Mono- and Trinuclear Phosphine-Phenolate Nickel(II) Complexes as Precatalysts in Ethylene/Acrylate Copolymerization

Abstract

Five new κ2-[phosphine-(di)phenolate]Ni(II)Me complexes with either mono- or trinuclear structures were synthesized, characterized, and utilized in the catalytic (co)polymerization of ethylene. These complexes were accessed from the complexation of (TMEDA)NiMe₂ (N,N,N′,N′-tetramethylethylenediamine nickel(II) dimethyl) with either a BINOL (1,1′-bi-2-naphthol)-based phosphine diphenol (P,O) ligand or an analogous ligand which features an ethyl ether in place of the nonortho phenol. Complexes which featured this ether differed by the labile monodentate ligand, pyridine, or triethylphosphine. The phosphine diphenol ligand yielded a mononuclear Ni(II)Me species with either triethylphosphine or pyridine as labile ligands or a labile ligand free trinuclear nickel complex. The mononuclear phenol containing complex was characterized by single-crystal X-ray diffraction (SC-XRD) and revealed an intramolecular hydrogen-bonding interaction in the solid state involving the coordinating phenolate and the phenol. The trinuclear complex was also characterized by SC-XRD and showcased the presence of two terminal κ2-[phosphine (di)phenolate]Ni(II)Me units and a central hexacoordinate nickel(II) center with two axial pyridine ligands. All complexes were active for ethylene homopolymerization (featuring activity up to 81.3 × 10⁵ g polymer × mol Ni^–1 × hr^–1 and M_n up to 4.2 kg/mol) and ethylene/methyl acrylate (MA) copolymerization (MA_mol % up to 8.3% at [MA] = 0.2 M).