Group 4 alkoxide complexes containing [NNO]-type scaffold: synthesis, structural characterization and polymerization studies

Abstract

A series of Ti(IV), Zr(IV) and Hf(IV) complexes are synthesized by complexation of [NNO]-type tridentate Schiff base ligands {2-amino-3-((E)-(2-hydroxybenzylidene)amino)maleonitrile}, L1(H)₂; {2-amino-3-((E)-(2-hydroxy-3,5-dimethylbenzyl-idene)amino)maleonitrile}, L2(H)₂ and {2-amino-3-((E)-(3-(tert-butyl)-2-hydroxy-5-methylbenzylidene)amino)maleonitrile}, L3(H)₂ with suitable group 4 metal alkoxides. The ligands are derived by the mono-condensation of diaminomaleonitrile and salicylaldehyde derivatives. All the complexes are found to be dinuclear on the basis of their NMR and MS spectral data. Three of the nine complexes are structurally characterized by single crystal X-ray diffraction studies. The crystal structures confirmed the dinuclear nature of these complexes. The metal centers of the dinuclear core are connected through two bridging alkoxy groups. The coordination environment of a metal center comprises one unit of a di-deprotonated ligand, one unit of a terminal alkoxide and two units of the bridging alkoxide groups, so as to form distorted octahedral complexes. The catalytic activities of these complexes were investigated towards the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) and rac-lactide (rac-LA) in bulk. These complexes were found to be highly active catalysts for the production of PCL [poly(caprolactone)] and PLA [poly(lactic acid)] with a high rate of conversion. The strong electron withdrawing cyano groups present in these complexes rationalized the high polymerization activity. The polymers exhibited narrow molecular weight distributions (MWDs). Furthermore, the isolated PLAs were remarkably heterotactic-rich with maximum P_r = 0.80, by using an Hf catalyst with tert-butyl substituent on the ortho-position of the phenolic moiety. These complexes are moderately active towards the ring-opening homopolymerization of rac-epoxides, such as rac-cyclohexene oxide (rac-CHO), rac-propylene oxide (rac-PO) and rac-styrene oxide (rac-SO). These complexes exhibited good activity in ethylene polymerization upon activation with co-catalyst MAO and with a bulky substituent on the ortho-position of the phenolic moiety of the catalysts.