Ligand and Metal Based Multielectron Redox Chemistry of Cobalt Supported by Tetradentate Schiff Bases

Julie Renée Michelle Andrez, Marinella Mazzanti, Rosario Scopelliti
2017
Journal paper

Abstract

We have investigated the influence of bound cations on the reduction of cobalt complexes of redox active ligands and explored the reactivity of reduced species with CO2. The one electron reduction of [CoII(Rsalophen)] with alkali metals (M = Li, Na, K) leads to either ligand-centered or metal-centered reduction depending on the alkali ion. It affords either the [CoI(Rsalophen)K] complexes or the [CoII2(bis-salophen)M2] (M = Li, Na) dimers that are present in solution in equilibrium with the respective [CoI(salophen)M] complexes. The two electron reduction of [CoII(OMesalophen)] results in both ligand centered and metal centered reduction affording the Co(I)–Co(II)–Co(I) [Co3(tris-OMesalophen)Na6(THF)6], 6 complex supported by a bridging deca-anionic tris-OMesalophen10– ligand where three OMesalophen units are connected by two C–C bonds. Removal of the Na ion from 6 leads to a redistribution of the electrons affording the complex [(Co(OMesalophen))2Na][Na(cryptand)]3, 7. The EPR spectrum of 7 suggests the presence of a Co(I) bound to a radical anionic ligand. Dissolution of 7 in pyridine leads to the isolation of [CoI2(bis-OMesalophen)Na2Py4][Na(cryptand)]2, 8. Complex 6 reacts with ambient CO2 leading to multiple CO2 reduction products. The product of CO2 addition to the OMesalophen ligand, [Co(OMesalophen-CO2)Na]2[Na(cryptand)]2, 9, was isolated but CO32– formation in 53% yield was also detected. Thus, the electrons stored in the reversible C–C bonds may be used for the transformation of carbon dioxide.

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Julie Renée Michelle Andrez, Marinella Mazzanti, Rosario Scopelliti
2017
Journal paper

Abstract

Official source

https://infoscience.epfl.ch/record/230051?ln=en

About this result

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Carbon dioxide reduction by lanthanide(iii) complexes supported by redox-active Schiff base ligands

Nadir Jori, Marinella Mazzanti, Rosario Scopelliti, Davide Toniolo

Here we have explored the ability of Schiff bases to act as electron reservoirs and to enable the multi-electron reduction of small molecules by lanthanide complexes. We report the reductive chemistry of the Ln(iii) complexes of the tripodal heptadentate Schiff base H(3)trensal (2,2 ',2 ''-tris(salicylideneimino)triethylamine), [Ln(III)(trensal)],1-Ln(Ln = Sm, Nd, Eu). We show that the reduction of the [Eu-III(trensal)] complex leads to the first example of a Eu(ii) Schiff base complex [{K(mu-THF)(THF)(2)}(2){Eu-II(trensal)}(2)],3-Eu. In contrast the one- and two-electron reduction of the [Nd-III(trensal)] and [Sm-III(trensal)] leads to the intermolecular reductive coupling of the imino groups of the trensal ligand and to the formation of one and two C-C bonds leaving the metal center in the +3 oxidation state. The resulting one- and two electron reduced complexes [{K(THF)(3)}(2)Ln(2)(bis-trensal)],2-Ln, and [{K(THF)(3)}(2){K(THF)}(2)Ln(2)(cyclo-trensal)],4-Ln(Ln = Sm, Nd) are able to effect the reductive disproportionation of carbon dioxide by transferring the electrons stored in the C-C bonds to CO(2)to selectively afford carbonate and CO. The selectivity of the reaction contrasts with the formation of multiple CO(2)reduction products previously reported for a U(iv)-trensal system.

ROYAL SOC CHEMISTRY2020

High-pressure NMR spectroscopic and calorimetric studies on formic acid dehydrogenation and carbon dioxide hydrogenation

Cornel Fink

Our current energy production is convenient and simple, but it is not sustainable. The negative impacts on our environment are omnipresent in the form of smog, extinction of species, and global warming. A major challenge of our generation is the transition to new, responsible methods of energy production. The maxim is renewable energy. Hydrogen could serve as an intermediary energy carrier. However, problems arise with the storage of hydrogen due to safety concerns and low volumetric energy density. The reduction of carbon dioxide (CO2) to formic acid (FA) is an elegant process to âcompressâ hydrogen by transforming it into a chemical which is easier and safer to handle. In this dissertation, catalytic FA dehydrogenation, CO2 reduction and calorimetric studies of the involved thermodynamics are described. The first chapter provides a global overview, beginning with general considerations about energy economy, then crossing over to hydrogen storage and infrastructure for a future hydrogen economy, before discussing FA as a hydrogen storage medium and summarizing the developments of FA dehydrogenation and CO2 hydrogenation. Chapter two describes different compounds we have studied as potential catalysts for FA dehydrogenation and CO2 hydrogenation. The focus is on complexes of Ir, Rh, and Ru regarding metals, and on Cp* and an ethylene-spaced diamine as ligands. Other motifs were also explored including Ru-phosphine complexes which afforded unexpected results. The most promising candidates are examined more closely. The third chapter summarizes our findings of formic acid dehydrogenation with homogeneous catalysts, presented in three parts. Each subchapter highlights a different structural feature or metal center. The first one focuses on Ir and Rh metal centers, equipped with Cp* and diamines. In the second section, we explore the catalytic performance of a RAPTA-type precatalyst, and the last one addresses our achievements with a rhodium catalyst containing Cp* and di(1-pyrazolyl)methane ligands. The catalysts were evaluated towards core properties such as stability (TON), activity (TOF) and recycling. Chapter four elucidates the interactions of FA with different solvents and additives, which influence the kinetics and thermodynamics of FA dehydrogenation and CO2 hydrogenation. NMR and FT-IR spectroscopy, conductometry, calorimetric measurements and computational methods were used to analyze the systems thoroughly. In chapter five, high-pressure calorimetric measurements are employed to quantify the actual energy balance during the endothermic process of FA dehydrogenation. For this purpose, we modified an off-the-shelf high-pressure reactor to suit our needs and constructed a thermo-controlled environment. Chapter six describes our research into catalytic CO2 hydrogenation to FA with two ruthenium-based phosphine pre-catalysts. One has three 1,4,7-triaza-9-phosphatricyclotridecane (CAP) ligands coordinated, which is a just recently described phosphine compound. The other complex is prepared via a straightforward synthesis route with an inexpensive, simple ligand. Both are stable in aqueous FA over weeks, making them potential catalysts for large-scale application.

EPFL2018

Molecular hydrogen is a promising candidate to replace fossil fuels as the energy carrier. Hydrogen does not exist in its molecular form on earth and must therefore be generated, starting from hydrogen-rich compounds. Water would be a renewable resource for hydrogen. It can be split into oxygen and hydrogen. Water splitting, however, needs electrical energy. If this energy is produced from renewable resources, such as solar or wind power, water splitting would be a sustainable and green process. One key step in water splitting is the reduction of protons to form hydrogen. To achieve a high efficiency in this reaction, catalysts are required. This dissertation is devoted to the development of hydrogen evolution catalysts that are made of earth-abundant and inexpensive materials. First, transition metal complexes of tetrathiotungstate and tetrathiomolybdate were studied as potential homogeneous catalysts for hydrogen evolution in organic or aqueous solutions (chapter 1). The redox activity of these complexes was investigated by cyclic voltammetry in the absence and presence of an acid in organic solutions. The first reduction peak of (Pr4N)2[Ni(MoS4)2] became more intense and was shifted to less negative potentials upon addition of an acid, e.g. p-cyanoanilinium tetrafluoroborate. This was considered as a sign for catalytic proton reduction. Electrolysis of a solution containing (Pr4N)2[Ni(MoS4)2] and an acid produced hydrogen. However, the complex turned out to be a precursor of the real catalytically active species, which was deposited on the working electrode’s surface during the electrochemical experiment. Consecutive cyclic voltammetric scans were found to be a good method to deposit the catalytically active film in a controllable manner from an aqueous solution of (Pr4N)2[Ni(MoS4)2]. Furthermore, a catalytically active film could be deposited from a solution of MoS42-, i.e. in the absence of Ni2+. Chapter 2 describes the investigation and characterization of films made using MoS42- as the precursor. The films consist of amorphous molybdenum sulfide. They are efficient hydrogen evolution catalysts in water. The films were characterized by XPS and electron microscopy. Whereas the pre-catalysts could be MoS3 or MoS2, the active form of the catalysts was identified as amorphous MoSx, with x close to 2. Significant geometric current densities were achieved at low overpotentials (e.g., ca. 15 mA/cm2 at η = 200 mV) using these catalysts. The catalysis was compatible with a wide range of pH values. The current efficiency for hydrogen production was quantitative. A Tafel slope of 39 mV/dec was observed, suggesting a rate-determining Heyrovsky step. The turnover frequency per active site was estimated.

EPFL2012

High-pressure NMR spectroscopic and calorimetric studies on formic acid dehydrogenation and carbon dioxide hydrogenation

Cornel Fink

EPFL2018

EPFL2012