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NMR study of sample A (HDA ligand)
sponds to a highly mobile molecule (D=9.7ꢂ10ꢁ10 m2 sꢁ1). This
can be explained by the presence of a transferred NOE (trNOE)
originating from HDA bound to the particle (and poorly
mobile) to free HDA in solution in the vicinity of the crystal.
This effect has been observed for other systems[10a,11b] and re-
flects a weak interaction mode between the ligands and the
nanoparticles in solution. On the same spectrum, negative
NOEs were also evidenced for the iPr-NH2 molecules thus indi-
cating a spatial proximity of this small ligand with the surface
of the nanoparticle. In addition, a supplementary correlation
(location d=2.88/1.35 ppm) indicates an intermolecular inter-
action between HDA and iPr-NH2 molecules, which reflects the
common proximity of these species and their swapping close
to the surface of the nanocrystals. This leads us to conclude
that the short iPr-NH2 moiety participates also to the stabili-
zation of the NPs. Conversely, the secondary amine, that is,
iPr-NH-Et only presents NOEs signals that plead for a small
molecule left free in solution and not playing any role in the
stabilization of the copper crystals. Finally, diffusion-filtered
1H NMR spectroscopy could detect a very broad signal cen-
tered at d=1.4 ppm (Figure 4b). This signal is characteristic for
the alkyl chain groups of the amine ligand bound to the sur-
face of the NPs, with very slow tumbling motion. Due to the
important width of this signal, it was not possible to measure
its diffusion coefficient.
Figure 3 presents the salient features visible on the 1H NMR
spectrum at room temperature of a colloidal solution of
copper NPs stabilized by 0.5 equivalents of HDA (sample A) in
[D8]toluene. The resonance of the a(CH2) protons of the HDA
moiety is characterized by a large multiplet centered at
1
Figure 3. Selected areas of the H NMR spectrum of a colloidal solution of
Cu NPs stabilized by 0.5 equivalents HDA in [D8]toluene obtained from the
hydrogenolysis of [Cu(Amd)] (A=HDA, B=iPr-NH2, and C=iPr-NH-Et).
1
After air exposure, the H NMR signals of the HDA and iPr-
NH2 species are larger indicating their even stronger coordina-
tion to the oxidized Cu NPs (Figure 5). We also observed trNOE
signals for these two species confirming the presence of a fast
exchange between the surface of the NPs and the solution
(Figure S3 in the Supporting Information). Nonetheless, the in-
termolecular correlation between HDA and iPr-NH2 has disap-
peared, indicating the loss of the spatial proximity between
both molecules. Diffusion-filtered 1H NMR experiments
(Figure 4c) also show the presence of long aliphatic chains
(i.e., HDA) associated to the surface of the crystal
(d=1.4 ppm). In this case, a better-defined signal allows to
measure a diffusion coefficient (D=1.0ꢂ10ꢁ10 m2 sꢁ1), which
corresponds to a hydrodynamic diameter close to 8 nm.
d=2.55 ppm, the CH3 end-group resonance is located at d=
0.96 ppm and the resonance of the alkene protons is located
at d=1.35 ppm. The signal of the a(CH2) protons of HDA is
broad and this is usually correlated with an interaction of the
ligand with the surface of the nanoparticle.[11b,10d] HDA is partic-
ipating to the stabilization of the copper nanocrystals through
the coordination of the amine function (nitrogen electron dou-
blet) on the metal atoms. This ligand coordination is confirmed
by a broader 13C NMR resonance of the same a(CH2) group of
HDA at d=42.5 ppm (see Figure S1 in the Supporting Informa-
tion). Interestingly, two unexpected species appear in solution.
One is identified as an isopropyl-amine noted as iPr-NH2 (CH3
doublet at d=0.93 ppm, CH of iPr heptuplet at d=2.88 ppm).
The second one is an isopropyl-ethyl-amine noted as iPr-NH-Et
(CH3 of iPr doublet at d=0.93 ppm, CH3 of ethyl triplet at d=
1.02 ppm, CH2 of ethyl doublet of triplet at d=2.52 ppm, CH
of iPr doublet of heptuplet at d=2.67 ppm). These species
must be originating from the cleavage of the amidine moiety
occurring during the hydrogenation process.
In the 13C NMR spectrum, the signals of the CH groups from
the isopropyl moieties of iPr-NH2 and iPr-NH-Et species (ap-
pearing at d=42.5 and d 48.6 ppm, respectively) are narrow.
This suggests, contrarily to HDA, that these species may pres-
ent either a weak or even no interaction with the surface of
the NPs. In order to clarify the role of the ligands, a 2D-NOESY
experiment was performed with a short mixing time of
100 ms. Negative NOEs were observed for the HDA resonances
(Figure S2 in the Supporting Information). However, the diffu-
sion coefficient of HDA, measured by DOSY experiments, corre-
In conclusion, during the growth of copper oxide on the
surface of the crystal, the same ligands species (i.e., iPr-NH2
and HDA) participate to the stabilization of the NPs.
NMR study of sample C (TDPA ligand)
The 1H NMR spectrum of a colloidal solution of copper NPs
stabilized by 0.5 equivalents TDPA in [D8]toluene, shows a set
of resonance signals located at d=4.94, 3.42 (NCH), 3.28 (NH),
and 2.23 ppm (central CH3) characteristic of the amidine
moiety (Figure 6). The resonance peaks between d=0.8 and
2.1 ppm are attributed to end-methyl and CH2 groups of the
aliphatic chain of the TDPA ligand. The chemical shifts for
these molecules are different from the ones of free molecules
in toluene both for amidine and TDPA (d=4.20, 3.47 (NCH),
2.90 (NH), and 1.39 ppm (central CH3), Figure S4 in the Sup-
porting Information). This discrepancy may be due to a partial
protonation of the amidine moiety (iPr-NH-C-NH+-iPr or iPr-N-
Chem. Eur. J. 2014, 20, 1 – 11
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