Browsing by Author "Toro-Labbe, Alejandro"

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    Contrasting the Mechanism of H2 Activation by Monomeric and Potassium-Stabilized Dimeric AlI Complexes: Do Potassium Atoms Exert any Cooperative Effect?
    (2021) Villegas-Escobar, Nery; Toro-Labbe, Alejandro; Schaefer, Henry F., III
    Aluminyl anions are low-valent, anionic, and carbenoid aluminum species commonly found stabilized with potassium cations from the reaction of Al-halogen precursors and alkali compounds. These systems are very reactive toward the activation of sigma-bonds and in reactions with electrophiles. Various research groups have detected that the potassium atoms play a stabilization role via electrostatic and cationMIDLINE HORIZONTAL ELLIPSIS pi interactions with nearby (aromatic)-carbocyclic rings from both the ligand and from the reaction with unsaturated substrates. Since stabilizing KMIDLINE HORIZONTAL ELLIPSISH bonds are witnessed in the activation of this class of molecules, we aim to unveil the role of these metals in the activation of the smaller and less polarizable H-2 molecule, together with a comprehensive characterization of the reaction mechanism. In this work, the activation of H-2 utilizing a NON-xanthene-Al dimer, [K{Al(NON)}](2) (D) and monomeric, [Al(NON)](-) (M) complexes are studied using density functional theory and high-level coupled-cluster theory to reveal the potential role of K+ atoms during the activation of this gas. Furthermore, we aim to reveal whether D is more reactive than M (or vice versa), or if complicity between the two monomer units exits within the D complex toward the activation of H-2. The results suggest that activation energies using the dimeric and monomeric complexes were found to be very close (around 33 kcal mol(-1)). However, a partition of activation energies unveiled that the nature of the energy barriers for the monomeric and dimeric complexes are inherently different. The former is dominated by a more substantial distortion of the reactants (and increased interaction energies between them). Interestingly, during the oxidative addition, the distortion of the Al complex is minimal, while H-2 distorts the most, usually over 0.77 Delta Edist not equal . Overall, it is found here that electrostatic and induction energies between the complexes and H-2 are the main stabilizing components up to the respective transition states. The results suggest that the K+ atoms act as stabilizers of the dimeric structure, and their cooperative role on the reaction mechanism may be negligible, acting as mere spectators in the activation of H-2. Cooperation between the two monomers in D is lacking, and therefore the subsequent activation of H-2 is wholly disengaged.
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    Cover Picture: Electron Spin-Dependent Electrocatalysis for the Oxygen Reduction Reaction in a Chiro-Self-Assembled Iron Phthalocyanine Device (Angew. Chem. Int. Ed. 4/2024)
    (2024) Scarpetta-Pizo, Laura; Venegas, Ricardo; Barrias, Pablo; Munoz-Becerra, Karina; Vilches-Labbe, Nayareth; Mura, Francisco; Mendez-Torres, Ana Maria; Ramirez-Tagle, Rodrigo; Toro-Labbe, Alejandro; Hevia, Samuel; Zagal, Jose H.; Onate, Ruben; Aspee, Alexis; Ponce, Ingrid
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    Decomposition of the electronic activity in competing [5,6] and [6,6] cycloaddition reactions between C60 and cyclopentadiene
    (2019) Villegas-Escobar, Nery; Poater, Albert; Sola, Miquel; Schaefer, Henry F., III; Toro-Labbe, Alejandro
    Fullerenes, in particular C-60, are important molecular entities in many areas, ranging from material science to medicinal chemistry. However, chemical transformations have to be done in order to transform C-60 in added-value compounds with increased applicability. The most common procedure corresponds to the classical Diels-Alder cycloaddition reaction. In this research, a comprehensive study of the electronic activity that takes place in the cycloaddition between C-60 and cyclopentadiene toward the [5,6] and [6,6] reaction pathways is presented. These are competitive reaction mechanisms dominated by sigma and fluctuating activity. To better understand the electronic activity at each stage of the mechanism, the reaction force (RF) and the symmetry-adapted reaction electronic flux (SA-REF, J(i)()) have been used to elucidate whether or sigma bonding changes drive the reaction. Since the studied cycloaddition reaction proceeds through a C-s symmetry reaction path, two SA-REF emerge: J(A)() and J(A)(). In particular, J(A)() mainly accounts for bond transformations associated with bonds, while J(A)() is sensitive toward sigma bonding changes. It was found that the [6,6] path is highly favored over the [5,6] with respect to activation energies. This difference is primarily due to the less intensive electronic reordering of the sigma electrons in the [6,6] path, as a result of the pyramidalization of carbon atoms in C-60 (sp(2) sp(3) transition). Interestingly, no substantial differences in the electronic activity from the reactant complex to the transition state structure were found when comparing the [5,6] and [6,6] paths. Partition of the kinetic energy into its symmetry contributions indicates that when a bond is being weakened/broken (formed/strengthened) non-spontaneous (spontaneous) changes in the electronic activity occur, thus prompting an increase (decrease) of the kinetic energy. Therefore, contraction (expansion) of the electronic density in the vicinity of the bonding change is expected to take place.
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    Digging on the mechanism of some Diels-Alder reactions: the role of the reaction electronic flux
    (2023) Hernandez-Mancera, Jennifer Paola; Vivas-Reyes, Ricardo; Gutierrez-Oliva, Soledad; Herrera, Barbara; Toro-Labbe, Alejandro
    Within the framework of the reaction force analysis, numerical data obtained from DFT calculations were used to characterize the mechanism of three Diels-Alder reactions involving three substituted furandione as dienophile, and a chiral anthracene, as diene. Then, the Marcus potential energy function and the activation strain model were used to rationalize the energetics of the reactions and to obtain physical insights on the nature of activation energies. It has been found that the activation processes are dominated by structural arrangements of reactants, basically due to the approach of the diene to the dienophile to start the reaction. Besides, the electronic activity taking place along the reaction coordinate have been analyzed through the reaction electronic flux. It has been found that the electronic activity that emerge more intensively within the transition-state region, is mainly due to electronic transfer effects, due to the breaking and forming & pi; bonds. Although polarization effects are also present but to a lesser extent.
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    Electron Spin-Dependent Electrocatalysis for the Oxygen Reduction Reaction in a Chiro-Self-Assembled Iron Phthalocyanine Device
    (2023) Scarpetta-Pizo, Laura; Venegas, Ricardo; Barrias, Pablo; Munoz-Becerra, Karina; Vilches-Labbe, Nayareth; Mura, Francisco; Mendez-Torres, Ana Maria; Ramirez-Tagle, Rodrigo; Toro-Labbe, Alejandro; Hevia, Samuel; Zagal, Jose H.; Onate, Ruben; Aspee, Alexis; Ponce, Ingrid
    The chiral-induced spin selectivity effect (CISS) is a breakthrough phenomenon that has revolutionized the field of electrocatalysis. We report the first study on the electron spin-dependent electrocatalysis for the oxygen reduction reaction, ORR, using iron phthalocyanine, FePc, a well-known molecular catalyst for this reaction. The FePc complex belongs to the non-precious catalysts group, whose active site, FeN4, emulates catalytic centers of biocatalysts such as Cytochrome c. This study presents an experimental platform involving FePc self-assembled to a gold electrode surface using chiral peptides (L and D enantiomers), i.e., chiro-self-assembled FePc systems (CSAFePc). The chiral peptides behave as spin filters axial ligands of the FePc. One of the main findings is that the peptides ' handedness and length in CSAFePc can optimize the kinetics and thermodynamic factors governing ORR. Moreover, the D-enantiomer promotes the highest electrocatalytic activity of FePc for ORR, shifting the onset potential up to 1.01 V vs. RHE in an alkaline medium, a potential close to the reversible potential of the O2/H2O couple. Therefore, this work has exciting implications for developing highly efficient and bioinspired catalysts, considering that, in biological organisms, biocatalysts that promote O2 reduction to water comprise L-enantiomers.
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    Elucidating the electronic synergetic effects in heteroatomic doped FeN4-C-N-R (R= -F, -Cl, -Br) oxygen reduction catalysts
    (2023) Escobar, Gonzalo; Venegas, Ricardo; Ponce, Ingrid; Toro-Labbe, Alejandro; Zagal, Jose H.; Recio, F. Javier; Munoz-Becerra, Karina
    The structural and electronic characteristics of FeN4 are the determining factors in the catalytic performance of heat-treated Fe-N-C materials, as they serve as active sites. The insertion of heteroatoms as co-dopants (B, S, halogens) can induce electronic effects in the carbon matrix that improves their ORR catalytic activity. Therefore, it has become essential to combine experimental studies with DFT approaches to rationally design this type of catalyst. In this work, we evaluated by means of first principle DFT approaches, the ORR activity for the Fe (phen)2N2 moiety including atoms/functionalities with different atomic radii and electronegativity, to resemble co-doped Fe-N-C-R catalysts. The results showed that the inclusion of halogens heteroatoms (-F, -Cl, and -Br) in the graphitic N-C surrounding the FeN4 core could improve its ORR activity in terms of Fe-O2 binding energy that is related to the Fe(III)/Fe(II) formal potential and, in consequence, with the on-set potential for the ORR. The high expected ORR activity is obtained for bromide co-doped FeN4 catalyst (FeN4-C-Br) since -Br atoms act synergistically, inducing long- and short-range electronic effects over both the FeN4 unit and N-pyridinic-like functions that change the electronic distribution over the aromatic N-C structure modulating the Fe acidity, FeO2 binding, and Fe-O2 orbital interaction.
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    Link between reaction force and DFT reactivity descriptors
    (2006) Gutierrez-Oliva, Soledad; Herrera Pisani, Barbara Andrea; Toro-Labbe, Alejandro
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    Using reactivity predictors for enhancing the electrocatalytic activity of MN4 molecular catalysts for the oxygen reduction reaction: The role of the N-pyridinium functional group in the porphyrazine-derivative ligands
    (2023) Scarpetta-Pizo, Laura; Venegas, Ricardo; Munoz-Becerra, Karina; Munoz, Lisa; Toro-Labbe, Alejandro; Darwish, Nadim; Matute, Ricardo; Onate, Ruben; Zagal, Jose H.; Ponce, Ingrid
    Using reactivity predictors to enhance or control the electrocatalytic activity of materials is a fascinating concept. This is especially true for the development of alternative platinum metal group-free materials as it facilitates the rational design of active catalytic materials for the oxygen reduction reaction (ORR). In previous work, we have found that the peripheral and non-peripheral electron-withdrawing effects and the electron-pull effect from axial extraplanar ligand in iron-phthalocyanine (FePc) are key factors in improving the binding energy between the active Fe site and O2 resulting in an increase of the electrocatalytic activity of FePcs for the ORR. In this work, we have utilized fundamental principles of electrocatalysis and DFT calculations to design and synthesize FeN4 molecular catalysts to increase their catalytic performance for the ORR through the "pull" effect. To achieve this, by chemical synthesis, we have incorporated pyridinium functional groups (N+py) in peripheral and non -peripheral positions into the porphyrazine cyclic ligands. In this fashion we obtain the porphyrazinium molec-ular catalysts, [Fe(II)2,3-(TMe)TPyPz]4+ and [Fe(II)3,4-(TMe)TPyPz]4+. Because these new compounds are not commercially available and, to the best of our knowledge, they have not been tested for ORR. In order to determine their effectiveness, we have compared porphyrazinium with neutral analog porphyrazine compounds (Fe(II)TPyPz) and perfluorinated and perchlorinated iron phthalocyanines, which are currently the best molecular catalysts for ORR. The electrocatalytic activity was determined for each molecular catalyst deposited on the edge plane of a graphite electrode (EPG) surface in an alkaline medium. Only for the purpose of comparison we include two Fe porphyrins studied previously, which show low activity for ORR. Although the DFT theoretical analysis of porphyrazinium complexes suggests a high activity for these catalysts, our experimental findings revealed the opposite trend. Therefore, this finding makes us reconsider the interfacial effects, such as the counter-ions effects on N+py that could influence the electron-pull effect, opening new insights for designing molecular catalysts considering interface engineering. Moreover we report for the first time, the reactivity linear relationship between the metal-centered redox potential gap (E degrees Fe(III)/(II) - E degrees Fe(II)/(I))) with the electrocatalytic activity for ORR for all catalysts studied, emerging this potential gap as a possible and promising new reactivity descriptor for ORR in MN4 catalyst.