Advent and features of pyriporphyrinoids: an overview of a pyridine-based porphyrin analogue

Pyriporphyrinoids have recently attracted a significant proliferation of attention due to their versatile characters, which stem from structural motifs in which the pyridine moiety is involved. The evolution of pyriporphyrin chemistry revealed the subtle modifications of the macrocyclic core that tweak the electronic structure as compared to the parental macrocycle. The amendment of π-electronic organization inside the core manifests exceptional photophysical and coordination properties that cover a vast range of seemingly contradictory fields. In fundamental chemistry, the pyridine unit acts as a modulator of π-conjugated porphyrinoid systems, resulting in aromaticity swapping. From the applied chemistry perspective, these macrocycles are primarily utilized as (i) sensors, (ii) NIR absorbing photoacoustic dyes, (iii) electrochemical catalysts, (iv) singlet biradicaloid generation and (v) contributors to generate metal complexes with intriguing binding modes. Surprisingly, despite their prominence, pyriporphyrinoids are inadequately investigated, while pyridine unit-embedded calixphyrin, calixpyridinopyrrole and calixpyridine are barely reported. This review article illustrates the controlled formation of specific porphyrinic scaffolds with pyridine unit(s) and diverse functionalized heterocyclic and/or carbocyclic building block(s), and demonstrates a substantial influence on the macrocyclic properties.

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.

Pyrrolylmethylene Appended Corrorin: Peripheral Coordination and Transformation to Pyridyl Incorporated Hemiporphycene Analogue

A pyrrolylmethylene appended corrorin 1 was synthesized and coordinated with [Rh(CO)₂Cl]₂ to afford 1-Rh with unique Rh^I-η²-CC bonding in addition to the coordination of the dipyrrin-like unit and a carbonyl ligand. Further oxidation of 1 afforded 2, exhibiting a hydrocorrorinone core, and it can be further transformed into pyrrolo[3,2-c]pyridine incorporated hemiporphycene analogue 3 upon treatment with HOAc. The side chain modifies the reactivity of corrorin and effectively tunes the NIR absorption of the resulting porphyrinoids.

m-Benziporphyrin(1.1.0.0)s as a Rare Example of Ring-Contracted Carbaporphyrins with Metal-Coordination Ability: Distorted Coordination Structures and Small HOMO-LUMO Gaps

m-Benziporphyrin(1.1.0.0) and m-pyreniporphyrin(1.1.0.0) were prepared as ring-contracted carbaporphyrins. While m-Benziporphyrin(1.1.0.0) was unstable, m-pyreniporphyrin(1.1.0.0) was fairly stable. Both of their Pd^II complexes showed distorted coordination structures with extremely short Pd-C bonds. As compared with the reported m-benziporphyrin Pd^II complexes, these Pd^II complexes showed considerably small HOMO-LUMO gaps, despite their smaller molecular size. Pd^II metalation of the m-pyreniporphyrin(1.1.0.0) dimer gave the corresponding Pd^II complex, which showed similar distorted coordination and a smaller HOMO-LUMO gap. Finally, Pd^II metalation of a pyrene-sharing formal p-benziporphyrin(1.1.1.1) dimer gave a nonaromatic Pd^II dimer, which rearranged to an aromatic Pd^II complex upon treatment with alumina.

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.

Metal Complexes of Porphyrinoids Containing Nonpyrrolic Heterocycles

The replacement of one or more pyrrolic building block(s) of a porphyrin by a nonpyrrolic heterocycle leads to the formation of so-called pyrrole-modified porphyrins (PMPs), porphyrinoids of broad structural variability. The wide range of coordination environments (type, number, charge, and architecture of the donor atoms) that the pyrrole-modified frameworks provide to the central metal ions, the frequent presence of donor atoms at their periphery, and their often observed nonplanarity or conformational flexibility distinguish the complexes of the PMPs clearly from those of the traditional square-planar, dianionic, N₄-coordinating (hydro)porphyrins. Their different coordination properties suggest their utilization in areas beyond which regular metalloporphyrins are suitable. Following a general introduction to the synthetic methodologies available to generate pyrrole-modified porphyrins, their general structure, history, coordination chemistry, and optical properties, this Review highlights the chemical, electronic (optical), and structural differences of specific classes of metalloporphyrinoids containing nonpyrrolic heterocycles. The focus is on macrocycles with similar "tetrapyrrolic" architectures as porphyrins, thusly excluding the majority of expanded porphyrins. We highlight the relevance and application of these metal complexes in biological and technical fields as chemosensors, catalysts, photochemotherapeutics, or imaging agents. This Review provides an introduction to the field of metallo-PMPs as well as a comprehensive snapshot of the current state of the art of their synthesis, structures, and properties. It also aims to provide encouragement for the further study of these intriguing and structurally versatile metalloporphyrinoids.

PtCl2 mediated peripheral transformation of carbatriphyrin(3.1.1) into a meso-fused β-β' dimer and its monomer analogue

An unprecedented formation of a meso-fused β-β' carbaporphyrin dimer and its monomer with a keto group was described. These analogues were synthesized from carbatriphyrin(3.1.1.) by a metal assisted strategy using PtCl2 salt in a single step without any prefunctionalized precursors. Upon dimerization, the monomeric ligand with a dianionic core is transformed into a dimeric structure with unique trianionic cores.

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.

21-Carbaporphyrin: a cyclopentadiene moiety entrapped into a porphyrin scaffold

This minireview underscores the chemistry of 21-carbaporphyrins containing a plain cyclopentadiene moiety. Thus the cyclopentadiene incorporation afforded two 21-carbaporphyrin series represented by meso-tetraaryl-21-carbaporphyrin and [Formula: see text]-alkylated 21-carbaporphyrin with their properties evidently controlled by the nature of perimeter substitution. The synthetic strategy, physicochemical characterization and some insight in coordination properties of 21-carbaporphyrins have been illustrated. The formation of palladium(II), rhodium(III) and gold(III) meso-tetraaryl-21-carbaporphyrins via unprecedented intramolecular contractions of meta-benzi- or para-benziporphyrins has been also addressed.

Expanded Porphyrin Contraction: From [22]Triphyrin(6.6.0) to [22]Triphyrin(6.5.0)

An expanded triphyrin containing a bipyrrole moiety and annulene links, namely tetraphenyl-[22]triphyrin(6.5.0), 2, has been synthesized. The synthesis proceeded by a postsynthetic transformation of tetraphenyl-[22]triphyrin(6.6.0), 1, in a metal-free unexpected and unprecedented ring contraction during column chromatography on alumina. The observed transformation, located at the hydrocarbon chain linking the pyrrole units, formally corresponds to a subtraction of one carbon atom from an annulene circuit. In contrast to the flexible substrate 1, the product 2 is conformationally rigid, and capable of chloride anion binding in its protonated form.

Organocopper(III) Phenanthriporphyrin-Exocyclic Transformations

5,6-Dimethoxyphenanthriporphyrin 1 and 5,6-dioxophenanthriporphyrin 2 act as suitable organometallic ligands for copper(III), adopting trianionic [CCNN] coordination cores. Under oxidizing conditions, in the presence of methanol, copper(III) phenanthriporphyrin 1-Cu undergoes transformation to copper(III) phenanthriporphodimethene with methoxy substituents attached to two trans meso positions. Addition of acids to 1-Cu yields two isomeric copper(III) isophenanthriporphyrins protonated on one of the meso carbon atoms. Protonation of copper(III) 5,6-dioxophenanthriporphyrin 2-Cu yields the aromatic diprotonated complex 2-Cu-H₂²⁺. In the presence of HBF₄ 2-Cu undergoes borylation at the carbonyl oxygen atoms, forming an aromatic exocyclic boron(III) complex.

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.

Synthesis and applications of rhodium porphyrin complexes

A review on rhodium porphyrin chemistry, ranging from synthesis and properties to reactivity and application.

Synthesis and Properties of Carbaporphyrin and Carbachlorin Dimethyl Esters Derived from Cyclopentanedialdehydes

Norbornenes with two ester substituents were prepared by Diels-Alder cycloadditions of cyclopentadiene with dimethyl fumarate and dimethyl 1,1-ethylenedicarboxylate. Oxidation with potassium permanganate gave good yields of related diols that were oxidatively ring-opened to afford cyclopentane dialdehydes. MacDonald-type "3 + 1" condensations with a tripyrrane, followed by oxidation with DDQ in refluxing toluene, gave carbaporphyrin or carbachlorin products in good yields. The macrocyclic products were highly diatropic and produced porphyrin-like UV-vis spectra. The carbaporphyrin was converted into silver(III) and gold(III) organometallic derivatives. Reaction with methyl iodide in the presence of potassium carbonate gave mono- and dialkylation products, and treatment of the former with Ni(OAc)₂ or Pd(OAc)₂ afforded nickel(II) and palladium(II) complexes. The free base carbaporphyrin and carbachlorin, and the nickel and palladium complexes, were characterized by X-ray crystallography. The carbachlorin also reacted with silver(I) acetate to give a silver(III) derivative. Carbaporphyrins and carbachlorins underwent deuterium exchange at the meso-positions with deuteriated TFA, and this observation indicates that protonation is occurring at the bridging carbons. The new route to carbaporphyrins and carbachlorins has enabled detailed studies on the properties of these systems and provides the foundations for future investigations.

Ruthenium(II) and Ruthenium(III) Complexes of p-Benziporphyrin: Merging Equatorial and Axial Organometallic Coordination

A diamagnetic ruthenium(II) complex of 5,10,15,20-tetraphenyl-p-benziporphyrin [Ru^II(p-BzP)(CO)Cl] was obtained via the insertion of ruthenium into p-benziporphyrin using triruthenium(0) dodecacarbonyl [Ru₃(CO)₁₂] as the metal source. The procedure applying dichloro(cycloocta-1,5-diene)ruthenium(II) (polymer, [Ru(COD)Cl₂]_n) afforded the paramagnetic six-coordinate ruthenium(III) p-benziporphyrin [Ru^III(p-BzP)Cl₂]. As shown by X-ray crystallography, the p-phenylene ring in both complexes is sharply tilted out of the N₃ plane, as reflected by the respective N₃ (pyrrole)-C₆ (p-phenylene) dihedral angle [Ru^II(p-BzP)(CO)Cl, 52.5°; Ru^III(p-BzP)Cl₂, 53.7°]. p-Phenylene is bound to the ruthenium cation in an η² fashion, revealing the shortest ever Ru-C distance in the series of p-benziporphyrin complexes [Ru^II(p-BzP)(CO)Cl, 2.275(2) Å; Ru^III(p-BzP)Cl₂, 2.324(5) Å]. The reaction of Ru^II(p-BzP)(CO)Cl with ArMgCl or AlkMgCl results in the formation of diamagnetic six-coordinate ruthenium(II) p-benziporphyrin complexes containing the apically coordinated σ-alkyl or σ-aryl ligands, where the metal ion simultaneously coordinates to three carbon centers respectively accommodating η² (phenylene) and σ (aryl and alkyl) modes. Reactions of σ-aryl (alkyl) carbanions with paramagnetic Ru^III(p-BzP)Cl₂ have been followed by ¹H NMR spectroscopy. The procedure afforded the six-coordinate paramagnetic ruthenium(III) p-benziporphyrin [Ru^III(p-BzP)(Ph)Cl], which binds one σ-aryl ligand, as reflected by the characteristic ¹H NMR spectra spread within the +120 to -120 ppm range. Both paramagnetic complexes Ru^III(p-BzP)(Ph)Cl and Ru^III(p-BzP)(p-Tol)Cl are formed as a mixture of two stereoisomers differentiated by two nonequivalent locations of σ-aryl with respect to the puckered macrocyclic ring. The paramagnetic shifts of σ-aryls are indicative of π-spin delocalization patterns. Analysis of the contact shifts and parallel density functional theory calculations of the spin density distribution in Ru^III(p-BzP)Cl₂, Ru^III(p-BzP)(Ar)Cl, and Ru^III(p-BzP)(Alk)Cl reflect the features of the d_xy²(d_xzd_yz)³ electronic ground state.

Rhodium(i), rhodium(iii) and iridium(iii) carbaporphyrins

Treatment of a benzocarbaporphyrin with [Rh(CO)2Cl]2 in refluxing dichloromethane gave a rhodium(i) dicarbonyl complex, and further reaction in refluxing pyridine afforded an organometallic rhodium(iii) derivative. The carbaporphyrin also reacted with [Ir(COD)Cl]2 and pyridine in refluxing p-xylene to generate a related iridium(iii) compound. These novel metalated porphyrinoids retained strongly diatropic characteristics and were fully characterized by XRD.