Organometallic Transition Metal Catalysis: A Holistic Approach to Understanding and Predicting their Mechanisms

Organometallic Transition Metal Catalysis: A Holistic Approach to Understanding and Predicting their Mechanisms

Organometallic Transition Metal Catalysis: A Holistic Approach to Understanding and Predicting their Mechanisms

Andrew R. Barron

MiDAS Green Innovations

2022

Cover image © 2014 by jcrosemann

Copyright © 2022 by Andrew R. Barron

All rights reserved. This book or any portion thereof may not be reproduced or used in any manner whatsoever without the express written permission of the publisher except for the use of brief quotations in a book review or scholarly journal.

First Printing: 2022

ISBN 978-1-4716-8423-4

MiDAS Green Innovation, Ltd

Swansea, SA1 8RD, UK

www.midasgreeninnovation.com

Dedication

To the memory of my PhD supervisor, Prof. Sir Geoffrey Wilkinson Nobel Laureate, for his guidance and inspiration.

Acknowledgements

I would like to thank the students who put up with my teaching of this subject over the last thirty-five years.

I would also like to acknowledge the following individuals:

The late Sir Geoffrey Wilkinson for his guidance during the years of my PhD and inspiring my interest in Inorganic Chemistry.

David M. Hoffman for his friendship and intellectual stimulation when we were both in the Harvard mill.

Simon Bott who has been a great friend for over 30 years, collaborator, and now colleague.

Last, but not least, I would like to thank my wife, Merrie, for her love and support as well as putting up with me during the COVID-19 pandemic.

Preface

In 1987 as a young assistant Professor at Harvard University I was assigned to teach Organometallic Chemistry (CHEM 105) to Graduate students and Seniors. Having never taught before I remember reading an article by my PhD supervisor, Sir Geoffrey Wilkinson about his time at Harvard where he discussed a similar problem.

"So, gathering what was then the inorganic bible, Sidgwick’s, The Chemical Elements and their Compounds, as well as Pauling’s, Nature of the Chemical Bond, the book I had been brought up on at Imperial College, Emeleus and Anderson’s, Modern Aspects’ of Inorganic Chemistry, and whatever else was available, such as, Yost, Russell and Garner’s books on rare earths and the Groups V and VI elements, I set to work to be at least one lecture ahead of the students." (G. Wilkinson, The iron sandwich. A recollection of the first four months, J. Organometal. Chem., 1975, 100, 273-278).

Taking his lead, I got my copy of the book I had been brought up on at Imperial College, the inorganic bible, Cotton and Wilkinson, Advanced Inorganic Chemistry: A Comprehensive Text, I also attempted to be at least one lecture-ahead of the students. Over the next seven years the course evolved with practice.

Moving to Rice University in 1995, I taught Transition Metal Chemistry, (CHEM 495) on my own for several years, and that gave me the luxury to develop the topics and methods for teaching the subject. It was decided that Lon Wilson and I would team teach the course. While, Lon spent 6 weeks teaching nothing but porphyrin chemistry (I do not exaggerate), it was up to me in the remaining 6 weeks to teach all of organometallic chemistry. This increased my focus since I realized that it was not necessary to teach everything, if the students could be made to understand the concepts and then apply them.

The concept of teaching a pared down course has been developed further in teaching Transition Metal Catalysis as a guest lecturer at Swansea University for my good friend and longtime collaborator Simon Bott, who is at the forefront of the ‘flipped’ method of teaching.

Hence the approach taken in this book, not to provide a Comprehensive Text like Cotton and Wilkinson (or as it now is, Cotton, Wilkinson, Murillo and Bochmann) and more of a Hitchhikers Guide to Organometallic Catalysis (apologies to the late Douglas Adams).

It is the results of this journey that are covered in the present book.

Introduction

Why Transition metals?

A transition metal is defined by IUPAC (International Union of Pure and Applied Chemistry) as an element "whose atom has a partially filled d sub-shell, or which can give rise to cations with an incomplete d sub-shell". An alternative definition is of the elements of Groups 4 (Ti, Zr, Hf) to 11 (Cu, Ag, Au), but with the addition of scandium and yttrium in Group 3, which have a partially filled d sub-shell in their metallic state.

The two key attributes that make transition metals of interest with regard to their reactivity and application as catalysis are:

Flexible coordination number.

Adoption of multiple oxidation states.

Simplicity is the ultimate sophistication

Leonardo da Vinci’s quote is possibly the most appropriate description of organometallic catalysis. Whereas organic chemistry is exemplified by hundreds of different reactions (many named after those who discovered them) transition metal inorganic and organometallic compounds only undergo eight metal centered reactions:

Ligand association

Ligand dissociation

Ligand insertion

Ligand elimination

Oxidative addition

Reductive elimination

Oxidation

Reduction

Of these, four are the reverse (equilibrium) reactions of four others, thus limiting further the possible reactions, that need to be considered in transition metal inorganic and organometallic chemistry, to just four:

Ligand association/ligand dissociation.

Ligand insertion/ligand elimination.

Oxidative addition/reductive elimination.

Electron transfer (redox).

This makes understanding and prediction of catalytic cycles simple, since any overall reaction must be a combination of these reactions. For added simplicity, electron transfer is a reaction more associated with metal containing biological catalyst pathways rather than organometallic catalysis; however, it is included here for completion.

What is a catalysis?

Catalysis is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst.