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

Abstract

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.