What Is the Right Level of Activation of a High-Spin {FeNO}7 Complex to Enable Direct N-N Coupling? Mechanistic Insight into Flavodiiron NO Reductases.

Flavodiiron nitric oxide reductases (FNORs), found in pathogenic bacteria, are capable of reducing nitric oxide (NO) to nitrous oxide (N2O) to detoxify NO released by the human immune system. Previously, we reported the first FNOR model system that mediates direct NO reduction (Dong, H. T.; J. Am. Chem. Soc. 2018, 140, 13429-13440), but no intermediate of the reaction could be characterized. Here, we present a new set of model complexes that, depending on the ligand substitution, can either mediate direct NO reduction or stabilize a highly activated high-spin (hs) {FeNO}7 complex, the first intermediate of the reaction. The precursors, [{FeII(MPA-(RPhO)2)}2] (1, R = H and 2, R = tBu, Me), were prepared first and fully characterized. Complex 1 (without steric protection) directly reduces NO to N2O almost quantitatively, which constitutes only the second example of this reaction in model systems. Contrarily, the reaction of sterically protected 2 with NO forms the stable mononitrosyl complex 3, which shows one of the lowest N-O stretching frequencies (1689 cm-1) observed so far for a mononuclear hs-{FeNO}7 complex. This study confirms that an N-O stretch ≤1700 cm-1 represents the appropriate level of activation of the FeNO unit to enable direct NO reduction. The higher activation level of these hs-{FeNO}7 complexes required for NO reduction compared to those formed in FNORs emphasizes the importance of hydrogen bonding residues in the active sites of FNORs to activate the bound NO ligands for direct N-N coupling and N2O formation. The implications of these results for FNORs are further discussed.

Direct Determination of Aerosol pH: Size-Resolved Measurements of Submicrometer and Supermicrometer Aqueous Particles.

Measuring the acidity of atmospheric aerosols is critical, as many key multiphase chemical reactions involving aerosols are highly pH-dependent. These reactions impact processes, such as secondary organic aerosol (SOA) formation, that impact climate and health. However, determining the pH of atmospheric particles, which have minute volumes (10-23-10-18 L), is an analytical challenge due to the nonconservative nature of the hydronium ion, particularly as most chemical aerosol measurements are made offline or under vacuum, where water can be lost and acid-base equilibria shifted. Because of these challenges, there have been no direct methods to probe atmospheric aerosol acidity, and pH has typically been determined by proxy/indirect methods, such as ion balance, or thermodynamic models. Herein, we present a novel and facile method for direct measurement of size-resolved aerosol acidity from pH 0 to 4.5 using quantitative colorimetric image processing of cellular phone images of (NH4)2SO4-H2SO4 aqueous aerosol particles impacted onto pH-indicator paper. A trend of increasing aerosol acidity with decreasing particle size was observed that is consistent with spectroscopic measurements of individual particle pH. These results indicate the potential for direct measurements of size-resolved atmospheric aerosol acidity, which is needed to improve fundamental understanding of pH-dependent atmospheric processes, such as SOA formation.