Reactivity of palladium olefin complexes with heteroditopic NHC–pyridine as spectator ligand toward olefin exchange

We have synthesized some derivatives bearing two strong sigma-donor ligands simultaneously coordinated to the palladium centre by exploiting the lability of the pyridine wing of the (1-methyl-3-((6-methylpyridine- 2-yl)methyl)-imidazole) (Me-NHC-CH2-Py) moiety as a spectator ligand in Pd(0) complexes stabilized by deactivated olefins. Unprecedented mixed complexes bearing the ligand L (L = triphenylphosphine (PPh3), 2,6-dimethylphenyl isocyanide (DIC), triethyl phosphite (P(OEt)3)) and the ligand Me–NHC–CH2–Py with the uncoordinated dangling pyridine wing were therefore obtained by reacting an equimolar amount of the ligands L with the complex [Pd(eta2-olefin)(Me–NHC–CH2–Py)] (olefin = tetramethylethenetetracarboxylate (tmetc), fumaronitrile (fn), maleic anhydride (ma), naphthoquinone (nq)). Moreover, we have determined the equilibrium constants KE(ol) for the olefin exchange in the complex [Pd(eta2-olefin)(Me–NHC–CH2–Py)] and the related olefin coordinative ability order.