Siamese-Twin Porphyrin Origami: Oxidative Fusing and Folding

Synthesis of Spirocyclopropane-Containing 4H-Pyrazolo[1,5-a]indoles via Alkylative Dearomatization and Intramolecular N-Imination of an Indole-O-(Methylsulfonyl)oxime

In this paper, we report the synthesis of spirocyclopropane-containing 4H-pyrazolo[1,5-a]indoles 6a-e via alkylative dearomatization and intramolecular N-imination of indole-O-(methylsulfonyl)oxime 11. Starting materials tryptophol (7) and 2-bromocyclopetanone (8) were reacted in the presence of HBF4·OEt2, providing 1,2,3,5,6,11-hexahydrocyclopenta[2,3]oxepino[4,5-b]indole (9) in a 63% yield. Compound 9 was reacted with hydroxylamine hydrochloride to afford oxime 10 (65% yield), which was subsequently bis-methanesulfonated to form 11 in a 85% yield. Heating 11 with various alcohols in the presence of N,N-diisopropylethylamine (DIPEA) triggered the alkylative dearomatization and intramolecular N-imination, forming the spirocyclopropane and 4H-pyrazolo[1,5-a]indole structures in the targets 6a-e with 67-84% yields.

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.

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.

Synthesis of Imidazole-Based [30]Heptaphyrin and Stable Figure-Eight [60]Tetradecaphyrins via [5 + 2] Condensations in One Pot

Derived from a Pd-catalyzed oxidative C-H/C-H coupling reaction, two giant imidazole-based [60]tetradecaphyrins adopting stable figure-eight geometry together with one [30]heptaphyrin are obtained by [5 + 2] MacDonald condensations in one pot. The directional imidazole is believed to play a vital role for the diverse cyclization and conformation stabilization.

Siamese-Twin Porphyrin Goes Platinum: Group 10 Monometallic, Homobimetallic, and Heterobimetallic Complexes

A series of Pt^II-based monometallic (H₂PtL), homobimetallic (Pt₂L), and heterobimetallic (NiPtL and PdPtL) group 10 complexes of the previously established expanded twin porphyrin (H₄L) were prepared. Structural characterization of the bimetallic Pt^II series (Pt₂L, NiPtL, and PdPtL) revealed their similar general structures, with slight differences correlated to the ion size. An improvement of the metal-ion insertion process also allowed efficient preparation of the known Pd₂L complex, and the novel heterobimetallic NiPdL complex was also structurally characterized. UV-vis spectroscopy, NMR spectroscopy, magnetic circular dichroism (MCD), and (spectro)electrochemistry were used to characterize the complexes; the electronic properties followed largely established lines for metal complexes of the twin porphyrin, except that the Pt^II-based systems exhibited more complex UV-vis spectral signatures. MCD spectra accompanied by density functional theory (DFT)/time-dependent DFT computations (TDDFT) rationalize the origins of the optical features of the twin porphyrin. The presence of the nonplanar, nonaromatic macrocyclic π system with conjugation pathways confined to each half of the molecule could be visualized. Significant pyrazole(π) → pyrrole(π*) charge-transfer character was predicted for several transitions in the visible region. This study adds to our fundamental understanding of the formation, structure, and electronic structure of bimetallic complexes of this class of expanded metalloporphyrins containing nonpyrrolic moieties.

Revisiting the intense NIR active bronzaphyrin, a 26-π aromatic expanded porphyrin: synthesis and structural analysis

Dithiabronzaphyrin, with intense absorption and fluorescence in the NIR region, was synthesized as its β-octaethyl analogue via a thiophene bridged terpyrrole. Solid state structural characterization obtained for the first time revealed inversion of the thiophene rings. Studies showed that there is no effect from protonation or temperature on the structural rigidity of the macrocycle. Crystal packing revealed four open dimeric trifluoroacetate [(CF3COO)2H]- moieties binding to the macrocycle in the protonated form via H bonding.

Reaching across the Divide: How Monometalation of One Binding Pocket Affects the Empty Binding Pocket in a Siamese-Twin Porphyrin Palladium Complex

Siamese-twin porphyrin is a pyrazole-containing expanded porphyrin incorporating two porphyrin-like binding pockets. The macrocycle, however, does not possess an aromatic π system but rather two separated conjugation pathways that are isolated by the pyrazole junctions. Mono- and bimetallic complexes of the Siamese-twin porphyrin are known. This work addresses in detail the electronic consequences that monometalation (with Pd^II) has on the electronic properties of the nonmetalated binding pocket by studying the solid-state structure, acid/base, and electrochemical properties of the monopalladium twin-porphyrin complex. Specifically, metalation leads to a switch of the protonation sites of the free-base pocket. The unusual location of the protons at adjacent pyrrolic nitrogen atoms was revealed using X-ray diffraction and 1D/2D NMR spectroscopy. The one-electron oxidation and reduction events are both ligand-centered, as derived by spectroelectrochemical and electron paramagnetic resonance measurements, but are located on different halves of the molecule. Single-electron oxidation (-0.32 V vs Fc/Fc⁺) generated an organic radical centered on the metal-coordinating side of the ligand, while single-electron reduction (-1.59 V vs Fc/Fc⁺) led to the formation of an organic radical on the free-base side of the macrocycle. Density functional theory calculations corroborated the redox chemistry observed. The possibility of selectively preparing the monometallic complexes carrying two distinct redox sites-a metal-containing oxidation site and a metal-free reduction site-further expands the potential of Siamese-twin porphyrins to serve as an adjustable platform for multielectron redox processes in chemical catalysis or molecular electronics applications.

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.

Flexible Porphyrinoids

This review underscores the conformational flexibility of porphyrinoids, a unique class of functional molecules, starting from the smallest triphyrins(1.1.1) via [18]porphyrins(1.1.1.1) and concluding with a variety of expanded porphyrinoids and heteroporphyrinoids, including the enormous [96]tetracosaphyrin(1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0). The specific flexibility of porphyrinoids has been documented as instrumental in the designing or redesigning of macrocyclic frames, particularly in the search for adjustable platforms for coordination or organometallic chemistry, anion binding, or mechanistic switches in molecular devices. A structural prearrangement to coordinate one or more metal ions has been outlined. The coverage of the topic focuses on representative examples of geometry or conformational rearrangements for each selected class of the numerous porphyrinoids accordingly categorized by the number of built-in carbo- or heterocycles.