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1. Group 13 ion coordination to pyridyl models NAD ⁺ reduction potentials

   https://doi.org/10.1039/D3CC02562D  

   Parsons, Leo_W T; Fettinger, James C; Berben, Louise A (July 2023, Chemical Communications)

   N-alkylation andN-metallation of pyridine are explored herein to understand how metal-ligand complexes can model NAD⁺redox chemistry. 

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2. A Redox‐Active Tetrazine‐Based Pincer Ligand for the Reduction of N‐Oxyanions Using a Redox‐Inert Metal

   https://doi.org/10.1002/chem.202101302  

   Beagan, Daniel_M; Maciulis, Nicholas_A; Pink, Maren; Carta, Veronica; Huerfano, I_J; Chen, Chun‐Hsing; Caulton, Kenneth_G (July 2021, Chemistry – A European Journal)

   Abstract The reaction chemistry of the bis‐tetrazinyl pyridine ligand (btzp) towards nitrogen oxyanions coordinated to zinc is studied in order to explore the reduction of the NO_x⁻substrates with a redox‐active ligand in the absence of redox activity at the metal. Following syntheses and characterization of (btzp)ZnX₂for X=Cl, NO₃and NO₂, featuring O−Zn linkage of both nitrogen oxyanions, it is shown that a silylating agent selectively delivers silyl substituents to tetrazine nitrogens, without reductive deoxygenation of NO_x⁻¹. A new synthesis of the highly hydrogenated H₄btzp, containing two dihydrotetrazine reductants is described as is the synthesis and characterization of (H₄btzp)ZnX₂for X=Cl and NO₃, both of which show considerable hydrogen bonding potential of the dihydrotetrazine ring NH groups. The (H₄btzp)ZnCl₂complex does not bind zinc in the pincer pocket, but instead H₄btzp becomes a bridge between neighboring atoms through tetrazine nitrogen atoms, forming a polymeric chain. The reaction of AgNO₂with (H₄btzp)ZnCl₂is shown to proceed with fast nitrite deoxygenation, yielding water and free NO. Half of the H₄btzp reducing equivalents form Ag⁰and thus the chloride ligand remains coordinated to the zinc metal center to yield (btzp)ZnCl₂. To compare with AgNO₂, experiments of (H₄btzp)ZnCl₂with NaNO₂result in salt metathesis between chloride and nitrite, highlighting the importance of a redox‐active cation in the reduction of nitrite to NO. 

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3. Bimetallic, Silylene‐Mediated Multielectron Reductions of Carbon Dioxide and Ethylene

   https://doi.org/10.1002/anie.202011489  

   Whited, Matthew T.; Zhang, Jia; Conley, Anna M.; Ma, Senjie; Janzen, Daron E.; Kohen, Daniela (November 2020, Angewandte Chemie International Edition)

   Abstract A metal/ligand cooperative approach to the reduction of small molecules by metal silylene complexes (R₂Si=M) is demonstrated, whereby silicon activates the incoming substrate and mediates net two‐electron transformations by one‐electron redox processes at two metal centers. An appropriately tuned cationic pincer cobalt(I) complex, featuring a central silylene donor, reacts with CO₂to afford a bimetallic siloxane, featuring two Co^IIcenters, with liberation of CO; reaction of the silylene complex with ethylene yields a similar bimetallic product with an ethylene bridge. Experimental and computational studies suggest a plausible mechanism proceeding by [2+2] cycloaddition to the silylene complex, which is quite sensitive to the steric environment. The Co^II/Co^IIproducts are reactive to oxidation and reduction. Taken together, these findings demonstrate a strategy for metal/ligand cooperative small‐molecule activation that is well‐suited to 3dmetals. 

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4. Conformational Switching through the One‐Electron Reduction of an Acridinium‐based, γ‐Cationic Phosphine Gold Complex

   https://doi.org/10.1002/chem.202100389  

   Liu, Wei‐Chun; Kim, Youngmin; Gabbaï, François_P (March 2021, Chemistry – A European Journal)

   Abstract Our efforts in the chemistry of gold complexes featuring ambiphilic phosphine‐carbenium L/Z‐type ligand have led us to consider the reduction of the carbenium moiety as a means to modulate the gold–carbenium interaction present in these complexes. Here, it was shown that the one‐electron reduction of [(o‐Ph₂P(C₆H₄)Acr)AuCl]⁺(Acr=9‐N‐methylacridinium) produces a neutral stable radical, the structure of which showed a marked increase in the Au–Acr distance. Related structural changes were observed for the phosphine oxide analogue [(o‐Ph₂P(O)(C₆H₄)Acr]⁺, the reduction of which interfered with the P=O→carbenium interaction. These structural effects, driven by a reduction‐induced change in the electronic and electrostatic characteristics of the compounds, showed that the charge and accepting properties of the carbenium unit can be modulated. These results highlight the redox‐noninnocence of carbenium Z‐type ligand, a feature that can be exploited to induce specific conformational changes. 

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5. Steric Tuning of Spin States and Redox Potentials in Tris(imidazole) Triazacyclononane Complexes of Fe ^2+/3+ and Co ^2+/3+

   https://doi.org/10.1002/ejic.202400739  

   Gaynor, Ryan B; Shy, William E; Williams, Emily B; Parks, Omri O; Ingram, Nicole E; Creutz, Sidney E (April 2025, European Journal of Inorganic Chemistry)

   Abstract A series of Co^2+/3+and Fe^2+/3+complexes is prepared using three variants of a hexadentate tris(imidazole)triazacyclononane ligand bearing different 4‐alkyl substituents on the imidazole rings. The steric bulk of the alkyl substituent (R=H,ⁱPr, or^tBu) alters the preferred size of the ligand binding cavity by inhibiting close approach of the imidazole donors with bulky substituents. The resulting changes in geometry, redox potentials, spin states, and optical properties are catalogued across the series, demonstrating redox potential tuning over at least 670 mV as well as spin state switching based on the choice of substituent. The ligand field splitting of the complexes decreases with increasing bulk of the substituents. Tuning of the steric bulk of the substituents in these positions therefore allows for the electronic properties of the complexes to be fine‐tuned in a manner orthogonal to the donor properties of the substituents. 

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