Synthesis and reactivity studies of cyclopentadienyl derivatives of ruthenium iridium and osmium iridium mixed metal clusters 4

Chapter 4
Subtitution chemistry of Cp*IrOs
3
(μ-H)
2
(CO)
10

4.1 Carbonyl substitution in metal carbonyl clusters
The ligand substitution chemistry of homometallic clusters have been very well
documented but ligand substitution in heterometallic clusters has been the subject of
relatively few reports. The varying constituent metals of the heterometallic cluster
core afford the possibility of not only metalloselectivity upon ligand substitution but
also site selectivity due to a decrease in molecular symmetry. To understand the
substitution reactions in heterometallic clusters, it will be useful to have some
knowledge on the substitution reactions of homometallic clusters.
4.1.1 Carbonyl substitution in trinuclear metal carbonyl clusters

There are two types of CO ligands in M
3
(CO)
12
(M= Ru, Os). Those in the same plane
as the M
3
triangle are referred to as equatorial and those that are perpendicular to the
plane are axial (Figure. 4.1).

a - axial
e - equatorial
M

M
M
aa
a
e
e
e
e
a
e
a
e
a
M
M
M
aa
a
e
ee
e
a
e
a
e
a


Figure 4.1. Substitution positions in trinuclear clusters.

126

When CO is substituted by a phosphine on a cluster, the coordination site adopted is
restricted by the steric and electronic requirements of the comparatively bulky
phosphine ligand. In trinuclear clusters, the first phosphine is generally found to
replace an equatorial ligand. In Os
3
(CO)
12
, this preference for equatorial substitution
has been accounted for in steric terms, since simple calculations on Os
3
(CO)
12-x
L
x

systems have shown that the equatorial sites in (approximately) anticuboctahedral
structures of ligands are less sterically hindered than axial [1-7].
Pomeroy and coworkers have given a detailed discussion on the influence of
phosphine substitution on Os
3
(CO)
11
PR
3
structures [8]. There has also been earlier
studies by Bruce and coworkers who

determined the structures of 12 different mono-

and disubstituted phosphine derivatives of Ru
3
(CO)
12
and Os
3
(CO)
12
to investigate the
steric and electronic influences of phosphine substitution in these clusters [9, 10]. The
following conclusions were obtained from these studies:
1. The group 15 ligand prefers equatorial coordination site in monosubstituted
derivatives. In the disubstituted derivatives, the two ligands take up positions
that are as far apart as possible; each occupying an equatorial site on adjacent
metal atoms.
2. Due to steric interactions between the group 15 ligand and the CO group cis to
it on the adjacent metal atom, the M-M bond cis to the phosphine ligand is the
longest of the three M-M separations; for the disubstituted derivatives, no such
pronounced lengthening of this M-M bond has been observed.
3. Introduction of two group 15 ligands into the M
3
(CO)
12
cluster results in a
twisting of the ML
4
groups about the M-M axis, distorting the original D
3h

symmetry of the parent cluster to D

3
symmetry.

127
4. In the monosubstituted derivatives, the M-P bond length increases with
increasing cone-angle while in the disubstituted derivatives, the M-P bond
lengths are almost the same as those in the corresponding monosubstituted
derivatives.
5. Axial M-CO bonds are longer than equatorial M-CO bonds.
4.1.2 Carbonyl substitution in heteronuclear tetrahedral clusters

Heteronuclear tetrahedral clusters are of interest as species in which to scrutinize
metalloselectivity and site selectivity. Basically there are three positions available for
substitution in tetrahedral M’M
3
clusters, namely axial, equatorial and apical (Figure
4.2).
M
M
M'
M
eq
eq
eq
eq
ax
ax
ax
eq
ap

ap
ax - axial
eq - equatorial
ap - apical
ap
eq
M
M
M'
M
eq
eq
eq
eq
eq
ax
ax
ax
eq
ap
ap
ap


Figure 4.2. Substitution sites in heterometallic tetrahedral clusters.
Studies on metalloselectivity of these clusters have revealed that the selectivity could
be influenced by factors such as the nature of the metals and the nature of both the
existing and incoming ligands. For example, the clusters MCo
3
(μ-H)(CO)

12
(M= Fe,
Ru) reacted with secondary and tertiary phosphines to afford monosubstituted
derivatives in which the phosphine ligand was always bonded to Co, while Ru
3
Rh(μ-
H)(CO)
12
reacted with phosphines

to produce monosubstituted derivatives in which

128
the PR
3
ligand was attached to Rh [11-14]. In the disubstituted product,
RuCo
3
(CO)
10
(μ-H)(PMe
2
Ph)
2
, the second PMe
2
Ph was substituted at Ru, whereas in
the case of RuCo
3
(μ-H)(CO)

10
(PPh
3
)
2
, the phosphines were bonded to one cobalt
atom each [15]. Pakkanen and coworkers have recently reported the structures of two
isomers of [Ru
3
Ir(μ-H)
3
(CO)
11
(PPh
3
)] (Figure 4.3) [16]. In one of the isomers, the
phosphine ligand was coordinated to an axial position in the Ru
3
basal triangle and in
the other isomer the phosphine was found coordinated apically to the iridium atom.
Ru
Ru
Ir
Ru
H
H
H
Ph
3
P

Ru
Ru
Ir
Ru
Ph
3
P
H
H
H

Figure 4.3. Isomeric structures of [Ru
3
Ir(μ-H)
3
(CO)
11
(PPh
3
)].

Table 4.1 summarizes the preferred site of substitution of phosphines for various
tetrahedral mixed-metal clusters reported in the literature. It can be noted that axial or
apical substitution was always observed for the first phosphine ligand, while the
second substitution could be equatorial or axial. For example, the first substitution by
PPh
3
in FeRu
3
(μ-H)

2
(CO)
13
occurred at an axial position on one of the basal
ruthenium atoms.
For tetrahedral mixed metal clusters possessing a Cp or Cp* ligand, the substitution
almost always occurred at the basal metal triangle. For example, the reaction of
CpRu
3
Rh(H)
2
(CO)
10
with phosphines afforded mono- and disubstituted phosphine
derivatives where the substitution occurred at the ruthenium triangle. This could be
attributed to the fact that in these clusters, the bulky Cp ligand was attached to the
unique heterometallic vertex. Proton NMR studies of the derivatives have revealed at

129
least three isomers for the monosubstituted derivatives and two isomers for the
disubstituted derivatives in solution (Figure 4.4).
Table 4.1. Preferred phosphine substitution sites for tetrahedral mixed-metal clusters.
Phosphine derivatives
Cluster
mono-substituted di-substituted
Ref.
FeRu
3
(μ-H)
2

CO)
13
axial (Ru) axial, equatorial (Ru, Ru) [17]
[IrRu
3
(μ-H)
2
(CO)
12
]⎯
apical (Ru) [18]
RuRh
3
(μ-H)(CO)
12
axial (Rh) diaxial (Rh, Rh) [15]
Os
3
(μ-H)
3
IrCO)
12
apical (Ir)
[19]
RuCo
3
(μ-H)(CO)
12
axial (Co) diaxial (Co, Co)
[18]

IrRu
3
(μ-H)
3
(CO)
12
apical (Ir)
axial (Ru)
[20]
CpRhRu
3
(μ-H)
2
(CO)
10
axial (Ru) [major]
equatorial (Ru) [minor]
axial, equatorial (Ru, Ru) [21]
Cp*RhRu
3
(μ-H)
2
(CO)
10
equatorial (Ru) [major] axial, equatorial (Ru, Ru) [21]
CpNiOs
3
(μ-H)
3
(CO)

9
diaxial (Os, Os) [22]

Ru
Ru
Rh
Ru
L
L
L=PPh
3
(a) axial
(b) equatorial cis to bridging CO
(c) equatorial trans to bridging CO
(d) axial,equatorial cis to bridging CO
(e) axial, equatorial trans to bridging CO
(a)
(b)
(c)
(e)
Ru
Ru
Rh
Ru
L
Ru
Ru
Rh
Ru
Ru

Ru
Rh
Ru
L
L
(d)
Ru
Ru
Rh
Ru
L
L
1
2
3
1
1
1
1
2
2
2
2
3
3
33

Figure 4.4. Isomeric structures of mono- and disubstitued phosphine derivatives of
CpRu
3

Rh(μ-H)
2
(CO)
10
.

130
In contrast, the most stable isomer of the monosubstituted phosphine derivative in the
Cp* analogue did not involve phosphine coordination at Ru(1); substitution occurred
either at Ru(2) or Ru(3).
The reaction of the mixed metal cluster CpNiOs
3
(μ-H)
3
(CO)
9
with phosphines in the
presence of TMNO afforded disubstituted derivatives in which the phosphines were
reported to be bound axially to two osmium atoms. Isomers were not observed in
solution.
In contrast to phosphines, the substitution chemistry with isocyanides is relatively
unexplored. Isocyanide ligands generally show a propensity for axial substitution in
derivatives of M
3
(CO)
12
(M=Ru, Os). In complexes of the type Os
3
(μ-
H)(H)(CO)

10
(CNR) (R = Me, Ph), only axial coordination was observed. However,
for R =
t
Bu, equatorial substitution occurred, as has been confirmed by X-ray
crystallographic analysis of Os
3
(μ-H)(H)(CO)
9
(CNBu
t
)

[23, 24]. Also, in
Os
3
(CO)
11
(CNBu
t
),


there is NMR evidence for equatorial/axial isomerisation of the
t
BuNC ligand [25]. In the related complex, Ru
3
(CO)
11
(CNBu

t
), both axial and
equatorial forms existed in solution. However, in the solid state only the axial isomer
was observed [26]. Therefore it appears that for CNR ligands, the substituting ligand
can occupy an equatorial or axial position depending on the steric requirements of the
R group. Although electronic factors favour axial coordination of isocyanide groups,
steric constraints may result in equatorial substitution. In addition to the normal η
1

terminal bonding mode exhibited, the isocyanides also showed some propensity for
μ
n
,η
2
C-N bridged bonding modes (Table 4.2).




131
Table 4.2. Bridged bonding modes in isocyanides.
Bonding mode cluster Ref.
μ
2
, η
2
Νi
4
(CNBu
t

)
7
[27]
μ
3
, η
2
Os
6
(CO)
18
(CNC
6
H
4
Me-4)
2
[28]
μ
4
, η
2
Ru
5
(CO)
14
(CNBu
t
)
2

[29]

Shawkataly et. al. have reported a triruthenium cluster, Ru
3
(CO)
6
(μ
3-
PPhCH
2
PPh
2
)( μ
3
-CNCy)(CNCy)
2
Ph, in which one of the isocyanide ligands acted as
a four-electron donor [30]. Os
3
Pt(μ-H)
2
(CO)
9
(PCy
3
)(CNCy) is one of the very few
reported tetrahedral heterometallic clusters with isocyanides [31]. The solid state
structure was not reported, but
1
H and

13
C

NMR studies have shown that addition of
CyNC at 195 K yielded principally only one isomer, Os
3
Pt(μ-
H)
2
(CO)
10
(PCy
3
)(CNCy), which has a butterfly structure with the CyNC bonded to
an osmium center. On the other hand, reaction at 273 K produced three isomers which
were in dynamic equilibrium. All the three isomers exhibited butterfly geometry for
the metal core, with the CyNC ligand bonded to an osmium center. Refluxing a
hexane solution of Os
3
Pt(μ-H)
2
(CO)
10
(PCy
3
)(CNCy) resulted in facile
decarbonylation to afford a tetrahedral 58 electron unsaturated cluster Os
3
Pt(μ-
H)

2
(CO)
9
(PCy
3
)(CNCy) in which the CyNC ligand was bonded to the platinum atom.
There was thus a transfer of the CyNC ligand from Os to Pt in the decarbonylation of
the butterfly adduct as it closed to form the tetrahedral unsaturated cluster (Scheme
4.1).

132
Os
Os
Pt
Os
Cy
3
P
H
H
Os
Os
Os
CNCy
H
Pt
H
PCy
3
Os

Os
Os
H
Pt
H
PCy
3
Os
Os
Os
Pt
H
H
reflux, hexane
Os
Os
Pt
Os
Cy
3
P
H
H
CNCy
CNCy
273K
CNCy
PCy
3
CyNC

isomer A
isomer B
isomer C

Scheme 4.1.

Reaction of CpWIr
3
(CO)
11
with stoichiometric amounts of isocyanides was reported
to afford the clusters [CpWIr
3
(CO)
11-n
(CNR)
n
] in 47-63% yields (Scheme 4.2) [32].
The solid state structure of CpWIr
3
(CO)
9
(CNXy) [Xy = C
6
H
3
Me
2
-2,6] showed that
both the isocyanides were bonded to the same iridium atom.

Ir
W
Ir
Ir
Ir
W
Ir
Ir
RNC
RNC
CpWIr
3
(CO)
8
(CNR)
3
CpWIr
3
(CO)
10
(CNR)
1 eq RNC
2 eq RNC
3 eq RNC
R = Xy = 60%
R =
t
Bu = 58%
R = Xy = 47%

R =
t
Bu = 58%
R = Xy = 63%
R =
t
Bu = 59%

Scheme 4.2.

133
4.2 Substitution type reactions of Cp*IrOs
3
(μ-H)
2
(CO)
10
, 3c
It is evident from the foregoing that clusters containing mixed metals are of interest
as sites to probe metallo selectivity and site selectivity. Although there has been some
reports on the synthesis of Cp- and Cp*- containing osmium-iridium clusters, there
have been no reports on the reactivity of these clusters. The following sections present
our studies on the substitution reaction of 3c

with

various group 15 donor substrates
like phosphines, phosphites, isocyanides and pyridine.
4.2.1 Reaction of Cp*IrOs
3

(μ-H)
2
(CO)
10
, 3c, with triphenylphosphine

Cluster 3c was found to undergo facile substitution in the presence of trimethylamine
N-oxide. Reaction of cluster 3c with PPh
3
in dichloromethane at ambient temperature
in the presence of TMNO (dropwise addition via a dropping funnel) led to gradual
deepening of the original orange-red solution to deep red over a period of 2 h.
Chromatographic separation of the reaction mixture on silica-gel TLC plates afforded
Cp*IrOs
3
(μ-Η)
2
(CO)
9
(PPh
3
), 17a, and Cp*IrOs
3
(μ-Η)
2
(CO)
8
(PPh
3
)

2
, 18a,
respectively. Both clusters have been completely characterized, including by single
crystal X-ray crystallographic studies. The ORTEP plot of 17a is shown in Figure 4.5.
The solid state structure of 17a revealed that the tetrahedral core of the parent cluster
was retained; the bridging carbonyl and the two bridging hydrides in the parent cluster
were also intact and one of the axial carbonyls attached to the osmium triangle was
substituted by a phosphine ligand. The
1
H NMR spectrum taken at 300 K in d
8
toluene consisted of four resonances; one doublet at δ -19.94 ppm (
2
J
P-H
= 9.0 Hz) and
three broad singlets. On lowering the temperature to 233 K, the
1
H NMR spectrum
consisted of well-resolved resonances at δ -16.65d (
2
J
PH
= 9.1 Hz), -17.44s, -19.67d
(
2
J
PH
= 10.7 Hz) and -20.01d (
2

J
PH
= 9.1 Hz) ppm (Figure 4.6).

134


Figure 4.5. ORTEP diagram of 17a. Thermal ellipsoids are drawn at 50% probability
level. Organic hydrogens are omitted for clarity.
Two singlets at δ 2.14 and 2.10 ppm were also observed, assignable to two groups of
Cp* methyl protons. Integration of the
1
H resonances supported the existence of two
isomers in the ratio 1:0.14 (at 233 K) in solution. A
31
P selective decoupling
experiment performed on 17a by irradiating at the phosphorous resonances confirmed
that the splitting of the hydrides was indeed due to coupling with phosphorous atoms
(Figure 4.7). The two doublets observed at δ -16.65 and -19.67 ppm which are of

135
equal intensity, and the Cp* signal at δ 2.14 ppm, could be assigned to the major
isomer. The magnitude of the coupling constants indicated that the phosphine was in a
relative cis-position to the hydrides. This suggested that the major isomer was that
obtained from the X-ray structural study. The singlet resonance at δ -17.44 ppm and a
well-resolved doublet at δ -20.01 ppm, which are of equal intensity, and the Cp*
signal at δ 2.07 ppm, could be assigned to the minor isomer. The coupling constants
further suggested that one of the hydride resonances was coupled to a phosphorous
(
2

J
P-H
= 9.1 Hz) whereas the other hydride resonance was not.

-
-20.4-20.0-19.6-19.2-18.8-18.4-18.0-17.6-17.2-16.8-16.4-16.0-15.6
(ppm)
300 K
273 K
253 K
233 K

Figure 4.6.
1
H VT NMR spectrum of 17a recorded in CDCl
3
.


136
-19.2-18.8-18.4-18.0-17.6-17.2-16.8-16.4-16.0-15.6-15.2-14.8
(ppm)
Before decoupling
irradiation at -39.8 PP
M
irradiation at -23.67 PPM

Figure 4.7. P resonance coupled and decoupled H NMR spectra of
31 1
17a recorded in

d -toluene.
8

It has been reported by Churchill et. al. and Gladfelter et. al. that osmium-osmium
bridging hydrides cis to a carbonyl bridge have
1
H NMR chemical shifts at higher
field than -20 ppm and osmium-osmium bridging hydrides not cis to a bridging
carbonyl have
1
H NMR chemical shifts at lower field than -20 ppm [33, 34]. It
appears that there is a preference for axial substitution by phosphines in clusters of
this general structure, even in minor isomers [21, 22, 35]. If the further assumption is
made that the relative arrangements of the bridging carbonyl and hydrides remain as
in all the solid-state structures obtained thus far, this arrangement being preserved
also for similar derivatives of the clusters CpRhRu
3
(μ-H)
2
(CO)
10
and Cp*RhRu
3
(μ-
H)
2
(CO)
10
, then the minor isomer would correspond to substitution at one of the axial
positions of either Os(3) or Os(4) (Figure 4.8) [21, 35]. A

31
P{
1
H} NMR spectrum
taken at 300 K in CDCl
3
showed two singlets, at δ 0.15 and δ 16.26 ppm in the ratio
1:0.14 which could be assigned to the phosphines of the major and minor isomers,
respectively.

137
Os Os
Ir
Os
H
H
Ph
3
P
Os Os
Ir
Os
H
H
PPh
3


Figure 4.8. Tentative structures of the minor isomer of 17a.

The IR spectra of 17a recorded in dichloromethane and as a KBr pellet showed two
broad peaks in the bridging carbonyl region; the latter spectrum is shown in Figure
4.9. This suggested that the isomers existed in solution as well as in the solid state and
that both the isomers had bridging carbonyls.

Figure 4.9. IR spectrum of
17a recorded as a KBr pellet.
The
13
C NMR spectrum of 17a recorded in d
8
-

toluene at 300 K showed the presence
of two singlets at δ 97.97 and δ 97.50 ppm, in the ratio 1:0.14, which could be
assigned to the Cp* ring carbons of the major and minor isomers, respectively.

138
Singlets at δ 10.93 and 10.77 ppm, could be assigned to the Cp* methyl carbon
signals of the major and minor isomers, respectively. No signals due to carbonyls
were observed and this can be attributed to the low intensities.
A
31
P NMR spin-saturation transfer experiment on 17a at 300 K indicated that the two
isomers were undergoing chemical exchange with each other as well (see Appendix).
The
1
H EXSY spectrum of 17a recorded at 273 K (τ
m
= 0.5 s) indicated that at that

temperature, there was only mutual chemical exchange of the hydride resonances of
the major isomer, i.e., a fluxional process (Figure 4.10). In the
1
H EXSY spectrum
recorded at 300 K, exchange cross peaks between all the hydrides were observed, i.e.,
there is an additional isomerisation process (Figure 4.11). These suggest that the
simple fluxional exchange within the major isomer is more facile than the
isomerisation process. The former can be understood in terms of the rocking motion
which has been described for the RhRu
3
analogues [21, 35]; this corresponds to an
incomplete merry-go-round involving the bridging carbonyl and the terminal
carbonyls B, F, A’ and D’ which effectively moves the bridging carbonyl from the
Ir(1)-Os(3) edge to the Ir(1)-Os(4) edge. For the isomerisation process, a hydride
migration will also be required (Scheme 4.3).


139

H
A
H
C
Int
r
amolecula
r
exchange c
r
osspeak

Figure 4.10.
1
H EXSY spectrum of 17a recorded at 273 K (τ
m
= 0.5 s).



140
HA

H
A
H
B
H
C
H
D
H
B
H
C
H
D
H
A
Intermolecular exchange crosspeaks

Figure 4.11.
1
H EXSY spectrum of 17a recorded at 300 K (τ
m
= 0.5 s).







141
Os
Os
Ir(1)
Os(2) H
A
H
C
(4)
(3)
Os
Os
Ir(1)
Os(3) H
D
PPh
3
(2)

(4)
H
B
minor
Os
Os
Ir(1)
Os(2) H
A
H
C
PPh
3
(4)
(3)
Os
Os
Ir(1)
Os(2) H
C
H
A
(4)
(3)
Isomerisation process
major
C
Fluxional process
PPh
3

C
D
PPh
3
D
A
F
E
B
D'
C'
A'
A
E
F
B
C'
A'
D'

Scheme 4.3.

142
The
1
H NMR spectrum of 18a consisted of well-resolved resonances. The singlet
resonance at δ 2.08 ppm could be assigned to the Cp* protons. The doublet of
doublets at δ -18.18 ppm could be assigned to the bridging hydride cis to the bridging
carbonyl and the doublet at δ -15.71 ppm could be assigned to the hydride trans to the
bridging carbonyl. The
31
P{
1
H} NMR spectrum showed two singlet resonances at δ -
11.02 and 14.94 ppm due to the two different phosphorous atoms. Both
1
H and
31
P{
1
H} NMR spectra did not suggest the presence of isomers in solution.
The IR spectrum of 18a suggested the presence of a bridging carbonyl (Figure 4.12).
Substitution of electron donating phosphines for the carbonyl ligands had resulted in
lowering of the ν
CO
in 17a compared to 3c and further still in 18a. The ORTEP
diagram of 18a is shown in Figure 4.13. The molecular structure of 18a consisted of a
tetrahedral framework of osmium-iridium atoms. Two phosphine ligands, P(5) and
P(6) were coordinated to two adjacent osmium atoms in an axial and equatorial
fashion, respectively.

Figure 4.12. IR spectrum of 18a recorded in dichloromethane.

143

Figure 4.13. ORTEP diagram of 18a. Thermal ellipsoids are drawn at 50% probability
level. Organic hydrogens are omitted for clarity.








144
4.2.2 Reaction of 3c with P(OMe)
3



Reaction of 3c with P(OMe)
3
in the presence of TMNO yielded the monosubstituted
derivative, Cp*IrOs
3
(μ-Η)
2
(CO)
9

P(OMe)
3
, 17b, and the disubstituted derivative
Cp*IrOs
3
(μ-Η)
2
(CO)
8
[P(OMe)
3
]
2
, 18b, respectively. The solution IR spectral pattern
of 17b was similar to that of 17a, suggesting axial coordination of the P(OMe)
3
ligand, which was further confirmed by a single crystal X-ray crystallographic study
(Figure 4.14). The overall structure of 17b was similar to 17a with the P(OMe)
3

ligand axially coordinated to Os(2) of the osmium triangle. No isomers were detected
in the
1
H and the
31
P{
1
H} NMR spectra.

Figure 4.14. ORTEP diagram of 17b. Thermal ellipsoids are drawn at 50% probability

level. Organic hydrogens are omitted for clarity.

145
The IR spectral profile of 18b was similar to that of 17b, suggesting a similar
substitution pattern. The structure was further confirmed by a single crystal X-ray
crystallographic study. The ORTEP plot of 18b is shown in Figure 4.15.


Figure 4.15. ORTEP diagram of 18b. Thermal ellipsoids are drawn at 50% probability
level. Organic hydrogens are omitted for clarity.


146
4.3 Solid state structures of mono and disubstituted PR
3
[R = Ph or
OMe] derivatives of 3c
The general structural features of the mono- and disubstituted PR
3
derivatives of 3c
were similar. Selected bond lengths and bond angles are listed in Table 4.3 together
with the common cluster numbering scheme.
Table 4.3. Selected bond lengths (Å) and bond angles (º) for PR
3
derivatives of 3c.
(3) Os(4)
Ir
Os(2) H
H
(3) Os

Ir
Os
H
H
P(5)
P(5)
P(6)
(1)
C(13)
O
(13)
(1)
(4)
(2)
C
(13)
O
(13)
Os
Os
(17) (18)

17a 17b 18a 18b
Ir(1)-Os(4) 2.7374(3) 2.7295(6) 2.7121(7) 2.7265(3)
Ir(1)-Os(3) 2.7642(3) 2.7768(6) 2.7454(7) 2.7395(3)
Ir(1)-Os(2) 2.7909(3) 2.7912(6) 2.7878(7) 2.7861(3)
Os(2)-Os(4) 2.8822(3) 2.8781(6) 2.9008(7) 2.8757(4)
Os(2)-Os(3) 2.9772(3) 2.9700(6) 3.0039(7) 2.9789(3)
Os(3)-Os(4) 2.8003(3) 2.8007(6) 28147(7) 2.8087(3)
Ir(1)-C(13) 1.888(5) 1.923(12) 1.903(12) 1.936(6)

Os(3)-C(13) 2.267(5) 2.275(13) 2.271(14) 2.157(6)
P(5)-Os(2)-Ir(1) 170.79(4) 163.37(7) 166.58(8) 166.37(5)
O(13)-C(13)-Ir(1) 146.6(4) 146.6(11) 148.4(12) 140.2(5)
O(13)-C(13)-Os(3) 130.5(4) 131.0(10) 129.3(11) 136.0(5)
Os(2)-P(5) 2.3462(13) 2.278(3) 2.348(3) 2.2727(17)
Os(3)-P(6) - - 2.356(3) 2.2856(18)


147
Both the mono- and disubstituted PR
3
derivatives possess a tetrahedral cluster core;
the bridging carbonyls and the bridging hydrides in the parent cluster are preserved. In
the monosubstituted derivatives, substitution occurred exclusively at the Os vertex
and it occupied an axial site. In the disubstituted derivatives, the two PR
3
ligands were
coordinated to two adjacent osmium atoms in an equatorial and an axial fashion.
The bridging carbonyl groups in all the four structures were asymmetric, with the
Ir(1)-C(13) and Os(3)-C(13) distances ranging between 1.888(5) Å to 1.936(6) Å,
and 2.157(6) to 2.275(13) Å, respectively. The asymmetry could have arisen from a
combination of two effects:
1. The different covalent radii of iridium and osmium [metal-metal bond
covalent radius of Ir = 1.34 Å as in Ir
4
(CO)
12
and Os = 1.41 Å as in
Os
4

(CO)
14
] .
2. A component from some semi-bridging nature of the carbonyl group. In
keeping with this the O(13)-C(13)-Ir(1) angles were approximately 21º larger
than the O(13)-C(13)-Os(1) in these structures. It has been reported that the
semi-bridging nature of the bridging carbonyl increases as M = Co < Rh < Ir
for the CpMOs
3
(μ-H)
2
(CO)
10
clusters; this increasing semi-bridging character
of the carbonyls lengthens the M-Os bond distances in the series proceeding
from cobalt to rhodium to iridium for the clusters CpMOs
3
(μ-H)
2
(CO)
10
, with
respect to the unbridged M-Os bonds. The CO-bridged metal-metal bond
distances for Co-Os, Rh-Os and Ir-Os were found to be 2.645(1) Å, 2.736(1)
Å and 2.798(1) Å respectively. The increase in the bond lengths in this series
is greater than the changes in the covalent radii of the atoms (Covalent radius
of Co = 1.26; Rh = 1.35 and Ir = 1.37) [36].

148
The Os-P bond distances in both the mono- and the disubstituted P(OMe)

3

derivatives, 17b and 18b were shorter than the corresponding PPh
3
derivatives, 17a
and 18a. Studies on the correlation between the PR
3
cone angles and the Os-P bond
lengths in Os
3
(CO)
11
(PR)
3
clusters have suggested that the larger the cone angle the
greater is the lengthening of the Os-P bonds and the cis Os-Os bonds. Furthermore, a
larger Tolman electronic parameter (χ) also led to shortening of the Os-P bond. [8].
The cone angle for P(OMe)
3
[θ = 107º] is smaller than that of PPh
3
[θ = 145º]

and the
Tolman electronic parameter for P(OMe)
3
ligand [χ = 2079.5 cm
-1
] is greater than that
of PPh

3
ligand [2068.9 cm
-1
][37]. Thus the observed shortening of Os-P bond lengths
in 17b and 18b compared to 17a and 18a are in accord with both the steric and
electronic factors. The Os-P bond distances in 17a and 18a were comparable to the
Os-P distance in Os
3
(CO)
11
(PPh
3
) (2.35 Å), and those in 17b and 18b were in
agreement with the Os-P bond distance in Os
3
(CO)
11
P(OMe)
3
(2.28 Å) [9, 38].
Although both the osmium-osmium bonds, Os(2)-Os(3) and Os(2)-Os(4), were cis to
the PR
3
group, the elongation of the bond distance was more pronounced in the
former. The PR
3
group was trans to the Ir(1)-Os(2) bond and this bond length was
considerably elongated compared to the other iridum-osmium bonds in these
structures, which may be attributed to a trans influence from the PR
3

group. Similar
trans bond lengthening has been reported in Os
3
(μ
3
-S)
2
(CO)
8
(PMe
2
Ph) [39].
Phosphines are believed to be better σ-donors than CO and the σ-inductive effect has
been argued to play a major role in the trans influence. Thus, a trans influence
strongly dominated by σ-inductive effects could be a plausible explanation for the
trans bond lengthening observed in these clusters.



149
4.3.1 Reaction of 3c with isocyanides

The reaction of 3c with
t
BuNC under TMNO activation at room temperature afforded
Cp*IrOs
3
(μ-Η)
2
(CO)

9
(CN
t
Bu), 19a, and Cp*IrOs
3
(μ-Η)
2
(CO)
8
(CN
t
Bu)
2
, 20a in good
yields. A similar reaction with CyNC afforded the monosubstituted derivative,
Cp*IrOs
3
(μ-Η)
2
(CO)
8
(CNCy), 19b, in 75% yield.
Diffraction-quality crystals were obtained for 19b by slow diffusion of hexane into a
dichloromethane solution; the ORTEP diagram and selected bond parameters are
shown in Figure 4.16.
The solid state structure reveals that one of the equatorial CO ligands attached to the
osmium triangle in 3c is substituted by the CyNC ligand and it is oriented cis to the
Os(4)-Os(3) bond. The CyNC ligand is slightly bent (<C(43)-N(43)-C(44) =
172.7(11)º) and leans more towards the Os(4)-Os(3) bond (<C(43)-Os(4)-Os(3) =
98.8(3)º.

The IR spectrum of 19b recorded in dichloromethane solution showed a broad peak at
2184 cm
-1
confirming the presence of the CyNC ligand. Diffraction-quality crystals
were not obtained for 19a. Its IR spectral pattern was almost similar to that of 19b
suggesting a similar structure for 19a. A FAB-MS spectrum of the crystals of 19a
showed a strong molecular ion peak at 1235.8. The formulation was further confirmed
by satisfactory elemental analysis.



150