Abstract
The P,O-type phosphinophenol proligands (1·H, 2-PPh2-4-Me-6-Me-C6H2OH; 2·H, 2-PPh2-4-Me-6-(t)Bu-C6H2OH) readily react with one equiv. of ZnEt2 to afford in high yields the corresponding Zn(II)-ethyl dimers of the type [(κ(2)-P,O)Zn-Et]2 (3 and 4) with two μ-OPh 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 [(κ(2)-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 (κ(2)-P,O)2Zn2(μ-OBn)(μ–κ(1):κ(1)-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.
Highlights
Aliphatic polyesters and polycarbonates such as polylactic acid (PLA), poly(ε-caprolactone) (PCL) and poly(trimethylene carbonate) (PTMC) are of current importance as biodegradable and bioassimilable materials and, as such, have already found a wide array of applications, spanning from their use in food packaging to biomedicine.1 The thermoplastic properties of PLA and PCL position such materials as possible biodegradable alternatives to petrochemically-sourced thermoplastic polyolefins
The phosphines 1·H and 28.9 (1·H) and −31.7 (2·H) were synthesized according to a literature procedure and their identity was confirmed by NMR analysis
In the case of 2·H, single crystals suitable for X-ray diffraction (XRD) studies could be grown from a saturated EtOAc–hexane mixture and subsequently analyzed through XRD, confirming its molecular structure (Fig. 1)
Summary
Aliphatic polyesters and polycarbonates such as polylactic acid (PLA), poly(ε-caprolactone) (PCL) and poly(trimethylene carbonate) (PTMC) are of current importance as biodegradable and bioassimilable materials and, as such, have already found a wide array of applications, spanning from their use in food packaging to biomedicine (as excipients in controlled drug delivery and bone tissue prosthesis components, for example).1 The thermoplastic properties of PLA and PCL position such materials as possible biodegradable alternatives to petrochemically-sourced thermoplastic polyolefins. The structural data for 2·H are similar to those of the other few XRD-characterized P,O phosphinophenol species.10c,12 complex 3, a dimeric [(P,O)-ZnEt]2 structure is proposed in solution on the basis of diffusion-ordered NMR spectroscopy (DOSY) analysis (CDCl3, room temp.), which yielded a hydrodynamic radius of 6.01 Å for 3 and an estimated molecular volume of 909 Å3 in solution (ESI – Fig. S9 and Table S2†). Complexes 5 and 6 were isolated as colorless solids and their formulation was deduced from elemental analysis and NMR data (ESI – Fig. S3 and S4†).
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