Monitoring of liquid-phase organic reactions by photoelectron spectroscopy

Article abstract of DOI:10.1002/anie.201107402

There are strings attached: After linking the reacting groups to head groups of ionic liquids to drastically lower the vapour pressures of the reactants, ordinary liquid-phase organic reactions can be monitored by in situ X-ray photoelectron spectroscopy. This approach is demonstrated for the nucleophilic substitution of an alkyl amine and an alkyl chloride moiety, which are attached to the cation and anion of ionic liquids, respectively. Copyright

Full text of DOI:10.1002/anie.201107402

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DOI: 10.1002/anie.201107402
Ionic Liquids
Monitoring of Liquid-Phase Organic Reactions by Photoelectron
Spectroscopy**
Claudia Kolbeck, Inga Niedermaier, Nicola Taccardi, Peter S. Schulz, Florian Maier,
Peter Wasserscheid, and Hans-Peter Steinrꢀck*
In recent years, surface-science studies of ionic liquids (ILs)
have attracted increasing scientific interest.[1–3] The properties
of ILs make them attractive for applications in catalysis,[4–6]
separation processes,[7] and electrochemistry.[8] ILs are
defined as salts with a melting point below 1008C, and they
are characterized by extremely low vapor pressures.[9,10] The
low volatility allows to address fundamental and mechanistic
questions by ultrahigh vacuum (UHV) methods that were
originally developed for solids. X-ray photoelectron spectros-
copy (XPS), also denoted as electron spectroscopy for
chemical analysis (ESCA), has been proven to be one of the
most powerful methods to quantitatively analyze the chem-
ical composition of the near-surface region of ILs.[2] Since
core-level binding energies are sensitive to the local chemical
environment, XPS also allows the determination of the
chemical state (e.g., oxidation state) of particular atoms in
the near-surface region of an IL.
In contrast to classical spectroscopy tools in chemistry, the
advantage of reaction monitoring with XPS (denoted “in situ
XPS”) is the ability to monitor all elements (apart from
hydrogen and helium) simultaneously with respect to their
quantity and their chemical state under well-defined (ultra-
clean) conditions. Moreover, XPS focuses only on the near-
surface region, which is not possible with most other methods.
The inherent surface sensitivity of XPS originates from the
small inelastic mean-free path of photoelectrons in matter[11]
which for ILs results in a typical information depth below
9 nm under an electron-emission angle of 08 (with respect to
the surface normal).[12] By changing the emission angle to 808
(angle-resolved XPS, ARXPS) the information depth can be
further lowered to approximately 1.5 nm. An increase of
a particular core level with an increasing detection angle, and
thus with increasing surface sensitivity, indicates a higher
concentration of the element in the topmost layers as
compared to the “bulk”, thus enabling conclusions on
orientation and/or enrichment of molecules at the surface.
The growing number of surface science studies of ILs
under UHV conditions has not only increased our specific
knowledge on molten salt surfaces but also opened new
routes toward the fundamental understanding of the surface
properties of liquids in general.
Herein, we now demonstrate that in situ XPS offers a new
and general approach for the monitoring and mechanistic
understanding of ordinary liquid phase organic reactions, if
the reacting groups have been linked before-hand to an ionic
head group to drastically lower the vapor pressures of the
reactants. Our contribution demonstrates this approach for
the nucleophilic substitution reaction of an alkyl amine and
an alkyl chloride moiety, which are attached to the cation and
anion of ionic liquids, respectively. This novel approach offers
fundamentally new possibilities to gain insight into the
reactions of organic liquid-phase systems, including informa-
tion on surface composition and orientation, involvement of
different oxidation states in the proceeding reaction, and
kinetic investigations under solvent-free ultraclean UHV
conditions.
To date, only a few experiments have been carried out by
using in situ XPS for IL systems, that is, to follow reactions or
dissolution processes in an XP spectrometer. Lovelock et al.
studied the in situ adsorption, dissolution, and desorption of
water on [C8C1Im][BF4] by temperature-programmed XPS.[13]
Licence and co-workers followed the in situ electrochemical
oxidation of metallic Cu to Cu+ in an IL[14] and the electro-
chemical reduction of Fe3+ to Fe2+ in an IL,[15] and Compton
et al. measured the electrodeposition of potassium metal from
an IL.[16] Kçtz et al. also dealt with in situ electrochemical
XPS, but concentrated on the effect of the applied potential
on the binding energy and stability of [C2C1Im][BF4].[17]
Finally, the partial exchange of CO molecules adsorbed on
Pd nanoparticles on an Al2O3/NiAl(110) substrate by the IL
[C4C1Im][Tf2N] was monitored by Sobota et al., who used IR
and in situ XP spectroscopy.[18] All of the above-mentioned
experiments dealt either with a reaction at an interface or
with an electrochemical reaction, and in all cases nonfunc-
tionalized ILs were employed.
[*] C. Kolbeck, I. Niedermaier, Dr. F. Maier, Prof. H.-P. Steinrꢀck
Lehrstuhl fꢀr Physikalische Chemie II
Friedrich-Alexander-Universitꢁt Erlangen-Nꢀrnberg
Egerlandstrasse 3, 91058 Erlangen (Germany)
E-mail: steinrueck@chemie.uni-erlangen.de
Dr. N. Taccardi, Dr. P. S. Schulz, Prof. P. Wasserscheid
Lehrstuhl fꢀr Chemische Reaktionstechnik
Friedrich-Alexander-Universitꢁt Erlangen–Nꢀrnberg
Egerlandstrasse 3, 91058 Erlangen (Germany)
[**] This work has been supported by the DFG through SPP 1191, grants
STE 620/7-2 and WA 1615/8-2 and by the Excellence Cluster
“Engineering of Advanced Materials” granted to the Universitꢁt
Erlangen–Nꢀrnberg. C.K. acknowledges also support by the Max-
Buchner-Stiftung. Th. Drewello is acknowledged for the ESI-MS
analysis.
For our study of in situ XPS reaction monitoring, we
selected the reaction of 1-ethyl-3-methylimidazolium 4-chlor-
obutylsulfonate [C2C1Im][ClC4H8SO3] (IL1) and 1-methyl-3-
(3’-dimethylaminepropyl)imidazolium trifluoromethylsulfo-
Supporting information for this article (including information on
experimental details and on synthesis and characterization of the
ionic liquids) is available on the WWW under http://dx.doi.org/10.
1002/anie.201107402.
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nate [(Me2NC3H6)C1Im][TfO] (IL2) as a model system. IL1
was obtained by reacting [C2C1Im]Cl with 1,4-butane sultone
according to a reported procedure,[19] IL2 was prepared by
reacting 3-chloropropyldimethylamino hydrochloride and N-
methylimidazole in water, followed by anion metathesis (see
the Supporting Information for details). In the course of the
reaction of IL1 and IL2, the amine functionality of IL2 is
alkylated by the 4-chlorobutylsulfonate anion of IL1
(Scheme 1) to form the new zwitterionic salt IL3, and
[C2C1Im][TfO] as the second product.
Figure 1. N 1s region of IL2 at room temperature in 08 (black) and 808
(green) emission; BE=binding energy; I=intensity in counts per
Prior to our XPS investigations, the reaction was carried
out under standard laboratory conditions and routine
1H NMR analysis was applied to analyze the product mixture.
After heating a 1:1 mixture of neat IL1 and IL2 to 1008C for
90 minutes and dissolving the final product in [D6]-DMSO,
the 1H NMR spectra showed only minor changes. For
instance, the methyl groups of NMe2 in IL2 (singlet signal at
2.2 ppm, presumably most affected by the alkylation) shifted
less than 0.1 ppm downfield, thus making an unambiguous
quantitative NMR analysis virtually impossible. To verify if
the alkylation indeed took place under the applied conditions,
an electrospray ionization (ESI) mass spectrum of the product
was recorded and clearly showed the presence of an m/z =
304.2 species, which can be unambiguously attributed to the
formation of the IL3 cation.
XPS requires UHV conditions and the exposition to X-ray
irradiation. Both these factors in combination with an
elevated temperature could affect the stability of primary
ionic liquids. Therefore, at first ARXPS measurements of
both neat-substrate ILs were conducted at elevated temper-
atures. For IL2 no changes in the XP spectra were observed
after 2 hours at 1008C, thus validating the stability of IL2
under the reaction conditions that were later used for the
nucleophilic substitution reaction (see below). Furthermore,
the analysis of the angle-resolved XP spectra showed
a preferred orientation of the amine-functionalized cation at
the IL2–vacuum interface: The corresponding N 1s spectra
(Figure 1, 08 (black) and 808 (green) emission) clearly show
two signals. The signal at 401.9 eV originates from the two
imidazolium nitrogen atoms (Nimid) and the signal at 399.2 eV
from the amine functionality (Namine). When increasing the
emission angle from 08 to 808, the Namine signal increases and
the Nimid signal decreases. This behavior is attributed to
a preferential orientation of the cation with its amine
second.
functionality pointing toward the vacuum and the imidazo-
lium ring into the bulk.
In contrast to IL2, IL1 showed pronounced signs of
decomposition of the 4-chlorobutylsulfonate anion in UHVat
1008C. The left-hand side of Figure 2 shows the Cl 2p and the
O 1s XP spectra at 08 emission, before (black) and after (red)
heating IL1 in situ for 2 hours at 708C and 1.5 hours at 1008C;
the corresponding C 1s, N 1s, and S 2p spectra are shown in
Figure S1 in the Supporting Information. Before heating, the
Cl 2p region shows a doublet structure, due to the spin-orbit
split 2p1/2 and 2p3/2 components. The Cl 2p3/2 signal has
a binding energy of 200.5 eV, which is representative for
covalently bound chlorine (Clcov).[19] Upon heating to 708C,
the XP spectra remain nearly unchanged (not shown).
However, upon further heating to 1008C for 1.5 hours, the
intensity of the original Cl doublet decreases to about 45%,
and simultaneously a new doublet arises, with the Cl 2p3/2
signal at 197.2 eV. The latter is assigned to a chloride species
(Clion
,
by comparison to XPS measurements of
[C8C1Im]Cl).[20] Furthermore, a concomitant decrease of the
intensities of O 1s (Figure 1) and S 2p (Figure S1) is observed.
These observations indicate that a reversal of the ring-
opening reaction, which was employed to prepare IL1, occurs
upon heating the IL in vacuum (see Scheme 2a). In situ mass
spectrometry during the heating step shows the characteristic
pattern of 1,4-butane sultone (in the gas phase), thus
Figure 2. Cl 2p and O 1s XP spectra of IL1 before (black) and after
heating to 708C for 2 hours and additional heating to 1008C for
1.5 hours (red) in 08 emission (left) and 808 emission (right); note the
identical intensity scales in 08 and 808 emission.
Scheme 1. a) Structure of IL1 and IL2, and b) Alkylation of the IL2
cation by the IL1 anion to form IL3 (and [C2C1Im][TfO]).
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1008C for 2 hours in the spectrometer are shown in Figure 3
(left-hand side); the corresponding C 1s and S 2p spectra are
shown in Figure S3 in the Supporting Information. Upon
heating to 1008C, the Cl 2p region clearly shows an almost
complete transformation ((91 Æ 5)%) of covalent chlorine
(Clcov; Cl 2p3/2 at 200.2 eV) to chloride (Clion; 197.0 eV) in the
near-surface region. From the Cl 2p region alone, however,
discrimination between alkylation (i.e., IL3 formation),
sultone formation, and chlorobutylsulfonate ester formation
is not possible, because the transformation of covalently
bound Cl to chloride occurs in all cases. The required
information can be derived from the N 1s region, which
shows two well-separated signals: the signal at 401.9 eV is
attributed to the nitrogen atoms (Nimid) of the imidazolium
moieties of IL1 and IL2, and the signal at 399.4 eV to the
amine functionality (Namine) of IL2. After heating, the Namine
signal intensity decreases to (44 Æ 5)%, thus suggesting
a (56 Æ 5)% conversion of IL2 to IL3. The loss in Namine
signal intensity is balanced by the broadening of the Nimid
signal, which is attributed to the emerging ammonium group
in IL3. By fitting the N 1s spectra to the three distinct nitrogen
species, a binding energy of (403.1 Æ 0.2) eV for this ammo-
nium group Nammon was found (see dashed line in Figure 3).
Scheme 2. a) Sultone formation and b) formation of a 4-chlorobutylsul-
fonate ester by self-reaction of the 4- chlorobutylsulfonate anion.
confirming the reverse reaction (see Figure S2 in the Sup-
porting Information).[21] A thermal gravimetric analysis
(TGA) of similar compounds with the same 4-chlorobutylsul-
fonate anion and different cations showed a decomposition of
the anion to 1,4-butane sultone and the corresponding
chloride salt between 210 and 2158C at standard pressure.[19]
However, in our UHVapparatus a much lower temperature is
sufficient for this decomposition to occur, since the sultone is
steadily removed from the equilibrium under the vacuum
conditions.
The comparison of the intensities in the Cl 2p and O 1s
regions shows that the intensity of the Clcov signal decreases to
(45 Æ 5)% and the intensity of the O 1s signal decreases only
to (58 Æ 5)%. This is a clear indication that not only sultone
formation (which would lead to an identical decrease), but
also an additional reaction occurs. A possible explanation is
the formation of a sulfonic acid ester by self-reaction of the
chlorobutylsulfonate (Scheme 2b). Because of the anionic
nature of the ester that is formed, it does not evaporate and
therefore contributes to the O 1s signal.
Information about the orientation and the enrichment of
the ions at the surface is again obtained from the 808 emission
spectra, which are shown on the right-hand side of Figure 2.
Before heating IL1 (spectra in black), an increase in Cl 2p
(Clcov) and a decrease in O 1s intensity is observed at 808, as
compared to 08, thus suggesting an orientation of the
[ClC4H8SO3]À anion with its chlorobutyl chain sticking out
into the vacuum while the sulfonate group points toward the
bulk. After heating at 1008C (spectra in red in Figure 2), the
Clcov signal at 808 is larger than at 08, whereas the Clion signal
shows the opposite behavior. This observation suggests
a surface enrichment of the unreacted [ClC4H8SO3]À anion
at the expense of the formed chloride, which depletes into the
bulk. Such a depletion of chloride has been seen before in
a solution of [Pt(NH3)4]Cl2 in [C2C1Im][EtOSO3] and was
explained by the smaller molecular volume and the lower
polarizability of chloride in comparison to [EtOSO3]À.[22] It is
noteworthy, that both IL1 and IL2 were free of any surface
impurities, such as Si impurities from dissolved silicon grease.
After investigating IL1 and IL2 separately, we turned to
the reactive IL1/IL2 mixture and to the observation of the
alkylation reaction of IL2 and IL1 by in situ XPS. As the
reaction hardly proceeds at room temperature, the binary 1:1
mixture of the reactant ILs could be prepared under ambient
conditions. This mixture was transferred into the XP spec-
trometer within 10 minutes and then heated, while XPS data
were recorded.
Figure 3. Detail XP spectra of Cl 2p, N 1s, O 1s, and F 1s regions of
IL1/IL2 mixture in 08 emission (left) and 808 emission (right) before
(black line) and after (red line) heating to 1008C for 2 hours.
The Cl 2p, N 1s, O 1s, and F 1s XP spectra in 08 emission
before (black) and after (red) heating the IL1/IL2 mixture at
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To get an independent confirmation of the alkylation
reaction in the spectrometer, the sample holder of the
spectrometer was washed with methanol right after the
above-described heating of the IL1/IL2 mixture, and the
collected solutions were analyzed by ESI-MS. As expected,
the m/z = 304.2 signal in the ESI-MS analysis unambiguously
proved the formation of the IL3 cation.
atom-specific, oxidation-state-specific, and surface-selective
analysis technique, a significant amount of mechanistically
relevant information on the surface region of reacting organic
systems will become accessible with this approach.
Received: October 20, 2011
Published online: December 23, 2011
The
56%
conversion
of
the
IL2
cation
[(Me2NC3H6)C1Im]+ suggests that it reacts with 56% of the
IL1 anion [ClC4H8SO3]À to form IL3. The additional con-
version of covalent chlorine Clcov to chloride Clion is due to
sultone or ester formation. The 9% Clcov signal left after
heating (see above) indicates that 18% of IL1 have reacted to
ester according to Scheme 2b (note that upon ester formation
two Clcov are converted to one Clcov and one Clion species). The
remaining 26% of IL1 are then converted to volatile sultone.
As both IL1 and IL2 contain one SO3 group, this should result
in 13% decrease in O 1s and S 2p intensities. However, upon
heating, only minor changes (< 5%) occurred in the O 1s
(Figure 3) and S 2p regions (see the Supporting Information,
Figure S3). At the same time, we observed an increase of the
F 1s intensity (by (18 Æ 5)%) and a concomitant decrease in
the total Cl 2p signal intensities (by (20 Æ 5)%) in the 08
spectra in Figure 3 upon heating (note that both effects are
even more pronounced at 808). This behavior indicates
a depletion of chloride from the surface-near region and
a concomitant enrichment of the more surface-active SO3-
containing [TfO]À anion. This enrichment roughly compen-
sates the loss of SO3 groups because of the formation of
volatile 1,4-butane sultone.
Keywords: ionic liquids · reaction monitoring ·
surface chemistry · X-ray photoelectron spectroscopy ·
zwitterions
.
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In conclusion, we have demonstrated that it is possible to
monitor an organic liquid-phase nucleophilic substitution
reaction by in situ XPS. The applied method, which is new for
the investigation of organic reactions, allows to follow the fate
of all elements present in the reaction mixture (except for
hydrogen) in a quantitative and oxidation-state-sensitive
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attach ionic head groups to the reacting organic molecules, in
order to reduce their volatility. Herein, we investigated the
nucleophilic substitution reaction between [C2C1Im]
[ClC4H8SO3] (IL1) and [(Me2NC3H6)C1Im][TfO] (IL2). We
found that the alkylation of the amine functionality of IL2 by
the 4-chlorobutylsulfonate anion of IL1 occurs in the near-
surface region with a conversion of 56% under the applied
UHV conditions. As parallel reactions, the formation of an
anionic sulfonate ester and the intramolecular elimination of
volatile sultone from the anion [ClC4H8SO3]À have been
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