Aggregation of nitroaniline in tetrahydrofuran through intriguing H-bond formation by sodium borohydride

Abstract

The participation of sodium borohydride (NaBH₄) in hydrogen bonding interactions and transient anion radical formation has been proved. Thus, the properties of NaBH₄ are extended beyond the purview of its normal reducing capability and nucleophilic property. It is reported that ortho- and para-nitroanilines (NAs) form stable aggregates only in tetrahydrofuran (THF) in the presence of NaBH₄ and unprecedented orange/red colorations are observed. The same recipe with nitrobenzene instead of nitroanilines (NAs) in the presence of NaBH₄ evolves a transient rose red solution due to the formation of a highly fluorescent anion radical. Spectroscopic studies (UV-vis, fluorescence, RLS, Raman, NMR etc.) as well as theoretical calculations supplement the J-aggregate formation of NAs due to extensive hydrogen bonding. This is the first report where BH₄⁻ in THF has been shown to support such an aggregation process through H-bonding. It is further confirmed that stable intermolecular hydrogen bond-induced aggregation requires a geometrical match in both the nitro- and amino-functionalities attached to the phenyl ring with proper geometry. On the contrary, meta-nitroaniline remains as the odd man out and does not take part in such aggregation. Surprisingly, Au nanoparticles dismantle the J-aggregates of NA in THF. Explicit hydrogen bond formation in NA has been confirmed experimentally considering its promising applications in different fields including non-linear optics.