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]-<i>N</i>,<i>N</i>,<i>N</i>-trimethylaniline ([<i>p</i>-(Mes₂B)­C₆H₄(NMe₃)]⁺, <b>1</b>, where “Mes” represents mesitylene and “Me” represents methyl) and 2-[bis­(2,4,6-trimethylphenyl)­boranyl]-<i>N</i>,<i>N</i>,<i>N</i>-trimethylaniline ([<i>o</i>-(Mes₂B)­C₆H₄(NMe₃)]⁺, <b>2</b>) 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 <b>1</b> and <b>2</b> are the most active centers for nucleophilic addition of anions. The negative magnitude of free energy changes (Δ<i>G</i>) reveals that out of CN^–, F^–, Cl^–, Br^–, NO₃^–, and HSO₄^– only the binding of CN^– and F^– with <b>1</b> and <b>2</b> is thermodynamically feasible and spontaneous. In addition, the calculated binding energies reveal that the CN^– is showing lesser binding affinity than F^– both with <b>1</b> and <b>2</b>, 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 <b>1</b> and <b>2</b> 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]-<i>N</i>,<i>N</i>,<i>N</i>-trimethylaniline cyanide ([<i>p</i>-(Mes₂B)­C₆H₄(NMe₃)] CN, <b>1CN</b>, the cyano form of <b>1</b>) and 4-[bis­(2,4,6-trimethylphenyl)­boranyl]-<i>N</i>,<i>N</i>,<i>N</i>-trimethylaniline fluoride ([<i>p</i>-(Mes₂B)­C₆H₄(NMe₃)] F, <b>1F</b>, the fluoro form of <b>1</b>) and fifth excited state (S₅) of 2-[bis­(2,4,6-trimethylphenyl)­boranyl]-<i>N</i>,<i>N</i>,<i>N</i>-trimethylaniline fluoride ([<i>o</i>-(Mes₂B)­C₆H₄(NMe₃)] F, <b>2F</b>, the fluoro form of <b>2</b>) 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 <b>1CN</b>, <b>1F</b>, and <b>2F</b> after a significant internal conversion (IC) from S₄ and S₅ to S<sub>1.