Hybrid Surfactants with N-Heterocyclic Carbene Heads as a Multifunctional Platform for Interfacial Catalysis

Article abstract of DOI:10.1002/chem.201703902

Processing of substrates with different solvent compatibility is a persistent problem in homogeneous catalysis, in particular when one starting compound is water soluble and the other is not. A promising concept reported in the literature is micellar catalysis. However, the process of developing catalysts that are surfactants at the same time is still in its early stages. We report the synthesis of a new surfactant system with an N-heterocyclic carbene (NHC) moiety as a head group. Characteristic surfactant properties such as the formation of micelles or liquid crystals is documented. The new surfactant ligand forms coordination compounds with various metals, most importantly Pd²⁺, in square planar geometry. In addition, the Pd-NHC compound shows surfactant features, and can be used successfully for C?C cross-coupling reactions (Suzuki, Heck). The boost in catalytic activity by one order of magnitude compared to analogous but non-amphiphilic species is reported.

Full text of DOI:10.1002/chem.201703902

A Journal of
Accepted Article
Title: Hybrid Surfactants with N-Heterocyclic Carbene Heads as a
Multifunctional Platform for Interfacial Catalysis
Authors: Sebastian Polarz, Adrian Donner, Kay Hagedorn, Lorenz
Mattes, and Markus Drechsler
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To be cited as: Chem. Eur. J. 10.1002/chem.201703902
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Hybrid Surfactants with N-Heterocyclic Carbene Heads as a
Multifunctional Platform for Interfacial Catalysis
Adrian Donner,[a] Kay Hagedorn,[a] Lorenz Mattes,[a] Markus Drechsler[b] and Sebastian Polarz*[a]
Abstract: Processing of substrates with different solvent compatibility
is a persistent problem in homogeneous catalysis, in particular when
one starting compound is water soluble and the other is not. A
tempting concept found in literature is micellar catalysis. However,
developing catalysts, which are surfactants at the same time, stands
at its beginning. We report the synthesis of a new surfactant system
with a N-heterocyclic carbene (NHC) moiety as a head group.
Characteristic surfactant properties such as the formation of micelles
or liquid crystals is documented. The new surfactant ligand forms
coordination compounds with various metals, most importantly Pd2+ in
square planar geometry. The Pd-NHC compound as well shows
surfactant features, and can be used successfully for C-C cross
coupling reactions (Suzuki, Heck). The boost in catalytic activity by
one order of magnitude compared to analogous, but non-amphiphilic
species is reported.
needs the metal-containing moiety becoming its integral part, for
instance as a head group.[4] A powerful approach towards the
generation of metallosurfactants is the synthesis of a ligand
modified by a long, hydrophobic chain, typically an alkyl group.[5]
The head group becomes or remains charged after coordination
of a wanted metal cation resulting in hybrid surfactants with
interesting, novel properties inaccessible for pure organic
compounds.[6] Good examples can be found for surfactants with
magnetic heads.[7] To prepare surfactants with catalytic heads is
more difficult, because one has to provide free coordination sites
at the metal. So, only a few papers have been published yet.[8] A
nice case was presented by Mancini and coworkers,[9] who have
asymmetrically modified a porphyrin ring. After coordination to
Mn2+ the amphiphilic compound was used for the catalytic
epoxidation of alkenes with aqueous H2O2. Deng et al. described
a Rh(I) containing surfactant, which was used for the asymmetric
hydrogenation of ketones in water.[10]
Converting two reactants with different solvent compatibility, one
hydrophilic the other lipophilic, is a classic problem in chemistry.
Existing solutions to this problem can be divided into two
categories. One method is widely known as phase-transfer
catalysis.[1] One component is moved into the unfavorable phase
by coordination to auxiliaries changing its solubility. Second, one
may increase the chance for a reaction by providing a large
interface of the immiscible phases. This can be done by creating
micelles or emulsion droplets. The current state in the associated
field 'micellar catalysis' was described in an excellent review
article from Scarso et al. in 2015.[2] The situation becomes even
more intricate, as soon as the catalysts itself is not soluble in the
desired medium. Many molecular catalysts are soluble in apolar
solvents due to bulky, organic ligands coordinating to the metal.
The application of those compounds in polar media, ultimately
water, requires extra efforts.[3] However, a major difficulty
remaining is, when the catalytically active species needs to be
compatible with two solvents of opposing miscibility at the same
time.
Another class of highly important, catalyzed transformations
is represented by C-C cross-coupling reactions. It is well-known
that complexes with carbene ligands, in particular N-heterocyclic
carbenes (NHC), represent powerful catalysts.[11] Since the first
report of a water soluble NHC complex in 1974,[12] more attention
was devoted to this subject just within the last 10 years.[13] Few
examples exist on attempts to prepare an NHC system with
amphiphilic properties. Mingotaud and coworkers have presented
a Hoveyda's type catalyst comprising a perflourinated C9 chain
attached to Ru via a carboxylate group in 2008.[14] It is also worth
mentioning the pioneering work of the Uozumi group,[15] who
made a Pd-NHC pincer ligand with one side modified by two alkyl
chains as hydrophobic moieties and (on the other side) two oligo
glycol chains for water solubility. In several papers, the authors
show that product selectivity in the Miyaura-Michael reaction can
be improved in comparison to using conventional, non-
amphiphilic catalysts.[16]
Surfactants with catalytic heads fill a gap between phase
transfer catalysis on the one side and micellar catalysis on the
other side, taking the best from two worlds. In particular a
surfactant containing an NHC moiety promises to be of high
potential in several C-C cross-coupling reactions like Hiyama,
Kumada, Negishi, Stille, Suzuki, Sonogashira or Heck. Thus, our
target (5) is a new class of surfactants with an NHC head group
as shown in Scheme 1. Followed by synthesis, we will examine
the capabilities of the surfactant on the complexation of different
metal cations and study their surfactant features. Finally, for the
Pd-containing species catalytic investigations will be performed.
Compounds, which can effectively mediate between water
and an oil phase, are of course well-known: Surfactants;
molecules comprising a water-soluble head and a hydrophobic
tail. Arming a surfactant with catalytic properties is desired and
[a]
[b]
A. Donner, K. Hagedorn, Prof. Dr. S. Polarz
Department of Chemistry
University of Konstanz
Universitätsstrasse 10, 78457 Konstanz, Germany
E-mail: sebastian.polarz@uni-konstanz.de
Dr. M. Drechsler
Laboratory for Soft-Matter Electron Microscopy
University of Bayreuth, 95440 Bayreuth, Germany
Supporting information for this article is given via a link at the end of
the document.
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parameter is slightly larger,[19] and the tendency to forming
structures with higher curvature can be seen from the occurrence
of a hexagonal phase (see Supporting Information Fig. S4).
Scheme 1. Synthesis of the NHC surfactants.
The synthesis of (5) is described in detail in the experimental
section. First, 2,6-dichloro-4-iodopyridine (2) was coupled to the
alkyl-substituted phenylboronic acid (1) by means of Suzuki
coupling, followed by the nucleophilic substitution of the chlorine
substituents in (3) with a N-substituted imidazole (4). Because the
flexible alkyl chain prevents growing single-crystals and
characterization by X-ray diffraction methods, electron-spray
ionization mass spectrometry (ESIMS) has proven to be a
valuable method for hybrid surfactants.[7b, 17] The ESIMS patterns
of (5a,b) are shown in Supporting Information Fig. S1.
Because of its molecular architecture with the two mono-
cationic heads separated by the pyridine spacer (5) is expected
to be a surfactant with so-called gemini characteristics.[18] The
surfactant character can indeed be confirmed by concentration
dependent surface tension measurements shown in Fig. 1a. The
critical micelle concentration (cmc) of 5a is 54 mg/l (7.9´10-5
mol/g). This is roughly one order or magnitude lower compared to
a classical surfactant such as CTAB (cetyltrimethyl ammonium
bromide; 9.2 ´10-4 mol/g), which we assign to the twofold charge
and the larger head preventing a dense packing on the air/water
interface. 5b has a cmc in a similar range to 5a, but the saturation
value for surface tension g (» 40 mN/m) is higher than for a
conventional surfactant. The iso-propyl groups in 5b presumably
prevent an effective occupancy of the aqueous interface. This
assumption can be confirmed by calculation of the surface excess
G and the minimum area per molecule at the air/water interface
(Am(5a) = 16.0Å2, Am(5a) = 24.6Å2) from the concentration
dependent g curves (see Supporting Information Fig. S2).
Figure 1. Amphiphilic behavior of surfactants with NHC head group 5. (a)
Surface tension measurements (5a (black squares); 5b (red circles)). (b) Size
distribution (DH @ hydrodynamic diameter) derived from DLS measurements for
5b (red) and 6bi (blue). Cryo-TEM micrograph (c) of a micellar solution of 5a
(scale = 100 nm). Polarization microscopy image (d) and SAXS pattern (e; red
line) of a liquid crystalline phase of 5a. The SAXS pattern after coordination of
Pd2+ (6ai) is also shown. The vertical lines tell the patterns expected for a
lamellar phase La (red) and a hexagonal phase P6/mm (blue).
The size of the aggregates in water at low concentration was
determined by dynamic light scattering (DLS) (Fig. 1b). Because
there is always some uncertainty in the absolute value of micellar
aggregates derived from DLS, transmission electron microscopy
(TEM) under cryogenic conditions (Fig. 1c) was also applied.
Such objects with a dimension of 5 nm fit well to conventional,
spherical micelles and considering the molecular extension of one
surfactant molecule is 2.4 nm (see Supporting Information Fig.
S3). At higher concentration we have also observed the formation
of liquid crystalline phases (LCs) as indicated by birefringence
patterns found in polarization microscopy (Fig. 1d). The structure
of the LCs was clarified by small-angle X-ray scattering (SAXS)
shown in Fig. 1e. The noted pattern for 5a is characteristic for a
lamellar phase (La) with a layer-to-layer distance of 4.9 nm.
Because the head group of 5b is more bulky, the packing
The true functionality of the surfactants with NHC head group
is their ability to coordinate to metal centers resulting in the
compounds 6,b(i-iv) (see scheme 1). Among other methods
summarized in Supporting Information Fig. S5 crucial
information is delivered by ESIMS shown in Fig. 2. The precise
agreement between the experimental data and the patterns
expected for the molecular ions [6a,bi]+ proves the successful
synthesis of the Pd2+ containing compounds. The proper binding
of Pd could also be confirmed by shifts in infrared spectroscopy
(IR) signals involving the vibrations of the pyridine and
imidazolium rings. The generation of the carbene species was
confirmed by characteristic changes in 1H-NMR spectroscopy,
most importantly the absence of the imidazolium proton at d =
10.8 ppm. For a d8-configuration as in Pd2+ in general a square
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planar coordination geometry is preferred, and this can be
confirmed by geometry optimization calculations performed using
density functional theory (DFT) calculations (Fig. 2). When the
coordination geometry of the metal center is different, for example
octahedral as for Fe2+ (6bii) or linear as for Ag+ (6biv) molecular
species containing two NHC ligands are obtained (see also
Supporting information Fig. S5). Similar coordination modes
were also found in the literature for conventional, non-amphiphilic
ligands.[20]
Scheme 2. Catalytic test reaction performed using 6bi as a surfactant catalyst
in comparison to a non-amphiphilic catalyst as a reference.
As a catalytic test reaction we selected the Suzuki coupling of
phenylboronic acid with 4-bromoacetophenone (see scheme 2).
As a reference we have also synthesized the non-amphiphilic
coordination compound (7), which was used for the catalytic
reaction under otherwise identical conditions. The reaction was
checked by gas-chromatography (GC) (Fig. 3). It can be seen the
surfactant compound 6bi is superior over 7 with a conversion of
over 99% within 15 minutes. It is obviously much more difficult for
the non-amphiphilic catalyst to process a substrate mixture
containing both, a hydrophobic (4-bromoacetophenone) and a
hydrophilic (boronic acid) compound. Thus, the micellar catalysis
route enabled by 6bi leads to a substantial increase in the turn
over frequency (TOF) from 2.5x10-3 s-1 for (7) to 3.6x10-2 s-1 (mole
product per mole catalyst and second), although the catalytic
center is the same. This is in a range also reported for
conventional catalyst systems reported in the literature.[21] For
proof of concept, we have also checked the activity of the Pd-NHC
surfactants in other C-C cross-coupling reaction such as the Heck
reaction (see Supporting Information Fig. S7).
Figure 2. ESIMS spectra of the Pd-containing, hybrid surfactants 6ai (red) and
6bi (blue); the theoretical isotope pattern for the ions [6ai]+ and [6bi]+ is included
as gray lines. The molecular structure of the surfactants determined from DFT
calculations is also shown.
The self-assembly behavior of the new, hybrid surfactants
6a,bi was investigated next. We learn from SAXS measurements
(Fig. 1e) the presence of the metal has a marked effect on the
structure of the LC phases. A hexagonal phase (P6/mm) is
adopted, which is a structure encountered often for lyotropic liquid
crystals formed by various amphiphiles. The periodicity of the
structure has decreased slightly (4.2 nm) compared to the
lamellar phase of the organic surfactant 5a, which might be
explained by the reduced repulsion of the head group, which is
now only singly charged. Unfortunately, recording TEM data was
hampered by the reduction of Pd in the electron beam (see
Supporting Information Fig. 6). When we tried to examine the
micellar state of 6ai, we faced enormous problems. The
compound is almost insoluble in any solvent, no matter polar or
unpolar. The square planar coordination of Pd2+ leads to a
planarization of the entire head group region of the surfactant as
can be seen from Fig. 2. This results in an effective orientation of
the four conjugated rings and induces a strong p-p interaction.
Obviously one has to disturb this interaction, and this is why we
have introduced the iso-propyl groups attached to the imidazolium
rings in 6bi. The iso-propyl groups introduce a bump to the
molecule according to the DFT calculations (Fig. 2). This reduces
the intermolecular head-group interaction, and as a result the
surfactant becomes now soluble in polar media. The occurrence
of micelles can be observed again (Fig. 1b). Like for the LC
phases, the coordination of Pd leads to a slight decrease in the
size of the self-assembled structure.
Our study verifies that an NHC-based system with surfactant
architecture opens new perspectives in micellar catalysis. The
presented case is open for a multiple number of adjustments. It
was already shown that (5) is compatible with diverse metals.
Therefore, we expect that analogous complexes with Pt or Au
cannot only be used for a range of other catalytic transformations
e.g. hydrosilylation reactions,[22] but they are also promising in
other applications like as an anti-tumor drug.[23] Further, additional
substituents at the phenyl ring might help to fine-tune the catalytic
activity and to adjust product selectivity. We had overcome the
problem of the undesired p-p interaction by attaching isopropyl
substituents to the imidazolium. We intend to use other, polar
groups in the future for increasing the solubility of the surfactant
in water even more. In particular a sulfonic acid group seems to
be tempting aiming at acid - cross-coupling tandem catalysis.[24]
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Figure 3. Monitoring the catalytic conversion (Suzuki coupling; scheme 2) by
GC-MS, using 1-butanol as an internal standard (black: amphiphilic catalyst 6bi,
grey: non-amphiphilic reference 7).
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Experimental Section
Experimental Details are given in the Suppporting Information.
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Acknowledgements
We thank the European Research council for funding (ERC
consolidator grant ISURF; project 614606). MD thanks the
Bavarian Polymer Instiute (BPI) and the collaborative research
centre SFB840 of the German Research Foundation (DFG) for
financial support.
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Keywords: Surfactants • Self-Assembly • Homogeneous
Catalysis • Cross-Coupling Reactions • Organic-Inorganic
Hybrids
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Entry for the Table of Contents
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Soapy catalyst in action. A
Author(s), Corresponding Author(s)*
organic-inorganic hybrid surfactant
containing a carbene-Pd complex as
a head is used for micellar catalysis
bridging the gap for substrates with
orthogonal solvent-compatibility.
Page No. – Page No.
Hybrid Surfactants with N-
Heterocyclic Carbene Heads as a
Multifunctional Platform for
Interfacial Catalysis
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