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

   https://doi.org/10.1039/D1SC02465E  

   Senft, Laura; Moore, Jamonica L.; Franke, Alicja; Fisher, Katherine R.; Scheitler, Andreas; Zahl, Achim; Puchta, Ralph; Fehn, Dominik; Ison, Sidney; Sader, Safaa; et al (January 2021, Chemical Science)

   null (Ed.)

   Reactivity assays previously suggested that two quinol-containing MRI contrast agent sensors for H 2 O 2 , [Mn( H2qp1 )(MeCN)] 2+ and [Mn( H4qp2 )Br 2 ], could also catalytically degrade superoxide. Subsequently, [Zn( H2qp1 )(OTf)] + was found to use the redox activity of the H2qp1 ligand to catalyze the conversion of O 2 ˙ − to O 2 and H 2 O 2 , raising the possibility that the organic ligand, rather than the metal, could serve as the redox partner for O 2 ˙ − in the manganese chemistry. Here, we use stopped-flow kinetics and cryospray-ionization mass spectrometry (CSI-MS) analysis of the direct reactions between the manganese-containing contrast agents and O 2 ˙ − to confirm the activity and elucidate the catalytic mechanism. The obtained data are consistent with the operation of multiple parallel catalytic cycles, with both the quinol groups and manganese cycling through different oxidation states during the reactions with superoxide. The choice of ligand impacts the overall charges of the intermediates and allows us to visualize complementary sets of intermediates within the catalytic cycles using CSI-MS. With the diquinolic H4qp2 , we detect Mn( iii )-superoxo intermediates with both reduced and oxidized forms of the ligand, a Mn( iii )-hydroperoxo compound, and what is formally a Mn( iv )-oxo species with the monoquinolate/mono- para -quinone form of H4qp2 . With the monoquinolic H2qp1 , we observe a Mn( ii )-superoxo ↔ Mn( iii )-peroxo intermediate with the oxidized para -quinone form of the ligand. The observation of these species suggests inner-sphere mechanisms for O 2 ˙ − oxidation and reduction that include both the ligand and manganese as redox partners. The higher positive charges of the complexes with the reduced and oxidized forms of H2qp1 compared to those with related forms of H4qp2 result in higher catalytic activity ( k cat ∼ 10 8 M −1 s −1 at pH 7.4) that rivals those of the most active superoxide dismutase (SOD) mimics. The manganese complex with H2qp1 is markedly more stable in water than other highly active non-porphyrin-based and even some Mn( ii ) porphyrin-based SOD mimics. 

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2. Use of Intramolecular Quinol Redox Couples to Facilitate the Catalytic Transformation of O ₂ and O ₂ -Derived Species

   https://doi.org/10.1021/acs.accounts.4c00645  

   Farnum, Byron H; Goldsmith, Christian R (January 2025, Accounts of Chemical Research)

   The redox reactivity of transition metal centers can be augmented by nearby redox-active inorganic or organic moieties. In some cases, these functional groups can even allow a metal center to participate in reactions that were previously inaccessible to both the metal center and the functional group by themselves. Our research groups have been synthesizing and characterizing coordination complexes with polydentate quinol-containing ligands. Quinol is capable of being reversibly oxidized by either one or two electrons to semiquinone or para-quinone, respectively. Functionally, quinol behaves much differently than phenol, even though the pKa values of the first O−H bonds are nearly identical. The redox activity of the quinol in the polydentate ligand can augment the abilities of bound redox-active metals to catalyze the dismutation of O2−• and H2O2. These complexes can thereby act as high-performing functional mimics of superoxide dismutase (SOD) and catalase (CAT) enzymes, which exclusively use redox-active metals to transfer electrons to and from these reactive oxygen species (ROS). The quinols augment the activity of redox-active metals by stabilizing higher-valent metal species, providing alternative redox partners for the oxidation and reduction of reactive oxygen species, and protecting the catalyst from destructive side reactions. The covalently attached quinols can even enable redox-inactive Zn(II) to catalyze the degradation of ROS. With the Zn(II)-containing SOD and CAT mimics, the organic redox couple entirely substitutes for the inorganic redox couples used by the enzymes. The ligand structure modulates the antioxidant activity, and thus far, we have found that compounds that have poor or negligible SOD activity can nonetheless behave as efficient CAT mimics. Quinol-containing ligands have also been used to prepare electrocatalysts for dioxygen reduction, functionally mimicking the enzyme cytochrome c oxidase. The installation of quinols can boost electrocatalytic activity and even enable otherwise inactive ligand frameworks to support electrocatalysis. The quinols can also shift the product selectivity of O2 reduction from H2O2 to H2O without markedly increasing the effective overpotential. Distinct control of the coordination environment around the metal center allows the most successful of these catalysts to use economic and naturally abundant first-row transition metals such as iron and cobalt to selectively reduce O2 to H2O at low effective overpotentials. With iron, we have found that the electrocatalysts can enter the catalytic cycle as either an Fe(II) or Fe(III) species with no difference in turnover frequency. The entry point to the cycle, however, has a marked impact on the effective overpotential, with the Fe(III) species thus far being more efficient. 

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3. Aerobic C–C and C–O bond formation reactions mediated by high-valent nickel species

   https://doi.org/10.1039/c9sc03758f  

   Smith, Sofia M.; Planas, Oriol; Gómez, Laura; Rath, Nigam P.; Ribas, Xavi; Mirica, Liviu M. (November 2019, Chemical Science)

   Nickel complexes have been widely employed as catalysts in C–C and C–heteroatom bond formation reactions. While Ni(0), Ni( i ), and Ni( ii ) intermediates are most relevant in these transformations, recently Ni( iii ) and Ni( iv ) species have also been proposed to play a role in catalysis. Reported herein is the synthesis, detailed characterization, and reactivity of a series of Ni( ii ) and Ni( iii ) metallacycle complexes stabilized by tetradentate pyridinophane ligands with various N-substituents. Interestingly, while the oxidation of the Ni( ii ) complexes with various other oxidants led to exclusive C–C bond formation in very good yields, the use of O 2 or H 2 O 2 as oxidants led to formation of appreciable amounts of C–O bond formation products, especially for the Ni( ii ) complex supported by an asymmetric pyridinophane ligand containing one tosyl N-substituent. Moreover, cryo-ESI-MS studies support the formation of several high-valent Ni species as key intermediates in this uncommon Ni-mediated oxygenase-type chemistry. 

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4. Manganese Salan Complexes as Catalysts for Hydrosilylation of Aldehydes and Ketones

   https://doi.org/10.3390/catal13040665  

   Almutairi, Nora; Vijjamarri, Srikanth; Du, Guodong (April 2023, Catalysts)

   Manganese has attracted significant recent attention due to its abundance, low toxicity, and versatility in catalysis. In the present study, a series of manganese (III) complexes supported by salan ligands have been synthesized and characterized, and their activity as catalysts in the hydrosilylation of carbonyl compounds was examined. While manganese (III) chloride complexes exhibited minimal catalytic efficacy without activation of silver perchlorate, manganese (III) azide complexes showed good activity in the hydrosilylation of carbonyl compounds. Under optimized reaction conditions, several types of aldehydes and ketones could be reduced with good yields and tolerance to a variety of functional groups. The possible mechanisms of silane activation and hydrosilylation were discussed in light of relevant experimental observations. 

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5. Phosphorous-substituted redox-active ligands in base metal hydrosilylation catalysis

   https://doi.org/10.1039/D1DT02879K  

   Sharma, Anuja; Trovitch, Ryan J. (November 2021, Dalton Transactions)

   This article highlights the utilization of phosphine-containing redox-active ligands for efficient hydrosilylation catalysis. Manganese, iron, cobalt, and nickel precatalysts featuring these chelates have been described and leading activities for carbonyl, carboxylate, and ester C–O bond hydrosilylation have been achieved. Mechanistic studies have provided insight into the importance of phosphine hemilability. 

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