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

Tuning the Reactivity of Copper(II)-Nitrite Core Towards Nitric Oxide Generation

Insights into the molecular mechanism and factors affecting nitrite-to-NO transformation at transition metal sites are essential for developing sustainable technologies relevant to NO-based therapeutics, waste water treatment, and agriculture. A set of copper(II)-nitrite complexes 1-4 have been isolated employing tridentate pincer-type ligands (quL, pyL, ClArOL-, PhOL-) featuring systematically varied donors. Although the X-ray crystal structures of the copper(II)-nitrite cores in 1-4 are comparable, electrochemical studies on complexes 1-4 reveal that redox properties of these complexes differ due to the changes in the σ-donor abilities of the phenolate/N-heterocycle based donor sites. Reactivity of these nitrite complexes with oxygen-atom-transfer (OAT) reagent (e. g. triphenyl phosphine Ph3P) and H+/e- donor reagent (e. g. substituted phenols ArOH) show the reduction of nitrite to NO gas. Detailed kinetic investigations including kinetic isotope effect (KIE), Eyring analyses for determining the activation parameters unfold that reduction of nitrite at copper(II) by Ph3P or ArOH are influenced by the CuII/CuI redox potential. Finally, this study allows mechanism driven development of catalytic nitrite reduction by ArOH in the presence of 10 mol % copper complex (1).

Reduction of Nitrite in an Iron(II)-Nitrito Compound by Thiols and Selenol Produces Dinitrosyl Iron Complexes via an {FeNO}7 Intermediate

Reaction of an Fe(II) complex, [Fe(6-COO--tpa)]1+ (1), with PhE- and NO2- produced [Fe(6-COO--tpa)(EPh)] (E = S, 2a; Se, 3) and [Fe(6-COO--tpa)(κ2-O,O'-NO2)] (4), respectively (6-COOH-tpa is bis(2-pyridylmethyl)(6-carboxyl-2-pyridylmethyl)amine). Treatment of 4 with 2 equiv of PhEH (E = S, Se) produced NO in ∼40% yields, respectively, along with 1 and the DNICs, [Fe(EPh)2(NO)2]1- (E = S, Se). Treatment of 4 with excess PhEH produced NO in similar yields, while 4 was converted to the same DNICs and 2a/3 (instead of 1). The DNICs have been proposed to be generated via the reaction of PhE- with an in situ generated, unstable {FeNO}7 intermediate, [Fe(6-COO--tpa)(NO)]1+ (6), which has also been synthesized separately. Compound 6 reacts with PhS- to generate [Fe(SPh)2(NO)2]1-, thus supporting the proposed reaction pathway. Finally, while the treatment of two unique compounds, featuring inbuilt proton sources, [Fe(6-COO--tpa)(S-C6H4-p-COOH)] (7) and [Fe(6-COO--tpa)(S-C6H4-o-OH)] (8), with 0.5 and 1 equiv of NO2- could produce NO only in 8-26% yields, treatment of 4 with HS-C6H4-p-COOH and HS-C6H4-o-OH produced NO in much higher yields (65-77%). The combined results delineated the importance of coordination of NO2- for the proton-assisted reduction of NO2- to generate NO.

Role of ancillary ligands in S-nitrosothiol and NO generation from nitrite-thiol interactions at mononuclear zinc(ii) sites

Generation of S-nitrosothiol (RSNO) and nitric oxide (NO) mediated by zinc(ii) coordination motifs is of prime importance for understanding the role of zinc(ii)-based cofactors in redox-signalling pathways. This study uniquely employs a set of mononuclear [L2ZnII]2+ cores (where L = Me4PzPz/Me2PzPy/Me2PzQu) for introducing subtle alterations of the primary coordination sphere and investigates the role of ligand tuning in the transformation of NO2 - in the presence of thiols. Single crystal X-ray diffraction (SCXRD) analyses on [L2ZnII-X](X) (where X = perchlorate/triflate) illustrate consistent changes in the bond distances, thereby showing variations of the metal-ligand interactions depending on the nature of the heterocyclic donor arms (pyrazole/pyridine/quinoline). Moreover, such tuning of the ligands affects the Lewis-acidity of the [L2ZnII]2+ cores as evaluated by 31P NMR and SCXRD studies on the 1 : 1 acid-base adducts [L2ZnII(OPEt3)]2+. Crystallographic and 15N NMR spectroscopic analyses on the nitrite complexes [L2ZnII(κ2-nitrite)](ClO4) reveal that the chemical environments of the nitrite anions in these complexes are nearly identical, despite the dissimilarity in the Lewis-acidity of the [L2ZnII]2+ cores. Interestingly, RSNO and NO generation from the reactions of [L2ZnII(κ2-nitrite)](ClO4) with 4-tert-butylbenzylthiol ( t BuBnSH) exhibits that the [(Me2PzQu)2ZnII]2+ core is the most efficient in promoting nitrite-thiol interactions due to the ease of available hemilabile coordination sites at the Lewis acidic [ZnII]. Detailed UV-vis studies in tandem with computational investigation, for the first time, provide an unambiguous demonstration of the nitrous acid (HNO2) intermediate generated through an intramolecular proton-transfer from thiol to nitrite at zinc(ii).

Reduction of nitrite to nitric oxide and generation of reactive chalcogen species by mononuclear Fe(II) and Zn(II) complexes of thiolate and selenolate

Comparative reactivity of a series of new Zn(II) and Fe(II) compounds, [(Py2ald)M(ER)] (E = S, R = Ph: M = Zn, 1aZn; M = Fe, 1aFe; E = S, R = 2,6-Me2-C6H3: M = Zn, 1bZn; M = Fe, 1bFe; E = Se, R = Ph: M = Zn, 2Zn; M = Fe, 2Fe), and [(Py2ald)M]22+ (M = Zn, 5Zn; M = Fe, 5Fe) is presented. Compound 1aZn could react with nitrite (NO2-) to produce [(Py2ald)Zn(ONO)] (3Zn), which, upon treatment with thiols and PhSeH (proton source), could regenerate either 1aZn/5Zn and 2Zn respectively, along with the production of nitric oxide (NO) where the yield of NO increases in the order tBuSH ≪ PhCH2SH < PhSH < PhSeH. In contrast to this, 1aFe, 2Fe and 5Fe could affect the direct reduction of NO2- in the absence of protons to generate NO and [{(Py2ald)(ONO)Fe}2-μ2-O] (8Fe). Moreover, 8Fe could regenerate 5Fe and 1aFe/2Fe upon treatment with 4 and 6 equiv. of PhEH (E = S/Se), respectively, along with the generation of NO. Finally, a comparative study of the mononuclear Zn(II) and Fe(II) compounds for the transfer of the coordinated thiolate/selenolate and the generation and transfer of reactive sulfur/selenium species (RES-, E = Se, S) to a series of organic substrates has been provided.

Nitrite and Nitric Oxide Interconversion at Mononuclear Copper(II): Insight into the Role of the Red Copper Site in Denitrification

Nitrite (NO2 - ) and nitric oxide (NO) interconversion is crucial for maintaining optimum NO flux in mammalian physiology. Herein we demonstrate that [L2 CuII (nitrite)]+ moieties (in 2 a and 2 b; where, L = Me2 PzPy and Me2 PzQu) with distorted octahedral geometry undergo facile reduction to provide tetrahedral [L2 CuI ]+ (in 3 a and 3 b) and NO in the presence of biologically relevant reductants, such as 4-methoxy-2,6-di-tert-butylphenol (4-MeO-2,6-DTBP, a tyrosine model) and N-benzyl-1,4-dihydronicotinamide (BNAH, a NAD(P)H model). Interestingly, the reaction of excess NO gas with [L2 CuII (MeCN)2 ]2+ (in 1 a) provides a putative {CuNO}10 species, which is effective in mediating the nitrosation of various nucleophiles, such as thiol and amine. Generation of the transient {CuNO}10 species in wet acetonitrile leads to NO2 - as assessed by Griess assay and 14 N/15 N-FTIR analyses. A detailed study reveals that the bidirectional NOx -reactivity, namely, nitrite reductase (NIR) and NO oxidase (NOO), at a common CuII site, is governed by the geometric-preference-driven facile CuII /CuI redox process. Of broader interest, this study not only highlights potential strategies for the design of copper-based catalysts for nitrite reduction, but also strengthens the previous postulates regarding the involvement of red copper proteins in denitrification.