Method of Establishing the Lewis Acidity of a Metal Fragment Based on the Relative Binding Strengths of Ar-BIAN Ligands (Ar-BIAN = Bis(aryl)acenaphthenequinonediimine)

The relative coordination strengths of a series of differently substituted Ar-BIAN ligands (Ar-BIAN = bis(aryl)acenaphthenequinonediimine) to a series of palladium complexes in both the formal 0 and 2 oxidation states have been determined. In all cases a good to excellent linearity of log K eq with respect to the Hammett σ constants of the substituents on the aryl fragments of the ligands was observed. The resulting ρ constant is proposed to be a good indication of the Lewis acidity of the metal fragment, a physical quantity for which experimental parameters have been determined only for a limited class of compounds. The obtained parameters allow a comparison not only of different olefin complexes among themselves but also with respect to different metal fragments such as Pd(OAc) 2, Pd(Me)Cl, and a π-allyl complex. The Lewis acidity of the olefin complexes is extremely variable and ranges from the less acidic (Pd(Ar-BIAN)(DMFU); DMFU = dimethyl fumarate) to two of the most acidic (Pd(Ar-BIAN)(TCNE) and Pd(Ar-BIAN)(FN); TCNE = tetracyanoethylene, FN = fumarodinitrile) complexes among those examined. A cationic π-allyl complex has the highest Lewis acidity among the complexes examined. The importance of steric effects is examined in some cases.