Cooperative effects of electron donors and acceptors for the stabilization of elusive metal cluster frameworks: Synthesis and solid-state structures of [Pt19(CO)24(μ4-AuPPh3) 3]- and [Pt<su

Article abstract of DOI:10.1021/ic302282y

The anionic cluster [Pt₁₉(CO)₂₂]^4- (1), of pentagonal symmetry, reacts with CO and AuPPh₃⁺ fragments. Upon increasing the Au:Pt₁₉ molar ratio, different species are sequentially formed, but

Full text of DOI:10.1021/ic302282y

Article
pubs.acs.org/IC
Cooperative Effects of Electron Donors and Acceptors for the
Stabilization of Elusive Metal Cluster Frameworks: Synthesis and
Solid-State Structures of [Pt19(CO)24(μ4‑AuPPh3)3]− and
[Pt19(CO)24{μ4‑Au2(PPh3)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
‡Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland
§Dipartimento di Scienze dell’Ambiente e del Territorio, Universita
di MilanoBicocca, piazza della Scienza 1, 20126 Milano, Italy
⊥Dipartimento di Chimica, Universita
̀
degli Studi di Milano, via Golgi 19, 20133 Milano, Italy
̀
̀
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 [Pt19(CO)22]4− (1), of pentagonal symmetry, reacts
+
with CO and AuPPh3 fragments. Upon increasing the Au:Pt19 molar ratio, different
species are sequentially formed, but only the last two members of the series could be
characterized by X-ray diffraction, namely, [Pt19(CO)24(μ4-AuPPh3)3]− (2) and
[Pt19(CO)24{μ4-Au2(PPh3)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.
fairly unstable products. For example, it is known that the
reactions of 1 with NO+ and H+ eventually lead to the
formation of [Pt38(CO)44]2−, but only [Pt19(CO)21NO]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.
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
Pt2 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 [Pt19(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
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
© 2013 American Chemical Society
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stable addition of gold electrophilic fragments. The first results
in this field, namely, the chemical and structural character-
ization of Pt−Au heterometallic clusters, are described here. In
sharp contrast with the pentagonal symmetry of 1 and other
heterometallic compounds, the metallic frameworks of
[Pt19(CO)24(μ4-AuPPh3)3]− (2) and [Pt19(CO)24{μ4-
Au2(PPh3)2}2] (3) are rare examples of close-packed Pt−Au
metal clusters stabilized by the cooperative effect of the
electron-donor Au fragment and the electron-acceptor CO
ligand.
units bonded to Pt in carbonyl clusters, ranging from 500 to
300 Hz.4,11 The absence of any signal in the 31P NMR spectrum
of 3 can be a consequence of its limited solubility or of dynamic
processes involving the four AuPPh3 groups. Besides
dissociation, two additional processes can be envisaged,
involving the exchange of nonequivalent Au sites within the
two Au2 units, and scrambling of the Au2 entities over the three
square faces available. Indeed, Au2(PR3)2 moieties in carbonyl
clusters are known to be highly fluxional.12
Crystal Structures. The solid-state structure of 2, including
all of the carbonyl and phosphine ligands, is shown in Figure 1;
the atom labeling of the metallic framework of 3 is reported in
Figure 2; their relevant structural parameters are compared in
Table 1.
RESULTS
■
Synthesis and Reactivity. The reaction of 1 with
PPh3AuCl was carried out in acetonitrile at ambient temper-
ature, under CO, or under an inert atmosphere: in both cases,
IR monitoring of the reaction mixture showed systematic
trends: (i) upon the addition of the Au complex, a progressive
shift of the CO bands to higher wavenumbers was observed;
(ii) at the same Au:Pt19 molar ratios, the products formed
under CO had IR maxima red-shifted about 10 cm−1 with
respect to the products formed under an inert atmosphere.
Under CO, when the Au:Pt19 molar ratio is 3:1, a brown
precipitate forms, which can be easily isolated by filtration and
dissolved in tetrahydrofuran (THF) for characterization and
crystallization. This species is stable under an inert atmosphere
and does not need to be handled under CO. Thus, crystals of
(NBu4)[Pt19(CO)24(AuPPh3)3][(NBu4)2] suitable for X-ray
determination were obtained by layering this THF solution
with 2-propanol.
The low solubility of the salts of 2 in acetonitrile hampered
further reactions with an excess of PPh3AuCl. Therefore, in
order to obtain clusters with a higher Au content, we reacted
THF solutions of pure 2 with the solvated cationic complex
[PPh3AuTHF]+, obtained in situ from PPh3AuCl and Ag+.
This reaction induced a definite shift of the IR carbonyl
bands and allowed isolation of the neutral cluster 3 by
precipitation with 2-propanol.
Thus, the whole series of reactions can be summarized with
the following chemical equations:
Figure 1. Solid-state structure of 2 from X-ray diffraction refinements.
[Pt19(CO)22]4− [ν(CO) 2005s, 1930sh, 1799m cm−1]
+ 2CO → [Pt19(CO)24 ]4− [not isolated,
The platinum framework is identical in the two compounds,
being composed of a closed-packed face-centered-cubic core
and four layers of three, seven, six, and three atoms,
respectively. All Pt atoms but one are located on the surface
and bear at least one carbonyl ligand, whereas Pt1 is fully
interstitial at the center of the second layer. The cluster can be
idealized as a cubic octahedron, with three square faces capped
by another three Pt atoms and face-fused with an octahedron
(see Figure 2). The upper part of the cluster is therefore
exposing three square faces and four triangular faces as a cubic
octahedron. The two structures differ, however, in the
coordination of AuPPh3: in 2, the three fragments are all
capping three square faces, whereas in 3, they form two
(AuPPh3)2 dumbbells that asymmetrically cap one square face.
The distribution of the carbonyl ligands is similar in 2 and 3.
Thus, the ideal symmetry is Cs in 3 and C3v in 2 (reduced to C1
and C3, respectively, if the out-of-plane rotation of the phenyl
rings is considered). In the crystal structures, 2 is mirror-
symmetric, whereas 3 is not sitting on any symmetry element.
The Pt−Au bond distances in 2 span the range 2.80−2.93 Å
and average 2.86 Å. They are systematically longer for the three
Pt3 atoms of the outer triangle (>2.85 Å) and shorter for the
ν(CO) 1996s, 1928sh, 1801w, br, 1752m cm−1]
[Pt19(CO)24]4− + 3AuPPh3Cl
→ [Pt19(CO)24 (AuPPh3)3]− + 3Cl−
[Pt19(CO)24 (AuPPh3)3]− + [PPh3AuTHF]+
→ [Pt19(CO)24 {μ ‐Au2(PPh3)2}2]
4
31P{1H} NMR spectra have been recorded for both clusters
in acetone-d6 at 183 K, but only for (PPh4+)2 we could detect a
signal, consisting of a sharp singlet (PPh4 ) and a few narrow
lines emerging from a broad band centered at 67 ppm. Even
3
assuming a J(31P−195Pt) value close to zero, a very complex
spectrum could be anticipated because the P3Au3Pt9 unit is a
12-spin system, with many magnetically inequivalent iso-
topomers, corresponding to the different clusters with zero to
1
nine NMR-active 195Pt (S = /2; natural abundance 34%).
Therefore, it was impossible to estimate the 2J(31P−195Pt) value
and to compare it with the few values reported for AuPPh3
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Figure 3. CV profile recorded at a platinum electrode in a THF
solution of (PPh4)2 (0.5 × 10−3 M): supporting electrolyte,
(NBu4)PF6 (0.2 M); scan rate, 0.2 V s−1.
could be beyond the experimental window, limited by solvent
discharge. Controlled-potential coulometric tests in corre-
spondence with the first reduction (Ew = −0.4 V) proved
that it involves one electron per molecule, while CV on the
exhaustively reduced solution indicates that also the addition of
one electron, in fact, causes partial decomposition of the cluster,
as suggested by the appearance of new small peaks and by the
general decrease of the current intensity.
Figure 2. Metallic framework of 3. The Pt13 cubic octahedron core is
highlighted in orange. The remaining Pt atoms are in light gray and the
four Au atoms in yellow.
Table 1. Averaged Pt−Pt Distances (in Å) in the Cores of
a
the Two Clusters
In spite of the incomplete stability of the electrogenerated
neutral and dianionic species, we have monitored by IR
spectroelectrochemistry the variation of ν(CO) of both the
terminal and bridging groups of this cluster. The experiment
has been conducted in situ step-by-step, in order to maintain
the isosbestic points, so that electrolysis was, in fact, not
completed. As expected, in both cases, the original spectrum is
only partially recovered on the backscan. Figure 4 illustrates the
IR spectral trend recorded in the 0/1− and 1−/2− passage.
Pt−Pt bond
Pt1−Pt2
Pt1−Pt3
Pt1−Pt4
Pt2−Pt2
Pt2−Pt3
Pt2−Pt4/Pt5
Pt3−Pt3
no. of bonds
2
3
6
3
2.796(6)
2.82(2)
2.77(1)
2.79(2)
2.76(1)
2.80(1)
2.81(2)
2.833(8)
2.79(4)
2.81(8)
2.76(4)
2.79(5)
2.79(5)
2.79(5)
2.81(3)
2.822(5)
3
6
6
12
3
Pt4/Pt5−Pt4/Pt5
9
a
Numbers in parentheses are the standard deviations from the mean.
Pt2 atoms of the inner hexagon (<2.85 Å), suggesting a slightly
stronger interaction. In 3 the Pt−Au distances are more
scattered, being included in the range 2.64 −2.97 Å, but the
average (2.84 Å) is almost unchanged. The shorter distances
are those connecting the two more external Au1 atoms, bound
to only two Pt atoms (see Figure 2).
In both crystal structures, the clusters leave large voids to
host solvent molecules and cations (in 2) which are not visible,
given the relative scarce diffraction and the likely disordered
arrangement around the clusters.
Electrochemical Studies. As illustrated in Figure 3, (PPh4)
2 undergoes one anodic process at +0.45 V, complicated by
subsequent chemical reactions. In fact, we are in the presence of
an ECE mechanism with the formation of a byproduct
undergoing two new oxidation processes at +0.73 and +0.88
V. By analogy with what has been observed for the series
[Pt19(CO)22]4−, [Pt24(CO)30]2−, and [Pt38(CO)44]2−, two pairs
of cathodic processes, separated by ∼300 mV, are also visible at
−0.31, −0.65, −1.66, and −1.90 V, only the first of which
manifesting features of chemical reversibility in the time scale of
cyclic voltammetry (CV). In fact, at lower scan rates, the
departure from the current ratio ip(reverse)/ip(forward) = 1 expected
for a chemical reversible process is observed for all of the
reduction steps, except for the first. A fifth reduction process at
−2.25 V is also detected, while the possible partner of this pair
Figure 4. IR spectral changes recorded in an optically transparent thin-
layer electrode cell upon the progressive oxidation (top) and reduction
(bottom) of 2: THF solution; supporting electrolyte, (NBu4)PF6 (0.2
M).
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The original spectrum shows two main absorptions: at 2027
cm−1, which is assigned to the stretching vibration of the
carbonyl groups terminally bound at the metal core, and at
1803 cm−1, which is assigned to the stretching vibration of the
bridging carbonyl groups. Upon oxidation, both bands are blue-
shifted by ∼15−20 cm−1 and appear at 2042 and 1821 cm−1,
while the contrary happens upon reduction, with two new
bands appearing at 2009 and 1780 cm−1. This is a common
trend followed by carbonyl clusters and indicates decreasing or
increasing dπ-Pt → π*-CO back-bonding induced by electron
removal or addition, respectively. It is interesting to note the
agreement between the ν(CO) trend and the first oxidation or
first reduction redox potential values of the pristine sample of 2
compared to the analogous values observed for [Pt19(CO)22]4−
(2001 cm−1 and −0.15 and −1.13 V, for the first oxidation and
first reduction, respectively).8 All of these values clearly indicate
how the {Pt194−} metallic core is much less electron-rich in 2
than in the parent 1.
a mixture of 2 and 3 is clearly revealed. A new couple of anodic
processes is visibly detectable at −1.21 and −1.41 V, so it is
questionable whether the true first couple of anodic processes
of 3 is partially hidden by the correspondent couple of anodic
processes of 2. As a matter of fact, two shoulders at −0.53 and
−0.78 V are visible behind the first couple of reductions of 2,
which could be possibly ascribed to 3. The low solubility and
redox instability of 3 probably prevent a more correct
interpretation. In conclusion, as a general trend, the addition
of heterometallic Au fragments reduces the electron content of
these Pt clusters and, at the same time, makes them much less
prone to the electron-sink behavior observed for the
homometallic related family of clusters.
EXPERIMENTAL SECTION
■
All of the solvents were purified and dried by conventional methods
and stored under nitrogen. All of the reactions were carried out under
an oxygen-free nitrogen atmosphere using the Schlenk-tube
technique.13 (PPh4)4[Pt19(CO)22]6 and PPh3AuCl14 were prepared
by literature methods. IR spectra in solution were recorded on a
Nicolet Avatar 360 FT-IR spectrophotometer, using calcium fluoride
cells previously purged with dinitrogen. Elemental analyses were
carried out by the staff of Laboratorio di Analisi of the Dipartimento di
Chimica.
As illustrated in Figure 5, the redox behavior of the neutral
cluster 3 seems somehow reminiscent of that of 2 because an
CV was performed in a three-electrode cell containing a gold or a
glassy carbon working electrode surrounded by a platinum spiral
counter electrode and an aqueous saturated calomel reference
electrode (SCE) mounted with a Luggin capillary. A BAS 100W
electrochemical analyzer was used as the polarizing unit. All of the
potential values are referred to the SCE. Under the present
experimental conditions, the one-electron oxidation of ferrocene
occurs at E°′ = +0.38 V in an acetonitrile solution and E°′ = +0.59 V in
a THF solution. Controlled-potential coulometry was performed in an
H-shaped cell with anodic and cathodic compartments separated by a
sintered-glass disk. The working macroelectrode was a platinum gauze;
a mercury pool was used as the counter electrode.
Figure 5. CV profile recorded at a platinum electrode in a THF
solution of 3 (0.4 × 10−3 M): supporting electrolyte, (NBu4)PF6 (0.2
M); scan rate, 0.2 V s−1.
Anhydrous 99.8% acetonitrile was an Aldrich product. Anhydrous
99.9% HPLC-grade THF (Aldrich) was distilled in the presence of
sodium before use. Fluka (NBu4)PF6 or (NEt4)PF6 (electrochemical
grade) were used as supporting electrolytes.
31P{1H} NMR spectra have been recorded on a 300 MHz Bruker
spectrometer, operating at 121.5 MHz and referenced to H3PO4 in
D2O.
oxidation (E° = +0.33 V) and a couple of reductions (at E° =
−0.34 and −0.67 V) are observed. A clear trait of the illustrated
CV is the general lower chemical reversibility of all of these
redox processes (ip(reverse)/ip(forward) at v = 0.2 V s−1 is 0.1 for the
oxidation and 0.5 and 0.2 for the first and second reductions,
respectively). Anyway, a closer inspection of the redox pattern
of 3, obtained by using a potential step technique, like, for
example, differential pulse voltammetry (DPV) shown in Figure
6, reveals a more complicated situation. In fact, the presence of
Synthesis of 2. A total of 720 mg (0.127 mmol) of
(PPh4)4[Pt19(CO)22] was dissolved with 20 mL of CH3CN in a
Schlenk tube; the brown solution was put under a CO atmosphere and
stirred for 1 h. Solid PPh3AuCl (188 mg, 0.381 mmol) was then added
in three subsequent portions. The solution was stirred overnight. A
brown powder gradually separated. It was recovered by filtration,
washed with 2-propanol (2 × 10 mL), and dried in a vacuum. ν(CO)
(THF): 2059w, 2027s, 1804w cm−1. Yield: 515 mg (66%). Elem. anal.
Found: C, 20.8; H, 1.20. Calcd: C, 20.10; H, 1.07.
(NBu4)2 used for X-ray diffraction was prepared analogously from
(NBu4)4[Pt19(CO)22] with comparable yields. Crystals were grown
from THF/2-propanol. Elem. anal. Found: C, 18.91; H, 1.53; N, 0.51.
Calcd: C, 18.81; H, 1.35; N, 0.23.
Synthesis of 3. A solution of (PPh4)2 (264 mg, 0.043 mmol) in 25
mL of THF was stirred under a CO atmosphere. In another Schlenk
tube, PPh3AuCl (22 mg, 0.044 mmol) and AgBF4 (9 mg, 0.046 mmol)
were mixed in 1 mL of THF, giving rise to a fine white powder (AgCl),
which was decanted. The colorless solution of [PPh3AuTHF]BF4 was
added in small subsequent portions to the cluster solution, and the
reaction was monitored via IR spectroscopy. The mixture was filtered,
and the precipitate was washed with THF until the washings were
colorless; 40 mL of 2-propanol were added dropwise, and the volume
of the solvent was reduced in a vacuum. The black precipitate was
collected by filtration, washed with several portions of 2-propanol and
cyclohexane, and finally dried under reduced pressure. It was extracted
Figure 6. DPV responses recorded at a platinum electrode in a THF
solution of 2 (0.5 × 10−3 M, dashed line) and 3 (0.4 × 10−3 M, full
line): supporting electrolyte, (NBu4)PF6 (0.2 M).
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to a small Schlenk tube with a minimum amount of THF and
cautiously layered with 2-propanol; well-shaped black crystals were
obtained. ν(CO) (THF): 2069w, 2038s, 1805w cm−1. Yield: 120 mg
(45%). Elem. anal. Found: C, 20.0; H, 1.65. Calcd: C, 18.55; H, 0.97.
Crystal Structure Determination of (NBu4)2 and 3. The X-ray
diffraction studies were carried out on a Bruker SMART 1K CCD area
detector (Mo Kα radiation, 50 kV and 30 mA generator settings). 2 is
monoclinic P21/m with a = 15.5619(16) Å, b = 21.250(2) Å, c =
18.164(2) Å, β = 91.633(3)°, and V = 6004.2(11) Å3. The final R1 and
wR2 values were 0.0783 and 0.2389, respectively. 3 is also monoclinic,
P21/c, with a = 16.5865(6) Å, b = 29.9739(10) Å, c = 25.4375(9) Å, β
= 98.604(1)°, and V = 12504.2(8) Å3. The final R1 and wR2 values
were 0.0731 and 0.2052, respectively. More details on data collection
and crystal structure refinement are given in the Supporting
Information.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Tables S1 and S2 (crystallographic data), ORTEP drawings of 2
and 3 with all atomic labels, and experimental details of
crystallographic analysis. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: alessandro.ceriotti@unimi.it (A.C.), piero.macchi@
dcb.unibe.ch (P.M.).
■
(7) Washecheck, D. M.; Wucherer, E. J.; Dahl, L. F.; Ceriotti, A.;
Longoni, G.; Manassero, M.; Sansoni, M.; Chini, P. J. Am. Chem. Soc.
1979, 101, 6110.
Notes
The authors declare no competing financial interest.
(8) Ceriotti, A.; Masciocchi, N.; Macchi, P.; Longoni, G. Angew.
Chem., Int. Ed. 1999, 38, 3724.
(9) Fedi, S.; Zanello, P.; Laschi, F.; Ceriotti, A.; El Afefey, S. J. Solid
State Electrochem. 2009, 13, 1497.
ACKNOWLEDGMENTS
This paper was financially supported by MIUR [PRIN 2008 (to
A.C. and F.F.d.B.) and PRIN 2007 (to R.D.P.)].
■
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dx.doi.org/10.1021/ic302282y | Inorg. Chem. 2013, 52, 1960−1964