Synthesis of Ar-BIAN ligands (Ar-BIAN = bis(aryl)acenaphthenequinonediimine) having strong electron-withdrawing substituents on the aryl rings and their relative coordination strength toward palladium(0) and -(II) complexes

The synthesis of Ar-BIAN ligands (Ar-BIAN = bis(aryl)acenaphthenequinonediimine) having strong electron-withdrawing substituents on the aryl ring is reported. Most of these derivatives had escaped isolation in a pure form up to now. A quantitative scale of coordination strength of the newly synthesized ligands in the complexes Pd-0(L)(DMFU) (DMFU = dimethylfumarate) and Pd-II(L)(OAc)(2) has been measured. The series also includes some previously known Ar-BIAN ligands, phenanthroline, bipyridine, and Ph-DAB (Ph-DAB = diphenyldiazabutadiene). A good correlation is observed for the Ar-BIAN ligands between the Hammet or constants of the substituents of the aryl group and the relative binding constant with respect to Ph-BIAN. The values for the p constant for the Pd(L)(DMFU) and Pd(L)(OAc)2 series are respectively -1.57 and -3.44, indicating that electron-rich ligands bind more strongly in both series, but the effect is much stronger in the Pd(II) series. The observed effect is relevant to the question of the effective oxidation state of palladium in the Pd(L)(DMFU) complex. Phenanthroline and bipyridine are stronger ligands than any Ar-BIAN compound, whereas Ph-DAB is the weakest ligand of all. The sterically hindered 2,6-(Pr2C6H3)-C-i-BIAN binds less strongly than its basicity would suggest.