Selective visible light reduction of carbon dioxide over iridium(III)-terpyridine photocatalysts

The CO₂ reduction reaction is an imperative piece of technology that closes the carbon cycle in many critical energy conversion and chemical manufacturing processes. Here, we report two new iridium (III) terpyridine-based photocatalysts capable of selective reduction of CO₂ to CO under visible light (λ >= 420 nm). The first photocatalyst, [Ir-COOH], was functionalized with the carboxyl group on the phenylpyridine, whereas the second, [Ir-PhCOOH], was attached to a phenyl spacer on the terpyridine. The [Ir-PhCOOH] was characterized by a higher extinction coefficient than [Ir-COOH], thus allowing more absorption of photons. Although both photocatalysts require two-electron activation, the [Ir-PhCOOH] is more readily activated as a result of the more negatively charged Ir center. These photo catalysts show exclusive selectivities in the production of CO. The turnover frequencies for [Ir-COOH] and [Ir-PhCOOH] were 19 and 10 h^-1, respectively, under visible light irradiation. The e-e-H-H pathway was identified as the most favorable, consisting of the rate-limiting step in the conversion of *COOH to *CO, and where the barrier is significantly lower for [Ir-PhCOOH] than for [Ir-COOH]. (C) 2021 Elsevier Ltd. All rights reserved.