Synthesis, Structure, and Bonding Properties of 5-Carbaphosphatranes: A New Class of Main Group Atrane

1-Hydro-5-carbaphosphatrane (1) and 1-methyl-5-carbaphosphatrane (2), the first 5-carbon
analogues of phosphatranes, were synthesized by a demethylation reaction of cyclic phosphinate 3. X-ray
analysis revealed that 1 has a typical trigonal bipyramidal structure with hydrogen and carbon atoms at the
apical position and three oxygen atoms at the equatorial positions, indicating that 1 is a phosphorane in
the perfectly “anti-apicophilic” arrangement. Apical P−C and P−H bond lengths were 1.921(2) and 1.38(2)
Å, respectively. The 1JPH value of 1 and the 1JPC(P−CH3) value of 2 were 852 and 215 Hz, respectively,
which are extraordinarily large for the apical coupling constants of phosphoranes, but close to those of the
reported phosphatranes with a 5-nitrogen atom. IR and Raman spectra are also reported. Force constant
calculations indicate the transannular bond in carbaphosphatrane is 3 times stronger than in silatrane, due
to its covalent character.