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1. N-Confused Porphyrin (NCP)


In 1994, Furuta and Latos-Grazynski's groups independently reported a new type of porphyrin isomer, N-confused porphyrin (NCP) or inverted porphyrin.

NCP has a same backbone structure as porphyrin(1,1,1,1). However, NCP 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 having a carbon-metal bond, is formed. On the other hand, the peripheral nitrogen atom serves as a hydrogen bonding donor/acceptor as well as a metal coordination site.

Reviews and Accounts:

1-1. Structures and Properties

NCP derivatives other than meso-tetraaryl-type are continuously synthesized (OL 2003, EJOC 2005). NCP framework is composed of four pyrrolic rings just like standard porphyrins, but the X-ray structure analyses have shown that the confused pyrrole ring is tilted by ca. 30 degree to the NCP mean plane, which is a marked contrast to those of porphyrins with high symmetry and planarity (JACS 1994). Reflecting the structural difference, the tautomerism of NCP differs largely from that of normal porphyrin. NCP tautomerism involves the peripheral nitrogen as well as inner nitrogen atoms, and two kinds of tautomers are observed by the 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 at the peripheral nitrogen (NCP-2H). The both structures were elucidated by X-ray diffraction analyses. NCP-3H is dominantly observed in the nonpolar solvents, whereas NCP-2H is highly stabilized in polar solvents, such as DMF and acetone, by the hydrogen bonding interaction at the periphery NH moiety. These two tautomers show different aromaticity, photochemical properties, and so on.

NCP can serve as both trivalent and divalent metal coordination ligands. Actually, Ag(III) and Sb(V) complexes were obtained with a trivalent NCP ligand, and Pd(II) and Pt(II) complexes were synthesized with a divalent NCP ligand, accordingly. Moreover, the peripheral nitrogen atom of the confused pyrrole can serve as a coordination site to form bis-Rh(I) NCP and dimeric Pd(II) complexes. The details on NCP metal complexes are shown in "1-3.Metal Complexes."


1-2. Reactivity

Owing to the existence of the confused pyrrolic ring, NCP possesses high reactivity compared to the porphyrin. For example, electrophilic substitution reactions (nitrarion, bromination, etc.) readily occur at β-CH position of the confused pyrrolic ring (CL 1997, JACS 1999, JACS 2000) and the α-CH moiety is easily oxidized to form an amide-NCP in the presence of SnCl2, O2 and H2O (ACIE 2006). Furthermore, treating NCP with Cu(II) in the presence of O2, the confused pyrrole ring was removed to afford a tripyrrinone (OL 2002).

Dibromo-NCP, which is obtained by bromination of NCP, can be converted into N-fused porphyrin (NFP), via inversion of the confused pyrrole and subsequent nucleophilic substitution of the adjacent pyrrolic nitrogen. Additionally, some kinds of nucleophiles (e.g., RO) attack the fused moiety of NFP and force the ring-opening to rebirth the NCP. NFP exhibits characteristic electro- and photochemical properties, such as NIR absorption and emission (JACS 2000). Apart from dibromo-NCP route, NFP skeleton can be synthesized directly by a treatment of NCP with Re2CO10 (CC 2004).


1-3. Metal Complexes

1-3-1. Monomeric Metal Complexes by Coordinations in the NCP Core

NCP is expected to exhibit different coordination modes from those of normal porphyrins. In principle, NCP is able to stabilize both +3 and +2 oxidation states of metals to form electronically neutral metal complexes when it coordinates a metal in the core. For instance, NCP-Ag(III) (IC 1999), NCP-Sb(V) (J. Organomet. Chem. 2000), and NCP-Sn(IV) (ACIE 2006) complexes were isolated as air-stable compounds, which involve carbon-metal bonds in the NCP core. Similarly, NCP-Pd(II) (IC 2000) and NCP-Pt(II) (ACIE 2003) complexes were synthesized as neutral metal complexes. These complexes were applicable to anion sensors and catalysis.

Furthermore, tetrakis(pentafluorophenyl)NCP can stabilized both Cu(II) and Cu(III) species. The two Cu complexes are interconvertable by oxidation and reduction (JACS 2003).

1-3-2. Polymeric Metal Complexes by Coordinations at the NCP Periphery

The peripheral nitrogen atom of NCP can also serve as a coordination site. NCP coordinates Rh(I) ion not only in the core but also at the peripheral nitrogen to afford NCP bis-Rh(I) complex (CC 2001) and does Ir(I) with a confused pyrrole inverted, resulting in formation of NCP bis-Ir(I) complex (IC 2006). The peripheral coordination also plays an important role to construct dimeric NCP complexes. Pd(II)- and Pt(II)-NCP dimer complexes having orthometallated meso-aryl groups were synthesized (IC 2000, ACIE 2003). A treatment of NCP with Zn(II) salts afforded tetranuclear- and dinuclear-NCP dimer complexes, where the two NCP's are mutually coordinated by the peripheral nitrogen atom and Zn(II) ion (JACS 2002, IC 2004). A dimer having a Rh cluster is also synthesized (IC 2006)

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