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

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 (Scheme 1). 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 of these complexes was studied demonstrating that tris-trifluoromethylation is an important substitution class, especially when Ga3+ is present (TON 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 benzyl bromide 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 positive shift of 116 mV and 126 mV per [CF3] group, respectively, compared to unsubstituted parent species 1-Ga and 1-Al. However, the absorbance values (λabs = 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) (Fig 5 (a)), 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.