Half-sandwich bis(tetramethylaluminate) complexes of the rare-earth metals: synthesis, structural chemistry, and performance in isoprene polymerization

The protonolysis reaction of Ln(AlMe(4))(3)] with various substituted cyclopentadienyl derivatives HCp(R) gives access to a series of half-sandwich complexes Ln(AlMe(4))(2)(Cp(R))]. Whereas bis(tetramethylaluminate) complexes with 1,3-(Me(3)Si)(2)C(5)H(3)] and C(5)Me(4)SiMe(3)] ancillary ligands form easily at ambient temperature for the entire Ln(III) cation size range (Ln=Lu, Y, Sm, Nd, La), exchange with the less reactive 1,2,4-(Me(3)C)(3)C(5)H(3)] was only obtained at elevated temperatures and for the larger metal centers Sm, Nd, and La. X-ray structure analyses of seven representative complexes of the type Ln(AlMe(4))(2)(Cp(R))] reveal a similar distinct AlMe(4)] coordination (one eta(2), one bent eta(2)). Treatment with Me(2)AlCl leads to AlMe(4)] --> Cl] exchange and, depending on the Al/Ln ratio and the Cp(R) ligand, varying amounts of partially and fully exchanged products {Ln(AlMe(4))(mu-Cl)(Cp(R))}(2)] and {Ln(mu-Cl)(2)(Cp(R))}(n)], respectively, have been identified. Complexes {Y(AlMe(4))(mu-Cl)(C(5)Me(4)SiMe(3))}(2)] and {Nd(AlMe(4))(mu-Cl){1,2,4-(Me(3)C)(3)C(5)H(2)}}(2)] have been characterized by X-ray structure analysis. All of the chlorinated half-sandwich complexes are inactive in isoprene polymerization. However, activation of the complexes Ln(AlMe(4))(2)(Cp(R))] with boron-containing cocatalysts, such as Ph(3)C]B(C(6)F(5))(4)], PhNMe(2)H]B(C(6)F(5))(4)], or B(C(6)F(5))(3), produces initiators for the fabrication of trans-1,4-polyisoprene. The choice of rare-earth metal cation size, Cp(R) ancillary ligand, and type of boron cocatalyst crucially affects the polymerization performance, including activity, catalyst efficiency, living character, and polymer stereoregularity. The highest stereoselectivities were observed for the precatalyst/cocatalyst systems La(AlMe(4))(2)(C(5)Me(4)SiMe(3))]/B(C(6)F(5))(3) (trans-1,4 content: 95.6 %, M(w)/M(n)=1.26) and La(AlMe(4))(2)(C(5)Me(5))]/B(C(6)F(5))(3) (trans-1,4 content: 99.5 %, M(w)/M(n)=1.18).