Synthesis and Characterization of the First Borosulfates of Magnesium

Article pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Synthesis and Characterization of the First Borosulfates of Magnesium, Manganese, Cobalt, Nickel, and Zinc Philip Netzsch, Peter Gross, Hirotaka Takahashi, and Henning A. Höppe* Lehrstuhl für Festkörperchemie, Universität Augsburg, Universitätsstraße 1, D-86159 Augsburg, Germany

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S Supporting Information *

ABSTRACT: The first magnesium, manganese, cobalt, nickel, and zinc borosulfates were synthesized employing solvothermal conditions starting from the superacid H[B(HSO4)4] and the respective metal powders (Mg, Ni, Zn) or oxides (MnO2, CoO). α-M4[B2O(SO4)6] (M = Mg, Mn, Co, Ni, Zn) crystallize isotypically in a new structure type in P3̅ (No. 147) with Z = 1, a = 793.59(4)−810.86(9) pm, and c = 743.98(4)−775.09(9) pm. The oligomeric anion comprises unprecedented dimeric open-branched quadruple tetrahedra {oB, 4t}[B2O(SO4)6]8−, which are connected via M2O9 dimers to give a three-dimensional network. Upon mild heating, we observed a phase change from α-Mg4[B2O(SO4)6] to β-Mg4[B2O(SO4)6], yielding a further new structure type in P3̅ (No. 147) with Z = 3, a = 1391.96(6) pm, and c = 748.54(3) pm. The reaction of MgB2 with SO3 yields Mg[B2(SO4)4] crystallizing in C2/c with Z = 4, a = 1744.28(10) pm, b = 531.45(3) pm, c = 1429.06(8) pm, and β = 126.323(2)° showing phyllosilicate topology. UV/vis spectroscopy on αTM4[B2O(SO4)6] (TM = Co, Ni) confirms the valence state of the TM and reveals that borosulfates are weakly coordinating host structures. Structure relationships between the presented crystal structures and similar borophosphates are shown. The results of vibrational spectroscopy as well as magnetic and thermal measurement investigations are discussed.

1. INTRODUCTION Our systematic search for novel materials with interesting absorption or luminescence properties focuses on silicateanalogous compounds. Such compounds comprise tetrahedral basic building units and form condensed oligo- or polymeric anions. Currently, we are especially interested in weakly coordinating host structures providing terminal atoms with rather weak covalent and electrostatic interactions, i.e., small nephelauxetic and ligand-field effects. One of our recent examples in this area is a fluorooxoborate doped with divalent europium, viz. Ba[B4O6F2]:Eu2+, where both effects are so low that even a 4f−4f emission was observed.1 The most common and obvious building unit in borosulfates so far is supertetrahedra formed by a tetrahedral BO4 backbone saturated by four SO 4 tetrahedra, so you might eye borosulfates by considering BO4 as tetrahedral centers T and SO4 as bridging or terminal moieties X with the ratio T:X determining the anions’ topology. These building units might condense to infinite chains as in K3[B(SO4)3]2 and rings as in Gd2[B2(SO4)6]3 (B:S ratio 1:3) or three-dimensional networks (B:S = 1:2), viz. Li[B(SO4)2]2 homeotypic with tridymite, or remain noncondensed as observed in K5[B(SO4)4]4 (B:S = 1:4). In some cases, these building units condense even tighter, forego the bridging sulfate tetrahedra and form direct B−O−B bridges between adjacent BO4 tetrahedra instead, as observed in Cs2[B2O(SO4)3],5 Ba[B2O(SO4)3],3 or B2O(SO4)2.6 Some© XXXX American Chemical Society

times, the TX4 units connect via common edges like in H3O[B(SO4)2],7 leading to chains, comparable to SiS2.8 As yet, all anions are built up solely by tetrahedra; thus we classify these structures as silicate-analogous ones. From previous contributions on borosulfates, we deduced that their condensation degree seems to be dependent from the reaction temperature. In the case of the potassium compounds, we observed a depolymerization of the anion with increasing temperature from K3[B(SO4)3] comprising a chain-shaped anion of condensed supertetrahedra, synthesized around 400 °C, toward K5[B(SO4)4] with noncondensed supertetrahedra, synthesized around 460 °C. In contrast, for the lithium compounds, we found an increasing condensation degree from Li[B(S2O7)2] synthesized at room temperature to Li[B(SO4)2], made around 300 °C.2 Within this context, we investigated the crystal chemistry of divalent ions of similar ionic radius, i.e., Mg, Mn, Co, Ni, and Zn. In this contribution, we elucidate the very first structure types found for magnesium and the transition metal ions Mn2+, Co2+, Ni2+, and Zn2+ containing borosulfates. Furthermore, we shed light on the optical properties to actually measure their coordination behavior for the first time. Received: May 5, 2018

A

DOI: 10.1021/acs.inorgchem.8b01234 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

2. EXPERIMENTAL SECTION

Table 1. MAPLE Calculations of the Borosulfates M4[B2O(SO4)6] (M = Mg, Mn, Co, Ni, Zn) and Mg[B2(SO4)4] and Their Respective Metal Sulfates and Boron Oxide Sulfate

Synthetic Procedures. M4[B2O(SO4)6]. In the first step, the superacid H5[B(SO4)4] was synthesized in a Schlenk flask by dissolving B(OH)3 (123.7 mg, 2.000 mmol) in 2.5 mL of H2SO4. The flask was flushed with N2 and heated to 200 °C for 1 h and afterward cooled down to 120 °C. Subsequently, 0.3 mL of oleum (65%) was added. For a typical synthesis, 2 mmol of the respective metal powder (Mg, Ni, Zn) or metal oxide (MgO, MnO2, CoO, ZnO) were added and the suspension was stirred for another hour at 120 °C. The resulting suspension was transferred into a silica glass ampule (outer diameter: 1.2 cm, wall thickness: 0.1 cm) and fused. The ampule was placed in a muffle furnace applying the following temperature program: heating to 350 °C with 100 °C/h, holding the temperature for 96 h, and cooling down to room temperature with 50 °C/h. Caution! The reaction of basic oxides like MgO gives a vigorous reaction, so only tiny portions should be added carefully to the acid. Mg[B2(SO4)4]. MgB2 (23.0 mg, 0.500 mmol) was given in a silica glass ampule. Subsequently, 0.8 mL of oleum (65%) were added. The ampule was fused and the following temperature program was applied: heating to 200 °C with 50 °C/h, holding the temperature for 72 h, and cooling down to room temperature with 100 °C/h. Several single crystals were formed above the acid as well as a polycrystalline bulk in the acid. Ampules were opened after cooling down with liquid nitrogen. The bulk excess of the acid was pipetted, whereas the adhesive acid was evaporated at 300 °C. The crystals are very sensitive to moisture and hence were stored under inert conditions. β-Mg4[B2O(SO4)6]. The β-modification for the magnesium compound was achieved by removing the excessive sulfuric acid at 300 °C for 24 h. Crystal Structure Determination. Immediately after opening the ampule, single crystals were transferred into perfluorated polyether and selected for single-crystal XRD. Diffraction data for all compounds were collected with a Bruker D8 Venture diffractometer using Mo-Kα radiation (λ = 0.71073 Å). The temperature was adjusted with a nitrogen flow (Oxford Cryosystems). The absorption correction was done with the multiscan method; then the crystal structures were solved with direct methods and refined by the full-matrix least-squares technique within the SHELXTL program.9 All crystals of M4[B2O(SO4)6] (M = Mg, Mn, Co, Ni, Zn) were non-merohedral twins and were refined with the twin matrix: 1 0 0 0 1 0 0 0 −1. Further details of the crystal structure investigations discussed in this contribution are listed in Table 3, and Tables S1−S14 in the Supporting Information, and may be obtained from the Fachinformationszentrum Karlsruhe, D-76344 EggensteinLeopoldshafen, Germany (e-mail: crysdata@fiz-karlsruhe.de) on quoting the depository numbers CSD-434485 (α-Mg4[B2O(SO4)6]), CSD-434486 (β-Mg4[B2O(SO4)6]), CSD-434488 (α-Mn4[B2O(SO4)6]), CSD-434489 (α-Co4[B2O(SO4)6]), CSD-434490 (αNi4[B2O(SO4)6]), CSD-434491 (α-Zn4[B2O(SO4)6]), and CSD434487 (Mg[B2(SO4)4]), the names of the authors, and citation of this publication. X-ray Powder Diffraction. The samples were ground and filled into a Hilgenberg glass capillary (outer diameter 0.3 mm, wall thickness 0.01 mm) inside a glovebox. The data were collected with a Bruker D8 Advance diffractometer with Cu-Kα radiation (λ = 1.54184) with a 1D LynxEye detector. The TPXRD patterns (temperature-programmed X-ray diffraction) were collected on a PANalytical Empyrean diffractometer equipped with a PIXcel3D 2 × 2 detector. The measurement was done in Bragg−Brentano geometry with an Anton Paar XRK 900 reaction chamber. Infrared Spectroscopy. The infrared spectra were recorded using a Bruker EQUINOX 55 FT-IR spectrometer equipped with a platinum ATR setup in a range of 4000−400 cm−1. Optical Spectroscopy. The optical reflection spectra were measured with a Varian Cary 300 Scan UV/vis spectrophotometer in the range of 200−800 nm.

α-Mg4[B2O(SO4)6]

4MgSO432 + B2S2O96 −1

MAPLE = 223957 kJ mol β-Mg4[B2O(SO4)6]

MAPLE = 222363 kJ mol−1 (Δ = 0.7%) 4MgSO432 + B2S2O96

MAPLE = 222822 kJ mol−1 MAPLE = 222363 kJ mol−1 (Δ = 0.2%) Mg[B2(SO4)4] MgSO432 + B2S2O96 + SO333 MAPLE = 147526 kJ mol−1 MAPLE = 146784 kJ mol−1 (Δ = 0.5%) α-Mn4[B2O(SO4)6] 4MnSO434 + B2S2O96 MAPLE = 222662 kJ mol−1 MAPLE = 221436 kJ mol−1 (Δ = 0.6%) α-Ni4[B2O(SO4)6] 4NiSO435 + B2S2O96 MAPLE = 223786 kJ mol−1 MAPLE = 222024 kJ mol−1 (Δ = 0.8%) α-Zn4[B2O(SO4)6] 4ZnSO436 + B2S2O96 MAPLE = 223424 kJ mol−1 MAPLE = 221913 kJ mol−1 (Δ = 0.7%) α-Co4[B2O(SO4)6] 4CoSO437 + B2S2O96 MAPLE = 223105 kJ mol−1 MAPLE = 221517 kJ mol−1 (Δ = 0.1%)

Magnetic Investigations. The magnetic susceptibility of Mn4[B2O(SO4)6] was measured with a Quantum Design MPMSXL superconducting quantum-interference device (SQUID) magnetometer between 1.8 K < T < 400 K with an external field of 1000 Oe. Thermal Analysis. The thermogravimetric analysis was done in alumina crucibles employing a NETZSCH STA 409 PC Luxx in a nitrogen atmosphere and a heating ramp of 5 K/min.

3. RESULTS AND DISCUSSION Synthetic Approach. Up to now, borosulfates were synthesized via solid state reactions starting from metal sulfates (e.g., K2SO4),4 metal sulfate hydrates (e.g., Li2SO4·H2O), metal hydrogensulfates (e.g., KHSO4), or metal disulfates (e.g., K2S2O7).2 Furthermore, syntheses were performed as an acid− base reaction under solvothermal conditions by reacting the superacid H[B(HSO4)4] with metal chlorides (e.g., BaCl2, GdCl3).3 Recently, also metal carbonates (e.g., CaCO3)10 and nitrates (e.g., AgNO3)11 were utilized. M4[B2O(SO4)6] are the first borosulfates synthesized by reaction of metal powders in the case of magnesium, nickel, and zinc or metal oxides for manganese and cobalt. Furthermore, the Mg and Zn compounds could also be achieved by the respective oxides, showing that both starting materials can be employed in this synthesis. In a typical synthesis, these starting materials were added to the superacid H[B(HSO4)4], which was characterized by Gillespie12 and Herrmann,13 followed by a solvothermal treatment according to eq 1: 2H[B(HSO4 )4 ] + 2SO3 + 4M → M4[B2O(SO4 )6 ] + 4SO2 + 5H 2O

(1)

All compounds were synthesized phase pure, as proven by PXRD (Figure 1a). Afterward, the excess of sulfuric acid was removed, which led to an unexpected phase transition in the case of α-Mg4[B2O(SO4)6] to give β-Mg4[B2O(SO4)6] (Figure 1b). B


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Inorganic Chemistry

Table 2. Selected Interatomic Distances (in pm) and Angles (in deg) in the Compounds α-M4[B2O(SO4)6] (M = Mg, Mn, Co, Ni, Zn), β-Mg4[B2O(SO4)6], and Mg[B2(SO4)4]a M−O ΣIR(M−O) S−Obr S−Oterm B−O M−M O−M−O O−S−O O−B−O

22

α-Mg4[B2O(SO4)6]

β-Mg4[B2O(SO4)6]

Mn4[B2O(SO4)6]

201.1(2)−214.2(2)

198.3(6)−218.2(5)

207 150.64(11)

207 149.8(2)−151.2(2)

210.06(14)− 224.34(13) 218 150.85(6)

143.5(2)− 148.08(10) 135.6(3)−147.5(2)

142.8(5)−147.9(2)

143.84(13)− 147.82(6) 135.44(18)− 149.26(11) 302.38(5) 74.63(5)− 95.90(6) 104.60(3)− 112.99(5) 105.36(10)− 113.33(9)

301.01(12) 75.49(10)− 96.61(10) 104.23(6)− 112.76(7) 106.86(18)− 111.97(16)

133.6(2)−150.8(3) 301.42(15) 73.39(17)−98.2(2) 103.65(13)− 113.66(17) 105.0(4)−113.0(4)

Co4[B2O(SO4)6]

Ni4[B2O(SO4)6]

Zn4[B2O(SO4)6]

203.3(4)− 217.4(6) 210 150.7(2)

199.5(3)− 211.1(4) 204 150.84(12)

200.7(2)− 216.1(3) 209 150.69(11)

143.2(5)− 148.7(2) 136.7(6)− 148.5(4) 301.52(12) 75.5(2)− 95.37(16) 104.66(11)− 113.70(13) 106.2(4)− 112.6(4)

142.1(3)− 148.80(11) 136.1(3)− 147.1(3) 292.65(5) 77.06(17)− 95.35(10) 104.50(6)− 113.58(8) 107.9(3)− 111.0(3)

142.2(3)− 148.22(11) 135.6(3)− 147.3(2) 307.29(5) 75.60(13)− 98.98(10) 104.92(6)− 113.21(8) 107.4(2)− 111.5(2)

Mg[B2(SO4)4] 202.49(10)− 205.85(11) 207 151.73(10)− 153.95(10) 141.22(11)− 144.10(11) 145.12(17)− 148.70(18) 86.73(4)− 93.07(5) 100.70(6)− 117.23(7) 106.94(12)− 112.19(11)

a

The standard deviations are given in parentheses.

The terminal oxygen atoms of the anions provide an octahedral environment for the metal cations, situated on Wyckoff site 2d with the M−O distances ranging between 201 and 214 pm (α-Mg), 210 and 224 pm (Mn), 203 and 217 pm (Co), 200 and 211 pm (Ni) as well as 201 and 216 pm (Zn) (Table 2). All of these values are close to the sum of respective ionic radii.22 These MO6 octahedra share common faces leading to M2O9 dimers with rather short M−M distances (Figure 5). Due to an apparent repulsion, the cations are shifted out of the octahedron centers. The crystal structure of α-M4[B2O(SO4)6] (M = Mg, Mn, Co, Ni, and Zn) is closely related to that of Mo4[Si2O(PO4)6], which contains Mo2O9 dimers and [Si2O(PO4)6]12− anions.18 However, both structures deviate from each other due to a different orientation of the anions. The structure type of α-Mg4[B2O(SO4)6] shows also an astonishing relationship to the borophosphates A2[B(PO4)3] (A = Cr, Fe, In).23−25 Structural relationships between borophosphates and borosulfates occur occasionally; sometimes, structures are isotypic like those of Ba3[B(PO4)3] and K3[B(SO4)3].2 In the borophosphates A2[B(PO4)3], the boron atoms are coordinated trigonally planar, yielding noncondensed anions [B(PO3)3]6−. Triangularly coordinated boron has not yet been found in borosulfates. Hence, the striving for tetrahedral coordination of boron in α-M4[B2O(SO4)6] agrees well with the linkage of two adjacent moieties leading to the described crystal structure. The high-temperature sister of the α-polymorph, βMg4[B2O(SO4)6], shows another new structure type (Figure 6). It comprises the same fundamental building units like the α-polymorph; however, in two-thirds of these [B2O(SO4)6]8− moieties, the six terminal oxygen atoms of the B2O7 backbone display an eclipsed conformation, whereas, in the remaining third of the [B2O(SO4)6]8− moieties, the terminal oxygen atoms remain in the staggered conformation of the αpolymorph. Accordingly, the unit cell contains three crystallographically independent anions as well as increased a and b axes (Figure S1 in the Supporting Information). The six sulfate tetrahedra in β-Mg4[B2O(SO4)6] show deviations of 0.03−0.27% and the three borate tetrahedra of 0.00−0.29% from tetrahedral symmetry.14,15 The magnesium atoms occupy the general Wyckoff site 6g; the Mg−O distances lie between 198 and 218 pm.

Mg[B2(SO4)4] was synthesized by reaction of oleum with MgB2.This is the first borosulfate synthesized from a boride, providing a further boron source besides boric acid and boron oxide. Crystal Structures. α-M4[B2O(SO4)6]. (M = Mg, Mn, Co, Ni, and Zn) crystallize in a new structure type in space group P3̅ (Figure 2). The structure comprises “layers” of edge-sharing [B2O(SO4)6]8− anions and MO6 octahedra. These octahedra condense to face-sharing M2O9 dimers and thus connect the layers toward a three-dimensional network (Figure 3). The anions [B2O(SO4)6]8− represent a novel structural motif for borosulfates (Figure 4) in which two anions of the superacid [B(SO4)4]5− condense under the cleavage of two SO3 molecules. Hence, it consists of a corner-sharing B2O7 dimeric backbone saturated with six sulfate tetrahedra. Such B−O−B bridges were already reported in borosulfates for B2O(SO4)2,6 Cs2[B2O(SO4)3],5 and Ba[B2O(SO4)3].3 However, in this case, a linear B−O−B bond is apparently present. Although the atomic displacement parameter of the bridging oxygen atom indicates a slight movement, the repulsion between the staggered sulfate tetrahedra enforces a linear B− O−B bridge. The deviation of the tetrahedra from the ideal symmetry was calculated by the method of Balic-Zunic and Makovicky.14,15 The single crystallographically independent sulfate tetrahedron shows deviations of 0.09% (α-Mg, Co) and 0.08% (Mn, Ni, Zn), the borate tetrahedron of 0.03% (α-Mg), 0.00% (Mn, Co), 0.10% (Ni), and 0.07% (Zn). To our experience, thus all tetrahedra can be safely classified as regular ones.16 According to the nomenclature of silicates suggested by Liebau,17 the anion is classified as {oB, 4t}[B2O(SO4)6]8− (open-branched quadruple tetrahedra chain). This topology was already reported for silicophosphates like Mo4[Si2O(PO4)6], in which also a linear Si−O−Si bridge was described.18 Furthermore, a structurally similar motif is present as part of polymeric structures, such as β-cristobalite19 or the sialon Sr2AlxSi12−xN16−xO2+x (x ≈ 2),20 just to name a few famous examples. If eyeing the topology using the TX formalism introduced in the Introduction, you will find an analogous one in the nitridosilicate SrSi6N8,21 in which direct Si−Si bonds were foundcomparable to the T-T backbone in [B2O(SO4)6]8−. Both of these aspects further underline the analogy to silicates. C


D

a

748.54(3) 1256.03(12) 3 2.821 1.117 1062 0.71073 Bruker D8 Venture multiscan 0.6416/0.7465 −19/19|−19/19|−10/10 2.927 < Θ < 29.983 21990 2448 1932 0.0848 164 0.0593 0.0990 w = 1/[s2(Fo2) + (0.0411P)2 + 2.8546P], where P = (Fo2 + 2Fc2)/3 1.073 −0.726/2.306

748.58(9)

416.62(11) 1 2.835

1.123

354 0.71073 Bruker D8 Venture multiscan 0.6486/.7505

−13/13|−13/13|−12/12

2.721 < Θ < 37.443 44220 1469

1441

0.0298 59 0.0258 0.0648 w = 1/[s2(Fo2) + (0.0105P)2 + 0.8298P], where P = (Fo2 + 2Fc2)/3 1.220 −0.594/2.268

β-Mg4[B2O(SO4)6] 280(2) 711.22 trigonal P3̅ (No. 147) hexagon 0.054 × 0.063 × 0.118 colorless 1391.96(6)

α-Mg4[B2O(SO4)6]

200(2) 711.22 trigonal P3̅ (No. 147) hexagon 0.184 × 0.293 × 0.332 colorless 801.65(9)

The standard deviations are given in parentheses.

GooF residual electron density (min./max.)/e− Å−3

temperature/K molar weight/g mol−1 crystal system space group crystal shape crystal size/mm3 color a/pm b/pm c/pm β/deg volume/106 pm3 Z calculated density Dx/g cm−3 absorption coefficient μ/mm−1 F(000) radiation (λ/Å) diffractometer absorption correction transmission factor (min./max.) index range h|k|l (min./max.) theta range/deg reflections collected independent reflections observed reflections (I > 2σ) Rint refined parameters R1 (all data) wR2 (all data) weighting scheme

Mn4[B2O(SO4)6]


3254

2.628 < Θ < 64.722 39406 3661

−16/16|−19/16|−15/16

406 0.71073 Bruker D8 Venture multiscan 0.4878/0.7525

3.647

441.34(11) 1 3.137

775.09(9)


Table 3. Crystal Data and Details of the Structure Refinementsa Co4[B2O(SO4)6]

0.0621 59 0.0419 0.0537 w = 1/[s2(Fo2) + (0.0086P)2 + 0.9511], where P = (Fo2 + 2Fc2)/3 1.110 −0.018/1.077

797

2.695 < Θ < 31.857 8286 974

−11/11|−11/11|−11/11


4.799

418.0(3) 1 3.376

755.9(3)

250(2) 849.70 trigonal P3̅ (No. 147) hexagon 0.062 × 0.076 × 0.092 pink 799.1(3)

Ni4[B2O(SO4)6]


1282

2.738 < Θ < 37.456 14901 1429

−13/13|−13/12|−12/12


5.495

405.77(5) 1 3.474

743.98(4)

200(2) 848.82 trigonal P3̅ (No. 147) hexagon 0.099 × 0.180 × 0.305 yellow 793.59(4)

Zn4[B2O(SO4)6]


1370

2.720 < Θ < 37.490 12781 1469

−13/13|−13/12|−12/12


6.619

414.81(8) 1 3.505

748.95(7)


Mg[B2(SO4)4]


1396

2.899 < Θ < 29.984 18476 1562

−24/24|−7/7|−20/20


1.059

200(2) 430.17 monoclinic C2/c block 0.680 × 0.215 × 0.175 colorless 1744.28(10) 531.45(3) 1429.06(8) 126.323(2) 1067.33(11) 4 2.677

Inorganic Chemistry Article


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Inorganic Chemistry

Figure 3. Linkage of the “layers” in α-Mg4[B2O(SO4)6] along [110] (color code as in Figure 2).

Figure 1. (a) PXRD patterns of α-M4[B2O(SO4)6] with M = Mn (blue), Co (pink), Ni (orange), and Zn (green) in comparison to the calculated pattern derived from single-crystal data of α-Mn4[B2O(SO4)6] (black). (b) PXRD pattern of β-Mg4[B2O(SO4)6] (red) in comparison to the calculated pattern derived from single-crystal data of β-Mg4[B2O(SO4)6] (gray).

Figure 2. Unit cell of α-Mg4[B2O(SO4)6] viewed along [001̅] (borate tetrahedra: green, sulfate tetrahedra: yellow, and magnesium-centered octahedra: red).

Figure 4. Anion [B2O(SO4)6]8− (B: green, S: yellow, O: red); ellipsoids are set to 70% probability.

Alternating staggered and eclipsed conformations of the anions are also present in the closely related crystal structure of Ti4[Si2O(PO4)6].26 Furthermore, a similar structure type has been found in borophosphate chemistry for the compound V2[B(PO4)3]27 in which the B2O7 backbone is formally replaced by two noncondensed BO3 triangles as already described for the α-polymorph and A2[B(PO4)3] (A = Cr, Fe, In). The thermal stability of β-Mg4[B2O(SO4)6] and a possible transformation to the α-phase was checked by a unit cell determination of a single crystal at −173 °C (Table S15 in the Supporting Information), as well as TPXRD up to 450 °C (Figure S1 in the Supporting Information). In both cases, the compound stays in the β-polymorph. α-Mg4[B2O(SO4)6]

shows a slightly higher density compared with β-Mg4[B2O(SO4)6] and should therefore be the more stable polymorph according to Ostwald’s rule; this seems reasonable as a staggered arrangement of oxygen atoms within the B2O7 backbone should be more favorable than an eclipsed one. Mg[B2(SO4)4] crystallizes in C2/c showing a further new structure type (Figure 7). The lower boron-to-sulfur ratio of 1:2 with respect to Mg4[B2O(SO4)6] indicates a higher condensation degree of the anionic substructure.2 Indeed, the structure contains infinite layers of condensed supertetrahedra B(SO4)4 and hence shows a phyllosilicate topology according to Liebau’s formalism.17 The layer is build up by alternating corner-sharing borate and sulfate tetrahedra forming vierer and zwölfer rings (Figure 8). Each sulfate E


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Inorganic Chemistry

Figure 8. Layers in Mg[B2(SO4)4] with vierer and zwölfer rings; the fundamental building unit is marked with red lines. Figure 5. Octahedral coordination sphere leading to Mg2O9 dimers; the Mg atoms are shifted out of center due to a metal−metal repulsion.

arrangement of the respective rings.10 Furthermore, a layered structure has been reported for B2S2O9.6 According to a B:S = 1:2 ratio, the TX formalism introduced in the Introduction would point to a three-dimensional network structure for the anion. However, the alternating condensation of two neighboring TX4 supertetrahedra via common edges (as in the vierer rings) and common corners (as in the zwölfer rings) shows both connecting pattern of threedimensional network like in SiO219 and the one-dimensional chain like in SiS2.8 Hence, a layered anion is possible even with this high condensation degree. Crystallographic Relationships. The symmetry relations between the crystal structures of the borophosphates Cr2[B(PO4)3] and the borosulfates α-Mg4[B2O(SO4)6] and βMg4[B2O(SO4)6], respectively, can be illustrated best by a group−subgroup relation scheme following the Bärnighausen formalism (Figure S1 in the Supporting Information).28 Cr2[B(PO4)3] crystallizes in space group P63/m (No. 176). The structure of α-Mg4[B2O(SO4)6] can be derived by a symmetry descent to the translationengleiche subgroup P3̅ (No. 147). Hence, the unoccupied special Wyckoff site 2b halfway between adjacent triangular BO3 units splits into the sites 1a and 1b, enabling an ordered occupation of site 1b by bridging oxygen atoms to form the B2O7 backbones of the anions. Additionally, the loss of the mirror plane allows the boron atoms to move out of the triangular plane to act as the center of the BO4 tetrahedra. From this crystal structure, the βmodification is derived by a further symmetry descent to the isomorphic subgroup P3̅ according to the transformation 2aα + bα, −aα + bα, cα. Within this enlarged unit cell in the a−b plane, the significant shift of oxygen atom O24 indicates the change of two-thirds of the staggered anions to the eclipsed ones (Figure S2 in the Supporting Information). Electrostatic Calculations. The electrostatic reasonability of the crystal structures was checked by calculations based on the MAPLE concept.29−31 Therefore, the MAPLE values of the new compounds were calculated and compared to the sum of the MAPLE values of respective metal sulfates and boron oxide sulfate (Table 1). The deviation for all presented structures is below 1%, which is the benchmark for electrostatic consistency. Furthermore, the calculations confirm the oxidation state +II for the respective metal ions. By comparison of the magnesium borosulfates, the βMg4[B2O(SO4)6] reveals a slightly smaller MAPLE value, which points to the fact that this is the slightly less stable modification. Infrared Spectroscopy. Figure 9 displays the infrared spectra of α-M4[B2O(SO4)6] (M = Mn, Co, Ni, Zn) and βMg4[B2O(SO4)6] (full spectra can be seen in Figure S3). All

Figure 6. Unit cell of β-Mg4[B2O(SO4)6] viewed along [001̅] (color code as in Figure 2).

Figure 7. Crystal structure of Mg[B2(SO4)4] along [01̅0] (color code as in Figure 2).

tetrahedron shares only two corners with borate tetrahedra, orientating the terminal oxygen atoms toward the layer surface. All tetrahedra can be classified as regular ones, with a deviation from the tetrahedral symmetry of 0.15−0.17% for the sulfate and 0.23% for the borate ones.14,15 The Mg2+ cations are located in between the layers on the Wyckoff site 4e; they are octahedrally coordinated with each three oxygen atoms of adjacent layers. The Mg−O distances range between 202 and 205 pm, which is in accordance with the sum of the ionic radii (207 pm).22 A rather similar structure has recently been published for Ca[B2(SO4)4], however, with a different F


Article

Inorganic Chemistry

the ratios Δ/B = 0.83 and C/B = 4.3, a Racah parameter B = 859 cm−1, and a ligand field splitting Δ = 713 cm−1. The spin-allowed transitions for α-Ni4[B2O(SO4)6] were recorded at λ = 433 nm (3A2 → 3T1(P)) and the position of the 3A2(F) → 3T1(F) transition could be assigned by a Gaussian fit to λ = 820 nm (Figure 11). Furthermore, three

Figure 9. Infrared spectra of α-M4[B2O(SO4)6] (M = Mn, Co, Ni, Zn) and β-Mg4[B2O(SO4)6].

relevant bands can be seen between 400 and 1450 cm−1. Bands between 1150 and 1420 cm−1 may be assigned to the asymmetric stretching vibrations of the SO4 tetrahedra; the asymmetric stretching vibrations of the BO4 tetrahedra probably occur between 980 and 1080 cm−1. Stretching vibrations of the S−O−B and B−O−B bridges are found at 750 and 820 cm−1, respectively, while bending vibrations of the BO4 and SO4 tetrahedra occur in the region between 430 and 660 cm−1.4,6,10 The simultaneous presence of a staggered and eclipsed conformation of the anion in β-Mg4[B2O(SO4)6] is reflected in a splitting of vibrational bands in the respective regimes. Optical Spectroscopy. The transition metal ions are present in the oxidation state +II in an octahedral ligand field, and we discuss the absorption spectra of α-Co4[B2O(SO4)6] and α-Ni4[B2O(SO4)6] here. The absorption spectrum of α-Co4[B2O(SO4)6] shows two intense transitions which can be assigned to the spin-allowed transitions 4T1(F) → 4T1(P) peaking at λ = 544 nm and 4 T1(F) → 4A2(F) at λ = 742 nm (Figure 10). The shoulder at λ = 481 nm occurs due to the spin-forbidden transition 4T1(F) → 2A2(F). These transitions lead to a high reflectance around 620 and 420 nm; it is thus in accordance with the pink body color of the compound. A Tanabe-Sugano analysis38−40 yields

Figure 11. Reflection spectrum (black) of Ni4[B2O(SO4)6] with a Gaussian fit (purple: 1E(G), 1T2(G), blue: 3T1(P), orange: 1T2(D), 1 A1(G), cyan: 1E(D), green: 3T1(F)) of the respective transitions and the overall sum of the fit (red).

spin-forbidden transitions can be identified at λ = 679 nm (3A2 → 1E), λ = 499 nm (3A2 → 1T2(D), 1A1(G)), and λ = 322 nm ( 3 A 2 → 1 E(G), 1 T 2 (G)) which is common for Ni 2+ compounds.41 The high reflectance between 530 and 620 nm causes the yellow body color of the compound. According to the ratios Δ/B = 0.92 and C/B = 4.8, a Racah parameter B = 799 cm−1 and a ligand field splitting Δ = 735 cm−1 results. Both, α-Co4[B2O(SO4)6] and α-Ni4[B2O(SO4)6], show a weak ligand field splitting in comparison to compounds like CoCl2 (Δ = 750 cm−1),42 β-CoSO4 (Δ = 730 cm−1),43 and NiCl2 (Δ = 755 cm−1).42 Hence, borosulfate ligands are weakly coordinating, which is expected due to the condensation of tetrahedra containing highly charged central atoms (S6+, B3+). Further details of the assignments of the observed bands can be found in the Supporting Information (Table S16). The absorptions of α-Mn4[B2O(SO4)6] werepresumably due to the almost regular octahedral surrounding and a thus strongly applying parity selection ruletoo weak; therefore, we performed a magnetic measurement on this compound to confirm the divalent state of the cation. Magnetic Measurements. To confirm the assumed oxidation state +II, the magnetic susceptibility of α-Mn4[B2O(SO4)6] was measured between 1.8 and 400 K at a magnetic field of 1000 Oe (Figure 12). The Curie−Weiss law is obeyed above 15 K with a Curie constant of C = 4.07 mol emu−1 K−1 and a Curie temperature of ΘP = −19 K pointing toward weak antiferromagnetic interactions. The corresponding effective magnetic moment of the Mn2+ ions is μeff = 5.71 μB and fits well to the theoretically expected value of μeff = 5.92 μB for a high-spin state of Mn2+, confirming the postulated oxidation state of +II.44 Thermal Analysis. The thermal stability of the title compounds was investigated using the example of β-Mg4[B2O(SO4)6] via TGA (Figure 13). The compound starts to decompose around 500 °C by presumable loss of two units of

Figure 10. Reflection spectrum (black) of α-Co4[B2O(SO4)6] with a Gaussian fit (blue: 2A2(F), green: 4T1(P), pink: 4A2(F)) of the respective transitions and the overall sum of the fit (red). G


Article

Inorganic Chemistry

Mg4[B2O(SO4)6], and Mg[B2(SO4)4]. Out of this series, the transition metal compounds and β-Mg4[B2O(SO4)6] could be obtained as phase-pure materials so far. In total, we identified three new structure types, some of which are in close crystallographic relationship to previously described ones in transition metal borophosphate chemistry. Moreover, we could prove the oxidation states of the transition metal compounds by a combination of single-crystal X-ray diffraction, electrostatic calculations, a magnetic measurement, andmost interestingoptical spectroscopy. The latter revealed nicely that borosulfates indeed provide weakly coordinating host structures, comparable to those of pure sulfates. Looking at the synthetic approaches and the respective temperatures employed within a quaternary chemical system demonstrates that the only difference between members of this system are normally stable binary and ternary compounds like SO3 or simple sulfates. These are usually formed upon heating, which explains the chemical relationships between compounds in the same chemical system. Depending on the stable compounds formed, a polymerization or a depolymerization of the borosulfate’s anion is observed. For instance, Mg4[B2O(SO4)6] is synthesized best at a maximum temperature of 350 °C, while the so far highest yield of Mg[B2(SO4)4] was achieved at 200 °C. The formal chemical difference between both are the stable binary compounds B2O3 and SO3; thus, Mg[B2(SO4)4] apparently only is an intermediate species toward the Mg4[B2O(SO4)6] phases. Therefore, it would be surprising if all of these compounds could be obtained as phase-pure compounds as these in some cases only act as precursors for the more stable ones. This also explains the polymerization and depolymerization relationships mentioned in the Introduction, when the polymerization of Li[B(S2O7)2] to Li[B(SO4)2] yields SO3, and the depolymerization of K3[B(SO4)3] to K5[B(SO4)4] is formally attended by the formation of K2SO4, SO3, and B2O3; both processes were experimentally observed. In the case of α-Mg4[B2O(SO4)6] and β-Mg4[B2O(SO4)6], the driving force behind the transition remains unclear as the α-polymorph only seems to be stable in the mother liquor and transforms readily into the β-polymorph upon drying; further investigations to elucidate this question are in train and will be presented elsewhere.

Figure 12. Temperature dependence of the inverse magnetic susceptibility of Mn4[B2O(SO4)6].

Figure 13. Thermogram of β-Mg4[B2O(SO4)6] until 1200 °C under nitrogen atmosphere.

SO3 per formula unit β-Mg4[B2O(SO4)6] according to reaction scheme (2): β‐Μg4[B2O(SO4 )6 ] → 4MgSO4 + B2O3 + 2SO3

(2)

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The expected mass loss of Δmtheo = 22.7% is very well reproduced by the experiment (Δm obs = 22.7%). A decomposition of the borosulfate to the respective sulfates and amorphous B2O3 was already observed for Ba[B2O(SO4)3]3 and was confirmed by powder XRD (Figure S5 in the Supporting Information). The second mass loss is presumably due to the decomposition of MgSO4 to MgO, which in situ reacts with the present B2O3 to Mg3B2O6 (Figure S6).45 The obtained mass loss of Δmobs = 44.5% is in very good accordance with the theoretically calculated one (Δmtheo = 45.0%).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01234. Synthetic procedure of the metal oxides, crystallographic tables of all presented compounds, TPXRD of βMg4[B2O(SO4)6], group-subgroup scheme, full IR spectra, assignment of optical transitions in Co4[B2O(SO4)6] and Ni4[B2O(SO4)6], TGA of α-M4[B2O(SO4)6], XRD patterns of the decomposed α-Mg4[B2O(SO4)6] after 700 and 1200 °C (PDF)

4. CONCLUSION The emphasis of our seventh publication on borosulfates lies on the extension of borosulfate chemistry toward the optically interesting divalent transition metal ions of Mn, Co, Ni as well as similarly sized ions Zn2+ and Mg2+. The second focus is on the optical properties of the obtained transition metal borosulfates and a third on an unexpected polymorphism within the series of magnesium compounds. In this contribution, we determined the isotypic crystal structures of α-M4[B2O(SO4)6] (M = Mg, Mn, Co, Ni, Zn), β-

Accession Codes

CCDC 1842794−1842800 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. H


Article

Inorganic Chemistry

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Henning A. Höppe: 0000-0002-8734-8258 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dana Vieweg (EP V, Universität Augsburg) for recording the magnetic measurements. Moreover, the authors thank the Deutsche Forschungsgemeinschaft (DFG) for financial support under the project HO 4503/5-1. P.N. thanks the Fonds der Chemischen Industrie (FCI) for a Ph.D. fellowship.

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Synthesis and Characterization of the First Borosulfates of Magnesium

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