An immobilised grubbs 2nd generation catalyst for application in flow-through devices

Article abstract of DOI:10.1002/adsc.201301182

An improved immobilised Grubbs 2^nd generation catalyst and its application in flow-through devices, shown for on-column reaction gas chromatography (ocRGC), has been studied. The coupling of a reaction capillary and a separation column in GC/MS

Full text of DOI:10.1002/adsc.201301182

UPDATES
DOI: 10.1002/adsc.201301182
An Immobilised Grubbs 2nd Generation Catalyst for Application
in Flow-Through Devices
Julia Gmeiner,a Max Seibicke,a Carolin Lang,a Ute Gꢀrtner,a and Oliver Trappa,*
a
Ruprecht-Karls-Universitꢀt Heidelberg, Organisch-Chemisches Institut, Im Neuenheimer Feld 270, 69120 Heidelberg,
Germany
Fax : (+49)-6221-544-904; e-mail: trapp@oci.uni-heidelberg.de
Received: December 31, 2013; Published online: May 28, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201301182.
Abstract: An improved immobilised Grubbs 2nd
generation catalyst and its application in flow-
through devices, shown for on-column reaction gas
chromatography (ocRGC), has been studied. The
coupling of a reaction capillary and a separation
column in GC/MS allows direct reaction monitoring
and analysis of conversion as well as reaction kinet-
ics. The presented permanently bonded N-heterocy-
clic carbene ligand shows a great stability and activ-
ity in ring closing metathesis reactions. A salt-free
approach was used to generate the carbene ligand,
which can be directly monitored by mass spectrom-
etry. The very flexible design of the immobilised
ligand system in reaction channels and capillaries of
flow through systems allows the preparation of vari-
ous catalysts using a broad variety of metal precur-
sors. This strategy of immobilised catalytically
active complexes offers a wide range of on-column
reactions combinable with fast reaction screening
by high throughput experimentation.
N-heterocyclic carbene (NHC) ligands[4] show an
even higher level of thermal and air stability com-
pared to phosphine ligands.[5] Many groups have in-
vestigated strategies for immobilisation of the Grubbs
catalysts of the second and third generations.[6] There
are various known routes to obtain a polymer bonded
ligand system.[7] Barrett et al., for example, immobi-
lised the Grubbs 2nd generation catalyst on vinyl
modified polystyrene via the alkylidene group.[8] The
results of the ring closing metathesis studies show that
the activity of the immobilised catalyst is independent
of the polymeric support, combining with the benefits
of a reusable catalytic system.[6]
Here we have realized the concept of a polymer-
bonded NHC precursor, which is permanently immo-
bilised on a fused silica micro capillary and therefore
suitable for application in flow-through devices, which
is demonstrated in the present contribution for on-
column reaction gas chromatography (ocRGC).[9]
Like we have demonstrated in several studies hither-
to, this concept of integrating catalytic activity and
separation selectivity in a single chromatographic
micro reactor system offers the opportunity of relia-
ble reaction screening under constant and defined re-
action conditions.[10,11] The here introduced ligand
system can easily be converted into a metathesis cata-
lyst based on the structural design of the Grubbs 2nd
generation catalyst. This mode of solid-state homoge-
Keywords: carbene ligands; heterogeneous cataly-
sis; high throughput screening; homogeneous catal-
ysis; metathesis
Recent developments in organic synthesis verify the neous catalysis opens an easy access to an improved
important role of homogeneous catalysis.[1] Yet, it is reaction screening system, which is not limited to
still challenging to completely remove the catalyst a particular catalyst but allows the screening of NHC
from the reaction solution. In particular this can trig- ligand-based catalysts.
ger manifold problems, when the remaining catalytic
The immobilisable pre-ligand was prepared by
complex causes undesirable side reactions, for exam- using the common strategy introduced by Blechert et
ple, decomposition of the product.[2] These difficulties al. which establishes the modification of the NHC
can be solved using immobilised ligands and com- backbone.[12] The advantage of this approach is that
plexes. For example, permanently bonded ruthenium- the steric and electronic properties[13] of the chemical
based catalysts are highly active in metathesis reac- environment of the carbene adjacent to the active
tions and show great stability in addition to the de- metal centre are not affected. As shown in Figure 1
sired recyclability.[3] Metathesis catalysts containing we started with the N,N-dimesityl-2,3-diaminopropane
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Julia Gmeiner et al.
Figure 1. Synthesis of the permanently immobilised diamine ligand 5 and conversion into the immobilised 1,3-dimesitylated
imidiazolidine-type carbene precursor 6. (a) 2,4,6-Trimethylaniline, 1208C, 15 h. (b) NaH, w-bromoalkene (n=1, 4, 6), 788C,
20 h. (c) Hydridomethyldimethylpolysiloxane (HMPS), Karstedtꢂs catalyst, THF, sonication, 3 h (n=1, 4, 6). (d) Fused silica
capillary (i.d. 250 mm). (e) Pentafluorobenzaldehyde, AcOH, THF, room temperature, 20 h.
2 to introduce alkene chains with variable spacer and constant nitrogen flow. Cyclisation of the bonded
lengths to investigate the influence of the linker in diamine 5 with pentafluorobenzaldehyde afforded 2-
the on-column reaction gas chromatographic set-up, (pentafluorophenyl)-imidazolidine 6.[16] At this stage
used as a precise flow-through device.
we took advantage of solid-phase chemistry using the
Compound 2 was prepared starting with the com- derivatisation reagent in great excess to achieve com-
mercially available reactants 2,3-dibromo-1-propanol plete conversion. In that respect the capillary was
1 and 2,4,6-trimethylaniline.[14] The reaction of 2 with flushed with a solution of the pentafluorobenzalde-
NaH and an w-bromoalkene results in the isolation of hyde in tetrahydrofuran. We used a syringe pump
the etherified product 3. Afterwards the modified di- with a speed rate of 0.25 mLhꢀ1 to accomplish the for-
ACHTUNGTRENNUNG
Here we present an improved synthetic protocol the use of a strong base to convert the corresponding
for the immobilisation reaction of the ligand 3 with imidazolium salt into the carbene. It was also shown
the polymer HMPS.[15] It is quite challenging to that the combination of a strong base and remaining
ꢀ
obtain a polymer with a proper concentration of re- Si H groups in the polymer will lead to the decompo-
maining Si H groups, which are required for perma- sition of the ligand system.[17] Therefore a salt-free
ꢀ
ꢀ
nent attachment to the fused silica surface via Si OH route was chosen.
groups of chips and fused silica capillaries, low
Finally the fused silica capillary coated with the
enough to avoid unintended reduction of the later in- permanently bonded imidazolidine 6 was coated once
troduced metal ion to form the catalytically active again with a solution of the Grubbs 1st generation cat-
(precursor) complex. We optimised the reaction time alyst in n-pentane to obtain the immobilised Grubbs
1
and conditions by systematic H NMR investigations 2nd generation catalyst 7. The on-column ligand ex-
(cf. the Supporting Information, Figure S9), providing change reaction can be controlled by GC/MS. Here,
ꢀ
reasonable certainty that a sufficient amount of Si H the in situ formation of the carbene was monitored by
groups are remaining for a permanent immobilisation continuous analysis of the generated pentafluoroben-
on the inner surface of the fused silica capillary via zene upon application of a temperature programme
a condensation reaction with surface silanol groups. to the coated capillary installed in
To achieve a permanent immobilisation the fused (Figure 2).
a GC/MS
silica capillary containing the ligand system was
Therefore the capillary is heated from 408C to
heated to 1908C at a rate of 0.5 Kminꢀ1 and main- 808C with a rate of 4 Kminꢀ1. The reaction starts
tained at this temperature for 62 h under a very slow after 15 min at a temperature of about 558C as shown
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An Immobilised Grubbs 2nd Generation Catalyst
UPDATES
Figure 2. Mass spectrometric reaction control of the on-column ligand exchange by in situ carbene formation to achieve
a permanently immobilised Grubbs 2nd generation catalyst 7. (a) (PCy3)2Cl2Ru=CHPh, n-pentane.
by recording the mass trace (Figure 2). The character- ligand system 3b (at 808C with n=4) and 3c (at 608C
istic MS (EI) fragmentation pattern of pentafluoro- with n=6).
benzene (m/z=168 Da) confirms the successful for-
In contrast to these results the allyl linker system
mation of the free carbene. Dissociated tricyclohexyl- 3a (n=1) shows a turnover of only 4% under these
phosphine (PCy3), formed by the ligand exchange reaction conditions and is therefore not the first
with the Grubbs 1st generation catalyst was not de- choice to be used to coat columns for flow-through
tected. This fact leads to the conclusion that the phos- experiments. In addition there is no observable tem-
phine remains on the fused silica column. It will be perature influence on the NHC complex 3a.
dissolved in the polymer and supports the stabiliza-
tion of the resting state of the immobilised catalyst with a spacer length n=4 affords the best results due
7.[18]
to a sufficient flexibility and minimised gauche inter-
For the ring closing metathesis the ligand system 3b
The synthesised and immobilised ligand system was actions with the polymeric framework.[20] Next we ex-
used to investigate the influence of the spacer length amined the turnover depending on the length of the
to ensure a sufficient flexibility of the ligand by re- reaction capillary, which decreases on shortening as
duced interactions with the polymeric backbone.[19] would be expected due to the reduced contact time.
We studied the system in the ring closing metathesis However, the 10 cm long catalyst column still shows
of N,N-diallyltrifluoroacetamide 8. The catalytically a yield of about 7% in the mentioned ring closing
active column (length=50 cm) was installed between metathesis, which demonstrates the great activity of
a pre-separation and a separation column in the GC/ the here presented system. It features stable measure-
MS. This set-up offers an in situ reaction monitoring ments over a period of 24 h as well as a wide temper-
of the ring closing metathesis of 8 injected onto this ature range (up to 1508C!) and emphasises the great
column arrangement. Figure 3 gives an overview of reproducibility in the here presented study. These
the examined diamine 3 in relation to the different temperature and long-term stabilities are remarkable
linker lengths (n=1, 4, 6). It can be shown that the characteristics of the here presented material, which
activity of the immobilised catalyst system strongly exceed by far those of the commonly used homogene-
depends on the length of the spacer. We observed ous system. We can explain this by three factors: (i)
under these reaction conditions with relatively short the catalytic centres are well separated by the here
contact times a maximum turnover of about 40%. employed polysiloxane avoiding metal-metal interac-
These high reaction rates are observed using the tions, (ii) the polymeric matrix stabilises the catalyst
and prevents oxidative decomposition by traces of air,
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Figure 3. (a) Experimental set-up: the reaction capillary is coupled between a pre-separation column (1 m, GE-SE-30) and
a separation column (25 m, GE-SE-52). (b) Elution profiles between 408C and 808C of the ocRGC ring closing metathesis
experiment using N,N-diallyltrifluoroacetamide 8 as model substrate (first eluted peak) converted to N-trifluoroacetamide-3-
pyrroline 9 (second eluted peak) using the catalyst column 7b (n=4). Helium was used as carrier gas (p=80kPa). (c) Turn-
over of the on-column ring closing metathesis of N,N-diallyltrifluoroacetamide in dependence on the length n of the linker
and the temperature T (p=80 kPa helium).
and (iii) the remaining tricyclohexylphosphine (PCy3) sulting activation energies of the on-column measure-
acts as stabiliser[18] and oxygen scavenger.
ments are summarised in Table 1.
We determined the activation parameters for the
ring closing metathesis of N,N-diallyltrifluoroaceta-
mide 8 on the basis of a permanently immobilised
Grubbs 2nd generation catalyst 7 with an optimised
spacer length (n=4).
Temperature-dependent measurements between 40
and 808C were used to calculate these parameters
¼
¼
(Figure 4):[9a] DG (298 K)=89.3 kJmolꢀ1, DH =
62.5ꢁ5.1 kJmolꢀ1 and D =ꢀ89ꢁ13 Jmolꢀ1 Kꢀ1. Re-
¼
action temperatures above 408C are necessary for the
elution of reactants and products. The negative value
¼
of DS refers to a highly ordered transition state. To
classify the determined parameters we studied the on-
column metathesis reaction of N,N-diallyltrifluoro-
ACHTUNGTRENNUNGacetACHTUNGTRENNUNG
amide 8 using Grubbs 1st and 2nd generation cata-
lysts as well as the Grubbs–Hoveyda catalyst. These
complexes were dissolved in dimethylpolysiloxane
and coated onto the inner surface of micro capillaries
Figure 4. Eyring plot to determine activation parameters of
the on-column ring closing metathesis of N,N-diallyltri-
ACHUTNGERNfNUG luoroACHTUNGTNERaNUGN cetamide 8 using the Grubbs catalyst column 7b (n=
by the static method introduced by Grob.[11,21] The re- 4).
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An Immobilised Grubbs 2nd Generation Catalyst
UPDATES
Table 1. Results of the on-column gas chromatographic ring
closing metathesis of N,N-diallyltrifluoroacetamide 8 using
different Grubbs-type catalysts. Gibbs activation energy
Experimental Section
N,Nꢀ-Dimesityl-2,3-diamino-1-propanol (2)
¼
DG is determined at 258C.
A solution of 2,3-dibromopropan-1-ol (2.36 mL, 23.0 mmol,
1.00 equiv.) and 2,4,6-trimethylaniline (8.36 mL, 60.0 mmol,
2.60 equiv.0) was stirred for 15 h at 1208C. Afterwards
400 mL of 1.7M NaOH and 400 mL of dichloromethane
were added. The organic layer was separated, washed with
distilled water and dried over MgSO4. After filtration the
solvent was removed under reduced pressure. The crude
product was purified by column chromatography (silica gel,
n-pentane/diethyl ether, v/v 3:1) to afford a colourless, crys-
talline solid: yield: 5.52 g (16.9 mmol, 74%). 1H NMR
(300 MHz, CDCl3): d=2.20–2.42 (m, 18H, Ar-CH3), 3.05
¼
¼
¼
Catalyst[a]
k[b]
DG
DH
DS
[10ꢀ3 sꢀ1] [kJmolꢀ1] [kJmolꢀ1] [Jmolꢀ1·Kꢀ1]
Grubbs 1st 7.6
Grubbs 2nd 4.8
Grubbs–
Hoveyda
7b
83.2
90.2
92.7
18.3ꢁ1.2 ꢀ218ꢁ29
24.9ꢁ0.7 ꢀ219ꢁ17
39.6ꢁ0.7 ꢀ178ꢁ22
0.6
6.4
89.3
62.5ꢁ5.1 ꢀ89ꢁ13
Grubbs 1st and 2nd generation catalysts as well as
Grubbs–Hoveyda catalyst were dissolved in polysiloxane
GE-SE-30;[11] catalyst 7b (n=4) was immobilised on hy-
dridomethyldimethylpolysiloxane.
Reaction rate constants reported at 508C and 80 kPa
helium.
[a]
2
3
(dd, JH,H =11.9 Hz, JH,H =4.3 Hz, 1H, NH-CH2-), 3.31 (dd,
3
2JH,H =12.0 Hz, JH,H =5.2 Hz, 1NH-CH2-), 3.41–3.56 (m, 1H,
[b]
NH-CH-), 3.71 (br. s, 3H, NH, OH), 3.80–3.91 (m, 1H,
-CH2-OH), 3.91–4.04 (m, 1H, -CH2-OH), 6.89 (d, 4J=
5.0 Hz, 4H, Ar-H); 13C NMR (126 MHz, CDCl3, 238C,
TMS): d=17.7, 18.7, 20.3, 20.4, 51.7, 56.9, 65.1, 128.7, 129.4,
129.6, 130.2, 130.6, 132.0, 141.6, 142.5; HR-MS: m/z=
327.2432, calcd. for C21H30N2O [M+H+]: 327.2436.
¼
The determined Gibbs activation energy DG of
catalyst 7b (n=4) is comparable with the dissolved
Grubbs 2nd generation catalyst (Table 1). The activa-
tion energy is about 6 kJmolꢀ1 higher than for the dis-
solved Grubbs 1st generation catalyst used in on-
column metathesis reactions. These experimentally
determined data are in very good agreement with pre-
General Procedure for the Synthesis of the
Immobilisable Diamine Ligands 3
Sodium hydride (2.0 equiv.) was suspended in anhydrous tet-
viously performed measurements and are also in ac- rahydrofuran at 08C. A mixture of 1.0 equiv. N,Nꢀ-dimesityl-
2,3-diamino-1-propanol 2 in anhydrous tetrahydrofuran was
added dropwise and the solution was allowed to stir for
30 min at 08C. Afterwards 1.1 equiv. w-bromoalkene, dis-
solved in anhydrous tetrahydrofuran was added dropwise.
The mixture was refluxed for 20 h under stirring. Subse-
quently it was quenched with a saturated solution of NH4Cl,
extracted with n-pentane and washed with distilled water.
The organic layer was dried over MgSO4, filtered and the
solvent removed under reduced pressure.
cordance with experimental findings by Grubbs
et al.[18] Additionally the observed reaction rate con-
stants using the permanently immobilised system 7b
(k=6.4·10ꢀ3 sꢀ1) are in agreement to reported results
obtained by NMR experiments of Grubbs et al. (k=
1.0·10ꢀ3 sꢀ1) and the given data of the dissolved cata-
lysts in Table 1. The higher activation barrier of the
Grubbs–Hoveyda catalyst indicates a slower initia-
tion, which is also in accordance with NMR studies.[22]
1-(Allyloxy)-N,N’-dimesityl-2,3-diamino-1-propanol (3a):
In conclusion, we have developed an advanced and The general procedure was used for the synthesis of 3a. The
crude product was purified by column chromatography
reproducible synthetic protocol for the permanent im-
mobilisation of Grubbs 2nd generation catalyst onto
the inner surface of reaction channels and provided
enhanced insights into the kinetics of flow-through re-
action systems. We can assume that using permanent-
ly immobilised ligand systems coated onto the inner
surface of fused silica capillaries offers a high poten-
tial for kinetic characterisation of catalytic reac-
tions,[23] in particular for NHC-carbene ligands, which
(silica gel, n-hexane/ethyl acetate, v/v 9:1) to obtain a yellow
oil; yield: 2.93 g (8.00 mmol, 87%). 1H NMR (300 MHz,
2
CDCl3): d=2.25–2.42 (m, 18H, Ar-CH3), 3.13 (dd, JH,H
=
=
2
11.9 Hz, 3JH,H =6.0 Hz, 1H, NH-CH2-), 3.39 (dd, JH,H
3
11.8 Hz, JH,H =5.6 Hz, 1NH-CH2-), 3.45–3.53 (m, 1H, NH-
2
CH-), 3.54–3.82 (m, 4H, NH, -CH2-O-), 4.02 (dd, JH,H
=
5.5 Hz, 3JH,H =1.4 Hz, 2H, -O-CH2-), 5.16–5.43 (m, 2H,
-CH=CH2), 5.87–6.06 (m, 1H, -CH=CH2), 6.90 (s, 4H, Ar-
H); 13C NMR (126 MHz, CDCl3, 238C, TMS): d=18.2, 18.7,
require tedious and time-consuming synthetic steps to 20.4, 20.5, 50.8, 56.5, 70.6, 72.1, 116.9, 129.2, 129.3, 129.5,
129.8, 140.7, 131.2, 134.7, 141.6, 143.5; HR-MS: m/z=
prepare sufficient material for classic reaction screen-
ing schemes. In our set-up the free carbene ligand can
be easily thermally generated and monitored, and we
are able to build bonded catalysts with various metal
precursors by on-column ligand exchange. This ad-
vantage could open the access to a broad variety of
flow-through catalytic processes which can be ana-
lysed in high-throughput measurements to kinetically
evaluate these catalysts and to elucidate reaction
mechanisms.
367.2745, calcd. for C24H34N2O [M+H+]: 367.2749.
1-(Hex-5-en-1-yloxy)-N,N’-dimesityl-2,3-diamino-1-propa-
nol (3b): The general procedure was used for the synthesis
of 3b. The crude product was purified by column chroma-
tography (silica gel, n-hexane/ethyl acetate, v/v 9:1) to
obtain
a yellow oil; yield: 1.67 g (4.09 mmol, 67%).
1H NMR (300 MHz, CDCl3): d=1.42–1.58 (m, 2H, -CH2-),
1.59–1.72 (m, 2H, -CH2-), 2.07–2.14 (m, 2H, -CH2-), 2.22–
3
2.46 (m, 18H, Ar-CH3), 3.06 (dd, 2JH,H =11.8 Hz, JH,H
=
5.8 Hz, 1H, NH-CH2-), 3.62–3.49 (m, 4NH-CH2-, NH-CH-,
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UPDATES
Julia Gmeiner et al.
-O-CH2-), 3.52–3.54 (m, 4H, NH, -CH2-O-), 4.93–5.18 (m,
On Column Ring Closing Metathesis Measurements
4
2H, -CH=CH2), 5.87–5.90 (m, 1 H, -CH=CH2), 6.85 (d, J=
3.9 Hz, 4H, Ar-H); 13C NMR (126 MHz, CDCl3, 238C,
TMS): d=18.3, 18.7, 20.5, 25.4, 29.1, 33.5, 50.8, 56.5, 71.1,
71.2, 114.6, 129.2, 129.4, 129.5, 129.8, 130.6, 131.2, 138.5,
141.7, 143.5; HR-MS: m/z=409.3214, calcd. for C27H40N2O
[M+H+]: 409.3219.
Ring closing metathesis experiments of the N,N-diallyltri-
fluoroacetamide were performed by on-column reaction gas
chromatography. The catalyst capillary was coupled between
a pre-separation column, coated with GE-SE-30 (1 m, i.d.
250 mm, 500 nm film thickness) and a separation column,
coated with GE-SE-52 (25 m, i.d. 250 mm, 250 nm film thick-
ness). Helium was used as carrier gas (p=80 kPa).
1-(Oct-7-en-1-yloxy)-N,Nꢀ-dimesityl-2,3-diamino-1-propa-
nol (3c): The general procedure was used for the synthesis
of 3c. The crude product was purified by column chromatog-
raphy (silica gel, n-hexane/ethyl acetate, v/v 9:1) to obtain
a yellow oil; yield: 3.04 g (6.97 mmol, 85%). 1H NMR
(300 MHz, CDCl3): d=1.24–1.51 (m, 6H, -CH2-), 1.53–1.67
(m, 2H, -CH2-), 2.05–2.09 (m, 2H, -CH2-), 2.19–2.51 (m,
Acknowledgements
2
3
We thank the European Research Council (ERC) for finan-
cial support of this research under Grant Agreement No. StG
258740 (AMPCAT).
18H, Ar-CH3), 3.07 (dd, JH,H =11.8 Hz, JH,H =5.9 Hz, 1H,
NH-CH2-), 3.25–3.47 (m, 4H, NH-CH2-, NH-CH-, -O-CH2-
), 3.53 (m, 2H, -CH2-O-), 4.84–5.22 (m, 2H, -CH=CH2),
4
5.70–5.96 (m, 1H, -CH=CH2), 6.85 (d, J=4.4 Hz, 4H, Ar-
H); 13C NMR (126 MHz, CDCl3, 238C, TMS): d=18.3, 18.7,
20.5, 26.1, 28.8, 28.9, 29.7, 33.7, 50.8, 56.5, 71.1, 71.4, 114.2,
129.5, 129.7, 130.7, 131.2, 139.0, 141.7, 143.5; HR-MS: m/z=
437.3527, calcd. for C29H44N2O [M+H+]: 437.3532.
References
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Fꢃrstner, (Eds), Adv. Synth. Catal. 2007, 349, 23–265
(special issue: olefin metathesis); b) R. H. Grubbs, in:
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3140–3172; Angew. Chem. Int. Ed. 2000, 39, 3012–3043;
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General Procedure for the Immobilised Diamine
Ligand 4
Hydridomethyldimethylpolysiloxane (HMPS, 1.0 equiv.,
ꢀ
10.2% Si H groups) and 0.10 equiv of the substrate 3 were
dissolved in anhydrous tetrahydrofuran and 10 mL of Kar-
stedtꢂs catalyst (2% by weight of platinum in xylene) were
added. The mixture was stirred in an ultrasonic bath for 3 h
at room temperature. The solvent was removed under re-
duced pressure and the residue passed through a short silica
column (dichloromethane/methanol, v/v 20:1). The solvent
was evaporated to give a pale yellow oil; yield: quantitative.
[4] T. Drçge, F. Glorius, Angew. Chem. 2010, 122, 7094–
7107; Angew. Chem. Int. Ed. 2010, 49, 6940–6952.
[5] W. A. Herrmann, Angew. Chem. 2002, 114, 1342–1363;
Angew. Chem. Int. Ed. 2002, 41, 1290–1309.
Coating of Fused Silica Columns
The immobilised compound 4 (24.0 mg) was dissolved in
3.00 mL anhydrous diethyl ether for use as coating solution.
Capillaries were coated by the static method of Grob.[21] The
fused silica column (i.d. 250 mm) was coated with this solu-
tion giving a 500 nm film at the inner wall of the capillary.
Immobilisation was achieved in a stream of nitrogen using
a temperature programme starting at 408C for 10 min, then
heating up to 1908C for 62 h at the rate of 0.5 Kminꢀ1.
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2087