Inverted porphyrins and expanded porphyrins: An overview

Porphyrins and metallopophyrins have attracted the attention of chemists for the past 100 years or more owing to their widespread involvement in biology. More recently, synthetic porphyrins and porphyrin-like macrocycles have attracted the attention of researchers due to their diverse applications as sensitizers for photodynamic therapy, MRI contrasting agents, and complexing agents for larger metal ions and also for their anion binding abilities. The number of π-electrons in the porphyrin ring can be increased either by increasing the numberof conjugated double bonds between the pyrrole rings or by increasing the number of heterocyclic rings. Thus, 22π sapphyrins, 26π rubyrins, 30π heptaphyrins, 34π octaphyrins and higher cyclic polypyrrole analogues containing 40π, 48π, 64π, 80π and 96π systems have recently been reported in the literature. These macrocycles show rich structural diversity where normal and different kinds of inverted structures have been identified. In this review, an attempt has been made to collect the literature of the inverted porphyrins and expanded porphyrins reported until December 2001. Since themeso aryl expanded porphyrins have tendency to form both inverted and non-inverted structures more emphasis has been given tomeso aryl expanded porphyrins.     

Gold-catalyzed Synthesis of Pyridine Containing Macrocycles, Related to Porphyrin

Porphyrins are very important substances used in a wide range of model systems in many areas, such as biomimic chemistry and material science. This kind of macrocycle generally consists of 5-membered ring to form a cyclic extended aromatic network. Recently much work have been done concerning the modification of porphyrins core structures, and many kinds of analogues have been recorded. One of the important aspects was the research of the porphyrinogen ligand.These macrocycles bear functional resemblance to certain kind of polydentate ligand, and provide a variety of tri-dimensional binding cavities for metal ions. In addition, some other kinds of analogues have been recorded, involving the introduction of a CH unit to replace one of the nitrogen atoms. We also notice the fact that pyridine is a very effective ligand, and it can coordinate with many kinds of metals. Based on these observations, we would like to design and synthesize a new porphyrinogen (5) analogue containing a NNNN core, two of the N atoms from pyrrole and the other two N from pyridine. Such core modifications may alter the electronic structure of the ring and provide variable cavity for metal coordination.     

Heterocorrole Conformations: Little Saddling,Much Ruffling

10‐Heterocorrole complexes with oxygen, sulfur, and selenium at position 10 of the macrocycle and with the divalent ions of nickel, copper, and palladium were prepared and investigated. The focus was set on the size adaptation and matching mechanisms of cavity size versus ionic radius in corrole‐type macrocycles. A full set of single‐crystal X‐ray analytical data revealed that in all but one case the N₄ binding site of the ring‐contracted tetrapyrrole was larger than necessary to bind the metal ion without deformation. In‐plane size adaptation through M−N bond‐length elongation by 2.5–3.2 % was effective, as well as pronounced out‐of‐plane ruffling of the macrocycle for those compounds with a more severe size mismatch. Such ruffling had been excluded for corroles previously, but is apparently the most efficient mechanism to adapt to small central ions.     

Heterocorrole Conformations: Little Saddling,Much Ruffling

10‐Heterocorrole complexes with oxygen, sulfur, and selenium at position 10 of the macrocycle and with the divalent ions of nickel, copper, and palladium were prepared and investigated. The focus was set on the size adaptation and matching mechanisms of cavity size versus ionic radius in corrole‐type macrocycles. A full set of single‐crystal X‐ray analytical data revealed that in all but one case the N₄ binding site of the ring‐contracted tetrapyrrole was larger than necessary to bind the metal ion without deformation. In‐plane size adaptation through M−N bond‐length elongation by 2.5–3.2 % was effective, as well as pronounced out‐of‐plane ruffling of the macrocycle for those compounds with a more severe size mismatch. Such ruffling had been excluded for corroles previously, but is apparently the most efficient mechanism to adapt to small central ions.     

Comparison between the electrocatalytic properties of different metal ion phthalocyanines and porphyrins towards the oxidation of hydroxide

This work reports on the electrocatalytic oxidation of hydroxide using different central metal ion phthalocyanines and porphyrins immobilized on gold electrodes. The apparent electrocatalytic activity of cobalt phthalocyanine or porphyrin modified electrodes was found to be the greatest among the present series of metal ion macrocycles investigated. Copper and unmetallated phthalocyanine or porphyrin modified electrodes show no electrocatalytic behaviour towards hydroxide, such as bare gold. A possible mechanism for the enhanced reactivity of cobalt ion macrocycles towards the oxygen evolution is given. It is also stated that the electrocatalytic activity towards an adsorbate involves several aspects, such as the coordination state of the central metal ion, the nature of the ligand, the stability of the complexes, the number of d electrons, the energy of orbitals and the strength of the bonding between the central metal ion and the axial ligand.     

Single molecule visualization of coordination-assembled porphyrin macrocycles reinforced with covalent linkings

Coordination-assembled porphyrin macrocycles reinforced with covalent bondings were deposited on a metal surface by a pulse injection method, and their scanning tunneling microscopy (STM) images were recorded under ultrahigh vacuum conditions at liquid nitrogen temperature. The decamer ring consisting of 30 porphyrins gave clear circular STM images with hollow structure, whereas that without covalent linking did not give clear circular images, showing that covalent linking of the coordination pairs by ring-closing metathesis reaction was effective to reinforce the supramolecular structure on a metal surface. This strategy will be applicable to a variety of supramolecular assemblies.     

Coordination assembled rings of ferrocene-bridged trisporphyrin with flexible hinge-like motion: selective dimer ring formation, its transformation to larger rings, and vice versa

Ferrocene-bridged trisporphyrin (2) was synthesized by two-steps condensation of corresponding aldehydes and dipyrromethanes, and its self-assembling behavior based on the complementary coordination motif of imidazolylporphyrinatozinc(II) was investigated in conjunction with hinge-like flexibility given by freely rotating cyclopentadienyl rings of ferrocene connector. Ferrocene-bridged trisporphyrin (2) spontaneously and exclusively generated the dimeric ring (7) upon simple zinc(II) insertion, indicating that the freely rotating hinge connector favored the smallest ring formation. Taking advantage of the unique hinge-like flexibility of ferrocene, we attempted to transform the dimer ring into a mixture of porphyrin macrocycles by reorganizing the structure cleaved once by pyridine. A series of porphyrin macrocycles from trimer to decamer can be separated into its components by preparative gel permeation chromatograms. Macrocycles obtained are kept stable in the absence of coordinating solvents. On the other hand, they were easily transformed to the dimer ring in the presence of coordinating solvents such as methanol, showing that the transformation is completely reversible and can be controlled by the choice of the solvent system. A series of porphyrin macrocycles was confirmed via covalent linking of each complementary coordination dimer pair by metathesis reaction in the presence of Grubbs's catalyst. The coordination behavior of the bidentate ligands with different spacer lengths toward the dimer ring revealed that only the bidentate ligand (15) with a spacer length that matched the facing central porphyrins was selectively accommodated inside the ring. Coordination assembled flexible rings with tunable cavities and multiple coordination sites will be used as versatile hosts for a wide variety of guest molecules.     

Double metal-ligand exchange in solvate complex-metal porphyrin systems

The kinetics and mechanism of one of the most complicated chemical reactions of coordination compounds, where two different porphyrin complexes exchange central metal ions and the corresponding ligands, were considered using a number of metal porphyrins as substrates.     

Syntheses and reactions of porphyrin and metalloporphyrin polymers

Various porphyrin functions such as protoporphyrin IX and chlorin a as well as metalloporphyrin functions such as Mg(II)– and Cu(II)–chlorophyllin a and Fe(III)– and Co(II)–protoporphyrin IX were incorporated into vinyl polymers by preparation and polymerization of their p-vinylbenzyl esters. The porphyrin function was also incorporated by reaction of poly-p-chloromethylstyrene with porphyrins or metalloporphyrins or by the reaction of p-aminostyrene polymers with chlorophyll b through Schiff-base formation. Mg(II)–porphyrin polymers were found to be remarkably effective as catalysts in photoredox systems; porphyrin polymers without central metal atoms were also effective to a lesser extent.     

Role of Mg2+ and Ca2+ in DNA bending: evidence from an ONIOM-based QM-MM study of a DNA fragment

The binding of hydrated Mg2+ and Ca2+ ions with a DNA fragment containing two phosphate groups, three sugar units, and a G.C base pair is modeled in the anion and dianion states using a three-layer ONIOM approach. A monodentate binding mode was the most stable structure observed for both the ions in the anion model. However, the interactions of Mg2+ and Ca2+ with the dianion model of the DNA fragment gave rise to a large structural deformation at the base pair region, leading to the formation of "ring" structures. In both anion and dianion models, Mg2+-bound structures were considerably more stable than the corresponding Ca2+-bound structures. This feature and the formation of ring structures in the dianion models strongly supported the higher coordination power of the Mg2+ toward DNA systems for its compaction. The charge of the DNA fragment appeared to be crucial in deciding the binding strength as well as the binding mechanism of the metal ions. To the best of our knowledge, this is the first theoretical investigation of the interaction of a comparatively larger DNA model system with the biologically important Mg2+ and Ca2+ ions.     

Cationic porphyrins are reversible proteasome inhibitors

The aim of this study is to verify if water-soluble porphyrins can be used as proteasome inhibitors. We have found that cationic porphyrins inhibit proteasome peptidase activities much more effectively than the corresponding anionic derivatives. The relevance of electrostatics in driving porphyin-proteasome interactions has been confirmed by the observation that the inhibitory efficiency of the cationic macrocycles decreases with the number of positive substituents. We have also investigated various metalloporphyrins, which differ due to the different propension of the central metal ion toward axial coordination. Our experimental results indicate that the naked cationic porphyrins are the most active in reversibly inhibiting the three main protease activities of the proteasome in the micromolar range. A spectroscopic characterization of porphyrin-proteasome interactions by UV-vis spectra parallels the results of inhibition assays: the higher the inhibitory effect the stronger the spectroscopic variations are. To interpret the action of porphyrins at a molecular level, we have performed calculations evidencing that cationic porphyrins may hinder the access to the canonical proteolytic site on the proteasome β5 subunit. In particular, an inspection of the top-scoring docking modes shows that the tetracationic porphyrin blocks the catalytic pocket, close to the N termini of the β5 proteasome subunit, more efficiently than its anionic counterpart. Proteasome inhibition activity of porphyrins unites their known anticancer properties making them suitable as a scaffold for the design of novel multitargeted molecules.     

Formation and stability of rare earth metalloporphyrin monolayer films on amino-terminated silanized quartz surfaces studied by ultraviolet-visible and X-ray photoelectron spectroscopy

We investigated the fabrication of self-assembled monolayers of 5,10,15,20-tetra-(p-chlorophenyl)-porphyrin metal hydroxyl compounds (MOH; M=Gd, Tb, Er, Lu) on amino-terminated silanized quartz surfaces using ultraviolet-visible (UV-vis) and X-ray photoelectron spectroscopy (XPS). The orientation of MOH molecules in the films, the kinetics of the adsorption of MOH from a chloroform solution on an amino-terminated quartz substrate, and the stability of the metalloporphyrin films under natural light, UV light, and acidic and basic conditions were studied by UV-vis spectroscopy. The results indicate that the central metal is crucial in the formation of self-assembled porphyrin films and that the stability of the MOH SAMs also depends on the central metal. Under natural and UV light irradiation conditions, the stability of the MOH SAMs depends on the strength of the M-N bonds between the central metal and pyrrole nitrogens or between the central metal and the axial ligand. In the acidic conditions, the UV-vis spectra of the MOH SAMs show decreases in the absorbance and blue shifts. These spectral changes lead us to suggest that the four M-N bonds between the central metal and the pyrrole nitrogens of the porphyrins are cleaved and the resulted porphyrins are protonated under acidic conditions. Thus, it is likely that the protonated porphyrins and MOH form pi-pi complexes with a parallel stacking of the macrocycles through the electrostatic attractive interaction in the SAMs, resulting in the blue shifts of the Soret bands of the SAMs.     

荧光光谱法研究卟啉与金属离子配位反应机理初探

本文用荧光光谱法初步研究了卟啉与金属离子配位反应机理和部分催化剂的催化机理.实验发现,在一定条件下,卟啉以一种与其主要存在形式不同的变形体H₂P^*存在,根据H₂P^*的存在和产生的条件,对卟啉与金属离子配位反应的一般条件作出了较为满意的阐述.     

Electrochemistry of [(TMpyP)M(II)]4+ (X-)4 (X- = Cl- or BPh4-) and [(TMpyP)M(III)Cl]4+ (Cl-)4 in N,N-dimethylformamide where M is one of 15 different metal ions

The electrochemistry of 16 different water-soluble porphyrins of the type [(TMpyP)M(II)]4+ (X-)4 or [(TMpyP)M(III)Cl]4+ (Cl-)4 is reported in nonaqueous media where TMpyP is the dianion of meso-tetrakis(N-methylpyridiniumyl)porphyrin and X- = Cl- or BPh4-. These studies were carried out to examine the effect of the metal ion and porphyrin counterion (X-) on the electrochemical properties of the TMpyP complexes with a special emphasis being given to the overall number of electrons added and the number of electrode processes upon reduction. All of the investigated compounds with electroinactive central metal ions undergo an overall addition of six electrons. This occurs for most compounds via three two-electron-transfer steps, but more than three processes are observed for porphyrins having metal ions with a low electronegativity (e.g., Cd(II)). The first reduction of each porphyrin having an M(II) ion or an electroinactive M(III) ion yields a porphyrin dianion which is characterized by an intense band located close to 800 nm, and this reversible reduction is followed by further reductions of the 1-methyl-4-pyridyl groups at more negative potentials. Four of the compounds with electroactive central metal ions, [(TMpyP)M(III)Cl]4+(Cl-)4 (M = Co, Fe, Mn, or Au), undergo an additional reversible M(III)/M(II) process prior to reactions involving the porphyrin pi-ring system and the 1-methyl-4-pyridyl substituents.     

Practical and efficient synthesis of various meso-functionalized porphyrins via simple ligand-free nickel-catalyzed C-O, C-N, and C-C cross-coupling reactions

Various meso-functionalized porphyrins were conveniently synthesized by direct reactions of meso-bromoporphyrins with oxygen-, nitrogen-, and carbon-based nucleophiles in moderate to high yields via practical, efficient, and ligand-free nickel-catalyzed C-O, C-N, and C-C bond-forming reactions. The central metal ions of the substrate porphyrin have much effect on the reactions. Introduction of Ni(II) as a central metal ion into the substrate porphyrin markedly accelerated the cross-coupling.     

Synthesis and Properties of 5-Chloro-8-hydroxyquinoline-Substituted Azacrown Ethers: A New Family of Highly Metal Ion-Selective Lariat Ethers

New 5-chloro-8-hydroxyquinoline (CHQ)-substituted aza-18-crown-6 (4), diaza-18-crown-6 (1), diaza-21-crown-7 (2), and diaza-24-crown-8 (3) ligands, where CHQ was attached through the 7-position, and aza-18-crown-6 (11) and diaza-18-crown-6 (10) macrocycles, where CHQ was attached through the 2-position, were prepared. Thermodynamic quantities for complexation of these CHQ-substituted macrocycles with alkali, alkaline earth, and transition metal ions were determined in absolute methanol at 25.0 degrees C by calorimetric titration. Two isomers, 1 and 10, which are different only in the attachment positions of the CHQ to the parent macroring, exhibit remarkable differences in their affinities toward the metal ions. Compound 1 forms very stable complexes with Mg(2+), Ca(2+), Cu(2+), and Ni(2+) (log K = 6.82, 5.31, 10.1, and 11.4, respectively), but not with the alkali metal ions. Ligand 10 displays strong complexation with K(+) and Ba(2+) (log K = 6.61 and 12.2, respectively) but not with Mg(2+) or Cu(2+). The new macrocycles and their complexes have been characterized by means of UV-visible and (1)H NMR spectra and X-ray crystallography. New peaks in the UV spectrum of the Mg(2+)-1 complex could allow an analytical determination of Mg(2+) in very dilute solutions in the presence of other alkali and alkaline earth metal cations. (1)H NMR spectral and X-ray crystallographic studies indicate that ligand 10 forms a cryptate-like structure when coordinated with K(+) and Ba(2+), which induces an efficient overlap of the two hydroxyquinoline rings. Such overlapping forms a pseudo second macroring that results in a significant increase in both complex stability and cation selectivity.     

Complexation of silver and co-recovered metals with novel aza-crown ether macrocycles by electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry (ESI-MS) is used to evaluate the metal binding selectivities of an array of novel caged macrocycles for silver, gold, copper, nickel, zinc, iron, lead, manganese and alkali metal ions. It is found that five of the new compounds display silver selectivity, and their relative affinities for various metals depend on the type, number, and arrangement of heteroatoms (N, O), the cavity size, and the presence of aromatic substituents. Alkali metal cation binding studies are used to evaluate the size-selectivities of the cavities of the macrocycles. Electronic structure calculation by B3LYP density function theory methods were used to model the metal complexes. The presence of nitrogen atoms in the macrocyclic ring is essential for silver selectivity over other transition metals and alkali metal ions, and the presence of aromatic groups also enhances silver avidity. Macrocycle 3, a triaza-18-crown-6 analog modified with two phenyl groups and a cage group, is capable of selective extraction of Ag+ from aqueous solutions in the presence of other transition metal ions and the most common alkali and alkaline earth metal ions.     

Determination of structure and properties of modified chlorophylls by using fast atom bombardment combined with tandem mass spectrometry

Tandem mass spectrometry (MS/MS) was used to investigate and compare the decompositions of radical cations (M^+.), radical anions (M^-.), [M + H]⁺ ions, and [M + Cat]⁺ ions (Cat = alkali metal ions) of chlorophylls. Included in this study are chlorophyll a, chlorophyll b, bacteriochlorophyll a, chlorophyll a allomers, and the corresponding pheophytins. Fast atom bombardment of chlorophyll a produces abundant M^+. ions, which decompose metastably and upon collisional activation to give fragment ions from losses of the phytyl chain and the β-keto group of ring V. In addition, previously unreported charge-remote fragmentations are useful for identification of branch points on the phytyl chain. Collisional activation of [M + Cat]⁺ ions produces fragment ions that are complementary to those of the M⁺ and are used to examine the intrinsic gas-phase reactivity of metal ions and chlorophylls. Peripheral metal ion attachment in chlorophyll a in the gas phase is suggested to be at C-9, and the β-keto ester group at C-10, of ring V. Examination of decompositions of chlorophyll dimers suggests that in the gas phase the interaction between monomers involves bonding of the Mg atom of one chlorophyll a molecule and the C-9 carbonyl oxygen of the other, which was also suggested for chlorophyll a dimers in solution.     

Syntheses and optical and electrochemical properties of porphyrin dimers linked by metal ions

The preparation, isolation, and characterization of several new peripherally functionalized monomeric porphyrins and metalloporphyrins and of porphyrin dimers are described. These dimers are obtained by linking with metal ions two monomeric porphyrins bearing at their periphery an enaminoketone chelate fully conjugated with the aromatic ring. Porphyrin dimers linked by metal ions display large interactions in the ground state as evidenced by their electronic spectra and their electrochemical behavior. Compared to the monomeric analogue, these dimers show absorption spectra with intensified red-shifted Q-bands and their first oxidation potentials are substantially lowered and split into two distinct redox steps.     

Origin of d‐π Interaction in Cobalt(II) Porphyrins under Synergistic Effects of Core Contraction and Axial Ligation: Implications for a Ligand Effect of Natural Distorted Tetrapyrrole

The reactivity of the metalloporphyrins was closely related to their ligand effect at axial position. The electronic properties of six model Co(II) porphyrins are investigated by spectral and electrochemical methods. Structural parameters of the Co(II) complexes are directly obtained from their crystal structures. We demonstrate that the unpaired 3d electron of low‐spin Co(II) ions in nonplanar Co(II) porphyrin complexes activated by core contraction of porphyrin macrocycles can be further activated by the axial ligation of imidazole. The activated electron can combine with a π orbital of the porphyrin ring to form a new d‐π orbital, which can induce the Q‐band of Co(II) porphyrins to visibly split. Addition of imidazole causes the Co(II)/Co(III) and Co(II)/Co(I) reactions to shift to more negative potential. Our results indicate that strong axial ligation and core contraction both play important roles in electron transfer in redox catalysis involving Co(II) complexes.