Structural evidence for Pπ-Pπ conjugation in aminosubstituted phosphaalkynes

The molecular structure of 2,2,6,6-tetramethylpiperidinophosphaalkyne was determined by the X-ray structural method. The main geometrical parameters are as follows: PC 1.559(2), N C(sp) 1.316(2) Å, PC N 178.9(1)°, with an almost planar trigonal bond configuration for the N atom and the chair conformation of the piperidine ring. Structural evidence for the nitrogen lone pair conjugation with the π-system of the triple bond was found to be different in phosphaalkynes PC-NR₂ and nitriles NC NR₂. Quantum-chemical ab initio calculations (HF/631G*) showed that this is caused by a different character of polarization of the PC and NC triple bonds.