Gold nanoparticles on wool in a comparative study with molecular gold catalysts

Article abstract of DOI:10.1007/s13404-012-0076-3

The catalytic activity of gold chloride nanoparticles is compared to the activity of two molecular gold(I) chloride phosphine complexes for the addition of methanol to 3-hexyne. The phosphines are triphenylphosphine and the bispidinone related bulky 6,8-bis-(4-dimethylamino-phenyl)-3-methyl-9-oxo-7- phenyl-3-aza-7-phospha-bicyclo[3.3.1]nonan-1,5-dicarboxylic acid dimethyl ester. Use of the bulky ligand made the addition reaction selective towards the enol product, meaning that no addition of methanol or water to alkenes, which were produced during the reaction, occurred. In contrast, use of triphenylphosphine gold(I) chloride resulted in the synthesis of a variety of products. The phosphines decomposed during reaction leading to the formation of gold nanoparticles, which were found to be catalytically inactive. Artificially produced gold nanoparticles also proved to be inactive. In contrast, gold chloride nanoparticles deposited on wool were active comparable to the gold phosphine-containing catalysts tested previously. Overall activities observed were low compared to results from the literature suggesting that the operating conditions chosen could be optimised.

Full text of DOI:10.1007/s13404-012-0076-3

Gold Bull (2013) 46:13–18
DOI 10.1007/s13404-012-0076-3
ORIGINAL PAPER
Gold nanoparticles on wool in a comparative study
with molecular gold catalysts
Thomas Borrmann & Teck Hock Lim & Hannah Cope &
Kerstin Lucas & Michael Lorden
Published online: 4 December 2012
#
The Author(s) 2012. This article is published with open access at SpringerLink.com
Abstract The catalytic activity of gold chloride nano-
particles is compared to the activity of two molecular
gold(I) chloride phosphine complexes for the addition of
methanol to 3-hexyne. The phosphines are triphenyl-
phosphine and the bispidinone related bulky 6,8-bis-(4-
dimethylamino-phenyl)-3-methyl-9-oxo-7-phenyl-3-aza-
7-phospha-bicyclo[3.3.1]nonan-1,5-dicarboxylic acid di-
methyl ester. Use of the bulky ligand made the addition
reaction selective towards the enol product, meaning
that no addition of methanol or water to alkenes, which
were produced during the reaction, occurred. In contrast,
use of triphenylphosphine gold(I) chloride resulted in
the synthesis of a variety of products. The phosphines
decomposed during reaction leading to the formation of
gold nanoparticles, which were found to be catalytically
inactive. Artificially produced gold nanoparticles also
proved to be inactive. In contrast, gold chloride nano-
particles deposited on wool were active comparable to
the gold phosphine-containing catalysts tested previous-
ly. Overall activities observed were low compared to
results from the literature suggesting that the operating
conditions chosen could be optimised.
Results and discussion
Gold has been shown to be catalytically active [1–4].
Several review papers provide an excellent overview of the
field, types of catalysts used and ligand effects [1–5].
In a preliminary study, we investigated the addition of
methanol to 3-hexyne using triphenylphosphine gold(I)
chloride, [(H6C5)3PAu]Cl. Triphenylphosphine was syn-
thesised by reacting dimethylsulfide gold(I) chloride with
triphenylphosphine (Scheme 1).
While the gold compound was catalytically active, it
decomposed during the reaction (Scheme 2) resulting in
the formation of gold nanoparticles. Such decomposition
was reported in the literature [1, 2]. One solution reported
is the addition of electron withdrawing ligands. As this route
has already been explored, it was not investigated.
Furthermore, the reaction was not selective resulting in a
mixture of various products, the main two being 3-methoxy
3-hexyne and 3,4-dimethoxy hexane. Consequently, other
catalysts were studied.
In attempt to increase selectivity of the catalytic reaction
and to increase the stability of the catalyst, a more stable and
more bulky phosphine was sought. Due to some positive
experiences in prior research [6] the ligand 6,8-bis-(4-dime-
thylamino-phenyl)-3-methyl-9-oxo-7-phenyl-3-aza-7-phos-
pha-bicyclo[3.3.1]nonan-1,5-dicarboxylic acid dimethyl
ester, C34H40N3O5P (Scheme 3) was chosen.
.
.
Keywords Gold-catalysed addition Gold nanoparticles
.
.
Gold chloride nanoparticles Gold phosphine Homogeneous
.
catalysis Heterogeneous catalysis
The reaction of the ligand C34H40N3O5P with dimethyl-
sulfide gold(I) chloride resulted in the formation of clear
yellow crystals (Scheme 4). The crystal structure and details
of the synthesis will be reported shortly in a separate article.
[C34H40N3O5PAu]Cl was tested in the catalytic addi-
tion of methanol to 3-hexyne. Results were compared
the ones achieved using [(H6C5)3PAu]Cl as catalyst. The
activity of [C34H40N3O5PAu]Cl was comparable to that
of [(H6C5)3PAu]Cl indicating that the bulk of the ligand
did not impede the reaction and hence that limiting step
in the proposed addition reaction mechanism is the
:
:
:
:
T. Borrmann T. H. Lim H. Cope K. Lucas M. Lorden
School of Chemical and Physical Sciences, Victoria University
of Wellington, PO Box 600, 6140 Wellington, New Zealand
Present Address:
T. Borrmann (*)
Othbergstr.10,
37632 Eschershausen, Germany
e-mail: aoc@gmx.li

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Gold Bull (2013) 46:13–18
AuCl3 + HCl
H[AuCl4]
[(H3C)2SAuCl3] + HCl
[(H3C)2SAu]Cl + (H3C)2SO
[(H5C6)3PAu]Cl + S(CH3)2
H[AuCl4] + S(CH3)2
O
N
[(H3C)2SAuCl3] + S(CH3)2 + H2O
[(H3C)2SAu]Cl + (H5C6)3P
O
P
O
Scheme 1 Synthesis of triphenylphosphine gold(I) chloride
O
dissociation of the product from the catalyst (step IV in
Scheme 5).
However, in terms of selectivity the use of [C34H40N3O5-
PAu]Cl resulted in only 3-methoxy 3-hexyne being pro-
duced. No other products were detected after completion
of the reaction. This indicates that the bulky ligand had a
directing effect and is in agreement with the assumption that
the dissociation of the product from the catalyst is the
determining step in terms of the reaction kinetics
It was hoped that introducing a bulky ligand would
improve the stability of the catalyst. This was found not to
be the case. As observed for [(H6C5)3PAu]Cl, the catalyst
decomposed during the reaction and the formation of gold
nanoparticles was observed. In return, the phosphines were
oxidised as indicated by 31P-NMR. The catalyst could be
used for three conversion reactions before it became com-
pletely inactive. The reason for the instability of even the
bulky gold compound [C34H40N3O5PAu]Cl was considered
to be due to its structure (Scheme 6, Fig. 1) adopting a linear
structure for ligand–gold–chloride, with the PAuCl angle
being 180° and no bonding to nitrogen being observed,
comparable to the structure found in other phoshines such
as [(H6C5)3PAu]Cl [7]. The linear structure in regards to the
environment around the gold centre of [C34H40N3O5PAu]Cl
was confirmed by single crystal X-ray analysis
A colorimetric and electron microscopic investigation
showed that the gold nanoparticles formed increased in size
during the reaction and over several uses of the catalyst.
This raised the possibility that the catalytic activity observed
was due to the presence of the nanoparticles or some inter-
mediate form. However, gold nanoparticles of various sizes
produced in-house [8] showed no measurable catalytic ac-
tivity. Gold nanoparticles were used suspended in solution
and supported on wool (Fig. 2). As reported in the literature
[1–4], gold(I) or gold(III) needs to be present for any cata-
lytic activity to occur.
N
N
Scheme 3 The ligand 6,8-bis-(4-dimethylamino-phenyl)-3-methyl-9-
oxo-7-phenyl–3-aza-7-phospha-bicyclo[3.3.1]nonan-1,5-dicarboxylic
acid dimethyl ester, C34H40N3O5P
A sample of 50-nm gold chloride nanoparticles on wool is
shown in Fig. 2.
The sample of gold chloride nanoparticles was also tested
in the addition of methanol to 3-hexyne. The nanoparticles
were found to be a lot less active per gold ion present
(Table 1) than the molecular catalysts tested before. They
were as selective as [C34H40N3O5PAu]Cl. In comparison to
[C34H40N3O5PAu]Cl and [(H6C5)3PAu]Cl, the gold chlo-
ride nanoparticles did not show any signs of decompos-
ing or changing during reaction. Even after eight cycles,
no change in activity was observed. A comparison of
the catalytic activities of the three catalysts tested is
shown in Fig. 3 (see also Table 1). First-order kinetics
are observed, which supported the proposed mechanism
shown in Scheme 5. As promising as the performance
of the gold chloride nanoparticles was, the overall activity of
this catalyst was poor compared to the phosphines investigated
earlier.
One fact has to be considered though: Only a few gold
centres in the gold nanoparticles were catalytically active.
To truly compare the performance of the gold nanoparticles
with the molecular gold catalysts, a few assumptions and
contemplations have to be made.
The amount of gold added in the form of molecules in the
other experiments involving [C34H40N3O5PAu]Cl and
[(H6C5)3PAu]Cl was calculated. Then it was attempted to
match this amount of active gold by calculating the surface
In another project [9], it was found that gold chloride
nanoparticles could be stabilised by depositing them on
wool. The gold oxidation state in these nanoparticles is +1.
Scheme 2 Catalytic addition of
methanol to 3-hexyne at 50 °C
in excess methanol using the
O
[(H5C6)3PAu]Cl
catalyst
O
catalyst [(H6C5)3PAu] Cl
+
2
O
methanol, 50 oC

Gold Bull (2013) 46:13–18
15
[(H3C)2SAu]Cl + C34H40O5N3P
[C34H40O5N3PAu]Cl + S(CH3)2
Cl-
O
N
Scheme 4 Synthesis of 6,8-bis-(4-dimethylamino-phenyl)-3-methyl-
9-oxo-7-phenyl–3-aza-7-phospha-bicyclo[3.3.1]nonan-1,5-dicarboxyl-
ic acid dimethyl ester gold(I) chloride [C34H40N3O5PAu]Cl
Au+
O
P
O
O
area of nanoparticles. 5.0 μmol of gold were used in the
cases of [C34H40N3O5PAu]Cl and [(H6C5)3PAu]Cl. The
particle size of the nanoparticles was 10 nm in diameter.
Particles were assumed to be spherical, which is only a
rough approximation as different shapes were observed by
electron microscopy. A 10 nm particle was calculated to
contain about 12,000 gold ions, of which about 1,500 were
located on its surfaces. A packing factor of 0.68 was con-
sidered based on the body-centred tetragonal crystal struc-
ture of AuCl. It was assumed that about half of the surface
gold will not be accessible due to facing the wool surface or
other obstructions. This meant that for each active gold,
there were assumed to be 15 inactive ones. One gram of
wool contained 0.016 g of gold equivalent to 81 μmol of
which 5.0 μmol were assumed to be active. This meant that
1 g of wool covered in gold nanoparticles should have been
required to match the activity of the molecular gold
catalysts used. Experiments showed that only 0.1 g of
gold(I) covered wool were required. This means that the
turn over frequency per catalytic centre of the nano-
particles is about 290 mol of product per mole active
centre per hour, which is actually quite comparable to
the results achieved for the phosphines.
N
N
Scheme 6 Structure of [C34H40N3O5PAu]Cl
and could have limited the activity observed significantly.
Smaller gold chloride nanoparticles (about 5 nm in
diameter) on wool should be able to achieve higher
turnover frequencies. Furthermore, Haruta found that
electron-rich supports in the form of oxide particles
are required to achieve good results in catalytic reac-
tions. It is likely that the wool acted as electron rich
substrate in this case, a fact that should be investigated
in future projects.
Overall, all catalytic conversions studied were not as
efficient as some of the ones reported in the literature [3,
4]. It is likely that the reaction conditions chosen were to
Findings by Haruta [10] suggest that the size of the
nanoparticles investigated in this study was not optimal
Scheme 5 Proposed 4-step
catalytic cycle for the addition
of methanol to 3-hexyne
employing a gold catalyst based
on transition states and inter-
mediates suggested by Asao et
al. [2, 3]
Step I
[Au]
[Au]
H
O
Step IV
H3COH
Step II
[Au]
[Au]
H+
O
Step III
O
H+
H

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Gold Bull (2013) 46:13–18
Fig. 1 Ortep diagram showing
the structure of
[C34H40N3O5PAu]Cl
some degree the limiting factor in the activity of the cata-
lysts. Further studies are required to optimise reaction
conditions.
solution. Two equal portions of S(CH3)2 (2×142 μL,
3.85 mmol) were added. A bright yellow precipitate was
observed immediately after the first portion of S(CH3)2
was added. The second portion was added 3 min after
the first portion and the resulting mixture was allowed
to stir for 15 min. Snowy white crystals precipitated,
which were recovered by vacuum filtration and washed
with ethanol (1.1 ml) twice. Recrystallization from ab-
solute ethanol gave a white solid 0.25 g (50 %). 1H
NMR (acetone–D6): δ (2.86, s). MS: m/z0553 (dimer
([Au(S(CH3)]Cl)2, −37Cl), m/z0555 (dimer ([Au(S(CH3)]
Cl)2, −35Cl).
Experimental
All reagents unless specifically noted were purchased
from Sigma Aldrich. All experiments unless otherwise
noted were carried out in purified solvents open to air
at ambient temperature. All spectra were acquired at
room temperature. Instruments: NMR, Varian UNITY
Inova Plus spectrometer operating at 300Hz; MS, PE
Biosystem Mariner 5158 TOF in positive ion mode; IR,
Bio-RAD FTS-7 (resolution of 4 cm−1). GC: HP 540,
Column DB-WAX. The Analytical Lab of Otago Uni-
versity carried out elemental analyses.
Synthesis of triphenylphosphine gold chloride
Triphenylphosphine (6.6 mg, 25 mmol) was dissolved in a
degassed solution of [Au(S(CH3)]Cl (7.4 mg, 25 mmol) in
methanol (10 ml). The resulting mixture was heated and
stirred under nitrogen and reflux for 1 h. Then it was
allowed to cool to room temperature. Yellow–white
needle-shaped crystals precipitated, which were recovered
by vacuum filtration and washed with cold methanol. 31P
Synthesis of [Au(S(CH3)]Cl [11]
HCl (150 μL, 1.75 mmol) was added to solid AuCl3 (0.49 g,
1.65 mmol) concentration followed by addition of water
(2.3 ml). Ethanol (11.5 ml) was added to the resulting
Fig. 2 Gold nanoparticles on
wool of various sizes; the
particle size decreases from the
left to the right for the first six
samples. The seventh sample,
on the far right is comprised of
gold chloride nanoparticles
deposited on wool

Gold Bull (2013) 46:13–18
17
Table 1 Overview of catalytic tests; conversion (percent) after 1 h
Catalyst
Cycle
TOF
1
2
3
4
5
6
7
8
[(H6C5)3PAu]Cl
16.2a
16.3a
16.9
1.51a
15.1a
16.1
0.00
286b
287b
18b
a
[C34H40N3O5PAu]Cl
4.3
0.00
16.2
Gold chloride nanoparticles on wool
17.0
17.1
16.5
16.7
16.7
a Formation of gold nanoparticles was noticed.
b TOF (based only on the first cycle) in mol product per mole of gold per hour. For the nanoparticles, this calculation is only partially valid (see
below).
NMR (CDCl3): δ, −5.05 (s)ppm. Elemental analysis in
percent—calculated (measured): Au, 39.81 (39.80); C,
43.70 (43.70); H, 3.06 (3.06); P, 6.26 (6.27); Cl, 7.15 (7.17).
by analysis of the residual solution (determination of the
gold uptake by atomic absorption).
Catalytic investigations
Preparation of [C34H40N3O5PAu]Cl
In a typical experiment, a catalyst was dissolved or sus-
pended in methanol (10.0 mL, 0.791 gcm−3, 0.247 mol) in
a two-neck round-bottom flask fitted with a condenser. n-
Decane (0.1 mL, 0.730 gcm−3, 0.51 mmol) was added as
internal standard. The resulting solution or mixture was
heated to 50 °C. Then 3-hexyne (1.00 mL, 0.723 gcm−3,
8.80 mmol) was added. Immediately, a sample (1 μL) was
taken and analysed by gas chromatography. In 10-min inter-
vals, further samples were taken and also analysed. After
2 h, the reaction was stopped. The catalyst was recovered by
distilling off the liquids under nitrogen or with a pair of
tweezers in the case of wool samples.
The ligand C34H40N3O5P was prepared as reported by
Vagana [6]. A solution of C34H40N3O5P (18.3 mg,
25 mmol) in chloroform (6 ml) was added dropwise
to a solution of [Au(S(CH3)]Cl (7.4 mg, 25 mmol) in
chloroform (5 ml). The resulting clear yellow solution
was allowed to stir for 2 h before the solvent was
removed to give a semitransparent yellowish glass-like
material, which was crystallised from a minimum of
chloroform. 1H NMR (CDCl3): δ, 2.8 [s, 12H, C14);
δ, 3.0 [d, JH−H 13 Hz, 2 H, C2/C4]; δ, 3.6 [s, 6 H,
C15]; δ, 3.8 [d, JH−H 13 Hz, 2 H, C2/C4]; δ, 4.4 [d,
JP−H 16 Hz, 2 H, C6/C8], δ6.5 [“d”, JP−H 7 Hz, 4 H,
C12]; δ (7.1–7.4) [m, C11/C18/C19]; δ, 7.6 [m, 2 H,
C12]ppm. IR, ν01,742 cm−1 C0O. 31P NMR (CDCl3):
δ, 42.3 (s)ppm.
[(H6C5)3PAu]Cl
Tripheylphosphine gold chloride (24.7 mg, 50.0 μmol) was
dissolved in methanol (0.500 mL, 0.791 gcm−3, 12.3 mmol).
Of this solution, 0.050 mL was used as catalyst.
Preparation of gold(I) chloride nanoparticles on wool [9]
Gold(III) chloride (2.46 mg, 81 μmol) were dissolved in
40 mg 1.0 molL−1 HCl. Ten millilitre distilled water
were added to this solution. Suspended in the solution
was 0.1 g wool. The suspension was shaken for 72 h
during which the nanoparticles developed. Then the pH
value of the reaction mixture was adjusted to pH11.2
using a 0.1 molL−1 solution of potassium hydroxide to
create an environment, where the nanoparticles remained
unaffected while the cysteine and cysteine moieties of
the wool were oxidised. This step was necessary to
prevent reduction of the gold chloride nanoparticles by
the wool. The mixture was heated and shaken at 50 °C
for 7 days to ensure conversion of all active groups
present in the wool. Then the wool was recovered by
filtration, washed extensively with water and air-dried.
The sample was analysed by electron microscopy and
Fig. 3 Catalytic activity of [C34H40N3O5PAu]Cl (Cat. 1),
[(H6C5)3PAu]Cl (Cat. 2) and gold chloride nanoparticles on wool
(Cat. 3)—a plot of conversion vs time

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Gold Bull (2013) 46:13–18
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Open Access This article is distributed under the terms of the Creative
Commons Attribution License which permits any use, distribution, and
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source are credited.
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