Abstract
There are numerous reports in the literature about the amount of hydrate water in sodium permanganate, which is said to be between one half and three water molecules per Na[MnO4]. Because no structural descriptions of the hydrate and the anhydrous compound can be found yet, this work reports the synthesis of anhydrous Na[MnO4] via the Muthmann method and its crystal structure. Na[MnO4] crystallizes as dark purple needles in the monoclinic space group P21/n with a = 572.98(5), b = 842.59(7), c = 715.47(6) pm, β = 92.374(3)° and Z = 4. As such and with its isotype to Ag[MnO4], Na[MnO4] completes the series of anhydrous alkali-metal permanganates, comprising Li[MnO4] (orthorhombic, Cmcm, Cr[VO4] type) and the isostructural heavier congeners A[MnO4] (A = K, Rb, Cs; orthorhombic, Pnma, Ba[SO4] type).
1 Introduction
Several permanganates of sodium have been reported in the literature [1], [2], [3], but all of them seem to contain water of hydration in varying amounts, ranging from Na[MnO4] · 1/2 H2O to Na[MnO4] · 3 H2O. Neither of these publications offers a detailed crystal structure of the reported products, however. The work reported shows a successful way to synthesize anhydrous Na[MnO4] and its crystal structure determination, which clearly reveals the isotype to Ag[MnO4] [4], [5], [6], exhibiting a sevenfold coordinated Na+ cation carrying seven independent, vertex-grafted, isolated [MnO4]− anions.
2 Results and discussion
We were successful in synthesizing anhydrous Na[MnO4], as opposed to the various hydrated products mentioned in the literature [1], [2], [3]. The thin, dark purple needles of Na[MnO4] adopt the monoclinic space group P21/n (no. 14) with a = 572.98(5), b = 842.59(7), c = 715.47(6) pm and β = 92.374(3)° for four formula units per unit cell (Table 1). In the crystal structure of Na[MnO4] (Table 2), the unique Na+ cations reside in a sevenfold oxygen coordination, resulting in a distorted pentagonal bipyramid (Fig. 1) as coordination sphere with d(Na–O) = 235–277 pm (Table 3). These [NaO7]13– polyhedra are connected to strands via two O2 atoms, which become further linked to adjacent strands through shared O3···O3 and O4···O4 edges. Because of the helical nature of a singular chain, these strands join together with other strands of this kind to form the achiral
Crystallographic data for Na[MnO4] and numbers pertinent to data collection and structure refinement.a
| Formula | Na[MnO4] |
| Molecular mass Mr | 141.93 |
| Crystal system | monoclinic |
| Space group | P21/n (no.14) |
| Formula units, Z | 4 |
| Lattice constants a, pm | 572.98(5) |
| b, pm | 842.59(7) |
| c, pm | 715.47(6) |
| β, deg | 92.374(3) |
| Calculated density Dx, g cm−3 | 2.732 |
| Molar volume Vm, cm3 mol−1 | 51.97(2) |
| Electron sum, F(000)/e | 272 |
| Diffractometer; radiation Wavelength λ, pm | IPDS-I (STOE); MoKαλ = 71.07c |
| Index range, ±h, ±k, ±l | 10, 12, 8 |
| θ range, deg | 2.8–28.3 |
| Absorption coefficient μ, mm−1 | 3.79 |
| Number of refined parameters | 56 |
| Refined extinction coefficient, g | 0.130(12) |
| Data corrections | Background, polarization and Lorentz factors; numerical absorption correction; program Habitus [7] |
| Collected vs. unique reflections | 4941, 1280 |
| Rint, Rσ | 0.049, 0.043 |
| Structure solution and refinement | Program package Shelxs/l-97 [8], [9] |
| Scattering factors | International tables, Vol. C [10] |
| R1, reflections with |Fo| > 4 σ(Fo) | 0.066; 879 |
| R1; wR2 for all reflections | 0.045; 0.122 |
| Goodness of fit (GooF) | 1.032 |
| Residual electron densities (ρ/e · 10−6 pm3), min; max | –0.60; 0.93 |
aFurther details of the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de, https://www.fiz-karlsruhe.de/en/leistungen/kristallographie/kristallstrukturdepot/order-form-request-for-deposited-data.html) on quoting the deposition number CSD-431105.
Fractional atomic coordinates and equivalent isotropic displacement parameters (Ueq, pm2) for Na[MnO4].
| Atom | x/a | y/b | z/c | Ueq, pm2 |
|---|---|---|---|---|
| Na | 0.2518(3) | 0.0364(2) | 0.1593(2) | 385(4) |
| Mn | 0.25709(9) | 0.18805(6) | 0.65959(7) | 267(2) |
| O1 | 0.2511(6) | 0.0616(4) | 0.4932(4) | 440(7) |
| O2 | 0.9932(6) | 0.2235(4) | 0.7159(4) | 414(6) |
| O3 | 0.8802(6) | 0.1504(3) | 0.0917(4) | 393(6) |
| O4 | 0.4049(6) | 0.1171(4) | 0.8381(4) | 400(6) |
All atoms occupy the general Wykoff sites 4e.
![Fig. 1: Coordination sphere of the [NaO7]13– polyhedron in Na[MnO4] (top) and its connectivity to seven vertex-grafted [MnO4]− tetrahedra (bottom).](/document/doi/10.1515/znb-2016-0099/asset/graphic/j_znb-2016-0099_fig_001.jpg)
Coordination sphere of the [NaO7]13– polyhedron in Na[MnO4] (top) and its connectivity to seven vertex-grafted [MnO4]− tetrahedra (bottom).
Motifs of mutual adjunction, selected interatomic distances (d, pm) and bond angles (∡, deg) in Na[MnO4].
| O1 | O2 | O3 | O4 | |
|---|---|---|---|---|
| Na | 1/1 | 2/2 | 2/2 | 2/2 |
| 239.8(4) | 247.5(3) | 236.7(3) | 235.5(3) | |
| 276.9(4) | 248.2(3) | 258.4(3) | ||
| Mn | 1/1 | 1/1 | 1/1 | 1/1 |
| 159.7(3) | 160.9(3) | 161.7(3) | 161.8(3) | |
| –O2: | –O3: | –O4: | –O1: | |
| 108.6(2) | 110.2(2) | 109.1(2) | 109.7(2) | |
| –O3: | –O4: | |||
| 109.5(2) | 109.7(2) |
![Fig. 2: View at the crystal structure of Na[MnO4] along [010]. The fusion of the [NaO7]13– polyhedra forms channels along the b axis, in which the isolated [MnO4]− tetrahedra reside.](/document/doi/10.1515/znb-2016-0099/asset/graphic/j_znb-2016-0099_fig_002.jpg)
View at the crystal structure of Na[MnO4] along [010]. The fusion of the [NaO7]13– polyhedra forms channels along the b axis, in which the isolated [MnO4]− tetrahedra reside.
![Fig. 3: The isolated permanganate anion [MnO4]− in Na[MnO4] shown as a tetrahedron with its immediate Na+ surrounding.](/document/doi/10.1515/znb-2016-0099/asset/graphic/j_znb-2016-0099_fig_003.jpg)
The isolated permanganate anion [MnO4]− in Na[MnO4] shown as a tetrahedron with its immediate Na+ surrounding.
3 Conclusions
Anhydrous Na[MnO4] with its sevenfold coordinated sodium cations crystallizes in the monoclinic space group P21/n and thus isotypically with Ag[MnO4] [6]. It fits well into the series of the anhydrous alkali-metal permanganates, which starts with Li[MnO4] [13] crystallizing in the orthorhombic space group Cmcm with the Cr[VO4]-type structure and displaying a sixfold coordination of oxygen around the Li+ cations. The permanganates A[MnO4] of the heavier homologues potassium, rubidium and cesium [14], [15], [16] all crystallize in the orthorhombic space group Pnma with the very flexible and adaptive baryte-type structure of Ba[SO4] allowing for an 8- to 12-fold coordination of the A+ cations (A = K, Rb, Cs) by oxygen atoms. As opposed to what can be found in the literature, where sodium permanganate occurs with from one half up to three water molecules per formula unit, the sodium permanganate now prepared via the Muthmann method crystallizes anhydrously just like the structurally prototypic Ag[MnO4], even though the samples are synthesized from aqueous solution.
4 Experimental section
Na[MnO4] was synthesized by adding 100 mmol Na2[SO4] dissolved in 50 mL demineralized water to a solution of 100 mmol Ba[MnO4]2 in 50 mL demineralized water, obtained through the Muthmann method described elsewhere in detail [17], [18]. After the complete metathesis and the removal of the precipitated Ba[SO4] from the deep purple solution, needle-shaped single crystals, a few millimeters in length, of anhydrous Na[MnO4] with the same color grew by fast isothermal evaporation in a P2O5-filled, evacuated desiccator. We were successful in synthesizing a single-phase product, from which we selected thin, needle-shaped, dark purple crystals for single-crystal X-ray diffraction.
Dedicated to: Professor Hanskarl Müller-Buschbaum on the occasion of his 85th birthday.
Acknowledgments
The financial support by the Deutsche Forschungsgemeinschaft (SFB 706: “Selective Catalytic Oxidation of C–H Bonds with Molecular Oxygen”), Bonn, and the Ministerium für Wissenschaft, Forschung und Kunst des Landes Baden-Württemberg, Stuttgart, is gratefully acknowledged.
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