Mimicking the C2 molecule: M2B2 and M3B2+ clusters (M = Li, Na) and the reactivity of the N-heterocyclic carbene bound Li2B2 complex

Abstract

C₂ has attracted considerable attention from the scientific community for its debatable bonding situation. Herein, we show that the global minima of M₂B₂ and M₃B₂⁺ (M = Li, Na) possess similar covalent bonding patterns to C₂. Because of strong charge transfer from M₂/M₃ to B₂ dimer, they can be better described as [M₂]²⁺[B₂]²⁻ and [M₃]³⁺[B₂]²⁻ salt complexes with the B₂²⁻ core surrounded perpendicularly by two and three M⁺ atoms, respectively. The energy decomposition analyses in combination with the natural orbital for chemical valence theory give four bonding components in C₂, M₂B₂, and M₃B₂⁺ clusters. However, the fourth component does not arise from a bonding interaction but from polarization/hybridization. Considering the effect of Pauli repulsion in σ-space, the attractive covalent interaction in these molecules mainly comes from the two π-bonds. We further presented stable N-heterocyclic carbene (NHC) and triphenylphosphine (PPh₃) ligands bound Li₂B₂(NHC)₂ and Li₂B₂(PPh₃)₂ complexes. A comparative study of reactivity towards L = CO₂, CO, and N₂ between Li₂B₂(NHC)₂ and B₂(NHC)₂ is also performed. L–Li₂B₂(NHC)₂ is highly stable against L dissociation at room temperature for L = CO₂ and CO, and the stability is markedly higher than that in L–B₂(NHC)₂. The larger B₂→L π-backdonation in L–Li₂B₂(NHC)₂ also makes L more activated than in L–B₂(NHC)₂.