Rhenium, Palladium, and Copper Pyridylalkoxide Complexes:  Synthesis, Structural Characterization, and Catalytic Application in Epoxidation Reactions

Reaction of bis(alkyl/aryl)-2-pyridylalcoholate ligands ( a − g ) with [ReOCl 4 ](NBu 4 ) results in the formation of metal-oxo complexes of the general formula [ReOCl 3 (L)](NBu 4 ) ( 2, 3 ) and [ReOCl(L) 2 ] ( 4, 5 ), with L = bis(alkyl/aryl)-2-pyridylalcoholate. Molecular structures of the complexes 2a, 4a, and 5a have been determined by single-crystal X-ray diffraction studies. Besides the expected crystal structure of 4a, where an alkoxide site of one of the ligands is coordinated trans to the rhenyloxo fragment, complex 5a shows a rare example of a trans chloro-oxo [ReOCl(L) 2 ] connectivity. Additionally, starting from Pd(OAc) 2 and [CuCl 2 ·2(H 2 O)] complexes of the form [M(L) 2 ] (M = Cu, Pd) ( 6a − g and 8a ) were prepared. For complexes 6a and 8a molecular structures have been determined by single-crystal X-ray diffraction studies, which showed the expected square planar geometry with the pyridine ring nitrogens situated mutually in trans position. Complexes 3, 4, 6, and 8 were tested in the epoxidation reaction of cyclooctene with TBHP ( tert -butylhydrogenperoxide).