Oxorhenium(V) Complexes with Phenolate–Pyrazole Ligands for Olefin Epoxidation Using Hydrogen Peroxide

Oxorhenium­(V) complexes of the general
formula [ReOCl2(PPh3)­(L)] (2a–c) and [ReOCl­(L)2] (3a–c) with L being monoanionic, bidentate phenolate–pyrazole
ligands 1a–c that bear substituents
with various electronic features on the phenol ring (1a Br, 1b NO2, 1c OMe) were prepared.
The compounds are stable toward moisture and air, allowing them to
be handled in a normal lab atmosphere. All complexes were fully characterized
by spectroscopic means and, in the case of 2b, 2c, 3b, and 3c, also by single-crystal
X-ray diffraction analyses. Electrochemical investigations by cyclic
voltammetry of complexes 3a–c showed
a shift to more positive potentials for the Re­(V)/Re­(VI) redox couple
in the order of 3b > 3a > 3c (R= NO2 > Br > OMe), reflecting the higher electrophilic
character of the Re atom caused by the ligands 1a–c. Complexes 2a–c and 3a–c display excellent catalytic activity
in the epoxidation of cyclooctene, where all six complexes give quantitative
conversions to the epoxide within 3 h if tert-butylhydroperoxide
(TBHP) is employed as oxidant. Moreover, they represent rare examples
of oxorhenium­(V) catalysts capable of using the green oxidant hydrogen
peroxide, leading to high yields up to 74%. Also, green solvents such
as diethylcarbonate can be used successfully in epoxidation reactions,
albeit resulting in lower yields (up to 30%).