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The reduction of nitrogen oxyanions is critical for the remediation of eutrophication caused by anthropogenic perturbations to the natural nitrogen cycle. There are many approaches to nitrogen oxyanion reduction, and here we report our advances in reductive deoxygenation using pre-reduced N-heterocycles. We show examples of nitrogen oxyanion reduction using Cr, Fe, Co, Ni, and Zn, and we evaluate the role of metal choice, number of coordinated oxyanions, and ancillary ligands on the reductive transformations. We report the experimental challenges faced and provide an outlook on new directions to repurpose nitrogen oxyanions into value-added products. 

A bidentate pyrazolylpyridine ligand (HL) was installed on divalent nickel to give [(HL) 2 Ni(NO 3 )]NO 3 . This compound reacts with a bis-silylated heterocycle, 1,4-bis-(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene (TMS 2 Pz) to simultaneously reduce one of the nitrate ligands and deprotonate one of the HL ligands, giving octahedral (HL)(L − )Ni(NO 3 ). The mononitrate species formed is then further reacted with TMS 2 Pz to doubly deoxygenate nitrate and form [(L − )Ni(NO)] 2 , dimeric via bridging pyrazolate with bent nitrosyl ligands, representing a two-electron reduction of coordinated nitrate. Independent synthesis of a dimeric species [(L − )Ni(Br)] 2 is reported and effectively assembles two metals with better atom economy. 

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.