Exchange Coupling Determines Metal-Dependent Efficiency for Iron- and Cobalt-Catalyzed Photochemical CO₂ Reduction

Abstract

Catalysts promoting multielectron charge delocalization offer selectivity for the CO2 reduction reaction (CO2RR) over the competing hydrogen evolution reaction. Here, we show metal–ligand exchange coupling as an example of charge delocalization that can determine the efficiency for photocatalytic CO2RR. A comparative evaluation of iron and cobalt complexes supported by the redox-active ligand tpyPY2Me establishes that the two-electron reduction of [Co­(tpyPY2Me)]2+ ([Co]2+ ) occurs at potentials 770 mV more negative than the [Fe­(tpyPY2Me)]2+ ([Fe] 2+ ) analogue by maximizing the exchange coupling in the latter compound. The positive shift in the reduction potential promoted by metal–ligand exchange coupling drives [Fe] 2+ to be among the most active and selective molecular catalysts for photochemical CO2RR reported to date, maintaining up to 99% CO product selectivity with total turnover numbers (TONs) and initial turnover frequencies exceeding 30,000 and 900 min–1, respectively. In contrast, [Co] 2+ shows much lower CO2RR activity, reaching only ca. 600 TON at 83% CO product selectivity under similar conditions accompanied by rapid catalyst decomposition. The spin density plots of the two-electron reduced [Co] 0 complex implicate a paramagnetic open-shell doublet ground state compared to the diamagnetic open-shell singlet ground state of reduced [Fe] 0 , rationalizing the observed negative shift in two-electron reduction potentials from the [M] 2+ species and lowered CO2RR efficiency for the cobalt complex relative to its iron congener. This work emphasizes the contributions of multielectron metal–ligand exchange coupling in promoting effective CO2RR and provides a starting point for the broader incorporation of this strategy in catalyst design.

Mechanistic Insights into Photochemical CO2 Reduction to CH4 by a Molecular Iron–Porphyrin Catalyst

“…In such a CO 2 reduction system, the catalyst is reduced after gaining electrons from the oxidative/reductive quenching process of the excited photosensitizer to generate the catalytically active species. Apart from the noble metal complexes (i.e., Re, − Ru, − Rh, and Ir complexes − ), many non-noble transition metal complexes, such as Mn, − Fe, − Co, − Ni, − and Cu complexes, − have also been used in photochemical CO 2 reduction. Modifying the ligands from neutral to charged groups can stabilize the critical M–CO 2 and M–CO intermediates through noncovalent electrostatic interactions, thus promoting the CO 2 RR toward target products.…”

Charge Transfer Chromophores Derived from 3d-Row Transition Metal Complexes

“…Quasi-reversible one-electron redox process, which is detected with significant displacement towards reductive steps for 1 and 2 , can be attributed to Co II → Co I reduction. Noticeably, the same metal-centered redox transformations were described in detail for Co II derivatives based on the polypyridyl multidentate ligand platforms [ 111 , 112 ]. Two consecutive one-electron reversible peaks are observed on the oxidative curve of the voltammogram.…”

“…Two consecutive one-electron reversible peaks are observed on the oxidative curve of the voltammogram. These processes can be attributed with equal probability to both the oxidation of catecholate ligands to o -semiquinone ones, and to the transition of Co II → Co III [ 112 ].…”

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

“…85 Notably, complex 36 showed remarkable activity towards the CO 2 RR also in a purely aqueous solution (0.1 M NaHCO 3 ) reaching FEs of 92% for CO. 75 This reactivity is also maintained under photochemical conditions using [Ru(bpy) 3 ] 2+ as the sensitizer and 1,3-dimethyl-2-phenyl-2,3-dihydro-1 H -benzoimidazole (BIH) as the electron donor. 76 Under continuous irradiation of an acetonitrile solution in the presence of 1 M phenol, CO is indeed delivered as the only reduction product (99% selectivity). The cobalt analogue 37 turns out to be effective as well in promoting the reduction of CO 2 to CO, although at larger overpotentials and with a lower selectivity (FE = 63.8%) than the iron complex.…”

“…The cobalt analogue 37 turns out to be effective as well in promoting the reduction of CO 2 to CO, although at larger overpotentials and with a lower selectivity (FE = 63.8%) than the iron complex. 76 Although the two-electron reduced species is similarly described as a [Co( ii )(L 2− )] involving a doubly-reduced ligand and a Co( ii ) centre, the different activity is explained on the basis of a lower metal–ligand orbital coupling in the case of the cobalt complex, thus highlighting the critical role of the metal–ligand combination in dictating the reactivity.…”