Distinctive Stereochemically Linked Cooperative Effects in Bimetallic Titanium Olefin Polymerization Catalysts

The complex (μ-Me2C-3,3′)­{(η5-cyclopentadienyl)­[1-Me2Si-(tBuN)]­(TiMe2)}2 (3) was prepared as a new binuclear catalyst motif for homogeneous olefin polymerization. Complex 3 exists as rac-3 and meso-3 diastereomers, which can be separated and characterized by solution NMR spectroscopy and single-crystal X-ray diffraction. While meso-3 has high thermal stability, rac-3 undergoes thermolysis in solution to quantitatively form the dimeric methylidene complex (μ-Me2C-3,3′)­{(η5-cyclopentadienyl)­[1-Me2Si­(tBuN)]­[(μ-CH2)­Ti]}2 (rac-4). Activation of rac-3 and meso-3 with 1 equiv of Ph3C+B­(C6F5)4– yields [(μ-CMe2-3,3′)­{(η5-cyclopentadienyl)­[1-Me2Si­(tBuN)]}2(μ-CH2)­(μ-CH3)­Ti2]+B­(C6F5)4– (5; rac-5 and meso-5, respectively). Interestingly, meso-5 is stable in the presence of an additional 1 equiv of Ph3C+B­(C6F5)4–, while rac-5 reacts to yield rac-[(μ-CMe2-3,3′)­{(η5-cyclopentadienyl)­[1-Me2Si­(tBuN)]}2(μ-CH2)­[(TiCH3)­(Ti-η1-Ph3C)]2+[B­(C6F5)4–]2 (rac-6) as indicated by multinuclear NMR spectroscopy and DFT computation. meso-3 reacts with 2 equiv of B­(C6F5)3 to yield meso-[(μ-CMe2-3,3′)­{(η5-cyclopentadienyl)­[1-Me2Si­(tBuN)]}2(μ-CH2)­(μ-CH3)­Ti2]+MeB­(C6F5)3– (meso-7) containing the same meso-5 cation but with a MeB­(C6F5)3–counteranion. These findings, along with catalytic results, indicate that rac-3 and meso-3 remain structurally intact during polymerization, consistent with the observed diastereoselectivity effects. Under identical ethylene/1-octene copolymerization conditions, only activated bimetallic rac-3 produces appreciable polymer, with meso-3 exhibiting low activity, but both yield polymer with a branch density >2× that of the monometallic control [(3-tBu-C5H3)­SiMe2NtBu]­TiMe2 (Ti1). In ethylene/styrene copolymerizations, rac-3 produces polymers with 3.1× higher Mn and 2.1× greater styrene incorporation versus Ti1, while meso-3 catalyzes only ethylene-free styrene homopolymerization. In 1-octene homopolymerizations, meso-3 + B­(C6F5)3 (i.e., meso-7) produces highly isotactic poly-1-octene (mmmm 91.7%), while rac-3 + Ph3C+B­(C6F5)4– (i.e., rac-5), rac-3 + B­(C6F5)3 (i.e., rac-7), and meso-3 + Ph3C+B­(C6F5)4– (i.e., meso-5) produce only atactic poly-1-octene. These bimetallic polymerization catalysts exhibit distinctive cooperative effects influencing product Mn, tacticity, and comonomer selection, demonstrating that binuclear catalyst stereochemical factors are significant.