Pyrazolate-Based Dinuclear alpha-Diimine-Type Palladium(II)and Nickel(II)Complexes-a Bimetallic Approach in Olefin Polymerisation

摘要

A series of compartmental pyrazole/imine dinucleating ligands HL~1-HL~4 have been prepared with different backbone substitutents and different steric bulk of the appending arylimine groups,and two of them have been structurally characterised.With PdCl_2 all ligands form the anticipated pyrazo-late-bridged bimetallic complexes LPd_2Cl_3(1a-4a)that feature square-planar metal ions,as is confirmed by the X-ray crystallographic analyses of L~1Pd_2Cl_3(1a)and L~3Pd_2Cl_3(3a).With NiBr2(dme),however,oligonuclear systems [LNi_2Br_3]_x(1b-4b)are formed according to mass spectrometric findings,which is corroborated by the crystallographic structure of [L~3Ni_2Br_3]_3.Complexes 1a-4a can be described as bimetallic versions of Brookhart-type alpha-diimine palladium complexes,where dissociation into mononuclear species is prevented by the dinucleating scaffold and the proximate metal ions are suitably positioned to work in concert during substrate transformation.Upon activation of the complexes with MAO and exposure to ethylene,polyethylene is formed.Whereas the palladium complexes display moderate activities,nickel complexes are very active.From structure/activity correlations it is evident that the presence of backbone substitutents at the pyrazolate scaffold as well as bulky ortho aryl substituents is advantageous for polymerisation.Overall,activities of the Ni complexes and the microstructure of the polymer obtained(total branching,T_m and molecular weights)are still rather similar to the data reported previously for mononuclear cationic diimine nickel complexes.