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N-Confused Porphyrin (NCP)

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In 1994, the Furuta and Latos-Grazynski groups independently reported a new type of porphyrin isomer, called N-confused porphyrin (NCP) or inverted porphyrin. NCP has the same backbone structure as a porphyrin (1,1,1,1), but possesses a "confused pyrrole," which is a pyrrolic moiety connected to the surrounding meso-carbons at the α- and β-positions. Owing to this unusual linkage, the positions of pyrrolic NH and β-CH are switched. As a result, NCP contains an NNNC core and an outward pointing N atom. When a metal is coordinated in the core, a kind of organometallic complex with a carbon-metal bond is formed. At the same time, the peripheral nitrogen atom serves as a hydrogen bonding donor/acceptor as well as a metal coordination site.

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CP derivatives other than meso-tetraaryl types are continuously synthesized. An NCP framework is composed of four pyrrolic rings, just like standard porphyrins, but X-ray structure analyses have shown that the confused pyrrole ring is tilted by ca. 30 degrees to the NCP mean plane, which is in marked contrast to those of porphyrins with high symmetry and planarity. Reflecting the structural difference, the tautomerism of NCP differs largely from that of a normal porphyrin. NCP tautomerism involves the peripheral nitrogen as well as inner nitrogen atoms, and two kinds of tautomers are observed by spectroscopic methods. One type of tautomer has three hydrogen atoms in the core (NCP-3H), and the other has two hydrogen atoms in the core and one hydrogen atom with the peripheral nitrogen (NCP-2H). Both structures have been elucidated by X-ray diffraction analysis. NCP-3H is predominantly observed in nonpolar solvents, whereas NCP-2H is highly stabilized in polar solvents, such as DMF and acetone, by the hydrogen bonding interaction at the peripheral NH moiety. These two tautomers show differing aromaticity, photochemical properties, and more.

NCP is expected to exhibit different coordination modes than those of normal porphyrins. In principle, NCP is able to stabilize both +3 and +2 oxidation states of a metal to form electronically neutral metal complexes when it coordinates a metal in the core. For instance, NCP-Ag(III), NCP-Sb(V), and NCP-Sn(IV) complexes were isolated as air-stable compounds, and involve carbon-metal bonds in the NCP core. Similarly, NCP-Pd(II) and NCP-Pt(II) complexes were synthesized as neutral metal complexes. These complexes have applications for anion sensors and catalysis.

Department of Applied Chemistry
Professor Hiroyuki Furuta
Associate Professor Soji Shimizu
Assistant Professor Motoki Toganoh
Assistant Professor Masatoshi Ishida
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