Introduction

1

1. Introduction

1.1. Overview

Organometallic chemistry, which involves metal complexes containing direct

metal-to-carbon bonds, has grown since the early 1950s at an almost exponential rate,

mostly owing to the development of an impressive array of highly sophisticated

apparatus of which in particular NMR and single-crystal X–ray equipments have been

invaluable [1]. Theoretical studies of the bonding in metal compounds and of the

course of reaction pathways have not only contributed to new knowledge, but also to

the purposeful design of complexes and their use in stoichiometric and catalytic

reactions.

Platinum metal complexes of unsymmetrical chelating ligands such as

functionalized phosphines have aroused much interest in recent time because of their

structural novelty, reactivity and catalytic applications, namely carbonylation [2–4],

hydrogenation [2c,5,6] and hydroformylation [2c,7,8] reactions. Homogeneous

catalysis by transition metal complexes has become a major synthetic tool in industrial

processes [2–8]. The potential of such catalytic reactions is due to the fact that most of

these reactions are highly selective, low energy processes and show high rate of

conversion. Among the platinum metals, rhodium, iridium and ruthenium complexes

have been given special attention in this thesis due to their versatile catalytic activity

and unique character.

1.2. Rhodium

Rhodium is one of the rarest and expensive precious metals. It has a typical

2

electronic configuration ([Kr]4d85s

1) and exhibits mainly the principal oxidation

states of +1 and +3. RhCl3.xH2O is a versatile starting material for the synthesis of

different rhodium(I) complexes. One of the most famous rhodium(I) complexes is the

Wilkinson’s catalyst, [RhCl(PPh3)3] [9a]. Great varieties of rhodium complexes are

prepared from RhCl3.xH2O and the methods for preparation of some selected starting

rhodium complexes are shown in Scheme 1.1.

Scheme 1.1. Important preparative routes for starting rhodium complexes

The primary use of rhodium is in automobiles as a catalyst converter, which

converts harmful emission like CO, unburned hydrocarbon (HC) and oxide of nitrogen

(NOx) from the engine into less harmful gases like CO2, N2 and H2O [10].

Rhodium(I)-complexes play an important role as industrial homogeneous catalysts for

the synthesis of value added products. A few important examples are: (i) Synthesis of

L-DOPA (a drug for curing Perkinson's disease) [11], (ii) Production of acetic acid by

RhCl3.3H2OCO, moisture, 100 oC

[RhCl(CO)2]2

trans-[RhCl(CO)(PPh3)2]

Ref. 9a

C2H5OH, CO, PPh3

Ref. 9b

PPh3, C2H5OH

Ref. 9c

[RhCl(PPh3)3]

COD, C2H5OH / H2O

Ref. 9d[RhCl(COD)]2

COE, Me2CHOH / H2O

Ref. 9e

[RhCl(COE)2]2

3

Monsanto’s process [12,13] (iii) hydroformylation of alkene to synthesize highly

selective linear to branch aldehydes [8,14], (iv) hydrosilylation crosslinking of

silicone rubber [15] and many other organic transformations [16].

1.3. Iridium

Iridium is the most corrosion-resistant metal, even at temperatures as high as

2000 °C. The electronic configuration of iridium is [Xe]4f14

5d76s

2, which exhibits

different oxidation states ranging from -3 to +6 and the most stable oxidation states

are +3 and +4 [5d]. Iridium forms a number of organometallic compounds used in

industrial catalysis, and in research [5d]. Some organometallic iridium(I) compounds

are notable enough to be named after their discoverers. Two important examples are:

(i) Vaska's complex, [IrCl(CO)(PPh3)2], which has the unusual property of binding to

the dioxygen molecule, O2 [17], and (ii) Crabtree's catalyst, [Ir(COD)(Pcy3)(Py)]+PF6

–,

a homogeneous catalyst for hydrogenation reactions [18]. These compounds are both

square planar, d8 complexes, with a total of 16 valence electrons, which accounts for

their reactivity [19]. Although iridium complexes are less frequently used than their

rhodium analogues, in some processes they may be more effective– the Cativa process

for carbonylation of methanol being an excellent example in the arena of bulk

chemistry [3a,b]. Perhaps the most important catalytic applications of iridium

complexes, however, are in the manufacture of fine chemicals, most notably in areas

of chemoselective and enantioselective hydrogenation [5d]. Great varieties of iridium

complexes are prepared from IrCl3.xH2O and the methods of preparation of a few

selected starting iridium complexes are shown in Scheme 1.2.

4

Scheme 1.2. Some important preparative routes for starting iridium complexes

1.4. Ruthenium

Ruthenium is a rare transition metal belonging to the platinum group of the

periodic table. It has an electronic configuration [Kr]4d75s

1 and can show a much

wider scope of oxidation states e.g. -2, 0, +2, +3, +4, +5, +6 and +8 valence states in

[Ru(CO)4]2-

, [Ru(CO)5], [Ru(H2O)6]2+

, [Ru(NH3)6]3+

, [RuO2], K[RuF6], [RuF6] and

[RuO4] respectively and various coordination geometries in each electronic

configurations [21]. For instance, in the principal oxidation states of +2 and +3,

ruthenium complexes normally prefer trigonal-bipyramidal and octahedral structures

respectively. Due to this variable oxidation states and higher economy of ruthenium

compared to the other platinum metals, ruthenium catalyzed processes have become

IrCl3.3H2O trans-[IrCl(CO)(PPh3)2]Ref. 20c

Ref. 20a

COD, C2H5OH / H2O

Ref. 20a[IrCl(COD)]2

COE, Me2CHOH / H2O

Ref. 20d

[IrCl(COE)2]2

PPh3, DMF

Cp*, alcohol[IrCl2(Cp*)]2

2-methoxyethanol,

120 oC, Ref. 20e

Et4NX/ / KI, CO

(X/ = Cl, Br)[Ph4As][IrX2(CO)2]

CO, Ref. 20bPPh3

[X = Cl, Br, I]

5

one of the most important and interesting methodologies in organic synthesis [21b,22–

24]. In particular, ruthenium complexes catalyzed hydrogenation reaction is one of the

most appealing routes for the reduction of various organic substrates [23]. The

development of Noyori’s transfer hydrogenation catalysts has attracted the attention of

the researchers towards the use of ruthenium catalysts in asymmetric hydrogenation

reactions [21b,22a,24c,25]. The ruthenium complexes also exhibit a variety of suitable

characteristics like high Lewis acidity, high electron transfer ability, low redox

potentials, stabilities of different reactive species etc. RuCl3.xH2O can be utilized for

the synthesis of ruthenium complexes of different ligands and the methods for

preparation of some selected starting ruthenium complexes [25d] are shown in

Scheme 1.3.

1.5. Carbon monoxide (CO) ligand

Carbon monoxide is the most common π-acceptor ligand in organometallic and

coordination chemistry. It plays a key role in many catalytic processes. Over the last

few decades, transition metal carbonyl complexes have become one of the most

important families of compounds in organometallic chemistry [27]. Rhodium and

iridium carbonyls are very active catalysts for carbonylation and hydroformylation

reactions to synthesize industrially important products [2,3,8,28]. Some prominent

examples are: (i) Rhodium catalyzed Monsanto’s process [12,13] for producing acetic

acid from methanol, (ii) A recent industrial catalytic process called ‘Cativa’ [3] where

[IrI2(CO)2]- species with ruthenium-complex activator are used for carbonylation of

6

methanol with high efficiency and (iii) Hydroformylation of alkenes to synthesize

highly selective linear to branch aldehydes [8,14].

Scheme 1.3. Some important preparative routes for ruthenium complexes [25d].

1.6. Functionalized phosphine

Tertiary phosphines functionalized with chalcogen donors like oxygen [29],

sulfur [30] and selenium [31] containing ‘Soft’ phosphorus and ‘Hard’ oxygen donors

or ‘Soft’ phosphorus and relatively less ‘Softer’ sulfur and selenium donor ligands

with distinctly different π-acceptor strength [5a] form a variety of metal complexes

RuCl3.nH2O

CO, Ethanol,

PPh3, CO,

[RuCl2(CO)2]n

[RuCl2(CO)(PPh3)2]

Ref. 26e,f[Ru3(CO)12]

CO, MeOH / CO, Zn

PPh3, EtOH[RuCl2(PPh3)3]

C5Me5H[RuCl2Cp*]n

1,5-C8H12, Zn[Ru(COD)(cot)]

Ref. 26h

Ref. 26d

Ref. 26g

PPh3, HCHO[RuH2(CO)(PPh3)3]

Ref. 26c

Reflux.

alc., Boil

1,5-C8H12, Zn [RuCl2(COD)]n

Ref. 26a

Ref. 26i

Ref. 26b

7

due to their different bonding abilities. Oxygen being a ‘Hard’ donor is capable of

stabilizing metal ions in higher oxidation states which can be ascribed due to the

absence of dπ-back bonding. On the other hand, ‘Soft’ phosphorus donors stabilize

metal ion in low oxidation state by dπ–σ* back bonding. The metal-oxygen bond is

weak and may dissociate reversibly to generate a vacant site for substrate binding and

such ligands are called ‘Hemilabile’. The M–S/Se bonds are comparatively labile than

the M–P bonds in the chelated complexes of P–S and P–Se donors ligands and thus

may permit facile generation of a ‘Vacant site’ at the metal centre [5a,32] and may

show interesting dynamic stereo-chemistry. Creation of such vacant coordination site

through a reversible ‘Opening and Closing’ mechanism [32b] is useful for

coordination and activation of the small molecules for further reaction with other

organic substrates (Scheme 1.4).

Scheme 1.4. Ring “Opening and Closing” mechanism (Hemilabile)

A thermodynamically stable metal complex will have the substitutionally

inert ligand group (X) binding to soft metals if X is a soft base, such as the case with

phosphorus based hemilabile ligands, or to hard metals if X is a hard base such as

oxygen or fluorine. These ligands were first described in a kinetic context as

‘Hemilabile ligands’ in 1979 by Jeffrey and Rauchfuss [32c].

LnM

X

Y

+ S

- SLnM

X

SY

M = Metal centre, Ln = Ligand, X = Inert group,

Y = Labile group, S = Solvent,

8

1.7. Rhodium, iridium and ruthenium complexes of functionalized phosphine: A

brief summary of the prior art.

Platinum metals particularly rhodium, iridium and ruthenium form a wide

variety of complexes with functionalized phosphine ligands. A brief summary of

different potential metal complexes are highlighted below:

1.7.1. Complexes of mono- and bidentate phosphines

A large number of coordination metal complexes of the first row transition

metals [33], lanthanides [34] and actinides [35] with phosphine-chalcogenide ligands

have been reported. However, the platinum metals like rhodium, iridium and

ruthenium complexes have got less attention. A few interesting neutral rhodium(I)

complexes of the type [Rh(COD)ClL] (L = Me3PS, Me2PhPS, Me2PhPSe, Me3AsS

[36]), [RhCl(CO)2(Cy3PO)] (Cy = C6H11) [37] and [RhCl(CO)2Ph3PX] (X = O, S, Se)

[38], and cationic complexes of the type [Rh(COD)(OER3)2]ClO4 and [Rh(CO)2-

(OER3)2]ClO4 ( E = P, As; R = Ph, p-MeC6H4, p–MeOC6H4 ) [39] have been reported.

There are also some reports on the rhodium(III) phosphineoxide complexes such as

[RhCl(ylide)2(OPR3)] [40], [Rh(dimethylglyoximate)2(OPPh3)] [41] etc. Reports on

the iridium complexes of chalcogen functionalized phosphines are limited except a

series of Vaska type complexes, [IrX(CO)L2], where X = Cl, Br, I and L = substituted

PPh3 [42]. The interesting monodentate ruthenium complexes reported so far are

[RuCl2(DMSO)3(PPh3O)] [43] and [RuCl2(CO)2(Ph3PX)x], (X = O, S, Se) [44].

During last two decades, increasing attention has been focused on the complex

chemistry of hemilabile ligands, particularly, bidentate tertiary phosphines

9

functionalized with oxygen [5b,32a,45], sulfur [46,47] and selenium [48] donors.

Reports on catalytic reactions of complexes containing sulfur or selenium donor are

scanty because of metal poisoning by sulfur atom [49a] and deselination of selenide

under the reaction condition [49b]. However, a few metal complexes containing P–S /

P–Se ligands have been reported to exhibit efficient catalytic activity in potential

organic synthesis [4e,50]. A few interesting metal complexes of unsymmetrical

chalcogen functionalized phosphines have been discussed with special emphasis on

bidentate phosphines.

There are different types of hemilabile P–O donor ligands such as ester-

phosphine [51], ether-phosphine [52], keto-phosphine [53], phosphine-acids [54],

diphosphine-monoxide [55] etc. An outstanding feature of these ligands is their

capability of coordinating to the metal centre either through a monodentate (ƞ1-P) or

bidentete (ƞ2-P,O) coordinated fashion depending upon the metal and its environment

The complexes [RhCl(CO)(ƞ2-P,O-BPMO)] (BPMO = dppmO, dppeO,

dpppO, and dppbO) [56a] and the iodo dppmO analogue [56b] are prepared by

reacting [RhX(CO)2]2 (X = Cl, I) with corresponding BPMOs (Eqn. 1). The five-

membered complex [RhCl(CO){ƞ2-P,O-Ph2PCH2P(O)Ph2}] does not react with

CO under low pressure (1 atm.), however, on increasing the pressure to 3 atm., a trace

amount of dicarbonyl complex cis-[RhCl(CO)2{ƞ1–P-Ph2PCH2P(O)Ph2}] is formed

[56c]. On the other hand, the six-membered complex [Rh(CO)Cl{ƞ2-P,O-Ph2PCH2-

CH2P(O)Ph2}] can reversibly be cleaved by CO even at low pressure (1–3 bar) to give

cis-[RhCl(CO)2{ƞ1-P-Ph2PCH2CH2P(O)Ph2}] (Scheme 1.5).

10

Scheme 1.5. ‘Hemilabile’ behaviour of P,O donor ligand.

Two complexes of the types [RhCl(CO)(2-Ph2PC6H4COOMe)] (ƞ2-P,O

chelate) and trans-[RhCl(CO)(2-Ph2PC6H4COOMe)2] (ƞ1-P coordinated) are reported

[4a]. The X-ray structure (Figure 1.1) of the latter complex indicates the presence of

‘Secondary’ Rh–O interactions with the ester groups of the phosphines, which exhibits

(CO) band at 1949 cm-1

indicating the lowest value so far reported of such

complexes.

P,S ligands show an interesting feature as [RhCl(CO)2]2 reacts with

Ph2PCH2SCH3 to produce a binuclear complex [Rh2Cl2(CO)2(µ-Ph2PCH2SCH3)2],

while with Ph2PCH2CH2SCH3 it yields a mononuclear complex [RhCl(CO)(Ph2P-

CH2CH2SCH3)] (Scheme 1.6) [57a].

Rh

OC

ClP

PPh2O

Ph2

Rh

OC

OC

Cl

P

PPh2O

Ph2

+ CO

[RhX(CO)2]2

CH2Cl2 / toluene

P

OP

Rh

OC X

[Eqn 1]

X = Cl; BPMO = dppmO, dppeO, dpppO, dppbOX = I; BPMO = dppmO

BPMO

11

Figure 1.1. X-ray structure of trans-[RhCl(CO)(2-Ph2PC6H4COOCH3)2] [Ref. 4a]

Scheme 1.6. Mono and binuclear rhodium carbonyl complexes of P,S donor ligands

Unsymmetrical phosphine–phosphine monoselenide ligands react with

[RhCl(CO)2]2 to form chelate [RhCl(CO)(ƞ2-P,Se-Ph2PCH2P(Se)Ph2)], [RhCl(CO)-

(ƞ2-P,Se-Ph2PN(CH3)P(Se)Ph2)] [4d] and non-chelate [RhCl(CO)2(ƞ

1-P-Ph2P-

(CH2)nP(Se)Ph2)] {n = 2–4} complexes depending on the nature of the ligands [4e].

L2: Ph2PCH2SCH3

L1: Ph2PCH2CH2SCH3

RhOC

Cl

Rh

OC

Cl

Ph2P SCH3

H3CS PPh2

L1L2

Rh

Cl CO

H3CS PPh2

OC

OCRh

Cl

ClRh

CO

CO

12

The molecular structure of [RhCl(CO)(ƞ2-P,Se-Ph2PN(CH3)P(Se)Ph2)] (Figure 1.2)

shows a stable five-member heterocyclic (Rh, P, N, P and Se) ring.

Figure 1.2. X-ray structure of [RhCl(CO){ƞ

2-P,Se-Ph2PN(CH3)P(Se)Ph2}] [Ref. 4d].

In addition to the several rhodium carbonyl complexes of unsymmetrical

phosphine-phosphine chalcogenides, the complexes of symmetrical phosphine-

phosphine chalcogenides are also reported [57b]. The preparation of rhodium carbonyl

complex [Rh(CO)2(dppmS2)][ClO4], where dppmS2 = Ph2P(S)CH2P(S)Ph2 is reported

[57b] and the molecular structure of which was established by single crystal X-ray

diffraction studies.

Some interesting complexes [Rh(CO)(PPh3){ƞ2-P,O-Ph2PNP(O)Ph2}],

[Rh(CO)2{ƞ2-Se,Se

/-Ph2P(Se)NP(Se)Ph2}] and [Rh(CO)(PPh3){ƞ

2-Se,Se

/-Ph2P(Se)-

NP(Se)Ph2}], are synthesized by stepwise reactions of CO and PPh3 with

[Rh(COD){ƞ2-P,O-Ph2PNP(O)Ph2}] and [Rh(COD){ƞ

2-Se,Se

/-Ph2P(Se)NP(Se)Ph2}]

respectively [57c]. The structure of the complex [Rh(CO)(PPh3){ƞ2-Se,Se

/-

Ph2P(Se)NP(Se)Ph2}] (Figure 1.3) exhibits a slightly distorted boat ring conformation.

13

Figure 1.3. The structure of [Rh(CO)(PPh3)(ƞ2-Se,Se-{Ph2P(Se)}2N)] [Ref. 57c].

Iridium also forms interesting complexes by the reaction of [IrL2]Cl (L =

Ph2PCH2CH2PPh2) with S8 or Se8 to yield [IrS2L2]Cl or [IrSe2L2]Cl, respectively;

where S2 and Se2 act as bidentate ligands [58a].

The neutral complex [IrCl(COD)(ƞ1-P-L)] (I) (L = dicyclohexyl(2-methoxy-

ethyl)phosphine), obtained from the reaction of [Ir(µ-Cl)(COD)]2 with L, reacts with

AgBF4 to undergo Cl- abstraction and subsequent occupation by the ether O atom at

the empty coordination site to produce cationic complex, [Ir(COD)(ƞ2-P,O-L)]Cl (II)

In the presence of H2 and L, the reactive Ir–O bond in II is cleaved with formation of

cis-[H2Ir(COD)L2] (III) (Figure 1.4) [58b].

A few more interesting iridium complexes are obtained by the reaction of

trans-[IrI(CO)(PEt3)2] with Y(SiH3)2 (Y = O, S, Se) in benzene to give [IrI(CO)H-

(PEt3)2(SiH2YSiH3)] or [IrI(CO)H(PEt3)2(SiH2)]2Y, although when Y = O the

formation of the former species is difficult to detect. The reaction of the complexes

with P(SiH3)3 leads to the formation of IrI(CO)H(PEt3)2[SiH2P(SiH3)2],

[IrI(CO)H(PEt3)2(SiH2)]2PSiH3, or [IrI(CO)H(PEt3)2(SiH2)]3P, depending on the

proportions of the reactants used [58c].

Rh

OC

Ph3P

Se

Se

PPh2

N

PPh2

14

Figure 1.4. Synthesis of different neutral and anionic iridium complexes of P,O donor

ligand.

Ruthenium forms various types of complexes containing chalcogen

functionalized phosphines as coordinating ligands. The polymeric complex

[RuCl2(CO)2]n reacts with Ph3PX; X = O, S and Se in an appropriate Ru : Ligand mole

ratio to yield five- and six-coordinated complexes of the types [RuCl2(CO)2(Ph3PX)]

and [RuCl2(CO)2(Ph3PX)2] respectively (Scheme 1.7) [44]. The ν(PX) bands of the

complexes show significantly lower wavenumber than the corresponding free ligands

indicating the formation of Ru–X bonds.

The reactions of [ƞ6-C6Me6)RuCl2]2 and [(ƞ

5-C5Me5)RhCl2]2 with the ligands L

= Ph2PCH2PPh2 or Ph2PCH2P(Se)Ph2 in benzene yield neutral complexes

[(ring)RuCl2(ƞ1-L)], [ring = (ƞ

6-C6Me6) or (ƞ

5-C5Me5)] [59]. However, cationic

complexes of the type [(ring)RuCl(ƞ2-L)]ClO4 can be prepared using acetone as

solvent in the presence of NaClO4 [59].

IrCl

P

H3CO

Ir

P

OAgBF4

H2 L

Ir

H L

H

L

(I)

(II)

(III)

P

H3CO

L :

+ BF4-

+ BF4-

15

Scheme 1.7. The reactions between [RuCl2(CO)2]n and Ph3PX (X = O, S, Se)

Interesting complex is formed by the reaction of diphosphine, oxygen-

functionalized diphosphine ligands and RuCl3 in the presence of formaldehyde (Figure

1.5) [60], where the Ru–P bond trans- to oxygen is considerably shorter (222 pm) than

the other two Ru–P bonds. The complex [RuCl2(P∩O)(P~O)2], formed by the reaction

between [RuCl2(PPh3)3] and P,O ligands, shows fluxional behaviour (Scheme 1.8)

[61].

Figure 1.5. Ruthenium complexes of P,P and P,O donor ligands [Ref. 60]

The bisphosphine monoxides (R)- and (S)-BINAPO(O), prepared by the mono-

oxidation of (R)- and (S)-BINAP respectively, form [(p-cymene)RuCl{ƞ2-P,O-

BINAPO(O)}]Cl and [(p-cymene)Ru{ƞ2-P,O-BINAPO(O)}](SbF6)2 complexes [62].

The BINAPO(O) ligand binds diastereo selectively, and form a single thermo-

[RuCl2(CO)2]n

n[RuCl2(CO)2(Ph3PX)2]n[RuCl2(CO)2(Ph3PX)]Ph3PX

2n mol Ph3PXn mol Ph3PX

X = O, S, Se

P

P

Ru

P

O P

Ph MeCl

Cl

Me Ph

Ph MePhMe

16

dynamically stable diastereomer of the two possible isomers. The compound [(p-

cymene)Ru{ƞ2-P,O-BINAPO(O)}](SbF6)2 is an efficient catalyst for the Diels-Alder

reaction of methacrolein and cyclopentadiene in moderate ee.

Scheme 1.8. Fluxional behaviour of trans-,cis-,cis- [RuCl2(P∩O)(P~O)2]

The hexa-coordinated chelate complex cis-[RuX2(CO)2(P∩S)] {P∩S= ƞ2-P,S-

coordinated, X = Cl, I} and penta-coordinated non-chelate complexes cis-[RuCl2-

(CO)2(P~S)] {P~S = ƞ1-P-coordinated} are synthesized [63a,b] by the reaction of

[RuX2(CO)2]n with equimolar quantity of the ligands Ph2P(CH2)nP(S)Ph2 (n = 1–4) at

room temperature (Scheme 1.9, 1.10). The bidentate nature of the ligand

Ph2PCH2P(S)Ph2 in the complex leads to the formation of five-member chelate ring

which confers extra stability to the complex. While, the reaction with 1:2 (Ru:L) mol

ratio affords the hexa-coordinated non chelate complexes cis-[RuX2(CO)2(P~S)2]

irrespective of the ligands.

[RuCl2(PPh3)3]

RuO

P3P1~O

P2~O

Cl

Cl

RuO

O3~P P1~O

P2

Cl

Cl[P,O : (CH3)2PCH2CH2OCH3]

P,O

17

Scheme 1.9. Synthesis of iodo-carbonyl ruthenium(II) complexes of P,S ligands.

Scheme 1.10. Synthesis of chloro carbonylruthenium(II) complexes of P,S ligands.

[RuI2(CO)2]n

n mol (i)

2n mol (i-iv)

n mol (ii-iv)

Ru

OC

IS

P

I

CO

Ru

OC

I P~S

I

P~S

CO

Ru

S~P

I CO

I

CO

n mol (ii-iv)n mol (i)

Ru

OC

OC S~P

P~S

I

I

Ru

CO

CO

I

I

Ligands: PPh2(CH2)xPPh2, x = 1(i), 2(ii), 3(iii) and 4(iv)

P~S : ƞ1-P coordinated or -P,S- coordinated; P∩S : ƞ

2-P,S coordinated

[RuCl2(CO)2]n

n mol (i)

2n mol (i-iv)

n mol (ii-iv)

Ru

OC

ClS

P

Cl

CO

Ru

OC

Cl P~S

Cl

P~S

CO

Ru

S~P

Cl CO

Cl

CO

n mol (ii-iv)n mol (i)

Ru

OC

S P

S

P

CO

(ClO4)2

2AgClO4

Ru

OC

Cl P

SCO

(ClO4)

AgClO4

Ligands: PPh2(CH2)xPPh2, x = 1(i), 2(ii), 3(iii) and 4(iv)

P~S : ƞ1-P coordinated; P∩S : ƞ

2-P,S coordinated

18

Neutral and cationic half-sandwich complexes of ruthenium, rhodium and

iridium with chiral [(S)-1-[(R)-2-(diphenylphosphino)ferrocenyl]ethyl]di-tbutyl

phosphine selenide (L) are reported [64]. The ligand L is the first example of a chiral

bisphosphine monoselenide prepared by direct monoselenation of a commercially

available chiral bisphosphine.

1.7.2. Complexes of poly-phosphines

Polyphosphines as ligands show several advantages like (i) excellent bonding

ability to metal, (ii) create high electron density at the metal centre (high

nucleophilicity), (iii) formation of stable metal-complexes, (iv) stabilize different

oxidation states of metals, (v) provide structural and bonding information due to the

metal-phosphorus and phosphorus-phosphorus coupling constants. Metal complexes

of such ligands are thermodynamically more stable than monophosphine analogues.

Tripod, HC(PPh2)3 with least steric constraints forms small rhodium cluster

having metal-metal bond (Figure 1.6) [65a].

Figure 1.6. Rhodium complex of HC(PPh2)3 containing Rh–Rh bond.

In metal-tripod [tripod: MeC(CH2PPh2)3] complex based catalytic reactions

like hydroformylation, one of the metal-ligand bonds dissociate to create vacant site

C

Rh Rh

Ph2PPPh2

H

S

S

OC

OC CO

2+

Ph2P

(S = solvent)

19

for substrate binding (Figure 1.7) [65b].

Figure 1.7. Reversible dissociation of Rh–P bond [Ref. 65b]

Though a large number of rhodium complexes of polyphosphine ligands have

been reported [65], there are only a few reports of rhodium complexes containing

chalcogen functionalized polyphosphine ligands (P–X). The cationic complex

[Rh(COD){ƞ3–P,S,S-(CH(PPh2)(P(S)Ph2)2}] is prepared as fluoroborate salts by the

reaction of the ligand CH(PPh2)(P(S)Ph2)2 with chlorobridged complex

[Rh2Cl2(COD)2] under mild conditions in presence of NaBF4 [66a]. The structure of

the complex is assigned by 31

P NMR spectroscopic studies indicating the formation of

tripodal ƞ3-P,S,S coordinated ligand.

The conventional oxidation of triphos with H2O2 is nonselective, but the ligand

can be mono-oxidized selectively [66b] in the form of its ƞ2-P,P-triphos metal

complex (Eqn. 2). The dicarbonyl rhodium complex that is originally formed loses a

molecule of CO upon bubbling argon through its solution.

The cationic complexes [(arene)RuCl{ƞ2-L}]A are synthesized by the reaction

between triphosphine CH(PPh2)3 or its mono / bichalcogenide derivatives with

suitable ƞ6-arene-Ru(II) species, where, arene = C6Me6, p-MeC6H4

iPr; L = CH(PPh2)3,

CH(PPh2)2{P(E)Ph2} (E = S, Se), CH(PPh2){P(S)Ph2}2 and A = PF6-, BF4

- [67a]. An

RhP

P

P

H

RhP

P

H

CO CO

OCP

CO

20

interesting complex [(arene)Ru{ƞ3-S,S,S-{P(S)Ph2}3C}]BF4; is prepared by deproto-

nation of the coordinated ligand [{P(S)Ph2}3CH], where the ligand is coordinated to

the ruthenium centre by tridentate ƞ3-S mode (Figure 1.8).

Figure 1.8. Synthesis of [(arene)Ru{ƞ3-S,S,S-{P(S)Ph2}3C}]BF4 [arene = p-

MeC6H4iPr] [Ref. 67a]

The complex [RuCl2(CO)2]n reacts with one mol equivalent of triphos to

afford [RuCl2(CO)2(ƞ2-P2-triphos)] (Scheme 1.11). However, the complex on setting

aside for about two weeks in DCM solution undergoes decarbonylation to yield a

tridentate monocarbonyl complex [RuCl2(CO)(ƞ3-P3-triphos)] (Scheme 1.12) [67b].

iPrMe

P P P

R

R

S

R

RS

RR

S

Ru

BF4

PPh2

PPh 2

Ph2P

Rh

Cl

CO

CO

PhC(OOH)Me2

CO

PPh2

PPh 2

Ph2P

Rh

Cl

CO

CO

O

-CO

[Eqn 2]PPh2

PPh 2

Ph2P

Rh

Cl

CO

O

21

Scheme 1.11. Synthesis of [RuCl2(CO)2(ƞ2-P2-triphos)].

Scheme 1.12. Decarbonylation of [RuCl2(CO)2(ƞ2-P2-triphos)]

1.8. Activation of small molecules

One important general problem in organometallic chemistry is the binding and

activation of the small molecules of nature. Thus, it is an interesting area to carry out

research to activate the less reactive molecules like CO, CO2, O2, H2, alkane etc. with

a goal to convert these relatively common compounds into useful organic chemicals

RuCl3.xH2OCO

Alcohol / Reflux[RuCl2(CO)2]n

PPPRu

Cl

CO

P

P

Cl

OC

P

CH3OH, reflux

3 h

P

P

P

P P P

=

RuCl

CO

P

P

OC

RuCl

CO

P

P

P

Cl-CO

Cl

P

P

P

PP P P

=

DCM/Hexane

Two weeks

22

[68]. For example, natural gas (methane) is wasted by being flared off in certain oil

fields, for lack of an economic method of transport. A method of turning this methane

into easily transportable liquids, such as methanol or higher alkanes, would be very

valuable. Therefore, much fundamental chemistry research has therefore been aimed

to address couple of questions such as:

How do metal ions coordinate to and modulate the reactivity of small, often

rather inert molecules?

What is the basis for the selectivity of natural and synthetic metal-containing

systems for specific small-molecule substrates?

Can one use knowledge of metal/small-molecule chemistry for the development

of new catalytic processes useful in the laboratory and / or in industry?

The activation of small molecules such as CO, CH3I, H2, O2, etc., by metal

complexes [69] is important and interesting due to their catalytic reactions; for

instance, OA of CH3I is an important step in the rhodium-catalyzed Monsanto [12,13]

and iridium-based Cativa [3a,b] process for acetic acid production. Activation of

molecular oxygen by metal complexes has attracted great interest in recent years for

producing model oxidation catalysts for chemical processes in terms of cost, atom

economy and the potential for limited by-products [69e, 70]. Similarly, the activation

of dihydrogen by metal centers is a fundamental step in nearly all metal catalytic

hydrogenation reactions [71].

23

1.9. Catalytic applications

Rhodium, iridium and ruthenium-catalyzed reactions have made a significant

contribution to the recent growth of organic synthesis particularly in carbonylation,

hydrogenation, hydroformylation, carbon-carbon bond formation etc. [3,5,11–14,16,

21–23]. In this section, two potential catalytic reactions, namely, carbonylation and

hydrogenation have been highlighted.

1.9.1. General aspects of carbonylation catalysis

Acetic acid is an important industrial commodity chemical, with a worldwide

demand of about 7–8 million tons per year. One of the largest and fastest growing uses

of acetic acid is in the production of vinyl acetate, an important industrial monomer.

The major part of the remaining acetic acid production is used to manufacture other

acetate esters. Cellulose acetate is used extensively in the preparation of fibers and

photographic films. Inorganic acetates are used in the textile, leather and paint

industries. Acetic acid is also used in the manufacture of chloroacetic acid and

teraphthalic acid [72].

The synthesis process of acetic acid has changed over the years with

changing technologies and several problems are associated due to their lack of

selectivity and formation of many byproducts. Novel acetic acid processes and

catalysts have been introduced, commercialized and improved continuously since the

1950s. The objective of the development of new acetic acid processes has been to

reduce raw material consumption, energy requirements and investment costs.

Significant cost advantages resulted from the use of carbon monoxide (derived from

24

natural gas) and of low-priced methanol (from synthesis gas) as feedstock. A few

carbonylation processes and their recent advances are highlighted below:

1.9.1.1. The cobalt-based BASF process

The first methanol-to-acetic acid carbonylation process was commercialized in

1960 by BASF. It used an iodide-promoted cobalt catalyst and required high pressures

(600 atm) as well as temperatures (230 °C), but gave acetic acid in ca. 90% selectivity

[73]. The process generates a few by-products like methane, acetaldehyde, ethanol and

ethers. The proposed mechanism of the process (Scheme 1.13) indicated that

[HCo(CO)4] is the active catalytic species to propagate the cycle. However, the

postulated intermediates are not well characterized [73].

Scheme 1.13. BASF catalytic cycle for the synthesis of acetic acid from methanol.

It is interesting to note that slightly lower CO pressures are possible in the

presence of Ru, Ir, Pd, Pt, or Cu salts as activators [74]. Cobalt catalysts can also be

used for the carbonylation of higher alcohols, such as benzyl alcohol [75].

[HCo(CO)4]

[CH3Co(CO)4]

[CH3COCo(CO)3]

[CH3COCo(CO)4]

CO

H2O

HICH3COOHCH3OH

CH3ICH3COI

HIHI

25

1.9.1.2. Nickel-based processes

Nickel carbonyl, as well as a variety of nickel compounds, is also catalytically

active for the carbonylation of methanol in the presence of iodine. Ni(CO)4 is formed

from NiI2 according to Eq. 3.

The hydrogen iodide formed in Eq. 3 is used to transform the alcohol into an

alkyl halide, which adds oxidatively to nickel, as described in Scheme 1.14 [76].

Scheme 1.14. Catalytic cycle of the nickel-catalyzed methanol carbonylation.

Nickel catalysts usually operate under high pressures and temperatures [77],

but with high methyl iodide concentrations, carbonylation occurs readily under milder

conditions [78]. Molar ratios of CH3I:CH3OH at least 1:10, pressures as low as 35 bar

can be applied at 150 °C, using Ni(OAc)2.4H2O and Ph4Sn as catalyst systems [79].

Although nickel systems can approach the reaction rates and selectivities of rhodium

based catalytic systems, and nickel is much cheaper than rhodium, commercialization

NiI2 + H2O + 5CO Ni(CO)4 + 2HI + CO2 [Eq. 3]

Ni(CO)4

CH3I

2CO

Ni(CO)2(CH3)I

CO

Ni(CO)2(COCH3)I

2CO

CH3COI

26

of these systems has not been achieved to date, which may be due to the very toxic

and volatile nature of the Ni(CO)4 compound.

1.9.1.3. The rhodium-based Monsanto process

The Monsanto acetic acid process is commercially an important process since

it produces over 50% of the world’s annual acetic acid demand, i.e. 7 million metric

tons [80]. The process involves the homogeneous catalysis in the carbonylation of

ethanol to yield acetic acid (Scheme 1.15).

Scheme 1.15. Catalytic cycle of the rhodium-catalyzed methanol carbonylation

Furthermore, the effect of the rhodium metal centre on the catalytic system is

very high, since the metal centre facilitates both the OA and reductive elimination

steps. The key reaction in the Monsanto process is the facial OA of methyl iodide,

(step A), to the square planar rhodium(I) metal centre to form the octahedral

CO

H2O

HICH3COOH CH3OH

CH3ICH3COI

Rh

CO

CO

I

I

-

Rh

CO

CO

I-I

I

Rh

CO

CO

I

I

-H3C

I

Oxidative addition(A)

CH3

Migratory insertion

(B)

Ligand addition(C)

Reductive elimination(D)

Rh

CO

CO

I-I

I

CH3

OC

27

rhodium(III) species. Carbon monoxide insertion (step B) into the cis- CH3–Rh bond

yields a five-coordinated acyl intermediate, which undergoes addition of carbon

monoxide (step C), reductive elimination of the acetyl iodide (step D) to yield the

original rhodium(I) complex. Hydrolysis of the acetyl iodide by water in the aqueous-

methanol feed gives acetic acid and HI, the latter reacts with methanol to regenerate

methyl iodide and the cycle repeats itself.

The mechanism presented, including all intermediates, was resolved after

extensive research [81] into the area and as recently as 1991 by Maitlis et al. [82], the

existence of the intermediate (after step A) was confirmed in low concentrations

utilizing FT-IR and FT-NMR spectroscopy.

1.9.1.4. Heterogenized rhodium catalysts

An important additional requirement for all homogeneous processes is that the

dissolved catalyst must be separated from the liquid product and recycled to the

reactor without significant catalyst loss [83]. In order to overcome the need for a

separation catalyst recycle step, the active rhodium complex was made heterogeneous

by using different support [80a,84]. Although the activity of these catalysts in the

liquid phase was comparable to Monsanto’s homogeneous catalyst, there were

problems with rhodium metal leaching from the support and the decomposition of the

support during operation at elevated temperature. Such problem was overcome by

Chiyoda by introducing a novel catalytic system containing pyridine resins where the

catalysts exhibited high activity, long catalyst life, and no significant rhodium loss

[85]. Based on this heterogeneous rhodium catalyst, Chiyoda and UOP (Universal Oil

28

Products) have jointly developed an improved methanol carbonylation process, called

‘Acetica process’, for the production of acetic acid. Until the recent development of a

commercial heterogeneous rhodium catalyst system by Chiyoda, no successful

demonstration of such a catalyst had been known [80a]. The heterogeneous catalyst

commercialized for the ‘Acetica process’ consists of rhodium complexes on a novel

polyvinyl pyridine resin [86] which is tolerant of elevated temperatures and pressures.

Very recently, Haynes et al. reported an alternative strategy for catalyst

immobilization, using ion-pair interactions between ionic catalyst complexes and

polymeric ion exchange resins [83]. These results demonstrated that the same

sequence of organometallic reactions occur for the polymer-supported rhodium

catalyst as already established for the homogeneous system.

1.9.1.5. The iridium-based Cativa process:

In 1996, BP chemicals introduced a new carbonylation process called ‘Cativa’

to produce acetic acid using iridium/iodide based catalyst, in combination with a

promoter metal such as ruthenium [3,87]. This catalytic system has high rates at

low water concentrations and exhibits high stability which allows for a wide range of

process conditions. The Cativa catalytic process is shown in Scheme 1.16.

The OA of CH3I to the activated iridium complex is faster than for similar

rhodium complexes. The equilibrium of this OA favors the Ir(III) state. Therefore the

OA is not the rate-determining step of the Cativa process. Instead the migratory

insertion of the methyl group to the co-ordinated CO has been found to be the slow

rate-determining step. The metal-to-carbon σ-bonds of third row metals are stronger,

more localized and more covalent than those in second-row metal complexes. A larger

29

amount of iridium is required to achieve an activity comparable to the rhodium

catalyst based processes, however the catalyst system is able to operate at lower water

levels (less than 8 wt.%). Hence, there is lower side-product formation and improved

carbon monoxide efficiency is achieved while the steam consumption is also

decreased.

Scheme 1.16. Iridium based Cativa process for the synthesis of acetic acid from

methanol.

The high stability of the iridium catalyst is one of its major advantages. It is

robust at low water concentrations (0.5 wt.%), which is significant and ideal for the

optimization of the methanol carbonylation process. The catalyst remains stable under

a wide range of experimental conditions that would cause similar rhodium analogues

to decompose completely to an inactive and largely unrecoverable rhodium salt.

Iridium is much more soluble than rhodium in the reaction medium which allows the

H2O

HICH3COOH

CH3OH

CH3ICH3COI

Ir

CO

CO

I

I

-

Ir

CO

CO

I

I

-H3C

I

Ir

CO

CO

ICOCH3

I

I-

CO

Ir

CO

CO

I

I

H3C

CO

I-

30

use of greater catalyst concentrations, making much higher reaction rates obtainable

[3c].

There are two groups of promoters which affect the reaction, namely simple

iodide complexes of zinc, cadmium, mercury, indium and gallium, and carbonyl

complexes of ruthenium, osmium, rhenium and tungsten [88]. The promoters are

highly successful when combined with iodide salts such as lithium iodide at low water

concentration [89]. The best rate is found at low water conditions. The selectivity of

acetic acid is greater than 99% based on methanol and there are fewer side products

than the Monsanto process.

1.9.1.6. Ligand-accelerated carbonylation of methanol

The migratory insertion reaction of CO into metal-alkyl bonds is a

fundamental step in the metal iodide-catalyzed carbonylation of methanol to acetic

acid (and also in hydroformylation reactions) [90]. The original [RhI2(CO)2]˗ catalyst,

developed at the Monsanto laboratories and studied in detail by Forster and co-

workers, [13] is largely used for the industrial production of acetic acid and anhydride.

However, the conditions used industrially (30–60 bar pressure and 150–200 °C) [91]

have spurred the search for new catalysts, which could work in milder conditions [92].

The rate-determining step of the rhodium-based catalytic cycle is the OA of CH3I, so

that catalyst design focused on the improvement of this reaction. The basic idea was

that ligands which increase the electron density at the metal should promote OA, and

consequently increase the overall rate of the reaction. For this purpose, other rhodium

31

compounds have been synthesized in the last few years, which show comparable or

better performance as compared to the Monsanto’s catalyst [92, 93].

1.9.2. Hydrogenation

The two important terms ‘Hydrogenation’ and ‘Transfer Hydrogenation’ have

distinct features i.e. in the former, hydrogen gas (H2) is involved while in the latter,

the hydrogen source must be different from H2.

The study of homogeneous hydrogenation reactions catalyzed by transition

metal complexes has been far more extensive than that of any other catalytic process

in solution [94]. One of the most popular homogeneous catalysts for hydrogenation

reaction is Wilkinson's catalyst, [RhCl(PPh3)3] [9a,95]. Recent survey shows that the

use of asymmetric hydrogenation for the production of fine chemicals is limited, but

expanding [96]. Two major factors that hamper its use are the cost of the catalysts,

and in particular the ligands that are often prepared in a multistep synthesis. However,

the demand for enantiomerically pure compounds with a desired biological activity is

growing rapidly in fine-chemical synthesis. An obvious reason for this development is

that one of the enantiomers of a chiral pharmaceutical or chemical has at best no

activity, or worse, causes side effects. One of the important success stories is the

synthesis of fine chemicals like L-DOPA (Scheme 1.17) by an asymmetric

hydrogenation of prochiral substrates with rhodium(I)-complexes containing optically

active ligand DiPAMP [16]

32

Scheme 1.17. Synthesis of L-DOPA

1.9.2.1. Transfer hydrogenation

Environmental concerns in chemistry have increased the demand for more

selective chemical processes with a minimum amount of waste (Green Chemistry).

Hydrogen transfer reactions are mild methodologies for reduction of ketones or imines

and oxidation of alcohols or amines in which a substrate-selective catalyst transfers

hydrogen between the substrate and a hydrogen donor or acceptor, respectively [97].

Furthermore, the donor (e.g. 2-propanol) and the acceptor (e.g. a ketone or a quinone)

are environmentally friendly and also easy to handle. Two main pathways have been

proposed for hydrogen transfer to ketones or aldehydes depending on the type of

metal used. Direct hydrogen transfer is claimed to occur with main group element and

was proposed for the Meerwein-Ponndorf-Verley reduction [98]. This is a concerted

process involving a six-member transition state, in which both the hydrogen donor and

the hydrogen acceptor are coordinated to the metal centre. The hydridic route includes

transition metal catalysts and gives rise to a characteristic metal hydride that involves

in the hydrogen transfer reaction [99].

COOH

NHAc

MeO

AcO

Rh(DiPAMP)

H2

COOH

NHAc

MeO

AcOH

* H3O+ COOH

NH2

HO

HOH

*

P P

OMe

MeO

DiPAMP

Conversion: 100%

Selectivity: 99.5%

[L-DOPA or S-DOPA]

33

Bäckvall et al. reported that the hydridic route can be further divided into

metal monohydride and metal dihydride pathways depending on the catalyst used

[100,101]. In the monohydride mechanism, the metal hydride arises purely from the α-

C–H of the hydrogen donor (e.g. 2-propanol), and this M–H is only transferred to the

carbonyl carbon of the substrate and thereby the hydride keeps its identity (Scheme

1.18, path A). In the dihydride pathway the metal hydride arise from both α-C–H and

the OH of the hydrogen donor, and therefore the catalyst does not distinguish between

the proton and the hydride (Scheme 1.18, path B).

Scheme 1.18. Hydridic routes of metal catalyzed transfer hydrogenation

There are two types of catalysts operating through the monohydride

mechanism that have been suggested to operate differently. The formation of the

metal monohydride from the hydrogen donor may involve the formation of a

transition metal alkoxide followed by β-elimination to give the M–H. Alternatively, it

may proceed through a concerted pathway with simultaneous transfer of proton and

hydride from 2-propanol to a basic site of the ligand and to the metal, respectively,

without coordination of the alcohol to the metal. In both pathways the metal hydride

gives rise to the α-C–H.

H1H2O

LnMX

Path A

LnM

Path B

O

O

LnMH1+ + H2X

LnMH1H2+R1 R2

O

R1 R2

O

H1H2O

R2R1

H1H2O

R2R1

H1H2O

R2R1

+

34

In the pathway via a transition metal alkoxide a coordinated carbonyl inserts

into the metal hydride 1 and forms metal alkoxide 2 (Scheme 1.19). This insertion

should proceed via π-bonded ketone (metal bound to the double bond). 2-Propanol

exchanges the alkoxide and releases the product. The metal isopropoxide then

undergoes β-elimination forming the metal hydride.

Scheme 1.19. Catalytic cycle via a metal alkoxide.

There is ample support for this alkoxide mechanism [102] and it has been

shown that transition metal hydrides are obtained from β-elimination of the corres-

ponding alkoxide complexes [103]. Even ligand assisted versions of this

transformation have been reported [104]. Many transition metal catalysts are

operating via a metal alkoxide in transfer hydrogenation, for example the Wilkinson

HM

R1 R2

OM

H

insertion

R1 R2

OM

HOHR1 R1

OH

OM

H

O

elimination

R1 R2

O

(1)

(2)

35

catalyst [RhCl(PPh3)3] [101]. A common feature of catalysts that are proposed to

operate through a concerted mechanism is that one of the donor atoms of the ligand

acts as a basic centre and activates the substrate. This mechanism was first proposed

by Noyori for 16-electron Ru complexes such as 3 (Scheme 1.20) [105,106].

Scheme 1.20. Catalytic cycle via a concerted six-member transition state.

The reaction cycle involves a simultaneous transfer of the proton and the

hydride from 4 to the carbonyl in a cyclic six-member transition state forming the

alcohol and 3. Then the proton and the hydride from 2-propanol are delivered to

Ru

H2N N

R R

Ts

H R1 R2

O

Ru

N N

R R

Ts

H

H

O

R2

H

R1

R1 R2

OH

Ru

HN N

R R

TsOH

Ru

N N

R R

Ts

H

H

O H

R1 R2

O

(4)

(3)

36

the ligand and the metal, respectively, in the same fashion forming 4 and acetone. It is

worth to note that the reaction is proposed to proceed without coordination of either

alcohol or ketone (aldehyde) to the metal (Scheme 1.20).

1.10. Aim and objectives of the Thesis

The literature survey presented here reveals that basic as well as applied

science of organometallic chemistry of platinum group metals particularly rhodium,

iridium and ruthenium have aroused much interest and importance in recent time. The

catalytic processes of carbonylation and hydrogenation reaction find tremendous

importance in industrial production of bulk and fine chemicals. Newer concepts of

ligand design and coordination of metal complexes proceeded promisingly in respect

of catalyst precursors. It is therefore important to synthesize and characterize novel

metal complexes and to determine their reactivities, kinetics of reactions and

applications in different catalytic processes. The specific objectives of the thesis are:

To synthesize and characterize Rh(I), Ir(I) and Ru(II) complexes of symmetrical

/ unsymmetrical chelating functionalized phosphine and nitrogen donor ligands

like P–P, P–X (X = O, S, Se) and N,O.

To study the reactivity [OA reactions, kinetics] of the suitable metal complexes

with the small molecules like CO, O2, CH3I, C2H5I, C6H5CH2Cl etc.

To study the hemilabile nature of different P–X (X = O, S) donor chelating

ligands.

To evaluate the catalytic efficiency of the synthesized complexes for

carbonylation of methanol and transfer hydrogenation of aldehydes and ketones.

37

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38

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39

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