Team exploits weak intermolecular interactions for photocatalysis

Photo Credit: Figure 1. (a) Proposed photocatalytic cycle for HDF. (b) Reduction potentials and density functional theory (DFT) computed electrostatic surface potential maps (inset) of Py and FA.
Figure 1. (a) Proposed photocatalytic cycle for HDF. (b) Reduction potentials and density functional theory (DFT) computed electrostatic surface potential maps (inset) of Py and FA.
by Department of Chemistry Thu, 12/01/2016 - 13:53

Figure 1. (a) Proposed photocatalytic cycle for HDF. (b) Reduction potentials and density functional theory (DFT) computed electrostatic surface potential maps (inset) of Py and FA.

Electron transfer (ET) plays a critical role in many organic transformations.[1] In photoredox catalysis, the ET usually follows light-induced non-adiabatic outer-sphere ET mode, which requires suitable overpotential to achieve efficient ET kinetics.[2] Inner-sphere ET[3], on the other hand, can be achieved adiabatically between closely bind species, resulting in significantly faster ET than that predicted by the outer-sphere mode[4] due to strong electronic coupling. It can be envisioned that fast inner-sphere ET can be achieved when photocatalyst closely binds with substrate, which, however, has not been developed yet.

Polyfluoroarene–arene (also known as “π-hole[5]–π”) interaction is a directional and non-covalent intermolecular force, attributed to the weak electrostatic interaction between polyfluoroarenes (positive surface potential due to the flipped quadruple moment) and arenes (negative surface potential).[6] Dr. Zhang and co-workers discovered that light-induced efficient inner-sphere ET between FA and photocatalyst can be achieved when photocatalyst is an electron-rich arene (for example, Py), owing to the formation of “π-hole–π” complex between FA and photocatalyst (Figure 1a). More importantly, this process proceeds smoothly against a large underpotential (Figure 1a) and subsequently promotes a HDF reaction to yield partially fluorinated arenes (Figure 1b).

The formation of “π-hole–π” complexes were proved by 1H NMR titration and were further confirmed by DFT-based structure optimization. Furthermore, the stronger electrostatic interaction in “π-hole–π” complexes than the typical π–π stacking force was revealed by the average interplane distances in single crystal analysis. The team applied the optimized reaction condition to a series of FA and successfully access to the HDF products with good to excellent yield up to 93%. They also demonstrated the potential utility of Py in the metal-free C-F reductive alkylation. This work points to the further development of the design paradigm for photoredox catalysis where the size and shape of photocatalyst can be fine-tuned to enhance the overall catalytic activity. This work also constitutes a new example of the utility of weak, non-covalent interaction in small molecule catalysis.

This work was just accepted by JACS.


[1] Houmam, A. Chem. Rev. 2008, 108, 2180

[2] Marcus, R. A. Angew. Chem. Int. Ed. 1993, 32, 1111

[3] Taube, H. Angew. Chem. Int. Ed. 1984, 23, 329

[4] Asahi, T.; Mataga, N. J. Phys. Chem. 1991, 95, 1956

[5] Bauza, A.; Mooibroek, T. J.; Frontera, A. ChemPhysChem 2015, 16, 2496

[6] Wang, H.; Wang, W.; Jin, W. J. Chem. Rev. 2016, 116, 5072