Self pH regulated iron(II) catalyst for radical free oxidation of benzyl alcohols

Abstract

Selective oxidation of various aromatic benzyl alcohols to benzaldehydes was found to be catalyzed with 90% conversion and 99% selectivity by an iron (II) catalyst herein designated as Fe-DDPA [DDPA = 3′-disulfanediyldipropionic acid]. The Fe-DDPA catalyst was prepared by a small loading of FeCl2 into a 2D sheet formed by the supramolecular assembling of DDPA. From both solid and liquid state nuclear magnetic resonance (NMR) spectroscopic study it was evident for the stabilization of the Fe(II) center through Fe-S interaction with the disulfide (S-S) unit of DDPA. DDPA was found to serve as an excellent support to maintain a pH that was required for a radical free oxidation of benzyl alcohol to aldehydes. The catalytic oxidation of benzyl alcohols was found to occur with excellent conversion and selectivity in acetonitrile (CH3CN) solvent in comparison to various other solvents. From various spectroscopic studies viz UV-vis, FT-IR and ESI-MS it was ascertained that the CH3CN interacted with Fe-DDPA to form a [(DDPA)2Fe(CH3CN)2]2+ species which then reacted with H2O2 to form an intermediate Fe-hydroxoperoxo, FeIII-OOH species. The Fe-OOH further got oxidized to the active FeIV=O species and was responsible for bringing the high selectivity in the oxidation reaction. The generation of highly unstable Fe-OOH species was further confirmed by electrochemical study, UV–vis, Raman and ESR spectroscopic analysis.