Two New Nonaborates with {B9} Cluster Open-Frameworks and Short Cutoff Edges

Two new nonaborates, Na2Ba0.5[B9O15]·H2O (1) and Na4Ca1.5[B9O16(OH)2] (2), have been solvothermally made with mixed alkali and alkaline-earth metal cationic templates. 1 and 2 are constructed by two different types of nonaborate clusters. In 1, the [B9O19]11− cluster is composed of three corner-sharing [B3O7]5− clusters, of which two of them interconnect to the 1D B3O7-based chains and are further bridged to the 3D framework with 7 types of 10-MR channels by another [B3O7]5− bridging cluster. The [B9O18(OH)2]11− cluster in 2 is made of four BO4-sharing [B3O8]7− clusters. As a 4-connected node, the interconnections of [B9O18(OH)2]11− construct the unpreceded 2D layer with large 14-MR windows, which are further joined by H-bonds to the 3D supramolecular framework. UV–Vis absorption spectra reveal that both 1 and 2 have short cutoff edges below 190 nm, exhibiting bandgaps of 6.31 and 6.39 eV, respectively, indicating their potential applications in deep UV (DUV) regions.


Introduction
Borates have long been the research focus of inorganic crystalline materials due to their mineralogical and industrial importance since last century [1][2][3][4]. In the past three decades, scientists have not only been dedicated to enriching borates' abundant structural diversity, but also explore their applications in fluorescence, catalysis, and nonlinear optics (NLO) [5][6][7][8][9]. As an oxyphilic element, boron atom can be triangular and tetrahedral coordinated with oxygen atoms, forming the fundamental building units of BO 3 and BO 4 . These two units can further polymerize through corner-/edge-sharing to produce series of oxoboron clusters with different numbers of B atoms and configurations [10,11], which contribute to the abundant structural diversities of borates. To date, most of the commonly seen monomeric oxoboron clusters have boron atoms ranging from 3 to 7 [12,13]. Larger oxoboron clusters are usually made by the combinations of these four main types of clusters and BO 3 [10]. The different combinations and linkages of these cluster units contribute to the different configurations of the oxoboron clusters and their extended structures [6,7]. Due to the steric hindrances and polyhydroxyl characters, larger oxoboron clusters commonly generate lower dimensional structures without metal nodes and other bridging groups (clusters, organic amines, and transition metal complexes) [19][20][21].
In the past decades, large efforts have been concentrated on the alkali and alkalineearth metal borates, of which the empty d and f orbitals in alkali-metal and alkaline-earth metal facilitate the transmittance of UV or even deep UV (DUV) light. Moreover, their asymmetric coordination geometries can reduce the symmetrical influences from templates to acentric oxoboron frameworks, enhancing the possibilities of acentric structures [22]. Large numbers of UV/DUV NLO borates have been obtained with alkali and alkaline-earth metal under different synthetic conditions, including the typical β-BaB 2 O 4 (BBO), LiB 3 O 5 (LBO), and CsLiB 6 O 10 (CLBO) [23][24][25], and the newly obtained LiBa 3 [27], and A 10 B 13 O 15 F 19 (A = K and Rb) [28]. From the view of the structure-property relationship, mixed alkali and alkaline-earth metal templates with different cationic radium are more likely to produce borates with compact structures, corresponding to the larger Second Harmonic Generation (SHG) response than single metal template, which are due to their different effects on the packing modes of anionic cluster originated from their different sizes and charges. Based on these considerations, our group has been working on the syntheses, structures, and properties of alkali and alkaline-earth metal borates under hydro(solvo)thermal conditions. In this work, by using mixed alkali-and alkaline-earth metal templates, we successfully synthesized two new borates containing two different types of nonaborate clusters, namely Na 2 Ba 0. 5  After stirring for 1 h, the resulting solution was sealed in a 25 mL Teflon-lined stainless-steel autoclave and heated at 230 • C for 7 days. After cooling down to room temperature spontaneously and being washed by distilled water, flake-like crystals were obtained in the yield of 37% (based on sodium perborate).

X-ray Crystallography
The single crystal diffraction data of 1 and 2 were collected on a Gemini A Ultra CCD diffractometer with graphite monochromated Mo Kα (Λ = 0.71073 Å) radiation at 293(2) K. The structures were solved by direct methods and refined by the full-matrix least-squares fitting on F 2 method with the SHELX-2008 program package [31]. Anisotropic displacement parameters were refined for all atomic sites except for hydrogen atoms. Basic crystallographic data and structural refinement data are listed in Table 1

General Procedure
All the reagents were analytical grade and used without any further purification. The powder X-ray diffraction (PXRD) patterns of the title compounds were both collected on a Bruker D8 Advance X-ray diffractometer (Bruker, Karlsruhe, Germany) with Cu Kα radiation (λ = 1.54056 Å), 2θ scanning from 5-50 • with a step size of 0.02 • at room temperature. UV-Vis absorption spectra were recorded on a Shimadzu UV3600 spectrometer (Shimadzu, Kyoto, Japan) with the wavelengths ranging from 190 to 800 nm. IR spectra were obtained on a Nicolet iS10 FT-IR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) with the wavenumbers ranging from 4000 to 400 cm −1 . Thermogravimetric analyses were carried out on a Mettler Toledo TGA/DSC 1100 analyzer (Mettler Toledo, Zurich, Switzerland), heating up from 25 to 1000 • C with a heating rate of 10 • C/min under air atmosphere.

Structure of 1
Single crystal X-ray analyses show that 1 crystalizes in the monoclinic space group P2 1 /c. Its asymmetric unit contains a [B 9 O 19 ] 11− cluster formed by three corner-sharing [B 3 O 7 ] 5− clusters, 2 Na + , 0.5 Ba 2+ and 1 water molecule (Figure 1a). In order to describe the construction of the structure, three corner-sharing [B 3 O 7 ] 5− clusters are named in the short-hand notations of B 3 -I, B 3 -II, and B 3 -III. Each of these three B 3 clusters is four-connected, bonding with two others in the same quantities (Figure 1b). B 3 -I and B 3 -II first interconnects to form the 1D [B 3 O 7 ] n 5n− chain along the b axis (Figure 1c), exhibiting two opposite orientations with an C 2 -symmetry axis. These two different orientated 1D chains alternately arrange in -ABAB-and -AAA-sequences along aand c-axes (Figure 1d), respectively, which are further bridged to 3D framework by the B 3 -III bridging cluster (Figure 1e-g). There exist seven different types of 10-MR channels in the 3D framework of 1 (Figure 1h,i). Along the a axis, 10-MR channels A and B show same shapes and delimitations (B5O 4 -B1O 3 -B2O 3 -B7O 4 -B6O 3 -B5O 4 -B1O 3 -B2O 3 -B7O 4 -B6O 3 ) but different orientations, they alternately arrange along the b axis. Another 10-MR channel C with different shape is delineated by 3 . Channels D, E, and F appear along the b axis, of which channel D and F have the same delimitations as channel C, but showing different shapes, while channel E is made from the alternations of B7O 4 -B2O 3 -B3O 4 -B4O 3 -B6O 3 -B7O 4 -B2O 3 -B3O 4 -B4O 3 -B6O 3 . Channel G along the c axis has the same components as those in A and B. These seven types of channels make up the intercommunicated channel system of 1. To the best of our knowledge, such complicated channel system is only found in the aluminoborate [H 3 Figure S1) [32], which exhibits the 3D porous layer with seven intercommunicated channel networks. Five-coordinated Na 1+ and seven-coordinated Na 2+ are located in the channels F, while twelve-coordinated Ba 2+ are located in the channels E, compensating the negative charges of the frameworks ( Figure S2a). It is worth noticing that each of the two Na 1+ , two Na 2+ , and one Ba 2+ alternate through corner-sharing, producing the rare [Na 4 BaO 28 ] pentamers ( Figure S2b).
In the structure, each [B 9 O 18 (OH) 2 ] 11− FBB links with four same ones to generate the 2D layer with large 14-MR windows (Figure 2b,c). Adjacent layers are arranged in -ABAB-manners along the c axis and show a curtain extend of dislocation (Figure 2d,e). Interlayered H-bond interactions join adjacent layers to the 3D supramolecular open framework (Table S1), containing three different types of supramolecular channels (Figure 2d,f). Charge-balancing cations all locate in the large 14-MR windows ( Figure S3a). Na 1+ and Na 2+ are 6-and 5-coordinated with Na-O bond length ranging from 2.312-2.695Å and 2.259-2.487Å, respectively, while Ca 2+ is 8-coordinated, corresponding to the bond length range of 2.354-2.840 Å. Na 1+ , Na 2+ , and Ca 2+ interconnect to construct the 3D metal-oxygen porous layer ( Figure S3b,c), which is rare in borates.

Structure Comparision
To date, there are three main types of nonaborate cluster FBBs with different configurations that have been obtained. According to the classifications and definitions of oxoboron clusters proposed by Christ, Clark and et al. [29,30], these nonaborate cluster FBBs can be divided into the following types:   (Figure 3f). The 2D layer in 2 is the first 2D layer built from the nonaborate clusters.

Powder XRD Patterns
The experimental PXRD patterns of 1 and 2 were all in good agreement with the simulated patterns obtained from single crystal data, which confirm the purities of the samples. The inconsistency of diffraction peaks' intensities between experimental and simulated patterns is attributed to the various preferred orientations of the samples ( Figure S4).

IR Spectra
The absorption bands and peaks in the IR spectra of 1 and 2 are similar in the range of 4000-400 cm  (Figure S5).

UV-Vis Absorption Spectra
UV-Vis absorption spectra of 1 and 2 were obtained in the wavelength range of 190-800 nm. As shown in Figure 4, both 1 and 2 have the short cutoff edges below 190 nm, showing the experimental band gaps of 6.31 eV (1, Figure 4a) and 6.39 eV (2, Figure 4b), respectively, indicating their potential applications in DUV regions. The short DUV cutoff edges and large bandgaps of 1 and 2 are mainly originated from the absences of d-d and f -f electron transitions of alkali-and alkaline-earth metal cations, which have been proved in other borates with DUV cutoff edges, including Li 2 CsB 7 O 10 (OH) 4 (6.35 eV) [27] and CsB 7 O 10 (OH) 2 (6.60 eV) [38]. No such cases were found in other borates without alkali and alkaline-earth metal cations.

Thermal Analysis
The thermal stabilities of 1 and 2 were examined in 25-1000 • C with the heating rate of 10 • C/min under the air atmosphere ( Figure S6). For 1, there was only one step weight loss of 4.49% (Cal: 3.83%) from 122 to 311 • C, corresponding to the removal of one lattice water molecule. For 2, the 3.64% (Cal: 3.33%) weight loss from 387 to 676 • C is assigned to one water molecule from the dehydration of two -OH groups.

Conclusions
In summary, two new nonaborates templated by mixed alkali-and alkaline earth metal cations, Na 2 Ba 0.5 [B 9 O 15 ]·H 2 O (1) and Na 4 Ca 1.5 [B 9 O 16 (OH) 2 ] (2), were successfully made under solvothermal conditions. 1 is made of three different [B 3 O 7 ] 5− clusters, of which two of them interconnects to make the 1D chains that are further joined to 3D framework by another [B 3 O 7 ] 5− clusters. There exist seven different types of intercommunicated channels in the framework of 1, which is first observed in 3D B-O frameworks. The nonaborate cluster in 2 is composed of four BO 4 -sharing [B 3 O 8 ] 7− clusters. It constructs the 2D monolayer with large 14-MR windows stacking in -ABAB-sequence, filling the blank of 2D layered nonaborates. H-bonds link adjacent layers to the 3D supramolecular open framework with three types of supramolecular channels. UV-Vis absorption spectra reveal that both 1 and 2 exhibit the short DUV cutoff edges below 190 nm, and the bandgaps are 6.31 and 6.39 eV, respectively, indicating that they have potential applications in DUV regions. Further work on exploring new DUV borates with excellent physical chemical properties is underway by using mixed alkali-and alkaline earth metal cationic templates under hydro(solvo)thermal conditions.