Synthesis and reactivity studies of cyclopentadienyl derivatives of ruthenium iridium and osmium iridium mixed metal clusters 3

Chapter 3
Reactivity of Cp*- containing ruthenium-iridium and
osmium-iridium mixed metal clusters towards alkynes
3.1 Introduction

Mixed-metal alkyne clusters have generated much interest in recent years largely due
to their catalytic potential in hydrogenation reactions [1]. They have been proven to
serve as models for the carbon-carbon triple bond activation on metal surfaces and
also for chemisorption of small molecules on metal surfaces [2-7]. Braunstein and co-
workers have recently found that silica-tethered alkyne mixed metal clusters obtained
by the sol-gel method have the potential to function as precursors to bimetallic nano
particles (Scheme 3.1) [8].

Scheme 3.1.
71

The reaction of tetrahedral clusters with internal and terminal alkynes often results in
cluster opening to give butterfly structures [9]. The butterfly clusters are quite
interesting because they represent an intermediate arrangement between the
tetrahedral clusters and the planar clusters. The relationship between tetrahedral,
butterfly and “spiked” triangular clusters is depicted in Scheme 3.2.

M
M
M
M
MM
MM
MM
MM
Tetrahedron

Butterfly
"Spiked" triangle
60-e
62-e
64-e

Scheme 3.2.

Sappa and coworkers have classified butterfly clusters into 13 classes (A-M).
Tetrahedral clusters are known to react with alkynes to give class B butterfly clusters
where the M
3
M’ skeleton takes the form of a butterfly and the alkyne C
2
unit bonds to
the metal framework in a μ
4
,η
2
fashion to form a quasi-octahedral M
3
M’C
2
skeleton
(Figure 3.1) [10-12]
. The acetylenic C≡C bond is disposed parallel to the “hinge”
metal–metal bond of the butterfly, and the alkyne interacts with all four metal atoms.
C
C
M'

M
M
M
Wing tip
metal atoms
π
σ
π
π
σ
π
Hinge
metal atoms

Figure 3.1. M
3
M’C
2
butterfly cluster core.
72
The alkyne is coordinated to the two “hinge” metal atoms via σ bonds and to the two
“wing-tip” metal atoms via its π bonds. The total electron count for these clusters
depends on the method used for counting. Considering the alkyne as a four-electron
donor, as is conventional in the EAN formalism, these clusters are 60-electron
systems. This gives an implication that they are electron deficient, since a M
3
M’
butterfly cluster consistent with the noble gas rule would require 62 electrons.
However, according to Wade’s system, they are considered as octahedral M
3

M’C
2
clusters, with each CR unit donating three electrons to the skeletal bonding. The
clusters are therefore 62-electron “electron precise” butterflies or 14-electron closo
octahedra [13].
The acetylenic C-C bond lengths in the coordinated “alkyne” in these clusters show
considerable variations, ranging from 1.34 to 1.56 Å, indicating considerable bond
lengthening and possible deactivation. Following a criterion used by Muetterties, the
elongation of the alkyne C≡C bond after coordination to more than one metal centre is
taken as a parameter of activation upon coordination and used for comparison with the
behaviour of these molecules on surfaces [14]. According to his hypothesis, the
stronger the interaction of the alkyne with the cluster, either for σ−bound or for σ-π-
bound alkynes, the greater is the probability of finding long C-C bond distances,
sometimes close to C-C single bond. The nature of the metal is also thought to play a
role in the activation process with the longest C-C bonds observed in the complexes
of the heaviest metals [9, 15, 16]. The dihedral angle between the two M’MM planes
usually lies in very narrow range between 112º and 118º, because of the restrictions
imposed on the metal framework by the coordinated alkyne.
Various isomers are possible for the M
3
M’C
2
skeleton and for the coordinated
alkynes. It can be seen that in Figure 3.2 (a) and (c) the alkyne is disposed parallel to a
73
Table 3.1 summarises some of the reactions of tetrahedral mixed metal clusters with
alkynes. It can be seen that most heterometallic tetrahedral clusters reacted with
alkynes to afford butterfly clusters. Most of these reactions afforded products
exhibiting either hinge-apex or alkyne, or both types of isomerisms. In some cases the
reactions were found to be highly stereoselective. For example, the reaction of

[CpMRu
3
(CO)
12
]⎯ (M=W, Mo), IrRu
3
(μ-H)(CO)
13
and [Ru
3
Ir(CO)
13
]
⎯
towards internal
alkynes afforded
M-Ru (M = W, Mo and Ir ) clusters with alkyne insertion into
the Ru-Ru bond as the only product. However, the reaction of [CpMRu
3
(CO)
12
]⎯
(M=W, Mo) clusters with phenyl acetylene afforded
M-Ru cis and trans isomers.
74
heterometallic MM’ bond (MM’) whereas in (b) the alkyne is disposed parallel to a
homometallic MM bond (MM) and thus they are related as hinge-apex isomers; (c)
differs from both (a) and (b) in the orientation of the alkyne and thus they are related
as alkyne isomers.
Figure 3.2. (a), (b) Metal hinge-apex isomers (c) alkyne isomer.

C
C
M'
M
M
M
R
R'
C
C
M
M
M
M'
R
R
'
C
C
M'
M
M
M
R'
R
(a)
(b)
(c)

75
Table 3.1. Reactions of tetrahedral mixed-metal clusters with alkynes.
Product
Cluster
Alkyne
Substrate
Isomerism
observed
Conditions
Major isomer Minor isomer
Ref.
CpRhRu
3
(μ-H)
4
(CO)
9

PhC≡CPh

EtC≡CEt
hinge apex
25 ºC THF
2-4 d
Ru-Ru

Ru-Ru
Ru-Rh

Ru-Rh

[17]
Cp*RhRu
3
(μ-H)
4
(CO)
9

PhC≡CPh -
50 ºC THF
2 h
Ru-Ru - [17]
FeRu
3
(μ-H)
2
(CO)
13
PhC≡CPh

MeC≡CMe

PhC≡CMe
hinge apex

hinge apex

hinge apex,
alkyne

Hexane reflux
15 min
Fe-Ru, 48%

Fe-Ru, 5%

Fe-Ru, (Ph cis to Fe) 31%
Fe-Ru, (Ph trans to Fe) 10%

Ru-Ru,
17%
Ru-Ru,
25%
Ru-Ru,
41%


[18]
[MoRu
3
Cp(CO)
12
][PPh
4
]
MeC≡CMe
PhC≡CH
-
alkyne THF reflux, 2 h
Mo-Ru, 65%

Mo-Ru (Ph cis to Mo)
Mo-Ru (Ph trans to Mo)

}50%
-
[19]
[WRu
3
Cp(CO)
12
][PPh
4
]
MeC≡CMe
PhC≡CH
-
alkyne THF reflux, 2 h
W-Ru, 70%
W-Ru (Ph cis to W)
W-Ru (Ph trans to W)

}45%
-
[19]
[RuCo
3
(CO)
12
]⎯
PhC≡CPh

PhC≡CH
-
alkyne THF reflux, 7 h
Co-Ru, 75%
Co-Ru (Ph cis to Co)
Co-Ru (Ph trans to Co)

}73%
-
[8]
IrRu
3
(μ-H )(CO)
13
PhC≡CPh
MeC≡CMe
-
Hex, 80 ºC, 1 h
Hex, 85 ºC, 1 h
Ru-Ir, 11%
Ru-Ir, 3%
-
[20]
[Ru
3
Ir(CO)
13
]⎯
PhC≡CPh
EtC≡CEt

- Hex, 90 ºC, 1 h
Ru-Ir, 85%
Ru-Ir, 75%
Ru-Ir, 80%
-

[21]
The reaction of FeRu
3
(μ-H)
2
(CO)
13
with alkynes is quite interesting as it afforded
hinge-apex as well as alkyne isomers (Figure 3.3).
C
C
R
R'
Ru
Ru
Fe
Ru
C
C
R'
Fe
Ru
Ru
Ru

ia: R = R' = Ph
iia: R = R' = Me
iiia: R = Ph; R' = Me
ib: R = R' = Ph
iib: R = R' = Me
iiib: R = Me R' = Ph
iiic: R = Ph; R' = Me
R

Figure 3.3. Hinge-apex and alkyne isomerism in FeRu
3
(CO)
12
(RCCR’) clusters.



In the reaction of both CpRhRu
3
(μ-H)
4
(CO)
9
and FeRu
3
(μ-H)
2
(CO)
13
towards

alkynes, hinge-apex isomerism was observed and the isomer with the heterometal
atom at the hinge (
Rh-Ru or Fe-Ru) was obtained as the major isomer from the
reaction. However, this isomer readily isomerised to the
Ru-Ru isomer with the
heterometal at the wingtip in both cases. In the case of CpRhRu
3
(μ-H)
4
(CO)
9
, the
isomerisation was observed on purification of the product on TLC plates while in the
case of FeRu
3
(μ-H)
2
(CO)
13
,

isomerisation occurred upon heating. The reverse
isomerisation was not facile in these systems. This suggested that for these two
systems, the isomer with the heterometal atom in the hinge was the kinetically favored
product and that with the heterometal atom in the wingtip was the thermodynamically
stable product.
The reactions of the neutral cluster IrRu
3
(μ-H)(CO)
13

, and the anionic cluster
[Ru
3
Ir(CO)
13
]
⎯
,

towards

alkynes were found to afford clusters exhibiting μ
3
,η
2

76
coordination

mode of the alkynes on a face of a tetrahedral metal framework, in
addition to the
μ
4
,η
2
coordination mode (Scheme 3.3). IrRu
3
(μ-H)(CO)
13
was found

to be an excellent catalyst for the hydrogenation of diphenylacetylene to stilbene and
the alkyne substituted clusters were found to represent side-channels of the catalytic
cycle.
Ru Ru
Ir
Ru
H
RCCR'
-2CO
Ru
Ru
Ru
Ir
C
C
R
R
H
C
C
R
R
Ir
Ru
Ru
Ru
C
C
R
H

+ 2 RCCR
+ RCCR
- 3CO
- CO
R = Ph or Me

Scheme 3.3.

Thus it can be observed from these reactions that both the nature of the metals and the
alkynes played an important role in determining the products and the isomers
obtained; the isomer with the heterometal atom in the hinge was the kinetically
favoured product in most of the cases.



77
3.2 Reaction of Cp*IrRu
3
(μ-H)
2
(CO)
10
, 3a, with internal alkynes
3.2.1 Reaction of 3a with RCCR (R = Ph, Et)

The thermal reaction of
3a with RCCR (R = Ph, Et) in hexane afforded a dark red
solution which after separation by TLC on silica-gel plates afforded three bands. In
both the reactions the fastest moving band 1 afforded red crystals which were
characterized by IR and

1
H NMR spectroscopy, microanalyses and further
characterized by single X-ray crystallographic studies; the compounds were identified
as the novel clusters, Cp*Ru
3
Ir(CO)
9
(RCCR) [R = Ph = 10a; R = Et = 10b]. The IR
spectrum of
10b recorded in hexane is shown in Figure 3.4 and the ORTEP diagram
of
10a is shown in Figure 3.5.



Figure 3.4. IR spectrum of
10b in hexane.



78


Figure 3.5. ORTEP diagram of Cp*Ru
3
Ir(CO)
9
(PhCCPh),10a. Thermal ellipsoids are
drawn at the 50% probability level. The phenyl hydrogens are omitted for clarity.

The molecular structure of
10a consisted of a butterfly skeleton; the butterfly
backbone consisted of ruthenium and iridium atoms which were bonded to two wing-
tip ruthenium atoms. Each of the three ruthenium atoms were bonded to 3 carbonyls
and the iridium atom was bonded to a Cp* ring. All carbonyls were terminal. The
PhCCPh ligand was coordinated to
the Ru
3
Ir metal core in a μ
4
,η
2
fashion. One of the
carbon atoms (C50) was
σ−bonded to Ir(1), and the second one (C60) was σ−bonded
to Ru(3). Both carbon atoms were
π-bonded to the two wingtip ruthenium atoms. The

79
C-C bond of the alkyne was disposed almost parallel to the hinge [(Ru(3)–Ir(1)] of the
butterfly.
The other two products from the reactions were the previously reported clusters,
Ru
3
(CO)
8
(C
4
R
4

) [R = Ph = 12a, R = Et = 12b], which were identified by their IR and
1
H NMR spectroscopic data [22], and the novel trinuclear clusters
Cp*IrRu
2
(CO)
7
C
2
R
2
[R = Ph = 11a, R = Et = 11b]. The latter were characterized by
IR and
1
H NMR spectroscopy, FAB-MS, microanalyses and single crystal X-ray
crystallographic studies.
The IR spectrum of
11b recorded in hexane is shown in Figure 3.6 and the ORTEP
diagram of
11a is shown in Figure 3.7. Selected bond lengths and bond angles for 11a
and
11b are presented in Table 3.2.



Figure 3.6. IR spectrum of
11b in hexane.


80

Figure 3.7. ORTEP diagram of Cp*IrRu
2
(CO)
7
C
2
(C
6
H
5
)
2
, 11a. Thermal ellipsoids are
drawn at 50% probability level. The phenyl hydrogens are omitted for clarity.
The overall molecular structures of 11a and 11b were similar, with the ruthenium and
iridium atoms forming a closed triangle. The clusters had the expected 48-electron
count for trinuclear clusters thus satisfying the noble gas rule. A comparison of both
structures showed that the distances Ru(2)-C(7) [2.094 (7) Å] and Ir(1)-C(6) [2.067
(6) Å] in
11b were similar to the analogous distances in 11a [Ru(2)-C(7) = 2.116(5)
and Ir(1)-C(6) = 2.090(6) (Å)] suggesting that changing the substituents on the alkyne
had not affected the metal-carbon bond distances significantly. Three terminal
carbonyls each were bonded to Ru(3) and Ru(2). An asymmetric bridging carbonyl

81
was found to span the Ir(1)-Ru(2) edge. The carbonyl bridged Ir(1)-Ru(2) bond was
longer than the unbridged Ir(1)- Ru(3) bond. The alkyne ligand was coordinated in a
μ
3

,η
2
fashion over the Ru
2
Ir triangle [21, 23]. The alkyne C(6)-C(7) bond was
disposed almost parallel to the Ir(1)-Ru(2) edge of the metal triangle; this type of
bonding mode has been given the notation
μ
3
,η
2
. It is generally considered to
donate four electrons to the cluster. This type of bonding mode has been observed in
Co
2
Ru(CO)
9
(C
2
Ph
2
), Ru
2
Ni(CO)
4
Cp
2
(PhCCPh), and OsW
2
(CO)

7
Cp
2
(TolCCTol) [24-
27].
Table 3.2. Selected bond lengths (Å) and bond angles (º) of compounds
11a and 11b.

11a 11b
Ir(1)-Ru(2) 2.7977(5) 2.8034(5)
Ir(1)-Ru(3) 2.7091(5) 2.7022(5)
Ru(2)-Ru(3) 2.6852(6) 2.6892(7)
Ir(1)-C(6) 2.090(6) 2.067(6)
Ru(2)-C(7) 2.116(5) 2.094(7)
Ru(3)-C(6) 2.196(5) 2.196(5)
Ru(3)-C(7) 2.250(5) 2.214(6)
C(6)-C(7) 1.393(8) 1.402(9)
Ir(1)-C(11) 1.880(6) 1.893(7)
Ru(2)-C(11) 2.422(6) 2.362(7)
Ir(1)-C(11)-O(11) 153.0(5) 150.6(6)
Ru(2)-C(11)-O(11) 126.9(5) 127.8(5)



82
All the metal-metal bond distances were different but were within the range of Ru-Ru
and Ru-Ir single bonds [28, 29]. The C(6) and C(7) carbon atoms of the alkyne were
σ−bonded to Ir(1) as well as Ru(2), and were both π-bonded to Ru(3). The
coordinated alkyne carbon-carbon bond length for
11a and 11b were found to be

1.393(8) Å and 1.402(9) (Å), respectively, which suggested that the formal bond order
was less than two. The C≡C bond lengths were slightly longer than those in the
related clusters, [IrRu
2
(CO)
9
(μ
3
-η
2
C
2
Ph
2
)]
⎯
, [IrRu
2
(CO)
9
(μ
3
-η
2
PhC
2
Me)]
⎯
and
[Co

2
Ru(CO)
9
(C
2
Ph
2
)], in which the C-C bond lengths measured 1.363(11) , 1.372(9)
and 1.370(3) Å, respectively [21, 24].
Prolonged heating of cluster
3a with PhCCPh and EtCCEt resulted in increased yields
of clusters
11a and 11b at the expense of 10a and 10b, respectively. This suggested
that
11a and 11b were derived from the butterfly alkyne clusters 10a and 10b,
respectively. To verify this, thermolyses of clusters
10a and 10b were carried out and
the reaction afforded
11a and 11b, thus confirming that the former were the
precursors to the latter.
Furthermore, photolysis of solutions of 10a and 10b under UV
also yielded
11a and 11b. This suggested that the butterfly alkyne clusters were not
very stable at high temperatures as well as under photolytic conditions and were prone
to undergo fragmentation to yield the stable triangular clusters (Scheme 3.4).
Thermolysis or photolysis of
10a and 10b did not yield the triruthenium clusters 12a
and 12b, which suggested that 10a and 10b were not their precursors and that the
formation of
12a and 12b probably followed a different pathway. Reactions of

Cp*Ir(CO)
2
with 12a or 12b were attempted under thermolytic and photolytic
conditions to see whether
10a and 10b were formed. IR monitoring of the reactions
did not indicate any reaction.


83
C
C
R
R'
Ir
Ru
Ru
Ru
UV
or
Ru
Ru
Ir
C
C
R
R'
R = R'= Et, Ph


Scheme 3.4.

3.2.2 Reaction of 3a with MeCCBu
t

Thermal reaction of
3a with 4, 4-dimethyl 2-pentyne afforded red crystals of 10c,
with IR spectroscopic characteristics similar to those of
10a and 10b. The identity of
10c has been confirmed by a single crystal X-ray structural study as
Cp*Ru
3
Ir(CO)
9
(MeCCBu
t
). The ORTEP plot of 10c is shown in Figure 3.8. The
overall structure of
10c was similar to 10a and 10b. However, it was noted that in this
case, the bonding mode appeared to be highly stereoselective, with the
t
Bu group in
the alkyne being positioned away from the iridium atom. The
1
H NMR spectrum did
not suggest the presence of isomers. IR monitoring of the reaction showed strong
peaks due to Cp*Ir(CO)
2
, suggesting fragmentation of the cluster. Trinuclear
ruthenium clusters were not isolated in this reaction, unlike with the other alkynes, as
most of the bands were not stable on the TLC plates.

84

Figure 3.8. ORTEP diagram of Cp*IrRu
3
(CO)
7
(MeCCBu
t
), 10c. Thermal ellipsoids
are drawn at 50% probability level. Organic hydrogens are omitted for clarity.
3.2.3 Reaction of 3a with R
3
SiCCSiR
3
(R = Me, Et)

The thermal reaction of
3a with R
3
SiCCSiR
3
(R = Me, Et) in hexane afforded two
products (Scheme 3.5). One of them were the known triruthenium clusters
Ru
3
(CO)
9

(μ-H)(C
2
SiR
3
) (R = Me , 13; R = Et , 14), obtained in ~ 30% yields as
yellow crystalline solids, identified by their IR and
1
H NMR spectroscopic data [30].
A second red, crystalline product was obtained in ~ 15% yields. The IR spectra
recorded in hexane solution were similar to
10a-c, suggesting a similar structure.

85
R
3
SiCCSiR
3
/ 60 °C
intramolecular -C-R bond
cleavage
C
C
Ru
Ir
Ru Ru
R
3
Si
H
Ru

Ru
Ru
C
R
3
Si
C
H
+
13 R = Me
14 R = Et
10d R = Me
10e R = Et
Ru Ru
Ir
Ru
H
3a
H

Scheme 3.5.
The molecular structures were determined by single crystal X-ray analyses and they
were identified to have the structural formula Cp*Ru
3
Ir(CO)
9
(R
3
SiCCH), (R = Me,
10d; R = Et, 10e). The ORTEP plot of 10d is shown in Figure 3.9.

The
1
H NMR and FAB-MS data were consistent with the suggestion that a
trialkylsilyl group was lost and replaced by hydrogen. The singlets at ~ δ 10.5 ppm
corresponded to the alkenic hydrogens on the alkynes and the values were in
agreement with similar compounds reported in the literature (
δ 9.18 ppm in
Ru
3
(CO)
10
HCCSiMe
3
and δ 10.56 ppm in Os
3
(CO)
10
HCCSiMe
3
, [31]).
1
H NMR
monitoring of the reaction with Me
3
SiCCSiMe
3
showed that the trimethylsilyl group
was lost during the course of reaction and not during workup procedure. GC analysis
of an aliquot of the crude reaction mixture suggested that the trimethylsilyl group was
lost as octamethyl trisiloxane, which suggested that the hydrogen might have

originated from trace amounts of water. However, the desilylation product was
observed in almost similar yields even when the reaction was carried out with

86
thoroughly dried solvents and glassware, which ruled out the above postulation. Since
Cp*Ir(CO)
2
is known to bring about C-H activation of hydrocarbons, it is also
possible that the Cp*Ir(CO) fragment in these clusters might be responsible for the
hydride exchange with the SiMe
3
group.

Figure 3.9. ORTEP diagram of Cp*Ru
3
Ir(CO)
9
(Me
3
SiCCH), 10d. Thermal ellipsoids
are drawn at 50% probability level. Organic hydrogens are omitted for clarity.
Desilylation has been reported by Schneider and coworkers in their attempt to
synthesize Fe-Ni heteronuclear clusters by the vaporisation of nickel atoms into a
solution of (bistrimethylsilyl)acetylene and Fe(CO)
5
[32]. The reaction had
unexpectedly afforded the cluster Fe
3
(CO)
9

(μ-H)(C≡CSiMe
3
), which was similar to
13 and 14 in that desilylation of one of the trimethylsilyl groups had occurred.
However, in their reactions they have not reported compounds similar to
10d and 10e
where one of the trialkylsilyl groups was replaced by a hydrogen, although it was

87
suggested that cleavage might have occurred during chromatographic work-up.
Vahrenkamp and coworkers have also reported desilylation in the reaction of
RuCo
2
(CO)
11
with Me
3
SiC≡CMe. The initial product, RuCo
2
(CO)
9
(μ
3
-
Me
3
SiC≡CMe), formed in the reaction underwent subsequent desilylation to give
RuCo
2
(CO)

9
(μ
3
-HC≡CMe) [33].
3.3 Reaction of 3a with terminal alkynes
3.3.1 Reaction of 3a with PhCCH

Reaction of cluster
3a with phenyl acetylene afforded three red crystalline products.
The major product, obtained in 28% yield, was identified as Cp*IrRu
3
(CO)
9
(PhCCH),
10f
3
; it was characterized spectroscopically and analytically, as well as by a single
crystal X-ray structure analysis. The ORTEP plot of
10f
3
is shown in Figure 3.10.
The other two products were obtained in very low yields. A single crystal X-ray
structural study on one of them showed it to be a hinge-apex isomer of
10f
3
. The
ORTEP diagram of this isomer,
10f
2
, is shown in Figure 3.11. The IR spectrum of the

third product had a pattern similar to
10a-e. Its
1
H NMR showed a multiplet between
δ 7.00 and 7.77 ppm due to aromatic protons, a singlet at δ 1.82 ppm which could be
assigned to Cp* methyl protons, and a singlet at δ 10.58 ppm due to the alkyne C-H.
Based on these spectroscopic characteristics it was tentatively identified as
10f
1
, a
third isomer of Cp*IrRu
3
(CO)
9
(PhCCH), 10f
3
.
The three isomers,
10f
1
, 10f
2
and 10f
3
, differed in the relative orientation of the alkyne
with respect to the butterfly cluster core; they are depicted in Scheme 3.6, together
with the notation employed.

88

Figure 3.10. ORTEP diagram of Cp*Ru
3
Ir(CO)
9
(PhCCH), 10f
3
. Thermal ellipsoids
are drawn at 50% probability level. Organic hydrogens are omitted for clarity.


89


Figure 3.11. ORTEP diagram of Cp*Ru
3
Ir(CO)
9
(PhCCH), 10f
2
. Thermal ellipsoids
are drawn at 50% probability level. Organic hydrogens are omitted for clarity.



90
C
C
Ph

H
Ir
Ru
Ru
Ru
Ru
Ru
Ir
Ru
H
H
C
C
Ph
H
Ru
Ru
Ru
Ir
PhCCH
-CO,-2H
Major product 10f
3
ll Ru-Ru isomer
3a
Minor product 10f
2
ll Ru-Ir cis isomer
C
C

H
Ph
Ir
Ru
Ru
Ru
Minor product 10f
1
ll Ru-Ir trans isomer


Scheme 3.6.

In the reaction of
3a with internal alkynes, only the Ru-Ir isomer was observed;
prolonged heating of this isomer did not lead to isomerisation but to cluster
fragmentation instead. In contrast, the reaction of
3a with phenyl acetylene yielded
the
Ru-Ru isomer (10f
3
) as the major product; the Ru-Ir cis and Ru-Ir trans
isomers were obtained as minor products. This suggested that there was an electronic
preference for the formation of the
Ru-Ru isomer; the extremely low yield of 10f
2
could be due to the steric effect of having both the bulky phenyl ring on the alkyne,
and the Cp* ring on the iridium, close to each other.
It was observed by the IR spectroscopy that on standing, a solution of
10f

3
converted
slowly to
10f
1
and 10f
2
. NMR monitoring of a deuterated benzene solution of 10f
3
at

ambient temperature indicated the formation of two new singlets at δ 1.82 ppm and δ
1.87 ppm, assignable to the Cp* signals of
10f
1
and 10f
2
,

respectively, and also two

91
singlets at

δ 10.58 ppm and δ 11.37 ppm, due to the respective C-H protons of the
phenyl acetylene. However, complete conversion to either
10f
2
or 10f
3

was not
observed even after a month. Prolonged heating of
10f
3
finally afforded a 1.2:1.0:5.4
equilibrium mixture of
10f
1
:10f
2
:10f
3.
This showed that the equilibrium was always
towards the
Ru-Ru isomer, 10f
3
,

indicating that it was the thermodynamically stable
isomer. This is in agreement with the earlier observations on the FeRu
3
and RhRu
3
systems [17, 18].
3.3.2 Reaction of 3a with
n
BuCCH

The reaction of
3a with 1-hexyne afforded a red solid in very low yield. The IR

spectral profile was similar to that of
10f
1
, suggesting a similar structure. Further
characterization was not possible because of the low yield. The compound was
tentatively identified as Cp*IrRu
3
(CO)
9
(
n
BuCCH), 10g.

3.3.3 Reaction of 3a with Me
3
SiCCH

The reaction of cluster
3a with Me
3
SiCCH afforded two products which were
identified as
10d (17% yield) and Ru
3
(CO)
9
(μ-H)(C
2
SiR
3

), 13 (30% yield). Thus the
reaction of
3a with both bis(trimethylsilyl)acetylene (an internal alkyne) and
trimethylsilyl acetylene (a terminal alkyne) yielded similar products.
3.3.4 Reaction of 3a with 1-hexene

The reaction of
3a with 1-hexene was investigated in an attempt to prepare cluster-
olefin complexes but none could be isolated. Instead, it was found that the cluster
catalysed isomerisation of 1-hexene to give a mixture of cis and trans 2-hexenes. The
reaction was carried out in an NMR tube and monitored by
1
H NMR (Figure 3.12)
However, complete isomerisation was not observed even after long hours. The

92
infrared spectrum of the reaction mixture after 7 h showed that
3a remained mainly
unreacted. However, further investigations were not carried out to isolate and quantify
the products.

Figure 3.12.
1
H NMR spectrum (δ 4.0 - 6.0 ppm region) of (a) 1-hexene (b)


(d)

(c)



(b)


(a)
immediately after mixing (c) after 3 h (d) 2-hexene.

93
3.4 Reaction of Cp*IrOs
3
(μ-H)
2
(CO)
10
, 3c and Cp*IrOs
3
(μ-H)
4
(CO)
9,
4b,

towards alkynes
The mixed metal clusters, Cp*IrOs
3
(μ-H)
2
(CO)
10
, 3c, and Cp*IrOs 4b, (μ-H)
3 4
(CO) ,
9
did not

react with excess alkyne even at 120 ºC. Photochemical activation (Hanovia

lamp, 450 W, quartz vessel) afforded some reaction but the products were in low
yields. Chemical activation of
3c and 4b with excess triethylamine afforded alkyne
substituted mixed metal clusters Cp*IrOs
(CO) (RCCR).
3 9
3.4.1 Reaction of 4b with RCCR (R = Ph, Et)

Reaction of
4b with RCCR (R = Ph, Et) in the presence of excess triethyl amine
afforded two products. The IR spectral pattern of one of the products were similar to
those of
10, in which the alkyne was disposed parallel to a Ru-Ir hinge. Further
characterization of one of them by single crystal X-ray analysis revealed the identity
as Cp*IrOs
3
(CO)
9
(PhCCPh), 15a
1
; The identity of Cp*IrOs
3
(CO)
9
(EtCCEt), 15b
1

was confirmed by its IR and FAB-MS characteristics. The ORTEP plot of
15a
1

is
shown in Figure 3.13. Its overall structure was similar to those of the
Cp*IrRu
3
(CO)
9
(RCCR’) clusters 10a-e, in that the alkyne was oriented parallel to the
Os-Ir hinge (
Os-Ir), with two osmium atoms occupying the wingtip positions of the
Os
3
Ir butterfly core.
The IR spectral profile of the other product in both the reactions was similar to that of
10f
3
in which the alkyne was oriented parallel to a Ru-Ru bond. Single crystal X-ray
structural studies confirmed their identities as Cp*IrOs
3
(CO)
9
(RCCR) [R = Ph, 15a
2
;
R = Et,
15b
2
], the hinge apex isomers of 15a
1
and 15b
1

,

respectively. The ORTEP
plot of
15a
2
is shown in Figure 3.14.

94

Figure 3.13. ORTEP diagram of Cp*Os
3
Ir(CO)
9
(PhCCPh), 15a
1
. Thermal ellipsoids
are drawn at 50% probability level. Phenyl hydrogens are omitted for clarity.


Figure 3.14. ORTEP diagram of Cp*Os
3
Ir(CO)
9
(PhCCPh), 15a
2
. Thermal ellipsoids
are drawn at 50% probability level. Phenyl hydrogens are omitted for clarity.

95