Exploiting the Reactivity of Fluorinated 2-Arylpyridines in Pd-Catalyzed C–H Bond Arylation for the Preparation of Bright Emitting Iridium(III) Complexes

Abstract

Pd-catalyzed C–H bond arylation applied to 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine (1) and 2-(3,5-difluorophenyl)-5-(trifluoromethyl)pyridine (5) allows the access to two families of Ir(III) complexes, charge-neutral and cationic species. The reaction is regioselective since only the C3- or C4-position of the fluorinated phenyl ring of 1 or 5 is readily functionalized - namely the C–H bond flanked by the two fluorine atoms which is the most acidic - which allows the electronic control of the reactive site. A range of electron-withdrawing (CN, CO₂Et, C(O)Me) substituents on the aryl group has been incorporated leading to the pro-ligands (1, Ar-2,4-dFppy; 2, Ar = p-C₆H₄–CN; 3, Ar = p-C₆H₄–CO₂Et; 4, Ar = p-C₆H₄–C(O)Me; 5, and Ar-3,5-dFppy; 6, Ar = p-C₆H₄–CO₂Et). The unsubstituted complexes F1/G1 and F1/G5 featuring 1 and 5, respectively, as C^N ligands are used as reference complexes. The families of five charge-neutral [Ir(C^N)₂(N^O)] complexes (C^N is 2-(5-aryl-(4,6-difluorophenyl)-5-(trifluoromethyl)pyridinato (F2–F4), and 2-(4-aryl-(3,5-difluorophenyl)-5-(trifluoromethyl)pyridinato (F5–F6), N^O = 2-picolinate) and five cationic [Ir(C^N)₂(N^N)]PF₆ complexes (N^N = dmbpy is 4,4′-dimethyl-2,2′-bipyridine) (G2–G6) were synthesized, and their structural and photophysical properties were studied with comparison to the unsubstituted analogues used as reference complexes. The appended aryl group provides large steric bulk as the biaryl fragment is twisted as shown by the X-ray crystal structures of F2, F5, F6, G3, and G5. These latter complexes display a wide variety of different Ir···Ir intermetallic distances in crystals, from 8.150 Å up to 15.034 Å. Moreover, the impact on the emission energy is negligible, as a result of the breaking of the conjugation between the two aryl groups. Charge-neutral complexes [Ir(C^N)₂(N^O)] (N^O = 2-picolinate) show bright luminescence: F2–F4 (λ_em = 495–499 nm) are blue-green emitters, whereas F5 and F6 (λ_em = 537, 544 nm), where the fluorine substituents are located at the C3- and C5-positions, emit in the green region of the visible spectrum. In all cases, a unitary photoluminescence quantum yield is found. The improvement of Φ might be explained by an increase of the radiative rate constant due to a higher degree of rigidity of these congested molecules, compared to the unsubstituted complex F1. The same trends are observed for the family of complexes G. Complexes G1–G4 exhibit blue photoluminescence, and G5 and G6 lead to a red-shifted emission band, as also found for the related complexes F5 and F6 due to the similar fluorine substitution pattern. Their emission quantum yields are remarkably high for charged complexes in the CH₂Cl₂ solution. These results showed that Pd-catalyzed C–H bond arylation is a valuable synthetic approach for designing efficient emitters with tunable photophysical properties.