Solvent-Dependent Crystallization of 1-Hydro-6-carbaphosphatrane and Its Tautomer

1-Hydro-6-carbaphosphatrane 2 was obtained as a mixture with
its tautomer, 3. Tautomers 2 and 3 were isolated by the
recrystallization from different solvents, respectively, and each
structure was determined by X-ray crystallographic analysis.
Variable-temperature NMR experiments revealed that pentacoordinate 2 and tricoordinate 3 are under the equilibrium, showing
that 3 is more stable. The theoretical calculations found a weaker
transannular PC bond in 2 than in 1-hydro-5-carbaphosphatrane,
1, which can be considered to be the reason 1-hydro-6-carba-phosphatrane 2 is less stable than its tautomer 3.