Divalent Palladium and Platinum Complexes Containing Rigid Bidentate Nitrogen Ligands and Electrochemistry of the Palladium Complexes1

Abstract

The synthesis and characterization of divalent palladium and platinum complexes of the type PdX2(Ar-BIAN) (X = Cl, Br, I, OC(O)Me), PdCl2(Ph-BIC) and PtCl2(Ar-BIAN) is described. These complexes contain the rigid bidentate nitrogen ligands bis(arylimino)acenaphthene (Ar-BIAN; Ar = Ph, p-MeC6H4, p-MeOC6H4, o,o‘-Me2C6H3, o,o‘-i-Pr2C6H3) or bis(phenylimino)camphane (Ph-BIC), which act as σ-donor ligands to the metal center, as was deduced from the observed shifts in the IR and NMR spectra of the complexes. Electrochemical reduction of PdCl2(Ar-BIAN) complexes in THF or DMF occurs via two one-electron reductions and affords the complex PdICl(Ar-BIAN)•-, which slowly produces some Pd0(Ar-BIAN) complex. PdICl(Ar-BIAN)•- reacts with iodomethane, whereas with iodobenzene or bromobenzene no reaction was observed. PdICl(Ar-BIAN)•- reacts with free Ar-BIAN or the alkenes dimethyl fumarate, dimethyl maleate, and methyl acrylate, giving complexes of the formulas PdICl(Ar-BIAN)2•- and PdICl(Ar-BIAN)(alkene)•-, respectively. A two-electron reduction of the latter afforded Pd0(Ar-BIAN)22- and Pd0(Ar-BIAN)(alkene)2-, respectively, whose further oxidation in two one-electron steps produces Pd0(Ar-BIAN)2 and Pd0(Ar-BIAN)(alkene). The Pd0(Ar-BIAN) complex which is slowly formed from PdICl(Ar-BIAN)•- reacts with alkene but directly affords the complexes Pd0(Ar-BIAN)(alkene)•- because Pd0(Ar-BIAN)(alkene) complexes are formed at a potential more negative than their first reduction potential. Reoxidation of Pd0(Ar-BIAN)(alkene)•- affords Pd0(Ar-BIAN)(alkene) complexes. The results of the electrochemical experiments corroborate earlier mechanistic proposals of exchange of Ar-BIAN ligands in Pd0(Ar-BIAN)(alkene) complexes and homogeneous hydrogenation of electron-poor alkenes by Pd0(Ar-BIAN)(alkene) complexes.