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

Although nitrite-to-NO transformation at various transition metals including Fe and Cu are relatively well explored, examples of such a reaction at the redox-inactive zinc(II) site are limited. The present report aims to gain insights into the reactivity of nitrite anions, nitrous acid (HONO), and organonitrite (RONO) at a dizinc(II) site. A phenolate-bridged dizinc(II)-aqua complex [LHZnII(OH2)]2(ClO4)2 (1H-Aq, where LH=tridentate N,N,O-donor monoanionic ligand) is illustrated to react with tBuONO to provide a metastable arene-nitrosonium charge-transfer complex 2H. UV-vis, FTIR, multinuclear NMR, and elemental analyses suggests the presence of a 2 : 1 arene-nitrosonium moiety. Furthermore, the reactivity of a structurally characterized zinc(II)-nitrite complex [LHZnII(ONO)]2 (1H-ONO) with a proton-source demonstrates HONO reactivity at the dizinc(II) site. Reactivity of both RONO (R=alkyl/H) at the phenolate-bridged dizinc(II) site provides NO+ charge-transfer complex 2H. Subsequently, the reactions of 2H with exogenous reductants (such as ferrocene, thiol, phenol, and catechol) have been illustrated to generate NO. In addition, NO yielding reactivity of [LHZnII(ONO)]2 (1H-ONO) in the presence of the above-mentioned reductants have been compared with the reactions of complex 2H. Thus, this report sheds light on the transformations of NO2-/RONO (R=alkyl/H) to NO/NO+ at the redox-inactive zinc(II) coordination motif. Nitric oxide from (in)organic nitrite at zinc(II): Reactions of a dizinc(II) coordination site with organonitrite or nitrous acid (RONO, R=tBu/amyl/H) result in the formation of a dark green colored arene-NO+ charge-transfer complex. One electron reductive transformation of the NO+ charge-transfer moiety and zinc(II)-nitrite complex towards NO, in the presence of exogenous reductants, have been illustrated.image