Synthesis and reactivity studies of cyclopentadienyl derivatives of ruthenium iridium and osmium iridium mixed metal clusters 2

Chapter 2
Synthesis of Cp- and Cp*- containing ruthenium-iridium
and osmium- iridium mixed metal clusters
2.1 Introduction
Although the chemistry of mixed metal platinum group metals have been extensively
studied, a greater proportion of these studies have involved clusters of Ru, Pd and Pt
mixed metal framework. Tetrahedral clusters containing Rh, Os, Ir or Ru mixed metal
framework have been less extensively studied. Very few clusters containing a Ru
3
Ir or
Os
3
Ir mixed metal frame work have been reported in the literature. Recently Suss-
Fink and coworkers have reported a very high yield synthesis of the carbonyl cluster
anions [Ru
3
Ir(CO)
13
]
-
and [Os
3
Ir(CO)
13
]
-
by a redox condensation reaction [1, 2]. The
reaction sequences are shown in Schemes 2.1 and 2.2, respectively.
Os
3
(CO)

12
+ [Ir(CO)
4
]
-
-3CO
Os
Os
Os
Ir
+H
+
Os
Os
Os
Ir
H
+H
2
-CO
Os
Os
Os
Ir
H
H
-
+H
-
Os

Os
Os
Ir
H
H
H
+H
2
-CO
-

Scheme 2.1.

27
Ru
3
(CO)
12
+ [Ir(CO)
4
]
-
-3CO
Ru
Ru
Ru
Ir
+H
+
Ru Ru

Ru
Ir
H
+H
2
-CO
-
+H
-
+H
2
-CO
-
Ru
Ru
Ir
RuH
H
+H
-
Ru
Ru
Ru
Ir
H
H
H

Scheme 2.2.
Another interesting synthesis reported involves salt elimination between

Ir((CO)Cl(PPh
3
)
2
and Na[Ru
3
H(CO)
11
] in THF at room temperature to yield several
Ru-Ir mixed metal clusters (Scheme 2.3)

[3, 4].
+
Na[Ru
3
H(CO)
11
]
[Ir(CO)Cl(PPh
3
)
2
]
Ru
4
H
2
(CO)
12
(PPh

3
)
[Ru
3
(CO)
12
]
Ru
2
IrH
2
(CO)
10
(PPh
3
)
2
Ru
4
H
4
(CO)
11
(PPh
3
)
Ru
3
IrH
3

(CO)
10
(PPh
3
)
2
Ru
2
IrH
3
(CO)
11
(PPh
3
)
Ru
3
IrH(CO)
12
(PPh
3
)
Ru
2
IrH
3
(CO)
11
(PPh
3

)
I
II
III
IV
V

Scheme 2.3.

28
The low temperature
1
H NMR of cluster III suggested the presence of two isomers
IIIa and IIIb in solution. In Cluster IIIa, (structurally characterized in the solid state)
two of the bridging hydrides were found to be equivalent but different from the third,
whereas in cluster IIIb, all three hydrides were found to be nonequivalent. The
structures of the two isomers are shown Figure 2.1.

IIIa IIIb
Figure 2.1. Two different isomeric forms of Ru
3
Ir(CO)
11
PPh
3
in solution.

Among the very few literature reports available on tetranuclear clusters containing the
Ru
3

Ir and Os
3
Ir mixed metal framework, only a handful are on clusters containing a
Cp or Cp* ligand. Table 2.1 summarizes the known tetranuclear Ru
3
Ir and Ru
3
Rh, as
well as Os
3
Ir and Os
3
Rh clusters possessing either a Cp or Cp* ligand, along with the
isolated yields and
1
H NMR chemical shifts.






29
Table 2.1. Known Cp and Cp* containing Ru
3
Ir and Ru
3
Rh tetranuclear clusters.
1
H NMR

δ ppm Cluster and yield reported
Shape & total
electron count
Cp*/Cp M-H-M
Ref.
Cp*IrRu
3
(μ-H)
4
(CO)
9
(Crystal structure not reported)
5%
Tetrahedral
60
2.20 -18.71 [5]
Ir
B
Ru
Ru
(OC)
3
Ru(CO)
3
(CO)
3
CO
H
H
H

(Two isomers reported) 15%
Open butterfly
or spike
triangle
64
1.94 -17.95
-18.64
[5]
Rh
B
Ru(CO)
3
H
H
Ru
Ru(CO)
3
H
(OC)
3
(Proposed structure) 40%
Butterfly
62
2.07 -20.29 [5]
Ru
Ru
Rh
H
Ru

HH
H

20%
Tetrahedral
60
1.71 -15.93 [6]

30
Ru
Ru
Rh
Ru
H
H

45%
Tetrahedral
60
5.16 -17.2 [6]
Ru
Ru
Rh
Ru
H
H

(Isomers reported) 15%
Tetrahedral
60

1.63, -13.0, -17.0
1.54 -20.3

[6]
(Cp*)
2
Rh
2
Ru
2
(CO)
7
(Crystal structure not reported)
30%
- 1.64 [6]
Os
Os
Ir
Os
CO


Planar
62
2.03 [7]
Os
Os
Ir
Os
H

H

21%
Tetrahedral
60
5.8 -18.95,
-21.80
[8]
Os
Os
Rh
Os
H
H

12%
Tetrahedral
60
-1.89 -16.53,
-20.08
[9]

31
Os Os
Rh
Rh
OC
H
H

7.2%
Tetrahedral
60
1.94, 1.92 -14.57,
-18.80
[9]
Os Os
Rh
Rh
OC
CO

40%
Tetrahedral
60
1.70 [9]
Os
Os
Rh
H
Os
HH
H

36%
Tetrahedral
60
1.94 -15.17,
-19.68
[9]

Os
Os
Rh
CO
Os
OC

20%
Tetrahedral
60
2.19 [10]
Os
Os
Rh
CO
Os
H
Cl

25%
Tetrahedral
60
1.57 -14.06 [10]


32
Tetranuclear clusters containing Os
3
Ir and Ru
3

Ir mixed metal framework possessing a
Cp or Cp* ligand seems to be unexplored. The next section of the thesis describes the
attempts made to synthesize Ru
3
Ir amd Os
3
Ir clusters containing a Cp or Cp* ligand.
2.2 Reactions of cyclopentadienyl iridium dicarbonyls with
Ru
3
(CO)
12

Attempts were made to synthesize Cp*IrRu
3
clusters by reacting various Cp*Iridium
complexes with triruthenium clusters. The reaction of Ru
3
(CO)
12
with acetonitrile in
the presence of TMNO led to the formation of the bis acetonitrile derivative of
ruthenium, Ru
3
(CO)
10
(CH
3
CN)
2.

, which was subsequently reacted with Cp*Ir(CO)
2
at
ambient temperature and also under photolytic conditions. IR monitoring of the
reactions did not suggest the formation of any new product. Decomposition of
Ru
3
(CO)
10
(CH
3
CN)
2
was observed during the reaction and the IR spectrum showed
strong peaks due to unreacted Cp*Ir(CO)
2
.

Ionic coupling between Cp*Ir(CH
3
COCH
3
)
3
]
2+
or [Cp*Ir(CH
3
CN)
3

]
2+
with
[Ru
3
(CO)
11
]
2-
afforded an unidentified brown solid. An attempt at chromatographic
separation of the product mixture yielded several bands in low yields which have not
been identified. Attempted purification by solvent extraction also afforded a mixture.
The reaction of Cp*Rh(CO)
2
with Ru
3
(CO)
12
in the presence of hydrogen has been
reported by Knobler and coworkers to yield Cp*RhRu
3
(μ-H)
2
(CO)
10
[6]. Attempts to
synthesize the iridium analogue by reacting Cp*Ir(CO)
2
and CpIr(CO)
2

with
Ru
3
(CO)
12
in the presence of molecular hydrogen afforded new ruthenium-iridium
mixed metal clusters in moderate yields. The synthetic procedures, yields and
characterization of the new clusters will be discussed in the following sections.

33
2.2.1 Reaction of Cp*Ir(CO)
2
with Ru
3
(CO)
12
and H
2
3a, and Cp*IrRuThe mixed–metal clusters Cp*IrRu
3
(μ-H)
2
(CO)
10, 3
(μ-H)
4
(CO)
9,
4a,
are formed in 40% and 13% yields, respectively, when H

2
is bubbled through
solutions of Ru (CO)
3 12
and Cp*Ir(CO) at 70-90 ºC (Scheme 2.4).
2

Ru
Ru
Ir
Ru
H
H
Ru
3
(CO)
12
+ Cp*Ir(CO)
2
H
2
1 atm
70 °C
+
1 2a 3a 4a
Ru
Ru
Ir
H
Ru

H
H
H

Scheme 2.4.
Cluster 3a is stable in air in the solid state for a few days. It is sparingly soluble in
hexane and completely soluble in dichloromethane. The IR spectrum of 3a in hexane
solution exhibited a peak at 1788 cm
-1
, showing the presence of bridging carbonyl.
The IR pattern was similar to that of the related cluster Cp*RhRu (H) (CO)
3 2 10
[6]. A
FAB-MS spectrum showed a very strong molecular ion peak at m/z = 911.8 and
fragment clusters of peaks corresponding to successive loss of up to 10 carbonyls.
Diffraction-quality crystals were obtained from hexane by slow cooling.
The molecular structure of 3a is shown in Figure.2.2. In the solid state structure, one
of the two hydrides is found bridging an edge of the Ru
3
triangle and the second
hydride is found bridging a ruthenium-iridium edge. The molecule is asymmetric and
the two hydrides are inequivalent. However, the ambient temperature proton NMR
spectrum shows a single sharp hydride signal at δ -18.21 ppm. A VT
1
H NMR
experiment showed broadening of the hydride resonance on cooling, but no
decoalescence was observed even down to 190 K.


34

Figure 2.2. ORTEP diagram of Cp*IrRu
3
(μ-H)
2
(CO)
10
, 3a. Thermal ellipsoids are
drawn at 50% probability level. Organic hydrogens are omitted for clarity.

The cluster 4a is stable in air for a short period of time. Slow decomposition to an
insoluble black solid was observed after a few hours. This cluster was previously
reported as a by product from the reaction of [Cp*IrCl ]
2 2
and
[(NPPh
3
)
2
][Ru
3
(CO)
9
(B
2
H )] by Galsworthy et.al. [5]; it was characterized by
1
5
H
NMR, IR and MS. However, the solid state structure of the compound was not

reported. We have determined the molecular structure of 4a by an X-ray
crystallographic analysis. The ORTEP plot of 4a is shown in Figure 2.3.


35

Figure 2.3. ORTEP diagram of Cp*IrRu
3
(μ-H)
4
(CO)
9
, 4a. Thermal ellipsoids are
drawn at 50% probability level. Organic hydrogens are omitted for clarity.

The solution IR spectrum recorded in hexane shows bands only in the terminal
carbonyl region, consistent with the solid-state structure. At room temperature, the
proton NMR spectrum in deuterated toluene showed a singlet at δ 1.79 ppm which
could be assigned to the Cp* ligand and another sharp singlet at δ -18.59 ppm
assignable to rapidly exchanging hydrides; on cooling the solution, the hydride signal
broadens, but no decoalescence was observed down to 200 K. A FAB-MS spectrum
of the crystals showed a very strong molecular peak at m/z = 887.


36
2.2.2 Reaction of CpIr(CO)
2
with Ru
3
(CO)

12
and H
2
The analogous reaction of Ru (CO)
3 12
with CpIr(CO)
2
in the presence of hydrogen
afforded only 3b in 46% yield; the tetrahydrido cluster, CpIrRu (μ-H) (CO)
43 9
,
analogous to 4a was not formed even after refluxing the reaction mixture for 10 h.
Replacing the Cp* ring with a Cp ring could have possibly influenced the reactivity.
Cp*Ir(CO)
2
being a relatively stronger nucleophile than CpIr(CO)
2
could have
probably favored further reactivity to afford 4a.
The solid state structure of 3b is similar to that of cluster 3a except for the position of
the bridging hydrides (Figure 2.4).



Figure 2.4. ORTEP diagram of CpIrRu
3
(μ-H)
2
(CO)
10

, 3b. Thermal ellipsoids are
drawn at 50% probability level. Organic hydrogens are omitted for clarity.

37
The related Cp- containing clusters, CpRhRu (H)
3 2
(CO) and CpIrOs (H) (CO)
10 3 2 10,
had
similar disposition of hydrides [6, 8]. The IR spectral profile of 3b in hexane was
similar to those of the above related clusters. The IR spectrum in hexane solution
exhibited a peak at 1820 cm
-1
, showing that the bridging carbonyl persisted in
solution. The
1
H NMR spectrum of 3b recorded at room temperature showed a singlet
at δ 4.84 ppm due to Cp protons and a singlet at δ -17.84 ppm due to the bridging
hydrides. It was not possible to observe the low temperature limiting NMR spectrum
even at 200 K as in the case with 3a and 4a. A FAB-MS spectrum showed a very
strong molecular ion peak at m/z = 842.4. The compound was highly unstable and
decomposed to a black insoluble compound even in the solid state after a few hours.
Reactions of cluster 3a with 1 atmosphere hydrogen for 6 h yielded cluster 4a in
about 60-70% yields. However, quantitative conversion was not observed even after
prolonged heating. The reverse reaction of 4a under 1 atmosphere CO resulted in
cluster fragmentation with the formation of Ru (CO)
3 12
and Cp*Ir(CO)
2
indicating that

the metal-metal bonds in these clusters are not very strong (Scheme 2.5). This type of
break down of cluster skeleton was previously reported in Cp*RhRu
(μ-H) (CO)
3 2 10

[6].
Ru Ru
Ir
Ru
H
70 °C /
Toluene
+ H
2
Ru Ru
Ir
Ru
H
H
H
H
+ CO
3a 4a
H

Scheme 2.5.

38
2.3 Reactions of cyclopentadienyl iridium dicarbonyl with triosmium
clusters

Triosmium based mixed metal clusters have been previously synthesized from the
formally unsaturated hydrido triosmium cluster Os
3
(μ-H)
2
(CO)
10
, by procedures that
exploit its Lewis acid character [11-16]. In the preparation of the mixed metal cluster,
FeOs
3
(μ-H)
2
(CO)
13
, it has been shown that Os
3
(μ-H)
2
(CO)
10
can also function both as
a Lewis acid and a Lewis base (Scheme 2.6) [17, 18].
Os
3
(μ-H)
2
(CO)
10
+ [Fe(CO)

4
]
2-
Os
3
(μ-H)
2
Fe(CO)
13
(1)

Os
3
(μ-H)
2
(CO)
10
+ [Fe
2
(CO)
9
]
(2)Os
3
(μ-H)
2
Fe(CO)
13

Scheme 2.6.

Shore and coworkers have synthesized mixed metal clusters of the type CpMOs
3
(μ-
H)
2
(CO)
10
(M=Co, Rh, Ir) by reacting the appropriate metal carbonyls CpM(CO)
2

with Os
3
(μ-H)
2
(CO)
10
under thermal or photolytic conditions [8, 9, 19]; the tetrahedral
cluster CpIrOs
3
(μ-H)
2
(CO)
10
, was synthesized in 21% yield. The Cp* analogue has
not been reported. Since we were interested in a comparative study of the chemistry
and reactivity of Cp*Ru
3
Ir and Cp*Os
3
Ir clusters, attempts were made to synthesize

Cp*- containing triosmium-iridium clusters. The results obtained along with the
characterization of the novel clusters will be discussed in the following sections.





39
2.3.1 Reaction of Cp*Ir(CO)
2
with Os
3
(μ-H)
2
(CO)
10
The reaction of Os
3
(μ-H)
2
(CO)
10
, 5, with Cp*Ir(CO) , 2a, in toluene at
2
120 ºC
afforded the mixed-metal cluster, Cp*IrOs (μ-H) (CO)
3 2 10
, 3c, as a red, air-stable solid
in

71% yield, and Cp*IrOs
4
(μ-H)
2
(CO)
13
, 7, in trace amounts (Scheme 2.7).
5 2a 3c 7
toluene
120 °C
Os
Os
Os
H
H
+
Ir
CO
CO
Os Os
Ir
Os
H
H
Os
Os
Os
Os
Ir
H

H
+

Scheme 2.7.
The infrared spectrum of cluster 3c exhibited a pattern similar to those of the related
clusters CpMOs (μ-H) (CO) (M = Co, Rh, Ir)
3 2 10
(Figure 2.5 and Table 2.2) [6, 8, 9,
19]. However, the terminal and bridging carbonyl stretching frequencies are much
lower in 3c, and may be attributed to the comparatively higher basicity

of the Cp*
ring The stretching frequency
.
values were comparable to the values of the closely
related cluster, Cp*RhOs
3
(μ-H) (CO)
2 10
[9]. The FAB-MS spectrum showed a very
strong molecular ion peak at m/z = 1180.8 and fragment clusters of peaks
corresponding to successive loss of up to 10 carbonyls. Crystals of suitable quality
were grown from hexane at -30 ºC to further confirm the structure by X-ray analysis.
The ORTEP plot of 3c is shown in Figure 2.6.

40

Figure 2.5. IR spectrum of 3c recorded in hexane.

Table 2.2. ν

CO
stretching frequencies of known CpMOs
3
clusters.
Cluster
ν
CO
cm
-1
CpIrOs
3
(μ-H)
2
(CO)
10
2091m, 2070m, 2044vs, 2004vs, 1992m(sh), 1974m,
1965m, 1824w
CpRhOs
3
(μ-H)
2
(CO)
10
2083m, 2063vs, 2042vs, 2010vs, 2000s(sh), 1982m,
1972m, 1819m
Cp*RhOs
3
(μ-H)
2
(CO)

10
2084m, 2064s, 2041s, 2002s, 1969w, 1962w, 1792m


41

Figure 2.6. ORTEP diagram of Cp*IrOs
3
(μ-H)
2
(CO)
10
, 3c. Thermal ellipsoids are
drawn at 50% probability level. Organic hydrogens are omitted for clarity.

The proton NMR spectrum of 3c at room temperature consists of a single sharp peak
at δ1.72 ppm which could be assigned to the Cp* group and two broad peaks in the
hydride region indicating their non-equivalence and also their fluxional nature. On
lowering the temperature to 233 K the two broad signals sharpen and appear at δ -
20.66 and -17.65 ppm for H
a
and H
b
(Figure 2.7). These assignments were made
based on related compounds where it has been observed that osmium-osmium
bridging hydrides cis to a carbonyl bridge experience chemical shifts at higher field
than -20 ppm whereas osmium-osmium bridging hydrides not cis to a bridging
carbonyl have chemical shifts at lower field than -20 ppm [14, 16, 20]. Presumably
the exchange mechanism is the same as that for the RhRu
3

analogues [21].

42
Os Os
Ir
Os
H
a
H
b
-20.66 ppm
-17.65 ppm

Figure 2.7. Tentative of
1
H NMR assignments for the bridging hydrides in 3c.
The pink band which was isolated from the column with hexane and dichloromethane
in a very low yield was identified as Cp*IrOs
4
(μ-H)
2
(CO)
13
, 7. The IR spectrum of 7
recorded in dcm solution showed the presence of terminal carbonyls. X-ray
diffraction-quality crystals were obtained by slow diffusion of hexane into a
dichloromethane solution at -30 ºC. The ORTEP plot of 7 showing the atomic
labeling scheme, and selected bond parameters, is shown in Figure 2.8.
The structure is similar to that of CpRhOs
4

(μ-H)
2
(CO)
13
reported by Shore and
coworkers [22]. The metal core of cluster 7 consists of a tetrahedral Os
4
metal
framework, edge bridged by a Cp*IrCO fragment. The cluster contains a total of 74
valence electrons which is consistent to that expected for an edge bridged tetrahedron
according to EAN rule. Three carbonyl ligands are attached to each osmium atom.
The iridium atom is attached to a Cp* ring and a carbonyl. All the carbonyls are
terminal in nature. Bridging hydrides were found to span the Os(2)-Os(4) and Os(2A)
- Os(4) edges which at 2.9459(3) Å are the longest Os-Os distances in this structure.
The osmiums which are bridged by iridium have the shortest Os-Os distance [Os(2)-
Os(2A) = 2.7764(4) Å].
The proton NMR spectrum is consistent with the molecular structure obtained in the
solid state. At 300 K (CDCl
3
)
,
the singlet at δ 1.85 ppm could be assigned to the Cp*
protons and the sharp singlet at δ -19.74 ppm to the 2 equivalent bridging hydrides.

43

Figure 2.8. ORTEP plot of Cp*IrOs
4
(μ-H)
2

(CO)
13
, 7 and selected bond parameters.
Thermal ellipsoids are drawn at 50% probability level. Organic hydrogens are omitted
for clarity. Os(1)-Os(2A) = 2.7764(4) Å; Os(2)-Os(3) = 2.8261(3) Å; Os(2)-Os(4) =
2.9459(3) Å; Os(3)-Os(4) = 2.7848(4) Å; Ir(1)-Os(2) = 2.8109(3) Å; Ir(1)-Os(2)-
Os(4) = 97.098(9)º; Ir(1)-C(11)-O(11) = 170.3(8)º; Ir(1)-Os(2)-Os(3) = 120.835(8)º;
Os(2)-Ir(1)-Os(2A) = 59.19(1)º; Os(2A)-Os(3)-Os(2) = 58.842(10)º.
(μ-H) (CO) is often present as an impurity in Os
The tetranuclear cluster Os
4 4 12 3
(μ-
H)
2
(CO)
10
and hence could be the precursor for 7. The reaction of Os
4
(μ-H) (CO)
4 12
with Cp*Ir(CO)
2
under similar conditions, however, afforded 7 only in trace amounts
thus ruling out this possibility. The reaction of cluster 3c with hydrogen (100 psi) in a
stainless steel autoclave at 120 ºC for 24 h afforded Cp*IrOs , 4b(μ-H) (CO)
3 4 9
as a
dark orange, air-stable solid in 90% yield after chromatographic separation (Scheme
2.8). This thus indicated that the formation of 7 did not proceed via 3c either.

44
Os Os
Ir
Os
H
H
120 °C
Toluene
+ H
2
Os Os
Ir
Os
H
H
H
H
+ CO
4b
3c

Scheme 2.8.
The IR spectral profile of 4b was similar to Cp*RhOs
3
(μ-H)
4
(CO)
9
[9]. The molecular
structure was further confirmed by X-ray crystallographic analysis; the ORTEP plot

of 4b is shown in Figure 2.9.

Figure 2.9. ORTEP diagram of Cp*IrOs
3
(μ-H)
4
(CO)
9
, 4b. Thermal ellipsoids are
drawn at 50% probability level. Organic hydrogens are omitted for clarity.
The proton NMR spectrum of 4b recorded at room temperature shows a singlet at δ -
19.27 ppm, suggesting the fluxional nature of the hydrides (Figure 2.10). On cooling
the sample to 195 K a total of five signals could be seen, of which two singlets of

45
equal intensity at δ -17.64 and δ -20.22 ppm could be assigned to structure (II). The
singlets at δ -18.36 and δ -20.44 and δ -20.92 ppm of relative intensities 1:2:1 could
be assigned to the structure (I), which is that observed in the solid state. In solution,
structure (II) appears to be the predominant isomer (Figure 2.11). The two isomers are
present in a 1.0:3.7 ratio in solution. The chemical shift assignments were made based
on observations by earlier workers that the chemical shift for a metal-hydride bridging
an Os-Os edge lies at a higher field than that bridging an Ir-Os edge [23, 24].
In the
1
H EXSY spectrum of 4b recorded in d
8
-toluene (τ
m
= 0.1 sec) at 200 K,
exchange cross peaks were seen between all the five resonances indicative of hydride

exchange that results in isomerisation between the two isomers as well as hydride
exchange within the same isomer (Figure 2.12). Since the system seemed too
complicated to derive thermodynamic and kinetic parameters, we have attempted to
propose a set of exchange pathways to account for the observed cross peaks in the
1
H
EXSY spectrum.
A plausible set of exchanges which involve either single hydride migration, or two
hydride migrations (either simultaneous or step-wise) that can account for the
observed exchange cross peaks is depicted in Scheme 2.9. The cross peak between Hb
and He can only be accounted for by a more complex exchange mechanism, and the
weaker intensity compared to the others is consistent with that.




46

Figure 2.10.
1
H VT NMR of 4b recorded in d
8
- toluene.

Os
Os
Ir
Ha
Os
Hc Hc

Ha
Os Os
Ir
Hd
Os He
Hb
Hd
structure(I) structure(II)
δ -18.36
δ -20.44
δ -20.92
δ -17.64
δ -20.22

Figure 2.11. Isomeric structures of cluster 4b.


47
He
He
Hd
Hd
Hb
Hb
Hc
Hc
Ha
Ha

Intramolecular exchange cross peaks

Intermolecular exchange cross peaks
Ha ↔ Hc

Ha ↔ He
Hb ↔ He

Ha ↔ Hd
Hb ↔ Hd

Ha ↔ Hb
Hd ↔ He

Hb ↔ Hc
Hc ↔ He

Hc ↔ Hd

Figure 2.12.
1
H EXSY spectrum of 4b recorded in d
8
-toluene at 200 K.


48
Os
Os
Ir

Ha
OsHc Hc
Ha
Os
Os
Ir
Hd
Os He
Hb
Hd
Os
Os
Ir
Hd
Os He
Hb
Hd
Intermolecular exchange pathways
Intramolecular exchange pathways
One hydride migration
Crosspeaks: a-b, a-d, c-d, c-e
Two hydride migrations
Os
Os
Ir
Ha
OsHc
Hc
Ha
Crosspeaks: b-c, a-e

Os
Os
Ir
Hd
Os He
Hb
Hd
Os
Os
Ir
Hb
OsHe
Hd
Hd
Crosspeaks: b-d, d-e
Os
Os
Ir
Hd
Os He
Hb
Hd
Crosspeak: b-d
Os
Os
Ir
Ha
OsHc Hc
Ha
Crosspeak: a-c

Scheme 2.9.
2.3.2 Reaction of Cp*Ir(CO)
2
with Os
3
(CO)
10
(CH
3
CN)
2

The mixed-metal cluster Cp*IrOs
3
(CO)
11
, 9 has been isolated in 20% yield by the
reaction of Os
3
(CO)
10
(CH
3
CN)
2
, 6,

with Cp*Ir(CO)

2
, 2a.

The IR spectrum of the
crude reaction mixture in dcm showed signals due to Cp*(CO)IrOs
3
(CO)
11
,
previously reported by

Pomeroy and coworkers from the reaction of Os
3
(CO)
10
(COE)
2

(COE = cyclooctene) with Cp*Ir(CO)
2
[7]. On subjecting the reaction mixture to TLC
on silica-gel plates, Cp*IrOs
2
(CO)
9
, 8, and Cp*IrOs
3
(CO)
11,
9, were obtained in

addition to several other bands in low yields which have not been identified. Cluster 8

49
has also been obtained in 28% yield by Pomeroy and coworkers from the reaction of
Cp*Ir(CO)
2
and Os (CO)
4
(COE) [25]. IR monitoring of the reaction indicated that 9 is
not formed directly in the reaction but has resulted from decomposition. Addition of
silica-gel to the crude reaction mixture and stirring for two hours did not show peaks
due to 9 thus ruling out the possibility of decomposition on silica-gel. It was observed
that the formation of 9 occurred when the solvent was removed under reduced
pressure from the crude reaction mixture thus suggesting that Cp*(CO)IrOs
3
(CO)
11
could be the precursor which subsequently undergoes decarbonylation under vacuum
to give 8 and 9 (Scheme 2.10).
Os
Os
Ir
CO
Os
OC
Os
3
(CO)
10
(CH

3
CN)
2
+ Cp*Ir(CO)
2
-CO
Os Ir
Os
+
9
Os
OsOs
Ir
8

Scheme 2.10.
Cluster 9 has been characterized spectroscopically as well as by a single crystal X-ray
diffraction study; the ORTEP plot is given in Figure 2.13. The molecular structure of
9 consists of a tetrahedral metal core, with six metal-metal bonds as expected from
polyhedral skeletal electron pair theory (PSEPT) for a 60 electron metal cluster
compound. Each osmium atom is linked to three terminal carbonyl ligands. Two
bridging carbonyl ligands are found, along the Os(2)-Ir and Os(2A)-Ir bonds. The
carbonyl bridged iridium-osmium bonds [Ir(1)-Os(2) = 2.7803(6) Å; Ir(1)-Os(2A) =
2.7803(6) Å] are longer than the unbridged bonds [Ir(1)-Os(3) = 2.7734(7) Å]. The

50
molecule contains a crystallographic mirror plane that passes through Ir(1), Os(3) and
the midpoint of the Os(2)-Os(2A) bond. The overall structure is similar to that of
Cp*RhOs
3

(CO)
11
[10]
.
The bond separation of Ir(1)-C(12) [2.108(9) Å] is
considerably longer than the Os(2)-C(12) [2.058(9) Å] bond, in contrast to that in
Cp*IrOs
3
(μ-H)
2
(CO)
10
, 3c, where the corresponding Ir-C bond length (connecting
the bridging carbonyl) is shorter than the Os-C bond length [Ir-C = 1.910(5) Å; Os-C
= 2.239(5) Å].

Figure 2.13. ORTEP diagram of Cp*IrOs
3
(CO)
11
, 9. Thermal ellipsoids are drawn at
50% probability level. Organic hydrogens are omitted for clarity. Ir(1)-Os(2) =
2.7803(6) Å; Ir(1)-Os(3) = 2.7734(7) Å; Os(2)-Os(3) = 2.7663(6) Å; Os(2)-C(12) =
2.058(9) Å; Ir(1)-C(12) = 2.108(9) Å; O(12)-C(12) = 1.172(11) Å.


51