Carboranes: Versatile synthetic building blocks for novel materials.

摘要

Carborane clusters are useful synthetic building blocks and find applications in materials science, medicine and organometallic chemistry. In my research I have used icosahedral carboranes for syntheses of a variety of boron-rich compounds that can find materials science and medicinal applications. Chapter 1 presents a concise introduction to the chemistry of carborane clusters, including their use in synthetic chemistry. Examples of their use in materials science and medicine such as the treatment of cancer via boron neutron capture therapy (BNCT) and the use of carborane clusters in syntheses of thermally stable polymeric materials and metal organic frameworks are discussed. Chapter 2 describes the facile synthesis of Bcage-appended symmetrical molecules via silicon tetrachloride mediated trimerization reaction. This approach involves the syntheses of carborane-appended aromatic ketones followed by their trimerization employing silicon tetrachloride/ethanol as the reagent system. These Bcage-appended trimers have the acidic hydrogens of the o-carborane clusters on the periphery for further functionalization. In Chapter 3 the trimerization approach has been further extended to synthesize carborane Ccage-appended symmetrical pi-conjugated compounds containing three to six peripheral o-carborane clusters. The presence of o-carborane clusters attached to the oligophenylene cores via a methylene moiety has dramatically enhanced the fluorescence intensity of these molecules. These compounds are also found to be extremely thermally stable. Chapter 4 reports the syntheses of carborane-appended 1,3,5-triphenylbenzene (TB) and 1,3,5-tris(biphenyl-4-yl)benzene (TBB) containing three ortho, meta and para-carborane clusters directly attached to the conjugated cores. The incorporation of icosahedral carboranes is associated with a red shift in the UV absorption spectrum of up to 13 nm as well as enhancement of emission intensity of up to 154%. The presence of o-carboranes showed the maximum red shift in the UV spectrum whereas the maximum enhancement of the emission intensity was observed in the presence of m-carborane clusters. The extension of pi-conjugation is the experimental evidence in support of the existence of three-dimensional aromaticity of icosahedral carboranes. Proton NMR of these carborane-appended trimers indicates that ortho- and meta-carboranes possess benzene-like aromatic characteristics. Chapter 5 deals with the syntheses of phenylene and triazine core based symmetrical cobaltabisdicarbollides. These compounds containing three to six cobaltabisdicarbollides were synthesized by using trifluoromethanesulfonic acid and silicon tetrachloride mediated cyclizations. These cobaltabisdicarbollides containing molecules are stable up to 700 °C. The final Chapter 6 deals with the synthesis of a carborane-containing star-shaped molecule via cobalt catalyzed [2+2+2] cycloaddition reaction. This boron-rich molecule is found to be highly thermally stable and 10% mass loss was observed close to 500 °C.