Optically active 4- and 5-coordinate transition metal complexes of bifurcated dipeptide Schiff bases

Symmetrical and unsymmetrical benzophenone Schiff bases of bifurcated dipeptides [e.g., Ar₂C=N-CHR¹CONH∼HNCO-CHR²-N= CAr₂] have been synthesized using Boc methodology. These ligands may be regarded as chiral porphyrin mimics because of the α-carbons of the amino acids. The Schiff bases function as effective ligands for transition metals, particularly the late transition metals Ni(II), Cu(II), and Zn. Upon metal insertion, there is loss of the amide protons, resulting in N₄ chelating ligands that retain the amino acid based chirality as well as newly generated metal-centered chirality, which for the Ni(II) complexes have been shown by X-ray analysis to be A (left-handed helix) if the amino acids are S. For Ni(II) and Cu(II), metal insertion results in highly colored complexes and is easily followed by UV-vis spectrophotometry. Several Ni(II) complexes were also characterized by ¹H NMR. Co(II) and Mn(II) complexes were characterized by CW EPR. Two Cu(II) complexes, 7f-Cu^II and 7k-Cu^II, were characterized by EPR (ENDOR and ESEEM), which clearly showed the pentacoordinate nature of 7k-Curr^II.