We report here the first purely organometallic fac‐[MnI(CO)3(bis‐MeNHC)Br] complex with unprecedented activity for the selective electrocatalytic reduction of CO2 to CO, exceeding 100 turnovers with excellent faradaic yields (η CO≈95 %) in anhydrous CH3CN. Under the same conditions, a maximum turnover frequency (TOFmax) of 2100 s−1 was measured by cyclic voltammetry, which clearly exceeds the values reported for other manganese‐based catalysts. Moreover, the addition of water leads to the highest TOFmax value (ca. 320 000 s−1) ever reported for a manganese‐based catalyst. A MnI tetracarbonyl intermediate was detected under catalytic conditions for the first time.

The reaction of [(PyNMe)Fe(CFSO)], 1, with excess peracetic acid at -40 °C generates a highly reactive intermediate, 2b(PAA), that has the fastest rate to date for oxidizing cyclohexane by a nonheme iron species. It exhibits an intense 490 nm chromophore associated with an S = 1/2 EPR signal having g-values at 2.07, 2.01, and 1.94. This species was shown to be in a fast equilibrium with a second S = 1/2 species, 2a(PAA), assigned to a low-spin acylperoxoiron(III) center. Unfortunately, contaminants accompanying the 2(PAA) samples prevented determination of the iron oxidation state by Mössbauer spectroscopy. Use of MeO-PyNMe (an electron-enriched version of PyNMe) and cyclohexyl peroxycarboxylic acid as oxidant affords intermediate 3b(CPCA) with a Mössbauer isomer shift δ = -0.08 mm/s that indicates an iron(V) oxidation state. Analysis of the Mössbauer and EPR spectra, combined with DFT studies, demonstrates that the electronic ground state of 3b(CPCA) is best described as a quantum mechanical mixture of [(MeO-PyNMe)Fe(O)(OC(O)R)] (∼75%) with some Fe(O)(OC(O)R) and Fe(OOC(O)R) character. DFT studies of 3b(CPCA) reveal that the unbound oxygen of the carboxylate ligand, O2, is only 2.04 Å away from the oxo group, O1, corresponding to a Wiberg bond order for the O1-O2 bond of 0.35. This unusual geometry facilitates reversible O1-O2 bond formation and cleavage and accounts for the high reactivity of the intermediate when compared to the rates of hydrogen atom transfer and oxygen atom transfer reactions of Fe(OC(O)R) ferric acyl peroxides and Fe(O) complexes. The interaction of O2 with O1 leads to a significant downshift of the Fe-O1 Raman frequency (815 cm) relative to the 903 cm value predicted for the hypothetical [(MeO-PyNMe)Fe(O)(NCMe)] complex.

Mechanistic understanding of electro-and photocatalytic CO2 reduction is crucial to develop strategies to overcome catalytic bottlenecks. In this regard, herein it is presented for a new CO2-to-CO reduction cobalt aminopyridine catalyst, a detailed experimental and theoretical mechanistic study toward the identification of bottlenecks and potential strategies to alleviate them. The combination of electrochemistry and in-situ spectroelectrochemistry together with spectroscopic techniques led us to identify elusive key electrocatalytic intermediates derived from complex [L N4 Co(OTf)2] (1) (L N4 =1-[2-pyridylmethyl]-4,7-dimethyl-1,4,7triazacyclononane) such as a highly reactive cobalt (I) (1 (I)) and cobalt (I) carbonyl (1 (I)-CO) species. The combination of spectroelectrochemical studies under CO2, 13 CO2 and CO with DFT disclosed that 1 (I) reacts with CO2 to form the pivotal 1 (I)-CO intermediate at the 1 (II/I) redox potential. However, at this reduction potential, the formation of 1 (I)-CO restricts the electrocatalysis due to the endergonicity of the CO release step. In agreement with the experimentally observed CO2-to-CO electrocatalysis at the Co I/0 redox potential, computational studies suggested that the electrocatalytic cycle involves striking metal carbonyls. In contrast, under photochemical conditions, the catalysis smoothly proceeds at the 1 (II/I) redox potential. Under the latter conditions, it is proposed that the electron transfer to form 1 (I)-CO from 1 (II)-CO is under diffusion control. Then, the CO release from 1 (II)-CO is kinetically favored, facilitating the catalysis. Finally, we have found that visible-light irradiation has a positive impact under electrocatalytic conditions. We envision that light-irradiation can serve as an effective strategy to circumvent the CO poisoning and improve the performance of CO2 reduction molecular catalysts. 1750 1950 2050 2150 Wavenumber (cm-1) Co I-CO Co 0-CO Co II-CO Theoretical 43 cm-1 Experimental under CO 2 Experimental under CO 1910 cm-1 44 cm-1 [L N4 Co I-CO] +

Metal oxides and oxyhydroxides exhibit state-of-the-art activity for the oxygen evolution reaction (OER); however, their reaction mechanism, particularly the relationship between charging of the oxide and OER kinetics, remains elusive. Here, we investigate a series of Mn-, Co-, Fe-, and Zn-doped nickel oxides using operando UV–vis spectroscopy coupled with time-resolved stepped potential spectroelectrochemistry. The Ni2+/Ni3+ redox peak potential is found to shift anodically from Mn- < Co- < Fe- < Zn-doped samples, suggesting a decrease in oxygen binding energetics from Mn- to Zn-doped samples. At OER-relevant potentials, using optical absorption spectroscopy, we quantitatively detect the subsequent oxidation of these redox centers. The OER kinetics was found to have a second-order dependence on the density of these oxidized species, suggesting a chemical rate-determining step involving coupling of two oxo species. The intrinsic turnover frequency per oxidized species exhibits a volcano trend with the binding energy of oxygen on the Ni site, having a maximum activity of ∼0.05 s–1 at 300 mV overpotential for the Fe-doped sample. Consequently, we propose that for Ni centers that bind oxygen too strongly (Mn- and Co-doped oxides), OER kinetics is limited by O–O coupling and oxygen desorption, while for Ni centers that bind oxygen too weakly (Zn-doped oxides), OER kinetics is limited by the formation of oxo groups. This study not only experimentally demonstrates the relation between electroadsorption free energy and intrinsic kinetics for OER on this class of materials but also highlights the critical role of oxidized species in facilitating OER kinetics.