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

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 kobs 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.