Encapsulation of Aromatic Guests in the Bisporphyrin Cavity of a Double-Stranded Spiroborate Helicate: Thermodynamic and Kinetic Studies and the Encapsulation Mechanism

Naoki Ousaka, Shinya Yamamoto, Hiroki Iida, Takuya Iwata, Shingo Ito, Rafael Souza, Yuh Hijikata, Stephan Irle, Eiji Yashima

Research output: Contribution to journalArticlepeer-review

5 Scopus citations

Abstract

A double-stranded spiroborate helicate bearing a bisporphyrin unit in the middle forms an inclusion complex with electron-deficient aromatic guests that are sandwiched between the porphyrins. In the present study, we systematically investigated the effects of size, electron density, and substituents of a series of aromatic guests on inclusion complex formations within the bisporphyrin. The thermodynamic and kinetic behaviors during the guest-encapsulation process were also investigated in detail. The guest-encapsulation abilities in the helicate increased with the increasing core sizes of the electron-deficient aromatic guests and decreased with the increasing bulkiness and number of substituents of the guests. Among the naphthalenediimide derivatives, those with bulky N-substituents at both ends hardly formed an inclusion complex. Instead, they formed a [2]rotaxane-like inclusion complex through the water-mediated dynamic B-O bond cleavage/reformation of the spiroborate groups of the helicate, which enhanced the conformational flexibility of the helicate to enlarge the bisporphyrin cavity and form an inclusion complex. Based on the X-ray crystal structure of a unique pacman-like 1:1 inclusion complex between the helicate and an ammonium cation as well as the molecular dynamics simulation results, a plausible mechanism for the inclusion of a planar aromatic guest within the helicate is also proposed.

Original languageEnglish
Pages (from-to)10501-10516
Number of pages16
JournalJournal of Organic Chemistry
Volume86
Issue number15
DOIs
StatePublished - Aug 6 2021
Externally publishedYes

Funding

This work was supported in part by JSPS KAKENHI (Grant-in-Aid for Specially Promoted Research, no. 18H05209 (E.Y.)). We thank Dr. Daisuke Taura (Nagoya University) for his help in the modeling of the helicates.

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