the cavity of the in-in isomer is large enough to encapsulate a chloride ion that is hydrogen bonded to the two N_H groups. The species thus formed is a cryptate, but differs from the cryptates discussed at p. 119 in that there is a negative rather than a positive ion enclosed. Even smaller ones (e.g., the 4,4,4 compound) have been shown to form mono-inside-protonated ions. In compound 88, which has four quaternary nitrogens, a halide ion has been encapsulated without a hydrogen being present on a nitrogen. This ion does not display in–out
isomerism. Out–in and in–in isomers have also been prepared in analogous allcarbon tricyclic systems.
It is known that chiral phosphanes are more pyramidal and that inversion is more difficult, usually requiring temperatures well over 100_C for racemization.