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-substitu-ents:nBu,H(1a);tBu,H(1b);nOc,H(1c); Bn, H (1d); andnBu,nBu (1e)] were successfully coordinated to the dioxomolybdicfragment, resulting in MoO2(R1R2SaAm)2complexes2a-e, whichwere characterized through elemental analysis, IR,1H- and13CNMR, ESI-HRMS, and XRD (for2a,b,e). All complexes are activecatalysts in the solvent-free epoxidation ofcis-cyclooctene withtert-butyl hydroperoxide in decane (TBHPdec), showing highturnover frequencies (TOF 1890 h-1for2b) at 1 % loading. Us-ing aqueous TBHP (TBHPaq) or H2O2, selectivity to cycloocteneoxide is always 100 %, although reactions are more sluggish.IntroductionDioxomolybdenum(VI) complexes have emerged as effectiveLewis acid/base catalysts, which have found application in anumber of organic transformations, ranging from acylation, ep-oxide-ring opening, carbamoylation, acetalization, and deoxy-genation reactions.[1,2]Furthermore, the acidic character of themetal center makes most of [MoO2]2+-based complexes capableof mediating the OAT (Oxygen-Atom Transfer) from bulk oxid-ants (e.g., alkylhydroperoxides, H2O2, and O2)toO-acceptors,such as sulfides, phosphines, and alkenes.[1,2]Owing to the highimportance of epoxides as intermediates in the synthesis ofvaluable organic compounds[3]and new materials,[4]the ep-oxidation of olefins has been widely investigated and an im-pressive number of dioxomolybdic complexes have been devel-oped over the last decades.[1,3,5-7,8]To date, the most studied (pre)catalysts have been essentiallyof three types: MoO2X2L2with X = Cl, Br and L =N,O-donorneutral mono- or L2= bidentate ligand [e.g., MoCl2O2(dmf)2,dmf =N,N-dimethylformamide; MoCl2O2(bipy), bipy = 2,2?-bi-pyridine],[5]MoO2(XL)2with (XL) = monoanionic bidentate li-[a]CNR-ICCOM, UOS Bari,Via Orabona 4, 70126 Bari, ItalyE-mail: annese@ba.iccom.cnr.it[b]Dipartimento di Chimica, Università di Bari "A. Moro",Via Orabona 4, 70126 Bari, Italy[c]CNR-ICCOM Florence Research Area,Via Madonna del Piano 10, 50019 Sesto Fiorentino (Fi), ItalySupporting information and ORCID(s) from the author(s) for this article areavailable on the WWW under https://doi.org/10.1002/ejic.201801096.Eur. J. Inorg. Chem.0000, 0-0© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim1The2c/TBHPaq system, which displays the best TOF (1070 h-1)at 0.25 % loading and 75 °C, allowed for the quantitative con-version oftrans-2-octene into its epoxide, while low epoxideselectivity was observed in the case of 1-octene, styrene, andmethyl oleate. In the latter case, 90 % epoxide selectivity at92 % conversion was achieved with the2b/TBHPdec system at55 °C, under solvent-free conditions. Compared to relatedMoO2X2(O-amide)-type complexes,2a-eexhibit increased cata-lytic performance under the greener conditions involving theuse of aqueous oxidants.