TiO2 Nanoparticles Functionalized with Non-innocent Ligands Allow Oxidative Photocyanation of Amines with Visible/Near-Infrared Photons

Abstract

Photosynthesis is an efficient mechanism for converting solar light energy into chemical energy. We report on a strategy for the aerobic photocyanation of tertiary amines with visible and near-infrared (NIR) light. Panchromatic sensitization was achieved by functionalizing TiO₂ with a 2-methylisoquinolinium chromophore, which captures essential features of the extended π-system of 2,7-diazapyrenium (DAP²⁺) dications or graphitic carbon nitride. Two phenolic hydroxy groups make this ligand highly redox-active and allow for efficient surface binding and enhanced electron transfer to the TiO₂ surface. Non-innocent ligands have energetically accessible levels that allow redox reactions to change their charge state. Thus, the conduction band is sufficiently high to allow photochemical reduction of molecular oxygen, even with NIR light. The catalytic performance (up to 90% chemical yield for NIR excitation) of this panchromatic photocatalyst is superior to that of all photocatalysts known thus far, enabling oxidative cyanation reactions to the corresponding α-cyanated amines to proceed with high efficiency. The discovery that the surface-binding of redox-active ligands exhibits enhanced light-harvesting in the red and NIR region opens up the way to improve the overall yields in heterogeneous photocatalytic reactions. Thus, this class of functionalized semiconductors provides the basis for the design of new photocatalysts containing non-innocent donor ligands. This should increase the molar extinction coefficient, permitting a reduction of nanoparticle catalyst concentration and an increase of the chemical yields in photocatalytic reactions.