Rhenium(III), (IV) and (V) complexes with 6-hydroxypicolinic acid

Abstract

The reaction of trans-[ReOBr3(PPh3)2] with 6-hydroxypicolinic acid (H2hpa) in ethanol led to the isolation of the ReIVReIV dimer (μ-O)(μ-hpa)2[Re2Br(OEt)(PPh3)2] (1). Each hpa2− anion acts as a bridging ligand with the coordination of a neutral pyridol oxygen to one rhenium ion, and the coordination of a carboxylate oxygen and a pyridinate nitrogen to the other rhenium ion. By using [ReOCl3(PPh3)2] as precursor in ethanol, two products were isolated, i.e. (μ-Cl)(μ-O)(μ-hpa)[ReIV2Cl2(OEt)(PPh3)2] (2) and [ReIIICl2(Hhpa)(PPh3)2] (3). The complexes cis-[ReOX2(Hhpa)(PPh3)] (X=Br (4); Cl (5)) were the only products formed by the reaction of [ReOX3(PPh3)2] with H2hpa in acetonitrile. The bromide equivalent of 3, i.e. [ReIIIBr2(Hhpa)(PPh3)2] (6), was obtained from the reaction of [ReOBr3(PPh3)2] with H2hpa in 2-propanol. Coordination of Hhpa− in 3–6 occurs through the carboxylate oxygen and neutral pyridyl nitrogen. In addition to the X-ray crystal structures, infra-red, 1H NMR, electrochemical and electronic properties are also reported.