Chemistry Reference
In-Depth Information
2.3.3 Post-reduction Phase
Mass spectrometry has been used to monitor ligand exchange reactions of thiolate-
protected gold nanoclusters. For example, Spivey et al. [
88
] used MALDI-TOF-MS
to show that treatment of glutathione-protected nanoclusters with dodecanethiol
(SC
12
H
25
) results in the transformation to monodisperse Au
38
(SC
12
H
25
)
12
. Bare
Au
38
clusters, formed via reduction of Au
38
(SC
12
H
25
)
12
by H
2
, were attached to
TiO
2
supports, and their role in catalysing CO oxidation was investigated.
Whetten et al. have used mass spectrometry to assign the number of ligands and
gold atoms of synthesised thiolate-protected gold nanoclusters. For instance, they
used ESI-MS to identify the various charge states of [Au
144
Cl
60
]
z
. This cluster was
synthesised from the well-known thiolate-protected cluster [Au
144
(SR)
60
]
z
via
2 and 4
+
charge
states yielded a highly symmetric (I
h
) cluster. Mass spectrometry was also used in
their study of the Au
67
(SR)
35
cluster via MALDI-TOF [
90
] as well as the study of
[Au
25
(SC
6
H
13
)
18
]
x
cluster via ESI-MS [
91
]. In this latter study, they concluded that
geometric rather than electronic factors are responsible for the stability of
[Au
25
(SC
6
H
13
)
18
]
x
(
x
ligand exchange with chloride [
89
]. They concluded that the
z
¼
1, 0, + 1).
Recently, Jin et al. [
92
] used ESI-MS and UV-Vis spectroscopy to monitor the
solution phase reactivity of monodisperse Au
38
(PET)
24
, (PET
¼
phenylethanethiol),
with an excess of 4-
tert
-butylbenzenethiol (TBBT) and sampled at various time
intervals, Fig.
6
. The excess addition of the bulkier ligand TBBT ultimately results
in the formation of monodisperse Au
36
(TBBT)
24
.
Given the data in Fig.
6
the evolution of the thiol-induced Au
36
(TBBT)
24
from
Au
38
(PET)
24
was divided into four stages (Fig.
7
): (1) ligand exchange reactions of
PET for TBBT occur, (2) ligand exchange reaction continues together with a
structural distortion of the cluster core as observed by the optical spectra, (3) dis-
proportionation reaction occurs as identified by ESI-MS whereby 2 equiv. of
Au
38
(TBBT)
m
(PET)
24
m
give Au
36
(TBBT)
m
(PET)
24
m
and Au
40
(TBBT)
m
+2
(PET)
24
m
, and (4) size focusing occurs, resulting in the formation of monodis-
perse Au
36
(TBBT)
24
.
¼
2.4 Phosphine Ligands
Gold nanoclusters protected by phosphine ligands have been studied in the con-
densed phase since the pioneering work of Malatesta in the 1960s [
93
-
97
]. These
early studies were motivated by the desire to develop models for cluster bonding
and thus a key aim was the isolation of crystalline material suitable for X-ray
diffraction studies [
57
,
98
-
108
]. A significant achievement was the isolation and
characterisation of a triphenylphosphine monolayer-protected clusters (MPCs) of
monodisperse Au
55
(PPh
3
)
12
Cl
6
by Schmid [
109
-
112
]. In recent years ESI-MS
has been used to monitor the role of bis(phosphino)alkane ligands of the type
