Iron(II) and Zinc(II) Complexes with Tetradentate Bis(pyrazolyl)methane Ligands as Catalysts for the Ring-Opening Polymerisation of rac-Lactide

A new subgroup of bis(pyrazolyl) methane ligands has been developed through the synthesis of the first N, N, N, N-tetradentate bis(pyrazolyl) methane compounds, namely, bis( pyrazolyl) bipyridinylmethane [HC(Pz)(2)Bipy, L1] and bis(3,5methylpyrazolyl) bipyridinylmethane [HC(3,5-MePz)(2)Bipy, L2]. From these ligands, a large variety of new mononuclear Fe-II and Zn-II complexes with counterions ranging from halides to carboxylates and sulfate were synthesised and characterised by X-ray crystallography. The complexes [Fe{HC(Pz)(2)Bipy}X-2] (X = Cl-, CF3CO2-, CF3 SO3-), [Fe{HC(3,5-MePz)(2)Bipy}X-2] (X = Cl-, CF3CO2-) and [Zn{HC(Pz)(2)Bipy}X-2] (X = Cl-, CF3CO2-) were tested in the melt ring-opening polymerisation of sublimed and unsublimed technical-grade rac-lactide at 150 degrees C. The complex [Fe{HC(Pz)(2)Bipy}(CF3CO2)(2)(-)] showed the most promising results and produced a conversion of 87 % after 30 h with molecular weights of M-w approximate to 30000 g mol(-1).