Karolina Kazmierczak

The First Borosulfate K5[B(SO4)4]

Sulfate anions SO4 2 do not show a strong tendency to form condensed oligoanions owing to the high formal charge of sulfur. A recent very interesting example for an oligosulfate is ReO2Cl(S2O7). [1] Disulfate tetrahedra share common corners with an adjacent ReO6 octahedron forming a cyclic moiety. Regarding oligosulfates, only chainlike anions, typically S3O10 2 or even S5O16 2 , were described. In K5[B(SO4)4], sulfate tetrahedra avoid direct condensation but form the unprecedented anion [B(SO4)4] 5 shown in Figure 1. In

Exploring a New Structure Family: Alkali Borosulfates Na5[B(SO4)4], A3[B(SO4)3] (A = K, Rb), Li[B(SO4)2], and Li[B(S2O7)2]

New alkali borosulfates were obtained by precipitation from oleum, solid-state reactions, or thermal decomposition. The crystal structures were characterized with single-crystal data. They are all based on corner-linked BO4 and SO4 tetrahedra with varying coordination of the alkali cations. According to the ratio of BO4 and SO4 tetrahedra, different frameworks are observed, i.e., noncondensed complex anions (1:4), one-dimensional chains (1:3), or three-dimensional (3D) networks (1:2). This is in analogy to silicates, where the ratio Si/O relates to the dimensionality also. For Na5[B(SO4)4], which exists in two different polymorphs, there are noncondensed pentameric units. Na5[B(SO4)4]-I: space group Pca21, a = 10.730(2) Å, b = 13.891(3) Å, c = 18.197(4) Å. Na5[B(SO4)4]-II: space group P212121, a = 8.624(2) Å, b = 9.275(2) Å, c = 16.671(3) Å. A3[B(SO4)3] (A = K, Rb) are isotypic with Ba3[B(PO4)3] adopting space group Ibca [K3[B(SO4)3], a = 7.074(4) Å, b = 14.266(9) Å, c = 22.578(14) Å; Rb3[B(SO4)3], a = 7.2759(5) Å, b = 14.7936(11) Å, c = 22.637(2) Å] with vierer chains of BO4tetrahedra based on two bridging and two terminal SO4 tetrahedra. Li[B(SO4)2] [space group Pc, a = 7.6353(15) Å, b = 9.342(2) Å, c = 8.432(2) Å, and β = 92.55(2)°] comprises a 3D network that is closely related to β-tridymite. Li[B(S2O7)2] [space group P212121, a = 10.862(2) Å, b = 10.877(2) Å, c = 17.769(4) Å] represents the first example of a disulfate complex with noncondensed [B(S2O7)2](-) units. Vibrational spectra were recorded from all compounds, and the thermal behavior was also investigated.

The Borosulfate Story Goes on—From Alkali and Oxonium Salts to Polyacids

The structural principles of borosulfates derived from the B/S ratio are confirmed and extended to new representatives of this class showing novel motifs. According to the composition, Na[B(S2O7)2] (P2(1)/c; a=10.949(6), b=8.491(14), c=12.701(8) Å; β=110.227(1)°; Z=4) and K[B(S2O7)2] (Cc; a=11.3368(6), b=14.662(14), c=13.6650(8) Å; β=94.235(1)°; Z=8) contain isolated [B(S2O7)2](-) ions, in which the central BO4 tetrahedron is coordinated by two disulfate units. The alkali cations have coordination numbers of 7 (Na) and 8 (K), respectively. The structure of Cs[B(S2O7)(SO4)] (P2(1)/c; a=10.4525(6), b=11.3191(14), c=8.2760(8) Å; β=103.206(1); Z=4) combines, for the first time, sulfate and disulfate units into a chain structure. Cs has a coordination number of 12. The same structural units were found in H[B(S2O7)(SO4)] (P2(1)/c; a=15.6974(6), b=11.4362(14), c=8.5557(8) Å; β=90.334(3)°; Z=8). This compound represents the first example of a polyacid. The hydrogen atoms were located and connect the chains to form layers through hydrogen-bonding bridges. H3O[B(SO4)2] (P4/ncc; a=9.1377(6), c=7.3423(8) Å; Z=4) is the first oxonium compound of this type to be found. The BO4 tetrahedra are linked by SO4 tetrahedra to form linear chains similar to those in SiS2. The chains form a tetragonal rod packing structure with H3O(+) between the rods. The structures of borosulfates can be classified following the concept described by Liebau for silicates, which was extended to borophosphates by Kniep et al. In contrast to these structures, borosulfates do not comprise B-O-B bonds but instead contain S-O-S connections. All compounds were obtained as colourless, moisture-sensitive single crystals by reaction of B2O3 and the appropriate alkali salt in oleum.