Synthesis of divalent ytterbium terphenylamide and catalytic application for regioselective hydrosilylation of alkenes

Article abstract of DOI:10.1039/c7dt01837a

The dimeric heteroleptic ytterbium amido complex [(2,6-(3,5-Me₂C₆H₃)₂C₆H₃NH)Yb(N(SiMe₃)₂)]₂ (1) has been prepared and characterized. This divalent terphenyl

Full text of DOI:10.1039/c7dt01837a

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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Synthesis of Divalent Ytterbium Terphenylamide and Catalytic
Application for Regioselective Hydrosilylation of Alkenes
Received 00th January 20xx,
Accepted 00th January 20xx
Yinghua Shi,a Jianfeng Li,*a Chunming Cui*a,b
DOI: 10.1039/x0xx00000x
The dimeric heteroleptic ytterbium amido complex [(2,6-(3,5-Me2C6H3)2C6H3NH)Yb(N(SiMe3)2)]2 (1) have been prepared
and characterized. This divalent terphenylamido complex enabled highly regioselective hydrosilylation of various terminal
alkenes with very low catalyst loading. Reaction of 1 with phenylsilane at high temperature led to the dehydrogenative
coupling of the silane with terphenylamide ligand and redistributions of amide ligands via a hypervalent silane
intermediate, which has been spectroscopically characterized.
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ytterbium amide with the significant agostic interactions between
ytterbium center and Si−H bonds.8 Herein we report the synthesis
and characterization of a divalent ytterbium amide with the
Introduction
Hydrosilylation is an important and indispensable chemical reaction
for the production of organosilanes, which are important
intermediates in silicon industry, synthetic chemistry and material
science.1 Transition metal-catalyzed hydrosilylation of alkenes have
been widely employed due to their high efficiency, chemoselectivity
and broad substrate scopes. Lanthanide catalysts supported by
various ligand frameworks for alkene hydrosilylation have also
attracted much attention for several decades.2-4 Compared with
transition metal catalysts, lanthanide alkyls and amides generally
offer clean reaction in the hydrosilylation of alkenes and side
reactions such as isomerization and dehydrogentaive silylation have
been rarely observed. In addition, the interaction between highly
electropositive lanthanide ions with alkenes is unique, leading to
different selectivity and reaction modes from transition metal
catalysts. As such, the development of highly regioselective and
efficient lanthanide catalysts is highly desirable.
terphenylamido ligand (1). 1 can catalyze hydrosilylation of alkenes
with high regioselectivity and high yields.
Results and discussion
Synthesis and molecular structure of [(ArNH)Yb(N(SiMe3)2)]2 (1)
The reaction of terphenylamine ArNH2 (Ar = 2,6-(3,5-Me2C6H3)2C6H3)
with Yb[N(SiMe3)2]2(THF)2 yielded the divalent ytterbium amide (1,
[(ArNH)Yb(N(SiMe3)2)]2) in 79 % yield as dark purple crystals in
toluene at room temperature (Scheme 1). Complex 1 is air- and
moisture-sensitive, but significantly thermal-stable. For example, 1
persisted for several hours in refluxing toluene. Complex 1 is well
soluble in THF and toluene and has been characterized by 1H, 13C
.
29Si NMR and IR spectroscopy In the 13C NMR spectrum of 1, the
resonances at δ 152.19 and 147.56 ppm were assigned to the C(ispo)
atom, indicating the Yb···arene interactions. The molecular
structure of 1 has been determined by single-crystal X-ray analysis.
Lanthanide amides have shown interesting features for silane
transformation. For example, La[N(SiMe3)2]3 has been reported as a
highly regioselective catalyst for hydrosilylation of alkene by
Livinghouse.5 Roesky reported the bis(phosphinimino)methanide
lanthanide amides for the highly efficient hydrosilylation.6 However,
the divalent lanthanide amides have rarely been explored for the
hydrosilylation of alkenes. Bulky terphenylamido ligand is important
ligand to stabilize highly active organometallic complexes with low
coordination number.7 Our group previously reported the
dehydrosilylation reaction of Yb[N(SiMe3)2]2(THF)2 with the bulky
Scheme 1. Synthesis of
1
G
Yb
Yb
H
N
2Yb[N(SiMe3)2]2(THF)2
N
H
NH2
2
toluene, r.t., 12 h
-2 HN(SiMe3)2
G
1 (79%), G = N(SiMe3
)
2
terphenylaminosilane ArNHSiH3 (Ar
= 2,6-(3,5-Me2C6H3)2C6H3),
Single crystals of 1 suitable for X-ray diffraction studies were
obtained by recrystallization from toluene solution at −40 oC. The
molecular structure of 1 is depicted in Figure 1 together with
selected bond lengths and angles. The X-ray single crystal analysis
of 1 disclosed a dimeric structure with ArNH bridged the two Yb
atoms, leading to a planar Yb2N2 ring. The average length (2.458(3)
Å) of bridging Yb−N bonds is similar to those reported in the
literature, for example, the complex [Yb{NPhSiMe3}2(THF)2] (2.497
leading to the formation of the first solvent-free monomeric
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Å), [NaYb{N(SiMe3)2}3] (2.46(2) Å) and the dimeric complexes solvent, leading to the final product 2.
[Yb2{N(SiMe3)2}4] (2.502(3) Å).9 The Yb···arene interactions are also
proved by the short Yb−C contacts of 2.804(5) Å and 2.805(5) Å.
Scheme 2. Reaction of complex 1 with phenylsilane
DME
Yb
SiMe3
N
2 PhSiH3
Yb
2
N
N
1
N
SiMe3
toluene,
r.t., 6 h
- H2
DME,
reflux, 3 h
Si
H
SiH2
Ph
Ph
A
(70%)
2 (25%)
ligand
redistribution
desilylative
cyclization
- Yb[N(SiMe3)2]2
- PhSiH3
Ph
H2Si
N
Yb
N
SiH2
Figure 1. ORTEP drawing of 1 with 50% probability ellipsoids. Hydrogen atoms
and methyl groups on SiMe3 have been omitted for clarity except the hydrogen
atoms on N of terphenylamine. Selected bond lengths (Å) and angles (deg):
Yb1−N2 2.434(3), Yb1−N1 2.486(3), Yb2−N2 2.477(3), Yb2−N1 2.436(3), Yb1−N3
2.293(3), Yb2−N4 2.305(3), Yb1−C34 2.804(5), Yb2−C16 2.805(5), N1−Yb1−N2
92.23(11), Yb1−N1−Yb2 87.54(10), N1−Yb2−N2 92.43(11), Yb1−N2−Yb2
87.78(10).
Ph
B
Complex 2 has been structurally characterized by H, 13C and 29Si
1
NMR and IR spectroscopy, elemental analysis and X-ray
crystallographic study. The molecular structure of 2 is depicted in
Figure 2 together with selected bond lengths and angles. Complex 2
featured the rare N−Yb−N−Si four-membered ring. The Yb···arene
interactions were suggested by the short Yb−C29 and Yb−C30
distances of 2.868(3)Å and 2.818(3)Å.13 The Yb−N bond lengths
(2.350(2) and 2.380(2)Å) are within the known range for ytterbium
amides in the literature.14 In addition, the two Yb−N−Si angles
97.77(9)° and 96.64(10)° are very close to right angles. Hill reported
the isolation of the similar magnesium bis(amido)silane complexe
by the reaction between magnesium amide and phenylsilane.12
The reaction of complex 1 with phenylsilane
The reaction of 1 and phenylsilane has been investigated, which
gave A as a dark green solid at room temperature after 6 hours
(Scheme 2). Attempts to grow single crystals of A under various
conditions were unsuccessful. However, the compound A can be
spectroscopically characterized. The absorption for the Si–H bonds
appeared at 2124 cm-1 in IR spectrum of A which is similar to the
normal Si−H absorption (2152 cm-1).8 The resonance of hydrogen
atoms on silicon was observed at 5.18 ppm with integration of two
1
in H NMR of A. This chemical shift is similar to those in f-element
scandium and thorium complexes with (aryl)N(SiH2Ph) ligands (5.53
and 5.59 ppm).10 The silane resonance of A displayed a triplet at
1
−36.5 ppm with JSiH of 205.3 Hz in the pronton-coupled 29Si NMR
spectrum, which indicated the one-bond coupling of Si nucleus with
two magnetic equivalent H nuclei. Based on these spectroscopic
data and experimental phenomena,11 it was proposed that PhSiH3
attacked the bridging nitrogen atoms, leading to the dissociation
and dehydrogenative Si−N coupling with the formation of
A
ArN(SiH2Ph)YbN(SiMe3)2 without Si−H agostic interactions with Yb.
Hill also suggested the similar complex in the reaction between
magnesium amide and phenylsilane.12
Heating a solution of A in 1,2-dimethoxyethane (DME) at 80 oC
for 3 hours yielded a black solution, from which the divalent
ytterbium silylamido complex [ArNSiH(Ph)NAr]Yb(DME) (2) was
isolated. In addition, NMR analysis of the crude mixture indicated
the formation of solvated Yb[N(SiMe3)2]2. These results indicated
the ligand redistribution took place upon heating. The proposed
mechanism for the formation of 2 is shown in Scheme 2. The ligand
redistribution of A would lead to the formation of B with the
elimination of Yb[N(SiMe3)2]2. This type of redistribution is very
common for lanthanide complexes. The next step may involve the
desilylative cyclization via a hypervalent silicon intermediate in DME
Figure 2. ORTEP drawing of 2 with 50% probability ellipsoids. Hydrogen atoms
have been omitted for clarity except the hydrogen atoms on Si. Selected bond
lengths (Å) and angles (deg): Yb1−N1 2.350(2), Yb1−N2 2.380(2), Si1−N1
1.713(2), Si1−N2 1.714(2), Yb−C29 2.868(3), Yb−C30 2.818(3), Yb1−Si1 3.0896(8),
N1−Yb1−N2 66.74(7), Yb1−N1−Si1 97.77(9), Yb1−N2−Si1 96.64(10), N1−Si1−N2
98.78(11).
2 | J. Name., 2012, 00, 1-3
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Catalytic hydrosilylation of alkene
We used three primary hydrosilanes: phenylsilane, n-hexylsilane
We investigated the catalytic behavior of 1 (0.5−1 mol % catalyst and cyclohexylsilane as the silane sources in this transformation. n-
loading) for alkene hydrosilylation. Among these catalysis, we find Hexylsilane and cyclohexylsilane have not been employed in the
that vinylarenes react with silanes can selectively (>95%) afford the hydrosilylation of alkenes with lanthanide catalysts previously. It
Markovnikov products in more than 95% GC yields (Table 1). Due to was found that the activities of the hydrosilylation reactions in the
the aryl-directed effect, 1, like the most of lanthanide catalysts, led order: phenylsilane
> n-hexylsilane > cyclohexylsilane. The
to different regioselectivity from transition metal catalysts in the hydrosilylation reactions of vinylarenes and aliphatic alkenes with
hydrosilylation of aryl-substituted alkenes.1a The aliphatic alkenes secondary silanes (Ph2SiH2, PhMeSiH2 and Et2SiH2) and tertiary
react with silanes can selectively (>95%) afford the anti- silanes (Ph3SiH, PhMe2SiH, Et3SiH, and (EtO)3SiH) did not occur in
Markovnikov products in more than 95% GC yields (Table 2). The the presence of 1 mol % of 1 at 70 °C.
heteroleptic amide 1 was found to be aslo active in the reaction of
olefins with bulky groups (3d, 3e) in high yields and regioselectivity.
Table 2. Hydrosilylation of aliphatic alkenesa
Furthermore, the hydrosilylation of nonconjugated 4-vinyl-
cyclohexene (3j) occurred chemoselectively at the terminal C=C
1
0.5-1 mol%
SiH2R
double bond to afford the anti-Markovnikov products in 99% GC
yields. Complex 1 also showed good activity in the hydrosilylation of
the nonconjugated double bond in 1,5-hexadiene (3m), the reaction
with two equivalents of silane yielded anti-Markovnikov products in
high regioselectivity (95%).
RSiH3
+
Alkyl
Alkyl
toluene, 70-90oC, 6-8h
3g-m
4g-m, 5g-m, 6g-m
T
time yield regiosel.
product
R
(oC)
(h)
(%)
(%)
4g Phb
70
70
90
70
70
90
70
70
90
70
70
90
70
70
90
70
70
90
70
70
6
90
97
b
Table 1. Hydrosilylation of vinylarenesa
5g C6H13
6g Cyc
8
8
6
8
8
6
8
8
6
8
8
6
8
8
6
8
8
6
8
95
90
90
95
90
70
92
93
90
95
93
75
95
95
80
95
95
70
80
97.5
97
99
98
97
98
98
99
99
98
99
97
98.5
98
97
98
98
95
97
4h Phb
b
5h C6H13
6h Cyc
4i Phc
T
time yield
regiosel.
(%)
product
R
(oC)
(h)
(%)
4a Phb
5a C6H13
6a Cyc
60
60
70
60
60
70
60
60
80
70
70
90
70
70
90
70
70
90
4
95
99.9
99.9
99.9
98
c
5i C6H13
6i Cyc
b
4
5
4
4
5
6
6
6
6
6
8
8
8
8
6
8
8
97
95
95
97
95
95
95
95
92
92
90
93
93
90
90
90
90
4j Phc
4b Phb
c
5j C6H13
b
5b C6H13
6b Cyc
98
6j Cyc
4k Phc
5k C6H13
6k Cyc
4l Phc
98
4c Phb
99.9
99
c
b
5c C6H13
6c Cyc
99.5
96
4d Phc
5d C6H13
6d Cyc
c
5l C6H13
c
98
6l Cyc
98
4m Phc
4e Phc
99
c
5m C6H13
6m Cyc
c
5e C6H13
6e Cyc
99
90
8
80
95
99.8
99
a Conditions: alkene (1.0 mmol), silane (1.0 mmol), 0.2 ml toluene. The yields
were isolated yields. The regioselectivity was determined by GC-MS analysis. b 0.5
mol % 1. c 1 mol % 1. d silane (2.0 mmol)
4f Phb
b
5f C6H13
6f Cyc
99
99
Complex 1 exhibited high regioselectivity (>95%) and high activity
(0.5−1 mol % catalyst loading) for these alkenes compared to the
known trivalent lanthanide catalysts.2 In general, lanthanide–
catalyzed hydrosilylation of alkenes requires ca 5 mol% loadings of
catalysts for high conversions. The comparable catalytic activity has
a Conditions: alkene (1.0 mmol), silane (1.0 mmol), 0.2 ml toluene. The yields
were isolated yields.The regioselectivity was determined by GC-MS analysis. b 0.5
mol % 1. c 1 mol % 1.
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only been reported with lanthanocene catalysts by Marks and co-
workers.2b The hydrosilylation of alkene catalyzed by divalent
lanthanide complexes is rare. Harder has reported the highly
regioselective hydrosilylation of 1,1-diphenylethylene catalyzed by
5 mol % Yb(II) hydride complex with nacnac ligand (nacnac =
CH{(CMe)(2,6-iPr2C6H3N)}2).15 Takaki reported the hydrosilylation of
aryl-substituted alkene in moderate activity by 10 mol % Yb(II)
imine complex.16 However, the anti-Markovnikov products were
obtained in these catalysis reported by Takaki, which were opposite
to the Markovnikov products generated by the most of lanthanide
catalysts.2
Experimental section
General Considerations. All operations on air- and moisture-
sensitive materials were carried out under an atmosphere of dry
argon by using modified Schlenk line and glove box techniques. All
solvents were freshly distilled from sodium and degassed
immediately prior to use. Elemental analyses were carried out on
an Elemental Vario EL analyzer. The 1H, 13C and 29Si NMR
spectroscopic data were recorded on a Bruker Mercury Plus AV400
spectrometer. Chemical shifts are referenced against external Me4Si
(1H, 13C, 29Si). Multiplicities are recorded as: s = singlet, d = doublet,
t = triplet, dd = doublet of doublets, br s = broad singlet, m =
multiplet. Infrared spectra were recorded on a Bio-Rad FTS 6000
spectrometer and a Bruker Alpha infrared spectrometer with ATR
operator station. GC-MS data were obtained with a Thermo
Scientific TRACE 1300 & ISQ QD system, equipped with a TG-5MS
column. Injection temperature 250 oC, helium flow 1.0 mL/min,
temperature program 100 oC for 2 min to 280 oC, rate 10 oC/min.
GC-MS data are reported as m/z (M+) for characteristic peaks.
Alkenes and dienes were vacuum-distilled over CaH2 and then were
The catalytic behaviors of 1 have been compared with those of
the simple divant ytterbium amide Yb[N(SiMe3)2]2(THF)2. 1 and
Yb[N(SiMe3)2]2(THF)2 have the similar activity and regioselectivity
for the hydroslilylation of less bulky aryl-substituted alkenes.
However, 1 has much higher activity than Yb[N(SiMe3)2]2(THF)2 for
the alkyl-substituted alkenes or bulky alkenes. For example, 95% GC
yield and 98% regioselectivity 4i can be produced by 1 mol % of 1 at
70 oC after 6 hours, but 60 % GC yield and 98% regioselectivity same
produt was affored by 1% of Yb[N(SiMe3)2]2(THF)2 in the same
catalytic conditions. And 96% GC yield and 99% regioselectivity 4e
can be produced by 1 mol % of 1 at 70 oC after 8 hours, but 33 % GC
yield and 98% regioselectivity same product was affored by 1% of
Yb[N(SiMe3)2]2(THF)2 in the same catalytic conditions. Furthermore,
complex 2 displayed very low activity for all the alkenes in Table 1
and 2 in the optimized catalytic conditions of 1.
17
degassed prior to use. The complexes Yb[N(SiMe3)2]2(THF)2 and
the terphenyl amine ArNH2 (Ar = 2,6-(3,5-Me2C6H3)2C6H3)18 were
prepared according to literature procedures. Phenylsilane, n-
hexylsilane and cyclohexylsilane were purchased from Aldrich and
dried over CaH2 prior to use. Other reagents and solvents were
purchased from chemical vendors and used as received unless
otherwise noted.
The mechanism of the hydrosilylation reaction has be
investigated. The byproduct PhSiH2N(SiMe3)2 can be observed in the
stoichiometric reaction of compound 1 with phenylsilane and
styrene in the 1H NMR spectrum and GC-MS analyses of the
reaction mixtures. Based on preliminary results, it is postulated that
the complex A underwent σ-bond metathesis in the presence of an
excess of RSiH3 to a hydride intermediate with the elimination of
RSiH2N(SiMe3)2 followed by alkene insertion into the Yb−H bond.
Unfortunately, attempts to isolate these postulated intermediates
were unsuccessful so far.
Crystal structure determinations: CCDC 1545003 (2) and
1545004 (1) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre.
Preparation of [(ArNH)Yb(N(SiMe3)2)]2 (1). ArNH2 (1.5 g, 5.0 mmol)
in toluene (10 mL) was added to a mixture of Yb[N(SiMe3)2]2(THF)2
(3.2 g, 5 mmol) in toluene (10 mL) at room temperature. The color
immediately changed from orange red to atropurpureus. The
mixture was stirred for 6 h to yield a purple black solution. The
solvents were removed under vacuum, and the remaining residue
was washed by hexane (10 mL) and dried under vacuum to give 1 as
a atropurpureus powder solid (2.5 g, 79%). M. P.: 190–192 °C. IR
Conclusions
1
(KBr cm-1): ν (N–H): 3383. H NMR (C6D6, 400 MHz, 298K): δ (ppm)
In summary, we have disclosed that the divalent ytterbium amide 1
0–0.31 (m, 36H, Si(CH3)3), 1.84–2.30 (m, 24H, Ar-CH3), 4.57 (br, 2H,
NH), 6.20–7.42 (m, 18H, Ar-H). 13C NMR (C6D6, 101 MHz, 298K): δ
(ppm) 5.7 (Si(CH3)3), 21.6 (Ar-CH3), 115.6, 125.0, 131.6, 132.5, 138.3,
142.3, 147.6, 152.2 (Ar-C). 29Si NMR (C6D6, 79 MHz, 298K): δ (ppm) –
12.72. Anal. Calcd. for C56H80N4Si4Yb2 (1268): C, 53.06; H, 6.36; N,
4.42. Found: C, 52.10; H, 6.05; N, 4.96. The solution of 1 in dilute
toluene stored at –40 oC for 48 h afforded atropurpureus crystals.
Preparation of [ArNSiH(Ph)NAr]Yb(DME) (2). PhSiH3 (0.22 g, 2.0
mmol) in toluene (5 mL) was added to a stirred solution of 1 (1.3 g,
1.0 mmol) in toluene (10 mL) at room temperature, and stirred for 6
h to yield a dark green solution. The solvents were removed under
vacuum, and the remaining oily residue was extracted with n-
hexane (50 mL). The exact was concentrated (ca.10 mL) and stored
at –40 oC for 24 h to give A (1.0 g, 70%) as a dark green solid. M. P.:
is capable of catalyzing regioselective hydrosilylation (>95%) of
vinylarenes and aliphatic alkenes. Complex
1 exhibited high
regioselectivity and activity compared to known lanthanide amide
catalysts probably due to the unique bonding of the
terphenylamido ligand.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
We are grateful to the National Natural Science Foundation of
China (Grant No. 21390401 and 21472098) for financial
support.
1
140 oC (decomp.). IR (KBr, cm-1): ν (Si-H): 2124. H NMR (C6D6, 400
MHz, 298K): δ (ppm) 0.16 (s, 18H, Si(CH3)3), 2.05 (br, 12H, Ar-CH3),
5.18 (br, 2H, SiH2), 6.58–7.62 (m, 14H, Ar-H). 13C NMR (C6D6, 101
4 | J. Name., 2012, 00, 1-3
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2245−2250.
MHz, 298K): δ (ppm) 3.5 (Si(CH3)3), 21.3 (Ar-CH3), 114.2, 129.3,
129.8, 130.0, 131.6, 133.8, 134.0, 135.9, 138.0, 139.4, 143.9, 153.0
(Ar-C). Proton-coupled 29Si NMR (C6D6, 79 MHz, 298K): δ (ppm) –
4
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1
36.3 (t, JSiH = 205.3 Hz, PhSiH2), 5.7 (m, Si(CH3)3). A (0.74 g, 1.0
mmol) dissolved in DME (10 mL) and then the mixture was heated
to 80 oC and stirred for 3 h to yield a black solution. The solvents
were removed under vacuum, and the remaining purple black oily
residue was extracted with toluene (10 mL). The exact was
concentrated (ca. 2 mL) and stored at –40 oC for 24 h to give 2 (0.24
5
6
7
-1
o
g, 25%) as a black solid. M.P.: 167–172 C. IR (KBr, cm ): ν (Si-H):
1798. 1H NMR (C6D6, 400 MHz, 298K): δ (ppm) 2.12 (s, 24H, Ar-CH3),
2.92 (br, 10H, DME-H), 4.10(s, 1H, SiH), 6.61–7.41(m, 23H, Ar-H). 13
C
NMR (C6D6, 101 MHz, 298K): δ (ppm) 21.7 (Ar-CH3), 59.5, 73.5
(DME-C), 116.8, 124.6, 130.6, 133.5, 137.8, 139.0, 145.2, 148.4,
151.3 (Ar-C). 29Si NMR (C6D6, 79 MHz, 298K): δ (ppm) –49.65. Anal.
Calcd. For C54H58N2O2SiYb(968.18): C, 66.99; H, 6.04; N, 2.89; Found:
C, 65.34; H, 6.50; N, 2.06. The solution of 2 in dilute toluene stored
at –40 oC for 24 h afforded black crystals.
General Procedures for the Catalytic Hydrosilylation Reactions. In
an Ar glove box, a Schlenk tube (10 ml) was charged with catalyst 1
(0.005–0.01 mmol) and toluene (0.2 mL). And then silane (1.0
mmol) and the appropriate alkene or diene (1.0 mmol) were added
by microsyringe. The Schlenk tube was quickly removed from the
glovebox. The reaction mixture was stirred 4–8 h at 60–90 oC using
oil bath, and then the resulting solution was concentrated in
vacuum. The crude product was purified by column
chromatography on silica gel with hexane as eluent.
8
9
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DOI: 10.1039/C7DT01837A
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ARTICLE
Synthesis of Divalent Ytterbium Terphenylamide and Catalytic Application
for Regioselective Hydrosilylation of Alkenes
Yinghua Shi, Jianfeng Li,* Chunming Cui*
Divalent ytterbium terphenylamide exhibited high regioselectivity and activity in hydrosilylation of alkenes with low
catalyst loading.
This journal is © The Royal Society of Chemistry 2013
J. Name., 2013, 00, 1-3 | 1