Chemistry Special Seminar - My Cao

Title: Chiral Phospha[n]ferrocenophanes: New Metallopolymers Through Ring-Opening Polymerizations

Chiral polymers have many important applications, including separation of racemic compounds, or as catalysts for asymmetric syntheses. There are many examples of chiral natural polymers, such as proteins, DNA, or cellulose. However, synthetic chiral polymers are rather rare. Our work concentrates on the synthesis of chiral phospha[n]ferrocenophanes and investigations of their ring-opening polymerization. A series of new enantiomerically pure phospha[1]ferrocenophanes (P[1]FCPs; 2a-e) with different substituents on phosphorus were prepared via a salt-metathesis reaction between Cl₂PR and the chiral dibromoferrocene 1, which was equipped with two iPr groups in α positions. In addition, species 1 was applied for a two-step synthesis and chiral diphospha[2]ferrocenophanes 4a-b were obtained. Compounds 4a and 4b are the first [2]FCPs which have both planar chirality on Cp rings and central chirality on phosphorus. Performance of thermal ring-opening polymerization (ROP) of 2c gave polymers and cyclic oligomers. Gel permeation chromatography (GPC) of the sulfurized polymer resulted in the molecular weight (M_w) of 24 kDa and a polydispersity index (PDI) of 1.5. Mass spectrometric analysis of the oligomers showed the presence of cyclic oligomers from dimers to hexamers. After sulfurization, preparative thin layer chromatography led to separation of five isomeric dimers. Structural characterization of one of these dimers by single-crystal X-ray analysis led to the conclusion that the Fe-Cp bond breaks during the thermal ROP process. Besides thermal ROP, attempted anionic ROP will also be discussed.