Wataru Kosaka

Shape-Memory Nanopores Induced in Coordination Frameworks by Crystal Downsizing

Size Affects Shape Porous molecular framework materials can adopt a different phase when guest molecules absorb and uniformly distort the framework. Usually the framework returns to its original shape when the guests desorb. Sakata et al. (p. 193) noted that because surface stress drives this process, it might be avoided in smaller crystals. Indeed, a flexible porous coordination polymer, [Cu2(dicarboxylate)2(amine)]n, could retain the structure induced by guest molecules such as methanol if crystallites were made sufficiently small (submicrometer scale) and did so to a greater degree as the crystallite dimensions decreased. A porous material retains its framework shape after guest molecules desorb if its crystallites are sufficiently small. Flexible porous coordination polymers change their structure in response to molecular incorporation but recover their original configuration after the guest has been removed. We demonstrated that the crystal downsizing of twofold interpenetrated frameworks of [Cu2(dicarboxylate)2(amine)]n regulates the structural flexibility and induces a shape-memory effect in the coordination frameworks. In addition to the two structures that contribute to the sorption process (that is, a nonporous closed phase and a guest-included open phase), we isolated an unusual, metastable open dried phase when downsizing the crystals to the mesoscale, and the closed phase was recovered by thermal treatment. Crystal downsizing suppressed the structural mobility and stabilized the open dried phase. The successful isolation of two interconvertible empty phases, the closed phase and the open dried phase, provided switchable sorption properties with or without gate-opening behavior.

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.

Vanadium Octacyanoniobate-Based Magnet with a Curie Temperature of 138 K

In this work, we prepared a three-dimensional vanadium octacyanoniobate-based magnet, K(0.10)V(II)(0.54)V(III)(1.24)[Nb(IV)(CN)(8)].(SO(4))(0.45).6.8H(2)O. This compound exhibits ferrimagnetism with a Curie temperature of 138 K, in which the sublattice magnetizations of V(II) (S = (3)/(2)) and V(III) (S = 1) are antiparallelly ordered to that of Nb(IV) (S = (1)/(2)). The estimated superexchange interaction constants of V(II)-Nb(IV) and V(III)-Nb(IV) are -51 and -25 cm(-1), respectively.

Selective NO Trapping in the Pores of Chain-Type Complex Assemblies Based on Electronically Activated Paddlewheel-Type [Ru2II,II]/[Rh2II,II] Dimers

The design of porous materials that undergo selective adsorption of a specific molecule is a critical issue in research on porous coordination polymers or metal-organic frameworks. For the purpose of the selective capture of molecules possessing an electron-acceptor character such as nitric oxide (NO), one-dimensional chain compounds possessing a high donor character have been synthesized using 4-chloroanisate-bridged paddlewheel-type dimetal(II, II) complexes with M = Ru and Rh and phenazine (phz) as the chain linker: [M2(4-Cl-2-OMePhCO2)4(phz)]·n(CH2Cl2) (M = Ru, 1; Rh, 2). These compounds are isostructural and are composed of chains with a [-{M2}-phz-] repeating unit and CH2Cl2 occupying the void space between the chains. Compounds 1 and 2 change to a new phase (1-dry and 2-dry) upon evacuating the crystallization solvent (CH2Cl2) and almost lose their pores in the drying process: no void space in 1-dry and 31.8 Å(3), corresponding to 2.9% of the cell volume, in 2-dry. Nevertheless, the compounds show a unique gas accommodation ability. Accompanied by a structural transformation (i.e., the first gate-opening) at low pressures of <10 kPa, both compounds show a typical physisorption isotherm for O2 (90 K) and CO2 (195 K), with the adsorption amount of ca. 2-4 gas molecules per [M2] unit. In addition, the adsorption isotherm for NO (121 K) involves the first gate-opening followed by a second gate-opening anomaly at NO pressures of ≈52 kPa for 1-dry and ≈21 kPa for 2-dry. At the first gate-opening, the absorbed amount of NO is ca. 4 molecules per [M2] unit, and then it reaches 8.4 and 6.3 for 1-dry and 2-dry, respectively, at 95 kPa. Only the isotherm for NO exhibits hysteresis in the desorption process, and some of the NO molecules are trapped in pores even after evacuating at 121 K, although it recovers to the original dried sample on heating to room temperature. The adsorbed NO molecules accrue a significant electron donation from the host framework even in the [Rh2] derivative, indicating that such simple porous compounds with electron-donor characteristics are useful for the selective adsorption of NO.

Carrier Concentration Dependent Conduction in Insulator-Doped Donor/Acceptor Chain Compounds

On the basis of the concept that the design of a mixed valence system is a key route to create electronic conducting frameworks, we propose a unique idea to rationally produce mixed valency in an ionic donor/acceptor chain (i.e., D(+)A(-) chain). The doping of a redox-inert (insulator) dopant (P) into a D(+)A(-) chain in place of neutral D enables the creation of mixed valency A(0)/A(-) domains between P units: P-(D(+)A(-))nA(0)-P, where n is directly dependent on the dopant ratio, and charge transfer through the P units leads to electron transport along the framework. This hypothesis was experimentally demonstrated in an ionic DA chain synthesized from a redox-active paddlewheel [Ru2(II,II)] complex and TCNQ derivative by doping with a redox-inert [Rh2(II,II)] complex.

Fully Electron-Transferred Donor/Acceptor Layered Frameworks with TCNQ2–

In a series of two-dimensional layered frameworks constructed by two electron-donor (D) and one electron-acceptor (A) units (a D2A framework), two-electron transferred systems with D(+)2A(2-) were first synthesized as [{Ru2(R-PhCO2)4}2(TCNQRx)]·n(solv) (R = o-CF3, Rx = H2 (1), R = o-CF3, Rx = Me2 (2), R = o-CF3, Rx = F4 (3), R = o-Me, TCNQRx = BTDA-TCNQ (4), R = p-Me, TCNQRx = BTDA-TCNQ (5), where TCNQ is 7,7,8,8-tetracyano-p-quinodimethane and BTDA-TCNQ is bis[1,2,5]dithiazolotetracyanoquinodimethane). The D(+)2A(2-) system was synthesized by assembling D/A combinations of paddlewheel-type [Ru2(II,II)(R-PhCO2)4] complexes and TCNQRx that possibly caused a large gap between the HOMO of D and the LUMO of A (ΔEH-L(DA)). All compounds were paramagnetic because of quasi-isolated [Ru2(II,III)](+) units with weakly antiferromagnetically coupled S = 3/2 spins via diamagnetic TCNQRx(2-) and/or through the interlayer space. The ionic states of these compounds were determined using the HOMO/LUMO energies and redox potentials of the D and A components in the ionization diagram for ΔEH-L(DA) vs ΔE1/2(DA) (= E1/2(D) - E1/2(A); E1/2 = first redox potential) as well as by previously reported data for the D2A and DA series of [Ru2]/TCNQ, DCNQI materials. The boundary between the one-electron and the two-electron transferred ionic regimes (1e-I and 2e-I, respectively) was not characterized. Therefore, another diagram for ΔEH-L(DA) vs |(2)E1/2(A) - (1)E1/2(A)|, where (2)E1/2(A) and (1)E1/2(A) are the second and first redox potentials of TCNQRx, respectively, was used because the 2e-I regime is dependent on on-site Coulomb repulsion (U = |(2)E1/2(A) - (1)E1/2(A)|) of TCNQRx. This explained the oxidation states of 1-5 and the relationship between ΔEH-L(DA) and U and allowed us to determine whether the ionic regime was 1e-I or 2e-I. These diagrams confirm that a charge-oriented choice of building units is possible even when designing covalently bonded D2A framework systems.

Electron-Transferred Donor/Acceptor Ferrimagnet with T(C) = 91 K in a Layered Assembly of Paddlewheel [Ru2] Units and TCNQ.

The donor (D)/acceptor (A) assembly reaction of the paddlewheel-type diruthenium(II,II) complex [Ru2(2,4,6-F3PhCO2)4(THF)2] (2,4,6-F3PhCO2(-) = 2,4,6-trifluorobenzoate; abbreviated hereafter as [Ru2]) with 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) in a p-xylene/CH2Cl2 solvent system led to the formation of a two-dimensional layered compound, [{Ru2(2,4,6-F3PhCO2)4}2(TCNQ)]·2(p-xylene)·2CH2Cl2 (1). As expected from this D/A combination, 1 has a one-electron-transfer ionic state with the D(0.5+)2A(-) formulation. This state formally derives a heterospin state composed of S = 1 for [Ru(II,II)2], S = 3/2 for [Ru(II,III)2](+), and S = ½ for TCNQ(•-), possibly causing intralayer ferrimagnetic spin ordering. Most of these types of compounds have an antiferromagnetic ground state because of the coupling of ferrimagnetically ordered layers in dipole antiferromagnetic interactions. However, 1 became a three-dimensional ferrimagnet with T(C) = 91 K because of the presence of interlayer ferromagnetic interactions.

Construction of an Artificial Ferrimagnetic Lattice by Lithium Ion Insertion into a Neutral Donor/Acceptor Metal-Organic Framework.

Construction of a molecular system in which the magnetic lattice exhibits long-range order is one of the fundamental goals in materials science. In this study, we demonstrate the artificial construction of a ferrimagnetic lattice by doping electrons into acceptor sites of a neutral donor/acceptor metal-organic framework (D/A-MOF). This doping was achieved by the insertion of Li-ions into the D/A-MOF, which was used as the cathode of a Li-ion battery cell. The neutral D/A-MOF is a layered system composed of a carboxylate-bridged paddlewheel-type diruthenium(II,II) complex as the donor and a TCNQ derivative as the acceptor. The ground state of the neutral form was a magnetically disordered paramagnetic state. Upon discharge of the cell, spontaneous magnetization was induced; the transition temperature was variable. The stability of the magnetically ordered lattice depended on the equilibrium electric potential of the D/A-MOF cathode, which reflected the electron-filling level.

Inclusion and dielectric properties of a vinylidene fluoride oligomer in coordination nanochannels.

The dynamics of oligo(vinylidene fluoride) (OVDF) confined in regular nanochannels of a porous coordination polymer (PCP) was studied by means of dielectric spectroscopy. The OVDF chains in the PCP nanopores showed two Arrhenius-type relaxation processes at lower temperatures than the relaxation temperature observed for the neat OVDF, showing the enhanced mobility of the confined OVDF.

Axial-Site Modifications of Paddlewheel Diruthenium(II, II) Complexes Supported by Hydrogen Bonding

The reactions of paddlewheel-type diruthenium(II, II) complexes, [Ru2(II,II)(x-FPhCO2)4(THF)2] (x-FPhCO2(-) = x-fluorobenzoate with x- = o-, m-, p-), with 2,6-diaminopyridine (dapy) and 7-azaindole (azain) afford axially capped discrete compounds, [Ru2(II,II)(x-FPhCO2)4(dapy)2] (x = o-, 1; m-, 2; p-, 3) and [Ru2(II,II)(o-FPhCO2)4(azain)2] (4), respectively. In these compounds, intramolecular hydrogen bonds are observed between NH2 groups for 1-3 or imine NH groups for 4 and oxygen atoms of carboxylate groups. In addition, hydrogen bonds of NH2···F are also observed for 1 and 4 with an o-positioned F atom on benzoate. This coordination mode, i.e., a dual bonding mode with σ-bonding and hydrogen bonding, should assist ligand coordination to the axial position of the [Ru2] unit. The Ru-N bond distance in 1-4 is shorter than that observed in related compounds reported previously. In a similar fashion, reactions with planar M(II) dithiobiuret (dtb) complexes, [M(II)(dtb)2] (M(II) = Pd(II) and Pt(II)), were carried out. One-dimensional alternating chains, [{Ru2(II,II)(o-FPhCO2)4}{M(II)(dtb)2}] (M(II) = Pd(II), 5; Pt(II), 6), were obtained, in which the hydrogen-bonding modes of NH2···O and NH2···F are present, as expected. DFT calculations for the [M(II)(dtb)2] unit revealed that the LUMO of [M(II)(dtb)2] lies at -2.159 and -1.781 eV for M = Pd and Pt, respectively, which is much higher than HOMO energy at -4.184 eV calculated for [Ru2(II,II)(o-FPhCO2)(THF)2], proving that the respective units are essentially electronically isolated in the chains.