Characteristics of Porous Crystalline Frameworks Composed of Nonanuclear AuI6CdII3 Complex Molecules

概要

Nowadays, the creation of porous materials is one of the trending topics in scientific fields, especially in material science and chemistry. Generally, the attractiveness of porous materials is caused by their characters, such as permeability for inducing guest molecules, high energy absorption, and large specific surface area. Since the first porous crystalline material that originated from the discovery of synthetic zeolites, hybrid materials composed of organic linkers and metal ions were developed to produce extended frameworks such as metal-organic framework (MOF). Recently, single-crystal X-ray diffraction (SCXRD) technique has been used not only for the structural determination but also for the investigation of a new insight in specific chemical reactions such as post synthetic modification (PSM), single-crystal-to-single-crystal (SCSC) transformation, and crystalline sponge. In some cases, porous crystalline frameworks can not maintain the crystallinity after being exposed to an external stimulus, which makes it impossible to use SCXRD.

In this doctoral thesis, the characteristics of the porous crystalline framework of [{AuI 6CdII 3(tdme)2(D-pen)6}12Cd4Na4](NO3)12 (1CdNa) composed of the nonanuclear AuI 6CdII 3 complex cations was investigated. The investigation scope is not finite to determine the crystal structure. Some specific phenomena for the crystalline porous material in the course of chemical reactions (e.g., metal exchange) under the control of temperature were also investigated in detail based on X-ray crystallographic evidences, together with related characterizations. The topics of this doctoral thesis are divided into three main chapters.

In Chapter II, the SCSC transmetallation reaction of 1CdNa was investigated. By soaking single crystals of 1CdNa in an aqueous Cu(NO3)2, the isostructural cage-of-cage of [{AuI 6Cu II 3(tdme)2(D-pen)6}12Cu8](NO3)16 (1Cu) was produced within 30 minutes as a kinetic product. Single crystal X-ray structure analysis of 1Cu revealed that CdII -to-CuII metal replacement proceeded not only in the NaI /CdII linker sites but also in the three Cd metal centers in the nonanuclear AuI 6CdII 3 complex cations. The metal substitution of CdII centers by CuII centers afforded the coordination geometry transformation from N2O2S2 octahedral to N2S2 square planar or N2OS2 square pyramidal geometries. Further soaking crystals for 3 days generated the more stable crystals of [{AuI 6Cu II 3(tdme)2(D-pen)6}12Cu8]0.5[{AuI 6Cu II 6(tdme)2(D-pen)6(H2O)6}12Cu8]0.5(NO3)52 (1’Cu). Compound 1’Cu contained two kinds of cage-of-cages, A and B. Type A has the same cage-of-cage structure as 1Cu with chemical formula of [AuI 6CuII 3(tdme)2(D-pen)6]. Type B has a structure slightly different from type A; three additional CuII exist on the outer position and each binds to a flipping D-pen ligand and two water molecules to give the chemical formula of [AuI 6CuII 6(tdme)2(D-pen)6(H2O)6] 6+ . Hydrogen-bonding interactions between cage-of-cage A and B are formed and provides a more robust framework, which is suitable for using it as a crystalline flask. It was found that 1’Cu underwent an anion exchange reaction of NO3 - with MoO4 2- to generate compound 2. In addition, a condensation of oxomolybdate species in 2 was induced by adjusting the pH to around 7 and 1 to form polyoxomolybdates, Mo7O24 6- (3) and β-type Mo8O26 4- (4), respectively, demonstrating an availability of 1’Cu as a crystalline flask.

In Chapter III, the SCSC transmetallation reaction of 1CdNa with aqueous Co(NO3)2, which proceeds in a moderate speed, was investigated by SCXRD snapshot technique. By soaking single crystals of 1CdNa in an aqueous Co(NO3)2, the isostructural cage-of-cage of [{AuI 6CoII 3(tdme)2(D-pen)6}12Co8](NO3)16 (1Co) was obtained after 6 days with retaining the single crystallinity. The crystal structure of 1Co showed the metal replacement of CdII by CoII proceeded at all NaI /CdII linkers and CdII metal centers in nonanuclear AuI 6CdII 3 complexes. In contrast to CdII -to-CuII metal replacement, the coordination geometry remained unchanged after the transmetallation, adopting an N2O2S2-octahedral geometry. The moderate rate of the SCSC transmetallation reaction enabled to observe the intermediate state by SCXRD snapshot technique. Based on the site occupancies on the M1-M5 sites of the intermediate states, two intermediate states, [{AuI 6CdII 3(tdme)2(D-pen)6}12Cd4Co4](NO3)16 (1CdCo 1h) and [{AuI 6CdIICoII 2(tdme)2(D-pen)6}12Cd4Co4](NO3)16(1CdCo 1d), were found during the SCSC transmetallation reaction of 1CdNa, the crystals of which were soaked in a Co(NO3)2 aqueous solution. Porous cavities in 1CdNa allows a smooth diffusion of Co species to induce the homogeneous CdII -to-CoII transmetallation event. This is the first example of the observation of intermediate states in the SCSC transmetallation reaction by X-ray crystallography evidence.

In Chapter IV, the formation of water cluster inside 1CdNa and its phase transition were deeply studied. In 1CdNa, there exist four kinds of cavity spaces that are occupied by solvated water molecules owing to the pseudo-primitive cubic packing of cage-of-cage complex cations. It was found that, by cooling a single crystal of 1CdNa rapidly, solvated water molecules in the cavity A were arranged in a specific pattern corresponding to that of ice Ic, which is difficult to be formed generally. In the cavity A, a total of 143 water molecules were found, which form a water cluster with an ice Ic arrangement. Although a large number of studies on the formation of ice Ic in porous materials was reported, this is the first successful finding and direct evidence of the formation of an ice Ic arrangement of water cluster by SCXRD technique. Here, three important structural factors are proposed, i) spherical shape, ii) large enough space, iii) symmetry of the cavity. Especially, the molecular arrangement in 432 crystallographic point group is essential for the formation of ice Ic arrangement in 23 crystallographic point group. In addition, from the screening of the temperature control, it was proposed that the cooling rate is crucial for the formation of an ice Ic arrangement of water cluster. While rapid cooling afforded well-ordered ice Ic arrangement, slow cooling gave less-ordered arrangement presumably owing to the difference between homogeneous and heterogeneous thermal conductions. By elevating the temperature, a phase transition was observed at 200 K from a drastic volume reduction. At the same time, SCXRD observed directly the disappearance of the water cluster with an ice Ic arrangement. The appearance of an endothermic peak in the DSC profile supported that the phase transition is a pseudo-melting from ice Ic to disordered water molecules. This is also the first observation of the phase transition from ice Ic to another phase of water cluster not via ice Ih. After the observation of the pseudo-melting, it was demonstrated that the arrangement of water molecules can be reverted back to ice Ic by cooling.

In conclusion, the structural changes in the porous single crystals of 1CdNa cage-of-cage complex, i) the transmetallation (CdII -to-CuII and CdII -to-Co II) ii) the condensation of oxomolybdates to form polyoxometalates, iii) the formation of ice Ic water cluster and its pseudo-melting were observed in this study. These phenomena found in this study are all distinct from those in non-porous materials because of the usage of porous spaces in the crystalline materials. This study demonstrated the importance of the design of highly symmetrical porous spaces that are large enough, providing a significant direction to develop porous materials for the study of chemical reactions and unique phenomena in the solid state.