Carbon Monoxide Induced Reductive Elimination of Disulfide in an N‑Heterocyclic Carbene (NHC)/ Thiolate Dinitrosyl Iron Complex (DNIC)

• Published in: Journal of the American Chemical Society Vol. 135; no. 22; pp. 8423 - 8430
• Main Authors: Pulukkody, Randara, Kyran, Samuel J, Bethel, Ryan D, Hsieh, Chung-Hung, Hall, Michael B, Darensbourg, Donald J, Darensbourg, Marcetta Y
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 05.06.2013
• Subjects: carbon monoxide; Carbon Monoxide - chemistry; Crystallography, X-Ray; cysteine; Disulfides - chemistry; glutathione; Heterocyclic Compounds - chemistry; iron; Iron - chemistry; Kinetics; ligands; Methane - analogs & derivatives; Methane - chemistry; methodology; Models, Molecular; Molecular Conformation; nitric oxide; Nitrogen Oxides - chemistry; oxidation; Oxidation-Reduction; Quantum Theory; Sulfhydryl Compounds - chemistry; temperature; Thermodynamics
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/ja403916v

Dinitrosyliron complexes (DNICs) are organometallic-like compounds of biological significance in that they appear in vivo as products of NO degradation of iron–sulfur clusters; synthetic analogues have potential as NO storage and releasing agents. Their reactivity is expected to depend on ancillary ligands and the redox level of the distinctive Fe(NO)2 unit: paramagnetic {Fe(NO)2}9, diamagnetic dimerized forms of {Fe(NO)2}9 and diamagnetic {Fe(NO)2}10 DNICs (Enemark–Feltham notation). The typical biological ligands cysteine and glutathione themselves are subject to thiolate-disulfide redox processes, which when coupled to DNICs may lead to intricate redox processes involving iron, NO, and RS–/RS•. Making use of an N-heterocyclic carbene-stabilized DNIC, (NHC)(RS)Fe(NO)2, we have explored the DNIC-promoted RS–/RS• oxidation in the presence of added CO wherein oxidized {Fe(NO)2}9 is reduced to {Fe(NO)2}10 through carbon monoxide (CO)/RS• ligand substitution. Kinetic studies indicate a bimolecular process, rate = k [Fe(NO)2]1[CO]1, and activation parameters derived from k obs dependence on temperature similarly indicate an associative mechanism. This mechanism is further defined by density functional theory computations. Computational results indicate a unique role for the delocalized frontier molecular orbitals of the Fe(NO)2 unit, permitting ligand exchange of RS• and CO through an initial side-on approach of CO to the electron-rich N–Fe–N site, ultimately resulting in a 5-coordinate, 19-electron intermediate with elongated Fe–SR bond and with the NO ligands accommodating the excess charge.