Role of the kinetic template effect in the syntheses of non symmetric Schiff base complexes

The kinetic template effect favours or not formation of a neutral non symmetric Schiff base complex, depending on the coordination of the nitrogen or oxygen atom of the pyrrole carboxaldehyde reacting ligand to the copper ion. The template effect involves two main effects, first the “kinetic template effect”, in which a metal ion is able to hold two reactive functions in a position that allows a reaction to occur, followed by the “thermodynamic template effect” that favours the formation of a thermodynamically more stable product resulting from the reaction. Through the preparation of a non symmetric Schiff base, we highlight the primordial role of the “kinetic template effect”. Very often, this kinetic effect is not easy to evidence, the resulting complex being too reactive to be isolated. In the present example, the use of an ambidentate ligand, pyrrole carboxaldehyde, has been quite interesting. Reaction of a cationic copper complex possessing a primary amine function coordinated to the copper ion with pyrrole carboxaldehyde can yield two different complexes, depending on the reaction conditions. In absence of a base, the structural determination shows that the copper ion is linked in apical position to the carbonyl oxygen atom of pyrrole carboxaldehyde and the two reactive aldehyde and amine functions are unable to react, as a consequence of their mutual coordination to copper. On the contrary, addition of a base to the reaction mixture induces deprotonation of the pyrrole function followed by its coordination to copper ion, leaving free the carbonyl carbon atom that can be positioned in the vicinity of the primary amine function, thus facilitating the formation of a new imine function. The resulting thermodynamically stable complex is characterized by X-ray diffraction.