Coordination of substitutionally inert phenolate ligands to lanthanide(II) and (III) compounds--catalysts for ring-opening polymerization of cyclic esters.

A new [ONO] tridentate phenolate ligand (H2LI) 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 (H2LI) or HOC6H2-(2,4-tBu)-6-CH2N(Me)CH2CH2N(Me)CH2-6-(2,4-tBu)-C6H2OH (H2LII) in hexanes to yield [Yb(LI)]2 () and [Yb(LII)]2 (), respectively. The third divalent ytterbium compound [Yb(L2)] () 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 was treated with silver triflate and phenol to form corresponding heteroleptic trivalent ytterbium phenolate complexes [Yb(LI)(O3SCF3)(THF)] () and [Yb(LI)(OPh)] (), 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 (H2LI) in THF {[La(LI)(N(SiMe3)2)(THF)2] (); [Sm(LI)(N(SiMe3)2)(THF)] (); [Yb(LI)(N(SiMe3)2)(THF)] ()}. These lanthanide(II) and (III) compounds were assessed as catalyst precursors towards the ring-opening polymerization of both l-lactide and epsilon-caprolactone. End-group analyses and detailed kinetics studies {rate law: -d[LA]/dt=k[LA]1[catalyst]1} of the most efficacious lanthanum compound () further corroborated the substitutionally inert characteristics of the new stationary ancillary [ONO] tridentate dianionic ligand.

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

Five new heteroleptic lanthanide(III) phenolate compounds have been synthesised in high yield, four via a transamination reaction between Ln(N(SiMe(3))2)3 and two equivalents of the phenol, HOC(6)H(2)(2,4-Bu(t))-6-CH(2)N(Me)CH(2)CH(2)NMe(2) [corrected] (LH) in thf {L(2)LnN(SiMe(3))2 where Ln = La (1); Nd (2); Sm (3); Yb (4)}. The fifth compound, [L(2)La][BPh(4)] 5 was formed by conversion of 1 by treatment with one equivalent of [Et(3)NH][BPh(4)] 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 C(4)D(8)O solution which was temperature dependent. All five compounds were assessed as catalyst precursors towards the ring-opening polymerisation of both L-lactide and epsilon-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(SiMe(3))2] initiating group in 1 with [BPh(4)] 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.