Intramolecular ligandligand interactions in mixed ligand complexes

Abstract

One of the most important properties of mixed ligand complexes is direct and metal ion-mediated ligandligand interactions, the latter being known as π-acidπ-base interactions around the central metal ion. Examples of direct intramolecular ligandligand interactions are ionic and hydrogen bonds between charged and/or polar side groups and hydrophobic interactions between coordinated aromatic rings and side chain aliphatic groups or aromatic rings (aromatic ring stacking). Ionic interactions were found to exist in aqueous solutions of low-molecular weight mixed ligand complexes, e.g., Cu(A)(B), where A refers to acidic amino acids (aspartate or glutamate) and B to monoprotonated basic amino acids (zwitterionic arginine, lysine, or ornithine) [1]. The ligandligand interactions in the M(L-A)(L-B) systems (MCu(II) or Pd(II) give rise to a CD spectral magnitude anomaly which is dependent on the solvent polarity and the ionic strength, and the rotamer populations calculated from the NMR coupling constants are affected by the interactions in the Pd(II) complexes [1,2]. On the other hand, the hydrogen bonding between the carboxylate group of histidinate (His) and the polar side group of asparaginate (Asn), serinate (Ser), threoninate (Thr), etc. (reffered to as AA) in the ternary systems M(AA)(His) was confirmed by evidence from synthetic and CD and NMR spectral studies [3]. The infrared spectra of the isolated complexes indicated the existence of geometric isomerism in the diastereomers of M(A)(B) and M(AA)(His). On the basis of the steric requirements for the intramolecular ligandligand interactions, the geometry of M(L-A)(L-B) and M(D-AA)(L-His) has been inferred to be trans and that of M(D-A)(L-B) and M(L-AA)(L-His) cis with respect to the amine nitrogens. this is further supported by our recent infrared spectral study of the crystalline 63Cu- and 65Cu(AA)(L-His) complexes (AA = L-Asn, D-Asn, L-Thr, L-Ser, orD-Ser), which shows that only Cu(L-AA)(L-His) exhibits a metal isotope shift ( ca. 1 cm −1) of the CuNH 2 symmetric stretching band. The finding is reasonably explained by the cis structure for Cu(L-AA)(L-His) as established for AA = Asn and Thr by X-ray analysis and the trans structure for Cu(D-AA)(L-His) where the isotope shift is not observed because the symmetric stretching mode does not involve the displacement of Cu. Possibilities of ligandligand interactions in mixed ligand heteronuclear complexes will also be discussed.