Coordination of substitutionally inert phenolate ligands to lanthanide(ii) and (iii) compounds—catalysts for ring-opening polymerization of cyclic esters

Abstract

A new [ONO] tridentate phenolate ligand (H₂L^I) containing an aliphatic alcohol as a side arm has been synthesized, deprotonated and attached to lanthanide(II) and (III) ions, which are employed as catalysts for ring-opening polymerization of cyclic esters. In contrast to many other mono-phenolate lanthanide compounds, these have been found to be inert to polymer incorporation during the polymerization reactions. Three new divalent ytterbium compounds have been synthesized in high yield containing ancillary ligands; two via a transamination reaction between [Yb(N(SiMe₃)₂)₂(THF)₂] and one equivalent of the phenols, HOC₆H₂-(2,4-^tBu)-6-CH₂N(Me)CH₂CH₂OH (H₂L^I) or HOC₆H₂-(2,4-^tBu)-6-CH₂N(Me)CH₂CH₂N(Me)CH₂-6-(2,4-^tBu)-C₆H₂OH (H₂L^II) in hexanes to yield [Yb(L^I)]₂ (1) and [Yb(L^II)]₂ (2), respectively. The third divalent ytterbium compound [Yb(L₂)] (3) was prepared by treatment of [Yb(N(SiMe₃)₂)₂(THF)₂] with two equivalents of a related monoanionic ancillary phenol, HOC₆H₂-(2,4-^tBu)-6-CH₂N(Me)CH₂CH₂NMe₂ (HL) in hexanes. Additionally, the oxidation chemistry of these divalent systems was explored where compound 1 was treated with silver triflate and phenol to form corresponding heteroleptic trivalent ytterbium phenolate complexes [Yb(L^I)(O₃SCF₃)(THF)] (4) and [Yb(L^I)(OPh)] (5), respectively. Finally, three new heteroleptic trivalent lanthanide silylamido compounds were synthesized via a ligand exchange transamination reaction between the homoleptic trivalent [Ln(N(SiMe₃)₂)₃] compound and one equivalent of the new dianionic ligand (H₂L^I) in THF {[La(L^I)(N(SiMe₃)₂)(THF)₂] (6); [Sm(L^I)(N(SiMe₃)₂)(THF)] (7); [Yb(L^I)(N(SiMe₃)₂)(THF)] (8)}. These lanthanide(II) and (III) compounds were assessed as catalyst precursors towards the ring-opening polymerization of both L-lactide and ε-caprolactone. End-group analyses and detailed kinetics studies {rate law: −d[LA]/dt = k[LA]¹[catalyst]¹} of the most efficacious lanthanum compound (6) further corroborated the substitutionally inert characteristics of the new stationary ancillary [ONO] tridentate dianionic ligand.