Catalytic CO₂ Reduction with Heptacoordinated Polypyridine Complexes: Switching the Selectivity via Metal Replacement

Abstract

The discovery of molecular catalysts for the CO2 reduction reaction (CO2RR) in the presence of water, which are both effective and selective towards the generation of carbon‐based products, is a critical task. Herein we report the catalytic activity towards the CO2RR in acetonitrile/water mixtures by a cobalt complex and its iron analog both featuring the same redox‐active ligand and an unusual seven‐coordination environment. Bulk electrolysis experiments show that the cobalt complex mainly yields formate (52% selectivity at an applied potential of −2.0 V vs Fc+/Fc and 1% H2O) or H2 (up to 86% selectivity at higher applied bias and water content), while the iron complex always delivers CO as the major product (selectivity >74%). The different catalytic behavior is further confirmed under photochemical conditions with the [Ru(bpy)3]2+ sensitizer (bpy = 2,2’‐bipyridine) and N,N‐diisopropylethylamine as electron donor, where the cobalt complex leads to preferential H2 formation (up to 89% selectivity), while the iron analog quantitatively generates CO (up to 88% selectivity). This is ascribed to a preference towards a metal‐hydride vs. a metal‐carboxyl pathway for the cobalt and the iron complex, respectively, and highlights how metal replacement may effectively impact on the reactivity of transition metal complexes towards solar fuel formation.

Bioinspired motifs in proton and CO2 reduction with 3d-metal polypyridine complexes

“…2 and 12). Interestingly, differently from its parent cobalt complex 3 for which H 2 is still the dominant product (see Table 2), the iron( ii ) analogue 35 behaves as a selective catalyst towards the CO 2 reduction to CO. 73 Catalysis mainly occurs via an EECC mechanism involving a rate-determining proton-assisted CO 2 binding at the two-electron reduced complex (formal Fe(0) species). A combination of spectroelectrochemistry and theoretical calculations suggests that the catalytically-active Fe(0) can be approximated as a [Fe( ii )(L 2− )] species involving an iron( ii ) centre and a two-electron reduced ligand (Fig.…”