Expanded Porphyrin with a Split Personality: A Hückel–Möbius Aromaticity Switch

Unveiling Möbius/Hückel Topology and Aromaticity in A Core-Expanded [10]Annulene at Different Oxidation States

The exploration of annulene's conformation, electronic properties and aromaticity has generated enduring interest over the years, yet it continues to present formidable challenges for annulenes with more than ten carbon atoms. In this study, we present the synthesis of a stable [10]cyclo-para-phenylmethine derivative (1), which bears a resemblance to [10]annulene. 1 can be readily oxidized into its respective cations, wherein electrons are effectively delocalized along the backbone, resulting in different conformations and aromaticity. Both 1 and its tetracation (14+  ⋅ 4SbF6 - ) exhibit a nearly planar conformation with a rectangular shape, akin to the E,Z,E,Z,Z-[10]annulene. In contrast, the radical cation (1⋅+  ⋅ SbCl6 - ) possesses a doubly twisted Hückel topology. Furthermore, the dication (12+  ⋅ 2SbCl6 - ) displays conformational flexibility in solution and crystalizes with the simultaneous presence of Möbius-twisted (1a2+  ⋅ 2SbCl6 - ) and Hückel-planar (1b2+  ⋅ 2SbCl6 - ) isomers in its unit cell. Detailed experimental measurements and theoretical calculations reveal that: (1) 1 demonstrates localized aromaticity with an alternating benzenoid/quinoid structure; (2) 1a2+  ⋅ 2SbCl6 - and 1b2+  ⋅ 2SbCl6 - with 48π electrons are weakly Möbius aromatic and Hückel antiaromatic, respectively; (3) 14+  ⋅ 4SbF6 - exhibits Hückel aromaticity (46π) and open-shell diradical character.

Stable Crystalline Nanohoop Radical and Its Self-Association Promoted by van der Waals Interactions

A stable nanohoop radical (OR3) combining the structures of cycloparaphenylene and an olympicenyl radical is synthesized and isolated in the crystalline state. X-ray crystallographic analysis reveals that OR3 forms a unique head-to-tail dimer that further aggregates into a one-dimensional chain in the solid state. Variable-temperature NMR and concentration-dependent absorption measurements indicate that the π-dimer is not formed in solution. An energy decomposition analysis indicates that van der Waals interactions are the driving force for the self-association process, in contrast with other olympicenyl derivatives that favor π-dimerization. The physical properties in solution phase have been studied, and the stable cationic species obtained by one-electron chemical oxidation. This study offers a new molecular design to modulate the self-association of organic radicals for overcoming the spin-Peierls transition, and to prepare novel nanohoop compounds with spin-related properties.

Synthesis of Twisted [n]Cycloparaphenylene by Alkene Insertion

Mono-alkene-inserted [n]cycloparaphenylenes 1 [(ene)-[n]CPP] with n=6, 8, and 10, mono-ortho-phenylene-inserted [6]CPP 2, and di-alkene-insertved [n]CPP 3 [(ene)₂ -[n]CPP] with n=4, 6, and 8 were synthesized by fusing CPP precursors and alkene or ortho- phenylene groups through coupling reactions. Single-crystal X-ray diffraction analyses reveal that the strips formed by the π-surfaces of 1 and 2 exhibited a Möbius topology in the solid state. While the Möbius topology in the parent 1 and 2 in solution was lost due to the free rotation of the paraphenylene unit even at low temperatures, ene-[6]CPP 4 with eight 1-pyrrolyl groups preserved the Möbius topology even in solution. Despite a twist, 1 has in-plane conjugation and possesses a unique size dependence of the electronic properties: namely, the opposite size dependency of the HOMO-LUMO energy relative to conventional π-conjugated molecules.

Synthesis and Properties of Nonaromatic Meta-Benziheptaphyrins and Aromatic Para-Benziheptaphyrins

Two examples of nonaromatic m -benziheptaphyrins and two examples of aromatic p -benziheptaphyrins were synthesized by [5+2] condensation of appropriate m -benzi pentapyrrane and p -benzi pentapyrrane respectively and bithiophene diol in CH 2 Cl 2 in the presence of one equivalent of TFA under inert conditions for 30 min followed by oxidation with DDQ in open air for 1 h. The 1 H NMR studies carried out at room temperature as well as at lower temperature indicated the nonaromatic nature of m -benziheptaphyrins with inversion of two thiophene rings and aromatic nature of p -benziheptaphyrins with inversion of one of the thiophene ring. The X-ray structure obtained for one of the p -benziheptaphyrins showed a planar conformation with alignment of one of the thiophene ring away from the macrocyclic inner core but maintained its coplanarity with the mean plane and supported the aromatic nature of the macrocycle. The absorption spectra of m -benziheptaphyrins resembled with the nonaromatic systems and showed two intense bands at 445 nm, 555 nm and a very broad band in the region of 600-1100 nm whereas the p -benziheptaphyrin showed three sharp intense bands at 534 nm, 585 nm and 832 nm due to their aromatic nature. The protonation of m -benziheptaphyrins and p -benziheptaphyrins resulted in significant bathochromic shifts in their absorption maxima and showed strong absorption in NIR region. The electrochemical studies indicated that m - & p -benziheptaphyrins undergo oxidations and reductions easily. DFT and TD-DFT studies were in agreement with the experimental observations.

Synthesis and Properties of Stable 20π Porphyrinoids

The 20π porphyrinoids are immediate higher homologues of 18π porphyrins and differ from porphyrins in aromaticity which in turn affects the structure, properties and chemical reactivities. Research over the years indicated that the 20π porphyrinoids can be stabilized as non-aromatic/anti-aromatic or Mobius aromatic macrocycles using different strategies such as core-modification of porphyrins, non-metal/metal complexation of porphyrins, peripheral modification of porphyrins and expanded porphyrinoids. The structural properties such as aromaticity of the macrocycle can be controlled by choosing the right synthetic strategy. This review will provide an overview of the development in the chemistry of 20π porphyrinoids giving emphasize on the synthesis, structure and electronic properties of these macrocycles which have huge potential for various applications.

Hückel- and Baird-type 3D Global Aromaticity in a Fully Conjugated Molecular Cage

Global aromaticity in 3D π-conjugated molecular cages remains largely unexplored. Herein, we report the facile synthesis of a fully conjugated molecular cage ( 1 ) containing two bridged triphenylamine units and three quinoidal bithiophene arms. X-ray crystallographic analysis, NMR/ESR measurements and theoretical calculations reveal that: ( 1 ) its dication ( 1 2+ ) has an open-shell singlet ground state and is 3D globally aromatic, with individual macrocycles being 2D Hückel aromatic; (2) its tetracation ( 1 4+ ) has a triplet ground state and is also 3D globally aromatic, with individual macrocycles being 2D Baird aromatic; and (3) its hexacation ( 1 6+ ) has a closed-shell nature and shows local aromaticity. The study revealed a close relationship between 2D Hückel/Baird aromaticity and 3D global π-aromaticity.

Möbius ZnII -Hexaphyrins Bearing a Chiral Coordinating Arm: A Chiroptical Switch Featuring P/M Twist Inversion Controlled by Achiral Effectors

By their conformational flexibility, Möbius aromatic hexaphyrins provide a dynamic chirality attractive to develop stimuli responsive systems such as chiroptical switches. A regular [28]hexaphyrin has been equipped with a chiral coordinating arm to achieve transfer of chirality from a fix stereogenic element to the dynamic Möbius one. The arm allows straightforward formation of labile monometallic Zn^II complexes with an exogenous ligand, either a carboxylato or an amino with opposite inwards/outwards orientations relative to the Möbius ring. As a corollary, the chiral coordinating arm is constrained over the ring or laterally, inducing opposite P and M Möbius configurations with unprecedented high stereoselectivity (diast. excess greater than 95 %). By tuning the transfer of chirality, these achiral effectors generate electronic circular dichroism spectra with bisignate Cotton effect of opposite signs. Switching between distinct chiroptical states was ultimately achieved in mild conditions owing to ligand exchange, with high robustness (10 cycles).

Carbazole-Containing Carbadecaphyrins: Non-aromatic Expanded Porphyrins that Undergo Proton-Triggered Conformational Changes

A pair of meso-unsubstituted expanded carbaporphyrins containing two carbazole moieties were prepared in high isolated yields (82 and 76 %, respectively). The two macrocycles, namely 3 and 4, differ with respect to their substitution at the carbazole N-atoms i. e. by H and i-Bu, respectively. As prepared in their free-base forms, macrocycles 3 and 4 adopt figure-of-eight conformations and are best characterized as 40 π-electron, non-aromatic species possessing a decaphyrin(1.1.0.0.0.1.1.0.0.0) skeleton. Protonation of 3 with either trifluoroacetic acid (TFA) or perchloric acid (HClO₄ ) produces a parallelogram-shaped structure. A similar structure is produced when N-functionalized system 4 is treated with TFA. In contrast, protonation of 4 with HClO₄ leads it to adopt a twisted Möbius strip-like structure in the solid state, thus allowing access to three distinct conformational states as a function of the conditions.

Synthesis and Studies of Stable Nonaromatic Dithia Pyribenzihexaphyrins

We report here one of the rare examples of expanded hexaphyrins named as dithia pyribenzihexaphyrin macrocycles containing six-membered rings such as pyridine and p-phenylene along with five-membered heterocycles such as pyrrole and thiophene as a part of a macrocyclic frame. Trifluoroacetic acid catalyzed [3 + 3] condensation of equimolar mixture of [10,10'-bis(p-tert-butyl phenyl)hydroxymethyl]-1,3-bis(2-thienyl)pyridine diol (2,6-pyri diol) and 1,4-bis(phenyl(1H-pyrrol-2-yl)methyl)benzene (p-benzidipyrrane) in CH₂Cl₂ followed by oxidation with DDQ afforded stable nonaromatic dithia 2,6-pyri-para-benzihexapyrins 1 and 2 in 6-8% yields. The macrocycles were characterized by high-resolution mass spectroscopy and 1D and 2D NMR spectroscopy. NMR studies revealed the nonaromatic nature of dithia 2,6-pyri-p-benzihexaphyrins and indicated that the para-phenylene ring prefers to be in quininoid form rather than in benzenoid form. The macrocycles displayed sharp absorption bands in the region of ∼380-500 nm and a broad band at ∼700 nm, reflecting their nonaromatic nature. Upon protonation, these macrocycles showed NIR absorption properties. The redox studies of macrocycles indicated their electron-deficient nature. The DFT/TD-DFT studies are in line with the experimental observations.

Kinetic versus Thermodynamic Control Over Multiple Conformations of Di-2,7-naphthihexaphyrin(1.1.1.1.1.1)

Di-2,7-naphthihexaphyrin(1.1.1.1.1.1), a non-aromatic carba-analogue of the hexaphyrin(1.1.1.1.1.1), incorporating two built-in 2,7-naphthylene moieties was synthesized as two separate, conformationally locked stereoisomers. Both conformers followed complex protonation pathways involving structurally different species, which can be targeted under kinetic and thermodynamic control. The neutralization of the ultimate dicationic product, accessible from both stereoisomers of the free base, allowed to realize the complex conformational switching cycle involving six structurally different species.

Expanded Carbaporphyrinoids

This Review outlines the progress in the field of synthetic expanded carbaporphyrinoids. The evolution of this topic is demonstrated with expanded porphyrin-inspired systems with a variety of incorporated entities that introduce one or more carbon atoms into the cavity. The discussion starts with platyrins-the macrocycles that were identified as parent molecules of not only the expanded carbaporphyrinoids, but the carbaporphyrinoid class in general. After historic considerations, the plethora of expanded porphyrin-like macrocycles containing N-confused or neo-confused pyrrole motifs and different carbocyclic subunits are presented. Special emphasis is given to applications of expanded carbaporphyrinoids in different areas, including organometallic chemistry, switching systems, or aromaticity, concluding with the demonstration of a covalent cage based on an expanded carbaporphyrinoid.

Conformation-Dependent Response to the Protonation of Diphenanthrioctaphyrin(1.1.1.0.1.1.1.0): A Route to Pseudorotaxane-Like Structures

Diphenanthrioctaphyrin(1.1.1.0.1.1.1.0), an expanded carbaporphyrinoid incorporating two phenanthrenylene moieties, exists as two separate, yet interconvertible, locked stereoisomers. These species demonstrate complex dynamic behavior upon protonation, consisting in multiple conformational rearrangements and anion-binding events. The formation of one of the final dicationic forms is accompanied by the inclusion of a complex anion(s) within the macrocyclic cavity yielding a pseudorotaxane-like host-guest complex. Protonation with trifluoroacetic or dichloroacetic acids followed by neutralization afforded a conformation-switching cycle, which involves six structurally different species. Analogous acidification with chiral 10-camphorsulfonic acid and subsequent neutralization generated one of the free base stereoisomers with enantiomeric excess. Therefore, it was shown that the simple acid-base chemistry of diphenanthrioctaphyrin can act as stimulus, inducing chirality into the system, allowing for the manipulation of the stereochemical information imprinted into the enantiomers of the macrocycle.

Bis-4,4'-biphenyl Ring Embedded Octaphyrin with Three Distinct Conformational Structures

Three distinct conformational structures of carbaoctaphyrins were prepared by incorporating bis-4,4'-biphenyl units in the macrocyclic core. The free-base form adopts a figure-eight conformation, whereas the protonation triggers a conformational change with a pyrrole ring inversion and acquires an open-framework structure. The insertion of bis-Rh^I metal ion in the macrocyclic core affords a singly twisted conformational structure. Furthermore, the local aromaticity in the bis-4,4'-biphenyl ring dominates the overall macrocyclic aromaticity in all three forms, and thus adopts nonaromatic characteristics. These results are supported by spectral as well as theoretical studies, and they are unambiguously confirmed by X-ray crystal analyses.

Cyclobis(7,8-(para-quinodimethane)-4,4'-triphenylamine) and Its Cationic Species Showing Annulene-Like Global (Anti)Aromaticity

π-Conjugated macrocycles containing all-benzenoid rings usually show local aromaticity, but reported herein is the macrocycle CBQT, containing alternating para-quinodimethane and triphenylamine units displaying annulene-like anti-aromaticity at low temperatures as a result of structural rigidity and participation of the bridging nitrogen atoms in π-conjugation. It was easily synthesized by intermolecular Friedel-Crafts alkylation followed by oxidative dehydrogenation. X-ray crystallographic structures of CBQT, as well as those of its dication, trication, and tetracation were obtained. The dication and tetracation exhibited global aromaticity and antiaromaticity, respectively, as confirmed by NMR measurements and theoretical calculations. Both the dication and tetracation possess open-shell singlet ground states, with a small singlet-triplet gap.

Homocarbaporphyrinoids: The m-o-m and p-o-p Terphenyl Embedded Expanded Porphyrin Analogues and Their RhI Complexes

Stable homocarbaporphyrinoids were successfully synthesized by incorporating the m-o-m and p-o-p terphenyl units into the porphyrin core. The distinct bonding modes of terphenyl in the macrocycle generated two structural isomers with two and four carbon atoms in the macrocyclic environment. The core was utilized to stabilize the Rh^I ion. The spectral and structural analyses revealed that the restricted (m-o-m) and allowed (p-o-p) conjugation in the macrocyclic core provide overall non-aromatic characteristics both to the free bases and their complexes.

24π Core-Modified Nonfused Pentaphyrin with Möbius Aromaticity

Syntheses of core-modified 24π pentaphyrin 7 and 20π homoporphyrin 8 are reported. The freebase form of 7 exhibits optical and ¹ H NMR spectroscopic behavior corresponding to a 4nπ nonaromatic topology at 298 K. However, upon lowering the temperature to 188 K, 7 exhibits weak aromaticity because of a small conformational change. Protonation of the imine pyrrolic nitrogen atoms results in sharpening of the absorption band with more than 100 nm red shift and two-fold increase in ϵ values. The ¹ H NMR spectra show large shielding and deshielding of specific protons accompanied by twisting of pyrrole and thiophene rings from the mean plane of the macrocycle. These observations suggest a major structural change upon protonation. The single-crystal X-ray structure of protonated 7 a reveals a severe twisting of the macrocycle, especially at the ethene bridge, and the two thiophene rings adjacent to the ethene bridge are pointed above and below the plane of the ethene bridge (49.07° and 49.08°). The nucleus-independent chemical shift (NICS) values calculated at the center of the macrocycle for the freebase and protonated derivatives of 7 a and 7 b are -5.6, -8.3 ppm and -5.3, -8.4 ppm, respectively. The anisotropy-induced current density (AICD) plots suggest a clockwise circulation of π electrons. Taken together, the changes occurring in the optical spectra, ¹ H NMR spectra, single-crystal X-ray structure and NICS calculation reflect a Möbius aromatic topology for protonated 7. However, the homoporphyrin 8 remains nonaromatic both in the freebase and protonated form.

Rhodium-Induced Reversible C-C Bond Cleavage: Transformations of Rhodium(III) 22-Alkyl-m-benziporphyrins

The structurally prearranged carbaporphyrins 22-methyl- and 22-ethyl-m-benziporphyrins provide the platform stabilizing aromatic rhodium(III) 22-(μ-methylene-m-benziporphyrin) and rhodium(III) 22-(μ-ethylidene-m-benziporphyrin). An intramolecular conversion facilitated by the m-phenylene reactivity and observed for both aromatic complexes efficiently leads to rhodium(III) 21-(μ-methylene)-21-carbaporphyrin and rhodium(III) 21-(μ-ethylidene)-21-carbaporphyrin. The distinctive macrocyclic environment of rhodium(III) 21-carbaporphyrin created an opportunity to trap unique organometallic transformations of inner core substituents affording the fulvene-like bond pattern or the rearrangement to 21-vinyl substituent. The one-electron reduction of the rhodium(III) carbaporphyrin anion π-radical with a (d_xy )² (d_xz )² (d_yz )² -(P^.- ) electronic configuration is demonstrated. The further process of reduction of paramagnetic species triggers the ethyl migration from carbon(22) to rhodium(III), affording the diamagnetic rhodium(III) meta-benziporphyrin containing the apically coordinated σ-ethyl ligand providing an example of reversible C(sp² )-C(sp³ ) bond cleavage.

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.

Phenylene-Bridged Expanded Porphyrazines

Novel expanded porphyrazines comprising o- and p-phenylene units were synthesized from self-condensation reactions of acyclic [2+1]-type precursors prepared from 1,3-diiminoisoindoline and the corresponding phenylenediamines. Möbius and Hückel topological structures of [4+2]-type and [3+2]-type expanded porphyrazines, respectively, were elucidated by X-ray crystallography. Despite the Möbius and Hückel [4n]π-electron systems, these expanded porphyrazines are virtually nonaromatic as a result of their flexible structures in solution, which may diminish intrinsic aromaticity and antiaromaticity.

Aromaticity and antiaromaticity of substituted fulvene derivatives: perspectives from the information-theoretic approach in density functional reactivity theory

Strong correlations among aromaticity descriptors and information-theoretic quantities are unveiled, providing novel insights about aromaticity and antiaromaticity from different perspectives.

A Doubly Zwitterionic Antiaromatic [28]Hexaphyrin Formed upon Deprotonation of 5,20-Di(N-methyl-4-pyridinium)-Substituted [28]Hexaphyrin

A rectangular 5,20-di(4-pyridyl) [26]hexaphyrin was reduced with NaBH₄ to give the corresponding twisted Möbius aromatic [28]hexaphyrin. Subsequent double N-methylation gave a dicationic 5,20-di(N-methyl-4-pyridinium) [28]hexaphyrin, which was converted to a doubly zwitterionic and Hückel antiaromatic [28]hexaphyrin upon deprotonation with sodium methoxide.

Duality of Orbital-Symmetry-Allowed Transition States for Thermal Sigmatropic Hydrogen Shifts in Transition Metal Compounds

Herein, the Zimmerman Möbius/Hückel concept is extended to pericyclic reactions involving transition metals. While sigmatropic hydrogen shifts in parent hydrocarbons are either uniquely antarafacial or suprafacial, we have shown by theoretical orbital topology considerations and quantum chemical computations at DFT level that both modes of stereoselectivity must become allowed in the same system as a consequence of Craig-Möbius-type orbital arrays, in which a transition metal d orbital induces a phase dislocation in metallacycles. This may have fundamental implications for the understanding of reactivity and bonding in organometallic chemistry.