Preparation of an Active Neodymium Catalyst for Regioselective Butadiene cis-Polymerization Supported by a Dianionic Modification of the 2,6-Diiminopyridine Ligand

Treatment of the 2,6-diiminopyridine ligand 2,6-{[2,6-(i-Pr)2C6H3]NC(CH3)}2(C5H3N) with 2 equiv of Me3SiCH2Li afforded the corresponding {[2,6-{[2,6-(i-Pr)2C6H3]N-C(CH2)}2(C5H3N)]2- (1) dianion via deprotonation of the two methyl groups attached to the two imine functions. Reaction of 1 with NdCl3(THF)3 yielded {[2,6-{[2,6-(i-Pr)2C6H3]N-C(CH2)}2(C5H3N)]Nd(THF)}(μ-Cl)2[Li(THF)2]·0.5(hexane) (2a), whose recrystallization from DME gave the corresponding ionic {[2,6-{[2,6-(i-Pr)2C6H3]N-C(CH2)}2(C5H3N)]NdCl2(DME)}{Li(DME)3](2b). With the exception of the reaction with (allyl)MgBr, which proceeded readily with 2b to form the allyl derivative {[2,6-{[2,6-(i-Pr)2C6H3]N-C(CH2)}2(C5H3N)]Nd(η3-C3H5)Br}{Li(DME)3} (3), compounds 2a,b are not suitable starting substrates for further alkylation reactions. A viable synthetic strategy for the preparation of alkyl derivatives of 2 consisted instead of the pretreatment of NdCl3(THF)3 with RLi [R = Me3SiCH2, CH3] at low T followed by treatment with either the diimine ligand or 1. According to this procedure, the terminally bound alkyl derivative [2,6-{[2,6-(i-Pr)2C6H3]N-C(CH2)}2(C5H3N)]Nd[CH2Si (CH3)3](THF) (4) was prepared and subsequently crystallographically characterized. The same synthetic procedure with MeLi afforded {[2,6-{[2,6-(i-Pr)2C6H3]N-C(CH2)}2(C5H3N)]Nd}(μ-CH3)2[Li(THF)2] (5) and {[2,6-{[2,6-(i-Pr)2C6H3]N-C(CH2)}2(C5H3N)]Nd}(μ-Cl)(μ-X)[Li (THF)2] (6) [X = Cl 53%, Me 47%] depending on the MeLi/Nd ratio. Both 5 and 6 as well as 2a are potent catalysts for the cis-polymerization of butadiene.