A Porphyrin with a C═C Unit at Its Center

Spectroscopic Manifestations of (Anti)Aromaticity in Oxidized and Reduced Porphyrin and Norcorrole

Aromaticity and antiaromaticity are foundational principes in organic chemistry, regularly invoked to explain stability, structure, and magnetic and electronic properties. There are ongoing challenges in assigning molecules as aromatic or antiaromatic using optical spectroscopy. Here we report spectroelectrochemical and computational analyses of porphyrin (18π neutral, aromatic) and norcorrole (16π neutral, antiaromatic), and their oxidized (16π porphyrin dication) and reduced (norcorrole 18π dianion) forms. Our results show that while the visible spectra are characteristic of (anti)aromaticity consistent with Hückel's rules, the IR spectra are much less informative, owing to the relative rigidity of norcorrole. The results have implications for the assignment of (anti)aromaticity in both ground-state and time-resolved spectra.

20π-Electron Antiaromatic Benziphthalocyanines with Absorption Reaching the Near-Infrared-II Region

Although second near-infrared (NIR-II, 1000-1500 nm) light has attracted considerable attention, especially for life sciences applications, the development of organic dyes with NIR-II absorption remains a formidable challenge. Herein we report the design, synthesis, and electronic properties of 20π-electron antiaromatic benziphthalocyanines (BPcs) that exhibit intense absorption bands in the NIR region. The strong, low-energy absorption of the antiaromatic BPcs is attributed to electric-dipole-allowed HOMO-LUMO transitions with narrow band gaps, enabled by the reduced structural symmetry of BPc compared with regular porphyrins and phthalocyanines. The combination of peripheral substituents and a central metal decreases the HOMO-LUMO energy gaps, leading to the extension of the absorption bands into the NIR-II region (reaching 1100 nm) under reductive conditions.

Cyclo[2]pyridine[8]pyrrole: An Expanded Porphyrinoid with Three Closed-Shell Oxidation States

We report here an expanded porphyrinoid, cyclo[2]pyridine[8]pyrrole, 1, that can exist at three closed-shell oxidation levels. Macrocycle 1 was synthesized via the oxidative coupling of two open chain precursors and fully characterized by means of NMR and UV-vis spectroscopies, MS, and X-ray crystallography. Reduction of the fully oxidized form (1, blue) with NaBH4 produced either the half-oxidized (2, teal) or fully reduced forms (3, pale yellow), depending on the amount of reducing agent used and the presence or absence of air. Reduced products 2 or 3 can be oxidized to 1 by various oxidants (quinones, FeCl3, and AgPF6). Macrocycle 1 also undergoes proton-coupled reductions with I-, Br-, Cl-, SO32-, or S2O32- in the presence of an acid. Certain thiol-containing compounds likewise reduce 1 to 2 or 3. This conversion is accompanied by a readily discernible color change, making cyclo[2]pyridine[8]pyrrole 1 able to differentiate biothiols, such as cysteine (Cys), homocysteine (Hcy), and glutathione (GSH).

Easily Switchable 18π-, 19π-, and 20π-Conjugation of Diazaporphyrin Double-Pincer Bispalladium Complexes

Ni^II 3,7,13,17-tetrapyridyl-5,15-diazaporphyrin serves as a double tridentate ligand to Pd^II ions to provide a pincer-type bispalladium complex. Electrochemical analysis revealed that the bispalladium complex shows excellent ability to accept electrons and reversible redox properties due to the coordination of the two cationic Pd^II centers to the meso-nitrogen atoms. We isolated and characterized one- and two-electron reduction species of the bispalladium complex. The 20π antiaromatic nature of the two-electron reduction species was confirmed by ¹ H NMR spectroscopy, UV/Vis-near-IR (NIR) absorption spectra, and density functional theory (DFT) calculations. X-ray diffraction revealed highly twisted structures for the bispalladium complexes regardless of the oxidation state.

A Switchable Near-Infrared-Absorbing Dye Based on Redox-Bistable Benzitetraazaporphyrin

Activatable near-infrared (NIR) dyes responsive to external stimuli are used in medical and other applications. Here, we describe the design and synthesis of bench-stable 18π- and 20π-electron benzitetraazaporphyrins (BzTAPs) possessing redox-switchable NIR properties. X-Ray, NMR, and UV/Visible-NIR analyses revealed that 20π-electron BzTAP 1 exhibits NIR absorption and antiaromaticity with a paratropic ring-current, while 18π-electron BzTAP 2 shows weakly aromatic character with NIR inertness. Notably, the NIR-silent BzTAP 2 was readily converted to the NIR-active BzTAP 1 in the presence of mild reducing agents such as amine. The intense NIR absorption band of BzTAP 1 is in sharp contrast to the very weak absorption bands of previously reported antiaromatic porphyrinoids. Molecular orbital analysis revealed that symmetry-lowering perturbation of the 20π-electron porphyrinoid skeleton enables the HOMO-LUMO transition of 1 to be electric-dipole-allowed. BzTAPs are expected to be useful for constructing activatable NIR probes working in reductive environments.

Structure and bonding of proximity-enforced main-group dimers stabilized by a rigid naphthyridine diimine ligand

The development of ligands capable of effectively stabilizing highly reactive main-group species has led to the experimental realization of a variety of systems with fascinating properties. In this work, we computationally investigate the electronic, structural, energetic, and bonding features of proximity-enforced group 13-15 homodimers stabilized by a rigid expanded pincer ligand based on the 1,8-naphthyridine (napy) core. We show that the redox-active naphthyridine diimine (NDI) ligand enables a wide variety of structural motifs and element-element interaction modes, the latter ranging from isolated, element-centered lone pairs (e.g., E = Si, Ge) to cases where through-space π bonds (E = Pb), element-element multiple bonds (E = P, As) and biradical ground states (E = N) are observed. Our results hint at the feasibility of NDI-E₂ species as viable synthetic targets, highlighting the versatility and potential applications of napy-based ligands in main-group chemistry.

Synthesis and Properties of Stable 20π Porphyrinoids

The 20π porphyrinoids are immediate higher homologues of 18π porphyrins and differ from porphyrins in aromaticity which in turn affects the structure, properties and chemical reactivities. Research over the years indicated that the 20π porphyrinoids can be stabilized as non-aromatic/anti-aromatic or Mobius aromatic macrocycles using different strategies such as core-modification of porphyrins, non-metal/metal complexation of porphyrins, peripheral modification of porphyrins and expanded porphyrinoids. The structural properties such as aromaticity of the macrocycle can be controlled by choosing the right synthetic strategy. This review will provide an overview of the development in the chemistry of 20π porphyrinoids giving emphasize on the synthesis, structure and electronic properties of these macrocycles which have huge potential for various applications.

Synthesis and Properties of an 18π Aromatic Norcorrole P(V) Complex

[16]Norcorrole is an antiaromatic ring-contracted porphyrinoid. Here, we show that the phosphorus insertion of the free-base [16]norcorrole switches its antiaromaticity to aromaticity. The treatment of a free-base meso-dimesityl[16]norcorrole with phosphorus tribromide in pyridine afforded an [18]norcorrole P(V) complex, which exhibited porphyrin-like absorption and fluorescence spectra. The phosphorus center adopted a distorted tetragonal pyramidal coordination geometry. The distinct aromatic nature of the [18]norcorrole P(V) complex was corroborated both experimentally and theoretically.

Doubly Strapped Redox-Switchable 5,10,15,20-Tetraaryl-5,15-diazaporphyrinoids: Promising Platforms for the Evaluation of Paratropic and Diatropic Ring-Current Effects

This paper presents a novel series of chemically stable and redox-switchable 20π, 19π, and 18π 5,10,15,20-tetraaryl-5,15-diazaporphyrinoids (TADAPs) that have two alkyl-chain straps above and below the diazaporphyrin ring. Three types of doubly strapped TADAPs were prepared as nickel(II) complexes using meso-N-(2,6-dihydroxyphenyl)-substituted TADAP and the corresponding aliphatic diacids as precursors. Theoretical calculations revealed that regardless of their oxidation states, all strapped TADAPs had essentially flat π-planes. It was found that the alkyl-chain straps slightly affected the optical and electrochemical properties of the DAP rings, particularly in the oxidized forms. ¹H NMR spectroscopy was used to evaluate the antiaromatic character of the 20π TADAPs and the aromatic character of the 18π TADAP dications, and it was observed that they displayed paratropic and diatropic ring-current effects, respectively, on the chemical shifts of methylene protons in the spatially separated alkyl chains. The degree of shielding and deshielding depended on the position of the methylene units; it decreased with increase in separation from the π-plane and central axis of the porphyrin ring. The NMR experiments also revealed that the degree of the diatropic ring currents was clearly related to the π-electron density of the porphyrin ring; the ring-current effects decreased as the charge increased from 0 to +2. These findings are also qualitatively supported by the nucleus-independent chemical shifts.

Porphyrin nanoribbon-based spin filtering devices

Advancement in molecular electronics opens up another new domain with a new possibility of realizing its spin-polarized version, which is called molecular spintronics. This novel domain has a range of applications such as high-capacity storage devices and quantum computers. Several contemporary researchers have considered porphyrin molecules and their derivatives as potential candidates for molecular devices. Herein, using the first-principles calculations, we propose a porphyrin nanoribbon-based system for spin-filtering applications. Such a system shows robust half-metallicity and also exhibits itinerant magnetism. Our calculated spin transport properties exhibit that our device can give 100% spin-polarizing efficiency, which is very promising for next-generation spin-filtering applications.

Facile Redox-Induced Aromatic-Antiaromatic Interconversion of a β-Tetracyano-21,23-Dithiaporphyrin under Ambient Conditions

Facile redox-induced aromatic-antiaromatic interconversions were accomplished by using β-tetracyano-21,23-dithiaporphyrin (CN₄ S₂ Por). Introduced cyano groups not only increased the reduction potential of the porphyrin core but also stabilized the antiaromatic isophlorin (CN₄ S₂ Iph) by π conjugation. The reduction of CN₄ S₂ Por with hydrazine in polar solvents quantitatively affords CN₄ S₂ Iph, even under ambient conditions. CN₄ S₂ Iph retains a nearly planar conformation and exhibits considerable antiaromaticity. Aerobic oxidation of CN₄ S₂ Iph to CN₄ S₂ Por occurs in nonpolar solvents. This study was conducted to contribute to the understanding of the structure-antiaromaticity relationship.

Site-Selective N-Methylation of 5,15-Diazaporphyrins: Reactive Cationic Porphyrinoids that Provide Isoporphyrin Analogues

N-Alkylation significantly changes the electronic and optical properties, as well as the reactivity of nitrogen-containing π-conjugated molecules. In this study, it is found that treating 5,15-diazaporphyrins with methyl triflate selectively affords the corresponding N-methyl-5,15-diazaporphyrinium cations in good yield. N-Methylation substantially alters the electronic properties and reactivity of diazaporphyrins. The electron-accepting properties of the N-methyl-5,15-diazaporphyrinium cations are enhanced due to their lowered LUMO level. Stabilization of the LUMO energy enables regio- and stereoselective Diels-Alder reactions of the cationic diazaporphyrin with cyclopentadiene. N-Methylation also enhances the acidity of the inner NH protons, and thus, allows facile deprotonation to provide nitrogen-substituted isoporphyrin analogues with only one NH group in the central cavity.

Aromaticity and Antiaromaticity in the Excited States of Porphyrin Nanorings

Aromaticity can be a useful concept for predicting the behavior of excited states. Here we show that π-conjugated porphyrin nanorings exhibit size-dependent excited-state global aromaticity and antiaromaticity for rings containing up to eight porphyrin subunits, although they have no significant global aromaticity in their neutral singlet ground states. Applying Baird's rule, even rings ([4 n] π-electrons) are aromatic in their lowest excited states, whereas the lowest excited states of odd rings ([4 n + 2] π-electrons) are antiaromatic. These predictions are borne out by density functional theory (DFT) studies of the nucleus-independent chemical shift (NICS) in the T1 triplet state of each ring, which reveal the critical importance of the triplet delocalization to the emergence of excited-state aromaticity. The singlet excited states (S1) are explored by measurements of the radiative rate and fluorescence peak wavelength, revealing a subtle odd-even alternation as a function of ring size, consistent with symmetry breaking in antiaromatic excited states.

Skeletal Rearrangement of Twisted Thia-Norhexaphyrin: Multiply Annulated Polypyrrolic Aromatic Macrocycles

A hybrid thia-norhexaphyrin comprising a directly linked N-confused pyrrole and thiophene unit (1) revealed unique macrocycle transformations to afford multiply inner-annulated aromatic macrocycles. Oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone triggered a cleavage of the C-S bond of the thiophene unit, accompanied with skeletal rearrangement to afford unique π-conjugated products: a thiopyrrolo-pentaphyrin embedded with a pyrrolo[1,2]isothiazole (2), a sulfur-free pentaphyrin incorporating an indolizine moiety (3), and a thiopyranyltriphyrinoid containing a 2H-thiopyran unit (4). Furthermore, 2 underwent desulfurization reactions to afford a fused pentaphyrin containing a pyrrolizine moiety (5) under mild conditions. Using expanded porphyrin scaffolds, oxidative thiophene cleavage and desulfurization of the hitherto unknown N-confused core-modified macrocycles would be a practical approach for developing unique polypyrrolic aromatic macrocycles.

Main-Group-Element Isophlorin Complexes Revisited: The Question of a Subvalent Central Atom

A careful density functional theory reexamination of the geometric and electronic structures of reduced main-group porphyrin complexes E(Por)L₂ (E = Si or Ge; L = pyridine or tetrahydrofuran), B₂(Por), and C₂(Por) has confirmed these as pure isophlorin derivatives with normal-valent coordinated central atoms. Only axially unligated Ge(Por) and the dications [B₂(Por)]²⁺ and [C₂(Por)]²⁺ feature aromatic porphyrin ligands. The calculations faithfully reproduce the strong bond-length alternation along the outer rim of the macrocycle in the reduced complexes, consistent with antiaromatic character, as well as much stronger ruffling in the reduced group 14 complexes relative to their nonreduced counterparts such as E(Por)X₂ (E = Si or Ge; X = F or Cl). The latter is thought to reflect the lower barrier to nonplanar deformation for the antiaromatic systems. In addition, unlike B₂(Por) and its dication, which are planar, C₂(Por) and its dication are predicted to be strongly ruffled, reflecting the smaller size of the central C₂ unit. The calculations also predict characteristically low ionization potentials and singlet-triplet gaps for the antiaromatic complexes. A brief exploratory study of the as-yet-unknown group 15 complexes E(TPP)(Ph)(py), where E = P and As, also indicated an antiaromatic isophlorin macrocycle coordinated to a pentavalent group 15 center.

Theoretical Study on Open-Shell Singlet Character and Second Hyperpolarizabilities in Cofacial π-Stacked Dimers Composed of Weak Open-Shell Antiaromatic Porphyrins

From the analysis based on the broken-symmetry density functional theory (DFT) calculations, we in this study propose a strategy to enhance the open-shell characters and third-order nonlinear optical (NLO) properties of π-stacked dimers composed of antiaromatic molecules with weak open-shell characters. For this purpose, we here constructed cofacial π-stacked dimer models composed of aromatic and antiaromatic Ni^II porphyrins in order to examine the π-π stacking distance (R) dependence of the diradical characters (y) and static second hyperpolarizabilities (γ). The antiaromatic porphyrin dimers are found to have intermediate y around R∼3.3 Å, the result of which originates in the unique intermolecular interactions between the antiaromatic monomers. Static γ along the stacking direction of such antiaromatic porphyrin dimers with intermediate diradical characters are shown to be enhanced significantly as compared to those of the isolated monomers and the aromatic porphyrin dimers.

A 1,5-Naphthyridine-Fused Porphyrin Dimer: Intense NIR Absorption and Facile Redox Interconversion with Its Reduced Congener

Pt^II -catalyzed cyclization of β-to-β ethynylene-bridged meso-amino Ni^II porphyrin dimer 4 followed by oxidation with PbO₂ afforded 1,5-naphthyridine-fused porphyrin dimer 5 in good yield. This dimer possesses a redox-active 1,4-diazabutadiene linkage that is interconvertible with its reduced 1,2-diaminoethene linkage upon treatments with NaBH₄ or PbO₂ . The dimer 5 exhibits an intense NIR absorption and a narrow HOMO-LUMO gap with a remarkably low reduction potential mainly due to effective bonding interactions in the LUMO through the 1,4-diazabutadiene linkage. In contrast, the reduced dimer 7 is fairly electron-rich with high HOMO energy and shows a relatively large HOMO-LUMO gap compared to that of 5.

Spin coupling interactions in C[double bond, length as m-dash]C or B-B-cored porphyrin-mimetic graphene patch nitroxide diradicals

In view of the unique structures and promising applications of porphyrins and their derivatives, exploration of their various properties has continued to a hot topic. In this work, we combine porphyrin-mimetic graphene patches which are core-modified by a C[double bond, length as m-dash]C or a B-B unit and two nitroxide radical groups to construct a series of novel diradical molecules (the CC-cored or BB-cored molecules). The spin coupling constants (J) of diradicals were calculated at the (U)B3LYP/6-311G(d,p) level by considering the different linking modes of two nitroxide groups. The results indicate that different core modification considerably affects the J values of such diradicals, and the linking modes can tune the sizes and signs of J, changing their magnetic coupling interactions with different magnitudes and the signs of J from antiferromagnetic to ferromagnetic or vice versa. More interestingly and importantly, the spin coupling interactions of the CC-cored molecules can also be tuned by stretching the core unit C-C bond, suggesting the possibility of activating specific vibrational modes of the CC-cored diradicals by energy pulses to yield variable J coupling magnitudes. On the other hand, for the BB-cored molecules, two-electron reduction can switch or tune their magnetism from ferromagnetic to antiferromagnetic. The essence of all observations is further analyzed from the structural effects and orbital and spin density distributions. The findings about magnetic regulation in these core-modified porphyrin-mimetic graphene patch nitroxide diradicals further expand the field of molecular magnets and provide a rational theoretical basis for designing novel building blocks of magnetic functional molecular materials.

Formation and Isolation of a Four-Electron-Reduced Porphyrin Derivative by Reduction of a Stable 20π Isophlorin

The two-electron reduction of a diprotonated dodecaphenylporphyrin derivative by Na₂ S₂ O₄ gave a corresponding isophlorin (Iph) selectively. Formation of Iph was confirmed by spectroscopic measurements and the isolation of tetramethylated Iph. Further reduction of Iph proceeded to form an unprecedented four-electron-reduced porphyrin (IphH₂ ), which was fully characterized by spectroscopic and X-ray crystallographic analysis. IphH₂ , with a unique conformation, could be oxidized to reproduce the starting porphyrin, resulting in a proton-coupled four-electron reversible redox system.

Syntheses, Properties, and Catalytic Activities of Metal(II) Complexes and Free Bases of Redox-Switchable 20π, 19π, and 18π 5,10,15,20-Tetraaryl-5,15-diazaporphyrinoids

In spite of significant advances in redox-active porphyrin-based materials and catalysts, little attention has been paid to 20π and 19π porphyrins because of their instability in air. Here we report the meso-modification of 5,10,15,20-tetraarylporphyrin with two nitrogen atoms, which led to redox-switchable 20π, 19π, and 18π 5,10,15,20-tetraaryl-5,15-diazaporphyrinoids (TADAPs). Three kinds of metal(II) complexes and free bases of TADAP were prepared by the metal-templated annulation of the corresponding metal-bis(dipyrrin) complexes. The inductive and resonance effects of the meso-nitrogen atoms on the aromatic, optical, electrochemical, and magnetic properties of the entire TADAP π-systems were assessed by using various spectroscopic measurements and density functional theory calculations. The aromaticity and π-π* electronic transition energies of the TADAPs varied considerably, and were shown to be dependent on the oxidation states of the π-systems. In contrast to the isoelectronic 5,10,15,20-tetraarylporphyrin derivatives, the 20π and 19π TADAPs were chemically stable under air. In particular, the 19π TADAP radical cations were extremely stable towards dioxygen, moisture, and silica gel. This reflected the low-lying singly occupied molecular orbitals of their π-systems and the efficient delocalization of their unshared electron spin. The capability of MgTADAP to catalyze aerobic biaryl formation from aryl Grignard reagents was demonstrated, which presumably involved a 19π/20π redox cycle.

Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds: Synthetic Routes, Properties, and Applications

Two-dimensionally extended, polycyclic heteroaromatic molecules (heterocyclic nanographenes) are a highly versatile class of organic materials, applicable as functional chromophores and organic semiconductors. In this Review, we discuss the rich chemistry of large heteroaromatics, focusing on their synthesis, electronic properties, and applications in materials science. This Review summarizes the historical development and current state of the art in this rapidly expanding field of research, which has become one of the key exploration areas of modern heterocyclic chemistry.

Chemo- and Regioselective Reduction of 5,15-Diazaporphyrins Providing Antiaromatic Azaporphyrinoids

Reagent-controlled chemo- and regioselective reduction of 5,15-diazaporphyrins has been developed. The selective reduction of carbon-carbon double bonds of diazaporphyrins provides 18 π aromatic isobacteriochlorin-type products, whereas the reduction of carbon-nitrogen double bonds leads to selective formation of 20 π N,N'-dihydrodiazaporphyrins in excellent yields. The distinct antiaromatic character of N,N'-dihydrodiazaporphyrins has been revealed. The free-base N,N'-dihydrodiazaporphyrin exhibits slower inner NH tautomerism than that in the corresponding 18 π porphyrins.

TiC2: a new two-dimensional sheet beyond MXenes

MXenes are attracting attention due to their rich chemistry and intriguing properties. Here a new type of metal-carbon-based sheet composed of transition metal centers and C2 dimers rather than individual C atom is designed. Taking the Ti system as a test case, density functional theory calculations combined with a thermodynamic analysis uncover the thermal and dynamic stability of the sheet, as well as a metallic band structure, anisotropic Young's modulus and Poisson's ratio, a high heat capacity, and a large Debye stiffness. Moreover, the TiC2 sheet has an excellent Li storage capacity with a small migration barrier, a lower mass density compared with standard MXenes, and better chemical stability as compared to the MXene Ti2C sheet. When Ti is replaced with other transition metal centers, diverse new MC2 sheets containing C=C dimers can be formed, the properties of which merit further investigation.

NiII tetrahydronorcorroles: antiaromatic porphyrinoids with saturated pyrrole units

While hydrogenated porphyrins are abundant in natural and synthetic compounds, antiaromatic hydrogenated porphyrinoids have not been synthesized to date. Here, we report bacteriochlorin-like Ni^II tetrahydronorcorrole complexes as the first examples of antiaromatic porphyrinoids that contain saturated pyrrole units.

Evolution of neutral organic super-electron-donors and their applications

In recent times, metal-free chemistry has received significant attention due to its inherent qualities and its potential savings in the costs of (i) reagents and (ii) environmental treatments of residues. In this context, recently developed neutral organic electron-donors have shown an ability to perform challenging reductions that are traditionally the preserve of reactive metals and metal-based complexes, under mild reaction conditions. Hence, this feature article is aimed at describing the evolution of neutral organic super-electron-donors and their rapidly developing applications in electron-transfer reactions.

Core chemistry and skeletal rearrangements of porphyrinoids and metalloporphyrinoids

Porphyrin core alteration allows for the exploration of porphyrin-like or porphyrin-unlike coordination chemistry and provides an insight into reactions inside particularly shaped macrocyclic architecture including metal-mediated structural transformations.

Isoquinolino[4,3,2-de]phenanthridine: synthesis and its use in 1,3-dipolar cycloadditions to form nitrogen-containing polyaromatic hydrocarbons

The synthesis of the novel azomethine ylide, isoquinolino[4,3,2-de]phenanthridine, and its use in 1,3-dipolar cycloaddition with various alkenes and alkynes to form the corresponding fused pyrrolidines and pyrroles is reported.

Discovery and development of organic super-electron-donors

Based on simple ideas of electron-rich alkenes, exemplified by tetrakis(dimethylamino)ethene, TDAE, and on additional driving force associated with aromatization, families of very powerful neutral organic super-electron-donors (SEDs) have been developed. In the ground state, they carry out metal-free reductions of a range of functional groups. Iodoarenes are reduced either to aryl radicals or, with stronger donors, to aryl anions. Reduction to aryl radicals allows the initiation of very efficient transition-metal-free coupling of haloarenes to arenes. The donors also reduce alkyl halides, arenesulfonamides, triflates, and triflamdes, Weinreb amides, and acyloin derivatives. Under photoactivation at 365 nm, they are even more powerful and reductively cleave aryl chlorides. They reduce unactivated benzenes to the corresponding radical anions and display original selectivities in preferentially reducing benzenes over malonates or cyanoacetates. Additionally, they reductively cleave ArC-X, ArX-C (X = N or O) and ArC-C bonds, provided that the two resulting fragments are somewhat stabilized.