Synthesis and characterisation of high-nuclearity osmium-silver mixed-metal clusters

Article abstract of DOI:10.1039/b709860j

The reaction of the triosmium cluster anion, [Os₃(μ-H)(CO) ₁₁][PPN] (PPN = [N(PPh₃)₂]⁺), with [AgPF₆] in the presence of [Ir(PPh₃)₂(CO)Cl] in THF at room temperature a

Full text of DOI:10.1039/b709860j

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Synthesis and characterisation of high-nuclearity osmium–silver mixed-
metal clusters{
Yui-Bing Lee and Wing-Tak Wong*
Received (in Cambridge, UK) 28th June 2007, Accepted 3rd August 2007
First published as an Advance Article on the web 23rd August 2007
DOI: 10.1039/b709860j
50%), [Os9Ag9(m3-O)2(CO)30][PPN] (2) (18.9 mg, 10%) and
[Ir(PPh3)2(CO)Cl(O2)] (3) (12.5 mg, 40%). The reaction scheme is
shown in Scheme 1.
The reaction of the triosmium cluster anion, [Os3(m-H)(CO)11]-
[PPN] (PPN = [N(PPh3)2]+), with [AgPF6] in the presence of
[Ir(PPh3)2(CO)Cl] in THF at room temperature affords two
new high-nuclearity osmium–silver clusters, [Os13Ag9(CO)48]-
[PPN] (1) and [Os9Ag9(m3-O)2(CO)30][PPN] (2), and an iridium
complex, [Ir(PPh3)2(CO)Cl(O2)] (3).
Red block crystals of 1 that were suitable for single-crystal
X-ray analysis were obtained from the saturated solution of
dichloromethane–diethyl ether–methanol at 220 uC after a week.
It was crystallized in its [PPN]+ salt with one dichloromethane
molecule and two water molecules as the solvents of crystallization
inside the crystal lattice.
Fig. 1 shows the molecular structure of [Os13Ag9(CO)48]2 (1).{§
The metal framework consists of three ‘‘Ag3’’ units that are
arranged in a slightly distorted ‘‘ABA’’ arrangement with a
maximum deviation of 8.965u from the average Ag3 plane. No
direct bonding is observed between Ag(2) and Ag(3)
The chemistry of high-nuclearity metal clusters, in particular
heterometallic clusters, has been of increasing interest, as they serve
as good catalysts for many economically important industrial
reactions, such as the hydrogenation, hydroformylation and
carbonylation of olefins.1 Attention has turned to this class of
compounds recently because they serve as good precursors in the
synthesis of mixed-metal nanostructures.2
…
To our knowledge, only a very limited number of osmium–silver
clusters have been prepared3,4 due to the high oxidizing power of
silver salts, which may induce the oxidation of osmium clusters to
osmium metal oxides. Lewis4 reports the reaction of
[Os3(m-H)(CO)11][PPN] with [AgPF6] in refluxing THF, which
yielded an osmium–silver cluster, [{Os3(m-H)(CO)10}2Ag][PPN].
In general, silver and its mixed-metal clusters are known to form
open-structure metal cores. Only a very few examples have
demonstrated a closely packed metal structure, with the central
silver metal atoms usually stabilized by closely packed carbonyl
ligated transition metal layers.5 Silver salts react to form large
reactive cationic clusters in the presence of reducing agents that
serve as the intermediates in the formation of high-nuclearity
mixed-metal clusters.6 On the other hand, Vaska’s complex,
[Ir(PPh3)2(CO)Cl], has been studied for decades and is well-known
for its highly reversible oxygenation behaviour.7 However, it has
not been fully studied as a reducing agent. Herein, we report the
synthesis and structural characterization of two high-nuclearity
osmium–silver clusters using Vaska’s complex as the reducing
agent and for its unique irreversible oxygenation.
˚
[Ag(2) Ag(3), 3.600(3) A] or between Ag(8) and Ag(9)
…
[Ag(8) Ag(9), 3.595(2) A]. Four triosmium units are bound at
˚
the four sides of the silver core, whereas the middle trisilver unit
Ag(4)–Ag(5)–Ag(6) is edge-bridged by another osmium atom
(Os(7)) that is lying on the same plane. A similar asymmetric metal
core has been observed in another high-nuclearity osmium–
rhodium cluster, [Os12Rh9(CO)44(m-Cl)].8 The average Os–Ag
˚
bond length observed is 2.904 A, which falls into the typical range
˚
of Os–Ag bonds (2.85 to 2.95 A) that have been reported in
previous studies.4 The mean Ag–Ag bond length observed is
˚
2.856 A, with two exceptionally longer bonds in Ag(2)–Ag(4) and
˚
Ag(5)–Ag(8), of 3.187(3) and 3.149(3) A, respectively. The average
9
˚
Ag–Ag bond is shorter than that of 2.89 A in bulk silver metal,
and Dahl suggests that this type of stabilisation in the silver core
could be a consequence of the cumulative effects of many weak
delocalised Ag–Ag interactions. A similar interaction was also
observed in [Ag16Ni24(CO)40],42 which has a ccp Ag16 kernel.5
The 1H NMR spectroscopy confirms that there are no
associated hydride species, as there is no signal detected
between 240 and 0 ppm. Such species are common in high-
nuclearity clusters. The metal core of 1 with a length of about 1 nm
shows the highest nuclearity of Os–Ag clusters that has ever been
reported.
[AgPF6] (35 mg, 0.1384 mmol) was allowed to stir with Vaska’s
complex, [Ir(PPh3)2(CO)Cl] (30 mg, 0.0384 mmol), in THF (10 mL)
at room temperature until white precipitate was observed. Then,
the filtrate was transferred to another flask that contained
[Os3(m-H)(CO)11][PPN] (200 mg, 0.1358 mmol) via cannula. The
mixture was allowed to stir for another 15 min and was then
separated by TLC to yield [Os13Ag9(CO)48][PPN] (1) (86 mg,
The observation of this type of cluster with a bare metal core
that is surrounded by triosmium carbonyl units is rather rare
because triosmium clusters usually react with mononuclear
complexes in a ‘‘3 + 1’’ manner. The structure of 1 suggests that
a cationic ‘‘Ag9’’ intermediate may be formed prior to its reaction
with the triosmium anion to form a large cluster, and the
triosmium carbonyl units can therefore stabilize the Ag9
intermediate. This is a reasonable suggestion, as we have also
isolated another osmium–silver mixed-metal cluster, 2, with a
similar Ag9 skeleton.
Department of Chemistry, The University of Hong Kong, Pokfulam
Road, Hong Kong SAR, Peoples Republic of China.
E-mail: wtwong@hkucc.hku.hk; Fax: 852-2547-2933;
Tel: 852-2859-2157
{ Electronic supplementary information (ESI) available: detailed synthetic
procedures, crystallization experiments and crystallographic data in CIF.
See DOI: 10.1039/b709860j
3924 | Chem. Commun., 2007, 3924–3926
This journal is ß The Royal Society of Chemistry 2007

View Article Online
Scheme 1 Reaction pathway (only metal cores are shown for 1 and 2 for clarity).
‘‘ABABA’’ fashion with a three-fold axis. The middle trisilver unit,
Ag(4), Ag(5), Ag(6), was edge-bridged by three osmium atoms,
Os(4), Os(5), Os(6), on the same plane. The unique geometry of 2
is that the metal atoms adopt a closed-packing structure, which is
rarely observed in mixed-metal clusters.10 The mean Os–Ag bond
˚
observed is 2.857 A, which suggests a stronger Os–Ag interaction
over 1 and a more favourable and stable geometry of the highly
symmetrical triangular column in 2. However, the mean Ag–Ag
˚
bond length observed is 2.959 A, which is significantly longer than
that of 1. The two triosmium units at the two ends are capped by
an oxygen atom that becomes the centre of the three-fold axis. This
type of face-capping oxo ligand is rarely observed in mixed-metal
˚
systems. The average Os–O bond length observed is 2.049 A,
which is similar to the Os–O bond lengths with m3-O coordination
11
mode that have been reported (2.02–2.20 A). The two oxo
˚
˚
ligands, O(31) and O(32), have out-of-plane distances of 1.220 A
˚
and 1.217 A, respectively.
Fig. 1 ORTEP diagram (50% probability thermal ellipsoids) of
[Os13Ag9(CO)48]2 of 1 with the atom numbering scheme omitting the
labels on the carbonyl ligands for clarity.
Air- and photo-stable crystals of 3 were obtained from its
saturated solution in dichloromethane–methanol. Fig. 3 shows the
molecular structure of 3, which adopts a trigonal bipyramidal
geometry, with the two triphenylphosphine ligands trans to each
other. Disorder is observed for the carbonyl ligand and the
chloride atom. Their occupancy were refined and then fixed for
subsequent refinements of the structure. It is highly reminiscent of
the structure that was reported by Vaska,7 except for the dioxygen
molecule. In the reported structure, the dioxygen molecule was
added reversibly to the iridium centre, and the adduct is highly
Suitable brown block crystals of [PPN]+ salt of 2 were obtained
from the saturated solution of its [PPN]+ salt in dichloromethane–
methanol at 220 uC after 10 days. Fig. 2 shows the molecular
structure of the metal core of complex 2. The metal core consists of
five layers of the M3 unit (M = Os or Ag) that are arranged in
Fig. 2 ORTEP diagram (50% probability thermal ellipsoids) of
[Os9Ag9(m3-O)2(CO)30]2 of 2 with the atom numbering scheme omitting
the labels on the carbonyl ligands for clarity.
Fig. 3 ORTEP diagram (50% probability thermal ellipsoids) of
[Ir(PPh3)2(CO)Cl(O2)] 3, showing the disorder carbonyl ligand and
chloride at their higher occupancy positions.
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 3924–3926 | 3925

View Article Online
We have successfully demonstrated the introduction of Vaska’s
complex as a reducing agent for silver salts to yield two new high-
nuclearity osmium–silver clusters and suggested the probable
mechanism that was involved in the reaction.
We gratefully acknowledge the financial support of the Hong
Kong Research Grants Council and The University of Hong
Kong. Y.-B. Lee acknowledges the receipt of a Postgraduate
Studentship from The University of Hong Kong.
Notes and references
{ Spectroscopic data: 1: IR [n(CO), CH2Cl2] 2112 m, 2072 m, 2036 s,
2000 m. m/z: 4790 (calc. 4790, M+). 1H NMR (CD2Cl2): d 7.4–7.8 (m, 30H,
PPN).
2: IR [n(CO), CH2Cl2] 2070 m, 2038 s, 2019 m, 1985 m. m/z: 3558 (calc.
3557, M+). 1H NMR (CD2Cl2): d 7.4–7.8 (m, 30H, PPN).
3: IR [n(CO), CH2Cl2] 2052 m [n(O–O), CH2Cl2] 860 s. m/z: 812 (calc.
812, M+). 1H NMR (CD2Cl2): d 7.2–7.4 (m, 30H, PPh3).
§ Crystallographic data: Refinements of all structures were based on full-
matrix least refinement on F. Detailed experimental procedures can be
found in the supporting information{.
Fig. 4 Schematic diagram showing the proposed mechanism.
photosensitive and will readily release oxygen even in the solid
˚
state. The reported bond length is 1.30 A, which suggests a typical
superoxide ion (O22) addition. For complex 3, however, the bond
CCDC 644530–644532. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/b709860j
˚
length observed is 1.475(6) A, which suggests a peroxide ion
(O222), and the addition is totally irreversible. The iodo analog of
3, [IrI(CO)(PPh3)2(O2)],7 which also possesses an irreversible
1 B. C. Gates, Chem. Rev., 1995, 95, 511; N. Toshima and T. Yonezawa,
New J. Chem., 1998, 22, 1179; A. K. Smith and J. M. Basset, J. Mol.
Catal., 1977, 2, 229; G. L. Geoffroy and W. L. Gladfelter, J. Am. Chem.
Soc., 1977, 99, 6775; P. J. Dyson, Coord. Chem. Rev., 2004, 248, 2458.
2 A. Siani, B. Captain, O. S. Alexeev, E. Stafyla, A. B. Hungria,
P. A. Midgley, J. M. Thomas, R. D. Adams and M. D. Amiridis,
Langmuir, 2006, 22, 5160; M. W. DeGroot, H. Rosner and
J. F. Corrigan, Chem.–Eur. J., 2006, 12, 1547; P. Jena, S. N. Khanna
and B. K. Pao, Clusters and Nano-assembles, Physical and Biological
systems, Richmond, Virginia, USA, 2003.
3 B. F. G. Johnson, J. Lewis and M. Monari, J. Chem. Soc., Dalton
Trans., 1990, 3525; B. F. G. Johnson, R. Khattar, J. Lewis and
P. R. Raithby, J. Chem. Soc., Dalton Trans., 1989, 1421; S. R. Drake,
B. F. G. Johnson and J. Lewis, J. Chem. Soc., Dalton Trans., 1989, 505;
S. R. Drake, B. F. G. Johnson and J. Lewis, J. Organomet. Chem., 1988,
340, C31; S. B. Colbran, C. M. Hay, B. F. G. Johnson, F. J. Lahoz,
J. Lewis and P. R. Raithby, J. Chem. Soc., Chem. Commun., 1986, 1766;
M. I. Bruce, E. Horn, J. G. Matisons and M. R. Snow, J. Organomet.
Chem., 1985, 286, 271; J. A. Ladd, H. Hope and A. L. Balch,
Organometallics, 1984, 3, 1838.
4 M. Fajardo, M. P. Gomez-Sal, H. D. Holden, B. F. G. Johnson,
J. Lewis, R. C. S. McQueen and P. R. Raithby, J. Organomet. Chem.,
1984, 267, C25.
5 J. Zhang and L. F. Dahl, J. Chem. Soc., Dalton Trans., 2002, 1269;
V. G. Albano, L. Grossi, G. Longoni, M. Monari, S. Mulley and
A. Sironi, J. Am. Chem. Soc., 1992, 114, 5708; B. K. Teo, H. Zhang and
X. Shi, Inorg. Chem., 1994, 33, 4086; B. K. Teo and H. Zhang, J. Cluster
Sci., 2001, 12, 349; B. K. Teo, H. Dang, C. F. Campana and H. Zhang,
Polyhedron, 1998, 17, 617.
6 J. Sinzig, L. L. de Jongh, A. Ceriotti, R. della Pergola, G. Longoni,
M. Stener, K. Albert and N. Rosch, Phys. Rev. Lett., 1998, 81, 3211.
7 J. A. McGinnety, R. J. Doedens and J. A. Ibers, Science, 1967, 155, 709;
L. Vaska, L. S. Chen and C. V. Senoff, Science, 1971, 174, 587;
L. Vaska, Science, 1963, 140, 809; S. J. La Placa and J. A. Ibers, J. Am.
Chem. Soc., 1965, 87, 2581; J. A. McGinnety, R. J. Doedeats and
J. A. Ibers, Inorg. Chem., 1967, 6, 2243.
8 S. Y. W. Hung and W. T. Wong, Chem. Commun., 1997, 2099.
9 J. Donohue, The Structure of Elements, J. Wiley & Sons, NY, 1974, 209.
10 K. F. Yung and W. T. Wong, Angew. Chem., Int. Ed., 2003, 42, 553;
O. A. Belyakova and Yu. L. Slovokhotov, Russ. Chem. Bull., 2003, 52,
2299.
11 J. S. Y. Wong andW. T. Wong, Organometallics, 2003, 22,4798;K.S.Y.
Leung and W. T. Wong, J. Chem. Soc., Dalton Trans., 1997, 4357.
12 D. J. Liston, C. A. Reed, C. W. Eigenbrot and W. R. Scheidt, Inorg.
Chem., 1987, 26, 2739; J. Peone, Jr. and L. Vaska, Angew. Chem., Int.
Ed. Engl., 1971, 10, 511.
˚
oxygen addition, has been isolated with a bond length of 1.50 A
for the bound O2.7
However, reactions between Vaska’s complex and other silver
salts such as [AgBF4], [AgNO3] and [Ag2O] have ended up with
halide-replaced cationic iridium species,12 and no silver clusters or
mixed-metal clusters of osmium–silver can be isolated. Replacing
the reaction medium with chlorinated solvents such as CH2Cl2,
CHCl3 or methanol also leads to some triosmium clusters instead
of the formation of high-nuclearity osmium–silver mixed-metal
clusters. Although the mechanistic detail of these cluster buildup
reactions is usually difficult to delineate, a few key steps that are
involved can be put forward. It is likely that [AgPF6] oxidized
Vaska’s complex to give a IrII species in THF. The successive
hydrolysis of this IrII intermediate through the trace amount of
water that was present led to the iridium dioxygen species 3. A
schematic drawing of this suggested mechanism is shown in Fig. 4.
An increase in C–O stretching frequency in the IR spectrum was
observed when [AgPF6] was mixed with Vaska’s complex (from
1967 cm21 to 2052 cm21). In contrast, the halide abstraction
reaction with other silver salts (AgBF4, AgNO3) on Vaska’s
complex gave only a slight increase in the C–O stretching
frequencies (from 1967 cm21 to 1989 cm21). The role of Vaska’s
complex–H2O as a reducing agent is crucial for the formation of
the [Ag9]n+ core. Replacing Vaska’s complex with its reversible
oxygen adduct, [Ir(PPh3)2(CO)Cl(O2)], or the irreversible iodo
oxygen adduct, [Ir(PPh3)2(CO)I(O2)], did not lead to any osmium–
silver mixed-metal clusters. The formation of 3 from hydrolysis is
evident in 18O-labelling studies. A 250 mL sample of 18O-enriched
water was introduced in a pre-dried THF solvent for the
preparation. Infrared spectroscopic study shows that 3 displayed
three absorption bands at 856, 833 and 808 cm21 that correspond
to the 16O–16O, 16O–18O and 18O–18O stretching vibrations,
respectively. The results are in good agreement with reports and
calculations13 in the literature and, therefore, confirm that the
oxygen atom in water was oxidized and bound to the iridium
centre to give complex 3.
13 K. Takao, Y. Fujiwara, T. Imanaka, M. Yamamoto, K. Hirota and
S. Teranishi, Bull. Chem. Soc. Jpn., 1970, 43, 2249.
3926 | Chem. Commun., 2007, 3924–3926
This journal is ß The Royal Society of Chemistry 2007