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

摘要

The bimetallic formamidinate complexes Ln(Form)(AlMe_4) _2 (Ln = Y, Form (ArNCHNAr) = EtForm (Ar = 2,6-Et_2C _6H_3), MesForm (Ar = 2,4,6-Me_3C 6H2), DippForm (Ar = 2,6-iPr_2C _6H_3), tBuForm (Ar = 2-tBuC_6H_4); Ln = La, Form = DippForm, tBuForm) were obtained in high yield by protonolysis reactions between formamidines (FormH) and homolepticrare-earth-metal tetramethylaluminates Ln(AlMe_4)_3. Y(Form)(AlMe _4)_2 (Form = EtForm, DippForm) were also prepared by treatment of Y(Form)[N(SiHMe_2)_2]_2(thf) with trimethylaluminum after the former were prepared by the protonolysis of Y[N(SiHMe_2)_2]_3(thf)_2 complexes with EtFormH or DippFormH. The monomeric six-coordinate complexes Ln(Form)(AlMe _4)_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 AlMe _4 ligands. The complex [La(EtFormAlMe_3)(AlMe _4)_2](C_7H_8)1.5 from a protonolysis reaction between La(AlMe_4)_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 η~1(N):η~6(arene) manner. From the reaction of La(AlMe_4)_3 with MesFormH, C-H bond activation of an o-methyl group of the mesityl moiety occurred, yielding [La{η~1(N):η~6(Ar)-Me_2CH_2FormAlMe_3} (AlMe_3)(AlMe_4)][La(Me_2CH _2FormAlMe_3)(AlMe_3)(AlMe_4)](C _6H_(14))_(1.5) (Me_2CH_2Form = MesForm-H(o-Me)), in which two linkage isomers of Me_2CH _2Form were observed. Investigations were carried out on the compounds [Ln(Form)(AlMe_4)_2] (Ln = Y, La; Form = EtForm, DippForm) as precatalysts activated by [Ph_3C][B(C_6F _5)_4] or [PhNMe_2H][B(C_6F _5)_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).