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

Abstract

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.