Crystal structure, thermal stability, spectroscopy, and DFT studies of quaternary salts of N-halomethylated ammonium tetrafluoroborate

Considering the great importance of quaternary ammonium tetrafluoroborate compounds due to their potential application in areas such as catalysis, electrochemistry, and materials chemistry, three new salts were prepared from analogous iodine salts using anion exchange reaction, in which the iodine salt was treated with AgBF₄ in methanol. The characterizations were performed by ¹H NMR and ¹³C NMR spectroscopy. Single crystals of C₄I⁺BF₄⁻ 1a and C₅I⁺BF₄⁻ 1b tetrafluoroborate salts were obtained by slow evaporation and their crystal structures were determined by single crystal X-ray diffraction. In 1a and 1b, the packing is mainly affected by Coulombic interactions and CH‧‧‧F hydrogen bonds. Interestingly, C-I‧‧‧F interactions were detected where I and fluorine atoms act as Lewis acid and base, respectively. The thermal behavior of the compounds C₄I⁺BF₄⁻ 1a, C₅I⁺BF₄⁻ 1b and C₆I⁺BF₄⁻ 1c was studied using TGA and DSC methods, showing excellent thermal stability up to 150 °C. Geometry optimizations and frequency calculations for 1a, 1b and 1c were performed at DFT level of theory by B3LYP method, employing the 6–311++G(d,p) and LANL2DZ mixed basis set. FT-IR vibrational spectra were analyzed for 1a, 1b and 1c, and the corresponding vibrational assignments were performed. Supporting information reports the complete calculated mode frequencies for each compound and the respective correlation with the observed modes in infrared (IR) spectroscopy is reported. Electronic molecular behavior was studied by the calculation of the frontier molecular orbitals and the global chemical reactivity descriptors were calculated, showing that the length of the alkyl chains influences their molecular reactivities. Electronic UV–Vis spectra were explained showing that mainly π → π* electronic transitions occur for these compounds. These calculations were contrasted with experimental data showing excellent congruence demonstrating that calculations were performed adequately.