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The electrocatalytic reduction of carbon dioxide (CO2) to carbon monoxide (CO) is explored for both rhenium and manganese complexes. Electrochemistry, X-ray crystallography, Infrared spectroelectrochemistry, and stopped-flow kinetics are employed in order to identify catalysts and probe their mechanism and selectivity. Two catalysts in particular, Re(bipy-tBu)(CO3(L) and Mn(bipy-tBu)(CO3L (where bipy-tBu = 4,4'-di-tert-butyl-2,2'-bipyridine and L = Cl−, Br− or (MeCN)(OTf)−), were studied extensively and displayed high activity, Faradaic efficiency, and selectivity for the reduction of CO2 to CO. The Re-Cl catalyst exhibits a turnover frequency of>200 s−1, one of the fastest reported rates for a catalyst with appreciable turnover number. The Mn catalysts, when Brönsted acid sources are added to the electrochemical solution, exhibit current densities rivaling those of the Re-Cl catalyst. Amazingly, these catalysts showed high selectivity for CO2 in both dry solvents and those with significant amounts of Brönsted acid added. Stopped-flow UV-Vis kinetics experiments showed that the reaction of the active form of the catalysts, [M(bipy-tBu)(CO)3]−1, is ca 50 times faster with CO2 than they do with protons. Stopped-flow IR kinetics experiments comparing the reactions [Re(bipy-tBu)(CO)3]−1 with CO2 and [Re(bipy)(CO)3]−1 with CO2 shows, that at equal CO2 concentrations, the bipy-tBu analog reacts ten times faster than the bipy analog. CO2 also appears to react with both complexes via a concerted, two-electron oxidative addition of CO2 to the metal center. The heterogenization of these catalysts was also explored with limited success. Intercalation, covalent bonds to gold, and covalent bonds to p-Si were all demonstrated, but none displayed activity towards the reduction of CO2. Future experiments are suggested to solve this issue.
Electrocatalytic reduction of carbon dioxide (CO2) is a profoundly challenging problem that is of interest, not only as a means of counteracting unsustainable emissions of CO2, but also as a method for the development of renewable fuels. Rhenium and manganese bipyridine tricarbonyl complexes are among the most active and robust catalysts for proton-coupled CO2 reduction to carbon monoxide (CO). X-ray Absorption Spectroscopy studies are reported to reveal the electronic ground state of the Re catalysts, which help explain origins for high selectivity for CO2 reduction over proton reduction. Stopped-flow mixing in tandem with rapid-scan IR spectroscopy is utilized to probe the direct reaction of the Re catalysts with CO2, observing, for the first time, the binding of CO2 to these catalysts. Manganese bipyridine catalysts are desirable, in comparison with their Re analogs, due to the earth-abundance of Mn and the ability for these catalysts to operate at lower overpotentials. One distinct difference between these Mn catalysts and their Re counterparts is a high tendency for dimerization after one-electron reduction, which contributes to the potential necessary to access their active state and to limiting their catalytic activity. Synthetic modification of the bipyridine ligand (by adding bulky mesityl groups) is used to completely eliminate dimerization for these Mn complexes, allowing the active catalyst to be generated at a 300 mV more positive potential than in typically Mn bipyridine complexes. CO2 reactivities in the presence of weak Brønsted acids, strong Brønsted acids, and Lewis acids have been explored in order to encourage this bulky Mn catalyst to reduce CO2 at low overpotentials. Mechanistic tools, including IR-spectroelectrochemistry, are described to gain insight into these unique catalytic processes. In order to further enhance stability and facilitate product separation, the use of metal-organic frameworks (MOFs) is explored as a means of anchoring molecular catalysts on a heterogeneous platform. A Mn bipyridine catalyst attached to a highly robust Zr(IV)-based MOF is used to enhance photochemical CO2 reduction. By utilizing an iron porphyrin catalyst, anchored into the linkers of a MOF thin film, we demonstrate, in a proof of principle, electrochemical CO2 reduction by this heterogenized molecular catalyst.
Efficient conversion of carbon dioxide, a major greenhouse gas, into liquid fuels and commodity chemicals will have a huge impact on the energy economy and environmental sustainability. Thermodynamic stability and kinetic inertness of the CO2 molecule pose a challenge for its conversion into useful products. Development of an efficient and selective catalyst for CO2 reduction is thus important. Rhenium and manganese-based complexes have been studied for their ability to catalyze the electrochemical reduction of CO2. Rhenium and manganese-based catalysts containing phenanthroline and substituted phenanthroline have been studied for their activity towards the reduction of CO2 to CO. Manganese carbonyl complexes containing the diimine ligand such as diphenylphenanthroline, dimethyldiphenylphenanthroline, and tetramethylphenanthroline catalyze the reduction of CO2 even in the absence of external proton source. Presence of external proton source such as water is a must for most manganese-based CO2 redcution (sic) electrocatalysts to complete the catalytic cycle. In chapter four, a new series of manganese-based catalysts containing phenylenediammine ligands is discussed. The generation of spectacular CO gas bubbles from the electrochemical reduction of CO2 was observed in the bulk electrolysis experiments. Effects of electron-donating and electron-withdrawing substituents in the diamine ligands were investigated. The role of acid on the catalytic rates was also studied. Chapter five presents the novel design, synthesis, and electrocatalytic studies of rhenium and manganese complexes containing [selecting]-donation and the steric feature resulting from the various substituents in the P and N ligands. Finally, few dinuclear and trinuclear manganese complexes were tested for their ability to catalyze the reduction of CO2. Di- and trinuclear systems investigated here did not display cooperative binding and the resulting increased catalytic current is due to the presence of multiple metal centers as active sites.
Increased levels greenhouse gases have been suggested to have the potential to alter climate on a global scale. Carbon dioxide (CO2) is a rapidly growing component to the atmosphere; the rate of CO2 production is such that the natural carbon cycle is no longer adequate to maintain pre-industrial revolution levels. Carbon capture and sequestration is a viable means of becoming carbon neutral by converting atmospheric CO2 into useable fuels. It is widely believed that efficient conversion of CO2 into useable fuels, or fuel feedstock, requires the use of molecular catalysts. To that end, two series of catalysts of the form Mn(L)(CO)5Br where L = 2 aminopyridine derivative or dipyrromethane moiety, have been proposed. Spectroscopy was used to characterize these complexes and cyclic voltammetry was used to determine catalytic rate. Eleven of twelve proposed complexes show catalytic activity. The reported complexes show catalytic currents and overpotentials that meet or exceed current manganese catalysts.
Filling the need for an up-to-date handbook, this ready reference closely investigates the use of CO2 for ureas, enzymes, carbamates, and isocyanates, as well as its use as a solvent, in electrochemistry, biomass utilization and much more. Edited by an internationally renowned and experienced researcher, this is a comprehensive source for every synthetic chemist in academia and industry.
Carbon dioxide reduction has been an increasingly popular research in the renewable energy development as it can be used to store the solar energy in the form of chemical energy in liquid fuels, like gasoline and diesel. There are two main catalytic approaches to overcome the thermodynamically unfavored conversion of carbon dioxide (CO2) to carbon-based species, such as carbon monoxide and format: photochemical reduction using direct sunlight, and electrochemical reduction using electricity generated by solar panels. In a typical photochemical system using rhenium or manganese bipyridine catalysts, previous work has been done on ligand modification to improve the quantum yield of carbon monoxide (CO) and other carbon species production. The work presented in this dissertation focuses on the structural modification of these catalysts to eliminate dimerization of manganese bipyridine catalyst upon first reduction and facilitate electron transfer from singly reduced photosensitizer to catalyst through non-covalent supramolecular assembly. In the former method, the bromide ligand of the manganese bipyridine catalyst (Mnbpy(CO)3Br) was replaced with a cyanide ligand (Mnbpy(CO)3CN) to reach an alternative reaction mechanism, in which disproportionation of two singly reduced manganese bipyridine catalyst occurs to give the active species without dimerization. In the latter method, electron transfer between the singly reduced photosensitizer and the catalyst is facilitated by the closer proximity of the two through non-covalent hydrogen bonding. Both method, unexpectedly, discovered the role of solvents in photocatalysis on product selectivity. One of the biggest obstacles of electrochemical reduction of carbon dioxide in large-scale application is the immobilization of catalysts onto electrode surface. Most of the attachment methods in the literature face the issues of catalysts detachment and deactivation, and poor electrical contact between the catalyst and the electrode. A novel solvent-free synthetic method was invented to embed a top carbon dioxide electrocatalyst iron porphyrin into covalent organic frameworks. The COF-modified electrode demonstrated good activity for production of CO under electrocatalytic conditions in acetonitrile (MeCN) compared to the control electrode with only adsorbed iron porphyrins.
Fossil fuels still need to meet the growing demand of global economic development, yet they are often considered as one of the main sources of the CO2 release in the atmosphere. CO2, which is the primary greenhouse gas (GHG), is periodically exchanged among the land surface, ocean, and atmosphere where various creatures absorb and produce it daily. However, the balanced processes of producing and consuming the CO2 by nature are unfortunately faced by the anthropogenic release of CO2. Decreasing the emissions of these greenhouse gases is becoming more urgent. Therefore, carbon sequestration and storage (CSS) of CO2, its utilization in oil recovery, as well as its conversion into fuels and chemicals emerge as active options and potential strategies to mitigate CO2 emissions and climate change, energy crises, and challenges in the storage of energy.
Manganese Catalysis in Organic Synthesis A must-read reference for anyone interested in catalyst design and sustainable organic synthesis In Manganese Catalysis in Organic Synthesis, distinguished researcher Jean-Baptiste Sortais delivers an insightful and robust overview of the use of manganese in homogenous catalysis. The editor includes papers from authoritative academics describing the organometallic precursors used to develop manganese catalysts and covers critical applications in organic synthesis, including reduction to oxidation reactions, C-C, C-N, C-X bond formation reactions, cross-coupling reactions, C-H bond activation to dihydroxylation and epoxidation reactions. Manganese Catalysis in Organic Synthesis is a practical resource for every organic chemist in academia and industry with an interest in non-noble metal catalysis, organic synthesis, and sustainable chemistry. It is intuitively and clearly organized, covering the most important synthetic procedures using homogenous manganese catalysts. It is also the ideal companion to works like Cobalt Catalysis in Organic Synthesis, Nickel Catalysis in Organic Synthesis, and Iron Complexes in Catalysis. Readers will also enjoy: Thorough introductions to organometallic manganese compounds in organic synthesis and manganese-catalyzed hydrogenation and hydrogen transfer reactions A comprehensive exploration of manganese-catalyzed hydrogen borrowing reactions and dehydrogenative coupling reactions Practical discussions of manganese-catalyzed hydrosilylation and hydroboration reactions and manganese-catalyzed electro- and photocatalysis transformations In-depth examinations of manganese-catalyzed C-H oxygenation reactions and manganese-catalyzed organometallic C-H activation Insightful treatments of manganese-catalyzed cross-coupling processes and manganese(III) acetate mediated cyclizations Perfect for catalytic, organic, and pharmaceutical chemists, Manganese Catalysis in Organic Synthesis deserves a place in the libraries of researchers and professionals interested in catalyst design and sustainable organic synthesis.
Porphyrins play a vital role in many biological functions including oxygen transport, electron transfer and catalyzing the incorporation of oxygen into other molecules. This current survey discusses the use of modern physical techniques to probe porphyrin structure and function. The authors review the data available through a particular technique and show what can be learned therefrom about the (electronic) structure and function of biologically important porphyrins. The techniques include magnetic circular dichroism, X-ray absorption fine structure (EXAFS) and Mössbauer spectroscopies. All contributors are well known in their respective fields, enjoying world-wide reputation.
The use of CO2 as a chemical feedstock has been the focus of much research in recent years due to the promise for carbon neutral fuel storage in chemical bonds. Specifically, complexes of the type Re(bpy)(CO)3Cl (bpy = 2,2'-bipyridine and analogues thereof) have been studied for their ability to electrocatalytically reduce CO2 to CO. These catalysts are among the most active, selective, and robust homogeneous catalysts for CO2 reduction in the literature. Previous work has focused on mechanistic studies and determining the inductive effect of bipyridine functional groups on catalysis. The work presented in this dissertation focuses on the structural and biomimetic modification of these catalysts. In order to determine the optimal point of modification, the CO2 reduction capabilities of a series of Re(n,n'-dimethyl-bpy)(CO)3Cl (n = 3, 4, and 5) catalysts with a bpy modified at the 3, 4, and 5 positions with methyl substituents were assessed. A decreased catalytic current response in the n = 3 case can be explained by steric hindrance in the 3,3'- substituted catalyst disfavoring optimal charge transfer in the catalytic cycle. A series of rhenium catalysts were synthesized with bpy substituents amenable to common surface- and bio-conjugation techniques. Specifically, aminomethyl, groups were found to contribute to competent catalysts and were interrogated further. Interestingly, before the complex was coupled to amino acids, simple acylated amines (mimicking peptide bonds) on the 4- and 4,4'- positions of the bpy ligand were found to alter the mechanism by the rhenium catalyst operated electrochemically. Instead of a single metal site catalyzing the proton-dependent reduction of CO2 to CO and H2O, the bimetallic site, templated by hydrogen-bonding of the peptide bonds, reduces CO2 to CO and CO32- at potentials up to 240 mV more positive than previously studied catalysts of this type. The catalysts were then incorporated into amino acids and short peptides to investigate the advantageous effects of adding proton sources (tyrosine) and readily modifiable platform (peptides) on catalysis. In addition to proton sources, Lewis-acids can serve to increase the rates of catalysis; however insoluble carbonates prevent the reaction from being catalytic with respect to the Lewis acid. Macromolecules were used to bind the metal Lewis acids to prevent metal-carbonate formation over the course of the reaction. Furthermore, hydrogen-bonding could be utilized to template these heterobimetallic interactions and the preliminary work is presented.