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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    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.