Syntheses and crystal structures of the manganese hydroxide halides Mn₅(OH)₆Cl₄, Mn₅(OH)₇I₃, and Mn₇(OH)₁₀I₄

2.1 Synthesis

The compounds Mn₅(OH)₆Cl₄, Mn₅(OH)₇I₃, and Mn₇(OH)₁₀I₄ were obtained via high-pressure/high-temperature syntheses in a hydraulic 1000 t press (mavo press LPR 1000-400/50, Max Voggenreiter GmbH, Mainleus, Germany) with a modified Walker-type module (also Max Voggenreiter GmbH). The chloride phase could be obtained according to Eq. (1) at 1173 K and 3 GPa from a 3:2 molar mixture of MnO (99%, Sigma Aldrich, Steinheim, Germany) and MnCl₂·4H₂O (purum, p.a., Fluka, Buchs, Switzerland).

Crystals of the iodide phases Mn₅(OH)₇I₃ and Mn₇(OH)₁₀I₄ occurred as side-phases from several reactions (all at 2.5 GPa/1273 K) of Mn (99+%, Sigma Aldrich, Steinheim, Germany) and BiOI with different molar ratios, hydrolyzed by wetting with water vapor. Therefore, BiOI was synthesized by adding BiI₃ (99.95%, Alfa Aesar, Karlsruhe, Germany) in distilled water and precipitated with a diluted KOH solution. The precipitate was filtered and washed with EtOH and H₂O until the resulting BiOI (ICDD card No. 00-73-2062 [31]) was found to be phase pure by PXRD. Possible reactions for the formation of the iodide phase are presented in Eqs. (2) and (3). Further attempts to synthesize phase pure samples of the iodide phases failed so far.

Reactions were carried out in molybdenum capsules (0.025 mm foil, 99.95%, Alfa Aesar, Karlsruhe, Germany) and transferred into crucibles made from h-BN (HeBoSint^® P100, Henze BNP GmbH, Kempten, Germany). The crucibles were built into 18/11 assemblies, which were compressed by eight tungsten carbide cubes (HA-7%Co, Hawedia, Marklkofen, Germany). Details on the construction of such assemblies can be found in the corresponding work [32].

Unlike the previously reported compounds Mn(OH)Br and Mn(OH)I [30], these phases, although slightly hygroscopic, showed reasonable air stability and did not require handling (but storage) in an inert atmosphere. The chloride phase shows the characteristic pale rose color of manganese(II) compounds, whereas the Mn₅(OH)₇I₃ and Mn₇(OH)₁₀I₄ platelets are initially clear, turning slightly yellow after several days of light exposure. The elemental bismuth formed during the reaction was present as small beads and could be separated from the hydroxide halides by hand.

2.2 Single-crystal X-ray diffraction (SCXRD)

For the single-crystal X-ray structure analysis, platelets of Mn₅(OH)₆Cl₄ and Mn₅(OH)₇I₃ were mounted on a Bruker D8 Quest diffractometer equipped with a Photon 100 detector and an Incoatec Microfocus source generator (Mo-K_α radiation with λ = 71.073 pm, multi-layered optics monochromatized) and intensity data was collected at 173 and 190 K, respectively. Single-crystals of Mn₇(OH)₁₀I₄ were sealed in a glass capillary (0.5 mm, Hilgenberg, Malsfeld, Germany) and SCXRD data were collected with a Bruker CCD Kappa APEXII diffractometer, also using Mo-K_α radiation. Multi-scan absorption corrections based on equivalent and redundant intensities were applied with the program Sadabs-2014/5 [33], [34]. The crystal structures of the hydroxide halides were standardized with the program Structure Tidy[35–37]. The chloride phase shows a substitutional disorder of the ligands Cl1/O1 and Cl3/O4, whereby the occupancy factors were refined close to 0.75 (Cl1 and O4) and 0.25 (O1 and Cl3) and fixed at these values. The protons binding to O1 and O4 could not be detected as they are situated close to the positions of Cl1 and Cl3. For all phases, the O–H distance was restrained to a value of 83 ± 2 pm. All non-hydrogen atoms were refined with anisotropic displacement parameters. Table 1 summarizes the experimental details of the data collection and evaluations. Tables 2–4 contain the positional parameters, anisotropic displacement parameters, and selected interatomic distances and angles of the chloride phase and Tables 5–7 those of the iodide compounds.

Additional information on the crystal structure investigations can be obtained from the joint CCDC/FIZ Karlsruhe deposition service on quoting the deposition number 1940191 (Mn₅(OH)₆Cl₄), 1940192 (Mn₅(OH)₇I₃), and 1940193 (Mn₇(OH)₁₀I₄).

2.3 Powder X-ray diffraction (PXRD)

A powdered sample containing the hydroxide halide Mn₅(OH)₆Cl₄ was additionally characterized using powder X-ray diffraction (PXRD) technique. Therefore, data were collected with a Stoe Stadi P powder diffractometer in transmission geometry with Mo-K_α1 (λ = 70.930 pm; Ge(111) primary beam monochromator) radiation and a Mythen2 1K detector (Dectris, Baden, Switzerland).

2.4 Vibrational spectroscopy

To confirm the hydroxide groups, single-crystal transmission FTIR spectra were measured with a Bruker Vertex 70 FTIR spectrometer (resolution 4 cm⁻¹), equipped with a KBr beam splitter, an LN-MCT (Mercury Cadmium Telluride) detector and a Hyperion 3000 microscope (Bruker, Vienna, Austria). The measurements were performed in absorption mode in the range of 600–4000 cm⁻¹ using a silicon carbide rod (Globar) as mid-infrared source, and a 15×  IR objective as focus was used. Mn₅(OH)₆Cl₄ was measured under a protective film of oil (Nujol for IR-spectroscopy, Alfa Aesar, Karlsruhe, Germany). About 260 scans of each single-crystal were acquired and a correction of atmospheric influences was performed with the software Opus 7.2 [38].

2.5 Bond-length bond-strength (BL/BS) concept

By applying the bond-length bond-strength concept [39], [40], the bond valence sums (∑ν) for all non-hydrogen atoms were calculated. The resulting values were compared to the formal ionic charges. The values for all oxygen atoms were calculated with and without the implication of a proton (O^2–/OH⁻) to affirm the presence of the hydroxide groups at each oxygen position.

3.1 Crystal structures

Table 1 summarizes the results of the SCXRD analysis of the phases Mn₅(OH)₆Cl₄, Mn₅(OH)₇I₃, and Mn₇(OH)₁₀I₄.

In contrast to γ-Mn(OH)Cl [29], which can be derived from the CdCl₂ (C19) structure type, Mn₅(OH)₆Cl₄ can be seen as a C6 structure with (AγB)(AγB) stacking sequence. The layers consist of edge-sharing distorted Mn(OH)_6−xCl_x (x = 1, 2, 3, 4) octahedra forming corrugated, uncharged layers parallel to the (100) plane (Figure 1). This undulation prevents the formation of polytypes. The octahedral coordination around the manganese(II) atoms show substitutional disorder of the ligands Cl1/O1 (occupancy: 0.75/0.25) and Cl3/O4 (occupancy: 0.25/0.75).

Figure 1:

The crystal structure of Mn₅(OH)₆Cl₄ viewed along the a- and c-axis and coordination of the manganese atoms (displacement ellipsoids represent 90% probability at 173 K). The occupancies of the disordered atom pairs O1/Cl1 and O4/Cl3 are 0.25/0.75 and 0.75/0.25, respectively.

The Mn–Cl distances (Table 4) range from 255.6(3) to 265.4(1) pm with average values of 262.0 (Mn1–Cl), 260.7 (Mn2–Cl), and 262.4 pm (Mn3–Cl). These bonds are longer than in MnCl₂ (254.8(2) pm [41]) and similar to the ones in γ-Mn(OH)Cl (average 260.9 pm [29]) The oxygen atoms of the hydroxide groups are situated at Mn–O distances ranging from 214.6(2) to 221.1(8) pm and in average (Mn1–O: 218.8, Mn2–O: 217.2, Mn3–O: 215.8 pm) shorter than the mean Mn–O distance in Mn(OH)₂ (219.62 pm [42]). This behavior indicates that the hydroxide groups in Mn₅(OH)₆Cl₄ are stronger ligands to manganese(II) than the chloride ions and can be attributed to hydrogen bonding. The Cl–Mn and O–H bonds are weakened as part of the valence is used for Cl binding to H. Correspondingly, the O–Mn bond strength increases with weaker O–H bonds.

The bond valence sums (∑ν) for all non-hydrogen atoms were calculated (Table 2), whereby oxygen atoms were treated without and with the implication of a proton (O^2–/OH⁻). The calculated values for the oxygen atoms are −1.08/−1.95 (O1), −1.12/−1.99 (O2), −1.13/−2.01 (O3), and −1.39/−2.27 (O4), confirming the presence of hydroxide groups. For the non-oxygen atoms the values of 1.93 (Mn1), 1.95 (Mn2, Mn3), −1.08 (Cl1), −0.76 (Cl2), and −1.12 (Cl3) correspond well to the formal ionic charges of the atoms.

The manganese hydroxide iodides Mn₅(OH)₇I₃ and Mn₇(OH)₁₀I₄ both crystallize in the triclinic space group P1‾ (no. 2) and similar to Mn(OH)I [30] and Mn₅(OH)₆Cl₄ described above, their crystal structure is related to the brucite (C6, Mg(OH)₂/CdI₂) structure. The single layers are formed by distorted Mn(OH)_6−xI_x (x = 1, 2) octahedra parallel to the (100)-plane (Figures 2 and 3). Within the octahedra having two iodine atoms, the halogen atoms occur both in cis and trans position. In contrast to Mn₅(OH)₆Cl₄, the iodides show no substitutional disorder of the ligands.

Figure 2:

Octahedral coordination spheres of the manganese(II) atoms in Mn₅(OH)₇I₃ (displacement ellipsoids represent 90% probability at 190 K) and polyhedral representation of the crystal structure viewed along the a- and b-axis.

Figure 3:

Octahedral coordination of the manganese(II) atoms in Mn₇(OH)₁₀I₄ (displacement ellipsoids represent 90% probability at 296 K) and polyhedral representation of the crystal structure viewed along the a- and b-axis.

The Mn–I distances (Table 7) range from 297.75(3) to 323.67(6) pm (average 303.91 pm) in Mn₅(OH)₇I₃, and 299.20(3) to 322.34(7) pm (average 306.01 pm) in Mn₇(OH)₁₀I₄. These bonds are considerably longer than in MnI₂ (292.02 pm [43]) and in MnI₂·4H₂O (292.16 pm [44]).

2The values for the Mn–O bond-lengths are between 210.8(3) and 221.7(3) pm (average 215.2 pm) for Mn₅(OH)₇I₃, and 211.6(3) and 222.7(3) pm (average 215.1 pm) in Mn₇(OH)₁₀I₄ and are shorter than the average Mn–O distances in Mn(OH)₂ (219.62 pm [42]). Like in Mn₅(OH)₆Cl₄, this behavior can be seen as a result of the existing hydrogen bonds.

Also for the iodides, bond valence sums (∑ν) for all non-hydrogen atoms were determined (Table 5). The non-oxygen atoms have values close to their formal ionic charges. The values for the oxygen atoms calculated as O^2– and OH⁻ are −1.14/−2.02 (O1), −1.18/−2.05 (O2), −1.12/−1.99 (O3, O4, O6), −1.07/−1.94 (O5), and −1.13/−2.00 (O7) for Mn₅(OH)₇I₃ and −1.11/−1.98 (O1), −1.10/−1.97 (O2, O4), −1.15/−2.02 (O3), and −1.13/−2.00 (O5) for Mn₇(OH)₁₀I₄. These values confirm the presence of hydroxide groups. The rather small bond-valence sums of the iodine atoms are presumably a result of the influence of the O–H··· X hydrogen bonds, which were not considered in the BLBS calculations. The IR spectra (vide infra) show the presence of stronger hydrogen bonding in Mn₅(OH)₇I₃ and Mn₇(OH)₁₀I₄ than in Mn₅(OH)₆Cl₄, thus having more influence on the I–Mn bond valence and leading to the underestimation of the valence sum.

3.2 Powder X-ray diffraction of Mn₅(OH)₆Cl₄

A sample containing the chloride phase was additionally analyzed by PXRD. The measured diffraction pattern was compared to the theoretical pattern derived from SCXRD data (Figure 4). Besides Mn₅(OH)₆Cl₄, which represented the main phase, the sample contained an unidentified side phase.

Figure 4:

The measured diffraction pattern of a sample containing Mn₅(OH)₆Cl₄ (top) compared to the simulated pattern from SCXRD data (bottom). Reflections marked with an asterisk originate from an unidentified side phase.

3.3 Group–subgroup relationships

The manganese hydroxide halides Mn₅(OH)₆Cl₄, Mn₅(OH)₇I₃, and Mn₇(OH)₁₀I₄ reported herein are crystal chemically closely related to the well-known layer structure of cadmium iodide or brucite (Mg(OH)₂). CdI₂ crystallizes with space group P3¯2/m1, which is a maximal subgroup of P6₃/mmc, the space group type of the hexagonal closest packing. In CdI₂, every other layer of octahedral voids of the hcp iodine packing remains empty. In the present case, we have distorted hexagonal packings of the hydroxide and halide ions and the octahedral voids are filled by Mn²⁺.

These close relationships readily call for group–subgroup schemes, relating the three new structures with the aristotype CdI₂. The first Bärnighausen tree (Figure 5) [45], [46], [47], [48] of the CdI₂ family was worked out by Meyer during his PhD work [49] in the Bärnighausen group. General considerations on subgroups for hcp packings with partial ordered occupancies of octahedral voids were published by Müller [48], [50], [51].

Figure 5:

Group–subgroup scheme for CdI₂ superstructures. The indices for the translationengleiche (t), klassengleiche (k), and isomorphic (i) symmetry reductions are given. The highlighted part of the scheme was reproduced from [49].

The Bärnighausen tree worked out by Meyer [49] has three different branches: (i) the structure of Na₂Sn(OH)₆, (ii) the structures of Li₂Pt(OH)₆, Bi₂Pt (h2), Na₂Pt(OD)₆ and Tl₂S, and (iii) the structure of CrBr₂ and its lower symmetric derivatives NbTe₂, AgAuCl₄, AgAuTe₄, and Cu₂Cl(OH)₃, nicely underpinning the broad crystal chemical diversity of the CdI₂ family. The third part of the Bärnighausen tree from Meyer is included in Figure 5, since the superstructures of the manganese hydroxide halides Mn₅(OH)₆Cl₄, Mn₅(OH)₇I₃, and Mn₇(OH)₁₀I₄ also derive from CrBr₂ [52]. The structure of chromium dibromide is monoclinic. The space group symmetry is reduced by a translationengleiche symmetry reduction of index 3 (t3) from P3¯2/m1 to C12/m1.

Each, Mn₅(OH)₆Cl₄, Mn₅(OH)₇I₃, and Mn₇(OH)₁₀I₄ deserve two subsequent steps starting from CrBr₂. This is paralleled with unit cell enlargements. In Figure 6, we present a projection of the MnI₂ structure along the trigonal axis. The unit cells of the three hydroxide halide are incorporated into this drawing for better comparison with the aristotype. The individual Bärnighausen trees along with the evolution of the atomic parameters are shown in Figures 7–9.

Figure 6:

Structure of MnI₂ [43] with view along the c-axis. The manganese and iodine atoms are drawn as gray and purple circles, respectively. The cell of MnI₂ is highlighted (gray) as well as the cells of the CdI₂ superstructures Mn₅(OH)₆Cl₄ (blue), Mn₇(OH)₁₀I₄ (green), and Mn₅(OH)₇I₃ (red).

Figure 7:

Group–subgroup scheme in the Bärnighausen formalism [45], [46], [47], [48] for the structures of Mn(OH)₂ [42], HP-CrBr₂ [52], and Mn₅(OH)₆Cl₄. The indices for the translationengleiche (t), klassengleiche (k), and isomorphic (i) symmetry reductions and the evolution of the atomic parameters are given.

Figure 8:

Group–subgroup scheme in the Bärnighausen formalism [45], [46], [47], [48] for the structures of MnI₂ [43], HP-CrBr₂ [52] and Mn₇(OH)₁₀I₄. The indices for the translationengleiche (t), klassengleiche (k), and isomorphic (i) symmetry reductions and the evolution of the atomic parameters are given.

Figure 9:

Group–subgroup scheme in the Bärnighausen formalism [45], [46], [47], [48] for the structures of MnI₂ [43], HP-CrBr₂ [52] and Mn₅(OH)₇I₃. The indices for the translationengleiche (t), klassengleiche (k), and isomorphic (i) symmetry reductions and the evolution of the atomic parameters are given.

We start with the structure of Mn₅(OH)₆Cl₄ (Figure 7). The subsequent i5 and k2 transitions lead to five crystallographically independent anion sites, which allow the hydroxide-halide ordering. Nevertheless, the structure shows some residual disorder. Especially these sites show the largest differences between the positional parameters calculated from the subcell and those refined from the X-ray data. The manganese cations also show small displacements from the ideal positions in order to account for the anion substructure (OH/Cl ordering).

For Mn₇(OH)₁₀I₄, the second step in symmetry reduction (Figure 8) is an isomorphic one from P1¯ to P1;¯ with the rare index seven (i7). Consequently, we obtain seven anion sites for the OH/I ordering. The latter was well resolved from the single-crystal X-ray diffraction data. Again, the anions show the largest displacements from the ideal subcell sites. This is expressed in the undulated layers of condensed octahedra, drastically deviating from planarity as in the aristotype.

Mn₅(OH)₇I₃ shows the most complicated of the three superstructures with 10 crystallographically independent anion sites, similar to Mn₇(OH)₁₀I₄ with complete OH/I ordering. The displacements of the different anions from the ideal subcell positions are similar to the two compounds described above and also results in undulated layers of condensed octahedra.

The strong undulations of the Mn(OH/X)₂ layers for all three compounds show the limits of group–subgroup relations. The strong shifts in the atomic parameters underpin the differences in the radii OH/Cl versus OH/I but also changes in chemical bonding, i.e., ionicity toward covalence. In that view, it is interesting to mention the intermetallic phase Bi₂Pt [53]. The latter also derives from the CdI₂ type and is included in the Bärnighausen tree in Figure 5. The evolution of the atomic parameters presented in [53] shows substantial displacements for both the platinum and bismuth atoms, leading to distorted PtBi_6/3 octahedra. Also, this example is a limit of a group–subgroup scheme. Besides the purely geometrical relationship, the chemical bonding pattern should match as close as possible, in order to have a close crystal chemical similarity.

3.4 FTIR-spectroscopy

Figures 10–12 show the experimental IR spectra of single-crystals of Mn₅(OH)₆Cl₄, Mn₅(OH)₇I₃, and Mn₇(OH)₁₀I₄ in the range from 600 to 4000 cm⁻¹ in absorbance mode. In the fingerprint region, the strong bands from 600 to 800 cm⁻¹ correspond to ν(Mn–O) stretching modes. In the range from 800 to 1200 cm⁻¹, O–H bending modes of the hydroxide groups can be found (very weak in Mn₅(OH)₆Cl₄, strong in the iodide phases). Marked peaks in Figure 10 are due to the protective Nujol film on the Mn₅(OH)₆Cl₄ crystal and the bands in the region from 1500 to 3000 cm⁻¹ in the spectra of the iodide compounds (Figures 11 and 12) originate from residual grease of the sample preparation procedure. In the spectrum of Mn₅(OH)₇I₃ (Figure 11), the band from 3000 to 3300 cm⁻¹ can be attributed to absorbed water and the small shoulder at around 1600 cm⁻¹ to the corresponding bending vibration of O–H groups in the absorbed water molecules.

Figure 10:

FTIR spectrum of a single-crystal of Mn₅(OH)₆Cl₄ under a protective layer of Nujol (bands marked with an asterisk).

Figure 11:

FTIR spectrum of a single-crystal of Mn₅(OH)₇I₃.

Figure 12:

FTIR spectrum of a single-crystal of Mn₇(OH)₁₀I₄.

All spectra show sharp ν(O–H) stretching vibrations at around 3500 cm⁻¹ (Mn₅(OH)₆Cl₄: 3550 cm⁻¹; Mn₅(OH)₇I₃: 3491, 3533, and 3573 cm⁻¹; Mn₇(OH)₁₀I₄: 3503, 3552, and 3595 cm⁻¹) and confirm the presence of the hydroxide groups. The red-shift of the values for these modes compared to the value for a free hydroxide group (3620 cm⁻¹ [54]) is an indication for the occurrence of hydrogen bonding of the type O–H···X (X = Cl, I) between the layers. The occurring peak splitting in the IR spectra of the iodide phases (Figures 11 and 12) can be interpreted as a result of existing hydrogen bonds of different strengths arising from different O–H···I distances.