Interaction of aryl tetrazolones with anions: proton transfer vs. hydrogen bonding

Abstract

In this study, the interaction of two monosubstituted aryl tetrazolones, namely 1-(4-nitrophenyl)-1,4-dihydro-tetrazol-5-one 1a and 1-(4-trifluoromethylphenyl)-1,4-dihydro-tetrazol-5-one 1b, with anions of varying basicity e.g. hydrogen sulfate (HSO₄⁻), bromide (Br⁻), nitrate (NO₃⁻), thiocyanate (NCS⁻), chloride (Cl⁻) and acetate (AcO⁻) was investigated in acetonitrile and DMSO. UV and NMR titrations indicated that tetrazolones interact through hydrogen bonding or weak electrostatic interactions with HSO₄⁻, Br⁻, NO₃⁻, NCS⁻, and Cl⁻. The association constants (K_a) were found to be in the order Cl⁻ > Br⁻ > NO₃⁻ > NCS⁻ > HSO₄⁻. Tetrazolones 1a,b exhibited almost 9 times higher selectivity towards Cl⁻ compared to Br⁻ which may be attributed to the higher basicity of the former. The affinity of 1a towards most anions was higher than 1b, except for HSO₄⁻. This was attributed to the higher electron withdrawing ability of the –NO₂ group compared to –CF₃ that renders the N–H proton more acidic for interaction with anions resulting in higher K_a values. Significant UV changes were observed upon addition of the AcO⁻ anion to the solution of 1a,b in acetonitrile as new bands formed and isosbestic points were observed indicating the formation of a new species. NMR titrations in DMSO-d₆ further confirmed that 1a,b underwent deprotonation with AcO⁻ owing to its higher basicity.