Syntheses and crystal structures of the manganese hydroxide halides Mn 5 ( OH ) 6 Cl 4 , Mn

Single-crystals of the newmanganese hydroxide halides Mn5(OH)6Cl4, Mn5(OH)7I3, and Mn7(OH)10I4 were obtained by means of high-pressure/high-temperature synthesis in a Walker-type multianvil apparatus. The chloride crystallizes in the monoclinic space group P21/c (no. 14) with the lattice parameters (single-crystal data) a = 592.55(8), b = 1699.7(2), c = 597.33(8) pm, and β = 112.58(1)°. The iodides crystallize in the triclinic space group P1 (no. 2) with a = 653.16(2), b = 905.98(3), c = 1242.98(4) pm, α = 114.21(1)°, β = 99.91(1)°, and γ = 94.37(1)° for Mn5(OH)7I3 and a = 656.90(3), b = 906.59(4), c = 909.32(4) pm, α = 119.29(1)°, β = 97.99(1)°, and γ = 95.03(1)° for Mn7(OH)10I4. The crystal structures consist of edge-sharing Mn(OH)6–nXn (X = Cl, I; n = 1, 2, 3) octahedra arranged in stacked sheets. Adjacent layers are connected by hydrogen bonds of type O–H···X, confirmed by further characterization of single-crystals by IR-spectroscopy. The crystal chemical relationshipwith the aristotype Mg(OH)2 (brucite) is discussed on the basis of Bärnighausen trees (group–subgroup relations).

As hydroxide halides of the magnetically active 3d transition-metals show intriguing properties, our recent works focused on the less investigated manganese phases of this material class. Here, only Mn 2 (OH) 3 Cl, naturally occurring as the mineral kempite, and its bromine counterpart were thoroughly characterized [4,18,28]. In the case of the other previously reported phases, i.e., the αand β-modification of Mn(OH)Cl as well as Mn 2 (OH) 3 I, the characterization is limited to their cell parameters and the isotypicity of Mn 2 (OH) 3 I to botallackite [4,7].
Poor crystallinity and a tendency for polymorphism, polytypism, and stacking faults make detailed structural characterizations of the manganese hydroxide halides a challenging task. Taking advantage of the provided opportunities for synthesis and improved crystallization behavior offered by the high-pressure/high-temperature method, we investigated this system under such conditions. In our preliminary work, we could already present a new modification of Mn(OH)Cl and the new compounds Mn(OH)Br and Mn(OH)I, successfully synthesized by this method [29,30]. In the course of our inquiries, it was possible to identify further representatives of this material class. In this contribution, the syntheses, structural characterizations through single-crystal X-ray diffraction, IR spectroscopy, and the structural relationships of the new phases Mn 5 (OH) 6    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 Mn 5 (OH) 7 I 3 and Mn 7 (OH) 10 I 4 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.

Single-crystal X-ray diffraction (SCXRD)
For the single-crystal X-ray structure analysis, platelets of Mn 5 (OH) 6 Cl 4 and Mn 5 (OH) 7 I 3 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 Mn 7 (OH) 10 I 4 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   Table : Atomic coordinates, occupancy, bond valence sums ∑ν (for oxygen atoms as O -/OH − ) and equivalent isotropic displacement parameters (U eq / − pm  ) of Mn  (OH)  Cl  . All atoms except Mn (b) have the Wyckoff position e. U eq is defined as one third of the trace of the orthogonalized U ij tensor (standard deviations in parentheses). were standardized with the program STRUCTURE TIDY [35][36][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   /OH − ) and equivalent isotropic displacement parameters (U eq /  − pm  ) of Mn  (OH)  I  and Mn  (OH)  I  . U eq is defined as one third of the trace of the orthogonalized U ij tensor (standard deviations in parentheses).

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.

Crystal structures
The Mn-Cl distances (Table 4)   P1 (no. 2) and similar to Mn(OH)I [30] and Mn 5 (OH) 6 Cl 4 described above, their crystal structure is related to the brucite (C6, Mg(OH) 2 /CdI 2 ) structure. The single layers are formed by distorted Mn(OH) 6−x I 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 Mn 5 (OH) 6 Cl 4 , the iodides show no substitutional disorder of the ligands.
The Mn-I distances (  [42]). Like in Mn 5 (OH) 6 Cl 4 , 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)

Powder X-ray diffraction of Mn 5 (OH) 6 Cl 4
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 Mn 5 (OH) 6 Cl 4 , which represented the main phase, the sample contained an unidentified side phase.

Group-subgroup relationships
The manganese hydroxide halides Mn 5 (OH) 6 Cl 4 , Mn 5 (OH) 7 I 3 , and Mn 7 (OH) 10 I 4 reported herein are crystal chemically closely related to the well-known layer structure of cadmium iodide or brucite (Mg(OH) 2 ). CdI 2 crystallizes with space group P32/m1, which is a maximal subgroup of P6 3 /mmc, the space group type of the hexagonal closest packing. In CdI 2 , 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 Mn 2+ .
These close relationships readily call for group-subgroup schemes, relating the three new structures with the aristotype CdI 2 . The first Bärnighausen tree ( Figure 5) [45][46][47][48] of the CdI 2 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].
The Bärnighausen tree worked out by Meyer [49] has three different branches: (i) the structure of Na 2 Sn(OH) 6 , (ii) the structures of Li 2 Pt(OH) 6 , Bi 2 Pt (h2), Na 2 Pt(OD) 6 and Tl 2 S, and (iii) the structure of CrBr 2 and its lower symmetric derivatives NbTe 2 , AgAuCl 4 , AgAuTe 4 , and Cu 2 Cl(OH) 3 , nicely underpinning the broad crystal chemical diversity of the CdI 2 family. The third part of the Bärnighausen tree from Meyer is included in Figure 5, since the superstructures of the manganese hydroxide halides Mn 5 (OH) 6 Cl 4 , Mn 5 (OH) 7 I 3 , and Mn 7 (OH) 10 I 4 also derive from CrBr 2 [52]. The structure of chromium dibromide is monoclinic. The space group symmetry is reduced by a translationengleiche symmetry reduction of index 3 (t3) from P32/m1 to C12/m1.
Each, Mn 5 (OH) 6 Cl 4 , Mn 5 (OH) 7 I 3 , and Mn 7 (OH) 10 I 4 deserve two subsequent steps starting from CrBr 2 . This is paralleled with unit cell enlargements. In Figure 6, we present a projection of the MnI 2 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  We start with the structure of Mn 5 (OH) 6 Cl 4 ( 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 Mn 7 (OH) 10 I 4 , 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.
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 Bi 2 Pt [53]. The latter also derives from the CdI 2 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.

FTIR-spectroscopy
Figures 10-12 show the experimental IR spectra of single-crystals of Mn 5 (OH) 6 Cl 4 , Mn 5 (OH) 7 I 3 , and Mn 7 (OH) 10 I 4 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 Mn 5 (OH) 6 Cl 4 , strong in the iodide phases). Marked peaks in Figure 10 are due to the protective Nujol film on the Mn 5 (OH) 6 Cl 4 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 Mn 5 (OH) 7 I 3 (Figure 11), the band from 3000 to 3300 cm −1 can be attributed to absorbed water and the  [45][46][47][48] for the structures of MnI 2 [43], HP-CrBr 2 [52] and Mn 7 (OH) 10 I 4 . The indices for the translationengleiche (t), klassengleiche (k), and isomorphic (i) symmetry reductions and the evolution of the atomic parameters are given.
small shoulder at around 1600 cm −1 to the corresponding bending vibration of O-H groups in the absorbed water molecules.
All spectra show sharp ν(O-H) stretching vibrations at around 3500 cm −1 (Mn 5 (OH) 6 Cl 4 : 3550 cm −1 ; Mn 5 (OH) 7 I 3 : 3491, 3533, and 3573 cm −1 ; Mn 7 (OH) 10 I 4 : 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.

Conclusions
Three new manganese(II) hydroxide halides were obtained by means of high-pressure/high-temperature synthesis using a multianvil apparatus. Single-crystals of the compounds Mn 5 (OH) 6 Cl 4 , Mn 5 (OH) 7 I 3 , and Mn 7 (OH) 10 I 4 were characterized via X-ray diffraction, showing layered crystal structures derived from the CdI 2 /Mg(OH) 2 crystal structure. The chloride phase crystallizes in the space group P2 1 /c and exhibits a substitutional disorder concerning half of the ligand positions, whereas the iodide phases both crystallize in P1 and do not exhibit any disorder. In contrast to the aristotype, these hydroxide halide phases show a distortion of the coordination polyhedra and strong Figure 9: Group-subgroup scheme in the Bärnighausen formalism [45][46][47][48] for the structures of MnI 2 [43], HP-CrBr 2 [52] and Mn 5 (OH) 7 I 3 . The indices for the translationengleiche (t), klassengleiche (k), and isomorphic (i) symmetry reductions and the evolution of the atomic parameters are given.
undulation of the layers that prevents the formation of polytypes. Additional FTIR spectroscopy affirm the hydroxide groups of the compounds and indicate the occurrence of hydrogen bonding of type O-H···X.