Reactivity of the sulfur center in rhodium-bound benzaldehyde thiosemicarbazones towards molecular oxygen. A theoretical investigation

The 4-R-benzaldehyde thiosemicarbazones (L-R) are known to react with [Rh(PPh3)3Cl] in refluxing ethanol in the presence of a base (NEt3) to afford organorhodium complexes (2-R), where the thiosemicarbazones are coordinated to rhodium as tridentate CNS donors with the sulfur atom oxidized by aerial oxygen to sulfone. Two triphenylphosphines and a hydride are also coordinated to the metal center. From the reaction with 4-nitrobenzaldehyde thiosemicarbazone, a second organorhodium complex (1-NO2) is obtained, in which the sulfur atom is not oxidized. Reaction of the 4-R-benzaldehyde thiosemicarbazones with [Rh(PPh3)3Cl] in refluxing ethanol in the absence of NEt3 affords another group of organorhodium complexes (3-R), in which the thiosemicarbazones are coordinated to rhodium as tridentate CNS donors, along with two triphenylphosphines and a chloride. In these 3-R complexes also the sulfur atom is not oxidized. Structures of all the complexes have been optimized by DFT calculations and compared with the already known X-ray crystallographic structures. Also the experimentally observed electronic absorption bands have been assigned to specific transitions based on the TDDFT studies. Molecular electrostatic potential (MESP) topographical analysis performed to find the deepest MESP point on the coordinated sulfur atom (Vmin) is used as a probe for assessing the oxidizability of the coordinated sulfur in 1-R and 3-R complexes. Energy differences between the three sets of complexes have been estimated and based on the results obtained, 3-R has been experimentally transformed into 2-R, via formation of 1-R as the intermediate.

Graphical abstractThe 4-R-benzaldehyde thiosemicarbazones (L-R) react with [Rh(PPh3)3Cl] in refluxing ethanol in the presence of NEt3 to afford organorhodium complexes 2-R, via formation of intermediate 1-R. Similar reaction in the absence of NEt3 affords complexes 3-R. DFT studies show that presence of hydride in 1-R makes it responsible for oxidation of the sulfur center by molecular oxygen to generate 2-R. Presence of chloride in 3-R makes the sulfur center inert towards further oxidation. Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Benzaldehyde thiosemicarbazones show interesting reactivity with [Rh(PPh3)3Cl]. ► In presence of NEt3 thiosemicarbazones undergo oxidation by molecular oxygen. ► In absence of NEt3 complexes containing unoxidized thiosemicarbazone are obtained. ► DFT calculations show the hydride intermediate is responsible for the oxidation.