Substituent Effects on Aluminyl Anions and Derived Systems: A High-Level Theory

Abstract

Aluminyl anions are low-valent aluminum species bearing a lone pair of electrons and a negative charge. These systems have drawn recent synthetic interest for their nucleophilic nature, allowing for the activation of σ-bonds, and have been proposed as a pathway to hydrogen energy storage. In this research, we provide high-level ab initio geometries and energies for both the simplest aluminyl anion (AlH2 –) and several substituted derivatives. Geometries are reported using the gold-standard CCSD­(T)/aug-cc-pV­(T+d)­Z level of theory. Energies were extrapolated to the complete basis set limit through the focal point approach, utilizing coupled-cluster methods through perturbative quadruples and basis sets up to five-ζ quality. Geometries were rationalized using electrostatic, steric, and orbital donation effects. The donation from substituents to Al is accompanied by back-donation effects, a property traditionally thought of in transition-metal systems. Stereoelectronic effects through the secondary orbital interaction play a fundamental role in stabilizing these low-valent aluminum compounds and would likely also affect the feasibility of their use within several industrial applications. The energetic analysis of the formation of each substituted anion is rationalized as the result of three energetic schemes. The effectiveness of these schemes for determining the relative formation energies is discussed.

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

“…28,29 All computations were performed in the S 0 potential energy surface since this state is 14 kcal mol −1 lower in energy than the T 1 state for the simplest AlH 2 − species, as previously reported. 26 Therefore, all computations demanded restricted Hartree-Fock wave functions (RCCSD(T)/aug-cc-pV(T+d)Z). Tight convergence standards were used for geometry optimizations.…”

“…The B3LYP exchange-correlation functional was employed because it yielded structures closer to the ones obtained at the coupled-cluster level of theory. 26 These computations were performed with the Gaussian 16 software package. 44 The most important driving forces in the activation of H 2 were analyzed using the distortion/interaction model based on symmetry-adapted perturbation theory (SAPT).…”

“…The minor strain energy from the aluminyl anions is due to the reordering of substituents attached to the Al center, from angular geometry to tetrahedral, and more directly accompanied by the widening of the R 1 −Al−R 2 angle in the low-valent structures. 26 Although hybridization does not strictly apply in heavier main group elements, the Al center shows an approximate sp 2 hybridization, explicitly s 0.86 p 1.45 on average and based on NBO results (Supporting Information).…”

“…Although a selection of iconic examples is presented in Figure , the lack of systematic studies of simplified systems is detrimental to a complete understanding of the ligand or substituent effect on aluminyl anions’ energetics and reactivity. Therefore, this fact boosted us to examine in a recent work the substituent effect on substituted aluminyl anions and the response in geometry, electronic structure, and energetics at high-level theory yielding subchemical accuracy . However, in the present research, we focus on examining the substituent effect on aluminyl anions toward the activation of molecular hydrogen.…”

“…Therefore, this fact boosted us to examine in a recent work the substituent effect on substituted aluminyl anions and the response in geometry, electronic structure, and energetics at high-level theory yielding subchemical accuracy. 26 However, in the present research, we focus on examining the substituent effect on aluminyl anions toward the activation of molecular hydrogen. This work aims to provide a quantitative basis to discriminate the more suitable substitution and a rational ligand design to be utilized in H 2 activation applications.…”