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1. Conversion of NO _x ¹⁻ ( x = 2, 3) to NO using an oxygen-deficient polyoxovanadate–alkoxide cluster

   https://doi.org/10.1039/C9CC08230A  

   Petel, Brittney E.; Matson, Ellen M. (January 2020, Chemical Communications)

   We report the activation of nitrogen-containing oxyanions using an oxygen-deficient polyoxovanadate–alkoxide cluster. Reduction of NO 2 1− and NO 3 1− results in near-quantitative oxygen atom transfer to the coordinatively unsaturated V III ion, and selective formation of NO. These results provide insight into possible mechanisms of oxyanion reduction by polyoxometalates. 

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2. Back donation, intramolecular electron transfer and N–O bond scission targeting nitrogen oxyanion reduction: how can a metal complex assist?

   https://doi.org/10.1039/D0DT03430D  

   Beagan, Daniel M.; Cabelof, Alyssa C.; Caulton, Kenneth G. (February 2021, Dalton Transactions)

   null (Ed.)

   A density functional theory exploration studies a range of ancillary coordinated ligands accompanying nitrogen oxyanions with the goal of promoting back donation towards varied nitrogen oxidation states. Evaluation of a suite of Ru and Rh metal complexes reveals minimum back donation to the κ 1 -nitrogen oxyanion ligand, even upon one-electron reduction. This reveals some surprising consequences of reduction, including redox activity at pyridine and nitrogen oxyanion dissociation. Bidentate nitrate was therefore considered, where ancillary ligands enforce geometries that maximize M–NO x orbital overlap. This strategy is successful and leads to full electron transfer in several cases to form a pyramidal radical NO 3 2− ligand. The impact of ancillary ligand on degree of nitrate reduction is probed by comparing the powerful o-donor tris-carbene borate (TCB) to a milder donor, tris-pyrazolyl borate (Tp). This reveals that with the milder Tp donor, nitrate reduction is only seen upon addition of a Lewis base. Protonation of neutral and anionic (TCB)Ru(κ 2 -NO 3 ) at both terminal and internal oxygens reveals exergonic N–O bond cleavage for the reduced species, with one electron coming from Ru, yielding a Ru III hydroxide product. Comparison of H + to Na + electrophile shows weaker progress towards N–O bond scission. Finally, calculations on (TCB)Fe(κ 2 -NO 3 ) and [(TCB)Fe(κ 2 -NO 3 )] – show that electron transfer to nitrate is possible even with an earth abundant 3d metal. 

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3. Functionalized Ferrocene Enables Selective Electrosorption of Arsenic Oxyanions over Phosphate─A DFT Examination of the Effects of Substitutional Moieties, pH, and Oxidation State

   https://doi.org/10.1021/acs.jpca.3c03826  

   Nwokonkwo, Obinna; Pelletier, Vivienne; Broud, Michael; Muhich, Christopher (September 2023, The Journal of Physical Chemistry A)

   Ferrocene (Fc)/ferrocenium (Fc+)-decorated carbon nanotube electrode materials have shown promise for selectively adsorbing arsenic (As) over dissimilar anions like Cl– and ClO4–, and isostructural transition-metal oxyanions for water remediation; however, the competition between same-group oxyanions (such as arsenate vs phosphate) is underexplored and poorly understood. We use ab initio calculations to examine the competitive binding of As(V), P(V), and As(III) to Fc/Fc+ with and without functional substitutions (OH, SH, NH2, COOH, CH3, C2H5, NO2, and Cl). This work aims to understand factors that induce the selective binding of toxic arsenic over phosphate. We find that neat Fc cannot distinguish the three oxyanions because physical forces (electrostatics and dispersion) dominate the Fc-oxyanion interactions. However, combined oxidation and substitution effects enable selectivity for As(V) over P(V). Oxidation of Fc to Fc+ allows the formation of Fc+-oxyanion covalent bonds with varying donor–acceptor character depending on the oxyanion. Additionally, NH2 and SH groups that donate charge to the base Fc+ molecule and H-bond to oxyanion induce an energetic preference for As(V) over P(V) by −0.23 and −0.13 eV, respectively. Differences in pKa between As(V)/P(V) and As(III) preclude any preference for As(III) over the other anions. Using the calculated energetics, we predict the pH-dependent binding selectivity of functionalized ferrocenium. These findings demonstrate the challenges of Fc/Fc+-oxyanion interaction for selective binding and provide a path for identifying other molecules and substituents for efficient metallocene adsorbent design. 

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4. Mechanistic Insights into the Selectivity for Arsenic over Phosphate Adsorption by Fe ³⁺ -Cross-Linked Chitosan Using DFT

   https://doi.org/10.1021/acs.jpcb.3c06838  

   Nwokonkwo, Obinna; Muhich, Christopher (February 2024, The Journal of Physical Chemistry B)

   Fe3+-cross-linked chitosan exhibits the potential for selectively adsorbing arsenic (As) over competing species, such as phosphate, for water remediation. However, the effective binding mechanisms, bond nature, and controlling factor(s) of the selectivity are poorly understood. This study employs ab initio calculations to examine the competitive binding of As(V), P(V), and As(III) to neat chitosan and Fe3+-chitosan. Neat chitosan fails to selectively bind As oxyanions, as all three oxyanions bind similarly via weak hydrogen bonds with preferences of P(V) = As(V) > As(III). Conversely, Fe3+-chitosan selectively binds As(V) over As(III) and P(V) with binding energies of −1.9, −1, and −1.8 eV for As(V), As(III), and P(V), respectively. The preferences are due to varying Fe3+–oxyanion donor–acceptor characteristics, forming covalent bonds with distinct strengths (Fe–O bond ICOHP values: – 4.9 eV/bond for As(V), – 4.7 eV/bond for P(V), and −3.5 eV/bond for As(III)). Differences in pKa between As(V)/P(V) and As(III) preclude any preference for As(III) under typical environmental pH conditions. Furthermore, our calculations suggest that the binding selectivity of Fe3+-chitosan exhibits a pH dependence. These findings enhance our understanding of the Fe3+–oxyanion interaction crucial for preferential oxyanion binding using Fe3+-chitosan and provide a lens for further exploration into alternative transition-metal–chitosan combinations and coordination chemistries for applications in selective separations. 

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

   https://doi.org/10.1002/chem.202101302  

   Beagan, Daniel_M; Maciulis, Nicholas_A; Pink, Maren; Carta, Veronica; Huerfano, I_J; Chen, Chun‐Hsing; Caulton, Kenneth_G (July 2021, Chemistry – A European Journal)

   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 NO_x⁻substrates with a redox‐active ligand in the absence of redox activity at the metal. Following syntheses and characterization of (btzp)ZnX₂for X=Cl, NO₃and 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 NO_x⁻¹. A new synthesis of the highly hydrogenated H₄btzp, containing two dihydrotetrazine reductants is described as is the synthesis and characterization of (H₄btzp)ZnX₂for X=Cl and NO₃, both of which show considerable hydrogen bonding potential of the dihydrotetrazine ring NH groups. The (H₄btzp)ZnCl₂complex 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 AgNO₂with (H₄btzp)ZnCl₂is shown to proceed with fast nitrite deoxygenation, yielding water and free NO. Half of the H₄btzp reducing equivalents form Ag⁰and thus the chloride ligand remains coordinated to the zinc metal center to yield (btzp)ZnCl₂. To compare with AgNO₂, experiments of (H₄btzp)ZnCl₂with NaNO₂result in salt metathesis between chloride and nitrite, highlighting the importance of a redox‐active cation in the reduction of nitrite to NO. 

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