Cationic Scandium Methyl Complexes Supported by a β-Diketiminato (“Nacnac”) Ligand Framework

The base-free dimethyl scandium complex supported by the bulky beta-diketiminato ligand ArNC((t)Bu)CHC((t)Bu)NAr (Ar = 2,6-(i)Pr(2)C(6)H(3), 1) reacts with various equivalencies of the strong organometallic Lewis acid B(C(6)F(5))(3) to give scandium alkyl cations. With 0.5 equiv, a monocationic mu-methyl dimer (2) was observed spectroscopically. Reaction with a further 0.5 equiv of borane gives the monomeric methyl cation 3, which was fully characterized, including via X-ray crystallography. This compound is fluxional on the NMR time scale via a "ligand flip" mechanism. Reaction with another equivalent of borane gives the unique dication 4, which exhibits a static structure on the NMR time scale. Dimethyl compound 1 is a highly active catalyst precursor for ethylene polymerization under borane or MAO-type activation. Activities for this group 3 metal based catalyst approach those observed for group 4 based metallocene systems.