Wilkinson's Catalyst

Professor Sir Geoffrey Wilkinson was one of if not the most influential of inorganic chemists in the post-war era. This article serves to highlight his contributions in original research and does not address his influence through his famous text books or the impact arising from the exceptionally large number of graduate students, co-workers, the children, grandchildren and great grandchildren etc. who have been inspired by working with him during the forty-five years that he held academic posts first at MIT and Harvard and subsequently, for 41 years as the Sir Edward Frankland Professor of Inorganic Chemistry at Imperial College.

Wilkinson was always very proud of the fact that he was a Yorkshireman and that all his forebears were Yorkshire people. His grandfather (also Geoffrey Wilkinson) came to Todmorden from Boroughbridge; his father Harry fought in the First World War, and was lucky to survive, having been left for dead in France at the battle of Bullecourt in 1917. Harry married a weaver, Ruth Crowther in 1920; members of the Crowther family had been weavers in the local mills for several generations. Geoffrey was born on July 14, 1921, the first of three children, in the village of Springside on the outskirts of Todmorden. Sadly there is little left of Springside now. The family moved to the centre of Todmorden in 1926 to no. 4 Wellington Road (the house is now marked by a blue plaque erected in 1990). Todmorden is a small industrial town at the junction of three deep valleys in the centre of the Pennines, the hills and moors rising 1000 feet above the town. The population today is about 13 000, about half of that which existed before the decline of the cotton industry. His brother John speaks of the strong local pride and sense of community which Todmorden had and still has, and Wilkinson remained very fond of Todmorden all his life. His eyes would light up when he spoke of it or remembered the superb countryside such as Hardcastle Crags close by. He often returned there and had friends in the local community: he always enjoyed walking on the moors near Todmorden and fell- walking in the Lake District with family and friends. His interest in chemistry came early in life, before he went to secondary school. He recalled being fascinated at the age of six or seven by watching his father,a house painter and decorator. Mixing his painting materials (his uncle John Willy Wilkinson was also a painter but had died at the age of 22 from accidental arsenic poisoning caused by a green copper–arsenical pigment, Paris Green, fashionable at that time). An uncle on his mother’s side managed a factory making Epsom and Glauber’s salts in Todmorden. Wilkinson recalled how he loved to go on a Saturday morning to tinker in the small laboratory at the factory in Todmorden, indeed the family hoped that he would eventually manage the factory, but his career was to be very different. His parents, like most people at that time, had left full-time education at the age of 12, they were determined that their children should have a better education, and Wilkinson never forgot this. He went to Roomfield School and then, having won a West Riding County Minor Scholarship in 1931, went to Todmorden Secondary School, later named Todmorden Grammar School and later still Todmorden High School. He made the most of his education there. The school had several other pupils who were later to become famous, including Sir John Cockcroft, who worked with Rutherford at Cambridge and was to become the first of the school’s two Nobel Laureates-to-be in 1951. Many of the school’s pupils were groomed for entrance to Cambridge but Wilkinson made exceptional progress and was entered for a Royal Scholarship at the Imperial College of Science and Technology, London University, because the entry scholarships there were held at an earlier date than for other View Article Online universities. He won the scholarship and after a two day practical analytical chemistry examination in London, joined the college in 1939.

Pioneering studies on homogeneous hydrogenation catalysis demonstrated the important role of solvent-stabilized catalytic intermediates comprising coordinated solvent molecules. Wilkinson’s catalyst is another highly important catalyst with a wide range of industrial applications and the implicit role of solvents has been delineated. A generic catalytic cycle for the hydrogenation of unsaturated compounds using Wilkinson’s catalyst is shown in Figure 1. In the first step the stable 16 valence electron compound, RhCl(PPh₃)₂, is converted to a highly reactive 14 valence electron active catalyst species, RhCl(PPh₃)₂, via the dissociation of a phosphine ligand. The productive part of the catalytic cycle continues with the oxidative addition of hydrogen, which was shown to be considerably faster than that determined for other species present in the reaction, e.g. is the starting compound RhCl(PPh₃)₃ and various dimers. The formation of a solvated species, RhCl(PPh₃)(solvent), has tentatively been proposed, undergoing oxidative addition of hydrogen with concomitant dissociation of the solvent ligand. Indeed, if the reaction is conducted in strong donor solvents, which tends not to be the case in practice, such an intermediate species is not unreasonable. NMR spectroscopy in C₆D₆ using pH2 revealed a number of previously unobserved species but none involving coordination of the solvent, as expected for this solvent. However, a kinetic analysis using electrospray ionization mass spectrometry revealed that the slowest step in the productive part of the catalytic cycle corresponds to the association of the unsaturated substrate. The slow ligand association step indirectly points to a solvent molecule needing to be displaced prior to the ligand association, which is consistent with the effect of donor solvents reducing the rate of reaction. Should the solvent be an ionic liquid then the cation and/or anion could interact with the catalyst and depending on which ions interact with the catalyst intermediate will further influence the reaction.

Figure 1 Catalytic cycle for the hydrogenation of unsaturated C-C bonds (typically alkenes and alkynes) using Wilkinson’s catalyst, RhCl(PPh₃)₃. The widely accepted intermediates in the catalytic cycle are shown in black in the main cycle and the solvated species that have been proposed are shown to the right of the main cycle. Solvent is denoted by S. The 16 valence electron solvated species RhCl(PPh₃)₂(solvent) and the 18 valence electron solvated intermediate RhCl(PPh₃)₂(H)₂(solvent) are present in donor solvents.

It should be noted that the active catalyst species/mechanism is not only influenced by the solvent, but in principle by the nature of the substrate and all reaction parameters. It has been shown, for example, that at high temperatures under the reducing hydrogen environment rhodium(0) nanoparticles can form and are excellent hydrogenation catalysts, especially for aromatic and heteroaromatic substrates. A remarkable solvent effect was observed with Wilkinson’s catalyst in the hydrogenation of alkenes containing aromatic nitro-groups (Figure 2) and other sensitive groups such as aryl iodide that cannot be rationalized by coordination of the solvent to the catalyst. In order to prevent the reduction of the nitro group or dehalogenation from taking place very mild reactions conditions are required. In benzene, under 1 atm of H₂and at room temperature no reaction was observed, whereas in MeOH the alkene bond was reduced to afford the saturated product in 80% yield and in THF the yield increases to 91%.51 However, the highest yields were obtained in MeOH-THF and tBuOH-THF (1:1) mixtures (93 and 95%, respectively). The study also shows the importance of using mixtures of solvents rather than pure solvents, even though the latter overwhelmingly dominate the literature.

Figure 14 Selective hydrogenation of a C=C double bond in the presence of a reducible nitro-functional group using Wilkinson’s catalyst.

It is unlikely that the rate of this reaction is correlated to the solubility of hydrogen gas in the different solvents evaluated (see above). However, the rate of hydrogenation does appear to be correlated with the β values of the solvents. Benzene, THF and methanol have approximately the same π* values and there is no clear trend with respect to the α parameters of the solvents. The parameter that differs between the solvents that facilitate catalytic activity under mild conditions and the solvents where the yield of the reduction is low or does not take place is β, i.e. the basicity or hydrogen-bond accepting ability of the solvent. How this parameter enhances the reaction remains a matter of speculation, but one possibility is that the alkene protons hydrogen-bond with the solvent thereby activating the unsaturated bond. These solvents would not be able to hydrogen-bond to (and activate) the nitro-group in the same way, hence endowing the system with high selectivity towards reduction of the C=C double bond. With both of the rhodium catalysts described above, the active catalyst species is generated in situ, in the case of [Rh(cod)(R,R-DIPAMP)]⁺ via hydrogenation and elimination of the cod ligand and with RhCl(PPh₃)₃ via dissociation of a phosphine. The electronically unsaturated species generated are stabilized by the solvent, but if too stable, then activity is reduced. Hence, a balance between stability and activity must be found.

If catalyst activation involves dissociation of an anionic halide ligand then the nature of the solvent has a profound influence. For example, ruthenium(II) complexes [RuCl(diphosphine)(arene)]⁺ (the diphosphine ligand = diphenylphosphinomethane, diphenylphosphinoethane, diphenylphosphinopropane and the highly water-soluble analogue 1,2-bis(di-4-sulfonatophenylphosphino)benzene, arene = p-cymene, benzene or [2.2]paracyclophane), are efficient catalyst precursors for the hydrogenation of unsaturated bonds in an aqueous solution. In other solvents, where the enthalpy of chloride solvation is low, reaction rates are much lower and in some cases catalysis is completely suppressed as dissociation of the chloride anion from the ruthenium center is suppressed. In other words, if the halide ion is poorly solvated, it is more nucleophilic, and will coordinate to the catalyst preventing generation of the active catalyst. Solvents have also been shown to strongly influence the outcome, e.g. C=C versus C=O selectivity, of hydrogenation reactions catalyzed by heterogeneous systems.

REFERENCE

Bennett, Martin A, Andreas A Danopoulos, P Griffith, and Malcolm L H Green. 1997. “The Contributions to Original Research in Chemistry by Professor Sir Geoffrey Wilkinson FRS 1921 – 1996 I . Early Life and Education.” : 3049–60.

Manuscript, Accepted. 2016. “Catalysis Science & Technology.”

Perea-buceta, Jesus E et al. 2015. “SI WK 270815 FINAL Diverting Hydrogenations with Wilkinson ’ s Catalyst towards Highly Reactive Rh - ( I ) Species General Procedures and Methods.” (December).

Prize, Nobel, and Winning Transition-metal Mediated. 2019. “Nobel Prize Winning Transition-Metal Mediated Processes.” 66(1): 6–10.

@fffaizzarul