Reactivity of Bridged and Nonbridged Zirconocenes toward Biorenewable Itaconic Esters and Anhydride

This work investigates the reactivity of neutral and cationic complexes of both bridged ansa-zirconocenes, rac-[C₂H₄(Ind)₂]­ZrMe­[OC­(OⁱPr)CMe₂] (1) and rac-[C₂H₄(Ind)₂]­Zr⁺(THF)­[OC­(OⁱPr)CMe₂]­[MeB­(C₆F₅)₃]⁻ (1⁺), and nonbridged zirconocenes, Cp*­(ⁿPrCp)­ZrMe­[OC­(OⁱPr)CMe₂] (13) and Cp*­(ⁿPrCp)­Zr­(THF)­[OC­(OⁱPr)CMe₂]⁺[MeB­(C₆F₅)₃]⁻ (13⁺), toward biorenewable itaconic dialkyl esters (itaconates) and anhydride (IA). Behaving similarly, both cationic complexes 1⁺ and 13⁺ react readily with itaconates to form cleanly single monomer addition products, eight-membered-ring metallacycles 2 and 15, respectively, and neutral enolate complexes 1 and 13 insert 1 equiv of IA to afford single-IA-addition products 5 and 17. Behaving differently, eight-membered-ring chelates 2 derived from the bridged metallocene framework undergo slow isomerization at room temperature via ligand exchange between the coordinated and uncoordinated ester groups to form thermodynamically favored seven-membered-ring chelates 4, while eight-membered chelates 15 derived from the sterically more crowded unbridged metallocene framework are stable at room temperature and do not undergo such isomerization. The above cationic complexes exhibit no reactivity toward further additions of itaconates. Replacing itaconates with more basic monomers such as N,N-dimethylacrylamide that can ring open the chelating resting intermediate, however, brings about effective and controlled polymerization by eight-membered Zr-itaconate metallacycles 2 and 15, but not seven-membered 4, producing either highly isotactic polymers (>99% mm, by 2) or polymers with narrow molecular weight distributions (Đ < 1.19, by 15). These results further highlight the ansa effects in the metallocene polymerization chemistry and the importance of the formation and ring opening of the eight-membered chelating intermediates involved in the metallocene-mediated conjugate-addition polymerization.