Coordination chemistry and reactivity of zinc complexes supported by a phosphido pincer ligand

摘要

The preparation and characterization of new Zn ~(II) complexes of the type (PPP)ZnR in which R=Et (1) or N(SiMe _3) _2 (2) and PPP is a tridentate monoanionic phosphido ligand (PPP-H=bis(2- diphenylphosphinophenyl)phosphine) are reported. Reaction of ZnEt _2 and ZnN(SiMe _3) _2 _2 with one equivalent of proligand PPP-H produced the corresponding tetrahedral zinc ethyl (1) and zinc amido (2) complexes in high yield. Homoleptic (PPP) _2Zn complex 3 was obtained by reaction of the precursors with two equivalents of the proligand. Structural characterization of 1-3 was achieved by multinuclear NMR spectroscopy (~1H, ~(13)C, and ~(31)P) and X-ray crystallography (3). Variable-temperature 1H and 31P NMR studies highlighted marked flexibility of the phosphido pincer ligand in coordination at the metal center. A DFT calculation on the compounds provided theoretical support for this behavior. The activities of 1 and 2 toward the ring-opening polymerization of ε-caprolactone and of L- and rac-lactide were investigated, also in combination with an alcohol as external chain-transfer agent. Polyesters with controlled molecular parameters (M _n, end groups) and low polydispersities were obtained. A DFT study on ring-opening polymerization promoted by these complexes highlighted the importance of the coordinative flexibility of the ancillary ligand to promote monomer coordination at the reactive zinc center. Preliminary investigations showed the ability of these complexes to promote copolymerization of L-lactide and ε-caprolactone to achieve random copolymers whose microstructure reproduces the composition of the monomer feed.