K2[Ga(B5O10)]·4H2O: The First Chiral Zeolite-like Galloborate with Large Odd 11-Ring Channels

Galloborates as ultraviolet nonlinear optical crystals: advances and perspectives

Metal borates are excellent source materials for exploring short-wavelength nonlinear optical (NLO) crystals. Galloborates show rich structural chemistry with various coordination configurations of Ga cation and B-O anionic units and are suitable candidates as ultraviolet NLO crystals. Up to now, the shortest cut-off edge of galloborates was reported to be down to 190 nm in KCs2Ga(B5O10)(OH), while the largest second harmonic generation (SHG) effect of galloborates was reported to be up to 4.6 times that of KH2PO4 (KDP) in Na5Ga[B7O12(OH)]2·2B(OH)3. Herein, we give a detailed summary of the recent progress in NLO inorganic galloborates, where these galloborates are grouped into two types in terms of their compositions: (1) alkali/alkaline earth metal galloborates and (2) alkali/alkaline earth metal galloborate halides. We discuss their structural features, band gaps, and SHG intensities. Finally, we give future perspectives in this field.

Magnesium cations as templates for the self-assembly of supramolecular luminescent {Mg@[B18φ34-35]}-clusters

Solvothermal reaction of magnesium nitrate and boron oxide in N,N-dimethylformamide produced a number of particularly complex supramolecular magnesium borates. Five topologically different types of negatively charged {Mg@[B18φ34-35]}-clusters, φ = O, OH, were observed with the magnesium cation as a core and octadecaborate anions as shells. The clusters assemble via common borate polyhedra forming 1D chains, a 2D mesoporous layer, and 3D mesoporous frameworks with an effective channel width of up to 16 Å. Topological analysis of the clusters in combination with the modular crystallography approach indicates that numerous new functional materials can be obtained by varying their assembly mode. At least one compound containing such clusters exhibits a very strong luminescence.

Synthesis and optical properties of N→Ga coordinated gallium boroxines

Reactions of the N,C,N-chelated organogallium amide LGa(NEt₂)₂ (1), where L is {2,6-(Me₂NCH₂)₂C₆H₃}^-, with organoboronic acids RB(OH)₂ yielded molecular gallium boroxines LGa(O₃B₂R₂) (2: R = OH, 3: R = Ph, 4: R = 4-MeO-C₆H₄, 5: R = 4-CHO-C₆H₄, 6: R = Fc), neutral analogues of gallaborates. The molecular structures revealed the presence of a six-membered central GaB₂O₃ ring. The film forming properties of 5 allowed the deposition of transparent thin films by a spin coating method. The thicknesses, refractive index, energy of the optical gap (Eoptg), activation energy of surface electrical conductivity (Esa) and pre-exponential factor (σ₀) of the thin layers of 5 were measured and they are close to those found for related oxygen glass. Finally, GBO 5 was also used as an additive to printing ink and a thin film of 5 was prepared by the gravure printing technique.

Crystal structure and pale-yellow emitting properties of K3Gd1-xDyxB6O12 solid solutions

A new potassium dysprosium polyborate, K₃DyB₆O₁₂, has been prepared via the high-temperature molten salt method and structurally characterized by single-crystal X-ray diffraction analysis. The structure can be described as a three-dimensional framework composed of isolated bicyclic [B₅O₁₀]^5- groups and Dy³⁺ and K⁺ ions. The Fourier transform IR (FT-IR) and ultraviolet-visible (UV-Vis) spectra were investigated. A series of K₃Gd_1-xDy_xB₆O₁₂ phosphors was prepared and their photoluminescence properties were studied. The K₃Gd_1-xDy_xB₆O₁₂ phosphors exhibit a strong yellow emission band at 577 nm (the ⁴F_9/2→⁶H_13/2 transition of Dy³⁺) under UV excitation of 275 nm (the ⁸S_7/2→⁶I_J transition of Gd³⁺), suggesting the occurrence of the energy transfer Gd³⁺→Dy³⁺. The optimized doping concentration of the Dy³⁺ ion was 8 mol%. We may expect that K₃Gd_1-xDy_xB₆O₁₂ is a promising pale-yellow emission phosphor for visual displays or solid-state lighting.

Why Some Noncentrosymmetric Borates Do Not Make Good Nonlinear Optical Materials: A Case Study with K3B5O8(OH)2

Synthesis of deep-ultraviolet (DUV) transparent materials, especially those with noncentrosymmetric structures, remains a great challenge in the solid-state chemistry community. A new DUV transparent borate, K₃B₅O₈(OH)₂, was discovered with a short absorption edge below 200 nm. The title compound crystallizes in a noncentrosymmetric, polar space group, Fdd2 (point group mm2), with the following cell parameters: a = 13.736(9) Å, b = 19.317(12) Å, c = 7.606(5) Å. The structure of K₃B₅O₈(OH)₂ features a 3D framework composed of [B₅O₈(OH)₂]^3- basic building units that are linked by contacts with K⁺ cations and O-H···O hydrogen bonds. Second harmonic generation (SHG) measurements were performed, and an SHG efficiency of 0.5 × SiO₂ was observed. Symmetry dictates that the χ₁₄ component of the macroscopic NLO susceptibility is equal to zero in mm2, which prevents the maximal component of the microscopic NLO susceptibility for [B₅O₈(OH)₂]^3- units (χ₁₄) from contributing to the macroscopic NLO susceptibility of the crystal and therefore limits the SHG efficiency. In contrast, large SHG effects can be observed from compounds containing [B₅O₁₀] units that crystallize in point groups with nonzero χ₁₄, such as 222. These findings provide insight into understanding the relationship between crystal structure and SHG efficiency in [B₅O₁₀]-based compounds and discovering other borate-based DUV materials.

RE(SO4)[B(OH)4](H2O), RE(SO4)[B(OH)4](H2O)2, and RE(SO4)[B(OH)4](H2O)·H2O: Rare-Earth Borate-Sulfates Featuring Three Types of Layered Structures

Using hydrothermal reactions, three series of rare-earth borate-sulfates, namely, RE(SO₄)[B(OH)₄](H₂O) (RE = La (1), Sm (2), Eu (3)), RE(SO₄)[B(OH)₄](H₂O)₂ (RE = Pr (4), Nd (5), Sm (6), Eu (7), Gd (8)), and RE(SO₄)[B(OH)₄](H₂O)·H₂O (RE = Tb (9), Dy (10), Ho (11), Er (12), Tm (13), Yb (14), Lu (15), Y (16)), have been synthesized, which represent the first rare-earth borate-sulfate mixed-anion compounds. All these compounds possess the same fundamental building anionic units of SO₄ and B(OH)₄ tetrahedra; however, they exhibit three different types of two-dimensional (2D) layered structures composed of 1D RE-B-O and RE-S-O chains. The rare-earth borate chains are similar in all compounds, while the rare-earth sulfate chains differ in each type of compound due to the various coordination modes of sulfate groups. On the basis of the measured UV-vis diffuse reflectance spectra, the optical band gaps of compounds 2, 3, 6, and 7 are estimated to be 4.66, 4.53, 4.62, and 4.50 eV, respectively. Luminescence studies show that compounds 2, 3, 6, and 7 exhibit strong emission in the orange or red regions. Furthermore, thermal analysis and magnetic susceptibility measurements for these four representative compounds have also been performed.

NH4B11O16(OH)2: a new ammonium borate with wavy-shaped polycyclic 2∞[B11O16(OH)2] layers

The ammonium borate NH₄B₁₁O₁₆(OH)₂ has been successfully synthesized by a mild hydrothermal method.

Ln2Ga[B3O6(OH)]2[B7O9(OH)2](CH3CO2)2 (Ln = Y, Sm, Eu, Gd, Dy): A Series of Lanthanide Galloborates Decorated by Acetate Anions

The first examples of mixed-anion lanthanide galloborates, namely, Ln2Ga[B3O6(OH)]2[B7O9(OH)2](CH3CO2)2 [Ln = Y (1), Sm (2), Eu (3), Gd (4), Dy (5)], have been obtained through hydrothermal synthesis. The title compounds are isomorphic and belong to monoclinic space group C2/c (No. 15). Their structures possess [B7O13(OH)2] borate layers further bridged with [B3O7] clusters to give a three-dimensional (3D) borate framework displaying two types of rhombus-like B14O14 14-membered-ring (14-MR) channels along the b axis. The Ga(3+) ions are octahedrally coordinated and located at one end of the B14O14 14-MR channels, forming small tunnels of B7Ga 8-MRs, which are filled by the Ln(III) ions. The Ln ions and Ga cations are further held together by bridging acetate anions. It is worth noting that in these compounds there are two different types of borate clusters and two types of anions that are uncommon in the borates reported. Luminescent studies revealed the characteristic emission bands of Ln ions for compounds 2-5, and the luminescent lifetimes are 3.6, 0.86, and 3.05 ns for compounds 2, 3, and 5, respectively. Magnetic measurements suggest that there are antiferromagnetic interactions between magnetic centers for compounds 2-5.

Structure modulations in nonlinear optical (NLO) materials Cs(2)TB4O9 (T = Ge, Si)

Incommensurately modulated borate structures of a new type were studied in detail in the nonlinear optical (NLO) materials Cs(2)TB4O9 (T = Ge, Si) using single-crystal X-ray diffraction techniques. The structures were solved by the charge-flipping algorithm in the superspace group I2(αβ0)0. The refinement results strongly suggest that the main structure modulation feature of Cs(2)TB4O9 is the ordering of the O atoms. With these modulated structure models, the unreasonable B-O distances in the average structures were explained as the ordering of BO4 and BO3.

N→Sn-Coordinated Stannaoxidoborates Containing a SnB4O6 Unit

We report here the synthesis of new N→Sn-coordinated stannaoxidoborates H[LSnB4O6R4] {L = [2,6-(Me2NCH2)C6H3](-) and R = Ph (6), 4-Br-Ph (7), 3,5-(CF3)2-Ph (8), and 4-CHO-Ph (9)} containing a nonsymmetric SnB4O6 unit. Compounds 6-9 represent new derivatives of the pentaborates [B5O6R4](-) in which the central boron is substituted by a tin atom. Compounds 6-9 were characterized by means of elemental analysis, electrospray ionization mass spectrometry, and NMR spectroscopy and in the case of 6-8 also by single-crystal X-ray diffraction analysis. The structures of N→Sn-coordinated stannaoxidoborates 6-8 consist of a spirobicyclic arrangement, with two six-membered SnB2O3 rings at the tin atom providing the new stannaoxidoborate [LSnB4O6R4](-) motif, which is compensated for by the proton atom coordinated to the Me2N group of the ligand L. The linear and thermal properties of 6-9 were studied with the help of electronic absorption spectra and differential scanning calorimetry. In addition, the presence of the nonsymmetric stannaoxidoborate SnB4O6 unit in 6, 7, and 9 prompted us to investigate their second-order nonlinear-optical properties.

Zn(1,3-DAP)[B4O7]: a rare chiral zeolitic framework constructed of four-connected [B4O9] clusters with a single-stranded helical channel

A zinc borate Zn(1,3-DAP)[B₄O₇] (1) was prepared. 1 possessed a rare chiral zeolitic framework based on a [B₄O₉] cluster. The combination of a [B₄O₉] cluster and Zn(1,3-DAP)O₃ chain produced a helical ribbon motif and gave rise to a single-stranded helical channel.

Cotemplating synthesis of a rare borate Ni(en)3·Hen·[B9O13(OH)4]·H2O with a chiral polyanonic chain constructed from two different clusters

Ni(en)3·Hen·[B9O13(OH)4]·H2O was prepared under the cotemplating effect of [Ni(en)3](2+) and Hen(+). It was the first example of a cotemplated borate with an unusual chiral polyanionic chain, which was constructed from [B5O8(OH)2] and [B4O7(OH)2] clusters.

Two novel metal borates with three-dimensional open-framework layers constructed from [M2B8O20(OH)2] (M = Al, Ga) cluster units

Two novel metal borates, Ba[MB4O8(OH)]·H2O (M = Al/Ga), with a three-dimensional open-framework layer have been synthesized under hydro(solvo)thermal conditions, in which the Al(3+)/Ga(3+) ions have been successfully introduced into the oxoboron cluster backbone and further linked together to form the open-framework layer composed of the [M2B8O20(OH)2] cluster units. The half-unit [MB4O11(OH)] can be viewed as analogous to the known [B5O9(OH)] cluster. Interestingly, the rare M-O-M linkages play a crucial role in forming the open-framework layer.

β-BaGa[B4O8(OH)](H2O) and Ba4Ga[B10O18(OH)5](H2O): new barium galloborates featuring unusual [B4O8(OH)]5− and [B10O18(OH)5]11− clusters

Centrosymmetric β-BaGa[B₄O₈(OH)](H₂O) with a layered structure and polar Ba₄Ga[B₁₀O₁₈(OH)₅](H₂O) with a novel 3D framework structure have been reported.

Incorporating [GaB4O11(OH)]8−clusters to construct luminescent galloborate with open-framework layers

A luminescent 5-connected galloborate with open-framework layers has been obtained under mild hydrothermal conditions by incorporating [GaB₄O₁₁(OH)]⁸⁻clusters.

Exploratory investigation of new SHG materials based on galloborates

Three new galloborates, namely, GaB(5)O(8)(OH)(2)(en)(2)·H(2)O (1), LiGa(OH)(BO(3))(H(2)O) (2), and Rb(2)Ga(B(5)O(10))(H(2)O)(4) (3), have been synthesized by hydrothermal reactions. Compound 1 is the first example of a galloborate that contains an organic component. It crystallizes in space group P2(1)/c, and its crystal structure exhibits an infinite zigzag chain consisting of [B(5)O(8)(OH)(2)](3-) anions and GaO(2)N(4) octahedra interconnected via corner sharing. Compound 2 crystallizes in space group P31c with a layered structure composed of GaO(4), LiO(4), and BO(3) building units. Compound 3 belongs to chiral space group C222(1); the basic building blocks of the structure are the [B(5)O(10)](5-) cluster anion and GaO(4) tetrahedron, which are interconnected to form a three-dimensional network with tunnels of Ga2B6 eight-membered rings (8-MRs) which are filled by Rb(+) cations and lattice water molecules. Interestingly, Rb(2)Ga(B(5)O(10))(H(2)O)(4) displays a moderate second-harmonic generation (SHG) response comparable to that of KH(2)PO(4) (KDP), and it is phase matchable. Band structure and optical property calculations for Rb(2)Ga(B(5)O(10))(H(2)O)(4) based on DFT methods were also performed.

Synthesis and structure of QD-6: a novel aluminoborate constructed from unprecedented [B@Al6O24] and polyborate clusters

A novel aluminoborate (NH(4))(6)[C(5)NH(12)](6)[Al(12)B(65)O(105)(OH)(33)]·(H(2)O)(15) (QD-6), has been synthesized under mild hydrothermal conditions and characterized by IR, elemental analysis, TGA, powder and single-crystal X-ray diffractions. This compound crystallizes in the rhombohedral space group R3 (No. 148) with the lattice constants a = 23.7421(2) Å, c = 24.7699(3) Å, V = 12091.9(2) Å(3), and Z = 3. QD-6 consists of two unprecedented aluminoborate clusters, [Al(6)B(34)O(54)(OH)(18)](6-) and [Al(6)B(31)O(51)(OH)(15)](6-), which are built from the same hexagon-like [B@Al(6)O(24)] clusters and [B(11)O(17)(OH)(6)] or [B(10)O(16)(OH)(5)] polyborates.