Munehiro Inukai

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.

Integration of Intrinsic Proton Conduction and Guest-Accessible Nanospace into a Coordination Polymer

We report the synthesis and characterization of a coordination polymer that exhibits both intrinsic proton conductivity and gas adsorption. The coordination polymer, consisting of zinc ions, benzimidazole, and orthophosphate, exhibits a degree of flexibility in that it adopts different structures before and after dehydration. The dehydrated form shows higher intrinsic proton conductivity than the original form, reaching as high as 1.3 × 10(-3) S cm(-1) at 120 °C. We found that the rearranged conduction path and liquid-like behavior of benzimidazole molecules in the channel of the framework afforded the high proton conductivity. Of the two forms of the framework, only the dehydrated form is porous to methanol and demonstrates guest-accessible space in the structure. The proton conductivity of the dehydrated form increases by 24 times as a result of the in situ adsorption of methanol molecules, demonstrating the dual functionality of the framework. NMR studies revealed a hydrogen-bond interaction between the framework and methanol, which enables the modulation of proton conductivity within the framework.

Dense Coordination Network Capable of Selective CO2 Capture from C1 and C2 Hydrocarbons

We elucidated the specific adsorption property of CO(2) for a densely interpenetrated coordination polymer which was a nonporous structure and observed gas separation properties of CO(2) over CH(4), C(2)H(4), and C(2)H(6), studied under both equilibrium and kinetic conditions of gases at ambient temperature and pressure.

Postsynthesis Modification of a Porous Coordination Polymer by LiCl To Enhance H+ Transport

A Ca(2+) porous coordination polymer with 1D channels was functionalized by the postsynthesis addition of LiCl to enhance the H(+) conductivity. The compound showed over 10(-2) S cm(-1) at 25 °C and 20% relative humidity. Pulse-field gradient NMR elucidated that the fast H(+) conductivity was achieved by the support of Li(+) ion movements in the channel.

Encapsulating Mobile Proton Carriers into Structural Defects in Coordination Polymer Crystals: High Anhydrous Proton Conduction and Fuel Cell Application

We describe the encapsulation of mobile proton carriers into defect sites in nonporous coordination polymers (CPs). The proton carriers were encapsulated with high mobility and provided high proton conductivity at 150 °C under anhydrous conditions. The high proton conductivity and nonporous nature of the CP allowed its application as an electrolyte in a fuel cell. The defects and mobile proton carriers were investigated using solid-state NMR, XAFS, XRD, and ICP-AES/EA. On the basis of these analyses, we concluded that the defect sites provide space for mobile uncoordinated H3PO4, H2PO4(-), and H2O. These mobile carriers play a key role in expanding the proton-hopping path and promoting the mobility of protons in the coordination framework, leading to high proton conductivity and fuel cell power generation.

Mixing of immiscible polymers using nanoporous coordination templates

The establishment of methodologies for the mixing of immiscible substances is highly desirable to facilitate the development of fundamental science and materials technology. Herein we describe a new protocol for the compatibilization of immiscible polymers at the molecular level using porous coordination polymers (PCPs) as removable templates. In this process, the typical immiscible polymer pair of polystyrene (PSt) and poly(methyl methacrylate) (PMMA) was prepared via the successive homopolymerizations of their monomers in a PCP to distribute the polymers inside the PCP particles. Subsequent dissolution of the PCP frameworks in a chelator solution affords a PSt/PMMA blend that is homogeneous in the range of several nanometers. Due to the unusual compatibilization, the thermal properties of the polymer blend are remarkably improved compared with the conventional solvent-cast blend. This method is also applicable to the compatibilization of PSt and polyacrylonitrile, which have very different solubility parameters.

Template-directed proton conduction pathways in a coordination framework

We present a strategy for creating coordination frameworks exhibiting proton conduction with thermal stability. The coordination framework, where template cations link 1-D chains via hydrogen bonds, has dynamic hydrogen bond networks where protons move without water support. Solid-state NMR and X-ray studies visualized the proton hopping mechanism, and revealed that the templates provide the bridging of the 1-D chains to attain proton conduction. The templates also enable the proton conductive networks to be maintained at 190 °C through multiple interactions between the templates and the 1-D chains.

Fe2+-based layered porous coordination polymers and soft encapsulation of guests via redox activity

2-D layer type porous coordination polymers containing redox active Fe2+ centers were synthesized. One of the compounds [Fe(isophthalate)(4,4′-bipyridyl)] showed structural flexibility via guest adsorption, and we observed reversible Fe2+ and Fe3+ switching by iodine insertion. The composite showed electric conductivity.