| Abstract |
The P, O-type phosphinophenol proligands (1 center dot H, 2-PPh2-4-Me-6-Me-C6H2OH; 2 center dot H, 2-PPh2-4-Me-6-Bu-t-C6H2OH) readily react with one equiv. of ZnEt2 to afford in high yields the corresponding Zn(II)ethyl dimers of the type [(kappa(2)-P, O) Zn-Et](2) (3 and 4) with two mu-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 [(kappa(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 8 via the transient formation of (kappa(2)-P, O)(2)Zn-2(mu-OBn)(mu-kappa(1):kappa(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 center dot H and 2 center dot 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), epsilon-caprolactone (epsilon-CL) and trimethylene carbonate (TMC). Species 5 is a well-behaved ROP initiator for the homo-, co- and terpolymerization 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. |
