Half-Sandwich Bis(tetramethylaluminate) Complexes of the Rare-Earth Metals: Synthesis, Structural Chemistry, and Performance in Isoprene Polymerization

The protonolysis reaction of [Ln(AlMe4)3] with various substituted cyclopentadienyl derivatives HCpR gives access to a series of half‐sandwich complexes [Ln(AlMe4)2(CpR)]. Whereas bis(tetramethylaluminate) complexes with [1,3‐(Me3Si)2C5H3] and [C5Me4SiMe3] ancillary ligands form easily at ambient temperature for the entire LnIII cation size range (Ln=Lu, Y, Sm, Nd, La), exchange with the less reactive [1,2,4‐(Me3C)3C5H3] 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(AlMe4)2(CpR)] reveal a similar distinct [AlMe4] coordination (one η2, one bent η2). Treatment with Me2AlCl leads to [AlMe4] → [Cl] exchange and, depending on the Al/Ln ratio and the CpR ligand, varying amounts of partially and fully exchanged products [{Ln(AlMe4)(μ‐Cl)(CpR)}2] and [{Ln(μ‐Cl)2(CpR)}n], respectively, have been identified. Complexes [{Y(AlMe4)(μ‐Cl)(C5Me4SiMe3)}2] and [{Nd(AlMe4)(μ‐Cl){1,2,4‐(Me3C)3C5H2}}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(AlMe4)2(CpR)] with boron‐containing cocatalysts, such as [Ph3C][B(C6F5)4], [PhNMe2H][B(C6F5)4], or B(C6F5)3, produces initiators for the fabrication of trans‐1,4‐polyisoprene. The choice of rare‐earth metal cation size, CpR 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(AlMe4)2(C5Me4SiMe3)]/B(C6F5)3 (trans‐1,4 content: 95.6 %, Mw/Mn=1.26) and [La(AlMe4)2(C5Me5)]/B(C6F5)3 (trans‐1,4 content: 99.5 %, Mw/Mn=1.18). Postmetallocene library: Depending on the LnIII metal center, the substituents on the cyclopentadienyl ancillary ligand, and the type of cocatalyst, complexes of the type [Ln(AlMe4)2(CpR)] display distinct performances in the polymerization of isoprene. Precatalyst–cocatalyst interactions result in the formation of stable but inactive chloride‐exchange compounds and are responsible for the high activity of borate/borane‐containing catalyst mixtures (see scheme).