Iridium assisted S–H and C–H activation of benzaldehyde thiosemicarbazones. Synthesis, structure and electrochemical properties of the resulting complexes

Abstract

Reaction of five 4- R-benzaldehyde thiosemicarbazones (R = OCH 3, CH 3, H, Cl and NO 2) with [Ir(PPh 3) 3Cl] in refluxing ethanol in the presence of a base (NEt 3) affords complexes of three different types, viz. 1- R, 2- R and 3- R. In the 1- R complexes the thiosemicarbazone is coordinated to iridium as a monoanionic bidentate N,S-donor forming a four-membered chelate ring. Two triphenylphosphines, a hydride and a chloride are also coordinated to the metal center. The 2- R complexes are very similar in composition and stereochemistry to the corresponding 1- R complexes, except that a second hydride is bound to iridium instead of the chloride. In the 3- R complexes, the thiosemicarbazones are coordinated to iridium as dianionic tridentate C,N,S-donors forming two adjacent five-membered chelate rings. Two triphenylphosphines and a hydride are also coordinated to the metal center. Structures of the 1- NO 2 , 2- NO 2 and 3- NO 2 complexes have been determined by X-ray crystallography. Reaction of the same 4- R-benzaldehyde thiosemicarbazones with [Ir(PPh 3) 3Cl] in refluxing toluene in the presence of NEt 3 affords complexes of two types, viz. 3- R and 4- R. The 4- R complexes are very similar in composition and stereochemistry to the corresponding 3- R complexes, except that a chloride is bound to iridium instead of the hydride. Structure of the 4- CH 3 complex has been determined by X-ray crystallography. In all the complexes the two PPh 3 ligands are trans. All the complexes show intense MLCT transitions in the visible region. Cyclic voltammetry on the complexes shows an Ir( III)–Ir( IV) oxidation on the positive side of SCE followed by an oxidation of the coordinated thiosemicarbazone. A reduction of the coordinated thiosemicarbazone is also observed on the negative side of SCE.