Amidinate bisborohydride complexes of rare-earth metals [6-Me-C5H3N-2-CH2C(NPri)2]Ln(BH4)2THF2 (Ln = Y, Nd): synthesis, structure, and catalytic activity in isoprene polymerization

Abstract

Reactions of equimolar amounts of RN=C=NR (R = Prⁱ, Cy) and 6-Me-C₅H₃N-2-CH₂Li prepared in situ by metallation of 2,6-dimethylpyridine with n-butyllithium afforded corresponding lithium amidinates [Li{6-Me-C₅H₃N-2-CH₂C(NPrⁱ)₂}]•1/3THF (1) and [Li{6-MeC₅H₃N-2-CH₂C(NCy)₂}]₄ (2) containing new tridentate amidinate ligands. The salt metathesis reactions of Ln(BH₄)₃(THF)₃ (Ln = Y, Nd) with 1 (1: 1 molar ratio, THF) result in neutral amidinate bisborohydride complexes [Ln{6-Me-C₅H₃N-2-CH₂C(NPrⁱ)₂}(BH₄)₂(THF)₂} (Ln = Y (3), Nd (4)). According to X-ray data, both compounds are monomeric, terminal borohydride ligands being coordinated to the rare earth metal atom in η³-fashion. Nitrogen atoms of the pyridine fragments of amidinate ligands are not involved in complexation with metal cations. Complexes 3 and 4 in combination with [Ph₃C][B(C₆F₅)₄] and AlBu₃ⁱ (1: 1: 10 molar ratio) exhibit catalytic activity in isoprene polymerization.