Water Exchange on Manganese(III) Porphyrins. Mechanistic Insights Relevant for Oxygen Evolving Complex and Superoxide Dismutation Catalysis

Abstract

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.

Electrochemical and Spectroelectrochemical Properties of Free‐Base Pyridyl‐ and N‐Alkyl‐4‐Pyridylporphyrins in Nonaqueous Media

“…The second, represented as [5] 2À (ii),i sg enerated in pyridine containing 0.1 m TBAP and is characterized as having an on-ion-paired N-methylpyridyl group. In each solvent/supporting electrolyte system, the first reduction proceeds throughar eversible two-electron transfer at about E 1/2 = À0.80 to À0.86 Vt og ive an electrogenerated product that can be further reduced in one of three separate Scheme4.Reductionm echanism of mono-N-methylpyridyl compounds 5a and 5b,where X À = I À or B(C 6 F 5 ) 4 À .…”

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More than three dozen different metal ions have been incorporated into the macrocyclic core of water-soluble tetra-N-methylpyridylporphyrins, and many of these complexes have been studied with respect to their physicochemical [1][2][3][4][5][6][7][8][9] and electrochemical [10][11][12][13][14][15][16][17][18][19][20][21][22] properties in both aqueous and nonaqueous media. These compounds are easily reducible in both water and nonaqueous solvents, and an umber of the complexes have been used in nuclear medicine, [21,[23][24][25][26] examples being given by tetra-N-methylpyridylporphyrins having the formula [M(TMPyP)] n + (X À ) n where TMPyP represents the porphyrin macrocycle with four meso-substituted N-methylpyridyl groups, n = 4o r5 ,X À = an anion and M = In II ,Mn III ,Fe III ,o rG d III .

The electrochemistry of [M(TMPyP)] n + (X À ) n in nonaqueous media has been characterizeda lmoste xclusively with respect to the reductionst hat can occur at the conjugated p-ring system of the macrocycle, the electroactive N-methylpyridyl substituents, and, in some cases, at the central metal ions.

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Detailed mechanism of the autoxidation of N-hydroxyurea catalyzed by a superoxide dismutase mimic Mn(iii) porphyrin: formation of the nitrosylated Mn(ii) porphyrin as an intermediate

“…Based on the activation parameters, the mechanism of the water exchange is assumed to be dissociative interchange (I d ). 18–19 A UV-vis photometric titration under anaerobic conditions was carried out also with the reduced Mn(II)TTEG: p K a3 = 9.8 ± 0.1 and p K a4 = 11.7 ± 0.1. The spectra of the corresponding species were calculated (Figure S1).…”

Synthesis and characterization of four novel manganese(II) chains formed by 4,4′-azobis(pyridine) and benzoate or nitrobenzoates: Stabilization of unusual ladder structures

“…Among the transition metal ions, manganese-based coordination compounds have received considerable interest in the fields of supramolecular chemistry and crystal engineering not only for their remarkable magnetochemical properties [21][22][23][24], but also for their rich biochemistry [25][26][27]. Furthermore, due to their low cost and toxicity, manganese coordination compounds are growing in importance as homogeneous catalysts [28][29][30][31]. The carboxylate groups, which can display a wide variety of coordination modes such as chelating (Scheme 1a), terminal (Scheme 1b) and bridging syn-syn (Scheme 1c), anti-anti (Scheme 1d) or syn-anti (Scheme 1e), are one of the most widely used bridging ligands for designing polynuclear complexes with interesting structures and topologies [3].…”