Neodymium borohydride complexes supported by diamino-bis(phenoxide) ligands: diversity of synthetic and structural chemistry, and catalytic activity in ring-opening polymerization of cyclic esters

Abstract

New heterobimetallic borohydrido neodymium complexes {[OONN]¹Nd(BH₄)(μ-BH₄)Li(THF)}₂ (1) and [OONN]³Nd(BH₄)(μ-BH₄)Li(THF)₂ (3) supported by diamino-bis(phenoxide) ligands ([OONN]¹ = {CH₂N(Me)CH₂-3,5-Me,t-Bu-C₆H₂O}₂; [OONN]³ = C₅H₄NCH₂N{CH₂-3,5-Me,t-Bu-C₆H₂O}₂) were synthesized by the reactions of Nd(BH₄)₃(THF)₂ with equimolar amounts of dilithium derivatives of diamino-bis(phenol)s Li₂[OONN]ⁿ and isolated in high yields. In the case of Li₂[OONN]² ([OONN]² = Me₂NCH₂CH₂N{CH₂-3,5-t-Bu-C₆H₂O}₂), the same synthetic procedure afforded the heterobimetallic bis(phenoxide) complex Li{Nd[OONN²]₂} (2). The structures of complexes 1–3 were established by X-ray diffraction studies. Compounds 1–3 act as single-site initiators for the ring-opening polymerization (ROP) of racemic lactide and racemic β-butyrolactone under mild conditions (20 °C), providing atactic polymers with controlled molecular weights and relatively narrow polydispersities (M_w/M_n = 1.07–1.82). While 1 and 3 initiate polymerizationvia their borohydride groups, ROP with 2 proceeds viainsertion into the Nd–O(ligand) bond.