Heteroleptic Tethered NHC Rare Earth Initiators for the Ring Opening Polymerisation of rac-Lactide

Abstract

The importance of catalyst design is shown by the contrasting reactivity of tethered NHC and Tp ligands with rare-earth metal centres highlighting that successful catalyst design is not as simple as linearly combining moieties with known catalytic activity. The heteroleptic fluorenyl-tethered-NHC rare-earth complexes LnLX₂ [Ln = Y, La, Ce, Nd; L = (η⁵-C₁₃H₈)C₂H₄N(κ-C)C₂H₂NMe; X = N″ or Br] were synthesised from [Ln{N(SiMe₃)₂}₃] (LnNʹʹ₃), with X selected by varying the precursor. Direct addition of LnN″₃ to the fluorene-tethered imidazolium salt LH₂Br proceeded with loss of two equivalents of HN(SiMe₃)₂ forming [LnL(N″)Br] complexes; alternatively, initial deprotonation of LH₂Br with ⁿBuLi or KCH₂Ph prior to addition of LnNʹʹ₃ yielded [LnL(Nʹʹ)₂] complexes. [LnL(Nʹʹ)Br] [Ln = Ce (3), Nd (4)] are dimeric through bridging Br ligands whereas [LnL(Nʹʹ)₂] [Ln = La (5), Ce (6), Nd (7)] are monomeric. These complexes were investigated for their activity and selectivity in the ring opening polymerization (ROP) of rac-lactide using benzyl alcohol as a co-initiator, showing high activity with full conversion typically being obtained in under 15 minutes at room temperature. Comparisons with the yttrium amide complex [Y(Tp)₂(Nʹʹ)] (8, Tp = hydrotris(1-pyrazolyl)borate) demonstrated the important role of the fluorenyl-tethered NHC ligand in generating an active catalyst, illustrating that simply having a rare-earth metal centre with a bis(trimethylsilyl)amide group is not enough to generate an effective (pre)catalyst. Finally, we hope to raise awareness that a trace impurity of lactic acid, sometimes present in commercial rac-lactide, greatly inhibited ROP and was not easily removed by sublimation or re-crystallisation.