Mechanistic Information on the Reversible Binding of NO to Mono‐ and Dinuclear Fe^II Complexes of a Biomimetic S₄N Ligand

Abstract

Structural and mechanistic information on the binding of NO to mono-and dinuclear [Fe II (S 4 NNEt 2 )] fragments as potential catalysts for the removal of NO from effluent gas streams have been obtained from concentration, temperature and pressure dependent kinetic measurements using stoppedflow techniques. The results indicate that the steric and electronic structure only affect the rate but not the nature of the binding mechanism by which NO coordinates to the selected complexes. Therefore, the sterically hindered dinuclear complex [Fe II (S 4 NNEt 2 )] 2 binds NO ca. eight times slower than

The nature of Ru–NO bonds in ruthenium tetraazamacrocycle nitrosyl complexes—a computational study

“…39 Although the slow NO release may narrow some biological applications, such as improving the population spike of neurons, 39 it can act as a prodrug for vasodilation with slow blood pressure decrease, as observed in hypertensive male Wistar rats. 19 On replacing cyclam with four amine ligands the resulting ruthenium tetraamine nitrosyl complexes, 3 ), 4-picoline (4-pic), 4-chloropyridine (4-Clpy), imidazole (imC or imN), 4-acetylpyridine (4-acpy) and L-histidine (L-hist)) and k −NO can be tuned by a judicious choice of the trans ligand L. [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] In previous theoretical studies we have addressed this issue. 54,55 Unlike Thus, N-(or C-) functionalization could be a suitable approach to control the reactivity of coordinated NO.…”

“…For example, when trans-[Ru II (P(OEt) 3 (NO)(NH 3 ) 4 )] (PF 6 ) 3 is reduced at −0.10 V (vs. SCE) NO is released with k −NO = 0.97 s −1 with formation of the trans-[Ru II (P(OEt 3 )(H 2 O)-(NH 3 ) 4 )] 2+ species. 53 The {RuNO} 6/7 redox potential of the tetraamine nitrosyl complexes (trans-[Ru II (L)(NO)(NH 3 ) 4 ] q+ ) (L = Cl − , isonicotinamide (isn), pyridine ( py), H 2 O, pyrazine ( pz), triethylphosphite (P(OEt) 3 ), 4-picoline (4-pic), 4-chloropyridine (4-Clpy), imidazole (imC or imN), 4-acetylpyridine (4-acpy) and L-histidine (L-hist)) and k −NO can be tuned by a judicious choice of the trans ligand L. [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] In previous theoretical studies we have addressed this issue. 54,55 Unlike trans-[Ru II (L)(NO)(NH 3 ) 4 ] q+ , the choice of ligand L trans to NO in trans-[Ru II Cl(NO)(cyclam)] 2+ is not easily made feasible due to the remarkable substitution inertness of chloride in the synthetic precursor.…”

Factors That Determine the Mechanism of NO Activation by Metal Complexes of Biological and Environmental Relevance

“…Since most kinetic studies regarding NO activation by transition metal complexes of biological relevance are devoted to interactions of NO with metalloporphyrins, the aim was to perform a detailed mechanistic study on the binding of NO to synthetic [Fe–S] enzyme models, which differ structurally and electronically from the well‐known heme model complexes. In order to evaluate the influence of steric and electronic factors of the sulfur‐containing ligand on the NO binding process, the nitrosylation reactions of different [Fe II (L)(S 4 NR)] {L = bridging thiolate σ‐donor S atom ( 1 ), CH 3 OH ( 2 ), and CO ( 4 ), Figure 3} fragments were carried out in methanol or toluene 43…”

“…The dinuclear complex, [Fe II S 4 N N Et 2 ] 2 , was characterized by EPR and Mössbauer spectroscopy as a high‐spin species. The binding of nitric oxide to the latter produces the mononuclear, low‐spin, diamagnetic complex, [Fe II (NO)S 4 N N Et 2 ] ( 3 ), which can formally exist as either [Fe I (NO + )S 4 N N Et 2 ] or [Fe II (NO · )S 4 N N Et 2 ] 43…”

“…Since reactive NO donors such as transition metal nitrosyls or S‐nitrosothiols in biological systems may act as a source for NO or NO + , mechanistic investigations on the reactivity of [Fe II (L)(S 4 NR)] fragments towards the nitrosonium ion, NO + , (nitrosation) have also been performed44 as a comparative study to that involving reactions with NO (nitrosylation) 43. The nitrosation reactions were performed using NOBF 4 salt as a source of NO + ion in acetonitrile under anhydrous conditions to prevent the hydrolysis of the employed salt in the presence of traces of water.…”