Crystal transformation and host molecular motions in CO2 adsorption process of a metal benzoate pyrazine (M(II) = Rh, Cu).

For the purpose of investigating the correlation between host gas adsorption ability and structural flexibility, the combination of metal benzoate complexes [M(II)(2)(bza)(4)] (M(II) = Rh (a), Cu (b); bza = benzoate) and pyrazine derivatives (pyz = pyrazine (1), 2-mpyz = 2-methylpyrazine (2), 2,3-dmpyz = 2,3-dimethylpyrazine (3)) yields a series of one-dimensionally assembled complexes. The study of the adsorption properties of this series was examined for CO(2), H(2), N(2), O(2), and Ar gases at 195 K (CO(2)) or at 77 K (all others). The adsorption manners of these complexes are similar for each gas, while the pressure at which adsorption started or rapidly grew increased with a rise in the number of methyl groups in the case of adsorbable gases. The maximum amount of adsorption was a positive integer, e.g., 3 molecules per M(2) unit for 1 and 2 and 2 molecules per M(2) unit for 3 in the case of CO(2) adsorption for all complexes at 0.1 MPa of adsorbable gases. Structural transformation was observed accompanying gas adsorption. This transformation was observed when the adsorption amount reached 1 molecule per M(2) unit, suggesting a correlation of the adsorption amount and dynamic adsorption behavior. Single-crystal X-ray analyses of nonincluded crystals and CO(2) inclusions for all hosts (1-3) revealed that large structural changes occurred through CO(2) adsorption to increase the inner space for adsorption gases, depending on the substituents on the pyrazine ring. These facts were confirmed as a transition by DSC measurements using a mixed CO(2)/N(2) gas atmosphere. Solid-state (1)H and (2)H NMR studies of the crystalline sample of 1a and its partially deuterated samples of 1a' (deuterated phenyl group) and 1a'' (deuterated pyrazine) revealed rapid 180 degree-flip motions of the aromatic rings of the host skeletons, which form the walls of the channels. These "rotating" motions would help the diffusion of CO(2) molecules through a narrow channel at relatively low pressure. Indeed, the motions of phenyl groups and methyl-substituted pyrazine moieties of phenyl deuterated 3a were confirmed to be very slow by solid-state (1)H and (2)H NMR spectra, where the amount of adsorbed gas molecules was small for 3a at 0.1 MPa of CO(2).

Structural and magnetic study of O2 molecules arranged along a channel in a flexible single-crystal host family.

We report the magnetic behaviors of O(2) molecules, which aligned in the channels of four types of single-crystal adsorbents, [M(2)(bza)(4)(pyz)](n) (bza = benzoate; pyz = pyrazine; M = Rh(II) (1a) and Cu(II) (1b)) and [M(2)(bza)(4)(2-mpyz)](n) (2-mpyz = 2-methylpyrazine; M = Rh(II) (2a) and Cu(II) (2b)). The X-ray single-crystal structures at various temperatures from 10 to 298 K were determined for O(2)-included crystals of 1a and 2a. All adsorbed O(2) molecules exhibited abnormal magnetic phases above 4 T in the temperature range around 55-105 K. The magnetic behaviors of adsorbed O(2) molecules between four inclusions were discussed. The existence of the metastable state, which was also suggested by hysteresis on the M-H curves, was revealed by analysis of the time course of the magnetization. Considering that the abnormal magnetic behavior occurred at relatively high temperatures and a low magnetic field, it was suggested that these behaviors were induced because of the changes of magnetic interaction of included O(2) aggregates involving transformation which is supported by the surrounding of the channels of the single-crystal hosts under the applied magnetic field.

Alcohol-vapor inclusion in single-crystal adsorbents [M(II)2(bza)4(pyz)]n (M = Rh, Cu): structural study and application to separation membranes.

The vapor absorbency of the series of alcohols methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol was characterized on the single-crystal adsorbents [M(II)2(bza)4(pyz)]n (bza = benzoate, pyz = pyrazine, M = Rh (1), Cu (2)). The crystal structures of all the alcohol inclusions were determined by single-crystal X-ray crystallography at 90 K. The crystal-phase transition induced by guest adsorption occurred in the inclusion crystals except for 1-propanol. A hydrogen-bonded dimer of adsorbed alcohol was found in the methanol- and ethanol-inclusion crystals, which is similar to a previous observation in 2 x 2EtOH (S. Takamizawa, T. Saito, T. Akatsuka, E. Nakata, Inorg. Chem. 2005, 44, 1421-1424). In contrast, an isolated monomer was present in the channel for 1-propanol, 1-butanol, and 1-pentanol inclusions. All adsorbed alcohols were stabilized by hydrophilic and/or hydrophobic interactions between host and guest. From the combined results of microscopic determination (crystal structure) and macroscopic observation (gas-adsorption property), the observed transition induced by gas adsorption is explained by stepwise inclusion into the individual cavities, which is called the "step-loading effect." Alcohol/water separation was attempted by a pervaporation technique with microcrystals of 2 dispersed in a poly(dimethylsiloxane) membrane. In the alcohol/water separation, the membrane showed effective separation ability and gave separation factors (alcohol/water) of 5.6 and 4.7 for methanol and ethanol at room temperature, respectively.

Structural and magnetic study of N2, NO, NO2, and SO2 adsorbed within a flexible single-crystal adsorbent of [Rh2(bza)4(pyz)]n.

The crystalline one-dimensional compound, [Rh(II)2(bza)4(pyz)]n (1) (bza = benzoate, pyz = pyrazine) demonstrates gas adsorbency for N2, NO, NO2, and SO2. These gas-inclusion crystal structures were characterized by single-crystal X-ray crystallography as 1 x 1.5 N2 (298 K), 1 x 2.5 N2 (90 K), and 1 x 1.95 NO (90 K) under forcible adsorption conditions and 1 x 2 NO2 (90 K) and 1 x 3 SO2 (90 K) under ambient pressure. Crystal-phase transition to the P1 space group that correlates with gas adsorption was observed under N2, NO, and SO2 conditions. The C2/c space group was observed under NO2 conditions without phase transition. All adsorbed gases were stabilized by the host lattice. In the N2, NO, and SO2 inclusion crystals at 90 K, short interatomic distances within van der Waals contacts were found among the neighboring guest molecules along the channel. The adsorbed NO molecules generated the trans-NO...NO associated dimer with short intermolecular contacts but without the conventional chemical bond. The magnetic susceptibility of the NO inclusion crystal indicated antiferromagnetic interaction between the NO molecules and paramagnetism arising from the NO monomer. The NO2 inclusion crystal structure revealed that the gas molecules were adsorbed in the crystal in dimeric form, N2O4.

Channel-switching crystal with guest stress drive.

By the addition of a "leverage" mechanism in a host component of a metal-organic crystal, a channel-switching property was developed in a porous single-crystal adsorbent driven by incorporated guest gas stress, which may provide new single-crystal devices with the active controllability of anisotropic guest diffusivity.

Generation of a plastic crystal including methane rotator within metal-organic cavity by forcible gas adsorption.

The structural determination of saturated adsorbed methane inside a metal-organic cavity by the forcible pressure swing adsorption method (ca. 13 MPa) through a gas-adsorption equilibrium state gives a methane inclusion crystal even at 298 K. The adsorbed methane molecules regularly locate in the pocket-like narrow corners of the necks of the 1-D channel without disorder. The thermal motion of the pseudo-spherical methane molecules seems to be effectively suppressed in its translation mode but allowed rotation. In cooling to 90 K, the crystal structure remained essentially unchanged while the thermal motion decreased, indicating that a lower temperature reduces the rotation of the adsorbed pseudo-spherical methane. The observed crystal structure could also be influenced by a reduction of the vibrational magnitude, and a phase transition from a static disordered structure to an ordered state might occur. The observed crystal state at a higher temperature should have a plastic crystal nature in terms of the randomness of the orientation of incorporated guests. The single-crystal adsorbent is effective for crystallographic observation of the thermal activated guest forced into regular alignment in the crystal lattice, which can be used as a model of the supercritical fluid.

Selective generation of organic aggregates included within metal-organic micropores through vapor adsorption.

The ethanol vapor adsorption behavior and the inclusion crystal structure of a 1D-transformable coordination polymer host were characterized. The adsorption jump was observed during phase transition or two-phase equilibrium with abnormal adsorption enthalpy caused by the nature of "mass induced phase transition." The included ethanol guests selectively form O-H...O hydrogen bonded pairs inside channels, suggesting selective construction of a specific cluster/aggregate in pores under control of thermodynamic factors and cooperative intermolecular interactions among the guest and channel surface.