High-Level Coupled-Cluster Study on Substituent Effects in H₂Activation by Low-Valent Aluminyl Anions

Abstract

The synthesis of novel aluminyl anion complexes has been well exploited in recent years. Moreover, the elucidation of the structure and reactivity of these complexes opens the path toward a new understanding of low-valent aluminum complexes and their chemistry. This work computationally treats the substituent effect on aluminyl anions to discover suitable alternatives for H2 activation at a high level of theory utilizing coupled-cluster techniques extrapolated to the complete basis set. The results reveal that the simplest AlH2 – system is the most reactive toward the activation of H2, but due to the low steric demand, severe difficulty in the stabilization of this system makes its use nonviable. However, the results indicate that, in principle, aluminyl systems with −C, −CN, −NC, and −N chelating centers would be the best choices of ligand toward the activation of molecular hydrogen by taking care of suitable steric demand to prevent dimerization of the catalysts. Furthermore, computations show that monosubstitution (besides −H) in aluminyl anions is preferred over disubstitution. So our predictions show that bidentate ligands may yield less reactive aluminyl anions to activate H2 than monodentate ones.

MQM 2022: The 10th Triennial Conference on Molecular Quantum Mechanics

“…32 Next, Villegas-Escobar, Hoobler, Toro-Labbe, and Schaefer at the Pontificia Universidad Catoĺica de Chile and the University of Georgia examined the reactivity of low-valent aluminyl ions toward H 2 through a series of coupled cluster calculations. 33 They found that aluminyl systems with carbonand nitrogen-based chelating centers would serve well to activate hydrogen without dimerization. Finally, according to a report by Datar, Wright, and Matthews at Southern Methodist University, the presence of electric fields yields shifts in absorption energies and intensities in X-ray Stark spectra that may be attributed to responses in the molecular orbitals, the dipole moment, and the geometry.…”

“…Using restricted active space configuration interaction (RASCI) wave functions, Rodríguez-Jiménez, Carreras, and Casanova at the Donostia International Physics Center in Spain explore the origin of these couplings and introduce a new definition of the SOC constant for molecular aggregates . Next, Villegas-Escobar, Hoobler, Toro-Labbé, and Schaefer at the Pontificia Universidad Católica de Chile and the University of Georgia examined the reactivity of low-valent aluminyl ions toward H 2 through a series of coupled cluster calculations . They found that aluminyl systems with carbon- and nitrogen-based chelating centers would serve well to activate hydrogen without dimerization.…”

Three Oxidative Addition Routes of Alkali Metal Aluminyls to Dihydridoaluminates and Reactivity with CO2

“…In an earlier study with the lighter alkali metal aluminyls [AM{Al(NON Dipp )}] 2 [AM=Li ( I‐ Li), Na ( I ‐Na), K ( I ‐K)], [6b,c] the efficiency of dissociating the H−H bond at 1.5 bar pressure illustrated a diminishing trend on descending the group, requiring elevated temperatures (100 °C) and long reaction times (Li=3 days, Na=14 days, K=25 days) to achieve complete conversion [6b] . There are possibly several factors behind the harsher reaction conditions required on descending group one, but the stronger electronegativity at the top of the group is likely to be a factor in the higher reactivity of the lighter metal complexes with H 2 [8h,i] . Hence, we postulated that alternative hydride sources containing breakable C−H and Si−H bonds such as in 1,4‐cyclohexadiene (1,4‐CHD) and phenylsilane (PhSiH 3 ) may provide other oxidative addition routes to the bimetallic hydrides.…”