In the view of physiological significance, the transition-metal-mediated routes for nitrite (NO2 –) to nitric oxide (NO) conversion and phenol oxidation are of prime importance. Probing the reactivity of substituted phenols toward the nitritocopper­(II) cryptate complex [mC]­Cu­(κ2 -O2N)­(ClO4) (1a), this report illustrates NO release from nitrite at copper­(II) following a proton-coupled electron transfer (PCET) pathway. Moreover, a different protonated state of 1a with a proton hosted in the outer coordination sphere, [mCH]­Cu­(κ2 -O2N)­(ClO4)2 (3), also reacts with substituted phenols via primary electron transfer from the phenol. Intriguingly, the alternative mechanism operative because of the presence of a proton at the remote site in 3 facilitates an unusual anaerobic pathway for phenol nitration.

The controlled generation of hydrogen sulfide (H2S) under biologically relevant conditions is of paramount importance due to therapeutic interests. Via exploring the reactivity of a structurally characterized phenolate-bridged dinuclear zinc­(II)-aqua complex {LZnII(OH2)}2(ClO4)2 (1a) as a hydrolase model, we illustrate in this report that complex 1a readily hydrolyses CS2 in the presence of Et3N to afford H2S. In contrast, penta-coordinated [ZnII] sites in dinuclear {(LZnII)2(μ–X)}­(ClO4) complexes (7, X = OAc; 8, X = dimethylpyrazolyl) do not mediate CS2 hydrolysis in the presence of externally added water and Et3N presumably due to the unavailability of a coordination site for water at the [ZnII] centers. Moreover, [ZnII]–OH sites present in the isolated tetranuclear zinc­(II) complex {(LZnII)2(μ–OH)}2(ClO4)2 (4) react with CS2, thereby suggesting that the [ZnII]–OH site serves as the active nucleophile. Furthermore, mass spectrometric analyses on the reaction mixture consisting of 1a/Et3N and CS2 suggest the involvement of zinc­(II)–thiocarbonate (3a) and COS species, thereby providing mechanistic insights into CS2 hydrolysis mediated by the dinuclear [ZnII] hydrolase model complex 1a.

Nitrous acid (HONO) plays pivotal roles in various metal‐free as well as metal‐mediated routes relevant to biogeochemistry, atmospheric chemistry, and mammalian physiology. While the metastable nature of HONO hinders the detailed investigations into its reactivity at a transition metal site, this report herein utilizes a heteroditopic copper(II) cryptate [oC]CuII featuring a proton‐responsive second‐coordination‐sphere located at a suitable distance from a [CuII](ONO) core, thereby enabling isolation of a [CuII](κ1‐ONO⋅⋅⋅H+) complex (2H‐NO2). A set of complementary analytical studies (UV‐vis, 14N/15N FTIR, 15N NMR, HRMS, EPR, and CHN) on 2H‐NO2 and its 15N‐isotopomer (2H‐15NO2) reveals the formulation of 2H‐NO2 as {[oCH]CuII(κ1‐ONO)}(ClO4)2. Non‐covalent interaction index (NCI) based on reduced density gradient (RDG) analysis on {[oCH]CuII(κ1‐ONO)}2+ discloses a H‐bonding interaction between the apical 3° ammonium site and the nitrite anion bound to the copper(II) site. The FTIR spectra of [CuII](κ1‐ONO⋅⋅⋅H+) species (2H‐NO2) shows a shift of ammonium NH vibrational feature to a lower wavenumber due to the H‐bonding interaction with nitrite. The reactivity profile of [CuII](κ1‐ONO⋅⋅⋅H+) species (2H‐NO2) towards anaerobic nitration of substituted phenol (2,4‐DTBP) is distinctly different relative to that of the closely related tripodal [CuII]‐nitrite complexes (1‐NO2/3‐NO2/4‐NO2).

Reductions of nitrate and nitrite (NO x –) are of prime importance in combatting water pollution arising from the excessive use of N-rich fertilizers. While examples of NO x – reductions are known, this report illustrates hydrazine (N2H4)-mediated transformations of NO x – to nitric oxide (NO)/nitrous oxide (N2O). For nitrate reduction to NO, initial coordination of the weakly coordinating NO3 – anion at [(mC)CuII]2+ cryptate has been demonstrated to play a crucial role. A set of complementary analyses (X-ray diffraction and Fourier-transform infrared spectroscopy (FTIR), UV–vis, and NMR spectroscopies) on NO3 –-bound metal-cryptates [(mC)MII(NO3)](ClO4) (1-M, M = Cu/Zn) demonstrates the binding of NO3 – through noncovalent (NH···O, CH···O, and anion···π) and metal–ligand coordinate interactions. Subsequently, reactions of [(mC)CuII(14/15NO3)](ClO4) (1-Cu or 1-Cu/ 15 N) with N2H4·H2O have been illustrated to reduce 14/15NO3 – to 14/15NO. Intriguingly, in the absence of the second-coordination-sphere interactions, a closely related coordination motif [(Bz 3 Tren)CuII]2+ (in 3-Cu) does not bind NO3 – and is unable to assist in N2H4·H2O-mediated NO3 – reduction. In contrast, nitrite coordinates at the tripodal CuII sites in both [(mC)CuII]2+ and [(Bz 3 Tren)CuII]2+ irrespective of the additional noncovalent interactions, and hence, the N2H4 reactions of the copper(II)-nitrite complexes [(mC)CuII(O14/15NO)]+ and [(Bz 3 Tren)CuII(O14/15NO)]+ (in 2-Cu/4-Cu) result in a mixture of 14/15NO and N14/15NO.

Transformations of nitrogen‐oxyanions (NOx−) to ammonia impart pivotal roles in sustainable biogeochemical processes. While metal‐mediated reductions of NOx− are relatively well known, this report illustrates proton‐assisted transformations of NOx− anions in the presence of electron‐rich aromatics such as 1,3,5‐trimethoxybenzene (TMB−H, 1 a) leading to the formation of diaryl oxoammonium salt [(TMB)2N+=O][NO3−] (2 a) via the intermediacy of nitrosonium cation (NO+). Detailed characterizations including UV/Vis, multinuclear NMR, FT‐IR, HRMS, X‐ray analyses on a set of closely related metastable diaryl oxoammonium [Ar2N+=O] species disclose unambiguous structural and spectroscopic signatures. Oxoammonium salt 2 a exhibits 2 e− oxidative reactivity in the presence of oxidizable substrates such as benzylamine, thiol, and ferrocene. Intriguingly, reaction of 2 a with water affords ammonia. Perhaps of broader significance, this work reveals a new metal‐free route germane to the conversion of NOx to NH3.