C-H bond activation and isoprene polymerization by rare-earth-metal tetramethylaluminate complexes bearing formamidinato N-ancillary ligands

The bimetallic formamidinate complexes Ln(Form)(AlMe₄)₂ (Ln = Y, Form (ArNCHNAr) = EtForm (Ar = 2,6-Et₂C₆H₃), MesForm (Ar = 2,4,6-Me₃C₆H₂), DippForm (Ar = 2,6-iPr₂C₆H₃), tBuForm (Ar = 2-tBuC₆H₄); Ln = La, Form = DippForm, tBuForm) were obtained in high yield by protonolysis reactions between formamidines (FormH) and homolepticrare-earth-metal tetramethylaluminates Ln(AlMe₄)₃. Y(Form)(AlMe₄)₂ (Form = EtForm, DippForm) were also prepared by treatment of Y(Form)[N(SiHMe₂)₂]₂(thf) with trimethylaluminum after the former were prepared by the protonolysis of Y[N(SiHMe₂)₂]₃(thf)₂ complexes with EtFormH or DippFormH. The monomeric six-coordinate complexes Ln(Form)(AlMe₄)₂ (Ln = Y, Form = EtForm, MesForm, DippForm, tBuForm; Ln = La, Form = DippForm, tBuForm) show similar molecular structures with distorted-octahedral geometry and bidentate (N,N') Form and AlMe4 ligands. The complex [La(EtFormAlMe₃)(AlMe₄)₂](C₇H₈)₁.₅ from a protonolysis reaction between La(AlMe₄)₃ and EtFormH has the EtForm ligand adopting a configuration in which one nitrogen and one aryl substituent are coordinated to the eight-coordinate lanthanum center in an η¹(N):η⁶(arene) manner. From the reaction of La(AlMe₄)₃ with MesFormH, C–H bond activation of an o-methyl group of the mesityl moiety occurred, yielding [La{η¹(N):η⁶(Ar)-Me₂CH₂FormAlMe₃}(AlMe₃)(AlMe₄)][La(Me₂CH₂FormAlMe₃)(AlMe₃)(AlMe₄)](C₆H1₄)₁.₅(Me₂CH₂Form = MesForm-H(o-Me)), in which two linkage isomers of Me₂CH₂Form were observed. Investigations were carried out on the compounds [Ln(Form)(AlMe₄)₂] (Ln = Y, La; Form = EtForm, DippForm) as precatalysts activated by [Ph₃C][B(C₆F5)₄] or [PhNMe₂H][B(C₆F5)₄] in isoprene polymerization. While the lanthanum complexes showed narrower molecular weight distributions (PDI < 1.2), a stereodirecting role was evidenced for the cocatalysts (trityl borate, maximum 87% trans-1,4-selectivity; anilinium borate, maximum 82% cis-1,4-selectivity)