Rhodium Assisted C−H Activation of Benzaldehyde Thiosemicarbazones and Their Oxidation via Activation of Molecular Oxygen

The benzaldehyde thiosemicarbazones are found to undergo oxidation at the sulfur center upon reaction with [Rh(PPh₃)₃Cl] in refluxing ethanol in the presence of a base (NEt₃). A group of organorhodium complexes are obtained from such reactions, in which the oxidized thiosemicarbazones are coordinated to rhodium as tridentate CNS donors, along with two triphenylphosphines and a hydride. From the reaction with <i>para</i>-nitrobenzaldehyde thiosemicarbazone, a second organometallic complex is obtained, in which the thiosemicarbazone is coordinated to rhodium as a tridentate CNS donor, along with two triphenylphosphines and a hydride. Reaction of the benzaldehyde thiosemicarbazones with [Rh(PPh₃)₃Cl] in refluxing ethanol in the absence of NEt₃ affords another group of organorhodium complexes, in which the thiosemicarbazones are coordinated to rhodium as tridentate CNS donors, along with two triphenylphosphines and a chloride. Structures of representative complexes of each type of complexes have been determined by X-ray crystallography. In all of the complexes, the two PPh₃ ligands are trans. All of the complexes show intense MLCT transitions in the visible region. Cyclic voltammetry on these complexes shows a Rh(III)−Rh(IV) oxidation on the positive side of SCE. Redox responses of the coordinated thiosemicarbazones are also displayed by all of the complexes.