Lithiated Calix[ n]arenes (n = 6 or 8): Synthesis, Structures, and Use in the Ring-Opening Polymerization of Cyclic Esters

Xing, Tian; Jiang, Chengying; Elsegood, Mark R. J.; Redshaw, Carl

Abstract

A variety of lithiated calix[n]arenes, for which n = 6 or 8, have been isolated, structurally characterized, and evaluated as catalysts for the ring-opening polymerization (ROP) of the cyclic esters ϵ-caprolactone (ϵ-CL), δ-valerolactone (δ-VL), and rac-lactide (r-LA). In particular, interaction of p-tert-butylcalix[6]areneH6 (L6H6) with LiOtBu in THF led to the isolation of [Li14(L6H)2(CO3)2(THF)6(OH2)6]·14THF (1·14THF), the core of which has a chain of five Li2O2 diamonds. Similar use of p-tert-butylcalix[8]areneH8 (L8H8) afforded [Li10(L8)(OH)2(THF)8]·7THF (2·7THF), where the core is composed of a six-rung Li-O ladder. Use of debutylated calix[8]areneH8 (deBuL8H8) led to an elongated dimer [Li18(deBuL8)2(OtBu)2(THF)14]·4THF (3·4THF) in which the calix[8]arenes possess a wavelike conformation forming bridges to link three separate LixOy clusters (where x and y = 6, ignoring the THF donor oxygens). Interaction of L8H8 with LiOH·H2O afforded [Li4(L8H4)(OH2)4(THF)6]·5.5THF (4·5.5THF), where intramolecular H-bond interactions involving Li, O, and H construct a cage in the core of the structure with six- and eight-membered rings. Lastly, addition of Me3Al to the solution generated from L8H8 and LiOtBu led to the isolation of [(AlMe2)2Li20(L8H2)2(OH2)4(O2-)4(OH)2(NCMe)12]·10MeCN (5·10MeCN) in which Li, O, Al, and N centers build a polyhedral core. These complexes have been screened for their potential to act as precatalysts in the ring-opening polymerization (ROP) of ϵ-CL, δ-VL, and r-LA. For the ROP of ϵ-CL, δ-VL, and r-LA, systems 1-4 exhibited moderate activity at 130 °C over 8 h. In the case of ROP using the mixed-metal (Li/Al) system 5, better conversions and high molecular weight polymers were achieved. In the case of the ROP of ω-pentadecalactone (ω-PDL), the systems proved to be inactive under the conditions employed herein.

Citation

Xing, T., Jiang, C., Elsegood, M. R. J., & Redshaw, C. (2021). Lithiated Calix[ n]arenes (n = 6 or 8): Synthesis, Structures, and Use in the Ring-Opening Polymerization of Cyclic Esters. Inorganic chemistry, https://doi.org/10.1021/acs.inorgchem.1c02192

• Journal Article Type: Article
• Acceptance Date: Sep 28, 2021
• Online Publication Date: Oct 1, 2021
• Publication Date: Oct 1, 2021
• Deposit Date: Oct 2, 2021
• Publicly Available Date: Oct 2, 2022
• Journal: Inorganic Chemistry
• Print ISSN: 0020-1669
• Electronic ISSN: 1520-510X
• Publisher: American Chemical Society
• Peer Reviewed: Peer Reviewed
• DOI: https://doi.org/10.1021/acs.inorgchem.1c02192
• Keywords: Ring-opening polymerization; Molecules; Oxygen; Solvents; Molecular structure
• Public URL: https://hull-repository.worktribe.com/output/3848655