An appreciation of Professor David Cole-Hamilton

It has been my privilege and pleasure to have been a friend of David Cole-Hamilton over many years, in fact since he started his research career with Tony Stephenson and Sir Geoffrey Wilkinson in the 1970's.

We have worked in very closely related areas of chemistry but, so far as I am aware, we have never published together. I have followed, with interest and great applause, his varied and wide-ranging studies, from blue-skies synthetic inorganic and organometallic chemistry, kinetics, ionic and supercritical liquids, and homogeneous catalysis, right through to very applied R&D in materials, the conservation of archaeological wooden artefacts, catalyst recycling and continuous-flow catalysis. His skills, energy and great enthusiasm have made him a major international figure in an impressively large number of areas, many of which share the thread of applying the theory and practice of organometallic chemistry.

Catalysis has huge importance in very diverse fields, from biology to cosmology, and from nanotechnology to oil refining. The applications in chemistry to molecular synthesis are especially noteworthy, as catalytic processes enable the efficient and ecologically responsible production of countless important pharmaceuticals, agrochemicals and basic commodity chemicals. One significant aspect is the catalysis of reactions in homogeneous solutions. By contrast to heterogeneous catalysis, homogeneously catalysed reactions are more readily tuned to afford greater activity and selectivity towards desired products.

David Cole-Hamilton has been one of the most productive and best-known scientists working on the discovery of new catalytic reactions. A drawback to homogeneous catalysis has been that the separation of the product from the catalyst and the solvent can be very difficult. David leads the world in pioneering new approaches to the problem of product separation. They include the use of supercritical fluids and ionic liquids that provided the very first examples of continuous flow homogeneous systems for relatively low volatility substrates. Here, the catalyst in the reactor is dissolved (and immobilised) in an involatile ionic liquid and the products flow through dissolved in supercritical CO₂. This world beating concept is the basis of a commercial process, and publications on this work have been very widely cited, for example, in articles in The Independent, in C&EN and on the radio.

David showed that the ionic liquid could be replaced by the substrate product mixture and pioneered the use of supercritical fluids (SCFs) for transporting substrates and product using supported ionic liquid phase (SILP) catalysts, where the catalyst is immobilised in a thin film of ionic liquid within the pores of silica. His application of the SILP-SCF process for alkene metathesis provided the first examples of continuous flow metathesis, overcoming the major problem of catalyst removal from the reaction products. His group was also the first to combine scCO₂ with solid-supported catalysts.

An alternative catalyst separation strategy used was to dissolve the catalyst in water, which does not mix with the organic reaction products. Although commercialised some years ago, this process had not been applied to hydrophobic substrates. David's discovery of additives that accelerate the reaction rate for these substrates by up to a thousand-fold whilst retaining rapid (<1 min) phase separation and low catalyst leaching (<0.5 ppm) was a ‘spectacular achievement’.

More recently he has disclosed remarkable catalysts that can be used in an organic solvent and then switched into water by bubbling CO₂. Phase separation, addition of a new substrate and bubbling nitrogen at 60 °C switches the catalyst back into the organic phase where it retains its activity. This work was featured in the C&EN Chemical Year in Review, 2009.

A further area where David has made a major contribution is in catalysts bound to dendrimers, which can provide multiple binding sites on their periphery and which can be separated by filtration through ceramic membranes. He was the very first to describe an unambiguous example of catalysis with such functionalised dendrimers, achieving a 13.9 : 1 linear : branched selectivity in hydroformylation products compared with a ratio of <5 for small-molecule analogues. He was also the first to build and demonstrate a fluorous biphasic reactor with full product separation and catalyst recycle.

He has cemented his world-leading position in catalyst recycling by co-editing the definitive book Catalyst Separation, Recovery and Recycling.¹

David has also been responsible for introducing some new and very important homogeneous catalytic reactions, including the hydrogenation of amides to amines. Many routes to pharmaceuticals involve the reduction of amides to amines using (pyrophoric) metal hydrides, produce substantial amounts of waste and are difficult to handle especially on a large scale. David's use of hydrogen as the reducing agent, with water as the only by-product, provided a significant breakthrough for these important reactions. Amines can also be made by reductive amination of carboxylic acids. David's group has adapted this chemistry to di-acids to provide a totally new and green route to N-heterocyclic compounds of importance to the pharmaceutical industry.

David also developed reactions in which double bonds are moved from the thermodynamically favoured middle of a carbon chain to give functionality at the end of the chain. This is especially important for unsaturated acids because it allows the formation of α,ω-diacids, which are precursors to polyesters and nylons. His pioneering work on using this reaction to form dimethyl 1,19-nonadecanedioate from naturally occurring methyl oleate has been a major breakthrough, since it represents the only way of getting highly selective α,ω-difunctionality.

His group has also been able to show that terminal alkynes can be converted into α,ω-diesters using a single catalyst, which promotes carbonylation, double bond isomerisation and carbonylation in precisely that order.

David was also the first to report cooperative ligand effects where different ligands provide enhanced selectivity at different steps of a catalytic cycle. This major discovery allows catalyst tuning in two dimensions. He has developed completely new reaction protocols where important steps occur at the metal and at a coordinated ligand (usually with P coordination), that have been used for the hydrogenation of highly substituted carboxylic acids and for the alkylation of phenols.

Other firsts include the catalytic thermal production of hydrogen from alcohols (an approach to bioderived hydrogen that has now become very popular) and the photochemical water–gas shift reaction (which allows this important industrial process to be carried out under very mild conditions).

David has, of course, played key roles in promoting chemistry nationally. He was involved closely with the founding of EaStCHEM, the joint research school of the Chemistry Departments of Edinburgh and St Andrews, in the East of Scotland, which has had great success and has demonstrated world-class breadth and depth in the chemical sciences. The great research quality, as judged by peer review, is a result of a sustained commitment by the Universities and the Scottish Funding Council in supporting excellence in chemistry. It also reflects the dedication and commitment of academics, support staff and their students in ground-breaking research over a wide range of challenging programmes.

I send David all good wishes for his retirement, secure in the belief that although he may slow down a bit in future, he will not entirely abandon thinking, lecturing and working in his long-standing love for chemistry and catalysis research, where he has already contributed so much.

References

1. Catalyst Separation, Recovery and Recycling, Ed. D. J. Cole-Hamilton and R. P. Tooze, Kluwer, Dordrecht, 2006.

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