Solid-Phase Quasi-Intramolecular Redox Reaction of [Ag(NH₃)₂]MnO₄: An Easy Way to Prepare Pure AgMnO₂

Abstract

Two monoclinic polymorphs of [Ag(NH 3 ) 2 ]MnO 4 containing a unique coordination mode of permanganate ions were prepared, and the high-temperature polymorph was used as a precursor to synthesize pure AgMnO 2 . The hydrogen bonds between the permanganate ions and the hydrogen atoms of ammonia were detected by IR spectroscopy and single-crystal X-ray diffraction. Under thermal decomposition, these hydrogen bonds induced a solid-phase quasi-intramolecular redox reaction between the [Ag(NH 3 ) 2 ] + cation and MnO 4 – anion even before losing the ammonia ligand or permanganate oxygen atom. The polymorphs decomposed into finely dispersed elementary silver, amorphous MnO x compounds, and H 2 O, N 2 and NO gases. Annealing the primary decomposition product at 573 K, the metallic silver reacted with the manganese oxides and resulted in the formation of amorphous silver manganese oxides, which started to crystallize only at 773 K and completely transformed into AgMnO 2 at 873 K.

AgNO3⋅NH4NO3 – an enigmatic double‐salt type “decomposition intermediate” of diamminesilver(I) permanganate

“…The deuterated compound 1 (1-D) was prepared in two different ways. Gradual deuteration of compound 1 in D 2 O with the complete evaporation and removal of the equilibrium protium content, using freshly prepared CaO as Ca(OH) x (OD) 1À x , resulted in gradual increase of deuterium level [7,16] in compound 1 and four deuteration steps were needed to reach almost complete deuteration of compound 1. For all these steps freshly prepared CaO had to be used in a closed desiccator.…”

“…Furthermore, there is no elementary silver formation and the permanganate ion completely disappear at 80°C, whereas, at 125°C, metallic silver forms. [7] The IR spectrum of the decomposition intermediate formed at 80°C (ESI Figure 14) shows the presence of nitrate (1314, 1213, 1038, 820 or 707 cm À 1 ), coordinated ammonia (3324, 3265, 3174 cm À 1 , 1745, 1366) and water (~3500 cm À 1 , ν OH ;~1600 cm À 1 , δ OH2 ) bands, and the complete absence of MnO 4 nÀ (n = 1-3) oxoanions (missing ν as (Mn À O) bands for M VII , M VI or M V species). [23] There is no MnO 2 either, only lower manganese oxides could be detected.…”

“…2) The compound 3 easily decomposes on heating above 100°C into metallic silver, N 2 and NH 3 , [28] which explains Ag appearance at 125°C reported in. [7] The decomposition intermediate of compound 2 contains compound 3 and MnO. The mixture of compound 3 and MnO decomposes on heating in air into Ag and MnO x (x = 1 (MnO), 1.34 (Mn 3 O 4 ) or 1.5 (Mn 2 O 3 ) mixture, which resulted in AgMnO 2 due to 1 : 1 Ag:MnO x ratio.…”

“…Compound 1 was also isolated from the aqueous leachate of the thermal decomposition product of [Ag(NH 3 ) 2 ]MnO 4 (compound 2), but the mechanism of its formation is unknown. [7] The selective oxidation of one of the coordinated ammonia ligand into ammonium nitrate during the thermal decomposition of [M-(NH 3 ) 4 ](MnO 4 ) 2 (M=Cu, Zn, Cd) complexes [8][9][10] is known, however, in the case of compound 1, it is questionable why only the double salt (compound 1) is formed instead of the expected ammonium nitrate. Furthermore, thermal decomposition features of compound 1 showed that compound 2 forms via an unknown intermediate (compound 3), which transforms into compound 1 during the aqueous leaching.…”

“…Furthermore, thermal decomposition features of compound 1 showed that compound 2 forms via an unknown intermediate (compound 3), which transforms into compound 1 during the aqueous leaching. [7] Neither AgNO 3 nor NH 4 NO 3 is present in the evaporated aqueous leachate, which raises that question how could form AgNO 3 and NH 4 NO 3 in 1 : 1 molar ratio?…”