Phthalocyanine metal complexes: Versatile catalysts for selective oxidation and bleaching

Rational design of organic ligands for metal-organic frameworks as electrocatalysts for CO2 reduction

Electrocatalytic carbon dioxide (CO2) reduction to valuable chemical compounds is a sustainable technology with enormous potential to facilitate carbon neutrality by transforming intermittent energy sources into stable fuels. Among various electrocatalysts, metal-organic frameworks (MOFs) have garnered increasing attention for the electrochemical CO2 reduction reaction (CO2RR) owing to their structural diversity, large surface area, high porosity and tunable chemical properties. Ligands play a vital role in MOFs, which can regulate the electronic structure and chemical environment of metal centers of MOFs, thereby influencing the activity and selectivity of products. This feature article discusses the strategies for the rational design of ligands and their impact on the CO2RR performance of MOFs to establish a structure-performance relationship. Finally, critical challenges and potential opportunities for MOFs with different ligand types in the CO2RR are mentioned with the aim to inspire the targeted design of advanced MOF catalysts in the future to achieve efficient electrocatalytic CO2 conversion.

[2+1] Cycloadditions Modulate the Hydrophobicity of Ni-N4 Single-Atom Catalysts for Efficient CO2 Electroreduction

Microenvironment regulation of M-N4 single-atom catalysts (SACs) is a promising way to tune their catalytic properties toward the electrochemical CO2 reduction reaction. However, strategies that can effectively introduce functional groups around the M-N4 sites through strong covalent bonding and under mild reaction conditions are highly desired. Taking the hydrophilic Ni-N4 SAC as a representative, we report herein a [2+1] cycloaddition reaction between Ni-N4 and in situ generated difluorocarbene (F2C:), and enable the surface fluorocarbonation of Ni-N4, resulting in the formation of a super-hydrophobic Ni-N4-CF2 catalyst. Meanwhile, the mild reaction conditions allow Ni-N4-CF2 to inherit both the electronic and structural configuration of the Ni-N4 sites from Ni-N4. Enhanced electrochemical CO2-to-CO Faradaic efficiency above 98 % is achieved in a wide operating potential window from -0.7 V to -1.3 V over Ni-N4-CF2. In situ spectroelectrochemical studies reveal that a highly hydrophobic microenvironment formed by the -CF2- group repels asymmetric H-bonded water at the electrified interface, inhibiting the hydrogen evolution reaction and promoting CO production. This work highlights the advantages of [2+1] cycloaddition reactions on the covalent modification of N-doped carbon-supported catalysts.

Binuclear Metal Phthalocyanines with Enhanced Activity in the Oxygen Evolution Reaction: A First-Principles Study

The rational design of efficient catalysts for the electrochemical oxygen evolution reaction (OER) critically relies on a comprehensive understanding of the reaction mechanisms. Herein, the alkaline OER on planar mononuclear metal phthalocyanines (MPc, where M = Mn, Co, Fe, and Ni) and binuclear metal phthalocyanines (bi-MPc) is studied using density functional theory (DFT) methods. Both FePc and bi-CoPc exhibit enhanced stability and OER activity, with the energy required for the leaching of central metal being as high as 2.28 and 2.45 eV and the overpotentials of the OER being 0.48 and 0.57 V, respectively. Through electronic structure analysis, it is found that, in the OER process of bi-MPc, the large macrocyclic ligand and metal ions not bonding with the intermediate can serve as hole reservoirs. Intermediate species are further stabilized by the dispersal of a positive charge, reducing the free energy. These findings underscore the significance of macrocyclic ligands in the rate-determining step of the OER catalyst.

Gas-Phase Electronic Structure of Phthalocyanine Ions: A Study of Symmetry and Solvation Effects

Research into and applications of phthalocyanines (Pc) are mostly connected to their intriguing electronic properties. Here, messenger-type UV-vis spectroscopy of two metal-free ions from the phthalocyanine family, cationic H2Pc+ and H2PcD+, along with their hydrates is performed. They show that the electronic properties of both ions can be traced to those in the conjugate base, Pc2-, however, they are affected by state splitting due to the reduced symmetry; in the H2Pc+ radical cation, a new band appears due to excitations into the singly-occupied molecular orbital. Quantum chemical spectra modeling reproduces all important features of the measured spectra and provides insight into the nature of electronic transitions. Hydration of the ions has only a mild effect on the electronic spectra, showing the stability of the electronic structure with respect to solvation effects.

Mechanistic Studies on the Hexadecafluorophthalocyanine-Iron-Catalyzed Wacker-Type Oxidation of Olefins to Ketones*

The hexadecafluorophthalocyanine-iron complex FePcF₁₆ was recently shown to convert olefins into ketones in the presence of stoichiometric amounts of triethylsilane in ethanol at room temperature under an oxygen atmosphere. Herein, we describe an extensive mechanistic investigation for the conversion of 2-vinylnaphthalene into 2-acetylnaphthalene as model reaction. A variety of studies including deuterium- and ¹⁸ O₂ -labeling experiments, ESI-MS, and ⁵⁷ Fe Mössbauer spectroscopy were performed to identify the intermediates involved in the catalytic cycle of the oxidation process. Finally, a detailed and well-supported reaction mechanism for the FePcF₁₆ -catalyzed Wacker-type oxidation is proposed.

Recent advances in electrocatalysis with phthalocyanines

Applications of phthalocyanines (Pcs) in electrocatalysis-including the oxygen reduction reaction (ORR), the carbon dioxide reduction reaction (CO₂RR), the oxygen evolution reaction (OER), and the hydrogen evolution reaction (HER)-have attracted considerable attention recently. Pcs and their derivatives are more attractive than many other macrocycles as electrocatalysts since, although they are structurally related to natural porphyrin complexes, they offer the advantages of low cost, facile synthesis and good chemical stability. Moreover, their high tailorability and structural diversity mean Pcs have great potential for application in electrochemical devices. Here we review the structure and composition of Pcs, methods of synthesis of Pcs and their analogues, as well as applications of Pc-based heterogeneous electrocatalysts. Optimization strategies for Pc-based materials for electrocatalysis of ORR, CO₂RR, OER and HER are proposed, based on the mechanisms of the different electrochemical reactions. We also discuss the structure/composition-catalytic activity relationships for different Pc materials and Pc-based electrocatalysts in order to identify future practical applications. Finally, future opportunities and challenges in the use of molecular Pcs and Pc derivatives as electrocatalysts are discussed.

Porphyrazines with annulated diazepine rings. 6. Synthesis and properties of the alkyl substituted derivative - MgII complex of tetrakis-2,3-(5,7-di-tert-butyl-6H-1,4-diazepino)porphyrazine

Novel MgII porphyrazine 3 bearing four appended seven-membered tert-butyl substituted 1,4-diazepine rings was prepared via Linstead macrocyclization of 5,7-di-tert-butyl-6[Formula: see text]-1,4-diazepine-2,3-dicarbonitrile 2, obtained by condensation of diaminomaleonitrile and 2,2,6,6-tetramethylheptanedione-3,5 (1) in anhydrous ethanol in the presence of P2O5. The structure of the dinitrile precursor 2 was established by single crystal X-ray work. 1H NMR study revealed that the 1,4-diazepine ring is present in the 6[Formula: see text] tautomeric form both in the dinitrile precursor 2 and the MgII complex 3. The inversion of the tert-butyl substituted diazepine ring occurs more easily than in the case of aryl and alkenyl substituted species. Although porphyrazine 3exhibit some tendency to aggregation in low-polar solvents the presence of bulky tert-butyl substituents stabilizes the monomeric form and hinders formation of H-bonded dimers characteristic for aryl and styryl substituted analogues.

A novel and low-cost CuPc@C catalyst derived from the compounds of sunflower straw and copper phthalocyanine pigment for oxygen reduction reaction

A series of carbon and phthalocyanine catalysts were prepared with uniform and stretchable sunflower straw biological materials as the carbon source and inexpensive copper phthalocyanine (CuPc) pigment as a nitrogen doping source by a facile high-temperature carbonization method. This kind of biomass carbon material sunflower straw with abundant pore structure and sponge-like expansion and contraction functions can not only be used as a source of porous carbon in biomass carbon materials, but also as a carbon carrier with high specific surface area to provide nanoparticle adhesion sites. When it was immersed in the copper phthalocyanine pigment solution, more active sites could be exposed, so that CuPc particles could be uniformly doped and distributed on the porous carbon material. As a result, thanks to the doping of nitrogen atoms and the improvement of graphitization degree, the composite catalyst treated at 800 °C (CuPc@C-800) exhibits a porous structure with a 38 mV lower on-set potential and a high stability of 87.4% compared to commercial Pt/C (20%) catalyst. These results demonstrate that CuPc@C series composite catalysts have a splendid electrochemical performance in oxygen reduction reaction catalysts, which can start a new direction for later workers to study combining the properties of biomass carbon material and the phthalocyanine series of catalysts.

A series of CuPc@C composite catalysts were prepared with uniform and stretchable sunflower straw biological materials as the carbon source and inexpensive copper phthalocyanine pigment as a nitrogen doping source.

Oxidation of diclofenac in the presence of iron(II) octacarboxyphthalocyanine

This paper presents the results of the research on the influence of catalytic activity of iron(II) octacarboxyphthalocyanines (FePcOC) on the transformation of diclofenac (DCF) which is the most popular anti-inflammatory analgesic. Diclofenac poses a serious threat to the natural environment. The paper demonstrates that diclofenac, in the presence a monomeric form of iron octacarboxyphthalocyanine and hydroxyl radicals (HO^•) (from H₂O₂), undergoes a transformation into diclofenac-2,5-iminoquinone (DCF-2,5-IQ), causing distinct changes in the UV-Vis absorption spectrum. In the presence of iron octacarboxyphthalocyanine and H₂O₂, the previously colourless diclofenac solution becomes intense orange. As a result, a new band at approx. 450 nm appears in the absorption spectrum. HPLC analysis has shown that the concentration of diclofenac decreases with time. TD-DFT calculations using the CAM-B3LYP/6-31+G (d, p) method have been conducted to confirm experimental data concerning the formation of a new band at λ_max = 450 nm.

Metallophthalocyanines as Catalysts in Aerobic Oxidation

The first remarkable property associated to metallophthalocyanines (MPcs) was their chemical “inertness”, which made and make them very attractive as stable and durable industrial dyes. Nevertheless, their rich redox chemistry was also explored in the last decades, making available a solid and detailed knowledge background for further studies on the suitability of MPcs as redox catalysts. An overlook of MPcs and their catalytic activity with dioxygen as oxidants will be discussed here with a special emphasis on the last decade. The mini-review begins with a short introduction to phthalocyanines, from their structure to their main features, going then through the redox chemistry of metallophthalocyanines and their catalytic activity in aerobic oxidation reactions. The most significant systems described in the literature comprise the oxidation of organosulfur compounds such as thiols and thiophenes, the functionalization of alkyl arenes, alcohols, olefins, among other substrates.

Significant role of high-valent iron-oxo species in the degradation and detoxification of indomethacine

A novel high-valent iron-oxo species (Fe(IV) = O) generated from Iron hexadecachlorophthalocyanine (FePcCl₁₆)-mediated peroxymonosulfate (PMS) activation under visible light illumination for the degradation of a special group of compounds, indomethacine (IDM), containing methoxy, carboxyl, chloro, and amide groups was investigated. The experimental results indicate that Fe(IV) = O was able to selectively attack the carbonyl C-N bond on twisted amide groups, which exerts a strong toxic effect, and could therefore, effectively degrade and detoxify IDM and its byproducts. Twelve byproducts were identified by HPLC/MS/MS and calculation of frontier electron densities (FEDs), with all amide-group breakage products detected, and the possible pathways were deduced, which mainly consisted of Fe(IV) = O-induced cleavage of amide groups and radicals-induced reactions. Ecological risk assessment further confirmed a decrease in toxicity towards IDM degradation, which provides a promising Fe(IV) = O species for selective oxidation and detoxification of destabilized ground-state amides in drinking-water and wastewater treatment.

Design and Fine-Tuning Redox Potentials of Manganese(II) Complexes with Isoindoline-Based Ligands: H2O2 Oxidation and Oxidative Bleaching Performance in Aqueous Solution

A series of divalent manganese complexes [MII(HL1–6)Cl2] with the 1,3-bis(2’-Ar-imino)isoindolines (HLn, n = 1–6, Ar = pyridyl, 4-methylpyridyl, imidazolyl, thiazolyl, benzimidazolyl and N-methylbenzimidazolyl, respectively) including the previously reported ligands (HL1–2, 4–6) and complexes ([MII(HL1,5)Cl2]) have been prepared and characterized by electrochemical and spectroscopic methods. In these complexes, it was possible to control the redox potential of the metal center by varying the aryl substituent on the bis-iminoisoindoline moiety, and investigate its effect in a catalase-like reaction, and oxidative bleaching process in buffered aqueous solution. The kinetics of the dismutation of H2O2 into H2O and O2, and the oxidative degradation of morin by H2O2 were investigated in buffered water, where the reactivity of the catalysts in both systems was markedly influenced by the redox and Lewis acidic properties of the metal centers and the concentration of the bicarbonate ions. Both the catalase-like and bleaching activity of the catalysts showed a linear correlation with the MnIII/MnII redox potentials. The E1/2 spans a 561 mV range from 388 mV (Ar = benzymidazolyl) to 948 mV (Ar = 4-methylpyridyl) vs. the SCE. The amount of bicarbonate is a critical issue for the in situ formation of peroxycarbonate as a versatile oxidant, and its participation in the formation of high valent MnIV = O species.

Peripherally substituted soluble nickel phthalocyanines: Synthesis, characterization, aggregation behavior and antioxidant properties

Tetra-zwitterionic-substituted nickel(II) phthalocyanine derivatives were newly synthesized starting from nonionic 2(3),9(10),16(17),23(24)-tetrakis-[2-([Formula: see text]-((3-dimethylamino)propyl)carbamate)oxyethyl)phthalocyaninato nickel (II). The novel compounds have been characterized by a combination of UV-vis, FT-IR and mass spectroscopies and elemental analysis. The critical micelle concentrations of the prepared compounds were measured, and the antioxidant activities were analyzed with radical scavenging ability of 1,1-diphenyl-2-picrylhydrazyl (DPPH) and with 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS). The zwitterionic molecules showed aggregated spectra in the UV-vis region, and they might be good surfactant candidates for the detergent industry with their appropriate critical micelle concentration (CMC) properties in water. The compounds exhibited ABTS radical scavenging activity and thus they have antioxidant activity.

A simple and facile bioinspired catalytic strategy to decolorize dye wastewater by using metal octacarboxyphthalocyanine particles

To explore the simple, facile, environmental friendly and low cost catalytic technique to decolorize harmful dye contaminants in solution and understand the mechanism is an interesting and practical research. In this paper, we provide a highly efficient and convenient method for fast decolorization of dyes (methylene blue and rhodamine B) in aqueous solution catalyzed by iron octacarboxyphthalocyanine (FeOCPc) or cobalt octacarboxyphthalocyanine (CoOCPc). Compared to the traditional methods, our method is very simple. The 30 mg/L methylene blue could be decolorized almost absolutely less than 30 min just by dispersing FeOCPc powders into the dye solution. The decolorization of rhodamine B at high concentration (30 mg/L) could be achieved to 100% decolorization degree less than 20 min in the presence of FeOCPc and tert-butyl hydroperoxide (BuOOH). Moreover, the ESR and HPLC-MS measurement were performed to determine the active radicals and various intermediates in decolorization processes and the possible catalytic mechanism was proposed. It is noted that both FeOCPc and CoOCPc catalysts show the different catalytic oxidation behaviors depending on the oxidant (O₂ or BuOOH). Our investigation provides a novel, low cost and convenient strategy to purify the environmental pollutions.

The efficient, fast and facile decolorization of organic dyes homogeneously catalyzed by iron octacarboxylic phthalocyanine

To remove the harmful dye contaminants via an efficient, facile and low energy consumption route is a grave challenge in current chemical industry. Though the great progresses of TiO₂ photocatalysis and enzymatic degradation have been witnessed, the strategy for satisfying the above requirements is still worth exploring. Herein, we develop a biomimetic catalysis strategy for the fast decolorization of organic dyes catalyzed by iron octacarboxylic phthalocyanine (FeOCPc) complexes assisted with tert-butyl hydroperoxide (BuOOH). Methyl orange (MO) and methylene blue (MB) were used as the model pollutants and experimental results show that the decolorization degree of 25 mg/L MO could achieve 100% within 20 min and 80% for 25 mg/L MB within 30 min. The molar ratio for FeOCPc/MO and FeOCPc/MB is 0.146 and 0.142, respectively. Interestingly, other than the high-valent iron-oxygen active species, tert-butyl peroxyl radicals and hydroxyl radicals were detected as the active species generated during the catalytic oxidation by the electron paramagnetic resonance (EPR) measurement. This work not only provides a distinctive biomimetic catalysis system of FeOCPc-BuOOH for the fast bleaching of dye pollutants, but also proposes the new insight on a mechanism based on the cooperation catalysis of iron-oxygen active species, tert-butyl peroxyl radicals and hydroxyl radicals.

Flexible oxidation of styrene using TBHP over zirconia supported mono-copper substituted phosphotungstate

A heterogeneous catalyst comprising mono-copper substituted phosphotungstate and hydrous zirconia was synthesized using wet impregnation method, characterized by various physico-chemical techniques and evaluated for solvent-free oxidation of styrene using TBHP as oxidant. Various reaction parameters like time, catalyst amount, amount of TBHP and temperature were optimized with focus on optimum selectivity of styrene-oxide. Further, the catalytic activity was compared with that of unfunctionalized PW₁₁Cu to understand the role of the support. Finally, the role of each component of the reaction was clearly elucidated by a detailed kinetic study of the reaction using both the catalysts.

Mono-copper substituted phosphotungstate was supported on hydrous zirconia by wet impregnation, characterized and evaluated for its catalytic activity in the solvent-free flexible oxidation of styrene using TBHP as oxidant.

Unsubstituted metallophthalocyanine catalysts for the removal of endocrine disrupting compounds using H2O2 as oxidant

Advanced oxidation processes have become increasingly important to treat non-biodegradable compounds entering environmental waters. In recent decades, water-soluble metallophthalocyanines have been shown to catalyse H₂O₂-containing oxidation reactions through the production of unique reactive species, nucleophilic metal-peroxo complexes. Few reports in the literature have examined water insoluble metallophthalocyanines (MPc). The oxidative catalytic activity of water insoluble manganese- and iron-phthalocyanine (MnPc, FePc) at pH 7 has been shown through the decolourisation of methylene blue and removal of bisphenol A. These studies expand on this previous study, exploring the catalytic activity of a range of metallophthalocyanines catalysts under both acidic and neutral conditions. FePc, while only active under neutral conditions, was the best performing catalyst. This activity was significantly improved upon by the addition of acetonitrile as a co-solvent, as well as increasing the ratio of H₂O₂ to catalyst. MnPc was catalytically active at both pH 3 and 7. FePc and MnPc catalysts showed the ability to remove bisphenol A in the presence of dam water. Reaction rates were reduced for bisphenol A removal with FePc as a catalyst but were unchanged in the presence of MnPc. The removal of 17β-estradiol, estrone, and coumestrol was successfully demonstrated, with greater than 96% removal of all tested EDC's achieved. This is the first reported study showing the removal of the phytoestrogen, coumestrol. Even though considerably lower concentrations of costly catalysts and oxidation reagents were used in our work, the removal extent of EDC's by the MPc-catalysed oxidation reactions achieved here compares favourably with literature.

Iron(III) phthalocyanine supported on a spongin scaffold as an advanced photocatalyst in a highly efficient removal process of halophenols and bisphenol A

This study investigated for the first time the degradation of phenol, chlorophenol, fluorophenol and bisphenol A (BPA) by the novel iron phthalocyanine/spongin hybrid material under various process conditions: hydrogen peroxide and UV irradiation. The heterogeneous catalyst, iron phthalocyanine/spongin (SFe), was produced by an adsorption process. The product obtained was investigated by a variety of spectroscopic techniques - X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and carbon-13 nuclear magnetic resonance (¹³C NMR) - as well as elemental and thermal analysis. The study confirmed the stable immobilization of the dye on the biopolymer. The results demonstrate that the degradation of phenols and BPA followed pseudo-second-order kinetics under different experimental conditions. The synergy of SFe, H₂O₂ and UV was found to produce a significant increase in the removal efficiency and resulted in complete removal of contaminants in a short time of 1 h. The reaction products were identified by high-performance liquid chromatography/mass spectrometry (HPLC-MS) and possible degradation pathways were proposed, featuring a series of steps including cleavage of CC bonds and oxidation.

Catalytic degradation of sulfaquinoxalinum by polyester/poly-4-vinylpyridine nanofibers-supported iron phthalocyanine

Iron (II) phthalocyanine (FePc) supported on electrospun polyester/poly-4-vinylpyridine nanofibers (PET/P4VP NFs) was prepared by stirring in tetrahydrofuran. The resulting product was confirmed and characterized by ultraviolet-visible diffuse reflectance spectroscopy, attenuated total reflection Fourier transform infrared spectra, X-ray photoelectron spectroscopy, gas chromatography/mass spectrometry, and ultra-performance liquid chromatography. More than 95% of sulfaquinoxalinum (SQX) could be removed by the activation of hydrogen peroxide in the presence of FePc-P4VP/PET with a PET and P4VP mass ratio of 1:1. This system exhibited a high catalytic activity across a wide pH and temperature range. The degradation rates of SQX achieved 100, 95, and 78% at a pH of 3, 7, and 9, respectively, and the degradation rates of SQX are more than 80% at the temperature ranging from 35 to 65 °C. DMSO₂ could be detected by gas chromatography/mass spectrometry after the addition of DMSO, suggesting the formation of the high-valent iron intermediates in this catalytic system. In addition, the electron paramagnetic resonance experiments proved that free radicals did not dominate the reaction in our system. Therefore, the high-valent iron intermediates were proposed to the main active species in the FePc-P4VP/PET/hydrogen peroxide system. In summary, the heterogeneous catalytic processes with non-radical catalytic mechanism might have better catalytic performance for the removal of organic pollutants, which can potentially be used in wastewater treatment.

Quantitative Immobilization of Phthalocyanine onto Bacterial Cellulose for Construction of a High-Performance Catalytic Membrane Reactor

We report the fabrication of a tetra-amino cobalt (II) phthalocyanine (CoPc)-immobilized bacterial cellulose (BC) functional nanocomposite, CoPc@BC, by quantitative immobilization of CoPc onto a BC membrane. Lab-cultured BC was oxidized by NaIO₄ to generate aldehyde groups on BC for the subsequent CoPc immobilization, the processing conditions were optimized by monitoring both the generated aldehyde content and the resulting CoPc loading. X-ray photoelectron spectroscopy (XPS) was employed to characterize the change of the element bonding environment during the functionalization processes. The CoPc@BC functional nanocomposite was utilized for the treatment of reactive red X-3B dye wastewater. The CoPc molecules in the CoPc@BC nanocomposite can function as an “antenna” to adsorb the target anionic dye molecules, the adsorption takes place both on the surface and in the interior of CoPc@BC. A catalytic membrane reactor (CMR) was assembled with the CoPc@BC nanocomposite, the performance of CMR was evaluated based on the catalytic oxidation behavior of reactive red X-3B, with H₂O₂ as an oxidant. Highly-reactive hydroxyl radical (OH) was involved in the catalytic oxidation process, as detected by electron paramagnetic resonance (EPR). Under optimal operating conditions of a flow rate of 6 mL/min, a reaction temperature of 50 °C, and an H₂O₂ concentration of 10 mmol/L, the decoloration rate of CMR was as high as 50 μmol⋅min⁻¹⋅g⁻¹.