Generation and Direct Observation of the 9-Fluorenyl Cation in Non-acidic Zeolites

Abstract

9-Alkyl-9-fluorenyl cations and the parent 9-fluorenyl cation are generated via photoheterolysis of the corresponding 9-fluorenols within alkali metal zeolites such as LiY, NaY and Naβ, and observed directly using nanosecond laser flash photolysis. The observation of the highly reactive and highly unstable 9-fluorenyl cation under these conditions provides a remarkable example of the extraordinary ability of non-proton exchanged zeolites to provide kinetic stabilisation for electrophilic guests. The reactivity of the carbocations is found to be highly influenced by the nature of the alkali metal counterion as well as the inclusion of cosolvents within the zeolite matrix. The availability of several reaction pathways, other than heterolysis, to photoexcited 9-fluorenols within the zeolite cavities is also demonstrated and shown to be modified by co-adsorbed protic reagents in a manner consistent with solvent-assisted dehydroxylation.