Biomedical subjects
Publications and source records attributed to Masaki Kawano.
A hollow coordination cage with a highly symmetric cavity was successfully self-assembled to form a 2D-network having a less symmetric cavity via networking of disordered guests with a metal connector.
The distinct properties of fluorous phases are practically useful for separation, purification, and reaction control in organic synthesis. Here, we report the formation of a liquid-like fluorous droplet, composed of 24 perfluoroalkyl chains confined in the interior of a 5-nanometer-sized, roughly spherical shell that spontaneously assembled in solution from 12 palladium ions and 24 bridging ligands. Crystallographic analysis confirmed the rigid shell framework and amorphous interior. Perfluoroalkanes can dissolve in this well-defined fluorous phase, whereas they can hardly dissolve in a surrounding polar organic solution, and their solubility (up to approximately eight perfluoroalkane molecules per spherical complex) can be finely controlled by tuning the length of perfluoroalkyl chains tethered to the shell.
Self-assembled chiral polymetallic complexes often demonstrate novel properties as asymmetric catalysts. We report the three-dimensional structures of two such asymmetric catalysts (crystals A and B) for Strecker alpha,alpha-disubstituted amino acid synthesis. These complexes are constructed via assembly of the same chiral modules derived from d-glucose, but their assembly modes differ. The enantioselectivity in the Strecker reaction was dramatically switched, depending on which assembly mode was used: the catalyst generated in situ whose structure is represented by crystal B, or by crystal A. These findings provide insight into the functional importance of higher-order structures of an artificial catalyst.
In situ crystallographic observation of an extremely labile coordinatively unsaturated transition-metal complex, Cp'Mn(CO)2 (Cp' = methylcyclopentadienyl), was achieved by generating and trapping it within the confined cavity of a self-assembled cage in the crystalline state at a low temperature. The labile target Cp'Mn(CO)2 was formed by the photodissociation of the CO ligand from stable Cp'Mn(CO)3, which was accommodated within the cavity of the coordination cage. Being trapped in the cage at a low temperature, Cp'Mn(CO)2 showed sufficient stability to be analyzed by X-ray crystallography. The diffraction study revealed that the 16-electron unsaturated Mn complex adopted a pyramidal geometry rather than a planar geometry.
Reported here is the preparation and property of 2D coordination networks composed of rodlike ligands with ethylene glycol side chains (1). Two 2D coordination networks, [[Co(1)2(H2O)2](NO3)2.1.5H2O]n and [[Ni(1)2(H(2)O)2](NO3)2.1.5H2O]n, have been synthesized and characterized by single-crystal X-ray diffraction, TG, DSC, UV-vis spectroscopy, and magnetic measurements. The structural analyses clarified that infinite 1D hydrogen-bond arrays composed of ethylene glycol chains contribute to the stabilization of 2D coordination frameworks, keeping the environment of substitution-active metal sites unchanged. They are more stable than a similar square-grid coordination network that does not possess an ethylene glycol chain on the ligand. We also succeeded in the direct observation of a reversible apical-ligand-exchange reaction at the cobalt(II) and nickel(II) ions in a single-crystal-to-single-crystal fashion because of the considerable stability as well as moderate flexibility of the framework. The cobalt-containing coordination network crystal showed chromic behavior depending on temperatures. Crystallographic and spectroscopic studies revealed that the color change of the crystal was attributed to the ligand-exchange process between H2O and a NO3 anion on the cobalt metal. Magnetic measurements indicated weak antiferromagnetic nearest-neighbor spin coupling between cobalt(II) ions.
Coordination self-assembly entities have been demonstrated to be useful and powerful alternatives for the construction of predefined and well-organized architectures. Self-assembled coordination cages, which possess an inner hydrophobic cavity, can be utilized to move substrates closer by encapsulation so enabling their interaction with each other and exhibiting of non-classical physical properties.
In situ crystallography reveals that the solid state [2 + 2] photodimerization of acenaphthylene in a coordination cage takes place smoothly without preorganizaiton of reaction centers at a preferred geometry, because the substrate tumbles thermally in the large hollow of the cage.
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The single binding pocket of a self-assembled Pd6L4 coordination cage recognizes oligopeptides in a highly sequence-selective fashion. In particular, the Trp-Trp-Ala sequence is strongly bound by the cavity (Ka >/=106 M-1). Tripeptides possessing the same residues but in different sequences (i.e., Trp-Ala-Trp and Ala-Trp-Trp) show much poorer affinity. Even singly mutated tripeptides with aromatic-aromatic-aliphatic sequences of the residues (e.g., Trp-Trp-Gly and Trp-Tyr-Ala) are not recognized efficiently. X-ray analysis and NMR reveal that all residues of the Trp-Trp-Ala sequence cooperatively interact with the cage via CH-pi and pi-pi interactions.
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An adamantanoid (H2O)10 cluster is formed within the hydrophobic cavity of a self-assembled coordination cage. This cluster is termed "molecular ice" because it is the smallest unit of naturally occurring Ic-type ice. X-ray structural analysis, coupled with neutron diffraction study, reveals that the molecular ice is formed not by a simple space-filling effect but by efficient molecular recognition within the cage via H2O:...pi interaction.
We present a new approach for manipulating spin-spin interaction by the self-assembled M6L4-coordination cage. Stable radicals are encapsulated by the cage to become a radical pair in triplet state in both solid and solution states, although the radicals themselves show doublet character without particular intermolecular interaction in solution. These results were confirmed by ESR spectroscopy and X-ray crystallography.
Single-crystal-to-single-crystal guest exchanges of large guest molecules [triphenylene (3a), anthracene (3b), perylene (3c), and triphenylphosphine oxide (3d)] were successfully performed in a large channel of a 3D coordination network (2) having a planar ligand, (1). Crystallographic analysis revealed efficient stackings between the planar guests (3a-c) and the ligand. The crystals of the inclusion complexes of 3a-c showed drastic color change because of strong donor-acceptor interactions between the electron-deficient ligand (1) and electron-rich guests (3a-c).
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A 3.5-nm coordination nanotube was self-assembled from 12 Pd(II) ions and four tape-shaped ligands with the aid of a 3.0-nm template. This tubular structure was unambiguously determined by the single-crystal X-ray analysis. Being cooperatively sustained by 24 Pd(II)-pyridine interactions, the framework was kinetically stable even when the template was removed. The empty tube was, of course, capable of binding other molecules in the cavity.
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