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

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.

Recent Advances in the Design and Coordination Chemistry of Heteroscorpionate Ligands Bearing Stereogenic Centres

“…This redistribution seems to be favoured by polar solvents and sterically less demanding ligands. In contrast, we recently proved [33] [34] even in refluxing toluene over several days. Furthermore, when the reaction was carried out with 2 equiv.…”

Heteroscorpionate Rare‐Earth Metal Zwitterionic Complexes: Syntheses, Characterization, and Heteroselective Catalysis on the Ring‐Opening Polymerization of rac‐Lactide

“…[13,27] However, many of them suffered low activity even at elevated temperatures. We then investigated the catalysis of our heteroscorpionate metal complexes 4-6 for the ROP of rac-lactide.…”

Lithium Quinolyl‐Amidinates Efficiently Promoting Ring‐Opening Polymerization of ε‐Caprolactone: Synthesis and 7Li NMR Spectroscopic Studies

“…The polymerization reactions were also studied without solvent and BnOH; a good conversion of 76 % was achieved for a [CL]/[Li] ratio of 500:1 in 10 min (Table , entry 17). All prepared polymers exhibited unimodal distribution curves with broad PDI values; this can be attributed to rapid propagation, compared with the initiation of the catalysts Li1 – Li5 in the polymerization process,[5c], [6b], and also to the multinuclear active species. The kinetic experiments were carried out by using Li4 at regular time intervals (2, 4, 6, 8, and 10 min); Li4 showed 75 % conversion for a [CL]/[Li] ratio of 250:1 at 2 min and further irregular increases were observed (Figure S1).…”

“…Among the phenolate‐supported lithium complexes, piperazinylaminephenolate lithium complexes have shown better activity in shorter reaction times and with higher conversion, for example, 99 % for a [CL]/[Li]/[BnOH] (CL = ε‐caprolactone) ratio of 300:1:1 in 7 min at 25 °C,[10a] and are more active than other lithium phenolate catalysts. [11b], [11c], , In comparison with the lithium phenolates,[10a], [11b], [11c], , sterically hindered amidinate‐based heteroscorpionate lithium salts [related to a bis(pyrazolyl)methane system] showed enhanced efficiencies toward the ROP of ε‐caprolactone, forming high–medium molecular‐weight polymers within minutes, whereas the coordination between Li and the ligands was in an N , N , N chelating fashion . Very recently, we reported amidinate‐based tridentate quinolyl ligand ( N , N , N )‐supported Mg and Al complexes for the ROP of ε‐caprolactone, in which both Mg and Al catalysts showed high activity with good conversion, up to a [CL]/[M] ratio of 800:1 .…”