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 para-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.