Hammett correlations as test of mechanism of CO-induced disulfide elimination from dinitrosyl iron complexes

Abstract

The displacement of RS˙ from [(NHC)(SPh)Fe(NO)₂] (NHC = N-heterocyclic carbene) by carbon monoxide follows associative kinetics, rate = k [CO]¹ [(NHC)(SPh)Fe(NO)₂]¹, resulting in reduction of the oxidized form of the dinitrosyliron unit, {Fe(NO)₂}⁹ (Enemark–Feltham notation) to {Fe(NO)₂}¹⁰. Thermodynamically driven by the release of PhS–SPh concomitant with formation of [(NHC)(CO)Fe(NO)₂], computational studies suggested the reactant dinitrosyliron unit serves as a nucleophile in the initial slanted interaction of the π* orbital of CO, shifting into normal linear Fe–CO with weakening of the Fe–SPh bond. The current study seeks to experimentally test this proposal. A series of analogous {Fe(NO)₂}⁹ [(NHC)(p-S–C₆H₄X)Fe(NO)₂] complexes, with systematic variation of the para-substituents X from electron donor to electron withdrawing groups was used to monitor variation in electron density at the Fe(NO)₂ unit via Hammett analyses. Despite the presence of non-innocent NO ligands, data from ν(NO) IR spectroscopy and cyclic voltammetry showed consistent tracking of the electron density at the {Fe(NO)₂} unit in response to the aryl substituent. The electronic modifications resulted in systematic changes in reaction rates when each derivative was exposed to CO. A plot of the rate constants and the Hammett parameter σ_p is linear with a negative slope and a ρ value of −0.831; such correlation is indicative of rate retardation by electron-withdrawing substituents, and provides experimental support for the unique role of the delocalized frontier molecular orbitals of the Fe(NO)₂ unit.