Overview and New Insights into the Thiol Reactivity of Coordinated NO in {MNO}^6/7/8 (M = Fe, Co) Complexes

Abstract

The reactivity of free NO (NO(+), NO(•), and NO(-)) with thiols (RSH) is relatively well understood, and the oxidation state of the NO moiety generally determines the outcome of the reaction. However, NO/RSH interactions are often mediated at metal centers, and the fate of these species when bound to a first-row transition metal (e.g., Fe, Co) deserves further investigation. Some metal-bound NO moieties (particularly NO(+), yielding S-nitrosothiols) have been more thoroughly studied, yet the fate of these species remains highly condition-dependent and, for M-NO(-), an unexplored field. Herein, we present an overview of thiol reactions with metal nitrosyls that result in N-O bond activation, ligand substitution on {MNO} fragments, and/or redox chemistry. We also present our results pertaining to the thiol reactivity of nonheme {FeNO}(7/8) complexes [Fe(LN4(pr))(NO)](-/0) (1 and 2) and the noncorrin {CoNO}(8) complex [Co(LN4(pr))(NO)] (3), an isoelectronic analogue of the {FeNO}(8) complex 1. Among other products, the reaction of 1 with p-ClPhSH affords [Fe2(μ-SPh-p-Cl)2(NO)4](-) (anion of 6), a reduced Roussin's red ester (rRRE), which was characterized by Fourier transform infrared (FTIR), UV-vis, electron paramagnetic resonance (EPR), and X-ray diffraction. Similarly, the reaction of 1 with glutathione in buffer affords the corresponding rRRE, which has also been spectroscopically characterized by EPR and UV-vis. The oxidation states of the metals and nitrosyls both contribute to the complex nature of these interactions, and as such, we discuss the varying product distribution accordingly. These studies shed insight into the products that may form through MNO/RSH interactions that lead to NOx activation and {MNO} redox.

Nitric Oxide Modulates Endonuclease III Redox Activity by a 800 mV Negative Shift upon [Fe4S4] Cluster Nitrosylation

“…Upon reduction by dithionite, the {Fe­(NO) 2 } 9 dinitrosyl iron complex is reduced into an EPR-silent {Fe­(NO) 2 } 10 complex, while the EPR-silent {Fe­(NO) 2 } 9 –{Fe­(NO) 2 } 9 Roussin’s red ester is reduced into the EPR-active {Fe­(NO) 2 } 9 –{Fe­(NO) 2 } 10 ( S = 1/2) species referred to as the reduced Roussin’s red ester (Scheme ). , Our assignment of the reduced Roussin’s red ester, [(μ-Cys) 2 Fe 2 (NO) 4 ] − , is supported by similar EPR resonances observed for synthetic and peptide-bound model complexes. ,, …”

“…22,43 Our assignment of the reduced Roussin’s red ester, [( μ -Cys) 2 Fe 2 (NO) 4 ] − , is supported by similar EPR resonances observed for synthetic and peptide-bound model complexes. 23,42,48 …”

“…22,43 Our assignment of the reduced Roussin's red ester, [(μ-Cys) 2 Fe 2 (NO) 4 ] − , is supported by similar EPR resonances observed for synthetic and peptide-bound model complexes. 23,42,48 It should be noted that the reduced Roussin's red ester signal with principal g-values of g ⊥ = 2.011 and g ∥ = 1.972 was originally assigned as a d 9 dinitrosyl iron complex having been reduced from a d 7 form. 19 However, compelling spectroscopic data of structurally well-characterized model complexes disagree with the initial assignment and instead support the assignment of a reduced Roussin's red ester.…”

Direct NMR detection of the unstable “red product” from the reaction between nitroprusside and 2-mercaptosuccinic acid

“…In general, well characterized transition metal coordination complexes containing RSNO ligands are extremely rare. 23,24 Among them the so-called "red products" (RPs), which are produced from the reaction between nitroprusside (NP, [Fe II (CN) 5 NO] 2− ) and organic thiolates (RS − ) in aqueous solution, are the most extensively studied. [25][26][27][28][29][30][31][32][33][34][35] It is commonly accepted that the RP is [Fe II (CN) 5 N(O)SR] 3− , where RSNO is coordinated to the Fe(II) center in the κ 1 -N binding mode.…”

Electronic Structure and Reactivity of Bioinspired Organometallic Iron Complexes Relevant to Small Molecule Activation

“…[213] ˙NO 2 can further react with additional ˙NO to give unstable N 2 O 3 . [23,230] Consequently, it can be proposed that NO 2 is formed in the headspace of the reaction and is then coordinating at the free axial position of the square pyramidal coordinated iron center in 17, accompanied by oxidation of the iron ion and formation of NO 2 − . [23] In order to obtain further support for this mechanistic idea, isotope labeling experiments using 18 O 2 were performed.…”