The new high-pressure borate Co7B24O42(OH)2·2H2O—Formation of edge-sharing BO4 tetrahedra in a hydrated borate

Recent Progress in Crystalline Borates with Edge-Sharing BO4 Tetrahedra

Crystalline borates have received great attention due to their various structures and wide applications. For a long time, the corner-sharing B-O unit is considered a basic rule in borate structural chemistry. The Dy₄B₆O₁₅ synthesized under high-pressure is the first oxoborate with edge-sharing [BO₄] tetrahedra, while the KZnB₃O₆ is the first ambient pressure borate with the edge-sharing [BO₄] tetrahedra. The edge-sharing connection modes greatly enrich the structural chemistry of borates and are expected to expand new applications in the future. In this review, we summarize the recent progress in crystalline borates with edge-sharing [BO₄] tetrahedra. We discuss the synthesis, fundamental building blocks, structural features, and possible applications of these edge-sharing borates. Finally, we also discuss the future perspectives in this field.

Toward the Coordination Fingerprint of the Edge-Sharing BO4 Tetrahedra

The K₃Sb₄BO₁₃ (KSBO) material undergoes an uncommon symmetry increase upon cooling, from triclinic symmetry at room temperature to monoclinic symmetry at low temperature. The first-order phase transition is accompanied by shrinkage of the unit cell, resulting in the transformation of every pair of head-to-tail triangular BO₃ groups into one B₂O₆ unit featuring unique edge-sharing BO₄ tetrahedra. This is the first material with B₂O₆ units formed through temperature lowering and exhibiting a B-O anionic framework composed uniquely of isolated edge-sharing BO₄ tetrahedra. Several techniques including single-crystal X-ray diffraction experiments, Raman and ¹¹B magic-angle-spinning NMR spectroscopies, and, for the first time, B K-edge electron energy loss spectroscopy were used to evidence the rare and discrete B₂O₆ units. The complete transformation of BO₃ units into B₂O₆ units makes the KSBO compound the perfect candidate to extract information about B₂O₆ units whose signal can be unambiguously assigned.

Oxidative dehydrogenation of light alkanes to olefins on metal-free catalysts

Metal-free boron- and carbon-based catalysts have shown both great fundamental and practical value in oxidative dehydrogenation (ODH) of light alkanes. In particular, boron-based catalysts show a superior selectivity toward olefins, excellent stability and atom-economy to valuable carbon-based products by minimizing CO2 emission, which are highly promising in future industrialization. The carbonaceous catalysts also exhibited impressive behavior in the ODH of light alkanes helped along by surface oxygen-containing functional groups. This review surveyed and compared the preparation methods of the boron- and carbon-based catalysts and their characterization, their performance in the ODH of light alkanes, and the mechanistic issues of the ODH including the identification of the possible active sites and the exploration of the underlying mechanisms. We discussed different boron-based materials and established versatile methodologies for the investigation of active sites and reaction mechanisms. We also elaborated on the similarities and differences in catalytic and kinetic behaviors, and reaction mechanisms between boron- and carbon-based metal-free materials. A perspective of the potential issues of metal-free ODH catalytic systems in terms of their rational design and their synergy with reactor engineering was sketched.

Pb2.28Ba1.72B10O19 featuring a three-dimensional B–O anionic network with edge-sharing [BO4] obtained under ambient pressure

The unique borate Pb_2.28Ba_1.72B₁₀O₁₉ containing a three-dimensional B–O anionic network with edge-sharing [BO₄] was obtained under ambient pressure, featuring an extremely large B–B interatomic distance (d_B–B) and small internal O–B–O angles (∠_{in(O–B–O)}).

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

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.

Ni6B22O39·H2O – extending the field of nickel borates

Ni6B22O39·H2O was synthesized in a high-pressure/high-temperature reaction at 5 GPa/900°C. It crystallizes in the orthorhombic space group Pmn21 (no. 31) with the lattice parameters a=7.664(2), b=8.121(2) and c=17.402(2) Å. The crystal structure is discussed with regard to the isotypic compounds M 6B22O39·H2O (M=Fe, Co) and the structurally related phase Cd6B22O39·H2O. Furthermore, the characterization of Ni6B22O39·H2O via X-ray powder diffraction and vibrational spectroscopy is reported.

β-Y(BO2)3 – a new member of the β-Ln(BO2)3 (Ln=Nd, Sm, Gd–Lu) structure family

β-Y(BO2)3 was synthesized in a Walker-type multianvil module at 5.9 GPa/1000°C. The crystal structure has been elucidated through single-crystal X-ray diffraction. β-Y(BO2)3 crystallizes in the orthorhombic space group Pnma (no. 62) with the lattice parameters a =15.886(2), b =7.3860(6), and c =12.2119(9) Å. Its crystal structure will be discussed in the context of the isotypic lanthanide borates β-Ln(BO2)3 (Ln =Nd, Sm, Gd–Lu).

High-Pressure Synthesis and Characterization of the Ammonium Yttrium Borate (NH4)YB8O14

The first high-pressure yttrium borate (NH₄)YB₈O₁₄ was synthesized at 12.8 GPa/1300 °C using a Walker-type multianvil module. The compound crystallizes in the orthorhombic space group Pnma (no. 62) with the lattice parameters a = 17.6375(9), b = 10.7160(5), and c = 4.2191(2) Å. (NH₄)YB₈O₁₄ constitutes a novel structure type but exhibits similarities to the crystal structure of β-BaB₄O₇. X-ray single-crystal and powder diffraction, EDX, vibrational spectroscopy as well as quantum chemical calculations were used to characterize (NH₄)YB₈O₁₄.

Solvent-controlled syntheses of mixed-alkali-metal borates exhibiting UV nonlinear optical properties

A series of mixed-alkali-metal borates have been solvothermally synthesized by using various solvents.