P,O-Phosphinophenolate zinc(ii) species: synthesis, structure and use in the ring-opening polymerization (ROP) of lactide, ε-caprolactone and trimethylene carbonate

Abstract

The P,O-type phosphinophenol proligands (1·H, 2-PPh₂-4-Me-6-Me-C₆H₂OH; 2·H, 2-PPh₂-4-Me-6-^tBu-C₆H₂OH) readily react with one equiv. of ZnEt₂ to afford in high yields the corresponding Zn(II)-ethyl dimers of the type [(κ²-P,O)Zn-Et]₂ (3 and 4) with two μ-O_Ph bridging oxygens connecting the two Zn(II) centers, as determined by X-ray diffraction (XRD) studies in the case of 3. Based on diffusion-ordered NMR spectroscopy (DOSY), both species 3 and 4 retain their dimeric structures in solution. The alcoholysis reaction of Zn(II) alkyls 3 and 4 with BnOH led to the high yield formation of the corresponding Zn(II) benzyloxide species [(κ²-P,O)Zn-OBn]₂ (5 and 6), isolated in a pure form as colorless solids. The centrosymmetric and dimeric nature of Zn(II) alkoxides 5 and 6 in solution was deduced from DOSY NMR experiments and multinuclear NMR data. Though the heteroleptic species 5 is stable in solution, its analogue 6 is instable in CH₂Cl₂ solution at room temperature to slowly decompose to the corresponding homoleptic species 8via the transient formation of (κ²-P,O)₂Zn₂(μ-OBn)(μ–κ¹:κ¹-P,O) (6′). Crystallization of compound 6 led to crystals of 6′, as established by XRD analysis. The reaction of ZnEt₂ with two equiv. of 1·H and 2·H allowed access to the corresponding homoleptic species of the type [Zn(P,O)₂] (7 and 8). All gathered data are consistent with compound 7 being a dinuclear species in the solid state and in solution. Data for species 8, which bears a sterically demanding P,O-ligand, are consistent with a mononuclear species in solution. The Zn(II) alkoxide species 5 and the [Zn(P,O)₂]-type compounds 7 and 8 were evaluated as initiators of the ring-opening polymerization (ROP) of lactide (LA), ε-caprolactone (ε-CL) and trimethylene carbonate (TMC). Species 5 is a well-behaved ROP initiator for the homo-, co- and ter-polymerization of all three monomers with the production of narrow disperse materials under living and immortal conditions. Though species 7 and 8 are ROP inactive on their own, they readily polymerize LA in the presence of a nucleophile such as BnOH to produce narrow disperse PLA, presumably via an activated-monomer ROP mechanism.