New chemistry of sterically-crowded carboranes

Abstract

Chapter one gives an introduction into heteroborane chemistry focussing on the areas of icosahedral and supraicosahedral (metalla)carboranes and their isomerisation mechanisms. Chapter two describes attempts at placing bulky substituents onto a 1,7-closo-C2B10 species so that upon reduction and subsequent oxidation the carbon atoms are unable to connect thus potentially preventing oxidation and allowing for easier capitation to form a supraicosahedral carborane. Initial efforts to synthesise such 1,7- bulky species, using tertiary alcohol substituents, did not lead to polyhedral expansion due to undesired reactivity. Compounds with bulky substituents incorporating cobaltacarborane clusters were successfully prepared but in insufficient yield for further expansion chemistry. Chapter three discusses the synthesis and characterisation of 1,7-closo-C2B10 species which incorporate a ferrocenyl unit on each substituent. Polyhedral expansion to form a 13-vertex carborane was unsuccessful, instead upon oxidation forming significantly sterically deformed 1,2 species with unprecedentedly elongated C-C connectivities. Chapter four investigates the redox chemistry of the ferrocenyl 1,2 and 1,7 species. The electronic spectrum of the 1,2 species were compared to that given by TD DFT calculations which suggest a degree of charge transfer. Associated electrochemistry suggests this may be a sterically-induced charge transfer within the 1,2 species. The steric deformation of the bis-ferrocenyl 1,2 species leads to a relatively lowtemperature isomerisation to the 1,7 species. This has renewed interest in the elucidation of the isomerisation mechanism of icosahedral carboranes. Chapter five details the synthesis and characterisation of labelled species with labels which can be reliably tracked upon isomerisation. A comparison of the experimental findings to the results of theoretical studies on the subject is examined. Chapter six contains experimental procedures and characterisation details for all the new compounds reported herein. Crystallographic data is listed in Appendices A and B (CD) along with structure solution and refinement details.