Browsing by Author "Villegas-Escobar, Nery"

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    Bulky and Electron-Deficient α-Iminocarboxamidato-Nickel(II) Complexes: A Study of the Steric and Electronic Effects on Ethylene Activation
    (2021) Skarjan, Leon; Villegas-Escobar, Nery; Correa, Sebastian A.; Daniliuc, Constantin G.; Matute, Ricardo A.; Rojas, Rene S.
    Two alpha-iminocarboxamidato-nickel(II) complexes containing ligands with several CF3 groups were synthesized and characterized by NMR spectroscopy, elemental analysis, and density functional theory (DFT) calculations. Surprisingly, the prepared complexes were inactive toward ethylene oligo/polymerization reactions upon activation attempts with common lewis acid co-catalysts such as B(C6F5)(3) and BF3. Quantum chemistry calculations were employed to reveal that adduct formation is thermodynamically favored for small Lewis acids such as BF3 due to the sterically demanding ligand environment of the complex, confirming the experimental findings. DFT results associate the lack of polymerization activity with a highly unfavorable steric environment, undesirable London dispersion interactions between the ligands, and a strong electrostatic stabilization caused by the employed ligands. Our findings should help future researchers to identify necessary electronic and steric requirements for the compounds to generate active Ni(II) catalysts for ethylene oligo/polymerization activated by suitable boron Lewis acids.
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    Catalytic Mechanism of H-2 Activation by a Carbenoid Aluminum Complex
    (2015) Villegas-Escobar, Nery; Gutiérrez Oliva, Soledad; Toro Labbé, 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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    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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    Toward a Neutral Single-Component Amidinate Iodide Aluminum Catalyst for the CO2 Fixation into Cyclic Carbonates
    (2021) Saltarini, Sebastian; Villegas-Escobar, Nery; Martinez, Javier; Daniliuc, Constantin G.; Matute, Ricardo A.; Gade, Lutz H.; Rojas, Rene S.
    A new iodide aluminum complex ({AlI(kappa(4)-naphbam)}, 3) supported by a tetradentate amidinate ligand derived from a naphthalene-1,8-bisamidine precursor (naphbamH, 1) was obtained in quantitative yield via reaction of the corresponding methyl aluminum complex ({AlMe(kappa(4)-naphbam)}, 2) with 1 equiv of I-2 in CH2Cl2 at room temperature. Complexes 2 and 3 were tested and found to be active as catalysts for the cyclic carbonate formation from epoxides at 80 degrees C and 1 bar of CO2 pressure. A first series of experiments were carried out with 1.5 mol % of the alkyl complex 2 and 1.5 mol % of tetrabutylammonium iodide (TBAI) as a cocatalyst; subsequently, the reactions were carried out with 1.5 mol % of iodide complex 3 as a single-component catalyst. Compound 3 is one of the first examples of a nonzwitterionic halide single-component aluminum catalyst producing cyclic carbonates. The full catalytic cycle with characterization of all minima and transition states was characterized by quantum chemistry calculations (QCCs) using density functional theory. QCCs on the reaction mechanism support a reaction pathway based on the exchange of the iodine contained in the catalyst by 1 equiv of epoxide, with subsequent attack of I- to the epoxide moiety producing the ring opening of the epoxide. QCCs triggered new insights for the design of more active halide catalysts in future explorations of the field.