Photocatalytic Aerobic Dehydrogenation of N-Heterocycles with Ir(III) Photosensitizers Bearing the 2(2'-Pyridyl)benzimidazole Scaffold

Abstract

Photoredox catalysis constitutes a very powerful tool in organic synthesis, due to its versatility, efficiency, and the mild conditions required by photoinduced transformations. In this paper, we present an efficient and selective photocatalytic procedure for the aerobic oxidative dehydrogenation of partially saturated N-heterocycles to afford the respective N-heteroarenes (indoles, quinolines, acridines, and quinoxalines). The protocol involves the use of new Ir(III) biscyclometalated photocatalysts of the general formula [Ir(C^N)₂(N^N')]Cl, where the C^N ligand is 2-(2,4-difluorophenyl)pyridinate, and N^N' are different ligands based on the 2-(2'-pyridyl)benzimidazole scaffold. In-depth electrochemical and photophysical studies as well as DFT calculations have allowed us to establish structure-activity relationships, which provide insights for the rational design of efficient metal-based dyes in photocatalytic oxidation reactions. In addition, we have formulated a dual mechanism, mediated by the radical anion superoxide, for the above-mentioned transformations.

Figure 1

Synthesis route and molecular structures of ligands L1–L5 and complexes [Ir1]Cl–[Ir5]Cl. Complexes were obtained as racemic mixtures but only Λ enantiomers are shown.

Figure 2

ORTEP diagrams for the molecular structures of (Λ)-[Ir1]⁺, (Λ)-[Ir3]⁺, (Λ)-[Ir4]⁺ and (Λ)-[Ir5]⁺ obtained by X-ray diffraction. Thermal ellipsoids are shown at the 30% probability level. The Δ enantiomers, the H atoms, the Cl^– or PF₆^– counterions, and the solvent molecules (MeOH for rac-[Ir1]Cl) have been omitted for the sake of clarity.

Figure 3

Schematic representation of the energies and the isovalue contour pictures calculated for the frontier molecular orbitals of [Ir(dfppy)₂(bpy)]⁺, [2]⁺, and [Ir3]⁺.

Figure 4

Energy diagram showing the calculated energy difference values between the lowest triplet excited state (T₁) and the singlet ground state, keeping the geometry of the respective triplet (S₀) for complexes [1]⁺, [2]⁺, and [Ir1]⁺–[Ir5]⁺.

Figure 5

(a) Overlaid UV–vis absorption spectra of [Ir1]Cl–[Ir5]Cl (10^–5 M) in CH₃CN at 25 °C along with the emission spectrum of the blue light used in the photocatalytic assays (left). (b) Overlaid emission spectra of of [Ir1]Cl–[Ir5]Cl (10^–5 M) in deoxygenated CH₃CN at 25 °C upon excitation with λ_ex = 405 nm (right).

Figure 6

Cyclic voltammograms of complexes [Ir1]Cl–[Ir5]Cl in acetonitrile solution (5 × 10^–4 M), using 0.1 M [nBu₄N][PF₆] as supporting electrolyte and recorded with scan rate of 0.10 V·s^–1.

Figure 7

Latimer diagram for [Ir3]Cl, with redox potentials determined by CV and the emission energy calculated from the photoluminescence spectrum. The redox potentials for [Ir3]⁺ and its excited state [Ir3*]⁺ are given in V versus Fc⁺/Fc. E_1/2(Ir^IV/Ir^III*) = E_1/2(Ir^IV/Ir^III) – E_1/2(Ir^III*/Ir^III) and E_1/2(Ir^III*/Ir^II) = E_1/2(Ir^III/Ir^II) + E_1/2(Ir^III*/Ir^III). All the potential values are given as reduction potentials regardless the sense of the arrows for the quenching cycles.

Figure 8

Stern–Volmer quenching experiments. (a) Emission quenching of [Ir4]Cl (0.07 mM in CH₃CN, 25 °C) upon incremental addition of substrate 3a (0.1–2 mM) under N₂ and λ_ir = 405 nm. (b) Stern–Volmer quenching plot, where I₀ = PL intensity of [Ir4]Cl at [3a] = 0 mM; I = PL intensity of [Ir4]Cl at different [3a]; I₀/I = 29.728 × [3a] + 1.0544; R² = 0.9996.

Figure 9

Pathways A and B for the oxidative dehydrogenation of 3a in the presence of the new Ir(III) PCs. Steps (2), (3) and (4), (5) etc. from species A are common for both pathways.