On-Surface Synthesis of Porphyrin-Complex Multi-Block Co-Oligomers by Defluorinative Coupling.

On-surface chemical reaction has become a very powerful technique to synthesize nanostructures by linking small molecules in the bottom-up approach. Given the fact that most reactants are simultaneously activated at certain temperatures, a sequential reaction in a controlled way has remained challenging. Here, we present an on-surface synthesis of multi-block co-oligomers from trifluoromethyl (CF3 ) substituted porphyrin metal complexes. The oligomerization on Au(111) is demonstrated with a combination of bond-resolved scanning probe microscopy and density functional theory (DFT) calculations. Even after the first oligomerization of single monomer unit, the termini of the oligomer keep the CF3 group, which can be used as a reactant for further coupling in a sequential order. Consequently, copper, cobalt, and palladium complexes of bisanthracene-fused porphyrin oligomers were linked with each other in a designed order.

Figure-eight Octaphyrin Bis-Ge(IV) Complexes: Synthesis, Structures, Aromaticity, and Chiroptical Properties.

Highly twisted structures of expanded porphyrin provide a prominent basis to unravel the relationship between aromaticity and chirality. Here we report the synthesis of bis-Ge(IV) complexes of [38]octaphyrin that display rigid figure-eight structures. Two bis-Ge(IV) [38]octaphyrin isomers with respect to the stereochemistry of the axial hydroxy groups on the germanium ions were obtained and found to be aromatic. Upon oxidation with MnO2 , these [38]octaphyrin complexes were converted to a single syn-type isomer of [36]octaphyrin with retained figure-eight conformation. The enantiomers have been successfully separated by HPLC equipped with a chiral stationary phase. While aromatic [38]octaphyrin Ge(IV) complexes showed quite large molar circular dichroism of up to Δϵ=1500 M-1 cm-1 with a dissymmetry factor gabs of 0.035, weakly antiaromatic [36]octaphyrin Ge(IV) complexes underscored moderate values; Δϵ=540 M-1 cm-1 with gabs of 0.023. Thus, the figure-eight octaphyrin scaffold has been proved to be an attractive platform for novel chiroptical materials with tunable aromaticity.

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.

Aromatic and Antiaromatic Cyclophane-type Hexaphyrin Dimers.

A peripherally strapped [28]hexaphyrin takes a rectangular conformation and exhibits antiaromatic character. A cyclophane-type dimer consisting of such [28]hexaphyrins was synthesized from hexakis(pentafluorophenyl) [26]hexaphyrin via SN Ar reaction with allyl alcohol, one-pot intra- and intermolecular olefin metathesis under improved Hoveyda-Grubbs catalysis, and final reduction with NaBH4 . The cyclophane-type structures of [26]- and [28]hexaphyrin dimers have been revealed by X-ray analysis. Studies on the structural, optical, and electronic properties have led to a conclusion that there is no favorable electronic interaction between the two [28]hexaphyrin segments and thus no indication of 3D aromaticity.

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.

Synthesis of (bis)Silicon Complexes of [38], [37], and [36]Octaphyrins: Aromaticity Switch and Stable Radical Cation.

Silicon complexation of a [38]octaphyrin (1) was accomplished by reaction with an excess amount of MeSiCl3 in the presence of N,N-diisopropylethylamine, thus giving an aromatic [38]octaphyrin bis(silicon) complex 2. This complex was interconvertible with an antiaromatic [36]octaphyrin congener (3) by oxidation with MnO2 and reduction with NaBH4 . Curiously, mild oxidation of 2 with ferrocenium hexafluorophosphate afforded a [37]octaphyrin bis(silicon) complex 4 as an stable radical cation that can be stored under ambient conditions in the solid state. Owing to the two NNNCC-five-coordinated Si atoms bearing trigonal bipyramidal geometry, these octaphyrin bis(silicon) complexes take on similar and rigid figure-of-eight structures with different consecutive numbers of conjugated π-electrons (38, 37, and 36), and are all stable.

Möbius Aromatic [28]Hexaphyrin Germanium(IV) and Tin(IV) Complexes: Efficient Formation of Triplet Excited States.

[28]Hexaphyrin GeIV and SnIV complexes were synthesized in high yields by reactions of [28]hexaphyrin with GeCl4 or SnCl4 in the presence of triethylamine. Both complexes display distinct 28π Möbius aromatic character and possess a trigonal bipyramidal geometry at the central GeIV or SnIV atom. The equatorial hydroxy group of the GeIV complex was smoothly exchanged with neutral nucleophiles, such as phenol derivatives and thiophenol, with retention of configuration. In the SnIV complex, intersystem crossing to the T1  state is remarkably enhanced owing to the effective heavy-atom effect, thus allowing the formation of the T1  state in high yield. The T1  states of the GeIV and SnVI complexes were found to be antiaromatic on the basis of the transient absorption features in line with the Baird rule.

Doubly N-Fused [24]Pentaphyrin Silicon Complex and Its Fluorosilicate: Enhanced Möbius Aromaticity in the Fluorosilicate.

Treatment of nonaromatic N-fused [24]pentaphyrin with trichloromethylsilane in the presence of a base afforded doubly N-fused [24]pentaphyrin and its silicon complex. Addition of fluoride ion to the silicon complex led to the formation of its fluorosilicate as an unprecedented monoanionic six-coordinated SiIV complex of porphyrinoid. Treatment of the fluorosilicate with acid led to the recovery of the silicon complex. The doubly N-fused pentaphyrin, the silicon complex, and the fluorosilicate were all characterized as distinct Möbius aromatic molecules by spectroscopic measurements and X-ray crystallographic analyses. Importantly, the second N-fusion reaction, Si-incorporation and fluoride addition to the Si-atom enhanced the aromaticity of doubly N-fused [24]pentaphyrins in this order. Tamao-Fleming oxidation of the silicon complex gave ß-keto doubly N-fused pentaphyrin and triply fused [24]pentaphyrin, which were nonaromatic and Hückel anti-aromatic, respectively.

meso-Aryl [28]Hexaphyrin Silicon Complexes Bearing Various Si-Substituents and 1,16-Dihydrohexaphyrin bis-Chlorosilicon Complex.

Herein, the synthesis of Möbius aromatic [28]hexaphyrin silicon complexes bearing various Si-substituents from reactions of [28]hexaphyrin 1 with suitable silicon sources in the presence of a base is reported. Si-substituents newly introduced are vinyl (4), phenyl (6), hydroxy (7), and hydride groups (8). X-ray crystallographic analysis of complexes 6 and 7 has shown trigonal bipyramidal penta-coordinated silicon atoms, which are favorable for the hexaphyrin ligands to take on smoothly twisted Möbius conformations. N-fused Si-vinyl complex 5 was also isolated and structurally well characterized. Through these studies, the Si-substituent effect has been shown to be relatively small. Reaction of 1 with HSiCl3 in CH2Cl2 gave 1,16-dihydrohexaphyrin bis-chlorosilicon complex 9, probably through intramolecular hydride transfer from the silane to the pyrrolic α-carbon.

SiIV incorporation into a [28]hexaphyrin that triggered formation of Möbius aromatic molecules.

Incorporation of Si(IV) into an expanded porphyrin has been achieved for the first time. Treatment of [28]hexaphyrin 1 with CH(3)SiCl(3) and N,N-diisopropylethylamine gave Si(IV) complex 2 and its N-fused product 4 that both have Möbius aromatic nature. In both complexes, the coordinated Si atom is satisfied in a typical trigonal bipyramidal coordination. Si(IV) incorporation induces conformational rigidification and redshifted absorption profiles due to σ-π conjugation between the Si atom and hexaphyrin macrocycle. Tamao-Fleming oxidation of 2 with H(2)O(2) gave ß-hydroxy [28]hexaphyrin 5, which exists as a ruffled rectangular shape in the solid state, yet it has been revealed to exist predominantly as a twisted Möbius aromatic conformer in CH(2)Cl(2).

Diprotonated [28]hexaphyrins(1.1.1.1.1.1): triangular antiaromatic macrocycles.

Protonation of meso-aryl [28]hexaphyrins(1.1.1.1.1.1) triggered conformational changes. Whereas protonation with trifluoroacetic acid led to the formation of monoprotonated Möbius aromatic species, protonation with methanesulfonic acid led to the formation of diprotonated triangular antiaromatic species. A peripherally hexaphenylated [28]hexaphyrin was rationally designed and prepared to undergo diprotonation to favorably afford a triangular-shaped antiaromatic species.