The Tandem Photoredox Catalysis Mechanism of [Ir(ppy)₂(dtb-bpy)]⁺ Enabling Access to Energy Demanding Organic Substrates

Abstract

We report the discovery of a tandem catalytic process to reduce energy demanding substrates, using the [Ir(ppy) 2 (dtb-bpy)] + (1 + ) photocatalyst. The immediate products of photoinitiated electron transfer (PET) between 1 + and triethylamine (TEA) undergo subsequent reactions to generate a previously unknown, highly reducing species (2). Formation of 2 occurs via reduction and semisaturation of the ancillary dtb-bpy ligand, where the TEA radical cation serves as an effective hydrogen atom donor, confirmed by nuclear magnetic resonance, mass spectrometry, and deuterium labeling experiments. Steady-state and time-resolved luminescence and absorption studies reveal that upon irradiation, 2 undergoes electron transfer or proton-coupled electron transfer (PCET) with a representative acceptor (N-(diphenylmethylene)-1-phenylmethanamine; S). Turnover of this new photocatalytic cycle occurs along with the reformation of 1 + . We rationalize our observations by proposing the first example of a mechanistic pathway where two distinct yet interconnected photoredox cycles provide access to an extended reduction potential window capable of engaging a wide range of energy demanding and synthetically relevant organic substrates including aryl halides.

Multi‐Photon Excitation in Photoredox Catalysis: Concepts, Applications, Methods

“…It is tempting to argue that photons are cheap and low η values can be counteracted by increasing the intensity and/or the duration of the photo‐irradiation, but this is delicate in view of possible (catalyst) photo‐degradation. As noted in the sections above, in selected cases, there was clear evidence for the conversion of catalysts into other catalytically active species, and this may be a more frequent phenomenon than one might commonly think.…”

“…Curiously, a change of the DIPEA concentration by a factor of 10 had essentially no influence on the product yield, which is unexpected in the framework of the mechanism of Figure because of the abovementioned balance between energy and electron‐transfer quenching of *PC. Under comparable experimental conditions with a closely related tertiary amine reductant, a similar Ir III complex undergoes photo‐driven ligand reduction, yielding a highly reducing new Ir species that was the actual photocatalyst in thermodynamically challenging reactions (Section 8) …”

“…The mechanism in Figure is relevant for the hydrodehalogenation of aryl and alkyl halides (Table ) . Irradiation of [Ir(ppy) 2 (dtbpy)] + (Table , left) in the presence of TEA leads to the formation of a new green‐emitting Ir species.…”

Multiphotonen‐Anregung in der Photoredoxkatalyse: Konzepte, Anwendungen und Methoden

“…Unter vergleichbaren experimentellen Bedingungen mit einem sehr ähnlichen tertiären Amin als Reduktionsmittel wurde für einen ähnlichen Ir III ‐Komplex über photochemische Ligandenreduktion berichtet, die eine stark reduzierende neue Ir‐Spezies erzeugt. Diese Spezies agierte als eigentlicher Photokatalysator bei thermodynamisch anspruchsvollen Reaktionen (Abschnitt 8) …”

“…Der Mechanismus aus Abbildung ist für die Hydrodehalogenierung von Aryl‐ und Alkylhalogeniden (Tabelle ) verwendet worden . Bei der Bestrahlung von [Ir(ppy) 2 (dtbpy)] + (Tabelle , links) in Gegenwart von TEA wird ein neuer, grün emittierender Ir‐Komplex gebildet.…”