A Stable Zn-MOF for Photocatalytic Csp3–H Oxidation: Vinyl Double Bonds Boosting Electron Transfer and Enhanced Oxygen Activation

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.