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.