Oxidation states of manganese porphyrins in water solutions

Abstract

The Mn oxidation state of two water soluble Mn porphyrins, MnTMPyP and MnTPPS 4 , was studied as a function of the aqua or hydroxo ligands of the Mn atom. In NaOH solutions, longlived O=Mn(IV) species were detected in the presence of O 2 . Conversely, the dihydroxo Mn(III) porphyrin reduces spontaneously to the Mn(II) species in the absence of O 2 . In alkaline solutions, these Mn porphyrins were able to electrocatalyze the 4-electron reduction of O 2 to H 2 O on a vitreous carbon electrode. 933397878 ∏ SPP full member in good standing Journal of Porphyrins and Phthalocyanines Published at http://www.u-bourgogne.fr/jpp/ J. Porphyrins Phthalocyanines 2005; 9: 436-443 Published on web 07/21/2005 J. Porphyrins Phthalocyanines 2005.09:436-443. Downloaded from www.worldscientific.com by EMORY UNIVERSITY on 08/05/15. For personal use only.

Studies of poly(3,4-ethylenedioxythiophene) (PEDOT) films containing cationic Mn porphyrins. A loading-dependent demetalation of Mn(III)TPP in PEDOT (Mn(III)TPP=5,10,15,20-tetraphenylporphyrinato manganese(III))

“…TPPS and MnTPPS were prepared by modifications of a previously published procedure [9,13,14]. The absorbance peaks and their extinction coefficients for MnTPPS agreed with published data [12]. TPP (0.1 M) was protonated by dissolution in dichloromethane, followed by the addition of a few drops of trifluoroacetic acid to the mixture to protonate the porphyrin.…”

“…It should be noted that such a photoreduction was not observed during the preparation of solutions for UV-Vis analysis with Mn(III)TPP alone. These studies involved a carefully degassed solvent [12,27,35], as was also required when the porphyrin was reduced in solution using sodium borohydride. Without an inert atmosphere, the porphyrin would quickly oxidize back to its Mn(III) ion upon even minute exposure to air oxygen.…”

Differential Coordination Demands in Fe versus Mn Water-Soluble Cationic Metalloporphyrins Translate into Remarkably Different Aqueous Redox Chemistry and Biology

“…8, 24, 48 The OH − axial ligand affects both thermodynamics and kinetics of O 2 •− dismutation. It neutralizes single charge on Fe site and in turn reduces favorable electrostatics with the tetracationic (OH)(H 2 O)FeP species relative to the pentacationic (H 2 O) MnP species.…”

“…As detailed under the Redox Property of Metal Site and Protonation Equilibria, the differences are the consequence of the vast difference in the electron-deficiency of the metal site, which results in different species present in solution under physiological conditions: (OH)(H 2 O)FePs vs (H 2 O) 2 MnP. 8,24,48 The OH − axial ligand affects both thermodynamics and kinetics of O 2…”

Structures Self‐Assembled from Anionic Graphene and Cationic Manganese Porphyrin: Characterization and Application in Artificial Photosynthesis

“…[46] Graphene oxide (GO) is characterized by two different carboxylic acidic groups: Those that are in close proximity to a phenol or hydroxy group and those that are not. Because at pH 13, Mn III TMPyP converts totally from an aqua-hydroxo into a dihydroxo species (pK a1 and pK a2 of 10.5 and 11.4, respectively), [51] this may be due to a decrease in the electrostatic attraction between CGO and Mn III TMPyP(OH -) 2 and the release of the latter into solution. [47] CGO-MnTMPyP was studied spectroscopically and electrochemically in the pH range 6-13, in which most of the carboxy groups are deprotonated.…”