In Pursuit of Novel Porphyrin Isomers. Aromatic Character and Relative Stability of Conjugated Tetrapyrroles with Two Neo-Confused Rings or with Mixed Neo-Confused and N-Confused Subunits

Aromatic Character and Relative Stability of Pyrazoloporphyrin Tautomers and Related Protonated Species: Insights into How Pyrazole Changes the Properties of Carbaporphyrinoid Systems

Pyrazoloporphyrins (PzPs), which are porphyrin analogues incorporating a pyrazole subunit, are examples of carbaporphyrin-type structures with a carbon atom within the macrocyclic cavity. DFT calculations were used to assess a series of 17 PzP tautomers, nine monoprotonated species and four related diprotonated PzP dications. The geometries of the structures were optimized using M06-2X/6-311++G(d,p), and the relative stabilities computed with the cc-PVTZ functional. Nucleus independent chemical shifts, both NICS(0) and NICS(1)_zz, were calculated, and the anisotropy of the induced current density (AICD) plots were generated for all of the species under investigation. The results for free base PzPs show that fully aromatic PzP tautomers are not significantly more stable than weakly aromatic cross-conjugated species. In addition, strongly aromatic structures with internal CH_2's are much less stable, a feature that is also seen for protonated PzPs. The degree of planarity for the individual macrocycles does not significantly correlate with the stability of these structures. The results allow significant aromatic conjugation pathways to be identified in many cases, and provide insights into the aromatic properties of this poorly studied system. These investigations also complement experimental results for PzPs and emphasize the need for further studies in this area.

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.

Creation from Confusion and Fusion in the Porphyrin World─The Last Three Decades of N-Confused Porphyrinoid Chemistry

Confusion is a novel concept of isomerism in porphyrin chemistry, delivering a steady stream of new chemistry since the discovery of N-confused porphyrin, a porphyrin mutant, in 1994. These days, the number of confused porphyrinoids is increasing, and confusion and associated fusion are found in various fields such as supramolecular chemistry, materials chemistry, biological chemistry, and catalysts. In this review, the birth and growth of confused porphyrinoids in the last three decades are described.

Telluracarbaporphyrins and a Related Palladium(II) Complex: Evidence for Hypervalent Interactions

The reaction of a carbatripyrrin with a tellurophene dicarbinol in the presence of BF₃·Et₂O, followed by oxidation with DDQ, afforded the first example of a telluracarbaporphyrin. Although this system exhibits strongly aromatic characteristics, it is prone to air oxidation, giving rise to a hydroxy derivative that was characterized by X-ray crystallography. The initial telluracarbaporphyrin reacted with palladium(II) acetate to give a stable organometallic complex, and X-ray crystallography showed that the palladium cation was coordinated to all four atoms in the CNTeN core. An oxacarbatripyrrin was also reacted with a tellurophene dialcohol to give an air-stable porphyrin analogue with a CNTeO core. Nonmetalated telluracarbaporphyrins showed relatively short Te-N separations that strongly implied the involvement of hypervalent tellurium interactions. Furthermore, despite the presence of a very large tellurium atom, the tellurophene subunit is not strongly pivoted away from the mean macrocyclic plane as would be expected in the absence of these interactions. The aromatic properties of heterocarbaporphyrins were assessed by proton NMR spectroscopy, NICS calculations, and AICD plots. In addition, the relative stability of hydroxytelluraporphyrins in comparison to their tellurophene oxide tautomers was investigated and the aromatic characteristics of these oxidized structures were evaluated.

Neo-Porphyrinoids: New Members of the Porphyrinoid Family

The four pyrrole rings and four meso carbons of tetrapyrrolic porphyrins can be arranged in different ways and the resulting porphyrin isomers exhibit very distinct electronic properties. The extensive research carried out on the porphyrins over the years has revealed that porphyrin can have several possible isomers and some of these have been identified and synthesized. Among the porphyrin isomers synthesized so far, porphycene and N-confused porphyrins have been investigated extensively whereas the other porphyrin isomers such as hemiporphycene, corrphycene and isoporphycene remain underdeveloped because of synthetic difficulties and their inherently unstable nature. Neoporphyrinoids are new members of the porphyrinoid family that were discovered serendipitously in 2011. Neoporphyrinoids are structural analogues of porphyrinoids with a confused pyrrole nitrogen linked to a meso carbon or the adjacent pyrrole carbon. Thus, neoporphyrinoids have an unusual structure in which pyrrole N is a part of a porphyrinoid framework and the lone pair of electrons on nitrogen participate in macrocyclic conjugation. It's been a decade since the discovery and different types of neoporphyrinoids, including regular, contracted and expanded neoporphyrinoids, have been synthesized by rational synthetic methodologies and their spectral, structural, aromatic and coordination properties have been studied. There is huge scope to develop different synthetic routes to produce new types of stable neoporphyrinoids to study their properties and potential applications. This article presents a brief overview of the synthesis, structure and properties of the neoporphyrinoids reported in this decade.

Origins of the Electronic Modulations of Bacterio- and Isobacteriodilactone Regioisomers

Advances in the utilization of porphyrinoids for photomedicine, catalysis, and artificial photosynthesis require a fundamental understanding of the relationships between their molecular connectivity and resulting electronic structures. Herein, we analyze how the replacement of two pyrrolic C_β═C_β bonds of a porphyrin by two lactone (O═C-O) moieties modulates the ground-state thermodynamic stability and electronic structure of the resulting five possible pyrrole-modified porphyrin isomers. We made these determinations based on density functional theory (DFT) and time-dependent DFT computations of the optical spectra of all regioisomers. We also analyzed the computed magnetically induced currents of their aromatic π-systems. All regioisomers adopt the tautomeric state that maximizes aromaticity, whether or not transannular steric strains are incurred. In all isomers, the O═C_β-O_β bonds were found to support a macrocycle diatropic ring current. We attributed this to the delocalization of nonbonding electrons from the ring oxa- and oxo-atoms into the macrocycle. As a consequence of this delocalization, the dilactone regioisomers are as-or even more-aromatic than their hydroporphyrin congeners. The electronic structures follow different trends for the bacteriochlorin- and isobacteriochlorin-type isomers. The presence of either oxo- or oxa-oxygens conjugated with the macrocyclic π-system was found to be the minimal structural requirement for the regioisomers to exhibit distinct electronic properties. Our computational methods and mechanistic insights provide a basis for the systematic exploration of the physicochemical properties of porphyrinoids as a function of the number, relative orientation, and degree of macrocycle-π-conjugation of β-substituents, in general, and for dilactone-based porphyrinic chromophores, in particular.

Aromatic Pathways in Porphycene Derivatives Based on Current-Density Calculations

Magnetically induced current densities have been calculated for porphycenes at the density functional theory level using gauge-including atomic orbitals to ensure gauge-origin independence and a fast basis-set convergence of the current densities. The current densities have been analyzed by using the gauge-including magnetically induced current (GIMIC) method. The porphycenes are aromatic, sustaining strong diatropic ring currents. The ring-current pathways have been determined by integrating the strength of the current density passing selected bonds. The calculations show that the ring current of the porphycenes splits into an outer and inner branch at the pyrrolic rings implying that the ring current involves all 26 π electrons of the porphycenes, which is similar to the ring current of porphyrins. The pyrrolic rings of the aromatic porphycenes do not sustain any significant local ring currents. Dihydroporphycene with four inner hydrogens is antiaromatic with weakly aromatic pyrrolic rings. The annelated benzoic rings in benzoporphycene sustain local paratropic ring currents, whereas the global ring current of dibenzoporphycene splits into an outer and inner branch at the benzoic rings. Comparison of calculated ¹H NMR shieldings with ring-current strengths shows that interactions between the inner hydrogen and the neighbor nitrogen is more significant for differences in the ¹H NMR shieldings than variations in global ring-current strengths. Calculated excitation energies show that the antiaromatic dihydroporphycene has a smaller optical gap than the aromatic porphycene, even though its HOMO-LUMO gap is larger.

Expanded, Contracted, and Isomeric Porphyrins: Theoretical Aspects

The use of cyclic polyene perimeter-model approaches, such as Gouterman's four-orbital model and Michl's perimeter model, to analyze trends in the electronic structures and optical properties of expanded, contracted, and isomeric porphyrins is described with an emphasis on the use of magnetic circular dichroism (MCD) spectroscopy to validate the results of TD-DFT calculations. Trends in the electronic structures and optical properties of isomeric porphyrins are examined by comparing the properties of porphycenes, corrphycenes, hemiporphycenes, isoporphycenes, N-confused and neoconfused porphyrins, and norroles, whereas those of ring-contracted porphyrins are examined by comparing the properties of subporphyrins, triphyrins, and vacataporphyrins. The ring-expanded compounds that are examined include cyclo[n]pyrroles, [22]pentaphyrins(1.1.1.1.1), sapphyrins, smaragdyrins, isosmaragdyrins, orangarins, ozaphyrins, [26]hexaphyrins(1.1.1.1.1.1), rubyrins, rosarins, amethyrins, isoamethyrins, bronzaphyrins, and doubly N-confused hexaphyrins.