Fluorine Chemistry at Leicester

In December, 1999 John Holloway and Eric Hope describe the history of fluorine chemistry in Leicester.

Descriptions of the Manhattan Project have generally celebrated the achievements of nuclear physics. The key role played by fluorine chemistry in the realisation of nuclear weapons has received scant attention, and yet, the separation of fissile U-235 on the requisite scale by gaseous diffusion of UF₆ was a key step. The British war-time research programme which facilitated this was progressed in a number of laboratories including those of I.C.I. General Chemicals. Some of the development of electrolytic fluorine generators at I.C.I. was carried out by A.J. Rudge and the establishment of fluorine chemistry at Leicester can be traced back to the influence of this man. He had been a research student at the University of Durham's King's College, Newcastle-upon-Tyne. In September, 1948 he was invited back to King's to talk about fluorine and Ray Peacock, who had graduated only that year, was appointed, as he put it, 'to be the dogsbody who did the experiments!' These included demonstrating the spontaneous burning of asbestos by ClF₃; a dangerous and frightening undertaking for a new graduate! Notwithstanding this literal 'baptism of fire', Ray was clearly intrigued and joined P.L. Robinson's research group to commence work on fluorine. His laboratory was housed in a conservatory attached to an old laboratory. There was no fume cupboard and unwanted fluorine was simply led out of the laboratory through a pipe passing through a hole Ray had drilled in the window frame!

Ray gained his Ph.D. in 1951, Harry Emeléus acting as his external examiner, and then spent a further two years at King's as a research assistant and one as a temporary lecturer before taking up an appointment at Imperial College, London. In 1958 he moved on to the University of Birmingham where he established an inorganic fluorine group alongside the already powerful organic fluorine team. This was a productive time yielding the first structures of transition metal pentafluorides and oxide fluorides, and some of the earliest work on noble-gas compounds. One of the students contributing to this work was John Holloway who, after gaining his Ph.D. in 1963, left to set up a fluorine group in the University of Aberdeen. Two years later, Ray was appointed to the Chair of Inorganic Chemistry at Leicester.

In Leicester, Ray continued noble-gas chemistry with a visiting Australian, Valda McRae, a significant achievement being the first structural determination of a xenon difluoride complex, [XeF][Sb₂F₁₁]^-, carried out in collaboration with David Russell. With Ray Kemmitt, Ray Peacock also began to try to make transition-metal carbonyl fluorides and, again with help from David Russell, solved the first crystal structure of such a species, [Ru(CO)₃F₂]₄. Novel chloride fluorides WF₅Cl and TeF₅Cl were also prepared and characterised. A long and productive joint research programme with John Burgess on heats of hydrolysis of fluorides provided information on fluoride ion affinities and electron affinities, as well as details of the charge distribution in a range of hexafluorometallates. Ray also engaged in a productive line of research involving reactions of MF_n species with Me₃SiX (X = N₃, NCO, NCS, CN) and succeeded in the difficult (and hazardous) synthesis of fluoroazides with John Fawcett.

In 1970, John Holloway was invited to apply for a lectureship in Leicester and to join Ray Peacock's group. When he arrived in January 1971, he brought with him expertise on working with metal and Kel-F vacuum systems and of doing fluorinations under pressure, plus some of the special skills required to carry out mass spectrometry and vibrational spectroscopy of the most highly reactive fluorides. Thus, the interests and activities of the group broadened. Collaboration on the fluorination of transition-metal carbonyls led to the successful characterisation of the novel low-valent carbonyl-fluoride compounds and placed them on a firm footing. It also showed the value of XeF₂ as a clean, mild fluorinating agent in solution. Its use has been a continuing theme in subsequent Leicester work, enabling John to discover a second new class of compounds, the transition-metal chalcogenide fluorides. Also, working with Professor Ebsworth in Edinburgh, the syntheses of a range of other low-valent transition-metal fluoro compounds including the first metal-PF₄ complex, metal complexes containing the new difluoroamido ligand and the first transition-metal SF₃ and fluoroacyl species were achieved.

Noble-gas chemistry was a consistent feature of John's early work in Leicester and, with his students, characterised a range of [XeF]⁺ and [Xe₂F₃]⁺ adducts. In collaboration with visiting academics Boris Frlec and Gary Schrobilgen, he contributed significantly to the understanding of the nature of fluorine bridging in xenon complexes, prepared and characterised the [KrF]⁺ and [Kr₂F₃]⁺ species and their complexes, and demonstrated the powerful low temperature fluorinating abilities of KrF₂ and its fluoro-cationic derivatives in solution. This included the first synthesis of AuF₅. Gary and John also carried out the first comprehensive n.m.r. study of a heavy nucleus, ¹²⁹Xe.

A long association with Joñef Stefan Institute in Slovenia, and in particular with Boris Frlec and the late Darja Gantar, produced a wealth of metal difluoride adducts including unusual cationic species such as [(AgF)_n]ⁿ⁺. There was also an extensive collaboration with David Brown at Harwell and Roland Bougon at the Centre D'Itudes Nuclaires de Saclay, in France on uranium fluorides and oxide fluorides.

In 1987, Eric Hope joined the group from Southampton University as an S.E.R.C. Nato Fellow. He was awarded an SERC Advanced Fellowship in 1989 and followed this up with a Royal Society Research Fellowship in 1994. With him came not only experience of fluorine chemistry, but also expertise in n.m.r. and matrix isolation spectroscopy. Eric's arrival coincided with the promotion of John to a Chair in inorganic chemistry and expansion of the group. This was followed by Ray's retirement in 1991, but the skills base of the group broadened considerably with the appointment of postdoctoral workers Wolfgang Dukat, Mattias Rieland, Paul Watson and Alan Brisdon. This allowed diversification to include selective fluorination of organic compounds, oxidative addition to low-valent metal centres, activation of main-group fluorides and, with colleagues at Southampton, the development of the structural characterisation of fluorides using EXAFS. Renewed efforts to develop further low valent transition-metal fluorine chemistry resulted in the group showing that, in these complexes, synthetic chemistry can be undertaken at the metal centre, the fluorine ligand or the coordinated ligand in a predictable way. This is currently being developed with our synthetically gifted post-doctoral colleague Howard Clark. Together with another excellent post-doctoral colleague, Graham Saunders, the influence of fluorine incorporated into phosphorus III ligands bound to metal centres has been studied. These investigations have shown that steric interactions dominate the coordination chemistry and contributions to the understanding of the processes underlying C-F bond activation have been made.

The interest in phosphine functionalisation, together with Eric Hope's interest in fluorous biphasic media and Pravat Bhattacharyya's dedication, culminated in the synthesis of ligands incorporating long-chain fluorocarbon 'ponytails,' such as P(CH₂CH₂C₆F₁₃)₃ and P(OC₆H₄C₈F₁₇)₃, and their use in the synthesis of fluorocarbon soluble complexes for catalytic systems where separation of product from catalysts is readily achieved. This work required genuine organofluorine chemistry input and Alison Stuart, trained by Paul Coe in Birmingham, joined the group as a post-doctoral fellow in 1996. Her appointment has allowed huge advances to be made in fluorous chemistry leading to patent protection for a hydroformylation catalyst and collaborations with groups in Bath, Cambridge, Liverpool and St. Andrews. Recently, Alison has been awarded a Lloyd's Tercentenary Fellowship to extend the fluorous project into organic synthesis using phase transfer catalysis.

Throughout all of this work, the support of David Russell and John Fawcett in the determination of the structures of a wide range of compounds has been crucial.

The fluorination of fullerenes with Roger Taylor, Professor Sir Harry Kroto, and their colleagues at the University of Sussex has proved to be exceedingly difficult giving rise to complex mixtures of compounds. However, with post-doctoral input from Adrian Adamson, a range of C₆₀ and C₇₀ fluorides and oxide fluorides have been synthesized and identified, and it has been shown that replacement of fluorine on the fullerene cages by a range of carbon, oxygen and nitrogen nucleophiles is possible.

The most recent collaborative venture has been with the microwave spectroscopist Professor Tony Legon at Exeter University. Using F-T microwave spectrometry, he and John Holloway have been able to observe the rotational spectra and determine the structures of a wide range of 'pre-reactive complexes,' formed in the supersonically expanded gas mixtures of F₂ or ClF with Lewis bases. In many cases the complexes observed involve combinations with bases such as NH₃, C₂H₄ and C₂H₂, which would normally react violently.

The Leicester Group has been fortunate in sustaining constant support from the research councils and a number of companies, including ICI Chemicals and Polymers, Johnson Matthey, B.P. and various parts of British Nuclear Fuels, especially F₂ Chemicals Limited. This has been much valued since it has not only provided essential underpinning for much curiosity-driven research but has provided interesting chemical problems related to the business needs and developments in a number of leading industrial organisations. These national and international collaborations evidence how the group has gone from strength-to-strength over the past 30 years. We hope it will continue to prosper well into the next millenium.

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