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

The half-sandwich imido tantalum dichloride (eta(5)-C5Me4H)Ta(=(NBu)-Bu-t)Cl-2 is prepared directly from the reaction of the neutral constrained-geometry ligand Me2Si(C5Me4H)((BuNH)-Bu-t) and TaCl5 in refluxing toluene. The dichloride is cleanly converted to the corresponding imido tantalum dimethyl complex (eta(5)-C5Me4H)Ta(=(NBu)-Bu-t)Me-2 (2). Using the same synthetic strategy, the neutral bulky chelating diamine ligand ArNH(CH2)(3)NHAr (Ar = 2,6-(Pr2C6H3)-Pr-i) is reacted with TaCl5 in toluene under mild reflux conditions, affording the six-coordinate tantalum complex [ArN(CH2)(3)NHAr]TaCl4, which is conveniently converted to the corresponding non-metallocene tantalum trimethyl complex [ArN(CH2)(3)NAr]TaMe3 (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 TaCl5 followed by addition of an excess of MeMgBr. Reaction of Cp2TaMe3 with 1 equiv of B(C6F5)(3) in benzene or toluene produces an oily mixture containing both Cp2TaMe2+CH3B(C6F5)(3)(-) and Cp2TaMe2+[(C6F5)(3)B-CH3-B(C6F5)(3)](-) in the solution phase. Subsequently, the reaction of Cp2TaMe3 with 2 equiv of Al(C6F5)(3) cleanly generates the tantalocene cation-binuclear anion ion pair Cp2TaMe2+[(C6F5)(3)Al-CH3-Al(C6F5)(3)]- (5) as colorless crystals. In bromobenzene-d(5), reactions of Cp2TaMe3 with Lewis acids M(C6F5)(3) (M = B, Al) in a 1:1 ratio afford the expected cationic species Cp2TaMe2+CH3M(C6F5)(3)(-) as clean products. Reaction of the non-metallocene 4 with M(C6F5)(3) is similar to that of Cp2TaMe3. When it is activated with 2 equiv of Al(C6F5)(3), Cp2TaMe3 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 x 10(6) g of polymer/((mol of metal) atm h)) for high-temperature (140 degreesC) olefin copolymerizations, producing low-density poly(ethylene-co-1-octene) copolymers with high molecular weight.