Synthesis of γ-amidine-functionalized dianionic β-diketiminato lanthanide amides and trianionic β-diketiminato Na/Sm heterobimetallic complexes and their reactivity in polymerization of l-lactide

Abstract

The readily accessible dianionic β-diketiminato lanthanide amido complexes LnLN(SiMe₃)₂(THF) (L = {(2,6-ⁱPr₂C₆H₃)NC(CH₂)CHC(CH₃)N(2,6-ⁱPr₂C₆H₃)}²⁻) show an unprecedented reactivity toward carbodiimides. The reaction with N,N′-dicyclohexylcarbodiimide (DCC) led via [4 + 2] cycloaddition to γ-amidine-functionalized dianionic β-diketiminato lanthanide amido complexes, LnL¹N(SiMe₃)₂ (L¹ = {[(NHC₆H₁₁)(NC₆H₁₁)C]HC[C(CH₂)N(2,6-ⁱPr₂C₆H₃)]₂}²⁻, Ln = Sm(1), Yb(2), Y(3), Gd(4)). Conversion of a mixture of SmLN(SiMe₃)₂(THF) and NaN(SiMe₃)₂ with carbodiimide furnished the heterobimetallic complexes of Sm/Na with a novel amidinate-functionalized trianionic β-diketiminate ligand, [Na(DME)₂](μ-L²)[SmN(SiMe₃)₂] (L² = {[C(NⁱPr)₂]HC[C(CH₂)N(2,6-ⁱPr₂C₆H₃)]₂}³⁻, DME = dimethoxyethane) (5) for N,N′-diisopropylcarbodiimide (DIC) and [Na(DME)₃]⁺[SmL³N(SiMe₃)₂]⁻ (L³ = {[C(NCy)₂]HC[C(CH₂)N(2,6-ⁱPr₂C₆H₃)]₂}³⁻) (6) for DCC. Molecular structures of complexes 1–6 were determined by an X-ray single crystal structure analysis. Complexes 1–4 were found to be highly active initiators of the ring-opening polymerization (ROP) of L-lactide (L-LA). The activity depended on the central metal with the increasing sequence of Yb < Y < Gd < Sm. Notably, the binary 1/BnOH (benzyl alcohol) system exhibited an “immortal” nature and proved able to convert 2000 equivalents of L-LA with up to 100 equivalents of BnOH per initiator. All the polylactides (PLAs) obtained showed monomodal, narrow molar mass distributions (M_w/M_n = 1.08–1.13) with the M_n (average number molar mass) decreasing with increasing amount of BnOH proportionally.