The electrochemical reduction of CO has been extensively investigated in recent years, with the expectation that a detailed mechanistic understanding could achieve the goal of finding a stable catalyst with high turnover frequencies and low reduction potentials. In the catalytic cycle of the carbon dioxide hydrogenase enzyme, it has been suggested that the reduced metal center reacts with CO to form a carboxylate intermediate that is stabilized by hydrogen bonding using a histidine moiety in the second coordination sphere. Using the well-known fac-Re(I)bipyridine(CO)Cl complex as a starting point, the bipyridine ligand was modified in the second coordination sphere with a thiourea tether that is known to form hydrogen bonds with carbonyl moieties. The resulting Re(I) catalyst was an excellent electrocatalyst for the selective reduction of CO to CO, with a turnover frequency of 3040 s. The binding of CO to the thiourea tether was observable by H NMR, and NOE experiments showed that the hydrogen atoms of the thiourea group were labile. Further experiments indicated that the thiourea moiety is also a local proton source and addition of an external proton source actually inhibits catalysis. The absence of a kinetic isotope effect was explained through DFT calculations that showed that the proton invariably jumps to the nearest CO oxygen atom to form a metal-carboxylic acid without going through any minimum or transition state. EPR and NMR spectroscopies were used to identify the various reduced intermediates. Thus, the thiourea tether in the second coordination sphere can bind CO, stabilize carboxylic acid reaction intermediates, and directly act as a local proton source, leading to a significantly more active catalyst.
The photochemical reduction of CO to CO requires two electrons and two protons that, in the past, have been derived from sacrificial amine donors that are also non-innocent in the catalytic cycle. Towards the realization of a water-splitting reaction as the source of electrons and protons for CO reduction, we have found that a reduced acidic polyoxometalate, H PW W O , is a photoactive electron and proton donor with visible light through excitation of the intervalence charge-transfer band. Upon linking the polyoxometalate to a dirhenium molecular catalyst, a cascade of transformations occurs where the polyoxometalate is electrochemically reduced at a relatively low negative potential of 1.3 V versus Ag/AgNO and visible light, a 60 W tungsten lamp, or a red LED is used to transfer electrons from the polyoxometalate to the dirhenium catalyst active for the selective reduction of CO to CO.
Research on the photochemical reduction of CO2, initiated already 40 years ago, has with few exceptions been performed by using amines as sacrificial reductants. Hydrocarbons are high‐volume chemicals whose dehydrogenation is of interest, so the coupling of a CO2 photoreduction to a hydrocarbon‐photodehydrogenation reaction seems a worthwhile concept to explore. A three‐component construct was prepared including graphitic carbon nitride (g‐CN) as a visible‐light photoactive semiconductor, a polyoxometalate (POM) that functions as an electron acceptor to improve hole–electron charge separation, and an electron donor to a rhenium‐based CO2 reduction catalyst. Upon photoactivation of g‐CN, a cascade is initiated by dehydrogenation of hydrocarbons coupled to the reduction of the polyoxometalate. Visible‐light photoexcitation of the reduced polyoxometalate enables electron transfer to the rhenium‐based catalyst active for the selective reduction of CO2 to CO. The construct was characterized by zeta potential, IR spectroscopy, thermogravimetry, scanning electron microscopy (SEM) and energy dispersive X‐ray spectroscopy (EDS). An experimental Z‐scheme diagram is presented based on electrochemical measurements and UV/Vis spectroscopy. The conceptual advance should promote study into more active systems.
The reductive activation of molecular oxygen catalyzed by iron-based enzymes toward its use as an oxygen donor is paradigmatic for oxygen transfer reactions in nature. Mechanistic studies on these enzymes and related biomimetic coordination compounds designed to form reactive intermediates, almost invariably using various “shunt” pathways, have shown that high-valent Fe(V)O and the formally isoelectronic Fe(IV)O porphyrin cation radical intermediates are often thought to be the active species in alkane and arene hydroxylation and alkene epoxidation reactions. Although this four decade long research effort has yielded a massive amount of spectroscopic data, reactivity studies, and a detailed, but still incomplete, mechanistic understanding, the actual reductive activation of molecular oxygen coupled with efficient catalytic transformations has rarely been experimentally studied. Recently, we found that a completely inorganic iron–tungsten oxide capsule with a keplerate structure, noted as {Fe30W72}, is an effective electrocatalyst for the cathodic activation of molecular oxygen in water leading to the oxidation of light alkanes and alkenes. The present report deals with extensive reactivity studies of these {Fe30W72} electrocatalytic reactions showing (1) arene hydroxylation including kinetic isotope effects and migration of the ipso substituent to the adjacent carbon atom (“NIH shift”); (2) a high kinetic isotope effect for alkyl CH bond activation; (3) dealkylation of alkylamines and alkylsulfides; (4) desaturation reactions; (5) retention of stereochemistry in cis-alkene epoxidation; and (6) unusual regioselectivity in the oxidation of cyclic and acyclic ketones, alcohols, and carboxylic acids where reactivity is not correlated to the bond disassociation energy; the regioselectivity obtained is attributable to polar effects and/or entropic contributions. Collectively these results also support the conclusion that the active intermediate species formed in the catalytic cycle is consistent with a compound I type oxidant. The activity of {Fe30W72} in cathodic aerobic oxidation reactions shows it to be an inorganic functional analogue of iron-based monooxygenases.
Host–guest solution chemistry with a wide range of organic hosts is an important and established research area, while the use of inorganic hosts is a more nascent area of research. In the recent past in a few cases, Keplerate-type molybdenum oxide-based porous, spherical clusters, shorthand notation {Mo 132 }, have been used as hosts for organic guests. Here, we demonstrate the synthetically controlled encapsulation of first-row transition metals (M = Mn, Fe, and Co) within a Keplerate cluster that was lined on the inner core with phosphate anions, {Mo 132 PO 4 }. The resulting M 2+ x ⊂{Mo 132 PO 4 } host–guest complexes were characterized by 31 P NMR and ENDOR spectroscopy that substantiated the encapsulation of the first-row transition metal guest. Magnetic susceptibility measurements showed that the encapsulation of up to 10 equiv showed little magnetic interaction between the encapsulated metals, which indicated that each guest atom occupied a single site. Visualization of the capsules and differentiation of the Mo atoms of the capsule framework and the encapsulated transition metal were possible using spherical and chromatic double aberration-corrected electron microscopy combined with energy-filtered TEM (EFTEM) elemental maps. In addition, use of visible light-induced XPS for chemically resolved electrical measurements (CREM) confirmed the successful encapsulation of M within {Mo 132 PO 4 } and furthermore showed photoinduced electron transfer from M to Mo. In the future, such targeted electron transfer between host {Mo 132 } and a transition metal guest could be used as photoinitiated switches using inorganic compounds and for single site photocatalytic reactions in confined space.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.