The mechanism of oxygenation of organic sulfide by oxo(salen)chromium(V) complexes has been studied experimentally and by the density functional theory (DFT) method. Spectral studies show DMSO ligands bind with the Cr center of oxidant and the adduct formed was responsible for the oxygenation reaction. The reaction was first order with respect to oxidant and substrate. Hammett plot shows the formation of positive charge over sulfur atom and the development of negative charge over the oxidant in the transition state (TS). For the substrate p values are in the range from-1.5 to -1.8. The geometry has been correctly predicted by the B3LYP function, and it gives better results for spin states, harmonic frequencies, and thermodynamic energies for the system. DFT results indicate the existence of oxo(salen)chromium(V) and Cr(III)-salen at doublet and quartet as ground state, respectively. Binding of donor ligand weakens the Cr-O bond. TS structures show an increase in the negative charge and spin density over Cr atom indicating the involvement of spin inversion and radical character. The low activation energy and high free energy change are responsible for the enhancement of the reaction rate and product yield in the presence of DMSO donor ligand, while the rebound mechanism and the direction of substrate approach perpendicular to the salen plane are responsible for their higher selectivity. From this combined study, a mechanism involving consecutive two-electron transfer from the oxo(salen)chromium(V) ion to the electron-rich sulfur atom is proposed.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry