Synthesis and study of novel zwitterionic transition metal complexes and their application as olefin polymerisation catalysts

Abstract

The synthesis, characterization and coordination chemistry of novel zwitterionic late transition metal complexes has been carried out, and an investigation of their ability to act as olefin polymerisation catalysts has been conducted. These systems are based on 6- aminofulvene-2-aldiminate ligands (R2AFA¯) which are capable of binding metal centres via two nitrogen donors, delocalising the negative charge into their cyclopentadienyl moiety, thus resulting in the formation of neutral zwitterionic complexes. Preparation and characterisation of mono- and di-substituted complexes such as Ph2AFACuPPh3, (Ph2AFA)2Zn, (Ph2AFA)2Co, (Ph2AFA)2Ni and (Ph2AFA)2Pd have revealed that this type of ligand has enough flexibility to distort upon coordination to the metal depending on geometrical or steric restrictions. As a result, when the ligand coordination involves narrow binding angle of the metal chelate, as it happens in the square-planar species, a severe loss of planarity of the ligand framework is observed, in contrast with the tetrahedral structures where such binding angles are wider. Although the coordination of the ligand primarily occurs through the nitrogen donors, once they have been occupied by the metal centre, it is possible to exploit the aromatic Cp ring for coordination to a Cp*Ru+ unit. In this way, the synthesis and characterisation of two- and tri-metallic complexes [(Cp*Ru)(Ph’2AFA)Pd(η3-C3H5)][BF4] (where Ph’= 2,4,6-trimethylphenyl) and [(Cp*Ru)2(Ph2AFA)2Pd][BF4]2 has been achieved, featuring the R2AFA¯ molecule acting as an ambidentate ligand, binding the palladium atom in a diimine fashion and the ruthenium centres by means of the C5 ring. The synthesis of the complex Cp*RuPh2AFA where the two N atoms are vacant was also achieved and it was found that this compound acts as a proton sponge in the presence of protic solvents. In an effort to prepare AFA-metal complexes which could be of potential use in olefin polymerisation catalysis, two novel species, [(Ph2AFA)Pd(Me)PPh3] and [(Ph’2AFA)Pd(C3H5)] (where Ph’= 2,4,6-trimethylphenyl) have been synthesised and characterised, and polymerisation tests with ethylene have been carried out. Some preliminary screening of other molecules as monomers for polymerisation catalysis has also been conducted.