Tetraaryldimethoxybenziporphyrins. At the Edge of Carbaporphyrinoid Aromaticity

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.

Synthesis, Spectroscopic Properties, and Metalation of 3-Alkoxybenziporphyrins

A series of 5-alkoxy-1,3-benzenedicarbaldehydes and related dimers were prepared in three steps from dimethyl 5-hydroxyisophthalate. Acid catalyzed condensation of the dialdehydes with a tripyrrane dicarboxylic acid, followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone, afforded good yields of 3-alkoxybenziporphyrins, although dimeric tetraaldehydes failed to give isolatable porphyrinoid products. Proton NMR spectroscopy gave no indication of an aromatic ring current, but addition of trifluoroacetic acid resulted in the formation of dications that exhibited weakly diatropic characteristics. Spectroscopic titration with TFA demonstrated that stepwise protonation took place, generating monocationic and dicationic species. 3-Alkoxybenziporphyrins reacted with nickel(II) or palladium(II) acetate to give the related nickel(II) or palladium(II) complexes. These stable organometallic derivatives showed increased diatropic properties that were most pronounced for the palladium(II) complexes. These unique porphyrinoids provide further insights into the properties of benziporphyrins.

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.

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.

Nonaromatic Organonickel(II) Phototheranostics

Earth-abundant metal-based theranostics, agents that integrate diagnostic and therapeutic functions within the same molecule, may hold the key to the development of low-cost personalized medicines. Here, we report a set of O-linked nonaromatic benzitripyrrin (C^N^N^N) macrocyclic organonickel(II) complexes, Ni-1-4, containing strong σ-donating M-C bonds. Complexes Ni-1-4 are characterized by a square-planar coordination geometry as inferred from the structural studies of Ni-1. They integrate photothermal therapy, photothermal imaging, and photoacoustic imaging (PAI) within one system. This makes them attractive as potential phototheranostics. Relative to traditional Ni(II) porphyrins, such as F₂₀TPP (tetrapentafluorophenylporphyrin), the lowest energy absorption of Ni-1 is shifted into the near infrared region, presumably as a consequence of Ni-C bonding. Ultrafast transient absorption spectroscopy combined with theoretical calculations revealed that, upon photoexcitation, a higher population of ligand-centered and ³MLCT states is seen in Ni-1 relative to NiTPBP (TPBP = 6,11,16,21-tetraphenylbenziporphyrin). Encapsulating Ni-1 in 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000] (DSPE-PEG₂₀₀₀) afforded nanoparticles, Ni-1@DSPE, displaying red-shifted absorption features, as well as good photothermal conversion efficiency (∼45%) in aqueous media. Proof-of-principle experiments involving thrombus treatment were carried out both in vitro and in vivo. It was found that Ni-1@DSPE in combination with 785 nm photo-irradiation for 3 min (0.3 W/cm²) proved successful in removing blood clots from a mouse thrombus model as monitored by photoacoustic imaging (PAI). The present work highlights the promise of organonickel(II) complexes as potential theranostics and the benefits that can accrue from manipulating the excited-state features of early transition-metal complexes via, for example, interrupting π-conjugation pathways.

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.

Synthesis, Characterization, and Spectroscopic Studies of Bis-(meso-4-methoxyphenyl)-Benziporphyrin and Its Pd-Metal Complex

Benziporphyrin systems are widely explored, yet alternative improved synthetic routes towards these systems are needed. Here, a fairly and efficient synthesis of the free base and its metal complex is well designed. Dimethoxybenzene dicarbinol intermediate was prepared in excellent yields by reacting 4-methoxyphenylmagnesium bromide with isophthaladehyde in diethyl ether. Reaction with equivalent pyrrole and pentafluorobenzaldehyde in the presence of trifluoroacetic acid (TFA), followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), provided good yields of bis-(meso-4-methoxyphenyl)-benziporphyrin. Metalation of the free base was performed using palladium(II) acetate salt in acetonitrile. All intermediates and the final products are fully characterized using NMR, HMRS, and UV-Vis spectroscopies and briefly discussed.

Synthesis, metalation and antiaromatic properties of 22-hydroxybenziporphyrins

Condensation of 5-alkyl-2-hydroxybenzene-1,3-dicarbaldehydes with a tripyrrane in the presence of trifluoroacetic acid (TFA), followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), gave two examples of 22-hydroxybenziporphyrins. Unlike a related meso-tetraphenyl-22-hydroxybenziporphyrin, these products were reasonably stable in solution. Dicationic species were obtained upon addition of TFA to CDCl3 solutions, and these exhibited weakly diatropic properties. However, the free base forms gave broad peaks for the macrocycle’s peripheral protons that fell into the range of 4.5–3.3 ppm, demonstrating that the system has paratropic characteristics that are consistent with the keto-tautomers. The porphyrin analogues gave broad, poorly resolved, UV-vis spectra of the type expected for nonaromatic or antiaromatic structures. Reaction of a hydroxybenziporphyrin with nickel(II), copper(II) or palladium(II) acetate gave the related coordination complexes. The nickel and palladium derivatives were structurally characterized by X-ray crystallography and exhibited buckled geometries with the phenolate units strongly pivoted away from the macrocyclic core.

Effects of Core Modification on Electronic Properties of para-Benziporphyrins

In contrast to the nonaromatic meta-benziporphyrins, the para-benziporphyrins possess aromatic character depending on the type of five-membered ring present in the macrocyclic core. The effects of changing the para-benziporphyrinic core from C₂N₃ to C₂NSN, C₂NSeN, and C₂NTeN by replacing the pyrrole with other five-membered heterocycles such as thiophene, selenophene, and tellurophene on aromatic properties of p-benziporphyrins are described here using spectral, electrochemical, X-ray, and density functional theory (DFT) studies. The missing core-modified p-benziporphyrins with C₂NSeN and C₂NTeN cores were synthesized by condensing 1 equiv of benzitripyrrane and 1,3-benzene-bis((4-phenyl)methanol with an appropriate diol such as 2,5-bis[(p-tolyl)hyroxymethyl]selenophene and 2,5-bis(hydroxymethyl)tellurophene under mild acid-catalyzed conditions at room temperature and characterized in detail by high-resolution mass spectrometry (HR-MS), one- & two-dimensional NMR, and X-ray crystallography of the one of the macrocycles, Selena p-benziporphyrin. The X-ray structure of Selena p-benziporphyrin revealed that the macrocycle was almost planar apart from the p-phenylene ring, which was deviated by 49.71° from the mean plane of the macrocycle defined by four meso carbons, unlike Selena m-benziporphyrin, which is relatively more distorted. NMR studies revealed that, as the core changes from C₂N₃ to C₂NSN, C₂NSeN, and C₂NTeN, the diatropic ring current decreases, indicating that the aromatic character also decreases in the same order. X-ray structure and DFT studies also revealed that the distortion in the macrocycle increases as the pyrrole ring of p-benziporphyrin was replaced with other heterocycles such as furan, thiophene, selenophene, and tellurophene and that the tellura p-benziporphyrin was the most distorted macrocycle among core-modified p-benziporphyrins. Absorption and electrochemical properties were in agreement with these observations. Our repeated attempts on metalation of these p-benziporphyrins resulted in the successful synthesis of a Pd(II) complex of tellura p-benziporphyrin. The Pd(II) complex was characterized by HR-MS and NMR techniques, and the structure was optimized by DFT. The studies indicated that the Pd(II) ion was bonded to one of the pyrrolic nitrogens, tellurophene, tellurium, and two chloride ions in distorted square-planar geometry.

Synthesis of Nonaromatic and Aromatic Dithia Benzisapphyrins

A series of [24π] dithia meta-benzisapphyrins and [22π] dithia para-benzisapphyrins were synthesized by 3 + 2 condensation of appropriate benzitripyrrane with bithiophene diol under mild acid catalyzed conditions. The dithia m-benzisapphyrins and dithia p-benzisapphyrins were thoroughly characterized by HR-MS, 1D and 2D NMR, absorption, and electrochemical techniques. Our studies showed that dithia m-benzisapphyrins are nonaromatic, whereas the dithia p-benzisapphyrins are aromatic in nature. Thus, we demonstrated here that the dithiabenzisapphyrins can be made aromatic by replacing the m-phenylene moiety of the benzisapphyrin macrocycle with a p-phenylene unit. Furthermore, the studies also indicated that the aromaticity of the dithia p-benzisapphyrins was relatively more compared to the reported heterosapphyrins. The structural and spectral characteristics including aromaticity of the m-benzisapphyrins and p-benzisapphyrins were also discussed with the help of DFT, NICS, and TD-DFT studies.

Synthesis of Tellurabenziporphyrin and Its Pd(II) Complex

An unprecedented tellurabenziporphyrin containing C, N, and Te donor atoms was synthesized by condensing benzitripyrrane and tellurophene diol under acid catalyzed conditions. The tellurabenziporphyrin readily forms a Pd(II) complex when treated with PdCl₂ in CHCl₃/CH₃CN. The crystal structures of tellurabenziporphyrin and its Pd(II) complex revealed that the benzene ring hinders the π-electron delocalization. An unusual five-membered ring formed inside the macrocycle due to the strong interaction between "Te" and "N" in the Pd(II) complex.

Late transition metal complexes of oxypyriporphyrin and the platinum(II) complex of oxybenziporphyrin

Even though oxypyriporphyrin, a pyridone-containing porphyrinoid system, has the equivalent coordination core to true porphyrins, its coordination chemistry has been little explored. In this study, the first examples of palladium(II), platinum(II) and silver(II) oxypyriporphyrins have been synthesized. Conventional conditions failed to result in the formation of a platinum(II) complex, but moderate yields were obtained when oxypyriporphyrin was reacted with platinum(II) chloride in refluxing mixtures of DMF and acetic acid containing potassium acetate. The palladium(II) and platinum(II) derivatives were characterized by X-ray crystallography. The structurally analogous carbaporphyrinoid system oxybenziporphyrin was also shown to react with platinum(II) chloride under the same conditions to give a platinum(II) hydroxybenziporphyrin complex. Unlike oxybenziporphyrin, this complex is only weakly diatropic. In the presence of base, the diatropic character was reasserted to afford an aromatic anionic species.

An alternative synthesis of benziporphyrins starting from isophthaloyl chloride

An alternative route to benziporphyrins has been developed. Reaction of an [Formula: see text]-unsubstituted pyrrole ethyl ester with isophthaloyl chloride in the presence of aluminum chloride afforded a diketone that underwent selective reduction with diborane to give a benzitripyrrane. Cleavage of the ethyl esters with sodium hydroxide in refluxing ethylene glycol, followed by acid catalyzed condensation with a pyrrole dialdehyde and oxidation with DDQ, generated the targeted benziporphyrin product. Spectrophotometric titration of the benziporphyrin with trifluoroacetic acid (TFA) demonstrated the sequential formation of mono- and dicationic species. At lower dilutions, the free base benziporphyrin showed unusually strong [Formula: see text] coupling between two adjacent methyl substituents, indicating that the connecting unit has substantial olefinic characteristics.

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.

Benziporphyrins, a unique platform for exploring the aromatic characteristics of porphyrinoid systems

Benziporphyrins and related systems exhibit a wide range of properties and may possess nonaromatic, strongly aromatic or even antiaromatic characteristics.

Oxatriphyrins(2.1.1) incorporating an ortho-phenylene motif

An understanding of fundamental aspects of archetypal organic structural motifs remains a key issue faced by the experimental and theoretical chemists. Two possible bonding modes for a disubstituted benzene ring, that is a meta and para, determines the π delocalization for oligomeric structures. When the less abundant ortho-substituted variant is introduced into a triphyrin(2.1.1) skeleton an aromatic molecule is obtained and the carbocyclic ring participates in the conjugation of the macrocycle. The two-electron reduction and introduction of boron(III) changes the aromatic character and results in an anti-aromatic structure which has been confirmed by single-crystal analysis and supported by theoretical calculations.

Relative stability of benziporphyrin and naphthiporphyrin tautomers and the emergence of macrocyclic diatropicity

The conformations and relative stabilities of a series of benziporphyrin and naphthiporphyrin tautomers were calculated and the diatropic properties of each of these species were assessed. The results were in good agreement with experimental observations and allow favorable delocalization pathways to be identified.

Origin of aromatic character in porphyrinoid systems

The aromatic nature of porphyrins is commonly attributed to the presence of an [18]annulene substructure. However, this viewpoint has been disputed. Drawing on a range of examples of carbaporphyrinoid systems from the author's own studies, the [18]annulene model is shown to be a self-consistent and insightful approach for considering the aromatic properties of these porphyrin analogs. Benziporphyrins provide a continuum of porphyrinoid structures that range from nonaromatic to highly aromatic species and the presence of 18π electron delocalization pathways provides an excellent explanation for the variations in their properties. The same type of analysis is applied to carbaporphyrins, tropiporphyrins, azuliporphyrins, N-confused porphyrins, pyrazoloporphyrins and dicarbaporphyrins. Nevertheless, these properties might also be explained by a 22π electron delocalization model proposed by Schleyer. The [18]annulene model gains further support from the properties of dideazaporphyrins which cannot take part in this type of 22π electron delocalization but nevertheless retain porphyrin-like aromatic properties. These results support the concept that porphyrins are bridged [18]annulenes.

The metal chemistry of the carbahemiporphyrazines

Over fifty years ago, Linstead and Elvidge described the syntheses of phthalocyanine-like macrocycles in which one or two of the interior metal-binding nitrogen atoms were replaced with aromatic C - H groups. We review here the coordination chemistry of two of these systems, dicarbahemiporphyrazine and benziphthalocyanine, and we emphasize the unusual reactivity displayed by these macrocycles and the varying extent to which the C - H groups are activated by metal-binding.