Uncoordinated Carboxyl Groups as Proton Sources in Polyoxometalate-Based Metal-Organic Frameworks Enhance Proton Conduction

To select appropriate organic ligands is an effective strategy to enhance the proton conductivities of polyoxometalate-based metal-organic frameworks (POMOFs). Two new Dawson-type POMOFs, named CUST-961 and CUST-962, have been designed and synthesized via combining Htzbc selected by hard and soft acid and base theory and density functional theory calculation, transition metal ions, alkali metal ions (Na+ and K+), and Dawson-type polyoxometalates ([P2W18]6-) under the hydrothermal method. Their stabilities under different temperatures and relative humidities (RHs) have been investigated through powder X-ray diffraction and thermogravimetric analysis. Both CUST-961 and CUST-962 exhibited excellent aqueous and thermal stabilities. The alternating current (AC) impedance spectrum tests revealed that the proton conductivity of CUST-961 could reach 1.4 × 10-4 S cm-1 at 95 °C and 98% RH, which is about 3 times that of CUST-962. The different proton conductivities between the two compounds are due to the fact that CUST-961 possesses more uncoordinated carboxylic acid groups, as confirmed by attenuated total reflection infrared spectroscopy and 1H solid-state nuclear magnetic resonance spectroscopy, which can not only act as the proton source but also establish a richer hydrogen bonding network to enhance proton conduction. This work provides a new strategy and insight for the design and preparation of polyoxometalate-based proton conductive materials.

Nanomaterial with Core-Shell Structure Composed of {P2W18O62} and Cobalt Homobenzotrizoate for Supercapacitors and H2O2-Sensing Applications

Designing and preparing dual-functional Dawson-type polyoxometalate-based metal-organic framework (POMOF) energy storage materials is challenging. Here, the Dawson-type POMOF nanomaterial with the molecular formula CoK₄[P₂W₁₈O₆₂]@Co₃(btc)₂ (abbreviated as {P₂W₁₈}@Co-BTC, H₃btc = 1,3,5-benzylcarboxylic acid) was prepared using a solid-phase grinding method. XRD, SEM, TEM et al. analyses prove that this nanomaterial has a core-shell structure of Co-BTC wrapping around the {P₂W₁₈}. In the three-electrode system, it was found that {P₂W₁₈}@Co-BTC has the best supercapacitance performance, with a specific capacitance of 490.7 F g^-1 (1 A g^-1) and good stability, compared to nanomaterials synthesized with different feedstock ratios and two precursors. In the symmetrical double-electrode system, both the power density (800.00 W kg^-1) and the energy density (11.36 Wh kg^-1) are greater. In addition, as the electrode material for the H₂O₂ sensor, {P₂W₁₈}@Co-BTC also exhibits a better H₂O₂-sensing performance, such as a wide linear range (1.9 μM-1.67 mM), low detection limit (0.633 μM), high selectivity, stability (92.4%) and high recovery for the detection of H₂O₂ in human serum samples. This study provides a new strategy for the development of Dawson-type POMOF nanomaterial compounds.

Porous polyoxotungstate/MXene hybrid films allowing for visualization of the energy storage status in high-performance electrochromic supercapacitors

Electrochromic supercapacitors (ECSCs) have recently received growing attention for potential smart energy storage components in intelligent electronics. However, in the development of ECSCs, the design and assembly of high-performance electrode materials remain ongoing challenges. In this study, Ti₃C₂T_x MXene and polyoxotungstate (P₂W₁₈) were deposited on TiO₂ nanowires to construct a unique three-dimensional (3D) porous hybrid film, NW@MXene/P₂W₁₈, via a convenient layer-by-layer self-assembly approach. The 3D porous structure of the nanocomposite reduced the aggregation and stacking of Ti₃C₂T_x MXene nanosheets during self-assembly, leading to the formation of unobstructed ion diffusion channels and interfacial charge transfer between adjacent layers, resulting in a good electrochemical performance. Compared to the tightly packed structure, the porous hybrid film demonstrated an enhanced electrochromic energy storage performance with a higher areal capacitance (i.e., 19.0 mF cm^-2 at a current density of 0.6 mA cm^-2), in addition to a high cycling stability (i.e., 90.7% retention rate after 2000 cycles), and an excellent color rendering efficiency. Subsequently, an asymmetric ECSC was fabricated using an NW@MXene/P₂W₁₈ film as the cathode and a TiO₂ nanowire film as the anode. This ECSC exhibited a high areal capacitance of 4.0 mF cm^-2 at a current density of 0.1 mA cm^-2 with a wide operating window of 4.5 V, whilst also achieving high-speed color switching between olive green and dark blue during the charge/discharge processes, ultimately offering new avenues for the development of electrochromic energy storage electrode materials and the design of novel devices.

Sterically Hindered Tellurium(IV) Catecholate as a Lewis Acid

Sterically hindered tellurium catecholate Te(Cat³⁶)₂ (Cat³⁶ = 3,6-di-tert-butyl-catecholate) was synthesized with the reaction of amorphous Te with 3,6-di-tert-butyl-o-benzoquinone. Adducts of Te(Cat³⁶)₂ with various O- and N-donors were isolated and characterized by means of single-crystal X-ray diffraction along with IR, UV-vis, and NMR (¹H, ¹³C, and ¹²⁵Te) spectroscopies. In the crystal structure of the adduct with 2,2'-bipyridine (bipy), the unprecedented μ-κ²N,N':κ²N,N'-bridging coordination mode of bipy was observed. Various intermolecular interactions Te...O, Te...N, and Te...C in adducts were analyzed using density functional theory calculations and quantum theory of atoms in molecules analysis. The estimated strength for appropriate short contacts varies from 0.9 to 5.3 kcal/mol, and they are attractive and purely non-covalent.

Unsymmetrical Coordination of Bipyridine in Three-Coordinate Gold(I) Complexes

The unsymmetrical coordination of gold(I) by 2,2'-bipyridine (bipy) in some planar, three-coordinate cations has been examined by crystallographic and computational studies. The salts [(Ph₃P)Au(bipy)]XF₆ (X = P, As, Sb) form an isomorphic series in which the differences in Au-N distances range from 0.241(2) to 0.146(2) Å. A second polymorph of [(Ph₃P)Au(bipy)]AsF₆ has also been found. Both polymorphs exhibit similar structures. The salts [(Et₃P)Au(bipy)]XF₆ (X = P, As, Sb) form a second isostructural series. In this series the unsymmetrical coordination of the bipy ligand is maintained, but the gold ions are disordered over two unequally populated positions that produce very similar overall structures for the cations. Although many planar, three-coordinate gold(I) complexes are strongly luminescent, the salts [(R₃P)Au(bipy)]XF₆ (R = Ph or Et; X = P, As, Sb) are not luminescent as solids or in solution. Computational studies revealed that a fully symmetrical structure for [(Et₃P)Au(bipy)]⁺ is 7 kJ/mol higher in energy than the observed unsymmetrical structure and is best described as a transition state between the two limiting unsymmetrical geometries. The Au-N bonding has been examined by natural resonance theory (NRT) calculations using the "12 electron rule". The dominant Lewis structure is one with five lone pairs on Au and one bond to the P atom, which results in a saturated (12 electron) gold center and thereby inhibits the formation of any classical, 2 e^- bonds between the gold and either of the bipy nitrogen atoms. The nitrogen atoms may instead donate a lone pair into an empty Au-P antibonding orbital, resulting in a three-center, four-electron (3c/4e) P-Au-N bond. The binuclear complex, [μ₂-bipy(AuPPh₃)₂](PF₆)₂, has also been prepared and shown to have an aurophillic interaction between the two gold ions, which are separated by 3.0747(3) Å. Despite the aurophillic interaction, this binuclear complex is not luminescent.

A Series of Compounds Based on [P2W18O62]6- and Transition Metal Mixed Organic Ligand Complexes with High Catalytic Properties for Styrene Epoxidation

Seven compounds based on [P₂W₁₈O₆₂]^6- ({P₂W₁₈}) were successfully prepared and carefully characterized. [HC₅H₅N][Cu(2,2'-bpy)₂]₂[HP₂W₁₈O₆₂]·2H₂O (bpy = bipyridine) (1) is constructed from {P₂W₁₈} bridged by [Cu(2,2'-bpy)₂]²⁺. [HC₅H₅N][Zn(2,2'-bpy)₂]₂[HP₂W₁₈O₆₂]·4H₂O (1a) is isostructural and isomorphous with compound 1. [Cu₄(2,2'-bpy)₃(nic)₃(OH)₂(H₂O)][H₃P₂W₁₈O₆₂]·0.5H₂O (nic = nicotinic acid) (2) is formed by {P₂W₁₈} and tetracopper transition metal mixed organic ligand complexes (TMMCs). [Cu₂(2,2'-bpy)₂(C₂O₄)]₃[P₂W₁₈O₆₂]·3H₂O (3) is made up of {P₂W₁₈} and bicopper TMMCs, [Cu₆(2,2'-bpy)₆(OH)₆][P₂W₁₈O₆₂]·2H₂O (4) is built up from {P₂W₁₈}, and hexacopper complexes of 2,2'-bpy and hydroxyls. [Cu(2,2'-bpy)(hnic)_0.5][Cu₃(2,2'-bpy)₃(hnic)₂(H₂O)₂][H₃P₂W₁₈O₆₂] (hnic = hydroxyl nicotinic acid) (5a) contains two different TMMCs. In addition, compound 5a is the first example of a compound that contains Cu-π interactions. [Cu₂(2,2'-bpy)₂(hnic)][H₄P₂W₁₈O₆₂]· xH₂O ( x ≈ 50) (5b) is based on {P₂W₁₈} and [Cu₂(2,2'-bpy)₂(hnic)]²⁺. We discuss the mechanisms for the formations of these compounds. All the catalytic performances of the compounds for styrene epoxidation to styrene oxide are high.

Efficient and robust photocatalysts based on {P2W18} modified by an Ag complex

Ag⁺ and the flexible ligand bimb were introduced into Dawson phosphomolybdate systems as linkage units to induce two fascinating 3-D inorganic–organic networks, which exhibit excellent electro- and photo-catalytic behavior.