Oxorhenium(V) Complexes with Pyrazole Based Aryloxide Ligands and Application in Olefin Epoxidation

We synthesized and characterized a set of new oxorhenium(V) complexes coordinated by various pyrazole containing phenol (L1−L3) and naphthol ligands (L4−L7). Depending on the starting material, we were able to selectively synthesize monosubstituded or disubstituted complexes of the type [ReOBr₂L(PPh₃)] (1−7; L = L1−L7) and [ReOClL₂] (L = L1 8; L2 9; L4 10; L6 11), respectively. All complexes are stable to air and moisture, both in solid state as well as in solution. Furthermore, the cationic oxorhenium(V) complex [ReO(L1)₂(NCMe)](OTf) (8a) was obtained upon chloride abstraction with silver triflate from 8. All new complexes were able to catalyze the epoxidation of cis-cyclooctene in yields up to 64%. The ease of preparation and their tolerance to air and moisture, as well as the simple ligand modifications, make them an interesting class of novel catalysts. An attempted reduction of perchlorate ClO₄⁻ with complex 8 was unsuccessful. Molecular structures of complexes 1, 4, 6, 7, 8, 8a, 10, and 11 were determined by single crystal X-ray diffraction analyses.