Dioxomolybdenum(VI) Complexes with Salicylamide Ligands: Synthesis, Structure, and Catalysis in the Epoxidation of Olefins under Eco-Friendly Conditions

Five salicylamides [R1R2SaAmH; R1, R2 = N-substituents: nBu, H (1a); tBu, H (1b); nOc, H (1c); Bn, H (1d); and nBu, nBu (1e)] were successfully coordinated to the dioxomolybdic fragment, resulting in MoO2(R1R2SaAm)2 complexes 2a-e, which were characterized through elemental analysis, IR, 1H- and 13C NMR, ESI-HRMS, and XRD (for 2a,b,e). All complexes are active catalysts in the solvent-free epoxidation of cis-cyclooctene with tert-butyl hydroperoxide in decane (TBHPdec), showing high turnover frequencies (TOF 1890 h–1 for 2b) at 1 % loading. Using aqueous TBHP (TBHPaq) or H2O2, selectivity to cyclooctene oxide is always 100 %, although reactions are more sluggish. The 2c/TBHPaq system, which displays the best TOF (1070 h–1) at 0.25 % loading and 75 °C, allowed for the quantitative conversion of trans-2-octene into its epoxide, while low epoxide selectivity was observed in the case of 1-octene, styrene, and methyl oleate. In the latter case, 90 % epoxide selectivity at 92 % conversion was achieved with the 2b/TBHPdec system at 55 °C, under solvent-free conditions. Compared to related MoO2X2(O-amide)-type complexes, 2a-e exhibit increased catalytic performance under the greener conditions involving the use of aqueous oxidants.