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

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 NOx− 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 NOx−1. A new synthesis of the highly hydrogenated H4btzp, containing two dihydrotetrazine reductants is described as is the synthesis and characterization of (H4btzp)ZnX2 for X=Cl and NO3, both of which show considerable hydrogen bonding potential of the dihydrotetrazine ring NH groups. The (H4btzp)ZnCl2 complex 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 AgNO2 with (H4btzp)ZnCl2 is shown to proceed with fast nitrite deoxygenation, yielding water and free NO. Half of the H4btzp 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 (H4btzp)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.

Modeling Reactivity of Nitrite and Nitrous Acid at a Phenolate Bridged Dizinc(II) Site: Insights into NO Signaling at Zinc

“…This study has illustrated that the 1e − reduction of nitrite‐to‐NO at a redox‐inactive [Zn II ] site involves persulfidation of thiol, where the sulfane sulfur species serve as the formal oxidant [15] . Notably, a previous report demonstrate that a reaction of ( H 4 btzp )ZnCl 2 (where, H 4 btzp =bis‐dihydrotetrazene pyridine) with AgNO 2 results in NO [16] . A detailed investigations including control experiments highlight that the dihydrotetrazene moieties from redox‐active ligand H 4 btzp plays an important role in the 1e − reduction of nitrite‐to‐NO along with concomitant reduction of Ag + to Ag, leaving the [Zn II ] site inactive.…”

“…Nevertheless, insights into the reactivity of zinc(II)‐aqua complex with NO 2 − /H + or RONO relevant to nitrite anhydrase activity remains unexplored. Moreover, the examples of nitrite‐to‐NO transformation at [Zn II ] sites in the presence of external reductants are rare [13,14,16] …”

“…[15] Notably, a previous report demonstrate that a reaction of (H 4 btzp)ZnCl 2 (where, H 4 btzp = bis-dihydrotetrazene pyridine) with AgNO 2 results in NO. [16] A detailed investigations including control experiments highlight that the dihydrotetrazene moieties from redox-active ligand H 4 btzp plays an important role in the 1e À reduction of nitrite-to-NO along with concomitant reduction of Ag + to Ag, leaving the [Zn II ] site inactive. Nevertheless, insights into the reactivity of zinc(II)-aqua complex with NO 2 À /H + or RONO relevant to nitrite anhydrase activity remains unexplored.…”

“…Moreover, the examples of nitriteto-NO transformation at [Zn II ] sites in the presence of external reductants are rare. [13,14,16] Aiming to understand the interactions of [Zn II ]-OH 2 species with nitrous acid and organonitrite (RONO, R = alkyl, H), we herein report reactions of a phenolate-bridged dizinc(II)-aqua [17] complex [L H Zn II (OH 2 )] 2 (ClO 4 ) 2 (1 H -Aq, where L H = tridentate N,N,O-donor monoanionic ligand) with t BuONO or NO 2 À /H + resulting in an arene-nitrosonium charge-transfer species (2 H ) (Figures 2 and 3). A wide range of complementary spectroscopic analyses including UV-vis, 14 N/ 15 N FTIR, and multinuclear ( 1 H, 15 N) NMR suggest the nature of complex 2 H .…”

Aqueous Zn-Tetrazine Batteries with Cooperative Zn2+/H+ Insertion

“…−0.09 V vs Zn 2+ /Zn (Figure A, red trace), rendering it impractical as cathode materials in AZIBs. However, the coordination of Lewis acidic ions, e.g., Sc 3+ , Fe 2+ , and Zn 2+ , to tetrazine has been shown to shift their redox potentials anodically. − …”