Hepta-coordinated heteroleptic derivatives of zirconium(IV): Synthesis, structural characterization and ring opening polymerization of ε-caprolactone

A series of new hepta-coordinated heteroleptic derivatives of zirconium(IV) of the types [(CH3COCHCOCH3)2ZrL] (1, 3, 5 and 7) and [(C6H5COCHCOC6H5)2ZrL] (2, 4, 6 and 8) (L = a dianionic [ONO]-tridentate ancillary ligand) was synthesized quantitatively by reacting Zr(OPri)4·PriOH with the tridentate Schiff’s base ligands H2Lx (H2L1 = C13H11NO2; H2L2 = C13H10BrNO2; H2L3 = C14H13NO3; H2L4 = C17H13NO2) and β-diketones (acetylacetone/dibenzoylmethane) in a 1:1:2 stoichiometry using dry benzene and ethanol as solvents. All these newly synthesized solid derivatives were fairly soluble in common organic solvents. They were characterized by elemental analyses, FTIR and NMR (1H and 13C) spectral studies. Single crystal XRD data of complexes 1, 2, 3 and 5 indicated the hepta-coordination and monomeric nature of these complexes. Furthermore, all these derivatives were tested for the ring opening polymerization of ε-caprolactone. The dibenzoylmethane derivatives were inactive in polymerizing ε-caprolactone unlike the acetylacetonate complexes. The molecular weight and polydispersity index values of polycaprolactone were obtained by GPC analysis. Kinetic studies for the rate of polymerization of ε-caprolactone by these catalytically active zirconium(IV) complexes authenticated the first order dependence of the polymerization rate on the monomer concentration. Among these, complex 3, with an electronegative bromine atom on the Schiff’s base moiety, exhibited the best catalytic activity with a conversion of >95% in 2 h.

A series of new metallo-organic complexes of hepta-coordinated zirconium(IV) have been obtained by using zirconium isopropoxide, acetylacetone/dibenzoylmethane and tridentate Schiff’s bases. Derivatives associated with the acetylacetonate moiety were remarkably active in the ring opening polymerization of ε-caprolactone, whereas the analogous dibenzoylmethane derivatives were reluctant towards it due to steric hindrance.Figure optionsDownload as PowerPoint slide