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

The CO 2 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 CO 2 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 photocatalysts 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]. • New iridium complexes show high activity and selectivity in the visible light reduction of CO 2 . • Addition of phenyl spacer to the terpyridine gave rise to higher absorbance and facile activation. • The e-e-H-H pathway is identified as the most favorable for CO 2 reduction. • Addition of phenyl spacer resulted in low-energy barrier in the conversion of ∗COOH to ∗CO.