Green metrics in mechanochemistry

The development of new green methodologies and their broader adoption for promoting sustainable development in chemistry laboratories and industry play a significant role in society, due to the economic importance of chemistry and its widespread presence in everyday life. Therefore, a sustainable approach to chemistry contributes to the well-being of the worldwide population and complies with the United Nations Sustainable Development Goals (UN SDGs) and the European Green Deal. The review highlights how batch and continuous mechanochemical methods are an eco-friendly approach for organic synthesis, with a lower environmental footprint in most cases, compared to solution-based procedures. The assessment is objectively based on the use of green metrics (e.g., atom and real atom economy, E-factor, process mass intensity, material parameter recovery, Eco-scale, stoichiometric factor, etc.) and indicators (e.g. DOZN tool and life cycle assessment, LCA, studies) applied to organic transformations such as synthesis of the amide bond, carbamates, heterocycles, active pharmaceutical ingredients (APIs), porphyrins, porous organic polymers (POPs), metal- or acid-catalysed processes, multicomponent and condensation reactions, rearrangements, etc. The generalized absence of bulk solvents, the precise control over the stoichiometry (i.e., using agents in a stoichiometrically rather than in excess), and the more selective reactions enabling simplified work-up procedures are the distinctive factors, marking the superiority of mechanochemical processes over solution-based chemistry.

Solvent-Driven Self-Organization of Meso-Substituted Porphyrins: Morphological Analysis from Fluorescence Lifetime Imaging Microscopy

A morphological analysis of different thin films of meso-tetra-p-(di-p-phenylamino)phenylporphyrin, H₂T(TPA)₄P, was made by fluorescence lifetime imaging microscopy (FLIM) and scanning electron microscopy (SEM). A comprehensive study of H₂T(TPA)₄P was undertaken through UV/vis absorption and fluorescence techniques in different solvents, solvent mixtures and in thin films. In solution, occurrence of intramolecular energy transfer from the triphenylamine (TPA) moieties to the porphyrin core, with quenching efficiencies in the order of 94-97%, is observed. The energy transfer rate constants are determined assuming Förster's dipole-dipole and Dexter's electron exchange mechanisms. In drop-cast-prepared thin films, from samples with different solvent mixtures, the photoluminescence (PL) quantum yield (Φ_PL) decreases ∼1 order of magnitude compared to the solution behavior. FLIM and SEM experiments showed the self-organization and morphology of H₂T(TPA)₄P in thin films to be highly dependent on the solvent mixture used to prepare the film. In chloroform, the solvent's evaporation results in the formation of elongated and overlapped microrod structures. Introduction of a cosolvent, namely, a polar cosolvent, promotes changes in the morphology of the self-assembled structures, with the formation of three-dimensional spherical structures and hollow spheres. H₂T(TPA)₄P dispersed in a polymer matrix shows enhanced Φ_PL values when compared to the drop-cast films. FLIM images showed coexistence of three different states or domains: aggregated, interface, and nonaggregated or less-aggregated states. This work highlights the importance of FLIM in the morphological characterization of heterogeneous films, together with the photophysical characterization of nano- and microdomains.

Increasing Reaction Rates of Water-Soluble Porphyrins for 64Cu Radiopharmaceutical Labeling

Searching for new compounds and synthetic routes for medical applications is a great challenge for modern chemistry. Porphyrins, natural macrocycles able to tightly bind metal ions, can serve as complexing and delivering agents in nuclear medicine diagnostic imaging utilizing radioactive nuclides of copper with particular emphasis on ⁶⁴Cu. This nuclide can, due to multiple decay modes, serve also as a therapeutic agent. As the complexation reaction of porphyrins suffers from relatively poor kinetics, the aim of this study was to optimize the reaction of copper ions with various water-soluble porphyrins in terms of time and chemical conditions, that would meet pharmaceutical requirements and to develop a method that can be applied for various water-soluble porphyrins. In the first method, reactions were conducted in a presence of a reducing agent (ascorbic acid). Optimal conditions, in which the reaction time was 1 min, comprised borate buffer at pH 9 with a 10-fold excess of ascorbic acid over Cu²⁺. The second approach involved a microwave-assisted synthesis at 140 °C for 1-2 min. The proposed method with ascorbic acid was applied for radiolabeling of porphyrin with ⁶⁴Cu. The complex was then subjected to a purification procedure and the final product was identified using high-performance liquid chromatography with radiometric detection.

Main Strategies for the Synthesis of meso-Arylporphyrins

meso-Arylporphyrins as most accessible tetrapyrrole macroheterocycles have always been the focus of attention from researchers concerned with practically useful properties of these compounds. The first syn­theses of meso-arylporphyrins date back to about 90 years ago. Up to now, the yields of these compounds have been improved from 5 to 80%. The present review analyzes different ways and strategies for the synthesis of meso-aryl-substituted porphyrins. The most efficient methods that can be scaled up to an industrial level have been identified.

Modern Methods for the Sustainable Synthesis of Metalloporphyrins

Metalloporphyrins are involved in many and diverse applications that require the preparation of these compounds in an efficient manner, which nowadays, also involves taking into consideration sustainability issues. In this context, we use ball milling mechanochemistry and sonochemistry for the rational development of synthetic strategies for the sustainable preparation of metalloporphyrins. Zinc, copper, cobalt and palladium complexes of hydrophobic porphyrins were obtained in high yields and under mechanical action with a moderate excess of the metal salt, without any solvent or additive. Sonochemistry prove to be a good alternative for the preparation of metal complexes of water-soluble porphyrins in good yields and short reaction times. Both strategies have good sustainability scores, close to the ideal values, which is useful in comparing and helping to choose the more adequate method.

Fast, facile, base-free microwave-assisted metallation of bacteriochlorophylls and corresponding high yield synthesis of TOOKAD

Naturally-derived metallo-bacteriochlorophylls have attracted much attention since their clinical approval for cancer photodynamic therapy. Their therapeutic properties are rooted in the metal complexation of bacteriochlorophylls, which endows them with optical properties favourable for biophotonic and biomedical applications, including near-infrared light-activated reactive oxygen species generation at therapeutic levels. Despite these advantages, the utility of these chromophores has been limited by synthetic challenges associated with bacteriochlorophyll metallation; specifically, a slow reaction rate and necessity of complex purification procedures remain barriers towards metalated bacteriochlorophyll synthesis. Here, these limitations are overcome through the development of a new fast, facile, efficient, base-free microwave heating metallation method for the synthesis of a series of metallo (Pd, Cu, Zn, Cd, Sn, In, Mn, Co) bacteriopyropheophorbides. The preparation and structural and optical spectral characterization of these complexes are presented. This microwave-enabled synthetic method is then applied to generate the clinical photosensitizer agent Pd-bacteriopheophorbide (TOOKAD) effectively and efficiently, followed by validation of its metallation-enhanced ROS generation.

Technology supporting green chemistry in chemical education

Changes in technology have affected the way we teach, the way students learn and the way chemical research is conducted. Rapid changes in technology have greatly improved laboratory instrumentation, data collection and treatment and have greatly enabled Green Chemistry. This chapter will trace the career of a 4-year college professor who began teaching as a high school teacher in the sixties and transcended to the collegiate level. She will describe how changes in technology changed the way we teach chemistry and how this has enabled us to introduce green chemistry at all levels to our students. This chapter will highlight changes in technology which have enabled educators both in teaching chemistry labs and conducting research to employ green chemistry.

meso-Thiophenium Porphyrins and Their Zn(II) Complexes: A New Category of Cationic Photosensitizers

A new category of cationic meso-thiophenium porphyrins are introduced as possible alternatives to the popular meso-pyridinium porphyrins. Combinations of cationic porphyrins bearing meso-2-methylthiophenium and meso-4-hydroxyphenyl moieties T₂(OH)₂M (A₂B₂ type) and T(OH)₃M (AB₃ type) along with their zinc(II) complexes T₂(OH)₂MZn and T(OH)₃MZn, are reported. The increase in the number of thienyl groups attached to the meso-positions of the porphyrin derivatives (A₂B₂ frame) has been shown to impart longer fluorescence lifetimes and stronger photocytotoxicity toward A549 lung cancer cells, as evident with T₂(OH)₂M and its corresponding diamagnetic metal complex T₂(OH)₂MZn. The photoactivated T₂(OH)₂MZn imparts an early stage reactive oxygen species (ROS) upregulation and antioxidant depletion in A549 cells and contributes to the strongest oxidative stress-induced cell death mechanism in the series. The DFT calculations of the singlet-triplet energy gap (ΔE) of all the four hydrophilic thiophenium porphyrin derivatives establish the potential applicability of these cationic photosensitizers as PDT agents.

Effect of the coordination of π-acceptor 4-cyanopyridine ligand on the structural and electronic properties ofmeso-tetra(para-methoxy) andmeso-tetra(para-chlorophenyl) porphyrin cobalt(ii) coordination compounds. Application in the catalytic degradation of methylene blue dye

To examine the influence of both the important π-acceptor character of the 4-cyanopyridine ligand and the nature of the para-substituted phenyls of meso-porphyrins on the electronic, electrochemical and structural properties of cobaltous metalloporphyrins, we prepared and fully characterized two coordination compounds: the (4-cyanopyridine)[meso-tetra(para-methoxyphenyl)porphyrinato]cobalt(ii) and the (4-cyanopyridine)[meso-tetra(para-chlorophenyl)porphyrinato]cobalt(ii) with the [Co^II(TMPP)(4-CNpy)] and [Co^II(TClPP)(4-CNpy)] formulas (complexes 1–2). The solution structures of compounds 1–2 were confirmed by ¹H NMR spectroscopy and mass spectrometry methods. They were further characterized by cyclic voltammetry and photoluminescence studies. The X-ray molecular structure data show that the Co-TClPP-4-NCpy derivative (2) exhibits high ruffling deformation compared to that of the Co-TMPP-4-CNpy species (1). Notably, the crystal packing of complex 1 shows the formation of Co⋯Co supramolecular dimers with a distance of 5.663 Å. As an application of our two cobaltous compounds, an investigation involving complexes 1–2 in the degradation of the methylene blue dye in the presence and absence of H₂O₂ in aqueous solutions was carried out. These promising results show that 1–2 can be used as catalysts in the degradation processes of dyes.

Preparation and UV/vis, IR, MS, ¹H NMR, cyclic voltammetry and molecular structures of two new Co(ii) complexes with para-methoxy-phenyl and para-chloro meso-porphyrins and 4-cyanopyridine ligand (1–2). Catalytic oxidation data of MB dye using 1–2.

Porphyrin synthesis using mechanochemistry: Sustainability assessment

Looking for sustainable synthetic methodologies, mechanochemistry as a new tool for one-step and two-step approaches for the synthesis of meso-substituted porphyrins was explored. The best results were obtained in a two-step procedure, under liquid-assisted grinding in the oxidation step using 2-methyltetrahydrofuran, an environmentally acceptable solvent, and MnO2 as a heterogeneous oxidant. The sustainability was assessed using two sustainability metrics, E-factor and EcoScale, which allow comparison between procedures and methods.

Synthesis and electronic properties of A3B-thienyl porphyrins: experimental and computational investigations

A facile synthesis ofmeso-thienyl porphyrins containing a phenyl substituent allows fine tuning of the frontier orbitals to suit applications in DSSC and photomedicine.

Structural and transport properties of neutral radical crystals of CoIII(tmp)(CN)2 (tmp = 5,10,15,20-tetramethylporphyrinato) and the CN-bridged polymer [CoIII(tmp)(CN)]n

An axially ligated Co(tmp) (tmp = 5,10,15,20-tetramethylporphyrinato) anion, [Co^III(tmp)(CN)₂]^-, has been prepared and subjected to electrochemical oxidation to obtain the open shell tmp π-ligand. With acetone as the solvent, solvent-free neutral radical crystals of Co^III(tmp)(CN)₂ are obtained, whereas solvent-inclusive crystals of Co^III(tmp)(CN)₂·2CH₃CN are obtained with CH₃CN as the solvent. In both crystals, the open shell tmp ring deforms into a ruffled form, which makes the π-π interactions in the crystal weak. Thus, the electrical conductivity is low, and the crystals behave as semiconductors with a room temperature resistivity of 10⁶ Ω cm and an activation energy of about 0.3 eV. When the solvent is acetone, non-oxidized crystals of the CN-bridged polymer {[Co^III(tmp)(CN)](acetone)}_n are obtained as a byproduct. The closed shell tmp ring deforms into a ruffled form, and there π-π interactions in the crystal are negligible. The room temperature resistivity is rather high at about 10⁸ Ω cm.

β-Formyl- and β-Vinylporphyrins: Magic Building Blocks for Novel Porphyrin Derivatives

Porphyrins bearing formyl or vinyl groups have been explored as starting materials to prepare new compounds with adequate features for different applications. In this review it is discussed mainly synthetic strategies based on the reaction of meso-tetraarylporphyrins bearing those groups at β-pyrrolic positions. The use of some of the obtained porphyrin derivatives for further transformations, namely via pericyclic reactions, is also highlighted.

Immobilized Lignin Peroxidase-Like Metalloporphyrins as Reusable Catalysts in Oxidative Bleaching of Industrial Dyes

Synthetic and bioinspired metalloporphyrins are a class of redox-active catalysts able to emulate several enzymes such as cytochromes P450, ligninolytic peroxidases, and peroxygenases. Their ability to perform oxidation and degradation of recalcitrant compounds, including aliphatic hydrocarbons, phenolic and non-phenolic aromatic compounds, sulfides, and nitroso-compounds, has been deeply investigated. Such a broad substrate specificity has suggested their use also in the bleaching of textile plant wastewaters. In fact, industrial dyes belong to very different chemical classes, being their effective and inexpensive oxidation an important challenge from both economic and environmental perspective. Accordingly, we review here the most widespread synthetic metalloporphyrins, and the most promising formulations for large-scale applications. In particular, we focus on the most convenient approaches for immobilization to conceive economical affordable processes. Then, the molecular routes of catalysis and the reported substrate specificity on the treatment of the most diffused textile dyes are encompassed, including the use of redox mediators and the comparison with the most common biological and enzymatic alternative, in order to depict an updated picture of a very promising field for large-scale applications.

Synthesis of meso-substituted porphyrins using sustainable chemical processes

The easy access and low price of pyrrole and aldehydes, conjugated with the simplicity in preparing meso-substituted porphyrins, makes this type of porphyrins very attractive for a broad range of applications. However, there is an increasing demand for the development of new synthetic processes involving more sustainable chemical principles substituting, whenever possible, dangerous organic solvents by alternative solvents, chromatographic purifications by precipitations and energy-intensive procedures by alternative energy sources such as microwaves and ultrasounds. In this review we will address some recent strategies to synthesize meso-substituted porphyrins using alternative energy sources, reaction media and catalysts, namely microwave irradiation, water as solvent, or solid microporous acid catalysts.

Microwave-assisted synthesis of fluorine substituted porphyrins and kinetics of formation of zinc porphyrin complexes in acetic acid

Three mono-fluorine substituted porphyrin compouds were synthesized under microwave irradiation. The effects of catalysts and oxidants on the yields of mono-fluorine substituted porphyrins also were investigated. The yields of 5,10,15,20-tetrakis(4-fluorophenyl)porphyrin, 5,10,15,20-tetrakis(3-fluorophenyl)porphyrin, 5,10,15,20-tetrakis(2-fluorophenyl)porphyrin in a use of nitrobenzene in propionic acid were 36%, 30% and 28%, respectively. In addition, the zinc(II) ion incorporation reactions into porphyrins to form the zinc porphyrin complexes have been kinetically investigated in acetic acid.

Characterization of porphyrin nanorods on fluorine doped tin oxide glass sheet

Porphyrin nanorods (PNR) have been fabricated by electrostatic self-assembly of two oppositely charged porphyrin molecules. The free base meso-tetra-(4-phenylsulphonate) porphyrin (TPPS[Formula: see text] served as negatively charged counterpart for the positively charged metallo meso-tetra(4-[Formula: see text]-methylpyridyl) porphyrins (MTM’PyP) with either Sn, Co, Mn or In as central metal M. Films of PNR were prepared on fluorine doped tin oxide glass sheets (FTO) by using a drop-dry method. The electronic spectra revealed J-aggregation of the charged molecules for the colloid PNR as well as for the films. Transmission electron microscopy confirmed the formation of porphyrin nanorods. The laser microscope and scanning electron microscope (SEM) images of the PNR/FTO films showed the formation of three kinds of structures in the films which consist of differently branched or linear needles with their main axis grown in the direction of the solvent flow during preparation. During cyclic voltammetry either applying negative potentials from 0.0 V to -1.0 V or positive potentials from 0.0 V to [Formula: see text]2.2 V irreversible reduction or oxidation reactions were detected for the films. Consistently, SEM images taken following cyclic voltammetry showed the disintegration of the PNR on the films into smaller subunits. Spectroelectrochemical measurements showed the formation of porphyrin anionic radicals during oxidation by a decrease in the absorption intensities and broadening of spectra with an additional band appearing around 900 nm. A similar trend was observed when negative potentials were applied but in this case the cationic radical was produced. In both cases the decrease of the intensity of the J-aggregate confirms a loss of intermolecular coupling, again consistent with the smaller subunits observed in SEM analysis.

Microwave-assisted synthesis of zinc 5-(4-carboxyphenyl)-10,15,20-triphenylporphyrin and zinc 5-(4-carboxyphenyl)-10,15,20-triphenylchlorin

The microwave-assisted synthesis of zinc 5-(4-carboxyphenyl)-10,15,20-triphenylporphyrin and zinc 5-(4-carboxyphenyl)-10,15,20-triphenylchlrorin are described and compared to classic conditions of synthesis in terms of time of reaction and yields obtained. The new experimental protocols are easy to implement required small amounts of reagents and solvents and lead to short reaction times. All compounds have been characterized by 1 H NMR, MS and spectroscopic techniques.

Solventless metallation of low melting porphyrins synthesized by the water/microwave method

A new ecofriendly methodology for low melting meso-substituted metalloporphyrin synthesis is described. This methodology displays one of the best sustainability classifications involving porphyrin synthesis.

Exploiting novel process windows for the synthesis of meso-substituted porphyrins under continuous flow conditions

Porphyrin synthesis by classical methods frequently involves harsh conditions yielding products with poor scalability. Herein, a continuous flow approach is described, thus demonstrating a scaled-up procedure in a safe and highly pure manner.

Ecofriendly porphyrin synthesis by using water under microwave irradiation

Water, under microwave irradiation and at a temperature of 473 K, reaches pressures above 16 bar, being capable to act as catalyst, without the use of organic solvents and oxidants, for meso-substituted porphyrin synthesis. Sustainability of the reaction is proved by E Factor=35 and EcoScale value of 50.5, the highest so far obtained for porphyrin synthesis. Methodology's wide versatility is clearly demonstrated by the good yields obtained for both aryl and alkyl substituted porphyrins. These reaction conditions represent a huge development, not only by using very high concentrations, minimizing organic solvent usage, but also by eradicating toxic expensive solvents and oxidants.

The small stones of Coimbra in the huge tetrapyrrolic chemistry building

Improvements over the Rothemund classical reaction methods have allowed the development of capacity to obtain a wide range of structures that were earlier unavailable. Performing the reaction in a mixed nitrobenzene/carboxylic acid medium allowed improvements of yield and purity of the porphyrin obtained, and in some cases the control of the oxidation level of the macrocycle. More recently, novel microwave synthetic methodologies were exploited to achieve important improvements in simple porphyrin chemistry. Tidy sulphonations of simple porphyrins opened the way to diverse desired, inexpensive structures in a very simple manner. The option to concentrate interests on improvements of synthetic methods to obtain simple and diversified tetrapyrrolic structures proved to be advantageous. Interesting biomimetic oxidation and photooxidation systems and PDT sensitizers were developed and significant knowledge was achieved in these specific and related areas. This review presents a view of our own results in the synthetic area, and outlines the contributions therefrom to the study of biomimetic and therapeutic processes.

5,10,15,20-tetrakis(pentafluorophenyl)porphyrin: a versatile platform to novel porphyrinic materials

5,10,15,20-tetrakis(pentafluorophenyl)porphyrin reacts with a range of nucleophiles (amines, alcohols, thiols, nitrogen heterocycles, and others) resulting in the nucleophilic aromatic substitution of the para-F atoms of the pentafluorophenyl groups. This reaction, which was fortuitously discovered by Kadish and collaborators in 1990, is now being extensively used to synthesize porphyrins bearing electron-donating substituents in the para-position of their meso-aryl groups. This mini-review highlights the methods of synthesis of 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin, the use of its metal complexes in catalysis and its reaction with nucleophiles to yield new monomeric porphyrins, porphyrins supported in polymers or new polymeric porphyrin matrices useful for heterogeneous catalysis.

Synthetic transformations of porphyrins – Advances 2004-2007

Recent developments in the synthesis and transformation of porphyrins and their derivatives are presented. In connection with the Fifth International Conference on Porphyrins and Phthalocyanines (ICPP-5) a survey of current method developments and reactivity studies is made. The review focuses on synthetic advances in porphyrin chemistry. A brief survey of important developments covers selectively the literature from 2004 to late 2007.

Solvent-free synthesis of meso- tetraarylporphyrins in air: product diversity and yield optimization

The scope and optimization of a solvent-free method for the rapid preparation and facile purification of technologically important meso-substituted aryl porphyrins, such as 5,10,15,20-tetraphenylporphyrin is presented. This one-step method involves heating the aromatic aldehyde to ~200°C in a vial fitted with a septum-lined cap, followed by addition of the pyrrole and maintaining the temperature for about 20 minutes. The dioxygen in air serves as the oxidant. Present results show that the addition of benzoic acid as a catalyst improves the yield of 5,10,15,20-tetraphenylporphyrin from 22% to 32% and of para halogenated phenylporphyrins from 10% to ~25%. Herein is also presented an examination of the many factors that influence the yield, the ease of purification, and the ability to scale up the reaction. Since the tarry by-products from this method are much less soluble than in most other synthetic strategies, much less solvent is required for purification; simple extraction is often sufficient. This method can be scaled in the lab to > 300 mg, and provides an attractive route to many meso-substituted porphyrins because of its minimal waste generation in terms of both solvent and chromatography support.

(5,10,15,20-Tetra-phenyl-porphyrinato-κN)cobalt(II)-18-crown-6 (1/1)

The asymmetric unit of the title compound, [Co(C(44)H(28)N(4))]·C(12)H(24)O(6), contains one half of a Co(II)(TPP) (TPP is tetra-phenyl-porphyrin) complex and one half of an 18-crown-6 mol-ecule of crystallization, both lying on inversion centers. The Co(II)(TPP) complex exhibits a nearly planar conformation of the porphyrinate core [maximum deviation = 0.069 (2) Å] with an average Co-N distance of 1.971 (4) Å. The distance between the Co atom and the closest O atom of the 18-crown-6 mol-ecule is 2.533 (2) Å, indicating a short non-bonded contact between the Co atom and the crown ether mol-ecule. An ethyl-ene group of the 18-crown-6 mol-ecule is disordered over two sites with occupancies of 0.565 (7) and 0.435 (7).

Microwave-assisted synthesis of a new series of resorcin[4]arene cavitand-capped porphyrin capsules

Capsule-shaped molecular receptors are fundamental to the modeling of biological host-guest interactions such as those characteristic of enzymatic processes. Among the more versatile and effective synthetic receptors are the hemicarcerand-type hosts. Herein we report the synthesis of six novel resorcin[4]arene cavitand-capped porphyrin capsules, in a new series of molecular capsules modeled on the classical hemicarcerands initially reported by Cram and co-workers. In the first reported instance of its utilisation in the preparation of supramolecularly-capped porphyrin host molecules, microwave (MW) irradiation was used (in conjunction with Adler conditions) in order to form the porphyrin cap in situ. Additionally, the yields obtained via these conditions exhibited a significant improvement, relative to the traditional refluxing protocol hitherto reported. Capsules 20-25 are indefinitely stable, and we observed that 22 and 25 possess the smallest rigidified cavities yet reported for resorcin[4]arene cavitand-capped porphyrin host molecules.