Interplay between β-Diimino and β-Diketiminato Ligands in Nickel Complexes Active in the Proton Reduction Reaction

Acord transformatiu CRUE-CSICTwo Ni complexes are reported with κ4-P2N2 β-diimino (BDI) ligands with the general formula [Ni(XBDI)](BF4)2, where BDI is N-(2-(diphenylphosphaneyl)ethyl)-4-((2-(diphenylphosphaneyl)ethyl)imino)pent-2-en-2-amine and X indicates the substituent in the α-carbon intradiimine position, X = H for 1(BF4)2 and X = Ph for 2(BF4)2. Electrochemical analysis together with UV-vis and NMR spectroscopy in acetonitrile and dimethylformamide (DMF) indicates the conversion of the β-diimino complexes 12+ and 22+ to the negatively charged β-diketiminato (BDK) analogues (1-H)+ and (2-H)+ via deprotonation in DMF. Moreover, further electrochemical and spectroscopy evidence indicates that the one-electron-reduced derivatives 1+ and 2+ can also rapidly evolve to the BDK (1-H)+ and (2-H)+, respectively, via hydrogen gas evolution through a bimolecular homolytic pathway. Finally, both complexes are demonstrated to be active for the proton reduction reaction in DMF at Eapp = -1.8 V vs Fc+/0, being the active species the one-electron-reduced derivative 1-H and 2-H

Redox Metal-Ligand Cooperativity Enables Robust and Efficient Water Oxidation Catalysis at Neutral pH with Macrocyclic Copper Complexes

Water oxidation catalysis stands out as one of the most important reactions to design practical devices for artificial photosynthesis. Use of late first-row transition metal (TM) complexes provides an excellent platform for the development of inexpensive catalysts with exquisite control on their electronic and structural features via ligand design. However, the difficult access to their high oxidation states and the general labile character of their metal–ligand bonds pose important challenges. Herein, we explore a copper complex (1²⁻) featuring an extended, π-delocalized, tetra-amidate macrocyclic ligand (TAML) as water oxidation catalyst and compare its activity to analogous systems with lower π-delocalization (2²⁻ and 3²⁻). Their characterization evidences a special metal–ligand cooperativity in accommodating the required oxidative equivalents using 1²⁻ that is absent in 2²⁻ and 3²⁻. This consists of charge delocalization promoted by easy access to different electronic states at a narrow energy range, corresponding to either metal-centered or ligand-centered oxidations, which we identify as an essential factor to stabilize the accumulated oxidative charges. This translates into a significant improvement in the catalytic performance of 1²⁻ compared to 2²⁻ and 3²⁻ and leads to one of the most active and robust molecular complexes for water oxidation at neutral pH with a k_(obs) of 140 s⁻¹ at an overpotential of only 200 mV. In contrast, 2²⁻ degrades under oxidative conditions, which we associate to the impossibility of efficiently stabilizing several oxidative equivalents via charge delocalization, resulting in a highly reactive oxidized ligand. Finally, the acyclic structure of 3²⁻ prevents its use at neutral pH due to acidic demetalation, highlighting the importance of the macrocyclic stabilization

Unravelling the Mechanistic Pathway of the Hydrogen Evolution Reaction Driven by a Cobalt Catalyst

Acord transformatiu CRUE-CSICA cobalt complex bearing a κ-NP ligand is presented (1 or Co(L), where L is (1E,1'E)-1,1'-(pyridine-2,6-diyl)bis(N-(3-(diphenylphosphanyl)propyl)ethan-1-imine). Complex 1 is stable under air at oxidation state Co thanks to the π-acceptor character of the phosphine groups. Electrochemical behavior of 1 reveals a two-electron Co/Co oxidation process and an additional one-electron reduction, which leads to an enhancement in the current due to hydrogen evolution reaction (HER) at E=−1.6 V vs Fc/Fc. In the presence of 1 equiv of bis(trifluoromethane)sulfonimide, 1 forms the cobalt hydride derivative Co(L)-H (2), which has been fully characterized. Further addition of 1 equiv of CoCp* (Cp* is pentamethylcyclopentadienyl) affords the reduced Co(L)-H (2) species, which rapidly forms hydrogen and regenerates the initial Co(L) (1). The spectroscopic characterization of catalytic intermediates together with DFT calculations support an unusual bimolecular homolytic mechanism in the catalytic HER with 1

Ru, Co and Ca-based catalysts for artificial photosynthesis

La fotosíntesis artificial ofrece una alternativa al panorama energético actual, basado en el consumo de combustibles fósiles. Tratando de emular la fotosíntesis de las plantas, este campo de estudio trata de utilizar la luz solar para producir electrones, protones y oxígeno a partir del agua, para más tarde utilizar los electrones para producir hidrógeno u otros combustibles, y almacenar de esta forma la energía solar en forma de enlaces químicos. Para hacer energéticamente viables estos dos procesos es necesario el uso de catalizadores, comúnmente basados en metales de transición. En el primer capítulo se introduce brevemente la motivación que ha llevado a la realización de la tesis, discutiéndose también los aspectos mecanísticos más relevantes de la catálisis de oxidación de agua, así como la reducción de protones, dando una visión general de los catalizadores en ambos campos más relevantes hasta la fecha. El segundo capítulo se centra en los objetivos de este trabajo. El objetivo principal de esta tesis es la síntesis y caracterización estructural y electroquímica de varios catalizadores de Ru, Cu y Co, y el posterior estudio de su reactividad en catálisis de oxidación de agua y/o reducción de protones. El objetivo final es comprender sus mecanismos de reacción y los factores que afectan a su actividad catalítica para ayudar al diseño futuro de catalizadores más eficientes y robustos. En el tercer capítulo se presenta la síntesis, caracterización y reactividad de una nueva familia de complejos de Ru. Un estudio espectroscópico y cinético detallado ha permitido identificar la formación de nuevas especies tras oxidación de los complejos en soluciones acuosas, las cuales tienen una implicación directa en su comportamiento en catálisis de oxidación de agua. El cuarto capítulo aborda la síntesis y caracterización de una nueva familia de complejos de Cu, estudiándose su reactividad en catálisis de oxidación de agua y comparándose con la de los complejos de cobre más relevantes reportados hasta la fecha en la bibliografía. Finalmente, en el quinto capítulo se presenta la desactivación de un cluster molecular de CoII/CoIII cuando este se somete a potenciales de reducción, dando lugar a la formación de nanopartículas de CoO o CoO(OH) depositadas sobre un electrodo de carbono. Dichas nanopartículas han sido probadas en catálisis de reducción de protones y su reactividad relacionada con su morfología y naturaleza. En el sexto capítulo se incluyen las conclusiones más relevantes del trabajo realizado. Finalmente, el último capítulo contiene un anexo que incluye otros trabajos realizados y publicados durante esta tesis relacionados con el tema principal de esta tesis.Artificial photosynthesis offers a viable alternative to the actual energetic model based mainly in the consumption of fossil fuels. Trying to emulate the photosynthesis process in higher plants, this area of study attempts to use sunlight in order to produce electrons, protons and oxygen from water, with the aim of using the released electrons for producing hydrogen or other useful fuels. Within this context, the use of catalysts usually based in transition metals is necessary to make these two processes viable. The first chapter contains a brief introduction about the motivation for the research presented in this thesis. The most relevant general mechanistic aspects for water oxidation (WO) as well as proton reduction catalysis are also presented, giving a general view of most relevant catalysts reported to date. The second chapter is focused in the objectives of this work. The main goal of this PhD thesis is the synthesis and the structural and electrochemical characterization of a series of Ru, Co and Cu-based catalysts and the ulterior study of their reactivity towards water oxidation and/or proton reduction catalysis. The final objective is to fully understand the mechanistic pathways and the factors that affect their catalytic performance for helping in the future rational design of more efficient and robust catalysts. In the third chapter, the synthesis, characterization and reactivity of a new family of Ru complexes is presented. A series of detailed electrochemical, spectroscopic and kinetic studies allows the identification of new species formed after oxidation of the complexes in aqueous solution that proved to be key for further understanding their catalytic behavior in water oxidation. The fourth chapter presents the synthesis and characterization of a new family of Cu complexes. Their reactivity towards water oxidation has been studied and compared with that of the most relevant Cu-based WO catalysts reported in the literature. Finally, in the fifth chapter we present the deactivation of a CoII/CoIII molecular cluster after application of reductive potentials, giving rise to the formation of CoO or CoO(OH) nanoparticles deposited onto a glassy carbon electrode. The ability of these nanoparticles for reducing protons has been tested, and their catalytic performance discussed on the basis of the nature of the species obtained and their morphology . In the sixth chapter the most relevant conclusions of this work are discussed. Finally, the last chapter includes an annex containing other works that have been carried out and published during this PhD thesis and that are closely related with the work carried out during the PhD

Ru, Co and Ca-based catalysts for artificial photosynthesis

La fotosíntesis artificial ofrece una alternativa al panorama energético actual, basado en el consumo de combustibles fósiles. Tratando de emular la fotosíntesis de las plantas, este campo de estudio trata de utilizar la luz solar para producir electrones, protones y oxígeno a partir del agua, para más tarde utilizar los electrones para producir hidrógeno u otros combustibles, y almacenar de esta forma la energía solar en forma de enlaces químicos. Para hacer energéticamente viables estos dos procesos es necesario el uso de catalizadores, comúnmente basados en metales de transición. En el primer capítulo se introduce brevemente la motivación que ha llevado a la realización de la tesis, discutiéndose también los aspectos mecanísticos más relevantes de la catálisis de oxidación de agua, así como la reducción de protones, dando una visión general de los catalizadores en ambos campos más relevantes hasta la fecha. El segundo capítulo se centra en los objetivos de este trabajo. El objetivo principal de esta tesis es la síntesis y caracterización estructural y electroquímica de varios catalizadores de Ru, Cu y Co, y el posterior estudio de su reactividad en catálisis de oxidación de agua y/o reducción de protones. El objetivo final es comprender sus mecanismos de reacción y los factores que afectan a su actividad catalítica para ayudar al diseño futuro de catalizadores más eficientes y robustos. En el tercer capítulo se presenta la síntesis, caracterización y reactividad de una nueva familia de complejos de Ru. Un estudio espectroscópico y cinético detallado ha permitido identificar la formación de nuevas especies tras oxidación de los complejos en soluciones acuosas, las cuales tienen una implicación directa en su comportamiento en catálisis de oxidación de agua. El cuarto capítulo aborda la síntesis y caracterización de una nueva familia de complejos de Cu, estudiándose su reactividad en catálisis de oxidación de agua y comparándose con la de los complejos de cobre más relevantes reportados hasta la fecha en la bibliografía. Finalmente, en el quinto capítulo se presenta la desactivación de un cluster molecular de CoII/CoIII cuando este se somete a potenciales de reducción, dando lugar a la formación de nanopartículas de CoO o CoO(OH) depositadas sobre un electrodo de carbono. Dichas nanopartículas han sido probadas en catálisis de reducción de protones y su reactividad relacionada con su morfología y naturaleza. En el sexto capítulo se incluyen las conclusiones más relevantes del trabajo realizado. Finalmente, el último capítulo contiene un anexo que incluye otros trabajos realizados y publicados durante esta tesis relacionados con el tema principal de esta tesis.Artificial photosynthesis offers a viable alternative to the actual energetic model based mainly in the consumption of fossil fuels. Trying to emulate the photosynthesis process in higher plants, this area of study attempts to use sunlight in order to produce electrons, protons and oxygen from water, with the aim of using the released electrons for producing hydrogen or other useful fuels. Within this context, the use of catalysts usually based in transition metals is necessary to make these two processes viable. The first chapter contains a brief introduction about the motivation for the research presented in this thesis. The most relevant general mechanistic aspects for water oxidation (WO) as well as proton reduction catalysis are also presented, giving a general view of most relevant catalysts reported to date. The second chapter is focused in the objectives of this work. The main goal of this PhD thesis is the synthesis and the structural and electrochemical characterization of a series of Ru, Co and Cu-based catalysts and the ulterior study of their reactivity towards water oxidation and/or proton reduction catalysis. The final objective is to fully understand the mechanistic pathways and the factors that affect their catalytic performance for helping in the future rational design of more efficient and robust catalysts. In the third chapter, the synthesis, characterization and reactivity of a new family of Ru complexes is presented. A series of detailed electrochemical, spectroscopic and kinetic studies allows the identification of new species formed after oxidation of the complexes in aqueous solution that proved to be key for further understanding their catalytic behavior in water oxidation. The fourth chapter presents the synthesis and characterization of a new family of Cu complexes. Their reactivity towards water oxidation has been studied and compared with that of the most relevant Cu-based WO catalysts reported in the literature. Finally, in the fifth chapter we present the deactivation of a CoII/CoIII molecular cluster after application of reductive potentials, giving rise to the formation of CoO or CoO(OH) nanoparticles deposited onto a glassy carbon electrode. The ability of these nanoparticles for reducing protons has been tested, and their catalytic performance discussed on the basis of the nature of the species obtained and their morphology . In the sixth chapter the most relevant conclusions of this work are discussed. Finally, the last chapter includes an annex containing other works that have been carried out and published during this PhD thesis and that are closely related with the work carried out during the PhD

Ru, Co and Ca-based catalysts for artificial photosynthesis /

La fotosíntesis artificial ofrece una alternativa al panorama energético actual, basado en el consumo de combustibles fósiles. Tratando de emular la fotosíntesis de las plantas, este campo de estudio trata de utilizar la luz solar para producir electrones, protones y oxígeno a partir del agua, para más tarde utilizar los electrones para producir hidrógeno u otros combustibles, y almacenar de esta forma la energía solar en forma de enlaces químicos. Para hacer energéticamente viables estos dos procesos es necesario el uso de catalizadores, comúnmente basados en metales de transición. En el primer capítulo se introduce brevemente la motivación que ha llevado a la realización de la tesis, discutiéndose también los aspectos mecanísticos más relevantes de la catálisis de oxidación de agua, así como la reducción de protones, dando una visión general de los catalizadores en ambos campos más relevantes hasta la fecha. El segundo capítulo se centra en los objetivos de este trabajo. El objetivo principal de esta tesis es la síntesis y caracterización estructural y electroquímica de varios catalizadores de Ru, Cu y Co, y el posterior estudio de su reactividad en catálisis de oxidación de agua y/o reducción de protones. El objetivo final es comprender sus mecanismos de reacción y los factores que afectan a su actividad catalítica para ayudar al diseño futuro de catalizadores más eficientes y robustos. En el tercer capítulo se presenta la síntesis, caracterización y reactividad de una nueva familia de complejos de Ru. Un estudio espectroscópico y cinético detallado ha permitido identificar la formación de nuevas especies tras oxidación de los complejos en soluciones acuosas, las cuales tienen una implicación directa en su comportamiento en catálisis de oxidación de agua. El cuarto capítulo aborda la síntesis y caracterización de una nueva familia de complejos de Cu, estudiándose su reactividad en catálisis de oxidación de agua y comparándose con la de los complejos de cobre más relevantes reportados hasta la fecha en la bibliografía. Finalmente, en el quinto capítulo se presenta la desactivación de un cluster molecular de CoII/CoIII cuando este se somete a potenciales de reducción, dando lugar a la formación de nanopartículas de CoO o CoO(OH) depositadas sobre un electrodo de carbono. Dichas nanopartículas han sido probadas en catálisis de reducción de protones y su reactividad relacionada con su morfología y naturaleza. En el sexto capítulo se incluyen las conclusiones más relevantes del trabajo realizado. Finalmente, el último capítulo contiene un anexo que incluye otros trabajos realizados y publicados durante esta tesis relacionados con el tema principal de esta tesis.Artificial photosynthesis offers a viable alternative to the actual energetic model based mainly in the consumption of fossil fuels. Trying to emulate the photosynthesis process in higher plants, this area of study attempts to use sunlight in order to produce electrons, protons and oxygen from water, with the aim of using the released electrons for producing hydrogen or other useful fuels. Within this context, the use of catalysts usually based in transition metals is necessary to make these two processes viable. The first chapter contains a brief introduction about the motivation for the research presented in this thesis. The most relevant general mechanistic aspects for water oxidation (WO) as well as proton reduction catalysis are also presented, giving a general view of most relevant catalysts reported to date. The second chapter is focused in the objectives of this work. The main goal of this PhD thesis is the synthesis and the structural and electrochemical characterization of a series of Ru, Co and Cu-based catalysts and the ulterior study of their reactivity towards water oxidation and/or proton reduction catalysis. The final objective is to fully understand the mechanistic pathways and the factors that affect their catalytic performance for helping in the future rational design of more efficient and robust catalysts. In the third chapter, the synthesis, characterization and reactivity of a new family of Ru complexes is presented. A series of detailed electrochemical, spectroscopic and kinetic studies allows the identification of new species formed after oxidation of the complexes in aqueous solution that proved to be key for further understanding their catalytic behavior in water oxidation. The fourth chapter presents the synthesis and characterization of a new family of Cu complexes. Their reactivity towards water oxidation has been studied and compared with that of the most relevant Cu-based WO catalysts reported in the literature. Finally, in the fifth chapter we present the deactivation of a CoII/CoIII molecular cluster after application of reductive potentials, giving rise to the formation of CoO or CoO(OH) nanoparticles deposited onto a glassy carbon electrode. The ability of these nanoparticles for reducing protons has been tested, and their catalytic performance discussed on the basis of the nature of the species obtained and their morphology . In the sixth chapter the most relevant conclusions of this work are discussed. Finally, the last chapter includes an annex containing other works that have been carried out and published during this PhD thesis and that are closely related with the work carried out during the PhD

Fate of the Molecular Ru–Phosphonate Water Oxidation Catalyst under Turnover Conditions

The present work uncovers the oxidative transformations of a recently reported polypyridyl phosphonate–phenoxo Ru-based water oxidation catalyst [RuIII(tPaO-κ-N2OPOC)(py)2]2–, 22– {tPaO5– is the 3-(hydroxo-[2,2′:6′,2″-terpyridine]-6,6″-diyl)bis(phosphonate)}, under turnover conditions. We show how the catalyst 22– suffers from oxidative degradation during water oxidation catalysis and generates the phosphonate–carboxylate Ru complex [RuII(Hbpc)(py)2], 3H, where bpc3– is 6′-phosphonato-[2,2′-bipyridine]-6-carboxylate. Complex 3H has been prepared by three different methods, and its oxidative transformations were also studied in detail. Under turnover conditions, complex 3H undergoes a series of transformations that can be monitored by electrochemical techniques including the generation of catalytically active molecular water oxidation catalyst intermediates and RuO2. In addition, catalytically inactive species such as [RuII(bpc-κ-N2OP)2] have also been detected

Effect of Ligand Chelation and Sacrificial Oxidant on the Integrity of Triazole-Based Carbene Iridium Water Oxidation Catalysts

We report the effect of replacing the pyridine group in the chelating trz Ir–water oxidation catalysts by a benzoxazole and a thiazole moiety. We have also evaluated if the presence of bidentate ligands is crucial for high activities and to avoid the decomposition into undesired heterogeneous layers. The catalytic performance of these benzoxazole/thiazole–triazolidene Ir-complexes in water oxidation was studied at variable pH using either CAN (pH = 1) or NaIO4 (pH = 5.6 and 7). Electrocatalytic experiments indicated that while CAN-mediated water oxidation led to catalyst heterogeneization irrespective of the triazolylidene substituent, periodate as sacrificial oxidant preserved a homogeneously active species. Repetitive additions of sacrificial oxidant indicates higher integrity of the Ir-complex with a thiazole-substituted triazolylidene compared to ligands featuring a benzoxazole as chelating donor or no chelating group at all. Rigid chelation of the thiazole group was also established from stability measurements under highly acidic, oxidizing, and high ionic strength conditions

Synthesis, Electrochemical Characterization and Water Oxidation Chemistry of Ru Complexes Containing the 2,6- Pyridinedicarboxylato Ligand

The tridentate meridional ligand pyridyl-2,6-dicarboxylato (pdc2-) has been used to prepare complexes RuII(pdc-ҡ3-N1O2)(DMSO)2Cl] (1II), RuII(pdc-ҡ3-N1O2)(bpy)(DMSO)] (2II) and {[RuIII(pdc-ҡ3-N1O2)(bpy)]2(-O)} (5III,III) where bpy: 2,2’-bipyridine. All complexes have been fully characterized through spectroscopic, electrochemical and single crystal X-ray diffraction techniques. Compounds 1II and 2II show SO linkage isomerization of the DMSO ligand upon oxidation from RuII to RuIII and thermodynamic and kinetic data have been obtained from cyclic voltammetry experiments. Dimeric complex 5III,III is a precursor of the monomeric complex [RuII(pdc-ҡ3-N1O2)(bpy)(H2O)] (4II) which is a water oxidation catalyst. The electrochemistry and catalytic activity of 4II has been ascertained for the first time and compared with related Ru-aquo complexes that are also active for the water oxidation reaction. It shows a TOFmax = 0.2 s-1 and overpotential of 240 mV in pH 1. The overpotential shown by 4II is one of the lowest reported in the literature and is associated to the role of the two carboxylato groups of the pdc ligand, providing high electron density to the ruthenium complex