Quinol-containing ligands enable high superoxide dismutase activity by modulating coordination number, charge, oxidation states and stability of manganese complexes throughout redox cycling

Abstract

Manganese complexes with polydentate quinol-containing ligands are found to catalyze the degradation of superoxide through inner-sphere mechanisms. The redox activity of the ligand stabilizes higher-valent manganese species.

A Macrocyclic Ligand Framework That Improves Both the Stability and T1-Weighted MRI Response of Quinol-Containing H2O2 Sensors

“…Complex 1 is also more stable than 2, which has a pMn of 7.25 under our standard conditions. 33 The stability of 1 also exceeds those of most other reported m o n o n u c l e a r M n ( I I ) -c o n t a i n i n g M R I c o n t r a s t agents. 9,14,37,39,40,45−48 The Gale and Caravan groups have recently reported a series of linear ligands with multiple carboxylate donors that coordinate tightly to Mn(II).…”

“…The gains in stability can be attributed largely to macrocyclic effects because H 4 qp2 and H 4 qp4 provide similar coordination spheres: four neutral nitrogen donors in addition to the two quinols/quinolates. Complex 1 is also more stable than 2 , which has a pMn of 7.25 under our standard conditions …”

Homogeneous catalysis of dioxygen reduction by molecular Mn complexes

“…73,143 Likewise, it is also possible that intrinsic factors that make Mn compounds excel at the disproportionation of superoxide and activation of hydrogen peroxide could be translated to the ORR. [144][145][146][147][148][149] In spite of the successes known from the effect of analogous modifications on Fe-and Co-based catalysts, development of comparable examples with Mn has lagged behind. For Mn, we speculate that its low activity results from its preference for open-shell configurations, which can facilitate the activation of O 2 beyond superoxide and the formation of intrinsically stable side-on modes, limiting kinetics.…”

Exceptional Manganese(II) Stability and Manganese(II)/Zinc(II) Selectivity with Rigid Polydentate Ligands**

“…Prominent examples are the oxidation of water to dioxygen in the oxygen‐evolving complex (OEC) in photosynthesis, the final reaction of the urea cycle catalyzed by arginase and the disproportionation of superoxide in the manganese‐based superoxide dismutase (SOD). [ 1 , 2 , 3 , 4 , 5 ] Manganese complexes are also increasingly important in medicinal applications, e.g., as SOD mimetics,[ 6 , 7 , 8 , 9 , 10 , 11 , 12 ] MRI contrast agents,[ 13 , 14 ] in chelation therapy of manganese overload and in‐cell fluorescence tracking of the labile manganese pool. [15] Essential for most of these applications are high kinetic inertness, high complex stability and selectivity with respect to other ubiquitous first‐row transition‐metal cations, specifically Zn II , the most relevant and abundant biological competitor.…”