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

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)ZnX₂ for X=Cl, NO₃ and 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 H₄ btzp, containing two dihydrotetrazine reductants is described as is the synthesis and characterization of (H₄ btzp)ZnX₂ for X=Cl and NO₃ , both of which show considerable hydrogen bonding potential of the dihydrotetrazine ring NH groups. The (H₄ btzp)ZnCl₂ complex does not bind zinc in the pincer pocket, but instead H₄ btzp becomes a bridge between neighboring atoms through tetrazine nitrogen atoms, forming a polymeric chain. The reaction of AgNO₂ with (H₄ btzp)ZnCl₂ is shown to proceed with fast nitrite deoxygenation, yielding water and free NO. Half of the H₄ btzp reducing equivalents form Ag⁰ and thus the chloride ligand remains coordinated to the zinc metal center to yield (btzp)ZnCl₂ . To compare with AgNO₂ , experiments of (H₄ btzp)ZnCl₂ with NaNO₂ result in salt metathesis between chloride and nitrite, highlighting the importance of a redox-active cation in the reduction of nitrite to NO.