engleski

Greener cyclooctene epoxidation with molybdenum complexes derived from aminobenzohydrazone ligands

INTRODUCTION Epoxides are undoubtedly the very important class of industrial chemicals, used as raw materials for epoxy resins, paints, surfactants and as intermediates in organic synthesis. Catalytic olefin epoxidation usually employs oxidants in excess, presence of organic solvents or ionic liquids. The future development of epoxidation systems is focused on environmentally friendly oxidants and highly economic processes. For that reason, eco- friendlier oxidants as O2, H2O2, or tert-butyl hydroperoxide (TBHP) have a great advantage. Among various catalysts, Mo complexes have been notably useful, particularly for the cyclic olefin epoxidation under mild reaction conditions.1, 2 A series of mononuclear, [MoO2(L)(MeOH)] and [MoO2(L)(EtOH)], and dinuclear, [MoO2(L)]2 complexes synthesized by using 2- or 4aminobenzoylhydrazone ligands derived from salicylaldehyde, 3-methoxy- and 4- methoxy- 2hydroxybenzaldehyde, were tested as catalysts. The hemilabile ligand-induced catalytic enhancement which was supported by density functional theory calculations. The molecular and crystal structures of mononuclear (and dinuclear complexes described in this work were determined based on single-crystal X-ray diffraction experiments. EXPERIMENTAL STUDY The catalytic activity of Mo complexes was investigated in the cyclooctene epoxidation with aqueous TBHP, without any addition of organic solvent, following and respecting eco- friendly conditions. Complexes were tested as catalysts in order to define the impact of the: (i) coordinated ligands, (ii) architecture of the complex (mononuclear vs. dinuclear ones), (iii) catalyst loading (0.05- 0.005 mmol) and (iv) reaction temperature. RESULTS AND DISCUSSION This study demonstrates the use of 2- or 4-aminoaroylhydrazone linking units for the formation of discrete Mo(VI) complexes. In these complexes, the cisMoO22+ core is coordinated to the bideprotonated ligand via the phenolate- oxygen, azomethine-nitrogen, and hydrazidato- oxygen. The sixth coordination site is occupied either by the nitrogen atom of the bridging aminobenzoylhydrazone moiety of the neighboring complex or by the oxygen atom from the coordinated alcohol molecules. Mo complexes with 2- aminobenzoylhydrazone ligands are much better catalysts than those containing 4aminobenzoylhydrazone in terms of activity and selectivity towards epoxide under proposed greener conditions. The position of the amino group on the phenyl ring strongly influences the catalytic behavior, suggesting that 2- aminobenzoylhydrazone derived complexes transform more easily to the pentacoordinated active species, crucial for the catalytic cycle. CONCLUSION The rational design of Mo(VI) catalysts with hemilabile aroylhydrazone ligands induced interesting catalytic results in terms of selectivity and conversion enhancement. The obtained catalytic results reveal that molybdenum complexes with this type of ONO hydrazones are efficient catalysts due to the peculiar noninnocent role of the NH2 substituent. Considering this is the first example of complexes with this type of ONO ligands providing excellent results operating under environmentally friendly conditions, further research will be directed in this regard.

molybdenum complexes ; hydrazones ; homogeneous catalysis ; organic solvent-free

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