Synthesis, characterization, and catalysis of recyclable new piperazine-bridged Mo(VI) polymers [MoO₂(Salen)(piperazine)]_n in highly selective oxygenation of alkenes and sulfides

Abstract

This study reports synthesis and characterization of polymeric [MoO₂(Salen)] complexes that combine MoO₂(acac)₂ and polymeric ligands bearing N₂O₂ coordination sites, abbreviated herein as [piperazinCH₂{MoO₂(Salen)}]_n (6), [piperazinCH₂{MoO₂(Salophen)}]_n (7), and [piperazinCH₂{MoO₂(Salpn)}]_n (8). The epoxidation of alkenes using tert-butyl hydroperoxide and oxidation of sulfides to sulfoxides by urea hydrogen peroxide were enhanced with excellent selectivity under the catalytic influence of these coordination polymers. No relevant difference in activity was found due to change in the diamine in the Schiff’s base ligands. The stability of polymeric catalysts was assessed by recycling a sample five times in small batch reactions in both epoxidation and sulfoxidation. In the case of cyclooctene epoxidation, catalytic activity of [piperazinCH₂{MoO₂(Salen)}]_n increased without losing the selectivity, whereas the catalytic activity and selectivity of the spent catalyst decreased in sulfoxidation of methyl phenyl sulfide in consecutive runs. Loss of Mo from the coordination polymer has been investigated using the filtrates as potential catalysts in sulfoxidation reactions. Molybdenum leaching degree decreases through five cycles. Moreover, the catalyst can be recovered from the reaction mixture unchanged as determined by IR and UV–Vis spectra.

Keywords:

• Oxidation
• Heterogeneous catalysis
• Molybdenum
• Polymeric complexes

Acknowledgment

Financial support of this study by the Research Council of Sharif University of Technology is greatly appreciated.