Oxidation of rhodium(I) by hydroxamic acids. Synthesis, structure, and electrochemical properties of bis(hydroxamate) complexes of rhodium(III)

There has been considerable interest in the coordination chemistry of the hydroxamic acids (1), because of their relevance in the physiological systems in general and because of their siderophoric activities in microbial transport of iron(3) and their theraputic applications in particular.(4) Hydroxamic acids are known to bind to metal ions usually as a bidentate O,O-donor forming a five-membered chelate ring (2).(5) However, we have recently observed an interesting chemical transformation of N-phenylbenzohydroxamic acids into their corresponding amides, brought about during their reaction with [Os(bPY)(2)Br-2], whereby the amides coordinate to osmium(III) as dianionic C,N-donors (3).(6) This has encouraged us to explore the interaction of the hydroxamic acids with other transition metal ions, preferably in their low oxidation states. For this study we selected rhodium(l) as the low-valent metal ion and N-phenylbenzohydroxamic acids as ligand. It may be mentioned here that though the chemistry of hydroxamate complexes of many transition metals has received considerable attention,(2) that of rhodium, hydroxamates appears to remain completely unexplored. As the source of rhodium(l), the Wilkinson's catalyst, viz. [Rh(PPh3)(3)Cl] was chosen because of its well-known ability to bring about catalytic transformation of organic molecules,7 as well as its efficiency as a synthon. for the preparation of mixed-ligand octahedral complexes of rhodium(III) via oxidative addition of incoming ligands.(8) Reaction of the, N-phenylbenzohydroxamic acids with [Rh(PPh3)(3)Cl] afforded a family of bis(hydroxamate) complexes of rhodium(III) along with the corresponding amides as the byproduct. The chemistry of the bis(hydroxamate) complexes of rhodium(M) is reported here with special reference to their synthesis, structure, and electrochemical properties.