ISOCYANIDE INSERTION INTO THE PALLADIUM-CARBON BOND OF COMPLEXES CONTAINING BIDENTATE NITROGEN LIGANDS - A STRUCTURAL AND MECHANISTIC STUDY

Insertion of the isocyanides 2,6-dimethylphenyl isocyanide (DIC), tert -butyl isocyanide (TIG) and tosylmethyl isocyanide (TosMIC) into the P d-Me bond of complexes (NN)Pd(Me)Cl (NN = 2,2'-bipyridine (bpy), 1,10- phenanthroline (phen), 2,2'-bipyrimidine (bpm)) afforded quantitativel y(NN)Pd(C(=N-R)Me)Cl(R = 2,6-Me2C6H3, t-Bu, CH(2)Tosyl). The course of the reaction has been shown to proceed via the intermediates [(NN)Pd( CN-R)(Me)]Cl(NN = bpy, phen; R = 2,6-Me2C6H3, t-BU, CH(2)Toysl), which have been characterized at 250 K by NMR and IR spectroscopies and con ductivity measurements. The mechanism oft;he insertion reaction involv es substitution of the halide by the isocyanide followed by a rate-det ermining methyl migration to the precoordinated isocyanide. Kinetic me asurements of the isocyanide insertion reaction, which provided the re action rate constants of the isocyanide association (k(1)), isocyanide dissociation (k(-1)), and methyl migration reaction (k(2)), have demo nstrated that the migration rate of the methyl group to the precoordin ated isocyanide increases with increasing electrophilicity of the isoc yanide. Methyl migration in the intermediate complexes [(NN)Pd(CN-R)(M e)]Cl(NN = bpy, phen; R = 2,6-Me2C6H3, t-Bu, CH(2)Tosyl) also occurs i n the solid state. The complexes (NN)Pd(C(=N-R)Me)X(NN = bpy, phen; R = t-Bu, CH(2)Tosyl; X = Cl, Br) show a fluxional behavior due to a sit e exchange of the nitrogen donor atoms. Spin saturation transfer H-1 N MR experiments showed that the mechanism of this process involves Pd-N bond breaking and subsequent isomerization via a Y-shaped intermediat e.