RIGID BIOENTATE NITROGEN LIGANDS IN ORGANOMETALLIC CHEMISTRY AND HOMOGENEOUS CATALYSIS .9. OXIDATIVE ADDITION OF ORGANIC HALIDES TO ZEROVALENT PALLADIUM COMPLEXES CONTAINING RIGID BIDENTATE NITROGEN LIGANDS

Zerovalent complexes of the type Pd(Ar-BIAN)(alkene), i.e. complexes containing the rigid bidentate nitrogen ligands bis(arylimino)acenaphthene (Ar = p-Tol, p-MeOC6H4, o-Tol,o,o'-Me2C6H3, o,o'-(i)Pr2C6H3) and an electron-poor alkene have been shown to react with a variety of (organic) halides RX, including methyl, benzyl, aryl, acyl and allylic halides, to give the corresponding square planar divalent Pd(R)X(Ar-BIAN) or [Pd(eta3-allyl)Ar-BIAN)]X complexes. The new complexes obtained have been fully characterized and their fluxional behaviour in solution studied by H-1 NMR spectroscopy. The rate of oxidative addition of iodomethane to Pd(p-Tol-BIAN)(alkene) complexes was found to decrease with increasing Pd-alkene bond strength, i.e. dimethyl fumarate > fumaronitrile, but oxidative addition to the fumaronitrile complex was accelerated by irradiation with a mercury lamp. Oxidative addition of allylic halides to Pd(p-Tol-BIAN)(alkene) complexes was instantaneous and independent of the coordinated alkene, whereas the analogous Pd(o,o'(i)Pr2C6H3-BIAN)(alkene) complexes react much more slowly. Mechanistic aspects are discussed on the basis of these observations. Some of organopalladium(II) complexes formed were treated with silver salts to give cationic complexes, e.g. [Pd(eta3-CH2Ph)(p-Tol-BIAN)]SO3CF3. When reactive (organic) halides such as acetyl chloride, benzyl bromide or dihalogens were employed, further oxidative addition occurred. In the case of dibromine, the intermediate Pd(IV)Br4(p-Tol-BIAN) complex was observed by H-1 NMR spectroscopy; in the other cases, a Pd(II)X2(p-Tol-BIAN) complex was isolated together with the organic coupling products R2.