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

The displacement of RS center dot 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 pi* 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 nu(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 sigma(p) is linear with a negative slope and a rho 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.