Tridentate ONS vs. ONO salicylideneamino(thio)phenolato [MoO2L] complexes for catalytic solvent-free epoxidation with aqueous TBHP

•S for O substitution in salicylideneaminophenolato gives a more active catalyst.•Efficient epoxidation catalysis under solvent-free conditions•Bartlett-like oxygen transfer mechanism from TBHP to the olefin substrate

Comparison of molybdenum precatalysts with salicylideneaminothiophenolato (SATP; a tridentate ONS ligand) and salicylideneaminophenolato (SAP; a tridentate ONO ligand) in the solvent-free epoxidation of cyclooctene with aqueous TBHP yields better activity for the former. The DFT study of the catalytic cycle shows that the SATP catalyst follows the same pathway recently established for the SAP analogue and with a slightly smaller activation barrier in agreement with the experimental results.