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.

三乙胺为一种挥发性液体，存在于大气中气相形态。作为一种有害空气污染物，它被认定为有毒空气污染物。因此，从该有毒物质的蒸汽态中生产氨（肥料生产中的关键化学品）是一项具有挑战性的任务。三乙胺蒸汽（C2H5）3N的扩散进入酸化水溶液中的钼酸钠，导致形成单一晶体的化合物[(C2H5)3NH]2[(C2H5)4N]­[NaMo8O26]（1）。值得注意的是，化合物1包含一个[(C2H5)4N]+阳离子，尽管相关的反应混合物并未经过任何四乙基铵盐的处理。在酸性水介质中，从(C2H5)3N中形成[(C2H5)4N]+阳离子的可能性仅在于整体反应中形成了铵离子(NH4+)：4(C2H5)3N + 4H+ = 3[(C2H5)4N]+ + [NH4]+。尽管通过奈斯勒试剂测试识别的NH4+阳离子并未包含在化合物1的晶体中，但它可以在化合物[NH4⊂B15C5]3[PMo12O40]·B15C5（2）（B15C5 = 苯-15-冠-5）的单晶分离过程中与冠醚相关联。对化合物2的晶体结构分析和1H NMR研究确立了化合物2中存在氢键结合的NH4+离子的存在，从而确立了重排反应。