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A pincer ligand composed of a pyridine with ortho positions substituted by a bulky phosphine arm and a pyrazole arm (PNNH) is installed on nickel(II) to yield the diamagnetic planar complex [(PNNH)NiCl]Cl. The chloride anion can be replaced by BPh₄⁻by a metathesis. The acidic pyrazole proton can be removed with LiN(SiMe₃)₂to yield the square planar neutral molecule (PNN⁻)NiCl. The coordinated chloride can be metathetically replaced by azide to yield diamagnetic (PNN⁻)Ni(N₃). To evaluate changing the phosphine donor for a phosphine sulfide, the corresponding pincer ligand SPNNH was synthesized and installed on NiBr₂. The reduced steric bulk from the more distant phosphorous keeps both halides coordinated in the paramagnetic molecular species (SPNNH)NiBr₂. Several attempts to dehydrobrominate this species result in synthesis and characterization of two unexpected products. One effort revealed that the electrophilic character of P(V) leaves the phosphorus atom in (SPNNH)NiBr₂vulnerable to nucleophilic attack, resulting in a P/C cleavage product which was characterized.

 

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.

 

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. 

The syntheses of (DIM)Ni(NO 3 ) 2 and (DIM)Ni(NO 2 ) 2 , where DIM is a 1,4-diazadiene bidentate donor, are reported to enable testing of bis boryl reduced N-heterocycles for their ability to carry out stepwise deoxygenation of coordinated nitrate and nitrite, forming O(Bpin) 2 . Single deoxygenation of (DIM)Ni(NO 2 ) 2 yields the tetrahedral complex (DIM)Ni(NO)(ONO), with a linear nitrosyl and κ 1 -ONO. Further deoxygenation of (DIM)Ni(NO)(ONO) results in the formation of dimeric [(DIM)Ni(NO)] 2 , where the dimer is linked through a Ni–Ni bond. The lost reduced nitrogen byproduct is shown to be N 2 O, indicating N–N bond formation in the course of the reaction. Isotopic labelling studies establish that the N–N bond of N 2 O is formed in a bimetallic Ni 2 intermediate and that the two nitrogen atoms of (DIM)Ni(NO)(ONO) become symmetry equivalent prior to N–N bond formation. The [(DIM)Ni(NO)] 2 dimer is susceptible to oxidation by AgX (X = NO 3 − , NO 2 − , and OTf − ) as well as nitric oxide, the latter of which undergoes nitric oxide disproportionation to yield N 2 O and (DIM)Ni(NO)(ONO). We show that the first step in the deoxygenation of (DIM)Ni(NO)(ONO) to liberate N 2 O is outer sphere electron transfer, providing insight into the organic reductants employed for deoxygenation. Lastly, we show that at elevated temperatures, deoxygenation is accompanied by loss of DIM to form either pyrazine or bipyridine bridged polymers, with retention of a BpinO − bridging ligand. 

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.