Direct evidence of charge separation in a metal-organic framework: efficient and selective photocatalytic oxidative coupling of amines via charge and energy transfer.

Caiyun Xu,Ji-Hu Su,Dandan Li,Hai-Long Jiang,Hang Liu

doi:10.1039/c7sc05296k

Abstract

The selective aerobic oxidative coupling of amines under mild conditions is an important laboratory and commercial procedure yet a great challenge. In this work, a porphyrinic metal-organic framework, PCN-222, was employed to catalyze the reaction. Upon visible light irradiation, the semiconductor-like behavior of PCN-222 initiates charge separation, evidently generating oxygen-centered active sites in Zr-oxo clusters indicated by enhanced porphyrin π-cation radical signals. The photogenerated electrons and holes further activate oxygen and amines, respectively, to give the corresponding redox products, both of which have been detected for the first time. The porphyrin motifs generate singlet oxygen based on energy transfer to further promote the reaction. As a result, PCN-222 exhibits excellent photocatalytic activity, selectivity and recyclability, far superior to its organic counterpart, for the reaction under ambient conditions via combined energy and charge transfer.

Highlights

Efficient and selective oxidation reactions based on molecular oxygen (O2) or even ambient air as the oxidant have received tremendous attention from the view of green chemistry
The porphyrin linkers in PCN-222 can be excited by visible light and the photogenerated electrons transfer to Zr-oxo clusters to form oxygen-centered active sites, leaving enhanced porphyrin p-cation radical signals by hole oxidation, both of which have been detected via electron spin resonance (ESR)
This hypothesis is supported by density functional theory (DFT) calculations (Fig. S3 and S4†) and cyclic voltammetry measurements (Fig. S5†)

Summary

Introduction

Efficient and selective oxidation reactions based on molecular oxygen (O2) or even ambient air as the oxidant have received tremendous attention from the view of green chemistry. The porphyrin linkers in PCN-222 can be excited by visible light and the photogenerated electrons transfer to Zr-oxo clusters to form oxygen-centered active sites, leaving enhanced porphyrin p-cation radical signals by hole oxidation, both of which have been detected via ESR.

Results

Conclusion