylphenyl) by oxidative addition of an Si–H
> 98
20
Si
SiH3
SiH3
SiH3
SiH3
H2
> 98
20
Si
H2
bond
to
form
the
corresponding
dihydrides
0b
Cp*(iPr2MeP)Fe(H)2SiH2R (2R, eq 1). Note that related half-
sandwich complexes have been reported previously.36,38,39
These silyl dihydride complexes do not promote hydrosilations
up to 80 °C, but they are converted to active cationic catalysts
by hydride abstraction.
N. R.
20
d
e
> 98
20
Si
H2
a isolated yield. bsilane redistribution observed at 80 ˚C.
Table 2: Olefin and alkyne scope for hydrosilation by PhSiH3.
Conditions: fluorobenzene, 0.1 mol % 1, 0.1 mol % [Ph3C][BArF ],
4
ambient temperature.
The cationic complex [Cp*(iPr2MeP)FeH2SiHPh)][BArF ] (3Ph;
Yielda
4
Alkene/Alkyne
Product
Time (h)
6
BArF = tetrakis(pentafluorophenyl)borate) was generated by
4
treatment of 1 with 1000 equiv of PhSiH3 followed by 1 equiv
> 98
Si
H2
[Ph3C][BArF ] in fluorobenzene at -35 °C. After warming to
f
4
ambient temperature, treatment with 1100 equivs of 1-octene
resulted in quantitative conversion of the silane substrate over
6 h to the linear product Ph(nOct)SiH2 (eq 2), corresponding to a
turnover frequency (TOF) of at least 170 h-1. This selectivity
toward primary silanes is reminiscent of the cationic Ru silylene
hydrosilation catalyst, but other selectivities (vide infra) suggest
that there are fundamental differences in the mechanistic
pathways.24,26,28,29 For this reaction the iron catalyst is more
active, as the ruthenium catalyst required a higher loading (1
mol %) and elevated temperature (80 °C) to achieve a
comparable conversion.26 This activity is in the range of the
most active iron catalysts for the hydrosilation of olefins by
primary silanes (Table S1).22,40–43
95
(97)
0.5
20
Si
H2
g
h
97
(98)
Si
H2
0
0
N. R.
20
20
i
j
N. R.
92
(97)b
20
5
k
l
H2Si
87
(95)
Si
H2
R'
R
89
(97)c
SiMe3
20
Si
m
H2
0
0
N. R.
N. R.
20
20
n
o
Aryl- and alkyl-substituted primary silanes are suitable
substrates for this catalysis (Table 1), but steric factors play an
important role. While (o-ethylphenyl)silane is a competent
substrate, hydrosilation of 4-methylpentene with mesitylsilane
did not proceed even at 80 °C. No hydrosilation products were
observed with secondary or tertiary silanes (e.g. PhMeSiH2 and
Et3SiH) up to 1 mol% catalyst loading over 20 h at ambient
temperature. Unlike the Ru silylenes, the Fe catalysts are highly
selective toward hydrosilation of terminal olefins (Table 2). This
difference in substrate tolerance may arise from the size
difference between ruthenium and iron, resulting in greater
steric demands from the Fe ancillary ligands in the hydrosilation
transition state, and is most dramatically illustrated by the
H
aisolated yield (NMR yield) bdiastereomeric excess = 15% cR = Me,
R’ = SiMe3, 87.5 %; R = SiMe3, R’ = Me, 12.5 %.
hydrosilation of limonene (entry k) which resulted in complete
selectivity for the terminal double bond.
The iron catalysts selectively give cis–hydrosilation of internal
alkyne substrates (entries l, m; Table 2, by 1H NMR
spectroscopy), which is consistent with direct Si–H addition. No
conversion was observed for diphenylacetylene (entry n),
presumably due to steric factors, and further evidence that
catalyst selectivity is driven by steric factors is seen with
propynyltrimethylsilane as substrate, to predominantly form 2-
Table 1: Silane scope for hydrosilation of 4-methylpentene.
Conditions: fluorobenzene, 0.1 mol % 1, 0.1 mol % [Ph3C][BArF ],
4
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phenylsilyl-1-trimethylsilylpropene (~90% selectivity). The may be regarded as a silylene dihydride. To wit, 3RuPh is
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DOI: 10.1039/D0SC01749C
intolerance for the terminal acetylenic substrate (entry o) is computed to possess an extremely short Ru–Si distance (2.25 Å;
most likely due to side reactions with the acetylenic C–H bond. FSR = 0.93), Si–H(bridge) distances which are elongated relative to
The selectivity of this catalyst system suggested the use of SiH4 those in 3Ph (1.67 Å), and short Ru–H interactions (1.75 Å; FSR =
to form secondary silanes (eq 3). For this reaction, the catalyst 1.11).
was generated by reaction of 2Mes with [Ph3C][BArF ] in The selectivity of the Fe catalysts with respect to the olefin
4
fluorobenzene, which was then treated with 400 equivs of 4- substrates prompted an investigation into the hydrosilation
methylpentene and exposed to 1 atm of 15 % silane in nitrogen. mechanism. For [Cp*(iPr3P)RuH2SiHR]+, the rate-limiting step in
While the yield of (4-Me-pentyl)2SiH2 was only 2.6 % after 18 h catalysis is the dissociative exchange of the product silane with
(based on olefin; TON = 52), the reaction is highly selective in silane substrate.26 For the corresponding osmium complexes,
that only the secondary silane was observed as product (by 1H catalytic activity was not observed, even though stoichiometric
NMR spectroscopy and TLC). The low yield is attributed to the Si–H addition to olefins proceeds rapidly.24,44 This suggests that
low concentration of SiH4; higher pressures of silane and longer dissociative silane exchange is even less favorable for Os. In light
reaction times would likely give greater turnover.
of this trend toward lower barriers to silane exchange from Os
to Ru, a reasonable hypothesis is that the barrier to silane
exchange for Fe is even lower, to the extent that the rate of
direct Si–H addition to the olefin may become competitive. This
would explain both the increased activity of the Fe catalysts for
unhindered substrates and the sterically-based selectivity for
terminal olefins.
Attempts to isolate the activated catalysts as crystalline solids
were not successful; in all cases, crystallization attempts
produced impure oils, prompting characterization in situ by
NMR spectroscopy. Complex 2Tol was treated with 1 equiv of
[Ph3C][BArF ] at -35 °C in fluorobenzene in an attempt to
4
generate [Cp*(iPr2MeP)FeH2SiH(p-Tol)]+ (3Tol, eq 4), which
resulted in a color change to blue. Integration against a C6Me6
internal standard indicated that 3Tol was formed in > 99% yield.
1
The H NMR spectrum at room temperature did not contain
obvious Fe–H–Si or Si–H resonances, suggesting the possibility
of exchange between such positions. Upon cooling a solution of
3Tol in fluorobenzene-d5 to -30 °C, Fe–H (-15.12 ppm, JHP = 20.9
Hz, JSi(µ-H) = 90 Hz) and Si–H (6.74 ppm, JSiH = 180 Hz) resonances
were observed. These resonances display cross peaks to a
downfield 29Si resonance at 188 ppm in the HMBC spectrum
(JSi(µ-H) 90 Hz).
Computations on the structure of 3Ph were found to be highly
dependent upon the choice of functional, particularly with
respect to the extent of Si–H activation at the Fe center. The
experimentally determined structure of the related Ru complex,
[Cp*(iPr3P)Ru(H)2=Si(H)Mes]+, was used to calibrate the choice
of functional, and the range-separated-hybrid functional
ωB97X-D3 was found to be in best agreement with this
structure. For the Fe complex, calculations at the ωB97X-
D3/def2-TZVP/SVP level of theory are consistent with the
interpretation that Si–H activation at Fe is modest. The
geometry of the molecule resembles an η3-silane structure
(Figure 1), with the hydrides symmetrically bridging the Fe and
Si atoms and minimal Si–H activation relative to free silane (dFeSi
= 2.20 Å; FSR = 0.95). The Si–H(bridge) distances are slightly longer
than the terminal Si–H distance (1.59 vs. 1.47 Å) and the Fe–H
distances average to 1.68 Å (FSR = 1.14). The FeH2Si unit is
planar (Σ∠ = 358°). In contrast, [Cp*(iPr2MeP)RuH2SiHPh)] (3RuPh
)
is computed to have proceeded significantly further along the
double Si–H activation continuum, and toward a structure that
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This hypothesis is supported by computational investigations of
the catalytic cycle using geometries calculated at the ωB97X-
D3/def2-TZVP/SVP level of theory, with single point energies
corrected at the DLPNO-CCSD(T)/def2-TZVP level of theory. The
computed energy profile for the olefin addition (Figure 1)
indicates that the transition state for Si–H addition to the olefin
(TS) is at a higher energy than that for the Ru system (ΔG‡ = 19.1
kcal mol-1 for Fe vs. 11.7 kcal mol-1 for Ru; previous
computations indicated a 8–15 kcal mol-1 barrier for Ru).28,29
Importantly, this barrier height is comparable to that of silane
exchange (vide infra), and a consequence is that the rate-
determining step for catalysis is substrate dependent. Thus, in
at least some cases the Si–H addition is slow enough (e.g., due
to steric factors) to render this step rate-determining. This
substrate-dependent reactivity could explain the novel
selectivity of this Fe system.
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DOI: 10.1039/D0SC01749C
Notably, while the 3Ph ground state involves bridging Fe–H–Si
interactions, the Si–H interactions are considerably weaker in
transition state TSFe, manifested in elongation of the Si–HFe
interactions from 1.59 to 1.89 Å, and shortening of the Fe–H
(from 1.68 to 1.52 Å; ΔFSR = -0.11) and Fe–Si (from 2.20 to 2.15
Å; ΔFSR = -0.02) distances, consistent with generation of an
electrophilic silylene in this transition state by a "double α-
migration".45
Figure 2. Computed energy profile for the dissociation of silane
from 3Ph. The singlet pathway is shown in red, and the triplet
pathway is shown in blue. Top left: structure of the MECP
(minimum-energy crossing point). Bottom right: structure of the
triplet state of I. Both structures were computed at the ωB97X-
D3/def2-TZVP/SVP level of theory.
A dissociative mechanism for silane exchange is supported by
VT NMR spectroscopy. The complex [Cp*(iPr2MeP)FeH2SiH(p-
Tol)][BArF ] (3Tol) was generated in fluorobenzene solution in
4
the presence of 1 equiv of p-tolylsilane, and the tolyl CH3 and
silane SiH3 1H NMR resonances were monitored from 246 to 330
K. Above 290 K, the resonances attributed to free and bound p-
TolSiH3 broaden and coalesce, consistent with chemical
exchange between the two species. Rate constants for the
exchange between p-tolylsilane and 3Tol were determined
based on lineshape analyses of the p-CH3 resonances; an Eyring
plot gave the activation parameters ΔH‡ = 27.0(4) kcal mol-1 and
ΔS‡ = 34(1) cal mol-1 K-1 (ΔG‡295 = 17(2) kcal mol-1 at 298 K; see
Supporting Information). These activation parameters are
consistent with a dissociative exchange mechanism similar to
that proposed for the Ru silylene system (ΔH‡ = 32(2) kcal mol-1
and ΔS‡ = 19(1) cal mol-1 k-1, ΔG‡ = 26(2) kcal mol-1). The
295
somewhat lower enthalpy of activation for Fe likely reflects the
weaker bonding of the η3-H3SiR ligand to the metal, and the
lower activation of the Si–H bonds in the Fe system relative to
Ru.
Computationally, the dissociative mechanism for silane
exchange
involving
the
coordinatively
unsaturated
intermediate Cp*(iPr2MeP)Fe+ (I, Figure 2) is reasonable. This
structure includes an agostic interaction with one of the
phosphine iPr substituents, as evidenced by a close contact with
a C-bound H atom (dFeH = 1.943 Å). Additionally, the triplet state
for I is lower in energy than the singlet state by 12.5 kcal mol-1
suggesting a two-state mechanism for dissociative silane
exchange at Fe in this system. In the triplet structure, the Fe–H
distance in the agostic interaction is shorter (dFeH = 1.893 Å).
Notably, this situation is not observed computationally for
[Cp*(iPr2MeP)Ru]+, for which the triplet state is higher in energy
than the singlet state by 18.1 kcal mol-1. A minimum energy
crossing point (MECP) was found along the reaction coordinate
Figure 1. Computed energy profile for addition of propylene to
3Ph (blue) or 3RuPh (red). Bottom left: Computed structure of 3Ph
Top right: computed structure of TSFe.
.
4 | J. Name., 2012, 00, 1-3
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This work was funded by the National Science Foundation
for silane dissociation from 3Ph. Note that the MECP computed
using the ωB97X-D3 functional most likely does not reproduce
either the energy or geometry of the MECP well, as this
functional overstabilizes triplet states in both the present and
related systems.37 Thus, using the B3LYP functional (for which
the singlet-triplet gap of I agrees much better with DLPNO-
CCSD(T)) the MECP is found to be 4.6 kcal mol-1 above 3Ph, and
still lower in energy than the fully dissociated triplet species. In
this structure, the Fe–Si bond has lengthened by ca. 0.2 Å, and
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under grant no. CHE-1566538. The UC Berkeley Molecular
DOI: 10.1039/D0SC01749C
Graphics and Computation Facility is supported by the National
Institutes of Health under grant no. S10-OD023532. We thank
Boulder Scientific Co. for a generous donation of chemicals.
Notes and references
1
B. Marciniec, Catalysis of Hydrosilylation of Carbon-Carbon
Multiple Bonds: Recent Progress. Silicon Chem. 2003, 1, 155–
175.
i
one of the Pr-Me groups from the phosphine has approached
the Fe center.
While we were unable to fully optimize a transition state
structure for silane dissociation, nudged elastic band (NEB)
calculations of the silane dissociation coordinate indicate a very
late transition state for the triplet state of I (Table S4). Thus, the
energetics of this intermediate can be used as a proxy for
activation energies for silane dissociation; these energies are
summarized in Table S3. For dissociation to the triplet state of I,
ΔH= 22.7 kcal mol-1 and ΔSis extremely positive (52.7 cal mol K-
1). However, since the true transition state for dissociation likely
involves a closely-associated silane⋯I pair, all the translational
and rotational entropy from dissociation will not have
manifested in the Eyring analysis. Additionally, MECPs prior to
the transition state can result in inefficient spin crossover,
which should lead to a decrease in the rate relative to that of an
analogous one-state process. These factors would result in a
less positive experimental ΔS‡ in an Eyring analysis.
2
3
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4
5
6
7
8
9
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hydrosilation of unhindered olefins with primary silanes. This
discovery provides the foundation for new catalyst design
strategies involving the first-row transition metals. Importantly,
the mechanistic pathway for these catalysts is non-oxidative in
character, in that distinct oxidative additions are not required.
The observed rate accelerations and enhanced selectivities
relative to a related Ru system highlight the distinctly different
catalysis that is possible for the 3d metals. In this case,
mechanistic studies suggest that the promising characteristics
of the iron catalysts relate to a modest degree of Si–H bond
activation in the silane substrate, and therefore weaker metal-
silane binding and a faster dissociative exchange of the product
silane for the silane substrate. The lower cost and greater
sustainability of Fe makes the present system highly attractive
for the synthesis of unsymmetrical secondary silanes, and
multiply-substituted silicon centers derived therefrom.
and
Bis(Imino)Pyridine
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Dialkyl
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Conflicts of interest
There are no conflicts to declare.
17 I. Buslov, J. Becouse, S. Mazza, M. Montandon-Clerc, X. Hu,
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Acknowledgements
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