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Dalton Transactions
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COMMUNICATION
Journal Name
DOI: 10.1039/C7DT01856H
Figure 1. TEM images of gold nanoparticles and size distribution (inlet): a) AuNp-
NHT TC6,C6, b) AuNP-NHTC14,C14, c) AuNP-NHTC18,C18
Scheme 2. Synthesis of gold complexes 3.
.
nanoparticles using alkyl- or aryl-substituted N-Heterocyclic
nanoparticles are very stable both in solution and in the solid
state, showing no degradation signs after exposing them to air
for prolonged periods of time, according to TEM images of the
gold nanoparticles stored in solution after ca. two months (see
ESI). Therefore, the interaction of the NHTs with the metal
atoms seems to be sufficiently strong to prevent
decomposition, the stability being comparable to that of gold
nanoparticles stabilized by neutral ligands such as N-
heterocyclic carbene and considerably superior to those
reported with thioethers.[10]
The UV-vis spectra are consistent with the mean size
observed by TEM. A strong absorption band at 526 nm,
characteristic of a plasmon band for nanoparticles bigger than
2 nm, have been recorded.[1h] On the other hand the good
solubility of the nanoparticles allowed to obtaining the 1H
NMR in which very broad signals spanning from 0 to 3 ppm are
observed in all cases (see ESI), consistent with the existence of
an interaction between the nanoparticles and the ligands, and
excluding the presence of molecular gold complexes, for which
well defined, narrow NMR signals are usually present. As
previously observed for metal nanoparticles stabilised by N-
heterocyclic carbene ligands,[4b,c,f] the olefinic protons at the
back-bone of the imidazole fragment are not discernible in the
1H NMR. However, in contrast to N-heterocyclic carbene
ligands, coordination of N-heterocyclic thiones to a metal
centre likely induces a bending of the imidazole fragment
towards the metal surface,[8] and therefore the relaxation
effect due to the proximity of the olefinic CH protons is likely
increased (Figure 2).
i
thiones (R = Me, Pr; Ar = Mes, 2,6-iPrC6H3, Scheme 2) using as
reducing agent KBEt3H, 9-BBN or NaBH4 led to complete
degradation of the starting material. However, TEM images of
the as-synthesised particles indicated the presence of gold
nanoparticles but, unfortunately, most of the metal present
was in the form of agglomerates with independence of the
reducing agent. Since this might be due to the lack of sufficient
ligand stabiliser present in solution, reactions in the presence
of variable amounts of the N-heterocyclic thiones (0.2 to 1
equiv with respect to the gold complex) were carried out.
Nevertheless, the impact of the excess of the ligand had nearly
no effect in preventing agglomeration. Although encouraging,
these results indicated that further stabilisation is required to
avoid formation of agglomerates. One way to prevent
precipitation of bulk-gold involves the use of ligands with long-
chain hydrocarbons to provide sufficient steric repulsions
between particles.[3c] Therefore, we carried out the reduction
of gold complexes 3, bearing alkyl-chains of different lengths
(NHTC6,C6, NHTC14,C14, NHTC18,C18), with KHBEt3 in THF at rt. A
fast reaction takes place leading to ruby-red solutions
characteristic of colloidal gold. No evidence for the
precipitation of black gold was observed. TEM analysis of the
as-prepared NPs indicated that well-disperse, spherical,
crystalline nanoparticles have been formed, with no
agglomerates present (Figure 1). The mean size of these Au-
NPs range from 2.9(0.7) to 3.5(1.0) nm (Figure 1), with a rather
narrow size dispersity. With the aim to determining the effect
of a single long-chain arm on the NHT, gold nanoparticles
bearing the unsymmetrical NHT (NHTC1,C14, NHTC1,C18) were
synthesised following an identical protocol. As can be seen in
Fig. S31 and S35 the TEM images also show the presence of
Au-NPs with a mean size varying from 3.7(0.7) to 3.8(0.8) nm
(see ESI).
The hydrodynamic diameters of the NPs were determined
by Dynamic Light Scattering (DLS). The values obtained (see
ESI) indicate a good relationship between the length of the
alkyl chain on the NHT and the diameter. For example, the
diameter was found to be 4.4(0.5), 8.5(0.3) and 8.6(0.4) nm for
the gold NPs NHTC6,C6, NHTC14,C14, NHTC18,C18, respectively.[11] As
mentioned before, the size distribution and morphology of the
nanoparticles do not change when exposed to air in solution
(see Figures S43 and S44). Similarly, DLS measurements of
solutions of the nanoparticles exposed to air for two weeks
clearly indicate that no agglomeration takes place, preserving
All these NPs are soluble in aromatic solvents, yet insoluble
in protic solvents such as ethanol. This allowed their
purification by simple washing with ethanol providing a way to
remove the N-heterocyclic thiones present in solution as well
as inorganic salts. The composition of the purified
nanoparticles obtained by Inductive Coupled Plasma (ICP)
indicates a metal content of 55-90 %. Additionally, these
2 | J. Name., 2012, 00, 1-3
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