DFT and TDDFT study on cation-π complexes of diboryne (NHC → B ≡ B←NHC)

In this study, density functional theory calculation on mono-cationic cation-π complexes of diborynes has been made to understand the interaction in cation-π complexes of diboryne. Results suggest that apart from the smaller cations Li⁺ and Na⁺, larger cation like K⁺ ion could also form complexes with diboryne compounds via cation-π interaction. From the calculated structural and spectroscopic analysis ¹¹B, ¹³C NMR (Nuclear Magnetic Resonance), FTIR (Fourier Transform Infra red) (force constant, value), and UV-vis spectra, it is found that the interaction between the cations and π-electron cloud of the diboryne is purely electrostatic. It is also observed that smaller cation (Li⁺) with high electronegativity interacts more strongly compared to larger cation (K⁺). Calculated interaction energy advocates that the π-electron cloud of the B₂ unit contributes more to the cation-π interaction than the two aromatic phenyl rings of the NHC (N-heterocyclic carbene) substituted with 2,6-diisopropylphenyl group. The aryl substituent at the NHC-ligands undergoes a change in spatial orientation with respect to the size of cations in order to provide suitable space to the cations for effective cation-π interaction. Quantum theory of atoms in molecules study clarifies further the nature and extent of B-B and B₂-cation interactions.¹¹B-NMR, ¹³C-NMR, and time dependent density functional theory analysis indicate that cation-π interaction annihilates the B → C (NHC) π-back donation and favours the B≡B bond formation.