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

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• 作者：Dominik Lieb ; Achim Zahl ; Tatyana E. Shubina ; Ivana Ivanovi-Burmazovi
• 刊名：Journal of the American Chemical Society
• 出版年：2010
• 出版时间：June 2, 2010
• 年：2010
• 卷：132
• 期：21
• 页码：7282-7284
• 全文大小：156K
• 年卷期：v.132,no.21(June 2, 2010)
• ISSN：1520-5126

文摘

In this work the rate constants (k_ex) and the activation parameters (ΔH^‡, ΔS^‡, and ΔV^‡) for the water exchange process on Mn^III centers have experimentally been determined using temperature and pressure dependent ¹⁷O NMR techniques. For the investigations the Mn^III porphyrin complexes [Mn^III(TPPS)S₂]ⁿ⁻ and [Mn^III(TMpyP)S₂]ⁿ⁺ (S = H₂O 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 ≈ 10⁷ 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−OH₂ bond found for a positively charged model porphyrin with protonated pyridine groups (2.446 Å) and for a simple model without any substituents on the porphyrin ring (2.437 Å). 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.