A Stable Zn-MOF for Photocatalytic C<sub>sp<sup>3</sup></sub>–H Oxidation: Vinyl Double Bonds Boosting Electron Transfer and Enhanced Oxygen Activation
Haitao Han, Xiangyu Zheng, Chengfang Qiao, Zhengqiang Xia, Qi Yang, Ling Di, Yang Xing, Gang Xie, Chunsheng Zhou, Wenyuan Wang, Sanping Chen
Abstract
Molecular oxygen activation has always been a difficult issue and challenge in heterogeneous photocatalytic aerobic oxidations due to the kinetically persistent or spin-forbidden nature of O2. In this work, a highly delocalized interpenetrated 3D MOF photocatalyst with high stability, Zn-TACPA (H3TACPA = tris(3-carboxybiphenyl)amine), based on vinyl-functionalized triphenylamine and bipyridine ligands has been fabricated and employed as a reactive oxygen species (ROS) generator to catalyze the photooxidative CDC/aromatization tandem reaction of glycine esters and styrenes. In comparison to a similar triphenylamine MOF (Zn-TCA), DFT calculations and extensive control experiments reveal that the introduction of functional vinyl double bonds not only optimizes the visible-light absorption and photoredox potential of triphenylamine ligand to powerfully activate O2 via a single-electron-transfer process but also improves the conjugation degree, charge-carrier separation, and migration efficiency of the MOF semiconductor for rapid O2 activation. Such an oxygen activation ability endows Zn-TACPA with a catalytic yield of up to 91%, 2.6 times higher than that of Zn-TCA. Furthermore, the crucial intermediates and activation processes were also properly captured and monitored by a series of experiments including ESI-MS, ESR, IR, and fluorescence analyses to better understand the possible catalytic mechanisms.