Modification of Visible-Light-Responsive Pb2Ti2O5.4F1.2 with Metal Oxide Cocatalysts to Improve Photocatalytic O2 Evolution toward Z-Scheme Overall Water Splitting.

The development of a highly active photocatalyst for visible-light water splitting requires a high-quality semiconductor material and a cocatalyst, which promote both the migration of photogenerated charge carriers and surface redox reactions. In this work, a cocatalyst was loaded onto an oxyfluoride photocatalyst, Pb2Ti2O5.4F1.2, to improve the water oxidation activity. Among the metal oxides examined as cocatalysts, RuO2 was found to be the most suitable, and the O2 evolution activity depended on the preparation conditions for Ru/Pb2Ti2O5.4F1.2. The highest activity was obtained with RuCl3-impregnated Pb2Ti2O5.4F1.2 heated under a flow of H2 at 523 K. The H2-treated Ru/Pb2Ti2O5.4F1.2 showed an O2 evolution rate an order of magnitude higher than those for the analogues without the H2 treatment (e. g., RuO2/Pb2Ti2O5.4F1.2). Physicochemical analyses by X-ray absorption fine-structure spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and time-resolved microwave conductivity measurements indicated that the optimized photocatalyst contained partially reduced RuO2 species with a particle size of ~5 nm. These partially reduced species effectively trapped the photogenerated charge carriers and promoted the oxidation of water into O2. The optimized Ru/Pb2Ti2O5.4F1.2 could function as an O2-evolving photocatalyst in Z-scheme overall water splitting, in combination with an Ru-loaded, Rh-doped SrTiO3 photocatalyst.

Three-Dimensional Visualization of Adsorption Distribution in a Single Crystalline Particle of a Metal-Organic Framework.

Many unique adsorption properties of metal-organic frameworks (MOFs) have been revealed by diffraction crystallography, visualizing their vacant and guest-loaded crystal structures at the molecular scale. However, it has been challenging to see the spatial distribution of the adsorption behaviors throughout a single MOF particle in a transient equilibrium state. Here, we report three-dimensional (3D) visualization of molecular adsorption behaviors in a single crystalline particle of a MOF by in situ X-ray absorption fine structure spectroscopy combined with computed tomography for the first time. The 3D maps of water-coordinated Co sites in a 100 µm-scale MOF-74-Co crystal were obtained with 1 µm spatial resolution under several water vapor pressures. Through the visualization of the water vapor adsorption process, 3D spectroimaging revealed the mechanism and spatial heterogeneity of guest adsorption inside a single particle of a crystalline MOF.

Structural diversity of copper(I)-ethylene complexes with 2,4-bis(2-pyridyl)pyrimidine directed by anions.

The 3 : 1 reaction of [Cu(C2H4)n]ClO4 with 2,4-bis(2-pyridyl)pyrimidine (bpprd) in Me2CO under C2H4 afforded yellow prism crystals of the dinuclear Cu(I)-C2H4 complex [Cu2(bpprd)(η2-C2H4)2(ClO4)2] (1). The 3 : 1 reaction of [Cu(C2H4)n]NO3 with bpprd in Me2CO under C2H4 afforded yellow plate crystals of the tetranuclear Cu(I)-C2H4 complex [Cu4(bpprd)2(η2-C2H4)4(µ-NO3)2](NO3)2 (2). The 10 : 1 reaction of [Cu(C2H4)n]BF4 with bpprd in Me2CO under C2H4 afforded yellow plate crystals of the dinuclear Cu(I)-C2H4 complex [Cu2(bpprd)(η2-C2H4)2(BF4)]BF4 (3). The 3 : 1 reaction of [Cu(C2H4)n]BF4 with bpprd in Me2CO under C2H4 afforded red prism crystals of the polymeric Cu(I)-C2H4 complex {[Cu6(bpprd)4(η2-C2H4)2(µ-η2:η2-C2H4)(µ-BF4)2](BF4)4}n (4). The X-ray crystal structures of complexes 1-4 have been determined. The structural diversity of Cu(I)-C2H4 complexes bridged by bpprd with different anions was demonstrated. The 1D Cu(I)-bpprd/C2H4 coordination polymer 4 bridged by unusual µ-η2:η2-C2H4 and the µ-BF4- anion is of particular significance. Complex 1 exhibited relatively well-resolved 1H NMR signals of bpprd and C2H4 (δ = 4.97 ppm) in (CD3)2CO at 23 °C.

Real-Space Observation of Ligand Hole State in Cubic Perovskite SrFeO3.

An anomalously high valence state sometimes shows up in transition-metal oxide compounds. In such systems, holes tend to occupy mainly the ligand p orbitals, giving rise to interesting physical properties such as superconductivity in cuprates and rich magnetic phases in ferrates. However, no one has ever observed the distribution of ligand holes in real space. Here, a successful observation of the spatial distribution of valence electrons in cubic perovskite SrFeO3 by high-energy X-ray diffraction experiments and precise electron density analysis using a core differential Fourier synthesis method is reported. A real-space picture of ligand holes formed by the orbital hybridization of Fe 3d and O 2p is revealed. The anomalous valence state in Fe is attributed to the considerable contribution of the ligand hole, which is related to the metallic nature and the absence of Jahn-Teller distortions in this system.

Inter-layer magnetic tuning by gas adsorption in π-stacked pillared-layer framework magnets.

Magnetism of layered magnets depends on the inter-layer through-space magnetic interactions (J NNNI). Using guest sorption to address inter-layer pores in bulk-layered magnets is an efficient approach to magnetism control because the guest-delicate inter-layer distance (l trans) is a variable parameter for modulating J NNNI. Herein, we demonstrated magnetic changes induced by the adsorption of CO2, N2, and O2 gases in various isostructural layered magnets with a π-stacked pillared-layer framework, , (M = Co, 1, Fe, 2, Cr, 3; Cp* = η5-C5Me5; 2,3,5,6-F4PhCO2 - = 2,3,5,6-tetrafluorobenzoate; TCNQ = 7,7,8,8-tetracyano-p-quinodimethane). Each compound had almost identical adsorption capability for the three types of gases; only CO2 adsorption was found to have a gated profile. A breathing-like structural modulation involving the extension of l trans occurred after the insertion of gases into the isolated pores between the [Ru2]2-TCNQ ferrimagnetic layers, which is more significant for CO2 than for O2 and N2, due to the CO2-gated transition. While adsorbent 1 with M = Co (S = 0) was an antiferromagnet with T N = 75 K, 1⊃CO2 was a ferrimagnet with T C = 76 K, whereas 1⊃N2 and 1⊃O2 were antiferromagnets with T N = 68 K. The guest-insertion effect was similarly confirmed in 2 and 3, and was characteristically dependent on the type of sandwiched spin in as M = Fe (S = 1/2) and Cr (S = 3/2), respectively. This study reveals that common gases such as CO2, O2, and N2 can serve as crucial triggers for the change in magnetism as a function of variable parameter l trans.

Isomerization-Controlled Proton-Electron Coupling in a π-Planar Metal Complex.

Proton-coupled electron transfer (PCET) is a ubiquitous and fundamental process in biochemistry and electrochemistry performed by transition-metal complexes. Most synthetic efforts have been devoted to selecting the components, that is, metal ions and ligands, to control the proton-electron coupling. Here, we show the first example of controlling the proton-electron coupling using the cis-trans metal-ligand isomerization in a π-planar platinum complex, Pt(itsq)2 (itsq1-: o-iminothiosemiquinonate). Both the isomers, which were obtained separately, were characterized by single-crystal X-ray diffraction, and the cis-to-trans isomerization was achieved by immersing in organic solvents. Theoretical calculations predicted that the proton-electron coupling evaluated from the energetic stabilization of the lowest unoccupied molecular orbital by protonation varies greatly depending on the geometrical configuration compared to the metal substitution.

Semiconductor-metal transition in Bi2Se3 caused by impurity doping.

Doping a typical topological insulator, Bi2Se3, with Ag impurity causes a semiconductor-metal (S-M) transition at 35 K. To deepen the understanding of this phenomenon, structural and transport properties of Ag-doped Bi2Se3 were studied. Single-crystal X-ray diffraction (SC-XRD) showed no structural transitions but slight shrinkage of the lattice, indicating no structural origin of the transition. To better understand electronic properties of Ag-doped Bi2Se3, extended analyses of Hall effect and electric-field effect were carried out. Hall effect measurements revealed that the reduction of resistance was accompanied by increases in not only carrier density but carrier mobility. The field-effect mobility is different for positive and negative gate voltages, indicating that the EF is located at around the bottom of the bulk conduction band (BCB) and that the carrier mobility in the bulk is larger than that at the bottom surface at all temperatures. The pinning of the EF at the BCB is found to be a key issue to induce the S-M transition, because the transition can be caused by depinning of the EF or the crossover between the bulk and the top surface transport.

Electron Density Analysis of Metal-Metal Bonding in a Ni4 Cluster Featuring Ferromagnetic Exchange.

We present a combined experimental and theoretical study of the nature of the proposed metal-metal bonding in the tetranuclear cluster Ni4(NPtBu3)4, which features four nickel(I) centers engaged in strong ferromagnetic coupling. High-resolution single-crystal synchrotron X-ray diffraction data collected at 25 K provide an accurate geometrical structure and a multipole model electron density description. Topological analysis of the electron density in the Ni4N4 core using the quantum theory of atoms in molecules clearly identifies the bonding as an eight-membered ring of type [Ni-N-]4 without direct Ni-Ni bonding, and this result is generally corroborated by an analysis of the energy density distribution. In contrast, the calculated bond delocalization index of ∼0.6 between neighboring Ni atoms is larger than what has been found for other bridged metal-metal bonds and implies direct Ni-Ni bonding. Similar support for the presence of direct Ni-Ni bonding is found in the interacting quantum atom approach, an energy decomposition scheme, which suggests the presence of stabilizing Ni-Ni bonding interactions with an exchange-correlation energy contribution approximately 50% of that of the Ni-N interactions. Altogether, while the direct interactions between neighboring Ni centers are too weak and sterically constrained to bear the signature of a topological bond critical point, other continuous measures clearly indicate significant Ni-Ni bonding. These metal-metal bonding interactions likely mediate direct ferromagnetic exchange, giving rise to the high-spin ground state of the molecule.

Observation of an Alternate Charge-Polarization State in a One-Dimensional Pt-Pt-I Chain Compound with a Bulky Pendant Ligand.

A one-dimensional (1D) halogen-bridged dinuclear-metal complex (MMX-chain) exhibits various electronic states based on a mixed-valence metal-dimer system. This report deals with the synthesis and physical properties of a new MMX-chain with a bulky pendant ligand, Pt2 (mcc-HexCS2 )4 I (mcc-HexCS2 =trans-4-(methoxycarbonyl)cyclohexanedithiocarboxylate). The steric hindrance caused by the bulky substituent induces a strain in its 1D chain, achieving at ambient condition the first pure alternate charge-polarization (ACP) state (-Pt2+ -Pt3+ -I- -Pt3+ -Pt2+ -I- -), a kind of spin-Peierls state, as confirmed by X-ray diffraction and its conducting and magnetic properties. The strain effect is also manifested as a very large linear thermal expansion along the chain direction, which is quite different from conventional MMX-chains without a bulky ligand. The design of low-dimensional materials with ligand variations is expected to lead to the emergence of new electron-lattice coupled electronic states.

Data-driven efficient synthetic exploration of anionic lanthanide-based metal-organic frameworks.

The synthesis of lanthanide metal-organic frameworks with terephthalate (Ln-BDC-MOFs) was investigated using a data-driven approach. Visually mapping the previously reported synthetic conditions suggested the existence of unexplored search spaces for novel Ln-BDC-MOFs. By focusing on the unexplored chemical reaction space, we successfully synthesized a series of new anionic Ln-BDC-MOFs, KGF-15, which demonstrated potential as luminescent sensors for Cu2+ ions. This synthetic exploration approach can significantly reduce the experimental effort required to discover new materials.

Tunable acetylene sorption by flexible catenated metal-organic frameworks.

The safe storage of flammable gases, such as acetylene, is essential for current industrial purposes. However, the narrow pressure (P) and temperature range required for the industrial use of pure acetylene (100 < P < 200 kPa at 298 K) and its explosive behaviour at higher pressures make its storage and release challenging. Flexible metal-organic frameworks that exhibit a gated adsorption/desorption behaviour-in which guest uptake and release occur above threshold pressures, usually accompanied by framework deformations-have shown promise as storage adsorbents. Herein, the pressures for gas uptake and release of a series of zinc-based mixed-ligand catenated metal-organic frameworks were controlled by decorating its ligands with two different functional groups and changing their ratio. This affects the deformation energy of the framework, which in turn controls the gated behaviour. The materials offer good performances for acetylene storage with a usable capacity of ~90 v/v (77% of the overall amount) at 298 K and under a practical pressure range (100-150 kPa).

BN-Substitution in Dithienylpyrenes Prevents Excimer Formation in Solution and in the Solid State.

Boron-nitrogen substitutions in polycyclic aromatic hydrocarbons (PAHs) have a strong impact on the optical properties of the molecules due to a significantly more heterogeneous electron distribution. However, besides these single-molecule properties, the observed optical properties of PAHs critically depend on the degree of intermolecular interactions such as π-π-stacking, dipolar interactions, or the formation of dimers in the excited state. Pyrene is the most prominent example showing the latter as it exhibits a broadened and strongly bathochromically shifted emission band at high concentrations in solution compared to the respective monomers. In the solid state, the impact of intermolecular interactions is even higher as it determines the crystal packing crucially. In this work, a thiophene-flanked BN-pyrene (BNP) was synthesized and compared with its all-carbon analogue (CCP) in solution and in the solid state by means of crystallography, NMR spectroscopy, UV-vis spectroscopy, and photoluminescence (PL) spectroscopy. In solution, PL spectroscopy revealed the solvent-dependent presence of excimers of CCP at high concentrations. In contrast, no excimers were found in BNP. Clear differences were also observed in the single-crystal packing motifs. While CCP revealed overlapped pyrene planes with centroid distances in the range of classical π-stacking interactions, the BNP scaffolds were displaced and significantly more spatially separated.

X-ray Electron Density Study of the Chemical Bonding Origin of Glass Formation in Metal-Organic Frameworks.

Glass-forming metal-organic frameworks (MOFs) have novel applications, but the origin of their peculiar melting behavior is unclear. Here, we report synchrotron X-ray diffraction electron densities of two zeolitic imidazolate frameworks (ZIFs), the glass-forming Zn-ZIF-zni and the isostructural thermally decomposing Co-ZIF-zni. Electron density analysis shows that the Zn-N bonds are more ionic than the Co-N bonds, which have distinct covalent features. Variable-temperature Raman spectra reveal the onset of significant imidazolate bond weakening in Co-ZIF-zni above 673 K. Melting can be controlled by tuning the metal-ligand and imidazole bonding strength as shown from thermal analysis of nine solid-solution Cox Zn1-x -ZIF-zni (x=0.3 to 0.003) MOFs, and a mere 4 % Co-doping into Zn-ZIF-zni results in thermal decomposition instead of melting. The present findings demonstrate the key role of the metal-ligand bonds and imidazolate bonds in controlling the delicate balance between melting and decomposition processes in this class of ZIF compounds.

Melamine-induced synthesis of a structurally perfect kagomé antiferromagnet.

We report here a structurally perfect kagomé lattice {[Cu3(bpy)6](SiF6)3(melamine)8}n (1), where bpy is 4,4'-bipyridine and [SiF6]2- is a hexafluorosilicate anion. In comparison to general 1D linear, 2D layered and 3D cubic metal-organic frameworks, by using Cu2+ nodes and bpy ligands, a perfect kagomé lattice was synthesized by introducing C3 symmetrical melamine molecules. Magnetic susceptibility and low-temperature heat capacity measurements indicated weak antiferromagnetic interactions between the spins and no long-range magnetic ordering to 0.7 K. Using C3 symmetrical melamine molecules can be considered as a challenging synthetic strategy to afford new topological materials.

Heterospin frustration in a metal-fullerene-bonded semiconductive antiferromagnet.

Lithium-ion-encapsulated fullerenes (Li+@C60) are 3D superatoms with rich oxidative states. Here we show a conductive and magnetically frustrated metal-fullerene-bonded framework {[Cu4(Li@C60)(L)(py)4](NTf2)(hexane)}n (1) (L = 1,2,4,5-tetrakis(methanesulfonamido)benzene, py = pyridine, NTf2- = bis(trifluoromethane)sulfonamide anion) prepared from redox-active dinuclear metal complex Cu2(L)(py)4 and lithium-ion-encapsulated fullerene salt (Li+@C60)(NTf2-). Electron donor Cu2(L)(py)2 bonds to acceptor Li+@C60 via eight Cu‒C bonds. Cu-C bond formation stems from spontaneous charge transfer (CT) between Cu2(L)(py)4 and (Li+@C60)(NTf2-) by removing the two-terminal py molecules, yielding triplet ground state [Cu2(L)(py)2]+(Li+@C60•-), evidenced by absorption and electron paramagnetic resonance (EPR) spectra, magnetic properties and quantum chemical calculations. Moreover, Li+@C60•- radicals (S = ½) and Cu2+ ions (S = ½) interact antiferromagnetically in triangular spin lattices in the absence of long-range magnetic ordering to 1.8 K. The low-temperature heat capacity indicated that compound 1 is a potential candidate for an S = ½ quantum spin liquid (QSL).

A Preinstalled Protic Cation as a Switch for Superprotonic Conduction in a Metal-Organic Framework.

Metal-organic frameworks (MOFs), made from various metal nodes and organic linkers, provide diverse research platforms for proton conduction. Here, we report on the superprotonic conduction of a Pt dimer based MOF, [Pt2(MPC)4Cl2Co(DMA)(HDMA)·guest] (H2MPC, 6-mercaptopyridine-3-carboxylic acid; DMA, dimethylamine). In this framework, a protic dimethylammonium cation (HDMA+) is trapped inside a pore through hydrogen bonding with an MPC ligand. Proton conductivity and X-ray measurements revealed that trapped HDMA+ works as a preinstalled switch, where HDMA+ changes its relative position and forms an effective proton-conducting pathway upon hydration, resulting in more than 105 times higher proton conductivity in comparison to that of the dehydrated form. Moreover, the anisotropy of single-crystal proton conductivity reveals the proton-conducting direction within the crystal. The present results offer insights into functional materials having a strong coupling of molecular dynamic motion and transport properties.

Synthesis and Magnetic Properties of a Dimerized Trinuclear Ni String Complex, [Ni6Cl2(dpa)8](I5)2·0.25I2 (dpa- = 2,2'-Dipyridylamide Anion).

Metal string complexes, linearly aligned transition metal arrays coordinated with the multidentate organic ligands, have gained much attention both in unique electronic/structural properties and in potential applications as conductive molecular nanowires. Here we report on a dimerized NiII trinuclear complex, [Ni6Cl2(dpa)8](I5)2·0.25I2 (dpa- = 2,2'-dipyridylamide anion). X-ray structural analysis revealed that two trinuclear moieties are bridged by a Cl anion to form a dimerized string structure. This is the first example of two Ni string complexes that are connected. In the electronic absorption and Raman spectra, characteristic absorption bands and a vibration mode based on the dimer string structure were observed. In the solid state, dimer complexes align in one dimension in an MMMXMMMX (M = metal, X = halogen) manner, leading to the intra- and interdimer antiferromagnetic interactions.

Correction: Dinuclear cobalt complexes with a redox active biphenyl bridging ligand [Co2(BP)(tqa)2](PF6)2 (H4BP = 4,4'-bis(3-tert-butyl-1,2-catechol), tqa = tris(2-quinolylmethyl)amine): structure and magnetic properties.

Correction for 'Dinuclear cobalt complexes with a redox active biphenyl bridging ligand [Co2(BP)(tqa)2](PF6)2 (H4BP = 4,4'-bis(3-tert-butyl-1,2-catechol), tqa = tris(2-quinolylmethyl)amine): structure and magnetic properties' by Yusaku Suenaga et al., Dalton Trans., 2021, DOI: .

Various Stacking Patterns of Two-Dimensional Molecular Assemblies in Hydrogen-Bonded Cocrystals: Insight into Competitive Intermolecular Interactions and Control of Stacking Patterns.

Control over the stacking patterns in 2D molecular assemblies is demonstrated using chemical modification. A target system is a hydrogen-bonded cocrystal (2:1) composed of 2-pyrrolidone (Py) and chloranilic acid (CA) (PyCA). X-ray crystallography showed that weak intersheet interactions give rise to a variety of metastable overlapping patterns comprised of the 2D assemblies mainly formed via hydrogen bonds, affording reversible and irreversible structural phase transitions. We prepared cocrystals of Py and anilic acids bearing different halogens, in which 2D assemblies isostructural with those observed in PyCA exhibit various overlapping patterns. The order of stability for each overlapping pattern estimated using calculations of the intermolecular interactions did not completely coincide with those indicated by our experimental results, which can be explained by considering the entropic effect: the molecular motion of Py as detected using nuclear quadrupole resonance spectroscopy.

Dinuclear cobalt complexes with a redox active biphenyl bridging ligand [Co2(BP)(tqa)2](PF6)2 (H4BP = 4,4'-bis(3-tert-butyl-1,2-catechol), tqa = tris(2-quinolylmethyl)amine): structure and magnetic properties.

The biscatechol, H4BP (4,4'-bis(3-tert-butyl-1,2-catechol)) that can directly connect two redox active catechol moieties was synthesized. Also, tris(2-pyridylmethyl)amine (tpa), bis(2-pyridylmethyl)(2-quinolylmethyl)amine (bpqa), (2-pyridylmethyl)bis(2-quinolyl methyl)amine (pbqa), and tris (2-quinolylmethyl)amine (tqa) were synthesized as terminal ligands of the tetracoordinated tripod. In total, five different dinuclear Co complexes were synthesized from H4BP with various terminal ligands as follows, [Co2(BP)(tpa)2](PF6)2 (1), [Co2(BP)(tpa)2](PF6)3 (2), [Co2(BP)(bpqa)2](PF6)2 (3), [Co2(BP)(pbqa)2](PF6)2 (4), and [Co2(BP)(tqa)2](PF6)2 (5). After a one-electron oxidation reaction of complex (1), complex (2), was isolated as a mixed valence state lsCoIII-[SQ-Cat]-lsCoIII, with an absorption intensity of about 1370 nm (intervalence charge transfer (IVCT) bands) in CH3CN solution. In addition, an investigation of the magnetic properties of the dinuclear Co complex (3) with SQUID showed that the χMT value gradually increased as the temperature increased from 280 to 380 K. Studies in the solid and solution states using electronic spectra, cyclic voltammetry and SQUID for the above complexes provide clear evidence for three different charge distributions: complexes (1) and (3) are CoIII-[Cat-Cat]-CoIII, complex (2) is CoIII-[Sq-Cat]-CoIII, complexes (4) and (5) are CoII-[Sq-Sq]-CoII. Of the five cobalt dinuclear complexes, only complex (3) shows evidence of the temperature dependence of the charge distribution, displaying a thermally induced valence tautomeric transition from the lsCoIII-[Cat-Cat]-lsCoIII to hsCoII-[Sq-Sq]-hsCoII in both solid and solution states. However, this valence tautomeric step is incomplete at 380 K, with the χMT value of hsCoII-[Sq-Sq]-hsCoII. This suggests that the steric hindrance of the quinolyl rings around the Co ion produces a coordination atmosphere that is weaker than that observed with pyridyl rings, which facilitates a change in the CoIII ions to CoII.