Visible-Light-Driven Photocatalytic Coupling of Benzylamine over Titanium-Based MIL-125-NH2 Metal-Organic Framework: A Mechanistic Study

Imines are important building blocks in organic chemistry. Titanium-based metal-organic framework (MOF) MIL-125-NH2(Ti) can photocatalyze, under visible light and at room temperature, the selective aerobic oxidation of benzylamine to N-benzylidenebenzylamine. We investigated the reaction mechanism using catalytic tests, ex situ infrared spectroscopy, and density functional calculations. In the dark, the presence of MIL-125-NH2(Ti) alone does not improve the reaction yield with respect to a blank experiment. This poor catalytic performance in the dark is associated with the absence of polarizing species on the MOF surface, as confirmed by acetonitrile adsorption. Excitation with different spectral regions evidenced the determinant role of the 450 < lambda < 385 nm range for catalyst photoactivation. The calculations show that the last step of the reaction would have an energy barrier of 206 kJ mol(-1) in anhydrous conditions, while it decreases to 88 kJ mol(-1) only if the mechanism is mediated by two water molecules.