Zinc complexes with guanidine-pyridine hybrid ligands - guanidine effect and catalytic activity

Two new series of guanidine-pyridine ligands have been synthesised by the reaction of various chloroformamidinium chlorides with 8-aminoquinoline or 2-picolylamine. The ligands were treated with either zinc chloride or zinc acetate to prepare the corresponding complexes, which were tested as initiators in the solvent-free ring-opening polymerisation of D, L-lactide. The effect of the guanidine ligand on the catalytic activity of the zinc complexes was investigated. A great number of the synthesised complexes showed appreciable polymerisation activity. 13 new zinc guanidine complexes have been structurally characterised, including the first examples of structures comprising a tetraethylguanidine unit. The experimental results were complemented by a density functional study, which has probed the influence of substituents at the guanidine unit. Especially for the tetraethylguanidine systems, insights into the strength of delocalisation within guanidine moieties are provided.