Structural characterisation of solution species implicated in the palladium-catalysed Heck reaction by Pd K-edge X-ray absorption spectroscopy: palladium acetate as a catalyst precursor

Abstract

Energy dispersive EXAFS (EDE), Quick EXAFS (QEXAFS), ¹³C NMR and X-ray crystallography have been used to probe the co-ordination sphere of palladium in the course of the phosphine free Heck reaction. [Pd₂I₆][NBu₃H]₂ has been isolated from the precatalytic solution and its crystal structure determined. EDE and QEXAFS spectra of the complexes Pd(OAc)₂, Pd(PPh₃)₄ and [Pd₂I₆][NEt₃H]₂ illustrated the value of the technique in structure elucidation. EXAFS of the precatalytic solution detects [Pd₂I₆]²⁻ and no co-ordinated carbon. EXAFS of the catalytic solution shows a first co-ordination sphere of 2 carbon atoms and a second of 2–2.5 iodines. A scheme involving an equilibrium between the oxidative addition product and the olefin co-ordination species, has been proposed to explain these results.