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

The synthesis and characterization of divalent palladium and platinum complexes of the type PdX 2 (Ar-BIAN) (X = Cl, Br, I, OC(O)Me), PdCl 2 (Ph-BIC) and PtCl 2 (Ar-BIAN) is described. These complexes contain the rigid bidentate nitrogen ligands bis(arylimino)acenaphthene (Ar-BIAN; Ar = Ph, p -MeC 6 H 4, p -MeOC 6 H 4, o, o ‘-Me 2 C 6 H 3, o, o ‘- i -Pr 2 C 6 H 3 ) 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 PdCl 2 (Ar-BIAN) complexes in THF or DMF occurs via two one-electron reductions and affords the complex Pd I Cl(Ar-BIAN) • -, which slowly produces some Pd 0 (Ar-BIAN) complex. Pd I Cl(Ar-BIAN) • - reacts with iodomethane, whereas with iodobenzene or bromobenzene no reaction was observed. Pd I Cl(Ar-BIAN) • - reacts with free Ar-BIAN or the alkenes dimethyl fumarate, dimethyl maleate, and methyl acrylate, giving complexes of the formulas Pd I Cl(Ar-BIAN) 2 • - and Pd I Cl(Ar-BIAN)(alkene) • -, respectively. A two-electron reduction of the latter afforded Pd 0 (Ar-BIAN) 2 2 - and Pd 0 (Ar-BIAN)(alkene) 2 -, respectively, whose further oxidation in two one-electron steps produces Pd 0 (Ar-BIAN) 2 and Pd 0 (Ar-BIAN)(alkene). The Pd 0 (Ar-BIAN) complex which is slowly formed from Pd I Cl(Ar-BIAN) • - reacts with alkene but directly affords the complexes Pd 0 (Ar-BIAN)(alkene) • - because Pd 0 (Ar-BIAN)(alkene) complexes are formed at a potential more negative than their first reduction potential. Reoxidation of Pd 0 (Ar-BIAN)(alkene) • - affords Pd 0 (Ar-BIAN)(alkene) complexes. The results of the electrochemical experiments corroborate earlier mechanistic proposals of exchange of Ar-BIAN ligands in Pd 0 (Ar-BIAN)(alkene) complexes and homogeneous hydrogenation of electron-poor alkenes by Pd 0 (Ar-BIAN)(alkene) complexes.