Synthesis of the first nickel borate nitrate

The novel potassium nickel borate nitrate K7Ni[B18O24(OH)9](NO3)6 · (H3BO3) was obtained from a simple hydrothermal synthesis in a stainless-steel autoclave at T = 513 K starting with nickel dichloride hexahydrate, and boric and nitric acid with the pH adjusted to 8 by KOH. Single-crystal X-ray diffraction data provided the basis for the structure analysis and refinement. The compound crystallizes in the trigonal space group R3̅ (no. 148) with the lattice parameters a = 1222.29(8) and c = 5478.4(4) pm. Generally, K7Ni[B18O24(OH)9] (NO3)6 · (H3BO3) is comprised of nitrate layers and complex nickel borate layers surrounded by boric acid, nitrate anions, and potassium cations.


Introduction
Borates incorporating cations from the transition metal group have long been of interest in our working group. This is especially true for high-pressure/high-temperature conditions as shown by several publications in the last decade. Borates allow for unique structural motifs incorporating not only [BO 3 ] 3− , but also [BO 4 [3][4][5]. Further intensive focus has been directed to the synthesis of novel nickel borates, as evidenced by the publications on γ-NiB 4 O 7 [6], NiB 3 O 5 (OH) [7], and Ni 3 B 18 O 28 (OH) 4 · H 2 O [8]. Another area of interest of our group has been the combination of simple metal borates with additional anionic groups like in Sn 3 [B 3 O 7 ]X (X = F, I) [9,10] and Pb[B 2 (SO 4 ) 4 ] [11]. The synthesis conditions could thereby be shifted from high-pressure/high-temperature conditions to ambient pressure in simple stainless-steel autoclaves and glass ampoules.
An extension of these investigations, while focusing on the synthesis of new transition metal containing compounds, resulted in the hitherto unknown potassium nickel borate nitrate. A simple hydrothermal synthesis in a stainless-steel autoclave could be utilized to prepare the potassium nickel borate nitrate K 7 Ni[B 18 O 24 (OH) 9 ] (NO 3 ) 6 · (H 3 BO 3 ). As the title compound could only be synthesized as a side phase, the characterization in this work is only focused on a X-ray single-crystal structure determination.

X-ray structure determination
Under a polarisation microscope, a suitable single-crystal of K 7 Ni[B 18 O 24 (OH) 9 ](NO 3 ) 6 · (H 3 BO 3 ) with a diameter of 60 μm was fixed on the tip of a MicroMount™ (MiTeGen, LLC, Ithaca, NY, USA) and immediately placed in the diffractometer. The intensity data was collected with a Bruker D8 Quest diffractometer (Bruker, Karlsruhe, Germany) equipped with a Photon 100 detector system and an Incoatec microfocus source generator (multi-layered optic, monochromatized MoKα radiation, λ = 71.073 pm). The collection strategy, concerning the ω and ϕ scans, was optimized using the Apex III [17] program package. Thus, a complete data set up to high angles with high redundancies was received. For data processing and data reduction, the program Saint [18] was employed. Thereafter, multi-scan absorption corrections were applied with the program Sadabs [19].
The structure solution and parameter refinement with anisotropic displacement parameters for all nonhydrogen atoms, except the nitrogen atom N5 and the   U eq is defined as one third of the trace of the orthogonalized U ij tensor (standard deviations in parentheses).
oxygen atom O19, was done utilising the Shelxs/l-2013 [20][21] software implemented in the program WinGX-2013.3 [22]. In the course of the refinement, the trigonal space group R3 ̅ was found to be correct. The nitrogen and oxygen atoms N5 and O19 were refined isotropically. Furthermore, the hydrogen atoms H5, H6, H8, and H12 were refined isotropically, while employing a bond restraint of 84(2) pm using the DFIX command. Relevant details of the data collection and evaluation are listed in Table 1, the atomic coordinates, Wyckoff positions, and the isotropic displacement parameters in Table 2, and the anisotropic displacement parameters in Table 3. Interatomic distances are shown in Table 4, bond angles in Table 5 and the hydrogen bond data in Table 6.
CCDC 1946591 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.

Crystal structure
The structure consists of seven crystallographically independent boron atoms, whereby two are surrounded by four oxygen atoms forming tetrahedral [BO 4 ] 5− units, 0.1249 (14) 0.041 (7) 0.0436 (7) 0.0111 (5)   Symmetry operations: a 1 + y-x, 1-x, +z; b 1-y, +x-y, +z; c 1-y, 1 + x-y, +z; d + y-x, 1-x, +z; e -y, +x-y, +z; f + y-x, -x, +z; g + y, 1-x + y, 1-z; h 1-y + x, +x, 1-z; i 2-x, 2-y, 1-z; j -1/3 + y, 1/3-x + y, 4/3-z; k 2/3-y + x, 1/3 + x, 4/3-z; l 2/3-x, 4/3-y, 4/3-z.  The nitrate group with the central nitrogen atom N4 allows for six different oxygen positions with N-O distances of 123.6(8) pm each. As the site occupancy of the oxygen atoms is 0.5, only three oxygen sites are simultaneously occupied, forming a trigonal planar conformation and thereby allowing for two distinct nitrate groups that differ in their orientation, as illustrated in Fig. 3 (left). For the nitrogen atom N5, two different NO 3 − groups are found, as illustrated in Fig. 3 (right). The nitrogen atom N5 is displaced from the trigonal planar conformation. Regarding the nitrogen atom N5A, the O19A oxygen atoms are closest with N-O distances of 116 (2)  Along the ab plane, the nickel borate complex is surrounded by three K1, three K2, and three K3 ions, as well as three H 3 BO 3 groups and six nitrate groups as illustrated in Fig. 2 (top). The crystals have a layer-like structure stacked along the c axis. As shown in Fig. 2 (bottom), a layer of N3-and N4-centred nitrate groups is followed by a layer of the nickel borate complex along with potassium cations, H 3 BO 3 molecules, and the N1-centred nitrate group, then again followed by a layer of N2-and N5-centred nitrate groups. With an additional translation of [1/3, 1/3, 0], an inverted layer of the central nickel borate units with their environment (K + , H 3 BO 3 , N1-centred nitrate groups) follows next. Due to the translation along the lattice and the inversion of every layer, six different layers can be identified. However, only two are depicted for clarity. For further clarification, red and blue circles are drawn in Fig. 2 to indicate the positioning directly above each other along the c axis.
To equalize the charges, four hydrogen atoms were found to be located in proximity to the oxygen atoms O5, O6, O8, and O12. For all positions, interconnecting hydrogen bonds were identified, which are listed in Table  6. Each hydrogen atom was fixed to the corresponding oxygen atom at 0.84(2) pm using the DFIX command.
To evaluate our single-crystal structure solution, bond valence sums were calculated for every crystallographically independent non-hydrogen atom utilizing the bondlength/bond-strength concept. The O-H distances were fixed at 95 pm according to Brown and Altermatt [29,30]. The obtained values agree well with the expected ones, at least within the accuracy of the method, with the exception of the nitrate group formed by the atom N5. Owing to the high degree of distortion of the nitrate anion, the calculated values are unusually high/low, depending on whether the short (A) or the long (B) N-O distances are considered. The exact values for the calculations are listed in Table 7.

Conclusion
The synthesis and crystallographic characterization of the novel compound K 7 Ni[B 18 O 24 (OH) 9 ](NO 3 ) 6 · (H 3 BO 3 ) is reported in this work. The title compound is one of the few structures to encompass both, the structural motifs of borates as well as of nitrates. The crystal structure consists of nitrate layers and complex nickel borate layers surrounded by boric acid molecules, nitrate anions, and potassium cations stacked along the c axis. Hence, the field of nickel borates could be expanded by the incorporation of nitrate groups leading to the first nickel borate nitrate. Fig. 3: The N4-nitrate appears either as the N4O18A 3 (green) or the N4O18B 3 (purple) unit (left). For the N5-nitrate, either the N5AO19A 3 or the N5BO19B 3 nitrate are formed (right). All intermolecular distances and bond angles are identical in both possible arrangements. The difference stems from their orientation and therefore their vicinity.