Synthesis and characterisation of lanthanide phenolate compounds and their catalytic activity towards ring-opening polymerisation of cyclic esters

Abstract

Five new heteroleptic lanthanide(III) phenolate compounds have been synthesised in high yield, four via a transamination reaction between Ln(N(SiMe₃)₂)₃ and two equivalents of the phenol, HOC₆H₄(2,4-Bu^t)CH₂N(Me)CH₂CH₂NMe₂ (LH) in thf {L₂LnN(SiMe₃)₂ where Ln = La (1); Nd (2); Sm (3); Yb (4)}. The fifth compound, [L₂La][BPh₄] 5 was formed by conversion of 1 by treatment with one equivalent of [Et₃NH][BPh₄] in toluene. Compound 3 was subjected to a single-crystal X-ray analysis and revealed a five-coordinate, distorted trigonal bipyramidal samarium(III) metal centre where each phenolate ligand is bidentate coordinating through the phenolate oxygen and nitrogen yielding six-membered chelate rings. Compound 1 exhibited fluxional behaviour in C₄D₈O solution which was temperature dependent. All five compounds were assessed as catalyst precursors towards the ring-opening polymerisation of both L-lactide and ε-caprolactone. These polymerisation studies revealed that catalysts containing larger lanthanide metals were more efficacious than those with smaller lanthanide metals. Furthermore, replacement of the [N(SiMe₃)₂] initiating group in 1 with [BPh₄] in 5 reduced catalytic activity by this compound. Detailed kinetics analysis of the ring-opening polymerisation of L-lactide by compound 1, the most efficacious catalyst precursor analysed in this study, revealed the following rate law: −d[LA]/dt = k[LA]²[1]¹ which is second order in lactide and first order in catalyst. End-group analysis by ESI mass spectrometry revealed the presence of phenolate end-groups and lactide cycles, the latter formed by intra-molecular, intrachain transesterification.