Salophen and salen oxo vanadium complexes as catalysts of sulfides oxidation with H2O2: Mechanistic insights

Abstract

a b s t r a c tThe application of V(V) catalysts in oxidation of sulfides with peroxides offers an efficient procedure, that is compatible with different functional groups, and leads to good yields and selectivities. However, the understanding of the factors affecting the reactivity of different catalysts is still far to be complete. An experimental and theoretical study on a series of V(V) complexes containing variously substituted salen and salophen ligands is reported with the aim to correlate the activity of the catalysts with the electronic character of the vanadium center. The results obtained indicate that steric factors play a major role in determining the outcome of the reaction, often overcoming the electronic effects. Theoretical results suggest the intervention in the catalytic cycle of an hydroperoxo vanadium species.

Investigation of VO–salophen complexes electronic structure

“…In agreement with this, theoretical calculations showed that salophen complexes reasonably react in a planar conformation, while the salen vanadyl hydroperoxo active species is more stable in a bent structure [9].…”

“…In fact, vanadium is currently used, mainly in its pentoxide form, to catalyze a number of industrial production processes or as a strengthening agent in alloys [8]. Inspired by the coordination chemistry of natural systems, among a wide range of synthetic ligands, vanadium has been successfully used to form stable complexes with salen and salophen ligands both in its IV and V oxidation states [9][10][11].…”

“…Vanadyl salen and salophen complexes are able to catalyze numerous reactions, among them oxidative processes in which the metal reacts with peroxides to form peroxometal species for the oxidation of organic substrates [9][10][11]. Moreover, they are also useful as model systems for the investigation of vanadium based biochemical processes.…”

“…Furthermore, the possibility to introduce a series of different substituents on the ligand scaffold would consent to tune the electronic and steric effects in the catalytic active center. Indeed, the different conformations observed for salen and salophen vanadyl complexes appeared to influence their reactivity; thus more or less rigid structures can be chosen, in order to enhance the desired activity [9].…”

“…Electronic effects can be modulated by introducing electron donor or withdrawing groups in the phenyl ring of the salicylaldehyde reagent [9,10,17]; on the other hand, the shape of the vanadyl complexes depends mainly on the nature of the diamine used. In agreement with this, theoretical calculations showed that salophen complexes reasonably react in a planar conformation, while the salen vanadyl hydroperoxo active species is more stable in a bent structure [9].…”