Heptannulated Perylene Diimides: Formation and Reactivity of Electron-Deficient Tropylium Cations and Heptafulvenes

The development of new π-conjugated motifs opens pathways to previously unexplored classes of organic semiconductors and functional dyes. In this study, five- and seven-membered carbocycles were fused at the ortho and bay regions of electron-deficient perylenes, starting from a common dialdehyde precursor. Structural analysis of the resulting perylene tetraesters, dianhydrides, and diimides (PDIs) revealed three distinct ring-fusion patterns and defined stereochemistry. The fused PDI cycloheptatrienes demonstrated susceptibility to acid-catalyzed transarylation, involving tropylium cation intermediates, which can be used preparatively. Under superacidic conditions, the PDI tropylium cations were directly observed and shown to undergo hydride-transfer reductions. Additionally, a fused PDI bis(heptafulvene) was synthesized by dehydrogenating a suitably substituted PDI cycloheptatriene. The final system contains two quinomethane units, which can be protonated to yield a stable tropylium-like dication.

Organometallic chemistry confined within a porphyrin-like framework

The world of modified porphyrins changed forever when an N-confused porphyrin (NCP), a porphyrin isomer, was first published in 1994. The replacement of one inner nitrogen with a carbon atom revolutionised the chemistry that one is able to perform within the coordination cavity. One could explore new pathways in the organometallic chemistry of porphyrins by forcing a carbon fragment from the ring or an inner substituent to sit close to an inserted metal ion. Since the NCP discovery, a series of modifications became available to tune the coordination properties of the cavity, introducing a fascinating realm of carbaporphyrins. The review surveys all possible carbatetraphyrins(1.1.1.1) and their spectacular coordination and organometallic chemistry.

Organometallic Chemistry within the Structured Environment Provided by the Macrocyclic Cores of Carbaporphyrins and Related Systems

The unique environment within the core of carbaporphyrinoid systems provides a platform to explore unusual organometallic chemistry. The ability of these structures to form stable organometallic derivatives was first demonstrated for N-confused porphyrins but many other carbaporphyrin-type systems were subsequently shown to exhibit similar or complementary properties. Metalation commonly occurs with catalytically active transition metal cations and the resulting derivatives exhibit widely different physical, chemical and spectroscopic properties and range from strongly aromatic to nonaromatic and antiaromatic species. Metalation may trigger unusual, highly selective, oxidation reactions. Alkyl group migration has been observed within the cavity of metalated carbaporphyrins, and in some cases ring contraction of the carbocyclic subunit takes place. Over the past thirty years, studies in this area have led to multiple synthetic routes to carbaporphyrinoid ligands and remarkable organometallic chemistry has been reported. An overview of this important area is presented.

Dancing with Azulene

It seems interesting to adopt the idea of dance in the context of the arrangement of molecular blocks in the building of molecular systems. Just as various dances can create various feelings, the nature and arrangement of molecular blocks in the generated molecular system can induce different properties. We consider obtaining such “dancing” systems in which the still little-known azulene moieties are involved. The dark blue nonbenzenoid aromatic azulene has one less axis of symmetry relative to the two axes of its isomer, i.e., the fully benzenoid naphthalene, acquiring valuable properties as a result that can be used successfully in technical applications. In a dancing system, the azulene moieties can be connected directly, or a more or less complex spacer can be inserted between them. Several azulene moieties can form a linear oligomer or a polymer and the involvement of azulene moieties in nonlinear molecules, such as crown ethers, calixarenes, azuliporphyrins, or azulenophane, is a relatively new and intensely studied topic. Some aspects are covered in this review, which are mainly related to obtaining the mentioned azulene compounds and less to their characterization or physico-chemical properties.

Phenanthrene cyclocarbonylation – Core post-synthetic modification of phenanthriporphyrin

The unique [Fe(CO)5]-induced cyclocarbonylation of the phenanthriporphyrin's core is an intriguing example of a post-synthetic core modification of the macrocycle. The reaction involves the activation of C(22)−H and C(25)−H bonds,...

Azulene Moiety as Electron Reservoir in Positively Charged Systems; A Short Survey

The non-alternant aromatic azulene, an isomer of alternant naphthalene, differs from the latter in peculiar properties. The large polarization of the π-electron system over the seven and five rings gives to azulene electrophile property a pronounced tendency to donate electrons to an acceptor, substituted at azulene 1 position. This paper presents cases in which azulene transfers electrons to a suitable acceptor as methylium ions, positive charged heteroaromatics and examples of neutral molecules that can accept electrons. The proposed product synthesis was outlined and the expected electron transfer was highlighted by analyzing the NMR, UV-Vis spectra and the pKR+ values.

Chemistry inside a Porphyrin Skeleton: Platinacyclopentadiene from Tellurophene

Platinum(II) binds to 21,23-ditelluraporphyrin forming a side-on complex, which can be easily transformed into an aromatic metallaporphyrin, that is, 21-platina-23-telluraporphyrin, with a platinacyclopentadiene unit built in the porphyrin skeleton in place of one pyrrole ring. The central platinum(II) ion with a CCNTe square-planar coordination sphere can be oxidized to platinum(IV) by chlorine, bromine, methyl iodide or allyl chloride to yield octahedral complexes. All platinatelluraporphyrins show dynamic behavior involving the platinum ion coordination sphere fluxionality and the porphyrin skeleton deformation, both in-plane and out-of-plane, as demonstrated by ¹ H NMR spectroscopy.

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.

Incorporation of a p-Phenylene Unit into the Azuliporphyrinogens Frame-Oxidation and Ruthenium Cluster Coordination

A porphyrinogen macrocycle incorporating two azulenes, phenylene and thiophene into the framework, joined by four C(sp³ ) atoms has been obtained as a mixture of six isomers. They were successfully separated and characterized spectroscopically. The identity of two of them was confirmed by X-ray crystallography. One isomer was tested in reaction with [Ru₃ (CO)₁₂ ] yielding exclusively π-complex with two clusters attached to azulenes. The partial oxidation of porphyrinogens yielded dication with two unmodified meso bridges. The stepwise oxidation followed by reaction with water as nucleophile afforded the dicationic species with two hydroxyl groups and a trication with one OH group. The hydroxy-dication can be reversibly transformed into hydroxy-trication by addition of HBF₄ etherate.

Coordination-Induced Molecular Tweezing: Ruthenium Clusters Docked at Azuliporphyrinogens

The reaction of [Ru₃(CO)₁₂] with a series of conformationally flexible thiaazuliporphyrinogens, namely, dithiadiazuli-, thiatriazuli- and tetraazuliporphyrinogen, yielded a series of complexes with two azulene rings coordinated by either Ru₂(CO)₅ and Ru₄(CO)₉ or two Ru₄(CO)₉ clusters. For dithiadiazuliporphyrinogen, three fundamental arrangements were detected, with two clusters being located at the same side or different sides of the meso plane. Coordination altered the electron density on the rings, allowing for dispersion-promoted interactions between coordinated azulenes. To create this face-to-face arrangement, the ligand framework performs a tweezing movement with coordinated azulenes approaching each other. The tweezer-like arrangement of the coordinated azulene rings assists the formation of azulene inclusion complexes as determined by ¹H NMR titration.

Out of the Blue! Azuliporphyrins and Related Carbaporphyrinoid Systems

First reported in 1997, azuliporphyrins have proven to be a truly remarkable family of porphyrin analogues. In this system, although the porphyrin framework is retained, one of the pyrrolic moieties has been replaced by an azulene unit. Azulene favors electrophilic substitution at the 1,3-positions, which are structurally analogous to the α-positions in pyrrole, and this property facilitates the construction of azulene-containing porphyrinoid systems. Azuliporphyrins were first prepared from tripyrranes and 1,3-azulenedicarbaldehyde using a "3 + 1" variant on the MacDonald reaction. Subsequently, azulenes were shown to react with acetoxymethylpyrroles under acidic conditions to generate azulitripyrranes that could be utilized in a back-to-front "3 + 1" methodology to form azuliporphyrins and related heteroporphyrinoids. In addition, the favorability of azulenes toward 1,3-substitution was applied to one-pot syntheses of tetraarylazuliporphyrins and calix[4]azulenes. Azuliporphyrins have significant diatropic character that is greatly enhanced upon protonation. They have been shown to form organometallic complexes with Ni(II), Pd(II), Pt(II), Ir(III), Rh(III), and Ru(II) and undergo selective oxidations at the internal carbon with copper(II) or silver(I) salts to afford 21-oxyazuliporphyrins. In addition, oxidative ring contractions readily occur under basic conditions in the presence of peroxides to give benzocarbaporphyrins, and this reactivity provides access to tetraarylbenzocarbaporphyrins and their organometallic derivatives. A diazulenylmethane dialdehyde has been shown to react with dipyrrylmethanes in the presence of HCl or HBr to give diazuliporphyrins that were isolated in a monoprotonated form, and metalation with palladium(II) acetate afforded a stable zwitterionic palladium(II) complex. Equally intriguing dicarbaporphyrinoids incorporating indene and azulene rings have been reported, and these systems exhibit significant aromatic character. Recent studies have demonstrated that calixazulenes form supramolecular complexes with quaternary ammonium salts and afford a 1:1 complex with C60. In addition, conjugated structures have been prepared from calixazulenes that are structurally related to quatyrin, the theoretically important hydrocarbon analogue of the porphyrins. Examples of expanded azuliporphyrinoids have also been described. These azulene-containing porphyrinoids exhibit unique and complex reactivity that compares favorably with better studied porphyrin analogue systems such as the N-confused porphyrins, and azuliporphyrin derivatives show promise in the development of new catalytic systems.

Annulation and arylation stabilize new porphyrinoids

More pi, please! A free-base, benzannelated triphyrin with a [2.1.1] bridging pattern (1) and an azulene-derived tetracationic all-carbon porphyrinoid (2) are discussed. Compound 1 can be used to form metal triphyrin complexes, and 2 shows potential in molecular electronics and as a receptor for weakly binding anions.