Collaboration between Trinuclear Aluminum Complexes Bearing Bipyrazoles in the Ring-Opening Polymerization of ε-Caprolactone

Someswara Rao Kosuru; Feng-Jie Lai; Yu-Lun Chang; Chen-Yu Li; Yi-Chun Lai; Shangwu Ding; Kuo-Hui Wu; Hsuan-Ying Chen; Yung-Han Lo

doi:10.1021/acs.inorgchem.1c01192

Collaboration between Trinuclear Aluminum Complexes Bearing Bipyrazoles in the Ring-Opening Polymerization of ε-Caprolactone

Inorg Chem. 2021 Jul 19;60(14):10535-10549. doi: 10.1021/acs.inorgchem.1c01192. Epub 2021 Jul 7.

Authors

Someswara Rao Kosuru¹, Feng-Jie Lai^{2

3}, Yu-Lun Chang¹, Chen-Yu Li¹, Yi-Chun Lai¹, Shangwu Ding^{1

4}, Kuo-Hui Wu⁵, Hsuan-Ying Chen^{1

4

6}, Yung-Han Lo⁷

Affiliations

¹ Department of Medicinal and Applied Chemistry, Drug Development and Value Creation Research Center, Kaohsiung Medical University, Kaohsiung 80708, Taiwan, Republic of China.
² Department of Dermatology, Chi Mei Medical Center, Tainan, Taiwan, Republic of China.
³ Center for General Education, Southern Taiwan University of Science and Technology, Tainan, Taiwan, Republic of China.
⁴ Department of Chemistry, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, Republic of China.
⁵ Department of Chemistry, National Central University, Taoyuan 32001, Taiwan, Republic of China.
⁶ Department of Medical Research, Kaohsiung Medical University Hospital, Kaohsiung 80708, Taiwan, Republic of China.
⁷ Department of Chemistry, Faculty of Science and Technology, Keio University, Minato City 108-8345, Tokyo, Japan.

PMID: 34232620
DOI: 10.1021/acs.inorgchem.1c01192

Abstract

Trinuclear aluminum complexes bearing bipyrazoles were synthesized, and their catalytic activity for ε-caprolactone (CL) polymerization was investigated. D^Bu₂Al₃Me₅ exhibited higher catalytic activity than did the dinuclear aluminum complex L^Bu₂Al₂Me₄ (16 times as high for CL polymerization; [CL]:[D^Bu₂Al₃Me₅]:[BnOH] = 100:0.5:5, [D^Bu₂Al₃Me₅] = 10 mM, conversion 93% after 18 min at room temperature). Density functional theory calculations revealed a polymerization mechanism in which CL first approached the central Al atom and then moved to an external Al. The coordinated CL ring was opened because the repulsion of two tert-butyl groups on the ligands pushed an alkoxide initiator on an external Al to initiate CL. In these trinuclear Al catalysts, the central Al plays a role in monomer capture and then collaborates with the external Al to activate CL, accelerating polymerization.