The molecule adamantane shares its name base with adamant reflecting its robust nature. In a recent publication, scientist Karl Irikura argues through in silico experimentation that the so-called in-isomer called in-1-H-F15-adamantane (the almost completely fluorinated derivative) is just as stable (DOI). In this molecule 15 hydrogen atoms are replaced by fluorine and the remaining hydrogen atom is inverted and pointing inwards instead of outwards. Agreed, isomerization to regular out-adamantane is a very exothermic process but the activation energy (the energy barrier the isomerization process has to overcome) is prohibitively high. The molecule also appears impervious to water and oxygen and is stable towards bimolecular reaction. Why perfluorinated? fluorine substitution creates stronger carbon-carbon bonds. The all-hydrogen in-adamantane has a chemical half-life of 2 weeks at dry ice temperature compared to 100 years for the fluorinated one at ambient temperature. To summarize, in-F15-adamantane is a viable molecule in the new HSS doctrine.
But synthesis represents a formidable task. For starters, selectively placing fluorine atoms on adamantane is not that easy. You can add up to three at targeted positions in selective organic reactions (DOI, DOI, DOI) or create perfluoroadamantane with all hydrogens replaced by fluorine in a single F2 aerosol blast (DOI) but even out-1-H-F15-adamantane is elusive let alone the in-isomer.
When it comes to in-molecules, fullerenes are much more likely candidates because of size and surface reactivity. The same people who have brought endohedral hydrogen fullerene or H2@C60 most recently have been able to incorporate ammonia into open-cages fullerenes as well (DOI). Surprisingly however, the hydrogenation of a fullerene from within has yet to be reported.