Lithium complexes of tri- and hexaanionic cyclophosphazenates, the impact of metal coordination on the ring conformation

Cyclophosphazenes (RNH)6P3N3 1 react with three and six equivalents of butyllithium in thf to give lithium complexes of tri- (2) and hexaanionic (3) phosphazenate ligands, respectively. A variety of lithium complexes 2 and 3 were prepared and structurally characterised. The degree of puckering of the (PN)3 ring systems correlates with increasing ligand charge. The trianions solely exhibit the chair conformation; their deprotonated side groups are positioned at equatorial sites. This conformation ensures that the charge of the ligand is most effectively distributed and it also provides three distinct coordination sites for the three lithium ions. Complexes of the trianion can be monomeric or dimeric. Aryl-N(exo) derivatives tend to form monomers, while alkyl derivatives form dimeric sandwich complexes. Complexes of the hexaanion fall into two categories. Binary complexes, which contain the ligand and lithium ions, form dimers; the (PN)3 ring in these complexes exhibit a chair conformation. Complexes which, in addition, contain small monodentate ions, such as chloride, fall into the second category; their ring systems adopt a boat conformation.