Cooperative Effects of Electron Donors and Acceptors for the
Stabilization of Elusive Metal Cluster Frameworks: Synthesis and
Solid-State Structures of [Pt
19
(CO)
24
(μ
4
‑AuPPh
3
)
3
]
−
and
[Pt
19
(CO)
24
{μ
4
‑Au
2
(PPh
3
)
2
}
2
]
Alessandro Ceriotti,*
,†
Piero Macchi,*
,‡
Annalisa Sironi,
§
Simona El Afefey,
†,⊥
Matteo Daghetta,
∥
Serena Fedi,
⊥
Fabrizia Fabrizi de Biani,
⊥
and Roberto Della Pergola
§
†
Dipartimento di Chimica, Universita ̀ degli Studi di Milano, via Golgi 19, 20133 Milano, Italy
‡
Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland
§
Dipartimento di Scienze dell’Ambiente e del Territorio, Universita ̀ di MilanoBicocca, piazza della Scienza 1, 20126 Milano, Italy
⊥
Dipartimento di Chimica, Universita ̀ degli Studi di Siena, via Aldo Moro, 53100 Siena, Italy
∥
Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta”, Politecnico di Milano, Piazza L. Da Vinci 32, 20133 Milano,
Italy
* S Supporting Information
ABSTRACT: The anionic cluster [Pt
19
(CO)
22
]
4−
(1), of pentagonal symmetry, reacts
with CO and AuPPh
3
+
fragments. Upon increasing the Au:Pt
19
molar ratio, different
species are sequentially formed, but only the last two members of the series could be
characterized by X-ray diffraction, namely, [Pt
19
(CO)
24
(μ
4
-AuPPh
3
)
3
]
−
(2) and
[Pt
19
(CO)
24
{μ
4
-Au
2
(PPh
3
)
2
}
2
](3). The metallic framework of the starting cluster is
completely modified after the addition of CO and AuL
+
, and both products display the
same platinum core of trigonal symmetry, with closely packed metal atoms. The three
AuL
+
units cap three different square faces in 2, whereas four AuL
+
fragments are
grouped in two independent bimetallic units in the neutral cluster 3. Electrochemical
and spectroelectrochemical studies on 2 showed that its redox ability is comparable with
that of the homometallic 1.
■
INTRODUCTION
Many Pt−Au mixed-metal clusters have been obtained in the
past, and most of them contain three to six metal atoms. These
compounds have assumed particular relevance because of their
peculiar bonding properties.
1
In the field of high-nuclearity
clusters (Pt + Au > 6), few examples are known, either rich in
Pt
2
or with a high Au content.
3
A few trimetallic clusters,
mainly containing other metals of groups 11 and 12 (Cu, Ag,
and Hg), were also reported,
4
and they are typically based on
icosahedral structures. Besides theoretical and structural
aspects, the combination of Pt and Au in molecular compounds
of well-defined composition may be relevant for the preparation
of bimetallic nanoparticles, which are known to possess
excellent activity in electrocatalysis,
5
and for the catalytic
oxidation of glycerol.
6
The anion [Pt
19
(CO)
22
]
4−
(1) has played a special role in the
field of carbonyl clusters because it remained for a long time the
largest structurally characterized compound of this kind.
Moreover, it is still one of the rare examples of high-nuclearity
carbonyl clusters possessing a framework of pentagonal
symmetry.
7
Despite these peculiarities, the studies on its
reactivity were hampered by the difficult characterization of the
fairly unstable products. For example, it is known that the
reactions of 1 with NO
+
and H
+
eventually lead to the
formation of [Pt
38
(CO)
44
]
2−
, but only [Pt
19
(CO)
21
NO]
3−
could be isolated as an intermediate.
8
Nevertheless, some
electrochemical investigations have been described, showing
that the cluster undergoes several couples of redox processes,
spanning reversibly the 8−/0 oxidation states.
9
One of us deeply investigated the behavior of the cluster
under a carbon monoxide (CO) atmosphere, observing a clean,
quantitative, and rapid reaction, but also in this case, the real
nature of the product could never be ascertained. In contrast to
what is normally observed,
10
the IR bands of the carbonylated
product shift to lower wavenumbers, indicating an increased
metal-to-ligand back-donation and suggesting that this
uncharacterized species would be a better electron donor
toward electrophiles.
For all of these reasons, in order to verify the donor
properties of the carbonylated product and to compare them
with those of 1, we performed new experiments, aiming at the
Received: October 18, 2012
Published: January 30, 2013
Article
pubs.acs.org/IC
© 2013 American Chemical Society 1960 dx.doi.org/10.1021/ic302282y | Inorg. Chem. 2013, 52, 1960−1964