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

Abstract

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.

Scheme 1. Schematic Representation of the Experimental Setup for the Synthesis of Compound 1 in a Gas–Liquid Interface Reaction

Figure 1

Nessler’s reagent test: (a) with ammonium chloride solution, (b) with filtrate containing HNO₃ acid, (c) with filtrate containing HCl acid, (d) with filtrate containing HNO₃ acid but without sodium molybdate, and (e) with solely water.

Figure 2

(a) Thermal ellipsoidal diagram of the asymmetric unit in the crystal structure of compound 1 at the 50% probability level. (b) Full molecule of compound 1 showing that two different halves of two different [Mo₈O₂₆]^4– clusters are coordinated to a central Na⁺ ion. Hydrogen atoms are not shown for clarity. Color code: Mo, purple; Na, yellow; O, red; N, blue; C, gray.

Figure 3

Thermal ellipsoidal diagram of the asymmetric unit in the crystal structure of compound 2 at the 50% probability level, representing the full molecule. Hydrogen atoms are omitted for clarity. Color code: Mo, cyan; O, red; C, gray; P, orange; N, blue.

Figure 4

Hydrogen bonding interactions around ammonium ions in the crystal structure of compound [NH₄⊂B15C5]₃[PMo₁₂O₄₀]·B15C5 (2). Purple dotted lines represent N–H···O hydrogen bonding interactions.

Figure 5

¹H NMR spectrum of the compound [NH₄⊂B15C5]₃[PMo₁₂O₄₀] ·B15C5 (2), showing shifts and splitting of the NH₄⁺ signals.