Excited-State Aromaticity of Gold(III) Hexaphyrins and Metalation Effect Investigated by Time-Resolved Electronic and Vibrational Spectroscopy.

Expanded porphyrins with appropriate metalation provide an excellent opportunity to study excited-state aromaticity. The coordinated metal allows the excited-state aromaticity in the triplet state to be detected through the heavy-atom effect, but other metalation effects on the excited-state aromaticity were ambiguous. Herein, the excited-state aromaticity of gold(III) hexaphyrins through the relaxation dynamics was revealed via electronic and vibrational spectroscopy. The SQ states of gold [26]- and [28]-hexaphyrins showed interconvertible absorption and IR spectra with those of counterparts in the ground-state, indicating aromaticity reversal. Furthermore, while the T1 states of gold [28]-hexaphyrins also exhibited reversed aromaticity according to Baird's rule, the ligand-to-metal charge-transfer state of gold [26]-hexaphyrins contributed by the gold metal showed non-aromatic features arising from the odd-number of π-electrons.

π-Ruthenium Complexes of Hexaphyrins(1.1.1.1.1.1): A Triple-Decker Complex Bearing Two Ruthenoarene Units.

Ruthenium(II) π-coordination onto [28]hexaphyrins(1.1.1.1.1.1) has been accomplished. Reactions of bis-AuIII and mono-AuIII complexes of hexakis(pentafluorophenyl) [28]hexaphyrin with [RuCl2 (p-cymene)]2 in the presence of NaOAc gave the corresponding π-ruthenium complexes, in which the [(p-cymene)Ru]II fragment sat on the deprotonated side pyrrole. A similar reaction of the bis-PdII [26]hexaphyrin complex afforded a triple-decker complex, in which the two [(p-cymene)Ru]II fragments sat on both sides of the center of the [26]hexaphyrin framework.

Internally Bridged Hückel Aromatic [46]Decaphyrins: (Doubly-Twisted-Annuleno)Doubly-Twisted-Annulene Variants.

Internally 3,3'-biphenyl-, 2,2',5,5'-bithiophene-, and 2,5-thieno[3,2-b]thiophene-bridged [46]decaphyrins were prepared. In addition to the global 46π-conjugated circuits, the internal core-modified [32]heptaphyrin- and [30]hexaphyrin moieties in 2,2',5,5'-bithiophene- and 2,5-thieno[3,2-b]thiophene-bridged [46]decaphyrins also possess doubly twisted topologies (|Lk |=2) and are regarded as (doubly-twisted-annuleno)doubly-twisted-annulene variants. All these decaphyrins display distinct Hückel aromaticity owing to the global 46π-electronic networks but the contributions of the local half circuits were almost negligible due to the perpendicular orientation of the bridges.

Quadruply Twisted Hückel-Aromatic Dodecaphyrin.

Molecular topology of π-conjugated circuits becomes increasingly important in the chemistry of aromatic and antiaromatic compounds. meso-Pentafluorophenyl-substituted 5,35-(1,4-phenylene)bridged [56]dodecaphyrin was synthesized by condensation of 1,4-phenylene-bridged dicarbinol dimer and 5,10,15-tris-(pentafluorophenyl)tetrapyrrane followed by oxidation with DDQ and was oxidized to its [54]- and [52]congeners in a stepwise manner. Metalation of the [52]dodecaphyrin with Pd2 (dba)3 gave two bis-PdII complexes that are isomers of metalation sites: anti and syn with regard to the 1,4-phenylene bridge. The anti-isomer was easily oxidized to its N-fused form, a quadruply twisted non-aromatic or weakly aromatic macrocycle. On the other hand, the syn-isomer was revealed to be the first example of Hückel aromatic molecule with a quadruply twisted structure.

A Stable Antiaromatic 5,20-Dibenzoyl [28]Hexaphyrin(1.1.1.1.1.1): Core AuIII Metalation and Subsequent Peripheral BIII Metalation.

5,20-Dibenzoyl [28]hexaphyrin(1.1.1.1.1.1) was synthesized as the first hexaphyrin bearing meso-aroyl substituents. The meso-dibenzoyl substituents are hydrogen-bonded with the pyrrolic protons to stabilize an antiaromatic dumbbell conformer. Core metalation of this hexaphyrin with AuIII afforded rectangular and aromatic [26]hexaphyrin bis-AuIII complexes, the major isomer of which was reduced with NaBH4 to give its antiaromatic 28π bis-AuIII complex. This complex allowed facile peripheral metalation with BIII owing to the peripheral benzoyl substituents.

Near-Infrared S2 Fluorescence from Deprotonated Möbius Aromatic [32]Heptaphyrin.

This study revealed S2 fluorescence from deprotonated meso-pentafluorophenyl-substituted Möbius aromatic [32]heptaphyrin(1.1.1.1.1.1.1) that was formed upon treatment of neutral antiaromatic [32]heptephyrin with tetrabutylammonium fluoride. Higher excited-state dynamics and emission were studied by fs-transient absorption spectroscopy and a broad-band fluorescence upconversion technique. This is the first S2 fluorescence from chromophores with twisted Möbius topology, and the observation of S2 fluorescence in the near-infrared region has been unprecedented. The higher excited-state dynamics of neutral and deprotonated [32]heptaphyrins were compared by ultrafast transient absorption spectroscopy to understand the S2 fluorescence origin. In the antiaromatic [32]heptaphyrin, a fast time component of 65 fs was assigned as an internal conversion process from the SB state to the SQ state, which occurs prior to relaxation to the optically dark, lowest electronic state (SD). Therefore, the SQ state of the antiaromatic [32]heptaphyrin acts as a trap state intervening radiative transitions from the SB state to the S0 state. In deprotonated [32]heptaphyrin, the internal conversion from the SB state to the SQ state proceeds with a slower time constant of 150 fs for owing to its rigid structure, helping the observation of its S2 fluorescence.

Internally 2,5-Thienylene-Bridged [46]Decaphyrin: (Annuleno)annulene Network Consisting of Möbius Aromatic Thia[28]hexaphyrins and Strong Hückel Aromaticity of its Protonated Form.

Internally 1,3-phenylene- and 2,5-thienylene-bridged [46]decaphyrins 2 and 3 have been synthesized. While 2 shows modest aromatic character derived from the global 46π-conjugated circuit, 3 displays larger aromatic character owing to the contribution of an (annuleno)annulene-type network consisting of two twisted Möbius aromatic thia[28]hexaphyrin segments in addition to the global 46π-network. Upon protonation, these [46]decaphyrins underwent large structural changes to acquire strong aromaticity. Protonated 3 has been revealed to take on a planar structure composed of fused two triangular thia[28]hexaphyrin segments.

[62]Tetradecaphyrin and Its Mono- and Bis-Zn(II) Complexes.

A coiled structure of meso-pentafluorophenyl-substituted [62]tetradecaphyrin 1 was revealed by X-ray structural analysis. Synthetic protocols were devised to form mono- and bis-Zn(II) complexes, 1 Zn and 1 Zn2 , selectively. The former displayed a trigonal-bipyramidal pentacoordinated Zn(II) ion as a rare case and a cyclic voltammogram exhibiting eleven reversible redox waves. The latter showed a Ci-symmetric structure with modest Hückel aromaticity owing to a 62 π-electronic circuit as the largest aromatic molecule to date.

Multifaceted [36]octaphyrin(1.1.1.1.1.1.1.1): deprotonation-induced switching among nonaromatic, Möbius aromatic, and Hückel antiaromatic species.

Deprotonation of nonaromatic octakis-(pentafluorophenyl)-substituted [36]octaphyrin(1.1.1.1.1.1.1.1) with tetrabutylammonium fluoride (TBAF) afforded monoanionic twisted Möbius aromatic species and dianinonic square Hückel antiaromatic species, depending upon the amount of TBAF.

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.

Group 9 Metal Complexes of meso-Aryl-Substituted Rubyrin.

Invited for the cover of this issue are Takanori Soya and Atsuhiro Osuka at Kyoto University. The image depicts Group 9 metal (Co, Rh, and Ir) complexes of meso-aryl-substituted rubyrin and a meteorite approaching to the atmosphere. A large amount of Iridium is often contained in meteorites. Read the full text of the article at 10.1002/chem.201501080.

Group 9 Metal Complexes of meso-Aryl-Substituted Rubyrin.

The metalation of meso-tetrakis(pentafluorophenyl)-substituted [26]rubyrin has been explored with Group 9 metal salts (Rh(I) , Co(II) , Ir(III) ), affording a Hückel aromatic [26]rubyrin-bis-Rh(I) complex with a highly curved gable-like structure, a Hückel antiaromatic [24]rubyrin-bis-Co(II) complex that displays intramolecular antiferromagnetic coupling between the two Co(II) ions (J=-4.5 cm(-1) ), and two Cp*-capped Ir(III) complexes; in one, the iridium metal sits on the [26]rubyrin frame with two IrN bonds, whereas the other has an additional IrC bond, although both Ir(III) complexes display moderate aromatic character. This work demonstrates characteristic metalation abilities of this [26]rubyrin toward Group 9 metals.

Stable [48]-, [50]-, and [52]dodecaphyrins(1.1.0.1.1.0.1.1.0.1.1.0): the largest Hückel aromatic molecules.

[52]Dodecaphyrin(1.1.0.1.1.0.1.1.0.1.1.0) was quantitatively oxidized with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) to the corresponding [50]dodecaphyrin. Further oxidation of [50]dodecaphyrin with MnO2 quantitatively afforded [48]dodecaphyrin. Of the three, [50]dodecaphyrin showed Hückel aromatic character as the largest aromatic molecule reported to date. Protonation of [50]dodecaphyrin with methanesulfonic acid (MSA) led to the formation of a planar tetraprotonated species that displayed a sharp and intense Soret-like band at 906 nm with ε=6.5×10(5) M(-1) cm(-1) and Q-band-like bands at 1346 and 1600 nm.

A Möbius aromatic [28]hexaphyrin bearing a diethylamine group: a rigid but smooth conjugation circuit.

The reaction of [26]hexaphyrin with triethylamine in the presence of BF3 ⋅OEt2 and O2 furnished a diastereomeric mixture of a diethylamine-bearing [28]hexaphyrin as a rare example of a Möbius aromatic metal-free expanded porphyrin. The Möbius aromaticity of these molecules is large, as indicated by their large diatropic ring currents, which are even preserved at 100 °C, owing to their internally multiply bridged robust structure with a smooth conjugation network. These molecules were reduced with NaBH4 to give an antiaromatic [28]hexaphyrin, and were oxidized with MnO2 to give aromatic [26]hexaphyrins, both through a Möbius-to-Hückel topology switch induced by a CN bond cleavage.

Ni(II) metalations of [40]- and [42]nonaphyrins(1.1.1.1.1.1.1.1.1): the largest doubly twisted Hückel antiaromatic molecule.

Ni(II) metalation of [42]nonaphyrin gave two Hückel aromatic [42]nonaphyrin Ni(II) complexes that possess different doubly twisted figure-eight conformations. While these complexes are conformers, the activation barriers for conformational interconversion were found to be exceptionally high. However, one-way isomerization from the higher-energy conformer to the lower-energy conformer can be induced upon treatment with trifluoroacetic acid (TFA). This conformational isomerization has been suggested to proceed via a protonation-induced caterpillar-like conformational rotation of the metal-free hexaphyrin-like segment by examining the similar isomerization of an A3 B6 -type [42]nonaphyrin Ni(II) complex. Rh(I) metalation of the lower-energy [42]nonaphyrin Ni(II) complex afforded a [42]nonaphyrin Ni(II) -Rh(I) hybrid complex, which was oxidized with p-chloranil to produce a [40]nonaphyrin Ni(II) -Rh(I) hybrid complex as the largest doubly twisted Hückel antiaromatic molecule to date.