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 N<inf>3</inf>P<inf>3</inf>Cl <inf>6-n</inf>(NHBut)<inf>n</inf> 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 N<inf>3</inf>P <inf>3</inf>Cl<inf>5</inf>(NHBut) and N<inf>3</inf>P <inf>3</inf>Cl<inf>2</inf>(NHBut)<inf>4</inf> have been determined at 120 K, and those of N<inf>3</inf>P<inf>3</inf>Cl<inf>6</inf> and N <inf>3</inf>P<inf>3</inf>Cl<inf>4</inf>(NHBut)<inf>2</inf> have been redetermined at 120 K. These are compared with the known structure of N <inf>3</inf>P<inf>3</inf>(NHBut)<inf>6</inf> studied at 150 K. Trends in molecular parameters [phosphazene ring, P-Cl and P-N(HBut) distances, PCl<inf>2</inf> 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. © 2006 International Union of Crystallography Printed in Great Britain - all rights reserved. © 2008 Elsevier B.V., All rights reserved.