Satoshi Horike

Size-Selective Lewis Acid Catalysis in a Microporous Metal-Organic Framework with Exposed Mn2+ Coordination Sites

Treatment of selected aldehydes and ketones with cyanotrimethylsilane in the presence of the microporous metal-organic framework Mn3[(Mn4Cl)3BTT8(CH3OH)10]2 (1, H3BTT = 1,3,5-benzenetristetrazol-5-yl) leads to rapid conversion to the corresponding cyanosilylated products. The transformation is catalyzed by coordinatively unsaturated Mn2+ ions that serve as Lewis acids and lead to conversion yields of 98 and 90% for benzaldehyde and 1-naphthaldehyde, the highest thus far for a metal-organic framework. Larger carbonyl substrates cannot diffuse through the pores of 1, and conversion yields are much lower for these, attesting to the heterogeneity of the reaction and its dependence on guest size. The Mukaiyama-aldol reaction, known to require much more active Lewis catalysts, is also catalyzed in the presence of 1, representing the first such example for a metal-organic framework. Conversion yields obtained for the reaction of selected aldehydes with silyl enolates reach 63%, on par with those obtained with zeolites. Size selectivity is demonstrated for the first time with this reaction through the use of larger silyl enolate substrates.

One-dimensional imidazole aggregate in aluminium porous coordination polymers with high proton conductivity

The development of anhydrous proton-conductive materials operating at temperatures above 80 degrees C is a challenge that needs to be met for practical applications. Herein, we propose the new idea of encapsulation of a proton-carrier molecule--imidazole in this work--in aluminium porous coordination polymers for the creation of a hybridized proton conductor under anhydrous conditions. Tuning of the host-guest interaction can generate a good proton-conducting path at temperatures above 100 degrees C. The dynamics of the adsorbed imidazole strongly affect the conductivity determined by (2)H solid-state NMR. Isotope measurements of conductivity using imidazole-d4 showed that the proton-hopping mechanism was dominant for the conducting path. This work suggests that the combination of guest molecules and a variety of microporous frameworks would afford highly mobile proton carriers in solids and gives an idea for designing a new type of proton conductor, particularly for high-temperature and anhydrous conditions.

Ion Conductivity and Transport by Porous Coordination Polymers and Metal–Organic Frameworks

Ion conduction and transport in solids are both interesting and useful and are found in widely distinct materials, from those in battery-related technologies to those in biological systems. Scientists have approached the synthesis of ion-conductive compounds in a variety of ways, in the areas of organic and inorganic chemistry. Recently, based on their ion-conducting behavior, porous coordination polymers (PCPs) and metal-organic frameworks (MOFs) have been recognized for their easy design and the dynamic behavior of the ionic components in the structures. These PCP/MOFs consist of metal ions (or clusters) and organic ligands structured via coordination bonds. They could have highly concentrated mobile ions with dynamic behavior, and their characteristics have inspired the design of a new class of ion conductors and transporters. In this Account, we describe the state-of-the-art of studies of ion conductivity by PCP/MOFs and nonporous coordination polymers (CPs) and offer future perspectives. PCP/MOF structures tend to have high hydrophilicity and guest-accessible voids, and scientists have reported many water-mediated proton (H(+)) conductivities. Chemical modification of organic ligands can change the hydrated H(+) conductivity over a wide range. On the other hand, the designable structures also permit water-free (anhydrous) H(+) conductivity. The incorporation of protic guests such as imidazole and 1,2,4-triazole into the microchannels of PCP/MOFs promotes the dynamic motion of guest molecules, resulting in high H(+) conduction without water. Not only the host-guest systems, but the embedding of protic organic groups on CPs also results in inherent H(+) conductivity. We have observed high H(+) conductivities under anhydrous conditions and in the intermediate temperature region of organic and inorganic conductors. The keys to successful construction are highly mobile ionic species and appropriate intervals of ion-hopping sites in the structures. Lithium (Li(+)) and other ions can also be transported. If we can optimize the crystal structures, this could offer further improvements in terms of both conductivity and the working temperature range. Another useful characteristic of PCP/MOFs is their wide application to materials fabrication. We can easily prepare heterodomain crystal systems, such as core-shell or solid solution. Other anisotropic morphologies (thin film, nanocrystal, nanorod, etc.,) are also possible, with retention of the ion conductivity. The flexible nature also lets us design morphology-dependent ion-conduction behaviors that we cannot observe in the bulk state. We propose (1) multivalent ion and anion conductions with the aid of redox activity and defects in structures, (2) control of ion transport behavior by applying external stimuli, (3) anomalous conductivity at the hetero-solid-solid interface, and (4) unidirectional ion transport as in the ion channels in membrane proteins. In the future, scientists may use coordination polymers not only to achieve higher conductivity but also to control ion behavior, which will open new avenues in solid-state ionics.

Nanochannels of Two Distinct Cross-Sections in a Porous Al-Based Coordination Polymer

A new aluminum naphthalenedicarboxylate Al(OH)(1,4-NDC) x 2 H2O compound has been synthesized. The crystal structure exhibits a three-dimensional framework composed of infinite chains of corner-sharing octahedral Al(OH)2O4 with 1,4-naphthanedicarboxylate ligands forming two types of channels with squared-shape cross-section. The large channels present a cross-section of 7.7 x 7.7 A(2), while the small channels are about 3.0 x 3.0 A(2). When water molecules are removed, no structural transformation occurs, generating a robust structure with permanent porosity and remarkable thermal stability. 2D (1)H-(13)C heteronuclear correlation NMR measurements, together with the application of Lee-Goldburg homonuclear decoupling, were applied, for the first time, to porous coordination polymers revealing the spatial relationships between the (1)H and (13)C spin-active nuclei of the framework. To demonstrate the open pore structure and the easy accessibility of the nanochannels to the gas phase, highly sensitive hyperpolarized (HP) xenon NMR, under extreme xenon dilution, has been applied. Xenon can diffuse selectively into the large nanochannels, while the small ones show no substantial uptake of xenon due to severe restrictions along the channels that prevent the diffusion. Two-dimensional exchange experiments showed the exchange time to be as short as 15 ms. Through variable-temperature HP (129)Xe NMR experiments we were able to achieve an unprecedented description of the large nanochannel space and surface, a physisorption energy of 10 kJ mol(-1), and the chemical shift value of xenon probing the internal surfaces. The large pore channels are straight, parallel, and independent, allowing one-dimensional anisotropic diffusion of gases and vapors. Their walls are composed of the naphthalene moieties that create an unique environment for selective sorption. These results prompted us to measure the storage capacity toward methanol, acetone, benzene, and carbon dioxide. The selective adsorption of methanol and acetone vs that of water, together with the permanent porosity and high thermal stability, makes this compound a suitable matrix for separation and purification.

Synthesis and Hydrogen Storage Properties of Be12(OH)12(1,3,5-benzenetribenzoate)4

The first crystalline beryllium-based metal-organic framework has been synthesized and found to exhibit an exceptional surface area useful for hydrogen storage. Reaction of 1,3,5-benzenetribenzoic acid (H(3)BTB) and beryllium nitrate in a mixture of DMSO, DMF, and water at 130 degrees C for 10 days affords the solvated form of Be(12)(OH)(12)(1,3,5-benzenetribenzoate)(4) (1). Its highly porous framework structure consists of unprecedented saddle-shaped [Be(12)(OH)(12)](12+) rings connected through tritopic BTB(3-) ligands to generate a 3,12 net. Compound 1 exhibits a BET surface area of 4030 m(2)/g, the highest value yet reported for any main group metal-organic framework or covalent organic framework. At 77 K, the H(2) adsorption data for 1 indicate a fully reversible uptake of 1.6 wt % at 1 bar, with an initial isosteric heat of adsorption of -5.5 kJ/mol. At pressures up to 100 bar, the data show the compound to serve as an exceptional hydrogen storage material, reaching a total uptake of 9.2 wt % and 44 g/L at 77 K and of 2.3 wt % and 11 g/L at 298 K. It is expected that reaction conditions similar to those reported here may enable the synthesis of a broad new family of beryllium-based frameworks with extremely high surface areas.

Inherent Proton Conduction in a 2D Coordination Framework

We synthesized a coordination polymer consisting of Zn(2+), 1,2,4-triazole, and orthophosphates, and demonstrated for the first time intrinsic proton conduction by a coordination network. The compound has a two-dimensional layered structure with extended hydrogen bonds between the layers. It shows intrinsic proton conductivity along the direction parallel to the layers, as elucidated by impedance studies of powder and single crystals. From the low activation energy for proton hopping, the conduction mechanism was found to be of the Grotthuss fashion. The hopping is promoted by rotation of phosphate ligands, which are aligned on the layers at appropriate intervals.

Confinement of Mobile Histamine in Coordination Nanochannels for Fast Proton Transfer

Proton-conducting solids, which act as the electrolyte of fuel cells, have received much attention. In particular, proton conductivity operating under anhydrous conditions and in the middle temperature region (> 100 8C) is regarded as a significant target. Heterogeneous hybridization of protonconductive molecules (or polymers) and solid supports, such as amorphous silica and porous materials, is one of the approaches for the preparation of proton-conductive hybrids. Porous coordination polymers (PCPs) or metal–organic frameworks (MOFs), built by metal ions with bridging organic ligands, represent a new class of porous materials with high designability in composition, structure, and function. To construct the proton conductors, we have focused on the hybridization of the proton carrier and PCP/MOFs on the molecular scale. Several works on proton conductivity with PCP/MOF materials under high-humidity conditions have been reported, and the composites show a remarkable drop of conductivity when dehydrated. Only two reports on PCP-based composites under anhydrous conditions have been published, including our previous work. 6] In both cases, incorporated proton carrier molecules transfer protons along the channels in ordered porous networks. However, the conductivities were not high enough to use the materials for practical systems. Therefore, other conductors having a conductivity above 10 3 S cm 1 under anhydrous conditions and in the middle temperature region are anticipated. In the work reported herein, we constructed the composite of aluminum-based microporous PCP and histamine, as the proton-donating molecule, and achieved a conductivity of over 10 3 Scm 1 at 150 8C in a completely anhydrous environment. [Al(OH)(ndc)]n (1, ndc = 1,4-naphthalenedicarboxylate), which has high thermo/chemo stabilities, was utilized as a support for the composite. In previous work we hybridized 1 with imidazole to give a conductivity of 10 5 Scm 1 at 120 8C. Compound 1 possesses one-dimensional channels with a 7.7 7.7 2 pore diameter, as shown in Figure 1a. Histamine was introduced as a proton-donating/ accepting molecule for hybridization. The melting point of histamine (83 8C) is lower than that of imidazole (89 8C), and three proton-donor/acceptor sites of an imidazole ring and an amine group act as the proton carrier (Figure 1b).

Coordination-network-based ionic plastic crystal for anhydrous proton conductivity.

An ionic coordination network consisting of protonated imidazole and anionic one-dimensional chains of Zn(2+) phosphate was synthesized. The compound possesses highly mobile ions in the crystal lattice and behaves as an ionic plastic crystal. The dynamic behavior provides a proton conductivity of 2.6 × 10(-4) S cm(-1) at 130 °C without humidity.

A solid solution approach to 2D coordination polymers for CH4/CO2 and CH4/C2H6 gas separation: equilibrium and kinetic studies

Gas separation properties for CH4/CO2 and CH4/C2H6 of flexible 2D porous coordination polymers under equilibrium gas conditions and mixed gas flow conditions were investigated and the gas separation efficiencies were optimized by precise tuning of the flexibility in ligand-base solid solution compounds.

Conformation and Molecular Dynamics of Single Polystyrene Chain Confined in Coordination Nanospace

Molecules confined in nanospaces will have distinctly different properties to those in the bulk state because of the formation of specific molecular assemblies and conformations. We studied the chain conformation and dynamics of single polystyrene (PSt) chains confined in highly regular one-dimensional nanochannels of a porous coordination polymer [Zn 2(bdc) 2ted] n ( 1; bdc = 1,4-benzenedicarboxylate, ted = triethylenediamine). Characterization by two-dimensional (2D) heteronuclear (1)H- (13)C NMR gave a direct demonstration of the nanocomposite formation and the intimacy between the PSt and the pore surfaces of 1. Calorimetric analysis of the composite did not reveal any glass transition of PSt, which illustrates the different nature of the PSt encapsulated in the nanochannels compared with that of bulk PSt. From N 2 adsorption measurements, the apparent density of PSt in the nanochannel was estimated to be 0.55 g cm (-3), which is much lower than that of bulk PSt. Results of a solid-state (2)H NMR study of the composite showed the homogeneous mobility of phenyl flipping with significantly low activation energy, as a result of the encapsulation of single PSt chains in one-dimensional regular crystalline nanochannels. This is also supported by molecular dynamics (MD) simulations.