Metallocene and other transition metal derivatives of phosphazene oligomers and polymers: A retrospective summary

Phosphazene rings and polymer chains possess a skeleton of alternating phosphorus and nitrogen atoms with two substituents linked to every phosphorus. Most of the known examples have organic or organosilicon side groups, but an increasing number have transition metal organometallic side units. Polymeric organometallic systems are of interest for the immobilization of catalysts or as electronic or optical materials and polymers with unique physical and engineering properties. Small molecule derivatives are valuable models for the synthesis of polymers and for understanding structural aspects. However, conversion of the small molecule information to the synthesis of metal-containing polymers is a major challenge. This brief account focuses on the linkage of metallocenes to the phosphazene platform and reviews some of the options available for access to other phosphazene-organometallic systems. It also emphasizes the importance of fluorophosphazenes in the development of organometallic derivatives.