Size-control in the synthesis of oxo-bridged phosphazane macrocycles via a modular addition approach

Xiaoyan Shi,Rakesh Ganguly,Jesús Díaz,Felipe García,Felix León,How Chee Ong

doi:10.1038/s42004-021-00455-9

Abstract

Inorganic macrocycles remain largely underdeveloped compared with their organic counterparts due to the challenges involved in their synthesis. Among them, cyclodiphosphazane macrocycles have shown to be promising candidates for supramolecular chemistry applications due to their ability to encapsulate small molecules or ions within their cavities. However, further developments have been handicapped by the lack of synthetic routes to high-order cyclodiphosphazane macrocycles. Moreover, current approaches allow little control over the size of the macrocycles formed. Here we report the synthesis of high-order oxygen-bridged phosphazane macrocycles via a “3 + n cyclisation” (n = 1 and 3). Using this method, an all-PIII high-order hexameric cyclodiphosphazane macrocycle was isolated, displaying a larger macrocyclic cavity than comparable organic crown-ethers. Our approach demonstrates that increasing building block complexity enables precise control over macrocycle size, which will not only generate future developments in both the phosphazane and main group chemistry but also in the fields of supramolecular chemistry.

Highlights

Inorganic macrocycles remain largely underdeveloped compared with their organic counterparts due to the challenges involved in their synthesis
Since conventional approaches have reached their limits for the synthesis of high-order cyclodiphosphazane macrocyclic species, as illustrated by the isolation of the first PIII pentameric species—
Our studies began with the synthesis of a trimeric acyclic poly-PIII2N2 cyclodiphosphazane via a “1 + 2” addition reaction, where 1 mol of [ClP(μNtBu)]2 (1), dissolved in THF, was reacted with water in the presence of Et3N, as a Brønsted base, to produce the intermediate [OP (H)(μ-NtBu)]2, which was subsequently deprotonated with nBuLi to generate [LiOP(μ-NtBu)]2 (2) in situ

Summary

Introduction

Inorganic macrocycles remain largely underdeveloped compared with their organic counterparts due to the challenges involved in their synthesis. Compound 4 exhibits an AB split pattern for the central and medial phosphorus atoms in the 31P{1H} spectra with two doublets (Δυ/JAB = 1.9)—at δ 140.1 and 139.2 ppm, and a broad singlet 119.2 ppm for the nitrogen substituted terminal phosphorus centres, respectively (Supplementary Fig. S3).

Results

Conclusion