Michael Zoller , Klaus Wurst and Hubert Huppertz

Synthesis of the first nickel borate nitrate K7Ni[B18O24(OH)9](NO3)6· (H3BO3)

De Gruyter | Published online: August 30, 2019

Abstract

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.

1 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 [BO3]3−, but also [BO4]5− units and even edge-sharing [BO4]5− tetrahedra [1]. Transition metal borates offer a large variety of compounds like Co7B24O42(OH)2·2H2O [2], and M6B22O39·H2O (M=Fe, Co, Ni, Cd) [3], [4], [5]. Further intensive focus has been directed to the synthesis of novel nickel borates, as evidenced by the publications on γ-NiB4O7 [6], NiB3O5(OH) [7], and Ni3B18O28(OH)4·H2O [8]. Another area of interest of our group has been the combination of simple metal borates with additional anionic groups like in Sn3[B3O7]X (X=F, I) [9], [10] and Pb[B2(SO4)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.

Similar to these syntheses, our group has prepared several new compounds containing borate and nitrate groups like K3Na[B6O9(OH)3]NO3 [12], Na3−xKx[B6O10]NO3 [13], Lu2B2O5(NO3)2·2H2O [14], and RE[B5O8(OH)(H2O)x]NO3·2H2O [RE=Pr (x=0.87), Nd (x=0.85), Sm (x=0)] [15], [16]. To the best of our knowledge, less than 20 compounds containing borate as well as nitrate functionalities are known to date.

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 K7Ni[B18O24(OH)9](NO3)6·(H3BO3). 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.

2 Experimental section

2.1 Synthesis

The compound K7Ni[B18O24(OH)9](NO3)6·(H3BO3) was synthesized under simple hydrothermal conditions in a stainless-steel autoclave (volume: 8 mL) fitted with a Teflon inlet. A mixture of 135.4 mg (0.57 mmol) NiCl2·6H2O (98%, abcr GmbH, Karlsruhe, Germany) and 64.6 mg (1.04 mmol) H3BO3 (≥99.8%, Roth GmbH+Co. KG, Karlsruhe, Germany) was thoroughly ground together in an agate mortar. To the mixture, 1 mL of water and 10 drops of nitric acid (65%, Roth GmbH+Co. KG, Karlsruhe, Germany) were added and the pH value was set to approximately 8 by addition of 2.8 mL KOH (1 mol L−1) solution. The reaction vessel was then placed in a furnace, heated up to a temperature of 513 K and kept there for 2 d. Afterwards, the reaction mixture was cooled down to room temperature with a cooling rate of 5 K h−1. A green product was obtained containing green, transparent crystals of K7Ni[B18O24(OH)9](NO3)6·(H3BO3). Suitable crystals were isolated and analyzed by single-crystal X-ray diffraction.

2.2 X-ray structure determination

Under a polarisation microscope, a suitable single-crystal of K7Ni[B18O24(OH)9](NO3)6·(H3BO3) 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 non-hydrogen atoms, except the nitrogen atom N5 and the 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 R3̅ 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.

Table 1:

Crystal data and structure refinement of K7Ni[B18O24(OH)9](NO3)6·(H3BO3) (standard deviations in parentheses).

Empirical formula K7Ni[B18O24(OH)9](NO3)6·(H3BO3)
Molar mass, gmol−1 1497.96
Crystal system Trigonal
Space group R3̅ (no. 148)
Single-crystal diffractometer Bruker D8 Quest Photon 100
Radiation; wavelength λ, pm Mo; 71.073
a, pm 1222.29(8)
c, pm 5478.4(4)
V, nm3 7.088(1)
Formula units per cell, Z 6
Calculated density, g cm−3 2.11
Crystal size, mm3 0.14×0.06×0.04
Temperature, K 297(2)
Absorption coefficient, mm−1 1.2
F(000), e 4452
2θ range, deg 4.9–56.0
Range in hkl ±16, ±16, ±72
Total no. of reflections 33867
Independent reflections/Rint 3811/0.0634
Data/restraints/parameters 3811/4/283
Absorption correction Multi-scan (Bruker Sadabs 2016/2)
Final R1/wR2 [I>2 σ(Io)] 0.0677/0.1973
Final R1/wR2 (all data) 0.0850/0.2077
Goodness-of-fit on Fi2 1.075
Largest diff. peak/hole, e Å−3 1.74/–1.28
Table 2:

Atomic coordinates, Wyckoff positions, and equivalent isotropic displacement parameters Ueq2) for K7Ni[B18O24(OH)9](NO3)6·(H3BO3) (space group R3̅).

Atom Wyckoff site S.O.F. x y z Ueq
Ni1 6c 1/3 2/3 1/3 0.58391(2) 0.0142(2)
K1 18f 1 0.7591(2) 0.7066(2) 0.47596(2) 0.0490(4)
K2 18f 1 0.6100(2) 0.7147(2) 0.64007(2) 0.0654(5)
K3 6c 1/3 0 0 0.60614(6) 0.0760(8)
B1 18f 1 0.7519(4) 0.5518(4) 0.55053(9) 0.0240(9)
B2 18f 1 0.6145(5) 0.6118(4) 0.53164(8) 0.0232(9)
B3 18f 1 0.5186(4) 0.4305(4) 0.55894(9) 0.0232(9)
B4 18f 1 0.5154(4) 0.4265(4) 0.60923(9) 0.0233(9)
B5 18f 1 0.7531(5) 0.5542(5) 0.61676(9) 0.0253(9)
B6 18f 1 0.3396(5) 0.2933(5) 0.63597(8) 0.027(1)
B7 6c 1/3 1/3 2/3 0.5902(2) 0.035(2)
N1 18f 1 0.7359(5) 0.8849(5) 0.58030(9) 0.050(2)
N2 6c 1/3 0 0 0.6656(1) 0.023(2)
N3 6c 1/3 1/3 2/3 0.5003(1) 0.021(1)
N4 3b 1/6 0 0 1/2 0.053(3)
N5 6c 1/6 1/3 2/3 0.6703(2) 0.016(2)
O1 18f 1 0.7356(3) 0.6366(3) 0.53673(6) 0.0305(7)
O2 18f 1 0.5112(3) 0.5175(3) 0.54256(5) 0.0265(6)
O3 18f 1 0.6487(2) 0.4561(2) 0.56229(5) 0.0182(5)
O4 18f 1 0.8676(3) 0.5649(3) 0.55176(5) 0.0224(6)
O5 18f 1 0.6062(3) 0.6866(3) 0.51429(6) 0.0315(7)
O6 18f 1 0.4751(3) 0.4512(3) 0.58430(5) 0.0275(6)
O7 18f 1 0.4216(3) 0.4095(3) 0.62685(6) 0.0286(7)
O8 18f 1 0.2552(3) 0.2853(3) 0.65315(6) 0.0332(7)
O9 18f 1 0.3427(3) 0.1864(3) 0.62947(6) 0.0341(7)
O10 18f 1 0.6352(3) 0.5354(3) 0.61592(5) 0.0238(6)
O11 18f 1 0.7874(2) 0.4739(3) 0.60597(5) 0.0193(6)
O12 18f 1 0.4334(4) 0.6461(4) 0.59054(8) 0.047(1)
O13 18f 1 0.7376(6) 0.8639(5) 0.60225(8) 0.076(2)
O14 18f 1 0.6348(4) 0.8220(5) 0.56854(8) 0.060(2)
O15 18f 1 0.8299(5) 0.9684(5) 0.5696(1) 0.077(2)
O16 18f 1 0.1129(4) 0.0288(5) 0.6661(1) 0.067(2)
O17 18f 1 0.2217(4) 0.6436(4) 0.5006(1) 0.061(2)
O18 18f 1/2 0.9107(8) 0.8936(8) 0.5046(2) 0.055(2)
O19 18f 1/2 0.434(2) 0.683(2) 0.6737(3) 0.107(4)
H5 18f 1 0.534(3) 0.659(5) 0.509(1) 0.04(2)
H6 18f 1 0.440(7) 0.496(6) 0.585(2) 0.08(2)
H8 18f 1 0.209(5) 0.210(3) 0.657(1) 0.05(2)
H12 18f 1 0.508(3) 0.712(4) 0.590(2) 0.07(2)

    Ueq is defined as one third of the trace of the orthogonalized Uij tensor (standard deviations in parentheses).

Table 3:

Anisotropic displacement parameters (Å2) of K7Ni[B18O24(OH)9](NO3)6·(H3BO3) (space group R3̅) with standard deviations in parentheses.

Atom U11 U22 U33 U23 U13 U12
Ni1 0.0147(3) U11 0.0132(4) 0 0 0.00733(14)
K1 0.046(7) 0.0899(10) 0.0315(6) −0.0091(6) −0.004(5) 0.0492(7)
K2 0.1249(14) 0.041(7) 0.0436(7) 0.0111(5) 0.035(8) 0.0515(8)
K3 0.0803(12) U11 0.0674(17) 0 0 0.0401(6)
B1 0.022(2) 0.019(2) 0.028(2) 0.0059(17) 0.0034(17) 0.0087(18)
B2 0.031(2) 0.02(2) 0.021(2) 0.0031(16) 0.001(17) 0.014(19)
B3 0.018(2) 0.022(2) 0.028(2) 0.0054(17) −0.003(17) 0.0086(17)
B4 0.022(2) 0.021(2) 0.028(2) 0.0007(17) 0.0066(17) 0.0114(18)
B5 0.024(2) 0.026(2) 0.027(2) −0.0069(18) −0.0018(18) 0.0136(19)
B6 0.024(2) 0.037(3) 0.023(2) 0.0045(19) 0.0054(17) 0.018(2)
B7 0.032(3) U11 0.041(5) 0 0 0.0158(14)
N1 0.054(3) 0.05(3) 0.047(3) 0.012(2) 0.008(2) 0.027(3)
N2 0.0258(18) U11 0.017(3) 0 0 0.0129(9)
N3 0.0192(16) U11 0.024(3) 0 0 0.0096(8)
N4 0.032(3) U11 0.095(10) 0 0 0.0162(17)
O1 0.025(15) 0.0255(15) 0.0399(17) 0.015(13) 0.0054(13) 0.0119(13)
O2 0.0266(15) 0.027(15) 0.029(15) 0.0093(12) 0.0005(12) 0.0157(13)
O3 0.017(13) 0.0175(12) 0.0208(13) 0.0034(10) 0.0022(10) 0.0092(11)
O4 0.0188(13) 0.0181(13) 0.0294(14) 0.0043(11) 0.0049(11) 0.0086(11)
O5 0.0319(17) 0.031(16) 0.0308(16) 0.0134(13) 0.0008(13) 0.0152(14)
O6 0.0314(16) 0.03(16) 0.0285(15) 0.0044(12) 0.004(12) 0.021(14)
O7 0.0288(16) 0.0313(16) 0.0293(15) 0.0017(12) 0.0104(12) 0.0176(13)
O8 0.0337(17) 0.0405(19) 0.0299(16) 0.0084(14) 0.0154(13) 0.0219(16)
O9 0.0307(16) 0.0302(16) 0.0422(18) 0.01(14) 0.0181(14) 0.0159(14)
O10 0.022(14) 0.0217(14) 0.0299(15) −0.0056(11) −0.0007(11) 0.0126(12)
O11 0.0157(12) 0.0198(13) 0.0212(13) −0.0043(10) −0.0028(10) 0.0079(10)
O12 0.037(2) 0.038(2) 0.071(3) 0.0082(19) 0.0066(19) 0.0218(18)
O13 0.095(4) 0.08(4) 0.04(2) 0.02(2) 0.01(2) 0.034(3)
O14 0.051(3) 0.057(3) 0.057(3) 0.01(2) 0.005(2) 0.015(2)
O15 0.053(3) 0.09(4) 0.07(3) 0.038(3) 0.009(2) 0.023(3)
O16 0.045(2) 0.055(3) 0.102(4) 0.013(3) 0.009(2) 0.026(2)
O17 0.041(2) 0.05(2) 0.091(3) 0.007(2) –0.001(2) 0.023(2)
O18 0.044(5) 0.034(4) 0.072(6) –0.008(4) –0.003(4) 0.008(3)

    The hydrogen atoms and the N5 nitrate (N5 and O19) have not been refined anisotropically.

Table 4:

Selected interatomic distances (pm) in K7Ni[B18O24(OH)9](NO3)6·(H3BO3) (standard deviations in parentheses).

Atoms Distance Atoms Distance
Ni1–O3 200.8(3) B7–O12 136.6(4)
Ni1–O3a 200.8(3) B7–O12c 136.7(4)
Ni1–O3b 200.8(3) B7–O12d 136.7(4)
Ni1–O11a 201.3(3) Ø=136.7
Ni1–O11b 201.3(3)
Ni1–O11 201.3(3) N1–O13 123.2(6)
Ø=201.1 N1–O15 123.7(7)
N1–O14 125.8(7)
B1–O4 134.4(5) Ø=124.2
B1–O1 137.5(5)
B1–O3 137.8(5) N2–O16 124.2(5)
Ø=136.6 N2–O16e 124.2(5)
N2–O16f 124.2(5)
B2–O2 135.1(5) Ø=124.2
B2–O5 135.8(5)
B2–O1 138.3(6) N3–O17 124.8(4)
Ø=136.4 N3–O17d 124.8(4)
N3–O17c 124.8(4)
B3–O4b 142.8(5) Ø=124.8
B3–O2 142.9(5)
B3–O3 147.1(5) N4–O18g 123.6(8)
B3–O6 155.3(6) N4–O18h 123.6(8)
Ø=147.0 N4–O18i 123.6(8)
Ø=123.6
B4–O7 143.2(5)
B4–O10 145.0(5) N5A–O19A 116(2)
B4–O11b 146.1(5) N5A–O19Ac 116(2)
B4–O6 153.2(6) N5A–O19Ad 116(2)
Ø=146.9 Ø=115.8
B5–O10 134.2(5) N5A–O19Bj 128(2)
B5–O11 137.9(5) N5A–O19Bk 128(2)
B5–O9a 138.0(6) N5A–O19Bl 128(2)
Ø=136.7 Ø=128.4
B6–O7 135.9(6)
B6–O8 136.3(5)
B6–O9 137.3(6)
Ø=136.5

    Symmetry operations: a1+yx, 1–x, +z; b1–y, +xy, +z; c1–y, 1+xy, +z; d+yx, 1–x, +z; ey, +xy, +z; f+yx, –x, +z; g+y, 1–x+y, 1–z; h1–y+x, +x, 1–z; i2–x, 2–y, 1–z; j–1/3+y, 1/3–x+y, 4/3–z; k2/3–y+x, 1/3+x, 4/3–z; l2/3–x, 4/3–y, 4/3–z.

Table 5:

Selected bond angles (deg) in K7Ni[B18O24(OH)9](NO3)6·(H3BO3) (standard deviations in parentheses).

Atoms Bond angle Atoms Bond angle
O4–B1–O1 118.6(4) O12–B7–O12c 119.98(3)
O4–B1–O3 122.7(4) O12–B7–O12d 119.98(3)
O1–B1–O3 118.8(4) O12c–B7–O12d 119.98(3)
Ø=120.0 Ø=119.98
O2–B2–O5 122.1(4) O13–N1–O15 121.8(6)
O2–B2–O1 122.7(4) O13–N1–O14 119.1(5)
O5–B2–O1 115.2(4) O15–N1–O14 119.1(5)
Ø=120.0 Ø=120.0
O4b–B3–O2 111.4(3) O16–N2–O16e 119.96(3)
O4b–B3–O3 111.9(3) O16–N2–O16f 119.96(3)
O2–B3–O3 112.6(3) O16e–N2–O16f 119.96(3)
O4b–B3–O6 108.0(3) Ø=120.0
O2–B3–O6 106.6(3)
O3–B3–O6 106.0(3) O17–N3–O17d 119.98(2)
Ø=109.4 O17–N3–O17c 119.98(3)
O17d–N3–O17c 119.98(3)
O7–B4–O10 109.3(3) Ø=120.0
O7–B4–O11b 112.9(3)
O10–B4–O11b 111.8(3) O18g–N4–O18i 116.0(3)
O7–B4–O6 108.5(3) O18g–N4–O18h 116.0(3)
O10–B4–O6 108.7(3) O18i–N4–O18h 116.0(3)
O11b–B4–O6 105.5(3) Ø=116.0
Ø=109.5
O19Ac–N5A–O19Ad 117.5(5)
O10–B5–O11 123.1(4) O19Ac–N5A–O19A 117.5(5)
O10–B5–O9a 118.5(4) O19Ad–N5A–O19A 117.5(5)
O11–B5–O9a 118.4(4) Ø=117.5
Ø=120.0
O19Bj–N5A–O19Bk 101(2)
O7–B6–O8 117.7(4) O19Bj–N5A–O19Bl 101(2)
O7–B6–O9 122.8(4) O19Bk–N5A–O19Bl 101(2)
O8–B6–O9 119.4(4) Ø=100.8
Ø=120.0

    Symmetry operations as in Table 4.

Table 6:

The hydrogen bond parameters (pm, deg) in K7Ni[B18O24(OH)9](NO3)6·(H3BO3).

Hydrogen bond D–H H…A D…A D–H…A
O5–H5…O17 83(2) 193(2) 275.6(6) 171(6)
O6–H6…O12 85(8) 190(8) 269.6(7) 155(7)
O8–H8…O16 84(2) 198(2) 281.1(6) 174(6)
O12–H12…O14 86(5) 186(6) 261.6(8) 145(4)

CCDC 1946591 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.

3 Results and discussion

3.1 Crystal structure

The structure consists of seven crystallographically independent boron atoms, whereby two are surrounded by four oxygen atoms forming tetrahedral [BO4]5− units, and four are surrounded by three oxygen atoms building up trigonal planar [BO3]3− units. These six borate units are linked via common corners as shown in Fig. 1a. The smallest crystallographically independent [B6O10(OH)3]5− unit can be subdivided into two dreier rings [23] consisting of two trigonal planar coordinated (B1, B2 and B5, B6, respectively) and one tetrahedrally coordinated (B3 and B4, respectively) borate unit interconnected via oxygen atoms (O1, O2, O3, and O7, O9, O11, respectively). The dreier rings are connected via the oxygen atom O8. This complete unit can be described via the descriptor 4Δ2□:⟨2Δ1□⟩⟨2Δ1□⟩ [24], [25] including all crystallographically independent boron and oxygen atoms. For the trigonal planar [BO3]3− units, the B–O distances range from 134.2(5) to 138.3(6) pm with an average B–O distance of 136.5 pm, which is in good agreement with other borates known from literature like NiB3O5(OH) (Ø=137.8 pm) [7] and NiB12O14(OH)10 (Ø=136.7 pm) [26]. The O–B–O angles range from 115.2(4) to 123.1(4)° with average values of 120.0°, exactly matching the theoretical value. Additionally, these findings are in good concordance with the mean values obtained by Zobetz (137.0(19) pm and 120.0(29)°) [27].

Fig. 1: The [B6O10(OH)3]5− anion forms the largest crystallographically independent structural unit (a). Three of those anions surround the central nickel atom forming an octahedral coordination sphere of the Ni atom by oxygen atoms (b). The central motif in K7Ni[B18O24(OH)9](NO3)6·(H3BO3) is the Ni[B18O24(OH)9]− complex (c), which incorporates two sechser rings built up by three dreier rings (blue, orange, purple).

Fig. 1:

The [B6O10(OH)3]5− anion forms the largest crystallographically independent structural unit (a). Three of those anions surround the central nickel atom forming an octahedral coordination sphere of the Ni atom by oxygen atoms (b). The central motif in K7Ni[B18O24(OH)9](NO3)6·(H3BO3) is the Ni[B18O24(OH)9] complex (c), which incorporates two sechser rings built up by three dreier rings (blue, orange, purple).

In the tetrahedrally coordinated [BO4]5− units, the B–O distances range from 142.8(5) to 155.3(6) pm with an average B–O distance of 147.0 pm, which is also in good agreement with values known from literature [5], [8]. The O–B–O angles range from 105.5(3) to 112.9(3)° with average values close to the theoretical value of 109.47° (B3: 109.4°, B4: 109.5°). For [BO4]5− units, a broader range of B–O distances is given by Zobetz (144.4–153.4 pm) [28]. The mean values for the B–O distances of 147.6(35) pm, as well as the O–B–O angles of 109.44(278)° are in good agreement with our findings.

Three of these polyborate units enclose a Ni(II) ion forming two sechser rings that are interconnected via the borate tetrahedra. The central Ni[B18O24(OH)9] complex is depicted with the nickel coordination sphere in Fig. 1b, c. The nickel ion is six-fold coordinated forming an octahedron with Ni–O distances ranging from 200.8(3) to 201.3(3) pm (Ø=201.1 pm), which is in good agreement with nickel borates known from literature like Ni3B18O28(OH)4·H2O (Ø=210.3 pm) [8], NiB3O5(OH) (Ø=206.2 pm) [7], and γ-NiB4O7 (Ø=208.2 pm) [6].

The seventh crystallographically independent boron atom forms a trigonal planar H3BO3 unit isolated from the central [B18O24(OH)9]3− complex. The B–O distances are 136.7(4) pm, which is similar to the values encountered in [BO3]3− units of the central complex. The O–B–O angles are 120°, which matches with a trigonal planar situation. Additionally, the central complex is surrounded by three crystallographically independent potassium ions with K–O distances ranging from 262.4(9) to 334.0(4) pm. Furthermore, the central nickel borate complex is surrounded by five crystallographically independent nitrate groups as shown in Fig. 2. The N1, N2, and N3 atoms of the nitrate groups show similar N–O distances ranging from 123.2(6) to 125.8(7) pm, with average values of 124.2, 124.2, and 124.8 pm, respectively, which are in good agreement with values known in literature [13], [14], [16]. The O–N–O bond angles range from 119.1(5) to 121.8(6)° with averages of 120.0° each.

Fig. 2: The central nickel borate complexes are isolated from each other and are each surrounded by six nitrate groups in the ab plane (top). Additionally, the nickel borate complex is surrounded by a layer-like structure of nitrates shown along the ac plane (bottom).

Fig. 2:

The central nickel borate complexes are isolated from each other and are each surrounded by six nitrate groups in the ab plane (top). Additionally, the nickel borate complex is surrounded by a layer-like structure of nitrates shown along the ac plane (bottom).

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 NO3 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) pm and O–N–O bond angles of 117.5(5)°. The other possible oxygen positions are further away with N–O distances of 128(2) pm and O–N–O bond angles of 101(2)°. The same is obviously true for the position N5B, with interchanged values. A statistical presence of the different possible nitrates seems to be most plausible.

Fig. 3: The N4-nitrate appears either as the N4O18A3 (green) or the N4O18B3 (purple) unit (left). For the N5-nitrate, either the N5AO19A3 or the N5BO19B3 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.

Fig. 3:

The N4-nitrate appears either as the N4O18A3 (green) or the N4O18B3 (purple) unit (left). For the N5-nitrate, either the N5AO19A3 or the N5BO19B3 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.

Along the ab plane, the nickel borate complex is surrounded by three K1, three K2, and three K3 ions, as well as three H3BO3 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, H3BO3 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+, H3BO3, 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 bond-length/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.

Table 7:

Charge distribution in K7Ni[B18O24(OH)9](NO3)6·(H3BO3) (space group R3̅, no. 148), calculated with the bond-length/bond-strength concept (ΣV)a [29].

Ni1 K1 K2 K3 B1 B2 B3 B4 B5 B6
ΣV 2.29 1.37 1.31 1.04 3.05 3.06 3.09 3.09 3.04 3.05
B7 N1 N2 N3 N4 N5A N5B
ΣV 3.04 5.01 5.01 4.93 5.10 6.29 4.48
O1 O2 O3 O4 O5 O6 O7 O8 O9 O10
ΣV 1.99 1.96 2.13 2.11 2.38 2.26 1.92 2.35 2.01 2.10
O11 O12 O13 O14 O15 O16 O17 O18 O19A O19B
ΣV 2.14 2.06 2.11 1.77 2.01 1.67 1.69 1.96 2.10 1.49

    aThe superscripts A,B refer to the short (A) and long (B) N5–O19 distances.

4 Conclusion

The synthesis and crystallographic characterization of the novel compound K7Ni[B18O24(OH)9](NO3)6·(H3BO3) 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.

Acknowledgements

We thank Assoc. Prof. Dr. G. Heymann for the collection of several sets of single-crystal data and Dr. H. Schwartz for fruitful discussions.

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Received: 2019-08-13
Accepted: 2019-08-20
Published Online: 2019-08-30
Published in Print: 2019-10-25

©2019 Hubert Huppertz et al., published by De Gruyter, Berlin/Boston

This work is licensed under the Creative Commons Attribution 4.0 Public License.