Controlling the regioselectivity of the hydrosilylation reaction in carbon nanoreactors

Article abstract of DOI:10.1002/chem.201201542

Hollow graphitized carbon nanofibres (GNF) are employed as nanoscale reaction vessels for the hydrosilylation of alkynes. The effects of confinement in GNF on the regioselectivity of addition to triple carbon-carbon bonds are explored. A systematic comparison of the catalytic activities of Rh and RhPt nanoparticles embedded in a nanoreactor with free-standing and surface-adsorbed nanoparticles reveals key mechanisms governing the regioselectivity. Directions of reactions inside GNF are largely controlled by the non-covalent interactions between reactant molecules and the nanofibre channel. The specific π-π interactions increase the local concentration of the aromatic reactant and thus promote the formation of the E isomer of the β-addition product. In contrast, the presence of aromatic groups on both reactants (silane and alkyne) reverses the effect of confinement and favours the formation of the Z isomer due to enhanced interactions between aromatic groups in the cis-orientation with the internal graphitic step-edges of GNF. The importance of π-π interactions is confirmed by studying transformations of aliphatic reactants that show no measurable changes in regioselectivity upon confinement in carbon nanoreactors. Nanoscale reaction vessels: Carbon nanoreactors are prepared by encapsulating catalytic Rh or RhPt nanoparticles in hollow graphitised nanofibres. Inside the nanoreactors, the pathways of the hydrosilylation reactions differ from those on the surface of nanofibres or in the bulk phase (see scheme). Copyright

Full text of DOI:10.1002/chem.201201542

FULL PAPER
DOI: 10.1002/chem.201201542
Controlling the Regioselectivity of the Hydrosilylation Reaction
in Carbon Nanoreactors
William A. Solomonsz, Graham A. Rance, Mikhail Suyetin, Alessandro La Torre,
[
a]
Elena Bichoutskaia, and Andrei N. Khlobystov*
Abstract: Hollow graphitized carbon
nanofibres (GNF) are employed as
nanoscale reaction vessels for the hy-
drosilylation of alkynes. The effects of
confinement in GNF on the regioselec-
tivity of addition to triple carbon–
carbon bonds are explored. A system-
atic comparison of the catalytic activi-
ties of Rh and RhPt nanoparticles em-
bedded in a nanoreactor with free-
standing and surface-adsorbed nano-
particles reveals key mechanisms gov-
erning the regioselectivity. Directions
of reactions inside GNF are largely
controlled by the non-covalent interac-
tions between reactant molecules and
the nanofibre channel. The specific p–
p interactions increase the local con-
centration of the aromatic reactant and
thus promote the formation of the E
isomer of the b-addition product. In
contrast, the presence of aromatic
groups on both reactants (silane and
alkyne) reverses the effect of confine-
ment and favours the formation of the
Z isomer due to enhanced interactions
between aromatic groups in the cis-ori-
entation with the internal graphitic
step-edges of GNF. The importance of
p–p interactions is confirmed by study-
ing transformations of aliphatic reac-
tants that show no measurable changes
in regioselectivity upon confinement in
carbon nanoreactors.
Keywords: carbon
nanostructures
regioselectivity · rhodium
·
catalysis
·
·
·
nanoreactor
Introduction
(with a tensile strength much higher than that of steel) than
any other molecular nanocontainers, the confinement of
[9–11]
[12–13]
Confinement of molecules and atoms inside hollow nano-
scale containers provides a powerful strategy for studying
structural and chemical properties of individual molecules at
small molecules
and other nanoscopic materials
in
CNT has been widely explored. Whereas the surfaces of
[14]
carbon nanostructures possess rich chemical reactivity, the
concave side of the nanotube is relatively chemically unreac-
tive, so that even aggressive chemical processes can be con-
tained within the CNT, which makes hollow carbon nano-
structures highly suitable for nanoreactor applications.
Indeed, it has been recently shown that composite super-
structures comprising metallic species encapsulated in
carbon nanotubes exhibit superior activity, chemoselectivity
and stereoselectivity as catalysts in chemical transforma-
[1–4]
the nanoscale.
Over the past three decades, a variety of
nanocontainers, including cyclodextrins, cavitands, calixar-
enes, cucurbiturils, supramolecular/coordination cages, bilay-
[5,6]
er vesicles and zeolites,
have been developed to replicate
the behaviour of enzymes in nature and probe the unique
way that molecules behave under intense spatial restrictions.
In these so-called “nanoreactor” systems, it has been experi-
[7]
[8]
mentally and theoretically demonstrated that both the ki-
netics and selectivity of chemical reactions are fundamental-
ly affected by confinement effects; these include altered
local concentrations and pressures, preferential alignment of
reactants and increased specific non-covalent interactions
lowering the activation barriers for reactions relative to the
bulk solution or gas phase. As hollow carbon nanostructures,
such as carbon nanotubes (CNT) and graphitised nanofibres
[
15]
tions, such as the conversion of syngas to ethanol,
the
and the addi-
tive-induced Pt catalysed asymmetric hydrogenation of a-
[16]
Fe O catalysed Fischer–Tropsch synthesis
2
3
[17]
ketoesters. Moreover, the use of carbon nanostructures in
catalysis permits examination of two distinct confinement
phenomena (Figure 1).
In very narrow containers, such as single-walled carbon
nanotubes (SWNT) and small-diameter multi-walled carbon
nanotubes (MWNT), encapsulated reactants and catalytic
centres are severely confined within the 1D channel (Fig-
ure 1a). The directions and rates of chemical processes in
such narrow nanoreactors will differ significantly from those
in the bulk phase, thus offering a powerful mechanism for
controlling reactions. However, a major drawback of the re-
stricted reaction volume is related to transport resistance in
(
GNF), are significantly more thermally stable (up to 7008C
in air and 28008C in vacuum) and mechanically more robust
[
a] W. A. Solomonsz, Dr. G. A. Rance, Dr. M. Suyetin, A. La Torre,
Dr. E. Bichoutskaia, Prof. Dr. A. N. Khlobystov
School of Chemistry, University of Nottingham
University Park, Nottingham, NG7 2RD (UK)
Fax : (+44)115-9513555
[18,19]
carbon nanostructures.
As the mean free path of mole-
E-mail: Andrei.khlobystov@nottingham.ac.uk
cules is often larger than the diameter of the carbonaceous
nanocontainer, diffusion of molecules in narrow CNT is
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201542.
Chem. Eur. J. 2012, 00, 0 – 0
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To explore the effects of a confined environment in
carbon nanoreactors on regioselectivity we have chosen the
transition-metal-catalysed hydrosilylation of alkynes, which
generates vinylsilanes that are particularly useful as synthet-
[22]
ic intermediates. The reaction proceeds in the presence of
[
23]
a variety of neutral and cationic metals, such as rhodium,
[24]
[25,26]
platinum and ruthenium
tion of products (Scheme 1).
and yields a broad distribu-
[27,28]
The specific ratio of the Markovnikov a-addition product
, the anti-Markovnikov b-addition (Z isomer) 2 and the b-
1
addition (E isomer) 3 regioisomers and the two dehydrogen-
ative hydrosilylation products (4 and 5) is critically depend-
ant on the specific nature of the catalyst and experimental
Figure 1. Schematic representation of the two types of carbon nanoreac-
tors: (a) carbon nanotubes (narrow, atomically smooth interior) and
(
b) graphitised carbon nanofibres (wider, corrugated interior). Circles
[29]
conditions. In this study, we assess the intermolecular hy-
represent the catalytic centres and arrows indicate the diffusion of reac-
tants in and products out of nanoreactors.
drosilylation of terminal alkynes using new rhodium-based
nanoparticle catalysts located at the internal step-edges of
graphitised carbon nanofibres and reveal mechanisms gov-
erning the regioselectivity of chemical transformations
within carbon nanoreactors.
often orders of magnitude lower than in conventional cataly-
sis. Therefore, assessment of the catalyst performance in
terms of kinetics is often inap-
propriate and consequently in-
vestigation of mechanisms con-
trolling selectivity of reactions
in confined environments can
be significantly more valuable.
Graphitised carbon nanofi-
bres have
a wider internal
volume than CNT, which large-
ly alleviates the problem of
transport resistance of reac-
tants/products and ensures that
the GNF cavity is always acces-
sible. Whereas GNF are signifi-
cantly less explored as nano-
[
12,20]
containers
tors
and nanoreac-
[
12]
than CNT, they possess
many of the functional charac-
teristics of nanotubes with the
added advantage of a larger in-
ternal volume. Furthermore,
Scheme 1. Generalised reaction scheme for the hydrosilylation of a terminal alkyne yielding a distribution of
products: 1=a-addition, 2=b-addition (Z isomer), 3=b-addition (E isomer), 4=vinyl product, 5=alkynylsi-
lane.
unlike carbon nanotubes, which consist of concentric tubes
of graphene, the structures of the inner and outer surfaces
of GNF are fundamentally different to each other. The
outer surface of GNF consists of continuous, atomically flat
cylindrical layers of graphene, but the internal surface pos-
sesses a succession of step-edges formed by rolled-up sheets
of graphene (3–4 nm high and spaced by 8–15 nm from each
other), which act as effective “anchoring points” for mole-
cules and nanoparticles. In general, GNF have the poten-
tial to be superior nanoreactors compared with CNT, as
their overall tubular topology can create local reaction envi-
ronments, different to the bulk phase, but without any sig-
nificant restriction of diffusion, whereas the corrugated in-
ternal structure of GNF opens up opportunities to probe
local spatial confinement effects in which reactivity and dy-
namics at the nanofibre–reactant and nanofibre–catalyst in-
terfaces become important.
Results and Discussion
To assess the effect of nanoscale confinement on the regio-
selectivity of the hydrosilylation of terminal alkynes, we syn-
thesised a series of novel catalysts using a modification to
the approach previously developed within our research
[21,30]
group (Scheme 2).
The rhodium- (RhNP) and rhodium–platinum (RhPtNP)
alloy nanoparticles used in this study were synthesised by
[21]
[
31]
adapting the procedure outlined by Murray et al. This ver-
satile preparation has been used to procure large quantities
of monodisperse nanoparticles of a variety of metals, includ-
[31]
[32]
[33]
[34]
ing gold,
silver,
platinum
and palladium;
in this
study we show that the same technique can be successfully
applied for rhodium and its intermetallic analogues. To min-
imise the possible effect of catalyst poisoning, we employed
sodium S-dodecylthiosulfate (SSDS) as the organic stabiliser
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of the catalyst, it was important
to establish that these attributes
were unaffected by the assem-
bly procedures and as can be
seen from the micrographs (Fig-
ure S4.1 in the Supporting In-
formation) all nanoparticles
were roughly spherical in mor-
phology and around 3 nm in
mean diameter (2.9Æ0.4, 2.7Æ
0
.4 and 2.9Æ0.3 nm for RhNP,
RhNP and RhNP , respec-
ext
int
Scheme 2. Schematic representation of the strategy used for the preparation of rhodium nanoparticles and
their adsorption on and encapsulation in graphitised carbon nanofibres. An analogous approach was employed
for the preparation of the rhodium–platinum nanoparticulate system.
tively). Due to the similar con-
trast of the RhNP and the GNF
walls it is difficult to definitive-
ly describe the location and
in our nanoparticle synthesis because it has been recently
density of the nanoparticles in the composite materials;
however, it was found that in excess of 80% of RhNP are
adsorbed on the exterior surface in RhNPext and more than
85% of RhNP are confined within nanofibres in RhNPint.
[35]
shown by Shon et al. that similar palladium nanoparticles
possessed superior catalytic ability relative to those pre-
pared using more conventional dodecanethiol, due to a re-
duction in the surface coverage
of ligand. This resulted in fewer
metal–sulphur
bonds
that
poison the surface of the nano-
particle and allowed more effi-
cient adsorption of organic sub-
strates, features that are critical
[36]
for efficient catalysis.
Identi-
cal, low quantities of the as-pre-
pared nanoparticles were then
supported on the external sur-
face (RhNP ) and confined in
ext
the inner cavity (RhNP ) of
int
graphitised carbon nanofibres
using conventional organic sol-
vents and a mixture of hexane/
carbon dioxide under supercriti-
cal conditions respectively and
the resultant materials charac-
terised by a range of comple-
mentary techniques (Figure 2
and S2–S4 in the Supporting In-
formation). A typical loading of
the metal catalyst was estimat-
ed to be 5–6 and 3–4 weight%
for the RhNP /RhNP
and
ext
int
RhPtNP /RhPtNP
samples,
ext
int
respectively. Systematic com-
parison of the catalytic activi-
ties of these systems enables
discrimination between the pos-
Figure 2. Bright-field transmission electron micrographs of the GNF nanoreactors. Image (a) shows the open-
end of a nanoreactor, where an array of RhNP resides at the internal step-edges. A section from the middle of
sible effects of catalyst support the nanoreactor is shown in (b) in which the rhodium nanoparticles appear as dark circles (three examples of
and confinement.
RhNP are highlighted by white arrows). Image (c) shows a high-magnification area of the step-edge of a nano-
reactor, although due to the low atomic weight of rhodium and thickness of GNF, nanoparticles are difficult to
distinguish at the step-edges; the black circles in (d) highlight the positions of the RhNP. By using thermal an-
nealing to increase the size of RhNP it was possible to ascertain their positions at the GNF step-edges, as
shown in (e–g). Scale bars are 20 nm (a)–(b) and 2 nm (c)–(g) (see Figure S4.2 in the Supporting Information
As nanoparticle-catalysed re-
actions are potentially sensitive
to composition (e.g., size, shape
and surfactant surface density) for micrographs of the corresponding RhPtNP analogues).
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A. N. Khlobystov et al.
Furthermore, it is significant to note that encapsulated nano-
particles seemingly reside at the internal step-edges of the
nanofibre, due to increased van der Waals interactions be-
tween the RhNP and the graphitic surface of the GNF inte-
[
20]
rior
and therefore the commensurate size of the RhNP
and the step-edge (Figure 2) permits us to investigate the
effect of local spatial confinement on chemical reactions.
The rhodium-catalysed hydrosilylation of phenylacetylene
using triethylsilane has been investigated by a number of re-
search groups and has been shown to proceed highly unse-
lectively yielding all five possible products, each of which is
1
readily identifiable by H NMR spectroscopy and GC-MS
[24,26]
(
Figure 3).
1
The H NMR spectra of products 1–4 have unique and
distinguishable shifts and multiplicities in the region of d =
H
5–7 ppm due to the presence of vinylic protons. Both of the
doublets corresponding to the a product (1) appear in this
region, although only one pair of the b-(Z) (2) and b-(E) (3)
doublets may be seen because their counterparts appear in
the crowded aromatic region beyond d =7 ppm. As 5 has
H
no vinylic protons that allow diagnosis in this way using
1
H NMR spectroscopy, the presence of this compound (and
all others) was confirmed by GC-MS.
Following optimisation of the reaction conditions, in
which it was observed that elevated temperatures (908C)
were necessary to overcome the tardy reaction kinetics asso-
ciated with the necessary extremely low loading of catalyst
employed in the reaction, all new catalytic materials were
tested in the hydrosilylation of a series of alkynes and the
effect of confinement assessed (Table 1; see also Table S6 in
Table 1. The effect of confinement in GNF with respect to the supported
catalysts. For full quantitative analysis please see Table S6 in Supporting
Information.
[
a]
Catalyst
R
R’
R’’
Regioselectivity
Change in the Z/E Change in the
isomeric ratio
amount of 4 and 5
RhNP
RhPtNP Ph
RhNP
RhNP
RhNP
Ph
Et
Et
Et
Et
5.8-fold decrease
no change
3.0-fold increase
1.6-fold increase
no change
no change
no change
Figure 3. Comparative techniques used to evaluate the distribution of
products in the hydrosilylation of phenylacetylene using triethylsilane:
(a) quantitative H NMR spectroscopy consistent with known literature
values (b) qualitative GC-MS (the corresponding mass spectra are shown
in Figure S5 in the Supporting Information).
1
Ph
Cy
Me Ph 2.6-fold increase
Et
Et
Et
no change
no change
n-Oct Et
[
a] If comparative values were within the determined 3% experimental
error, we concluded that no change was apparent.
yield of the b-(E) (3) product of the triethylsilane addition
to phenylacetylene increases by a factor of 5.8 as compared
with its geometric isomer b-(Z) (2) and a greater than 10-
fold increase in selectivity of this product inside nanoreac-
tors as compared with the bulk solution phase (Table S6 in
the Supporting Information). Confinement in nanoreactors
also promotes a 3.0-fold increase in products of dehydrogen-
ative silylation (Table 1). A similar trend was observed for a
different catalytic system, RhPtNP, whereby confinement of
the catalyst also resulted in a promotion of the dehydrogen-
ative silylation products. However, this effect was less pro-
nounced as the presence of Pt measurably enhances the se-
lectivity of this reaction even without confinement (Table S6
the Supporting Information). It is important to note that our
control experiments with catalytic nanoparticles adsorbed
on the outer surface of GNF (RhNPext and RhPtNP ) ex-
ext
hibit regioselectivity intermediate between free-standing
and confined catalysts. To emphasise the effects of spatial
confinement within nanofibres, the selectivity of different
reactions carried out in GNF is calibrated to that on GNF
surface.
Our observations clearly indicate that the regioselectivity
of the hydrosilylation reaction is significantly affected by
spatial confinement in carbon nanoreactors. Inside GNF, the
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in the Supporting Information), as expected for catalysis by
platinum.
lation using trialkylsilane was observed. The main effect of
the introduction of a phenyl group in the silane reactant
[37]
This apparent enhanced regioselectivity inside nanoreac-
tors was an astonishing and unexpected result. However, to
confirm that the observed trends were related to confine-
ment of the catalyst numerous control experiments were
conducted (see S7 in the Supporting Information). As this
(e.g., HSiMe Ph) is that upon confinement in GNF the re-
gioselectivity of addition changes in the opposite direction
2
to trialkylsilanes (e.g., HSiEt ). Indeed, when using a RhNP
3
nanoreactor, the formation of the b-(Z) product (2) be-
comes more favourable for the addition of HSiMe Ph to
2
[29]
reaction is so sensitive to the nature of the catalyst,
a
phenylacetylene, at the expense of the b-(E) and no promo-
tion of the dehydrogenative silylation products is observed
(Table 1). These observations can be rationalised on the
basis of mechanistic considerations (Scheme 3).
number of compositional parameters of the metallic nano-
particles were varied in an attempt to account for the above
findings. From these control experiments, it was confirmed
that nanoparticle size, loading and density of the capping
ligand do not significantly affect the product distribution in
such a way that would account for the observations made.
Furthermore, experimental conditions, such as temperature
in the range of 75–1058C relevant to our studies, have mini-
mal effect on the product distribution. It is significant to
note, however, that changes in the stoichiometry of reac-
tants had a noticeable effect on the regioselectivity (Table
S7.5 in the Supporting Information), hence a 1:1 molar ratio
of the starting alkyne and silane were strictly maintained in
all experiments comparing the regioselectivity of hydrosily-
lation.
A number of previous literature reports have described
the mechanism for the formation of specific products within
[24]
the catalytic cycle, however, a comprehensive mechanism
accounting for all five of the products observed in our ex-
periments has yet to be established. The scheme we propose
combines known mechanistic concepts and is based primari-
ly on a modified Chalk–Harrod cycle (also referred to as the
Crabtree–Ojima cycle) in which silylmetallation in the initial
step leads to a mixture of products (Scheme 3). The key
step in the mechanism appears to be when intermediate A
either relieves steric repulsion between the metal-containing
group and the silyl R’/R’’ groups to form intermediate B, or
transforms into the thermodynamically most stable b-(E)
product (3). Intermediate B can also undergo metal hydride
b-elimination, which subsequently leads to the formation of
the dehydrogenative silylation products (Scheme 3).
Supramolecular interactions of reactant molecules with
nanoreactors are often responsible for the altered reactivity
[6]
observed in confined reactions. Considering the graphitic
nature of nanofibres, phenylacetylene is expected to have a
higher affinity for GNF than triethylsilane due to specific p–
p interactions between the former reactant and the GNF. In
this context, we define p–p interactions as the dispersion
forces between the quadrupole moment of the small aromat-
ic molecule with the highly polarisable and delocalised p-
The experimental data shows that the percentage of prod-
ucts formed from intermediate B (1, 2, 4 and 5) relative to
intermediate A remains constant when R=Ph and R’=
R’’=Et, which is indicative of no change in the relative sta-
bilities of these intermediates upon confinement in GNF.
However, the relative distribution of products formed from
B is significantly affected by confinement (a 17-fold and a 4-
fold decrease in the b-(Z):4+5 ratio was observed using
RhNP and RhPtNP in GNF, respectively). This implies that
metal hydride b-elimination to produce 4 and 5 is favoured
over the formation of 2, commensurate with a higher local
concentration of phenylacetylene. However, as the intercon-
version between A and B is a reversible process, bulky R
and R’ groups may destabilise B in favour of intermediate
A thus enhancing the yield of the thermodynamically most
[38]
system of the carbon nanofibre.
This would lead to a
higher local concentration of the acetylene bearing the aro-
matic phenyl group within the nanoreactor which can affect
the course of the reaction. Control measurements with free-
standing RhNP carried out for different alkyne/silane molar
ratios, clearly demonstrate that increased concentration of
alkyne favours the formation of dehydrogenative silylation
products at the expense of the b-(Z) isomer (Table S7.5 in
the Supporting Information). This result is consistent with
the observed changes of regioselectivity inside GNF
[28]
(
Table 1), confirming that confinement increases the local
stable E isomer of the b-addition product. It is, therefore,
a remarkable result that when R=Ph and R’=Me, R’’=Ph,
the less stable Z isomer (2) is actually favoured inside a
GNF. In this case, the percentage of products formed from
intermediate B (1, 2, 4 and 5) as compared with intermedi-
ate A increases, indicative of a difference in the relative sta-
bilities of A and B and accounts for the regioselective
switching inside GNF. Hence, the effects of local concentra-
tion, expected to be significant in this system as both sub-
strates bear aromatic moieties, are negated, that is, no pro-
motion of products 4 and 5 upon encapsulation was ob-
served and the relative stabilities of intermediates must be
responsible.
concentration of alkyne.
To test this mechanism, additional control experiments
using cyclohexylacetylene or 1-decyne as the alkynyl compo-
nent were conducted (Table 1 and Table S5 in the Support-
ing Information) which showed no changes in regioselectivi-
ty upon spatial confinement in nanofibres for the reactions
of molecules without aromatic moieties. Aromatic interac-
tions of the reactants with the nanofibre interior leading to
higher local concentrations is an important factor in such re-
actions, particularly because steric arguments cannot explain
the difference in the behaviours of phenylacetylene and cy-
clohexylacetylene that have comparable sizes and shapes.
When both reactants (acetylene and silane) possess aro-
matic groups a trend qualitatively different to the hydrosily-
To probe this further, the calculated total energies of the
optimised geometries of A and B when R=Ph, R’=Me and
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A. N. Khlobystov et al.
Scheme 3. The mechanism of the hydrosilylation reaction. Isolatable reaction products (1–5) and key reaction intermediates (A and B) are highlighted
by solid- and dashed-line boxes, respectively.
R’’=Ph were compared. Our density functional theory
(
DFT) calculations of the relative geometries and energetics
of the two intermediates under standard conditions assist in
our explanation of experimental observations, showing that
intermediate A is more stable than B (by ca. 1 eV), which is
consistent with the fact that intermediate A generates the
thermodynamically more favourable E isomer. However,
the geometries of these structures are very different: B is
splayed with the two phenyl rings lying co-planar to each
other at opposite ends of the molecule, whereas A adopts a
pearl-shell like structure with the two aromatic moieties en-
compassing the rhodium catalyst (Figure 4). Depending on
the specific nature and consequent steric bulk of the active
catalyst, which could be either a low coordination rhodium
atom at the vertex of the nanoparticle surface or a leached
[39]
species formed during catalysis, the geometry of B in the
real system may be slightly more splayed. Irrespective of the
degree of splaying, it is evident that attractive interactions
between each of the intermediates and the underlying
graphitic surface of the nanoreactor step-edge are greater
for B than A, as the more open shape of B results in a
larger area of surface contact with the GNF step-edge.
Thus, the orientation of the phenyl groups in the case of
the b-(Z) isomer and its precursor intermediate B facilitates
maximal attractive p–p interactions with the graphitic step-
edge. When both reactants carry aromatic groups, the local
nanoscale morphology of the inner surface of GNF step
Figure 4. DFT optimised geometries of intermediates A (top) and B
(bottom).
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edges mitigates the steric repulsion between R on the
alkyne and R’/R’’ on the silane and promotes the formation
of the thermodynamically less stable isomer. Furthermore,
this implies that enhanced specific interactions between the
reactants and the nanofibre at the nanoparticle–nanofibre
interface play a role as significant as the increased local con-
centrations of reactants for reactions in a carbon nanocon-
tainer.
v/v, “aqua regia”) and rinsed thoroughly with deionised water, cleaned
with potassium hydroxide in isopropyl alcohol and finally rinsed thor-
oughly with deionised water before use. H NMR spectra were obtained
using a Bruker DPX-300 (300.13 MHz) spectrometer at 298 K using
3
CDCl as the solvent. Integration was carried out to quantify the product
1
distribution based on the most upfield 1-proton doublet(s). The overall
distribution of products was then quantified by calculating the integral
value for a product as a percentage figure of the sum of all integrals. GC-
+
MS was performed using a VG Autospec in EI mode. Thermogravimet-
ric analysis was performed using a TA Instruments SDT Q600 under a
À1
À1
flow of oxygen at a rate of 100 mLmin at a heating rate of 108C min
from room temperature to 9008C. Transmission electron microscopy was
performed using a JEOL 2100F TEM (field emission gun source, infor-
mation limit <0.19 nm) at room temperature. Samples were typically
prepared by drop-drying onto a copper grid mounted “holey” or continu-
ous carbon films. Average particle diameters (dNP) were calculated on the
basis of counting at least 100 particles from different micrographs, using
Gatan Digital Micrograph software and ImageJ. UV/Vis spectra were re-
corded in toluene using 1 cm quartz cells on a Perkin–Elmer Lambda 25
Conclusion
We report the first observation of regioselectivity switching
of a preparative chemical reaction due to spatial confine-
ment in carbon nanostructures. The precise structures of re-
actant molecules crucially determine the effects of confine-
ment, with the balance of aliphatic and aromatic moieties
being the most important parameter. Depending on the
nature of reactants, three main trends are observed in
carbon nanoreactors: 1) when neither alkyne nor silane
bears aromatic moieties, no favourable attractive interac-
tions exist with the interior of nanoreactor and therefore no
confinement effect is observed; 2) when only the alkyne is
aromatic, a characteristic increase in the amount of dehy-
drogenative silylation products and decrease in the b-(Z): b-
À1
UV/Vis spectrophotometer at a scan rate of 480 nmmin over the range
1
90–1100 nm. IR spectra were measured in the solid state using a Bruker
À1
Tenser 27 ATR FT-IR spectrometer over the range 400–4000 cm . All
filtrations were carried out using Whatman 0.2 mm polytetrafluoroethy-
lene (PTFE) membranes. Where high pressure techniques were required,
all equipment was leak-tested immediately prior to use.
Catalyst preparation: The preparation of rhodium and rhodium-platinum
alloy nanoparticles is based on a modified two-phase Brust–Schiffrin re-
[
42]
duction. A typical synthesis for the preparation of rhodium nanoparti-
cles is described as follows: to a stirred solution of potassium hexachloro-
rhodate (157 mg, 0.36 mmol, 2.7 equiv) in deionised water (12.5 mL) was
(
E) isomers ratio is observed, which is related to the in-
added
a separate solution containing tetraoctylammonium bromide
creased local concentration of the aromatic reactant (phe-
nylacetylene) within the GNF cavity; 3) when both alkyne
and silane possess aromatic groups, maximised aromatic
stacking interactions between phenyl groups on the reactive
intermediates and the nanofibre at the nanoparticle-nanofi-
bre interface are sufficient to overcome the effects of local
concentrations and favour the formation of the thermody-
namically less stable b-(Z) regioisomer.
(
550 mg, 1.0 mmol, 7.5 equiv) in toluene (25 mL) and the resultant bipha-
sic mixture was vigorously stirred for 10 min at room temperature. The
phases were separated, the aqueous phase discarded and to the remain-
ing organic phase was added tetraoctylammonium bromide (550 mg, 1.0
mmmol, 7.5 equiv) and a separate solution of sodium S-dodecylthiosul-
fate (40 mg, 0.13 mmol, 1 equiv) in water/methanol (10 mL, 3:1 v/v) and
the mixture stirred for 10 min at room temperature. To this was added a
separate solution containing sodium borohydride (150 mg, 5 mmol,
37.5 equiv) in deionised water (7.5 mL) and the combined mixture vigo-
rously stirred for 16 h at room temperature. The phases were separated,
the aqueous phase discarded and the organic phase retained, washed
with deionised water (3ꢁ100 mL), dried over anhydrous magnesium sul-
phate, filtered and the solvent removed concentrated in vacuo. To this
was added ethanol (900 mL) and the product precipitated at À308C over
16 h, collected by vacuum filtration (0.20 mm, PTFE), washed with etha-
nol (300 mL) and acetone (300 mL) and dried in vacuo to yield a dark
solid (17.2 mg). The as-prepared nanoparticles were supported on pris-
As the understanding of the physicochemical properties
of carbon nanostructures as nanoscale containers and sub-
strates under a variety of different conditions, including
[40,41]
those relevant to biological systems,
is advancing rapid-
ly, carbon cavities become increasingly important for con-
trolling the structure and reactivity of encapsulated mole-
cules. Our approach for catalyst assembly and experimental
methodology for monitoring the selectivity of reactions in
carbon nanostructures are general and applicable for a wide
variety of chemical transformations. In this study we have
established mechanisms for controlling the regioselectivity
of reactions in confinement which will guide further devel-
opment towards the preparative applications of carbon
nanoreactors.
[
30]
tine nanofibres at room temperature under ultrasonication
fined within the same nanofibres using a mixture of hexane/CO
supercritical conditions (see S1–S3 in the Supporting Information).
and con-
under
2
[13]
Hydrosilylation of terminal alkynes with hydrosilanes: The catalyst
(0.30 mg and 0.15 mg metal content for RhNP and RhPtNP systems, re-
spectively) and the hydrosilane (18 mmol) were added to an argon-flush-
ed Schlenk tube. The alkyne (18 mmol) was then added dropwise. The
mixture was homogenised with brief ultrasonication at room temperature
and stirred at the required reaction temperature of 908C. The progress of
1
the reaction was monitored primarily by H NMR spectroscopy, taking
aliquots (0.25 mL) of the mixture at regular intervals to provide quantita-
tive analysis of reaction progress and product distribution. GC-MS was
used for qualitative analysis of the composition of the crude mixture.
Experimental Section
General procedures: All reagents and solvents were purchased from
Sigma–Aldrich, UK and used without further purification. Water was pu-
rified (>18 MW cm) using a Barnstead NANOPure II system and toluene
was distilled over calcium hydride. The PR24 GNF were purchased from
Applied Science, USA and produced via chemical vapour deposition. All
glassware was cleaned with a mixture of hydrochloric and nitric acid (3:1
Molecular modelling: The geometry optimisations of the molecules have
been obtained with the DFT/B3LYP level of theory and the Grimme em-
[
43]
pirical dispersion corrections as implemented in the Q-Chem quantum
[
44]
chemistry package. The effective core potential LANL2DZ basis set
has been used for the rhodium atom, and the 6–31G* basis set for all
other atoms.
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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A. N. Khlobystov et al.
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Received: May 3, 2012
Published online: && &&, 2012
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Carbon Nanoreactors
FULL PAPER
Nanostructures
W. A. Solomonsz, G. A. Rance,
M. Suyetin, A. La Torre,
E. Bichoutskaia,
A. N. Khlobystov* ........... &&&& — &&&&
Controlling the Regioselectivity of the
Hydrosilylation Reaction in Carbon
Nanoreactors
Nanoscale reaction vessels: Carbon
nanoreactors are prepared by encapsu-
lating catalytic Rh or RhPt nanoparti-
cles in hollow graphitised nanofibres.
Inside the nanoreactors the pathways
of the hydrosilylation reactions differ
from those on the surface of nanofi-
bres or in the bulk phase (see scheme).
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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These are not the final page numbers!