Positive shift in corrole redox potentials leveraged by modest β-CF₃-substitution helps achieve efficient photocatalytic C–H bond functionalization by group 13 complexes

Abstract

Tris- and tetrakis-β-trifluoromethylated gallium (3CF₃-Ga, 4CF₃-Ga) and aluminum (3CF₃-Al, 4CF₃-Al) corrole systems were synthesized by a facile "one-pot" approach from the respective tri- and tetra-iodo starting compounds using the FSO₂CF₂CO₂Me reagent. The isolated 5,10,15-(tris-pentafluorophenyl)corrole-based compounds set the groundwork for another important β-substituent study in inorganic photocatalysis. As seen previously, -CF₃ group substitution leads to red shifts in both the absorption and emission spectra compared to their unsubstituted counterparts (X. Zhan, et al., Inorg. Chem., 2019, 58, 6184-6198). All CF₃-substituted corrole complexes showed strong fluorescence; 3CF₃-Al possessed the highest fluorescence quantum yield (0.71) among these compounds. The photocatalytic production of bromophenol by way of these photosensitizing complexes was studied demonstrating that tris-trifluoromethylation is an important substitution class, especially when Ga³⁺ is present (experimental TON value in parentheses): 3CF₃-Ga (192) > 4CF₃-Ga (146) > 3CF₃-Al (130) > 4CF₃-Al (56) > 1-Ga (43) > 1-Al (18). The catalytic performance (turn-over number, TON) for benzylbromide formation (from toluene) was found to be: 3CF₃-Ga (225) > 1-Ga (138) > 3CF₃-Al (130) > 4CF₃-Ga (126) > 1-Al (95) > 4CF₃-Al (89); in these trials, benzaldehyde was also detected as a product in which 3CF₃-Ga outperforms the other compounds (TON = 109). The tetra-CF₃-substituted 4CF₃-Ga and 4CF₃-Al species exhibit a dramatic formal positive shift of 116 mV and 126 mV per [CF₃] group, respectively, compared to the unsubstituted parent species 1-Ga and 1-Al. However, the absorbance values (λ_abs = 400 nm) of these corrole complexes (all equally concentrated: 4.0 × 10^-6 M) were 3CF₃-Al (0.23) > 3CF₃-Ga (0.22) > 1-Al (0.21) > 1-Ga (0.20) > 4CF₃-Al (0.19) > 4CF₃-Ga (0.15), which helps rationalize why 3CF₃-Ga performs the best among these catalysts. These new photosensitizers were carefully characterized by ¹H and ¹⁹F NMR spectroscopy to help verify the number and position (symmetry) of the CF₃ groups; 3CF₃-Ga and 3I-Al were structurally characterized. Distortions in the corrole macrocycle imposed by the multiple β-substitution were quantified.