Manganese Porphyrin Heterodimers and -trimers in Aqueous Solution

Fluorinated Mn(III)/(II)-Porphyrin with Redox-Responsive 1 H and 19 F Relaxation Properties

A new fluorinated manganese porphyrin, (Mn-TPP-p-CF3 ) is reported capable of providing, based on the Mn(III)/Mn(II) equilibrium, dual 1 H relaxivity and 19 F NMR response to redox changes. The physical-chemical characterization of both redox states in DMSO-d6 /H2 O evidenced that the 1 H relaxometric and 19 F NMR properties are appropriate for differential redox MRI detection. The Mn(III)-F distance (dMn-F =9.7-10 Å), as assessed by DFT calculations, is well tailored to allow for adequate paramagnetic effect of Mn(III) on 19 F T1 and T2 relaxation times. Mn-TPP-p-CF3 has a reversible Mn(II)/Mn(III) redox potential of 0.574 V vs. NHE in deoxygenated aqueous HEPES/ THF solution. The reduction of Mn(III)-TPP-p-CF3 in the presence of ascorbic acid is slowly, but fully reversed in the presence of air oxygen, as monitored by UV-Vis spectrometry and 19 F NMR. The broad 1 H and 19 F NMR signals of Mn(III)-TPP-p-CF3 disappear in the presence of 1 equivalent ascorbate replaced by a shifted and broadened 19 F NMR signal from Mn(II)-TPP-p-CF3 . Phantom 19 F MR images in DMSO show a MRI signal intensity decrease upon reduction of Mn(III)-TPP-p-CF3 , retrieved upon complete reoxidation in air within ~24 h. 1 H NMRD curves of the Mn(III)/(II)-TPP-p-CF3 chelates in mixed DMSO/water solvent have the typical shape of Mn(II)/Mn(III) porphyrins.

Electrocatalytic reduction of dioxygen by Mn(iii) meso-tetra(N-methylpyridinium-4-yl)porphyrin in universal buffer

The electrochemical characterization of manganese(iii) meso-tetra(N-methylpyridinium-4-yl)porphyrin pentachloride ([Mn(TMPyP)Cl][Cl]₄) via cyclic voltammetry (CV) and UV-vis spectroelectrochemistry (UV-vis SEC) was performed across the entire pH domain in aqueous buffered conditions. Assessment of the homogeneous electrocatalytic efficiency for the oxygen reduction reaction (ORR) from pH 3 to 6 using rotating-ring disk electrode experiments (RRDE) found it to be selective for water (82 to 93%). The observed efficiency for water is in contrast to previous reports on electrocatalytic ORR activity by Mn porphyrins in aqueous systems, which identified H₂O₂ as the primary product using indirect RDE methods only. The results described here are consistent with recent reports on the electrocatalytic behavior of Mn porphyrins under nonaqueous conditions, where the similar selectivity for water was also determined by RRDE methods. At pH 1, UV-vis SEC experiments also revealed that decomposition was occurring; free-base porphyrin was observed after the application of reducing potentials.

A biocompatible redox MRI probe based on a Mn(ii)/Mn(iii) porphyrin

A water-soluble fluorinated Mn^III/II porphyrin responds reversibly to ascorbate redox state as a turn-on MRI probe.

Nuclearity versus oxidation state in the catalytic efficiency of MnII/III azo Schiff base complexes: computational study on supramolecular interactions and phenoxazinone synthase-like activity

Supramolecular interactions of a mononuclear Mn(iii) and a tetranuclear Zn(ii)–Mn(ii) complexes and their comparative bio mimetic catalytic activity have been reported.

Solid-state Porphyrin Interactions with Oppositely Charged Peripheral Groups

The crystallization and the crystal and molecular structure of a very slightly soluble electrostatically interacting pair of porphyrins is described. The tetra-anion 5,10,15,20-tetrakis-(4-sulfonatophenyl)-21,23H-porphyrin [H2TPPSO3]4- and the tetra-cation 5,10,15,20-tetra(N-methylpyridyl)21H,23H-porphyrin [H2TMePyP]4+ are found to form an alternating one-dimensional stack that is stabilised by electrostatic interactions between the porphyrin rings but also by π-π interactions between all substituted phenyl rings in the ensemble. The resulting interactions between the porphyrins is exceptionally tight.

Electrochemistry and spectroelectrochemistry of 5,10,15,20-tetrakis(1,3-benzodioxole) porphyrin and its manganese and iron complexes

Cyclic voltammetry and thin-layer spectroelectrochemical properties of the free base 5,10,15,20-tetrakis(1,3-benzodioxole)porphyrin ( H 2 P ), and its manganese and iron complexes ( MnP and FeP , respectively) were studied in dichloromethane with tetra-butyl ammonium hexafluorophosphate (TBAPF6) as supporting electrolyte. The free base undergoes two monoelectronic oxidations at +0.71 and +0.96 V vs.  Ag / Ag +, corresponding to the formations of the π-cation radical and dication. Two reductions were observed at -1.53 and -1.87 V corresponding to the formation of π-anion and dianion species. The cyclic voltammograms of the Mn III and Fe III complexes also showed a reversible process centered in the metal ion with E1/2 values at -0.61 and -0.59 V, respectively. In a typical experiment, in situ spectroelectrochemical response showed a decrease of the Soret band at 422 nm, a red shift of the Q-bands, and the new low intensity bands between 800–950 nm range. Structural features and spectroscopic assignments were proposed and discussed based on semi-empirical (molecular mechanics MMFF and ZINDO/S (Zerner method of intermediate neglect of differential overlap for spectroscopy) calculations.

Water exchange on manganese(III) porphyrins. Mechanistic insights relevant for oxygen evolving complex and superoxide dismutation catalysis

In this work the rate constants (k(ex)) and the activation parameters (DeltaH(double dagger), DeltaS(double dagger), and DeltaV(double dagger)) for the water exchange process on Mn(III) centers have experimentally been determined using temperature and pressure dependent (17)O NMR techniques. For the investigations the Mn(III) porphyrin complexes [Mn(III)(TPPS)S(2)](n-) and [Mn(III)(TMpyP)S(2)](n+) (S = H(2)O and/or OH(-)) have been selected due to their high solution stability in a wide pH range, enabling the measurements of water exchange in the case of both diaqua and aqua-hydroxo complexes. We have experimentally demonstrated that the water exchange on Mn(III) porphyrins is a fast process (k(ex) approximately = 10(7) s(-1)) of an I(d) to I mechanism, strongly influenced by a Jahn-Teller effect and as such almost independent of a porphyrin charge and a trans ligand. This is also supported by our DFT calculations which show only a slight difference in an average Mn(III)-OH(2) bond found for a positively charged model porphyrin with protonated pyridine groups (2.446 A) and for a simple model without any substituents on the porphyrin ring (2.437 A). The calculated effective charge on the Mn center, which is significantly lower than its formal +3 charge (ca. +1.5 for diaqua; +1.4 for aqua-hydroxo), also contributes to its substitution lability. The herein presented results are discussed in connection to a possible fast exchanging substrate binding site in photosystem II and corresponding inorganic model complexes, as well as in the context of a possible inner-sphere catalytic pathway for superoxide dismutation on Mn centers.

H-aggregation of cationic palladium-porphyrin as a function of anionic surfactant studied using phosphorescence, absorption and RLS spectra

Room temperature phosphorescence (RTP) was used as a useful analytical tool to investigate the interaction behavior between tetracationic meso-tetrakis (4-trimethylaminophenyl) porphyrin palladium (Pd-TAPP) and anionic sodium dodecyl sulfate (SDS). UV-vis absorption and resonance light scattering (RLS) were further applied to characterize the system. It was presumably suggested that nonspecific self-aggregates among porphyrins formed considering the relatively high concentration of Pd-TAPP. Furthermore, Pd-TAPP changed from free monomer/nonspecific aggregate to H-aggregate and then to out-micellized monomer/nonspecific aggregate as a function of SDS concentration. The fact that RTP signal enhanced obviously and excitation spectrum was blue-shifted by 1580cm(-1) in energy with respect to energy of free Pd-TAPP monomer demonstrated that 1:4 electrostatic interaction between Pd-TAPP and SDS led to the formation of the premicellar porphyrin-surfactant H-aggregates. The RLS spectrum reviewed that the formed H-aggregates were multiple porphyrin units, and UV-vis spectra revealed that cationic groups of monomers/nonspecific aggregates of Pd-TAPP were electrostatically attracted in favor of the surface of anionic micelles but were not encapsulated within apolar regions of SDS micelles when the concentration of SDS was above its critical micelle concentration (CMC).

Study on aggregation of palladium-porphyrins using room temperature phosphorescence

This paper reported room temperature phosphorescence (RTP) behaviors of meso-tetra-(4-sulfonatophenyl) porphyrin palladium (Pd-TSPP) and meso-tetra-(4-trimethylaminophenyl) porphyrin palladium (Pd-TAPP) in bovine serum albumin (BSA) medium. It was found that Pd-TSPP self-aggregated with its increasing concentration and hetero-aggregated with Pd-TAPP when they were mixed together. The self-aggregation of Pd-TSPP resulted in the remarkably splitting of excitation spectra because of the strongly excitonic coupling and phosphorescence quenching excited by Soret band, while Q band always kept the increase in intensity. The hetero-aggregation was out of the ground-state interaction stronger than the former one owing to its electrostatic-interaction nature. It was also indicated that inorganic salts like KCl would be an aid to hetero-aggregation. The equilibrium constants of both kinds of aggregation were estimated, namely, K(hom)=1.9 x 10(5) l/mol (homo-aggregation), and K(het)=1.06 x 10(7) l/mol (hetero-aggregation).

Non-covalent synthesis in aqueous solution of discrete multi-porphyrin aggregates with programmable stoichiometry and sequence

This unprecedented calixarene templated, noncovalent synthesis of multi-porphyrin complexes in aqueous solution makes it possible to plan the nature, stoichiometry, and sequence of porphyrins. The proposed method presents the same accuracy of the covalent approach and a 100% yield, but it is not as time-consuming as the latter method.