Chapter 1 Inorganic
Fluorine
Chemistry
Toward the 21st Century 1
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Joseph S. Thrasher and Steven H. Strauss
1Department of Chemistry, The University of Alabama, P.O. Box 870336, Tuscaloosa, AL 35487-0336 2Department of Chemistry, Colorado State University, Fort Collins, CO 80523 Fluorine and fluorinated substituent groups play an increasingly important role in modern inorganic chemistry. Many advances in solid-sate chemistry, coordination chemistry, main group element chemistry, and organometallic chemistry rely on the unique physical and chemical properties of the most electronegative of the chemical elements. This chapter lists the important monographs and reviews that have been published during the last twenty-five years in this area as well as some thoughts about where the field of inorganic fluorine chemistry is heading.
The unusual properties exhibited by numerous materials upon the incorporation of either fluorine or fluorine-containing substituents are no longer just topics of idle curiosity for the academician, but are the driving force behind developments in the field of fluorine chemistry. Many advances have either recently found or await commercialization as witnessed by the increasing diversity of industrial applications of fluorine compounds (7-7). Two illustrative examples of the current importance of inorganic fluorine chemistry to industry are the development of both chlorofluorocarbon (CFC) alternatives and new precursors for chemical vapor deposition (CVD) of inorganic materials. Although the CFC alternatives will almost certainly be organofluorine compounds, the processes being developed to produce these materials cannot escape the use of inorganic fluorine chemistry in terms of catalysts (8-15), halogen exchange reagents (16-20), and the like. The references given here serve only as representative examples; the current patent literature is full of references in light of the global concern to help halt the depletion of stratospheric ozone. The second aforementioned case falls into the increasingly important area of materials science. For example, a number of groups have recently taken advantage of the well-known increase in volatility associated with fluorine incorporation to make a series of volatile metal alkoxides which serve as new precursors for CVD of metal fluorides (21-28). Separate chapters on both of these topics of industrial importance appear later in this book. 0097-6156/94/0555-0001$08.54/0 © 1994 American Chemical Society Thrasher and Strauss; Inorganic Fluorine Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
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INORGANIC FLUORINE CHEMISTRY: TOWARD THE 21ST CENTURY
A number of other topical areas that are of current interest in inorganic fluorine chemistry and promise to be of continued interested towards the 21st century will be surveyed in the following pages. Many of these topics, but not all, are then described in more detail in later chapters.
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Coordination Numbers Greater than Six Among Fluorides and Oxofluorides The recent and continuing explosion in terms of the number of examples, as well as our understanding of their structures and bonding, of fluorides and oxofluorides with coordination numbers greater than six can be attributed almost solely to the development of a synthetic procedure for truly anhydrous tetramethylammonium fluoride by Christe and co-workers (29). With this source of more active fluoride, the groups of Christe, Schrobilgen, and Seppelt have been able to prepare and characterize the following remarkable species: XeF5" (30), C u V (31), BrF6" (32-34), IF6" (32,35), TeF7" (36-38), TeF6(OCH3)" (38), TeF5(OCH3)2" (38), TeOF6 " (39), IOF6" (40), TeF8 " (36), and IF8" (36,41). The concept of coordination numbers greater than six has also been extended to d- and /-block metal fluorides, namely M0F7", WF7-, and UF7" (42). In addition, other novel species such as the PF4" anion (43), which was previously only known in a low-temperature matrix (44), have now been prepared from tetramethylammonium fluoride. All of the aforementioned anions, as well as the ones remaining to be discovered, will almost certainly become the textbook examples of the future (45). The current viewpoint on the structure and bonding in these novel species is the topic of two chapters in this monograph. 2
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From Naked Fluoride to Least Coordinating Anion Obviously, the fluoride ion cannot be totally naked in a real chemical environment; however, successes such as those mentioned above can be attributed totally to more basic or nucleophilic sources of fluoride such as cesium fluoride, tetramethylammonium fluoride, and the phosphazenium fluoride [{(CH3)2N}3P=N=P{N(CH3)2}3] F- (46). In each case, a bulky cation is used in order to maximize the cation-anion distance and thus minimize their interaction. In addition, it is important to have a cation that is as stable as possible towards oxidation. Furthermore, it is necessary for the fluoride source to be anhydrous. It is with these reasons in mind, and the well-known stabilization of a complex anion by the use of a bulky cation (47,48), that Thrasher and co-workers prepared a potentially "naked" SF5- salt using Cs([18]crown-6)2 as the cation. Although the closest cation-anion contacts in this salt were over 7 À, the SF5" anion in this salt failed to act as a nucleophile when reacted with methyl iodide (49). Seppelt and co-workers have recently published the structures of an SeFs" salt and several TeFs salts in which they describe weak interactions between the chalcogen atom of one anion with the fluorine atom of a neighboring anion (50). Just as bulky cations have been used to disperse the positive charge over the cation and minimize cation-anion interactions, so has the search continued for more and more weakly coordinating anions. Again fluorine chemistry has played an +
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Thrasher and Strauss; Inorganic Fluorine Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
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THRASHER AND STRAUSS
Inorganic Fluorine Chemistry
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important role in this search, not only in terms of aromatic C-F bonds in tetraarylborates, but also in terms of the pentafluorooxotellurate, OTeF5", and related anions (51,52). For example, Strauss and co-workers have recently extended their search to anions such as Ti(OTeF5)6 ", Nb(OTeF5)6", Ta(OTeFs)6~, and Sb(OTeF5)6" in which the negative charge(s) is(are) spread primarily over thirty fluorine atoms. The high solubility of the silver salts of these anions in solvents like CFC-113 shows that appreciable concentrations of metal ions can be produced in solvents that are even more weakly coordinating than dichloromethane if the counterion is large enough and weakly coordinating enough (57). Winter and co workers, as described in a later chapter, have successfully utilized more or less the same concept to produce a series of highly soluble organotitanium Lewis acids (53,54). Strauss' search has allowed the preparation and characterization of the first isolable silver carbonyls Ag(CO) and Ag(CO)2 (55,56) where the would-be coordinatively unsaturated metal cation binds to weak ligands such as carbon monoxide. Aubke and co-workers have also recently been able to stabilize the Au(CO)2 cation either in highly acidic media (57) or in the solid state with the weakly basic fluoro anion Sb2Fi \~ (58). These silver and gold carbonyl cations are particularly interesting in that they possess C-0 stretching frequencies higher than that in free CO which is an indication of an increase in bond order between the carbon and oxygen upon coordination to silver or gold. A more detailed overview of these complexes and their structures and bonding is included in two separate chapters in this monograph. In terms of what the future holds, the intriguing question has been posed "Carbon monoxide. A ligand for main group elements?" (59). 2
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Fluorinated Allotropes of Carbon From the initial report of synthetic quantities of buckminsterfullerene, one could realistically think of the possibility of preparing better lubricants than the graphite fluorides (60). Several research groups have undertaken the fluorination of C60 and other fullerenes (61-65), and although there may be some debate as to the degree of fluorination, highly fluorinated C60 is rather susceptible to nucleophilic substitution which precludes its use as a lubricant (66-68). On the other hand, the reactivity of fluorinated C60 provides a potential route to many new fullerene derivatives (66-69), and high quality thinfilmsof fluorinated C60 have already been prepared and studied (70). In addition, buckyballs have recently been coated with perfluoroalkyl groups by a team of scientists at Du Pont (77). As far as the other allotrope of carbon is concerned, Margrave's group has been studying the fluorination of diamond for a number of years (72-75). More recently, Margrave and co-workers have been looking at the halogen-assisted CVD of diamond (76-79), including the potential disposal of CFCs and Halons via this technique (80). It will be interesting to examine the economics of this latter proposal when done on a large scale. Novel Bond Types to Noble Gases It has been almost twenty years since DesMarteau reported the first example of a compound with a xenon-nitrogen bond (81) and approximately ten years since
Thrasher and Strauss; Inorganic Fluorine Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
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INORGANIC FLUORINE CHEMISTRY: TOWARD THE 21ST CENTURY
Schrobilgen published the first structural evidence for this bond type (82). In the intervening time, the majority of new compounds with a xenon-nitrogen bond have resulted from the exploitation of the Lewis acid properties of noble gas cations by Schrobilgen and co-workers (83-88). These workers have further exploited these properties to prepare the first compounds with a krypton-nitrogen bond (87,89). The majority of these new Xe-N and Kr-N compounds are solution species; they have been characterized predominately by multinuclear NMR and vibrational spectroscopy and are generally too unstable to be isolated in a pure form. In 1979, Lagow and co-workers reported the first evidence for a compound with a bond between xenon and carbon, namely Xe(CF3)2 (90). While the evidence for this compound still receives much scrutiny, the groups of Frohn and Naumann independently synthesized the first definitive examples of compounds with a xenoncarbon bond (91-96). Until recently all of the examples of xenon-carbon bonds were cationic species. However, earlier this year Frohn reported the preparation, characterization, and structure of C6F5Xe02CC6F5, the first truly covalent compound with not only a xenon-carbon bond, but a xenon-oxygen bond as well (97). The recently prepared Kr(OTeFs)2 represents the first compound with krypton-oxygen bonds (98). Again, this compound is only a solution species which has been characterized by its F and 0 NMR spectra. Much technical expertise on updated procedures for noble gas fluorides has come from the groups of Zemva and Kinkead (99-103). A detail discussion of the preparation of one of these noble gas fluorides, namely KrF2, is given in a following chapter. In addition, these research groups have led the way recently in the synthesis of new
