N-Heterocyclic Carbenes E-Book

• Herausgeber: Wiley-VCH
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 876

Veröffentlichungsjahr: 2014

Beschreibung

This comprehensive reference and handbook covers in depth all major aspects of the use of N-heterocyclic carbene-complexes in organic synthesis: from the theoretical background to characterization, and from cross-coupling reactions to olefin metathesis.
Edited by a leader and experienced scientist in the field of homogeneous catalysis and use of NHCs, this is an essential tool for every academic and industrial synthetic chemist.

Leseprobe

CONTENTS

Cover

Related Titles

Title Page

Copyright

Dedication

List of Contributors

Preface

Chapter 1: N-Heterocyclic Carbenes

1.1 Introduction

1.2 Structure and Properties of NHCs

1.3 Abnormal Carbenes

1.4 Why Are NHCs Stable?

1.5 Bonding of NHCs to Metal Centers

1.6 Quantifying the Properties of NHCs

1.7 N-Heterocyclic Carbenes in the Context of Other Stable Carbenes

1.8 Synthesis of NHCs

1.9 Salts and Adducts of NHCs

1.10 Summary

References

Chapter 2: Tuning and Quantifying Steric and Electronic Effects of N-Heterocyclic Carbenes

2.1 Introduction

2.2 Steric Effects in NHC ligands

2.3 Electronic Effects in NHC Ligands

2.4 Conclusions

References

Chapter 3: Chiral Monodendate N-Heterocyclic Carbene Ligands in Asymmetric Catalysis

3.1 Introduction

3.2 NHC–Ru

3.3 NHC–Rh

3.4 NHC–Ir

3.5 NHC–Ni

3.6 NHC–Pd

3.7 NHC–Cu

3.8 NHC–Ag

3.9 NHC–Au

3.10 Conclusion

References

Chapter 4: (N-Heterocyclic Carbene)-Palladium Complexes in Catalysis

4.1 Introduction

4.2 Cross-Coupling Reactions

4.3 Chelates and Pincer Ligands

4.4 Asymmetric Catalysis

4.5 Oxidation Reactions

4.6 Telomerization, Oligomerization and Polymerization

4.7 Anticancer NHC–Pd Complexes

References

Chapter 5: NHC Platinum(0) Complexes: Unique Catalysts for the Hydrosilylation of Alkenes and Alkynes

5.1 Introduction

5.2 Hydrosilylation of Alkenes: The Beginning

5.3 Initial Results with Phosphine Ligands

5.4 NHC Platinum(0) Complexes: The Breakthrough

5.5 Hydrosilylation of Alkynes

5.6 Conclusions

References

Chapter 6: Synthesis and Medicinal Properties of Silver-NHC Complexes and Imidazolium Salts

6.1 Introduction

6.2 Silver–NHC Complexes as Antimicrobial Agents

6.3 Silver–NHC Complexes as Anticancer Agents

6.4 Conclusions

References

Chapter 7: Medical Applications of NHC-Gold and -Copper Complexes

7.1 Introduction

7.2 Gold Antimicrobial Agents

7.3 Metals as Antitumor Reagents

7.4 Copper Complexes as Antitumoral Reagents

7.5 Conclusion

References

Chapter 8: NHC-Copper Complexes and their Applications

8.1 Introduction

8.2 History of NHC–Copper Systems

8.3 Hydrosilylation

8.4 Allene Formation

8.5 1,4-Reduction

8.6 Conjugate Addition

8.7 Hydrothiolation, Hydroalkoxylation, Hydroamination

8.8 Carboxylation and Carbonylation (via Boronic Acids, CH Activation): CO2 Insertion

8.9 [3 + 2] Cycloaddition Reaction: Formation of Triazole

8.10 Allylic Substitution

8.11 Carbene and Nitrene Transfer

8.12 Boration Reaction

8.13 Olefination of Carbonyl Derivatives

8.14 Copper-Mediated Cross-Coupling Reaction

8.15 Fluoride Chemistry

8.16 Other Reactions

8.17 Transmetalation

8.18 Conclusion

References

Chapter 9: NHC-Au(I) Complexes: Synthesis, Activation, and Application

9.1 Introduction

9.2 Synthesis of NHC–Gold(I) Chlorides

9.3 Activation of NHC–Au(I) Chlorides

9.4 Applications of NHC–Au(I) Catalysts

9.5 Conclusion

References

Chapter 10: Recent Developments in the Synthesis and Applications of Rhodium and Iridium Complexes Bearing N-Heterocyclic Carbene Ligands

10.1 Introduction

10.2 Rh– and Ir–NHC-Based Complexes: Structural and Electronic Features

10.3 Catalytic Applications of Rhodium and Iridium NHC-Based Complexes

10.4 Abbreviations

References

Chapter 11: N-Heterocyclic Carbene-Ruthenium Complexes: A Prominent Breakthrough in Metathesis Reactions

11.1 Introduction

11.2 Variations of NHC in Ruthenium Complexes

11.3 Modifications in Imidazol- and Imidazolin-2-ylidene Ligands

11.4 Influence of Symmetrically 1,3-Substituted N-Heterocyclic Carbene in Metathesis

11.5 Unsymmetrically N,N′-Substituted N-Heterocyclic Carbenes

References

Chapter 12: Ruthenium N-Heterocyclic Carbene Complexes for the Catalysis of Nonmetathesis Organic Transformations

12.1 Introduction

12.2 Transfer Hydrogenation

12.3 Direct Hydrogenation (and Hydrosilylation)

12.4 Borrowing Hydrogen

12.5 Alcohol Racemization

12.6 Arylation

12.7 Reactions of Alkynes

12.8 Isomerization of CC Bonds

12.9 Allylic Substitution Reactions

12.10 Miscellaneous Reactions

12.11 Conclusions

References

Chapter 13: Nickel Complexes of N-Heterocyclic Carbenes

13.1 Introduction

13.2 Nickel–NHC Catalysts

13.3 Cross-Coupling Reactions

13.4 Oxidation/Reduction Reactions

13.5 Hydrosilylation

13.6 Cycloadditions

13.7 Isomerization

13.8 Reductive Coupling

13.9 Conclusions and Outlook

References

Chapter 14: Coordination Chemistry, Reactivity, and Applications of Early Transition Metal Complexes Bearing N-Heterocyclic Carbene Ligands

14.1 Introduction

14.2 Group 3 Metal Complexes

14.3 Group 4 Metal Complexes

14.4 Group 5 Metal Complexes

14.5 Group 6 Metal Complexes

14.6 Group 7 Metal Complexes

14.7 Conclusion

References

Chapter 15: NHC Complexes of Main Group Elements: Novel Structures, Reactivity, and Catalytic Behavior

15.1 Introduction

15.2 Structures of Common NHCs for Main Group Chemistry

15.3 NHC Complexes of Group 1 Elements

15.4 NHC Complexes of Group 2 Elements

15.5 NHC Complexes of Group 13 Elements

15.6 NHC Complexes of Group 14 Elements

15.7 NHC Complexes of Group 15 Elements

15.8 NHC Complexes of Group 16 Elements

15.9 NHC Complexes of Group 17 Elements

15.10 NHC Reactivity with Protic Reagents

15.11 Cyclic Alkyl Amino Carbenes: Closely Related Cyclic Cousins to NHCs with Similar and Differing Reactivities

15.12 Summary and Outlook

References

Chapter 16: Catalysis with Acyclic Aminocarbene Ligands: Alternatives to NHCs with Distinct Steric and Electronic Properties

16.1 Introduction

16.2 Metalation Routes of Acyclic Carbene Ligands

16.3 Ligand Properties of Acyclic Carbenes

16.4 Catalytic Applications

16.5 Frontiers in Acyclic Carbene Chemistry

16.6 Conclusion

References

Index

End User License Agreement

List of Tables

Table 5.1

Table 5.2

Table 5.3

Table 5.4

Table 5.5

Table 5.6

Table 5.7

Table 5.8

Table 7.1

Table 7.2

Table 7.3

Table 7.4

Table 7.5

Table 7.6

Table 7.7

Table 7.8

Table 7.9

Table 7.10

Table 7.11

Table 10.1

Table 12.1

Table 12.2

Table 12.3

Table 12.4

Table 12.5

Table 12.6

Table 12.7

Table 14.1

Table 16.1

Table 16.2

List of Illustrations

Figure 1.1

Scheme 1.1

Scheme 1.2

Scheme 1.3

Figure 1.2

Scheme 1.4

Figure 1.3

Figure 1.4

Figure 1.5

Figure 1.6

Figure 1.7

Figure 1.8

Figure 1.9

Figure 1.10

Figure 1.11

Figure 1.12

Figure 1.13

Figure 1.14

Figure 1.15

Figure 1.16

Figure 1.17

Figure 1.18

Figure 1.19

Figure 1.20

Figure 1.21

Figure 1.22

Figure 1.23

Figure 1.24

Scheme 1.5

Scheme 1.6

Scheme 1.7

Scheme 1.8

Scheme 1.9

Figure 2.1

Figure 2.2

Figure 2.3

Figure 2.4

Figure 2.5

Figure 2.6

Figure 2.7

Figure 2.8

Figure 2.9

Figure 2.10

Figure 2.11

Figure 2.12

Scheme 3.1

Scheme 3.2

Scheme 3.3

Scheme 3.4

Scheme 3.5

Scheme 3.6

Scheme 3.7

Scheme 3.8

Scheme 3.9

Scheme 3.10

Scheme 3.11

Scheme 3.12

Scheme 3.13

Scheme 3.14

Scheme 3.15

Scheme 3.16

Scheme 3.17

Scheme 3.18

Scheme 3.19

Scheme 3.20

Scheme 3.21

Scheme 3.22

Scheme 3.23

Scheme 3.24

Scheme 3.25

Scheme 3.26

Scheme 3.27

Scheme 3.28

Scheme 3.29

Scheme 3.30

Scheme 3.31

Scheme 3.32

Scheme 3.33

Scheme 3.34

Scheme 3.35

Scheme 3.36

Scheme 3.37

Scheme 3.38

Scheme 3.39

Scheme 3.40

Scheme 3.41

Scheme 3.42

Scheme 3.43

Scheme 3.44

Scheme 3.45

Scheme 3.46

Scheme 3.47

Scheme 3.48

Scheme 3.49

Scheme 3.50

Scheme 3.51

Scheme 3.52

Scheme 3.53

Scheme 3.54

Scheme 3.55

Scheme 3.56

Scheme 3.57

Scheme 3.58

Scheme 3.59

Scheme 3.60

Scheme 3.61

Scheme 3.62

Scheme 3.63

Scheme 3.64

Scheme 3.65

Scheme 3.66

Scheme 3.67

Scheme 3.68

Scheme 3.69

Scheme 3.70

Scheme 3.71

Scheme 3.72

Scheme 3.73

Scheme 3.74

Scheme 3.75

Scheme 3.76

Scheme 3.77

Figure 4.1

Figure 4.2

Figure 4.3

Figure 4.4

Figure 4.5

Figure 4.6

Figure 4.7

Figure 4.8

Figure 4.9

Figure 4.10

Figure 4.11

Figure 4.12

Figure 4.13

Figure 4.14

Figure 4.15

Figure 4.16

Figure 4.17

Figure 4.18

Figure 4.19

Figure 4.20

Figure 4.21

Figure 4.22

Figure 4.23

Figure 4.24

Figure 4.25

Figure 4.26

Figure 4.27

Figure 4.28

Figure 4.29

Figure 4.30

Figure 4.31

Figure 4.32

Figure 4.33

Figure 4.34

Scheme 5.1

Figure 5.1

Scheme 5.2

Scheme 5.3

Figure 5.2

Figure 5.3

Figure 5.4

Figure 5.5

Figure 5.6

Scheme 5.4

Scheme 5.5

Figure 5.7

Scheme 5.6

Figure 5.8

Figure 5.9

Scheme 5.7

Figure 5.10

Scheme 5.8

Scheme 5.9

Figure 5.11

Figure 5.12

Figure 5.13

Figure 5.14

Scheme 5.10

Scheme 5.11

Figure 5.15

Figure 5.16

Figure 5.17

Figure 5.18

Scheme 5.12

Scheme 5.13

Scheme 5.14

Figure 5.19

Figure 5.20

Scheme 5.15

Scheme 5.16

Scheme 5.17

Scheme 6.1

Scheme 6.2

Scheme 6.3

Scheme 6.4

Scheme 6.5

Scheme 6.6

Scheme 6.7

Scheme 6.8

Scheme 6.9

Figure 6.1

Figure 6.2

Figure 6.3

Figure 6.4

Figure 6.5

Scheme 6.10

Figure 6.6

Figure 6.7

Figure 6.8

Figure 6.9

Figure 7.1

Figure 7.2

Figure 7.3

Figure 7.4

Figure 7.5

Figure 7.6

Figure 7.7

Figure 7.8

Figure 7.9

Figure 7.10

Figure 7.11

Figure 7.12

Figure 7.13

Figure 7.14

Figure 7.15

Figure 7.16

Figure 7.17

Figure 7.18

Figure 7.19

Figure 7.20

Scheme 8.1

Scheme 8.2

Scheme 8.3

Scheme 8.4

Scheme 8.5

Scheme 8.6

Scheme 8.7

Scheme 8.8

Scheme 8.9

Scheme 8.10

Figure 8.1

Scheme 8.11

Scheme 8.12

Scheme 8.13

Scheme 8.14

Scheme 8.15

Scheme 8.16

Scheme 8.17

Scheme 8.18

Scheme 8.19

Scheme 8.20

Scheme 8.21

Scheme 8.22

Scheme 8.23

Scheme 8.24

Scheme 8.25

Scheme 8.26

Scheme 8.27

Scheme 8.28

Scheme 8.29

Scheme 8.30

Scheme 8.31

Scheme 8.32

Figure 8.2

Scheme 8.33

Scheme 8.34

Scheme 8.35

Scheme 8.36

Scheme 8.37

Scheme 8.38

Scheme 8.39

Scheme 8.40

Scheme 9.1

Scheme 9.2

Scheme 9.3

Scheme 9.4

Scheme 9.5

Scheme 9.6

Scheme 9.7

Scheme 9.8

Scheme 9.9

Scheme 9.10

Scheme 9.11

Scheme 9.12

Scheme 9.13

Scheme 9.14

Scheme 9.15

Scheme 9.16

Scheme 9.17

Scheme 9.18

Scheme 9.19

Scheme 9.20

Scheme 9.21

Scheme 9.22

Scheme 9.23

Scheme 9.24

Scheme 9.25

Scheme 9.26

Scheme 9.27

Scheme 9.28

Scheme 9.29

Scheme 9.30

Scheme 9.31

Scheme 9.32

Scheme 9.33

Figure 10.1

Figure 10.2

Figure 10.3

Figure 10.4

Figure 10.5

Figure 10.6

Figure 10.7

Figure 10.8

Figure 10.9

Figure 10.10

Figure 10.11

Figure 10.12

Figure 10.13

Scheme 10.1

Scheme 10.2

Scheme 10.3

Figure 10.14

Scheme 10.4

Scheme 10.5

Scheme 10.6

Figure 11.1

Scheme 11.1

Figure 11.2

Figure 11.3

Figure 11.4

Scheme 11.2

Figure 11.5

Scheme 11.3

Figure 11.6

Figure 11.7

Figure 11.8

Figure 11.9

Scheme 11.4

Figure 11.10

Figure 11.11

Figure 11.12

Figure 11.13

Figure 11.14

Figure 11.15

Figure 11.16

Figure 11.17

Scheme 11.5

Figure 11.18

Figure 11.19

Figure 11.20

Scheme 11.6

Figure 11.21

Figure 11.22

Figure 11.23

Scheme 11.7

Figure 11.24

Figure 11.25

Figure 11.26

Figure 11.27

Figure 11.28

Scheme 11.8

Figure 11.29

Figure 11.30

Figure 11.31

Figure 11.32

Figure 11.33

Figure 11.34

Figure 11.35

Figure 11.36

Scheme 12.1

Scheme 12.2

Scheme 12.3

Scheme 12.4

Scheme 12.5

Scheme 12.6

Scheme 12.7

Scheme 12.8

Scheme 12.9

Scheme 12.10

Scheme 12.11

Figure 13.1

Figure 13.2

Figure 13.3

Figure 13.4

Figure 13.5

Figure 13.6

Figure 13.7

Figure 13.8

Figure 13.9

Figure 13.10

Figure 13.11

Figure 13.12

Figure 13.13

Figure 13.14

Figure 13.15

Figure 13.16

Figure 13.17

Figure 13.18

Figure 13.19

Figure 13.20

Figure 13.21

Figure 13.22

Figure 13.23

Figure 13.24

Figure 13.25

Figure 13.26

Figure 13.27

Figure 13.28

Figure 13.29

Figure 13.30

Figure 13.31

Figure 13.32

Figure 13.33

Figure 13.34

Figure 13.35

Figure 13.36

Figure 13.37

Figure 13.38

Figure 13.39

Figure 13.40

Figure 13.41

Figure 13.42

Figure 13.43

Figure 13.44

Figure 13.45

Figure 13.46

Scheme 14.1

Scheme 14.2

Scheme 14.3

Scheme 14.4

Scheme 14.5

Figure 14.1

Scheme 14.6

Scheme 14.7

Scheme 14.8

Figure 14.2

Scheme 14.9

Scheme 14.10

Scheme 14.11

Scheme 14.12

Scheme 14.13

Figure 14.3

Scheme 14.14

Scheme 14.15

Scheme 14.16

Scheme 14.17

Scheme 14.18

Scheme 14.19

Scheme 14.20

Scheme 14.21

Scheme 14.22

Scheme 14.23

Scheme 14.24

Scheme 14.25

Scheme 14.26

Scheme 14.27

Scheme 14.28

Scheme 14.29

Scheme 14.30

Scheme 14.31

Scheme 14.32

Scheme 14.33

Scheme 14.34

Scheme 14.35

Scheme 14.36

Scheme 14.37

Figure 14.4

Scheme 14.38

Figure 15.1

Figure 15.2

Figure 15.3

Figure 15.4

Figure 15.5

Figure 15.6

Figure 15.7

Figure 15.8

Figure 15.9

Figure 15.10

Figure 15.11

Figure 15.12

Scheme 15.1

Figure 15.13

Figure 15.14

Figure 15.15

Figure 15.16

Figure 15.17

Scheme 15.2

Scheme 15.3

Figure 15.18

Scheme 15.4

Figure 15.19

Scheme 15.5

Scheme 15.6

Figure 15.20

Scheme 15.7

Scheme 15.8

Scheme 15.9

Figure 15.21

Figure 15.22

Figure 15.23

Figure 15.24

Figure 15.25

Scheme 15.10

Scheme 15.11

Figure 15.26

Scheme 15.12

Scheme 15.13

Figure 15.27

Figure 15.28

Figure 15.29

Figure 15.30

Figure 15.31

Figure 15.32

Figure 15.33

Scheme 15.14

Figure 15.34

Scheme 15.15

Figure 15.35

Scheme 15.16

Figure 15.36

Figure 15.37

Figure 15.38

Scheme 15.17

Figure 15.39

Scheme 15.18

Figure 15.40

Figure 15.41

Figure 15.42

Scheme 15.19

Figure 15.43

Scheme 15.20

Figure 15.44

Figure 15.45

Figure 15.46

Scheme 15.21

Figure 15.47

Scheme 15.22

Figure 15.48

Figure 15.49

Figure 15.50

Scheme 15.23

Scheme 15.24

Figure 15.51

Scheme 15.25

Scheme 15.26

Figure 15.52

Scheme 15.27

Scheme 15.28

Scheme 15.29

Figure 15.53

Scheme 15.30

Scheme 15.31

Scheme 15.32

Figure 15.54

Figure 16.1

Scheme 16.1

Scheme 16.2

Figure 16.2

Figure 16.3

Scheme 16.3

Figure 16.4

Scheme 16.4

Figure 16.5

Scheme 16.5

Scheme 16.6

Scheme 16.7

Scheme 16.8

Scheme 16.9

Scheme 16.10

Scheme 16.11

Scheme 16.12

Figure 16.6

Scheme 16.13

Scheme 16.14

Scheme 16.15

Scheme 16.16

Figure 16.7

Figure 16.8

Figure 16.9

Scheme 16.17

Figure 16.10

Guide

Cover

Table of Contents

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Chapter 1

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Edited by Steven P. Nolan

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Dedication

“To Carl: friend, mentor and not so old afterall …And to Catherine, Maëlys and Kaelia, for always being there.”

List of Contributors

Abdullah Mohamed Asiri

King Abdulaziz University

Center of Excellence for Advanced Materials Research (CEAMR)

Jeddah 21589

P.O. Box 80203

Saudi Arabia

Stéphane Bellemin-Laponnaz

Université de Strasbourg-CNRS

Institut de Physique et Chimie des Materiaux de Strasbourg

23 rue du Loess

67034 Strasbourg Cedex 2

France

and

Université de Strasbourg-CNRS

Institut de Chimie de Strasbourg

1 rue Blaise Pascal

67000 Strasbourg

France

Luigi Cavallo

King Abdullah University of Science and Technology (KAUST)

Chemical and Life Sciences and Engineering

Kaust Catalysis Center

Thuwal 23955-6900

Saudi Arabia

Catherine S.J. Cazin

University of St Andrews

EaStCHEM School of Chemistry

Purdie Building, North Haugh

St Andrews, Fife KY16 9ST

UK

Jason A.C. Clyburne

Saint Mary's University

Department of Chemistry

The Atlantic Centre for Green Chemistry

Halifax

Nova Scotia B3H 3C3

Canada

Samuel Dagorne

Université de Strasbourg-CNRS

Institut de Physique et Chimie des Materiaux de Strasbourg

23 rue du Loess

67034 Strasbourg Cedex 2

France

and

Université de Strasbourg-CNRS

Institut de Chimie de Strasbourg

1 rue Blaise Pascal

67000 Strasbourg

France

Steve Dierick

Université Catholique de Louvain

Laboratoire de Chimie Organique et Médicinale

Place Louis Pasteur, 1, bte L4.01.02

B-1348 Louvain-la-Neuve

Belgium

Reto Dorta

University of Western Australia

School of Chemistry and Biochemistry

35 Stirling Highway

6009 Crawley, WA

Australia

Laura Falivene

Università di Salerno

Dipartimento di Chimica e Biologia

Via Ponte don Melillo

84084 Fisciano

Italy

Daniel Guest

University of Sussex

Department of Chemistry

Brighton

BN1 9QJ

UK

Gregorio Guisado-Barrios

Universitat Jaume I

Departamento de Química Inorgánica y Orgánica

Avda. Vicente Sos Baynat s/n

Castellón, E-12071

Spain

A. Stephen K. Hashmi

Ruprecht Karls University Heidelberg

Institute of Organic Chemistry

Im Neuenheimer Feld 270

69120 Heidelberg

Germany

M. Taylor Haynes II

University of Michigan

Department of Chemistry

930 N. University

Ann Arbor, MI 48109-1055

USA

Mario Hoyos

CSIC

Instituto de Ciencia y Tecnología de Polímeros

Juan de la Cierva, 3

28006 Madrid

Spain

Evan P. Jackson

University of Michigan

Department of Chemistry

930 N. University

Ann Arbor, MI 48109-1055

USA

Faïma Lazreg

University of St Andrews

EaStCHEM School of Chemistry

Purdie Building, North Haugh

St Andrews, Fife KY16 9ST

UK

N. Gabriel Lemcoff

Ben-Gurion University

Department of Chemistry

P.O. Box 653

Beer-Sheva 84105

Israel

István E. Markó

Université Catholique de Louvain

Laboratoire de Chimie Organique et Médicinale

Place Louis Pasteur, 1, bte L4.01.02

B-1348 Louvain-la-Neuve

Belgium

Jason D. Masuda

Saint Mary's University

Department of Chemistry

The Atlantic Centre for Green Chemistry

Halifax

Nova Scotia B3H 3C3

Canada

Sudheendran Mavila

Ben-Gurion University

Department of Chemistry

P.O. Box 653

Beer-Sheva 84105

Israel

John Montgomery

University of Michigan

Department of Chemistry

930 N. University

Ann Arbor, MI 48109-1055

USA

Luke J. Murphy

Saint Mary's University

Department of Chemistry

The Atlantic Centre for Green Chemistry

Halifax

Nova Scotia B3H 3C3

Canada

Oscar Navarro

University of Sussex

Department of Chemistry

Brighton

BN1 9QJ

UK

David J. Nelson

University of St Andrews

EaStCHEM School of Chemistry

North Haugh, St Andrews

Fife KY16 9ST

UK

Steven P. Nolan

University of St Andrews

EaStCHEM School of Chemistry

St Andrews

KY16 9ST

United Kingdom

Matthew J. Panzner

The University of Akron

Department of Chemistry

190 E Buchtel Commons

Akron, OH 44325-3601

USA

Eduardo Peris

Universitat Jaume I

Departamento de Química Inorgánica y Orgánica

Avda. Vicente Sos Baynat s/n

Castellón, E-12071

Spain

Albert Poater

Universitat de Girona

Institut de Química Computacional i Catàlisi

Departament de Química

Campus de Montilivi

E-17071 Girona

Spain

Macarena Poyatos

Universitat Jaume I

Departamento de Química Inorgánica y Orgánica

Avda. Vicente Sos Baynat s/n

Castellón, E-12071

Spain

Katherine N. Robertson

Saint Mary's University

Department of Chemistry

The Atlantic Centre for Green Chemistry

Halifax

Nova Scotia B3H 3C3

Canada

Alvaro Salvador

University of Western Australia

School of Chemistry and Biochemistry

35 Stirling Highway

6009 Crawley, WA

Australia

Leonid Schwartsburd

University of Bath

Department of Chemistry

Claverton Down

Bath BA2 7AY

UK

Kerri L. Shelton

The University of Akron

Department of Chemistry

190 E Buchtel Commons

Akron, OH 44325-3601

USA

LeGrande M. Slaughter

University of North Texas

Department of Chemistry

1155 Union Circle # 305070

Denton, TX 76203-5070

USA

Claire A. Tessier

The University of Akron

Department of Chemistry

190 E Buchtel Commons

Akron, OH 44325-3601

USA

Patrick O. Wagers

The University of Akron

Department of Chemistry

190 E Buchtel Commons

Akron, OH 44325-3601

USA

Michael K. Whittlesey

University of Bath

Department of Chemistry

Claverton Down

Bath BA2 7AY

UK

Linglin Wu

University of Western Australia

School of Chemistry and Biochemistry

35 Stirling Highway

6009 Crawley, WA

Australia

Thomas Wurm

Ruprecht Karls University Heidelberg

Institute of Organic Chemistry

Im Neuenheimer Feld 270

69120 Heidelberg

Germany

Wiley J. Youngs

The University of Akron

Department of Chemistry

190 E Buchtel Commons

Akron, OH 44325-3601

USA

Preface

It has been eight years since the first monograph on N-heterocyclic carbenes (NHC) appeared [1]. In this rather short timespan the uses of NHCs in their many incarnations have contributed to advances in numerous areas of synthetic chemistry. These initial curiosities [2] have become workhorses in synthesis and homogeneous catalysis [3]. Catalytic studies nowadays almost always include the testing of a NHC or NHC precursor as part of a ligand/catalyst screening.

The thinking involving NHC in catalysis has also evolved considerably since the original in situ catalyst generation protocols and tertiary phosphine mimic analogies. The stabilizing effects of NHCs on organometallic complexes have permitted lower catalyst loading operations and unique mechanistic insights. The area has evolved in a very “green” direction and this trend I hope will continue as well-defined systems have been identified that belong within or sit off-cycle as catalyst reservoirs in important catalytic reactions. The use of these now almost ubiquitous ligands has helped the field better understand fundamental transformations, such as C–H activation and more interestingly C–H bond functionalization [4].

The present monograph presents important developments in metal-mediated transformations. I hope established and younger researchers alike will find here inspiration to take the past discoveries as a foundation to design novel scaffolds with original properties and deploy these in known and undiscovered catalysis and synthetic uses.

I will not use this preface to perform an exercise in crystal-ball-gazing and pontificate on what should or should not be explored as performing research in this area has made me a more humble and regularly amazed researcher. As many have, I came to this field by accident. Serendipity does and continues to play an important role in a number of developments in this now very fruitful (some may call it mature) area of research. I truly believe many more surprises are in store for us facing the pleasures and frustrations of exploring this fascinating area of Science.

I would like to thank Mrs. Carolyn Busby and Dr. David Nelson for their editorial assistance and I am grateful to the contributing authors who are true authorities in this still very rapidly evolving field.

St Andrews

Steven P. NolanFebruary 2014

References

1.

Nolan, S.P. (ed.) (2006)

N-Heterocyclic Carbenes in Synthesis

, John Wiley & Sons, New York.

2.

For the first NHC, see: Arduengo, A.J., III, Harlow, R.L., and Kline, M. (1991)

J. Am. Chem. Soc.

,

113

, 361 and for an earlier example of carbene; Igau, A., Grutzmacher, H., Baceiredo, A., and Bertrand, G. (1988)

J. Am. Chem. Soc.

,

110

, 6463.

3.

Díez-González, S., Marion, N., and Nolan, S.P. (2009)

Chem. Rev.

,

109

, 3612.

4.

For two examples of TM-NHC-catalyzed C–H bond carboxylation, see: (a) Boogaerts, I.I.F. and Nolan, S.P. (2010)

J. Am. Chem. Soc.

,

132

, 8858; (b) Boogaerts, I.F.F., Fortman, G.C., Furst, M.R.L., Cazin, C.S.J., and Nolan, S.P. (2010)

Angew. Chem., Int. Ed.

,

44

, 8674.

1N-Heterocyclic Carbenes

David J. Nelson and Steven P. Nolan

1.1 Introduction

Over the past few decades, stable carbenes have received a great deal of attention from a number of researchers [1]. In the singlet carbene compounds, a carbon center bears a lone pair of electrons in an sp2 hybridized orbital while a p orbital remains vacant (Figure 1.1a). Triplet carbenes are also known, where each of the two electrons occupy a degenerate p orbital (Figure 1.1b).

Figure 1.1 (a) Singlet carbenes; (b) triplet carbenes.

N-Heterocyclic carbenes (NHCs) are a specific form of this class of compound, where the carbene is located on an N-heterocyclic scaffold. While these species were initially not widely applied in chemistry, they have now been employed in a broad range of fields, including organocatalysis [2] and organometallic chemistry [3]. Hundreds of different NHCs are known in the literature, and much has been learned about their properties and reactivity. Various experimental and theoretical techniques have been applied toward this aim, including density functional theory (DFT) studies, which have allowed an insight into the bonding and orbital arrangements in NHCs. This chapter details the discovery and isolation of stable NHCs, the characterization of the electronic nature of this species, the factors that render them stable, and the nature of their bonding to metal centers. In addition, some of the ways in which the electronic and steric properties of these species can be explored and quantified will be discussed.

1.2 Structure and Properties of NHCs

Prior to the isolation of stable NHCs, some information was known about the properties of these species. As early as the 1960s, researchers such as Wanzlick were active in probing the reactivity of NHCs generated in situ from, for example, the thermolysis of the corresponding dimers [4]. In this way, the nucleophilic reactivity of these species with a number of reagents was characterized (Scheme 1.1). In addition, reaction with HCl yielded the corresponding imidazolium chloride salts. Metal–carbene complexes were also prepared by Wanzlick and Schönherr, without isolation of the free carbene itself (Scheme 1.2) [5].

Scheme 1.1 Early studies of the reactivity of N-heterocyclic carbenes. [4].

Scheme 1.2 Synthesis of an NHC-mercury complex [5].

The isolation of stable NHCs was a key event in the chemistry of this valuable class of compound, as this allowed the preparation of material for detailed characterization. In addition, many modern syntheses of NHC–metal complexes rely on the use of isolated NHCs. In 1991, when Arduengo et al. exposed imidazolium chloride 1 to NaH and catalytic DMSO in THF, stable carbene 2 was isolated (Scheme 1.3) [6]. This species, also known as IAd, could be characterized by various methods, including X-ray crystallography and NMR spectroscopy.

Scheme 1.3 Synthesis of IAd [6].

Initially, it was unclear whether steric or electronic effects were the source of the stability of 2. A subsequent publication from Arduengo et al. reported a further four stable carbenes 3–6 (3 is typically referred to as ITME, and 4 as IMes) with various N-substituents, which were prepared in the same manner as 2(Figure 1.2) [7]. Notably, these were far less sterically hindered, suggesting that the origin of their stability was electronic, rather than steric, or was a combination of these factors.

Figure 1.2 Stable NHCs isolated by Arduengo et al. [7].

The aromatic nature of the imidazolium ring was thought to be critical to the stability of NHCs. However, in 1995, NHC 7 (SIMes) bearing a saturated backbone and bulky mesityl N-substituents was obtained by deprotonation of the corresponding imidazolium chloride 8, and was characterized by Arduengo et al. (Scheme 1.4) [8].

Scheme 1.4 Synthesis of SIMes [8].

With a robust route to synthesize and isolate free carbenes in hand, several researchers applied a number of tools to investigate their properties and reactivity. A thorough understanding of these properties is essential to understand how these species can be applied in chemistry, and to inform the rational design of new NHCs. X-ray photoelectron spectroscopy (XPS) and DFT studies of a model carbene 9 () confirmed the presence of a lone pair of electrons in the plane of the imidazolylidene ring, and an empty p orbital on the same carbon center [9]. However, there was initially some debate as to whether the carbene was best considered as a carbene or as an ylide (Figure 1.3); that is, whether a resonance contribution from the lone pair centered on nitrogen was a part of the bonding arrangement in NHCs. Understanding this aspect of the structure of NHCs was important in order to understand both how the structure of the NHC might affect reactivity, as well as allowing for the tuning of reactivity via structural modifications.

Figure 1.3 Carbenic and ylidic resonance forms of N-heterocyclic carbenes (9 () is pictured).

Initial studies by Dixon and Arduengo suggested that the ylidic form was not a major contributor to the structure of imidazolium-based NHCs [10]. Subsequent electron distribution mapping of a model carbene, 3-d12 (ITME-d12) using X-ray and neutron radiation also suggested very little contribution from the ylidic form; these methods relied on mapping the electron distribution 0.7 Å above the plane of the NHC, which the authors proposed should indicate whether pπ–pπ delocalization occurred. The lengthened C—N bonds in imidazolylidenes compared to the corresponding imidazolium salts were proposed to be further evidence of negligible interaction of the nitrogen lone pairs with the empty p orbital at the carbene. Visualization of the electron density using this method showed π electron density between C4 and C5 corresponding to the double bond, and the p electrons of the nitrogen; no evidence for an ylidic form was found. In addition, the shielding tensor σ11 was revealed to be negative, suggesting that the carbenic resonance form was dominant [11]. However, later work by Boehme and Frenking suggested that the method of electron density mapping that was employed was not appropriate, as it suggested negligible π-delocalization in pyridine and pyrrole, which are known to be aromatic [12]. In silico calculations by these authors, particularly those involving natural bond order (NBO) calculations, strongly suggested that pπ–pπ delocalization was significant in both imidazol-2-ylidenes and imidazolidin-2-ylidenes, but more pronounced in the former.

A detailed study, published at the same time by Heinemann et al., explored this pπ–pπ delocalization in NHCs, and aimed to understand whether imidazol-2-ylidenes were aromatic species [13]. Three key characteristics of NHCs were explored: thermodynamic stability, geometric structure, and the charge distribution. Isodesmic calculations on acyclic carbenes and aminocarbenes showed that even when conjugation was not possible, the carbene was stabilized by adjacent amino groups due to their σ electron-withdrawing properties. Conjugation further increased stabilization; imidazolidin-2-ylidenes were more stable again, while imidazol-2-ylidenes were most stable (Figure 1.4). Similarly, structural data were consistent with π-delocalization. Calculated magnetic susceptibility anisotropies (Δχ) were suggestive of cyclic π-delocalization, but to a lesser extent than in benzene. All of these results strongly suggest the involvement of pπ–pπ delocalization from the nitrogen lone pair into the empty orbital at the carbene, and that imidazol-2-ylidenes show some aromatic character. In a later study by Bielawski and coworkers, it was shown that the electronic properties of acyclic diaminocarbenes (as probed using the infrared spectra of [IrCl(CO)2(L)] complexes 10 and 11, shown later) were dependent on the ligand conformation, due to differing degrees of pπ–pπ delocalization (Figure 1.5) [14].

Figure 1.4 Stability of some carbenes, as determined by isodesmic calculations for the reaction of each carbene with methane to generate NHC·H2 and dihydrocarbene [13].

Figure 1.5 [IrCl(CO)2(L)] complexes in which the differing degrees of pπ–pπ conjugation affects the electronic properties of the metal center [14].

Clearly, this delocalization is a key component of the bonding in NHCs, and for this reason NHCs are typically drawn with the inclusion of a curve between the nitrogen atoms in order to emphasize this aspect of their electronic structure.

1.3 Abnormal Carbenes

While the majority of reports of imidazolylidenes bound to metal centers involve coordination via the C2 position (i.e., imidazol-2-ylidenes), there has been recent and growing interest in so-called abnormal carbenes, often referred to as aNHCs, where binding occurs via the C4 or C5 position (Figure 1.6) [15]. Often the substitution pattern is chosen to block the C2 position. The resulting imidazolylidenes are stabilized by only one nitrogen moiety, as the π-donating and σ-accepting properties of the second nitrogen atom are greatly reduced. Such species provide great scope for achieving different properties from so-called normal carbenes.

Figure 1.6 Normally versus abnormally bound imidazolylidenes.

These species tend to show quite different electronic properties to their normally bound congeners. They are considerably more electron-donating due to the reduced σ-withdrawal from the carbene center, as evidenced by calculated Tolman electronic parameter (TEP) [16] values (shown later) for a range of these species [17]. In addition, they have been shown to be more π-accepting (due to reduced pπ–pπ delocalization), as determined by analysis of the 31P chemical shifts of the corresponding phosphinidene adducts [18]. The different properties of these ligands will naturally confer different properties and reactivity to the metal centers to which they are coordinated.

Other species with reduced heteroatom stabilization are also known; these include isomers of imidazolylidenes (e.g., 1,2-imidazol-3-ylidenes, 1,2-imidazol-4-ylidenes), 1,2,3-triazoly-4-lidenes, and pyrimidazolylidenes (Figure 1.7) [15].

Figure 1.7 NHC species with reduced heteroatom stabilization [15].

1.4 Why Are NHCs Stable?

Prior to the isolation of NHCs, various studies were carried out on species generated in situ [4]. However, once Arduengo succeeded in isolating a series of stable species, attention naturally turned to identifying why some species were stable and isolable, while others were not.

Heinemann and Thiel [19] and Carter and Goddard [20] both applied theoretical methods to investigate the singlet–triplet gap in prototypical carbene compounds, showing that this factor was key in the stability of NHCs. Triplet carbenes are known to be much less stable than singlet species [21]. Some of the factors affecting the singlet–triplet gap have been established for some time, such as the influence of the geometry of the carbene and the presence of neighboring π systems [22]. Importantly, the two otherwise degenerate empty px and py orbitals on the carbene must be rendered different in energy (e.g., by bending the carbene in one plane), and the energy gap between the singlet and triplet forms must be as large as possible.

Bertrand and coworkers have discussed, with the use of illustrative examples, the various means by which carbenes can be stabilized [1]; the three examples employed are reproduced here (Figure 1.8). In Figure 1.8a, which is a typical arrangement in NHCs, electron density from the lone pair of an adjacent heteroatom is donated into the empty p orbital, while the inductive σ-electron-withdrawing nature of the heteroatom reduces the electron density at the carbene center. In Figure 1.8b, the (linear) carbene is stabilized by donation of electron density from the carbene into adjacent π-accepting heteroatoms (such as boron), which are σ-donating. In Figure 1.8c, the combination of π-donating/σ-withdrawing and π-withdrawing/σ-donating heteroatoms acts to stabilize the linear carbene.

Figure 1.8 Bonding arrangements in some stable carbene systems [1].

Boehme and Frenking probed the stability difference between saturated and unsaturated carbene species, considering various factors such as the optimized geometries (MP2/6-31G(d)), energies of hydrogenation and NBO analysis of imidazol-2-ylidene and imidazolidin-2-ylidene [12]. The enthalpy of hydrogenation of imidazol-2-ylidene was lower than that of imidazolidin-2-ylidene (about −20 and −40 kcal mol−1, respectively), in agreement with the higher thermodynamic stability of the former. NBO analysis showed that, despite the longer C2—N bonds in the former, the pπ(C2) occupancy was higher, suggesting a greater contribution from the nitrogen lone pair. The contribution of σ donation to the electronics of C2 was shown to be similar in both systems, suggesting that the difference in stability must result from greater π donation from the nitrogen atoms in unsaturated analogs.

Cavallo and coworkers have shown that the stability of a singlet carbene (with respect to dimerization) can be ascertained via calculation of two key properties of a carbene [23]. The steric bulk is quantified using percent of buried volume, %Vbur (shown later) [24], while the electronic nature is quantified using the singlet–triplet energy gap (ES–T). It was shown that a linear correlation (R2 = 0.93 or 0.88, for gas-phase or THF data, respectively) exists between the energy of dimerization (Edim) and a linear combination of %Vbur and ES–T (Equation 1.1). Importantly, the value of Edim can be used to predict the behavior of a carbene. For carbenes where Edim was predicted to be ≤ −20 kcal mol−1, the NHC was found (experimentally) to be stable as a dimer. In cases where Edim ≥ 0 kcal mol−1, the NHC was found to be stable as a monomer. For intermediate cases, the NHC exists as a mixture of the free carbene and dimer at equilibrium.

(1.1)

1.5 Bonding of NHCs to Metal Centers

One of the major applications of NHCs is as ligands for transition and main group metal centers. Therefore, understanding the way in which NHCs bond to and influence the properties of metal(loid) centers is of great importance. In this section, the nature of bonds to NHCs will be discussed, highlighting how this important characteristic has been probed experimentally and theoretically. Section 1.6 deals with the characterization of NHCs, and includes a discussion of the various metrics that can be used to quantitatively describe the way in which different NHCs can affect the properties of species to which they are bound.

N-Heterocyclic carbenes and phosphine ligands can bind metal centers in a somewhat similar fashion: via dative coordination using a lone pair of electrons. Therefore, phosphines are often considered to be the closest neighbors to NHCs in terms of organometallic chemistry. However, the properties of these two classes of compounds can be quite different, and can yield, for example, catalysts with quite different reactivity. The use of NHCs in olefin metathesis [25] and in palladium catalysis [26], for example, has allowed the preparation of highly active and stable species that are now at the forefront of their fields. Various studies have been conducted to explore the nature of bonding between NHCs and metal centers. Díez-González and Nolan reviewed some aspects of NHC coordination to metal centers in 2007 [27].

One of the key issues was the degree of σ and π bonding between the NHC and the metal center. Initially, NHCs were thought to be purely σ donors, but later studies revealed contributions from π-bonding also. Structural data for NHC–Cu complexes suggested C—Cu bond lengths that were shorter than those that would be expected for purely σ bonds [28]. Later studies by Hu et al. revealed the structure of molecular orbitals in a number of NHC–Ag complexes [29]. Initially, significant π interactions were observed between the pπ orbitals of the carbene (i.e., perpendicular to the plane of the imidazolylidene ring) and the metal dxz and dyz orbitals [29a], suggesting that π-backbonding comprised a significant part of the NHC–Ag bond. This contribution was later quantified at 15–30% of the total orbital interaction energy, while Frenking and coworkers calculated a value of about 20% [30], underlining its importance in NHC–metal bonding [29b].

Hu et al. also used structural data for [Z(IMes)2] (Z = Ni0, AgI, I+) compounds (12–14) to illustrate the importance of π backbonding (Figure 1.9) [29b]. In the nickel example, the metal is relatively electron rich (d10), and so can donate electron density from the d orbitals into the C—N π* orbital of the NHC, lengthening the C—N bond. In the silver example, the metal center is less electron rich (and is cationic), so it is less able to participate in d → π* backbonding, and the C—N bond is less elongated as a result. Iodine does not have filled d orbitals at all, and so the C—N bond in the NHC is shorter still, being unaffected by d → π* backbonding.

Figure 1.9 Probing the effect of π-backbonding in bis(NHC) compounds [29b].

Scott et al. have explored the bonding in some bis(NHC) iridium and rhodium complexes in which the NHCs are present in a trans arrangement [31]. The metal–carbene bonding in complex [] 15 was found to involve π donation from the NHC to the metal (Figure 1.10). The bond lengths from the iridium center to the carbenes differed by about 0.2 Å, both by X-ray crystallographic analysis and by density functional theory studies. In addition, NBO analyses showed that the bonds had quite different orders (0.62 and 0.51). These different bond lengths were rationalized by visualizing the molecular orbitals of the complex; the shorter bond involves partial π → d donation from the highest occupied molecular orbital (HOMO) of the NHC (a π orbital) to the empty d orbitals of the electron-deficient metal center.

Figure 1.10 Ir—C bond lengths in [IrCl(ItBu)2] [31].

A later study by Jacobsen and coworkers systematically evaluated the various contributions of π donation and π backbonding in a variety of metal complexes of imidazol-2-ylidene (Figure 1.11) [32].

Figure 1.11 Model complexes studied by Jacobsen and coworkers to evaluate the contributions of π-donation and π-backbonding to metal—carbene bonds [32].

The systems studied covered a wide range of metals, d-configurations, geometries, and oxidation states. The authors studied the enthalpies of the metal–NHC bonds by assessing the energies of the complexes versus systems in which the NHCs were separated from the metal; however, the new fragments were not allowed to rearrange and so the bond enthalpy in each case was termed BEsnap, that is, only the energy required to “snap” the bond. Applying an energy decomposition analysis to the results allowed the separation of σ and π bonding, and the separation of the latter into π donation and π backbonding. Orbital interactions were found to dominate, with stronger bonds found in cationic species and those with high d-electron counts. Furthermore, bond energies were found to change when ligands with different π-donor or π-acceptor properties were used, strongly suggesting that π bonding to the metal is a significant factor in the NHC–metal bond. In a further step, π donation and π backbonding were separated in two subsequent calculations by removing the empty metal d orbitals and then the empty NHC π orbitals, respectively. The authors therefore concluded that three types of bonding were significant: σ bonding from the lone pair into the , π donation, and π backbonding between the π system localized on the NHC and the dxz (or dyz orbitals) (Figure 1.12). This is quite a complex bonding arrangement, and means that the M–NHC interaction is dependent not only on the structure and properties of the NHC, but also on the electronic arrangement at the metal center. Surprisingly, even metal centers that are formally d0 exhibited considerable π backbonding, with this contribution increasing with d electron count.

Figure 1.12 Contributions to the metal—carbene bond: (a) π-backbonding, (b) σ-donation, and (c) π-donation.

Figure 1.13 Characterization of cis-[PtCl2(DMSO)(NHC)] complexes [33].

Three key systems have been studied experimentally to investigate the contribution of π bonding to metal–NHC bonding. Fantasia et al. prepared a series of cis-[PtCl2(DMSO)(NHC)] complexes 18–21 (Figure 1.13), in which the NHC was the only potential π-accepting ligand [33]. Analysis of the NMR chemical shifts and the coupling constants reveal information about the influence of the carbene ligand on the properties of the metal center. Larger coupling constants are indicative of more electron density in the σ bond between carbene and metal; therefore, the larger constants observed for complexes bearing unsaturated NHCs suggested that these bind the platinum center with more σ character than their saturated congeners. However, the chemical shifts revealed that Pt centers coordinated to saturated NHCs were more electron rich (lower δPt). This was rationalized using computational studies, which supported the experimental evidence that saturated NHCs are both better σ donors and π acceptors, and form stronger bonds to the Pt center.

Figure 1.14 Spectroscopic investigation of π-backbonding in [RhCl(COD)(NHC)] and [RhCl(CO)2(NHC)] complexes.

Bielawski and coworkers utilized [RhCl(COD)(NHC)] and [RhCl(CO)2(NHC)] complexes to investigate π backbonding by spectroscopic means (Figure 1.14). Of particular interest were NHCs 22 and 23, which contain spectroscopic probes (carbonyl and nitrile, respectively) for IR analysis. The frequency of signals corresponding to these probes can be altered by changing the degree of π backbonding from the metal center. The corresponding [RhCl(COD)(NHC)] complexes were exposed to CO to produce [RhCl(CO)2(NHC)] complexes, in which the COD ligand was replaced with two CO ligands, which are better π acceptors. An increase in the stretching frequency of the probes was found to occur, indicative of strengthening of the bonds from the weaker π backbonding from the Rh center in the dicarbonyl species.

Figure 1.15 NHC-phosphinidene adducts, and rotational barriers at room temperature for two examples.

Most recently, phosphinidene adducts have been used to probe the nature of π bonding to NHCs [18]. The bonding in these adducts can be considered to be somewhere between dative coordination to the phosphorus atom, and double bond character between the carbene and the phosphorus (Figure 1.15). Adducts bearing the IPr and SIPr NHCs were studied; the former showed free rotation around the C—P bond (equivalent iso-propyl moieties) at room temperature, while the latter did not. Variable temperature NMR studies allowed the barrier to rotation in each species to be quantified, revealing a greater degree of double-bond character between phosphorus and the saturated NHC. This was put forward as further evidence of the contribution of π bonding in bonds to NHCs. Furthermore, the chemical shift was shown to vary considerably (about −60 to 130 ppm), depending on the ability of the NHC under examination to undergo d → π backbonding.

Figure 1.16 Tuning the properties of NHC ligands through structural confinement of the nitrogen substituents.

While the various contributions from σ and π bonding are a function of the electronic configuration of the metal center, they also depend on the structure of the NHC ligands. Bertrand and coworkers reported an NHC ligand 24 where one of the nitrogen atoms was confined in an orientation where π-delocalization into the empty p orbital was restricted (Figure 1.16) [34]; therefore, the carbene is stabilized by the σ-inductive withdrawing effects of two nitrogen atoms, but only accepts π electron density from one. The new ligand was as nucleophilic and σ-donating as analogs such as 25, but was found to have a smaller singlet–triplet energy gap and was more π-accepting, as a result of the interaction with only one nitrogen lone pair.

Figure 1.17 Percent of buried volume for characterizing the steric properties of NHC ligands [24].

1.6 Quantifying the Properties of NHCs

With such a vast range of NHCs known from the literature and a wider still range accessible using established organic chemistry methodologies [35], it becomes important to be able to compare this catalog of ligands quantitatively. To achieve this aim, a number of metrics have been employed. A fuller discussion of the latest models that can be used to describe NHCs can be found in a subsequent chapter, but a brief discussion is presented here, divided into steric impact and electronic properties. Given the similarity of NHCs to phosphanes, in that both are neutral two-electron σ-donor ligands, some of these metrics bear relation to those employed in the chemistry of phosphanes. Other metrics are newer, and take advantage of recent increases in the capabilities of analytical equipment and molecular modeling software. However, the inherent simplicity of quoting a single number for steric impact and a single number for electronic properties render %Vbur and TEP [16] the current metrics of choice for describing NHC ligands.

1.6.1 Steric Impact

For phosphanes, the Tolman cone angle, defined as a cone extending from a metal center (2.28 Å from the phosphorus of the phosphine ligand) that encompasses the substituents, provides the most common measure of steric impact [16]. However, NHCs are typically a very different shape from phosphanes. Initially, a metric based on the “wedge” shape of NHCs was proposed, using two parameters to describe the size of an NHC [36]. However, this was quickly superseded by the concept of percent of buried volume (%Vbur) [24], which is defined as the proportion of a sphere, centered on the metal, which is occupied by the ligand (Figure 1.17). This scale can be used to describe NHCs and phosphanes, and in theory any other ligands also. Cavallo and coworkers have developed a simple Web-based software that allows %Vbur to be calculated quickly from crystallographic data [37].

Figure 1.18 Comparison of %Vbur using different metal complexes.

The percent of buried volume is heavily dependent on both the nature of the ligand under investigation and on the geometry of the complex to which it is coordinated. It is therefore very difficult to compare values of %Vbur obtained using different families of complexes. For example, NHCs 26 (IiPr) and 27 (IiPrMe) differ only in the methylation of the imidazolylidene backbone. When the linear two-coordinate [AuCl(NHC)] complexes are considered, there is a significant difference in %Vbur (23.5% versus 33.9%; where the M–NHC distance is 2.28 Å) (Figure 1.18) [24]. However, in square planar [Ir(COD)(NHC)(OH)] complexes, the difference is far less stark (27.1 and 28.4%), but still sufficient to change the way in which the ligand and metal interact [38]; IiPrMe was found to add only once to [IrCl(COD)]2 to form [IrCl(COD)(IiPrMe)], while IiPr added twice, yielding [Ir(COD)(IiPr)2][Cl]. For this reason, the steric properties of new NHCs are typically evaluated on at least two systems, and often three; for example, linear two-coordinate [AuCl(NHC)], square planar [IrCl(CO)2(NHC)], and tetrahedral [Ni(CO)3(NHC)], where the IR spectrum of the latter complex is also used to determine the TEP.

Figure 1.19 Systems commonly used to determine TEP for NHCs: (a) [Ni(CO)3(NHC)], (b) [IrCl(CO)2(NHC)], and (c) [RhCl(CO)2(NHC)].

However, %Vbur does have some limitations as a metric. Values are typically calculated from X-ray diffraction data or DFT-derived structures, which represent the solid state and gas phase, respectively. It can therefore be difficult to infer the structure in the solution phase; the vast majority of the most commonly used computational solvation models do not take into account specific solvation effects, and instead apply an electric field to simulate the effect of solvent. In addition, %Vbur is a static measure, and only takes into account the steric impact of the specific conformation examined. In solution, many ligands will adopt a number of conformations. Although it is possible to conduct very elegant and detailed studies assessing the dynamic behavior of ligands in specific environments [39], these studies are time-consuming and computationally expensive.

1.6.2 Electronic Properties

The TEP is the typical method used to investigate the electronic properties of phosphanes [16], and has been extended to describe NHCs [40].

This parameter is determined by the preparation and analysis of the corresponding [Ni(CO)3(NHC)] complex. Electron-rich ligands will increase the π donor ability of the metal center, leading to donation into the π*CO antibonding orbital. As a result, the C—O bond distance lengthens and the stretching frequency νCO, observable by IR at ~2000–2100 cm−1