“To Twist or Not to Twist”: Figure‐of‐Eight and Planar Structures of Octaphyrins

Amongst the various porphyrinoids, octaphyrin has attracted significant attention owing to its diverse syntheses, conformations, and metal‐ligation properties. Octaphyrin is a higher homologue of porphyrin and is formed by linking together heterocycles such as pyrrole, furan, thiophene, and selenophene through α‐α or α‐meso carbon bonds. The planar conformation is mainly achieved through inversion of the heterocyclic units from the center of macrocycle; avoiding meso ‐bridges; introducing a para ‐quinodimethane bridge; employing a neo‐confusion approach; protonation; and by generating dianionic species. In this Focus Review, recent synthetic advancements in the field of octaphyrins are summarized. The twisted conformation of the octaphyrin binds to two metal ions in a tetracoordinate geometry. The diphosphorus complex of octaphyrin represents the first example of a stable expanded isophlorin.     

Synthesis of a Tetrabenzotetraaza[8]circulene by a “Fold‐In” Oxidative Fusion Reaction

Tetrabenzotetraaza[8]circulene ( 1 ) has been synthesized in good yield by a “fold‐in” oxidative fusion reaction of a 1,2‐phenylene‐bridged cyclic tetrapyrrole. X‐ray diffraction analysis of 1 has revealed a planar square structure with a central cyclooctatetraene (COT) core that shows little alternation of the bond lengths. Despite these structural features, 1 shows aromatic‐like character, such as sharp absorption bands, high fluorescence quantum yields (Φ_F=0.55 in THF), and a single exponential fluorescence decay with τ_F=3.8 ns. These observations indicate a dominant contribution of an [8]radialene‐like π conjugation and hence aromatic character of the local aromatic segments in 1 .     

Comment on “The electron‐pair origin of anti‐aromaticity: Spectroscopic manifestations”

In the article by Zilberg and Haas, “The Electron‐Pair Origin of Anti‐aromaticity: Spectroscopic Manifestations,” the relative sign of the two Kekulé valence bond functions, R and L, in conjugated cyclic hydrocarbons was discussed. It was proposed that in the ground‐state wave function of aromatic compounds, the two functions contribute with like sign, while in the ground state of anti‐aromatic compounds, the two functions contribute with opposite sign. In this Comment, it is shown that the two functions enter with like sign also into the ground‐state wave function of anti‐aromatic compounds. Furthermore, it was argued that resonance tends to (de)stabilize a symmetric ground‐state geometry in case of the (anti‐)aromatic compounds. The expression derived by Zilberg and Haas for the stabilization energy shows an unusual dependence on the ring size and distortion coordinate. An alternative formula is derived for the stabilization energy, in which the energy depends quadratically on the distortion coordinate. Without further numerical calculations, it is not possible to predict whether this term will (de)stabilize a symmetric geometry of the ground state of (anti‐)aromatic molecules. Rather, we are led to believe that the influence of term in question on the geometric stability may be small, thus not providing the main reason for the geometric distortion of anti‐aromatic compounds. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Negative Charge as a Lens for Concentrating Antiaromaticity: Using a Pentagonal “Defect” and Helicene Strain for Cyclizations

Incorporation of a five‐membered ring into a helicene framework disrupts aromatic conjugation and provides a site for selective deprotonation. The deprotonation creates an anionic cyclopentadienyl unit, switches on conjugation, leads to a >200 nm red‐shift in the absorbance spectrum and injects a charge into a helical conjugated π‐system without injecting a spin. Structural consequences of deprotonation were revealed via analysis of a monoanionic helicene co‐crystallized with {K⁺(18‐crown‐6)(THF)} and {Cs⁺₂(18‐crown‐6)₃}. UV/Vis‐monitoring of these systems shows a time‐dependent formation of mono‐ and dianionic species, and the latter was isolated and crystallographically characterized. The ability of the twisted helicene frame to delocalize the negative charge was probed as a perturbation of aromaticity using NICS scans. Relief of strain, avoidance of antiaromaticity, and increase in charge delocalization assist in the additional dehydrogenative ring closures that yield a new planarized decacyclic dianion.     

Global versus Local Aromaticity in Porphyrinoid Macrocycles: Experimental and Theoretical Study of “Imidacene”, an Imidazole‐Containing Analogue of Porphycene

The synthesis, spectroscopic properties, and computational analysis of an imidazole‐based analogue of porphycene are described. The macrocycle, given the trivial name “imidacene”, was prepared by reductive coupling of a diformyl‐substituted 2,2′‐biimidazole using low‐valent titanium, followed by treatment with 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone. Imidacene displays a porphyrin‐like electronic structure, as judged by its ¹H NMR, ¹³C NMR, and UV/Vis spectral characteristics. Despite a cyclic 18 π‐electron pathway, dichloromethane or ethyl acetate solutions of imidacene were found to undergo rapid decomposition, even in the absence of light and air. A series of high‐level theoretical calculations, performed to probe the origin of this instability, revealed that the presence of a delocalized 18 π‐electron pathway in both imidacene and porphycene provides less aromatic stabilization energy than locally aromatic 6 π‐electron heterocycles in their reduced counterparts. That reduction of imidacene occurs on perimeter nitrogen atoms allows it to maintain its planarity and two stabilizing intramolecular hydrogen bonds, thereby distinguishing it from porphycene and, more generally, from porphyrin. Despite the presence of both 18 π‐ and 22 π‐electron pathways in the planar, reduced form of imidacene, aromaticity is evident only in the 6 π‐electron five‐membered rings. Our computational analysis predicts that routine ¹H NMR spectroscopy can be used to distinguish between local and global aromaticity in planar porphyrinoid macrocycles; the difference in the chemical shift for the internal NH protons is expected to be on the order of 19 ppm for these two electronically disparate sets of ostensibly similar compounds.     

Computationally Designed Families of Flat,Tubular, and Cage Molecules Assembled with “Starbenzene” Building Blocks through Hydrogen‐Bridge Bonds

Using density functional calculations, we demonstrate that the planarity of the nonclassical planar tetracoordinate carbon (ptC) arrangement can be utilized to construct new families of flat, tubular, and cage molecules which are geometrically akin to graphenes, carbon nanotubes, and fullerenes but have fundamentally different chemical bonds. These molecules are assembled with a single type of hexagonal blocks called starbenzene (D_6h C₆Be₆H₆) through hydrogen‐bridge bonds that have an average bonding energy of 25.4–33.1 kcal mol^?1. Starbenzene is an aromatic molecule with six π electrons, but its carbon atoms prefer ptC arrangements rather than the planar trigonal sp² arrangements like those in benzene. Various stability assessments indicate their excellent stabilities for experimental realization. For example, one starbenzene unit in an infinite two‐dimensional molecular sheet lies on average 154.1 kcal mol^?1 below three isolated linear C₂Be₂H₂ (global minimum) monomers. This value is close to the energy lowering of 157.4 kcal mol^?1 of benzene relative to three acetylene molecules. The ptC bonding in starbenzene can be extended to give new series of starlike monocyclic aromatic molecules (D_4h C₄Be₄H₄^2?, D_5h C₅Be₅H₅^?, D_6h C₆Be₆H₆, D_7h C₇Be₇H₇⁺, D_8h C₈Be₈H₈^2?, and D_9h C₉Be₉H₉^?), known as starenes. The starene isomers with classical trigonal carbon sp² bonding are all less stable than the corresponding starlike starenes. Similarly, lithiated C₅Be₅H₅ can be assembled into a C₆₀‐like molecule. The chemical bonding involved in the title molecules includes aromaticity, ptC arrangements, hydrogen‐bridge bonds, ionic bonds, and covalent bonds, which, along with their unique geometric features, may result in new applications.     

A Neutral “Aluminocene” Sandwich Complex: η1‐ versus η5‐Coordination Modes of a Pentaarylborole with ECp* (E=Al,Ga; Cp*=C5Me5)

The pentaaryl borole (Ph*C)₄BXyl^F [Ph*=3,5‐tBu₂(C₆H₃); Xyl^F=3,5‐(CF₃)₂(C₆H₃)] reacts with low‐valent Group 13 precursors AlCp* and GaCp* by two divergent routes. In the case of [AlCp*]₄, the borole reacts as an oxidising agent and accepts two electrons. Structural, spectroscopic, and computational analysis of the resulting unprecedented neutral η⁵‐Cp*,η⁵‐[(Ph*C)₄BXyl^F] complex of Al^III revealed a strong, ionic bonding interaction. The formation of the heteroleptic borole‐cyclopentadienyl “aluminocene” leads to significant changes in the ¹³C NMR chemical shifts within the borole unit. In the case of the less‐reductive GaCp*, borole (Ph*C)₄BXyl^F reacts as a Lewis acid to form a dynamic adduct with a dative 2‐center‐2‐electron Ga?B bond. The Lewis adduct was also studied structurally, spectroscopically, and computationally.     

[32]π Fused Core‐Modified Heptaphyrin with Möbius Aromaticity

A new fused core‐modified 32π heptaphyrin with Möbius aromatic character is reported. The ¹H NMR data indicated a weak Möbius aromaticity at 298 K; however, at 213–183 K, the molecule predominates [4n]π Möbius conformation with strong diatropic ring current, which was further confirmed by X‐ray analysis. The protonation experiment led to preservation of the Möbius aromaticity at 298 K. Nevertheless, the experimental results were further supported by theoretical studies. Overall, this study represents the first example of Möbius aromatic fused core‐modified expanded porphyrin.     

(Rg = He ∼ Rn,n = 1–4): In quest of the potential trapping ability of the aromatic ring

A new series of divalent boron‐rare gas cations (Rg = He ∼ Rn, n = 1–4) have been predicted theoretically at the B3LYP, MP2, and CCSD(T) levels to present the structures, stability, charge distributions, bond natures, and aromaticity. The Rg B bond energies are quite large for heavy rare gases and increase with the size of the Rg atom. Because of steric hindrance new Rg atoms introduced to the B₄ ring will weaken the Rg B bond. Thus in the Rg B bond has the largest binding energy 90–100 kcal/mol. p‐ has a slightly shorter Rg B bond length and a larger bond energy than o‐ . NBO and AIM analyses indicate that for the heavy Rg atoms Ar ∼ Rn the B Rg bonds have character of typical covalent bonds. The energy decomposition analysis shows that the σ‐donation from rare gases to the boron ring is the major contribution to the Rg B bonding. Adaptive natural density partitioning and nuclear‐independent chemical shift analyses suggest that both and have obvious aromaticity.     

Efficient Light‐Induced pKa Modulation Coupled to Base‐Catalyzed Photochromism

Photoswitchable acid–base pairs, whose pK_a values can be reversibly altered, are attractive molecular tools to control chemical and biological processes with light. A significant, light‐induced pK_a change of three units in aqueous medium has been realized for two thermally stable states, which can be interconverted using UV and green light. The light‐induced pK_a modulation is based on incorporating a 3‐H‐thiazol‐2‐one moiety into the framework of a diarylethene photoswitch, which loses the heteroaromatic stabilization of the negatively charged conjugate base upon photochemical ring closure, and hence becomes significantly less acidic. In addition, the efficiency of the photoreactions is drastically increased in the deprotonated state, giving rise to catalytically enhanced photochromism. It appears that protonation has a significant influence on the shape of the ground‐ and excited‐state potential energy surfaces, as indicated by quantum‐chemical calculations.