Narrow Results

Cyclodextrins are versatile building blocks for a variety of macromolecules due to the inclusion complexes that are formed with hydrophobic organic molecules. Cyclodextrin-porphyrin interactions are of particular interest since cyclodextrins can serve as a non-covalent binding pocket while metalloporphyrins could serve as the heme analogs in the construction of heme protein model compounds. Various approaches to the design and assembly of biomimetic porphyrin constructs are compared and contrasted in this minireview with a particular emphasis on self-assembled and porphyrin-cyclodextrin systems. Several recent advances from our laboratories are described in this context. A sensitive fluorescent binding probe, 6^A-N-dansyl-permethylated-β-cyclodextrin (Dan-NH-TMCD), was found to form 2:1 complexes with the meso-tetraphenylporphyrins Mn(III)TCPP, Mn(III)TPPS and Mn(III)TF₄TMAP with high binding constants. A perPEGylated cyclodextrin, heptakis(2,3,6-tri-O-2-(2-(2-methoxyethoxy)ethoxy)ethyl)-β-cyclodextrin (TPCD), has been shown by ¹H NMR spectroscopy to form a 1:1 complex with H₂TCPP with a binding constant above 10⁸M^-1. Such a strong binding constant is the largest found for a 1:1 complex between a monomeric cyclodextrin and a guest. TPCD was also found to bind Mn(III)TCPP with a binding constant of 1.2 × 10⁶M^-1. A novel, self-assembled hemoprotein model, hemodextrin is also described. The molecular design is based on a PEGylated cyclodextrin scaffold that bears both a heme-binding pocket and an axial ligand that binds an iron porphyrin. The binding constant for Fe(III)TPPS (iron(III) meso-tetra(4-sulfonatophenyl)porphyrin) by py-PPCD was determined to be 2 × 10⁶M^-1. The pyridyl nitrogen of py-PPCD was shown to ligate to the iron center by observing signal changes in the Fe(II)-porphyrin ¹H NMR spectrum. This hemodextrin ensemble, a minimalist myoglobin, was shown to bind dioxygen reversibly and to form a stable ferryl species.

Oxidation of naphthalene was performed with various manganese and iron porphyrins as catalysts and metachloroperbenzoic acid, tert-butylhydroperoxide, and hydrogenperoxide as oxidants, in the presence of imidazole as cocatalyst. With m-CPBA, in low concentration of catalyst, iron porphyrin shows a higher catalytic activity than manganese porphyrin, while a greater amount of manganese porphyrin is needed for having a good catalyst. In the presence of excess amount of H₂O₂ oxidant in CH₃OH solvent, manganese porphyrin acts better than iron porphyrin; however with the same oxidant and in aprotic solvent such as CH₃CN:CH₂Cl₂ again iron shows a higher activity than manganese. Moreover, the effects of metalloporphyrin concentration on the selectivity, activity and stability of catalyst were studied in H₂O₂-catalytic oxidation.

In this study we describe the electrochemical behavior of 5,10,15,20-tetrakis(2'-aminophenylporphyrin)manganese(III) chloride supported on a glassy carbon electrode, as well as the electrochemical preparation and characterization of thin films based on pyrrole-3-carboxylic acid. The electrocatalytic action of the electrode modified with the Mn(III) porphyrin toward an azo dye was tested, and the characteristic strong interaction between the incorporated metalloporphyrin and RR120 dye was verified.

Reaction of chloro(tetraphenylporphyrinato)manganese(III) with tetra-n-butylammonium hydrogen monopersulfate (n-Bu₄NHSO₅) in the presence of high ratio of imidazole was followed by UV-Vis spectroscopy in CH₂Cl₂ at room temperature. The variation in the UV-vis spectra shows that the reaction goes through Mn(III), high valent Mn(V) or Mn(IV), Mn(III), low valent Mn(II) and again Mn(III) complexes. The Mn(III) complex before the addition of n-Bu₄NHSO₅ could not be reduced to Mn(II), but Mn(III) complex which was obtained after decay of high valent Mn complexes was reduced to Mn(II), probably because its spin state is different from the first one. Addition of methanol to the reaction mixture (in the presence or absence of ferrocene) shows that two high valent Mn complexes [(Mn^V(TPP)-(O)(X)] (X = Im^- or OMe^-) and [Mn^IV(TPP)(OMe)₂] are involved in the course of the reaction. Using meta-choloroperbenzoic acid instead of (n-Bu₄NHSO₅) as oxidant showed that reduction of Mn(III) to Mn(II) is taking place only in the presence of HSO₄.

Various nitrogenous bases, such as imidazoles, pyridines and amines were employed as axial ligands in epoxidation reaction of cyclooctene bytetra-n-butylammonium hydrogen monopersulfate (n-Bu₄NHSO₅), in the presence of Mn(III)-tetrakis(2,3-dimethoxyphenyl)porphyrin-acetate (T(2,3-OMeP)PorMnOAc). T(2,3-OMeP)PorMnOAc is a fairly stable catalyst, with the ability of producing hydrogen bonding. High epoxidation yield of 85 ± 6% was obtained in the presence of imidazole axial ligand with 100% selectivity in 30 min. Higher conversion of around 100% was obtained by pyridine axial base, while selectivity was reduced to 69%. Further epoxidation reactions were also performed using Mn(III)-Tetrakis(2,3-dihydroxyphenyl)porphyrin-acetate (T(2,3-OHP)PorMnOAc) as catalyst. In addition to the usual electronic and steric effects, it is proposed that the catalytic activity depends on the existence and kind of hydrogen bonding between the axial base and the ortho-methoxy or hydroxy groups on the phenyl rings of manganese porphyrin. The cis to trans ratio of cis-stilbene oxide formed by imidazole and pyridine axial bases were obtained as 7.5 and 2.5 respectively. In addition GC-Ms and UV-vis studies were employed to find the nature of active species and product formation. Our DFT calculations disclosed that pyridine hydrogen bonding with moiety of the macrocycle rings strongly affects the relative energies of S/Q spin states in [T(2,3-OMeP)PorMn^V(O)(Py)]⁺, in that it results in the longer Mn–O bond and reactivity toward substrates.

In the present research meso-tetrakis(4-carboxyphenyl)porphyrinatomanganese(III) acetate (Mn(TCPP)OAc) nanoparticles have been prepared for the first time without any stabilizing agent or supporting matrix under ultrasonic irradiation. Scanning electron microscopy (SEM) was used to characterize and investigate the nanocatalyst. Rapid, efficient, facile and highly selective oxidation of a wide range of olefins by tetra-$n$-butylammonium hydrogen monopersulfate over the prepared manganese nanocatalyst was investigated. Also oxidation of sulfides and alkanes in the presence of the (Mn(TCPP)OAc) nanoparticles were studied. The catalyst is recovered by filtration and reused at least five times.

A heterogenized meso-tetrakis(2,3-dihydroxyphenyl)porphyrinatomanganese(III) acetate at zeolite imidazolate framework-8 (T(2,3-OHP)PorMn@ZIF-8) is investigated for the catalytic olefin epoxidation reactions at room temperature. Heterogenization is accomplished through a non-classical hydrogen bond proposed between T(2,3-OHP)PorMn bearing O–H groups and C–H of the 2-methylimidazolate linkers in the ZIF-8 structure. The aforementioned compound is characterized by X-ray powder diffraction (XRD), atomic absorption spectroscopy (AAS), nitrogen adsorption−desorption, FT-IR spectroscopy, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and Raman spectroscopy. The catalytic system with rather high potential of reusability is proposed as a fairly efficient epoxidation catalyst compared to reports in homogeneous media.

As functional molecules, the cationic manganese (III) tetrakis-5, 10, 15, 20-(N-methyl-4-pyridyl) porphyrins (MnTMPyP) were used to assemble with the exfoliated Ca₂Nb₃O${}_{10}^{-}$ nanosheets, which were obtained via treating the acidified compounds HCa₂Nb₃O${}_{10}$ of perovskite composites KCa₂Nb₃O${}_{10}$ with the tetrabutyl ammonium hydroxide (TBA${}^{+}$OH${}^{-})$ aqueous solution. Several analytic techniques, such as XRD, SEM, TEM, FT-IR, UV-Vis, were employed to investigate the morphology and structure of KCa₂Nb₃O${}_{10}$, HCa₂Nb₃O${}_{10}$ and intercalation nanocomposites MnTMPyP/Ca₂Nb₃O${}_{10}$. The stability and uniformity of the obtained Ca₂Nb₃O${}_{10}^{-}$ nanosheets colloidal dispersion were proved by the surface charge of $-42.2$mV, which was obtained by a Zetasizer Nano instrument. Meanwhile, the electrochemical properties of MnTMPyP/Ca₂Nb₃O${}_{10}$ nanocomposites modified glass carbon electrode (GCE) was tested by using the cyclic voltammetry (CV) measurements and the differential pulse voltammetry (DPV) measurements. Results show that the intercalation nanocomposites have a good electrocatalysis towards oxidation of nitrite with a peak potential decrease from 1.125V to 0.916V. There is a good linear correlation between the reduction peak current and the square root of the scan rate, indicating a diffusion controlled process. In the end, an estimate of the detection limit was $8.987\times 10^{-5}$M at a signal-to-noise ratio of 3.0 with a concentration range of $9.995\times 10^{-5}$M to $5.825\times 10^{-3}$M.