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Four Decades of Fluorine Chemistry at McMaster

Schrobilgen group - Fall '99. (Left to Right - Top row): Michael Gerken, John Lehmann, Michael Becher, Karsten Koppe, Prof. Gary Schrobilgen
(Left to Right - Bottom row): Neil Vasdev, Dr. Hélène Mercier-Schrobilgen, Barbara Fir and Bernard Pointner.

Being asked to write this article comes at an opportune time because it coincides with the 75th birthday of Professor Emeritus Ronald J. Gillespie. Ron Gillespie was my Ph.D. supervisor at McMaster (1971-73). I wanted to relate the evolution of my research interests, particularly those in fluorine chemistry and NMR spectroscopy, and the role that Ron Gillespie has played in that process. The intense interest in synthetic and structural main-group chemistry and in superacidic solvent media prevalent in Ron Gillespie's laboratory at the time I joined his research group provided an ideal research environment in which to pursue a range of topics in main-group inorganic fluorine chemistry. This experience has endured over the two decades that I have been at McMaster and has led to the synthesis and characterization of a significant number of novel and structurally interesting high-oxidation state fluorine species and a high proportion of the noble-gas compounds that are presently known.

My present interest in fluorine chemistry developed from an early encounter with, and subsequently acquired interest in, NMR spectroscopy. Shortly before coming to Canada in March, 1969, I had been called up for military service, but was deemed incapable of meeting the requirements for combat by the U.S. Army medical examiners because I was near-sighted and have ever since regarded near-sightedness as a positive physical attribute. I knew that the U.S. Selective Service Board was not necessarily finished with me and could redraft me at any time as a non-combatant into the Army Chemical Corps. Because a one year deferral was normally possible upon being called up, I elected to do an M.Sc. degree and went to work with Prof. J.S. (Steve) Hartman at Brock University. As Steve's first graduate student, I was assigned to an NMR-intense research project. The project was concerned with the synthetic and structural aspects of mixed boron trihalide adducts of organic nitrogen, oxygen, sulfur and selenium bases as well as with the characterization of the mixed tetrahaloborates. Included in the synthetic program were the mixed fluoro-derivatives. Because we only had access to an NMR spectrometer (Varian A-60) at Brock that ran protons, Steve and I regularly commuted to McMaster to use the Chemistry Department's NMR facility to obtain routine boron and fluorine NMR spectra. Steve, a former student of Ron Gillespie's, was very familiar with the facility which then consisted of three Varian electromagnet systems (HR-100, DP-60 and A-60). The DP-60 system was one of the very first NMR spectrometers in Canada, the one we used routinely at McMaster and that which I continued to use for my subsequent Ph.D. research at McMaster.

The establishment of an NMR facility at McMaster is in itself an interesting story and I digress to outline the role Ron Gillespie played in its creation. Conditional on acceptance of an offer to join the McMaster Chemistry Department in 1958 from University College, London, where he was a Lecturer, Ron Gillespie had stipulated that a commercial NMR spectrometer, capable of running ¹⁹F and ¹H spectra, and a Raman spectrometer be purchased for his use at McMaster. The NMR spectrometer, a Varian HR-60, operating at 56.4 MHz for ¹⁹F was one of the first commercial NMR spectrometers in Canada, and was installed during the summer of 1959. It was equipped with the latest plotting technology, a "hot-wire" plotter which used thermally sensitive paper and a resistively heated stylus to literally burn the spectrum onto a narrow strip chart. While being installed in the basement of the McMaster Engineering Building, the two-ton electromagnet was dropped outside the building, creating a large indentation in the concrete pavement which, though since filled in, is still visible. Despite its early trauma, the instrument performed to specifications until 1967 when it was upgraded to a DP-60 spectrometer by the addition of field flux stabilization and was used by me as a graduate student in Ron Gillespie's group for early ¹⁹F NMR studies of noble-gas species from 1971-73. The instrument was finally decommissioned in 1978. Almost simultaneous with the arrival of the Chemistry Department's first NMR spectrometer, Ron Gillespie offered the first NMR course at McMaster in the fall of 1959 and continued to do so in subsequent years. The facility was unique in eastern Canada and, through Ron Gillespie's generosity, it was made available to George Olah, then at Dow Chemical in Sarnia, who carried out some of his earliest investigations on carbocations in superacid media using the McMaster HR-60 spectrometer. Olah was awarded the 1994 Nobel Prize in Chemistry for his lifetime work on carbocation systems. Ron Gillespie was also responsible for introducing Raman spectroscopy as a routine tool for structure determination in inorganic chemistry into Canada shortly after his arrival at McMaster. This technique also remains as an essential structural probe in all our current research programs.

After a year and a half with Steve Hartman at Brock and the eventual loss of interest on the part of the Selective Service in reclassifying me for military service (I had lost interest several years prior),

X-ray crystal structure of FXeN(SO2F)2 showing the first observed Xe-N bond.

I was allowed to transfer my NRC postgraduate scholarship to McMaster and to continue my Ph.D. studies with Ron Gillespie in relative peace. When Ron proposed several research projects to me, I was pleased to find nothing but fluorine-related topics - but none related to noble-gas chemistry. I initially elected to investigate the solvolytic behavior of IO₄^- in anhydrous HF, which leads to the pseudo-octahedral cis- and trans-IO₂F₄^- anions. The study was mainly carried out using a combination of conductivity measurements in anhydrous HF solvent and ¹⁹F NMR spectroscopy. I also began to investigate IO₂F₃ chemistry, which I returned to later in my career.

Ron Gillespie's interest in noble-gas chemistry was expressed, shortly after the discovery of noble-gas reactivity by Neil Bartlett in 1962, when he used the valence shell electron pair repulsion (VSEPR) rules to rationalize and predict the molecular geometries of then known and unknown xenon fluorides and oxide fluorides. He presented his findings at the first conference on noble-gas chemistry held at Argonne National Laboratory in April, 1963. Just prior to my arrival at McMaster, Ron's interests in noble-gas chemistry began to take an experimental turn. A program was begun to synthesize and structurally characterize new xenon species with the view to adding further weight to the validity of the VSEPR rules. Ron Gillespie's early dedication to the use of ¹⁹F NMR spectroscopy for the characterization of fluoro-species in superacids provided a ready-made means for the structural characterization of noble-gas species and, in particular, noble-gas fluoride and oxide fluoride cations in strong acid media.

Shortly after I arrived at McMaster in January, 1971, I began to follow with considerable interest the work of Aaron Netzer, a postdoctoral fellow in Ron's research group (from the Hebrew University, Jerusalem) who was a specialist on anhydrous HF, and from whom I was to learn a great deal of practical fluorine chemistry. Aaron was investigating the solvolytic behaviors of xenon fluorides by means of ¹⁹F NMR spectroscopy in anhydrous HF solvent in the presence of strong fluoride ion acceptors such as AsF₅ and SbF₅ as well as in liquid SbF₅, HSO₃F and HSO₃F/SbF₅ superacid mixtures. Aaron and a graduate student, Ben Landa, had been trying to prepare XeF₄ by the accepted procedure of reacting F₂ and Xe at high pressures and high temperatures. The material was initially assumed to be XeF₄, but the ¹⁹F NMR spectrum that resulted upon dissolution of XeF₄ in HSO₃F did not seem to make much sense. At about this time Aaron was nearing the end of his stay with Ron, so I asked Ron if I could pursue the project and convinced him I knew what was going on. I speculated that the spectrum resulted from XeF₆ (present in equilibrium with XeF₄ in the high-temperature gas-phase synthesis) and noted that it could be assigned to the square pyramidal (AX₄E VSEPR arrangement) XeF₅⁺ cation. The ionization of XeF₆ leading to the XeF₅⁺ cation in HF solutions of XeF₆ had been postulated earlier to give rise to the high conductance of these solutions. Upon Aaron Netzer's departure from the group, I was reassigned to the noble-gas fluoride project and inherited Aaron's glass/metal vacuum (fluorine) line. Unfortunately, the glass liquid nitrogen cold trap of that vacuum system had been left ladened with an XeF₆/XeF₄ mixture. In those days there were no rigorous procedures in place like now to dispose of these materials, which hydrolyze rapidly in air and react more slowly with glass to yield shock-sensitive and highly explosive XeO₃. The ad hoc and "accepted procedure" was to suit up with face shield, neoprene gauntlet gloves and a full length rubber apron over your lab coat, and disassemble the trap while maintaining the contents at liquid nitrogen temperature.

Once disassembled, the trap was quickly plunged into an aqueous NaOH solution, where the fluorides hydrolyzed and disproportionated to stable sodium perxenate, Na₄XeO₆, and Xe gas in a quiescent manner. Aaron had departed on the Labor Day weekend (1971) and it was Labor Day morning when I came to add liquid nitrogen to the trap and found our liquid nitrogen storage dewar was empty and none was to be had in the entire department until the following day. I knew that the trap would warm to "criticality" over the next several hours, allowing XeF₆ to react with the glass trap to form highly shock sensitive XeO₃ which would likely detonate. Rather than risk an unattended incident, I suited up, shut down the vacuum pump, quickly yanked the dewar from the trap, slammed the door of the walk-in fumehood shut and hit the floor. The ensuing blast, accompanied by a brilliant blue flash of light, occurred within seconds, blowing out the tempered glass fumehood doors and reducing the glass portions of the fluorine line to a mélange of shards and powder. Upon surveying the damage, it occurred to me that this experience was almost as hazardous as being drafted into the U.S. Army. The next morning, Ron took the whole incident in stride and agreed that a new line should be constructed immediately and equipped with proper metal soda lime traps for neutralizing volatile, reactive fluorides in a safe, reliable and more discreet manner.

I subsequently worked in collaboration with Ben Landa whose primary interest was in Raman spectroscopy. At that time there was somewhat of a mystique concerning XeF₄ and its apparent inability to donate a fluoride ion to a strong fluoride ion acceptor and to accept fluoride from an alkali metal fluoride source. We eventually managed to make pure XeF₄ and I proceeded to look at the ¹⁹F NMR spectrum of XeF₄ using liquid SbF₅, the strongest Lewis acid known, as the solvent. The spectrum, with its accompanying ¹²⁹Xe satellites, was readily assigned to the novel T-shaped (AX₃E₂ VSEPR arrangement) XeF₃⁺ cation. We eventually characterized XeF₃⁺ by Raman spectroscopy and X-ray crystallography (Figure 2) and then proceeded to synthesize two new xenon(VI) cations, XeOF₃⁺ and XeO₂F⁺, starting from XeF₆ and to characterize them by ¹⁹F NMR and Raman spectroscopy.

During this time I was also continuing with Aaron Netzer's work and finished off a solution NMR study of the xenon(II) cations, XeF⁺ and Xe₂F₃⁺ (V-shaped and fluorine bridged) and their solvolytic behaviors in strong acid media. As a result of this study I thought about the possibility of Kr(II) analogues, and indeed there was already some evidence in the literature that KrF₂ formed complexes with SbF₅. Ron arranged for me to visit Argonne National Laboratory, where the first direct syntheses of xenon fluorides from xenon and fluorine were carried out and where Bartlett's historic reaction Xe + PtF₆ -> "Xe⁺PtF₆^-" was also repeated in 1962. While at Argonne, I discussed building a replica of their glow discharge apparatus for generating KrF₂ at liquid oxygen temperature (­183 ^oC) and did so upon my return. The krypton work proved to be even more exciting, and I am grateful to Ron for having allowed me to pursue this interest. In addition to a definitive characterization of the KrF⁺ cation, which is still the strongest chemical oxidizer known, I also characterized the Kr₂F₃⁺ cation and showed that the third known example of a Br(VII) species, octahedral BrF₆⁺, could be synthesized by oxidative fluorination of BrF₅ with a KrF⁺ salt such as KrF⁺AsF₆^-. After my Ph.D. defense in December, 1973, I spent the next two years as an NRC (now NSERC) postdoctoral fellow in the U.K. where I continued to deepen my interests in fluorine chemistry with Prof. John H. Holloway at Leicester University. It was during this time that I commuted regularly to Wissembourg and Nancy, France to carry out multi-nuclear magnetic resonance studies at Bruker France and the University of Nancy, resulting in the first extensive ¹²⁹Xe NMR study and compilation of ¹²⁹Xe NMR parameters. My return to North America was greeted by academic job uncertainty and although I was eventually offered positions in the U.S., I continued to favor the research climate at McMaster and the general access to instrumentation that the Chemistry Department afforded. Upon receiving an NSERC University Research Fellowship and junior faculty appointment at McMaster in 1980, I developed a new line of chemistry concerned with the polyanions of the main-group elements (Zintl anions) which spanned both classically bonded and electron-deficient species. The initial interest in this field was spawned by an interest in heavy element nuclear spin-spin couplings and the effects of relativity on their magnitudes. The interest first arose in the course of work with the ¹⁹F and ¹²⁹Xe NMR spectra of xenon compounds where the magnitudes of Xe(II)-F couplings often exceeded 5000 Hz, and from our solution multi-NMR characterizations of a large number of polyatomic cations of group 16 and of mercury that had been previously synthesized in Ron Gillespie's laboratory. Included among these studies was the linear Hg₃²⁺ cation, which displayed the world's largest spin-spin (¹⁹⁹Hg-¹⁹⁹Hg) coupling (139,600 Hz). Because the Zintl anions we observe in solution are often mixtures and not always representative of the solids that crystallize from these solutions and their X-ray crystal structures, multi-NMR remains an important complimentary structural probe in our present-day non-fluorine programs in synthetic main-group chemistry.

Shortly after joining the McMaster Chemistry Department, I also initiated a program to document the first true Xe-N bond by low-temperature X-ray crystallography, and obtained the first Xe-N bond distance for FXeN(SO₂F)₂ (Figure 3). At about this time we also embarked on programs to prepare derivatives of ligands whose group electronegativities approached that of fluorine. These programs led to a significant extension of the chemistry of the OTeF₅ group and the first derivatives of the -­OIOF₄ group. Both research areas

have increased our understanding of the factors that contribute to the stabilization of high oxidation states.

Funding from a contract provided by the U.S. Air Force HEDM (High Energy Density Materials) Program at Edwards AFB, California allowed the pursuit of synthetic strategies that led to other novel noble-gas species. Because no bonds to krypton other than the Kr-F bonds in KrF₂, KrF⁺ and Kr₂F₃⁺ were known at the time, the formation of the thermodynamically and kinetically unstable HCN-KrF⁺ cation was attempted and verified using ¹H, ¹⁹F, ^14,15N and ¹³C NMR spectroscopy and Raman spectroscopy as well as several other perfluorinated nitriles adducted to the KrF⁺ cation. The discovery of the first Kr-N bonds shortly thereafter led us to synthesize the first Kr-O bonded compound, Kr(OTeF₅)₂.

Our interests in noble-gas chemistry continue and we remain one of very few laboratories in the world where this chemistry can be pursued. Early interests in Xe-N bond formation have, more recently, led to the use of XeF⁺ salts such as XeF⁺AsF₆^- as Lewis acids and have resulted in the isolation and full structural characterizations of adduct cations such as HCN-XeF⁺, Bu^tCN-XeF⁺, C₆F₅N-XeF⁺ and s­C₃F₃N₂N-XeF⁺ by multi-NMR spectroscopy and low-temperature single crystal X-ray diffraction.

Many of the fluorine chemistry programs we developed in the past remain active as is exemplified by the chemistry of the OTeF₅ ("teflate") group and the recent synthesis of a series of oxidatively resistant weakly coordinating anions, M(OTeF₅)₆^- (M = As, Sb and Bi). The strong fluorinating ability of XeF₆, and the extreme oxidizing strengths of KrF₂ and KrF⁺ are being exploited more and more for the syntheses of the highest oxidation states of the elements, and are leading to a fuller understanding of the factors that stabilize the fluorides and oxide fluorides of Au(V), Tc(VII), Re(VII), I(VII), Os(VIII) and Xe(VIII). Of recent applied interest has been our syntheses of the missing oxide fluorides of technetium, TcO₂F₃ and TcOF₅ using XeF₆ and KrF₂ as synthetic reagents. It has recently become apparent that TcO₂F₃ (and possibly TcOF₅) plays a significant role in the uranium re-enrichment cycle. A recent excursion into naked fluoride ion chemistry was made possible when truly anhydrous tetramethylammonium fluoride became available and allowed us to answer an old question relating to the Lewis acidity of XeF₄. The interaction of XeF₄ with N(CH₃)₄⁺F^- in rigorously anhydrous CH₃CN gave the XeF₅^- salt in quantitative yield. The pentagonal planar XeF₅^- anion (Figure 4) represented the first example of an AX₅E₂ VSEPR arrangement. The approach was extended to other weakly basic systems such as the IOF₆^- anion which was shown to have a pentagonal bipyramidal geometry with the oxygen in the axial position. Our present holy grails in fluorine chemistry are many and include the characterization of new Kr-O bonds, extending the now limited chemistries of Br(VII) and Xe(VIII), the first oxygen(IV) compounds (OF₄ and OF₃⁺) and the synthesis of the first argon compound.

Like many who do fundamental chemistry in a university setting, I am sometimes asked about the applications of what I do and if one were to pursue this chemistry in graduate school what is the likelihood that a job will be found that is relevant to the skills that have been acquired. There is no question that fluorine chemistry is challenging on several fronts, but its demands are rewarding not only because its pursuit has revealed exciting and, often times, unexpected new chemical species, but because it utilizes a wide range of structural characterization techniques and requires the mastery of the rigors of synthetic, air-sensitive chemistry. Consequently, these research programs provide participants with broad research experiences and challenges that bolster self confidence and have enabled them to innovate problem solving strategies in a variety of technical environments they encounter in industry and academia. It is primarily for these reasons that I have seldom felt it was necessary to justify the pursuit of fundamental chemistry in a university setting, particularly since these research experiences have produced individuals who have been highly successful in both academic and industrial settings.

Fluorine and its chemistry often have been described by the adjectives I have used throughout this article, i.e. exciting, exotic, unusual, unexpected, novel, highly reactive and challenging. I am sure that I speak for Ron Gillespie when I say that fluorine chemistry has lived up to all these adjectives and more throughout both our careers and remains for me an exciting field of synthetic and structural challenges and for Ron, in his "retirement years", an abundant source of examples he is using to examine and explain the theoretical basis of the VSEPR rules.

Gary Schrobilgen

Related Links: G.J. Schrobilgen Research Group R.J. Gillespie Research Group Ronald Gillespie: A Lifetime in Chemistry McMaster NMR Facility

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