Transition-metal-like bonding behaviors of a boron atom in a boron-cluster boronyl complex [(η7-B7)-B-BO]−

Boron displays many unusual structural and bonding properties due to its electron deficiency. Here we show that a boron atom in a boron monoxide cluster (B₉O⁻) exhibits transition-metal-like properties. Temperature-dependent photoelectron spectroscopy provided evidence of the existence of two isomers for B₉O⁻: the main isomer has an adiabatic detachment energy (ADE) of 4.19 eV and a higher energy isomer with an ADE of 3.59 eV. The global minimum of B₉O⁻ is found surprisingly to be an umbrella-like structure (C_6v, ¹A₁) and its simulated spectrum agrees well with that of the main isomer observed. A low-lying isomer (C_s, ¹A′) consisting of a BO unit bonded to a disk-like B₈ cluster agrees well with the 3.59 eV ADE species. The unexpected umbrella-like global minimum of B₉O⁻ can be viewed as a central boron atom coordinated by a η⁷-B₇ ligand on one side and a BO ligand on the other side, [(η⁷-B₇)-B-BO]⁻. The central B atom is found to share its valence electrons with the B₇ unit to fulfill double aromaticity, similar to that in half-sandwich [(η⁷-B₇)-Zn-CO]⁻ or [(η⁷-B₇)-Fe(CO)₃]⁻ transition-metal complexes. The ability of boron to form a half-sandwich complex with an aromatic ligand, a prototypical property of transition metals, brings out new metallomimetic properties of boron.

The global minimum of the B₉O⁻ cluster is found to have an umbrella-like structure, where the central B atom exhibits transition-metal-like bonding properties, coordinated by a η⁷-B₇ ligand on one side and a BO ligand on the other.     

Reply to the ‘Comment on “Uncommon structural and bonding properties in Ag16B4O10” by A. Lobato,Miguel Á. Salvadó, and J. Manuel Recio,Chem. Sci., 2021, 12, DOI: 10.1039/D1SC02152D

Where are the excess electrons in Ag₁₆B₄O₁₀?

Ag₁₆B₄O₁₀ features an exotic scheme of chemical bonding and extends the growing family of subvalent silver oxides. These findings constitute a new general and intrinsic facet of the chemistry of silver, which has not been fully understood, yet, and definitely deserves to be analysed from different perspectives. Against this background, we distinctly appreciate the efforts made by A. Lobato, Miguel Á. Salvadó, and J. Manuel Recio (LSR) in studying these phenomena at the example of the title compound.¹ While the computational results presented in the Comment article well comply with those published in our original paper,² the interpretations follow different routes. Whereas LSR focus on the analogy of pattern of the Electron Localization Function (ELF) in position space in the title compound with those found in elemental silver, we interpreted the electronic structure of Ag₁₆B₄O₁₀, both in position and reciprocal space, also considering the interactions between cationic and anionic partial structures.     

Idiosyncratic Ag7Pt2O7: An Electron Imprecise yet Diamagnetic Small Band Gap Oxide

The seminal qualitative concepts of chemical bonding, as presented by Walter Kossel and Gilbert Newton Lewis back in 1916, have lasting general validity. These basic rules of chemical valence still serve as a touchstone for validating the plausibility of composition and constitution of a given chemical compound. We report on Ag₇Pt₂O₇, with a composition that violates the basic rules of chemical valence and an exotic crystal structure. The first coordination sphere of platinum is characteristic of tetravalent platinum. Thus, the electron count corresponds to Ag₇Pt₂O₇*e⁻, where excess electrons are associated with the silver substructure. Such conditions given, it is commonly assumed that the excess electrons are either itinerant or localized in Ag−Ag bonds. However, the material does not show metallic conductivity, nor does the structure feature Ag-Ag pairs. Instead, the excess electrons organize themselves in 2e−4c bonds within the silver substructure. This subvalent silver oxide reveals a new general facet pertinent to silver chemistry.     

A New Synthetic Methodology in the Preparation of Bimetallic Chalcogenide Clusters via Cluster-to-Cluster Transformations

A decanuclear silver chalcogenide cluster, [Ag₁₀(Se){Se₂P(OⁱPr)₂}₈] (2) was isolated from a hydride-encapsulated silver diisopropyl diselenophosphates, [Ag₇(H){Se₂P(OⁱPr)₂}₆], under thermal condition. The time-dependent NMR spectroscopy showed that 2 was generated at the first three hours and the hydrido silver cluster was completely consumed after thirty-six hours. This method illustrated as cluster-to-cluster transformations can be applied to prepare selenide-centered decanuclear bimetallic clusters, [Cu_xAg_10-x(Se){Se₂P(OⁱPr)₂}₈] (x = 0–7, 3), via heating [Cu_xAg_7−x(H){Se₂P(OⁱPr)₂}₆] (x = 1–6) at 60 °C. Compositions of 3 were accurately confirmed by the ESI mass spectrometry. While the crystal 2 revealed two un-identical [Ag₁₀(Se){Se₂P(OⁱPr)₂}₈] structures in the asymmetric unit, a co-crystal of [Cu₃Ag₇(Se){Se₂P(OⁱPr)₂}₈]_0.6[Cu₄Ag₆(Se){Se₂P(OⁱPr)₂}₈]_0.4 ([3a]_0.6[3b]_0.4) was eventually characterized by single-crystal X-ray diffraction. Even though compositions of 2, [3a]_0.6[3b]_0.4 and the previous published [Ag₁₀(Se){Se₂P(OEt)₂}₈] (1) are quite similar (10 metals, 1 Se²⁻, 8 ligands), their metal core arrangements are completely different. These results show that different synthetic methods by using different starting reagents can affect the structure of the resulting products, leading to polymorphism.     

Au3-to-Ag3 coordinate-covalent bonding and other supramolecular interactions with covalent bonding strength

An efficient strategy for designing charge-transfer complexes using coinage metal cyclic trinuclear complexes (CTCs) is described herein. Due to opposite quadrupolar electrostatic contributions from metal ions and ligand substituents, [Au(μ-Pz-(i-C₃H₇)₂)]₃·[Ag(μ-Tz-(n-C₃F₇)₂)]₃ (Pz = pyrazolate, Tz = triazolate) has been obtained and its structure verified by single crystal X-ray diffraction – representing the 1^st crystallographically-verified stacked adduct of monovalent coinage metal CTCs. Abundant supramolecular interactions with aggregate covalent bonding strength arise from a combination of M–M′ (Au → Ag), metal–π, π–π interactions and hydrogen bonding in this charge-transfer complex, according to density functional theory analyses, yielding a computed binding energy of 66 kcal mol⁻¹ between the two trimer moieties – a large value for intermolecular interactions between adjacent d¹⁰ centres (nearly doubling the value for a recently-claimed Au(i) → Cu(i) polar-covalent bond: Proc. Natl. Acad. Sci. U.S.A., 2017, 114, E5042) – which becomes 87 kcal mol⁻¹ with benzene stacking. Surprisingly, DFT analysis suggests that: (a) some other literature precedents should have attained a stacked product akin to the one herein, with similar or even higher binding energy; and (b) a high overall intertrimer bonding energy by inferior electrostatic assistance, underscoring genuine orbital overlap between M and M′ frontier molecular orbitals in such polar-covalent M–M′ bonds in this family of molecules. The Au → Ag bonding is reminiscent of classical Werner-type coordinate-covalent bonds such as H₃N: → Ag in [Ag(NH₃)₂]⁺, as demonstrated herein quantitatively. Solid-state and molecular modeling illustrate electron flow from the π-basic gold trimer to the π-acidic silver trimer with augmented contributions from ligand-to-ligand’ (LL′CT) and metal-to-ligand (MLCT) charge transfer.

A stacked Ag₃–Au₃ bonded (66 kcal mol⁻¹) complex obtained crystallographically exhibits charge-transfer characteristics arising from multiple cooperative supramolecular interactions.     

Comment on “Uncommon structural and bonding properties in Ag16B4O10” by A. Kovalevskiy,C. Yin,J. Nuss,U. Wedig,and M. Jansen,Chem. Sci., 2020, 11, 962

A thorough systematic study of the Electron Localization Function (ELF) in fcc silver metal, the deficient vacant-type Ag₁₆□₄ structure, and the Ag₁₆B₄O₁₀ title compound of the Chem. Sci., 2020, 11, 962 edge article leads to a further understanding of the sub-valent characteristics of silver in the silver borate compound. By visualizing the process in three consecutive steps, (fcc)_eq-Ag → (fcc)_ex-Ag → Ag₁₆□₄ → Ag₁₆B₄O₁₀, the electron reduction of Ag atoms can be traced to be due to (i) the expansion (ex) of the host metallic array from its equilibrium (eq) geometry and (ii) the vacancy creation and subsequent insertion of guest borate clusters. Our ELF analysis also allows us to identify to what extent metallic features remain in the title compound, providing an alternative explanation of why Ag₁₆B₄O₁₀ is not a conductor whereas pure silver is.

ELF superbasins (dark-yellow stick and ball circuits) reveal how borate clusters (green and red spheres) play the role of crystal defects interrupting the electric conductivity of metallic silver (grey spheres).     

Delocalization tunable by ligand substitution in [L2Al]n− complexes highlights a mechanism for strong electronic coupling

Ligand-based mixed valent (MV) complexes of Al(iii) incorporating electron donating (ED) and electron withdrawing (EW) substituents on bis(imino)pyridine ligands (I₂P) have been prepared. The MV states containing EW groups are both assigned as Class II/III, and those with ED functional groups are Class III and Class II/III in the (I₂P⁻)(I₂P²⁻)Al and [(I₂P²⁻)(I₂P³⁻)Al]²⁻ charge states, respectively. No abrupt changes in delocalization are observed with ED and EW groups and from this we infer that ligand and metal valence p-orbitals are well-matched in energy and the absence of LMCT and MLCT bands supports the delocalized electronic structures. The MV ligand charge states (I₂P⁻)(I₂P²⁻)Al and [(I₂P²⁻)(I₂P³⁻)Al]²⁻ show intervalence charge transfer (IVCT) transitions in the regions 6850–7740 and 7410–9780 cm⁻¹, respectively. Alkali metal cations in solution had no effect on the IVCT bands of [(I₂P²⁻)(I₂P³⁻)Al]²⁻ complexes containing –PhNMe₂ or –PhF₅ substituents. Minor localization of charge in [(I₂P²⁻)(I₂P³⁻)Al]²⁻ was observed when –PhOMe substituents are included.

Organo-aluminum mixed-valent complexes combine properties of both organic and transition element mixed-valent compounds. This supports delocalized electronic structures that are structurally and electronically tunable.     

Salt Stress Mitigation via the Foliar Application of Chitosan-Functionalized Selenium and Anatase Titanium Dioxide Nanoparticles in Stevia (Stevia rebaudiana Bertoni)

High salt levels are one of the significant and major limiting factors on crop yield and productivity. Out of the available attempts made against high salt levels, engineered nanoparticles (NPs) have been widely employed and considered as effective strategies in this regard. Of these NPs, titanium dioxide nanoparticles (TiO₂ NPs) and selenium functionalized using chitosan nanoparticles (Cs–Se NPs) were applied for a quite number of plants, but their potential roles for alleviating the adverse effects of salinity on stevia remains unclear. Stevia (Stevia rebaudiana Bertoni) is one of the reputed medicinal plants due to their diterpenoid steviol glycosides (stevioside and rebaudioside A). For this reason, the current study was designed to investigate the potential of TiO₂ NPs (0, 100 and 200 mg L⁻¹) and Cs–Se NPs (0, 10 and 20 mg L⁻¹) to alleviate salt stress (0, 50 and 100 mM NaCl) in stevia. The findings of the study revealed that salinity decreased the growth and photosynthetic traits but resulted in substantial cell damage through increasing H₂O₂ and MDA content, as well as electrolyte leakage (EL). However, the application of TiO₂ NPs (100 mg L⁻¹) and Cs–Se NPs (20 mg L⁻¹) increased the growth, photosynthetic performance and activity of antioxidant enzymes, and decreased the contents of H₂O₂, MDA and EL under the saline conditions. In addition to the enhanced growth and physiological performance of the plant, the essential oil content was also increased with the treatments of TiO₂ (100 mg L⁻¹) and Cs–Se NPs (20 mg L⁻¹). In addition, the tested NPs treatments increased the concentration of stevioside (in the non-saline condition and under salinity stress) and rebaudioside A (under the salinity conditions) in stevia plants. Overall, the current findings suggest that especially 100 mg L⁻¹ TiO₂ NPs and 20 mg L⁻¹ Cs–Se could be considered as promising agents in combating high levels of salinity in the case of stevia.     

Fused 1,2-Diboraoxazoles Based on closo-Decaborate Anion–Novel Members of Diboroheterocycle Class

The novel members of the 1,2-diboraoxazoles family have been obtained. In the present work, we have carried out the intramolecular ring-closure reaction of borylated iminols of general type [B₁₀H₉N=C(OH)R]⁻ (R = Me, Et, ⁿPr, ⁱPr, ^tBu, Ph, 4-Cl-Ph). This process is conducted in mild conditions with 83–87% yields. The solid-state structures of two salts of 1,2-diboraoxazoles were additionally investigated by X-ray crystallography. In addition, the phenomena of bonding interactions in the 1,2-diboraoxazole cycles have been theoretically studied by the Quantum Theory of Atoms in Molecules analysis. Several local and integral topological properties of the electron density involved in these interactions have been computed.     

Ba4Ca(B2O5)2F2: π-conjugation of B2O5 in the planar pentagonal layer achieving large second harmonic generation of pyro-borate

The nonlinear optical (NLO) crystals that can expand the wavelength of the laser to the deep-ultraviolet (DUV) region by the cascaded second harmonic generation (SHG) are of current research interest. It is well known that borates are the most ideal material class for the design of new DUV NLO crystals owing to the presence of good NLO genes, e.g., BO₃ or B₃O₆ groups. However, the NLO pyro-borates with the B₂O₅ dimers as the sole basic building units are still rarely reported owing to their small SHG responses. In this communication, by constructing a planar pentagonal [Ca(B₂O₅)]_∞ layer, the NLO pyro-borate Ba₄Ca(B₂O₅)₂F₂ with a large SHG response (∼2.2 × KDP, or ∼7 × α-Li₄B₂O₅) and a DUV transparent window has been designed and synthesized. The first-principles calculations show that the large SHG response of Ba₄Ca(B₂O₅)₂F₂ mainly originates from the better π-conjugation of the coplanar B₂O₅ dimers in the [Ca(B₂O₅)]_∞ layer. In addition, the planar pentagonal pattern in the [Ca(B₂O₅)]_∞ layer provides an ideal template for designing the new DUV NLO crystals, apart from those in known DUV borates, e.g., the [Be₂BO₃F₂]_∞ layer in KBe₂BO₃F₂ (KBBF).

A new deep-UV NLO pyro-borate Ba₄Ca(B₂O₅)₂F₂ was synthesized by solid-state reactions. The better π-conjugation of B₂O₅ dimers in the planar pentagonal layer achieves a large SHG response (∼2.2 × KDP), which is the largest among all the known DUV transparent borates with B₂O₅ units.

Deep-ultraviolet (DUV, λ < 200 nm) coherent lights with high photon energy, high spatial resolution, and a small heat-affected zone are of significance for applications in photolithography, high-resolution spectroscopy, laser cooling, and scientific equipment.^1–4 However, it is difficult or well-nigh impossible for solid-state lasers to directly radiate the DUV coherent lights. In contrast, relying on the process of second harmonic generation (SHG) of nonlinear optical (NLO) crystals is a more effective way to generate the DUV coherent lights and causes much attention.^5,6 Therefore, the NLO crystal has become an important material basis of solid-state lasers, which seriously affects the development of all-solid-state laser technology. However, it is still a great challenge to rationally design and synthesize DUV NLO crystals because of the extremely rigorous requirements of structural symmetry and properties.^7–10 Structurally, the DUV NLO crystals must crystallize in the noncentrosymmetric (NCS) space groups which are the prerequisite for the materials to exhibit SHG responses. Moreover, it should possess a broad transparency window, a largely effective NLO coefficient (d_eff ≥ 0.39 pm V⁻¹), and a moderate birefringence (0.05–0.10@1064 nm) to achieve the phase-matching (PM) conditions in the DUV region.¹⁰ Based on these requirements, borates have been considered as the ideal material class for DUV NLO crystals because of their special structure and properties'' virtues, including the rich acentric structural types, large band gaps, and stable physical and chemical properties.⁸ To date, the commercialized borate-based UV NLO crystals consist of β-BaB₂O₄ (BBO), LiB₃O₅ (LBO), CsLiB₆O₁₀ (CLBO),^9,10 and the practical DUV NLO crystal KBe₂BO₃F₂ (KBBF). Especially for KBBF, it has become the sole material that can generate DUV coherent laser light (177.3 nm) by a direct SHG method.⁷ Other excellent borate-based UV NLO crystals also consist of K₃B₆O₁₀Cl,¹¹ SrB₅O₇F₃,¹² Li₂B₆O₉F₂,⁵ CsAlB₃O₆F,¹³ M₂B₁₀O₁₄F₆ (M = Ca, Sr),¹⁴ NH₄B₄O₆F,¹⁵ NaSr₃Be₃B₃O₉F₄,¹⁶ AB₄O₆F (A = K, Rb, and Cs),¹⁷etc.The above borate-based materials have achieved great success as UV and DUV NLO crystals, which are mainly attributed to the ability of boron atoms to coordinate with three or four oxygen anions forming trigonal-planar or tetrahedral building blocks.^18,19 For example, the first borate-based NLO crystal, KB₅O₈·4H₂O (KB₅), has the basic building units (BBUs) of [B₅O₁₀], while the BBUs of β-BBO, LBO, and KBBF are [B₃O₆], [B₃O₇], and isolated [BO₃], respectively.^7,8 Remarkably, although various borate crystals with different types of borate groups have been explored during the past decades, the pyro-borate NLO crystals with B₂O₅ groups as the sole BBUs are rarely reported owing to their weak SHG responses.^20–23 For example, the SHG response of the DUV transparent α-Li₄B₂O₅ (ref. 23) is only ∼0.3 × KDP, which is far smaller than the expected value (0.39 pm V⁻¹, 1 × KDP).Actually, the flexible B₂O₅ groups which are composed of two π-conjugated BO₃ units through corner-sharing may also be capable of generating excellent optical performance if they have benign arrangements. In recent research, Pan''s group has indicated that the B₂O₅ dimers are perfect for the design of DUV birefringent crystals. By the synergistic combination, they have successfully designed a potential pyro-borate birefringent crystal, Li₂Na₂B₂O₅, with a short UV cut-off edge (181 nm) and large birefringence (0.095@532 nm).²¹ And they have also grown Ca(BO₂)₂ crystals exhibiting a short UV cut-off edge and larger birefringence (169 nm; 0.2471@193 nm). Based on the analysis of the structure–property relationship of Ca(BO₂)₂, they stated that the polymerized planar B_nO_2n+1 groups, e.g., B₂O₅, could generate a larger anisotropy than isolated BO₃.²² However, their opposite arrangements of B–O groups make them crystallize in the centrosymmetric (CS) space groups, which limit their further development as NLO compounds. Thus, it is clear that pyro-borates exhibiting a large birefringence and a short UV cut-off edge would also be promising DUV NLO crystals if their SHG responses can be enhanced.Based on the above-mentioned ideas, a systematical investigation has been performed on DUV pyroborates. And finally, we successfully synthesized a new NCS pyro-borate, Ba₄Ca(B₂O₅)₂F₂, which can exhibit not only a large SHG response (∼2.2 × KDP and ∼7 × α-Li₄B₂O₅) but also a short UV cut-off edge (<190 nm). Analyzing its structure, one can find that its excellent NLO properties mainly originate from the unique planar pentagonal [Ca(B₂O₅)]_∞ layer, where the B₂O₅ groups adopt the almost coplanar configurations that favor the structure to generate large SHG response and birefringence,²¹ meanwhile the terminal O atoms of B₂O₅ groups are also linked by the Ca²⁺ cations, which eliminate the dangling bonds of B₂O₅ groups and further blue-shift the UV cut-off edge. More importantly, the adjacent [Ca(B₂O₅)]_∞ layers in Ba₄Ca(B₂O₅)₂F₂ are linked by other B₂O₅ groups to form a 3D framework, which will be favorable for the material to avoid the layer habit that KBBF suffers from. In this sense, the planar pentagonal [Ca(B₂O₅)]_∞ layer is similar to the [Be₂BO₃F₂]_∞ layer in KBBF, and it can be seen as a new structure template for the design of new DUV NLO crystals, especially for the DUV pyro-borates. Herein, we will describe the synthesis, experimental and computational characterization as well as the functional properties of the new DUV NLO material, Ba₄Ca(B₂O₅)₂F₂.A polycrystalline sample of Ba₄Ca(B₂O₅)₂F₂ was synthesized by the conventional solid-state reaction and the purity was confirmed by powder X-ray diffraction (XRD) (Fig. S1^†). With the polycrystalline sample, the thermal behavior of Ba₄Ca(B₂O₅)₂F₂ was studied by the thermogravimetric (TG) and differential scanning calorimetry (DSC) measurements. The heating DSC curve shows a sharp endothermic peak at 815 °C with no obvious weight loss in the TG curve (Fig. S2^†), suggesting that Ba₄Ca(B₂O₅)₂F₂ has good thermal stability. To further investigate the thermal behavior of Ba₄Ca(B₂O₅)₂F₂, the polycrystalline sample was calcined at 840 °C and the XRD analysis showed that the calcined sample was Ba₄Ca(B₂O₅)₂F₂, Ba₂Ca(BO₃)₂ (PDF #01-085-2268), Ba₂CaB₆O₁₂ (PDF #01-075-1401) and other unknown phases (Fig. S3^†). These results illustrate that Ba₄Ca(B₂O₅)₂F₂ melts incongruently and the suitable flux is necessary for the crystal growth.With the Na₂O–PbF₂–B₂O₃ as the flux, millimeter-sized block crystals of Ba₄Ca(B₂O₅)₂F₂ were grown for the single-crystal XRD structure determination. Ba₄Ca(B₂O₅)₂F₂ crystallizes in the NCS and polar space group, P2₁ (Table S1^†). In the asymmetric unit, there are four unique Ba, one Ca, four B, ten O, and two F atom(s), which all fully occupy the 2a Wyckoff positions (Table S2^†). All B atoms are coordinated to three oxygen atoms to form the BO₃ triangles with the B–O distances ranging from 1.312(17) to 1.460(16) Å and O–B–O angles varying from 108.0(13) to 130.2(15)°. The BO₃ triangles are further connected to form two types of B₂O₅ dimers, i.e. plane B(1,3)₂O₅ and twisted B(2,4)₂O₅, which are the BBUs of Ba₄Ca(B₂O₅)₂F₂. The Ca atoms are coordinated to six oxygen atoms to form CaO₆ octahedra with the Ca–O distances ranging from 2.285(9) to 2.325(13) Å. For the Ba²⁺ cations, they exhibit three different coordination environments, Ba(1,2)O₆F₂, Ba(3)O₈F₂, and Ba(4)O₇F₂ (Fig. S4^†) with the Ba–O distances ranging from 2.585(9) to 3.250(11) Å and the Ba–F bond lengths ranging from 2.635(8) to 2.736(8) Å. Remarkably, for the F⁻ anions, each unique fluorine atom serves as a common vertex for four Ba atoms to form the FBa₄ polyhedra (Fig. S5a^†), which could be treated as fluorine-centered secondary building units (SBUs). The Ba–F–Ba angles vary from 99.0 (2) to 120.2 (3)°. The bond valence sum (BVS) calculations show the values of 1.67–1.97, 2.45, 2.88–3.10, 1.78–2.13, and 0.95–1.09, for Ba²⁺, Ca²⁺ B³⁺, O²⁻, and F⁻, respectively (Table S2^†). The BVSs of atoms are consistent with their expected oxidation states except the one from the Ca²⁺ cations. The larger BVSs of Ca²⁺ cations can be attributed to six shorter Ca–O bond lengths, which are also observed in other Ca²⁺-containing borates, such as YCa₃(VO)₃(BO₃)₄ (2.44),²⁴ Rb₂Ca₃B₁₆O₂₈ (2.29), and Cs₂Ca₃B₁₆O₂₈ (2.30).²⁵The structure of Ba₄Ca(B₂O₅)₂F₂ is shown in Fig. 1. In the structure, the plane B(1,3)₂O₅ dimer is first connected with four CaO₆ octahedra, meanwhile, each CaO₆ octahedron is also linked by four B(1,3)₂O₅ dimers through sharing their four equatorial O atoms to form a unique planar pentagonal [Ca(B₂O₅)]_∞ layer in the b–c plane (Fig. 1a, b). Then, these [Ca(B₂O₅)]_∞ layers are further linked by the twisted B(2,4)₂O₅ dimers to construct a 3D framework with Ba²⁺ cations maintaining the charge balance (Fig. 1c). Remarkably, for the arrangements of the Ba²⁺ cations and the F⁻ anions, the fluorine-centered SBU FBa₄ polyhedra are linked to construct the 2D [F₂Ba₄] infinite layer (Fig. S5b^†) with the same orientation, which further fills the apertures in the [Ca(B₂O₅)₂] framework (Fig. S5c^†). The existence of fluorine-centered SBUs would certainly have a strong influence on the local coordinate environments, and finally on the whole structure.²⁶Open in a separate windowFig. 1(a) The [Ca(B₂O₅)] layer is composed of B₂O₅ dimers and CaO₆ octahedra. (b) The planar pentagonal topology layer. The comparison of structures between (c) Ba₄Ca(B₂O₅)₂F₂ and (d) KBBF.It is very interesting that Ba₄Ca(B₂O₅)₂F₂ contains a planar pentagonal [Ca(B₂O₅)]_∞ layer, which is similar to the [Be₂BO₃F₂]_∞ layer in KBBF. The structural evolution from KBBF to Ba₄Ca(B₂O₅)₂F₂ is also shown in Fig. 1c and d. In KBBF, the BBUs are the planar BO₃ triangles, which are connected with BeO₃F in the a–b plane by strong covalent bonds to form the [Be₂BO₃F₂]_∞ layers (Fig. S6c^†) and the [Be₂BO₃F₂]_∞ layers have achieved excellent NLO properties of the KBBF crystal.⁷ However in Ba₄Ca(B₂O₅)₂F₂, the BO₃ triangles are changed into the B₂O₅ dimers, and the BeO₃F tetrahedra are substituted by the CaO₆ polyhedra. These B₂O₅ dimers are also connected by the CaO₆ polyhedra to form the interesting planar pentagonal [Ca(B₂O₅)]_∞ layer (Fig. S6d^†). More importantly, in KBBF, the adjacent [Be₂BO₃F₂]_∞ layers are connected by the weak K⁺-F⁻ ionic bonds that results in the strong layer habit of the KBBF crystals, whereas in Ba₄Ca(B₂O₅)₂F₂, the [Ca(B₂O₅)]_∞ layers are bridged by the strong covalent B–O bonds to form a stable 3D framework, which will greatly overcome the layering tendency of the KBBF crystal and facilitate the crystal growth.In addition, we also notice that the planar pentagonal [Ca(B₂O₅)]_∞ layer maybe helpful for enhancing the SHG responses of pyro-borates because small SHG responses of pyro-borates are attributed to the typical twisted configurations of the B₂O₅ groups, which are unfavorable for forming the π-conjugation and the superposition of the microscopic SHG response. For example, α-Li₄B₂O₅, a DUV transparent pyro-borate with sole B₂O₅ units as the BBUs, has a weak SHG response, which may be derived from the twisted B₂O₅ groups and non-planar arrangements (Fig. S7a^†). However, in Ba₄Ca(B₂O₅)₂F₂, the planar configuration of the pentagonal layers can assist the B₂O₅ groups to adopt a nearly coplanar arrangement (Fig. S7b^†) and effectively enhance the π-conjugation of B₂O₅ groups. The better π-conjugation of the planar B₂O₅ groups in the planar pentagonal [Ca(B₂O₅)]_∞ layer has also been confirmed by the electron orbital calculation based on the first-principles calculations.²⁷ The calculated result is shown in Fig. 2. Clearly, the prominent conjugated interactions are observed in the nearly coplanar B(1,3)₂O₅ dimers of Ba₄Ca(B₂O₅)₂F₂ (Fig. 2a), whereas it does little in the twisted B(2,4)₂O₅ dimers of Ba₄Ca(B₂O₅)₂F₂ (Fig. 2b) and two types of twisted B₂O₅ dimers in α-Li₄B₂O₅ (Fig. 2c and d). It can be expected that the nearly coplanar B₂O₅ dimers are more conducive to the large SHG response than the twisted B₂O₅ dimers. Remarkably, the similar pentagonal layers are also observed in other pyro-phosphates, such as Ba₂NaClP₂O₇, K₂Sb(P₂O₇)F, Rb₃PbBi(P₂O₇)₂, and Rb₃BaBi(P₂O₇)₂. Clearly, as pyro-phosphates are the non-π-conjugated systems, the planar pentagonal layers are only helpful for the orientation of anion groups.^28–31 However, they cannot form the better π-conjugation. Therefore, the better π-conjugation of the nearly coplanar B₂O₅ groups in planar pentagonal layers of pyro-borate Ba₄Ca(B₂O₅)₂F₂ would have a different contributing mechanism to the SHG effect with other non-π-conjugated pyro-phosphates.Open in a separate windowFig. 2The orbitals of the nearly coplanar B(1,3)₂O₅ (a) and twisted B(2,4)₂O₅ dimers (b) in Ba₄Ca(B₂O₅)₂F₂. The orbitals of two twisted B₂O₅ dimers (c and d) in α-Li₄B₂O₅.The presence of BO₃ triangles in Ba₄Ca(B₂O₅)₂F₂ is confirmed by the IR spectral measurements (Fig. S8^†). The peaks at 1362 cm⁻¹ and 1208 cm⁻¹ can be attributed to the asymmetric stretching of BO₃ groups.³² A strong band at 1069 cm⁻¹ in the IR spectrum may be associated with the stretching vibration of B–O–B in B₂O₅.^33,34 The weak absorption bands at 950, and 810 cm⁻¹ correspond to the symmetrical stretching vibrations of BO₃ and B–O–B in B₂O₅, respectively. The peaks at 751 and 615 cm⁻¹ can be attributed to the out-of-plane bending of the BO₃ groups.³⁴ Further, the UV-vis-NIR diffuse reflectance spectrum was also measured (Fig. S9^†), which shows that Ba₄Ca(B₂O₅)₂F₂ is transparent down to the DUV region with a UV cut-off edge less than 190 nm (corresponding to a large band gap of 6.2 eV), which is comparable to the newly developed NLO-active borates, such as RbB₃O₄F₂ (<190 nm), CsZn₂BO₃X₂ (X₂ = F₂,Cl₂, and FCl)) (<190 nm) and so on.^35–38 The short cut-off edge demonstrates the potential application of Ba₄Ca(B₂O₅)₂F₂ as a DUV NLO crystal.As Ba₄Ca(B₂O₅)₂F₂ crystalizes in the NCS space group P2₁, it possesses the SHG response, which was measured by the Kurtz-Perry method with the well-known NLO material KH₂PO₄ (KDP) as a reference.³⁹ As shown in Fig. 3, the SHG intensities of Ba₄Ca(B₂O₅)₂F₂ increase with the increase of particle sizes, indicating that Ba₄Ca(B₂O₅)₂F₂ is type-I phase-matchable. The SHG intensity of Ba₄Ca(B₂O₅)₂F₂ at the particle size of 150–212 μm is about 2.2 times that of KDP, and is larger than that of KBBF (1.2 × KDP) or comparable with those newly reported UV NLO crystals, i.e. γ-Be₂BO₃F (2.3 × KDP),⁶ β-Rb₂Al₂B₂O₇ (2 × KDP),⁴⁰ Li₄Sr(BO₃)₂ (2 × KDP),⁴¹ CsB₄O₆F(∼1.9 × KDP).² In addition, as we know, the SHG response of Ba₄Ca(B₂O₅)₂F₂ is the largest among all the known DUV transparent borates with B₂O₅ units (Table S4^†). Its SHG response (2.2 × KDP) is about seven times larger than that of α-Li₄B₂O₅ (0.3 × KDP), another DUV transparent pyro-borate with sole B₂O₅ units.Open in a separate windowFig. 3(a) Phase-matching curve, i.e., particle size vs. SHG intensity, data for Ba₄Ca(B₂O₅)₂F₂ and KH₂PO₄ (KDP) as reference. The solid curve is a guide for the eye, not a fit to the data. (b) Oscilloscope traces showing SHG intensities for Ba₄Ca(B₂O₅)₂F₂ and KDP.To understand the origin of the excellent optical properties of Ba₄Ca(B₂O₅)₂F₂, we also carried out the first-principles calculations.²⁷ It shows that Ba₄Ca(B₂O₅)₂F₂ has an indirect band gap of 6.34 eV (Figures S10a^†), which is in accordance with the experimental results. The valence band maximum (VBM) of Ba₄Ca(B₂O₅)₂F₂ is mainly composed of the orbitals in Ba, and O atoms, while the conduction band minimum (CBM) is dominantly composed of the orbitals in Ba, B, and O atoms. Therefore, the band gap of Ba₄Ca(B₂O₅)₂F₂ is mainly determined by Ba atoms and B₂O₅ groups. Based on the calculated electron structure, the NLO coefficients of Ba₄Ca(B₂O₅)₂F₂ are also calculated. The largest NLO coefficient of Ba₄Ca(B₂O₅)₂F₂ is d₂₂ = −0.524 pm V⁻¹, which is about 5 times lower than that of α-Li₄B₂O₅ (d₂₄ = −0.102 pm V⁻¹) (Table S5a^†), which is matched with the experimental one. Further, the SHG-weighted density maps of Ba₄Ca(B₂O₅)₂F₂ are shown in Fig. 4. These reveal that B₂O₅ dimers make the dominant contribution (72.7%) to the total SHG effect (Table S5b^†). The band-resolved SHG analysis can also conclude that B–O orbitals in Ba₄Ca(B₂O₅)₂F₂ contribute more to the SHG response than those in α-Li₄B₂O₅ (Fig. S10b, S10c^†), indicating that the arrangements of B₂O₅ dimers in Ba₄Ca(B₂O₅)₂F₂ is more beneficial for the large SHG response. And different from α-Li₄B₂O₅, F-centered secondary building units (SBUs) exist in the structure of Ba₄Ca(B₂O₅)₂F₂, and they are further linked to construct 2D [F₂Ba₄]_∞ infinite layers, which could help B₂O₅ groups arrange in a planar pattern (Fig. S5^†).²⁶ So, based on the above analysis, we can conclude that the nearly coplanar B₂O₅ dimers in the planar pentagonal layer and the SBU FBa₄ tetrahedra make a significant contribution to the SHG response of Ba₄Ca(B₂O₅)₂F₂.Open in a separate windowFig. 4The SHG-weighted density maps of the virtual electron process (a) and virtual hole process (b) of d₂₂ for Ba₄Ca(B₂O₅)₂F₂.     

The first silver bismuth borate,AgBi2B5O11

The first silver bismuth borate, AgBi₂B₅O₁₁ (silver dibismuth pentaborate), has been prepared via glass crystallization in the Ag₂O–Bi₂O₃–B₂O₃ system and characterized by single‐crystal X‐ray diffraction. Its structure is derived from that of centrosymmetric Bi₃B₅O₁₂ by ordered substitution of one Bi³⁺ ion for Ag⁺, which results in the disappearance of the mirror plane and inversion centre. Second harmonic generation (SHG) measurements confirm the acentric crystal structure. It is formed by [Bi₂B₅O₁₁]_∞ layers stretched along c and comprised of vertex‐sharing B₅O₁₀ and BiO₃ groups which incorporate the Ag⁺ cations. The new compound was characterized by thermal analysis, high‐temperature powder X‐ray diffraction, and vibrational and UV–Vis–NIR (near infrared) spectroscopy. Its thermal expansion is strongly anisotropic due to the presence of rigid B₅O₁₀ groups aligned in a parallel manner. The minimal value is observed along their axis [parallel to c, α_c = 3.1 (1) × 10^?6 K^?1], while maximal values are observed in the ab plane [α_a = 20.4 (2) and α_b = 7.8 (2) × 10^?6 K^?1]. Upon heating, AgBi₂B₅O₁₁ starts to decay above 684 K due to partial reduction of silver; incongruent melting is observed at 861 K. According to density functional theory (DFT) band‐structure calculations, the new compound is a semiconductor with an indirect energy gap of 3.57 eV, which agrees with the experimental data (absorption onset at 380 nm).     

Novel metal cluster complexes synthesized by matrix deposition of mass-selected clusters

Metal complexes produced by depositing size selected Fe and Ag cluster cations in N₂ and O₂ matrices respectively are studied by infrared spectroscopy. Unknown species such as Fe(N₂)x, Fe₃ (N₂)x and Ag₃(O₂)x are observed. The IR spectra of Ag⁺, Ag ₂ ⁺ and Ag ₉ ⁺ in excess O₂ indicate that no complexes involving molecular oxygen are formed. However, the strong silver cluster UV-visible absorptions detected in Ar matrices disappear in the oxygen matrices, suggesting that silver-oxygen complexes are formed with dissociated oxygen.     

Silbermercaptide und -mercaptokomplexe

The complex formation of silver(I) has been studied with the anions of simple mercaptans RSH which have been rendered soluble by replacing some H in the substituent R by OH. All equilibria constants refer to a solvent of ionic strength μ = 0,1 and 20°C. Monothioglycol HO? CH₂? CH₂? SH (pK = 9.48) forms an amorphous insoluble mercaptide {AgSR} (s), ionic product [Ag⁺] [SR^?] = 10^?19.7. The solution in equilibrium with the solid contains the molecule AgSR at a constant concentration of 10^?6.7 M which furnishes the formation constant of the 1:1-complex: K₁ = 10^{13. 0}. The solid is soluble in excess of mercaptide (AgSR+SR^? → Ag(SR)₂^?: K₂ = 10^{4. 8}) as well as in an excess of silver ion (AgSR + Ag⁺ → Ag₂SR⁺K ≈? 10⁶). With the bulky monothiopentaerythrite (HO? CH₂? )₃C? CH₂? SH (pK = 9.89) no precipitation occurs with silver when the mercaptan concentration is below 10^{?3. 2}M. A single polynuclear Ag₁₀(SR)₉⁺ (β_10.9 = 10¹⁷⁵) is formed in acidic solutions which breaks up with the formation of Ag₂SR⁺ (β_2.1 = 10^19.0) when an excess of silver ion is added. Below the mononuclear wall ([RS]_total < 10^?6) Ag₂SR⁺ is formed via the mononuclear AgSR (K₁ = 10¹³). At higher mercaptan concentrations ([RS]_tot > 10^?3.2) an amorphous precipitate is formed which has almost the same solubility product as silver thioglycolate ([Ag⁺] [SR^?] = 10^?19.1). Apparently silver(I) forms with mercaptans always the complexes Ag₂SR⁺, AgSR and Ag(SR)₂^?. Above the mononuclear wall, these species condense to chain-like polynuclears which are cations Ag(SRAg)_n⁺ in presence of an excess of Ag⁺, and anions SR (AgSR)_n^? when the concentration [RS^?] is larger than [Ag⁺]. Usually n becomes rapidly very large as soon as the condensation starts (n → ∞: precipitate). The decanuclear Ag(SRAg)₉⁺ formed with thiopentaerythrit is somewhat more stable than the shorter chains (n < 9) and larger chains (n > 9), because it can tangle up to a ball by coordination of bridging mercapto-sulfur to the terminal silver ions (figure 12, page 2179). This ball seems to be further stabilized by hydrogen bonds between the many alcoholic OH groups of the substituent R = (HO? CH₂)₃C? CH₂? . The stability of the bonds Ag? S, however, is little influenced by the substituent R which carries the mercaptide sulfure.     

Boron(iii) β-diketonate-based small molecules for functional non-fullerene polymer solar cells and organic resistive memory devices

A class of acceptor–donor–acceptor chromophoric small-molecule non-fullerene acceptors, 1–4, with difluoroboron(iii) β-diketonate (BF₂bdk) as the electron-accepting moiety has been developed. Through the variation of the central donor unit and the modification on the peripheral substituents of the terminal BF₂bdk acceptor unit, their photophysical and electrochemical properties have been systematically studied. Taking advantage of their low-lying lowest unoccupied molecular orbital energy levels (from −3.65 to −3.72 eV) and relatively high electron mobility (7.49 × 10⁻⁴ cm² V⁻¹ s⁻¹), these BF₂bdk-based compounds have been employed as non-fullerene acceptors in organic solar cells with maximum power conversion efficiencies of up to 4.31%. Moreover, bistable resistive memory characteristics with charge-trapping mechanisms have been demonstrated in these BF₂bdk-based compounds. This work not only demonstrates for the first time the use of a boron(iii) β-diketonate unit in constructing non-fullerene acceptors, but also provides more insights into designing organic materials with multi-functional properties.

Boron(iii) β-diketonates have been demonstrated to serve as multi-functional materials in NFA-based OPVs and organic resistive memories.     

Boron Hydrogen Compounds: Hydrogen Storage and Battery Applications

About 25 years ago, Bogdanovic and Schwickardi (B. Bogdanovic, M. Schwickardi: J. Alloys Compd. 1–9, 253 (1997) discovered the catalyzed release of hydrogen from NaAlH₄. This discovery stimulated a vast research effort on light hydrides as hydrogen storage materials, in particular boron hydrogen compounds. Mg(BH₄)₂, with a hydrogen content of 14.9 wt %, has been extensively studied, and recent results shed new light on intermediate species formed during dehydrogenation. The chemistry of B₃H₈⁻, which is an important intermediate between BH₄⁻ and B₁₂H₁₂²⁻, is presented in detail. The discovery of high ionic conductivity in the high-temperature phases of LiBH₄ and Na₂B₁₂H₁₂ opened a new research direction. The high chemical and electrochemical stability of closo-hydroborates has stimulated new research for their applications in batteries. Very recently, an all-solid-state 4 V Na battery prototype using a Na₄(CB₁₁H₁₂)₂(B₁₂H₁₂) solid electrolyte has been demonstrated. In this review, we present the current knowledge of possible reaction pathways involved in the successive hydrogen release reactions from BH₄⁻ to B₁₂H₁₂²⁻, and a discussion of relevant necessary properties for high-ionic-conduction materials.     

Negative and linear positive magnetoresistance effect in silver-rich silver selenide

In the non-magnetic semiconductor silver selenide (Ag_2+δSe) with a minor heterogeneous silver excess (0.79 × 10⁻⁴ < δ ≤ 1 × 10⁻²) we measured either a saturating negative magnetoresistance (MR) effect, a linear positive MR effect or a superposition of both. This complex MR behavior depends on the amount of the silver metal excess, but is also strongly influenced by the thermal treatment of the samples. Excess silver that cannot be accommodated homogeneously in the silver selenide lattice creates structural heterogeneities and forms a microstructure which is controlled by the thermal annealing procedure. We suggest that small silver segregations at the grain boundaries of the silver selenide matrix (“nanofilms”) are responsible for the negative MR effect, whereas nanoscale silver particles within the silver selenide matrix (nanoparticles) cause a linear positive effect.     

Reactions of organic peroxy radicals,RO2, with substituted and biogenic alkenes at room temperature: unsuspected sinks for some RO2 in the atmosphere?

Until now the reactions of organic peroxy radicals (RO₂) with alkenes in the gas phase have been essentially studied at high temperature (T ≥ 360 K) and in the context of combustion processes, while considered negligible in the Earth''s atmosphere. In this work, the reactions of methyl-, 1-pentyl- and acetylperoxy radicals (CH₃O₂, C₅H₁₁O₂, and CH₃C(O)O₂, respectively) with 2-methyl-2-butene, 2,3-dimethyl-2-butene and for the first time the atmospherically relevant isoprene, α-pinene, and limonene were studied at room temperature (298 ± 5 K). Monitoring directly the radicals with chemical ionization mass spectrometry led to rate coefficients larger than expected from previous combustion studies but following similar trends in terms of alkenes, with (in molecule⁻¹ cm³ s⁻¹) = 10⁻¹⁸ to 10⁻¹⁷ × 2/2 and = 10⁻¹⁴ to 10⁻¹³ × 5/5. While these reactions would be negligible for CH₃O₂ and aliphatic RO₂ at room temperature, this might not be the case for acyl-, and perhaps hydroxy-, allyl- and other substituted RO₂. Combining our results with the Structure–Activity Relationship (SAR) predicts k^II(298 K) ∼10⁻¹⁴ molecule⁻¹ cm³ s⁻¹ for hydroxy- and allyl-RO₂ from isoprene oxidation, potentially accounting for up to 14% of their sinks in biogenic-rich regions of the atmosphere and much more in laboratory studies.

The reactions of organic peroxy radicals with alkenes, overlooked until now, could be more significant than expected for some RO₂ in the atmosphere.     

Closed and open-shell atomic oxygen on silver: two distinct patterns of the O<Subscript>1s</Subscript> binding energy and X-ray absorption O <Emphasis Type="Italic">K</Emphasis>-edge spectra as revealed by density functional theory

The electronic structure of atomic oxygen adsorbed species is studied by means of the density functional theory in the context of the ethylene epoxidation on the silver surface. The adsorbed oxygen species are modeled by the Ag₂O molecule either in its closed (¹A₁) or open-shell states (³B₁ and ¹B₁). In both open-shell states the 1s level appears to be lower than that in ¹A₁ by about 2 eV. This is apparently a sequence of the separation of electron pair, occupying the *-type highest occupied molecular orbital (HOMO), decreasing the electron density at the oxygen center. Such variation of the O_1s level for closed and open-shell Ag₂O states seems to explain the X-ray photoelectron spectroscopy (XPS) data concerning two distinct atomic oxygen species on silver surface having the O_1s binding energy of about 528 and 530 eV, called nucleophilic and electrophilic oxygen, respectively. The X-ray absorption O K-edge spectra (XANES) calculated for two types of the Ag₂O states by means of multiple-scattered-X-based approach appears to be in a qualitative agreement with those experimentally recorded for nucleophilic and electrophilic oxygen.