The first binuclear sandwich-like complexes based on the aromatic tetraatomic species

Abstract

The first binuclear sandwich-like complexes based on the aromatic tetraatomic species with formula M₂(η⁴-E₄)₂ (M = Al, Ga; E = N, P, As) have been studied by density functional theory (DFT). The stable conformer for each M₂(η⁴-E₄)₂ is the staggered one with D_4d symmetry except for Ga₂(η²-N₄)₂ with C_2v symmetry. Natural bonding orbital (NBO) analysis indicates that the metal–metal bonds of Al₂(η⁴-E₄)₂ (E = N, P, As) and Ga₂(η⁴-E₄)₂ (E = P, As) are all σ single bonds, which are derived mostly from the s and p_z orbitals of the metal atoms by molecular orbital (MO) analysis. For M₂(η⁴-E₄)₂ (M = Al, Ga; E = P, As), the metal–ligand interactions are covalent, while for Al₂(η⁴-N₄)₂ the interactions between the Al atoms and the N₄²⁻ ligands are ionic. According to the calculated dissociation energies for breaking metal–metal bonds, the Al–Al and Ga–Ga bonds are very strong indicating that these stable sandwich-like compounds Al₂(η⁴-E₄)₂ (E = N, P, As) and Ga₂(η⁴-E₄)₂ (E = P, As) may be synthesized in future experiments. The nitrogen-rich compounds Al₂(η⁴-N₄)₂ and Ga₂(η²-N₄)₂ may be used as potential candidates of high energy density materials (HEDMs). Nucleus-independent chemical shifts (NICS) values reveal that the E₄²⁻ rings in the Al₂(η⁴-E₄)₂ (E = N, P, As) and Ga₂(η⁴-E₄)₂ (E = P, As) species possess conflicting aromaticity (σ antiaromaticity and π aromaticity) and with the same ligands, the E₄²⁻ ligands in Ga₂(η⁴-E₄)₂ have more aromaticity than those in Al₂(η⁴-E₄)₂.