Polyhedral boron chemistry

December 9, 1919-April 14, 2011), Emeritus Professor at Harvard University and the winner of the 1976 Nobel Prize for Chemistry. Professor Lipscomb, one of the founders of theoretical and structural polyhedral boron chemistry, had dedicated much of his career to promoting boron science, which is the subject of this book. Much of the progress in these endeavors rests on Professor Lipscomb s original work on bonding in boranes. His dedication to our global society is hereby recognized and saluted. We miss him. 

NMR is a crucial tool in polyhedral boron chemistry the literature is extensive and detailed excellent comprehensive compilations of the data published to mid-1981 are in Refs. 3, 6, and 7, but unfortunately no recent critical and comparative comprehensive account is available. Citations given should themselves be considered as sources of further references. 

The discovery of polyhedral boranes and polyhedral heteroboranes, which contain at least one atom other than in the cage, initiated a new era in boron chemistry.1-4 Most commonly, of the three commercially available isomeric dicarba-closo-dodecaborane carboranes(l,2-, 1,7-, and 1,12-), the 1,2-isomer 1 has been used for functionalization and connection to organic molecules. The highly delocalized three-dimensional cage bonding that characterizes these carboranes provides extensive thermal and kinetic stabilization as well as photochemical stability in the ultraviolet and visible regions. The unusual icosahedral geometry of these species provides precise directional control of all exopolyhedral bonds. 

In the First Edition of Regular Polytopes, Coxeter stated, ... the chief reason for studying regular polyhedra is still the same as in the times of the Pythagoreans, namely, that their symmetrical shapes appeal to one s artistic sense [23], The success of modem molecular chemistry does not diminish the validity of this statement. On the contrary. There is no doubt that aesthetic appeal has much contributed to the rapid development of what could be termed polyhedral chemistry. One of the pioneers in the area of polyhedral borane chemistry, Earl Muetterties, movingly described his attraction to the chemistry of boron hydrides, comparing it to M. C. Escher s devotion to periodic drawings [24], Muetterties words are quoted here [25] ... 

The chemistry described above exemplifies the great similarity between aromatic polyhedral borane chemistry and the aromatic branch of organic chemistry. Modular syntheses with carboranes and the discovery of a variety of camouflaged derivatives clearly reveals the tentacles of organic chemistry reaching into the polyhedral borane field. Thus, the conflux of boron and carbon chemistries is broadened. 

Formally subvalent compounds of boron containing a boron-boron single bond are intermediate in structural complexity between simple monoboron derivatives and the polyhedral electron-deficient compounds of the element. The properties of such compounds, particularly the simple derivatives of the B2X4 type, have attracted the attention of several groups of workers since Stock s initial discovery of BjCL some 45 years ago (96). These materials provide the simplest examples of catenation in boron chemistry and offer suitable systems in which to study the properties of the covalent B—B bond and the characteristic chemistry of compounds containing this linkage. 

Although the silicon atom has the same outer electronic structure as carbon its chemistry shows very little resemblance to that of carbon. It is true that elementary silicon has the same crystal structure as one of the forms of carbon (diamond) and that some of its simpler compounds have formulae like those of carbon compounds, but there is seldom much similarity in chemical or physical properties. Since it is more electro-positive than carbon it forms compounds with many metals which have typical alloy structures (see the silicides, p. 789) and some of these have the same structures as the corresponding borides. In fact, silicon in many ways resembles boron more closely than carbon, though the formulae of the compounds are usually quite different. Some of these resemblances are mentioned at the beginning of the next chapter. Silicides have few properties in common with carbides but many with borides, for example, the formation of extended networks of linked Si (B) atoms, though on the other hand few silicides are actually isostructural with borides because Si is appreciably larger than B and does not form some of the polyhedral complexes which are peculiar to boron and are one of the least understood features of boron chemistry. 

Adams, L., S. N. Hosmane, J. E. Eklund, J. Wang, and N. S. Hosmane. 2002. Novel route to boron-10 enriched pentaborane(9) from boric acid and conversion to nido- °BioHi4 and anti- BigH22 Synthetic advance in polyhedral borane chemistry and in BNCT research. In Proceedings of the First International Boron Symposium, ed. K. Erarslan, pp. 129-133. Kiitahya, Turkey Dumlupinar University Press. 

The propensity of boron to form polyhedral structures is reflected also in the structures of elemental boron and boron-rich metal borides. In hydrocarbon chemistry, benzene is characterized by its extra stability the thermodynamically most stable allotrope of carbon, namely, graphite is formed by the condensation of benzene units. This beautiful relationship between compounds and allotropes exists in boron chemistry as well, where the stable allotropes of elemental boron and many of the boron-rich metal borides are made up of icosahedral subunits. 

In the chemistry of polyhedral boron hydrides, boron-centered cations were postulated to be key intermediates of an electrophile-induced nucleophilic substitution mechanism that is responsible for the formation of a variety of boron-substituted derivatives [14], Such boron-centered cations can be easily generated by abstraction of a hydride by the treatment of polyhedral boron hydrides with Lewis or Bronsted acids [15], Similar to the classical chelate-restrained borinium cations based on 3-coordinate boron, these species, which we called quasi-borinium cations, have an unstabilized p orbital and are strong electrophiles (Scheme 6.1). Such quasi-borinium cations are highly reactive and react with even weak nucleophiles, such as ether or nitrile solvent molecules giving the corresponding oxonium and nitrilium derivatives whose properties are close to those of similar complexes of transition metals [15-17]. 

The applications of boron in medicine are not limited to the development of new chemotherapeu-tics, but also include diagnostics and the modulation of many metabolic processes through dietary boron. The discovery of polyhedral boron compounds added new dimensions to the medicinal chemistry of boron, facilitating the quest for biologically active molecules containing boron clusters rather than only a single boron atom per molecule. Thus, boron is an essential element for life and living. 

One of the most important and exciting events in the chemistry of the twentieth century was a discovery of polyhedral boron hydrides at the end of 1950s. It was shown that boron atoms in boron hydrides are linked by unusual three-centered two-electron bonds or multicentered bonds. The establishment of three-centered two-electron bonds made a true revolution in the theory of chemical bonding and William Lipscomb was awarded the Nobel Prize in 1976 for his studies on the structure of boranes illuminating problems of chemical bonding [1]. 

In contrast to the previously known boron hydrides, the polyhedral boron hydrides were shown to be exceptionally stable. One of the most striking features of the carboranes is the capability of the two carbon atoms and ten boron atoms to adopt the icosahedral geometry in which the carbon and boron atoms are hexacoordinated. This feature of carboranes gives rise to the unusual properties of these molecules and their carbon and boron derivatives. Due to these features of icosahedral carboranes, their chemistry has developed extensively, and the results obtained are summarized in R. Grimes monograph and review [5]. Investigation of the properties of polyhedral boron hydrides resulted in the conclusion that these compounds have aromatic properties. It was the first example of nonplanar three-dimensional aromatic compounds and resulted in the development of the eon-cept of three-dimensional aromaticity that is generally accepted at the present time [6,7]. 

Besides the interest in the basic aspects of the chemistry of polyhedral boron compounds, considerable effort has been put in the development of high-energy fuels for aircraft and rocket propulsion [6]. Other potential applications of the boron cluster compounds are presented in the review by J. PleSek [8]. 

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