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1. N ₃ -Ligated nickel( ii ) diketonate complexes: synthesis, characterization and evaluation of O ₂ reactivity

   https://doi.org/10.1039/d0dt01338b  

   Elsberg, Josiah G. ; Peterson, Austin ; Fuller, Amy L. ; Berreau, Lisa M. ( January 2020 , Dalton Transactions)

   Interest in O 2 -dependent aliphatic carbon–carbon (C–C) bond cleavage reactions of first row divalent metal diketonate complexes stems from the desire to further understand the reaction pathways of enzymes such as DKE1 and to extract information to develop applications in organic synthesis. A recent report of O 2 -dependent aliphatic C–C bond cleavage at ambient temperature in Ni( ii ) diketonate complexes supported by a tridentate nitrogen donor ligand [(MBBP)Ni(PhC(O)CHC(O)Ph)]Cl ( 7-Cl ; MBBP = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine) in the presence of NEt 3 spurred our interest in further examining the chemistry of such complexes. A series of new TERPY-ligated Ni( ii ) diketonate complexes of the general formula [(TERPY)Ni(R 2 -1,3-diketonate)]ClO 4 ( 1 : R = CH 3 ; 2 : R = C(CH 3 ) 3 ; 3 : R = Ph) was prepared under air and characterized using single crystal X-ray crystallography, elemental analysis, 1 H NMR, ESI-MS, FTIR, and UV-vis. Analysis of the reaction mixtures in which these complexes were generated using 1 H NMR and ESI-MS revealed the presence of both the desired diketonate complex and the bis-TERPY derivative [(TERPY) 2 Ni](ClO 4 ) 2 ( 4 ). Through selective crystallization 1–3 were isolated inmore »analytically pure form. Analysis of reaction mixtures leading to the formation of the MBBP analogs [(MBBP)Ni(R 2 -1,3-diketonate)]X (X = ClO 4 : 5 : R = CH 3 ; 6 : R = C(CH 3 ) 3 ; 7-ClO4 : R = Ph; X = Cl: 7-Cl : R = Ph) using 1 H NMR and ESI-MS revealed the presence of [(MBBP) 2 Ni](ClO 4 ) 2 ( 8 ). Analysis of aerobic acetonitrile solutions of analytically pure 1–3 , 5 and 6 containing NEt 3 and in some cases H 2 O using 1 H NMR and UV-vis revealed evidence for the formation of additional bis-ligand complexes ( 4 and 8 ) but suggested no oxidative diketonate cleavage reactivity. Analysis of the organic products generated from 3 , 7-ClO4 and 7-Cl revealed unaltered dibenzoylmethane. Our results therefore indicate that N 3 -ligated Ni( ii ) complexes of unsubstituted diketonate ligands do not exhibit O 2 -dependent aliphatic C–C bond clevage at room temperature, including in the presence of NEt 3 and/or H 2 O.« less
2. Critical computational analysis illuminates the reductive-elimination mechanism that activates nitrogenase for N ₂ reduction

   https://doi.org/10.1073/pnas.1810211115  

   Raugei, Simone ; Seefeldt, Lance C. ; Hoffman, Brian M. ( October 2018 , Proceedings of the National Academy of Sciences)

   Recent spectroscopic, kinetic, photophysical, and thermodynamic measurements show activation of nitrogenase for N₂→ 2NH₃reduction involves the reductive elimination (re) of H₂from two [Fe–H–Fe] bridging hydrides bound to the catalytic [7Fe–9S–Mo–C–homocitrate] FeMo-cofactor (FeMo-co). These studies rationalize the Lowe–Thorneley kinetic scheme’s proposal of mechanistically obligatory formation of one H₂for each N₂reduced. They also provide an overall framework for understanding the mechanism of nitrogen fixation by nitrogenase. However, they directly pose fundamental questions addressed computationally here. We here report an extensive computational investigation of the structure and energetics of possible nitrogenase intermediates using structural models for the active site with a broad range in complexity, while evaluating a diverse set of density functional theory flavors. (i) This shows that to prevent spurious disruption of FeMo-co having accumulated 4[e⁻/H⁺] it is necessary to include: all residues (and water molecules) interacting directly with FeMo-co via specific H-bond interactions; nonspecific local electrostatic interactions; and steric confinement. (ii) These calculations indicate an important role of sulfide hemilability in the overall conversion ofE₀to a diazene-level intermediate. (iii) Perhaps most importantly, they explain (iiia) how the enzyme mechanistically couples exothermic H₂formation to endothermic cleavage of the N≡N triple bond in a nearly thermoneutralre/oxidative-addition equilibrium, (iiib) while preventing the “futile”more »generation of two H₂without N₂reduction: hydrideregenerates an H₂complex, but H₂is only lost when displaced by N₂, to form an end-on N₂complex that proceeds to a diazene-level intermediate.

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3. 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 progressmore »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.« less
4. Rapid hydrolysis of tertiary isoprene nitrate efficiently removes NO _x from the atmosphere

   https://doi.org/10.1073/pnas.2017442117  

   Vasquez, Krystal T. ; Crounse, John D. ; Schulze, Benjamin C. ; Bates, Kelvin H. ; Teng, Alexander P. ; Xu, Lu ; Allen, Hannah M. ; Wennberg, Paul O. ( December 2020 , Proceedings of the National Academy of Sciences)

   The formation of a suite of isoprene-derived hydroxy nitrate (IHN) isomers during the OH-initiated oxidation of isoprene affects both the concentration and distribution of nitrogen oxide free radicals (NO_x). Experiments performed in an atmospheric simulation chamber suggest that the lifetime of the most abundant isomer, 1,2-IHN, is shortened significantly by a water-mediated process (leading to nitric acid formation), while the lifetime of a similar isomer, 4,3-IHN, is not. Consistent with these chamber studies, NMR kinetic experiments constrain the 1,2-IHN hydrolysis lifetime to less than 10 s in deuterium oxide (D₂O) at 298 K, whereas the 4,3-IHN isomer has been observed to hydrolyze much less efficiently. These laboratory findings are used to interpret observations of the IHN isomer distribution in ambient air. The IHN isomer ratio (1,2-IHN to 4,3-IHN) in a high NO_xenvironment decreases rapidly in the afternoon, which is not explained using known gas-phase chemistry. When simulated with an observationally constrained model, we find that an additional loss process for the 1,2-IHN isomer with a time constant of about 6 h best explains our atmospheric measurements. Using estimates for 1,2-IHN Henry’s law constant and atmospheric liquid water volume, we show that condensed-phase hydrolysis of 1,2-IHN can account for this lossmore »process. Simulations from a global chemistry transport model show that the hydrolysis of 1,2-IHN accounts for a substantial fraction of NO_xlost (and HNO₃produced), resulting in large impacts on oxidant formation, especially over forested regions.

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5. Quantitative constraints on autoxidation and dimer formation from direct probing of monoterpene-derived peroxy radical chemistry

   https://doi.org/10.1073/pnas.1812147115  

   Zhao, Yue ; Thornton, Joel A. ; Pye, Havala O. T. ( November 2018 , Proceedings of the National Academy of Sciences)

   Organic peroxy radicals (RO₂) are key intermediates in the atmospheric degradation of organic matter and fuel combustion, but to date, few direct studies of specific RO₂in complex reaction systems exist, leading to large gaps in our understanding of their fate. We show, using direct, speciated measurements of a suite of RO₂and gas-phase dimers from O₃-initiated oxidation of α-pinene, that ∼150 gaseous dimers (C_16–20H_24–34O_4–13) are primarily formed through RO₂cross-reactions, with a typical rate constant of 0.75–2 × 10⁻¹²cm³molecule⁻¹s⁻¹and a lower-limit dimer formation branching ratio of 4%. These findings imply a gaseous dimer yield that varies strongly with nitric oxide (NO) concentrations, of at least 0.2–2.5% by mole (0.5–6.6% by mass) for conditions typical of forested regions with low to moderate anthropogenic influence (i.e., ≤50-parts per trillion NO). Given their very low volatility, the gaseous C_16–20dimers provide a potentially important organic medium for initial particle formation, and alone can explain 5–60% of α-pinene secondary organic aerosol mass yields measured at atmospherically relevant particle mass loadings. The responses of RO₂, dimers, and highly oxygenated multifunctional compounds (HOM) to reacted α-pinene concentration and NO imply that an average ∼20% of primary α-pinene RO₂from OH reaction and 10% from ozonolysis autoxidize at 3–10 s⁻¹and ≥1more »s⁻¹, respectively, confirming both oxidation pathways produce HOM efficiently, even at higher NO concentrations typical of urban areas. Thus, gas-phase dimer formation and RO₂autoxidation are ubiquitous sources of low-volatility organic compounds capable of driving atmospheric particle formation and growth.

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