Selective Complexation of Cyanide and Fluoride Ions with Ammonium Boranes: A Theoretical Study on Sensing Mechanism Involving Intramolecular Charge Transfer and Configurational Changes

The anion binding selectivity and the recognition mechanism of two isomeric boranes, namely, 4-[bis­(2,4,6-trimethylphenyl)­boranyl]-N,N,N-trimethylaniline ([p-(Mes₂B)­C₆H₄(NMe₃)]⁺, 1, where “Mes” represents mesitylene and “Me” represents methyl) and 2-[bis­(2,4,6-trimethylphenyl)­boranyl]-N,N,N-trimethylaniline ([o-(Mes₂B)­C₆H₄(NMe₃)]⁺, 2) has been investigated using density functional theory (DFT) and time dependent-density functional theory (TD-DFT) methods. Natural population analysis indicates that the central boron atoms in 1 and 2 are the most active centers for nucleophilic addition of anions. The negative magnitude of free energy changes (ΔG) reveals that out of CN^–, F^–, Cl^–, Br^–, NO₃^–, and HSO₄^– only the binding of CN^– and F^– with 1 and 2 is thermodynamically feasible and spontaneous. In addition, the calculated binding energies reveal that the CN^– is showing lesser binding affinity than F^– both with 1 and 2, while other ions, viz. NO₃^–, HSO₄^–, Br^–, and Cl^–, either do not bind at all or show very insignificant binding energy. The first excited states (S₁) of 1 and 2 are shown to be the local excited states with π → σ* transition by frontier molecular orbital analysis, whereas fourth excited states (S₄) of 4-[bis­(2,4,6-trimethylphenyl)­boranyl]-N,N,N-trimethylaniline cyanide ([p-(Mes₂B)­C₆H₄(NMe₃)] CN, 1CN, the cyano form of 1) and 4-[bis­(2,4,6-trimethylphenyl)­boranyl]-N,N,N-trimethylaniline fluoride ([p-(Mes₂B)­C₆H₄(NMe₃)] F, 1F, the fluoro form of 1) and fifth excited state (S₅) of 2-[bis­(2,4,6-trimethylphenyl)­boranyl]-N,N,N-trimethylaniline fluoride ([o-(Mes₂B)­C₆H₄(NMe₃)] F, 2F, the fluoro form of 2) are charge separation states that are found to be responsible for the intramolecular charge transfer (ICT) process. The synergistic effect of ICT and partial configuration changes induce fluorescence quenching in 1CN, 1F, and 2F after a significant internal conversion (IC) from S₄ and S₅ to S_1.