Mono- and binuclear chiral N, N, O-scorpionate zinc alkyls as efficient initiators for the ROP of rac-lactide. Issue 43 (25th October 2017)

• Record Type: Journal Article
• Authors: Otero, Antonio; Fernández-Baeza, Juan; Sánchez-Barba, Luis F.; Sobrino, Sonia; Garcés, Andrés; Lara-Sánchez, Agustín; Rodríguez, Ana M.
• Is Part Of: Dalton transactions. Volume 46:Issue 43(2017)
• Page Start: 15107
• Page End: 15117
• Publication Date: 2017-10-25
• Subjects: Chemistry, Inorganic -- Periodicals; Chemistry, Physical and theoretical -- Periodicals; Chemistry, Inorganic -- Periodicals; 546.05
• DOI: 10.1039/c7dt03045b
• Languages: English
• ISSNs: 1477-9226

Abstract

Abstract : Activity enhancement in the ROP of rac -LA through the cooperative effect of two remote zinc metals in N, N, O-scorpionate binuclear trisalkyls. Abstract : The preparation of new chiral bis(pyrazol-1-yl)methane-based N, N, O-donor scorpionate ligands in the form of the alcohol compounds bpzampeH (1 ) {2, 2-bis(3, 5-dimethylpyrazol-1-yl)-1-[4-(dimethylamino)phenyl]ethanol}, bpzaepeH (2 ) {2, 2-bis(3, 5-dimethylpyrazol-1-yl)-1-[4-(diethylamino)phenyl]ethanol}, and bpzimeH (3 ) {2, 2-bis(3, 5-dimethylpyrazol-1-yl)-1-[1-methyl-1 H -imidazol-2-yl]ethanol} has been carried out by the 1, 2-addition reactions of a series of aldehydes. These new chiral heteroscorpionate ligands reacted with [ZnR2 ] (R = Me, Et, CH2 SiMe3 ) in a 1 : 1 molar ratio in toluene to give the mononuclear monoalkyl zinc complexes [Zn(R)(κ 3 -NNO)] (4–12 ). When these reactions were carried out in a 1 : 2 molar ratio the binuclear trisalkyls [Zn(R)(κ 2 -NNμ-O)Zn(R)2 ] (13–18 ) were obtained. The structures of these complexes were elucidated by 1 H and 13 C{ 1 H} NMR spectroscopy and the X-ray crystal structures of4 and5 were also established. Interestingly, alkyl-containing zinc complexes4–13, 15 and17 act as efficient single-component initiators for the ring-opening polymerization of rac -lactide at 20 °C to afford PLA materials with low molecular weights in a few hours. The dinuclear trisalkyls showed higher activity in comparison with the mononuclear zinc counterparts, suggesting a cooperative effect of the two remote metals. The narrow dispersity ranges ( M w / M n = 1.05) of the isolated polymers in conjunction with the linear nature of the number average molecular weight versus conversion plot provided evidence for living behavior. Inspection of the kinetic parameters showed that the propagations have the usual pseudo-first-order dependence on rac -lactide and catalyst concentration. End-group analysis and MALDI-TOF mass spectrometry confirmed that the initiation occurs through nucleophilic attack of the alkyl on the lactide monomer. Microstructural analysis of poly( rac -lactide)s revealed that the most sterically hindered ligand on the alkoxide fragment exerts a moderate influence on the degree of stereoselectivity, with heteroenriched-PLAs ( P s = 0.68) produced at room temperature.