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 [LH ZnII (OH2 )]2 (ClO4 )2 (1H -Aq, where LH =tridentate N,N,O-donor monoanionic ligand) is illustrated to react with t BuONO 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 [LH ZnII (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 [LH ZnII (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.

Nitrate and Nitrite Reductions at Copper(II) Sites: Role of Noncovalent Interactions from Second-Coordination-Sphere.

Reductions of nitrate and nitrite (NOx-) are of prime importance in combatting water pollution arising from the excessive use of N-rich fertilizers. While examples of NOx- reductions are known, this report illustrates hydrazine (N2H4)-mediated transformations of NOx- 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/15N) 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 [(Bz3Tren)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 [(Bz3Tren)CuII]2+ irrespective of the additional noncovalent interactions, and hence, the N2H4 reactions of the copper(II)-nitrite complexes [(mC)CuII(O14/15NO)]+ and [(Bz3Tren)CuII(O14/15NO)]+ (in 2-Cu/4-Cu) result in a mixture of 14/15NO and N14/15NO.

Metal-free Transformations of Nitrogen-Oxyanions to Ammonia via Oxoammonium Salt.

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)2 N+ =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 [Ar2 N+ =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 .

Phenol Reduces Nitrite to NO at Copper(II): Role of a Proton-Responsive Outer Coordination Sphere in Phenol Oxidation.

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.

H2S Generation from CS2 Hydrolysis at a Dinuclear Zinc(II) Site.

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.