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

Evans, John, O'Neill, Lynn, Kambhampati, Vijaya L., Rayner, Graham, Turin, Sandra, Genge, Anthony, Dent, Andrew J. and Neisius, Thomas (2002) 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. Journal of the Chemical Society, Dalton Transactions, (10), 2207-2212. (doi:10.1039/b200617k).

Abstract

Energy dispersive EXAFS (EDE), Quick EXAFS (QEXAFS), C-13 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.

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