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Heteroleptic Silylamido Phenolate Complexes of Calcium and the Larger Alkaline Earth Metals: β-Agostic Ae⋅⋅⋅SiH Stabilization and Activity in the Ring-Opening Polymerization of L-Lactide.
MedLine Citation:
PMID:  22492467     Owner:  NLM     Status:  Publisher    
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 [{LO(i) }CaNu(thf)(n) ] supported by mono-anionic amino ether phenolate ligands (i=1, {LO(1) }(-) =4-(tert-butyl)-2,6-bis(morpholinomethyl)phenolate, Nu(-) =N(SiMe(2) H)(2) (-) , n=0, 4; i=2, {LO(2) }(-) =2,4-di-tert-butyl-6-{[2-(methoxymethyl)pyrrolidin-1-yl]methyl}phenolate, Nu(-) =N(SiMe(2) H)(2) (-) , n=0, 5; i=4, {LO(4) }(-) =2-{[bis(2-methoxyethyl)amino]methyl}-4,6-di-tert-butylphenolate, Nu(-) =N(SiMe(2) H)(2) (-) , n=1, 6; Nu(-) =HCCCH(2) O(-) , n=0, 7) and those of the related [{LO(3) }AeN(SiMe(2) H)(2) ] ({LO(3) }(-) =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Si internal β-agostic interactions, which play a key role in the stabilization of [{LO(i) }AeN(SiMe(2) H)(2) ] complexes against ligand redistribution reactions, in contrast to regular [{LO(i) }AeN(SiMe(3) )(2) ]. Pulse-gradient spin-echo (PGSE) NMR measurements showed that 1, 4, 6, and 7 are monomeric in solution. Complexes 1-7 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>4≈5). 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CCH(2) O(-) initiating group (as in 7) proved superior to N(SiMe(2) H)(2) (-) or N(SiMe(3) )(2) (-) (as in 6 or [{LO(4) }CaN(SiMe(3) )(2) ] (B), respectively).
Bo Liu; Thierry Roisnel; Jean-Paul Guégan; Jean-François Carpentier; Yann Sarazin
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Publication Detail:
Type:  JOURNAL ARTICLE     Date:  2012-4-11
Journal Detail:
Title:  Chemistry (Weinheim an der Bergstrasse, Germany)     Volume:  -     ISSN:  1521-3765     ISO Abbreviation:  -     Publication Date:  2012 Apr 
Date Detail:
Created Date:  2012-4-11     Completed Date:  -     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  9513783     Medline TA:  Chemistry     Country:  -    
Other Details:
Languages:  ENG     Pagination:  -     Citation Subset:  -    
Copyright Information:
Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Catalyse et Organométalliques, UMR CNRS 6226 Sciences Chimiques de Rennes, Université de Rennes 1, Campus de Beaulieu, Avenue du Général Leclerc, 35042 Rennes Cedex (France).
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