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(3,5‐methylpyrazolyl)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 – , CF 3 CO 2 – , CF 3 SO 3 – ), [Fe{HC(3,5‐MePz) 2 Bipy}X 2 ] (X = Cl – , CF 3 CO 2 – ) and [Zn{HC(Pz) 2 Bipy}X 2 ] (X = Cl – , CF 3 CO 2 – ) were tested in the melt ring‐opening polymerisation of sublimed and unsublimed technical‐grade rac ‐lactide at 150 °C. The complex [Fe{HC(Pz) 2 Bipy}(CF 3 CO 2 ) 2 ] showed the most promising results and produced a conversion of 87 % after 30 h with molecular weights of M w ≈ 30000 g mol –1 .