Aromaticity Of Porphyrinoids Research Articles

Effects of Electron-Withdrawing and Electron-Donating Groups on Aromaticity in Cyclic Conjugated Polyenes.

Determining the aromaticity of various fluorinated benzenes is challenging as easily obtained experimental aromaticity [Δδ(Houter - Hinner)] necessitates the chemical shifts of inner and outer protons. This issue was addressed in porphyrinoids by replacing the electron-withdrawing (E.W.) groups at the meso-positions of porphyrins and allyliporphyrins. Electronic effects on aromaticity in porphyrinoids have not been thoroughly examined in the literature. In porphyrins, the effect of E.W. groups is minimal, making it difficult to establish a clear relationship between the aromaticity strength and E.W. groups. Conversely, in allyliporphyrins, stronger E.W. groups, such as indandione (IND) derivatives, significantly reduce the aromaticity of the parent structure. The IND derivatives disrupted the aromatic pathway of allyliporphyrin more effectively than those attached to porphyrins. This is attributed to the absence of β-carbons in allyliporphyrins. The effect of electron-donating (E.D.) groups on porphyrins and allyliporphyrins was further investigated. Contrary to the initial assumption that the E.D. groups might enhance aromaticity owing to their ability to increase electron density, as the strength of the E.D. groups increased, the aromaticity of the porphyrinoids decreased. Despite the modest reduction in aromaticity, any form of electron perturbation reduces aromaticity. The aromaticity of various fluorinated benzenes is expected to parallel our observations of porphyrinoids as representative aromatic polyenes.

β-Trioxopyrrocorphins: pyrrocorphins of graded aromaticity.

Octaethyltrioxopyrrocorphins unexpectedly show macrocycle-aromatic properties, even though they contain the macrocyclic π-system of the non-aromatic pyrrocorphins (hexahydroporphyrins). Two of the four possible triketone regioisomers were first reported in 1969 by one-pot oxidation of octaethylporphyrin but remained essentially unexplored since. We detail here the targeted preparation of the remaining two triketone isomers and the optical and NMR spectroscopic properties of all isomers. All four regioisomers possess unique electronic properties, including broadly varying degrees of diatropicity that were experimentally determined using 1H NMR spectroscopy and computationally verified. Structural patterns modulating the aromaticity were recognized. These differences highlight the regioisomerically differentiated influences of the three β-oxo-functionalities. We also present the solid state structure of the two most common isomers (in their free base form or as zinc complexes), allowing further conclusions to be made about the resonance structures present in these triketones. Remarkably, also, the halochromic properties of the triketones differ sharply from those of regular (hydro)porphyrins, providing further support for the proposed 16-membered, 18 π-electron aromatic ring-current. The work conceptually expands the understanding of tris-modified hydroporphyrinoid analogues and the factors that enable and control porphyrinoid aromaticity.

Local and macrocyclic (anti)aromaticity of porphyrinoids revealed by the topology of the induced magnetic field.

The aromaticity in porphyrinoids results from the π conjugation through two different annular perimeters: the macrocyclic ring and the local heterocyclic rings appended to it. Analyses, based on aromatic stabilization energies (ASE), indicate that the local circuits (6π) are responsible for the significant aromatic stabilization of these systems. This local aromaticity can be coupled with the one from 4n + 2π macrocyclic circuit. It can either compensate for the destabilization due to a 4n π macrocyclic circuit, or be the only source of aromatic stabilization in porphyrinoids with macrocycles without π-conjugated bonds. This "multifaceted" aromatic character of porphyrinoids makes it challenging to analyze their aromaticity using magnetic descriptors because of the intricate interaction of local versus macro-cyclic circulation. In this contribution, we show that the analysis of the bifurcation of the induced magnetic field, Bind, allows clear identification and quantification of both local, and macrocyclic aromaticity, in a representative group of porphyrinioids. In porphyrin, bifurcation values accurately predict the local and macrocyclic contribution rate to overall aromatic stabilization determined by ASE.

Rhodium Complexes of Carbaporphyrins, Carbachlorins, adj-Dicarbaporphyrins, and an adj-Dicarbachlorin.

The macrocyclic cavities in carbaporphyrins are well suited for the formation of metalated derivatives. A carbaporphyrin diester and a naphthocarbaporphyrin reacted with [Rh(CO)2Cl]2 to give good-to-excellent yields of rhodium(I) complexes, and these were fully characterized by X-ray crystallography. Both rhodium(I) derivatives were converted into rhodium(III) complexes in refluxing pyridine, albeit in moderate yields. Carbachlorins also formed rhodium(I) complexes, but these could not be further transformed into rhodium(III) products. The rhodium(III) complexes incorporate two axial pyridine ligands, which exhibit strongly shielded resonances in their 1H NMR spectra, and the rhodium(III) carbaporphyrin diester was further characterized by X-ray crystallography. adj-Dicarbaporphyrins also formed rhodium(I) complexes, but these reactions involved the relocation of a proton to generate an internal methylene unit. The environments associated with the two faces of the resulting macrocycles are very different from one another, and this results in the 1H NMR chemical shifts for the two internal methylene protons being separated by well over 3 ppm. Although the diatropicities of rhodium(I) complexes for monocarbaporphyrins and carbachlorins are comparable to those of the parent ligands, the chemical shifts for rhodium(I) dicarbaporphyrins are consistent with a significant reduction in the porphyrinoid aromaticity. A dicarbachlorin also gave a rhodium(I) complex, but this species fully retained the diatropic characteristics of the parent ligand. Nevertheless, the internal CH2 unit still gave two widely separated doublets indicative of radically differing environments for the two faces of the macrocycle. Rhodium(I) dicarbaporphyrin and dicarbachlorin complexes were further characterized by X-ray crystallography.

New electron delocalization tools to describe the aromaticity in porphyrinoids.

The role of aromaticity in porphyrinoids is a current subject of debate due to the intricate structure of these macrocycles, which can adopt Hückel, Möbius and even figure-eight conformers. One of the main challenges in these large π-conjugated structures is identifying the most conjugated pathway because, among aromaticity descriptors, there are very few that can be applied coherently to this variety of conformers. In this paper, we have investigated the conjugated pathways in nine porphyrinoid compounds using several aromaticity descriptors, including BLA, BOA, FLU and HOMA, as well as the recently introduced AV1245 and AVmin indices. All the indices agree on the general features of these compounds, such as the fulfillment of Hückel's rule or which compounds should be more or less aromatic from the series. However, our results evince the difficulty of finding the most aromatic pathway in the macrocycle for large porphyrinoids. In fact, only AVmin is capable of recognizing the annulene pathway as the most aromatic one in the nine studied structures. Finally, we study the effect of the exchange in DFT functionals on the description of the aromaticity of the porphyrinoids. The amount of exact exchange quantitatively changes the picture for most aromaticity descriptors, AVmin being the only exception that shows the same qualitative results in all cases.

Control and Switching of Aromaticity in Various All-Aza-Expanded Porphyrins: Spectroscopic and Theoretical Analyses.

Modification of aromaticity is regarded as one of the most interesting and important research topics in the field of physical organic chemistry. Particularly, porphyrins and their analogues (porphyrinoids) are attractive molecules for exploring various types of aromaticity because most porphyrinoids exhibit circular conjugation pathways in their macrocyclic rings with various molecular structures. Aromaticity in porphyrinoids is significantly affected by structural modification, redox chemistry, NH tautomerization, and electronic states (singlet and triplet excited states). Conversely, aromaticity significantly affects the spectroscopic properties and chemical reactivities of porphyrinoids. In this context, considerable efforts have been devoted to understanding and controlling the aromaticity and antiaromaticity of porphyrinoids. Thus, a series of porphyrinoids are in the limelight, being expected to shed light on this field because they have some advantages to demonstrate the switching of aromaticity; it is possible to control the aromaticity by lowering the temperature, adding and removing the protons of expanded porphyrins, changing the chemical environment, and switching the electronic states (triplet and singlet excited states) by photoexcitation. In this regard, this Review describes the control of aromaticity in various expanded porphyrins from the spectroscopic point of view with assistance from theoretical calculations.

ChemInform Abstract: Structure Modification and Spectroscopic Properties of Artificial Porphyrinoids

AbstractReview: 48 refs.

ChemInform Abstract: Aromaticity Switching in Porphyrinoids

AbstractReview: 95 refs.

Structure Modification and Spectroscopic Properties of Artificial Porphyrinoids

AbstractIn this review article, we summarize our recent progress in rational structure modification and spectroscopic studies of artificial porphyrinoids. Structural modifications, including meso aryl substitution, peripheral aromatic ring fusion, ring contraction and expansion, and core modification with heterocyclic subunits other than pyrrole, perturb the electronic structures and aromaticity of porphyrinoids, thus regulating their electronic properties. The relationship between structure and spectroscopic properties as well as the applications of these porphyrinoids are also reviewed.

Porphyrinoid Aromaticity Induced by the Interaction between Oxidized and Reduced Pyridine Subunits

Coupling of 3-hydroxypyridine-2,6-dicarbaldehyde with a tripyrrane-like precursor bearing a central pyridine ring afforded a new porphyrinoid with two pyridine-related subunits Spectroscopic and theoretical analyses demonstrate that the aromaticity of this porphyrinoid is fulfilled by the cooperative interaction between the two pyridinoid rings.

Aromaticity Switching in Porphyrinoids.

The aromaticity of porphyrinoids can be substantially altered by reversible modification of their original electronic structures. Well-defined modulators can be used as a means to initiate these modifications, including redox processes, acid-base chemistry, and conformational phenomena. This Focus Review emphasizes the situation for which a single macrocyclic frame alternatively adopts diatropic and paratropic features and both situations are readily and mutually exchangeable. Eventually, such a porphyrinoid transformation can be explored as a suitable element to construct switchable optoelectronic materials.

Deciphering aromaticity in porphyrinoids via adaptive natural density partitioning.

The adaptive natural density partitioning (AdNDP) method has been applied for the first time to porphyrinoids in order to describe their aromaticity. The analysis of π-electron system reveals that aromaticity of annulene originates from 6-π-electron delocalization, while aromaticity of porphyrin can be better described in terms of local aromaticities of the appended 6-π-electron pyrrolic heterocycles and 6-π-electron central fragment. The patterns of chemical bonding for porphyrinoids obtained by AdNDP are consistent with chemical intuition and lead to unique and compact graphic formulas.

ChemInform Abstract: Carbaporphyrins and Related Systems. Synthesis, Characterization, Reactivity and Insights into Porphyrinoid Aromaticity

AbstractReview: 478 refs.

Description of Aromaticity in Porphyrinoids

Like the larger nonplanar Möbius rings, porphyrinoid aromaticity is not due primarily to the macrocyclic π conjugation of the corresponding annulene perimeters. The block-localized wave function (BLW)-derived aromatic stabilization energies (ASE) of several porphyrinoids reveal that, on a per atom basis, the appended 6π electron heterocycles of porphyrinoids confer aromaticity much more effectively than the macrocyclic 4n+2 π electron conjugations. There is no direct relationship between thermochemical stability of porphyrinoids and their macrocyclic 4n or 4n+2 π electron counts. Porphyrinoids having an "antiaromatic" macrocyclic 4n+2 π electron conjugation pathway (e.g., 4) as well as those having no macrocyclic conjugation (e.g., 9) can be stabilized by aromaticity. Computed nucleus independent chemical shifts (NICS) and the anisotropy of the induced current density (ACID) disclose the intricate local versus macrocyclic circulation interplay for several porphyrinoids.

Retro‐Diels–Alder Approach to the Synthesis of π‐Expanded Azuliporphyrins and Their Porphyrinoid Aromaticity

Bicyclo[2.2.2]octadiene (BCOD) fused azuliporphyrins were synthesized by 3+1 porphyrin synthesis of azulitripyrranes with diformylpyrroles. Subsequent retro-Diels-Alder reaction of the BCOD-fused azuliporphyrins afforded azulibenzo-, azulidibenzo-, and azulitribenzoporphyrins 1-5. NMR and UV/Vis spectra, as well as nucleus-independent chemical shift (NICS) calculations revealed that 1-5 and their diprotonated dications exhibit relatively low porphyrinoid aromaticity, which was dependent on the position and number of fused benzene rings present.

The origin of global and macrocyclic aromaticity in porphyrinoids

The global and macrocyclic aromaticity of porphyrinoids was characterized using our graph theory of aromaticity. The sequential line plots of topological resonance energy (TRE) against the number of π-electrons (N(π)) for different porphyrinoids are similar with four major extrema to those for five-membered heterocycles. This supports the view that five-membered rings are the main origin of global aromaticity in porphyrinoids. Macrocyclic circuits contribute significantly to macrocyclic π-circulation but modestly to global aromaticity. Macrocyclic aromaticity/antiaromaticity in oligopyrrolic macrocycles can be predicted by formally applying Hückel's [4n + 2] rule to an annulene-like main macrocyclic conjugation pathway (MMCP). This bridged annulene model can be justified by examining the contribution of individual macrocyclic circuits to macrocyclic aromaticity. A Hückel-like rule of macrocyclic aromaticity was found for porphyrinoid species.

ChemInform Abstract: Figure Eights, Moebius Bands, and More: Conformation and Aromaticity of Porphyrinoids

AbstractReview: 285 refs.

Figure Eights, Möbius Bands, and More: Conformation and Aromaticity of Porphyrinoids

The aromatic character of porphyrins, which has significant chemical and biological consequences, can be substantially altered by judicious modifications of the parent ring system. Expansion of the macrocycle, which is achieved by introducing additional subunits, usually increases the so-called free curvature of the ring, leading to larger angular strain. This strain is reduced by a variety of conformational changes, most notably by subunit inversion and p surface twisting. The latter effect creates a particularly convenient access to Möbius aromatic molecules, whose properties, predicted over 40 years ago, are of considerable theoretical importance. The conformational processes occurring in porphyrin analogues are often coupled to other chemical phenomena, and can thus be exploited as a means of constructing functional molecular devices. In this Review, the structural chemistry of porphyrinoids is discussed in the context of their conformational dynamics and p-electron conjugation.

π Conjugation Through a Ferrocenothia-porphyrin Macrocycle

The authors reveal a novel facet of porphyrinoid aromaticity, namely the possibility of including a d-electron metallocene subunit in the π-conjugation pathway.

Synthesis and Characterization of Tetraphenyl-21,23-dideazaporphyrin: The Best Evidence Yet That Porphyrins Really Are the [18]Annulenes of Nature

McMurry coupling of a pyrrole bisacrylaldehyde afforded a dipyrrolic macrocycle that is structurally midway between 1,10-diaza[18]annulene and the porphyrins. The diprotonated dication of this system retains porphyrin-like properties and provides insights into the nature of porphyrinoid aromaticity.