Angewandte
Communications
Chemie
Modified Nanoparticles
Modular Bidentate Hybrid NHC-Thioether Ligands for the
Stabilization of Palladium Nanoparticles in Various Solvents
Andreas Rühling+, Kira Schaepe+, Lena Rakers, Benjamin Vonhçren, Patricia Tegeder,
Dedicated to Professor Hans-Joachim Galla
Abstract: The synthesis of four different bidentate hybrid
NHC-thioether ligands is presented. The corresponding palla-
dium nanoparticles are stable in various solvents, depending on
the ligand used, and show high chemoselectivity in the
hydrogenation of olefins. The solubility of the nanoparticles
can be switched multiple times depending on the pH value of
the solvent. XPS analysis (which shows a subtle shift in the
binding energy) was identified as a convenient tool to establish
the binding mode of NHC ligands.
bidentate ligand would allow us to exploit the properties of
both ligand classes in a synergistic fashion, thereby resulting
in enhanced stability of the NPs.[9] Such a combinatorial
approach simplifies the synthesis of the ligand, and in
combination with the great variety of thioether motifs, one
can quickly adapt the polarity and therefore solubility of the
NPs on demand. In terms of stability, the NHC as a stronger
donating ligand would bind to the NP through a covalent
bond, whereas the thioether would bind through a weaker
dative interaction (Scheme 1).
D
uring the last decade, the importance of nanoparticles
(NPs) has grown significantly, especially in the fields of
organometallic drugs, materials, and catalysis.[1] However,
a common challenge in these research fields is stabilization of
the NPs, since agglomeration would render them “inactive”
for their designed purpose. NPs are normally stabilized with
ligands such as thiols, phosphines, amines, and thioethers.[2]
However, N-heterocyclic carbenes (NHCs),[3] which are
superb ligands in organometallic chemistry, are an emerging
class of ligands for NP stabilization.[4–6] Based on the few
existing reports on NHC-stabilized NPs, it can be argued that
the long-term stability of these systems is not on a par with
those stabilized by thiols. In 2014, we reported a tailor-made
NHC, bearing two long alkyl chains in the backbone, which
stabilized palladium NPs (PdNPs) for four months, presum-
ably through steric repulsion of the alkyl chains.[7] The
drawback of this system is the difficulty of adapting this
design to different polarities (e.g., polyethylene glycol as
a polar and water-soluble group), which is needed for
solubility and stability in polar media.[8] Consequently,
a broader field of applications could be addressed with
a more flexible and modular design in which an alternative
stabilizing motif is implemented. We envisioned that the
combination of an NHC ligand with a thioether as a hybrid
Scheme 1. Hybrid ligand concept: retrosynthetic disconnections and
elements of the modular design. PEG3=triethyleneglycol.
To test our concept of stabilizing NPs with a hybrid NHC-
thioether bidentate ligand, we synthesized four ligand pre-
cursors with different chains attached to the sulfur atom.
Since these chains should influence the solubility of the
resulting NPs the most, we decided to vary this chain from
nonpolar (1a) to polar (1c). The “PEGylated” (1b) and
longer-chain carboxylic acid (1d) should have intermediate
polarity. The connection between the NHC core and the
sulfur atom was set to be a propyl chain. As shown in our
related work, small substituents on the nitrogen are beneficial
for the stability of the NPs and hence we chose a methyl group
in this position.[7] It should be noted that in principle any other
group can be placed there. All ligands could be synthesized in
moderate to high yields within a convenient two-step syn-
thesis. The corresponding PdNPs were synthesized through
ligand exchange, using a solution of the in situ generated free
NHCs and thioether stabilized PdNPs (Scheme 2).[4f, 7] For 1a,
we used tetraethyleneglycol thioether stabilized PdNP with
a size of 4.5 (Æ 0.4) nm. In the case of 1b–d we used
[*] A. Rühling,[+] K. Schaepe,[+] L. Rakers, B. Vonhçren, P. Tegeder,
Prof. Dr. B. J. Ravoo, Prof. Dr. F. Glorius
Westfälische Wilhelms-Universität Münster
Organisch-Chemisches Institut
Corrensstraße 40, 48419 Münster (Germany)
E-mail: b.j.ravoo@uni-muenster.de
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
5856
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 5856 –5860