Synthesis and characterization of zirconium and hafnium aryloxide compounds and their reactivity towards lactide and ε-caprolactone polymerization

Reactions of 2 equiv. pyrrole with [C 4 H 3 N(CH 2 NMe 2 )-2] 2 M(NEt 2 ) 2 in toluene generated tetra-pyrrolyl metal compounds [C 4 H 3 N(CH 2 NMe 2 )-2] 2 M(C 4 H 4 N) 2 (3, M = Zr; 4, M = Hf) in moderate yields. Similarly, treatment of metal amides [C 4 H 3 N(CH 2 NMe 2 )-2] 2 M(NEt 2 ) 2 with 2,6-dimethylphenol or 2,6-diisopropylphenol in heptane resulted in the elimination of diethylamine along with the formation of the corresponding metal alkoxides [C4H3N(CH2NMe2)-2]2M(OR)2 (5, M = Zr, R = C 6 H 3 -2,6-Me 2 ; 6, M = Hf, R = C 6 H 3 -2,6-Me 2 ; 7, M = Zr, R = C 6 H 3 -2,6-iPr 2 ; 8, M = Hf, R = C 6 H 3 -2,6-iPr 2 ) in moderate yields. All the new compounds were characterized by 1 H and 13 C NMR spectroscopy and the structures of 3, 4, 6, 7, and 8 have also been determined by X-ray crystallographic studies. The aryloxides and the substituted pyrrolyl ligands in both compounds 5 and 6 show fluxionality as observed by 1 H NMR signals. A kinetic study on the ring-opening polymerization of lactide exhibits a first-order reaction of lactide monomer with compound 8. The catalytic properties of all the metal complexes have been studied for the ring-opening polymerization of e-caprolactone.