The first nonmetal-centered binuclear sandwich-like complexes based on the tetraatomic species E 2−4 (E = N, P, As, Sb, Bi) and boron atoms

Abstract

The first nonmetal-centered binuclear sandwich-like complexes, B₂(η⁴-E₄)₂ (E = CMe, N, P, As, Sb, Bi; Me = CH₃), have been investigated by density functional theory (DFT) to explore whether the nonmetal element can be at the center to form stable sandwich-like complexes. The stable conformer for each species is the D_4d staggered one, in which there exists strong interaction between the two boron atoms. Natural bonding orbital (NBO) analysis indicates that the boron–boron bonds are all σ single bonds, which are predicted to be derived mostly from the s and p_z orbitals of the boron atoms. The boron–boron bond dissociation energies are higher than that of the Zn–Zn bond in the synthesized complex Cp*ZnZnCp* (Cp* = C₅Me₅; Me = CH₃) suggesting that these nonmetal-centered sandwich-like complexes may be synthesized in future experiments. According to the energy decomposition analysis (EDA), the boron–boron bond is much weaker than the boron–ligand bond and the ability of the E²⁻₄ ligands to stabilize the boron–boron bonds is in the order of N²⁻₄ > P²⁻₄ > As²⁻₄ > Sb²⁻₄ > Bi²⁻₄. Nucleus-independent chemical shift (NICS) values reveal that E²⁻₄ (E = CMe, N, P, As, Sb) rings of B₂(η⁴-E₄)₂ exhibit characteristics of π aromaticity at 1.0 Å above the ring center, whereas Bi²⁻₄ rings of B₂(η⁴-Bi₄)₂ possess antiaromaticity. In addition, the differences in the bonding nature between the nonmetal-centered and metal-centered binuclear sandwich-like complexes were also investigated. In these nonmetal-centered complexes B₂(η⁴-E₄)₂, the interactions between the boron atoms and the E²⁻₄ ligands are more than half covalent, while in the metal-centered binuclear sandwich-like complexes, the metal–ligand interactions are mainly ionic.