A Rearrangement Reaction to Yield a NH₄⁺ Ion Driven by Polyoxometalate Formation

Triethylamine is a volatile liquid and exists in the atmosphere in the gas phase. It is a hazardous air pollutant and identified as a toxic air contaminant. Thus, producing ammonia (a vital chemical for fertilizer production) from the vapor state of this toxic substance is a challenging task. Diffusion of the vapor of triethylamine, (C₂H₅)₃N, into an acidified aqueous solution of sodium molybdate results in the formation of single crystals of compound [(C₂H₅)₃NH]₂[(C₂H₅)₄N]­[NaMo₈O₂₆] (1). Notably, compound 1 includes a [(C₂H₅)₄N]⁺ cation, even though the concerned reaction mixture was not treated with any tetraethylammonium salt. The formation of the [(C₂H₅)₄N]⁺ cation from (C₂H₅)₃N in an acidic aqueous medium is logically possible only when an ammonium cation (NH₄⁺) is formed in the overall reaction: 4­(C₂H₅)₃N + 4H⁺ = 3­[(C₂H₅)₄N]⁺ + [NH₄]⁺. Although the resulting NH₄⁺ cation (identified by Nessler’s reagent test) is not included in the crystals of compound 1 as a cation, it can be made associated with a crown ether in the isolation of single crystals of compound [NH₄⊂B15C5]₃[PMo₁₂O₄₀]·B15C5 (2), (B15C5 = benzo-15-crown-5). Crystal structure analysis and ¹H NMR studies of compound 2 have established the presence of an H-bonded NH₄⁺ ion in compound 2, thereby established the rearrangement reaction.