Palladium nanoparticles supported on graphene as catalysts for the dehydrogenative coupling of hydrosilanes and amines

Article abstract of DOI:10.1039/c4cy01486c

Palladium nanoparticles (Pd NPs) were supported on undoped and N- or B-doped graphenes (Gs) and these materials have been used as catalysts for the dehydrogenative coupling of hydrosilanes and amines to form silazanes. Working under optimal conditions, a conversion over 99% and a selectivity of 84% were achieved for the reaction of dimethylphenylsilane with morpholine. In contrast, copper (Cu NPs) or nickel nanoparticles (Ni NPs) supported on G did not promote the formation of the corresponding Si-N coupling product. It was found that Pd/G performed better for this coupling than analogous catalysts, in which Pd NPs were supported on active carbon, multiwall carbon nanotubes or diamond NPs. Pd/G as a catalyst has a wide substrate scope, including aliphatic and aromatic amines and mono or dihydrosilanes. Pd/G undergoes a gradual deactivation due to the growth and partial agglomeration of Pd NPs and the aggregation of G sheets, as observed by TEM. This journal is

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ARTICLE TYPE
Palladium nanoparticles supported on graphene as catalyst for the
dehydrogenative coupling of hydrosilanes and amines.
Juan Francisco Blandez, Iván Esteve-Adell, Mercedes Alvaro and Hermenegildo García*
Instituto Universitario de Tecnología Química CSIC-UPV and Departamento de Química, Univ. Politécnica de Valencia, Av de los Naranjos s/n, 46022
5
Valencia, Spain.
E-mail: hgarcia@qim.upv.es
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
10 Palladium nanoparticles (Pd NPs) have been supported on undopped and Nꢀ or Bꢀdoped graphenes (Gs)
and these materials have been used as catalysts for the dehydrogenative coupling of hydrosilanes and
amines to form silazanes. Working under optimal conditions, a conversion over 99 % and selectivity of
84 % were achieved for the reaction of dimethylphenylsilane with morpholine. In contrast, copper (Cu
NPs) or nickel nanoparticles (Ni NPs) supported on G did not promote the formation of the corresponding
15 SiꢀN coupling product. It was found that Pd/G performed better for this coupling than analogous catalysts
in where Pd NPs were supported in active carbon, multiwall carbon nanotubes or diamond NPs. Pd/G as
catalyst has a wide substrate scope, including aliphatic and aromatic amines and mono or dihydrosilanes.
Pd/G undergoes a gradual deactivation due to the growth and partial agglomeration of Pd NPs and
aggregation of the G sheets, as observed by TEM.
45 In this context, the present manuscript reports the catalytic
activity of doped G supported Pd NPs for the dehydrogenative
coupling of hydrosilanes and amines (Eq. 1).
20 1. Introduction
Due to the unique properties derived from its 2D morphology and
the oneꢀatom thickness, graphene (G) offer unique properties as
support for metal NPs.1ꢀ7 The interest of G and related material as
supports in catalysis derives from its large specific surface area
25 that can be as large as 2500 m2×gꢀ1 8 as well as from the strength
,
of the interaction with the supported metal NPs arising from the
overlap of the extended π orbital of G with the d orbitals of
transition metal atoms at the interface of G and the metal NP. G is
constituted by one atom thick layer of sp2 carbons in hexagonal
30 arrangement and, therefore, the use of this type of material as
support represents the physical limit of miniaturization
corresponding ideally to the minimum thickness for surface
area.9ꢀ11 In addition, the properties of G as support can be
modified and its electronic effects tuned by the presence of
35 defects, oxygenated functional groups and heteroatoms.12ꢀ17
Theoretical calculations at the DFT level with G models with one
carbon atom vacancy interacting with transitions metals has
shown that this type of defect characterized by dangling bonds on
G is particularly suited to interact with metal atoms.18 Similar
40 type of computational calculations also indicate that heteroatoms
on the G sheet can stabilize metal NPs. The so far reported data in
the literature support that G is a unique material offering as
support many opportunities to enhance the catalytic activity of
metal NPs.
This reaction between hydrosilanes and amines has been reported
50 in the literature with homogeneous catalysts using mainly Pd
20
complexes.19,
Related to the dehydrogenative coupling of
hydrosilanes and amines, the analogous reaction between
hydrosilanes and alcohols has been recently reported to be
performed using heterogeneous catalysts.21 In particular, it has
55 been found that supported Au NPs can efficiently act as
heterogeneous catalysts for the reaction between hydrosilanes and
alcohols, giving rise to the corresponding siloxane. Considering
these precedents, it could also be that the coupling of
hydrosilanes and amines can equally be promoted by supported
60 metal NPs, Pd NPs being an obvious choice considering the
known catalytic activity of Pd complexes for this reaction.19, 20
The commercial interest of the resulting silazanes stems from
22ꢀ25
their applications as silylating agents
material surfaces.
and for coating of
65
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Fig.1 a) and b) TEM images of Pd/G at different magnifications. The
inset shows the Pd particle size distribution. c) AFM image of a layer of
exfoliated Pd/G catalyst and d) its section profile.
2. Results and Discussion
A series of different Gs were prepared and used as supports of
metals NPs. These materials were obtained, as reported, by
pyrolysis under inert atmosphere of alginate or chitosan
conveniently modified or not by boric acid.26ꢀ28 The resulting
carbonaceous residues after pyrolysis can be exfoliated without
the need of additional chemical oxidation to form single and few
layer G suspensions in different solvents. This G material was
used as support of metal NPs that were prepared by the polyol
30 Fig. S1 in the supporting information also shows a highꢀ
resolution image of Pd NPs in which the expected lattice spacing
of 0.27 nm corresponding to the (1.1.1) facets of Pd can be
clearly observed, as well as the different orientations of the
planes and the existence of nanodomains. Analysis of a
35 statistically relevant number of these Pd shows that these metal
NPs do not present a preferential facet. Supporting information
provides the selected area electron diffraction pattern of one of
these Pd NPs.
5
10 method, using ethyleneglycol as reducing agent.29 Scheme 1
illustrates the process followed for the synthesis of metal NPs
supported on Gs.
Although TEM shows the expected wrinkles and light contrast
40 expected for single layer G, this technique is generally not
considered as conclusive with respect to the single layer
morphology of Gs. The single and few layers configuration of the
suspended G platelets is more safely determined by AFM
measurements of the vertical height of a statistically relevant
45 number of G platelets (Fig. 1). The G structure and the presence
of defects was established by Raman spectroscopy in where the
characteristic graphitic (G) and defect (D) peaks appearing at
about 1600 and 1350 cmꢀ1 were observed. In agreement with the
predominant singleꢀlayer morphology of G, the presence of a
50 weak band at about 2690 cmꢀ1 was also observed. A quantitative
estimation of the relative importance of defects for the various G
samples can be obtained from the relative intensity ratio of the G
versus the D bands.30ꢀ33 ID/IG ratio of 0.84 was measured that is a
value notable much lower than those typically reported in the
55 literature for G samples obtained by reduction of graphene
i)
Graphene (G)
Polysaccharide
Graphene +
Cu/Graphene
ii)
iii)
Reaction
mixture
Ethylene glycol
+ metal solution
Ni/Graphene
Pd/Graphene
Scheme 1 Procedure employed for the preparation of the G materials
15 under study. Corresponding characterization data are provided in the
o
35
oxide34, that are about 1.4. This indicates that the G sample
supporting information. i) pyrolysis at 900 C under Ar, ii) sonication for
1 h, iii) reduction for 24 h in reflux at 120 oC.
used here has lower density of defects. The surface area of G
suspended in aqueous solution was estimated by the methylene
37
blue (MB) method.36, In these measurements, the amount of
Specifically the doping content and the distributions of the doped
20 element in two different sites were determined by quantitative
XPS analysis.26 The layer morphology and the hexagonal
arrangement of the C atoms were established by high resolution
TEM imaging together with selected area electron diffraction.
Fig. 1 shows representative images of the samples to illustrate the
25 morphology of G and the particle size distribution of Pd NPs.
60 MB adsorbed on G is determined by the decrease of the intensity
of the visible absorption band of MB upon adsorption on G and
removal of MBꢀG conjugate. The specific surface area was
estimated 1550 m2×gꢀ1, in the range, but lower, of the theoretical
maximum area of completely exfoliated G materials.8 Overall, all
65 the available characterization data of the G materials employed in
the present study are in agreement with reported literature data
that has previously established the formation of undoped or
doped G suspensions following the procedure employed in the
present study.26
70
In the initial stage of our work and in order to optimize the
reaction conditions and select the most active catalyst, the
dehydrogenative coupling of dimethylphenylsilane and
morpholine was chosen as model reaction. The influence of the
75 reaction at temperature from 60 to 120 oC, as well as the nature of
coꢀsolvent and catalyst was studied.
2
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Table
1
Results
of the dehydrogenative
coupling
of
dimethylphenylsilane (1a) and morpholine (2a) in the presence of
different catalysts with or without metal NPs supported on G.
Catalyst
T (ºC)
t (h)
Conversion (%)b Selectivity (%)b
1
2
3
4
5
6
7
8
9
G
60
60
60
60
60
60
60
120
120
120
120
24
24
24
24
24
24
3
24
24
0.5
48
20
22
7
17
30
6
70
0
67
99
47
0
0
0
0
0
0
67
0
2
Cu/G
Cu/(N)G
Cu/(B)G
Cu/D
Ni/G
Pd/G
ꢀ
Fig. 2 Plot of the logarithm of the reaction rate constant (k) vs the inverse
of the absolute temperature (T) for the dehydrogenative coupling of
45 dimethylphenylsilane (1a) and morpholine (2a). The inset shows the
Cu/G
o
o
temporal profile of product formation at 80 C (
♦
), 100 C (ꢀ) and 120
10 Pd/G
11
89
12
oC (▲).
G
12 Pd/NH2G 120
3
99
74
a Reaction conditions: 1a (5 mmol), 2a (10 mmol), catalyst (0.05 mol%),
Ar atmosphere. b Conversion and selectivity were determined by GC
using internal standard.
The activation energy (Ea) estimated from the Arrhenius plot
(Fig. 2) was 22.9 kJ×molꢀ1 that is a low Ea value, but in the range
50 of those determined for the Cuꢀcatalyzed dehydrogenative
coupling of hydrosilane and alcohols.37 Therefore, a suitable
reaction temperature was 120 oC. Working at this temperature
and a Pd/substrate mol ratio of 0.05 %, full conversion 1a was
obtained in 30 min (Table 1, entry 10).
55 With regard to the influence of coꢀsolvents, it was observed that
the presence of 1,4ꢀdioxane or toluene (Table 2, entries 4 and 5)
is highly detrimental for the catalytic activity that disappear
completely when any of these two solvents was present. Thus,
reactions were carried out in the absence of coꢀsolvents. The
60 morpholineꢀhydrosilane mol ratio was also optimized. Initially,
an amineꢀhydrosilane mol ratio of 2 was selected, considering
that the amine should play the role of reactant and solvent. But
also, a 1:1 (Table 2, entry 7) and 1:2 mol ratios between
morpholine and hydrosilane (Table 2, entry 8) were evaluated. It
65 was found that a decrease in the mol ratio of morpholine leads to
a decrease in the yield of the silazane coupling product.
5
Table 1 summarizes the main results obtained in this screening
tests. Gs in the absence of metals NPs (Table 1, entry 11)
10 catalyses exclusively disiloxane formation, the yield and the
selectivity toward the desired product, 3a, being negligible even
after much longer times than those employed later. Also Cu NPs
and Ni NPs at metal to substrate molar ratio of 0.05 mol% fail to
promote the dehydrogenative coupling. At high temperatures
o
15 (120 C) and long reaction time (24 h), a notable hydrosilane 1a
conversion was observed using Cu/G as catalyst (Table 1, entry
9). However, in spite of the high 1a conversion, only trace
amounts of the desired silazane 3a was detected under these
conditions. This negative result contrasts with the catalytic
20 activity recently found for Cu/G in the analogous
dehydrogenative coupling of hydrosilanes with primary alcohols
to render alcoxysilanes.37 Apparently the different properties of
alcohols and amines in terms of nucleophilicity of the heteroatom
and acidity of the hydrogens bonded to the heteroatom should be
25 responsible for this notable difference in the catalytic behavior of
Cu/G for these two related reactions.
In contrast, Pd containing catalysts promote the SiꢀN coupling
with high selectivity to product 3a at high substrate conversion. It
was observed that, at the same Pd loading, the nature of the G and
30 the presence of dopant element plays a certain influence on the
catalytic activity. It was found that Nꢀdoping was detrimental for
the catalytic activity, while the presence of B does not increase
the activity with respect to undoped G. Accordingly Pd/G (Table
1, entry 10) is apparently the best choice to promote this
35 coupling. As expected, the reaction temperature played a notable
influence on the conversion and selectivity that increased as the
temperature increased from 80 to 120 oC (see Table 2, entries 1, 2
and 3).
Table 2 Optimization of the reaction conditions for the dehydrogenative
coupling of 1a and 2a catalyzed by Pd/G.
1a
2a
T
t
Convers Selectivit
Solvent
(mmol) (mmol) (ºC) (h) ion (%)b y (%)b
1
2
3
ꢀꢀ
ꢀꢀ
ꢀꢀ
1
1
1
2
2
2
80
0.5 28
0
7
89
100 0.5 77
120 0.5 99
1,4ꢀ
4
1
2
120 24
120 24
120 0.5 99
120 0.5 75
0
0
0
Dioxane
Toluene
ꢀꢀ
5
7
8
1
1
2
2
1
1
0
70
43
ꢀꢀ
aReaction conditions. Catalyst Pd/G (0.05 mol%), Ar atmosphere.
b
70
Conversion and selectivity were determined by GC using internal
standard.
An additional experiment was carried out at 120 oC in the
absence of coꢀsolvent and 1b-2a mol ratio of 1ꢀ2, but using ten
75 times lower amount of Pd/G (Pdꢀ1b mol ratio 0.003 mol%). Full
1b conversion was also achieved in this experiment with very
high selectivity to 3d, reaching a TON of product molecules per
This influence of temperature on the initial reaction rate has
40 allowed us to estimate the activation energy of the process from
the slope of the Arrhenius plot of log (k) versus 1/T (Fig. 2).
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Pd atom of 33,300 although requiring much longer reaction time
(24 h).
o
With the optimal conditions for the model reaction (120 C, no
coꢀsolvent, amineꢀsilane mol ratio 2) the catalytic activity of
Pd/G was compared to that of other Pd NPs supported in different
carbon materials. The results are presented in Table 3.
5
Table 3 Catalytic activity of Pd NPs supported on different carbon
materials.
Catalystb
T (oC)
t (h)
Conversion (%)c Selectivity (%)b
1
2
3
4
5
6
Pd/G
120
120
120
120
120
120
0,5
0,5
0,5
0,5
0,5
0,5
99
84
96
99
99
92
89
74
87
81
84
63
Pd/(N)G
Pd/(B)G
Pd/C
Pd/MWCNT
Pd/DH
Fig.3 Temporal profiles of 1a conversion after two consecutive uses of
the same Pd/G and Pd/Gꢀ(NH2) sample as catalyst. Reaction conditions:
ratio between 1a and 2a 1:2, catalyst (0.05 mol%), Ar atmosphere. First
aReaction conditions: 1a (5 mmol), 2a (10 mmol), catalyst (0.05 mol%),
55 used of Pd/G (
), second used of Pd/G (ꢀ), and first used of Pd/Gꢀ
10 Ar atmosphere. bMWCNT: multiꢀwalled carbon nanotubes, DH:
hydrogenꢀtreated diamond nanoparticles. cConversion and selectivity
were determined by GC using internal standard.
(NH2) ( ) and second used of Pd/Gꢀ(NH2) (X).
As can be seen in Fig. 3, a gradual decrease in activity was
observed upon reuse, particularly between the second and the
third use. Analysis of Pd in the liquid phase indicates that only
60 the 0.12 % of the initial Pd has leached from G to the liquid phase
and that four times reused Pd/G still contains more than 99 % of
the Pd content of the fresh sample. This analytical data indicates
that the loss of Pd cannot be the cause of the catalyst deactivation
observed in Fig. 3. This negligible percentage of Pd leaching is in
65 favor of a strong Pd/G interfacial interaction. We also performed
a hot filtration test in which the reaction starts in the presence of
catalyst, but then the catalyst is removed by filtration while the
suspension is still hot at a conversion of about 30% (5 min). As it
can be seen in Fig. 4, the reaction stops when Pd/G is removed
70 from the reaction mixture. This indicates that there is no
significant contribution of homogenous catalysis due to the small
Pd leaching. These hot filtration data are in accordance with the
low amount of Pd leaching previously described.
Specifically in the present study the catalytic activity of Pd NPs
supported on multiwall carbon nanotubes (MWCNTs) and
15 reduced diamond nanoparticles (DH) was also checked.
MWCNTs have many similarities with the structure of G and, as
expected, the catalytic activity obtained using MWCNT as
support was very similar (Table 3, entry 5), the main difference
should derive from the curvature of the G wall forming to the
20 nanotube. In contrast, it has been found that DH offers a highly
inert environment to metal NPs, due to the lack of chemical
reactivity and presence of functional groups on the diamond
surface. In this case 1a conversion was very similar to that
achieved with Pd/G, but the selectivity to the desired product was
25 very low (Table 3, entry 6). Besides allotropic carbon forms,
active carbons have been favorite supports in catalysis due to the
large surface area and high adsorption capacity of these materials.
The comparison of catalytic data shows that Pd NPs supported on
active carbons (Table 3, entry 4) exhibit also a good performance,
30 but with lower yield than that achieved with Pd/G. This
comparative study shows the superiority of G as support for this
dehydrogenative coupling with respect to other carbon supports.
In the literature there are precedents also showing the higher
efficiently of supported metals NPs on G compared to other
35 related carbons materials.37, 38 This better performance has been
attributed to the combination of three factors, namely, large
surface area of G, strong metal NPsꢀG interaction that may
change the electronic density of the metal atoms at the metal NPꢀ
G interface and the presence of an extended π orbital favoring
40 adsorption of substrates and reagents near the metal NPs.38, 39 It
can be assumed that the same effects could also operate in the
present case.
Stability of Pd/G as catalyst was studied by performing
consecutive reuses of the same Pd/G sample and determining the
45 timeꢀconversion plots for each run (Fig. 3). After each reaction,
the catalyst was recovered by filtration, washed with hexane,
dried at the ambient and submitted to another consecutive run. In
addition, Pd content in the liquid phase after removal of the
catalyst was also determined by ICP analysis to assess the
50 possibility of Pd leaching from Pd/G to the liquid phase.
75 Fig.4 Timeꢀconversion plots for the reaction of 1a and 2a without (black)
and with hot filtration at 5 min allowing the reaction to continue in the
absence of Pd/G.
After four consecutive reuses, Pd/G was characterized by TEM
(Fig. 5). Statistical analysis of the particle size distribution shows
80 that after reuse of Pd/G the dimension of Pd NPs is between 3ꢀ7
nm. However, these images revealed also the formation during
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the course of the reaction of some particles of much bigger size.
amino groups could increase the affinity of Pd NPs for G and this
could eventually lead to a reusable G catalyst. To test this
30 possibility a sample of graphene oxide was treated with an
aqueous NH3 solution at 60 oC to lead to NH2G with a N content
determined by combustion chemical analysis of 8.1 %.
Deposition of Pd NPs on NH2G leads to a catalyst that also shows
a notable catalytic activity for SiꢀN coupling, although lower than
35 that of Pd/G (Table 1 compare entries 12 and 10). However, in
spite of the presence of NH2 groups on G, reuse of Pd/NH2G also
led to a significant degree of deactivation from 99 to 68 %
conversion in 3 h and, thus, the catalytic activity of Pd/NH2G was
not further explored.
40 The scope of Pd/G as catalyst for the coupling of hydrosilanes
and amines was expanded by studying the reactivity of other
aromatic and aliphatic mono and dihydrosilanes with primary and
secondary aliphatic amines, as well as, aniline and acetamide.
The results obtained are shown in Table 4 and supporting
45 information provides spectroscopic data of the resulting silazanes.
As it can be seen in Table 4, high conversions respect to the
hydrosilane were obtained in all cases with selectivity values
toward the expected silazane from good to high.
Fig.5 TEM images at two different magnifications of Pd/G catalyst after
four uses. The inset shows the particle size distribution of Pd NPs.
5
The appearance of these large agglomerated Pd particles was
observed in the almost deactivated catalyst, indicating that
particle size increase is, probably, one of the main causes of Pd/G
deactivation. Also in some areas the expected 2D morphology of
G was not observed, and it seems that some G sheets become
10 corrugated forming 3D objects. This change in the morphology of
G should also lead to catalyst deactivation due to the decrease in
the surface area of the G layers.
In view of Pd/G catalyst deactivation and Pd NP agglomeration,
it is pertinent to comment that the hot filtration tests have to be
15 taken cautiously in catalysis by Pd NPs.40 Prior work in the field
has shown that minor amounts of Pd leached from the solid to the
solution during the course of the reaction can be responsible for
the observed conversion.40 This leached Pd reꢀdeposit on the
support, mainly in an inactive, resting form upon cooling.40 This
20 mobilization of Pd species and reꢀdeposition will cause the
growth of the average particle size and the gradual inactivation of
the catalyst. In this context, it is worth noting that trace amounts
of Pd in the ppm level have been detected in solution during
catalysis in different types of CꢀC and Cꢀ.X bond formation and
25 considered to be the catalytically active species.40
Due to the low boiling point, coupling with propylamine (2b) was
50 carried out at 50 oC, allowing the reaction to run for much longer
time (Table 4, entries 2 and 5), achieving for 1a and triethylsilane
(1b) high conversion and selectivity. Diphenylsilane (1c) also
undergoes coupling with propylamine and the product for the
double coupling could be obtained with very high yields when the
o
55 reaction was performed at 120 C. Aniline (2c) as substrate can
also undergo dehydrogenative coupling with 1a and 1b to form
the corresponding Nꢀphenylsilazanes 3b and 3c. Even acetamide
(4b) can react under the present conditions. In this case, bisꢀO,Nꢀ
disilane was the corresponding product observed in moderate
60 yield at 120 oC. BisꢀO,Nꢀdisilane and related derivatives are
among the preferred silylating reagents to perform the
derivatisation of OH groups in sugars.41
An attempt was made to develop a G supported catalyst with
higher reusability. It was reasoned that the presence on G of
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Table 4. Activity data for the dehydrogenative coupling of hydrosilanes and amines catalysed by Pd/G.
Silane
(1)
Amine
(2)
Product
(3)
Entry
1
Time (h)
0.25
Temperature (oC)
120
Conv. (%)b Selec. (%)b
SiH
SiH
HN
O
Si N
O
94
99
99
87
80
70
a
a
a
a
a
Si N
H
b
H2N
2
3
7
50
b
H
SiH H2N
Si N
0.25
120
c
c
SiH
HN
H2N
H2N
O
Si N
O
4
5
6
0.25
120
50
97
87
96
92
99
82
a
b
d
H
SiH
Si N
7
b
b
e
H
SiH
Si N
0.25
120
c
b
f
NH
NH
SiH2
Si
H2N
7
20
120º
99
99
b
c
g
Si
O
O
SiH
8b
0.5
120
80
62
NH2
d
N
Si
d
h
a Indicate optimal reaction conditions, b The selectivity and the conversion were determined by GC with internal standard.b The Selectivity and the
conversion were determined by 1HꢀNMR spectroscopy.
doped and undoped G, Pd NPs supported on G is a suitable
catalyst to promote the dehydrogenative coupling of hydrosilanes
10 and primary or secondary amines with high conversion and good
or high selectivity to the corresponding mono or disilazanes. The
reaction takes place even for aromatic amines and acetamide. In
contrast, Cu NPs and Ni NPs do not promote this reaction. The
use of G as support is advantageous with respect to other forms of
5
3. Conclusions
In the present manuscript it has been found that from the various
6
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carbons including active carbons, MWCNTs and DH NPs. Pd/G
undergoes a gradual deactivation upon use, the most probable
causes being Pd NP agglomeration and aggregation of the G
sheets.
20. L. H. Sommer and J. D. Citron, The Journal of Organic Chemistry,
1967, 32, 2470ꢀ2472.
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This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 7

Catalysis Science & Technology
Page 8 of 8
View Article Online
DOI: 10.1039/C4CY01486C
TABLE OF CONTENT GRAPHICS
5
R1
R1
H
R2
R2
SiH
R3
+
H2N R4
Si N R4
R3
Pd-NPs
..
.
.
.
. .
.
.
.
Graphene
8
| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]