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, CO 2 Et, C(O)Me) substituents on the aryl group has been incorporated leading to the pro-ligands (Ar-2,4-dFppy; 2, Ar = p-C 6 H 4 -CN; 3, Ar = p-C 6 H 4 -CO 2 Et; 4, Ar = p-C 6 H 4 -C(O)Me; and Ar-3,5-dFppy; 6, Ar = p-C 6 H 4 -CO 2 Et). The unsubstituted complexes F/G-1 and F/G-5 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)]PF 6 complexes [N^N = dmbpy is 4,4'-dimethyl-2,2'-bipyridine](G2-G6) were synthesized and their structural and photophysical properties 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 = 2picolinate) 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  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 leads 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 CH 2 Cl 2 solution. These results showed that Pd-catalyzed C-H bond arylation is a valuable synthetic approach for designing efficient emitters with tunable photophysical properties.

Palladium‐Catalyzed C−H Bond Arylation of Cyclometalated Difluorinated 2‐Arylisoquinolinyl Iridium(III) Complexes

“…Evidently, the substituents conferred by the Pd(II)‐catalyzed C−H bond arylation of the cyclometalated difluorinated 2‐arylisoquinoline ligands only slightly shift the oxidation couples of 5 a and 6 – 8 for 0–0.05 V from the unsubstituted complex 3 . Among the complexes under study, 4 and 7 show identifiable but irreversible reduction wave at around 2.0 V vs. Fc 0/+ , and hence the HOMO‐LUMO gaps could be estimated to be around 2.6 eV, which is slightly smaller than that (2.7–2.8 eV) for the phenylpyridyl Ir(III) analogues [13b] …”

“…Complexes 3 , 4 , 5 a , 6 , and 8 show reversible oxidation waves at 0.60, 0.49, 0.63, 0.65, and 0.60 V vs. Fc 0/+ , respectively, while complex 7 with a benzoyl group, exhibits a quasi‐reversible oxidation wave at 0.61 V vs. Fc 0/+ . These oxidation waves are negatively shifted from the Ir(III)/(IV) redox couple that is usually at around 1.0 V vs. Fc 0/+ , [13b] and hence could be tentatively assigned to be the oxidation of the difluorinated 2‐arylisoquinoline ligand. The oxidation couple of 3 (0.60 V) is positively shifted from that of 4 (0.49 V), which is in line with our previous report on the phenylpyridyl Ir(III) analogues [13b] and signifies the importance of the positions of the fluorine atoms, i. e. their electronic effects on the cyclometalated ligands for Ir(III) complexes.…”

“…These oxidation waves are negatively shifted from the Ir(III)/(IV) redox couple that is usually at around 1.0 V vs. Fc 0/+ , [13b] and hence could be tentatively assigned to be the oxidation of the difluorinated 2‐arylisoquinoline ligand. The oxidation couple of 3 (0.60 V) is positively shifted from that of 4 (0.49 V), which is in line with our previous report on the phenylpyridyl Ir(III) analogues [13b] and signifies the importance of the positions of the fluorine atoms, i. e. their electronic effects on the cyclometalated ligands for Ir(III) complexes. Evidently, the substituents conferred by the Pd(II)‐catalyzed C−H bond arylation of the cyclometalated difluorinated 2‐arylisoquinoline ligands only slightly shift the oxidation couples of 5 a and 6 – 8 for 0–0.05 V from the unsubstituted complex 3 .…”

Metal-catalyzed silylation of sp3C–H bonds

“…Thus these reactions should allow new functionalizations leading to natural products but also to modify in one pot reaction monomers for futher polymerization or polymers themselves, [93][94] or the physical properties of molecular materials, 95 including luminescence properties. [96][97] One can even expect that in the near future that catalytic sp 3 C-H bond silylation can be applied to directly modify metal-complexes and their optical properties, 98 or to produce more efficient catalysts.…”

MOF‐Stabilized Perfluorinated Palladium Cages Catalyze the Additive‐Free Aerobic Oxidation of Aliphatic Alcohols to Acids

“…However, despite both suitable features, perfluorinated pyridines have not been profited in catalysis, as consequence of their inability to bind strongly enough metal cations and stabilize the resulting complex. Thus, it is difficult to find any literature example of a catalytically efficient perfluorinated pyridine complex, despite all the precedents suggest that, if conveniently formed and stabilized, they may show an enhanced catalytic activity respect less fluorinated derivatives for selected organic reactions [27,28] …”