Discrete Heteroscorpionate Lithium and Zinc Alkyl Complexes. Synthesis, Structural Studies, and ROP of Cyclic Esters

The reaction of bis(3,5-di- tert -butylpyrazol-1-yl)methane (bdtbpzm) with Bu n Li and carbodiimide derivatives, namely, N, N′ -diisopropyl and 1- tert -butyl-3-ethyl carbodiimides, gives rise to the new sterically hindered lithium acetamidinate [Li(tbp t amd)(THF)] ( 1 ) [tbp t amd = N- ethyl- N′ - tert -butylbis(3,5-di- tert -butylpyrazol-1-yl)acetamidinate] and [Li(pbp t amd)(THF)] ( 2 ) [pbp t amd = N, N′ -diisopropylbis(3,5-di- tert -butylpyrazol-1-yl)acetamidinate]. Subsequent hydrolysis of 1 and 2, and the recently reported heteroscorpionate lithium salts [Li(tbpamd)(THF)] [tbpamd = N- ethyl- N′ - tert -butylbis(3,5-dimethylpyrazol-1-yl)acetamidinate] and [Li(pbpamd)(THF)] [pbpamd = N, N′ -diisopropylbis(3,5-dimethylpyrazol-1-yl)acetamidinate] with NH 4 Cl/H 2 O in ether cleanly affords the corresponding amidine ligands Htbpamd ( 3 ), Hpbpamd ( 4 ), Htbp t amd ( 5 ), and Hpbp t amd ( 6 ) in very good yields. The X-ray diffraction molecular structure of 3 was obtained. Reaction of the amidine-heteroscorpionate ligands 3 – 6 with 1 equiv of ZnR′ 2 proceeds in very high yields to give the neutral heteroscorpionate alkyl zinc complexes [Zn(R′)(NNN)] (NNN = tbpamd, R′ = Me 7, Et 8, CH 2 SiMe 3 9; NNN = pbpamd, R′ = Me 10, Et 11, CH 2 SiMe 3 12; NNN = tbp t amd, R′ = Me 13, Et 14; NNN = pbp t amd, R′ = Me 15, Et 16 ). The single-crystal X-ray structures of the derivatives 8, 12, 15, and 16 confirm a four-coordinative structure with the zinc metal center in a distorted tetrahedral geometry and the heteroscorpionate ligands arranged in κ 3 -coordination mode. The new lithium salts 1 and 2 and the alkyls 7 – 9, 13, and 14 can act as efficient single-component initiators for the ring-opening polymerization of ϵ-caprolactone and lactides over a wide range of temperatures. ϵ-Caprolactone is polymerized within minutes to give high-medium molecular weight polymers with medium broad values of polydispersities. Lactide afforded PLA materials with medium molecular weights and polydispersities as narrow as M w / M n = 1.05. Additionally, polymerization of l -lactide occurred without racemization in the propagation process and offered highly crystalline, isotactic poly( l -lactides) with high melting temperatures ( T m = 165 °C). rac -Lactide polymerization also produces enriched levels of heterotactic poly(lactide). Polymer end group analysis shows that the polymerization mediated by alkyl zinc complexes is initiated by alkyl transfer to monomer.