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.     

[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.     

Mechanochemically Triggered Topology Changes in Expanded Porphyrins

A hitherto unexplored class of molecules for molecular force probe applications are expanded porphyrins. This work proves that mechanical force is an effective stimulus to trigger the interconversion between Hückel and Möbius topologies in [28]hexaphyrin, making these expanded porphyrins suitable to act as conformational mechanophores operating at mild (sub-1 nN ) force conditions. A straightforward approach based on distance matrices is proposed for the selection of pulling scenarios that promote either the planar Hückel topology or the three lowest lying Möbius topologies. This approach is supported by quantum mechanochemical calculations. Force distribution analyses reveal that [28]hexaphyrin selectively allocates the external mechanical energy to molecular regions that trigger Hückel–Möbius interconversions, explaining why certain pulling scenarios favor the Hückel two-sided topology and others favor Möbius single-sided topologies. The meso-substitution pattern on [28]hexaphyrin determines whether the energy difference between the different topologies can be overcome by mechanical activation.     

ESI‐MS/MS of expanded porphyrins: a look into their structure and aromaticity

Electrospray mass spectrometry/mass spectrometry was used to investigate the gas‐phase properties of protonated expanded porphyrins, in order to correlate those with their structure and conformation. We have selected five expanded meso‐pentafluorophenyl porphyrins, respectively, a pair of oxidized/reduced fused pentaphyrins (22 and 24 π electrons), a pair of oxidized/reduced regular hexaphyrins (26 and 28 π electrons) and a regular doubly N‐fused hexaphyrin (28 π electrons). The gas‐phase behavior of the protonated species of oxidized and reduced expanded porphyrins is different. The oxidized species (aromatic Hückel systems) fragment more extensively, mainly by the loss of two HF molecules. The reduced species (Möbius aromatic or Möbius‐like aromatic systems) fragment less than their oxidized counterparts because of their increased flexibility. The protonated regular doubly fused hexaphyrin (non‐aromatic Hückel system) shows the least fragmentation even at higher collision energies. In general, cyclization through losses of HF molecules decreases from the aromatic Hückel systems to Möbius aromatic or Möbius‐like aromatic systems to non‐aromatic Hückel systems and is related to an increase in conformational distortion. Copyright © 2016 John Wiley & Sons, Ltd.     

Viability of Möbius Topologies in [26]‐ and [28]Hexaphyrins

Recently, hexaphyrins have emerged as a promising class of π‐conjugated molecules that display a range of interesting electronic, optical, and conformational properties, including the formation of stable Möbius aromatic systems. Besides the Möbius topology, hexaphyrins can adopt a variety of conformations with Hückel and twisted Hückel topologies, which can be interconverted under certain conditions. To determine the optimum conditions for viable Möbius topologies, the conformational preferences of [26]‐ and [28]hexaphyrins and the dynamic interconversion between the Möbius and Hückel topologies were investigated by density functional calculations. In the absence of meso substituents, [26]hexaphyrin prefers a planar dumbbell conformation, strongly aromatic and relatively strain free. The Möbius topology is highly improbable: the most stable tautomer is 33 kcal mol^?1 higher in energy than the global minimum. On the other hand, the Möbius conformer of [28]hexaphyrin is only 6.5 kcal mol^?1 higher in energy than the most stable dumbbell conformation. This marked difference is due to aromatic stabilization in the Möbius 4n electron macrocycle as opposed to antiaromatic destabilization in the 4n+2 electron system, as revealed by several energetic, magnetic, structural, and reactivity indices of aromaticity. For [28]hexaphyrins, the computed activation barrier for interconversion between the Möbius aromatic and Hückel antiaromatic conformers ranges from 7.2 to 10.2 kcal mol^?1, in very good agreement with the available experimental data. The conformation of the hexaphyrin macrocycle is strongly dependent on oxidation state and solvent, and this feature creates a promising platform for the development of molecular switches.     

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.     

Topology Switching in [32]Heptaphyrins Controlled by Solvent,Protonation, and meso Substituents

The switching of topology between “figure‐eight”, Möbius, and untwisted conformations in [32]heptaphyrins(1.1.1.1.1.1.1) has been investigated by using density functional theory calculations. Such a change is achieved by variation of one internal dihedral angle and, if properly controlled, can provide access to molecular switches with unique optical and magnetic properties. In this work, we have explored different conformational control methods, such as solvent, protonation and meso substituents. Despite its antiaromatic character, most of the [32]heptaphyrins (R=H, CH₃, CF₃, Ph, C₆F₅) adopt a figure‐eight conformation in the neutral state, owing to their more‐effective hydrogen‐bonding interactions. The aromatic Möbius topology is only preferred with dichlorophenyl groups, which minimize the steric hindrance that arises from the bulky chlorine atoms. The conformational equilibrium is sensitive to the solvent, so polar solvents, such as DMSO, further stabilize the Möbius conformation. Protonation induces a conformational change into the Möbius topology, irrespective of the meso‐aryl groups. In the triprotonated species, the conformational switch is blocked and a non‐twisted conformer becomes much more stable than the figure‐eight conformation. We have shown that the relative energies of the protonated [32]heptaphyrins are dominated by aromaticity. Importantly, this topology switching induces a dramatic change in the magnetic properties and reactivity of the macrocycles, as revealed by several energetic, magnetic, structural, and reactivity indices of aromaticity.     

Disclosure of Ground-State Zimmerman-Möbius Aromaticity in the Radical Anion of [6]Helicene and Evidence for 4π Periodic Aromatic Ring Currents in a Molecular “Metallic” Möbius Strip

In 1966, Zimmerman proposed a type of Möbius aromaticity that involves through-space electron delocalization; it has since been widely applied to explain reactivity in pericyclic reactions, but is considered to be limited to transition-state structures. Although the easily accessible hexahelicene radical anion has been known for more than half a century, it was overlooked that it exhibits a ground-state minimum and robust Zimmerman-Möbius aromaticity in its central noose-like opening, becoming, hence, the oldest existing Möbius aromatic system and with smallest Möbius cycle known. Despite its overall aromatic stabilization energy of 13.6 kcal mol⁻¹ (at B3LYP/6-311+G**), the radical also features a strong, globally induced paramagnetic ring current along its outer edge. Exclusive global paramagnetic currents can also be found in other fully delocalized radical anions of 4N+2 π-electron aromatic polycyclic benzenoid hydrocarbons (PAH), thus questioning the established magnetic criterion of antiaromaticity. As an example of a PAH with nontrivial topology, we studied a novel Möbius[16]cyclacene that has a non-orientable surface manifold and a stable closed-shell singlet ground state at several density functional theory levels. Its metallic monoanion radical (0.0095 eV band gap at HSE06/6-31G* level) is also wave-function stable and displays an unusual 4π-periodic, magnetically induced ring current (reminiscent of the transformation behaviour of spinors under spatial rotation), thus indicating the existence of a new, Hückel-rule-evading type of aromaticity.     

Expanded porphyrins: intriguing structures, electronic properties, and reactivities

The chemistry of expanded porphyrins, which are higher homologues of porphyrins, has been intensively explored for the last three decades. Expanded porphyrins exhibit structures, electronic properties, coordination chemistry, and reactivities that are entirely different from those of porphyrins. Through these studies, it has become increasingly apparent that expanded porphyrins are attractive in views of aromaticity and multimetal coordination, or as functional dyes, nonlinear optical materials, ion receptors, or stable organic radicals. As such, we have continuously witnessed the emergence of expanded porphyrins that exhibit unprecedented structures and properties, as is highlighted by the facile realization of Möbius aromatic and even antiaromatic systems with twisted molecular structures. In this Review, the recent progress of the chemistry of expanded porphyrins after the seminal Review by Sessler and Seidel in 2003 is presented.     

Exploiting the Aromatic Chameleon Character of Fulvenes for Computational Design of Baird‐Aromatic Triplet Ground State Compounds

Due to the reversal in electron counts for aromaticity and antiaromaticity in the closed‐shell singlet state (normally ground state, S₀) and lowest ππ* triplet state (T₁ or T₀), as given by Hückel's and Baird's rules, respectively, fulvenes are influenced by their substituents in the opposite manner in the T₁ and S₀ states. This effect is caused by a reversal in the dipole moment when going from S₀ to T₁ as fulvenes adapt to the difference in electron counts for aromaticity in various states; they are aromatic chameleons. Thus, a substituent pattern that enhances (reduces) fulvene aromaticity in S₀ reduces (enhances) aromaticity in T₁, allowing for rationalizations of the triplet state energies (E_T) of substituted fulvenes. Through quantum chemical calculations, we now assess which substituents and which positions on the pentafulvene core are the most powerful for designing compounds with low or inverted E_T. As a means to increase the π‐electron withdrawing capacity of cyano groups, we found that protonation at the cyano N atoms of 6,6‐dicyanopentafulvenes can be a route to on‐demand formation of a fulvenium dication with a triplet ground state (T₀). The five‐membered ring of this species is markedly Baird‐aromatic, although less than the cyclopentadienyl cation known to have a Baird‐aromatic T₀ state.     

Hückel and Möbius Expanded para‐Benziporphyrins: Synthesis and Aromaticity Switching

The four expanded p‐benziporphyrins A,C‐di‐p‐benzi[24]pentaphyrin(1.1.1.1.1), N‐fused A‐p‐benzi[24]pentaphyrin, A,D ‐di‐p‐benzi[28]hexaphyrin(1.1.1.1.1.1), and A,C‐di‐p‐benzi[28]hexaphyrin(1.1.1.1.1.1) were obtained in three‐component Lindsey‐type macrocyclizations. These compounds were explored as macrocyclic ligands and as potential aromaticity switches. A BODIPY‐like difluoroboron complex was obtained from the A,C‐di‐p‐benzi[24]pentaphyrin, whereas A,C‐di‐p‐benzi[28]hexaphyrin yielded a Möbius‐aromatic Pd^II complex containing fused pyrrole and phenylene subunits. Conformational behavior, tautomerism, and acid‐base chemistry of the new macrocycles were characterized by means of NMR spectroscopy and DFT calculations. Free base N‐fused A‐p‐benzi[24]pentaphyrin showed temperature‐dependent Hückel–Möbius aromaticity switching, whereas the A,C‐di‐p‐benzi[28]hexaphyrin formed a Möbius‐aromatic dication.     

An Unusually Small Singlet–Triplet Gap in a Quinoidal 1,6‐Methano[10]annulene Resulting from Baird’s 4n π‐Electron Triplet Stabilization

Within the continuum of π‐extended quinoidal electronic structures exist molecules that by design can support open‐shell diradical structures. The prevailing molecular design criteria for such structures involve proaromatic nature that evolves aromaticity in open‐shell diradical resonance structures. A new diradical species built upon a quinoidal methano[10]annulene unit is synthesized and spectroscopically evaluated. The requisite intersystem crossing in the open‐shell structure is accompanied by structural reorganization from a contorted Möbius aromatic‐like shape in S₀ to a more planar shape in the Hückel aromatic‐like T₁. This stability was attributed to Baird’s Rule which dictates the aromaticity of 4n π‐electron triplet excited states.     

meso‐Aryl [20]π Homoporphyrin: The Simplest Expanded Porphyrin with the Smallest Möbius Topology

An unstable conjugated homoporphyrin was successfully stabilized by introducing meso ‐aryl substitutents. It was evident from the moderate diatropic ring current found by NMR analysis that the newly formed 20π conjugated free base and its protonated form exhibited Möbius aromatic character. Furthermore, complexation as a ligand with an Rh^I ion afforded a unique binding mode and retained the Möbius aromaticity. Overall, these compounds are the smallest Möbius aromatic molecules, as confirmed by spectral and crystal‐structure analysis and supported by theoretical studies.     

Interplay of Hückel and Möbius Aromaticity in Metal-Metal Quintuple Bonded Complexes of Cr,Mo, and W with Amidinate Ligand: Ab initio DFT and Multireference Analysis**

The aromaticity of metal-metal quintuple bonded complexes of the type M₂L₂ (M=Cr, Mo, and W; L=amidinate) are studied employing gauge including magnetically induced ring current (GIMIC) analysis and electron density of delocalized bonds (EDDB). It is found that the complexes possess two types of aromaticity: i) Hückel aromaticity through delocalization of ligand π electrons with metal-metal δ-bond-forming 6 conjugated electrons (4π and 2δ) ring; ii) Craig-Möbius aromaticity through delocalization of π electrons of both the ligands with metal d-orbitals in Craig type orientation forming 10π electrons ring with a double twist. Extended transition state natural orbital chemical valence (ETS-NOCV) and canonical molecular orbital natural chemical shielding (CMO-NCS) analysis confirm the Craig-Möbius type arrangement of the orbitals. Furthermore, the unprecedented Hückel and Möbius type aromaticity is confirmed from the plot of the current pathways using 3D line integral convolution (3D-LIC) plots. The metal-metal bond order also increases down the group as justified from the complete active space self-consistent field (CASSCF) analysis. Due to an increase in the π and δ electron conjugation, both the Hückel and Möbius aromaticity increase down the group.     

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 CH₃SiCl₃ 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 H₂O₂ gave β‐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 CH₂Cl₂.     

A Fully Conjugated Porphyrin-[36]Octaphyrin-Porphyrin Hybrid Tape Exhibiting Möbius Aromaticity

Directly meso-meso linked porphyrin-tetrabromo[36]octaphyrin-porphyrin hybrid trimer 10 was successfully synthesized via acid-catalyzed condensation reaction and subsequent oxidation. Zn^II-metalation of 10 induced transannular meso-meso bond formation to give Möbius aromatic bis-Zn^II octaphyrin 11 , which was oxidized by DDQ/Sc(OTf)₃ to provide fully conjugated porphyrin-[36]octaphyrin-porphyrin hybrid tape 12 as the first example of porphyrin tape exhibiting Möbius aromaticity. Hybrid tape 12 displays significantly red-shifted absorption and small electrochemical HOMO-LUMO gap, indicating the effective conjugation through the whole chromophores.     

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 S_Q 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.     

PdII Complexes 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.     

PdII Complexes 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.