Yasuo Tanaka

Protonated [4n]π and [4n+2]π Octaphyrins Choose Their Möbius/Hückel Aromatic Topology

Protonation-triggered conformational changes of meso-octakis(pentafluorophenyl) [36]octaphyrin and [38]octaphyrin have been investigated. The X-ray crystal structures and (1)H NMR analyses revealed that the protonation process cuts off intramolecular hydrogen bonds between aminic and iminic pyrrole units and, at the same time, produces intermolecular hydrogen-bond network between aminic pyrrole unit and counter-anions. Such a replacement induces some pyrrole inversion, leading to Mobius aromatic conformation for [36]octaphyrin and to Huckel aromatic conformation for [38]octaphyrin. These protonated octaphyrins show similar structures only with a subtle difference in tilted pyrrole angles, which results in their different topologies. This feature strongly suggests that the macrocycles control their topologies by pyrrole rotation to gain [4n]pi Mobius or [4n+2]pi Huckel aromatic stabilization, depending on the number of pi-electrons. Detailed photophysical properties such as absorption/fluorescence, excited singlet/triplet state lifetimes, and two-photon absorption cross-section values have been presented for both protonated [36] and [38]octaphyrins in conjunction with their Mobius or Huckel aromaticity.

Metathesis‐like Splitting Reactions of Metallated [36]Octaphyrins(1.1.1.1.1.1.1.1): Experimental and Computational Investigations

meso-Octakis(pentafluorophenyl)-substituted [36]octaphyrin(1.1.1.1.1.1.1.1) (1) is a figure-of-eight nonaromatic macrocycle that serves as a unique platform to induce a metathesis-like splitting reaction upon bis-Cu(II) metalation. To get a better understanding of this splitting reaction, we examined the metalation of 1 with several metal ions. In contrast with the smooth and quantitative splitting reaction of bis-Cu(II) complex 1-CuCu, free-base 1, mono-Cu(II) complex 1-Cu, and bis-Zn(II) complex 1-ZnZn do not undergo the splitting reaction. Mono-Co(II) complex 1-Co was selectively produced from metalation with the Co(II) ion, from which hybrid complex 1-CoCu was synthesized. The hybrid complex 1-CoCu undergoes the splitting reaction to give 2-Co and 2-Cu quantitatively. Activation parameters of the splitting reactions were determined: E(a)=104 kJ mol(-1), DeltaH(double dagger)=101 kJ mol(-1), DeltaS(double dagger)=-25.0 J mol(-1) K(-1), and DeltaV(double dagger)=18 cm(3) mol(-1) for 1-CuCu and E(a)=105 kJ mol(-1), DeltaH(double dagger)=102 kJ mol(-1), and DeltaS(double dagger)=-29.9 J mol(-1) K(-1) for 1-CoCu. A marked difference between the splitting reaction reactivity of 1-CuCu and 1-ZnZn has been examined by DFT calculations at the B3LYP/631SDD//B3LYP/LANL2DZ level, which revealed that the reaction proceeds through a stepwise route involving initial C1-C20 bond formation to give INT1 as the rate-determining step and subsequent C21-C40 bond formation to give a spirocyclobutane intermediate (INT2), followed by a radical reverse cycloaddition reaction to give two metalloporphyrins. Inherent instability of 1-CuCu, which may arise from its strongly distorted structure, was indicated to be a main factor in the smooth splitting reaction. Finally, a new bis-Pd(II) complex (5-PdPd) was isolated in the metalation of 1 with Pd(OAc)(2) in a 9:1 mixture of 2,2,2-trifluoroethanol and methanol as a manifestation of the transannular electronic interaction in metalated octaphyrin complexes. Collectively, these results underscore the importance of the transannular electronic interactions that are enhanced by metalation, depending upon the coordinated metal ions.

A Stable Organic π‐Radical of a Zinc(II)–Copper(I)–Zinc(II) Complex of Decaphyrin

A Zn(II) -Cu(I) -Zn(II) heterotrimetal complex of decaphyrin was synthesized by stepwise metalations: metalation of a [46]decaphyrin with Zn(II) ions to produce a 46π decaphyrin bis(Zn(II) ) complex and its subsequent metalation with Cu(II) ion. In the second metalation step, it has been shown that Cu(II) ion is reduced to a Cu(I) ion in the complex and a dianionic bis(Zn(II) ) containing [46]decaphyrin ligand is oxidized to the corresponding monoanionic [45]decaphyrin ligand, indicating a non-innocent nature of the decaphyrin ligand. Despite the radical nature, the heterotrimetal complex is fairly stable under ambient conditions and exhibits almost no intermolecular magnetic interaction, owing to extensive delocalization of an unpaired electron in the large π-conjugated circuit of decaphyrin moiety.

The Extension of Baird's Rule to Twisted Heteroannulenes: Aromaticity Reversal of Singly and Doubly Twisted Molecular Systems in the Lowest Triplet State

We have investigated the aromaticity of singly twisted Möbius aromatic and doubly twisted Hückel antiaromatic bis(palladium(II)) [36]octaphyrins in the lowest triplet state (T1 ) by spectroscopic measurements and quantum calculations. The T1 state of the singly twisted Möbius [36]octaphyrin shows broad and weak absorption spectral features that are analogous to those of antiaromatic expanded porphyrins while the T1 state of the doubly twisted Hückel [36]octaphyrin exhibits intense and distinct spectral features, indicating the aromatic nature. These results along with theoretical calculations support the hypothesis that the aromaticity is reversed in the T1 state. Furthermore, we show that the degree of structural smoothness affects the aromaticity reversal in the T1 state.