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a slightly modified procedure relative to that previously
reported by Chechik and co-workers for metal NPs.[4b] After
precipitation with acetonitrile, centrifugation, and subsequent
washing with acetonitrile, pure C11-IMe@UCNPs were
obtained and could be redispersed in toluene or chloroform.
It is important to note that other NHC ligands lacking the
long alkyl chains for steric repulsion (IMe) or with increased
steric bulk at the nitrogen atom (C11-IPr) resulted in
irreversible precipitation or precluded ligand exchange. This
finding is in agreement with our previous report, thus
underlining the key features of the design.[7] 1H and
13C NMR spectroscopy was employed (Figure 2) to assess if
(136.9 ppm).[17] The width of the 13C carbene signal was the
same as observed in previous studies.[7,9,11]
PFG-NMR studies were conducted to verify the binding
and the proximity between the NHC and UCNP surface. It
was found that the unbound NHC salt—an air- and moisture-
stable NHC model—at 5.6 ꢀ 10À10 m2 sÀ1 and free OA at 6 ꢀ
10À10 m2 sÀ1 have a diffusion coefficient that is two magnitudes
higher than their NP-bound versions. The NP-bound OA had
a diffusion coefficient of 4 ꢀ 10À12 m2 sÀ1 and the NP-bound
NHC 5.2 ꢀ 10À12 m2 sÀ1. This observation strongly supports the
binding of the NHC to the UCNPs, as binding to the UCNP
slows down the diffusion (see the Supporting Information).
Further control experiments were performed to distin-
guish between carbene binding and an electrostatic interac-
tion. The addition of C11-IMe-HI directly to the dispersion of
OA@UCNPs in toluene led to a ligand exchange occurring,
but the NMR spectra showed a signal for the acidic hydrogen
atom of the NHC salt, that is, different from the spectrum of
C11-IMe@UCNPs. In this case the UCNP stabilization likely
occurs by an electrostatic interaction of the NHC salt with the
NP surface. The use of C11-IMe-MeI, which contains a methyl
group instead of an acidic hydrogen atom in the C2-position,
led to an exchange but, in contrast to C11-IMe@UCNPs, only
traces of poorly redispersable NPs were obtained. Presum-
ably the electrostatic interaction between C11-IMe-MeI and
the UCNP is very weak. These results were confirmed when
the cationic surfactant CTAB was employed instead of the
NHC salt, which resulted in only the free surfactant being
isolated and strongly supports the C11-IMe-carbene binding.
Additional TGA measurements with OA@UCNPs and
C11-IMe@UCNPs revealed that the NHC started to dissociate
from 2508C, whereas OA@UCNPs dissociated at temper-
atures as low as 1008C (see the Supporting Information).
These results are consistent with a covalent bond between the
NHC and the UCNP, not with an electrostatic interaction.
The organic content was found to be 29% for OA@UCNPS
and 36% for C11-IMe@UCNPs, which is in good agreement to
reported NHC-stabilized Pd NPs.[7] Based on these results,
and assuming the UCNP to have the same density as NaYF4,
the number of C11-IMe ligands on one UCNP was calculated
to be between 1339 and 1476.[18] For OA, the number of
ligands was calculated to be between 1398 and 1536 (see the
Supporting Information), which leads to the conclusion that
roughly one OA is replaced by one C11-IMe.
Figure 2. 1H (top) and 13C NMR spectra (bottom). Comparison of the
ligand precursor C11-IMe-HI, OA@UCNPs, and C11-IMe@UCNPs. In
the bottom spectra, 13C-enriched C11-IMe-HI was used.
the ligand exchange completely replaces the precursor ligand
and whether the NHC indeed binds through the carbene
carbon atom (and not through electrostatic interactions of the
reprotonated NHC). Indeed, complete ligand exchange was
observed after stirring the reaction mixture overnight.
Neither the olefinic protons from oleic acid nor the acidic
imidazolium proton could be detected and a new broad signal
appeared around 4 ppm, which corresponds to the methyl
group on the NHC nitrogen atom.
In addition, the two undecyl chains in the backbone give
rise to a broad signal between 0.5 and 1.5 ppm. All the signals
are significantly broadened due to the proximity to the NP
surface and the, therefore, limited degree of rotational and
translational freedom. The 13C NMR spectra of the same C11-
IMe@UCNPs stabilized by 13C-labeled NHCs revealed a car-
bene signal around 170 ppm, which is about 20–25 ppm lower
compared to known NHC-yttrium complexes (190.3 and
194 ppm) and about 33 ppm higher than the NHC salt
XPS analysis was conducted to further probe the binding
of the NHC to the UCNP. XPS is an effective means to
examine the binding mode of the NHC without the need for
isotope labeling; it instead uses the chemical shift of the N(1s)
signal.[11] Besides the observed N(1s) signal of C11-
IMe@UCNPs, a subtle shift towards higher binding energies
compared to the NHC salt is observed (see the Supporting
Information). The NHC salt showed a binding energy of
401.2 eV and that of the NHC to the UCNP 401.7 eV, with
a standard deviation of 0.1 eV. The higher binding energy can
be explained by the coordination of the NHC to the strong
Lewis acid YIII, which withdraws a substantial amount of
electron density from the NHC and, therefore, from the
nitrogen atom. Consequently, relative to the NHC salt, more
energy is required to remove an electron from the N(1s)
2
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Angew. Chem. Int. Ed. 2017, 56, 1 – 6
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