1




Chapter One
Homo- and Heterobi- or Polymetallic Transition Metal
Complexes Based on Pincer Complexes, Phosphido-
and Oxalato- Ligands
Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer
Complexes, Phosphido- and Oxalato- Ligands
2
Chapter One
Homo- and Heterobi- or Polymetallic Transition
Metal Complexes Based on Pincer Complexes,
Phosphido- and Oxalato- Ligands
1.1 Introduction
The synthesis and reactivity of the homo-
1
and heterometallic
2
transition
metal complexes is a research topic that has been widely studied by inorganic
chemists. These complexes are particularly interesting because they often show
different reactivity compared to the mononuclear complexes. This difference is
due to the cooperative effect between the metals within a molecule.
3
Such an
effect has an implication in both stoichiometric
3, 4
and catalytic

5
reactions
involving the homo- and heterometallic complexes. These homo- and
heterometallic complexes are normally prepared from a cationic metal complex
(Lewis acid) and a metalloligand (Lewis base).

This thesis is aimed at the preparation of the homo- and heterometallic
complexes based on three types of selective and representative chemical
complexes, namely the homo- and heterometallic based on pincer [PCP]
complexes and the heterometallic complexes supported by the phosphido- as well
as the oxalato- ligands. An overview of these complexes is given in this chapter in
order to provide the background information of the complexes under study. The
Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer
Complexes, Phosphido- and Oxalato- Ligands
3
homo- and heterometallic based on pincer complexes will be introduced in the
next section, followed by the homo- and heterometallic complexes bridged by the
phosphido- and the oxalato- ligands. The objective and scope of this study will be
defined in the final section of the chapter.

1.2 Oligonuclear Homo- and Heterometallic Transition
Metal Complexes from the Pincer [ECE] Type
Ligands (C = Carbanion, E = Nitrogen, Phosphorus,
Sulfur)
The pioneering studies by Shaw
6
and van Koten
7
on the pincer complexes
have inspired many research groups to work on this type of complexes. The pincer

compounds are well known for their application in catalysis.
8
Apart from that,
they have been used to prepare the polynuclear homo- and the heterometallic
complexes.
9
These homo- and heterometallic complexes have various structures
and some of them show interesting properties.
10
These properties make them
potentially useful as catalysts,
11
electronic and optical devices,
12
metal templates
in the construction of macrocyclic rings
13
as well as building blocks and cross-
linkers for polygons
14
and polymers.
15
The oligonuclear pincer complexes play an
important role in the chemistry of dendritic molecules.
16
The dendrimers
incorporated with pincer complexes have shown to be useful in catalysis,
17
green
chemistry,

18
material chemistry
19
and gas sensing technology.
20
However, the
research that is specifically focussed on the oligonuclear pincer complexes is
rare.
9

Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer
Complexes, Phosphido- and Oxalato- Ligands
4
In this section, the oligonuclear pincer complexes of transition metals with
different types of ligand are reviewed. The discussion will be focussed on the
complexes with cyclometallated pincer ligands (Figure 1-1), where C refers to
either an aliphatic (1.1) or an aromatic (1.2) carbanion, while E refers to the donor
atoms like N, P and S. Other complexes such as those contain pincer [EC’E] (C’ =
carbene), EEE or EE’E (E ≠ E’) type ligands fall outside of the scope of current
discussion and will be excluded.


M
ER
n
ER
n
L
(
1.1

)
M
ER
n
ER
n
L
(
1.2
)
M

=

T
r
a
n
s
i
t
i
o
n

M
e
t
a
l

s
;

E

=

S
,

n

=

1
;

E

=

N
,

P
,

n

=


2

Figure 1-1: Chemical structures of the pincer [ECE] type complexes.
8



1.2.1 Oligonuclear Pincer Complexes Obtained by Coordination
of Bridging Ligands to Metal Vacant Sites
It is known that the halide-ligand trans to the M-C σ bond in a pincer
[ECE] complex can be easily replaced by a solvent molecule or a weakly
coordinating ligand.
21
This active site is used for the coordination of other
substrates. The following examples show how a homo- and heterometallic
complex can be prepared by using the metal active site. These complexes are
Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer
Complexes, Phosphido- and Oxalato- Ligands
5
prepared by either bridging the two pincer moieties using a bridging ligand or by
coordinating a metalloligand to the metal centre.

The anionic ligands such as halo,
22
hydrido,
23
acetato
24
and cyano

25
ligands
are known to serve as both terminal
26
and bridging ligands
22-25
in the coordination
complexes. The complexes with these ligands have been used as catalyst
precursors in the organic reactions.
27
Some of these complexes have been
identified as an intermediate in the chemical reactions.
28
It has been known that
the oligonuclear pincer complexes are readily formed from the halo,

hydrido,

acetato and cyano

ligands. Some of these examples are given in this section with
their significances highlighted.

The preparation of dinuclear pincer type complexes was first reported by
van Koten and co-workers.
29
In this report, a series of the monohalo-bridged
dinuclear Pd(II) and Pt(II) pincer complexes based on the tridentate ligand [{o,o’-
(Me
2

NCH
2
)
2
C
6
H
3
}]
-
were prepared (Scheme 1-1). The chemical structure of
complex 1.3a was confirmed by the single crystal X-ray diffraction technique.
The heterometallic complex 1.5 was also synthesised. This pioneering work has
led to the preparation of other similar compounds.
30

Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer
Complexes, Phosphido- and Oxalato- Ligands
6

NMe
2
NMe
2
M X
Me
2
N
Me
2

N
MH
2
O
X'
+
- H
2
O
Me
2
N
M'
NMe
2
X
M
Me
2
N
Me
2
N
M = M' = Pd; X = Cl (1.4a), Br (1.4b), I (1.4c) ; X' = BF
4
M = Pd, M' = Pt; X = Br; X' = BF
4
(1.5)
M = M' = Pt; X = Cl (1.3a), Br (1.3b), I (1.3c); X' = BF
4

X'

Scheme 1-1: Preparation of the monohalo-bridged dinuclear pincer [NCN] complexes of
Pd(II) and Pt(II).
29


The stability of the monochloro-bridged dinuclear pincer complex vs the
mononuclear aqua pincer complex was studied (Scheme 1-2).
31
The results
suggested that the product selectivity was apparently affected by the different
counter anions. Crystallographic and DFT calculation also suggested the existence
of hydrogen bonding in the mononuclear aqua complex. The DFT calculation was
previously performed on the dinuclear complexes 1.4a, 1.6a, 1.6b and the
mononuclear complex 1.7. The results showed a smaller energy difference (∆E
e
)
between the BF
4
-
complexes (∆E
e
= + 11 kcal/mol) and the [B{C
6
H
4
(SiMe
3
)}

4
]
-

complexes (∆E
e
= + 30 kcal/mol). It was believed that the formation enthalpy of
AgCl or NaCl played a role in shifting the equilibrium to the mononuclear aqua
complex 1.7.
31


Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer
Complexes, Phosphido- and Oxalato- Ligands
7
NMe
2
Pd
NMe
2
OH
2
BF
4
NMe
2
Pd
NMe
2
Cl

NMe
2
Pd
NMe
2
X'
Me
2
N
Pd
Me
2
N
Cl
[B{C
6
H
4
(SiMe
3
)}
4
1.6a,
[B{C
6
H
4
(SiMe
2
CH

2
CH
2
C
6
F
1
3
)}
4
] 1.6b
X' = BF
4
1.4a (acetone only)
(i)
(ii)
1.7

Scheme 1-2: Synthetic pathways of dinuclear complexes 1.4a, 1.6a, 1.6b and
mononuclear complex 1.7. (i) AgBF
4
, (CH
3
)
2
CO/H
2
O. (ii) M[B{C
6
H

4
(SiMe
2
R)}
4
] (M =
Na, Ag; R = Me, CH
2
CH
2
C
6
F
13
), (CH
3
)
2
CO/H
2
O or (CH
3
)
2
CO.
31


The dichloro-bridged homo- and heterometallic Rh(I) complexes were
prepared from the aqua complex 1.8.

32
The reactions of complex 1.8 with
[MCl(COD)]
2
(M = Rh, Ir) led to the dichloro-bridged dirhodium complex 1.9a
and the mixed metal Rh/Ir complex 1.9b (Scheme 1-3). On the other hand,
reaction of complex 1.8 with PdCl
2
(COD) and CuCl
2
gave Rh
2
M (M = Pd, Cu)
trinuclear complexes 1.10a and 1.10b. It was proposed that the two isomers of
complex 1.10b existed in the solution, giving rise to two sets of signal in its
1
H
NMR spectrum. The phosphine complex that is analogous to complex 1.9 was
also prepared and analysed by the X-ray crystallography.
11h

Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer
Complexes, Phosphido- and Oxalato- Ligands
8

Rh OH
2
NMe
2
N

Me
2
Rh
Cl
NMe
2
N
Me
2
Cl
Cl
RhCl
Me
2
N
N
Me
2
Cl
Cl
M
Rh
Cl
NMe
2
N
Me
2
Cl
Cl M

[MCl(COD)]
2
PdCl
2
(COD) or
CuCl
2
M = Rh (
1.9a
), Ir (
1.9b
)
M = Cu (
1.10a
), Pd(
1.10b
)
C
l
Cl
1.8

Scheme 1-3: Preparation of complexes 1.9 and 1.10 from complex 1.8.
32



Studies of the monohydrido-bridged complexes are of particular
importance since they have been identified as possible intermediates in the
hydride transfer reaction.

28c
The monohydrido-bridged complexes 1.11a and
1.11b, as well as the dihydrido bridged complexes 1.12a and 1.12b were first
prepared by van Koten and co-workers (Figure 1-2).
29
The dinuclear pincer [PCP]
hydrido Pd(II) and Pt(II) complexes 1.13a and 1.13b were prepared via the
reaction of an (CH
3
)
2
CO coordinated precursor complex with NaO
2
CH (Scheme
1-4).
33
The monohydrido-bridged heterometallic complex 1.13c was prepared
similarly as complex 1.3.
29



Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer
Complexes, Phosphido- and Oxalato- Ligands
9
Me
2
N
Pt
NMe

2
H
Pt
Me
2
N
Me
2
N
X'
X' =BF
4
(
1.11a
), OTf (
1.11b
)
NMe
2
NMe
2
M
BF
4
H
H
Pt
PPh
3
PPh

3
M = Pd (
1.12a
), Pt (
1.12b
)

Figure 1-2: Chemical structures of complexes 1.11 and 1.12.
29



PPh
2
PPh
2
M Cl
PPh
2
PPh
2
M O
CH
3
CH
3
OTf
Ph
2
P

PPh
2
M
H Ph
2
P
Ph
2
P
M'
AgOTf, acetone
- AgCl
NaO
2
CH, CH
3
OH
M = M' = Pd (1.13a)
PPh
2
PPh
2
Pt H
M = M' = Pt (1.13b)
M = Pd,M' = Pt (1.13c)
PPh
2
PPh
2
M O

CH
3
CH
2
+
OTf
OTf

Scheme 1-4: Preparation of the dinuclear complexes 1.13a, 1.13b and 1.13c.
33



The dinuclear Pd(II) acetato-bridged complex 1.14 was prepared by Soro
and co-workers (Figure 1-3).
30
This complex crystallised as ethanol solvate of
Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer
Complexes, Phosphido- and Oxalato- Ligands
10
[Pd
2
(NCN)
2
(μ-CH
3
CO
2
)]
2

[Hg
2
Cl
6
] with the dipincer cation, [Hg
2
Cl
6
]
2-
and ethanol
in 2:1:1 ratio. The two Pd(II) pincer moieties were found to stack on each other
with a Pd-Pd separation of 3.069(1) Å. The monomeric cation is stacked on
another dinuclear pincer Pd(II) unit with a Pd-Pd distance of 3.253(1) Å, forming
a Pd(II) chain [Pd-Pd-Pd angle = 164.7(1)˚].


N
N
N
N
Pd
Pd
BAr'
4
O
O
C CH
3


Figure 1-3: Chemical structure of complex 1.14.
30



The trinuclear pincer [NCN] carbonato Pd(II) complex 1.15 was isolated
by Protasiewicz and co-workers in an attempt to purify the iodo Pd(II) complex
1.16 (Scheme 1-5).
34
The X-ray structure of complex 1.15 showed three Pd(II)
pincer moieties bridged by a μ
3
-carbonato ligand with I
3
-
as counteranion. The
three pincer moieties in each individual trimer have the same chirality. The
molecule has a pseudo-three fold symmetry axis that passes through the carbon of
the carbonato linker and perpendicular to the plane of the carbonato ligand
bridging the pincer moieties. Complex 1.15 was believed to be formed by a
reaction of complex 1.16 with CO
2
(from atmosphere) and OH
-
(from water or
base).
34

Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer
Complexes, Phosphido- and Oxalato- Ligands

11
Pd
N
N
CH
3
H
3
C
H
3
C
CH
3
Pd
N
N
=
Pd
N
N
I
CH
3
H
3
C
H
3
C

CH
3
Pd
N
N
O C
O
O
Pd
N
N
Pd
N
N
3 I
-
3
CO
2
from atmosphere
OH
-
from water or base
1.15
1.16
3+

Scheme 1-5: Formation of complex 1.15 from complex 1.16.
34




The cyano-bridged dinuclear complexes 1.17 and 1.18 were synthesised in
the mid-1980s (Figure 1-4).
29a
They were prepared in a similar fashion to the
monohalide-bridged complex 1.3. It was proposed that the chemical structures of
complexes 1.17 and 1.18 were analogous to that of the complex 1.3. However,
their crystal structures have not been reported.


Me
2
N
M
NMe
2
CN
M
Me
2
N
Me
2
N
X'
M = Pt; X' =BF
4
(
1.18a

), OTf (
1.18b
)
M = Pd; X' =BF
4
(
1.17
)

Figure 1-4: Chemical structure of the cyano-bridged complexes 1.17 and 1.18.
29a


Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer
Complexes, Phosphido- and Oxalato- Ligands
12
The pincer [PCP] Rh(I) and Ir(I) complexes have been identified as
potential catalysts for dehydrogenation of alkanes.
35
They are more stable and
give higher TON than the known catalysts.
36
These complexes can be used in the
reaction involving different substrates. Furthermore, they are able to catalyse the
dehydrogenation reaction without a hydrogen acceptor.
37


A problem faced by the pincer [PCP] Rh(I) and Ir(I) catalysts in the
dehydrogenation reaction is that the catalysts are inhibited by nitrogen gas

(Scheme 1-6). It was found later that the catalyst inhibition is due to the formation
of a stable dinitrogen complex.
38
This was proven by the isolation of the Ir(I)
complex 1.19, which was crystallographically characterised. The observed N≡N
bond length [1.176(1) Å] in complex 1.19 is longer than the N≡N bond length in
free N
2
[1.098(2) Å].
39
The crystal structure showed that the two pincer units are
nearly perpendicular to each other [The dihedral angle between the planes defined
by P-Ir-P is 89.5˚]. This could be a factor that contributed to the stability of the
dinuclear Ir(I) complex. A recent study showed that complex 1.19 existed in
equilibrium with a terminal dinitrogen complex.
40
The analogous Rh(I) dinitrogen
complex was prepared and its crystal structure was reported.
41
The isolation of
complex 1.19 has led to the preparation of other dinitrogen bridged pincer type
complexes.
42


Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer
Complexes, Phosphido- and Oxalato- Ligands
13
Ir
P

t
Bu
2
P
t
Bu
2
N N Ir
Ir
P
t
Bu
2
P
t
Bu
2
H
H
H H
H CMe
3
+
Ir
P
t
Bu
2
P
t

Bu
2
H
HH
Ir
P
t
Bu
2
P
t
Bu
2
H
H
CMe
3
Ir
P
t
Bu
2
P
t
Bu
2
H
H
2
C

C
H
2
CMe
3
CH
3
CH
2
CMe
3
N
2
H
CMe
3
H
Fast
Slow
t
Bu
2
P
t
Bu
2
P
1
.
1

9

Scheme 1-6: Oxidative addition of tert-butylethylene (tbe) to the pincer [PCP] Ir(I)
catalyst and the catalyst inhibition by molecular nitrogen.
38


The di- and oligonuclear pincer complexes can be easily prepared by
bridging the mononuclear precursors with the commonly used bridging ligands.
43

The pincer [NCN] Pt(II) complexes bridged by dppm (1.20a and 1.21a),
deprotonated pyrazole (1.20b and 1.21b) and deprotonated imidazole ligands
(1.20c and 1.21c) were synthesised by reacting their respective mononuclear
chloro-precursors with the bridging ligands, followed by an anion exchange
reaction using LiClO
4
(Figure 1-5).
43a

Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer
Complexes, Phosphido- and Oxalato- Ligands
14
N
N
N
N
Pt
Pt
O

O
CH
3
O
O
H
3
C
E
E
N
N
N
N
Pt
Pt
O
O
O
O
E
E
C
12
H
25
C
12
H
25

ClO
4
E - E = dppm, n = 2 (1.20a)
E - E = deprotonated pyrazole, n = 1 (1.20b)
E - E = deprotonated imidazole, n = 1 (1.20c)
E - E = dppm, n = 2 (1.21a)
E - E = deprotonated pyrazole, n = 1 (1.21b)
E - E = deprotonated imidazole, n = 1 (
1
.
2
1
c
)
n
n+
n+
ClO
4
n

Figure 1-5: Chemical structures of the dinuclear pincer Pt(II) complexes 1.20 and 1.21.
43a


Recently Lang et al reported the synthesis of a heterometallic pincer
complex containing a bifunctional pyridine acetylide bridging ligand (Scheme 1-
7).
44
The Pt/Ru complex 1.22 was obtained from a ferrocenyl pincer metalloligand

and an unsaturated Ru(II) complex that formed upon elimination of the dinitrogen
bridging ligand. Complex 1.22 featured a redox active ferrocenyl unit and
conjugation across the two metal centres via the pyridine acetylide ligand.

Fe
Pt
N
M
e
2
NMe
2
C CC C N
N Ru
N
M
e
2
NMe
2
N N
NRu
M
e
2
N
Me
2
N
+

- N
2
Fe
Pt
NMe
2
NMe
2
C CC C N NRu
Me
2
N
Me
2
N
Cl
Cl
1
.
2
2
2
2


Scheme 1-7: Preparation of Pt/Ru complex 1.22.
44


Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer

Complexes, Phosphido- and Oxalato- Ligands
15
The electron-rich metal centre in the pincer [NCN] Pt(II) complex 1.23
makes it a good precursor to the heterometallic complexes with a M-M bond.
45

The reaction of complex 1.23 with metal complexes of Ag(I) or Hg(II) resulted in
the formation of a series of Pt/M (M = Ag or Hg) complexes (Scheme 1-8).
N
N
Pt
Ag N
N
Y
R
p-tol
B
r
(i)
(ii)
Y = N; R = Me (
1.25a
), Et (
1.25b
),
i
Pr (
1.25c
)
Y = CH; R = Me (

1.26a
), Et (
1.26b
),
i
Pr (
1.26c
)
Y = CH, R = Me (
1.24a
), Et (
1.24b
),
i
Pr (
1.24c
) , p-tol (
1.24d
)
Y = N; R = Me (
1.27a
), Et (
1.27b
),
i
Pr (
1.27c
)
1.23
N

N
Pt Br
N
N
Pt
Hg N
N
Y
R
p-tol
Cl
Br

Scheme 1-8: Preparation of complexes 1.24-1.27 from complex 1.23. (i) (1/n)[Ag(p-
tolylNYNR)]
n
. (ii) [Hg(p-tolylNYNR)Cl].
45


The structures of this series of compounds were represented by the crystal
structure of complex 1.26c.
45a
The five-coordinated Pt(II) centre has a nearly
square-pyramidal geometry. It is coordinated to the NCN tridentate ligand, the N
atom of the formamidino ligand and the Hg(II) metal. The Hg(II) centre has a
pseudotetrahedral geometry with a Pt-Hg-N angle that deviates from 90˚ [Pt-Hg-N
angle = 81.5(3)˚]. The distance between the Pt(II) and Hg(II) metals [2.833(7) Å]
is longer than those observed in other Pt-Hg bonded complexes [in the range of
2.576(2) Å and 2.609(3) Å].

46
This interaction can be viewed as a Pt(II) to Hg(II)
donor bond.
Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer
Complexes, Phosphido- and Oxalato- Ligands
16
1.2.2 Preparation of the Oligonuclear Transition Metal
Complexes from the Multipincer Ligands
Apart from using the vacant site on the metal, the oligonuclear pincer
complexes can also be prepared by using a multipincer ligand. This type of pincer
ligands has more than one tridentate site for metal coordination.
47
The additional
tridentate coordination sites can be introduced through a ligand modification. The
modification can be done either on the aryl ring (Figure 1-6, complex 1.28)
48
or
the donor arms (Figure 1-6, complex 1.29).
49
Complexes containing up to six
metal centres have been successfully prepared.
50


S
S
O
O
O
O

n n
O
O
O O
n
n
S
S
Pd NCCH
3
Pd
2 [BF
4
]
-
H
3
CCN
NMe
2
NMe
2
Pd
Me
2
N
Me
2
N
Pd

ClCl
1
.
2
8
1
.
2
9
2+

Figure 1-6: Chemical structures of the dinuclear Pd(II) complexes 1.28 and 1.29.
48,49



1.2.3 Summary
It has been demonstrated that the di- and oligonuclear complexes can be
constructed from a mononuclear pincer complex by applying different synthetic
strategies. Simple ligands such as halo- and hydrido ligands as well as more
complex bidendate ligands (N-heterocycles, phosphines or pyridine acetylide) can
be used as linkers to the pincer moieties. Many di- and oligonuclear pincer
complexes containing aromatic carbon backbones are known. However, those
Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer
Complexes, Phosphido- and Oxalato- Ligands
17
complexes with aliphatic carbon backbones have not been reported. It is
noteworthy that a number of mononuclear pincer complexes which can be used to
synthesise these di- and oligonuclear complexes are available.
51

Study of the
formation of these complexes is significant as it gives a better understanding on
the mechanism involved in chemical reactions catalysed by a pincer type complex.
An example has been given in scheme 1-6 where the isolation of dinitrogen
bridged Ir(I) complex 1.19 provided useful information to understand how a
pincer [PCP] Ir(I) catalyst is deactivated in the catalytic dehydrogenation reaction
of alkene.
38


The following discussion is focussed on another class of compounds,
namely the homo- and heterometallic complexes bridged by the phosphido ligands.
The chemical properties of a phosphido ligand and its complexes are different
from the ligands and their corresponding complexes discussed in the previous
section. Therefore, it is important to understand its chemistry so as to gain a
clearer view on the similarities and differences between the phosphido ligand and
the bridging ligands mentioned earlier, as well as the complexes supported by
these ligands.

1.3 Homo- and Heterometallic Complexes Bridged by the
Phosphido Ligands
Phosphido ligand [(PR
2
)
-
] is an important supporting ligand in homo-
52

and heterometallic
53

complexes. It often serves as a four-electron donor ligand
bridging across the homo-
54
or heterometals
55
(Figure 1-7, compounds 1.30 –
Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer
Complexes, Phosphido- and Oxalato- Ligands
18
1.35). Although the transition metal complexes bridged by the phosphido ligand
have been known long ago, their new chemistry continues to reveal throughout the
years. There are examples where the phosphido ligand acts as a terminal ligand
56

or a capping ligand between three metal centers.
57
An example is given in Scheme
1-9. In these complexes, one of the phenyl rings in the phosphido ligand acts as an
additional support to the metals (Compounds 1.37 and 1.38).
57



(OC)
3
Fe
Pt
P
h
2

P
SiR
3
NC
t
Bu
R = OMe 1.30a, Ph 1.30b
(OC)
4
Re
Re(CO)
4
P
H
t
Bu
t
Bu
Pt
Ph
2
P
Pt
P
Ph
2
Cl
Cl
P(p-Tolyl)
3

(p-Tolyl)
3
P
Pt
Ph
2
P
P
Ph
2
C
6
F
5
C
6
F
5
M
M
PPh
3
PPh
3
M = Ag 1.33a, Au 1.33b
Pd
R
2
P
Pd

Pd
Pt
Ph
2
P
Pt
P
Ph
2
C
6
F
5
Br
C
6
F
5
Ph
2
P
Pt
Ph
2
P
Br
Pt
C
6
F

5
C
6
F
5
2 [NBu
4
]
+
1.31
1.32
1.35
R
2
P
PR
2
Cl
Cl
Cl
R = N
i
Pr
2
1.34a, N(Cy)
2
1.34b
BuCN
t
2 -



Figure 1-7: Complexes supported by the phosphido ligand.
52-54


Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer
Complexes, Phosphido- and Oxalato- Ligands
19

Pt
Ph
2
P
M
NCCH
3
NCCH
3
P
Ph
2
C
6
F
5
C
6
F
5

M = Pd
M = Pt
- 4 NCCH
3
Pt
Pd
P
Ph
2
C
6
F
5
C
6
F
5
P
Ph
Pd
P
Ph
2
Pt
C
6
F
5
P
Ph

C
6
F
5
Pt
Ph
2
P
Pt
Pt
P
P
Ph
2
C
6
F
5
C
6
F
5
Ph
C
6
F
5
Pt
PPh
2

C
6
F
5
1.36
1.37
1
.
3
8
2


Scheme 1-9: Pd(II) and Pt(II) complexes with the phosphido ligands in a non-classical
coordination mode.
57

1.3.1 Synthesis and Chemical Reactivity of the Complexes
Bridged by the Phosphido Ligands
The complexes bridged by the phosphido ligands are typically obtained
either by an activation of the P-X bond in the ligand precursor PPh
2
X (X = H,
58

Cl
54c, 59
), the P-C bond of a phosphine ligand
60
or the P-P bond

61
of the dimeric
compound (PPh
2
)
2
in the presence of a metal complex. Scheme 1-10 shows the
synthetic pathways of a Mo/Ni (1.39)
62
and a Cr/Co (1.40)
63
heterometallic
complex obtained via a P-C bond cleavage of a phosphine ligand, whereas the
chromium complexes (1.41, 1.42 and 1.43) shown in Scheme 1-11 were obtained
through a desulfurisation reaction of a thiophosphinito ligand.
64


Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer
Complexes, Phosphido- and Oxalato- Ligands
20

Ph
2
P
Ni Mo
O
C
CO
C

O
+
excess PPh
2
(CH
2
CH=CH
2
)
refluxing
THF, 2 days
Mo
Mo
Ni
CO
Ph
2
P
CO
CO
+

Cr
Co
Co
C
Ph
OC
OC
OC

CO
CO
CO
CO
CO
O
O
O
Ph
2
P
PPh
2
Cr
Co
Co
C
Ph
CO
OC
CO
OC
PPh
2
O
O
O
1.39
1
.

4
0
CH
2
Cl
2
, reflux
CO


Scheme 1-10: Formation of the phosphido complexes 1.39 and 1.40 via a P-C bond
cleavage.
62, 63




Cr
OC
OC
S
P
R R
+
Cr
Cr
CO
OC
OC
OC

CO
CO
Cr Cr
CO
OC
CO
CO
H
P
H
3
C CH
3
Cr Cr
OC
CO
P
P
H
3
C
CH
3
H
3
C CH
3
Cr Cr
OC
S

P
H
3
C
CH
3
Cr
CO
+
+
[CpCr(CO)
2
]
2
+
[Cp
4
Cr
4
S
4
]
+
1.41
1.42
1.43
toluene,110 C
°



Scheme 1-11: Formation of the phosphido complexes 1.41, 1.42 and 1.43 via a
desulfurisation reaction of a thiophosphinito ligand.
64


Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer
Complexes, Phosphido- and Oxalato- Ligands
21
The di- and polynuclear complexes supported by the phosphido ligand are
resistant to fragmentation due to the strong M-P bond.
65
Scheme 1-12 shows a
series of reactions between the dinuclear molybdenum complex 1.44 and various
reagents.
65b
The Mo-P bonds were not affected at the end of the reactions.
65b


Mo
Cy
2
P
Mo
H CO
OC
Mo
Cy
2
P

Mo
H
CO
OC
OC
CO
Mo
C
y
2
P
Mo
C
H
CO
OC
N
t
Bu
CO
CN
t
Bu
Mo
2
Cp
2
(CO)
6
Mo

Cy
2
P
Mo
Sn
Ph
3
CO
OC
HSnPh
3
MnCp'(CO)
2
Mo
Cy
2
P
Mo
Mo
CO
photolysis
CO
OC
CO
OC
Mo
Cy
2
P
Mo

Mn
CO
H
OC
CO
OC
1.44
1.45
1.46
1.47
1
.
4
8
1.49

Scheme 1-12: Reactivity of the dinuclear molybdenum complex 1.44 towards various
reagents.
65b


Strong M-P-M cores allow the complexes to be used as precursors to the
homo- and heterometallic complexes.
66
Metal(s) can be incorporated into the low
Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer
Complexes, Phosphido- and Oxalato- Ligands
22
nuclearity complexes without breaking the M-P-M core, as shown in Scheme 1-
13.

66a, 66b


CO
Cp
Cp
Cp
Cp
Cp
Fe
C
y
2
P
Fe
H
Cp
OC
CO
trans 1.50
Cy
2
P
FeCp(CO)
(CO)CpFe
Li
LiBu
Fe
C
y

2
P
Fe
Au
Cp
OC
CO
cis 1.52a, trans 1.52b
P
i
Pr
3
(P
i
Pr
3
)AuCl
Mo
Cy
2
P
Mo
C
H
2
Cp
OC
CO
H
- H

2
- CO
Mo
Cy
2
P
Mo
Cp
OC
CO
C
H
(OC)
5
Mo
Mo
Cy
2
P
Mo
OC
Cp
Fe
2
(CO)
8
C
H
Fe
(CO)

3
Fe(CO)
3
Mo(CO)
6
photolysis
1.51
1
.
5
3
1
.
5
4
1
.
5
5
photolysis

Scheme 1-13: The homo- and heterometallic complexes generated from complexes 1.50
and 1.53.
66a, 66b



Although the complexes supported by a phosphido-ligand are generally
thought to be stable, there were reports on the transformation of phosphido
ligand

67
or the cleavage of M-P bond.
68
As shown in Scheme 1-14, the insertion of
sulfur into the Mo-P bonds in complex 1.54 gave dithiol complexes 1.55 and
1.57.
67a
Another example shown in Scheme 1-15 depicts the formation of Pt/Os
heterometallic complexes 1.60 and 1.61 which involves the cleavages of the Pt-P
and the Os-P bonds in complex 1.59.
68a


Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer
Complexes, Phosphido- and Oxalato- Ligands
23

(OC)
2
W
P
h
2
P
Mo(CO)
4
+
S
8
CH

2
Cl
2
, reflux, 40 h
W
(CO)
3
Ph
2
P
Mo(CO)
5
W
S
W
SS S
W
S
Mo
S
S
S
S
S
PPh
2
Mo
S
Mo
S

S S
W
S
S
P
Ph
2
S
PPh
2
S
S
S
Ph
2
P
+
+
1.54
1.55
1.56
1
.
5
7
1.58
+

Scheme 1-14: Sulfur insertion into the Mo-P bonds of the phosphido complex 1.54.
67a




H
Pt Os
P
t
Bu
t
Bu
Pt
Os
Os
C
C
Me
Me
Bu
3
P
174
o
C
2h
Os
Os
Os
Pt
P
Bu

3
P
t
Bu
t
Bu
C
C
C
H
Me
H
Pt Pt
P
t
Bu
t
Bu
Pt Pt
Os
Os
P
Os
Bu
t
Bu
+
1
.
5

9
1
.
6
0
1
.
6
1
t
t
t

Scheme 1-15: Formation of the Pt/Os complexes 1.60 and 1.61 from the vinylidene Pt/Os
complex 1.59, which involves the cleavages of the Pt-P and Os-P bonds.
68a



The phosphido ligand can be used as a bridging ligand to link two
relatively bulky moieties. The mononuclear phosphido complex 1.62 was used as
a precursor to the dinuclear pincer type complex 1.63 (Scheme 1-16).
69
The
heterometallic Pt/Pd phosphido-bridged complex 1.64 has been prepared in a
Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer
Complexes, Phosphido- and Oxalato- Ligands
24
similar way by reacting [Pd(C
6

H
4
CH
2
NMe
2
-2)(CH
3
CN)
2
][BF
4
] with complex 1.62.
However, the analytical data showed the product to be the aqua complex 1.65
(Figure 1-8). It was believed that complex 1.65 was formed via a substitution of
the CH
3
CN ligand in complex 1.64 by an aqua ligand. The crystal structure of
complex 1.65 showed that the Pt- P bond length [2.439(2) Å] is longer than those
observed for the two molecules of the NCN Pt(II) diphenylphosphine complex
1.66 [The bond lengths observed for two molecules in the unit cell are 2.346(2) Å
and 2.360(2) Å]. The angle between the two metal coordination planes is 71.1(2)˚.

NMe
2
NMe
2
Pt PPh
2
Me

2
N
Pt
NMe
2
P
Pt
Me
2
N
Me
2
N BF
4
P
h
P
h
Me
2
N
Pt
NMe
2
P
Pd
Ph
Ph
N
Me

2
H
3
CCN
(i)
(ii)
1.63
1
.
6
4
1.62
BF
4

Scheme 1-16: Preparation of complexes 1.63 and 1.64 from complex 1.62. (i)
[Pd(OH
2
)(NCN)][BF
4
]. (ii) [Pd(C
6
H
4
CH
2
)NMe
2
-2)(CH
3

CN)
2
][BF
4
].
69




Chapter One: Homo- and Heterobi- or Polymetallic Transition Metal Complexes Based on Pincer
Complexes, Phosphido- and Oxalato- Ligands
25

NMe
2
NMe
2
Pt PHPh
2
Me
2
N
Pt
NMe
2
P
Pd
BF
4

Ph
Ph
N
Me
2
H
2
O
1
.
6
5
1.66

Figure 1-8: Chemical structures of complexes 1.65 and 1.66.
69


1.3.2 Recent Work on the Phosphido-bridged Complexes
There are continuing efforts to study the reactivity of the phosphido-
bridged complexes.
70
Ruiz et al have investigated the reactivity of the unsaturated
complexes [M
2
Cp
2
(µ-COMe)(µ-PR
2
)

2
]BF
4
[M = W, R = Ph (1.67a); M = Mo, R =
Et (1.67b) ].
70a
Reactions of the complexes with carbonyl, isocyanide, bidentate
phosphines and diazoalkane gave rise to a series of compounds as shown in
Scheme 1-17.