Acyclic phosphazenes or iminophosphoranes continue to be of interest because of at least three reasons, One, they are good ligands for binding to transition metal ions as well as lanthanide (and some actinide) metal ions. Two, they can be extremely strong bases. Lastly, many of these compounds find applications either in the form of their metal complexes or as free ligands in a multitude of catalytic reactions ranging from various polymerization reactions to asymmetric synthesis. This section deals with different types of acyclic phosphazenes including the assembly of some new acyclic phosphazenes.^1–37

Benzo-1,2,3-thiazaphosphole (1) was synthesized by annulation of ortho-phosphinoarene sulfonyl fluorides with trimethylsilyl azide. This reaction proceeds through a Staudinger-type iminophosphorane intermediate followed by intramolecular trapping with sulfonyl fluoride.¹  The Staudinger reaction was employed to synthesize a novel coumarin-based iminophosphorane (2).²  2 functions as an excellent fluorescent probe. The absorption and fluorescence properties of 2 can be fine-tuned by stereo-electronic modulation of the aryl groups present on the phosphine ligands.²