Combinatorial Synthesis of Acacen-Type Ligands and Their Coordination Compounds

A highly
modular synthetic method for the preparation of acacen-type
ligands and their coordination compounds was developed. A series of
46 acacen-type ligands were synthesized by a combinatorial acid-catalyzed
transamination between six primary diamines and eight enaminones.
The bis-enaminone products were used as tetradentate ligands for coordination
of copper­(II), nickel­(II), cobalt­(II), and palladium­(II). Dependence
of the preferred E- or Z-configuration
of the enaminone ligand on the α-substituent of the enaminone
moiety in solution was determined by NMR and confirmed by X-ray diffraction.
The copper­(II) complexes were tested for their suitability as catalysts
in 3 + 2 cycloaddition of azomethine imine to methyl propiolate.