Synthesis of mixed Ar,Ar′-BIAN ligands (Ar,Ar′-BIAN = bis(aryl)acenaphthenequinonediimine). Measurement of the coordination strength of hemilabile ligands with respect to their symmetric counterparts

Abstract

The synthesis of Ar,Ar′-BIAN ligands (Ar,Ar′-BIAN = bis(aryl)acenaphthenequinonediimine) having different aryl groups bound to the two nitrogen atoms is reported for the first time. The ligands were obtained by two different strategies: (i) by a transimination reaction starting from a symmetric Ar,Ar-BIAN ligand having aryl groups bearing strongly electron-withdrawing substituents or (ii) by a two-step–one-pot sequence. The ligands synthesized have been chosen so that the electronic difference between the two aryl groups is very large, but the steric difference is variable and, in one case, the ligand is almost sterically symmetric. The coordination strength of the new ligands towards three palladium complexes has been measured by a competition experiment following a protocol previously described by us. The coordination strength of the mixed ligands is the mean of those of the corresponding symmetric counterparts. The X-ray crystal structure of a palladium π-allyl complex of the electronically asymmetric–sterically symmetric ligand (3,5-(CF₃)₂C₆H₃),(3,5-Me₂C₆H₃)-BIAN has been solved, together with those of the two symmetric analogues to allow a comparison. Despite the fact that the dodecafluorinated ligand has a K_eq value about three orders of magnitude lower than the non-fluorinated counterpart, no notable difference is observed in the N–Pd and Pd–C_allyl distances in the three complexes. Calculations at the density functional level confirm that Pd–BIAN distances are not strictly correlated to the coordination energies, which are in qualitative agreement with the spectroscopic evidence. The bond length is thus not a good indication of the bond strength in these cases.