Craig A. Wheaton

Cationic zinc complexes: a new class of catalyst for living lactide polymerization at ambient temperature

Cationic zinc complexes of a bis(phosphinimine) pincer ligand have been prepared. Methylzinc and zinc-lactate complexes have been structurally characterized, and the latter is the first cationic metal complex to promote coordination-insertion polymerization of lactide at ambient temperature. This novel catalyst system is remarkably active and also exhibits living character. A detailed investigation of the kinetics and mechanism of the polymerization process has been undertaken.

Cationic organozinc complexes of a bis(phosphinimine) pincer ligand: synthesis, structural and polymerization studies

Cationic organozinc complexes of a neutral bis(phosphinimine) pincer ligand (L) have been prepared and structurally characterized. This recently introduced ligand was constructed from a dibenzofuran (dbf) framework with symmetric attachment of phosphinimine groups at the 4 and 6 positions. Starting from protonated derivatives [LH][B(C(6)F(5))(4)] (1a), [LH][BPh(4)] (1b), or [LH(2)][BPh(4)](2) (1c), the complexes [LZnCH(3)][B(C(6)F(5))(4)] (2a), [LZnCH(3)][BPh(4)] (2b), and [LZnOAc][BPh(4)] (3), were prepared via protonolysis of an appropriate alkylzinc precursor. The complex [LZnPh][BPh(4)] (4) is generated as a side-product in the synthesis of 2b. Solid-state structural studies have revealed the compounds to be charge separated cationic zinc species with 3-coordinate trigonal planar geometry. Preliminary studies have shown these complexes to be inactive for the polymerization of lactide. Upon modification of the initiating group to a methyl-(S)-lactate, however, complex [LZnOCH(Me)CO(2)Me][B(C(6)F(5))(4)] (5) demonstrated significant polymerization activity at 60 degrees C. Additionally, NMR and mass spectrometry data confirmed a coordination-insertion mechanism was operative for this catalyst.

Exploring the versatility of a bis(phosphinimine) pincer ligand: effect of sterics on structure and lactide polymerization activity of cationic zinc complexes

Cationic zinc complexes of a neutral pincer framework 4,6-(ArNPPh2)-dibenzofuran (L111: Ar = 2-iPrPh; L22: Ar = o-tolyl; L33: Ar = Ph), have been prepared and characterized. Crystallographic and NMR studies of the methylzinc complexes [LZnCH3+][BAr4−] (4a–6a: Ar = m-(CF3)2–C6H3; 4b–6b: Ar = Ph) demonstrated that the steric demands of the ligand dramatically affect the solid-state geometry. The cationic zinc–lactate complexes [LZnOR+][B(m-(CF3)2–C6H3)4−] (7: L = L22; 8: L = L33; R = CH(Me)CO2Me) were also studied, and their efficacy as lactide polymerization catalysts was examined. Polymerization using 7 requires heating to 60 °C, while complex 8 displays high activity at ambient temperature. The difference in activity can be attributed to κ2versus κ3 binding modes of the ligand, providing important insight into structure–activity relationships for this system. Complex 8 gives modestly heteroenriched PLA (Pr = 0.70), which represents the best stereocontrol yet achieved by a cationic metal catalyst.

DESIGNING CATIONIC ZINC AND MAGNESIUM CATALYSTS FOR COORDINATION–INSERTION POLYMERIZATION OF LACTIDE

This review presents a synopsis of the development of cationic zinc and magnesium metal complexes as catalysts for the polymerization of lactones, with the major focus being directed toward the polymerization of lactide. By utilizing an electron-rich, neutral bis(phosphinimine) pincer ligand, cationic complexes with high ambient temperature activity for polymerization of lactide were obtained for the first time. A number of important structure–activity relationships have been established for this new class of catalyst. Recent progress toward P-stereogenic analogues of these cationic catalysts is also summarized.

Triamidoamine-supported zirconium: hydrogen activation, Lewis acidity, and rac-lactide polymerization

Investigation of a triamidoamine-supported zirconium hydride intermediate, important to a range of catalytic reactions, revealed the potential Lewis acidity of [κ5-N,N,N,N,C-(Me3SiNCH2CH2)2NCH2CH2NSiMe2CH2]Zr (1). A preliminary study of 1 as a precursor for the polymerization of rac-lactide showed modest activity but indicated that five-coordinate zirconium complexes with tetra-N donor ligands may be an avenue for further development in group 4 metal lactide polymerization catalysis.

Synthesis of Sterically Demanding Bis(phosphinimine) Dibenzofuran Ligands and Subsequent Zinc Metalation

In light of previous success surrounding the use of bis(phosphinimine)dibenzofuran ligands for zinc-mediated lactide polymerization, a series of sterically demanding P=N pincer compounds have been prepared with important steric and electronic modifications at both P- and N-sites (L, 3a–d). These systems are highly crystalline and have been extensively characterized using multinuclear NMR spectroscopy, elemental analysis, and X-ray diffraction. The ligands can be transformed into their protonated analogues [HL][BArF4] (4a–d, [BArF4] = [B(m-(CF3)2-C6H3)4]) by reaction with Brookhart’s acid, and subsequently coordinated to zinc via an alkane elimination reaction with [ZnEt2] at ambient temperature to afford the corresponding [LZnEt][BArF4] cationic complexes 5a–d. In addition, an unusual chloridozinc species [LZnCl][BArF4] (5c′) has been isolated and structurally characterized, providing comparisons to previously established ligand sets with similar geometries.