The reaction of the 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 1 equiv of RMgCl proceeds to give very high yields of the neutral heteroscorpionate alkyl magnesium complexes [Mg(R)(NNN)] (NNN = tbpamd, R = C3H5 1, t Bu 2, CH2SiMe3 3; NNN = pbpamd, R = C3H5 4, t Bu 5, CH2SiMe3 6). On heating toluene solutions of complexes 1 − 3, 5, and 6, a ligand redistribution process occurs to give the corresponding 6-coordinated sandwich complexes [Mg(tbpamd)2] (7) and [Mg(pbpamd)2] (8). Interestingly, the allyl derivative 4 can be easily transformed to 8 at room temperature. In addition, the cationic sandwich complex [Mg(tbpamdH)2]Cl2 (9) [tbpamdH = N-ethyl-N‘-tert-butylbis(3,5-dimethylpyrazol-1-yl)acetamidine] was obtained from 7 by means of a protonation process. Finally, alkyl-containing complexes 1 − 6 can act as highly effective single-component living initiators for the ring-opening polymerization of ε-caprolactone and lactides over a wide range of temperatures. ε-Caprolactone is polymerized within seconds to give high molecular weight polymers with narrow 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 = 160 °C). Polymer end-group analysis shows that the polymerization process is initiated by alkyl transfer to the monomer.

No abstract

The reaction of bis(3,5-di-tert-butylpyrazol-1-yl)methane (bdtbpzm) with BunLi and carbodiimide derivatives, namely, N,N′-diisopropyl and 1-tert-butyl-3-ethyl carbodiimides, gives rise to the new sterically hindered lithium acetamidinate [Li(tbptamd)(THF)] (1) [tbptamd = N-ethyl-N′-tert-butylbis(3,5-di-tert-butylpyrazol-1-yl)acetamidinate] and [Li(pbptamd)(THF)] (2) [pbptamd = 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 NH4Cl/H2O in ether cleanly affords the corresponding amidine ligands Htbpamd (3), Hpbpamd (4), Htbptamd (5), and Hpbptamd (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, CH2SiMe3 9; NNN = pbpamd, R′ = Me 10, Et 11, CH2SiMe3 12; NNN = tbptamd, R′ = Me 13, Et 14; NNN = pbptamd, 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 κ3-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.