Referenced Summary of Main Features of Research Work and Publications List (Papers)

Xenon Chemistry

The first noble-gas compound was prepared in the Summer of 1962 and John Holloway, then a student with R.D. Peacock, achieved the first synthesis of XeF₄ at atmospheric pressure (1) soon afterwards. Later, working alone in Aberdeen, he showed that XeF₂ could be easily prepared on a large scale by the photochemical combination of xenon and fluorine with sunlight (7,8). Since then, he has investigated the reactions of xenon difluoride with a wide variety of reagents (5,12,15,16,28

31, 36-38,41,43-45,47,49,51,54,56,58,62,66,69,92,96, 106,107,116,126,132,150,152,161,170,173, 174,180,210). These studies not only resulted in the characterization of the first xenon fluoride adducts (5) but have also led to the preparation of the new classes of compounds, carbonyl fluorides and transition-metal thio-fluorides, the preparation of the first metal fluoroacyl complex, a method of attaching pentacoordinated phosphorus to a metal, the identification of novel types of uranium fluoride derivatives and fluorinated Buckminsterfullerene (see later).

Studies on the bonding in xenon difluoride adducts by vibrational spectroscopic methods (12,36,37) and nuclear quadrupole resonance (31) showed that the solid-state structures can be interpreted in terms of ionic formulations involving [Xe₂F₃]⁺ and [XeF]⁺ cations and [MF₆]- and [M₂F₁₁]- anions but that the [XeF]⁺ compounds contain weak covalent interactions through fluorine bridging between the anions and cations. There is also a gradation of covalent character dependent upon the Lewis Acid strength of the pentafluoride. This has been confirmed by studies of the enthalpies of formation (33). It has also been demonstrated by vibrational spectroscopy (44) and conductivity measurement (49) that the nature of the bonding in the melts is similar. When adducts between XeF₂ and stronger fluoride-ion bases are prepared, Raman spectroscopy and 19F and 129Xe nuclear magnetic resonance studies (36,38,43,56,66,69 and 92) have shown that the ionic contribution, both in the solid state and in solution, is reduced and the structures are best formulated as covalent with fluorine bridges. For XeF₂.WOF₄ this has been confirmed by a crystal structure (38).

Further information on the bonding in a wide range of solid xenon species has been obtained from Mössbauer measurements (62) [in collaboration with H. de Waard in Groningen] and from solution studies by 19F and 129Xe Fourier Transform n.m.r. (43,56,66,69) [in collaboration with G.J. Schrobilgen of McMaster University and P. Granger in the University of Rouen]. The solution work on the KrF₂ and XeF₂ adducts with WOF₄ has been interpreted in terms of equilibria involving Ng-O-W and Ng---F---W (Ng = Noble gas) bridge species (66,69) although, where crystallisation takes place, only F-bridged species are observed. One of the solution studies (56), a 129Xe Fourier Transform n.m.r. examination involving 25 compounds, was one of the first and most compre-hensive n.m.r. investigations on a heavy nucleus which has been carried out.

Photoionisation mass spectrometric studies on XeF₂, XeF₄ and XeF₆ (24) (in collaboration with

J. Berkowitz at Argonne National Laboratory) have provided values for the average bond enthalpies and first ionisation energies which differ from thermochemical values. These prompted investigation into what exactly is measured in the two techniques. Studies on the photoionisation mass spectrometry of KrF₂ (59) were also subsequently completed.

Novel adducts of XeF₆ have been characterized (27,30) and the structure of the [XeF₈]^2- ion has been established as an Archimedean antiprism (27). The structure of the [(XeOF₄)₃F]^- anion in the salt Cs+[(XeOF₄)₃F]- has been determined and the unusual trigonally bonded fluorine observed (99). High-pressure experiments have shown that the existence of XeF₈ should not be completely ruled out (20).

Krypton Chemistry

Only the laboratories of Professor Gillespie in Canada and John Holloway in Leicester have been successful in developing the chemistry of krypton. The work in Leicester revealed a plethora of new compounds. The [KrF]⁺ and [Kr₂F₃]⁺ cations were fully characterized for the first time (29) and it has been demonstrated that, with pentafluorides, adducts of a similar nature to those of xenon difluoride can be prepared (29,32,41,46,50,69,135). However, adducts with transition-metal oxide tetrafluorides, which are obtained as crystalline solids with XeF₂ (36,38,66) can only be obtained in solution if KrF₂ is used (69). The structures of many of these species in the solid state by Raman spectroscopy (29,32,41,46,69,135) and Mössbauer spectroscopy (50), and in solution by Raman and 19F and 129Xe n.m.r. spectroscopy (69) have been obtained. The low temperature oxidative fluorinating ability of KrF₂ has been demonstrated and exploited by the preparation of IF₇ and XeF₆ (29) and the first synthesis of AuF₅ (41). More recently new routes to the preparation of KrF₂ have been discovered (119,148), a new crystalline modification of KrF₂ has been obtained and the spectro-scopic and structural relationship between the two forms has been determined (135).

Conducting Polymers

By looking at the doping of polyacetylene with related series of dopants it has been shown that, for example, the metal hexafluorides and pentafluorides cause doping to the metallic state and inverse correlations between the maximum conductivities attainable and the electron affinities of the hexafluorides or the fluoride ion affinities of the pentafluorides occur (78,84,86,93). Work in collaboration with the e.s.r. spectroscopist, J.B. Raynor, has shown that the early stages of doping of cis-trans mixtures of polyacetylene causes the cis-isomer to be converted to trans and the charge distribution over the polyacetylene chains has been determined (109). This work promised to provide fundamental information on the doping process and the mechanism of conduction, but did not progress through lack of funding.

Transition-Metal Fluoride Chemistry

John Holloway was the first to prepare RhF₅ (6) and AuF₅ (41) and was also responsible for the full characterizations of RuF₅ (2) and OsF₅ (25). Full, single-crystal X-ray structure determinations were carried out on RuF₅ and OsF₅ (2,25), and these tetrameric structures, along with those of A.J. Edwards, were the keys to the understanding of the nature of transition-metal pentafluorides and the first examples of fluorine-bridged, tetrameric metal fluorides. More recently, a wide range of mixed-metal pentafluorides have been obtained and their structures determined by X-ray crystallography and EXAFS methods (129,156). The first trimeric platinum metal pentafluoride (RuF₅)₃ has also been obtained (143). Efforts have been made to extend knowledge of the vapour-phase structures of pentafluorides and by vapour density (48), electron diffraction (163) and thermochemical measurements and this is continuing using matrix isolation in combination with photolysis methods designed to understand the spectroscopy of pentafluoride monomers.

Examination of the chemistry of these pentafluorides led to the syntheses of related compounds such as RuF₄ (3), RuOF₄ (2) [which has recently been re-examined (162)] and CsRhF₆ (6) for the first time.

Work with the transition-metal pentafluorides prompted investigation of the related oxide-fluorides, especially with respect to their weaker Lewis Acid properties. This was exploited in much of the work with noble gas fluoride adducts (36,38,66,69) and adducts with SbF₅ (68) which have provided novel solid state (68) and solution phase (66,69) structures and insights into the nature of the bridge-bonding and fluorine-bridge/oxygen-bridge equilibria (66,69). More recently (in collaboration with Prof. E.A.V. Ebsworth) it has been discovered that XeF₂ reacts smoothly and in high yield with Ir(CO)Cl₂ (PEt₃)2(P'F₂) to give Ir(CO)Cl₂(PEt₃)2(P'F₄), the first metal-PF₄ complexx (96, 107). This led to the preparation of a number of new transition-metal complexes such as Ir(CO)Cl₂(PEt₃)₂(NF₂) containing the novel difluoro amido ligand (105), the first transition metal -SF₃ complex (108) and the first metal fluoroacyl complex (116) as well as new -CF₃ complexes (119) and a transition-metal complex containing the novel -TeF₃ ligand (137).

However, perhaps the most important contributions to transitionmetal chemistry lie in the discovery and characterization of two new classes of compound, the transition-metal carbonyl fluorides and the transition-metal chalcogenide fluorides and their derivatives. The transition-metal carbonyl fluorides prepared (28,45,47,52,57,58,106,132,152,153,170,173,174,180,189,210,213,214) which were long sought by organometallic chemists are now a large class of compounds. Particularly important was the discovery of hydrolytically stable carbonyl fluorides (213) which means that this type of chemistry is now within the realms of regular inorganic chemistry and is not the preserve of the fluorine specialist. The new transition-metal chalcogenide fluorides (51,55,61,77,83,85,87, 90,95,102,117), are of significant structural interest.

The carbonyl fluorides are unusual in that many consist of mixed oxidation-state species such as

ReI^I (CO)₅F.Re^VF₅ (52) and [Ru^II(CO)₃F₂. Ru^VF₅]₂ (45) which permit the the powerfully oxidizing ligand, fluorine, and the reducing ligand, CO, to co-exist in the same molecule. An understanding of the nature of these molecules via their X-ray structures (28,52,58) and vibrational, n.m.r. and mass-spectral data (45,47,52,58) provided an excellent basis for predicting the nature of related compounds yet to be prepared (57). Since then, using mainly n.m.r. methods, new advances have been made in this area (132,152,153,170,173,174,180,189,210,213,214) with a range of new iridium (152,180,189), and osmium and ruthenium (170,173,174,189,210,213,214) compounds having been prepared. Currently, the preparation and characterization of a wide range of metal carbonyl phosphine fluorides are being made.

The transition-metal chalcogenide fluorides are of interest because they are related both to the transition-metal chalcogenide, chlorides, bromides and iodides and to metal oxide-fluorides and fluorides. They are also of interest because the chalcogenides themselves are so readily fluorinated that it is surprising to find them bound with fluorine to another element. A range of compounds, WSF₄ (51,90), WSeF₄ (55,102), MoSF₄, MoSeF₄ (61), ReSF₃, ReSF₄ and ReSF₅ (77,90) have been prepared. The first X-ray single crystal structure determinations on examples of this novel class of simple compounds [WSF₄ and ReSF₄ (87,90)] and their adducts [WSF₄.CH₃CN and WSF₄.SbF₅ (87,101)] and the first gas-phase electron diffraction studies on this type of species [WSF₄ and WSeF₄ (95,102)] have been obtained.

Development of manipulative methods for handling the more reactive fluorides have matrix isolation spectroscopic studies on hexafluorides (115,122,124) and hexafluoro anions (147) in collaboration with J.S. Ogden and W. Levason at Southampton University. EXAFS studies on CrO₂F₂ and MnO₃F (129,133), some pentafluorides (130), hexafluorides (133,138,146,157) and related fluoroanions (146) and osmium VIII oxide fluorides (158) have been given valuable information not available by other means.

Some of the more recent work in Leicester has been an effort to prepare novel organometallic compounds containing fluorinated ligands, the fruits of which are useful insights into C-F bond activation with concomitant C-C bond formation (164,168,175,176,183,186,191,201,209,212,221-223).

Actinide Fluoride Chemistry

Work in this area covers the preparation and chemistry of actinide pentafluorides, the chemistry of uranium oxide fluorides and the synthesis of new uranium chloride fluorides.

Neptunium pentafluoride was first characterized by John Holloway in 1969 and has now been prepared and shown to be isostructural with the high-temperature form of a-UF₅ (80). The chemistry of NpF₅ has been investigated and shown to differ from that of UF₅ and PaF₅ (80,81). In investigating these pentafluorides, considerable progress was made in achieving good synthetic routes and also new preparations of other uranium penta-halides such as UBr₅ and UCl₅ (75,80,89).

The first evidence of Lewis base character in UOF₄ and UO₂F₂ has been provided by reactions of these oxide fluorides with a series of penta-fluorides (60,63,76,79,82,88) and unusual complex ring and chain species have been characterized in the structures of UOF₄.2SbF₅ (63) and UO₂F₂.3SbF₅ (76). The acetonitrile and triphenylphosphine oxide derivatives of these adducts (97) and their UF₅ analogues (97) have been shown to have monomeric structures. Complexes of UF5 with Group V pentafluorides and SF₄ have also been prepared and characterized (98). Efforts to prepare and characterize uranium chloride fluorides (67,75,89,103) and their complexes (121) have led to interesting and unusual species (125).

Main Group Fluoride Chemistry

Two notable advances have been made in this area. The unusual antimony in trifluoride-pentafluoride adducts Sb₁₁F₄₃ (34) and Sb₆F₂₇ (123) has been prepared and the structure of the latter has shown it to be (SbF₂⁺)(SbF₂⁺)₂(SbF₆^-)₄ (123). The recent preparation of a series of metal difluoride arsenic pentafluoride adducts [in collaboration with Prof. B. Frlec and the late Dr. Gantar in Ljubljana] (64,70-73,91,111,112,113) and study of their properties indicates that they contain novel polymeric cations. Perhaps the most exciting work in the Main Group fluoride area has been the recent characterization of C_6OF_6O (126) for the first time and the studies of its chemistry (140,148,154,159,160,172,211,225).

Pre-chemical Intermediates

Recently (in collaboration with Prof. A.C. Legon) we have demonstrated that pre-chemical complexes involving substances, which would react vigorously under normal conditions, can be observed in a pulsed-nozzle, Fourier-transform microwave spectrometer and their structures studied (177-179,182,187,192-200,202,203,205-207,215-218,224,226,227). Particularly exciting has been the detection of the symmetric-top isotopomer H₃N...F₂ (179).

The views expressed in this document are those of the document owner.
If you are an authorised user you may edit this document through your Web browser.