Metalloporphyrins as models for the cytochromes p-450

A Porphyrin-Based Covalent Organic Framework as Metal-Free Visible-LED-Light Photocatalyst for One-Pot Tandem Benzyl Alcohol Oxidation/Knoevenagel Condensation

A porphyrin-based covalent organic framework (COF), namely Porph-UOZ-COF (UOZ stands for the University of Zabol), has been designed and prepared via the condensation reaction of 5,10,15,20-tetrakis-(3,4-dihydroxyphenyl)porphyrin (DHPP) with 1,4-benzenediboronic acid (DBBA), under the solvothermal condition. The solid was characterized by spectroscopic, microscopic, and powder X-ray diffraction techniques. The resultant multifunctional COF revealed an outstanding performance in catalyzing a one-pot tandem selective benzylic C-H photooxygenation/Knoevenagel condensation reaction in the absence of additives or metals under visible-LED-light irradiation. Notably, the catalytic activity of the COF was superior to individual organic counterparts and the COF was both stable and reusable for four consecutive runs. The present approach illustrates the potential of COFs as promising metal-free (photo) catalysts for the development of tandem reactions.

Porous polymers from octa(amino-phenyl)silsesquioxane and metalloporphyrin as peroxidase-mimicking enzyme for malathion colorimetric sensor

Rapid and efficient pesticide detection methods are particularly important due to the growing problems of pesticide residues. Here, a new azo-based porous organic polymer, Azo(Fe)PPOP, was prepared from octa(amino-phenyl)silsesquioxane (OAPS) and iron(III) 5,10,15,20-tetrakis(4-nitrophenyl)porphyrin (FeTPP(NO₂)₄) via a simple coupling reaction without the participation of metal catalysts. The inorganic cage units of OAPS endowed Azo(Fe)PPOP a porous framework, high surface area, favorably thermal and chemical stability. In Azo(Fe)PPOP, iron(III) porphyrin units were individually isolated in a fixed location, which could effectively avoid dimerization or self-oxidation as happens as in the case of porphyrin monomers. Such a unique structure made Azo(Fe)PPOP exhibit an excellent peroxidase-like catalytic performance in the presence of H₂O₂ and 3,3',5,5'-tetramethylbenzidine (TMB). Because of these advantages, we established a selective, facile, and sensitive colorimetric platform for direct detection of malathion within a very short time (3 min) with a low detection limit (8.5 nM). In addition, the recognition mechanism between Azo(Fe)PPOP and malathion was verified using X-ray photoelectron spectroscopy spectra. The practicality of the constructed platform was further executed by the detection of the pesticide in soil and food samples.

Robust Mn(III) N-pyridylporphyrin-based biomimetic catalysts for hydrocarbon oxidations: heterogenization on non-functionalized silica gel versus chloropropyl-functionalized silica gel

Two classes of heterogenized biomimetic catalysts were prepared and characterized for hydrocarbon oxidations: (1) by covalent anchorage of the three Mn(iii) meso-tetrakis(2-, 3-, or 4-pyridyl)porphyrin isomers by in situ alkylation with chloropropyl-functionalized silica gel (Sil-Cl) to yield Sil-Cl/MnPY (Y = 1, 2, 3) materials, and (2) by electrostatic immobilization of the three Mn(iii) meso-tetrakis(N-methylpyridinium-2, 3, or 4-yl)porphyrin isomers (MnPY, Y = 4, 5, 6) on non-modified silica gel (SiO2) to yield SiO2/MnPY (Y = 4, 5, 6) materials. Silica gel used was of column chromatography grade and Mn porphyrin loadings were deliberately kept at a low level (0.3% w/w). These resulting materials were explored as catalysts for iodosylbenzene (PhIO) oxidation of cyclohexane, n-heptane, and adamantane to yield the corresponding alcohols and ketones; the oxidation of cyclohexanol to cyclohexanone was also investigated. The heterogenized catalysts exhibited higher efficiency and selectivity than the corresponding Mn porphyrins under homogeneous conditions. Recycling studies were consistent with low leaching/destruction of the supported Mn porphyrins. The Sil-Cl/MnPY catalysts were more efficient and more selective than SiO2/MnPY ones; alcohol selectivity may be associated with hydrophobic silica surface modification reminiscent of biological cytochrome P450 oxidations. The use of widespread, column chromatography, amorphous silica yielded Sil-Cl/MnPY or SiO2/MnPY catalysts considerably more efficient than the corresponding, previously reported materials with mesoporous Santa Barbara Amorphous No 15 (SBA-15) silica. Among the materials studied, in situ derivatization of Mn(iii) 2-N-pyridylporphyrin by covalent immobilization on Sil-Cl to yield Sil-Cl/MnP1 showed the best catalytic performance with high stability against oxidative destruction and reusability/recyclability.

A facile biomimetic catalytic activity through hydrogen atom abstraction by the secondary coordination sphere in manganese(III) complexes

This paper describes the synthesis and structural characterization of four new manganese(iii) complexes (1-4) derived from N3O donor Schiff base ligands and their biomimetic catalytic activities related to catechol oxidase and phenoxazinone synthase. X-ray crystallography reveals that the Schiff bases coordinate the metal centre in a tridentate fashion, leaving the pendant tertiary amine nitrogen atom either protonated or free to balance the charge of the system, and these pendant triamines participate in strong hydrogen bonding interactions in the solid state. The hydrogen bonding ability of the pendant triamines at the second coordination sphere plays a crucial role in the substrate recognition and the stability of the complex-substrate intermediates. The effect of substitution at the phenolate ring towards the redox potential of the metal centre and the catalytic activity of these complexes has been observed. Detailed kinetic studies further disclose the deuterium kinetic isotope effect in which the transfer of the proton along the hydrogen bond from the substrates to the pendant triamine group at the secondary coordination sphere occurs at the key step in the catalytic reaction. The present reactivity nicely resembles the biochemical reactivities in the natural system in which a concerted electron and proton transfer to different species is usually observed. Remarkably, although some sort of influence of the secondary coordination sphere on catalytic activity has been reported mimicking the function of these metalloenzymes, such a direct participation of the secondary coordination sphere, particularly in modelling phenoxazinone synthase, has not been observed to date.

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

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

Manganese(ii) complex of an oxygen–nitrogen donor Schiff base ligand showing efficient catechol oxidase activity: synthesis, spectroscopic and kinetic study

A Mn(ii) complex containing two tridentate O,N,O-donor semicarbazone ligands shows very high catalytic activity for the aerial oxidation of 3,5DTBCH₂ with k_cat = 3.10 × 10⁶ h⁻¹, and k_cat/K_M = 3.25 × 10⁸ h⁻¹ M⁻¹.

5,5,15,15-Tetraoxo-5,15-Dithiaporphyrin as a Highly Electron-Deficient Porphyrinic Ligand

Oxidation of 5,15-dithiaporphyrin with meta-chloroperbenzoic acid afforded the corresponding S,S-tetraoxide in good yield. The resultant 5,5,15,15-tetraoxo-5,15-dithiaporphyrin exhibited the highly electron-deficient nature as elucidated by the electrochemical analysis and theoretical calculations. Treatment of tetraoxodithiaporphyrin with zinc(II) acetate and nickel(II) acetate provided the corresponding metal complexes efficiently. Owing to its enhanced Lewis acidity of the metal center by the electron-deficient ligand, the nickel complex underwent facile axial ligation to form pentacoordinate and hexacoordinate high-spin (S=1) complexes in solution and solid, respectively. The binding constant of pyridine to the Ni^II center was significantly higher than those of conventional porphyrin Ni^II complexes. Temperature-dependent magnetic susceptibility measurements of the high-spin Ni^II complex revealed the presence of weak ferromagnetic interactions.

Second-Generation Manganese(III) Porphyrins Bearing 3,5-Dichloropyridyl Units: Innovative Homogeneous and Heterogeneous Catalysts for the Epoxidation of Alkenes

The synthesis, characterisation and homogeneous catalytic oxidation results of two manganese(III) porphyrins of the so-called second-generation of metalloporphyrin catalysts, containing one or four 3,5-dichloropyridyl substituents at the meso positions are reported for the first time. The catalytic efficiency of these novel manganese(III) porphyrins was evaluated in the oxidation of cyclooctene and styrene using aqueous hydrogen peroxide as the oxidant, under homogeneous conditions. High conversions were obtained in the presence of both catalysts, obtaining the corresponding epoxide as the major product. The asymmetric metalloporphyrin, chloro[5,10,15-tris(2,6-dichlorophenyl)-20-(3,5-dichloropyridin-4-yl)porphyrinate]manganese(III), CAT-4, evidences a similar activity to that obtained with the well-known and highly efficient second-generation metalloporphyrin catalyst, chloro[5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrinate]manganese(III), CAT-2. CAT-4 was covalently attached onto Merrifield resin and 3-bromopropylsilica supports. The solid materials obtained were characterized by several techniques including diffuse reflectance, UV—VIS spectrophotometry, SEM and XPS. The catalytic results for the oxidation of cyclooctene and styrene using the immobilized catalysts are also presented. The Merrifield-supported catalyst showed to be very efficient, leading to five catalytic cycles in the oxidation of cyclooctene, using tert-butyl hydroperoxide as the oxidant.

Synthesis of Salicylaldimine Schiff Bases Cobalt (II) Complexes with Benzo-10-Aza-15-Crown-5 Pendant

The azacrown-ether-substituted salicylaldimine Schiff bases ligands HL1–HL5 have been prepared by the condensation of salicylaldehyde (or its derivatives) with the azacrown-ether-containing arylamine from benzo-10-aza-15-crown-5 (BN15C5). The cobalt (II) complexes CoL21–CoL25 are readily synthesised by treatment of HL1–HL5 with cobalt acetate; their structures were characterised by IR, MS, 1H NMR, molar conductance and elemental analysis.

Kinetics and mechanistic studies on the formation and reactivity of high valent MnO porphyrin species: mono-ortho or para-substituted porphyrins versus a di-ortho-substituted one

High valent Mn(O) species of a series of electron-rich and -deficient meso-tetra(aryl)porphyrins (aryl = phenyl, 2-Cl-phenyl, 2-nitrophenyl, 2-Me-phenyl, 2-Br-phenyl, 2,6-di-Cl-phenyl 4-OMe-phenyl, 4-Me-phenyl, 4-Cl-phenyl and 4-pyridyl) were prepared at 273 K.

Lewis-Acid-assisted Hydrogen Atom Transfer to Manganese(V)-Oxo Corrole through Valence Tautomerization

The kinetics of formation of the valence tautomers (tpfc^⋅)Mn^IV(O−LA)]ⁿ⁺ [where LA=Zn^II, Ca^II, Sc^III, Yb^III, B(C₆F₅)₃, and trifluoroacetic acid (TFA); tpfc=5,10,15‐tris(pentafluorophenyl) corrole] from (tpfc)Mn^V(O) were followed by UV/Vis spectroscopy, giving second‐order rate constants ranging over five orders of magnitude from 10⁻² for Ca to 10³  m⁻¹ s⁻¹ for Sc. Hydrogen atom transfer (HAT) rates from 2,4‐di‐tert‐butyl phenol (2,4‐DTBP) to the various Lewis acid valence tautomers of manganese oxo corrole complexes were evaluated and compared. For LA=TFA, Sc^III, or Yb^III, the rate constants of HAT were comparable to unactivated (tpfc)Mn^V(O). However, with LA=B(C₆F₅)₃, Zn^II, and Ca^II, 6‐, 21‐, and 31‐fold rate enhancements were observed, respectively. Remarkably, [(tpfc^⋅)Mn^IV(OCa)]²⁺ gave the most enhancement despite its rate of formation being the slowest. Comparisons of HAT rate constants among the various Lewis acid tautomers revealed that both size and charge are important. This study underscores how valence may affect the reactivity of high‐valent manganese‐oxo compounds and sheds light on nature's choice of Ca in the activation of Mn‐oxo in the oxygen‐evolving complex.

Progress of Mimetic Enzymes and Their Applications in Chemical Sensors

The need to develop innovative and reformative approaches to synthesize chemical sensors has increased in recent years because of demands for selectivity, stability, and reproducibility. Mimetic enzymes provide an efficient and convenient method for chemical sensors. This review summarizes the application of mimetic enzymes in chemical sensors. Mimetic enzymes can be classified into five categories: hydrolases, oxidoreductases, transferases, isomerases, and induced enzymes. Potential and recent applications of mimetic enzymes in chemical sensors are reviewed in detail, and the outlook of profound development has been illustrated.

Metal organic frameworks mimicking natural enzymes: a structural and functional analogy

In this review, we have portrayed the structure, synthesis and applications of a variety of biomimetic MOFs from an unprecedented angle.

Strong Inhibition of O-Atom Transfer Reactivity for Mn(IV)(O)(π-Radical-Cation)(Lewis Acid) versus Mn(V)(O) Porphyrinoid Complexes

The oxygen atom transfer (OAT) reactivity of two valence tautomers of a Mn(V)(O) porphyrinoid complex was compared. The OAT kinetics of Mn(V)(O)(TBP8Cz) (TBP8Cz = octakis(p-tert-butylphenyl)corrolazinato(3-)) reacting with a series of triarylphosphine (PAr3) substrates were monitored by stopped-flow UV-vis spectroscopy, and revealed second-order rate constants ranging from 16(1) to 1.43(6) × 10(4) M(-1) s(-1). Characterization of the OAT transition state analogues Mn(III)(OPPh3)(TBP8Cz) and Mn(III)(OP(o-tolyl)3)(TBP8Cz) was carried out by single-crystal X-ray diffraction (XRD). A valence tautomer of the closed-shell Mn(V)(O)(TBP8Cz) can be stabilized by the addition of Lewis and Brønsted acids, resulting in the open-shell Mn(IV)(O)(TBP8Cz(•+)):LA (LA = Zn(II), B(C6F5)3, H(+)) complexes. These Mn(IV)(O)(π-radical-cation) derivatives exhibit dramatically inhibited rates of OAT with the PAr3 substrates (k = 8.5(2) × 10(-3) - 8.7 M(-1) s(-1)), contrasting the previously observed rate increase of H-atom transfer (HAT) for Mn(IV)(O)(TBP8Cz(•+)):LA with phenols. A Hammett analysis showed that the OAT reactivity for Mn(IV)(O)(TBP8Cz(•+)):LA is influenced by the Lewis acid strength. Spectral redox titration of Mn(IV)(O)(TBP8Cz(•+)):Zn(II) gives Ered = 0.69 V vs SCE, which is nearly +700 mV above its valence tautomer Mn(V)(O)(TBP8Cz) (Ered = -0.05 V). These data suggest that the two-electron electrophilicity of the Mn(O) valence tautomers dominate OAT reactivity and do not follow the trend in one-electron redox potentials, which appear to dominate HAT reactivity. This study provides new fundamental insights regarding the relative OAT and HAT reactivity of valence tautomers such as M(V)(O)(porph) versus M(IV)(O)(porph(•+)) (M = Mn or Fe) found in heme enzymes.

Mononuclear manganese(iii) complexes of bidentate NO donor Schiff base ligands: synthesis, structural characterization, magnetic and catecholase studies

We have synthesized four mononuclear manganese(iii) complexes (1–4) of four closely related bidentate NO donor Schiff-base ligands, out of which three (2–4) were structurally characterized.

New highly brominated Mn-porphyrin: a good catalyst for activation of inert C–H bonds

Polybrominated Mn-porphyrin can act as a good catalyst for C–H activation. The oxidation of cyclohexane takes place with excellent selectivity and formation of 2-adamantanol is increased in adamantine oxidation.

Oxoiron(iv)-mediated Baeyer–Villiger oxidation of cyclohexanones generated by dioxygen with co-oxidation of aldehydes

Mechanistic studies on the Fe(ii)-catalyzed Baeyer–Villiger oxidation are described, including the formation and the reactivity of the trapped oxoiron(iv) intermediate.

Catalysis of μ-oxo-bis[porphyriniron(III)] for toluene oxidation with molecular oxygen

Toluene oxidation with molecular oxygen as the sole oxidant, and μ-oxo-bis[tetraphenylporphinatoiron(III)] as the catalyst, are reported. Under the reaction conditions of 438 K and 0.8 MPa, the molar total yields for the products benzaldehyde and benzyl alcohol and the turnover number of the catalyst are 4.35% and 21,830 (based on the metal ion), respectively. Compared with the reaction catalyzed by the corresponding monometalloporphyrin TPPFe III Cl , the total yields of the oxidation products and the catalyst turnover number by the dimeric iron porphyrin were almost twice those by the former. A possible reaction mechanism of the toluene oxidation by μ-oxo-bis[tetraphenylporphinatoiron(III)] is proposed.

Spectroscopic properties of p/p and o/o type iron(III)-metal-free porphyrin dimers and their catalysis as Cytochrome P450 model

Two series of p/p and o/o type iron(III)-metal-free porphyrin dimers were synthesized and characterized. The dimers consist of a ferric porphyrin and a free-base porphyrin, covalently linked with an alkoxy chain - O ( CH 2)n O -(n = 2–10) at the para and ortho position of two phenyl rings. The ferric paramagnetic effect and ring current effect on these dimers were discussed by 1 H NMR study; vibration modes sensitive to the conformations of the dimers were investigated from IR spectra; the electron density and spin state of ferric ion were examined using XPS and EPR spectroscopy. The catalytic activities of p/p and o/o type dimers on the hydroxylation of cyclohexane were studied under mild conditions. As a chemical mimic model of Cyto. P450, these porphyrins exhibit higher catalytic activities than the corresponding monomer FeTPPCl . With increase of the carbon numbers of alkoxy chain, the catalytic activity order of p/p dimers is C 2 < C 3 < C 4 < C 5 < C 6 > C 8 > C 10, the corresponding order of o/o dimers is C 2 > C 4 > C 6 > C 8 > C 10. Catalytic results of dimers are shown to strongly depend on their special conformation equilibrium, which affect the steric hindrance and electron transfer between two porphyrin rings.

Tetra(n-butyl)ammonium Dicyanotetraphenyl-porphyrinatomanganate(III); Crystal Structure and Electronic Resonance Raman and Absorption Spectra

The title complex salt has been isolated from a solution of chlorotetraphenylporphyrinatomanganese(III) and tetra(n-butyl)ammonium cyanide in dichloromethane. The complex salt crystallizes as a hydrate-dichloromethane solvate in the orthorhombic space group Pbca (no. 61) with cell parameters as follows: a = 21.491(5) Å, b = 21.706(5) Å, c = 28.489(5) Å, V = 13 290(5) Å3, Z = 8. The structure is that expected for a hexa-coordinated low-spin Mn ( III ) tetraphenylporphyrinate (magnetic moment: μ eff = 3.1 ± 0.2 μ B at 273 K). The Mn atom is in the centre of the slightly ruffled porphyrinato core. The Mn - C CN distance of 2.01(2)/2.02(2) Å and the Mn - N p distance, varying between 1.972(13) and 2.016(14) Å, are normal values. The Mn - C CN - N group is bent (171(2)/174(2) °). At low temperatures, a molecular electronic resonance Raman effect is detected at 198 and 344 cm−1. It is attributed to intraconfigurational transitions Γ 1g → Γ 4g and Γ 1g → Γ 3g Γ 5g , respectively, arising from the spin-orbit split 3 T 1g ground state of low-spin Mn ( III ), assuming pseudo-octahedral symmetry. Weak Q and intense B (Soret) transitions at 17 300 and 23 200 cm−1, respectively, as well as four weak trip–multiplet transitions are observed at 8650, 12 000, 14 900 and 19 900 cm−1.

Synthesis, electrochemistry, spectroscopy and photophysical properties of a series of meso-phenylpyridylporphyrins with one to four pyridyl rings coordinated to [Ru(bipy)2Cl]+ groups

A series of meso-phenylpyridylporphyrins and their respective supermolecular species obtained by the coordination of [ Ru ( bipy )2 Cl ]+ groups to the pyridyl substituents was synthesized and characterized. Their spectroscopic and electrochemical behavior were similar to that observed for the meso-tetra(4-pyridyl)porphyrin derivative, but the redox potential of the Ru (III/II) redox pair was about 70 mV more negative. The porphyrin centered fluorescence quantum yield exhibited a linear decrease as a function of the number of pyridyl substituents on the porphyrin ring. Efficient intramolecular energy transfer processes from the 3 MLCT state of the ruthenium complexes to the porphyrin center were observed at 77K in ethanol glass.

Effects of methoxy-substituted metalloporphyrins in catalytic alkene epoxidation by n-Bu4NHSO5

TPPMnOAc and four different kinds of manganese tetraphenylporphyrin acetates were synthesized using different numbers of methoxy substituents in various positions of the phenyl rings. These porphyrins were used as catalysts in the epoxidation of various alkenes with tetra-n-butylammonium hydrogen monopersulfate (n- Bu 4 NHSO 5) as the oxidant and imidazole as the axial base. The following order of catalytic activity was obtained: TPPMnOAc ≥ T (2,3- OMeP ) PMnOAc > T (4- OMeP ) PMnOAc > T (3,4- OMeP ) PMnOAc > T (2,4,6- OMeP ) PMnOAc . By studying the UV-vis spectra in the reaction solution, the stability of the applied methoxy porphyrins and the effect of this factor on obtained yields were investigated. Lower catalytic activity in some of the methoxy porphyrins emphasized steric effects and special hydrogen bonding among the reaction elements. However, the stability of T (2,3- OMeP ) PMnOAc under our reaction condition was considerable and high activity was observed. By adding small amounts of alcohol to the reaction solution, the effect of the solvent mixture was previewed and steps were taken to identify the active intermediate of the catalyst in these conditions.

Perhalogenated 2-pyridylporphyrin complexes: synthesis, self-coordinating aggregation properties, and catalytic studies

The synthesis of 2, 3, 7, 8, 12, 13, 17, 18-octabromo-meso-tetrakis(2-pyridyl)porphyrin, H 2 Br 8 T 2 PyP , is described, including the comparison of four attempted methods for the demetallation of ZnBr 8 T 2 PyP . One of the methods represents a strategy of demetallation based on the acid-base properties of the macrocycle, the solvent-dependent kinetics of metal insertion into porphyrins and the pH -dependent solubility of the 2-pyridylporphyrin derivatives in water. Self-coordinating aggregation of ZnBr 8 T 2 PyP in non-coordinating solvents was verified by 1 H NMR spectroscopy. The Mn(III)/Mn(II) redox potential for MnBr 8 T 2 PyPCl is 0.38 V higher than the reduction potential measured for its first-generation analogue, MnT 2 PyPCl . Cyclohexane hydroxylation by iodosylbenzene was performed in CH3 CN catalyzed by MnBr 8 T 2 PyPCl and MnT 2 PyPCl . MnBr 8 T 2 PyPCl was highly active, even at low concentration (5 × 10−5 M ), but perhalogenation did not account for oxidative robustness. At such a low catalyst concentration, MnT 2 PyPCl exhibited no activity as inferred by comparison to blank experiments.

Microwave-assisted synthesis of porphyrins and metalloporphyrins: a rapid and efficient synthetic method

The effective 'one-pot' microwave-assisted synthesis of several substituted 5,10,15,20-tetraarylporphyrins is reported. The microwave-assisted insertion of five different transition metals into the 5,10,15,20-tetraphenylporphyrin, 5,10,15,20-tetrakis(4-chlorophenyl)porphyrin and 5,10,15,20-tetrakis(3-hydroxyphenyl)porphyrin core was also achieved with high yields. In addition to their simplicity, both these straightforward, experimental protocols were also characterized by extremely short reaction times and quite small quantities of solvents employed.