A Three-Dimensionally Extended Metal-Organic Ladder Compound Exhibiting Proton Conduction.

Ladder systems situated in the dimensional crossover region have attracted much attention because their electronic states and physical properties depend strongly on the electronic correlations among the constituent legs. Generally, two-/three-legged transition metal-oxide ladder compounds are studied as representative ladder systems, but two-/three-dimensional (2D/3D) extensions based on such ladder systems with a few numbers of legs are difficult because of the extreme synthesis conditions. Here, for the first time, we report the successful creation of a 3D extended two-legged ladder compound, [Pt(en)(dpye)I]2(NO3)4 ⋅ 2H2O (en=ethylenediamine; dpye=1,2-Di(4-pyridyl)ethane), which is obtained by simple oxidative polymerization of a small Pt macrocyclic complex using elemental I2. The unique 3D extended lattice consists of 1D mixed-valence halogen-bridged metal chains (⋅⋅⋅Pt-I-Pt-I⋅⋅⋅) and helically arranged macrocyclic units as the constituent legs and rungs, as confirmed by single-crystal X-ray diffraction. Diffuse X-ray scattering analyses and optical measurements revealed that the out-of-phase mixed-valence Pt2+/Pt4+ arrangement arises from the weak interchain correlation among adjacent legs. In addition, this compound shows an increase in proton conductivity by a factor of up to 1000, depending on humidity.

A novel threefold interpenetrated zirconium metal-organic framework exhibiting separation ability for strong acids.

We report on the synthesis and selective adsorption property of a novel threefold interpenetrated Zr-based metal-organic framework (MOF), [Zr12O8(OH)8(HCOO)15(BPT)3] (BPT3- = [1,1'-biphenyl]-3,4',5-tricarboxylate) (abbreviated as Zr-BPT). This MOF shows a high tolerance to acidic conditions and has permanent pores, the size of which (approx. <5.6 Å) is the smallest ever reported among porous Zr-based MOFs with high acid tolerance. Zr-BPT selectively adsorbs aryl acids due to its strong affinity for them and exhibits separation ability, even between strong acid molecules, such as sulfonic and phosphonic acids. This is the first demonstration of a MOF exhibiting selective adsorption and separation ability for strong acids.

Self-Assembled Crystalline Bundles in Soluble Metal-Organic Nanotubes.

The use of nanotubes in the solution state is crucial not only for the exploration of physical and chemical behaviors at the molecular level but also for application such as thin-film fabrication. Surface modification is generally used to solubilize carbon nanotubes (CNTs) and various synthetic nanotubes; however, this method may affect the surface properties of the original nanotubes, and the detailed crystal structure obtained after modification is unclear. Here, we report the synthesis of a crystalline and soluble metal-organic nanotube consisting of a cationic tubular framework and an anion with a long alkyl chain. The nanotubular structures are formed not only in the solid state but also in the solution state, as confirmed by an X-ray structural analysis, optical measurements, and electron microscopy studies. This nanotube system is realized in different states without any surface modification, which is quite different from typical CNTs and synthetic nanotubes. In addition, self-assembled crystalline bundles are directly observed using transmission electron microscopy (TEM) for the first time in a metal-organic nanotube system. The bundle structures are also confirmed by atomic force microscopy (AFM) observations of thin nanotube films. We envisage a systematic design of such soluble metal-organic nanotubes that will enable direct observation of mass transport behavior in channels of bundles or a single nanotube, as well as a wide range of thin-film applications.

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.

Dibenzodiazapyracylenes: Doubly N-Doped Cyclopenta-Fused Polycyclic Molecules That Exhibit High Carrier Mobility.

Pyracylene is a unique cyclopenta-fused polycyclic aromatic hydrocarbon (CP-PAH) that exhibits dual aromatic characteristics. Herein we report the synthesis of doubly N-doped benzannulated pyracylenes, namely dibenzodiazapyracylenes, by oxidative N-N linking reaction of [2.2](2,5)pyrrolophane-type precursors. Dibenzodiazapyracylenes displayed well-ordered π-stacked molecular packing in the solid state, which were feasible for effective hole-transporting along the stacking direction. High carrier mobility was estimated by microwave conductivity measurements as compared to dibenzoullazine. The high HOMO level of dibenzodiazapyracylene was verified by electrochemistry and its persistent radical cation species has been detected.

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.

Practical Asymmetric Synthesis of Chiral Sulfoximines via Sulfur-Selective Alkylation.

Chiral sulfoximines have recently been considered as promising bioisosteres in medicinal chemistry. However, methods for preparing chiral sulfoximines in a stereoselective manner are underdeveloped. Herein, we demonstrate an asymmetric synthesis of chiral sulfoximines through a stereospecific S-alkylation of readily accessible chiral sulfinamides under practical conditions. A key to establishing the practical conditions was the identification of the intermediate structure in our previously reported S-alkylation by X-ray crystallographic analysis.

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.

Strain-Controlled Spin Transition in Heterostructured Metal-Organic Framework Thin Film.

Metal-organic framework (MOF) thin films have recently attracted much attention as a new platform for surface/interface research, where unconventional structural and physical properties emerge. Among the many MOFs as candidates for fabrication of thin films, Hofmann-type MOFs {Fe(pz)[M(CN)4]} [pz = pyrazine; M = Ni (Nipz), M = Pt (Ptpz)] are attractive, because they undergo spin transitions with concomitant structural changes. Here, we demonstrate the first example of a strain-controlled spin transition in heterostructured MOF thin films. The spin transition temperature of Ptpz can be controlled in the temperature range of 300-380 K by fabricating a nanometer-sized heterostructured thin film with a Nipz buffer layer, where the smaller lattice of Nipz causes epitaxial compressive strain to the Ptpz layer. The fabricated heterostructured thin film exhibited a remarkable increase in spin transition temperature with a dynamic structural transformation, confirmed by variable-temperature (VT) X-ray diffraction and VT Raman spectroscopy. By verifying interfacial strain in a heterostructured thin film, we can rationally control the characteristics of MOFs-not only spin transition but also various physical properties such as gas storage, catalysis, sensing, proton conductivity, and electrical properties, among others.

Ni@onion-like carbon and Co@amorphous carbon: control of carbon structures by metal ion species in MOFs.

We first report the facile synthesis of metal-carbon composites consisting of metal nanoparticles (NPs) and different types of carbon species: onion-like and amorphous carbon, Ni@onion-like carbon and Co@amorphous carbon. By simply changing the metal species in an isostructural metal-organic framework, thermal decompositions of MOF-74 directly afforded different types of metal NPs and carbon composites, which exhibited good electrical conductivity. In particular, the Ni@onion-like carbon, having a well-ordered carbon structure, had high electrical conductivity (σ = 5.3 Ω-1 cm-1 at 295 K), explained by a modified model of the Efros-Shklovskii variable range hopping.

Surface morphology-induced spin-crossover-inactive high-spin state in a coordination framework.

Here we report a surface morphology-induced spin state control in ultrathin films of a spin-crossover (SCO) material. The surface microstructure of film domains exhibited selectivity, to stabilize the SCO-active high-spin (HS) or SCO-inactive high-spin (HS2) states. To date, the latter has only been confirmed in the bulk counterpart at gigapascal pressure.

Crystal Size Effect on the Spin-Crossover Behavior of {Fe(py)2[Pt(CN)4]} (py = Pyridine) Monitored by Raman Spectroscopy.

The spin-crossover phenomenon in nanomaterials has been the subject of exploratory investigations for stimuli-responsive switching properties at nanoscale. Using variable-temperature Raman spectroscopy, we investigated changes in the temperature-driven spin-transition property of {Fe(py)2[Pt(CN)4]} (py = pyridine) induced by a size reduction from a bulk polycrystalline powder to an ultrathin film (crystallite size, 15 nm). When the crystallite size was reduced, the spin-transition temperature shifted lower and the spin-transition curves became less steep. In the thin-film form, the residual high-spin fraction was found to be about 20%.

A mixed-valent metal-organic ladder linked by pyrazine.

We report the synthesis, characterization, and electronic state of a novel mixed-valent metal-organic ladder (MOL) linked by pyrazine (pz). Single-crystal x-ray studies revealed that the MOL has a two-legged ladder-shaped framework, which is composed of a pz-connected Pt dimer with bridging Br ions. The electronic state of the MOL was investigated using x-ray and spectroscopic techniques; the MOL was found to have an electronic state that corresponds to the mixed-valence state of PtIIand PtIV. Furthermore, the intervalence charge transfer energy of the MOL has lower than that expected from the tendency of a similar halogen-bridged mixed-valence MOL owing to its unique 'zig-zag'-shaped legs. These results provide a new insight into the physical and electronic properties of MOL systems.

Remarkably enhanced proton conduction of {NBu2(CH2COOH)2}[MnCr(ox)3] by multiplication of carboxyl carrier in the cation.

The proton conduction of {NBu2(CH2COOH)2}[MnCr(ox)3] (dic-MnCr) is studied in comparison with its analogous {NBu3(CH2COOH)}[MnCr(ox)3] (moc-MnCr). The proton conductivity is enhanced remarkably by the multiplication of the carboxyl carrier in the cation, from 5.2 × 10-7 S cm-1 at 90% RH (25 °C) in moc-MnCr to 1.8 × 10-3 S cm-1 at 95% RH (25 °C) in dic-MnCr.

Confined water-mediated high proton conduction in hydrophobic channel of a synthetic nanotube.

Water confined within one-dimensional (1D) hydrophobic nanochannels has attracted significant interest due to its unusual structure and dynamic properties. As a representative system, water-filled carbon nanotubes (CNTs) are generally studied, but direct observation of the crystal structure and proton transport is difficult for CNTs due to their poor crystallinity and high electron conduction. Here, we report the direct observation of a unique water-cluster structure and high proton conduction realized in a metal-organic nanotube, [Pt(dach)(bpy)Br]4(SO4)4·32H2O (dach: (1R, 2R)-(-)-1,2-diaminocyclohexane; bpy: 4,4'-bipyridine). In the crystalline state, a hydrogen-bonded ice nanotube composed of water tetramers and octamers is found within the hydrophobic nanochannel. Single-crystal impedance measurements along the channel direction reveal a high proton conduction of 10-2 Scm-1. Moreover, fast proton diffusion and continuous liquid-to-solid transition are confirmed using solid-state 1H-NMR measurements. Our study provides valuable insight into the structural and dynamical properties of confined water within 1D hydrophobic nanochannels.

A Phosphate-Based Silver-Bipyridine 1D Coordination Polymer with Crystallized Phosphoric Acid as Superprotonic Conductor.

Phosphate-based silver-bipyridine (Ag-bpy) 1D coordination polymer {[{Ag(4,4'-bpy)}2 {Ag(4,4'-bpy)(H2 PO4 )}]⋅2 H2 PO4 ⋅H3 PO4 ⋅5 H2 O}n (1) with free phosphoric acid (H3 PO4 ), its conjugate base (H2 PO4 - ) and water molecules in its lattice was synthesized by room-temperature crystallization and the hydrothermal method. An XRD study showed that coordinated H2 PO4 - , lattice H2 PO4 - anions, free H3 PO4 and lattice water molecules are interconnected by H-bonding interactions, forming an infinitely extended 2D H-bonded network that facilitates proton transfer. This material exhibits a high proton conductivity of 3.3×10-3  S cm-1 at 80 °C and 95 % relative humidity (RH). Furthermore, synthesis of this material from commercially available starting materials in water can be easily scaled up, and it is highly stable under extreme conditions of conductivity measurements. This report inaugurates the usage and design principle of proton-conducting frameworks based on crystallized phosphoric acid and phosphate.

Network-Selectivity, Magnetism, and Proton Conduction of 2-D and 3-D Metal-Organic Frameworks of the Constituents {P(CH2OH)4}+/MII (MnII, FeII, or CoII)/[CrIII(ox)3]3.

Two types of metal-organic frameworks (MOFs), including {P(CH2OH)4}6[MnII3{CrIII(ox)3}4]·10H2O·2EtOH (1) and {P(CH2OH)4}[MIICrIII(ox)3]·H2O (M = Fe (2), and Co (3)), are selectively formed by a reaction of {P(CH2OH)4}Br, MCl2, and (NH4)3[Cr(ox)3]. 1 has the 3-D anionic framework, [Mn3{Cr(ox)3}4]n6n-, consisting of the Mn4Cr4 octagonal and Mn6Cr6 dodecagonal channels, in which {P(CH2OH)4}+ ions and the solvation molecules (H2O and EtOH) are accommodated. 2 and 3 have a 2-D network of oxalate-bridged bimetallic sheets with an intercalation of {P(CH2OH)4}+ ions. Magnetic studies revealed ferromagnetic ordering in all the MOFs, with TC = 5.6 K for 1, 10.6 K for 2, and 12.2 K for 3. The {P(CH2OH)4}+ ions in the MOFs promote proton transfer via the vehicle mechanism, which gives rise to a significant room-temperature conductivity of 1.03 × 10-4 S cm-1 at 70% RH for 1, 4.60 × 10-6 S cm-1 at 60% RH for 2, and 3.04 × 10-5 S cm-1 at 70% RH for 3.

Modular Cavities: Induced Fit of Polar and Apolar Guests into Halogen-Based Receptors.

Neutral triangular macrocyclic compounds, [PdX2(4,7-phen)]3·(DMF)3·Et2O (X = Cl, Br; 4,7-phen = 4,7-phenanthroline; DMF = N, N'-dimethylformamide; Et2O = diethyl ether), were synthesized, and their molecular structures were characterized. Solution-state 1H NMR results suggested the formation of metal-ligand bonds, and single-crystal X-ray crystallography revealed clear triangular structures. A detailed examination of the structures indicated the formation of two kinds of cavities in the solid state, where a triangular unit works as a halogen-based receptor for polar and apolar solvents through weak hydrogen-bonding and dipole-dipole interaction.

Compression of a Flapping Mechanophore Accompanied by Thermal Void Collapse in a Crystalline Phase.

Mechanical control of the molecular energy landscape is an important issue in modern materials science. Mechanophores play a unique role in that the mechanical responses are induced against the activation barrier for intramolecular transformation with the aid of external forces. Here we report an unprecedented activation process of a flexible flapping mechanophore. Namely, thermal void collapse in a crystalline phase triggers mechanophore compression in a definite proportion. Unfavored conformational planarization of the flapping mechanophore is compulsorily induced by packing force, leading to a total energy gain in crystal packing. Fluorescence chromism indicates extended π conjugation resulting from the mechanophore compression, giving rise to an energy transfer from the unpressed to compressed conformers.

A highly crystalline oriented metal-organic framework thin film with an inorganic pillar.

We report a step-by-step route to fabricate the first example of a crystalline oriented metal-organic framework thin film having an anionic inorganic pillar ligand, {Cu(4,4'-bipyridyl)2(SiF6)}. X-ray study and sorption analysis revealed its high crystallinity and porous character.