Max García-Melchor

Homogeneously dispersed, multimetal oxygen-evolving catalysts

Modulating metal oxides The more difficult step in fuel cells and water electrolysis is the oxygen evolution reaction. The search for earth-abundant materials to replace noble metals for this reaction often turns to oxides of three-dimensional metals such as iron. Zhang et al. show that the applied voltages needed to drive this reaction are reduced for iron-cobalt oxides by the addition of tungsten. The addition of tungsten favorably modulates the electronic structure of the oxyhydroxide. A key development is to keep the metals well mixed and avoid the formation of separate phases. Science, this issue p. 333 The addition of tungsten to iron cobalt oxides lowers the overpotential required for the evolution of oxygen from water. Earth-abundant first-row (3d) transition metal–based catalysts have been developed for the oxygen-evolution reaction (OER); however, they operate at overpotentials substantially above thermodynamic requirements. Density functional theory suggested that non-3d high-valency metals such as tungsten can modulate 3d metal oxides, providing near-optimal adsorption energies for OER intermediates. We developed a room-temperature synthesis to produce gelled oxyhydroxides materials with an atomically homogeneous metal distribution. These gelled FeCoW oxyhydroxides exhibit the lowest overpotential (191 millivolts) reported at 10 milliamperes per square centimeter in alkaline electrolyte. The catalyst shows no evidence of degradation after more than 500 hours of operation. X-ray absorption and computational studies reveal a synergistic interplay between tungsten, iron, and cobalt in producing a favorable local coordination environment and electronic structure that enhance the energetics for OER.

Computational Perspective on Pd-Catalyzed C–C Cross-Coupling Reaction Mechanisms

Palladium-catalyzed C-C cross-coupling reactions (Suzuki-Miyaura, Negishi, Stille, Sonogashira, etc.) are among the most useful reactions in modern organic synthesis because of their wide scope and selectivity under mild conditions. The many steps involved and the availability of competing pathways with similar energy barriers cause the mechanism to be quite complicated. In addition, the short-lived intermediates are difficult to detect, making it challenging to fully characterize the mechanism of these reactions using purely experimental techniques. Therefore, computational chemistry has proven crucial for elucidating the mechanism and shaping our current understanding of these processes. This mechanistic elucidation provides an opportunity to further expand these reactions to new substrates and to refine the selectivity of these reactions. During the past decade, we have applied computational chemistry, mostly using density functional theory (DFT), to the study of the mechanism of C-C cross-coupling reactions. This Account summarizes the results of our work, as well as significant contributions from others. Apart from a few studies on the general features of the catalytic cycles that have highlighted the existence of manifold competing pathways, most studies have focused on a specific reaction step, leading to the analysis of the oxidative addition, transmetalation, and reductive elimination steps of these processes. In oxidative addition, computational studies have clarified the connection between coordination number and selectivity. For transmetalation, computation has increased the understanding of different issues for the various named reactions: the role of the base in the Suzuki-Miyaura cross-coupling, the factors distinguishing the cyclic and open mechanisms in the Stille reaction, the identity of the active intermediates in the Negishi cross-coupling, and the different mechanistic alternatives in the Sonogashira reaction. We have also studied the closely related direct arylation process and highlighted the role of an external base as proton abstractor. Finally, we have also rationalized the effect of ligand substitution on the reductive elimination process. Computational chemistry has improved our understanding of palladium-catalyzed cross-coupling processes, allowing us to identify the mechanistic complexity of these reactions and, in a few selected cases, to fully clarify their mechanisms. Modern computational tools can deal with systems of the size and complexity involved in cross-coupling and have a continuing role in solving specific problems in this field.

Palladium round trip in the Negishi coupling of trans-[PdMeCl(PMePh2)2] with ZnMeCl: an experimental and DFT study of the transmetalation step.

Compared with the detailed mechanistic knowledge of the Stille reaction, little is known about the Negishi reaction. Recently, we experimentally uncovered the complicated behavior of the transmetalation of transACHTUNGTRENNUNG[PdRfClACHTUNGTRENNUNG(PPh3)2] (Rf= 3,5-dichloro-2,4,6-trifluorophenyl) with ZnMe2 or ZnMeCl, showing that each methylating reagent afforded stereoselectively a different isomer (trans or cis, respectively) of the coupling intermediate [PdRfMe ACHTUNGTRENNUNG(PPh3)2].[4] Moreover, the study revealed the occurrence of undesired transmetalations, such as those shown in Scheme 1, which could eventually produce homocoupling products; the corresponding undesired intermediates were detected and identified by NMR spectroscopy techniques. The formation of undesired intermediates in related reactions with aryl zinc derivatives was later observed by Lei et al. Herein, we report an experimental mechanistic study of the reaction of trans-[PdClMe ACHTUNGTRENNUNG(PMePh2)2] (1) with ZnMeCl, which affords the first experimental determination of thermodynamic parameters of a Negishi transmetalation. This is complemented with a theoretical DFT study, which provides a detailed view of the reaction pathway, consistent with the experimental parameters. The reactions of 1 with ZnMeCl were carried out (with one exception) in 1:20 ratio, simulating catalytic conditions with 5 % Pd, in THF at different temperatures. At room temperature, the only product observed was cis[PdMe2ACHTUNGTRENNUNG(PMePh2)2] (2), in equilibrium with the starting material 1. In these conditions, complex 2 undergoes slow decomposition (reductive elimination) to give ethane. When the reaction was monitored by P NMR spectroscopy at 223 K (Figure 1 a), the coupling rate to give ethane became negligible and the formation of trans-[PdMe2ACHTUNGTRENNUNG(PMePh2)2] (3), as well as cis-[PdMe2ACHTUNGTRENNUNG(PMePh2)2] (2), was observed. The trans isomer 3 seemed to be formed first and then disappeared. The same reaction, carried out at 203 K in 1:1 ratio to get a slower rate of transformation, confirmed that 3 is formed noticeably faster than 2 (Figure 1 b). Thus, the observation of the cis isomer at room temperature is deceptive for the stereoselectivity of the transmetalation. Snapshots of two moments of the transmetalation reaction at 203 K, as seen by P NMR spectroscopy, are shown in Figure 1 c. The behavior of 3 is typical of a kinetic product of noticeably lower stability than the thermodynamic product (2): eventually it disappears from observation as the reaction proceeds and gets closer to the equilibrium concentrations, where the concentration of 3 is very small. In effect, during the progress of the reaction at 223 K (Figure 1 a), the concentration of 2 increases continuously; in contrast, a small accumulation of 3 is produced initially and then its concentration decreases, so that in 300 min 3 has practically disappeared. After about 10 h at 223 K, the system has reached equilibrium between the starting complex 1 and the final thermodynamic product 2 ([1]=5.8 10 3 mol l , [2]=4.4 10 3 mol l , and Keq =2.0 10 ); the concentration of 3 is below the limit of NMR observation. [a] B. Fuentes, Dr. J. A. Casares, Prof. P. Espinet IU CINQUIMA/Qu mica Inorg nica Facultad de CienciasUniversidad de Valladolid 47071 Valladolid (Spain) Fax: (+34) 983423231/ ACHTUNGTRENNUNG(+34) 983186336 E-mail : casares@qi.uva.es espinet@qi.uva.es [b] M. Garc a-Melchor, Prof. A. Lled s, Prof. F. Maseras, Dr. G. Ujaque Qu mica F sica, Edifici C.n, Universitat Aut noma de Barcelona 08193 Bellaterra, Catalonia (Spain) Fax: (+34) 935812920 E-mail : gregori@klingon.uab.es [c] Prof. F. Maseras Institute of Chemical Research of Catalonia (ICIQ) Av. Pa sos Catalans, 16, 43007 Tarragona, Catalonia (Spain) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201001332. Scheme 1.

Cationic Intermediates in the Pd-Catalyzed Negishi Coupling. Kinetic and Density Functional Theory Study of Alternative Transmetalation Pathways in the Me–Me Coupling of ZnMe2 and trans-[PdMeCl(PMePh2)2]

The complexity of the transmetalation step in a Pd-catalyzed Negishi reaction has been investigated by combining experiment and theoretical calculations. The reaction between trans-[PdMeCl(PMePh(2))(2)] and ZnMe(2) in THF as solvent was analyzed. The results reveal some unexpected and relevant mechanistic aspects not observed for ZnMeCl as nucleophile. The operative reaction mechanism is not the same when the reaction is carried out in the presence or in the absence of an excess of phosphine in the medium. In the absence of added phosphine an ionic intermediate with THF as ligand ([PdMe(PMePh(2))(2)(THF)](+)) opens ionic transmetalation pathways. In contrast, an excess of phosphine retards the reaction because of the formation of a very stable cationic complex with three phosphines ([PdMe(PMePh(2))(3)](+)) that sequesters the catalyst. These ionic intermediates had never been observed or proposed in palladium Negishi systems and warn on the possible detrimental effect of an excess of good ligand (as PMePh(2)) for the process. In contrast, the ionic pathways via cationic complexes with one solvent (or a weak ligand) can be noticeably faster and provide a more rapid reaction than the concerted pathways via neutral intermediates. Theoretical calculations on the real molecules reproduce well the experimental rate trends observed for the different mechanistic pathways.

Edge reactivity and water-assisted dissociation on cobalt oxide nanoislands

Transition metal oxides show great promise as Earth-abundant catalysts for the oxygen evolution reaction in electrochemical water splitting. However, progress in the development of highly active oxide nanostructures is hampered by a lack of knowledge of the location and nature of the active sites. Here we show, through atom-resolved scanning tunnelling microscopy, X-ray spectroscopy and computational modelling, how hydroxyls form from water dissociation at under coordinated cobalt edge sites of cobalt oxide nanoislands. Surprisingly, we find that an additional water molecule acts to promote all the elementary steps of the dissociation process and subsequent hydrogen migration, revealing the important assisting role of a water molecule in its own dissociation process on a metal oxide. Inspired by the experimental findings, we theoretically model the oxygen evolution reaction activity of cobalt oxide nanoislands and show that the nanoparticle metal edges also display favourable adsorption energetics for water oxidation under electrochemical conditions.

Understanding and Tuning the Intrinsic Hydrophobicity of Rare-Earth Oxides: A DFT+U Study.

Rare-earth oxides (REOs) possess a remarkable intrinsic hydrophobicity, making them candidates for a myriad of applications. Although the superhydrophobicity of REOs has been explored experimentally, the atomistic details of the structure at the oxide-water interface are still not well understood. In this work, we report a density functional theory study of the interaction between water and CeO2, Nd2O3, and α-Al2O3 to explain their different wettability. The wetting of the metal oxide surface is controlled by geometric and electronic factors. While the electronic term is related to the acid-base properties of the surface layer, the geometric factor depends on the matching between adsorption sites and oxygen atoms from the hexagonal water network. For all the metal oxides considered here, water dissociation is confined to the first oxide-water layer. Hydroxyl groups on α-Al2O3 are responsible for the strong oxide-water interaction, and thus, both Al- and hydroxyl-terminated wet. On CeO2, the intrinsic hydrophobicity of the clean surface disappears when lattice hydroxyl groups (created by the reaction of water with oxygen vacancies) are present as they dominate the interaction and drive wetting. Therefore, hydroxyls may convert a intrinsic nonwetting surface into a wetting one. Finally, we also report that surface modifications, like cation substitution, do not change the acid-base character of the surface, and thus they show the same nonwetting properties as native CeO2 or Nd2O3.

Theoretical Evaluation of Phosphine Effects in Cross-Coupling Reactions

Cross-coupling reactions are one of the most useful reactions in organic synthesis. Among all the transition metal complexes developed as catalysts for this reaction those based on Pd are by far the most utilized ones. The most common stoichiometry of this family of catalyst is PdL2 with L = phosphine ligands. The effects of the phosphine ligands on the reaction mechanism evaluated by means of theoretical calculations are reviewed in these lines. How the nature of the phosphine ligand affects each of the elementary processes involved in a cross-coupling reaction, namely oxidative addition, transmetalation and reductive elimination will be exposed separately. The transmetalation process has its own particular mechanistic details depending on the cross-coupling reaction; those for the Suzuki–Miyaura and Stille reactions will be described here. The dichotomy between the monophosphine and bisphosphine reaction pathways will be also discussed.

Computationally Probing the Performance of Hybrid, Heterogeneous, and Homogeneous Iridium‐Based Catalysts for Water Oxidation

An attractive strategy to improve the performance of water oxidation catalysts would be to anchor a homogeneous molecular catalyst onto a heterogeneous solid surface to create a hybrid catalyst. The idea of this combined system is to take advantage of the individual properties of each of the two catalyst components. We use DFT calculations to determine the stability and activity of a model hybrid water oxidation catalyst that consists of a dimeric Ir complex attached on the IrO2(1 1 0) surface through two oxygen atoms. We find that homogeneous catalysts can be anchored to oxide surfaces without a significant loss of activity. Hence, the design of hybrid systems that benefit from both the high tunability of the activity of homogeneous catalysts and the stability of heterogeneous systems seems feasible.

Two-Dimensional Materials as Catalysts for Energy Conversion

Although large efforts have been dedicated to studying two-dimensional materials for catalysis, a rationalization of the associated trends in their intrinsic activity has so far been elusive. In the present work we employ density functional theory to examine a variety of two-dimensional materials, including, carbon based materials, hexagonal boron nitride (h-BN), transition metal dichalcogenides (e.g. MoS2, MoSe2) and layered oxides, to give an overview of the trends in adsorption energies. By examining key reaction intermediates relevant to the oxygen reduction, and oxygen evolution reactions we find that binding energies largely follow the linear scaling relationships observed for pure metals. This observation is very important as it suggests that the same simplifying assumptions made to correlate descriptors with reaction rates in transition metal catalysts are also valid for the studied two-dimensional materials. By means of these scaling relations, for each reaction we also identify several promising candidates that are predicted to exhibit a comparable activity to the state-of-the-art catalysts.Graphical AbstractScaling relationship for the chemisorption energies of OH* and OOH* on various 2D materials.

How Theoretical Simulations Can Address the Structure and Activity of Nanoparticles

Theoretical simulations in the field of heterogeneous catalysis started about two decades ago when the main goal was to understand the activation of small molecules on infinite surfaces. The improvements in the accuracy and the large availability of computers with increasing power have raised the quality of the calculations, the reliability of the results and prompted the interest in their predictions. Such changes have also allowed the study of nanoparticles by the combined investigation of different facets or by taking into account the complete structures. As for the reactivity, theoretical simulations allow the comparison of different synthetic conditions within the same approximation. Consequently, large systematic studies with the same theoretical models can provide databases for properties, structures, prove and disprove hypothetical reaction paths, identify intermediates, and complete the understanding of reaction mechanisms. In some cases, simulations support and give explanations to experiments but new emerging aspects such as the prediction of new properties or the analysis of complex systems are possible. Several challenges are ahead the simulations of reactions on nanoparticles: (i) how to drive the synthesis to achieve the desired architectures and (ii) how to stabilize the active phase under reaction conditions.