Construction of a monoanionic S,N,S-pincer ligand with a pyrrole core by sequential [1,2] phospho-Fries rearrangement. Characterization of palladium and silver coordination complexes

Abstract

The synthesis of an S,NH,S-pincer ligand possessing a pyrrole core and two O,O-diethylthiophosphonyl groups to design PS lateral coordination sites is reported. The synthetic procedure to produce this ligand makes use of the nitrogen atom of the pyrrole heterocycle to successively functionalize the ortho-positions with the two O,O-diethylthiophosphonyl moieties. The ortho-functionalization arises from a repetition of a [1,2] base-induced rearrangement allowing the transformation of O,O-diethyl-N-pyrollylthiophosphoramidate to ortho-thiophosphonate. The coordination properties of this tridentate S,NH,S-ligand have been investigated with silver and palladium metals. Reaction of the monoanionic S,N,S-pincer ligand 4 (O,O,O,O-tetraethylpyrrol-2,5-yl-dithiophosphonate) with PdCl₂(MeCN)₂ at room temperature in the presence of triethylamine gives rise to the formation of the η³-complex [Pd(η³-4′)Cl] 5. Ligand 4 also reacts with silver oxide in dichloromethane to produce complex 6 which, in the solid state, exists as a tetramer involving an almost linear arrangement of four silver atoms.