Paul G. Hayes

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.

Toward stereoselective lactide polymerization catalysts: cationic zinc complexes supported by a chiral phosphinimine scaffold.

The P-stereogenic phosphinimine ligands (dbf)MePhP═NAr (7: Ar = Dipp; 8: Ar = Mes; dbf = dibenzofuran, Dipp = 2,6-diisopropylphenyl, Mes = 2,4,6-trimethylphenyl) were synthesized as racemates via reactions of the parent phosphines (rac)-(dbf)MePhP (6) with organoazides. The ligands 7 and 8 were protonated by Brønsted acids to afford the aminophosphonium borate salts [(7)-H][BAr(4)] (9: Ar = C(6)F(5); 11: Ar = Ph) and [(8)-H][BAr(4)] (10: Ar = C(6)F(5); 12: Ar = Ph). The protonated ligands 9 and 10 were active toward alkane elimination reactions with diethylzinc and ethyl-[methyl-(S)-lactate]zinc to give the heteroleptic complexes [{(dbf)MePhP═NAr}ZnR][B(C(6)F(5))(4)] (Ar = Dipp, 13: R = Et; 15: R = methyl-(S)-lactate; Ar = Mes, 14: R = Et; 16: R = methyl-(S)-lactate). By contrast, reaction of the tetraphenylborate derivative 11 with diethylzinc yielded a phenyl transfer product, [(dbf)MePhP═NDipp]ZnPh(2) (17). Complex 15 was found to catalyze the ring-opening polymerization of rac-lactide.

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.

A Hydrogen-Substituted Osmium Stannylene Complex : Isomerization to a Metallostannylene Complex via an Unusual α-Hydrogen Migration from Tin to Osmium

An osmium complex bearing a terminal hydrogen-substituted stannylene ligand, Cp*((i)Pr(3)P)(H)Os=SnH(trip) (1) (trip = 2,4,6-triisopropylphenyl), has been prepared by stannylene extrusion, and the complex has been structurally characterized. Complex 1 coordinates Lewis bases and activates the O-H bonds of water and methanol. Most interestingly, 1 converts to the metallostannylene complex Cp*((i)Pr(3)P)(H)(2)OsSn(trip) (2) thermally or photochemically by what appears to be a radical process.

The Osmium–Silicon Triple Bond: Synthesis, Characterization, and Reactivity of an Osmium Silylyne Complex

The first silylyne complex of a metal beyond group 6, [Cp*((i)Pr3P)(H)Os≡Si(Trip)][HB(C6F5)3], was prepared by a new synthetic route involving hydride abstraction from silicon. NMR and DFT computations support the presence of a silylyne ligand, and NBO and ETS-NOCV analysis revealed the nature of this Os-Si interaction as a triple bond consisting of a covalent σ bond and two strong π back-donations. The discovery of this complex allowed observations of the first cycloadditions involving a silylyne complex, and terminal alkynes are shown to react via C-H bond additions across the Os≡Si bond.

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.

Rare Earth Pincer Complexes: Synthesis, Reaction Chemistry, and Catalysis

The research field surrounding rare earth pincer complexes has reached a stage where a comprehensive review about the reactivity and catalytic behavior of these species is justified. In this contribution, we begin with a brief introduction on common strategies for the preparation of rare earth pincer complexes, continuing with a section devoted to the versatile reactivity observed for this class of compound. Thereafter, several types of compounds are discussed, including extremely reactive hydrides, cationic species, and intriguing scandium imido complexes. Finally, the last portion of this chapter sums up the hitherto reported catalytic studies, including discussions on ring-opening polymerization of cyclic esters, polymerization of olefins and hydroamination reactions, as well as several examples of more infrequently encountered catalytic processes.

Crystal structure of a dimeric β-diketiminate magnesium complex.

The crystal structure of a dimeric β-diketiminate magnesium(II) complex crystallizes as two independent molecules, each with 2/m crystallographic site symmetry, located at Wyckoff sites 2c and 2d. These have symmetry-equivalent magnesium atoms bridged by μ-iodide ligands with very similar Mg—I distances.