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

Two monoclinic polymorphs of [Ag(NH3)2]MnO4 containing a unique coordination mode of permanganate ions were prepared, and the high-temperature polymorph was used as a precursor to synthesize pure AgMnO2. 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(NH3)2]+ cation and MnO4– anion even before losing the ammonia ligand or permanganate oxygen atom. The polymorphs decomposed into finely dispersed elementary silver, amorphous MnOx compounds, and H2O, N2 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 AgMnO2 at 873 K.


Correlation analysis for polymorphs LT-1 and HT-1
The isolated permanganate ion has T d symmetry, and accordingly, four Raman and two IR (F 2 species) active modes can be expected: ν 1 (symmetric stretching), ν 2 (symmetric bending), ν 3 (antisymmetric stretching) and ν 4 (antisymmetric bending). The symmetric bending mode is doubly (E), whereas the ν 3 and ν 4 modes are triply (F 2 ) degenerate under the molecular symmetry (T d ). In principle, after removing the degeneracy levels completely, 9 internal modes of permanganate ion can be expected. The compound LT-1 has two different permanganate ion types, thus the number of vibrations is double than that for compound HT-1. The hindered rotations and translations of the permanganate anions are both triply degenerate, under T d symmetry. The total number of factor-group modes, due to the external MnO 4 − vibrations (hindered translations and hindered rotations), is equivalent with 2 x12=24 and 12 degrees of freedom for compounds LT-1 and HT-1, respectively.

S17
The complex cation (Ag(NH 3 ) 2 + ) modes are decomposed into components of ammonia as ligand (C 3v ) modes and to the translation of central silver ions . For the ammonia as a ligand, ν 1 is the symmetric stretching, ν 2 is the symmetric bending, ν 3 is the antisymmetric stretching and ν 4 is the antisymmetric bending modes. Both ν 3 and ν 4 are doubly degenerated (E) vibrations under C 3v . The total number of factor-group modes, due to the internal vibrations and 4 or 2 types of crystallographically different ammonia ligands, are 4 x 12 = 48 and 2 x 12 = 24 resulting in 48 and 24 vibrational degrees of freedom in compound LT-1 and HT-1, respectively. The external modes (T xy and R xy ) are doubly degenerate modes under C 3v . The total number of factor-group modes due to the external vibrations is doubled and quadrupled (a consequence of two and four crystallographic types of NH 3 ) and are equivalent with 4 x 12 = 48 and 2 x 12 = 24 vibration degrees of freedom for compounds LT-1 and HT-1, respectively.
Regarding the Ag + -ions, there are 3 modes of acoustic origin out of the total of 48 (compound LT-1) and 24 (compound HT-1) external modes, which belong to species A u + 2B u . 45 and 21 optical modes of translational origin, 72 and 36 optical modes of rotational origin and 84 and 42 optical modes due to internal vibrations for compounds LT-1 and HT-1, respectively.

UV spectroscopic results
The diffuse UV reflectance spectra of the solid solution of compound HT-1 (1 %) in HT-1-ClO 4 had a very broad band system, which might contain the orbital allowed ( 1 A 1 -1 T 2 )(t 1 -2e) ( 480-580 nm) and ( 1 A 1 -1 T 2 )(t 1 -t 2 ) (320-380 nm). 29 The permanganate ( 1 A 1 -1 T 2 )(3t 2 -2e) transition is expected to be a weak band between 380 and 320 nm. 30 The site symmetry of permanganate ion in compound HT-1-ClO 4 is C s , thus 1 T 2 level splits into three states of symmetry, A", A'(1) and A'(2), and this splitting can further complicate the band system. In the UV range below 260 cm -1 the Ag 4d-5s transition and the N p -Ag 5s or O p -Ag 5s LMCT transitions could give signal.

Polymorphic crystal structures of diammine silver(I) permanganate measured at 100 K and 180 K.
Single crystal structures of two polymorphic modifications of [Ag(NH 3 ) 2 ]MnO 4 complex were determined at 100 K (LT-1) and 180 K (HT-1) respectively using MoK α radiation. Both modifications crystallize in the monoclinic crystal system. The low temperature modification (LT-1) has the lower P2/m symmetry, which is a maximal non-isomorphic symmetry subgroup of the space group of the high temperature modification (HT-1) which is I2/m. LT-1 is isomorphous to the known structure of [Ag(NH 3 ) 2 ]ClO 4 complex. The asymmetric unit of LT-1 contains four quarter silver(I) cations, four half ammonia ligands and two half permanganate anions. The contents of the asymmetric unit of HT-1 are one half of that of LT-1 due to its higher symmetry. The unit cells of the two modifications are quite similar (ESI Table 1  The silver ions form infinite chains parallel to the b crystallographic axis in both structures. The Ag-Ag distance is 3.010 Å for LT-1 and 3.034 Å for HT-1. The silver cations have two coordinated ammonia molecules and the ammonia hydrogens are disordered over two positions in both structures with 1:1 occupancy. Two permanganate anions are coordinated to the silver in both structures giving rise to an octahedral coordination around the silver ion ( Figure S30) and to a three-dimensional coordination network. The atomic planes of nitrogen and oxygen atoms are perpendicular to the silver chain in all cases. the N-Ag-O angles are listed in ESI Table 2. In the HT-1 structure, the Ag-N bonds are lying on the a and c unit cell axes while in LT-1, the Ag-N bonds are tilted from the unit cell axis directions.
The Ag-Ag chains coincide with the two-fold rotation axes, the Ag-N bonds are lying on mirror planes and the Ag ions sit on inversion centres in both structures. All of the permanganate anions are cut in half by mirror planes. In the high temperature modification parallel to the 2-fold rotation axes, 2-fold screw axes appear, which connects the Ag-Ag chains to each other. Besides, between every two mirror planes, a glide plane appears which S30 connects the Ag coordination sphere to each other. Thus, the permanganate anions are related by inversion centres.   Grinberg determined a parameter for ammine complexes based on the values of δ s (NH 3 ) between the uncoordinated (gaseous) ammonia (968 cm −1 ) with RBS=0 and the most stable ammine complex ([Pt(NH 3 ) 4 ]Cl 2 ) (1370 cm −1 ) with RBS=100. The RBS 0 is defined as the lowest experimental found RBS among the diamminesilver complexes which is supposed to the value without contribution of the hydrogen bond interaction (RBS 0 =53.0).