ω-Thioacetylalkylphosphonium salts: Precursors for the preparation of phosphonium-functionalised gold nanoparticles

Article abstract of DOI:10.1016/j.jorganchem.2008.08.009

Two new ω-thioacetylalkylphosphonium salts that function as masked cationic alkanethiolate ligands for the stabilisation of gold nanoparticles have been prepared. Both (3-thioacetylpropyl)triphenylphosphonium bromide and (6-thioacetylhexyl)triphenylphosphonium bromide were shown to form water-soluble gold nanoparticles of ca. 5-10 nm in size that are stable for up to six months. The related (3-thioacetylpropyl)diphenylphosphine oxide was also prepared but did not act as a stabilising ligand in gold nanoparticle formation.

Full text of DOI:10.1016/j.jorganchem.2008.08.009

Journal of Organometallic Chemistry 693 (2008) 3504–3508
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
x
-Thioacetylalkylphosphonium salts: Precursors for the preparation
of phosphonium-functionalised gold nanoparticles
*
Yon Ju-Nam, David W Allen, Philip H.E. Gardiner, Neil Bricklebank
Biomedical Research Centre, Sheffield Hallam University, City Campus, Sheffield S1 1WB, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 July 2008
Received in revised form 1 August 2008
Accepted 5 August 2008
Available online 11 August 2008
Two new
x-thioacetylalkylphosphonium salts that function as masked cationic alkanethiolate ligands for
the stabilisation of gold nanoparticles have been prepared. Both (3-thioacetylpropyl)triphenylphospho-
nium bromide and (6-thioacetylhexyl)triphenylphosphonium bromide were shown to form water-solu-
ble gold nanoparticles of ca. 5–10 nm in size that are stable for up to six months. The related (3-
thioacetylpropyl)diphenylphosphine oxide was also prepared but did not act as a stabilising ligand in
gold nanoparticle formation.
Keywords:
Phosphonium salt
Phosphine oxide
Thioalkyl
Ó 2008 Elsevier B.V. All rights reserved.
Nanoparticle
Gold
.
1. Introduction
to the surface of nanoparticles is an attractive proposition for
improving the aqueous solubility of the resulting MPCs and facili-
Studies of the synthesis and applications of monolayer-pro-
tected clusters (MPCs) of metal, metal oxide and semiconductor
nanoparticles continue to attract considerable attention [1–4].
The organic monolayer which is introduced to the surface of the
nanoparticle improves the stability of the particle by preventing
uncontrolled aggregation and may also be used to furnish the
nanoparticle with additional functionality, such as catalytic, sen-
sor, biomolecular recognition or transport properties [5,6]. The
most widely investigated family of ligands are thiolates (RSÀ,
R = organic moiety), frequently derived from organic thiols and
disulfides. Our own contribution to this area has been the synthesis
of a series of phosphonioalkylthiosulfate zwitterions 1 that behave
as masked thiolate ligands for the formation of cationically-func-
tionalised MPCs [7]. The term ‘masked thiolate’ is used here for
alkylthiolate species which are ‘protected’ as another group, in this
case thiosulfate, and which, under reductive conditions or contact
with metal surfaces, undergo cleavage of the sulfur–sulfur bond,
generating the thiolate anion which attaches to the metal surface.
Masked thiolates are easier to handle and less prone to oxidation
than conventional thiol ligands. In related studies, we have also re-
cently reported the synthesis of two new masked phosphonioalk-
ylselenolate ligands 2 and explored their ability to stabilise gold
nanoparticles [8]. The attachment of charged or hydrophilic groups
tating biomolecular recognition through non-covalent interactions
[5]. Perhaps not surprisingly, the most widely studied cationic spe-
cies have been ammonium groups. However, phosphonium groups
also offer a range of advantages including the availability of a wide
range of organic derivatives, which allows the possibility of creat-
ing a range of functionalised derivatives, and most importantly,
biocompatibility; the ability of the triphenylphosphonium group
to travel across cell membranes is well established. The latter
property has led to the use of phosphonium compounds as anti-
cancer agents [9], transport vectors for targeting mitochondria
[10–15], and as agents for tumour imaging and diagnostics
[16–20].
Although there are many studies of the use of organophospho-
rus ligands, notably phosphines and phosphine oxides [2,21–23],
for passivating the surface of nanoparticles, there have been far
fewer studies of the use of phosphorus ligands to impart function-
ality to nanoparticles. Tris(hydroxymethyl)phosphine has been
used to synthesise gold nanoparticles which readily form conju-
gates with DNA [24,25], and recently CdSe nanoparticles capped
with the phosphonium ionic liquid trihexyl(tetradecyl)phospho-
nium bis(2,4,4-trimethylpentylphosphinate) have been reported
[26].
Herein we describe our synthesis of two new
x-thioacetylalkyl-
phosphonium salts together with a related phosphine oxide ligand
and report their reactions with gold(III) salts for the formation of
cationically functionalised gold nanoparticles.
* Corresponding author.
E-mail address: n.bricklebank@shu.ac.uk (N. Bricklebank).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.08.009

Y. Ju-Nam et al. / Journal of Organometallic Chemistry 693 (2008) 3504–3508
3505
Alkaline hydrolysis of hydroxypropylphosphonium salt 3 (n = 1)
gave the (hydroxypropyl)diphenylphosphine oxide 8, as described
by Okuma et al. [28]. This compound was dissolved in HBr (48%)
and heated under reflux for 5 h to obtain the corresponding
bromopropylphosphine oxide 9. The (3-thioacetylpropyl)diphenyl-
phosphine oxide 10 was then obtained by the reaction of the latter
with potassium thioacetate (1.5 mol) in a mixture of ethanol and
water at room temperature. The reaction mixture was left stirring
overnight under nitrogen. The progress of the reaction was moni-
tored by TLC, using 10% methanol: 90% dichloromethane as the
mobile phase. The (3-thioacetylpropyl)diphenylphosphine oxide
10 was obtained by dichloromethane extractions of the reaction
mixture, and purified by trituration with dry diethyl ether to yield
a yellow oil. Treatment of 10 with aqueous ammonia solution, fol-
lowed by exposure to air, led to the formation of the bis(phos-
phinylalkyldisulfide) 12 as a pale yellow solid, implying the
intermediate formation and subsequent oxidation of the thiol 11
from 3 (Scheme 3). Both phosphine oxides 10 and 12 were charac-
terised by by 31P- and 1H NMR studies and by high resolution mass
spectrometry. When studied by ESMS in positive ion mode, ions
corresponding to the respective [M+H] cations were observed
and characterised under high resolution.
2. Results and discussion
2.1. Ligand synthesis
The starting compounds in our synthetic sequence are the
hydroxyalkylphosphonium salts 3, obtained through the quaterni-
sation of triphenylphosphine with the corresponding bromoalco-
hol (Scheme 1). Treatment of
3 with HBr produces the
x
-bromoalkyltriphenylphosphonium bromides 4 which are re-
acted with potassium thioacetate in a mixture of ethanol and water
at room temperature, followed by dilution in aqueous potassium
bromide solution and solvent extraction with dichloromethane.
Trituration with dry diethyl ether produces the
x-thioacetylpro-
pylphosphonium salt as pale yellow solid and the x-
5
a
thioacetylhexylphosphonium salt 6 as a yellow oil (Scheme 1).
Both 5 and 6 were characterised by 31P- and 1H NMR studies
and by high resolution mass spectrometry. When studied by ESMS
in positive ion mode, ions corresponding to the respective cations
were observed and characterised under high resolution. Both com-
pounds also exhibited
C@O = 1680 cmÀ1 due to the presence of the carbonyl group.
Treatment of the salt 5 with sodium borohydride, followed by
iodomethane, resulted in the formation of the -methylthiopro-
a significant IR absorption band at
t
x
2.2. Synthesis of functionalised gold nanoparticles
pylphosphonium salt 7 (Scheme 2), indicating that the thioacetyl
group is cleaved in the presence of a mild reducing agent, as sub-
sequently used in the synthesis of the phosphonioalkylthiolate-
capped gold nanoparticles (vide infra) to generate the phospho-
nioalkylthiolate ligand. The thioacetylalkylphosphonium salts
therefore behave in the same way as our earlier reported phospho-
nioalkylthiosulfate zwitterions under reductive conditions [7,8].
In view of the ability of phosphine oxides to form hydrogen-
bonded adducts with a variety of OH and NH donors, thereby pro-
viding the possibility of molecular recognition [27], we were also
interested to explore the properties of masked alkylthiolates bear-
The ability of the above new
x-thioacetylalkylphosphonium
salts 5 and 6 and the related phosphine oxide 10 to act as li-
gands for the stabilisation of gold nanoparticles has been inves-
tigated. Previous studies by Ashwell and coworkers have shown
that thioacetate derivatives of organic dyes are useful precursors
for the formation of self-assembled monolayers (SAMs) on gold
substrates; in the presence of weak base the acetyl group is lib-
erated generating a thiolate anion which attaches to the gold
surface [29–31].
For
x-thioacetylalkylphosphonium salts 5 and 6 a solution of
ing an
x-diphenylphosphinyl substituent as ligands for the stabil-
the ligand was prepared in dichloromethane (DCM) and solid
isation of gold nanoparticles.
R3P
(CH2)n
SSO3
SeCN
1 R = Ph,n = 1-7;
R = nBu,n = 1,4
Ph3P
(CH2)n
2,n = 1,4
Br
Ph3P
(CH2)n
OH
Br
Ph3P + Br(CH2)nOH
3
HBr
O
Br
Ph3P
i. KSC(O)CH3
ii.KBraq
Br
(CH2)n
Ph3P
(CH2)n
S
4
5,n = 1
6,n = 4
Scheme 1.

3506
Y. Ju-Nam et al. / Journal of Organometallic Chemistry 693 (2008) 3504–3508
O
Br
i) NaBH4
ii) MeI
Br
S
Ph3P
S
Ph3P
5
7
Scheme 2.
Br
Ph3P
OH-
Ph2P
OH
Ph2P
Br
OH
8
3
9
O
O
O
Ph2P
S
Ph2P
SH
11
O
O
10
Ph2P
S
S
PPh2
O
O
12
Scheme 3.
O
i) K[AuCl4](s)
ii) NaBH4(aq)
Au
Ph3P
(CH2)n
S
Ph3P
(CH2)n
S
5, n =1
6, n =4
Scheme 4.
potassium tetrachloroaurate (0.5 mol equiv.) was then added to
the solution. This was vigorously stirred during the dropwise addi-
tion of a freshly prepared aqueous solution of sodium borohydride
(Scheme 4). Stirring was continued for 24 h, after which the DCM
layer was colourless and the aqueous phase was dark red-purple,
indicating that phosphonioalkylthiolate-capped nanoparticles
were present in the aqueous phase, with no evidence of
aggregation.
Evidence for the formation of gold nanoparticles was provided
by UV–Visible spectroscopy. A broad band centred at 520 nm
was observed in the aqueous phase in the dark red-purple solu-
tions containing gold nanoparticles functionalised with the salts
5 or 6, indicating that the particle sizes were between 5 and
10 nm in diameter, according to the values for other thiolate-
capped gold nanoparticles reported in the literature [32]. The band
positions were identical to those recorded for the nanoparticles
stabilised using the corresponding triphenylphosphonioalkylthio-
sulfate zwitterions 1 [7]. The solutions remained stable over a
six-month period (Fig. 1). The functionalised particles can also be
freeze-dried, producing grey coloured powders, which can be re-
suspended in a variety of solvents.
Analysis of the functionalised particles using scanning trans-
mission electron microscopy (STEM) shows highly uniform spher-
1.6
1.4
time = 0
1 month
1.2
3 months
6 months
1
0.8
0.6
0.4
0.2
0
400 450 500 550 600 650 700 750 800 850 900
Wavelength (nm)
Fig. 1. UV–Vis spectra of
a solution of AuNPs functionalised with triphen-
yl(3-thiolatopropyl)phosphonium bromide 5, after 1, 3, and 6 months.

Y. Ju-Nam et al. / Journal of Organometallic Chemistry 693 (2008) 3504–3508
3507
gold nanoparticles which should display biocompatibility. We are
currently investigating the interaction of phosphonium-functional-
ised gold nanoparticles with biological molecules and studying
their up-take by cell mitochondria. The results of these experi-
ments will be reported in due course.
The apparent inability of the phosphine oxide ligand 10 to sta-
bilise gold nanoparticles was disappointing. Other workers have
shown that comparatively simple phosphine oxides can stabilise
gold nanoclusters although in these cases the particles are usually
prepared in a single organic solvent and it is the phosphinyl group
which bonds to the gold surface [21–23]. Previous studies have
shown that combinations of different capping ligands, such as
phosphine oxides and amines, can be used to control the size
and morphology of gold nanoparticles [22], and that the phosphine
oxides can be exchanged by competitive displacement with more
strongly binding ligands [21]. It is possible that the thiolate deriv-
ative of ligand 10 can effectively act as an ambidentate ligand,
bonding through both the SÀ and P+–OÀ moieties, which leads to
the eventual agglomeration of the particles.
Fig. 2. STEM micrograph of the gold nanoparticles functionalised with triphenyl(3-
thiolatopropyl)phosphonium bromide 5.
Size = 5.0 2.1 nm
60
50
40
30
20
10
0
4. Experimental
4.1. General
1H and 31P NMR spectra were obtained in CDCl3 and in
CDCl3/DMSO mixture on a Bruker DMX 250 (250 MHz) spectrome-
ter. Electrospray mass spectra were recorded using an Applied
Biosystems ‘‘QStar-Pulsar-i” hybrid quadrupole time of flight
LCMS-MS instrument. Accurate mass measurements were made
using a MALDI-TOF instrument in positive ion mode. Analytical thin
layer chromatography (TLC) was performed on Merck silica gel
60F254 plates using 90:10 dichloromethane: methanol as the eluent.
UV–Vis spectra of aqueous colloidal solutions were obtained on an
ATI UNICAM UV2 spectrometer. A Carl Zeiss STM SUPRATM 40VP
GEMINITM FE-SEM with a Multi-Mode STEM (30.00 kV) detection
system was used to determine the average particle size for the sam-
ple studied. Sample for STEM was prepared by placing one drop of
the phosphoniopropylthiolate-capped gold nanoparticle solution
on a carbon-coated TEM copper grid, and then the sample was al-
lowed to dry at room temperature overnight.
0
1
2
3
4
5
6
7
8
9
Particle size (nm)
Fig. 3. Size distribution histogram of gold nanoparticles functionalised with
triphenyl(3-thiolatopropyl)phosphonium bromide 5.
ical nanoparticles of ca. 5 nm (Fig. 2). As seen in Fig. 3, the size dis-
tribution of these gold nanoparticles is narrow.
When (3-thioacetylpropyl)diphenylphosphine oxide 10 was
used as the protecting ligand in the synthesis of gold nanoparticles,
following the addition of sodium borohydride, a dark blue organic
phase and a transparent aqueous phase were observed. It would
appear that the gold nanoparticles functionalised with this phos-
phine oxide-containing ligand have more affinity for the dichloro-
methane phase, which is opposite to the other cases previously
described. However, after 2 h, complete aggregation and visible
(blue) particles were observed at the bottom of the flask, making
it impossible to use the solutions for further analysis and
applications.
4.2. Synthesis of the
x-thioacetylalkylphosphonium salts (5, 6) and
related phosphine oxide ligands (10, 12)
The phosphonium salts 5 and 6 were generated by the reaction
of the appropriate
x-bromoalkyltriphenylphosphonium bromide
(2 mmol) with potassium thioacetate (3 mmol) in a mixture of eth-
anol and water at room temperature. The reaction mixture was left
stirring overnight under nitrogen. Progress of the reaction was
monitored by TLC, using methanol–dichloromethane (10:90% v/v)
as mobile phase. The
x-thioacetylalkylphosphonium salts were
3. Conclusions
isolated by dilution in aqueous potassium bromide solution and
solvent extraction with dichloromethane, before final purification
by trituration with dry diethyl ether.
The
x-thioacetylalkylphosphonium salts reported here extend
further the family of masked thiolate ligands that we have devel-
oped. Masked thiolates offer a number of advantages over conven-
tional thiol ligands including greater stability to aerobic oxidation
and ease of handling. Furthermore, the thioactetate ligands re-
ported here have greater aqueous solubility than the correspond-
ing phosphonioalkylthiosulfate zwitterions which we have
4.2.1. (3-Thioacetylpropyl)triphenylphosphonium bromide (5)
Isolated as a pale yellow solid, 91% yield, m.p. 85–89 °C. d31P
NMR (CDCl3) = 23.8 ppm; d1H NMR (CDCl3) = 1.9 (2H,m), 2.2
(3H,s), 3.2 (2H,m), 3.8 (2H,m), 7.6–7.8 (15H,m) ppm. FT-IR: mC@O
1680 cmÀ1. MALDI-TOF accurate mass analysis. Found 379.1307
[M+]; cation C23H24OPS requires 379.1285 [M+]. ESMS 380 [M+H]+.
reported previously. Under reductive conditions the
x-thi-
oacetylalkylphosphonium salts readily lose the acetyl groups gen-
erating thiolate anions which readily coordinate to the surface of
the gold nanoparticles as they grow during the in situ reduction
of gold(III) salts. The presence of the triphenylphosphonium
head-groups yields cationic functionalised monolayer-protected
4.2.2. (6-Thioacetylhexyl)triphenylphosphonium bromide (6)
Isolated as a pale yellow oil, 85% yield. d31P NMR (CDCl3) =
23.91 ppm, d1H NMR (CDCl3) = 1.14–1.59 (8H,m), 2.19 (3H,s),

3508
Y. Ju-Nam et al. / Journal of Organometallic Chemistry 693 (2008) 3504–3508
2.72 (2H,t), 3.68 (2H,m), 7.6–7.8 (15H,m) ppm. MALDI-TOF accu-
rate mass analysis. Found 421.1742 [M+]; cation C26H30OPS re-
quires 421.1755 [M+].
methane and final trituration with diethyl ether. 82% yield. d31P
NMR (CDCl3) = 31.93 ppm; d1H NMR (CDCl3) = 1.86–2.09 (4H,m),
2.28–2.40 (4H,m), 2.66 (4H,t), 7.40–7.50 (10H,m) ppm, 7.67–7.75
(10H,m). MALDI-TOF accurate mass analysis. Found 551.13746
[M+H+]; cation (C30H32O2P2S2 + H) requires 551.13973 [M+H+].
4.2.3. Reduction and in situ methylation of (3-
thioacetylpropyl)triphenylphosphonium bromide
Acknowledgement
(3-Thioacetylpropyl)triphenylphosphonium bromide (5), (0.5
mmol) was dissolved in methanol (3 cm3). A freshly prepared
aqueous solution of sodium borohydride (5 mmol) was then added
dropwise to the reaction flask, in order to allow formation of the
zwitterion Ph3P+(CH2)3SÀ. The mixture was stirred for 3 h at room
temperature, and then iodomethane (5 mmol) was added. The
resulting mixture was then stirred overnight. Progress of the reac-
tion was monitored by TLC, using methanol–dichloromethane
(10:90% v/v) as a mobile phase. The resulting mixture was treated
with an excess of aqueous potassium iodide solution and then ex-
tracted with dichloromethane, the organic phase was collected and
after removing the solvent, the resulting solid was initially purified
by trituration with dry diethylether to give 3-(methylthiopro-
pyl)triphenylphosphonium iodide (7), identical with the com-
pound isolated previously from the analogous reduction/
methylation of triphenylphosphoniopropylthiosulfate [7]. M.p.
136–138 °C. d31P NMR (CDCl3) = 24.3 ppm; d1H NMR (CDCl3) = 1.9
(3H,s), 2.8 (2H,t), 3.3 (2H,m), 3.8 (2H,m), 7.6–7.8 (15H,m) ppm.
MALDI-TOF accurate mass analysis. Found 351.1342 [M+]; cation
C22H24PS requires 351.1312 [M+].
We are grateful to Sheffield Hallam University for funding and
for the provision of a bursary for Y.J.N.
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(3-Hydroxypropyl)diphenylphosphine oxide (8), prepared as
described by Okuma et al. [28] was then dissolved in HBr (48%)
and heated under reflux for 5 h to obtain the corresponding
bromopropylphosphine oxide (9). After isolation by solvent extrac-
tion into dichloromethane, this was then treated with potassium
thioacetate (1.5 mol) in a mixture of ethanol and water at room
temperature. The reaction mixture was left stirring overnight un-
der nitrogen. The progress of the reaction was monitored by TLC,
using 10% methanol: 90% dichloromethane as the mobile phase.
The (3-acetylthiopropyl)diphenylphosphine oxide (10) was ob-
tained by dichloromethane extractions of the reaction mixture,
and purified by trituration with dry diethyl ether to yield a yellow
oil. d31P NMR (CDCl3) = 31.42 ppm, d1H NMR (CDCl3) = 1.76–1.92
(2H,m), 2.22 (3H,s), 2.24–2.33 (2H,m), 2.90 (2H,t), 7.35–7.50
(5H,m) ppm, 7.63–7.71 (5H,m). ESMS: 244 [M-SCOCH3], 319
[M+H+], 341 [M+Na+]. MALDI-TOF accurate mass analysis. Found
319.1136 [M+H+]; cation (C17H19O2PS + H) requires 319.1143
[M+H+].
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4.2.5. 3,3’-Bis(diphenylphosphinylpropyl)disulfide (12)
Treatment of (3-acetylthiopropyl)diphenylphosphine oxide (10)
with aqueous ammonia solution, followed by exposure to air over a
24 h period, led to the formation of the bis(phosphinylalkyldisul-
fide) (12) as a pale yellow solid, following extraction into dichloro-