Biomedical subjects
Publications and source records attributed to Michito Yoshizawa.
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.
Self-assembled, hollow molecular structures are appealing as synthetic hosts for mediating chemical reactions. However, product binding has inhibited catalytic turnover in such systems, and selectivity has rarely approached the levels observed in more structurally elaborate natural enzymes. We found that an aqueous organopalladium cage induces highly unusual regioselectivity in the Diels-Alder coupling of anthracene and phthalimide guests, promoting reaction at a terminal rather than central anthracene ring. Moreover, a similar bowl-shaped host attains efficient catalytic turnover in coupling the same substrates (although with the conventional regiochemistry), most likely because the product geometry inhibits the aromatic stacking interactions that attract the planar reagents to the host.
A mixed-valence state of the cation radical tetrathiafulvalene dimer, [(TTF)2]+*, is generated by the electrochemical oxidation of a stacked TTF dimer accommodated within an organic-pillared coordination cage. This mixed-valence species is remarkably stable (t1/2 = approximately 1 day at room temperature in aqueous solution under air) and clearly characterized by cyclic voltammogram and electronic absorption spectroscopy.
Two molecules of planar MII(acac)2 complexes (M = Pt, Pd, and, Cu; acac = acetylacetonato) are efficiently stacked within an organic-pillared coordination cage, exhibiting characteristic spectroscopies (for M = Pt and Pd) and electron spin-spin coupling (for M = Cu) attributable to metal-metal interaction.
Compound 1a, which possesses a triphenylene core and six tetraethyleneoxide side chains, shows efficient ionization of M(II)-containing (M=Pd, Pt) complexes in electrospray ionization mass spectrometry (ESI-MS). The molecular ion peaks [M]+, which are hardly detected under common ESI-MS conditions, are clearly observed as their [M x (1a)n]+ (n=1-4) adducts. UV-visible and NMR studies reveal that the electron-rich triphenylene core of 1a binds to the electron-deficient frameworks of the M(II) complexes in solution, giving rise to charge transfer (CT) complexes. We suggest that 1a stabilizes the complexes and promotes efficient ionization through unique donor-acceptor molecular recognition.
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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.
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The photoexcitation of a self-assembled M6L4-type coordination cage accommodating photochemically inert alkane guests (e.g., adamantane and cyclooctane) led to the regioselective oxidation of the guest within the cage. Under anaerobic conditions, the guest oxidation was accompanied by the stable radical formation as indicated by ESR spectrometry (g = 2.002) and change in solution color (from colorless to blue). These phenomena were shown to be characteristic of the self-assembled molecular systems: i.e., from the M6L4 supersetGn assembly, none of the components (M, L, or G) can be eliminated for the unusual oxidation and/or radical formation.
cis-Decalin (2) and perylene (3) are co-enclathrated by a self-assembled M6L4-coordination cage (1) to give 1 superset(2.3) although each of them is not enclathrated. This phenomenon is termed as AND bimolecular recognition because enclathration occurs only if 2 and 3 coexist. Cage 1 also coenclathrates azulene (8) and 1,4-naphthoquinone (9) to give 1 superset(8.9). In this case, 1 superset(8)2 and 1 superset(9)2 are formed by treating 1 with 8 and 9 individually. This case is termed as OR bimolecular recognition because enclathration occurs if 8 or 9 exists. Accordingly, we have shown that the self-assembled cage 1 experiences both AND and OR bimolecular recognition.
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Despite their structural similarity, triangular tetradentate ligands 2b and 2c experience different assembly pathways on complexation with (en)Pd(NO3)2 to give M8L4 tetrahedral (3) and open cone (4) structures, respectively, due to steric restriction by side chains at the corner or on the edge of the ligands.
Herein reported is the highly selective [2 + 2] cross-photodimerization of olefins within a self-assembled coordination cage that acts as a molecular flask in an aqueous medium. An M(6)L(4) coordination nanocage that self-assembles from six Pd(II) complexes and four tridentate ligands accommodates two different kinds of large olefin molecules such as acenaphthylene and 5-ethoxynaphthoquinone in a pairwise selective fashion. This prerequisite recognition mode makes possible the selective [2 + 2] cross-photodimerization of the olefins within the cavity. The reaction is extremely efficient in terms of reaction rate, stereoselectivity, and, most importantly, pairwise selectivity. Maleimide derivatives, which themselves are photochemically inactive under ordinary conditions, are also cross-dimerized with acenaphthylene or dibenzosuberenon quite efficiently. These results are in sharp contrast to those of common photodimerization in organic media, where the yields and selectivities are generally poor to moderate. The key step of the exclusive formation of the cross-dimers stems from the selective formation of ternary complexes before irradiation, which is governed by the size compatibility of the guests with the confined space of the cavity.
Self-assembly of pre-designed organic ligands with transition metal atoms is a powerful method for construction of novel supramolecular architectures. Particularly, various discrete 3-D hollow structures such as cages, cones, capsules and boxes have been obtained by multicomponent self-assembly of exo-multidentate ligands with cis-protected square planar metal complexes, [(L)M](NO(3))(2) (where L is ethylenediamine or 2,2'-bipyridine and M is Pd or Pt). Furthermore, these hollow structures act as molecular flasks to encapsulate guest molecules and regulate/promote specific reactions; for example, oligomerization of silanetriols and [2+2] intermolecular photodimerization of olefins.
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