One-pot synthesis and immobilisation of sulfonate-tethered N-heterocyclic carbene complexes on polycationic dendrimers

Article abstract of DOI:10.1002/chem.200901719

The development of an in situ transmetallation-immobilization strategy for the construction of non-covalent NHC- metallodendrimers starting from a common polycationic dendrimer in combination with an anion-tethered NHC ligand and different metal precursor

Full text of DOI:10.1002/chem.200901719

COMMUNICATION
DOI: 10.1002/chem.200901719
One-Pot Synthesis and Immobilisation of Sulfonate-Tethered N-Heterocyclic
Carbene Complexes on Polycationic Dendrimers
Morgane A. N. Virboul,[a] Martin Lutz,[b] Maxime A. Siegler,[b] Anthony L. Spek,[b]
Gerard van Koten,[a] and Robertus J. M. Klein Gebbink*[a]
Since their first isolation by Arduengo and co-workers in
1991, N-heterocyclic carbenes (NHCs) have been widely
used as versatile ligands in organometallic chemistry and ho-
mogeneous catalysis.[1] Due to their strong s-donor charac-
teristics, these compounds are known to form robust transi-
tion-metal complexes. As a consequence, the use of NHCs
as ligands allows the synthesis of highly functionalised or-
ganometallic complexes that combine catalytic properties of
the metal center with a variety of other functional group
properties.[2] The stability of NHC carbene complexes has
attracted our interest from the viewpoint of catalyst immo-
bilisation, where loss of functional metal sites from the con-
struct by metal leaching should be minimal.
situ transmetallation–immobilisation strategy for the con-
struction of non-covalent NHC-metallodendrimers starting
from a common polycationic dendrimer in combination with
an anion-tethered NHC ligand and different metal precur-
sors.
The zwitterionic imidazolium 1 was synthesised in a
single-step procedure by nucleophilic addition of mesityl
imidazole to 1,4-butane sultone (Scheme 1) and was ob-
tained in excellent yield.[5] Treatment of 1 with an excess of
Ag2O in acetonitrile at reflux temperature for 6 h resulted
in the formation of the corresponding bis-carbene silver
complex 2, which was isolated in 87% yield.
1
The H NMR spectrum of 2 displayed clear upfield shifts
We and others have previously proven the utility of carbo-
silane[3] and polycationic dendrimers[4] as supports for the
immobilisation of transition-metal complexes through cova-
lent and non-covalent attachment, respectively. In particular,
the use of polycationic dendrimers allows the easy (reversi-
ble) loading of dendrimers with larger numbers of functional
molecules and renders the synthesis of catalytically active
and recoverable dendritic assemblies feasible. Following
these investigations we have turned our attention to the syn-
thesis of functionalised NHC complexes that are suitable for
the preparation of non-covalently immobilised homogene-
ous catalysts. Here, we report on the development of an in
of the signals corresponding to the protons of the imidazoli-
um ring and lacked the signal corresponding to the C2
proton. Characterisation of compound 2 by 13C{1H} NMR
spectroscopy confirmed coordination of silver to the ligand
by the presence of a typical signal at d=180.2 ppm for the
C2 carbon. Single crystals of complex 2 suitable for X-ray
structure determination were grown by slow evaporation of
a solution of 2 in dichloromethane/benzene. The molecular
structure shows that 2 is a bis(imidazol-2-ylidene)silver com-
plex, in which a linear Ag bis-carbene fragment is present
(C11-Ag1-C12 178.55(14)8; Figure 1).[10] The silver–carbon
bond lengths (range 2.073–2.083 ꢀ, the individual s.u.ꢁs are
0.004 ꢀ) in this moiety are consistent with those reported in
literature, which are normally found in the range of 2.067-
2.117 ꢀ for bis-carbene silver complexes with non-coordi-
nating anions.[6b]
[a] M. A. N. Virboul, Prof. Dr. G. van Koten,
Prof. Dr. R. J. M. Klein Gebbink
Chemical Biology & Organic Chemistry
Debye Institute for Nanomaterials Science
Faculty of Science, Utrecht University
Padualaan 8, 3584 CH Utrecht (The Netherlands)
Fax : (+31)30-252-3615
The overall charge neutrality in crystals of 2 is achieved
by way of a second silver ion (Ag2). This counterion is
bound to three sulfonate groups from three different Ag–
E-mail: r.j.m.kleingebbink@uu.nl
À
bis-carbene moieties through Ag O bridges to form one-di-
[b] Dr. M. Lutz, Dr. M. A. Siegler, Prof. Dr. A. L. Spek
Crystal and Structural Chemistry
mensional (1D) polymeric chains along the crystallographic
c axis. Non-coordinated water molecules are also part of the
1D polymeric chains by forming hydrogen bonds between
two sulfonate moieties (see Figure S1 in the Supporting In-
formation).
Bijvoet Centre for Biomolecular Research
Faculty of Science, Utrecht University
Padualaan 8, 3584 CH Utrecht (The Netherlands)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901719.
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Scheme 1. Synthesis and metallation of functionalised ligand.
methods (see the Supporting Information). The formation
of a monomeric, molecular (NHC)–AuI complex as opposed
to the polymeric AgI complex 2 was confirmed by the crystal
structure of [PPh4][3] (Figure 2).[10] The molecular structure
Figure 1. Displacement ellipsoid plot (50% probability level) at
150(2) K) of a sulfonate-appended bis(imidazol-2-ylidene)silver complex,
that is the monomeric unit of one independent polymeric chain of 2. Hy-
drogen atoms and the non-coordinated water molecules are omitted for
the sake of clarity. Selected bond lengths [ꢀ] and angles [8] (second mol-
À
À
ecule in square brackets): Ag1 C11 2.078(4) [2.079(4)], Ag1 C12
2.073(4) [2.083(4)]; C11-Ag1-C12 178.55(14) [176.48(14)], N11-C11-N21
104.0(3) [103.9(3)], N12-C12-N22 104.0(3) [103.9(3)].
Following the pioneering work of Lin et al.,[6a] we focused
our attention on the synthesis of sulfonate-tethered mono-
carbene metal complexes, in which the metal centre bears
no formal charge, through transmetallation from 2 used as
carbene transfer agent. Starting from the halide-free Ag–
carbene complex 2, this strategy requires two equivalents of
an overall neutral metal precursor to form two neutral
metal carbene moieties. In addition, two equivalents of an
organic halide salt are necessary to provide each of the two
sulfonate groups with a counterion and to allow the forma-
tion of the two equivalents of silver halide.
Figure 2. Displacement ellipsoid plot (50% probability level) of the
asymmetric unit of [PPh4][3] at 150(2) K. Hydrogen atoms are omitted
À
for clarity. Selected bond lengths [ꢀ] and angles [8]: C1 Au1 1.976(2),
À
Au1 Cl1 2.2699(6); C1-Au1-Cl1 178.17(6), N1-C1-N2 110.71(18).
confirmed the presence of a non-interacting tetraphenyl-
phosphonium cation as counterion for the sulfonate group.
À
The Au C distance of 1.976(2) ꢀ and the nearly linear C-
Au-Cl angle (178.17(6)8) in [PPh4][3] are consistent with the
features of similar (NHC)–AuI complexes reported in the
literature (range between 1.942 and 1.998 ꢀ).[7]
This principle was first tested using [AuCl
G
To show the wider applicability of our strategy, we applied
À
metal precursor and PPh4Cl as the halide source, strictly fol-
lowing the stoichiometry of one equivalent of silver for one
this synthetic pathway to the synthesis of sulfonate (NHC)
Rh complexes by the transmetallation reaction of a silver
complex and a suitable Rh precursor, as previously de-
scribed by Crabtree et al.[8] It appeared that the sulfonate
RhI–carbene complexes could be readily obtained under
conditions similar to those described for [PPh4][3] (see
Scheme 3) from compound 2
equivalent of PPh4Cl and [AuCl
hydrothiophene).
ACHTUNGTNER(NUNG tht)] (Scheme 2; tht=tetra-
The resulting sulfonate-tethered AuI–carbene complex
[PPh4][3] was fully characterised by classical spectroscopic
using [{RhClACHTNUTRGNE(UNG cod)}2] (cod=cy-
clooctadiene) and an additional
halide source in the appropriate
stoichiometry.
Characterisation of [NBu4][4]
by 13C{1H} NMR spectroscopy
in CD2Cl2 showed that the sin-
glet corresponding to C2 of 2
had disappeared and had been
Scheme 2. Transmetallation of 2 with [AuClACTHNUTRGNEU(GN tht)] in the presence of an external halide source.
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Chem. Eur. J. 2009, 15, 9981 – 9986

Immobilisation of N-Heterocyclic Carbene Complexes
COMMUNICATION
previously investigated in our
group.[4c,f,g] The anchoring of
anionic guests was typically
achieved by means of a biphasic
ion exchange protocol that
proved to be efficient in many
cases. Yet, this protocol re-
quires a two-phase aqueous sol-
vent system that might not be
Scheme 3. Transmetallation of 2 with [{RhClACHTNUGTRENUNG(cod)}2] and an external halide source.
applicable to systems that are
replaced by a sharp doublet at d=181.3 ppm with a coupling
constant JRh–C =51.5 Hz, indicating coordination of the car-
sensitive to hydrolysis.
1
The strategy for the synthesis of the sulfonate mono-car-
bene complexes 3À and 4À uses dry conditions, avoiding any
possible hydrolysis. This strategy can be seen as the sum of
two separate reactions occurring simultaneously: a transme-
tallation, which is the transfer of the carbene ligand from
silver to gold or rhodium, and an ion exchange in which the
Ag+ ion is replaced by an organic cation. This strategy has
proven to be versatile in allowing ionic exchange with differ-
ent organic chloride salts as the source of chloride ions.
Dendrimer [5]Cl8 has been previously used for non-cova-
lent immobilisation and is an octacationic dendrimer bear-
ing eight ammonium groups with halides as counterions
(Scheme 4). Therefore, this dendrimer could in principle
combine the functions of dendritic support, as well as of or-
ganic halide salt. By taking advantage of these features, we
bene ligand to Rh. The 1H NMR spectrum measured at
298 K showed splitting of the proton signals corresponding
to the ligand. This diastereotopicity of the protons of the
NHC ligand can be explained by a hindered rotation around
the metal–NHC bond, inducing differentiation of the pro-
tons.[8] [PPh4][4] showed spectroscopic characteristics very
similar to those of [NBu4][4], indicating that there is no in-
fluence of the counter cation on the formation of the
I
À
(NHC) Rh complex. It is worth mentioning that in the case
of [PPh4][3] no splitting of the aliphatic signals of the car-
bene ligand was observed, which is consistent with its higher
molecular symmetry as compared to [NBu4][4].
The immobilisation of transition-metal complexes on
polycationic dendrimers by means of ionic interactions was
Scheme 4. Transmetallation of 2 with Au and Rh and immobilisation of the resulting NHC complexes through the formation of metallodendritic assem-
blies [5][4]8 and [5][3]8.
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R. J. M. Klein Gebbink et al.
synthesised and immobilised sulfonate-tethered metal–car-
bene complexes in a one-pot procedure. Indeed, overnight
stirring of four equivalents of compound 2 in the presence
Integration of the peaks corresponding to the supported
metal complexes and those of the dendrimer indicated that
eight (NHC)–metal complexes are present within a single
dendritic assembly. The Rh complex [NBu4][4] shows a pro-
nounced feature in its UV/Vis spectrum at l=397 nm (e=
1.64 LmolÀ1 cmÀ1). Based on this feature, the stoichiometry
of the Rh complex and dendritic support in [5][4]8 is 8.2.
Metallodendritic assemblies [5][3]8 and [5][4]8 were further
characterized by electrospray mass spectrometry (ESI-MS)
operating in the positive-ion mode (see Figure 4). Cations
were generated by dissociation of anionic (NHC)–RhI com-
plexes from the assembly to give rise to multiply charged
ions. Molecular ion peaks at m/z 3313.7 and 2019.5 were at-
of eight equivalents of [AuClACTHNUTRGNEUGN(tht)] and one equivalent of
the octacationic dendrimer [5]Cl8 in dichloromethane result-
ed in the formation of the corresponding metallodendritic
assembly [5][3]8 in good yield (Scheme 4). The use of
[{RhClACHTUNGTRENNUNG(cod)}2] as metal source in this one-pot transmetalla-
tion–immobilisation protocol gave the corresponding den-
dritic assembly [5][4]8 in good yield. The metallodendritic
assemblies were purified by filtration over Celite, followed
by passive dialysis.
As described for [NBu4][4] and [PPh4][4], the 13C{1H}
2+
NMR spectrum of [5][4]8 showed a characteristic doublet
tributed to the cations [5][4]6 and [5][4]53+, respectively,
1
for C2 at d=181.3 ppm with a coupling constant of JRh–C
=
which have calculated m/z values of 3314.0 and 2020.0, re-
spectively. In the case of [5][3]8, molecular peaks at m/z
À
51.5 Hz consistent with the presence of a C Rh bond. Simi-
2+
3+
larly, the 13C{1H} NMR spectrum of [5][3]8 exhibits a signal
3272.3 and 1996.4 were assigned to [5][3]6 and [5][3]5 ,
À
at d=170.9 ppm coherent with the presence of a C Au
bond as observed with [PPh4][3]. The H NMR spectra of
respectively, which have calculated m/z values of 3271.7 and
1996.5, respectively.
1
[5][3]8 (see Figure 3 for an example) and [5][4]8 showed the
presence of the typical signals corresponding to both the
metal complexes and the dendrimer, though with significant
shifts and sharpening of peaks compared to [5]Cl8, which
highlights a strong interaction between dendritic host and
NHC guest.[9]
These combined analytic data unequivocally showed the
successful formation of metallodendritic assemblies [5][3]8
and [5][4]8 in a single-step procedure that involves both the
transmetallation of eight carbene ligands, the formation of
eight equivalents of AgCl and the assembly of eight anions
with an octacationic support.
Figure 3. 1H NMR spectra of [5][3]8 and [5]Cl8 measured in CD2Cl2 at 298 K. (For the labelling of the dendrimer signals, see Scheme 4).
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Immobilisation of N-Heterocyclic Carbene Complexes
COMMUNICATION
Figure 4. ESI-MS spectra of [5][4]8 and [5][3]8.
Initial tests on the catalytic performances (Table 1) of
compounds [NBu4][4] and [5][4]8 were carried out for the
catalytic hydrosilylation of ketones. Both catalysts were able
Acknowledgements
This work was financially supported by the Council for the Chemical Sci-
ences of The Netherlands Organization for Scientific Research (CW-
NWO) through a VIDI scholarship to R.J.M.K.G.
Table 1. Rhodium-catalysed hydrosilylation of cyclohexanone.
Keywords: carbene ligands · dendrimers · immobilization ·
noncovalent interactions · transmetallation
Catalyst
[NBu4][4]
[5][4]8
Solvent
Time [h]
Yield [%][a]
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DCM
THF
DCM
THF
3
20
3
72
63
43
34
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20
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to convert cyclohexanone into cyclohexanol but with a
lower yield in the case of the metallodendritic catalyst. The
activity of the Rh–carbene catalyst is influenced both by the
type of solvent in which the reaction is carried out, as well
as by the immobilisation on the dendrimer. Further catalytic
studies are currently being undertaken to evaluate the activ-
ity of such metallodendrimers as catalysts.
In summary, we have shown that sulfonate-functionalised
metal mono-carbene complexes can be very easily and effi-
ciently immobilised on a dendritic support by means of ionic
interactions in a one-pot transmetallation–immobilisation
procedure. Quite remarkably, this procedure involves both
transmetallation and ion exchange, which constitutes selec-
tive metal complex synthesis and metal complex immobilisa-
tion in one single synthetic operation. Currently, we are de-
veloping this synthetic immobilisation protocol for the de-
velopment of new metallodendritic assemblies and are in-
vestigating their catalytic properties.
Chem. Eur. J. 2009, 15, 9981 – 9986
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Received: June 22, 2009
Published online: September 8, 2009
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