Synthesis and characterization of the lead borate Pb₆B₁₂O₂₁(OH)₆

2.1 Synthesis

The lead borate Pb₆B₁₂O₂₁(OH)₆ was synthesized hydrothermally in a Teflon-lined stainless steel autoclave (volume: 8 mL). The lead metaborate Pb(BO₂)₂ · H₂O [241.13 mg; 0.802 mmol, p.a., Alfa Aesar (Karlsruhe, Germany)] and KNO₃ [90.98 mg; 0.880 mmol, p.a., Carl Roth GmbH (Karlsruhe, Germany)] at a molar ratio of 1 : 1.1, H₂O (3 mL), and a few drops of a 2 M aqueous KOH solution (pH value ~ 10–11) were mixed. The mixture was ground together, heated to and kept at 513 K in an autoclave for 3 days in a circulating air oven, and slowly cooled down to 373 K (1 K h⁻¹). Afterwards, the autoclave was removed from the oven and cooled down to room temperature. Large, transparent, and colorless needles of Pb₆B₁₂O₂₁(OH)₆ were obtained.

The same lead borate Pb₆B₁₂O₂₁(OH)₆ was obtained by mixing the starting materials Pb(BO₂)₂ · H₂O [224.90 mg (0.801 mmol), p.a., Alfa Aesar (Karlsruhe, Germany)] and NaNO₃ [88.98 mg; 1.03 mmol, p.a., Fluka AG (Darmstadt, Germany)] at a molar ratio of 1.55 : 2 and H₂O (3 mL) with the pH value adjusted to 11.0 by the addition of a few drops of a 2 M aqueous KOH solution. The same heating program was used as previously described. In this synthesis, smaller, transparent, colorless needles were obtained.

Single-crystals were isolated from both experiments and measured by single-crystal X-ray diffraction. Those crystals obtained from the synthesis with KNO₃ as a starting material led to the highest accuracy in the solution and refinement of the single-crystal data. Therefore, crystals from this batch were also used for the FT-IR characterization of this compound.

2.2 X-ray structure determination

The powder X-ray diffraction pattern of Pb₆B₁₂O₂₁(OH)₆ was collected with a STOE Stadi P powder diffractometer in transmission geometry with MoK_α1 (λ = 70.93 pm) radiation applying a focusing Ge(111) primary beam monochromator and a linear PSD detector. Figure 1 (top) shows the experimental powder pattern of Pb₆B₁₂O₂₁(OH)₆ with the theoretical pattern simulated from the single-crystal data [Fig. 1 (center)]. For comparison, the theoretical pattern stemming from the single-crystal data of the structure reported by Yu et al. [8] is also given in Fig. 1 (bottom). On the basis of the two simulated powder patterns, no differentiation can be made between both structures. The reflections of the powder pattern of Pb₆B₁₂O₂₁(OH)₆ were indexed and refined [16]. The lattice parameters fit well with the parameters obtained from the single-crystal data (see Table 1).

Fig. 1:

Top: Experimental powder pattern of the product of the hydrothermal synthesis Pb₆B₁₂O₂₁(OH)₆. Center: theoretical powder pattern obtained from single-crystal data of the compound Pb₆B₁₂O₂₁(OH)_6. Bottom: theoretical powder pattern obtained from single-crystal data of the compound Pb₆B₁₁O₁₈(OH)₉ of Yu et al. [8].

For the single-crystal structure analysis, the sample was examined with a polarization contrast microscope and a suitable crystal was chosen and isolated mechanically. For the collection of the data at room temperature, a Bruker D8 Quest diffractometer (Photon 100) equipped with an Incoatec Microfocus source generator (multi-layered optics-monochromatized MoK_α radiation, λ = 71.073 pm) was used. Multi-scan absorption corrections were applied with the program Sadabs-2014/5 [17]. According to the systematic reflection conditions, the space groups P3₁ (no. 144) and P3₂ (no. 145), which are enantiomorphic space groups, were derived. After the structure solution and parameter refinement with anisotropic displacement parameters for all non-hydrogen atoms using the Shelxs/l-97 software suite [18, 19], the space group P3₂ was found to be correct for the single-crystal examined. The hydrogen atoms could not be refined. The residual electron density was located around the lead atoms. All relevant details of the data collections and evaluations are given in Table 1. The atomic coordinates and isotropic as well as anisotropic displacement parameters, interatomic distances, and angles are listed in Tables 2–6.

Further details of the crystal structure investigations may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de, https://icsd.fiz-karlsruhe.de/search/basic.xhtml) on quoting the deposition number CSD-431212.

2.3 Vibrational spectroscopy

A Bruker Vertex 70 FT-IR spectrometer (resolution 4 cm⁻¹) equipped with a KBr beam splitter and attached to a Hyperion 3000 microscope and an LN-MCT (Mercury Cadmium Telluride) detector was used in absorption mode to obtain the spectrum of a single-crystal of Pb₆B₁₂O₂₁(OH)₆ in the range of 400–4000 cm⁻¹. As mid-infrared source, a Globar (silicon carbide) rod and a 15× IR objective as focus was used. 320 scans of the single-crystal placed on a BaF₂ sample holder were acquired. A correction of atmospheric influences was performed with the software Opus 6.5 [20].

3.1 Crystal structure

The title compound Pb₆B₁₂O₂₁(OH)₆ was obtained through a hydrothermal synthesis and found to crystallize in the trigonal, noncentrosymmetric space group P3₂ (no. 145). The structure contains infinite helical [B₄O₇(OH)₂]^4– chains along the c axis (Fig. 2). The fundamental building blocks (FBBs) are Dreierringe [21] built up from three corner sharing BO₄ tetrahedra and one additional corner sharing BO₃ group connected to this ring (Fig. 3). Two corner sharing tetrahedra of two different Dreierringe are also connected to the latter BO₃ group forming an additional Dreierring. A three-fold screw axis is the central axis of each helical chain.

Fig. 2:

Crystal structure of Pb₆B₁₂O₂₁(OH)₆ viewed along [001]. O^2–: small blue spheres at the corners of the tetrahedra and of the trigonal-planar groups; B³⁺: centers of tetrahedra and BO₃ groups (depicted without polyhedra) as small red spheres; Pb²⁺: green spheres.

Fig. 3:

Anionic borate chain surrounded by Pb²⁺ cations (green). The pink spheres at vertices of the BO₄ tetrahedra represent the hydroxyl groups and the fundamental building block (FBB) is encircled in red.

The Pb²⁺ cations are arranged between neighboring chains of anions (Figs. 2 and 3). The pink spheres are shown as part of the BO₄ tetrahedra representing hydroxyl groups, which could be assigned by a bond valence analysis (see Table 7). The localization of the protons is based on the bond-length/bond-strength concept (BLBS) [22, 23] (vide infra) in contrast to the work of Yu et al. [8].

The boron positions B2–B4, B6–B8, and B10–B12 are tetrahedrally coordinated by four oxygen anions. The B–O distances in the tetrahedra range between 143.4(2) and 150.0(2) pm fitting well to the known average value of 147.6 pm [24, 25]. The corresponding bond angle values range from 104.6(7) to 112.8(7)° with an average value of 109.5°, very close to the ideal tetrahedral angle of 109.47°. The positions B1, B5, and B9 are threefold coordinated by oxygen atoms forming BO₃ groups. The bond angle values range from 117.6(9) to 121.5(9)° and the B–O distances from 135.7(2) to 141.7(2) pm with an average value of 137.3 pm.

In this lead borate structure, there are six crystallographically different lead atoms. Pb1–Pb3, Pb5, and Pb6 are surrounded by four oxygen atoms with Pb–O bond lengths in the range of 233.2(7)–272.0(7) pm. Pb4 is enclosed by five oxygen atoms in the range of 235.1(6)–275.9(7) pm. Altogether, the Pb–O distances range from 233.6(7) to 275.9(7) pm (average value: 251.0 pm). The average value fits very well with Pb–O distances found in the literature, where the irregular coordination polyhedra around lead cations are described [6, 9]. Figure 4 shows the coordination of the six Pb²⁺ cations by the O^2– anions. The irregular coordination spheres of the lead cations originate from the stereochemically active lone pairs of the Pb²⁺ cations.

Fig. 4:

Coordination spheres of the lead cations in the crystal structure of Pb₆B₁₂O₂₁(OH)₆.

From BLBS (bond-length/bond-strength, ΣV) calculations [22, 23], it can be concluded that the six oxygen positions O8, O9, O17, O18, O26, and O27 in the BO₄ tetrahedra are linked to hydrogen atoms representing hydroxyl groups. Without the additional protons, these oxygen positions would show unusually low bond valence sums of approximately –0.77 (see Table 7, bold values). Taking the hydrogen atoms bonded to these oxygen atoms into account, the calculated formal charges agree well with the expected ionic values within the accuracy of the method.

A comparison of the results of the single-crystal measurements of the two lead borates Pb₆B₁₂O₂₁(OH)₆ and “Pb₆B₁₁O₁₈(OH)₉” [8] is presented in Table 2. The lattice parameters of both compounds are nearly identical, exhibiting a deviation only in the decimal place. It is obvious that the principal difference between Pb₆B₁₂O₂₁(OH)₆ and “Pb₆B₁₁O₁₈(OH)₉” lies within the structural building blocks of the anionic boron–oxygen chains present in both structures. The here presented compound Pb₆B₁₂O₂₁(OH)₆ consists exclusively of [B₄O₇(OH)₂]^4– chains as displayed in Fig. 5 designated as chain a, b, and c. The only difference between both compounds Pb₆B₁₂O₂₁(OH)₆ and “Pb₆B₁₁O₁₈(OH)₉” is the missing boron position B2 in “Pb₆B₁₁O₁₈(OH)₉”, which is encircled in red in the structural presentation of Pb₆B₁₂O₂₁(OH)₆ (Fig. 5). In both compounds, the number of oxygen anions is identical and the missing three positive charges of B³⁺ in “Pb₆B₁₁O₁₈(OH)₉” are counterbalanced by three additional hydrogen atoms at three oxygen atoms surrounding the above mentioned and encircled (Fig. 5) tetrahedral position. The fourth oxygen atom surrounding this tetrahedral position belongs to a BO₃ group possessing no additional hydrogen atom resulting in a much too low bond valence sum of –1.09 in the crystal structure of “Pb₆B₁₁O₁₈(OH)₉”; another hint that this structural detail is not correct. A view on the crystal data and structure refinement of “Pb₆B₁₁O₁₈(OH)₉” [8] reveals relatively large residual values of R1 = 0.0681 and wR2 = 0.1170 [I > 2 σ(I)] and a large difference peak and hole with values of 4.56 and –3.76 e Å⁻³, respectively. This indicates that one boron position has most likely been overlooked in the structure refinement of “Pb₆B₁₁O₁₈(OH)₉,” most likely due to the presence of the heavy lead cations. For the structure refinement of Pb₆B₁₂O₂₁(OH)₆ presented here, the residual values are R1 = 0.0230 and wR2 = 0.527 [I > 2 σ(I)] with the difference peak and hole possessing values of 1.34 and –3.68 e Å⁻³ located near the Pb²⁺ cation. In fact, two helical chains in “Pb₆B₁₁O₁₈(OH)₉” are identical to those found in Pb₆B₁₂O₂₁(OH)₆. The addition of one boron atom into the tetrahedrally coordinated position of the oxygen atoms O1, O2, O4, and O6 of “Pb₆B₁₁O₁₈(OH)₉” (see encircled area in Fig. 5) including the elimination of three hydrogen atoms leads to three nearly identical chains, resulting in the reported structure of Pb₆B₁₂O₂₁(OH)₆.

Fig. 5:

Three infinite helical chains of [B₄O₇(OH)₂]^4– fragments in the borate Pb₆B₁₂O₂₁(OH)₆. The pink oxygen atoms are assigned to hydroxyl groups. Encircled in red is the boron atom B2, which is missing in the structure of “Pb₆B₁₁O₁₈(OH)₉”.

3.2 FT-IR spectroscopy

Figure 6 shows the FT-IR spectrum of Pb₆B₁₂O₂₁(OH)₆ in the range of 400–4000 cm⁻¹. It shows a sharp band at 3500 cm⁻¹, which can be assigned to O–H stretching vibrations. One strong broad band exists in the range between 800 and 1500 cm⁻¹. In this range, normally the stretching vibrations of BO₃ groups (1100–1500 cm⁻¹) and the stretching vibrations of BO₄ (800–1000 cm⁻¹) groups are found. The existence of trigonal BO₃, tetrahedral BO₄, and OH groups in the structure of Pb₆B₁₂O₂₁(OH)₆ is therefore confirmed.

Fig. 6:

FT-IR absorbance spectrum of a single-crystal of Pb₆B₁₂O₂₁(OH)₆ in the range of 400–4000 cm⁻¹.