The first nonmetal-centered binuclear sandwich-like complexes based on the tetraatomic species E2−4 (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 D4d 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 pz 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 A 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.