Carbon supported hybrid catalysts for controlled product selectivity in the hydrosilylation of alkynes

Article abstract of DOI:10.1039/d0cy02136a

A series of Rh- and Ir-hybrid catalysts with varying tether lengths has been prepared by immobilization of Rh^I, Rh^IIIand Ir^IIIcomplexes on carbon blackviaradical grafting. The performance of the different catalysts was ass

Full text of DOI:10.1039/d0cy02136a

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Carbon supported hybrid catalysts for controlled
product selectivity in the hydrosilylation of alkynes†
Cite this: Catal. Sci. Technol., 2021,
11, 1888
ab
c
b
Max Roemer,
Indrek Pernik,
*
Vinicius R. Gonçales,
Sinead T. Keaveney,
a
c
d
Jiaxin Lian, James Downes,
c
and Barbara A. Messerle *bc
J. Justin Gooding
A series of Rh- and Ir-hybrid catalysts with varying tether lengths has been prepared by immobilization of
I
III
III
Rh , Rh and Ir complexes on carbon black via radical grafting. The performance of the different catalysts
was assessed for the hydrosilylation of phenylacetylene with Et SiH. The efficiency of the catalysts was
3
III
III
dependent on the length of the tethers to the surface. The Rh - and Ir hybrids afforded the β(Z)-
III
vinylsilanes, as observed for the analogous homogeneous Rh catalyst. No distinct product selectivity was
I
III
Received 3rd November 2020,
Accepted 30th December 2020
observed when using the homogeneous Rh precursors as catalysts. However, on using the Rh hybrid
I
catalysts derived from the Rh precursors to promote hydrosilylation, the major products were the
α-vinylsilanes and the origin of the difference in reactivity was found to be a chemical modification of the
catalysts during immobilization. Substrate scope is demonstrated for a number of alkynes, and feasible
mechanisms supported by DFT calculations are proposed.
DOI: 10.1039/d0cy02136a
rsc.li/catalysis
interest are carbon supports such as graphene, glassy carbon
and carbon black (CB), as they are relatively inert supports
that are suitable for catalyst immobilization.
Introduction
5
–9
Surface immobilized organometallic catalysts, viz. hybrid
catalysts, are a young class of materials with potential for
Rh- and Ir based hybrid catalysts on carbon supports have
7
application
in
industrially
that combine
important
the
chemical
of
been applied in the promotion of hydroamination, hydrogen
9
6,10
transformations
advantages
borrowing and hydrosilylation reactions.
Hydrosilylation
homogeneous and heterogeneous catalysts. A major challenge
of homogeneous catalysis is the ready separation of the
of alkynes is an excellent way to gain access to organosilicon
compounds, which are important synthons in organic
catalyst from the product and hence the recyclability of the
chemistry and industrially highly relevant due to stability and
1
11
catalyst.
While these challenges are addressed by
ease of modification, low cost and toxicity.
However,
2
heterogeneous catalysts, fine tuning the reactivity is difficult.
Hybrid catalysts aim to combine the advantages of
homogeneous and heterogeneous catalysts being comprised
of typically soluble homogeneous catalysts immobilized on
gaining control over the distribution of formed isomers can
be challenging, particularly towards the α-isomer. Typically,
this has been achieved in homogeneous catalysis by tailoring
the interplay between metal, ligand and substrate, altering
3
,4
11–18
insoluble supports.
This immobilization renders the
the accessibility of intermediates in the catalytic cycle.
catalyst insoluble, making removal and recycling
straightforward while reactivity is maintained. Of particular
Further recent examples include a silica supported Rh
1
9
20
catalyst and Pt-clusters. Supporting catalysts on surfaces
potentially adds another dimension to controlling the
catalysis outcomes.
a
School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia.
Packing and headgroup orientation are well known to be
influenced by the alkyl chain length in monolayer systems of
inorganic compounds linked to surfaces, affecting their
E-mail: max.roemer@sydney.edu.au
b
Department of Molecular Sciences, Macquarie University, Sydney, NSW 2109,
Australia. E-mail: barbara.messerle@mq.edu.au
2
1,22
c
physical properties.
This in turn could influence the
School of Chemistry and the Australian Centre for NanoMedicine, The University
of New South Wales, Sydney, NSW 2052, Australia
outcomes of catalysis when using surface immobilized
catalysts, both in efficiency and/or selectivity. We were
interested in establishing the dependence of catalyst
structure on the performance of organometallic catalysts that
comprise Rh- and Ir-complexes tethered covalently to CB via
linkers. We were particularly interested in understanding
d
Department of Physics and Astronomy, Macquarie University, Sydney, NSW 2109,
Australia
†
Electronic supplementary information (ESI) available: Experimental details,
spectral and catalysis data (PDF), crystallographic data in cif-format for 3a, 6a
and 6b. CCDC: 1961819–1961821. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/d0cy02136a
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relative efficiency, depending on the linker length, and
stereoselectivity in the promotion of the hydrosilylation of
phenylacetylene and diphenylacetylene, both important
model reactions (Fig. 1).
Here, we present a series of Rh- and Ir-hybrid catalysts
attached to carbon via alkyl linkers where the length of the
linker was varied to modulate the packing of the layer, the
flexibility of the headgroup and the catalyst–surface distance,
with the aim of optimizing the catalytic efficiency. We
investigated the efficiency and selectivity of the catalysts for
the hydrosilylation of a series of alkynes. Intriguingly, the
I
catalysts derived from the Rh precursors afforded a quite
different selectivity following surface immobilization by the
well-established diazonium route to both that of the
I
III
homogeneous Rh precursors, and to that of the Rh - and
III
Scheme 1 Syntheses of ligands 6a/b and their molecular structures
obtained from X-ray single crystal diffraction (H-atoms are omitted for
clarity).
Ir catalysts. This dichotomy led us to investigate the active
species in more detail. In order to understand the
relationship between hybrid catalyst structure and catalytic
reactivity, we employed X-ray absorption spectroscopy (XAS)
and conducted a number of control experiments, and
using an Ir-catalyzed ring opening reaction has been reported
2
5
propose
feasible
reaction
mechanisms
for
the
just recently, while no procedure for the synthesis of 3b has
been reported thus far. Ligands 6a/b, prepared in three steps
from 3a/b, were used to synthesise the new Rh- and Ir-
catalysts, following established procedures (Scheme S1,
transformations, supported by DFT calculations.
Results and discussion
Syntheses of ligands and metal complexes
2
6
ESI†).
The ligands were reacted with [Rh(CO) Cl] , [RhCp*Cl ]
2 2
2
2
F
We prepared the catalysts by coordination of metal
precursors with pyrazole–triazole (PyT) ligands. Ligands with
different length alkyl chain linkers were obtained in several
steps from compound 1 (Scheme 1). Tin mediated Friedel–
2 2 4
or [IrCp*Cl ] , followed by chloride abstraction with NaBAr .
Rh C and Ir Cp*C were prepared in an analogous fashion.
We furthermore prepared two control compounds, Rh C Ph
and Rh Cp*C Ph, which do not bear the terminal NH -
I
III
7
0
0
I
0
III
0
2
2
6,27
Crafts acylation of
2
gives access to the p-bromo-
group.
alkylbenzenes 3a/b which are challenging to obtain by other
methods. This approach is analogous to that used in the
preparation of 1′-alkyl-substituted iodoferrocenes.
Surprisingly, many halobenzenes with long functionalized
alkyl chains have not yet been prepared. The synthesis of 3a
2
3,24
Surface immobilization and characterization
We immobilized the complexes on CB by in situ radical
6
,7
generation from the anilines
and formation of covalent
C–C bonds to the CB support (Fig. 2). The immobilization via
binding of a phenyl radical to the carbon support, generated
2
from a terminal aniline by treatment with HCl/NaNO ,
I
resulted for the Rh precursors simultaneously in surface
attachment and in situ generation of a modified catalyst
III
head-group by the generation of a Rh species with a
different ligand environment. Our results also show that the
III
III
Rh
following immobilization.
We initially varied the quantity of Rh C and Rh C
0
and Ir
catalyst head-groups remain unchanged
I
I
1
1
precursors with the goal to optimize the surface coverages/
III
metal loadings on the hybrid catalysts, Rh C
0
CB and
III
Rh C CB. In each case, the ratio of complex to CB was
1
1
−
1
varied from 0.07 to 1.4 μmol mg . The samples were
analysed by energy dispersive X-ray spectroscopy (EDX) and
X-ray photoelectron spectroscopy (XPS). Both characterization
methods show an increase in metal concentration on the
surface in accordance with increasing amounts of complex
added to the grafting mixture (Fig. 3 and S19–S22, ESI†). The
Fig. 1 Control over regio- and stereoselectivity in hydrosilylation
reactions of terminal alkynes using Rh and Ir based hybrid catalysts.
III
Rh loadings for Rh C CB were slightly higher compared to
0
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III
III
Fig. 2 a) Surface immobilization of Ir and Rh complexes groups affording hybrid catalysts with pyrazole–triazole (PyT)/Cp*/Cl head groups,
III
III
I
abbreviated Ir Cp* and Rh Cp*. The inset illustrates the diazonium grafting. b) Surface immobilization of Rh complexes groups affording hybrid
catalysts with a modified head group bearing two PyT groups and two co-ligands. The control compounds in panel a and b bear the same Rh
catalyst function, but no aniline which would generate a radical under the grafting conditions.
III
those of Rh C CB (Fig. 3), which is likely due to the relative
bind more efficiently as they can access more available sites
while larger flexible alkyl chains may also result in bent
structures. Furthermore, SEM/EDX shows a homogeneous
distribution of Rh on the surface for each sample. The
optimization approach indicates that using ∼0.7 μmol of
catalyst per mg of CB gives a layer that is near to the
1
1
size and structure of the molecules. Smaller molecules may
III
maximum surface coverage, i.e. 0.96 at% Rh for Rh C CB
1
1
III
0
and 1.22 at% Rh for Rh C CB (XPS).
We considered this to be the optimal loading and applied
the same procedure for immobilization of all other catalysts,
which were characterized in an analogous fashion (ESI,†
III
III
n
Table S3). The Rh Cp*C11- and Ir Cp*C -hybrid catalysts
III
showed consistently lower loadings than the Rh C CB
n
III
catalysts, i.e. 0.6 at% (XPS) for Rh Cp*C11CB and 0.2–0.3
III
n
at% (XPS) for Ir Cp*C CB (n = 0, 6, 11), which is in
III
7
agreement with a previous example of Ir Cp*C CB. The XPS
0
survey scans show the expected signals of complexes and CB.
The Rh : N ratios were near to 1 : 5, which agrees with the
F
4
molecular structures of the precursors. The BAr anion bears
24 F and thus a Rh : F ratio of 1 : 24 was expected, however,
the detected ratio was typically 1 : 3 or less.
Additionally, Cl signals were present in the XPS and EDX
for all samples. The presence of Cl signals was not expected
III
I
for the Rh C
n
CB hybrid catalysts as the Rh precursors do
not contain Cl. The detected Rh : Cl ratios were ∼2 : 1 (EDX)
III
III
Fig. 3 Rh loadings for Rh C11CB and Rh C
0
CB determined by EDX
III
and ∼1.3 : 1 (XPS) for the different Rh C
n
CB catalysts. On
and XPS for varying amounts of catalyst applied for immobilization.
The error bars represent the standard deviation of a minimum of five
different areas analyzed on a single surface. The lines serve as guides
treating a control sample of blank CB with HCl under the
applied grafting conditions, no significant Cl content was
III
−
for the eye only. SEM image and EDX maps (C, Rh, Cl) of Rh C11CB.
detected (ESI,† S25) indicating that any Cl is bound to the
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2
immobilized molecules. This suggests that catalyst structure
may have been altered, and hence further characterization
was warranted.
To probe the local electronic structure of the metals, we
recorded extended X-ray absorption fine structure spectra
Fourier transformed EXAFS data (k weighted) and
corresponding fitted curves obtained using the known X-ray
structures of the PyT head-groups and fitting parameters for
all homogeneous- and hybrid catalysts and are reported in
the SI (Fig. S24–S27, Tables S25–S28†). In particular, the
levels of correlation between the curves for the M Cp*C CB
n
III
(EXAFS) at the Rh K edge for the Rh based materials and at
the Ir L edge for the Ir based materials (Fig. 4). Comparison
derivatives are high with only little difference between the
3
of the X-ray absorption near edge (XANES) regions of the
curves obtained for the complexes before and after
I
I
spectra for the compounds Rh C
n
(n = 0, 6, 11; before
0, 6, 11, after
immobilization. On the other hand the fits for Rh C
n
and
III
III
immobilization), and Rh C CB (n
=
Rh C CB (n = 0, 6, 11) exhibited a considerable amount of
n
n
immobilization on CB), indicates a change in oxidation state
after immobilization (Fig. 4a). The energy of the Rh K edge
for the Rh Cn (n = 0, 6, 11) series is at 23.242 keV, in
misfit to the calculated data from X-ray crystallography of
Rh (CO) PyT. This was unsurprising for Rh C CB as the
2 n
I
III
I
oxidation state changed from Rh(I) to Rh(III) after
immobilization and the coordination sphere around the
metal centre changed too. However, for the homogeneous
I
28
agreement with other reported Rh examples. However, for
III
Rh C
23.250 keV). This effect is consistent across the whole series
of different chain lengths and points towards an increase in
n
CB (n = 0, 6, 11), the edge is shifted to a higher energy
I
I
(
Rh complexes (i.e. Rh C ) this was not expected as the
n
structure of these complexes was confirmed using NMR
spectroscopy, mass spectrometry and elemental analysis. We
postulate that the observed misfit originates mainly in a non-
physical interatomic distance signal at approximately 1 Å,
which does not correspond to a bond length (Fig. 4a, inset;
red traces). Overall, EXAFS supports our structural proposals
for the herein described catalysts.
I
III
n
oxidation state. Strikingly, when comparing Rh C and Rh -
CnCB (n = 0, 6, 11), not only the edge shifts about +0.008 keV
after immobilization, but nearly all features of the spectra
change. This change is also reflected in the radial
III
distribution (Fig. 4a, inset). In contrast, the EXAFS of Rh
III
and Ir examples did not show any significant edge shifts
when comparing data from before and after immobilization
(
Fig. 4b and c, insets). The Rh K edge was for both
III
III
Control experiments
Rh Cp*C11 and Rh Cp*C11CB at 23.244 keV while the Ir L
edge was for Ir Cp*C
1
3
III
III
n
and Ir Cp*C
n
CB (n = 0, 6, 11) at
To test the integrity of the hybrid complexes formed on using
I
1.218 keV, with no observed changes in radial distribution.
We also conducted analyses of the involved scattering
the Rh precursor catalysts, we conducted a series of control
I
experiments. Specifically, we treated the Rh precursors as if
paths by comparison of models generated from FEFF
we were immobilising them, but in the absence of CB. We
treated Rh C and Rh C with HCl/NaNO in MeNO , in the
11 0 2 2
absence of CB and analysed the resulting materials using
2
9
I
I
calculations using known X-ray structures of derivatives of
the catalysts, i.e. the simple pyrazole–triazole based
catalysts, that do not feature the alkyl chain tether or
NMR- and IR spectroscopy and mass spectrometry. We were
F
surface anchoring groups. Results strongly suggest that for
only able to unambiguously identify BAr counterions in the
4
III
1
−
the surface immobilized species Rh C
n
CB the CO ligands
two product samples for the H NMR- and ESI mass spectra.
F
1
are absent, as a major scattering path at 2.5 Å observed for
4
Signals other than those of BAr in the H NMR spectra
I
Rh C is absent after immobilization. This path was
appeared broad, thus inconclusive as to the exact nature of
these protons. IR spectroscopy confirmed that the CO ligands
n
assigned with the aid of FEFF calculations to the CO
III
I
ligands, suggesting that CO is no longer present in Rh C
n
-
are absent after treatment of the Rh precursors with HCl/
−
1
CB (Fig. S23, ESI†).
NaNO as the CO stretching bands at 2108 and 2048 cm for
2
I
III
III
n n
Fig. 4 EXAFS spectra recorded at the Rh K edge and Ir L3 edge of the catalysts and hybrid catalysts; (a) Rh C and Rh C CB, (b) Rh Cp*C11 and
III
III
III
2
n n
Rh Cp*C11CB and (c) Ir Cp*C and Ir Cp*C CB. The insets show the k weighted radial distributions.
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patterns shown in Fig. S34–S36 (ESI†). These different species
likely form through ligand exchanges in the reactive mixture
of aqueous HCl/NaNO
2
,
thus resulting in different
compositions depending on the reaction time. The Rh bound
Cl is also manifested in the detected Cl by EDX and XPS in
the hybrid catalyst samples after surface immobilization.
I
We propose the transformation of the Rh complexes on
attachment to the CB surfaces occurs via the reaction as
I
I
shown in Scheme 2 for Rh C
0
Ph. In the first step, the Rh
III
center is oxidized to Rh , on release of CO, by the oxidative
mixture of HCl/NaNO . The coordinatively unsaturated Rh
III
2
intermediate that is formed is stabilized by reaction with
I
remaining Rh starting material under loss of CO and one
equivalent of Rh, to yield the new catalytically active species.
The metal complex that forms the hybrid catalyst head-group
can have a number of configurations in the case of the
different stereoisomers as illustrated in Scheme 2. The
−
1
I
Fig. 5 Infrared spectra in the ranges of 2200–900 cm of Rh C11 and
I
Rh C
0
Ph, and after treatment of the compounds with HCl/NaNO
2
in
III
various possible three-dimensional structures of the new Rh
nitromethane.
products with two PyT-ligands, without surface anchoring
group, and with two Cl co-ligands were modelled (Scheme 2).
DFT calculations show that the isomer with PyT-ligands in
anti-configuration and Cl co-ligands in para configuration
−
1
RhC11 and 2110 and 2049 cm for RhC , are absent, while
0
−
1
most other significant features of the IR spectra are retained
Fig. 5 and S28–S29, ESI†). This confirms removal of the CO
ligands during the immobilization procedure, but otherwise
there were no major structural changes to the PyT-ligand.
As a further control we investigated the behaviour of the
has the lowest relative energy (0 kcal mol ) amongst the
−
1
(
other possible isomers (1.2 to 3.1 kcal mol ). If the two co-
ligands X and X (Scheme 2) are not identical, the number
1
2
of possible isomers increases significantly. Thus, the
generated mixture is complex, which results in the observed
complexity in the NMR spectra of the materials generated.
Furthermore, this explains why the only significant change in
the IR spectra is the absence of the CO stretching vibrations,
as those ligands ae removed, whereas the PyT-ligands remain
unchanged.
I
control compound Rh C Ph, which does not bear an NH -
0
2
group, under grafting conditions. As the grafting relies on
formation of the phenyl radical via the reaction of an NH
substituent and elimination of N , this control should not
2
-
2
generate
a phenyl radical under the applied grafting
conditions. We again observed the disappearance of the CO
Based on the results above that provide the unexpected
−
1
III
stretching vibrations at 2108 and 2049 cm (Fig. 5 and
structures of the Rh C CB hybrid species, the complexes
n
I
1
I
S30†). For Rh C
0
Ph, we were able to obtain a complex
H
generated from the parent Rh C
n
precursors bearing the
NMR spectrum (Fig. S31, ESI†), which suggests the material
obtained following the grafting procedure, even in the
absence of the key NH -group, is a mixture of compounds.
2
aniline should have two anchoring groups, given the reaction
generating the phenyl radical goes to completion for both
binding ligands. The in situ generated species bearing two
PyT ligands may bind more efficiently to the surfaces
compared to the counterparts with only a single PyT ligand,
due to the two available anchoring groups. In addition,
bidentate binding to the carbon surface may also occur.
Indeed, this effect appears to be visible for the whole series
We analysed different batches of this control reaction at
different time points, which showed a rising complexity of
1
the H NMR spectra as the time points became longer.
+
Analysis by ESI mass spectrometry (Fig. S32–S40, ESI†)
showed a number of different ions are present with the
highest intensity singly charged mass fragments being 623.1
I
in the current work as the hybrid catalysts based on the Rh
(
after 1.5 h reaction time) and 634.1 (after 16 h reaction
complexes consistently show higher metal loading on the
III
III
time). These ions were assigned by high resolution
measurements (ESI ) to [RhCl
(
surface compared to the monodentate Rh and Ir hybrid
catalysts. In addition, this effect is reflected in the isolated
+
+
2
(PyT)
2
]
2
and [RhCl(NO )
+
I
PyT)
2
] , respectively. A doubly charged species with a mass
yields of the hybrid catalysts: samples based on the Rh
of 276.5 was detected for both samples and assigned to
precursors gave consistently about 20% higher yields
2
+
III
III
[Rh(PyT)
2
] .
compared to the Rh and Ir hybrid catalysts. With these
new insights in mind, we revise also the earlier reported
structures of mono- and heterobimetallic layers based on the
Intriguingly, this analysis has confirmed that the original
I
Rh complexes are converted during the immobilization
III
I
7
treatment to Rh species bearing two PyT ligands and
Rh C
0
precursor.
III
combinations of Cl, NO
2
, NO
3
, OH as two additional co-
The thermostability of the hybrid catalysts Rh C CB was
n
ligands. Each of these species were detected and assigned by
mass spectrometry, with the experimental and calculated MS
tested by thermogravimetric analysis (TGA) (Fig. S41, ESI†).
The TGA curves indicate that all hybrid catalysts are stable
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I
Scheme 2 Treatment of the control compound Rh C
0
Ph with HCl/NaNO
2
to yield different bis-pyrazole–triazole Rh complexes and calculated
III
−1
structures of the five isomers of Rh Cl
2
and their relative energies reported in kcal mol
.
>300 °C with no significant weight loss occurring up to that
Application in catalysis – hydrosilylation of alkynes
temperature. Weight loss starts at around 350 °C, with
one weakly pronounced process for all samples. This is
followed by another process at around 500 °C for the Rh
samples, which show more overall weight loss than the Ir
samples.
Having established the formation of hybrid catalysts with
high metal loadings and their composition, the surface-
bound catalysts, and their homogeneous analogues, were
tested for the promotion of the hydrosilylation of
Table 1 Summary of the hydrosilylation reaction of phenylacetylene with triethylsilane promoted by the Rh- and Ir catalysts
a
Catalyst
Metal [mol%]
Loading EDX [at%]
Loading XPS [at%]
Conversion [%]
Ratio β(E)/β(Z)/α
Hybrid
III
Rh
Rh
Rh
C
C
C
11CB
1.0
1.0
1.0
0.49
0.52
0.82
0.96
0.97
1.22
96
95
56
11/4/85
11/4/85
10/8/82
III
III
6
CB
CB
0
Homogeneous
I
Rh C11
1.0
1.0
1.0
—
—
—
—
—
—
98
94
100
39/22/39
42/22/36
41/22/37
I
Rh C
6
I
Rh C
0
Controls
I
Rh C
0
Ph
2
1.0
1.0
1.0
—
—
—
—
—
—
98
99
100
42/21/37
10/4/86
9/2/89
III
b
Rh Cl
III
b
2
Rh
C
11Cl
Hybrid
III
Rh Cp*C11CB
0.5
0.5
0.5
0.5
0.25
0.13
0.11
0.08
0.63
0.24
0.28
0.22
91
98
60
85
1/96/3
2/95/3
2/95/3
2/95/3
III
Ir Cp*C11CB
III
Ir Cp*C
6
CB
CB
III
Ir Cp*C
0
Homogeneous
III
Rh Cp*C11
1.0
1.0
—
—
—
—
Traces
Traces
Complex
Complex
III
Ir Cp*C
n
Control
III
Rh Cp*C
0
Ph
1.0
—
—
93
2/96/2
a
3
Reaction conditions: 1 mL THF under Ar, 0.1 mmol scale and 1.8 eq. of Et SiH, 1 h, 60 °C. Calculated mol% metal applied in the reaction,
I I
b
0
based on the loading obtained from EDX. These homogeneous control compounds were obtained by treating Rh C Ph and Rh C11 with
grafting conditions under absence of CB.
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phenylacetylene (Table 1). The full set of the hybrid catalysts
Linker length
are efficient catalysts, which can operate under mild
conditions. The catalysts Rh C CB (n = 0, 6, 11) all afford
n
the α-isomer as the major product (>80%), followed by
III
III
III
III
n 11 6
Of all the Rh C CB series, the Rh C CB and Rh C CB are
the most efficient in promoting the hydrosilylation reactions,
followed by Rh C CB which is the least efficient catalyst
(Fig. 6). Similarly, concerning the phenylacetylene substrate,
Ir Cp*C CB performs best among the Ir -series, followed
III
minor amounts of the β(E)- and β(Z)-isomers. In contrast, the
0
I
precursor Rh C
n
(n = 0, 6, 11) used as homogeneous catalysts
III
III
in solution do not promote distinct product selectivity, while
11
III
III
I
by Ir Cp*C CB and Ir Cp*C CB. These results demonstrate
0 6
the control compound Rh C Ph without the NH -group, gave
0
2
how the length of the tether can influence the performance
of the catalyst. The catalysts with longer linkers tend to be
more effective. This effect is more pronounced at early time
points, i.e. at 20 min for the substrate phenylacetylene
(Fig. 6a) and at 15 min for diphenylacetylene (Fig. 6b).
The origin of the longer linker hybrid catalysts being the
better catalysts compared to the shorter linker analogues may be
due to the higher flexibility of the linker and consequently
greater potential mobility of the head-group, allowing the
substrates to bind more readily to the catalyst. Furthermore, the
longer linker examples could potentially align with head-groups
pointing towards the surface instead of away from the surface,
which could also affect the catalytic activity through sterics and/
or electronics resulting from the consequent interactions of the
headgroup with the carbon surface. However, a densely packed
layer, which we obtained by optimisation of the surface grafting
conditions, would likely reduce such effects as the layer is more
rigid. Additionally, the packing of the catalyst layer on the
surface may depend on the chain lengths of the catalysts and it
is possible that the layers obtained from the longer chain
derivatives differ in structure from the layers obtained from the
shorter chain derivatives.
2
the same result as the derivatives bearing an NH -group. As
the Rh precursors are modified during immobilization, we
I
I
also tested the materials obtained from treatment of Rh C
1
1
2
I
III
0 2
and Rh C Ph with NaNO /HCl for catalytic activity. Rh Cl
I
III
(
derived from Rh C Ph) and Rh C Cl (derived from
Rh C ), both reproduce nicely the observed α-selectivity of
the Rh C CB hybrid catalysts. Thus, the observed effect is
n
0 11 2
I
1
1
III
due to the chemical modification of the head-group. As
discussed above, all systems with the in situ modified
catalytically active group bear mixed species co-ligands (Cl,
NO , NO , OH) which originate in the grafting process. We
2
3
have used the systems as prepared and note that co-ligand
variation would need to be taken be into account to assess
performance in dependence of the coligands.
III
n
Each of the M Cp*C CB (M = Rh, n = 11; M = Ir, n = 0, 6,
1
1) hybrid catalysts promoted the formation of the β(Z)-
isomer with high selectivity, which is analogous to the
III
reported outcome on using
a homogeneous Cp*Rh
3
0
III
III
catalyst.
The Rh Cp*C11 and Ir Cp*C
n
examples in
solution also afford the β(Z) isomer. However, a significant
amount of starting material was present in all cases, and
side-product formation was also observed. Thus, the
homogeneous catalysts are inferior to the hybrid catalysts. In
III
0
contrast, application of the control compound Rh Cp*C Ph,
without NH -group, resulted in a clean reaction with near
2
Recyclability
III
quantitative conversion, reproducing the observed reactivity
We tested the recyclability of Rh C CB for a number of
1
1
of the hybrid analogues. This suggests that the divergent
consecutive runs for hydrosilylation of phenylacetylene by
removing the catalyst by centrifugation from the reaction
mixture, washing it and re-using it. The reactivity is
maintained over the course of seven repeated catalytic
experiments (Fig. 7). XPS analysis of a recycled sample of
III
reactivity of the M
catalysts, when not immobilized,
originates from the presence of the NH -group. We
2
furthermore tested the whole set of hybrid catalysts in the
hydrosilylation of diphenylacetylene (Table S31, S72, ESI†).
I
III
The catalysts based on the Rh precursors afford the β(E)
Rh C CB showed a Rh content of 0.86 at% after five runs,
1
1
III
isomer, while the M Cp* (M = Rh, Ir) hybrid catalysts do not
compared to 0.96 at% for the freshly prepared hybrid catalyst
show catalytic competency for this transformation.
(Fig. S44, ESI†).
III
III
Fig. 6 Performance of the Rh C
n
CB hybrid catalysts in terms of
Fig. 7 Recyclability of Rh C11CB for seven consecutive runs for the
overall conversion to the products in the hydrosilylation reaction of
phenylacetylene and diphenylacetylene with triethylsilane at selected
time points. The reaction temperatures were 60 °C (a) and 75 °C (b).
hydrosilylation of phenylacetylene. The product ratio of isomers for
the first run was at β(E)/β(Z)/α = 10/2/88 and remained constant for
the subsequent runs (2–7) at 10/1/89.
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We furthermore tested catalytic activity of the soluble
rich, electron deficient, ortho- and para-substituted aromatic
systems as well as a cyclohexene and two alkyl derivatives. As
the reactions proceeded slower for some substrates compared
to phenylacetylene, we tested the whole set of substrates at a
slightly higher temperature. We were delighted to see that
both hybrid catalysts reproduce the observed product
selectivity for the parent phenylacetylene for the vast majority
of tested alkynes (Table 2). Conversions are excellent with
most examples showing conversions of 90% or greater for
both catalysts. Exceptions are the alkynes 7 and 9 with
substituents in the ortho-position, which give somewhat
3
1
fraction of a reaction solution by a hot filtration test, with
the aliquot after filtration showing no catalytic activity,
suggesting the catalyst is of heterogeneous nature (ESI,† S69).
Scope
In the next step we examined the scope of the catalysed
hydrosilation reaction for a number of terminal alkyne
III
substrates using two of the hybrid catalysts, Rh C CB and
1
1
III
Rh Cp*C CB. To test the compatibility of the
1
1
III
III
transformation with various substrates we chose electron
lower conversion for both Rh Cp*C11CB and Rh C11CB. We
III
III
Table 2 Scope of the hydrosilylation of terminal alkynes with triethylsilane promoted by the hybrid catalysts Rh C11CB and Rh Cp*C11CB
Products
Product ratio
Terminal alkyne
Hybrid catalyst
β(E)/β(Z)/α
Conversion [%]
β(E)/β(Z)/α
III
Rh
C
11CB
7a/7b/7c
85 (1 h)
100 (4 h)
37 (1 h)
51 (4 h)
5/0/95
4/0/96
0/93/7
0/95/5
7
a/7b/7c
7a/7b/7c
a/7b/7c
III
Rh Cp*C11CB
7
III
Rh
C
11CB
8a/8b/8c
a/8b/8c
8a/8b/8c
a/8b/8c
90 (1 h)
100 (4 h)
73 (1 h)
93 (4 h)
5/0/95
4/0/96
0/93/7
0/95/5
8
III
Rh Cp*C11CB
8
III
Rh
C
11CB
9a/9b/9c
a/9b/9c
9a/9b/9c
a/9b/9c
35 (1 h)
67 (4 h)
70 (1 h)
98 (4 h)
12/2/86
4/0/86
1/97/2
0/98/2
9
III
Rh Cp*C11CB
9
III
Rh
C
11CB
10a/10b/10c
0a/10b/10c
10a/10b/10c
0a/10b/10c
99 (1 h)
100 (4 h)
56 (1 h)
91 (4 h)
4/2/94
7/6/87
6/91/3
3/95/2
1
III
Rh Cp*C11CB
1
III
Rh
C
11CB
11a/11b/11c
1a/11b/11c
11a/11b/11c
19 (1 h)
95 (4 h)
97 (1 h)
14/0/86
10/4/86
18/80/2
1
III
Rh Cp*C11CB
III
Rh
C
11CB
12a/12b/12c
40 (1 h)
100 (4 h)
81 (1 h)
99 (4 h)
12/4/84
7/3/90
7/91/2
4/93/3
1
2a/12b/12c
12a/12b/12c
2a/12b/12c
III
Rh Cp*C11CB
1
III
Rh
C
11CB
13a/13b/13c
3a/13b/13c
13a/13b/13c
3a/13b/13c
51 (1 h)
89 (4 h)
73 (1 h)
97 (4 h)
10/2/88
10/1/89
0/97/3
0/96/4
1
III
Rh Cp*C11CB
1
III
Rh
C
11CB
14a/14b/14c
4a/14b/14c
14a/14b/14c
4a/14b/14c
40 (1 h)
96 (4 h)
82 (1 h)
97 (4 h)
28/5/67
27/5/68
25/75/0
6/94/0
1
III
Rh Cp*C11CB
1
III
Rh
C
11CB
15a/15b/15c
5a/15b/15c
15a/15b/15c
69 (1 h)
100 (4 h)
100 (1 h)
15/9/76
16/3/81
0/100/0
1
III
Rh Cp*C11CB
III
Rh
C
11CB
16a/16b/16c
16a/16b/16c
100 (1 h)
100 (1 h)
17/6/77
0/100/0
III
Rh Cp*C11CB
3
Reaction conditions: 1.8 eq. of Et SiH, 75 °C in THF under Ar in a sealed Schlenk tube. Catalyst loading: 1.0 mol%. Conversions were
determined after 1 h or 4 h reaction by integration of the vinylic proton resonances of the combined isomeric product mixture vs. remaining
1
alkyne in the H NMR spectrum before purification.
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isolated most products as isomeric mixtures, and
the selectivity is directed to α. Furthermore, the non-
immobilized Rh derivatives show no distinct selectivity with all
1
13
I
characterized the mixtures using H- and
C
NMR
spectroscopy and mass spectrometry, or compared the
obtained H NMR spectra to the literature. Isolated yields for
isomers (β(E), β(Z), α) being produced. The selectivity of the
1
different catalysts is determined by the structure of the catalyst
I
III
most examples are good to excellent (ESI,† S73–S80).
head-group – that is, whether it is Rh (CO) PyT, Rh ClCp*PyT
2
III
III
III
Application of the Rh C11CB catalyst resulted in a selectivity
or Rh (X
1
2 2
)(X )(PyT) .
As the homogeneous Rh
control
of 68–99% for formation of the α-isomer with 10 out of 11
examples, giving a product selectivity of greater than 82%.
The cyclohexene derivative 14 shows a noteworthy decrease
in selectivity towards the β-isomer (68%), producing
significantly more β(E)-isomer (27%) compared to the other
compounds studied. The methoxy and phenoxy substituted
compounds 11 and 12, as well as compound 13, gave the
compounds without amine and alkyl tethers reproduce the
selectivity as the hybrid catalysts, the homogeneous control
complexes Rh C Ph and Rh Cl (Scheme 2 and Fig. 2) may
0 2
III
III
serve as tentative models for the hybrid catalysts. Furthermore,
I
the Rh C Ph control compound also promotes the formation of
0
I
the product mixture as observed for the non-immobilized Rh C
n
thus is may serve as well as model for the reactions using the
III
I
β(E)-isomer in ∼10%. On using the Rh Cp*C CB catalyst,
series of homogeneous catalysts Rh C (Table 1).
1
1
n
the β(Z)-isomer formed with a selectivity of 75–100%. Most
alkynes showed a selectivity of >90%, with the hexyne
derivatives 15 and 16 showing quantitative conversion to the
β(Z)-isomer. Noteworthy, the reaction involving the hexynes
showed better conversions compared to the aromatic
systems, which is presumably due to steric hindrance as the
To probe feasible reaction mechanisms, we performed
DFT calculations on the model systems, using CPCM (THF)
M06L/def2TZVP//wB97XD/6-31G(d)+SDD level of theory.
1
1,32–34
Results show that a Chalk–Harrod type mechanism,
involving oxidative addition of the silane to Rh followed by
insertion of the alkyne into the Si–Rh bond, possible cis–trans
isomerisation, product formation and regeneration of the
III
Rh Cp*C11CB bears a bulky Cp* ligand which makes it less
I
accessible for the substrates with substituents in close
proximity.
catalysts, is feasible for the Rh species in solution with all
required intermediates readily located in the calculations for
I
Rh C
0
Ph (Scheme S4, S82, ESI†).
III
III
Mechanism
In contrast, for the Rh Cp*C Ph and Rh Cl model
0 2
systems attempts to locate a H–Rh–Si intermediate were
unsuccessful, suggesting that a step-wise Chalk–Harrod type
mechanism is unlikely. However, intermediates where Rh
has coordinated to the silane via a bridging hydrogen were
The hybrid catalysts lead to two different outcomes on catalysis
III
III
–
on using the Rh Cp*C CB (and Ir Cp*C CB) catalyst the
11 n
III
product selectivity is directed to β(Z) while on using Rh C CB
n
III
Scheme 3 Proposed mechanisms and calculated structures of involved intermediates and transition states; a) for the Rh Cp*C
0
Ph catalyst
III
−1
2
leading to β(Z)-product formation. b) For the Rh Cl catalyst leading to α-product formation. Energies are reported in kcal mol .
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−
1
III
located (13.5 kcal mol for Rh Cp*C Ph and −7.0 kcal
isomers with high levels of selectivity. Proposed mechanisms
are supported by DFT calculations. The hybrid catalysts are
robust, selective and recyclable, thus providing a green approach
towards important industrially relevant building blocks.
0
−
1
3
III
mol
for Rh Cl , similar to that reported in other
2
). The formation of these Rh–H–Si intermediates
5,36
work
suggests that oxidative addition and alkyne insertion occurs
in a concerted fashion for the Rh catalysts, as reported
III
II 12,13,36
for Ru .
Author contributions
III
As such, for the mechanism involving Rh Cp*C Ph
0
M. R. synthesized the compounds and hybrid catalysts,
characterized the surfaces by XAS, EDX, TGA and applied the
hybrid catalysts in the catalytic transformations. V. R. G, J. L.
and J. J. G. characterized the surfaces by XPS. S. T. K.
performed the DFT calculations and characterized the
surfaces by XAS. I. P. and J. D. characterized the surfaces by
XAS. B. A. M. and M. R. conceived the experiments. All
authors discussed the data, and contributed to writing the
manuscript.
(
Scheme 3a) we postulate that the less sterically congested
anti-Markovnikov type transition state is favoured, leading to
selective generation of C -(E). The formed C -(E)
intermediate (65.6 kcal mol ) is significantly higher in
energy than C -(Z) (43.8 kcal mol ), thus isomerization of
C -(E) to C -(Z) is highly likely. This is consistent with the
reported isomerization of C -(E) to C -(Z) promoted by the
bulky Cp* ligand. Alternatively, considering the high energy of
C -(E), direct formation of C -(Z) through concerted oxidation
addition–silyl insertion may occur, as suggested previously for
Si
Si
−
1
Si
−1
Si
Si
Si
Si
37
Si
Si
38
Si
Conflicts of interest
Ir(III).
The C -(Z)-intermediate subsequently undergoes
reductive elimination to selectively generate the β(Z) isomer.
The authors declare no conflicts of interest.
III
Lastly, the α selective Rh Cl
Scheme 3b). Modelling a potential reaction mechanism
showed reaction involving low energy Rh–H–Si
2
catalyst will be considered
(
Acknowledgements
a
a
−
1
intermediate (−7.0 kcal mol ) is feasible with only one PyT-
ligand coordinated to the Rh. Surprisingly, it was difficult to
locate any of the key intermediates generated after alkyne
insertion, with the only intermediate located being a C -α
type species where interaction between Cl and an alkenyl C
results in lengthening of the alkene C–C bond (1.44 Å). A low
energy reductive elimination transition state (18.8 kcal mol )
from C -α was located, confirming that α-product formation
using Rh Cl is feasible. As the intermediates and reductive
elimination transition states that would lead to β(E) or β(Z)
product formation were not able to be located, it is likely that
the α-product is directly generated through a Markovnikov
type concerted oxidative addition–insertion pathway, as
previously reported for Ru .
of metallacyclopropene intermediates for the system under
investigation here would require involvement of +V oxidation
states (or higher) for Rh, thus making this pathway less
likely.
Financial support from The University of Sydney and
Macquarie University is gratefully acknowledged. J. J. G. and
V. R. G. acknowledge the financial support from the
Australian Research Council Australian Laureate Fellowship
Si
(
FL150100060). S. T. K. acknowledges the support of
Macquarie University through the receipt of a Macquarie
University Research Fellowship (MQRF).
−
1
Si
We gratefully acknowledge beamtime at the Australian
Synchrotron (Projects 14297 and 15780) and thank Dr Peter
Kappen and Dr Jessica Hamilton for scientific and technical
assistance with X-ray absorption spectroscopy (XAS)
measurements at the XAS beamline. We acknowledge the
scientific and technical assistance of the NMR and chemical
analysis facilities at Macquarie University, the Australian
Proteome Analysis Facility (APAF) and Sydney Analytical at
the University of Sydney. We thank Dr Mohan Bhadbhade,
Mark Wainwright Analytical Centre, for the X-ray structure
determinations, Dr Timothy Murphy for assisting with SEM/
EDX measurements and Dr Nicholas Proschogo for scientific
and technical assistance with mass spectrometry
experiments. This project was undertaken with the assistance
of resources and services from the National Computational
Infrastructure (NCI), which is supported by the Australian
Government.
III
2
II 12,13,36
Furthermore, involvement
3
6
Conclusions
In summary, we have prepared a series of carbon black based
Rh- and Ir-hybrid catalysts with varying surface linker
lengths. The hybrid catalysts were characterized by XPS, EDX
and synchrotron based EXAFS spectroscopy. The catalysts
efficiently catalyse the hydrosilylation of terminal alkynes
under mild conditions. The hybrid catalysts with longer
chain surface linkers achieved a more rapid transformation
of the alkynes compared to those with shorter chain surface
linkers. The surface immobilization of the described Rh
catalysts induces a chemical modification of the active
catalyst, which renders the complexes α-selective. In contrast,
the Rh precursor catalysts show no selectivity when applied
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