A versatile method for the preparation of carbon-rhodium hybrid catalysts on graphene and carbon black

Article abstract of DOI:10.1039/c5sc03787e

Strategies for combining the selectivity and efficiency of homogeneous organometallic catalysts with the versatility of heterogeneous catalysts are urgently needed. Herein a direct and modular methodology is presented that provides rapid access to well-de

Full text of DOI:10.1039/c5sc03787e

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A versatile method for the preparation of carbon–
rhodium hybrid catalysts on graphene and carbon
black†
Cite this: DOI: 10.1039/c5sc03787e
Chin Min Wong,ab D. Barney Walker,ab Alexander H. Soeriyadi,a J. Justin Goodinga
ab
*
and Barbara A. Messerle
Strategies for combining the selectivity and efficiency of homogeneous organometallic catalysts with the
versatility of heterogeneous catalysts are urgently needed. Herein a direct and modular methodology is
presented that provides rapid access to well-defined carbon–rhodium hybrid catalysts.
A pre-
synthesized Rh(I) complex containing a carbene-triazole ligand was found to be stable for direct
immobilization onto unactivated graphene, carbon black and glassy carbon electrodes. Characterization
of the heterogeneous systems using X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis
(TGA), inductively coupled plasma-optical emission spectroscopy/mass spectrometry (ICP-OES/MS),
Raman spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM)
confirmed the well-defined nature of the hybrid catalysts. The hybrid catalysts show excellent activity,
comparable to that of the homogeneous system for the hydrosilylation of diphenylacetylene, with
turnover numbers ranging from 5000 to 48 000. These catalysts are the best reported to date for the
hydrosilylation of diphenylacetylene. In common with conventional heterogeneous catalysts, high
reusability, due to a lack of Rh metal leaching, was also observed for all carbon–rhodium complexes
under investigation.
Received 6th October 2015
Accepted 7th December 2015
DOI: 10.1039/c5sc03787e
www.rsc.org/chemicalscience
occurs via at least two steps, where initially a precursor ligand is
attached to the surface then – in a second and separate step –
Introduction
a transition metal centre is introduced. The rst step is nor-
mally dictated by the nature of the functional groups on the
solid surface. For example, reactive Si–OH moieties on silica
surfaces can be effectively used to initiate ligand immobiliza-
tion.13,14 However, certain supports require additional surface
activation in order to achieve the functional groups that are
needed for ligand modication. This is a common strategy for
functionalization of graphene surfaces, whereby pre-oxidation
of graphene to graphene oxide (GO) is oen necessary prior to
functionalization with an appropriate ligand motif.15–17 At
times, reduction back to reduced GO is also performed aer
ligand immobilization and complexation with the metal
precursor.18–20 This complicated multistep assembly to the
hybrid system is not ideal for pristine materials, such as pure
graphene, where it is desirable that its regular, conjugated
surface is retained.21–24 The second step (complexation of metal
with the immobilized ligand) in the stepwise assembly of hybrid
catalysts could create an additional complication, as incom-
plete complexation of the immobilized ligand can occur.
Furthermore, the common use of an excess of metal precursors
increases the chance of nonspecic adsorption of metals onto
the surface.
The assembly of highly efficient and selective homogeneous
systems onto heterogeneous solid materials is a facile route to
the development of optimized and recyclable hybrid systems
with well-dened active sites.1–3 While a variety of catalysts
covalently tethered to carbon based scaffolds (e.g. activated
carbon, graphene/graphene oxide and carbon nanotubes) and
other well known supports (e.g. aluminosilicates, organic resins
and metal organic frameworks) have been reported,4–12 the
fabrication of these hybrid systems is oen complicated,
requiring multiple reaction steps.
The development of hybrid catalysts consisting of well-
dened organometallic complexes on solid supports can be
established using both non-covalent interactions or covalent
bonding. In terms of producing hybrid catalysts that are cova-
lently bonded to the solid support, the synthesis route typically
aDepartment of Chemistry, University of New South Wales, Sydney, NSW 2052,
Australia
bDepartment of Chemistry and Biomolecular Sciences, Macquarie University, Sydney,
NSW 2109, Australia. E-mail: barbara.messerle@mq.edu.au
† Electronic supplementary information (ESI) available: Additional schemes,
gures and tables, characterization of the complexes and hybrid complexes on
carbon surfaces, XPS, TGA, ICP-MS/OES, IR, Raman, SEM, TEM and
electrochemical data and calculations are included. Full catalysis and recycling
tables are also included. See DOI: 10.1039/c5sc03787e
In designing new hybrid catalysts, it would be highly desir-
able to have a simple, general method for attaching optimized
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Scheme 1 Illustration of the direct attachment of catalyst to G, CB or
GC without the need for oxidatively introduced functional groups.
metal complexes directly to a carbon support without the need
for surface pre-activation or post-immobilization treatment
with excess precious metal precursors. While direct function-
alization of graphene is possible using aryl diazonium-based
precursors,25–28 the harsh conditions required by this reaction
are typically not compatible with nely tuned catalytically-active
metal complexes. Herein we describe a modular protocol for the
direct hybridization of a pre-formed Rh(I) catalyst with the
surface of unactivated carbon materials (Scheme 1). We
demonstrate the potential widespread utility of this method by
functionalizing two different high surface area carbon allo-
tropes, graphene (G) and carbon black (CB) and show that these
are highly efficient catalysts. Furthermore, it is demonstrated
that even low surface area glassy carbon (GC) electrodes can be
readily transformed into functionalized, recyclable catalytic
systems using this efficient methodology.
Scheme
2Rh(CO)2].
2
Synthesis of [GC-2Rh(CO)2], [CB-2Rh(CO)2] and [G-
On application as a catalyst for hydroamination, our previ-
ously employed carbon supported bis-pyrazole system29,30
(Fig. 1) delivered remarkably high turnover numbers29 but
underwent some leaching of the Rh metal centre over time. The
novel Rh(I) catalyst attached to the carbon surface here contains
a carbene-triazole ligand system that has a high binding affinity
for the Rh ion in order to create recyclable hybrid catalysts that
exhibit minimal metal leaching.
While [GC-2Rh(CO)2] was characterized using X-ray photo-
electron spectroscopy (XPS), [CB-2Rh(CO)2] and [G-2Rh(CO)2]
were characterized using XPS, thermogravimetric analysis
(TGA), inductively coupled plasma-optical emission spectros-
copy/mass spectrometry (ICP-OES/MS), Raman spectroscopy,
scanning electron microscopy (SEM) and/or transmission elec-
tron microscopy (TEM).
Results and discussion
Here, we introduce our novel homogeneous catalyst, [2Rh(CO)2]
BPh4 (Fig. 1) onto the carbon supports by directly immobilizing
the pre-synthesized Rh(I) complex containing an aniline-func-
tionalized bidentate carbene-triazole ligand, [1Rh(COD)]BPh4
(COD ¼ cyclooctadiene) via the in situ generation of the diazo-
nium moiety (Scheme 2). The aniline functionalized complex
[1Rh(COD)]BPh4, in an acidic solution of nitromethane and
sodium nitrite was added to a GC electrode, or stirred with CB
or G, for 24 h. The carbon materials were then washed repeat-
edly, and suspended in a mixture of dichloromethane and
pentane under an atmosphere of CO. The GC electrode was
washed thoroughly and dried under a gentle ow of nitrogen to
give [GC-2Rh(CO)2]. [CB-2Rh(CO)2] or [G-2Rh(CO)2] were iso-
lated as black ꢀpowders aer copious washing and drying under
vacuum at 25 C overnight.
In the XP spectra31 of [G-2Rh(CO)2] (Fig. 2), the appearance of
the N1s peak at ca. 402 eV 32 and Rh3d5/2 peak at ca. 310 eV were
observed. N1s and Rh3d peaks at similar binding energies were
Fig. 1 Homogeneous Rh(I) complexes containing carbene-triazole (2) Fig. 2 XP survey scan of [G-2Rh(CO)2] and selected narrow scan of
and bis-pyrazole (3) ligands.
N1s and Rh3d.
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observed in the XP spectra of [GC-2Rh(CO)2], [CB-2Rh(CO)2] and spectra of [CB-2Rh(CO)2] and [G-2Rh(CO)2] showed a slight
the unattached complex [2Rh(CO)2]BPh4 (see all XPS gures of increase in the defects (D) band compared to the graphite (G)
hybrid complexes, unmodied carbon surfaces and homoge- band following immobilization of the complex onto the
neous complex in the ESI, section SI-2†), consistent with anal- surfaces (see ESI, section SI-7†), which indicates that both
ogous hybrid Rh(I) complexes on GC and CB reported carbon–rhodium hybrids contained covalent linkages to the
previously.29,30 The atomic ratio of N : Rh for [GC-2Rh(CO)2] and carbon black or graphene surface respectively, consistent with
[G-2Rh(CO)2] was found to be close to 5 : 1 as expected, which observations for similar covalent functionalization on carbon
statistically means that only individual complexes are immo- materials.33,34 An increase in the defects (D) band compared to
bilized on the surfaces. In the case of [CB-2Rh(CO)2], the ratio of the graphite (G) band of the carbon samples can be observed
N : Rh was found to be 3.8 : 1 suggesting that there may be when comparing the peak intensity ratio of the D to G band of
a small amount of adventitious physisorbed Rh on the CB unmodied graphene, which was 0.79. Aer immobilization to
surface.
form [G-2Rh(CO)2], the peak intensity ratio of the D to G band
The thermal stability of [CB-2Rh(CO)2] and [G-2Rh(CO)2] was increased to 0.82. Similarly, the peak intensity ratio of the D to G
analyzed against that of unmodied CB and G using TGA under band for [CB-2Rh(CO)2] was 1.0 which was higher when
an atmosphere of nitrogen (Fig. 3). The thermogram of compared to the peak intensity ratio of D to G band of the
[CB-2Rh(CO)2] displayed two weight loss processes at ca. unmodied carbon black of 0.92.
100–470 and 470–490 ꢀC giving a total weight loss of 13% while
To summarize the surface characterization, we have
the thermogram for [G-2Rh(CO)2] displayed one weight loss demonstrated a straight-forward and effective method for the
ꢀ
process between ca. 350–650 C with a total weight loss of 7% direct immobilization of Rh(I) complexes containing a mixed
(see ESI, section SI-3†). Additionally, we observed that the bidentate carbene-triazole ligand system onto carbon surfaces.
thermogram of [2Rh(CO)2]BPh4 only and
a mixture of The well-dened complexes were covalently anchored on GC,
[2Rh(CO)2]BPh4 and carbon black (see ESI, Fig. S12†) showed CB and G, showing that this method is versatile enough to be
a two weight loss trend from 100–380 ꢀC due to the burn-off of performed on various carbon surfaces with different surface
the unattached metal complex similar to what was observed areas and properties.
from the thermogram of [CB-2Rh(CO)2].
The quantitative amount of Rh on CB and G obtained using
Catalysis
ICP-OES was found to be 2.5 wt% for [CB-2Rh(CO)2] and
Following characterization of the hybrid catalysts, the efficiency
0.84 wt% for [G-2Rh(CO)2]. This corresponds well to the esti-
of homogeneous complex [2Rh(CO)2]BPh4 and hybrid
mated amount of Rh in [CB-2Rh(CO)2] (2.6 wt%) and
complexes [GC-2Rh(CO)2], [CB-2Rh(CO)2] and [G-2Rh(CO)2]
were evaluated as catalysts for the hydrosilylation of dipheny-
lacetylene (4) and triethylsilane at 50 ꢀC and/or 25 ꢀC using
[G-2Rh(CO)2] (1.3 wt%), calculated from the total weight loss
obtained from TGA. For [GC-2Rh(CO)2], cyclic voltammetry was
used to determine the quantity of Rh complexes immobilized
0.02 mol% of Rh relative to the quantity of 4 (Scheme 3).35
on the GC electrode to be 3.4 ꢁ 10ꢂ10 mol cmꢂ2 or 1.2 ꢁ
10ꢂ4 wt% cmꢂ2 (see ESI, section SI-5†).
Transition metal-catalyzed hydrosilylation of alkynes allows the
formation of alkenylsilanes in a straightforward, atom efficient
SEM and TEM images of [G-2Rh(CO)2] showed that the
and convenient method. Alkenylsilanes are versatile organo-
silicon compounds that can be used for further transformations
in many organic synthesis reactions.
At 50 ꢀC, [CB-2Rh(CO)2] and [2Rh(CO)2]BPh4 were both
highly efficient catalysts, with the reaction reaching complete
conversion of substrate 4 to product 5 in just 0.25 h (Fig. 4).
While [G-2Rh(CO)2] was slightly less efficient compared to
[CB-2Rh(CO)2] and [2Rh(CO)2]BPh4, the reaction still reached
56% conversion in 0.5 h and complete conversion in 1 h. The
integrity and morphology of the 2D structure is retained aer
treatment for immobilization of the Rh complex, and is similar
to images of unmodied G (see ESI, section SI-8/9†). The Raman
ꢀ
hydrosilylation reaction was repeated at 25 C (to decrease the
reaction rate to a more readily monitored rate) with [2Rh(CO)2]
BPh4 and [CB-2Rh(CO)2] for better comparison. [CB-2Rh(CO)2]
and [2Rh(CO)2]BPh4 each displayed very similar efficiency as
catalysts for the reaction (Fig. 4). The reaction using
Fig. 3 TGA curves for unmodified CB and G, [G-2Rh(CO)2] and [CB- Scheme
3
Hydrosilylation reaction of diphenylacetylene
4 and
2Rh(CO)2] recorded under an atmosphere of nitrogen.
triethylsilane.
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Table 1 Control experiments for the hydrosilylation reaction
Catalyst
Conv.a (%)
Time (h)
1
2
3
Unmodied CB only
Unmodied G only
G + [2Rh(COD)]BPh4 aer
synthesis treatment
No catalyst
0
0
0.08
24
24
24
4
0
24
a
Determined using 1H NMR spectroscopy.
on the ligand, a covalent hybrid catalyst could not be assembled
using the established synthesis.
Recyclability of hybrid catalysts
Initially, [GC-2Rh(CO)2] was investigated for the hydrosilylation
reaction over the course of three reaction cycles. Aer each run,
the GC electrode was analyzed by XPS. The ratio of N : Rh on the
surface of the GC remained constant at ca. 4.5 : 1 (see ESI,
Table S15†), and this conrmed that the Rh complex remained
on the GC surface even aer multiple runs.
Fig. 4 Graph depicting the % conversion at times 0.25, 0.5, 1, 2 and 48
h
for the hydrosilylation reaction using [2Rh(CO)2]BPh4, [CB-
2Rh(CO)2], [G-2Rh(CO)2] and [GC-2Rh(CO)2] at 50 ꢀC or 25 ꢀC.
The low level of conversion to product 5 (<11%) obtained
over an extended reaction time (48 h) when using [GC-
2Rh(CO)2] led to the investigation of the efficiency of hybrid
catalysts on G and CB. In the recycling experiments for [CB-
2Rh(CO)2] (1.6 mol%) and [G-2Rh(CO)2] (0.3 mol%), the hybrid
catalysts were recycled over three 24 h cycles, and the ltrate
and washings aer each run were analyzed by ICP-MS to
determine the Rh content. Both [G-2Rh(CO)2] and [CB-
2Rh(CO)2] had excellent recycling ability (Fig. 5(a)), with
complete conversion to product 5 aer each run. The excellent
recyclability was reinforced by ICP-MS analysis of the washings
(Fig. 5(b)). When using [G-2Rh(CO)2] as the catalyst, the ICP-MS
showed hardly any leaching aer each run, with less than 6%
loss of Rh from the G surface aer three runs. When using [CB-
2Rh(CO)2] as catalyst instead, an initial 12% loss of Rh from the
CB surface was observed aer the rst cycle (Fig. 5(b)), consis-
tent with the initial loss of the physisorbed Rh. This phys-
isorbed Rh could not be removed prior to catalysis by normal
washing procedures, and most likely requires the reaction
conditions of the catalysis reaction to enable its' removal. Aer
the second and third cycle, there was only a total of 3% loss of
[CB-2Rh(CO)2] as catalyst was slightly slower, reaching 77%
conversion to product 5 in 1 h compared to the complete
conversion obtained using [2Rh(CO)2]BPh4 in the same time
frame. This slightly lower efficiency of [CB-2Rh(CO)2] could be
due to the time required for diffusion of substrate to the carbon
black support compared to the direct mixing of the two in the
homogeneous reaction mixture. On completion, all reactions
had turnover numbers (TON) close to 5000 (see ESI, Table S14†).
These catalysts are the most efficient reported to date for the
hydrosilylation of disubstituted alkyne, both homogeneous36–40
and heterogeneous.41–45
On immobilizing the complex on a GC support ([GC-
2Rh(CO)2]), it was found that although the reaction was slow
and showed only 8% conversion to product 5 in 48 h (Fig. 4), the
TON for this reaction reached the very high value of 48 000 in
that time. This correlates well with the fact that the GC electrode
has a plate-like at structure with a limited surface area
(2.4 cm2) and hence holds only a small number of moles of
immobilized catalyst relative to the substrate whereas there was
a much higher mole ratio of catalyst to substrate immobilized
on the high surface area CB and G. The high TON observed here
for [GC-2Rh(CO)2] is very similar to the turnover numbers we
reported previously for GC supported catalysts.29
ꢀ
In a series of control experiments at 50 C, when using no
catalyst or unmodied CB and G as the catalyst, the hydro-
silylation reaction did not proceed even aer 24 h (Table 1). In
order to conrm that the carbon surfaces are covalently
attached to the Rh(I) complexes, the homogeneous complex
[2Rh(COD)]BPh4, which lacks the aniline functional group that
[1Rh(COD)]BPh4 contains for the purpose of attachment, was
mixed together with some unmodied G. The mixture was then
treated using the same approach used to synthesize the hybrid
catalysts, and the isolated G powder was tested for the hydro-
silylation reaction. There was <0.08% conversion observed aer
24 h, conrming that without the aniline functionalized group
Fig. 5 Graphs depicting: (a) the % conversions of the hydrosilylation
reaction for three reaction cycles when using [CB-2Rh(CO)2] and
[G-2Rh(CO)2)], which are shown on the same graph and (b) the %
rhodium remaining on the carbon surfaces after each reaction cycle.
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5000 to 48 000. These catalysts are the most efficient of the
homogeneous and heterogeneous catalysts reported to date for
promoting the hydrosilylation of disubstituted alkyne. Finally,
the recycling experiments also showed that the hybrid catalysts
could be recycled for at least 10 times with hardly any loss of Rh
from the surface. [G-2Rh(CO)2] was found to be slightly more
recyclable compared to [CB-2Rh(CO)2]. All the results demon-
strate the stability and reusability of the hydrid Rh(I) complexes
containing the carbene-triazole mixed bidentate ligand system.
Fig. 6 Graphs depicting: (a) the % conversions of the hydrosilylation
reaction for 10 reaction cycles when using [CB-2Rh(CO)2] and (b) the
% conversions of the hydrosilylation reaction for 10 reaction cycles
when using [G-2Rh(CO)2].
Experimental
General experimental and materials
Unless otherwise stated, all manipulations were carried out
Rh observed. Again, such results conrm the stability of the under inert atmosphere using standard Schlenk techniques or
immobilized Rh complex on the CB surface. These results also in a nitrogen or argon-lled Braun glovebox. All reagents were
suggest that the use of the mixed bidentate carbene-triazole purchased from Aldrich Chemical Company Inc. or Alfa Aesar
ligand system allows an enhanced capacity to recycle the Rh(I) Inc. and used as received unless otherwise noted. Graphene46
complexes compared to our previously described hybrid was contributed by Professor John Stride (UNSW). Glassy
system.29,30
carbon electrodes (1 ꢁ 1 ꢁ 0.1 cm3) were purchased from
To further investigate the recyclability of the hybrid catalysts Goodfellow. Carbon black (XC-72R) was supplied by Cabot
on G and CB, [CB-2Rh(CO)2] and [G-2Rh(CO)2] were reused over Corporation. RhCl3$xH2O was purchased from Precious Metals
10 cycles of the catalysis reaction. Each cycle was tested aer 4 h Online PMO P/L. For the purposes of air sensitive manipula-
at 25 ꢀC (Fig. 6). [G-2Rh(CO)2] was found to be an extremely tions and in the preparation of air sensitive metal complexes,
recyclable heterogeneous catalyst and complete (>98%) tetrahydrofuran, dichloromethane, acetonitrile and pentane
conversions were obtained for all 10 cycles (Fig. 6(b)). When were dispensed from a PuraSolv solvent purication system.
recycling [CB-2Rh(CO)2], complete conversion (>98%) of the Methanol was distilled from magnesium methoxide under an
reaction was obtained for the rst 7 cycles. Over the following atmosphere of nitrogen or argon. The bulk compressed gases
8–10 cycles, there was a slight 10–20% reduction in product argon (>99.999%) and carbon monoxide (>99.5%) were ob-
formation (Fig. 6(a)), which suggests that [CB-2Rh(CO)2] is tained from Air Liquide and used as received. Nitrogen gas for
slightly less recyclable compared to [G-2Rh(CO)2]. The high Schlenk line operation comes from in-house liquid nitrogen
recyclability of the hybrid catalysts shown here supports the boil-off. 1-Azido-4-nitrobenzene,47 [Rh(COD)Cl]2,48 1-mesityl-3-
benet of immobilizing the highly efficient and well-dened (prop-2-yn-1-yl)-1H-imidazol-3-ium bromide49 and 1-mesityl-3-
homogeneous catalyst [2Rh(CO)2]BPh4 on the surface. ((1-phenyl-1H-1,2,3-triazol-4-yl)methyl)-1H-imidazol-3-ium tet-
[2Rh(CO)2]BPh4 is also shown to be a highly efficient homoge- raphenylborate49 were synthesized using literature procedures.
neous catalyst for the hydrosilylation of diphenylacetylene as it 1H and 13C{1H} NMR spectra were recorded on Bruker DMX400,
can catalyze up to 5 cycles of extra substrate addition before DMX500 and DMX600 spectrometers operating at 400, 500 and
reaching a saturation point in the reaction mixture (see ESI, 600 MHz (1H) respectively and 100, 125 and 150 MHz (13C)
1
Fig. S22†). The development of the hybrid catalysts from the respectively. H and 13C NMR chemical shis were referenced
optimized homogeneous catalyst allows us to exploit the reus- internally to residual solvent resonances. Unless otherwise
ability of the homogeneous catalyst, providing us with highly stated, spectra were recorded at 298 K and chemical shis (d),
1
recyclable hybrid catalyst that also allows for easy product with uncertainties ꢃ0.01 Hz for H and ꢃ0.05 Hz for 13C, are
separation.
quoted in parts per million, ppm. Coupling constants (J) are
quoted in Hz and have uncertainties of ꢃ0.05 Hz for 1H–1H and
ꢃ0.5 Hz for 13C–103Rh and 13C–11B. Numbering and gures
corresponding to NMR characterization data are located in the
Conclusions
In conclusion, during the investigations of a series of hybrid ESI, Table S1.† Deuterated solvents were purchased from
complexes on G, CB and GC as catalysts for the hydrosilylation Cambridge Stable Isotopes and used as received. Air sensitive
of diphenylacetylene, it was found that [CB-2Rh(CO)2] was the NMR samples were prepared in an inert gas glovebox or by
most efficient catalyst and [G-2Rh(CO)2] was slightly less effi- vacuum transfer of deuterated solvents into NMR tubes tted
cient than [CB-2Rh(CO)2]. While [GC-2Rh(CO)2] did not reach with a Young's Teon valve. For air sensitive NMR samples,
the same conversion levels over time, the very high TON ach- dichloromethane-d2 and chloroform-d were distilled over
ieved demonstrated excellent efficiency of this catalyst as well. calcium hydride. Microanalyses were carried out at the Camp-
The results also showed hybrid complex [CB-2Rh(CO)2] was as bell Micro-analytical Laboratory, University of Otago, New Zea-
efficient as the homogeneous complex [2Rh(CO)2]BPh4 in land or at the Research School of Chemistry, The Australian
catalyzing this reaction. The catalysts investigated here could be National University, Canberra, Australia. Mass spectra were
utilized in very small amounts to give high TONs ranging from acquired using a Thermo LTQ Orbitrap XL located in the
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Bioanalytical Mass Spectrometry Facility (BMSF) in UNSW. M is p-CH3 of Mes), 2.05 (s, 6H, o-CH3 of Mes) ppm; 13C{1H} NMR
dened as the molecular weight of the compound of interest or (100 MHz, CDCl3): d 147.5 (Cq of ArNH2), 141.7 (p-CCH3 of Mes),
cationic fragment for cationic metal complexes.
140.7 (Tz-C40), 137.9 (Im-C2), 134.3 (o-CCH3 of Mes), 130.7 (ipso-
C of Mes), 130.04 (m-CH of Mes), 128.2 (ipso-C to NH2 of ArNH2),
123.8 (Tz-C50), 123.3 (Im-C5), 122.94 (Im-C4), 122.3 (o-CH of
ArNH2), 115.4 (m-CH of ArNH2), 44.7 (CH2), 21.2 (p-CH3 of Mes),
17.8 (o-CH3 of Mes) ppm; HRMS (ESI+, MeOH): m/z (%): calcu-
lated for [C21H23N6]+ ¼ [M]+ ¼ 359.1979; found [M]+ ¼ 359.1974
(100%) amu; elemental analysis: found C, 57.70; H, 5.55; N,
19.03; calculated for C21H23BrN6: C, 57.41; H, 5.28; N, 19.13%.
1-Mesityl-3-((1-(4-nitrophenyl)-1H-1,2,3-triazol-4-yl)methyl)-
1H-imidazol-3-ium bromide, nitro functionalized ligand
precursor to ligand 1
1-Mesityl-3-(prop-2-yn-1-yl)-1H-imidazol-3-ium bromide (1020
mg, 3.33 mmol) was reacted with 1-azido-4-nitrobenzene (547
mg, 3.33 mmol) in a deoxygenated solution of isopropanol : -
water (2 : 1, 50 mL). Aer stirring for 5 minutes, sodium
ascorbate (132 mg, 0.666 mmol, 20 mol%) followed by copper
sulfate pentahydrate (41.3 mg, 0.165 mmol, 5 mol%) was added
to the orange mixture. The reaction mixture was stirred for 24
hours to give an orange-red precipitate in an orange solution.
The precipitate was collected via ltration and washed thor-
oughly with a saturated solution of Na2EDTA until the ltrate
turned from green to colourless. The precipitate was then dried
under vacuum to give the ligand as a red solid. The ligand can
be recrystallized from dichloromethane : pentane as red crys-
tals (712 mg, 46%). 1H NMR (400 MHz, CDCl3): d 10.31 (br s, 1H,
[1Rh(COD)]BPh4
Ligand 1 (189 mg, 0.429 mmol) was dissolved partially as
a suspension in acetone (25 mL). Ag2O (87.6 mg, 0.378 mmol)
was added to the suspension prior to heating the reaction
mixture at reux under argon for one hour. Aer cooling the
reaction mixture to room temperature, [Rh(COD)Cl]2 (106 mg,
0.214 mmol) was added. The reaction mixture was heated again
at reux for another one hour and then stirred at room
temperature overnight under argon. The mixture was ltered
through Celite® which was washed thoroughly with excess
acetone. The solvent was removed in vacuo and the crude
product was dissolved in dichloromethane (15 mL). NaBPh4
(147 mg, 0.430 mmol) was added and white solid precipitates
immediately. The reaction mixture was stirred vigorously at
room temperature for 45 minutes to ensure complete reaction.
The mixture was ltered through Celite® which was washed
thoroughly with dichloromethane. The solution was concen-
trated in vacuo to ca. 2 mL which was added slowly dropwise to
a vigorously stirred solution of pentane (30 mL) in an ice bath.
Yellow precipitate forms immediately. Aer ve minutes of
stirring, the solvent was ltered and the precipitate dried under
vacuum to give [1Rh(COD)]BPh4 as a bright yellow solid
(323 mg, 85%). 1H NMR (400 MHz, CD2Cl2): d 7.40 (m, 8H, o-CH
3
Im-H2), 9.63 (s, 1H, Tz-H50), 8.38 (d, J ¼ 9.3 Hz, 2H, m-CH of
3
3
ArNO2), 8.13 (d, J ¼ 9.3 Hz, 2H, o-CH of ArNO2), 8.08 (t, J ¼
1.5 Hz, 1H, Im-H5), 7.14 (t, 3J ¼ 1.5 Hz, 1H, Im-H4), 6.98 (s, 2H,
m-CH of Mes), 6.22 (s, 2H, CH2), 2.32 (s, 3H, p-CH3 of Mes), 2.04
(s, 6H, o-CH3 of Mes) ppm; 13C{1H} NMR (100 MHz, CDCl3):
147.5 (Cq of PhNO2), 142.0 (Tz-C40), 141.7 (p-CCH3 of Mes), 140.9
(ipso-C to NH2 of PhNH2), 138.1 (Im-C2), 134.2 (o-CCH3 of Mes),
130.6 (ipso-C of Mes), 130.0 (m-CH of Mes), 125.6 (m-CH of
ArNO2), 124.9 (Tz-C50), 123.4 (Im-C5), 123.3 (Im-C4), 120.9 (o-CH
of ArNO2), 44.5 (CH2), 21.2 (p-CH3 of Mes), 17.8 (o-CH3 of Mes)
ppm; HRMS (ESI+, MeOH): m/z (%): calculated for
[C21H23N6O2]+ ¼ [M]+ ¼ 389.1721; found [M]+ ¼ 389.1718
(100%) amu; elemental analysis: found C, 51.85; H, 4.68; N,
17.21; calculated for C21H23BrN6O2$0.25CH2Cl2: C, 52.03; H,
4.42; N, 17.13%.
3
of BPh4), 7.24 (d, JH–H ¼ 8.9 Hz, 2H, o-CH of ArNH2), 7.04 (m,
2H, m-CH of Mes overlapped with Tz-H50), 7.04 (s, 1H, Tz-H50
overlapped with m-CH of Mes), 7.00 (t, 3JH–H ¼ 7.5 Hz, 8H, m-CH
of BPh4), 6.89 (d, 3JH4–H5 ¼ 1.9 Hz, 1H, Im-H5), 6.85 (t, 3JH–H
¼
3-((1-(4-Aminophenyl)-1H-1,2,3-triazol-4-yl)methyl)-1-mesityl-
1H-imidazol-3-ium bromide, (1)
The nitro functionalized ligand precursor above (605 mg, 6.72 (d, 3JH–H ¼ 8.9 Hz, 2H, m-CH of ArNH2), 4.92 (m, 2H, CH of
1.29 mmol) was dissolved in methanol (30 mL). Pd/C (10% w/w, COD), 4.76 (s, 2H, CH2), 4.02 (s, 2H, NH2), 3.64 (m, 2H, CH of
54.4 mg, 0.0511 mmol, 4 mol%) was then added to the orange COD), 2.37 (s, 3H, p-CH3 of Mes), 2.25 (m, 2H, CH2 of COD), 2.10
solution. Aer stirring for 5 minutes, hydrazine hydrate (1.3 mL, (s, 6H, o-CH3 of Mes), 1.97 (m, 6H, CH2 of COD) ppm; 13C{1H}
7.1 Hz, 3H, p-CH of BPh4), 6.75 (d, 3JH4–H5 ¼ 1.9 Hz, 1H, Im-H4),
1
26.8 mmol) was added slowly to the reaction mixture. The red- NMR (100 MHz, CD2Cl2): d 176.2 (d, JRh–C ¼ 51.2 Hz, Im-C2),
1
black mixture was heated at reux under argon overnight. The 164.4 (q, JB–C ¼ 49.3 Hz, o-CH of BPh4), 149.0 (Cq of ArNH2),
mixture was ltered through Celite® which was washed thor- 140.1 (p-CCH3 of Mes), 140.0 (Cq of Tz), 136.3 (br s, o-CH of
oughly with excess methanol. The ltrate was dried with anhy- BPh4), 135.9 (o-CCH3 of Mes), 135.5 (ipso-C of Mes), 129.5 (m-CH
drous magnesium sulfate, ltered and the solvent was of Mes), 127.1 (ipso-C to NH2 of ArNH2), 126.2 (q, 4JB–H ¼ 2.6 Hz,
evaporated to give the crude product as a yellow oil. The crude m-CH of BPh4), 123.1 (Im-C4), 122.6 (o-CH of ArNH2), 122.5 (Im-
product was recrystallized from a dichloromethane : pentane C5), 122.3 (p-CH of BPh4), 121.2 (Tz-C50), 115.3 (m-CH of ArNH2),
2
2
mixture to yield ligand 1 as a uffy pale yellow solid (526 mg, 96.7 (d, JRh–C ¼ 7.9 Hz, 2C, CH of COD), 79.1 (d, JRh–C ¼ 12.4
93%). 1H NMR (400 MHz, CDCl3): d 10.4 (br s, 1H, Im-H2), 8.93 Hz, 2C, CH of COD), 45.1 (CH2), 32.6 (2C, CH2 of COD), 29.2 (2C,
(s, 1H, Tz-H50), 8.03 (apparent t, 3J ¼ 1.5 Hz, 1H, Im-H5), 7.50 (d, CH2 of COD), 21.2 (p-CH3 of Mes), 18.6 (o-CH3 of Mes) ppm;
3J ¼ 8.8 Hz, 2H, o-CH of ArNH2), 7.10 (apparent t, J ¼ 1.5 Hz, HRMS (ESI+, MeOH): m/z (%): calculated for [C29H34N6Rh]+ ¼
3
1H, Im-H4), 7.00 (s, 2H, m-CH of Mes), 6.74 (d, 3J ¼ 8.5 Hz, 2H, [M]+ ¼ 569.1900; found [M]+ ¼ 569.1893 (100%); calculated for
m-CH of ArNH2), 6.23 (s, 2H, CH2), 3.93 (s, 2H, NH2), 2.34 (s, 3H, [M ꢂ C8H11Rh]+ ¼ 359.1979; found [M ꢂ C8H11Rh]+ ¼ 359.1977
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(78%) amu; elemental analysis: found C, 71.62; H, 6.09; N, 9.44; m-CH of Ph), 7.44 (m, 8H, o-CH of BPh4), 7.06 (s, 2H, m-CH of
calculated for C53H54BN6Rh: C, 71.62; H, 6.12; N, 9.46%.
Mes), 6.98 (t, 3JH–H ¼ 7.5 Hz, 8H, m-CH of BPh4 overlapped with
3
Im-H4), 6.97 (d, JH4–H5 ¼ 1.8 Hz, 1H, Im-H4 overlapped with
3
m-CH of BPh4), 6.93 (d, JH4–H5 ¼ 1.8 Hz, 1H, Im-H5), 6.82 (t,
[2Rh(COD)]BPh4
3JH–H ¼ 7.2 Hz, 4H, p-CH of BPh4), 6.70 (s, 1H, Tz-H50), 4.41 (s,
2H, CH2), 2.38 (s, 3H, p-CH3), 2.06 (s, 6H, o-CH3) ppm; 13C{1H}
NMR (125 MHz, CD2Cl2): d 184.7 (d, 1JRh–C ¼ 55.9 Hz, CO), 184.6
Sodium ethoxide (14.7 mg, 0.225 mmol) was added to a stirred
suspension of [Rh(COD)Cl]2 (53.5 mg, 0.109 mmol) in ethanol
(10 mL). The orange-yellow mixture was stirred at room
temperature for 10 minutes. A solution of 1-mesityl-3-((1-
1
1
(d, JRh–C ¼ 70.6 Hz, CO), 172.1 (d, JRh–C ¼ 46.9 Hz, Im-C2),
164.5 (q, 1JB–C ¼ 49.3 Hz, ipso-C of BPh4), 141.2 (p-CCH3 of Mes),
140.8 (Cq of Tz), 136.2 (br s, o-CH of BPh4), 136.1 (o-CCH3), 135.7
(ipso-C of Ph), 135.2 (ipso-C of Mes), 131.1 (p-CH of Ph), 130.5 (o-
CH of Ph), 129.9 (m-CH of Mes), 126.4 (q, 4JB–C ¼ 2.6 Hz, m-CH of
BPh4), 124.1 (Im-C5), 123.4 (Im-C4), 122.5 (p-CH of BPh4), 122.3
(Tz-C50), 121.4 (m-CH of Ph), 44.6 (CH2), 21.3 (p-CH3 of Mes),
18.4 (o-CH3 of Mes) ppm; HRMS (ESI+, MeOH): m/z (%): calcu-
phenyl-1H-1,2,3-triazol-4-yl)methyl)-1H-imidazol-3-ium
tetra-
phenylborate ligand4 (144 mg, 0.217 mmol) in tetrahydrofuran
(20 mL) was added slowly dropwise into the stirred suspension.
The reaction mixture was stirred at room temperature for three
hours. The solvent was removed in vacuo to give the crude
product as a bright yellow solid. The crude product was dis-
solved in dichloromethane and ltered through Celite® which
was washed thoroughly with excess dichloromethane. The
solvent was concentrated to ca. 2 mL. Pentane (25 mL) was then
added slowly and a bright yellow solid precipitates. The solvent
was ltered off and the solid was dried under vacuum to yield
[2Rh(COD)]BPh4 as a bright yellow solid (126 mg, 64%). 1H NMR
(400 MHz, CDCl3): d 7.55 (m, 8H, o-CH of BPh4), 7.48 (m, 3H, o-
lated for [C23H21N5O2Rh]+ ¼ [M]+ ¼ 502.0750; found [M]+
¼
502.0703 (100%) amu; FTIR (solid) nCO ¼ 2083, 2019 (s) cmꢂ1
;
elemental analysis: found C, 66.42; H, 4.95; N, 8.49; calculated
for C47H41BN5O2Rh$0.5CH2Cl2: C, 66.03; H, 4.90; N, 8.52%.
Immobilization of [CB-2Rh(CO)2] and [G-2Rh(CO)2]
and p-CH of Ph), 7.38 (m, 2H, m-CH of Ph), 7.02 (s, 2H, m-CH of The complex [1Rh(COD)]BPh4 (24.8 mg, 0.0279 mmol) was dis-
Mes overlapped with m-CH of BPh4), 7.00 (t, 3JH–H ¼ 7.2 Hz, 8H, solved in a degassed solution of nitromethane (10 mL). The
3
m-CH of BPh4 overlapped with m-CH of Mes), 6.84 (t, JH–H
¼
mixture was cooled in an ice bath for 15 minutes before
7.3 Hz, 4H, p-CH of BPh4), 6.76 (d, 3JH4–H5 ¼ 1.8 Hz, 1H, Im-H5), concentrated hydrochloric acid (0.5 mL, 0.5 M) was added
6.65 (d, 3JH4–H5 ¼ 1.8 Hz, 1H, Im-H4), 6.58 (s, 1H, Tz-H50), 4.89 dropwise into the stirred yellow solution while keeping the
(m, 2H, CH of COD), 4.44 (s, 2H, CH2), 3.63 (m, 2H, CH of COD), mixture between 0–5 ꢀC. Sodium nitrite (1.90 mg, 0.0279 mmol)
2.38 (s, 3H, o-CH3 of Mes), 2.25 (m, 2H, CH2 of COD), 2.09 (s, 6H, was then added and the mixture was stirred vigorously for
p-CH3 of Mes), 1.96 (m, 6H, CH2 of COD) ppm; 13C{1H} NMR another 5 minutes. Aer taking the mixture out of the ice bath,
(100 MHz, CDCl3): d 175.3 (d, 1JRh–C ¼ 52.7 Hz, Im-C2), 164.4 (q, carbon black or graphene (50 mg) was added while the mixture
1JB–C ¼ 49.6 Hz, ipso-C of BPh4), 140.1 (Cq of Tz), 139.8 (p-CCH3 was stirred vigorously to ensure that the carbon powders was
of Mes), 136.3 (br s, o-CH of BPh4), 135.8 (o-CCH3 of Mes), 135.6 diffused in solution properly. The mixture was allowed to
(ipso-C of Ph), 135.2 (ipso-C of Mes), 130.1 (p-CH of Ph), 130.0 (o- gradually warm up to room temperature. The mixture was stir-
CH of Ph), 129.4 (m-CH of Mes), 126.0 (q, 4JB–C ¼ 2.8 Hz, m-CH red under nitrogen at room temperature overnight. The modi-
of BPh4), 122.8 (Im-C5), 122.6 (Im-C4), 122.1 (p-CH of BPh4), ed carbon powder was separated from the reaction media via
121.8 (Tz-C50), 120.8 (m-CH of Ph), 96.3 (d, 2JRh–C ¼ 7.8 Hz, 2C, centrifugation (4000 rpm) for 5 minutes. It was consecutively
2
CH of COD), 78.9 (d, JRh–C ¼ 12.5 Hz, 2C, CH of COD), 44.6 washed and stirred vigorously with a methanol : MilliQ water
(CH2), 32.4 (2C, CH2 of COD), 29.0 (2C, CH2 of COD), 21.2 (p- (50 : 50, 20 mL) and methanol (5 ꢁ 10 mL) followed by centri-
CH3 of Mes), 18.6 (o-CH3 of Mes) ppm; HRMS (ESI+, MeOH): m/z fugation aer each washing. The washed powder was dried
(%): calculated for [C29H33N5Rh]+ ¼ [M]+ ¼ 554.1786, found [M]+ under vacuum in a dessicator for 24 hours. The modied carbon
¼ 554.1776 (100%) amu; elemental analysis: found C, 69.76; H, powders was then dispersed with vigorous stirring in a dichlor-
6.19 and N, 7.80; calculated for C53H53BN5Rh$0.5CH2Cl2: C, omethane : pentane solution (1 : 1, 20 mL). The mixture was
70.13; H, 5.94 and N, 7.64%.
degassed via three cycles of freeze–pump–thaw and then placed
under an atmosphere of carbon monoxide in a balloon (1 atm).
The mixture was le to react for one hour before the modied
powder was separated from the reaction solvent via centrifuga-
tion. The modied powder was washed and stirred vigorously
with pentane (3 ꢁ 10 mL) and methanol (3 ꢁ 10 mL) followed by
centrifugation aer each washing. The washed powder was then
dried under vacuum in the dessicator for 24 hours to give either
[CB-2Rh(CO)2] (37.2 mg, 72%) and [G-2Rh(CO)2] (32.0 mg, 68%).
[2Rh(CO)2]BPh4
[2Rh(COD)]BPh4 (117 mg, 0.133 mmol) was dissolved in
dichloromethane (15 mL). The yellow solution was degassed via
three cycles of freeze–pump–thaw. The reaction mixture was
placed under an atmosphere of carbon monoxide gas in
a balloon. The solution turned pale yellow in colour aer stir-
ring at room temperature for one hour. Pentane (45 mL) was
added and yellow solid precipitates in solution with vigorous
stirring. The solvent was ltered and the solid was washed with
Immobilization of [GC-2Rh(CO)2]
pentane (2 ꢁ 2 mL). The precipitate was dried in vacuo to yield The glassy carbon electrode was polished sequentially in
1
[2Rh(CO)2]BPh4 as a pale yellow solid (75.6 mg, 70%). H NMR alumina slurries (distilled water suspension) of 1.0, 0.3 and
(500 MHz, CD2Cl2): d 7.59 (m, 3H, o- and p-CH of Ph), 7.53 (m, 0.05 mm on micropolishing cloth (Buehler, IL, USA) and rinsed
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with distilled water between each step. It was then washed with methanol thoroughly (6 mL) and the electrode was dried
thoroughly with ethanol and soaked for 30 min in dichloro- under a gentle ow of nitrogen for 1 minute and reused
methane. The complex [1Rh(COD)]BPh4 (24.8 mg, 0.0279 mmol) subsequently. The carbon–rhodium catalysts on carbon black
was dissolved in a degassed solution of nitromethane (10 mL). and graphene was washed with methanol (3 ꢁ 2 mL) and
The mixture was cooled in an ice bath for 5 minutes before centrifuged to remove the solvent used in between each cycle.
concentrated hydrochloric acid (0.5 mL, 0.5 M) was added The carbon–rhodium catalyst was then dried under vacuum and
dropwise into the stirred yellow solution while keeping the reused in the next cycle. All supernatant and washings were
mixture between 0–5 ꢀC. Sodium nitrite (1.90 mg, 0.0279 mmol) collected and analyzed by ICP-MS.
was then added followed by the glassy carbon (GC) electrode.
The mixture was le to sit in the ice bath for a further hour
before allowing the mixture to gradually warm up to room
Acknowledgements
temperature and remained at room temperature overnight. The
Financial support from the University of New South Wales and
modied GC electrodes was separated from the reaction media
the Australian Research Council are gratefully acknowledged.
and washed with MilliQ water (5 mL). It was then consecutively
This research was supported under the Australian Research
washed with a methanol : MilliQ water (50 : 50, 20 mL) and
Council's Discovery Projects funding scheme (project numbers
methanol (5 ꢁ 10 mL). The washed modied GC electrode was
DP130101838 and DP150103065). We are grateful to Prof. John
dried under a gentle ow of nitrogen. The modied GC elec-
Stride for providing the graphene and Cabot Corporation for the
trode was then dispersed in a dichloromethane : pentane
donation of the carbon black (Vulcan XC-72R). We thank Dr
solution (1 : 1, 20 mL). The mixture was degassed via three
Karen Privat of the Mark Wainwright Analytical Centre (UNSW)
cycles of freeze–pump–thaw and then carbon monoxide was
for collecting the SEM images and Mr Fei Han for collecting the
slowly bubbled into the mixture for 15 minutes. The modied
TEM images. CMW thanks the Australian Government for the
GC electrode was washed with pentane (3 ꢁ 10 mL) and then
award of an International Postgraduate Research Scholarship
methanol (3 ꢁ 10 mL). The washed powder was then dried
(IPRS).
under a gentle ow of nitrogen to give with [GC-2Rh(CO)2].
Notes and references
Procedure for hydrosilylation reaction using the
homogeneous catalyst, glassy carbon (GC), carbon black (CB)
and graphene (G) hybrid catalysts
1 Heterogenized Homogeneous Catalysts for Fine Chemicals
Production, ed. P. Barbaro and F. Liguori, Springer,
The reactions were performed in glass v-vials tted with
a magnetic stirring bar and a Teon screw-caps. For the glassy
carbon hybrid catalyst, the electrode (1 ꢁ 1 ꢁ 0.1 cm), diphe-
nylacetylene (0.1 M), triethylsilane (0.2 M) and tetrahydrofuran
(2 mL) were weighed into the glass vial in the glovebox. For the
homogenous catalyst (0.3 mg), carbon black (0.8 mg) or gra-
phene (0.5 mg) hybrid catalysts, the catalyst (0.02 mol% Rh
relative to amount of diphenylacetylene) and diphenylacetylene
(3.3 M) were weighed in air into the vial. The vials was then
brought into the glovebox and triethylsilane (6.6 M) and tetra-
hydrofuran (0.06–0.6 mL) was added. The reactions vials were
stirred and heated in an oil bath at 50 ꢀC for 0.25–24 hours. At
set time intervals until completion of the reaction, the reaction
vial was removed from the oil bath and immediately cooled in
an ice bath to room temperature.
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Chem. Sci.