[Ir(tpy)(bpy)Cl] as a Photocatalyst for CO 2 Reduction under Visible‐Light Irradiation

Mononuclear iridium(III) terpyridine (tpy) 2,2′‐bipyridine (bpy) [Ir(tpy)(bpy)Cl] 2+ photocatalysts (denoted [Ir(bpy)]) were developed for selective CO 2 reduction to HCOOH under visible light. The CO 2 reduction product could be changed dramatically by substituting a 2‐phenylpyridine (ppy) ligand with bpy, with the mononuclear Ir ppy complex ([Ir(tpy)(ppy)Cl] + ) acting as a photocatalyst for selective CO 2 reduction to CO. A mechanistic study showed a structural change in [Ir(bpy)] during the photocatalytic reaction. The [Ir(bpy)] complex was transformed into an iridium–hydride complex ([Ir(tpy)(bpy)H] 2+ ) during an early stage of the photocatalytic reaction. However, [Ir(tpy)(bpy)H] 2+ did not function as a key intermediate in the photochemical CO 2 reduction because an additional structural change occurred. The tpy ligand of the Ir complex was reduced to piperidine‐2,6‐di‐2‐pyridine ligand during the photocatalytic reaction, resulting in the production of [Ir(piperidine‐2,6‐di‐2‐pyridine)(bpy)H] 2+ , which was the actual photocatalyst for HCOOH production.