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Physical Coordination Chemistry

  • Masaaki Ohba, Professor
  • Tomomi Koshiyama, Assistant Professor
In the field of molecular materials, one of the important targets is the development of multi-functional material combining different properties and functions in a synergic way. Our group focuses on “coordination polymers” and “liposome” as a meso-scale platform for interlocking various functions and properties. The coordination polymers can provide functional space that consists of flexible, highly ordered and designable frameworks based on coordination bonds. The frameworks can be incorporated magnetic, electrical, optical and other properties, and also can adsorb molecules in the void of structure. The liposome, a spherical vesicle composed of a phospholipid bilayer, provides hierarchical composite with selectively incorporating different functional molecules into the hydrophilic inner water phase, hydrophobic lipid bilayer, and inner and outer surfaces. We are exploring advanced functions using such spaces with synergically linking plural different properties of components.

1. Chemo-responsive Coordination Polymers

Porous coordination polymers (PCPs) are one of the promising compounds to achieve a combination of physical properties and porous functions, because the coordination frameworks are flexible and sensible for chemical stimulus. We have systematically studied on chemo-switching of magnetic properties and correlation between the magnetic and porous properties in polycyanomatallate-based PCPs showing magnetic ordering or spin-crossover (SCO).

Figure 1
Structure of {Fe(pz)[M(CN)4]} with functional components.

Hofmann-type coordination polymers {Fe(pz)[MII(CN)4]} (pz = pyrazine; MII = Pt (1), Pd (2), Ni (3)) form a 3-D pillared-layer-type porous framework incorporating a SCO subunits and guest interactive sites. Compounds 1–3 exhibit cooperative spin transition near room temperature (Tcup = 304 K and Tcdown = 284 K, and adsorb various guest molecules, such as hydroxylic solvents, aromatic molecules, gases, I2, etc. Guest molecules are confined in the pores, interacting weakly with the pz pillar ligands or the MII centers. The size and shape of guest molecules, host–guest interactions and rotation of pz ligand cause structural change with changing ligand field strength around Fe centers and cooperativity, which results in changing the magnetic properties of framework. The magnetic properties can be reversibly switched by the uptake, release and exchange of guest molecules. We have elucidated the correlation between the physical properties (spin transition temperature, hysteresis width, electrical conductivity, dielectric and optical properties) and guest molecules (arrangement, motion, diffusion in the pore etc.) by in-situ physical measurements under guest vapor, e.g. magnetization, quasi-elastic neutron scattering, solid state 2H-NMR, UV-Vis, IR and Raman spectra, and synchrotron X-ray powder diffraction.

2. Synergetic Transition in a Magnetically Bistable Porous Coordination Polymer

Figure 2
The synergetic transition of the ST (fast dynamics) and the disorder‒order transition of confined I2 (slow dynamics) in 2⊂I.

Hysteretic behavior generated by cooperativity in bistable molecular materials is a key phenomenon for development of molecular devices and advanced nanotechnology. Establishment of a strategy on how to control the cooperativity is an important subject. For this purpose, we propose a new strategy introducing the concept of synergetics by employing an intrachannel ordering of guest molecules as an additional order parameter in spin transition (ST) in porous coordination polymers (PCPs). An iodine clathrate compound {Fe(pz)[Pd(CN)4]·0.95I2} (2⊂I) showed two different transitions, ST and order–disorder transition in the position of confined I2, and anomalously wide thermal hysteresis of 94 K was observed. A variable temperature simultaneous measurement of microscopic Raman scattering and synchrotron X-ray powder diffraction corroborated the different dynamic behaviours of I2 and the host framework in 2⊂I. This synergetic transition of the ST in the host framework (fast dynamics) and the guest arrangement (slow dynamics) is a key for enhancing the cooperativity of STs.

3. Fast Ortho–Para Conversion of H2 in a Coordination Nanospace

Figure 3

Hydrogen molecule H2 has two different nuclear-spin isomers, para (p) and ortho (o), due to requirement of symmetry of wave function. As interconversion between these isomers is a forbidden process in an isolated molecule, it takes a long time constant of 100 hours. The o–p conversion is an exothermic process and causes a boil-off problem in a cryogenic H2 storage. We found a less than 10 minutes o–p conversion of H2 adsorbed in a Hofmann-type PCP, {Fe(pz)[Pd(CN)4]} (2) by temperature dependence of Raman spectra. Charge density study using synchrotron radiation X-ray powder diffraction revealed the electric field generated in the coordination nano-space in the PCP. The electric field excites the confined H2 molecules and induce the fast catalytic hydrogen o–p conversion.

4. Functional Space Integrated Functional Coordination Compounds and Liposome

Figure 4
Schematic diagram of incorporation of lipophilic metal complexes on the liposome surface.

To construct a highly efficient functional spaces, we incorporated functional metal complexes (e.g., water oxidation catalyst) into the specific liposome space. For the functionalization of liposome surface, we designed new lipophilic metal complexes using cholesterol derivatives having high affinity for phospholipid or phospholipid derivatives. For instance, we incorporated lipophilic Ru complexes including [Ru(terpy)(bpy)(H2O)]2+ (terpy = 2,2′;6′,2″-terpyridine, bpy = 2,2′-bipyridine) unit and cholesterol derivatives into the liposome surface as water oxidation catalysts. Its catalytic activity was improved by modulating surrounding environment of the Ru complexes: (1) types of phospholipids and (2) length of linker between bpy and cholesterol. We also succeeded in encapsulation of Prussian Blue (PB) nanoparticles in the inner aqueous phase by a direct synthesis through transmembrane channels. The composite exhibited higher Cs+ adsorption ability than PB particle with avoiding flocculation of suspended PB particles.