POLYMERS AND CERAMICS BASED ON ICOSAHEDRAL CARBORANES - MODEL STUDIES OF THE FORMATION AND HYDROLYTIC STABILITY OF ARYL ETHER, KETONE, AMIDE AND BORANE LINKAGES BETWEEN CARBORANE UNITS

The synthesis and hydrolysis to free carborane (R-CB10H10C-H) and a carboxylate under alkaline but not neutral conditions, of five 1,2- and 1,7-carboranyl ketones (R-CB10H10C-CO-R') is described in a study of the optimum sequence of ether, ketone, carborane and phenylene groups for poly(ether ketone) (PEK) polymers. 1,2-Dicarba-closo-dodecaborane (12) behaves as a better leaving group than the 1,7 isomer from a proposed intermediary anionic adduct of hydroxide to carbonyl carbon 1-(4-Fluorophenyl)-1,2-dicarba-closo-dodecaborane (12) (F-C6H4-CB10H10C-H) suffers cage degradation and not nucleophilic substitution with sodium phenoxide, precluding polymerisation by this method, but 1,2-carboranebenzoic acids (R-CB10H10C-C6H4-CO2H) condense cleanly with aryl ethers in trifluoromethanesulfonic acid (TFSA), as do phenoxyphenyl-1,2-carboranes (R-CB10H10C-C6H4-O-C6H5) with benzoic acid. 1,2- and 1,7-carboranyl N-monosubstituted amides (R-CB10H10C-CO-NHR') hydrolyse relatively slowly to free carboranes (R-CB10H10C-H) and amines in alkali, suggesting initial deprotonation at NH rather than nucleophilic addition. Synthesis of four model boranes is described; those with 1,2-carborane bonded to boron are readily hydrolysed by water, bis[1,7-dicarba-closo-dodecaborane(12)yl]phenylborane [(H-CB10H10C-)2B-C6H5] reacts slowly, but 1,7-bis(diphenylboryl) 1,7-dicarba-closo-dodecaborane(12) [(C6H5)2B-CB10H10C-B(C6H5)2] required alkaline conditions. Formation of free carboranes and B-hydroxy compounds is consistent with a mechanism analogous to hydrolysis of the corresponding ketones. The relative reactivities of the boranes correspond to the leaving-group abilities of the isomeric carboranes and the build-up of steric compression in a proposed tetrahedral borate intermediate. The most promising polymer structures are inferred from the hydrolytic experiments.