Half-sandwich (η5-Cp*)Rh(iii) complexes of pyrazolated organo-sulfur/selenium/tellurium ligands: efficient catalysts for base/solvent free C–N coupling of chloroarenes under aerobic conditions

Abstract

Three new pyrazolated chalcogenoether ligated Rh(III) half-sandwich complexes (1–3) were synthesised by the thermal reaction of chalcogenoether (S, Se and Te) substituted 1H-pyrazole ligands (L1–L3) and [(η⁵-C₅Me₅)RhCl]₂ in methanol. The complexes were fully characterised by various spectroscopic techniques, and the molecular structures of complexes 1 and2 were also established through single crystal X-ray crystallographic analysis, which indicates a pseudo-octahedral half-sandwich piano-stool geometry around the rhodium metal. All three complexes were found to be thermally stable and insensitive towards air and moisture. One mol% of Rh(III) complexes (1–3) along with 10 mol% of Cu(OAc)₂ were explored for the Buchwald–Hartwig type C–N coupling reactions of amine and aryl chloride. Good to excellent yields (89–92%) of the coupling products were obtained with seleno- and thio-ether functionalised pyrazolated Rh(III) complexes (1 and 2), while an average yield (39%) was obtained with the telluro-ether functionalised complex (3). In contrast to the previously reported C–N coupling reactions the present reaction works under solvent- and base-free conditions, and the coupling reaction is accomplished in just 6 h with a high yield of the coupling product. The present methodology was also found to be efficient for a wide variety of functionalised aryl halides, and aliphatic or aromatic amines (1° and 2°). Moreover, the reaction also enables the C–N coupling of electron-withdrawing substrates and base-sensitive functionalities.