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)]⁺ (<b>1</b>^<b>+</b>) photocatalyst. The immediate products of photoinitiated electron transfer (PET) between <b>1</b>^<b>+</b> and triethylamine (TEA) undergo subsequent reactions to generate a previously unknown, highly reducing species (<b>2</b>). Formation of <b>2</b> 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, <b>2</b> undergoes electron transfer or proton-coupled electron transfer (PCET) with a representative acceptor (<i>N</i>-(diphenylmethylene)-1-phenylmethanamine; <b>S</b>). Turnover of this new photocatalytic cycle occurs along with the reformation of <b>1</b>^<b>+</b>. 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.