Syntheses and catalysis of transition metal complexes with hemilabile ligands 1

Low Valent Nickel Complexes Supported by [P, N] ferrocenyl Ligands And Their Catalytic Applications in Ethylene Oligomerization 3.1.. SUMMARY The aim of the project is the development o

SYNTHESIS AND CATALYSES OF

TRANSITION-METAL COMPLEXES WITH HEMILABILE LIGANDS

TEO SHIHUI

(B Sc (Merit), NUS)

A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

DEPARTMENT OF CHEMISTRY

NATIONAL UNIVERSITY OF SINGAPORE

2010

Table of Contents

Abbreviations and Symbols vii

Summary viii

List of Publications x

List of Figures xi

List of Tables xv

Acknowledgements xvii

1 Chapter 1 General Introduction 1

1.1 Transition metal catalysts 1

1.2 Advantages of incorporating ferrocene in catalysts’ design 1

1.3 Types of mixed donor hybrid ligands 5

1.3.1 Bidentate [P, O] ligands 6

1.3.1.1 Phosphine-ether ligands 7

1.3.1.2 Phosphine-ester and phosphine-ketone ligands 11

1.3.1.3 Phosphine-alcohol and phosphine-phenol ligands 12

1.3.1.4 Phosphine-phosphonate ligands 13

1.3.2 Bidentate [P, N] ligands 13

1.3.2.1 Phosphine-oxazoline ligands 14

1.3.2.2 Phosphine-pyridine ligands 15

1.3.2.3 Phosphine-amine ligands 15

1.3.2.4 Phosphine-imine ligands 17

1.3.3 Bis(phosphine)amine ligands 18

1.4 Complexes with mixed donor hybrid ligands of interest 19

1.4.1 Reported complexes with [P, O] ligands 19

1.4.1.1 Ni complexes 19

1.4.1.2 Pd complexes 20

1.4.1.3 Cr complexes 21

1.4.1.4 Rh complexes 22

1.4.2 Reported complexes with [P, N] ligands 23

1.4.2.1 Ni complexes 23

1.4.2.2 Pd complexes 24

1.4.2.3 Cr complexes 25

1.4.2.4 Rh complexes 25

1.4.3 Complexes with bis(phosphine)amine ligands 26

1.5 Future Perspective 27

1.6 Objectives 28

2 Chapter 2 Synthesis of Bidentate Mixed donor Ligands 29

2.1 Synthesis of 1,1’-disubstituted phosphine-imine and phosphine-ether ferrocenyl ligands (L1 and L2) 29

2.1.1 Synthesis of phosphine-imine ferrocenyl ligands (L1) 31

2.1.2 Synthesis of phosphine-ether ferrocenyl ligands (L2) 34

2.2 Synthesis of phenyl iminophosphines 35

2.3 Synthesis of bis(phosphino)amine ligands 36

2.4 Conclusion 38

3 Chapter 3 Low Valent Nickel Complexes Supported by [P, N] ferrocenyl Ligands And Their Catalytic Applications in Ethylene Oligomerization 3.1 Introduction 39

3.2 Earlier work on nickel catalyzed ethylene oligomerization 41

3.2.1 SHOP catalysts 42

3.2.2 Ni(II) α-diimine and related [N, N] catalysts 43

3.2.3 [N, O] chelating neutral nickel catalysts 44

3.2.4 [P, N] chelating nickel catalysts 45

3.2.5 Development of low valent nickel complexes and catalytic contributions towards ethylene oligomerization 47

3.3 Objectives of this work 49

3.4 Results and discussion 50

3.4.1 Synthesis and characterization of new low valent nickel catalysts 3.4.1.1 Reaction of Ni(COD)2 with ferrocene iminophosphane ligands and diphenylacetylene 50

3.4.1.2 Reaction of Ni(COD)2 with ferrocene iminophosphane ligand under CO 53

3.4.1.3 Reaction of Ni(COD)2 with ferrocene iminophosphane ligand and AlEtCl2 or AlCl3 57

3.4.2 Catalytic Studies 60

3.5 Conclusion 64

4 Chapter 4 Chromium Precursors And [PNP] Ligands: Complexation and Potential Catalytic Applications in Ethylene Selective Tetra-, Oligo- and Polymerization 4.1 Introduction 4.1.1 Global demand for 1-octene 65

4.1.2 Ethylene tetramerization 66

4.2 Objectives 70

4.3 Results and discussion 71

4.3.1 Chromium(III) [PNP] complexes syntheses and structures 4.3.1.1 Reaction of [PNP] ligand L4d with CrCl3(THF)3 and RCN 71

4.3.1.2 Reaction of reported binuclear Cr(III)-PNP complexes with acetonitrile 76 4.3.1.3 Hydrolytic disintegration and ligand transformation of [PNP] ligand 79 4.3.2 Catalytic studies 85

4.3.2.1 Potential applications of compounds C5 – C10 and L5 to ethylene

oligomerization and polymerization 85

4.3.2.2 Potential applications of [PNP] ligand L4 towards catalyzing Selective

Ethylene Tetramerization in the presence of Cr(acac)3 89 4.4 Conclusion 92

5 Chapter 5 Palladium Catalyzed C-C bond formations

5.1 Introduction 93 5.1.1 Palladium catalyzed Suzuki-Miyaura cross coupling reactions 93 5.1.2 Palladium catalyzed 1,2-addition of aryl boronic acids to aldehydes 95

5.1.3 Palladium catalyzed 1,4-conjugate addition of organoboronic acids to

α,β-unsaturated ketones 97 5.2 Results and Discussion 99 5.2.1 Catalytic performance and studies

5.2.1.1 Suzuki cross coupling reactions of aryl halides and organoboronic

acids 99 5.2.1.2 1,2- and 1,4-addition reactions of organoboronic acid to carbonyl

compounds 105 5.2.2 Coordination studies of potential catalytic active intermediates formed

between the [P, N] ferrocenyl ligands and Pd(0) precursors 112

5.2.2.1 [P, N] palladium complexes formed from oxidative addition in

Suzuki-Miyaura cross coupling reactions 112 5.2.2.2 Contribution of chloroform in directing palladium catalyzed 1,2

addition of phenylboronic acid to aldehydes 118

5.3 Conclusion 120

6 Chapter 6 Potential Applications of Chiral 1,1’-Iminophosphine Ferrocenyl Ligands in Asymmetric Rhodium-catalyzed Hydrosilylation of ketones to alcohols 6.1 Introduction 121

6.1.1 Importance of optically active secondary alcohols 121

6.1.2 Asymmetric rhodium-catalyzed hydrosilylation of ketones to alcohols

122

6.1.3 Reported ligands in rhodium-catalyzed asymmetric hydrosilylation of ketones to alcohols 124

6.1.3.1 Nitrogen-based ligands 124

6.1.3.2 Diphosphine ligands 125

6.1.3.3 Heterobidentate [P, N] ligands 126

6.2 Objectives 129

6.3 Results and Discussion 129

6.3.1 Catalytic evaluation of [P,N] ferrocenyl ligands with Rh and Ir precursors towards asymmetric hydrosilylation of ketones to alcohols 129

6.3.2 Synthesis of Rh/ligand complexes 133

6.3.3 Catalytic performance of Rh/complex towards asymmetric hydrosilylation of ketones 136

6.4 Conclusion 138

7 Experimental 139

8 References 169

9 Appendix 187

ABBREVIATIONS AND SYMBOLS

FAB Fast atom bombardment

SUMMARY

The aim of the project is the development of ligands with mixed hybrid donors and their corresponding metal (namely nickel, chromium, palladium and rhodium) complexes Structural and catalytic studies on these transition metal complexes towards ethylene oligomerization, C-C bond formations and asymmetric reduction are also performed This thesis contains six chapters

Chapter 1 is a general introduction on the importance of mixed hybrid multi-dentate

ligands and transition metal catalysts in industries and laboratories The ability of these multidentate mixed hybrid ligands to bind in more than one fashion to the metal centre, which is known as the hemilabile behaviour, has greatly improved both catalytic and organometallic model reactions

Chapter 2 focuses on the synthesis of new mixed donor hybrid ligands namely:

1,1’-disubstituted iminophosphino ferrocenes, 1,1’-1,1’-disubstituted phosphino-ether ferrocenes, phenyl iminophosphines and bis(phosphino)amines

Chapter 3 describes the development of Ni(0), Ni(I) and Ni(II) catalysts towards

ethylene oligomerization to produce linear alpha-olefins which are versatile intermediates for production of co-polymers, plasticizers, alcohols, detergents, synthetic lubricants and surfactants Through the use of ferrocenyl iminophosphine ligands, appropriately stabilized Ni(0) and binuclear Ni(I)-Ni(0) complexes are synthesized, structurally identified and activated for catalyzing ethylene oligomerization The use of an alkyne-stabilized Ni(0) catalyst could be viewed as a mimic for olefin entry to nickel-promoted olefin oligomerization and thus, enhance the catalytic activity and oligomeric selectivity The isolation of binuclear Ni(I)-Ni(0) complex highlights a balance between coordinative ability, complex stability and catalytic activity can be achieved through the use of mixed-donor hybrid ligands

Chapter 4 is about developing suitable catalysts for producing 1-octene which is a

challenge for industrial linear alpha-olefin production Promising bis(phosphino)amine ligands with functional side arm at the N-site are developed These ligands display good

activities (TON: 9000 – 61000) in the chromium catalyzed selective ethylene tetramerization in the presence of Cr(acac)3, toluene and MAO, under 30 bar of ethylene pressure at 80°C Attempts to understand the coordination nature of these bis(phosphino)amine ligands with chromium precursors have led to an unusual ligand destruction followed by reconstruction process which the ligand is reconfigured from a bidentate [PNP] to a tridentate [ONO] donor set This discovery has revealed a direct pathway to prepare new type of hybrid ligand

Chapter 5 concerns the application of palladium precursors and the [P, N] mixed hybrid

ligands we have developed as catalysts for Suzuki-Miyaura cross coupling reactions, and 1,2- & 1,4- addition reactions of organoboronic acids to carbonyl compounds Further attempts to understand the contribution of the [P, N] ligands towards the Palladium intermediates in the Suzuki-Miyaura cross coupling reaction and 1,2-addition reaction of arylboronic acids to aldehydes have been made Potential useful spectroscopic data of the palladium intermediates has also been obtained

Chapter 6 describes our attempts to apply chiral 1,1’-iminophosphine ferrocenes in

asymmetric rhodium-catalyzed hydrosilylation of ketones Among them, the ferrocenyl

ligand which contains (R)-ethylnaphthylimine and the diphenylphosphine group and its

related rhodium complex catalyzed the asymmetric hydrosilylation of ketones efficiently under mild conditions The related square-planar rhodium complexes have been synthesized and structurally characterized

LIST OF PUBLICATIONS

1 S Teo, Z Weng, T S A Hor, Iminophosphine Ligands in Palladium Catalyzed

Addition Reactions of Arylboronic Acids to Carbonyl Compounds, Manuscript in

preparation

2 S Teo, Z Weng, T S A Hor, Substituent effect of ferrocenyl Iminophosphine in

Pd(II) catalyzed Suzuki coupling, J Organomet Chem 2011, submitted

3 S Teo, C.-Y Tan, Z Weng, T S A Hor, Complexation of chiral

1,1’-Iminophosphine Ferrocene to [RhI(COD)] and their catalytic effects on

asymmetric hydrosilylation to ketones to alcohols, Manuscript in preparation

4 S Teo, Z Weng, T S A Hor, Unusual Ligand Transformation Mediated by Chromium(III): Hydrolytic Disintegration of a [PNP] Hybrid Ligand with CH3CN

Insertion, Organometallics, 2008, 27, 4188

5 Z Weng, S Teo, T S A Hor, Metal Unsaturation and Ligand Hemilability in

Suzuki Coupling, Acc Chem Res 2007, 40, 676

6 Z Weng, S Teo, T S A Hor, Chromium(III) catalyzed ethylene tetramerization

promoted by bis(phosphino)amines with an N-functionalized pendant, Dalton

Trans 2007, 3493

7 Z Weng, S Teo, T S A Hor, Stabilization of Nickel (0) by hemilabile P, Ferrocene Ligands and their ethylene oligomerization Activities

N-Organometallics, 2006, 25, 4878

8 Z Weng, S Teo, L L Koh, T S A Hor, A structurally characterized Ni-Al

methyl-bridged complex with catalytic ethylene oligomerization activity, Chem

Comm 2006, 1319

LIST OF FIGURES

Figure 1.1 Ferrocenes and chirality 2

Figure 1.2 1,1’-disubstituted phosphine-imine and phosphine-ether ferrocenyl ligands we synthesized 4

Figure 1.3 General classifications of [P, O] ligands reported 6

Figure 1.4 General types of ether moieties for reported [P, O] ligands 7

Figure 1.5 Structure of o-diphenylphosphinoanisole and its related Ru complex 8

Figure 1.6 Other examples of phosphine-ether ligands with phenyl backbone reported 8

Figure 1.7 Some examples of MOP and its analogues 9

Figure 1.8 Examples of phosphine-ether ligands with alkyl backbone used in catalyses 10

Figure 1.9 Examples of ferrocene phosphine ether ligands reported 11

Figure 1.10 Some examples of phosphine-alcohol and phosphine-phenol ligands 12 Figure 1.11 Some examples of phosphine-phosphonate ligands reported 13

Figure 1.12 Some examples of phosphine-oxazoline ligands reported 14

Figure 1.13 Phosphine-pyridine ligands reported 15

Figure 1.14 Some examples of phosphine-amines ligands reported 16

Figure 1.15 Representative examples of phosphine-imine ligands 18

Figure 1.16 [P, O] coordinated Ni(II) complexes reported in ethylene oligomerization 20

Figure 1.17 Some examples of reported Rh complexes 22

Figure 1.18 Some nickel(II) complexes with [P, N] ligands synthesized 24

Figure 1.19 Some examples of palladium complexes supported by [P, N] ligands 24

Figure 1.20 Examples of metal complexes with [P, N, P] ligands 26

Figure 2.1 The two types of disubstituted unsymmetrical ferrocene ligands 30

Figure 2.2 Molecular structure of L4d (H atoms and the minor disorder

components are omitted for clarity) Thermal ellipsoids are drawn

at the 40% probability level 37

Figure 3.1 Some examples of nickel complexes with coordination numbers ranging

from four to six 41

Figure 3.2 Some examples of SHOP’s catalysts developed for ethylene

oligomerization 42

Figure 3.3 Some examples of [N, N] chelating nickel complexes 43

Figure 3.4 Some examples of [N, O] chelating neutral nickel catalysts reported 45

Figure 3.5 Examples of low-valent Ni(I) complexes reported 48

Figure 3.6 Catalytically active low valent Ni complexes 124-127 synthesized by

our group 49

Figure 3.7 Ligand L1b and L1d used as models for our studies on Ni(0) complexes

50

Figure 3.8 Molecular structure of complex C1 (H atoms are omitted) Thermal

ellipsoids are drawn at the 40% probability level 52

Figure 3.9 Molecular structure of complex C3 (H atoms are omitted) Thermal

ellipsoids are drawn at the 40% probability level 56

Figure 3.10 Molecular structure of complex C4 (H atoms are omitted) 59

Figure 4.1 Sasol [PNP] catalyst system for the selective tetramerization of

ethylene 67

Figure 4.2 BP catalyst for the trimerization of ethylene developed by Wass 67

Figure 4.3 Modified [PNP] ligand/Cr system for ethylene tetramerization catalyst

Figure 4.7 Molecular structure of C5 (H atoms are omitted) Thermal ellipsoids

are drawn at the 40% probability level 72

Figure 4.8 Molecular structure of C7a and C7b (H atoms are omitted) Thermal

ellipsoids are drawn at the 40% probability level 75

Figure 4.9 ORTEP diagram of complex C8 (R = Cy) with thermal ellipsoids at

the 40% probability level Hydrogen atoms have been omitted for

clarity 78

Figure 4.10 ORTEP diagram of complex C9 with thermal ellipsoids at the 40%

probability level 80

Figure 4.11 ORTEP diagram of complex L5 83

Figure 5.1 Some examples of diaryl methanols as intermediates for the

synthesis of medically active compounds 96

Figure 5.2 Phosphapalladacyclic complex (138), anionic palladacycle (139) and

thioether-imidazolinium chloride (140)/[Pd(allyl)Cl]2 for the

palladium catalyzed 1,2-addition of arylborons to carbon-heteroatom

double bonds 97

Figure 5.3 [P, N] ferrocenyl ligands synthesized in this study 99

Figure 5.4 The 31P NMR spectra of complex {{η-C5H4CH=N[CH3(CH)C10H7]}-

Fe[η-C5H4P(t-Bu)2]Pd(I)(C6F5)}2, C13a at room temperature (a) and the

decomposition of complex {{η-C5H4CH=N[CH3(CH)C10H7

]}Fe[η-C5H4P(t-Bu)2]Pd(I)(C6F5)} at 60°C (b) in the presence of dba 116

Figure 5.5 The variable temperature 31P NMR study of {{η-C5H4CH=N[CH3

(CH)-C10H7]}Fe[η-C5H4P(t-Bu)2]}2Pd(Br)(C10H7), C14 at room temperature

(a), 60°C (b) and back to room temperature (c) 117

Figure 6.1 Some useful optically active secondary alcohols 121

Figure 6.2 Some applications of chiral alcohols as building blocks or precursors for

syntheses of chiral compounds 122

Figure 6.3 Reported N-based ligands for Rh-catalyzed asymmetric hydrosilylation

124

Figure 6.4 Some reported diphosphines used in Rh-catalyzed asymmetric

hydrosilylation of ketones to alcohols 125

Figure 6.5 Rported phosphine-oxazoline ligands for enantioselective reduction of

simple ketones 126

Figure 6.6 Some reported bidentate [P, N] ligands for Rh-catalyzed asymmetric

hydrosilylation of carbonyl derivatives 127

Figure 6.7 ORTEP diagram of complex C16a with thermal ellipsoids at the 40%

probability level The H atoms are not shown for clarity except the one on C1

carbon atom 134

Figure 6.8 ORTEP diagram of complex C16b with thermal ellipsoids at the 40%

probability level The H atoms are not shown for clarity except the one on C1

carbon atom 135

LIST OF TABLES

Table 2.1 Summary of iminophosphino ferrocenyl ligands, L1a-o synthesized 33

Table 2.2 Selected bond lengths (Å) and angles (º) of L4d 38

Table 3.1 Selected bond lengths (Å) and Angles (º) of C1 53

Table 3.2 Selected bond lengths (Å) and Angles (º) of C3 56

Table 3.2 Selected bond lengths (Å) and Angles (º) of C4 59

Table 3.4 Activities of complexes C1 – C4 towards nickel catalyzed ethylene oligomerization 61

Table 4.1 Selected bond lengths (Å) and angles (º) of C5, C7a and C7b 76

Table 4.2 Selected bond lengths (Å) and angles (º) of C8 78

Table 4.3 Selected bond lengths (Å) and angles (deg) of complex C9 80

Table 4.4 Selected bond lengths (Å) and angles (°) of ligand L5 83

Table 4.5 Activities and selectivities of different catalytic systems towards polymerisation and oligomerisation of ethylene 86

Table 4.6 Key ethylene oligomerization data using Cr(III) and ligand L4 as catalyst

91

Table 5.1 Ligand Effect on the Suzuki cross-coupling reactions of 1-bromo-2- methylnaphthalene and 2-methylnaphthyl-1-boronic acid 100

Table 5.2 sp2-sp2 Suzuki cross-coupling of aryl bromides and aryl boronic acids catalyzed by Pd2(dba)3 with ligand L1o 102

Table 5.3 sp2-sp2 Suzuki cross-coupling of aryl chlorides and aryl boronic acids catalyzed by Pd2(dba)3 with ligand L1o 103

Table 5.4 sp2-sp3 Suzuki cross-coupling of aryl halides and alkyl boronic acids catalyzed by Pd2(dba)3 with ligand L1o 104

Table 5.5 1,2-addition of arylboronic acids to aldehydes catalyzed by Pd2(dba)3 with ligand L1k 105

Table 5.6 Effect of metal precursor towards 1,2-addition of phenylboronic acid to benzaldehyde 106

Table 5.7 Solvent effect towards 1,2-addition of phenylboronic acid to

Table 5.10 1,4-Conjugate addition of arylboronic Acids to trans-chalcone catalyzed by

Pd2(dba)3·CHCl3 with mixed-donor hybrid ligand 112

Table 6.1 Effect of ligand bulk on asymmetric hydrosilylation of acetophenone with

[Rh(COD)Cl]2 130

Table 6.2 Effect of silanes on asymmetric hydrosilylation of acetophenone catalyzed

by Rh/(R)-L1j 132

Table 6.3 Effect of Rh- or Ir- precursors on asymmetric hydrosilylation of

acetophenone using ligand (R)-L1j 133

Table 6.4 Selected bond lengths and bond angles for complex C16a-b 135

Table 6.5 Asymmetric hydrosilylation of simple ketones with catalyst C16 137

ACKNOWLEDGEMENTS

First of all, I am most grateful to my supervisors, Dr Zhiqiang Weng and Prof Tzi Sum Andy Hor, for their support and patience throughout the course of the project In addition, special mention must be made to chromatography lab officer, Mrs Frances Lim for her selfless help and guidance in the chromatography equipments upgrade & troubleshooting, sample preparations and method developments to make my project possible Thanks are due to the staff of CMAC (X-ray, microanalytical, NMR and Mass spectroscopy laboratories) for their technical support

Special thanks are also due to the National University of Singapore for granting me the research scholarship to give me the opportunity to carry out the research for this thesis

Last but not least, my sincere thanks to my family, members in Dr Weng and Prof Hor groups especially Mrs Sheau Wei Chien abnd Mr Gabriel Quek for their great help, Ms Aida Mardianah Kinam and Mr Raymond Gan Ching Ruey for their careful proof-reading

of my thesis and my good friends, Ms Serene Chai Xuemin, Ms Melissa Koh Baobao and

Ms Chen Hsiao Wei for sharing ideas and offering encouragement

Chapter 1 – General Introduction

1.1 Transition metal catalysts

Transition metal catalysts have been long used in industries and laboratories ranging from direct/indirect productions of major organic chemicals and petroleum products to green chemistry.1 There are many reviews on homogeneous catalysts with various transition metals.2 For transition metals, the d-orbitals present allow ligands such as hydrides, CO, and alkenes to be bound in such a way that they are activated towards further reactions.1b

In order for a reactant to be activated by a transition metal in homogeneous catalysis, prior coordination/interaction of the reactant to the metal centre is necessary.1c A loose coordination of the reactants to the central metal atom and facile release of products from the coordination sphere are required during the molecular transformation.1c Both processes must proceed with an activation Gibbs energy that is as low as possible and thus extremely labile metal complexes are required.1c-d The lability of the metal complexes can be optimized by varying the electronic and steric properties of the ligands which influence the catalyst activity and selectivity.1c Therefore, tailoring and developing ligands to obtain catalysts that are efficient in turnover numbers (TONs), turnover frequencies (TOFs) and selectivity of chemical transformations become a primary goal in the design of catalysts for industrial use and other applications

1.2 Advantages of incorporating ferrocene in catalysts’ design

Ferrocene has been playing a significant role in both academic and industrial world.3 Through the academic study of ferrocene, many applications of ferrocene derivatives have been developed and implemented in part of industrial processes such as fuel

additives,4 liquid crystals,5 medicinal production6 etc In catalytic applications, the ferrocene occupies an important position in ligand design Various donor atoms such as P,

N, O & S can be introduced on the ferrocene skeleton.7 The ferrocenyl core plays the important role of a spectator in giving these donors the desirable coordinative mobility and serving as an electronic reservoir needed for pendant/donating switches.8 The redox-active Fe(II) provides an additional electronic buffer that allows the catalytic metal to remain active in different redox stages.8

Another interesting feature of ferrocene-based ligands is the chirality induced by substitutions at the Cp rings (Figure 1.1).9 The planar chirality is obtained by 1,2

substitution of one Cp ring as in 1 If the ferrocene is substituted with a chiral side chain, planar and central chirality can be combined as in 2 When the ferrocene is 1,1’- substituted, a conformation with an axial chirality could be obtained as in 3 In chiral

ferrocene ligand design, these multi-chirality features (central, planar, and axial) could be installed all onto the backbone.9d This design greatly improves the current asymmetric catalytic applications such as Hartwig-Buchwald aminations and hydrosilylation of ketones.9d The benefits of using ligands with ferrocene backbone are supported by their applications in current practical applications such as catalysis.7

In a catalytic system, a ligand’s adaptability to enter the catalytic cycle without going through a high energy barrier and yet support the metal system’s geometric and coordinative changes along the cycle is highly desirable.10 Therefore, developing ligands that have hybrid and difunctional properties to support different catalytic intermediates formed within a single system is appealing To fully exploit the hemilability function exhibited by mixed donor hybrid ligands, the desirable ligand should have the following features:

1) It must be a functional ligand with at least two basic sites that have significantly

different donating abilities

2) The electronic character of each donor atom can be tuned by chemical alteration

of its attached or nearby substituents

3) The two donating sites are separated by a metallocenyl moiety that is

stereogeometrically flexible and redox-active

4) Its potential as a unidentate, chelating and bridging ligand must be demonstrated 5) It can support unsaturated metal through its electronic and spatial effects

On the basis of these considerations, a strategy that we have developed recently is the complementary use of electronically and coordinatively tuneable ferrocene-based ligands (Figure 1.2)8 and reactive low-coordinate metals.11 These phosphine-imine and

phosphine-ether ferrocenyl ligands L1 and L2 are sensitive to the dynamic needs of the

metal at different stages of a catalytic cycle.8 The use of these hybrid ligands has helped

us to trap a number of saturated and unsaturated species under both stoichiometric and catalytic conditions that are mechanistically significant in palladium catalyzed Miyaura-

Suzuki cross coupling (Scheme 1.1 and Scheme 1.2) These results encourage us to

extend the benefits of hemilability of L1 and L2 to other catalytic systems The catalytic

systems of interest are namely ethylene oligomerization & polymerization, addition reactions and asymmetric syntheses The details will be discussed in the subsequent chapters of the thesis

Figure 1.2 1,1’-disubstituted phosphine-imine and phosphine-ether ferrocenyl ligands we

synthesized.8

C=N-Ph

P(t-Bu)2

Pd Ph

Ph

O Ph-N=C

P(t-Bu)2

Pd

C=N-Ph P

Scheme 1.2 Different oxidative addition products from L2.8e

1.3 Types of mixed donor hybrid ligands

Containing soft and hard donor groups, the phosphorus-oxygen and phosphorus – nitrogen based ligands represent some of the extensively studied class of ligands.10 One of the virtues of these mixed-donor ligands is that they possess the ability to coordinate in more than one fashion to the metal centre, depending on the hardness or softness of the different heteroatoms.10 They provide open coordination sites at the metal during reaction that are “masked” in the ground-state structure, as well as stabilize the reactive intermediates This feature very often leads to an improvement in both catalytic and organometallic model reactions These ligands were first described in a kinetic context as

“hemilabile ligands” in 1979 by Jeffrey and Rauchfuss.12 Descriptions of these ligands are summarized below:

1.3.1 Bidentate [P, O] ligands

[P,O] ligands are of considerable interest as they have exhibited unusual selectivity enhancing effect in metal-catalyzed ethylene oligomerization & polymerization as well as other homogeneous catalyses like carbonylation & hydrocarbonylation of methanol to acetic acid, acetaldehyde, ethanol or ethylidene diacetate.8 Moreover, they have been used

in hydrogenation, hydroformylation, ethylene/CO co-polymerization and stereoselective reactions.13 In phosphorus-oxygen ligands, the oxygen functional groups include alcohol, ether, ketone, ester, and phosphonate groups have been reported (Figure 1.3).14 Although the various oxygen functions will impart significant changes in the coordination properties and hemilability of the corresponding [P, O] ligands, it is generally observed that phosphorus – oxygen ligands are the most weakly chelating type of ligand.12Extremely long metal-oxygen bond lengths in these complexes are a common observation.11 The weak metal-oxygen bonds are observed to aid in the stabilization of catalytic intermediates and provide reversible protection of one or more coordination sites.11

Figure 1.3 General classifications of [P, O] ligands reported.14

1.3.1.1 Phosphine-ether ligands

In many cases, the oxygen donors of the phosphine-ether ligands are part of simple acyclic or cyclic ether moieties (Figure 1.4).14 Mechanistic effects of phosphine-ether ligands in both catalytic and organometallic model reactions have been reviewed.15 The capacity of the weakly coordinated ether function to afford empty coordination sites on the metal centres is dictated by steric and conformational factors of the ligand backbone.15The backbones can be classified mainly into aryl, alkyl and ferrocenyl skeletions

Figure 1.4 General types of ether moieties for reported [P, O] ligands.14

Phosphine-ether ligands with aryl backbone

The first type of ligand to be termed hemilabile was o-diphenylphosphinoanisole (Figure

1.5), which is a phosphine-ether ligand investigated by Jeffrey and Rauchfuss in 1979.13

The ligand 11 was found to be able to stabilize its ruthenium derivative 12 (Figure 1.5) in

different forms of chelating modes during carbonylation (Scheme 1.3), allowing Ru catalytic intermediates to be reactive and yet stable to oxygen and heat.13 Similar phosphine-ether ligands with a slight difference in the steric loading on the ether group

have also been reported (Figure 1.6).16 Of great importance is the appearance of chirality

in ether-phosphine ligands 14 – 16 due to the asymmetric carbon or phosphorus atoms

These ligands are reported to be active in asymmetric reactions.16

PPh2

OCH3

11

Ru Cl

Cl

O O

Figure 1.5 Structure of o-diphenylphosphinoanisole and its related Ru complex

Scheme 1.3 Different chelating modes of 11 in stabilizing 12 during CO addition.13

Figure 1.6 Other examples of phosphine-ether ligands with phenyl backbone reported.16

Phosphine-ether ligands with binaphthyl backbone

2,2’-Substituted 1,1’-binaphthyls are particularly good candidates as chiral ligands for asymmetric catalysis due to their highly stable configuration.13n In 1991, Hayashi

developed 2-(diphenylphosphino)-2’-methoxy-1,1’-binaphthyl, 17a, which exhibited high

catalytic activity, regioselectivity and enantioselectivity in palladium catalyzed hydrosilylation.17 Many analogues with varying size of ether group have been synthesized (Figure 1.7) and employed in many enantioselective catalytic reactions, including palladium-catalyzed hydrosilylation of simple terminal and cyclic olefins, reduction of allylic esters and carbonates with formic acids.18

Figure 1.7 Some examples of MOP and its analogues

Phosphine-ether ligands with alkyl backbone

Phosphine-ether ligands with alkyl backbone have been synthesized and investigated as models for very reactive intermediates of catalytic reactions (Figure 1.8).19 One significant example is the use of rhodium and iridium based complexes with ether-phosphine ligands as a catalytic model for rhodium catalyzed hydrogenation of olefins (Scheme 1.4).20 It was shown that the complexes show the same behaviour as suggested for the coordinatively unsaturated species of typical Rh catalyst: PhClH2(PPh3)2(solvent) which is generally postulated to be an active intermediate in the catalysis.20 Recently,

ligand 18a has also been immobilized on alkoxysilanes to be used in solid-supported

20

Figure 1.8 Examples of phosphine-ether ligands with alkyl backbone used in catalyses.19

Scheme 1.4 Ligands 18a, 19 and 20 used as catalytic Rh and Ir models for

metal-catalyzed hydrogenation of olefins.21

Phosphine-ether ligands with ferrocenyl backbone

Though rare, ferrocenyl phosphine-ether ligands reported such as 21,22 and 2223 were found to be remarkably active towards asymmetric Pd-and Ni-catalyzed amination of halides, Grignard reactions, Suzuki cross-coupling reactions and associated syntheses

(Figure 1.9) In 2004, first 1,1’-unsymmetrical ferrocenyl ether ligand, 2324 reported by Gibson and Long was applied in Pd-catalyzed Suzuki cross-coupling reactions of aryl bromides and arylboronic acids They compared well with the aromatic-based [P,O]

ligand 13 that showed promise in Suzuki and asymmetric Heck reaction.16 The vast different electronic and steric characters of P and O donors, coupled with the conformationally flexible and coordinatively adaptive ferrocene backbone makes these systems attractive The rich underlying coordination chemistry with other transition metals like nickel and rhodium awaits for further investigations

Figure 1.9 Examples of ferrocene phosphine ether ligands reported.22-24

1.3.1.2 Phosphine-ester and phosphine-ketone ligands

Apart from the phosphine-ethers, [P, O] ligands with weakly bonding C=O moieties are the second most common type of hemilabile phosphorus-oxygen ligands.14 Phosphine-ester and phosphine-ketone ligands generally follow general structures of

R2PCH2C(O)OR’ and R2PCH2C(O)R’ respectively, where R = Cy, Ph, and R’ = Fc, Me,

H etc Similar to phosphine-ethers, these ligands have been shown to function either as unidentate or as bidentate ligands.14 The ligands and their related metal complexes are also capable of undergoing conversion to enolate moieties upon deprotonation by a base (TlOEt or NaH) or reaction with a metal species.25 An example is listed in equation 1.1.25bThese resultant metal complexes often give rise to interesting applications in coordination chemistry and catalysis.25

Equation 1.1 Reaction of phosphine-ester and phosphine-ketone ligands with

Ni(COD)2.25b

1.3.1.3 Phosphine-alcohol and phosphine-phenol ligands

In alcohol containing [P, O] ligands, they can be sub-divided into two groups, alcohols and phosphine-phenols (Figure 1.10).26 Both phosphine-alcohols and phosphine-phenols are easily deprotonated to the respective alkoxides and phenoxides This makes their syntheses and handling a challenge Their potential in catalytic application is the

phosphine-recent report on the capability of ligand 27-30 in accelerating nickel-catalyzed

cross-coupling reactions of unreactive aryl electrophiles and Grignard reagents.26c

Figure 1.10 Some examples of phosphine-alcohol and phosphine-phenol ligands.26

1.3.1.4 Phosphine-phosphonate ligands

In recent years, increasingly attention has been paid to the syntheses of phosphonate or bisphosphinomonoxides (BMPOs) due to better water solubility (Figure 1.11).27 It makes them ideal for catalyst recovery through aqueous extraction or for catalytic transformation in water.28 Applications involving complexes of BMPOs include polymerization,27d,29 olefin hydration30 and sulfoxidation.31

phosphine-Figure 1.11 Some examples of phosphine-phosphonate ligands reported.27

1.3.2 Bidentate [P, N] ligands

[P, N] ligands have been successfully applied in metal-catalyzed reactions including asymmetric transformation.32 Their good performance depends on the steric factors and the electronic differentiation due to the presence of phosphorus and nitrogen atoms in the ligand.33 The most successful classes of [P, N] ligands are classified as follows:

• Phosphine-oxazoline ligands

• Phosphine-pyridine ligands

• Phosphine-amine ligands

• Phosphine-imine ligands

1.3.2.1 Phosphine-oxazoline ligands

In the past decades, phosphine-oxazoline ligands (Figure 1.12) have been developed.9,34

The chiral analogues 38 – 41 allow a more selective regiocontrol compared to other [P, P]

and [N, N] ligands.34 They are proven to be highly effective in palladium-catalyzed asymmetric allylic substitution and Heck reactions as well as iridium-catalyzed asymmetric hydrogenation of tri-substituted alkenes and imines.34

N

N

OPPh2

ON

Ph2PPPh2 N

PR2

Fe

NO

PR2Fe

NO

1.3.2.2 Phosphine-pyridine ligands

Phosphine-pyridine ligands (Figure 1.13) have been applied to numerous metal-catalyzed transformations including asymmetric hydrogen transfer, carbonylation, hydroformylation, cross-coupling, allylic substitution and ethylene oligomerization.35

Figure 1.13 Phosphine-pyridine ligands reported.35

Figure 1.14 Some examples of phosphine-amines ligands reported. 7,10,36

1.3.2.4 Phosphine-imine ligands

Generally, it is observed that the lability of the nitrogen moiety of [P, N] ligand is lower than that of the oxygen moiety of [P,O] ligands.12,15 However, among the reported [P, N] ligands, phosphine-imine type ligands often display hemilabile behaviour due to a relatively weaker metal-nitrogen bond.39 An example is the presence of both η1- and η2-

coordination modes of a palladium(II)/ligand 61 complexes during coordination with

amines (Scheme 1.5).40 Due to this potential hemilability which gives the ligand the needed coordinative flexibility to respond to the metal changes during the catalytic cycle, phosphine-imine ligands are of our attention.40-41 Figure 1.15 lists the representative examples of the phosphine-imine ligands reported. 40-41

PPh2 N-tBu

PdCl2

CH3CN

PdPN

ClCl

PdCl

PPdN

Figure 1.15 Representative examples of phosphine-imine ligands.7,40-41

1.3.3 Bis(phosphino)amine ligands

The aminophosphines RN[(CH2)nPR’2]2 (n≥0) (PNP) are bidentate ligands with nitrogen bridged between the two phosphine ligands Qadir and co-workers have examined the

ligand effects of palladium based catalysts with tunable [P, N, P] ligands 70 in Heck

arylation reaction between aryl halides with methyl acrylate (Scheme 1.6).42 They found out that the nature of the hemilabile nitrogen donor have a large effect on the turnover of the catalytic reaction Thus, the continuous search for ligands with increased inherent selectivity had persuaded us to prepare new N-donor-functionalized [P, N, P] ligands

Scheme 1.6 Heck arylation reaction between aryl halides with methyl acrylate with

Pd(OAc)2 and 70.42

1.4 Complexes with mixed donor hybrid ligands of interest

Most of the transition metal catalytic applications reported consist of the combination use

of mixed donor hybrid ligands and metal precursors in situ.2 However, synthesis and isolation of metal complexes coordinated with mixed donor hybrid ligands are of interest, mainly for exploring and understanding the catalytic processes Among the transition metal catalysts, chromium, nickel, palladium and rhodium based complexes are of our interest due to their catalytic efficacies in ethylene oligomerization & polymerization, addition reactions and asymmetric syntheses.2

1.4.1 Reported complexes with [P, O] ligands

1.4.1.1 Ni complexes

Coordination complexes of Ni complexes have attracted much attention due to their potential catalytic significance For example, [P, O] coordinated Ni(II) catalytic active species reported in ethylene oligomerization (Figure 1.16).43 Ni(II) complex 74 has also been synthesized as part of the coordination studies on the ligand 22 (Equation 1.2).13k

Figure 1.16 [P, O] coordinated Ni(II) complexes reported in ethylene oligomerization.43

Equation 1.2 Reaction of NiBr2(DME) and [P, O] ligand 22.13k

1.4.1.2 Pd complexes

[P, O] complexes of Pd exhibit rather straightforward coordination chemistry due to its square-planar geometry Coordination of [P, O] ligands occurs readily with Pd precursors like PdCl2(COD), Pd(dba)2 and Pd2(dba)3. 13k,14,16a The coordination study has contributed greatly to understand the oxidative addition in Pd-catalyzed carbon-carbon bond formations like Suzuki-Miyaura cross coupling and Heck-type reaction.16 A general example has been shown in Scheme 1.2 Typically, upon reaction with 2 equivalents of [P, O] ligands and PdCl2(COD), cis-and trans-isomers are obtained (Equation 1.3).14 The stabilization of the isomers strongly depends on the nature of [P, O] ligands Also, depending on the nature of [P, O] ligands, the use of 1 equivalent with respect to

palladium salts leads to the formation of either a mononuclear bi-chelating Pd complex or

a mono-chelating Pd dimer (Scheme 1.6).13k,14,16a

Equation 1.3 Reaction of 2 equivalents of [P, O] ligands and PdCl2(COD).14

Scheme 1.7 Reaction of 1 equivalent of [P, O] ligand with palladium complex.13k,14,16a

1.4.1.3 Cr complexes

Although a large number of chromium compounds with oxygen or phosphorus donor ligands have been described,14 chromium complexes containing chelating [P, O] system

are rare Though chromium complex containing ligand 30 has been reported,

low-catalytic performance in ethylene polymerization is displayed.44

1.4.1.4 Rh complexes

Among all transition metals, those belonging to rhodium group have attracted great attention Several stable Rh complexes with [P, O] ligands exist (Figure 1.17).45 They can undergo insertion and oxidation addition readily (Scheme 1.7 & 1.8).14 This makes them highly efficient in catalytic hydrogenation and hydrocarbonylation.8

Figure 1.17 Some examples of reported Rh complexes.45

Scheme 1.7 Reactions of rhodium complexes with phosphine-ether ligands.14