A Brief Summary of Current Projects in the Hawthorne Research Group
The element boron neighbors carbon in the periodic table, exists in plentiful supply throughout the Universe and reaches a long arm across the periodic table in order to form stable compounds with a wide variety of other elements. The most important chemical property shared by carbon and boron is the ability of both these elements to form large families of discrete structures by bonding to themselves. Thus, boron atoms form stable bonds to other boron atoms when forming polyhedral borane clusters. Carbon, of course, adopts the same behavior while creating organic chemistry. Rapid advances made during the past fifty years have now established such boron clusters as the basis of a nearly infinite number of new species containing elements from throughout the periodic table. This new science supports an ever increasing scope of molecular structures having extraordinary chemical, biological, thermal and photochemical stabilities. Such properties provide unique applications not possible with other elements including carbon. While borane and hydrocarbon derivatives share many related structural features and functions, borane species are apparently stable in the presence of enzyme systems which have supported the evolution of life (at least on this planet) and they appear, at this time, to be inert to enzymatic degradation reactions. Another unique feature of boron is its isotopic distribution of 10B (20%) and 11B (80%) accompanied by the very great propensity of 10B to capture a slow neutron and fission to cytotoxic 4He and 7Li nuclei. This process is accompanied by the liberation of about 2.4 MeV of kinetic energy and a 0.5 MeV gamma-photon (the boron neutron capture reaction). This nuclear reaction provides the basis of boron neutron capture therapy of cancer (BNCT) and the nonmalignant disease, arthritis.
The research projects pursued by, and the members of, the Hawthorne Research Group are as diverse and eclectic as the field of polyhedral borane and carborane cluster chemistry. Of specific interest to the group are new structures and reactions which find applications in supramolecular chemistry, nanotechnology, molecular recognition and biomedicine. A separate link is provided for the latter subject entitled, "Biomedical Polyhedral Borane Chemistry."
Biomedical Polyhedral Borane Chemistry
Biomedical polyhedral borane chemistry is a new field which combines polyhedral borane chemistry with organic chemistry and aspects of biomedicine. New structures and reactions commonplace in the broad area of polyhedral organoborane chemistry provide the bases of novel derivatives and reactions designed for use in the medically important areas of disease diagnosis and therapy, drug delivery, pharmaceutical discovery and biosensors. It should be pointed out that nearly all of the chemical research under way in the Hawthorne Group is in some way applicable to biomedical science.
Functional Assemblies of Carborane-containing Modules
"Functional assemblies of carborane-containing modules" can be further subdivided into two categories. These are rod-like assemblies (carborods) and cyclic assemblies (carboracycles and mercuracarborands).
We are currently evaluating the rod-like constructs for application as Langmuir-Blodgett films, ordered functional monolayers on solid surfaces, and as liquid-crystalline materials. A variety of different linear groups for use between the carborane cages and carborane cage substituents are under investigation in order to control the length, rigidity, shape, reactivity and solubility of these materials. These rods carry an assortment of versatile terminal substituents which can be adapted to a variety of applications.
The cyclic assemblies under investigation are composed of two different types of cycles, depending on the type of linker used to join the carborane modules. When electrophilic mercury is used as the bridge between the carborane modules, mercuracarborands are formed. These species provide the basis of a variety of unique applications including anion complexation, a very robust and sensitive microelectrode for the determination of serum chloride ion in real time, electrophilic catalysts for organic reactions such as Diels-Alder and, when complexed with chloride ion, the resulting complex is a weakly-coordinating anion potentially useful in Ziegler-Natta catalysis of alkene polymerization. Another macrocyclic assembly is formed if the linkage is based upon a bifunctional arylene or alkylene hydrocarbon segments with or without ancillary functional groups. These carboracycles are being evaluated as hosts for host-guest chemistry in both organic and aqueous solvent environments. An assortment of linking groups is being investigated to adjust the size of the interior cavity, cavity functionality, and solubility properties. The carborane modules incorporated in carboracycles may also be converted to metallacarborane components by well-known reactions with transition metals, lanthanides, and main group metals.
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