Synthesis and derivative chemistry of icosahedral carboranes

Abstract

A series of icosahedral carboranes including, ortho-1-methyl-ortho- and 1-phenyl-ortho-carborane were prepared from decaborane and the appropriately substituted acetylenes. An investigation of the preparative route to ortho-carborane showed that improvements could be made to the literature method employed. Meta-carborane was obtained in high yields by the thermal isomerization of ortho-carborane. The structurally-characterised compounds Li(C(_2)B(_10)H(_10)Me) (PMDETA) and Mg(C(_2)B(_10)H(_10)Me)(_2)(C(_4)H(_8)O(_2))(_2). C(_2) H(_8) contain unprecedented examples of group I and II metals covalently bonded to six-co-ordinate carbon atoms. Features of the structures are compared with those of other organolithium and organo-magnesiura compounds and used to calculate the cone angles and steric requirements of icosahedral carboranyl and methyl-carboranyl ligands attached to metal atoms of various sizes. Series of, carboranyl ketones of formulae RθCOR' and (Rθ)(-2)CO (where R = H, Me, Ph and R' = Ph), carboranyl amides of formulae RNHCOθCONHR and RNHCOθ'CONHR (where R = Ph, Me,(^t) Bu) and boranyl-carboranes of formulae RθBR'(_2) and (Rθ')(_2)BR'(where R = H, Me and R' = Ph), were synthesized and an investigation carried out to assess their relative stabilities to hydrolytic degradation. These studies revealed interesting trends within each series. A series of C-hydroxy-derivatives including MeθOH, PhθOH, HθOH, Hθ'OH and H0θ0H were successfully prepared by the reactions of the mono- or dilithio-carboranes with oxygen or with benzoyl peroxide. All of the hydroxy- carboranes with the exception of H0θ0H were crystalline solids. A number of tertiary ammonium salts of the ortho-carboranyl-C-hydroxy derivatives were prepared in high yields. No salts of the meta-derivative, Hθ’OH were obtained. All of the salts were white crystalline solids, their infra-red, (^1)H, (^11)B and COSY n.m.r. spectra showing interesting and significant differences to those of their corresponding hydroxy-derivatives, many of these differences, presumably attributable to increases in the C-0 bond order and an increase in electron density within the cage.The previously unreported MeθOSiMe(_3) was prepared by the reaction of MeθOHNEt. with Me(_3)SiCl and isolated as a stable, crystalline solid.