2-Arylbenzo[d]xazolyl based iridium(III) complexes as highly efficient photoredox catalysts

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The [Ir(C^N)2(N^N)]+ complexes are one of the most employed family of photocatalyst. The substituent effect of C^N ligand, specifically 2-arylbenzo[d]oxazole (pbo) and 2-arylbenzo[d]thiazole (pbt), on complexes was discussed in this work. For this purpose, a series of Ir(III) complexes (A1∼C9) with differently substituted pbo/pbt ligands were designed, synthesized and characterized. Crystal structures show that A2 and A6 have a C2-symmetric trans-configuration while B8 possesses C1-symmetric cis-isomeric structure. These complexes are highly photoluminescent with quantum yield up to 69% in dilute solution. The maximum emission wavelength span from 485 to 615 nm. The substituent electronic effects on the para-position of phenyl have the greatest impact on the fluorescence properties of the complexes. Electron withdrawing nitro group on the 4-site of phenyl make the greatest red shift in the emission spectra while fluorination leads to blue shifts. The electrochemical measurements were combined with fluorescence data to estimate the excited-state potentials (ESP) of these complexes. Both oxidative and reductive ESP are in a good range compared to the widely applied [Ir(dF(CF3)ppy)2dtbbpy]PF6, suggesting that they are high-potential in photoredox catalysis. The complexes A1∼C9 were further applied in the deoxygenation-addition synthesis of N-dimethyl-4-oxo-4-(p-tolyl)butanamide to test the catalytic activity. Excellent yields (up to 91%) were obtained with catalyst A2, A8 and B8. Substitution on substrates was also tolerated in a degree.