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Keywords:

  • agostic interaction;
  • alkaline earth metals;
  • calcium;
  • phenolate ligands;
  • ring-opening polymerization

Abstract

The factors governing the stability and the reactivity towards cyclic esters of heteroleptic complexes of the large alkaline earth metals (Ae) have been probed. The synthesis and stability of a family of heteroleptic silylamido and alkoxide complexes of calcium [{LOi}Ca[BOND]Nu(thf)n] supported by mono-anionic amino ether phenolate ligands (i=1, {LO1}=4-(tert-butyl)-2,6-bis(morpholinomethyl)phenolate, Nu=N(SiMe2H)2, n=0, 4; i=2, {LO2}=2,4-di-tert-butyl-6-{[2-(methoxymethyl)pyrrolidin-1-yl]methyl}phenolate, Nu=N(SiMe2H)2, n=0, 5; i=4, {LO4}=2-{[bis(2-methoxyethyl)amino]methyl}-4,6-di-tert-butylphenolate, Nu=N(SiMe2H)2, n=1, 6; Nu=HC[TRIPLE BOND]CCH2O, n=0, 7) and those of the related [{LO3}Ae[BOND]N(SiMe2H)2] ({LO3}=2-[(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)methyl]-4,6-di-tert-butylphenolate Ae=Ca, 1; Sr, 2; Ba, 3) have been investigated. The molecular structures of 1, 2, [(4)2], 6, and [(7)2] have been determined by X-ray diffraction. These highlight Ae⋅⋅⋅H[BOND]Si internal β-agostic interactions, which play a key role in the stabilization of [{LOi}Ae[BOND]N(SiMe2H)2] complexes against ligand redistribution reactions, in contrast to regular [{LOi}Ae[BOND]N(SiMe3)2]. Pulse-gradient spin-echo (PGSE) NMR measurements showed that 1, 4, 6, and 7 are monomeric in solution. Complexes 17 mediate the ring-opening polymerization (ROP) of L-lactide highly efficiently, converting up to 5000 equivalents of monomer at 25 °C in a controlled fashion. In the immortal ROP performed with up to 100 equivalents of exogenous 9-anthracenylmethanol or benzyl or propargyl alcohols as a transfer agent, the activity of the catalyst increased with the size of the metal (1<2<3). For Ca-based complexes, the enhanced electron-donating ability of the ancillary ligand favored catalyst activity (1>6>45). The nature of the alcohol had little effect over the activity of the binary catalyst system 1/ROH; in all cases, both the control and end-group fidelity were excellent. In the living ROP of L-LA, the HC[TRIPLE BOND]CCH2O initiating group (as in 7) proved superior to N(SiMe2H)2 or N(SiMe3)2 (as in 6 or [{LO4}Ca[BOND]N(SiMe3)2] (B), respectively).