Tantalum(V)-Based Metallocene, Half-Metallocene, and Non-Metallocene Complexes as Ethylene−1-Octene Copolymerization and Methyl Methacrylate Polymerization Catalysts

Abstract

The half-sandwich imido tantalum dichloride (η 5 -C 5 Me 4 H)Ta( N t Bu)Cl 2 is prepared directly from the reaction of the neutral constrained-geometry ligand Me 2 Si(C 5 Me 4 H)( t BuNH) and TaCl 5 in refluxing toluene. The dichloride is cleanly converted to the corresponding imido tantalum dimethyl complex (η 5 -C 5 Me 4 H)Ta( N t Bu)Me 2 ( 2 ). Using the same synthetic strategy, the neutral bulky chelating diamine ligand ArNH(CH 2 ) 3 NHAr (Ar = 2,6- i Pr 2 C 6 H 3 ) is reacted with TaCl 5 in toluene under mild reflux conditions, affording the six-coordinate tantalum complex [ArN(CH 2 ) 3 NHAr]TaCl 4, which is conveniently converted to the corresponding non-metallocene tantalum trimethyl complex [ArN(CH 2 ) 3 NAr]TaMe 3 ( 4 ) on treatment with an excess of MeMgBr. Alternatively, 4 can be prepared in better than 70% overall yield based on the diamine ligand from a one-pot synthesis by mixing the neutral diamine ligand with TaCl 5 followed by addition of an excess of MeMgBr. Reaction of Cp 2 TaMe 3 with 1 equiv of B(C 6 F 5 ) 3 in benzene or toluene produces an oily mixture containing both Cp 2 TaMe 2 + CH 3 B(C 6 F 5 ) 3 - and Cp 2 TaMe 2 + [(C 6 F 5 ) 3 B−CH 3 −B(C 6 F 5 ) 3 ] - in the solution phase. Subsequently, the reaction of Cp 2 TaMe 3 with 2 equiv of Al(C 6 F 5 ) 3 cleanly generates the tantalocene cation−binuclear anion ion pair Cp 2 TaMe 2 + [(C 6 F 5 ) 3 Al−CH 3 −Al(C 6 F 5 ) 3 ] - ( 5 ) as colorless crystals. In bromobenzene- d 5, reactions of Cp 2 TaMe 3 with Lewis acids M(C 6 F 5 ) 3 (M = B, Al) in a 1:1 ratio afford the expected cationic species Cp 2 TaMe 2 + CH 3 M(C 6 F 5 ) 3 - as clean products. Reaction of the non-metallocene 4 with M(C 6 F 5 ) 3 is similar to that of Cp 2 TaMe 3 . When it is activated with 2 equiv of Al(C 6 F 5 ) 3, Cp 2 TaMe 3 is active for syndiospecific MMA polymerization but inactive for olefin polymerization. In contrast, the non-metallocene 4 has no activity for MMA polymerization but is active for olefin polymerization. Most interestingly, the half-metallocene 2, when activated with a suitable activator, is very active (with a catalytic efficiency of 1.2 × 10 6 g of polymer/((mol of metal) atm h)) for high-temperature (140 °C) olefin copolymerizations, producing low-density poly(ethylene- co -1-octene) copolymers with high molecular weight.