Tomoki Yoneda

Solvent-Dependent Aromatic versus Antiaromatic Conformational Switching in meso-(Heptakis)pentafluorophenyl [32]Heptaphyrin

We have investigated conformational switching dynamics of meso-heptakis(pentafluorophenyl) [32]heptaphyrin(1.1.1.1.1.1.1) in various solvents using steady-state, time-resolved, and temperature-dependent spectroscopy. Absorption and fluorescence spectra of [32]heptaphyrin are quite sensitive to solvent environments. In nonpolar toluene, the antiaromatic figure-of-eight conformation is dominant, as seen in the X-ray crystallography, based on broad and weak absorption bands without any fluorescence and moderate paratropic ring current. On the other hand, a well-resolved sharp absorption spectrum, strong fluorescence, and diatropic ring current in the 1H NMR spectrum in slightly polar THF indicate that most of [32]heptaphyrin molecules take significantly distorted Möbius conformation with aromatic character. By using transient absorption (TA) spectroscopy, the lowest singlet excited-state lifetimes have been revealed to decay biexponentially with the time constants of 5 and 65 ps for figure-of-eight and Möbius conformations, respectively. Based on these results along with vertical excitation energy calculations, we are able to assign two conformers as Hückel antiaromatic and Möbius aromatic species, respectively; it shoulf be noted that the aromaticity/antiaromaticity does not change with temperature variation. Interestingly, in moderately polar solvent, ethyl ether, we find out that these two conformational isomers coexist with a dynamic equilibrium, as revealed by excitation-wavelength-dependent TA, temperature-dependent absorption and 1H NMR spectra. Through our findings, we have demonstrated that the conformational switching dynamics between Hckel antiaromatic and Möbius aromatic conformers in [32]heptaphyrin(1.1.1.1.1.1.1) are strongly affected by solvent medium as well as temperature.

PdIIComplexes of [44]- and [46]Decaphyrins: The Largest Hückel Aromatic and Antiaromatic, and Möbius Aromatic Macrocycles

Reductive metalation of [44]decaphyrin with [Pd2(dba)3] provided a Hückel aromatic [46]decaphyrin Pd(II) complex, which was readily oxidized upon treatment with DDQ to produce a Hückel antiaromatic [44]decaphyrin Pd(II) complex. In CH2Cl2 solution the latter complex underwent slow tautomerization to a Möbius aromatic [44]decaphyrin Pd(II) complex which exists as a mixture of conformers in dynamic equilibrium. To the best of our knowledge, these three Pd(II) complexes represent the largest Hückel aromatic, Hückel antiaromatic, and Möbius aromatic complexes to date.

Synthesis and Catalytic Activities of Porphyrin‐Based PCP Pincer Complexes

2,18-Bis(diphenylphosphino)porphyrins undergo peripheral cyclometalation with group 10 transition-metal salts to afford the corresponding porphyrin-based PCP pincer complexes. The porphyrinic plane and the PCP-pincer unit are apparently coplanar, with small strain. The catalytic activities of the porphyrin-based pincer complexes at the periphery were investigated in the allylation of benzaldehyde with allylstannane and in the 1,4-reduction of chalcone to discover the electronic interplay between the inner metal and the outer metal in catalysis.

Ensemble and Single-Molecule Spectroscopic Study on Excitation Energy Transfer Processes in 1,3-Phenylene-Linked Perylenebisimide Oligomers

1,3-Phenylene-bridged perylenebisimide dimer (PBI2) and trimer (PBI3) were prepared along with monomer reference (PBI1) using perylene imide-anhydride 5 as a key precursor. 3,3-Dimethylbut-1-yl substituents were introduced at the 2,5-positions of perylenebisimide (PBI) to improve the solubilities of PBI oligomers. Actually, no serious aggregation of PBI2 and PBI3 was detected in their dilute CH(2)Cl(2) solutions. Under these conditions, intramolecular electronic interactions among PBI chromophores have been revealed by measuring the photophysical properties at their ensemble and single-molecule levels. The excitation energy transfer times of PBI2 (0.16 ps) and PBI3 (0.60 ps) were determined from the two different observables, anisotropy depolarization, and singlet-singlet annihilation, respectively, which are considered as the incoherent Förster-type energy hopping (EEH) times as compared with the EEH time constant (1.97 ps) calculated on the basis of the Förster mechanism. The relatively short EEH times compared to similar PBI oligomers can be attributed to 1,3-phenylene linker, which assures a short distance between the chromophores and, as a consequence, makes it hard to treat the PBI unit as a point dipole. The limitation of point-dipole approximation to describe the PBI oligomers and additional through-bond type interactions can be attributed as the causes of the discrepancies in excitation energy transfer times. Considering these photophysical properties, we can suggest that 1,3-phenylene-linked PBI oligomers have potentials as molecular photonic devices including the artificial light-harvesting system.

Synthesis of a [26]Hexaphyrin Bis‐PdII Complex with a Characteristic Aromatic Circuit

Not one, but two: Metalation of a mono-Pd(II) hexaphyrin (1) with [Pd(OCOCF3)2] gave a bis-Pd(II) complex (2), which possesses a characteristic 26 π-aromatic circuit with two outer amino-pyrroles within a rectangular molecular framework. Complex 2 was readily deprotonated to afford a dianion, which was regioselectively methylated to give a methyl derivative. A similar methylation reaction of 1 produced a skeletally rearranged product that contained an N-confused pyrrole.

Palladium(II)‐Triggered Rearrangement of Heptaphyrins to N‐Confused Porphyrins

A recent surge in the chemistry of expanded porphyrins is largely due to their attractive optical, electrochemical, and coordination properties, which arise from their large p-conjugated frameworks. The unique chemical reactivity of expanded porphyrins means that they often undergo metalation-induced skeletal rearrangements, as metalation reactions require specific coordination structures and expanded porphyrins have flexible electronic systems that are reactive toward such structural distortions. 3] The first examples of expanded porphyrins were the octaphyrins(1.1.1.0.1.1.1.0) reported by Vogel et al. , which were followed by our reports on 1) the splitting reaction of a bis(Cu octaphyrin) into two Cu porphyrins, 2) the boron(III)-triggered splitting reaction of Cu heptaphyrins to Cu porphyrins and B subporphyrins, and 3) the boron(III)-induced rearrangement of a hexaphyrin(1.1.1.1.1.1) to form a hexaphyrin(2.1.1.0.1.1). These skeletal rearrangements are intriguing in making “chemical connections” between important porphyrinoids such as octaphyrins, heptaphyrins, hexaphyrins, porphyrins, and subporphyrins. Despite these results, N-confused porphyrins (NCPs), which have a key position in porphyrin chemistry, 5] have not been included in this approach to date. Herein, we report that an NCP-type conjugated system is formed from [32]heptaphyrin(1.1.1.1.1.1.1) upon rearrangement triggered by metalation with Pd ions. meso-Aryl-substituted [32]heptaphyrins(1.1.1.1.1.1.1) are interesting macrocycles in terms of their flexible conformations, multiple N-fusion reactions, facile formation of twisted M bius aromatic species upon protonation, formation of three-coordinated Cu complexes, and oxidative ring opening to form conjugated helical molecules. We have previously reported that a M bius aromatic complex, 2, was formed in 88 % yield from meso-heptakis(2,6-dichlorophenyl)-substituted [32]heptaphyrin 1 upon treatment with Pd(OAc)2 in CH2Cl2. [7c] Careful examination of this reaction led to an isolation of brown complex 3 as a side-product in 9% yield. When the metalation reaction was carried out in acetone, the yield of 3 was improved to 22 % at the expense of the yield of 2 (Scheme 1). High-resolution electrospray ionization time-of-flight mass spectroscopy (HR-ESI-TOF MS) indicated the parent ion peak of 3 at m/z 1659.7423 (calcd for C77H34N7Cl14Pd [M H] : 1659.7485). The structure of 3 was unambiguously determined by single-crystal X-ray

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.

Conformational Fixation of a Rectangular Antiaromatic [28]Hexaphyrin Using Rationally Installed Peripheral Straps

A rectangular [28]hexaphyrin bearing outer straps at the long side has been synthesized by SN Ar reaction of [26]hexaphyrin with allyl alcohol, intramolecular olefin metathesis by using Hoveyda-Grubbs second-generation catalyst, and reduction with NaBH4 . The peripheral straps enforce a rectangular conformation for the [28]hexaphyrin, which shows Hückel antiaromatic character, as confirmed by its planar X-ray structure, a strong paratropic ring current, characteristic UV/Vis/NIR absorption features, small electrochemical HOMO-LUMO gap, and very fast S1 decay.

The Marriage of Peripherally Metallated and Directly Linked Porphyrins: Bromidobis(phosphine)platinum(II) as a Cation-Stabilizing Substituent on Directly Linked and Fused Triply Linked Diporphyrins

A meso-bromidoplatiniobis(triphenylphosphine) η(1)-organometallic porphyrin monomer was prepared by the oxidative addition of meso-bromoZnDPP (DPP=dianion of 5,15-diphenylporphyrin) to a platinum(0) species. The meso-meso directly linked dimeric porphyrin (5) was prepared from this monomer by silver(I)-promoted oxidative coupling and planarized to give a triply linked dizinc(II) porphyrin dimer (8). Acidic demetallation of 8 afforded the bis(free base) 9. Dimer 5 was demetallated then remetallated with nickel(II) to give the dinickel(II) analogue 10, the X-ray crystal structure of which showed a twisted molecule with ruffled, orthogonal NiDPP rings, terminated by square-planar trans-[Pt(PPh3)2Br] units. New compounds were fully characterized spectroscopically, and the fused diporphyrin exhibited a broad, low-energy, near-IR electronic absorption band near 1100 nm. Electrochemical measurements of this series indicate that the organometallic fragment is a strong electron donor towards the porphyrin ring. The triply linked organometallic diporphyrin has a substantially lowered first one-electron oxidation potential (-0.35 V versus the ferrocene/ferrocenium couple (Fc/Fc(+))) and a narrow HOMO-LUMO gap of 0.96 V. Solutions prepared for NMR spectroscopy slowly decompose with degradation of the signals, which is attributed to partial oxidation to the cation radical. This paramagnetic species can be reduced in situ by hydrazine to restore the NMR spectrum to its former appearance. The combined influence of the two [Pt(PPh3)2Br] electron-donating substituents is sufficient to make dimer too aerobically unstable to allow further elaboration.

Utility of heme analogues to intentionally modify heme-globin interactions in myoglobin.

Myoglobin reconstitution with various synthetic heme analogues was reviewed to follow the consequences of modified heme-globin interactions. Utility of dimethyl sulfoxide as the solvent for water-insoluble hemes was emphasized. Proton NMR spectroscopy revealed that loose heme-globin contacts in the heme pocket eventually caused the dynamic heme rotation around the iron-histidine bond. The full rotational rate was estimated to be about 1400 s(-1) at room temperature for 1,4,5,8-tetramethylhemin. The X-ray analysis of the myoglobin containing iron porphine, the smallest heme without any side chains, showed that the original globin fold was well conserved despite the serious disruption of native heme-globin contacts. Comparison between the two myoglobins with static and rotatory prosthetic groups indicated that the oxygen and carbon monoxide binding profiles were almost unaffected by the heme motion. On the other hand, altered tetrapyrrole array of porphyrin dramatically changed the dissociation constant of oxygen from 0.0005 mm Hg of porphycene-myoglobin to ∞ in oxypyriporphyrin-myoglobin. Heme-globin interactions in myoglobin were also monitored with circular dichroism spectroscopy. The observation on several reconstituted protein revealed an unrecognized role of the propionate groups in protoheme. Shortening of heme 6,7-propionates to carboxylates resulted in almost complete disappearance of the positive circular dichroism band in the Soret region. The theoretical analysis suggested that the disappeared circular dichroism band reflected the cancellation effects between different conformers of the carboxyl groups directly attached to heme periphery. The above techniques were proposed to be applicable to other hemoproteins to create new biocatalysts. This article is part of a Special Issue entitled Biodesign for Bioenergetics--the design and engineering of electronic transfer cofactors, proteins and protein networks, edited by Ronald L. Koder and J.L. Ross Anderson.