Yuh Hijikata

Gas detection by structural variations of fluorescent guest molecules in a flexible porous coordination polymer

The development of a new methodology for visualizing and detecting gases is imperative for various applications. Here, we report a novel strategy in which gas molecules are detected by signals from a reporter guest that can read out a host structural transformation. A composite between a flexible porous coordination polymer and fluorescent reporter distyrylbenzene (DSB) selectively adsorbed CO₂ over other atmospheric gases. This adsorption induced a host transformation, which was accompanied by conformational variations of the included DSB. This read-out process resulted in a critical change in DSB fluorescence at a specific threshold pressure. The composite shows different fluorescence responses to CO₂ and acetylene, compounds that have similar physicochemical properties. Our system showed, for the first time, that fluorescent molecules can detect gases without any chemical interaction or energy transfer. The host-guest coupled transformations play a pivotal role in converting the gas adsorption events into detectable output signals.

Selective sorption of oxygen and nitric oxide by an electron-donating flexible porous coordination polymer

Porous coordination polymers are materials formed from metal ions that are bridged together by organic linkers and that can combine two seemingly contradictory properties—crystallinity and flexibility. Porous coordination polymers can therefore create highly regular yet dynamic nanoporous domains that are particularly promising for sorption applications. Here, we describe the effective selective sorption of dioxygen and nitric oxide by a structurally and electronically dynamic porous coordination polymer built from zinc centres and tetracyanoquinodimethane (TCNQ) as a linker. In contrast to a variety of other gas molecules (C2H2, Ar, CO2, N2 and CO), O2 and NO are accommodated in its pores. This unprecedented preference arises from the concerted effect of the charge-transfer interaction between TCNQ and these guests, and the switchable gate opening and closing of the pores of the framework. This system provides further insight into the efficient recognition of small gas molecules. Porous coordination polymers can form materials that are both crystalline and flexible, creating regular yet dynamic channels that are promising for guest sorption. Guest selectivity is difficult to achieve, however, and typically relies on size- or shape-recognition. A framework has now been assembled that combines charge-transfer interactions and structural flexibility to only accommodate O2 and NO.

Self-Accelerating CO Sorption in a Soft Nanoporous Crystal

Soft, Selective CO Sorption Many industrial processes produce CO, which could be used as a chemical feedstock, but separation of CO from other gases, especially N2, is too difficult to be economically viable. Sato et al. (p. 167, published online 12 December 2013) now report that a porous coordination polymer containing Cu2+ ions can selectivity bind CO through serial structural changes reminiscent of allosteric effects in proteins. The separation of CO-N2 mixtures can be achieved with a low input energy for CO desorption. A soft nanoporous crystalline solid exhibits self-accelerating, selective carbon monoxide adsorption. Carbon monoxide (CO) produced in many large-scale industrial oxidation processes is difficult to separate from nitrogen (N2), and afterward, CO is further oxidized to carbon dioxide. Here, we report a soft nanoporous crystalline material that selectively adsorbs CO with adaptable pores, and we present crystallographic evidence that CO molecules can coordinate with copper(II) ions. The unprecedented high selectivity was achieved by the synergetic effect of the local interaction between CO and accessible metal sites and a global transformation of the framework. This transformable crystalline material realized the separation of CO from mixtures with N2, a gas that is the most competitive to CO. The dynamic and efficient molecular trapping and releasing system is reminiscent of sophisticated biological systems such as heme proteins.

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).

Porous coordination polymer with pyridinium cationic surface, [Zn(2)(tpa)(2)(cpb)].

We have synthesized a porous coordination polymer containing a pyridinium cation as an organic linker and have investigated the methanol absorptive ability of the pyridinium cationic surface. The result implies that the pyridinium cationic surface participates in the strong adsorption of methanol.

Functionalization of coordination nanochannels for controlling tacticity in radical vinyl polymerization.

Systematic functionalization of porous coordination polymers (PCPs), [Cu(2)(L)(2)(ted)](n) (where L = dicarboxylates and ted = triethylenediamine), by introducing various substituents onto the component organic ligand, L, was performed to regulate the radical polymerization of methyl methacrylate (MMA) in the nanochannels. The effect of the substituent groups on stereoregularity of the resulting poly(methyl methacrylate) (PMMA) was observed, where the tacticity of the PMMA strongly depended on the number and position of the substituent. In particular, polymerization of MMA in [Cu(2)(2,5-dimethoxyterephthalate)(2)(ted)](n) gave PMMA with high isotactic and heterotactic triad fractions, which is one of the most effective systems for changing the tacticity of PMMA in radical polymerization. To understand the mechanism of this drastic stereoregularity change, a variety of experimental and theoretical analyses, such as IR, N(2) adsorption, a statics study, and molecular dynamics (MD) calculations, were performed. Accurate MD calculations were helpful to determine the most plausible structures of [Cu(2)(L)(2)(ted)](n) and revealed that the specific channel shape of [Cu(2)(2,5-dimethoxyterephthalate)(2)(ted)](n) induces the large tacticity change of the resulting PMMA.

A Switchable Molecular Rotator: Neutron Spectroscopy Study on a Polymeric Spin-Crossover Compound

A quasielastic neutron scattering and solid-state (2)H NMR spectroscopy study of the polymeric spin-crossover compound {Fe(pyrazine)[Pt(CN)(4)]} shows that the switching of the rotation of a molecular fragment--the pyrazine ligand--occurs in association with the change of spin state. The rotation switching was examined on a wide time scale (10(-13)-10(-3) s) by both techniques, which clearly demonstrated the combination between molecular rotation and spin-crossover transition under external stimuli (temperature and chemical). The pyrazine rings are seen to perform a 4-fold jump motion about the coordinating nitrogen axis in the high-spin state. In the low-spin state, however, the motion is suppressed, while when the system incorporates benzene guest molecules, the movements of the system are even more restricted.

Relationship between channel and sorption properties in coordination polymers with interdigitated structures.

Porous coordination polymers constructed from Zn(2+) and isophthalate with linear bipyridyl-type ligands were synthesized. [Zn(ip)(bpb)](n) (CID-21; ip=isophthalate, bpb=1,4-bis(4-pyridyl)benzene), [Zn(ip)(bpt)](n) (CID-22; bpt=3,6-bis(4-pyridyl)-1,2,4,5-tetrazine), and [Zn(ip)(bpa)](n) (CID-23; bpa=1,4-bis(4-pyridyl)acetylene) all have interdigitated structures of layers and similar void volumes (≈27%). In these compounds, 1D bottleneck-type channels run along the perpendicular direction of the layer stacking and their properties are strongly dominated by the dipyridyl linker ligands. Because of the difference in packing of 2D layers, CID-21 and CID-22 have relatively rigid porous structures, whereas CID-23 has greater flexibility, as indicated by the results of powder X-ray diffraction studies. The micropores of CID-22 surrounded by tetrazine moieties adsorb polar molecules, such as methanol and water. The higher affinity of CID-22 for water than CID-21 is supported by a theoretical study. The 1D channel of CID-23 is wider than that of the other two compounds, which enables the incorporation of aromatic molecules. This is because the shape of the bpa linker ligand generates a wider pore diameter (8.6 Å). Only CID-23 can adsorb a benzene molecule and the isotherm of benzene has a gate-opening-type profile. This offers proof of the guest accommodation process through large structural transformation from a nonporous to a porous structure. The flexibility and restricted pore space of CID-23, at 298 K, allows only benzene, but not cyclohexane, to enter the channels. The porous structure exhibits clear selectivity for these similar guests. The incorporation of an elongated dipyridyl linker ligand in the 2D coordination layers provides a strategy for the design of microporous compounds with different flexibilities, microporous environments, and separation abilities.

Design of Flexible Lewis Acidic Sites in Porous Coordination Polymers by using the Viologen Moiety

Considerable effort has been devoted to the design of metal– organic architectures and a variety of frameworks have emerged through self-assembly processes involving metal ions and organic linkers. The synthesis of coordination polymers with a channel structure, polymers which have been called porous coordination polymers (PCPs) or metal– organic frameworks (MOFs), 3] are of great interest because of their unique functions, such as gas storage, separation, and catalysis. 7] Among the PCPs, frameworks having Lewis acidic sites have been highlighted because of their gascapturing properties or catalytic activities. The main strategy for preparing the Lewis acidic sites is to introduce open metal sites (OMSs). In contrast, we have achieved the fabrication of charged organic surfaces (COSs) using a pyridinium moiety in the porous framework. Because the guestaccessible interior of the pore is mainly organized by the organic moiety, these COSs interact effectively with guest molecules. Another noteworthy point of the introduction of COSs is flexibility, which is based on the flexible nature of the PCP framework. 10] PCPs often show a flexible contraction/ expansion of the framework through guest accommodation, and if we could incorporate COSs onto the flexible network, the obtained framework would show induced-fit capture of guests at the COSs, a process which is difficult to achieve with the use of OMSs. For the purpose of the construction of flexible COSs, we employed a viologen derivative as an organic linker because of its intrinsic Lewis acidity and the dynamic motion of the aromatic rings. Herein we report the synthesis of a PCP bearing a viologen motif, the strength of the Lewis acidity, and the adsorption properties. The reaction of Zn(NO3)2·6 H2O with 1,4-naphthalenedicarboxylic acid (1,4-H2ndc) and 1,1-bis(4-carboxybenzyl)4,4’-bipyridinium bis(hexafluorophosphate) (H2bcbpy·2PF6) in N,N’-dimethylformamide (DMF) affords the PCP {[Zn(1,4ndc)(bcbpy)]·(0.5DMF)(3.5 H2O)}n (1 0.5DMF·3.5H2O) (Scheme 1). The bcbpy, which is a zwitterionic ligand, acts as a neutral organic linker and 1 0.5DMF·3.5H2O does not include any counter anions. The crystal structure of 1 0.5DMF·3.5H2O was determined by single-crystal X-ray crystallography at 223 K. The Zn ion is tetrahedrally coordinated by two 1,4-ndc ligands and two bcbpy ligands (Figure 1a) to give two-dimensional (2D) interpenetrated layers along the ac plane (Figure 1 b). The 2D layers are of the 4-sql topology (see Figure S1 in the Supporting Information). The interpenetrated 2D layers are stacked along the b axis to form a 3D structure because of the p–p interaction between the 1,4-ndc and the viologen moiety of the bcbpy (Figure 1c). The 1 0.5DMF·3.5H2O possesses 1D channels along the c axis with a cross-section of 4.7 4.1 2 (Figure 1d). The 0.5DMF and 3H2O sit in the cavity and the 0.5H2O is between the 2D sheets. The pore surface is formed by bcbpy and 1,4-ndc ligands (Figure 1e). The carbonyl oxygen atom of the DMF is located 2.48 and 2.64 from the two a-hydrogen atoms of the viologen moiety (Figure 2 a), and it forms a C Scheme 1. Synthesis of 1 0.5DMF·3.5H2O.

Porous coordination polymers with ubiquitous and biocompatible metals and a neutral bridging ligand

The design of inexpensive and less toxic porous coordination polymers (PCPs) that show selective adsorption or high adsorption capacity is a critical issue in research on applicable porous materials. Although use of Group II magnesium(II) and calcium(II) ions as building blocks could provide cheaper materials and lead to enhanced biocompatibility, examples of magnesium(II) and calcium(II) PCPs are extremely limited compared with commonly used transition metal ones, because neutral bridging ligands have not been available for magnesium(II) and calcium(II) ions. Here we report a rationally designed neutral and charge-polarized bridging ligand as a new partner for magnesium(II) and calcium(II) ions. The three-dimensional magnesium(II) and calcium(II) PCPs synthesized using such a neutral ligand are stable and show selective adsorption and separation of carbon dioxide over methane at ambient temperature. This synthetic approach allows the structural diversification of Group II magnesium(II) and calcium(II) PCPs.