Geometry Change in a Series of Zirconium Compounds during Lactide Ring-Opening Polymerization

A new series of zirconium compounds, (salfen)Zr((OBu)-Bu-t)(2) (salfen = N,N'-bis(2,4-di-tert-butylphenoxy)-1,1'-ferrocenediimine), (salfen)Zr((OPr)-Pr-i)(2), and (salfen)Zr((OPr)-Pr-n)(2), was synthesized and characterized, featuring a cis-beta coordination geometry for the major isomer found in solution. An isomerization between the trans and cis-beta coordination geometry was observed at ca. 100 degrees C in solution. This isomerization was also observed in the presence of L-lactide (LA) before its ring-opening polymerization could occur. The isolation of a (salfen)Zr((OPr)-Pr-n)(2) lactide intermediate supports the change to the trans coordination geometry necessary before polymerization.