A new
iodide aluminum complex ({AlI(κ4-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(κ4-naphbam)}, 2) with 1 equiv of I2 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 °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.
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 σ-bonds and in reactions with electrophiles. Various research groups have detected that the potassium atoms play a stabilization role via electrostatic and cation� � � p interactions with nearby (aromatic)-carbocyclic rings from both the ligand and from the reaction with unsaturated substrates. Since stabilizing K⋯H 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)] À (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 À 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 DE 6 ¼ dist . 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.
The catalytic mechanism of H2 activation by a carbenoid
aluminum compound is analyzed in great detail. On the basis of the
reaction force analysis, the electronic activity that takes place
during the chemical reaction was identified and characterized through
the reaction electronic flux and rationalized in terms of chemical
events that drive the reaction. Successive transformation of the nucleophilic
or electrophilic character of the reagents along the reaction coordinate
monitored through the dual descriptor allows us to obtain a very complete
and detailed description of the reaction mechanism that proceeds through
a two-stage mechanism in a one-kinetic-step process.
Fullerenes, in particular C60, are important molecular entities in many areas, ranging from material science to medicinal chemistry. Decomposition of the electronic activity to understand competing regioselectivity in the cycloaddition between C60 and cyclopentadiene is studied in great detail.
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