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 (BuLi)-Li-n 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-tertbutylpyrazol-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-M-tert-butylbis(3,5-dimethylpyrazol1-yl)acetamidinate] and [Li(pbpamd)(THF)] [pbpamd N,N'-diisopropylbis(3,5-dimethylpyrazol-1-yl)acetamidinate] with NH4Cl/H2O 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 ZnR2' proceeds in very high yields to give the neutral heteroscorpionate alkyl zinc complexes [Zn(R')(NNN)] (NNN tbpamd, R' = Me 7, Et 8, CH2SiMe3 9; NNN = pbpamd, R' = Me 10, Et 11, CH2SiMe3 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 kappa(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 E-caprolactone and lactides over a wide range of temperatures. c-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 degrees 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.