Structural and mechanistic information on the nitrosation of model Fe(ii) complexes containing a biomimetic S4N chelate

Abstract

In order to provide insight into the reaction pathways of nitrogen oxide redox species with [Fe–S] models that may parallel those existing in biology, the reactivity of the iron-sulfur species, {[Fe^II(S₄NEt₂N)]}₂ (1) and [Fe^II(CH₃CN)(S₄NEt₂N)] (2), where (S₄NEt₂N)²⁻ = 2,6-bis(2-mercaptophenylthiomethyl)-4-diethylaminopyridine(2−), towards NO⁺ (nitrosation) has been studied mechanistically in acetonitrile and compared with the corresponding reactions with NO (nitrosylation). For the nitrosation of 1, the reaction takes place in two steps that correspond to the nitrosation of the mononuclear (2) and dinuclear (1) complexes, respectively. For the corresponding carbonyl complex [Fe^II(CO)(S₄NEt₂N)] (3), the nitrosation reaction occurs in a single step. The relative reactivity of the iron-sulfur species is approximately (1)/(2)/(3) = 1/20/10. Activation parameters for the nitrosation of 1 (ΔH^# = 27 ± 1 kJ mol⁻¹, ΔS^# = −111 ± 2 J K⁻¹ mol⁻¹, and ΔV^# = −19 ± 2 cm³ mol⁻¹), 2 (ΔH^# = 46 ± 2 kJ mol⁻¹, ΔS^# = −22 ± 7 J K⁻¹ mol⁻¹, and ΔV^# = −9.7 ± 0.4 cm³ mol⁻¹) and 3 (ΔH^# = 38 ± 1 kJ mol⁻¹, ΔS^# = −44 ± 4 J K⁻¹ mol⁻¹, and ΔV^# = −7.8 ± 0.3 cm³ mol⁻¹) were determined from variable temperature and pressure studies. The significantly negative ΔS^# and ΔV^# values found for the nitrosation reactions are consistent with an associative mechanism. A comparative study of the reactivity of the iron-sulfur species 1 to 3 towards NO⁺ and NO is presented.