The crystal structures of NSF3 and (NSF2N(CH3)CH2–)2: How short is the ‘Crystallographic’ NS triple bond?

The synthesis of the first unequivocally characterised bis(difluorothiazyne), [NSF2N(CH3)CH2–]2 is reported. The crystal structures of this and NSF3 are also reported. NSF3 has the same geometrical parameters, within error, as it does in the gas phase. PIXEL calculations show that the principal interactions in its crystal structure are SN⋯SN dipolar contacts, which form chains with S⋯N = 3.533(2) Å. These contacts are reminiscent of those observed in the crystal structures of ketones. The exchange of a fluorine by a dialkylamino group has almost no influence on the NS bond distance while the SF bonds are significantly elongated. This behaviour is explained by negative hyperconjugation and confirmed by experimental data (as far as available) and quantum chemical calculations for NSFn(NMe2)3−n and NSFnPh3−n (n = 1–3).

NSF3 is considered the archetype of a compound with a sulfur(VI)–nitrogen triple bond. We report the first determination of its crystal structure. The SN bond distance in [NSF2N(Me)–CH2–]2 is essentially the same as that in NSF3, but the SF bonds are longer. Ab initio calculations reveal that this can be explained by negative hyperconjugation.Figure optionsDownload as PowerPoint slide