Well-defined, solvent-free cationic barium complexes: Synthetic strategies and catalytic activity in the ring-opening polymerization of lactide

Abstract

Well-defined, solvent-free cationic barium complexes of the type [{LnX}Ba]+·[H2N{B(C6F5)3}2]− stabilized by multidentate amino-ether phenoxide or fluorinated amino-ether alkoxide ligands {LnX}− are available according to original, general and high-yield protocols. These cations have been prepared by (i) hydrolysis of heteroleptic complexes {LnX}BaN(SiMe2H)2 stabilized by Ba···H−Si interactions with [H(OEt2)2]+·[H2N{B(C6F5)3}2]−, or (ii) reaction of {Ba[N(SiMe2H)2]2}n with the doubly acidic pro-ligands [{LnX}HH]+·[H2N{B(C6F5)3}2]−. The solid-state structures of [{LO2}Ba(THF)2]+·[H2N{B(C6F5)3}2]− ({LO2}H=2-[(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)methyl]-4,6-di-tert-butylphenol) and [{RO2}Ba]+·[H2N{B(C6F5)3}2]− {RO2}H=2-[(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)methyl]-1,1,1,3,3,3-hexafluoropropan-2-ol) are described, highlighting the key role of internal secondary Ba···F−C interactions in these highly electrophilic species. In combination with an excess of an external nucleophile (chosen from benzyl alcohol, 1,3-propanediol, benzyl amine or an hydroxyl-functionalized alkoxy-amine) as a co-initiator, some of these Ba cations provide extremely efficient catalysts for the immortal ring-opening polymerization of l-lactide in the temperature range 0–30°C, converting rapidly up to 5000equiv. of monomer in a controlled fashion and with excellent end-group fidelity.