Ultra-small sugar-substituted N-heterocyclic carbene-protected palladium nanoparticles and catalytic activity

Article abstract of DOI:10.1002/aoc.4942

Ultra-small Pd nanoparticles (UNPs) represent a distinctive type of nanomaterial making them very attractive for a range of applications. Herein, chiral sugar-substituted N-heterocyclic carbenes (NHCs) with various lengths of alkyl chain (sugar-NHCs-ⁿC_nH_2n+1) were first used to prepare water-soluble Pd@NHCs-sugar UNPs via an efficient ligand-exchange strategy, which can be handled under air and are stable over 10?months. The Pd@NHCs-sugar UNPs were highly monodisperse, with tunable core diameters from 1.7 to 2.1?nm, and an effect of the particle size on the N-substituted aliphatic chains was observed. To investigate the accessibility of the surface, the Pd@NHCs-sugar UNPs were studied as catalysts for C–C coupling reaction in eco-friendly ethanol aqueous solution without any precipitation of metallic Pd. The presence of the longest aliphatic group in the Pd@NHCs-sugar UNPs imparts to them the highest catalyst activity (turnover number and turnover frequency up to 196?000 and 3?920?000?h^?1).

Full text of DOI:10.1002/aoc.4942

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Received: 29 January 2019
DOI: 10.1002/aoc.4942
Revised: 19 March 2019
Accepted: 19 March 2019
F U L L P A P E R
Ultra‐small sugar‐substituted N‐heterocyclic carbene‐
protected palladium nanoparticles and catalytic activity
Zhonggao Zhou
| Mei Li | Guangsheng Liu | Guohai Xu | Jun Xue
College of Chemistry and Chemical
Engineering, Gannan Normal University,
Ganzhou 341000, China
Ultra‐small Pd nanoparticles (UNPs) represent
a distinctive type of
nanomaterial making them very attractive for a range of applications. Herein,
chiral sugar‐substituted N‐heterocyclic carbenes (NHCs) with various lengths
of alkyl chain (sugar‐NHCs‐nCnH2n+1) were first used to prepare water‐
soluble Pd@NHCs‐sugar UNPs via an efficient ligand‐exchange strategy, which
can be handled under air and are stable over 10 months. The Pd@NHCs‐sugar
UNPs were highly monodisperse, with tunable core diameters from 1.7 to
2.1 nm, and an effect of the particle size on the N‐substituted aliphatic chains
was observed. To investigate the accessibility of the surface, the Pd@NHCs‐
sugar UNPs were studied as catalysts for C–C coupling reaction in eco‐
friendly ethanol aqueous solution without any precipitation of metallic Pd.
The presence of the longest aliphatic group in the Pd@NHCs‐sugar UNPs
imparts to them the highest catalyst activity (turnover number and turnover
frequency up to 196 000 and 3 920 000 h−1).
Correspondence
Zhonggao Zhou and Jun Xue, College of
Chemistry and Chemical Engineering,
Gannan Normal University, Ganzhou
341000, China.
Email: zhgzhou@foxmail.com;
xuejun8657@foxmail.com
Funding information
Gannan Normal University; Key Labora-
tory of Jiangxi University for Functional
Materials Chemistry, Grant/Award Num-
ber: FMC16201; National Natural Science
Foundation of China, Grant/Award
Number: 21562002
KEYWORDS
ligand exchange, palladium nanoparticle, sugar‐substituted NHCs, ultra‐small, water‐soluble
1 | INTRODUCTION
known shapes and sizes.[17] Richter et al.[17] and Planellas
et al.[18] have synthesized Pd NPs stabilized by NHCs with
Metal nanoparticles (NPs) have been studied extensively
due to their numerous applications in green chemistry,
catalysis, biology and medicine.[1,2] Such metal NPs are
usually stabilized with ligands including phosphines,
amines, thiols and thioethers.[3–7] The neutral, electron‐
rich N‐heterocyclic carbenes (NHCs) are an emerging
class of protected anchor for metal NPs due to their
strong σ‐donation,[2,8–12] since Arduengo et al. reported
the isolation of free NHCs in 1991.[13] Most recently,
NHCs were used as alternative anchors for metal (includ-
ing Au, Pd, Pt, Ru, Ag, etc.) surface modifications and so
provided many notable properties, which might make
them surpass thiol‐based systems.[2,12] Just a few exam-
ples of NHC‐protected Pd NPs are reported in the litera-
ture,[9,14–20] and the exchange strategy is an outstanding
method because it employs preformed Pd NPs with
a size distribution around 4.0 nm. Glorius and co‐
workers[20] have reported the preparation of NHC‐
protected water‐stable Pd NPs with a size distribution of
4.0–4.9 nm. Jesús and co‐workers have synthesized a
series of water‐soluble NHC‐stabilized Pd NPs with differ-
ent shapes and sizes, ranging from 1.3 to 7.0 nm.[9,19] Pd‐
based nanocatalysts have attracted considerable attention
because of their superior catalytic activity in many types
of reactions. The smaller Pd NPs expose more low‐
coordination sites, and ultra‐small nanoparticles tend to
have unexpectedly distinctive properties.[21] The best
advantage of using NHCs is the practical modification
of groups in the heterocycle which can give rise to a tun-
able metal surface. Furthermore, specific properties of
metal NPs capped with chiral compounds have attracted
a great deal of attention.[15,22]
Appl Organometal Chem. 2019;e4942.
wileyonlinelibrary.com/journal/aoc
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https://doi.org/10.1002/aoc.4942

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ZHOU ET AL.
The biological roles of sugars as recognition markers
and signaling effectors are associated with specific molec-
ular recognition in which proteins, DNA, RNA or other
sugars are involved.[23,24] Their excellent catalytic perfor-
mance is one of the important highlights of sugar‐based
NHCs and their metal complexes.[23,25–28] For example,
a series of sugar‐based mono‐NHCs‐Pd complexes were
used for Suzuki reactions with low Pd loading of 0.05–
1.0 mol%.[29–34] We also reported several sugar‐based
NHC precursors for Suzuki reactions at a Pd loading of
0.1–0.05 mol%.[35–37] However, the catalytic activity of
sugar‐based Pd@NHCs NPs was not investigated. In a
continuation of our previous work based on sugar‐based
NHCs, we were interested in synthesizing the first exam-
ple of sugar‐based Pd@NHCs NPs, and the catalytic activ-
ity of these Pd NPs may be very attractive.
In the work reported herein, we designed four sugar‐
based chiral NHC precursors with various lengths of ali-
phatic chains directly attached to the N atom of imidazole
heterocycle. Pd@NHCs‐sugar ultra‐small NPs (UNPs) sta-
bilized by these sugar‐based chiral NHC ligands were
synthesized via an efficient ligand‐exchange procedure,
and the catalytic activity was investigated in C–C cou-
pling reactions in eco‐friendly ethanol aqueous solution
(turnover number (TON) up to 196 000 and turnover
frequency (TOF) up to 3 920 000 h−1) without any aggre-
gation of Pd NPs.
FIGURE 1 Synthesis of sugar‐substituted NHC precursors
atom in the sugar ring (Figure S1). The chemical shift
of 1‐position C atom in precursors 1‐Br–4‐Br corre-
sponds to 83.77, 83.75, 83.62 and 83.56 ppm, respec-
tively (Figure S1).
Displacement of weakly binding didodecylsulfide
(DDS) ligands from the surface of DDS@Pd NPs by
stronger binding NHCs is a universal procedure for
attaching functional NHCs to Pd@NHCs NPs.[20] Herein,
Pd NPs stabilized by DDS were obtained according to a
literature method and have a size distribution of 4.0
( 0.3) nm.[41] Then Pd@NHCs‐sugar UNPs (5–8) were
synthesized by aliphatic chains and sugar‐substituted
chiral NHCs 1‐Br–4‐Br with an efficient ligand‐exchange
method (Figure 2). The free sugar‐NHCs were generated
in situ by deprotonation of the corresponding sugar‐
based NHCs precursors 1‐Br–4‐Br with KOtBu in
dimethylformamide (DMF). After the addition of
DDS@Pd NPs, a phase transfer of the Pd NPs from non-
polar hexane into the more polar DMF phase could be
seen with the naked eye, indicating the successful
sugar‐NHC exchange on the surface of Pd NPs. The
unbound DDS ligand can be removed by simple extrac-
tion after the exchange process. The Pd@NHCs‐sugar
UNPs could be isolated as a brown solid after purifica-
tion by repeated washing, precipitation and dialysis
against water. To confirm the crucial role of the alkyl
chains, N‐substituted C2H5, C4H9, C8H17 and C16H33
NHCs were also investigated as stabilizer. We were able
to synthesize the first example of ultra‐small, water‐
soluble chiral Pd NPs using sugar‐based NHCs with the
ligand‐exchange method. It should be pointed out that
the Pd@NHCs‐sugar UNPs can be handled in air and
are stable over 10 months.
2 | SYNTHESIS
In order to obtain ultra‐small, water‐soluble Pd NPs, we
synthesized a library of sugar‐substituted NHC precur-
sors, 1‐Br–4‐Br, in a four‐step sequence from D‐glucose
(Figure 1).[38] The sugar‐substituted NHC precursors
1‐Br–4‐Br have never been applied in the synthesis of
metal complexes and metal NPs. The NHC precursors
usually start from N‐substituted imidazole with
halogenoarene or haloalkane.[30,33,39] Herein, sugar‐
substituted NHCs precursors 1‐Br–4‐Br could be
obtained in high yields (above 95%) with a convenient
one‐step method directly from the reaction of bromi-
nated alkanes, including bromoethane, bromobutane,
bromoheptane and hexadecyl bromide with 1‐(2,3,4,6‐
tetra‐O‐acetyl‐β‐D‐glucopyranosyl)imidazole following a
literature procedure.[36]
We find the chemical shift of NCHN proton in
imidazolium heterocycle at relatively low field (10.38,
10.83, 10.30 and 10.76 ppm) (Figure S1) after forming
NHC precursors 1‐Br–4‐Br.[40] The substituent effect of
the sugar‐substituted NHC precursors can be observed
from the 1H NMR and 13C NMR spectra (Figure S1),
especially from the 13C NMR signal of 1‐position C

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ZHOU ET AL.
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FIGURE 2 Synthesis of Pd@NHCs‐sugar UNPs with ligand‐exchange procedure
FIGURE 3 (a–d) HR‐TEM images and
(e) XRD patterns of Pd@NHCs‐sugar
UNPs

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ZHOU ET AL.
TABLE 1 Selected parameters of Pd@NHCs‐sugar UNPs
Water δNCHN
elemental analysis, C:H:N ratios were in agreement with
the theoretical calculated values for these Pd@NHCs‐
sugar UNPs, confirming that the Pd NPs are stabilized
by the sugar‐based NHCs exclusively. It is worth men-
tioning that the Pd‐to‐NHCs‐sugar ratio is in principle
sufficiently high to enable the coordination of all sugar‐
based NHC molecules, except for steric factors, since the
calculated number of Pd surface atoms exceeds the esti-
mated number of ligands (up to around 3 times for
Pd@NHCs‐sugar UNPs).
Pd
contact (ppm)a NHCs/Pd Pd
Size
Entry UNPs angle (°)
(%)
(nm)
1
2
3
4
5
6
7
8
10.4
10.8
11.1
13.2
10.38
10.83
10.30
10.76
3.02:1
3.16:1
3.42:1
3.70:1
60.32 2.1 0.4
59.89 2.0 0.3
59.00 1.8 0.3
58.52 1.7 0.4
aThe NCHN proton signal of four sugar‐based NHC precursors.
1
The H NMR spectra of the Pd@NHCs‐sugar UNPs
clearly showed that all four acetyl groups remained. The
lack of observation of the NCHN proton signal in the
1H NMR spectrum may be because of the formation of
Pd–NHC bond (Figure S6); moreover it can also be due
to the widening of the resonance due to the interaction
with the Pd NP surface.
Recent research results suggested that hydrophilic
Pd@NHCs NPs containing water‐soluble functional
groups show efficient catalytic activity in neat water.[19]
Incorporation of sugar groups into Pd@NHCs NPs not
only introduces chirality but also improves water solubil-
ity. Herein, as‐cast films of Pd@NHCs‐sugar UNPs 5–7
exhibit contact angles of 10.4°, 10.8° and 11.1°, respec-
tively (Figure 4a(a–c)). Pd@NHCs‐sugar UNPs 8 exhibits
a relatively large contact angle of 13.2° (Figure 4a(d),
which may be attributed to the longer aliphatic chain.
The water contact angles of these Pd@NHCs‐sugar UNPs
films are lower than 15°, indicating that all of them are
fairly hydrophilic.[42]
Water‐soluble Pd NPs can catalyze chemical reactions
in aqueous solution.[19] After the successful synthesis
and characterization of Pd@NHCs‐sugar UNPs, we were
interested in investigating in detail whether these
Pd@NHCs‐sugar UNPs systems are catalytically active
or whether the aliphatic chains shield the active sites.
With high hole mobility, fluorene, triphenylamine, carba-
zole and fluorine moieties have been identified as
3 | RESULTS AND DISCUSSION
Purification was performed by dialysis to remove some
impurities. The purified Pd@NHCs‐sugar UNPs were re‐
dispersed in ethanol and examined using high‐resolution
transmission electron microscopy (HR‐TEM). The
dynamic light scattering size distribution histograms
and HR‐TEM images are shown in Figure 3a–d (standard
deviations given in parentheses). According to the HR‐
TEM results, Pd@NHCs‐sugar UNPs consist of monodis-
perse and well‐distributed NPs with an ultra‐small size of
1.7 (0.4), 1.8 (0.3), 2.0 (0.3) and 2.1 (0.4) nm (Table 1),
respectively, indicating formation without aggregation
during the thioether ligand‐exchange process. The size
distribution of Pd@NHCs‐sugar UNPs 6–8 remained
essentially unchanged over a period of 10 months under
air. Some coalescence was observed over longer periods
for Pd@NHCs‐sugar UNPs 5 because of the shortest
ethyl chain. Powder X‐ray diffraction (XRD) analyses
(Figure 3e) of the Pd@NHCs‐sugar UNPs clearly indi-
cated that the Pd@NPs were highly crystalline.
Thermogravimetric analysis (TGA; Figure 4b) and ele-
mental analysis were used to determine the composition
of the Pd@NHCs‐sugar UNPs. The Pd content was
around 60%, which is consistent with a Pd‐to‐NHCs‐sugar
ligand ratio of 3.02:1 (60.32%) for 5, 3.16:1 (59.89%) for 6,
3.42:1 (59.00%) for 7 and 3.70:1 (58.52%) for 8. From the
FIGURE 4 (a) Water contact angles and (b) TGA curves of of Pd@NHCs‐sugar UNPs