The effect of nucleophilic cis-coligands on the iron(III)-iodosylbenzene mediated sulfoxidation reactions: Kinetic and DFT study

A series of complexes with cis -XFe III (OIPh) (where X = Cl − , Br − , I − , and NCO − ) motif were obtained by treating the in situ generated parent compound, [Fe III (OIPh)(4R-PBI)(4R-PBI -H )(MeCN)] 2+ ( 3 MeCN , 4 MeCN ) supported by bidentate PBI ligands (PBI = 2-(2-pyridyl)benzimidazole; 4Me-PBI = 2-(4′-mehtyl-2′-pyridyl)-benzimidazole) and a labile MeCN molecule, with the corresponding TBA-X salts. The coordination of the anionic ligands (X = Cl − , Br − , I − , NCO − ) are evidenced by blue shifts of the characteristic near-IR ligand-field band (750 nm) to 668, 688, 717 and 728 nm, respectively. These complexes were compared for their oxo-transfer ability to PhSMe, and based on detailed kinetic and mechanistic studies including linear free energy relationship (log k rel vs. σ) with ρ = −1.13 to −1.25, and Marcus plot (log k rel vs. Ε 0 ) with m = −2.17 to −2.42, lead to the conclusion that the anionic coligands had no effect on the previously proposed direct oxygen atom transfer (DOT) mechanism, only on the reaction rates. The second order rate constants showed differences of up to 10-fold and 13-fold, depending on the amount of TBA-X (1 or 2 equiv.), and decreased in the following order: MeCN > Cl − > Br − > I − > NCO − , consistent with the increasing nucleophilicity of the anions and decreasing electrophilicity of the cis -XFe III (OIPh) species. DFT and TDDFT calculations were performed to investigate the formation of the active oxidative species as well as the exchange by coligands in terms of their spin states and spectral changes. The increasing nucleophilicity of the anions and the decreasing electrophilicity of the cis -XFe III (OIPh) species reduce the rate of the OAT. • Formation of [Fe III (OIPh)(PBI)(PBI -H )(MeCN)] 2+ intermediate is energetically favored. • Ligand substitution reactions of cis -(MeCN)Fe III (OIPh). • Equatorial ligand effects on the electronic structure. • The simulated UV–Vis spectra showed good agreement with the experiments. • Equatorial ligand effects on the cis -(X)Fe III (OIPh) mediated sulfoxidation reactions.