The compound (NH₄)_0.64B_4.36O_6.72(OH)_0.28(NH₃)_0.36 representing the first ammine/ammonium borate

2.1 Synthesis

For the synthesis of (NH₄)_0.64B_4.36O_6.72(OH)_0.28(NH₃)_0.36, the starting materials B₂O₃ (99.9 %, Strem Chemicals, Newburyport, Massachusetts, U.S.A.), h-BN (≥99 %, Strem Chemicals, Newburyport, Massachusetts, U.S.A.), and H₃BO₃ (≥99.8 %, Carl Roth GmbH & Co. KG, Karlsruhe, Germany) were mixed in a stoichiometric ratio of 1:1:1.36, thoroughly ground in an agate mortar and encapsulated in platinum foil (0.027 mm, 99.9 %, Chem-PUR, Karlsruhe, Germany). The capsule was placed in a crucible composed of h-BN (HeBoSint, P100, Henze Boron Nitride Products AG, Kempten, Germany), closed with a h-BN lid and arranged in an octahedral “14/8” assembly within a tubular graphite resistance heater (FE 254, Schunk Group GmbH, Vienna, Austria). The assembly was placed in the center of eight beveled tungsten carbide cubes (HA-7 %Co, Hawedia, Marklkofen, Germany). The synthesis was performed in a modified Walker-type multianvil device (mavo press LPR 1000–400/50, Max Voggenreiter GmbH, Mainleus, Germany). A more detailed description of the experimental setup can be found in the literature. ^{19 , 20 , 21} The maximum pressure of 12.0 GPa was reached within 317 min, and the maximum temperature of 1,300 °C within 10 min and then kept constant for 50 min, before it was decreased to room temperature within 60 min. Finally, the sample was decompressed to ambient conditions within 938 min. The reaction product was a colorless solid.

2.2 X-ray powder diffraction analysis and Rietveld refinement

The reaction product was analyzed with a STOE Stadi P powder diffractometer (STOE & Cie GmbH, Darmstadt, Germany), which was equipped with a Mythen 1 K microstrip detector (Dectris, Baden-Daettwil, Switzerland). The measurement was carried out with Ge(111)-monochromatized MoKα ₁ radiation (λ = 0.7093 Å) in transmission geometry with a flat sample holder across a 2θ range of 2.0–60.5°. Figure 1 shows a Rietveld refinement ²² of a sample containing the title compound, performed with the program Topas 4.2. ²³ At a 2θ value of about 19° a clear difference can be observed between the measured and calculated curves. The most likely explanation for this discrepancy is the presence of an unidentified side-phase.

Figure 1:

Rietveld refinement of the powder X-ray diffraction pattern (MoKα ₁ radiation) of the sample containing (NH₄)_0.64B_4.36O_6.72(OH)_0.28(NH₃)_0.36 and the additional phases B₂O₃ ²⁴ and NH₄[B₅O₆(OH₄)]·2H₂O. ²⁵

2.3 Crystal structure analysis

For the single-crystal structure analysis, a Bruker D8 Quest diffractometer (Bruker Corporation, Billerica, Massachusetts, U.S.A.) was used, which was equipped with an Incoatec microfocus X-ray tube (monochromatized MoKα radiation, λ = 0.7107 Å) and a Photon 100 CMOS detector. Data collection and processing was performed with the programs Saint (V8.38A) ²⁶ and Apex III ²⁷ and a multi-scan absorption correction with Sadabs (2016/2). ²⁸ Structure solution and parameter refinement were carried out with Shelxt (2018/2) ²⁹ and Shelxl (2018/3) ³⁰ implemented in the program system Olex2-1.5. ³¹ The title compound was solved and refined in the orthorhombic space group Pmn2₁ (no. 31). This non-centrosymmetric space group is supported by failed attempts to solve and refine the structure in the centrosymmetric space group Pmmn (no. 59). (Programs used: Shelxt (2018/2); ²⁹ Direct Methods via Shelxs (2013/1)). ^{32 , 33} A check with the program Platon (version 170613) ³⁴ clearly confirmed the non-centrosymmetric space group. Furthermore, we excluded a possible inversion twinning because this did not improve the R1 value in the structure refinement. Therefore, the Flack parameter of 0.4(3) (see Table 1) might be caused by the only presence of light elements in the structure. Moreover, the experimental and theoretical transmission coefficients deviate, but since a correction might only have a marginal influence for light elements (B, H, N, O), we retained the structure refinement in its present form.

With the exception of H, all atoms could be refined anisotropically; the part of the H atoms, which could be localized, had to be refined isotropically. In fact, not all hydrogen atoms were localizable (vide infra). Details of the data collection can be found in the synoptical Table 1. Tables 2 and 3 give the positional and displacement parameters, the Wyckoff positions, and site occupancy factors.

CSD-2410970 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

2.4 IR spectroscopy

A transmission FT-IR spectrum of a single crystal of the title compound (NH₄)_0.64B_4.36O_6.72(OH)_0.28(NH₃)_0.36, whose lattice parameters had previously been determined using a single-crystal diffractometer, were measured on a Vertex 70 FT-IR spectrometer in a spectral range of 600–4,000 cm⁻¹ with a spectral resolution of 4 cm⁻¹. The instrument was equipped with a KBr beam splitter, a liquid nitrogen cooled MCT (Mercury Cadmium Telluride) detector and a Hyperion 3000 microscope (Bruker Corporation, Billerica, Massachusetts, U.S.A.). A Globar rod (made of SiC) was used as mid-IR source and a 15 × IR objective as focus. The sample was placed on a BaF₂ slide and measured including 300 scans. Atmospheric influences were corrected with the software Opus 7.2. ³⁵

3.1 Crystal structure

The compound (NH₄)_0.64B_4.36O_6.72(OH)_0.28(NH₃)_0.36 crystallizes in the orthorhombic space group Pmn2₁ (no. 31) with two formula units in the unit cell and with the lattice parameters a = 10.7902(4), b = 4.5949(2), c = 4.2280(2) Å, and V = 209.62 (2) ų, exhibiting a slightly modified variant of the α-SrB₄O₇ structure type. ^{6 , 7} In fact, (NH₄)_0.64B_4.36O_6.72(OH)_0.28(NH₃)_0.36 is homeotypic to α-SrB₄O₇, ^{6 , 7} PbB₄O₇, ^{7 , 8 , 9} EuB₄O₇, ¹⁰ β-HgB₄O₇, ¹ β-SnB₄O₇, ¹¹ β-CaB₄O₇, ² and LnB_3.6O₇ (Ln = La, Pr, Nd). ^{4 , 5}

Similar to the homeotypic compounds, ^{1 , 2 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11} the anionic framework is built up by two crystallographically different boron atoms (B1 and B2), which are tetrahedrally coordinated by oxygen atoms. By alternately connecting these corner-sharing boron tetrahedra, vierer-rings ([B₄O₇] building blocks) are formed (see Figure 2, one building block is colored in yellow and orange).

Figure 2:

Visualization of the borate network [B₄O₇]_∞ of (NH₄)_0.64B_4.36O_6.72(OH)_0.28(NH₃)_0.36. The three crystallographically different boron centered tetrahedra are dyed in various shades of blue (light: [B1O₄], dark: [B2O₄], cyan: [B3O₃N]). One single [B₄O₇] building block is highlighted in yellow and orange. Either [NH₄] or [B3NH₃] units fill the interstices within the borate framework. All tetrahedra point towards viewing direction [001], which further confirms the non-centrosymmetric space group.

These building blocks are linked by corner sharing tetrahedra to a three-dimensional [(B₄O₇)^2–]_∞ borate network, showing sechser-, vierer-, and dreier-rings. Hence, cages within the borate framework are formed (see Figure 3), which serve as locations for two different tetrahedral structure motifs, [NH₄] and [BNH₃] units (see Figures 3 and 4). The [NH₄] unit is linked to the borate framework solely by hydrogen bonds to O1, O2, and O3, the [BNH₃] unit by hydrogen bonds to O1 and a covalent bond between B3 and an O atom of the [(B₄O₇)^2–]_∞ network (see Figure 3 and Table 6). The [BNH₃] and [NH₄] units are introduced into the [(B₄O₇)^2–]_∞ network, leading formally either to a “[B₅O₇NH₃]⁺” unit or to an arrangement, where the ammonium group is located in a void of the borate network and stabilized via hydrogen bonds in a “[(NH₄)⁺(B₄O₇)^2–]^–” unit. Apart from the atoms H1ab, H1b, and B3, all sites are fully occupied and therefore present in both units (see Table 2 and Figure 4). H1ab is also present in both units, but only one of the two positions can be occupied at the same time, which decreases the occupation to 0.5. H1b is solely in the “[(NH₄)⁺(B₄O₇)^2–]^–” constellation (see Figure 4a) with a site occupancy factor of 0.32. For H1b, also two positions are possible, of which only one can be occupied simultaneously. Atom B3 is exclusively present in the “[B₅O₇NH₃]^+” unit (see Figure 4b) and has an occupancy of 0.36. Therefore, the [NH₄] and [BNH₃] units both have two atoms of H1aa and one of H1ab and differ by the presence of H1b in [NH₄] and B3 in [BNH₃]. Based on all the atoms and their site occupancy factors in the structure refinement and their common charges (+3 for B, +1 for H, −3 for N, and −2 for O), the sum formula would be “(B_4.36H_3.64NO₇)^0.28−”. Hence, for charge compensation we assume the presence of an additional number of protons distributed over oxygen atoms of the network leading formally to O–H groups, which cannot be detected in the difference Fourier map. Therefore, the structure of the compound has to be taken as tentative, but since there is no evidence of another charge compensation and of an exclusion criterion, we are sure, that our structure model is the most reasonable one. Including these additional protons for charge compensation, the atoms are in the ratio “B_4.36H_3.92NO₇”. However, this does not reflect the actual connectivity of the atoms and so we decided to choose a sum formula that considers the assignment of the hydrogen atoms to specific atoms. Therefore, based on this structural analysis (single-crystal data), the sum formula of this new compound can be written as (NH₄)_0.64B₄O_6.72(OH)_0.28[B(NH₃)]_0.36. Thereby, the alternating units [NH₄] and [B(NH₃)] are in the ratio of 0.64:0.36, which is based on the site occupancy factors of 0.36 of B3, and of 0.32 of H1b (solely in [NH₄] unit), which is half of 0.64 because only half of the theoretically H1b sites can be occupied in the entire unit. There are seven O atoms which are partially loaded with protons: “O_6.72(OH)_0.28”. The sum formula (NH₄)_0.64B₄O_6.72(OH)_0.28[B(NH₃)]_0.36 resembles MB₄O₇, with 0.28 O being connected to H, and M is formally replaced by (NH₄)_0.64 and [B(NH₃)]_0.36.

Figure 3:

Visualization of the “cages”, that are surrounding either (a) the ammonium group [NH₄] or (b) the [BO₃NH₃] units depending on the occupation of the B3 site. The hydrogen bonding situation is marked with dashed lines.

Figure 4:

Depiction of the [NH₄] (a) and [B3O₃NH₃] (b) units. Atoms H1ab and H1b are drawn nearly transparent due to the fact that only one of the two positions can be occupied at the same time (according to their site occupancy factors). A mirror plane passes perpendicularly through the N atom in both (a) and (b).

The B1–O and B2–O distances are in the range between 1.414(2) and 1.552(2) Å with average values of 1.49, and 1.46 Å, respectively, which corresponds to the mean value of 1.476(35) Å investigated by Zobetz et al. ³⁶ for B–O distances in [BO₄] tetrahedra and to values of the homeotypic compounds α-SrB₄O₇ ^{6 , 7} and LnB_3.6O₇ (Ln = La, Pr, Nd). ^{4 , 5} The B3–O distances are in the range between 1.604(6) and 1.645(3) Å with a mean value of 1.63 Å. This increase may be explained by defective atoms inside the cage and an averaging of the [NH₄] and [B–NH₃] units in the structure refinement. The B3–N distance has a value of 1.678(6) Å, which is also increased compared to the ammine borate Cd(NH₃)₂[B₃O₅(NH₃)]₂ (1.605(3) Å) ¹⁵ and the oxonitridoborate tetrahedra (between 1.553(3) and 1.608(3) Å) ^{16 , 17} documented in the literature, and may be explained by the defective atoms as well. The N–H distances (hydrogen bond donor) are in the range between 0.96(4) and 1.03(8) Å (see Tables 4 and 6), which agrees well with typical data in the literature (e.g. values between 0.83(2) and 0.99(2) Å in HP-(NH₄)B₃O₅ ¹² and between 0.84(2) and 0.88(2) Å in [BF₃NH₃]·3NH₃ ³⁷ ). The hydrogen bond acceptors have distances between 1.82(4) and 1.99(8) Å, which is in agreement to values between 1.92(2) and 2.13(2) Å in HP-(NH₄)B₃O₅. ¹²

All the mean tetrahedral angles are in accordance with the expected regular value of 109.5°. The single angle values for O–B1–O, O–B2–O, O–B3–O, O–B3–N, and B–N–H are also close to the regular value (±6 % deviation). The single angle values for H–N–H are in the range of ±22 % (without consideration of the standard deviation), which is due to a distortion of the [NH₄] and [BNH₃] tetrahedra (see Table 5). This deviation from C _3ν symmetry is caused by the close proximity of the hydrogen atoms to the mirror plane (see Figure 4) and/or that the common nitrogen atom for [NH₄] and [BNH₃] does not exactly coincide with the occupancy disorder, which leads to a cigar-shaped deflection of the temperature factor ellipsoids. Additionally, we performed structure refinement attempts in the monoclinic space group Pn, which led to a decrease of the symmetry and an omitting of the mirror plane. In that model, we obtained twice the coordinate set with a fully occupied N atom, and the partial disorder of the H atoms was not present, but the R1 value increased from 0.020 to 0.026 and so the original structure refinement in the space group Pmn2₁ (no. 31) was retained (Tables 1–6).

3.2 CHARDI and BLBS

The results of the crystal structure refinement were further confirmed by calculations of the bond-length/bond-strength ³⁸ values (BLBS, see ΣV in Table 7), and the charge distribution ³⁹ (CHARDI, see ΣQ in Table 7). The calculated BLBS and CHARDI values for B1, B2 and all the O atoms showed only minimal deviations compared to the regular values of +3 for B, and −2 for O. The atoms inside the cavities showed a deviation of either the BLBS or the CHARDI result compared to the expected values, which are –3 for N, and +1 for H. If we additionally consider the site occupancy factors of 0.36 for B3, 0.5 for H1ab and 0.32 for H1b, all the CHARDI values fit well. The deviating BLBS values (written in italics in Table 7) can be explained by the disorder of atoms inside the cavities.

3.3 IR spectroscopy

In Figure 5, an IR spectrum of a single crystal of (NH₄)_0.64B_4.36O_6.72(OH)_0.28(NH₃)_0.36 in the spectral region of 600–4,000 cm⁻¹ is depicted. To ensure that the crystal corresponds to the title compound, the lattice parameters of the chosen crystal were determined, which confirmed its identity. The assignment of the vibration bands of the borate network has already been discussed for the homeotypic compounds α-SrB₄O₇, ⁴⁰ β-CaB₄O₇, ² LaB_3.6O₇, ⁴ and PrB_3.6O₇ ⁴ and was also followed for the title compound. Typically, vibration bands ranging from 850 to 1,100 cm⁻¹ correspond to the stretching modes of [BO₄] tetrahedra. ⁴¹ The BO₃N group of the title compound shows similar absorption bands in this region (e. g. peak at 908 cm⁻¹). At lower wavenumbers, there is an increase in contributions from bending modes and more intricate lattice vibrations. ⁴¹ Absorption bands between 1,200 and 1,500 cm⁻¹ (e. g. peaks at 1,248, 1,414 and 1,551 cm⁻¹ in (NH₄)_0.64B_4.36O_6.72(OH)_0.28(NH₃)_0.36) are commonly associated with the stretching modes of triangular [BO₃] units, ⁴² or with three-fold coordinated oxygen atoms and their corresponding [OB₃] units, because of the similar geometry and force constants. ^{2 , 4} However, in 2013 Kaindl et al. investigated possible band positions of [OB₃] units with quantum chemical calculations and showed that vibrations in these ranges are caused by stretching and bending modes of the borate network, whereas peaks of [OB₃] were at values below 830 cm⁻¹. ⁴³ As described by Sohr et al. for Cd(NH₃)₂[B₃O₅(NH₃)]₂, ¹⁵ stretching of the N–H bonds of ammonia occur at 3,000–3,500 cm⁻¹ and the bending of ammonia at 1,580–1,650 and 1,000–1,050 cm⁻¹. ¹⁵ Bending of B–N–H groups occurs at 800–900, 1,100–1,120, and 1,470–900 cm⁻¹. ¹⁵ We observed similar bands, which further confirms the structure refinement of [NH₄] and [BNH₃] groups. In detail, the broad band around 3,200 cm⁻¹ probably corresponds to stretching vibrations of N–H/O–H and the peak at 1,643 cm⁻¹ to bending vibrations of ammonia. However, the N–H/O–H stretching band is more extended in the title compound and is in the range between 2,500 and 3,700 cm⁻¹, which can be explained by the strength of the hydrogen bonds. ⁴⁴ The hydrogen bonds in the title compound with a distance between donor and acceptor in the range between 2.5 and 3.2 Å (see Table 6) can be assumed as moderately strong, ⁴⁵ and occur as a broad band in the before mentioned range. ⁴⁴ Typically, O–H stretching modes occur in the range 2,000–4,000 cm⁻¹, ^{46 , 47 , 48} but could not be assigned with certainty, because of the large number of ammine/ammonium groups in the structure model which show peaks in the same range. For a more detailed assignment of the vibration modes (e. g. the exact position of stretching of O–H and an explanation for the peak at about 1,712 cm⁻¹) quantum chemical calculations for the title compound would be required, but could not be sufficiently resolved in this complex vibration mode pattern.

Figure 5:

Single crystal IR spectrum of (NH₄)_0.64B_4.36O_6.72(OH)_0.28(NH₃)_0.36 in the spectral region of 600–4,000 cm⁻¹.