Kinetics studies provide mechanistic insight regarding the formation of dinitrosyl iron complexes (DNICs) now viewed as playing important roles in the mammalian chemical biology of the ubiquitous bioregulator nitric oxide (NO). Reactions in deaerated aqueous solutions containing FeSO4, cysteine (CysSH), and NO demonstrate that both the rates and the outcomes are markedly pH dependent. The dinuclear DNIC Fe2(μ-CysS)2(NO)4, a Roussin's red salt ester (Cys-RSE), is formed at pH 5.0 as well as at lower concentrations of cysteine in neutral pH solutions. The mononuclear DNIC Fe(NO)2(CysS)2(-) (Cys-DNIC) is produced from the same three components at pH 10.0 and at higher cysteine concentrations at neutral pH. The kinetics studies suggest that both Cys-RSE and Cys-DNIC are formed via a common intermediate Fe(NO)(CysS)2(-). Cys-DNIC and Cys-RSE interconvert, and the rates of this process depend on the cysteine concentration and on the pH. Flash photolysis of the Cys-RSE formed from Fe(II)/NO/cysteine mixtures in anaerobic pH 5.0 solution led to reversible NO dissociation and a rapid, second-order back reaction with a rate constant kNO = 6.9 × 10(7) M(-1) s(-1). In contrast, photolysis of the mononuclear-DNIC species Cys-DNIC formed from Fe(II)/NO/cysteine mixtures in anaerobic pH 10.0 solution did not labilize NO but instead apparently led to release of the CysS(•) radical. These studies illustrate the complicated reaction dynamics interconnecting the DNIC species and offer a mechanistic model for the key steps leading to these non-heme iron nitrosyl complexes.

Described is a kinetics and computational study of the reaction of NO with the intramolecular bridged P/B frustrated Lewis pair (FLP) endo-2-(dimesitylphosphino)-exo-3-bis(pentafluorophenyl)boryl-norbornane to give a persistent FLP-NO aminoxyl radical. This reaction follows a second-order rate law, first-order in [FLP] and first-order in [NO], and is markedly faster in toluene than in dichloromethane. By contrast, the NO oxidation of the phosphine base 2-(dimesitylphosphino)norbornene to the corresponding phosphine oxide follows a third-order rate law, first-order in [phosphine] and second-order in [NO]. Formation of the FLP-NO radical in toluene occurs with a ΔH(‡) of 13 kcal mol(-1), a feature that conflicts with the computation-based conclusion that NO addition to a properly oriented B/P pair should be nearly barrierless. Since the calculations show the B/P pair in the most stable solution structure of this FLP to have an unfavorable orientation for concerted reaction, the observed barrier is rationalized in terms of the reversible formation of a [B]-NO complex intermediate followed by a slower isomerization-ring closure step to the cyclic aminoxyl radical. This combined kinetics/theoretical study for the first time provides insight into mechanistic details for the activation of a diatomic molecule by a prototypical FLP.