Structural investigations of phosphorus-nitrogen compounds. 7. Relationships between physical properties, electron densities, reaction mechanisms and hydrogen-bonding motifs of N3P3Cl(6-n)(NHBut)n derivatives

Abstract

A series of compounds of the N3P3Cl(6 - n)(NHBut)(n) family (where n = 0, 1, 2, 4 and 6) are presented, and their molecular parameters are related to trends in physical properties, which provides insight into a potential reaction mechanism for nucleophilic substitution. The crystal structures of N3P3Cl5(NHBut) and N3P3Cl2(NHBut)(4) have been determined at 120 K, and those of N3P3Cl6 and N3P3Cl4(NHBut)(2) have been redetermined at 120 K. These are compared with the known structure of N3P3(NHBut) 6 studied at 150 K. Trends in molecular parameters [phosphazene ring, P-Cl and P-N(HBut) distances, PCl2 angles, and endo- and exocyclic phosphazene ring parameters] across the series are observed. Hydrogen-bonding motifs are identified, characterized and compared. Both the molecular and the hydrogen-bonding parameters are related to the electron distribution in bonds and the derived basicities of the cyclophosphazene series of compounds. These findings provide evidence for a proposed mechanism for nucleophilic substitution at a phosphorus site bearing a PCl(NHBut) group.