Crystal structure of MgK0.5[B6O10](OH)0.5·0.5H2O, poly[dimagnesium potassium bis(hexa-borate) hy-droxide monohydrate].

The solvothermal reaction of H3BO3, KCF3SO3, Mg(CF3SO3)2 and pyridine led to a new alkali- and alkaline-earth-metal borate, MgK0.5[B6O10](OH)0.5·0.5H2O. Its structure features an intricate three-dimensional framework built from [B6O13]8- clusters, thus resulting in a six-connected achiral net with high symmetry. Each [B6O13]8- building block is composed of three trigonal BO3 and three tetra-hedral BO4 units, with these BO4 units being further connected to neighboring BO3 units, giving rise to an oxoboron cluster of the general formula [B6O10]2-. © Qiu and Song 2022.

Chemical context

As inorganic materials, borates are an important class of non-linear optical crystals, mainly because they can easily crystallize in non-centrosymmetric space groups and such structures often show a large second-harmonic generation response (Qiu et al., 2021a ▸; Qui & Yang, 2021a ▸). The combination of BO3-trigonal and BO4-tetra­hedral units makes it possible to form a variety of isolated anionic clusters. Extended chains, layers and three-dimensional frameworks can be formed between clusters through the dehydration and condensation of the terminal hydroxyl groups of oxoboron clusters (Wang et al., 2017 ▸). In addition, negatively charged oxoboron clusters can also combine with a variety of counter-cations, making the structure of borates more complex and diverse. Here, single crystals of MgK0.5[B6O10](OH)0.5·0.5H2O with alkali- and alkaline-earth metals have been obtained under solvothermal conditions.

Structural commentary

The asymmetric unit of the title compound consists of 2 B, 10/3 O, 1/3 Mg, 1/6 K, 1/6 OH, and 1/6 H2O. The Mg, K, O4, O5 and O6 atoms are located on special positions with occupancy of 1/3 or 1/6, while the remaining B and O atoms are located at general positions with an occupancy of 1. Bond-valence-sum calculations show that Mg, K and B are consistent with the expected oxidation states (Brown & Altermatt, 1985 ▸; Brese & O’Keeffe, 1991 ▸). Three BO4 units are joined together through corner-sharing of the O4 atom and three BO4 units are connected with three neighboring BO3 units to form a [B6O13]8− oxoboron cluster (Fig. 1 ▸). To the best of our knowledge, this is the first example of a mixed alkali- and alkaline-earth-metal borate crystal with the [B6O13]8− cluster anion. In this cluster, the B—O4 bonds are unique because their bond distances [1.529 (2) Å] are longer than other B—O bonds [1.359 (2)–1.453 (2) Å] in the BO3 and BO4 units. Each [B6O13]8− unit is further connected to six other clusters by corner-sharing O atoms, resulting in a three-dimensional framework (Fig. 2 ▸).

Figure 1

The asymmetric unit of the [B6O13]8− oxoboron cluster. Displacement ellipsoids are drawn at the 50% probability level.

Figure 2

View of the three-dimensional supra­molecular framework along the [100] direction. Color code: BO3 trigonal, yellow, orange and brown; BO4 tetra­hedral, blue.

Supra­molecular features

In the title compound, the Mg and K atoms are six-coord­inated, with Mg—O and distances in the range 2.332 (1)–2.374 (1) Å and K—O = 2.845 (1) Å. The three-dimensional structure is stabilized by a water cluster formed by O5—H5⋯O5, O5—H5⋯O6 and O6—H6A⋯O2 hydrogen bonds involving the water mol­ecule, hydroxyl group and oxoboron cluster (Table 1 ▸). The channels of the compound are filled with ions/mol­ecules (Mg2+, K+, OH− and H2O). The title structure is similar to previously reported analogues NH4NaB6O10 (Wang et al., 2014 ▸), K0.5[B6O10]·H2O·1.5H3O (Qiu & Yang, 2021b ▸), and NaRb0.5[B6O10]·0.5H3O (Qiu et al., 2021b ▸), so the simultaneous use of NH4 and Na or K or Na and Rb or Mg and K cations as templates has no effect on the crystallization of the oxoboron three-dimensional framework. However, after the introduction of Cl (Wu et al., 2011 ▸) or Br (Al-Ama et al., 2006 ▸), the new compounds crystallize in the trigonal space group R3m with a large second-harmonic generation response. The introduction of different anions can therefore play a key role in changing the crystalline structure to a non-centrosymmetric system.

Table 1

Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O5—H5⋯O5i	0.85	1.67	2.42 (3)	145
O6—H6A⋯O2i	0.85	2.58	3.276 (15)	140
O5—H5⋯O6i	0.85	1.78	2.484 (19)	139
O5—H5⋯O5ii	0.85	2.30	3.06 (3)	150

Symmetry codes: (i) ; (ii) .

Database survey

A search of the Cambridge Structural Database (CSD, version 5.43, update June 2022; Groom et al., 2016 ▸) for the [B6O13]8− oxoboron cluster gave 23 hits. The terminal oxygen atoms of this type of [B6O ] unit can be completely deprotonated [B6O13]8−, partially protonated [B6O11(OH)2]6− or completely protonated [B6O7(OH)6]2−. Among the above 23 compounds, most of them are inorganic–organic hybrid solids, which contain transition-metal complexes and the [B6O7(OH)6]2− cluster (refcodes: CAFYIV, CAFYOB, Altahan et al., 2021 ▸; CECWEM, Heller & Schellhaas, 1983 ▸; EMEHIP, Li et al., 2016 ▸; HIXNAF, Jamai et al., 2014 ▸; JOCCUC, JOCDAJ, Altahan et al., 2019a ▸; JUZLIC, Altahan et al., 2020 ▸; MEBQUI, MEBRET, Altahan et al., 2017 ▸; POJVIW, POJVOC, Altahan et al., 2019b ▸; TAFROI, Natarajan et al., 2003 ▸; VUVLOP, Jemai et al., 2015 ▸; BATCUY, Jamai et al., 2022 ▸; SAZVEY, Xin et al., 2022 ▸). It is worth noting that this oxoboron cluster contains too many active hydroxyl groups and therefore tends to form isolated structures. In the crystal of [Cd(1,2-dap)]·[B6O11(OH)2]·H2O (1,2-dap = 1,2-di­amino­propane, refcode: LOZZUY, Deng et al., 2020 ▸) and Cd3[B6O9(OH)2]2·2NO3·4H2O (refcode: ZUXLIQ, He et al., 2020 ▸), partially protonated [B6O11(OH)2]6− was successfully extended to layered structures via B—O—B bonds. In the crystal of NaRb0.5[B6O10]·0.5H3O (refcode: UCEXOT, Qiu et al., 2021b ▸), each completely deprotonated [B6O13]8− unit was linked to six nearest neighbors by bridging O atoms, leading to a 3D framework, similar to that of the title compound.

Synthesis and crystallization

A mixture of H3BO3 (0.618 g, 10 mmol), KCF3SO3 (0.188 g, 1 mmol) and Mg(CF3SO3)2 (0.322 g, 1 mmol) was added to pyridine (3.0 mL). After stirring for 20 min, the resulting mixture was sealed in a 25 mL Teflon-lined stainless steel autoclave, heated at 488 K for 9 d, and then slowly cooled to room temperature and colorless block-shaped crystals MgK0.5[B6O10](OH)0.5·0.5H2O were obtained (yield 56% based on H3BO3). Infrared (KBr pallet, cm−1): 3190vs, 1631s, 1360s, 1268m, 1188m, 1134m, 1099m, 964s, 845m, 781m, 741m, 718m, 630w, 564w, 540w, 480w, 455w.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2 ▸. Hydrogen atoms were positioned geometrically (O—H = 0.85 Å) and refined as riding with U iso(H) 1.2U eq(O).

Table 2

Experimental details

Crystal data
Chemical formula	Mg2K[B6O10]2(OH)·H2O
M r	572.46
Crystal system, space group	Cubic, P a
Temperature (K)	296
a (Å)	12.2966 (2)
V (Å3)	1859.32 (9)
Z	4
Radiation type	Mo Kα
μ (mm−1)	0.47
Crystal size (mm)	0.10 × 0.08 × 0.08
Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Krause et al., 2015 ▸)
T min, T max	0.762, 0.936
No. of measured, independent and observed [I > 2σ(I)] reflections	23808, 952, 828
R int	0.056
(sin θ/λ)max (Å−1)	0.714
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.039, 0.116, 1.16
No. of reflections	952
No. of parameters	72
H-atom treatment	H-atom parameters constrained
Δρmax, Δρmin (e Å−3)	0.66, −0.64

Computer programs: APEX2 and SAINT (Bruker, 2006 ▸), SHELXT2018/3 (Sheldrick, 2015a ▸), SHELXL2018/3 (Sheldrick, 2015b ▸), and SHELXTL (Sheldrick, 2008 ▸).

Crystal structure: contains datablock(s) I, global. DOI: 10.1107/S2056989022008982/tx2057sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989022008982/tx2057Isup2.hkl

CCDC reference: 2205808

Additional supporting information: crystallographic information; 3D view; checkCIF report

MgK0.5[B6O10](OH)0.5·0.5H2O	Mo Kα radiation, λ = 0.71073 Å
Mr = 572.46	Cell parameters from 5324 reflections
Cubic, Pa3	θ = 2.9–30.3°
a = 12.2966 (2) Å	µ = 0.47 mm−1
V = 1859.32 (9) Å3	T = 296 K
Z = 4	Block, colorless
F(000) = 1128	0.10 × 0.08 × 0.08 mm
Dx = 2.045 Mg m−3

Bruker APEXII CCD diffractometer	828 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube, Bruker (Mo) X-ray Source	Rint = 0.056
φ and ω scans	θmax = 30.5°, θmin = 3.3°
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	h = −16→17
Tmin = 0.762, Tmax = 0.936	k = −16→16
23808 measured reflections	l = −16→17
952 independent reflections

Refinement on F2	0 restraints
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.039	H-atom parameters constrained
wR(F2) = 0.116	w = 1/[σ2(Fo2) + (0.0658P)2 + 0.9552P] where P = (Fo2 + 2Fc2)/3
S = 1.16	(Δ/σ)max < 0.001
952 reflections	Δρmax = 0.66 e Å−3
72 parameters	Δρmin = −0.64 e Å−3

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. The structure was solved by direct methods and refined by the full-matrix least-squares method on F2 using the SHELXL programs (Bruker, 2006; Sheldrick, 2015a). All non-hydrogen atoms in the complex were refined anisotropically.

	x	y	z	Uiso*/Ueq	Occ. (<1)
Mg	0.33884 (5)	0.33884 (5)	0.33884 (5)	0.0205 (3)
K	0.500000	0.500000	0.500000	0.0257 (3)
O1	0.52065 (8)	0.28854 (8)	0.29782 (8)	0.0110 (3)
O2	0.22982 (9)	0.18931 (8)	0.38027 (8)	0.0118 (3)
O3	0.36386 (8)	0.68000 (8)	0.55091 (8)	0.0113 (3)
O4	0.18887 (7)	0.18887 (7)	0.18887 (7)	0.0057 (3)
O5	0.4745 (13)	0.1216 (14)	0.5078 (12)	0.066 (4)	0.1667
H5	0.504419	0.061354	0.523614	0.099*	0.1667
O6	0.544 (2)	0.0524 (17)	0.5575 (10)	0.080 (6)	0.1667
H6A	0.548070	0.065436	0.625259	0.121*	0.1667
H6B	0.587560	0.097246	0.528419	0.121*	0.1667
B1	0.21526 (12)	0.22026 (12)	0.48534 (12)	0.0084 (3)
B2	0.16682 (11)	0.13303 (11)	0.29771 (11)	0.0067 (3)

	U11	U22	U33	U12	U13	U23
Mg	0.0205 (3)	0.0205 (3)	0.0205 (3)	0.0007 (2)	0.0007 (2)	0.0007 (2)
K	0.0257 (3)	0.0257 (3)	0.0257 (3)	0.0071 (2)	0.0071 (2)	0.0071 (2)
O1	0.0060 (4)	0.0113 (5)	0.0156 (5)	0.0015 (3)	−0.0031 (3)	−0.0047 (4)
O2	0.0141 (5)	0.0157 (5)	0.0055 (4)	−0.0039 (4)	0.0005 (3)	−0.0023 (3)
O3	0.0138 (5)	0.0134 (5)	0.0066 (4)	0.0060 (4)	−0.0017 (4)	−0.0027 (3)
O4	0.0057 (3)	0.0057 (3)	0.0057 (3)	0.0007 (3)	0.0007 (3)	0.0007 (3)
O5	0.058 (8)	0.074 (11)	0.067 (9)	0.005 (7)	0.011 (7)	−0.018 (8)
O6	0.123 (19)	0.101 (16)	0.017 (5)	0.029 (12)	−0.018 (7)	−0.002 (6)
B1	0.0094 (6)	0.0091 (6)	0.0065 (6)	0.0007 (5)	−0.0008 (5)	−0.0009 (5)
B2	0.0067 (6)	0.0067 (6)	0.0067 (6)	0.0003 (4)	0.0006 (4)	0.0005 (5)

Mg—O2i	2.3319 (12)	O2—B1	1.3587 (16)
Mg—O2ii	2.3319 (12)	O2—B2	1.4525 (16)
Mg—O2	2.3319 (12)	O3—B1vii	1.3570 (18)
Mg—O1i	2.3738 (10)	O3—B2viii	1.4519 (16)
Mg—O1ii	2.3738 (10)	O4—B2i	1.5285 (15)
Mg—O1	2.3738 (10)	O4—B2ii	1.5285 (15)
Mg—B1	2.7714 (15)	O4—B2	1.5285 (15)
Mg—B1i	2.7715 (15)	O5—O6ix	0.96 (2)
Mg—B1ii	2.7715 (15)	O5—O6x	1.27 (3)
Mg—K	3.4324 (11)	O5—O6	1.35 (3)
K—O3	2.8450 (10)	O5—H5	0.8500
K—O3iii	2.8450 (10)	O5—H6B	1.4451
K—O3i	2.8450 (10)	O5—H6Aix	0.70 (3)
K—O3iv	2.8450 (10)	O6—O6ix	1.20 (3)
K—O3v	2.8450 (10)	O6—O6xi	1.20 (3)
K—O3ii	2.8450 (10)	O6—H5	0.6476
O1—B1ii	1.3821 (18)	O6—H6A	0.8500
O1—B2vi	1.4531 (16)	O6—H6B	0.8500
O2i—Mg—O2ii	81.06 (5)	B1ii—O1—B2vi	123.77 (11)
O2i—Mg—O2	81.06 (5)	B1ii—O1—Mg	91.19 (8)
O2ii—Mg—O2	81.06 (5)	B2vi—O1—Mg	144.28 (8)
O2i—Mg—O1i	112.48 (4)	B1—O2—B2	136.70 (11)
O2ii—Mg—O1i	132.64 (4)	B1—O2—Mg	93.59 (8)
O2—Mg—O1i	58.46 (3)	B2—O2—Mg	121.98 (8)
O2i—Mg—O1ii	58.46 (3)	B1vii—O3—B2viii	122.23 (11)
O2ii—Mg—O1ii	112.48 (4)	B1vii—O3—K	125.09 (8)
O2—Mg—O1ii	132.64 (4)	B2viii—O3—K	110.32 (7)
O1i—Mg—O1ii	112.97 (3)	B2i—O4—B2ii	117.97 (4)
O2i—Mg—O1	132.64 (4)	B2i—O4—B2	117.97 (4)
O2ii—Mg—O1	58.46 (3)	B2ii—O4—B2	117.97 (4)
O2—Mg—O1	112.48 (4)	O6ix—O5—O6	59 (2)
O1i—Mg—O1	112.96 (3)	O6x—O5—O6	89.1 (19)
O1ii—Mg—O1	112.96 (3)	O6ix—O5—H5	47.9
O2i—Mg—B1	93.22 (5)	O6x—O5—H5	67.1
O2ii—Mg—B1	109.26 (5)	O6—O5—H5	22.1
O2—Mg—B1	29.29 (4)	O6ix—O5—H6B	94.6
O1i—Mg—B1	29.91 (4)	O6x—O5—H6B	104.1
O1ii—Mg—B1	123.25 (4)	O6—O5—H6B	35.2
O1—Mg—B1	121.16 (4)	H5—O5—H6B	50.4
O2i—Mg—B1i	29.30 (4)	O5xi—O6—O6ix	71 (3)
O2ii—Mg—B1i	93.22 (5)	O5xi—O6—O6xi	77 (3)
O2—Mg—B1i	109.26 (5)	O6ix—O6—O6xi	101 (2)
O1i—Mg—B1i	121.16 (4)	O5xi—O6—O5xii	114 (2)
O1ii—Mg—B1i	29.90 (4)	O6ix—O6—O5xii	136.9 (16)
O1—Mg—B1i	123.25 (4)	O6xi—O6—O5xii	45.7 (12)
B1—Mg—B1i	114.20 (3)	O5xi—O6—O5	107 (3)
O2i—Mg—B1ii	109.26 (5)	O6ix—O6—O5	43.9 (9)
O2ii—Mg—B1ii	29.30 (4)	O6xi—O6—O5	133.5 (16)
O2—Mg—B1ii	93.22 (5)	O5xii—O6—O5	134.5 (17)
O1i—Mg—B1ii	123.25 (4)	O5xi—O6—H5	104.1
O1ii—Mg—B1ii	121.16 (4)	O6ix—O6—H5	33.1
O1—Mg—B1ii	29.91 (4)	O6xi—O6—H5	103.9
B1—Mg—B1ii	114.20 (3)	O5xii—O6—H5	117.5
B1i—Mg—B1ii	114.20 (3)	O5—O6—H5	29.6
O2i—Mg—K	131.38 (3)	O5xi—O6—H6A	44.8
O2ii—Mg—K	131.38 (3)	O6ix—O6—H6A	101.0
O2—Mg—K	131.38 (3)	O6xi—O6—H6A	103.2
O1i—Mg—K	74.30 (3)	O5xii—O6—H6A	111.4
O1ii—Mg—K	74.30 (3)	O5—O6—H6A	111.4
O1—Mg—K	74.30 (3)	H5—O6—H6A	130.2
B1—Mg—K	104.19 (4)	O5xi—O6—H6B	149.2
B1i—Mg—K	104.19 (4)	O6ix—O6—H6B	122.2
B1ii—Mg—K	104.19 (4)	O6xi—O6—H6B	122.0
O3—K—O3iii	65.411 (16)	O5xii—O6—H6B	76.7
O3—K—O3i	114.589 (16)	O5—O6—H6B	78.5
O3iii—K—O3i	180.0	H5—O6—H6B	95.3
O3—K—O3iv	65.410 (16)	H6A—O6—H6B	104.5
O3iii—K—O3iv	114.590 (15)	O5xi—O6—H5xi	58.1 (15)
O3i—K—O3iv	65.410 (15)	O6ix—O6—H5xi	74 (3)
O3—K—O3ii	114.590 (16)	O6xi—O6—H5xi	28.4 (18)
O3iii—K—O3ii	65.410 (15)	O5xii—O6—H5xi	74 (3)
O3i—K—O3ii	114.590 (15)	O5—O6—H5xi	114 (3)
O3iv—K—O3ii	180.0	H5—O6—H5xi	87.3
O3v—K—O3ii	65.410 (16)	H6A—O6—H5xi	97.8
O3—K—Mgv	76.32 (2)	H6B—O6—H5xi	148.0
O3iii—K—Mgv	103.68 (2)	O3xiii—B1—O2	123.87 (12)
O3i—K—Mgv	76.32 (2)	O3xiii—B1—O1i	122.17 (12)
O3iv—K—Mgv	103.68 (2)	O2—B1—O1i	113.96 (12)
O3v—K—Mgv	103.68 (2)	O3xiii—B1—Mg	166.22 (10)
O3ii—K—Mgv	76.32 (2)	O2—B1—Mg	57.12 (7)
O3—K—Mg	103.68 (2)	O1i—B1—Mg	58.91 (7)
O3iii—K—Mg	76.32 (2)	O3xiv—B2—O2	112.28 (11)
O3i—K—Mg	103.68 (2)	O3xiv—B2—O1xv	109.48 (11)
O3iv—K—Mg	76.32 (2)	O2—B2—O1xv	110.35 (11)
O3v—K—Mg	76.32 (2)	O3xiv—B2—O4	109.12 (10)
O3ii—K—Mg	103.68 (2)	O2—B2—O4	107.66 (11)
Mgv—K—Mg	180.000 (17)	O1xv—B2—O4	107.83 (10)
O6ix—O5—O6—O5xi	−36 (3)	B1—O2—B2—O3xiv	21.1 (2)
O6x—O5—O6—O5xi	−94.9 (17)	Mg—O2—B2—O3xiv	−119.29 (10)
O6x—O5—O6—O6ix	−59 (2)	B1—O2—B2—O1xv	−101.35 (17)
O6ix—O5—O6—O6xi	52 (3)	Mg—O2—B2—O1xv	118.27 (10)
O6x—O5—O6—O6xi	−7 (5)	B1—O2—B2—O4	141.21 (14)
O6ix—O5—O6—O5xii	117 (3)	Mg—O2—B2—O4	0.84 (12)
O6x—O5—O6—O5xii	58 (4)	B2i—O4—B2—O3xiv	45.6 (2)
B2—O2—B1—O3xiii	16.3 (2)	B2ii—O4—B2—O3xiv	−162.41 (10)
Mg—O2—B1—O3xiii	163.52 (12)	B2i—O4—B2—O2	−76.54 (15)
B2—O2—B1—O1i	−163.39 (13)	B2ii—O4—B2—O2	75.49 (15)
Mg—O2—B1—O1i	−16.21 (12)	B2i—O4—B2—O1xv	164.40 (10)
B2—O2—B1—Mg	−147.18 (17)	B2ii—O4—B2—O1xv	−43.6 (2)

D—H···A	D—H	H···A	D···A	D—H···A
O5—H5···O5xii	0.85	1.67	2.42 (3)	145
O6—H6A···O2xii	0.85	2.58	3.276 (15)	140
O6—H5···O6ix	0.65	0.74	1.20 (3)	119
O6—H5···O6x	0.65	1.23	1.84 (2)	158
O6—H5···O6xvi	0.65	1.82	2.19 (3)	118
O6—H5···O5ix	0.65	1.79	2.34 (3)	143
O6—H5···O5x	0.65	2.09	2.484 (19)	121
O6—H5···O5	0.65	0.85	1.35 (3)	128
O5—H5···O6xii	0.85	1.78	2.484 (19)	139
O5—H5···O6xi	0.85	1.49	2.34 (3)	179
O5—H5···O6xvi	0.85	1.82	2.30 (2)	114
O5—H5···O5xii	0.85	1.67	2.42 (3)	145
O5—H5···O5xi	0.85	1.28	1.88 (3)	122
O5—H5···O5xvi	0.85	2.30	3.06 (3)	150