Ternary α-Amino Acid-Palladium(II) Complexes with Ligand-Ligand Hydrogen Bonding

Abstract

Abstract Histidinate(His)-containing ternary α-amino acid-palladium(II) complexes have been studied by synthetic and spectroscopic methods. The following complexes with imidazolate (im), l- and d-His, l-asparaginate (l-Asn), and l-glutaminate (l-Gln) have been isolated as crystals: [Pd(im)2]·0.5H2O; [Pd(l-His) (d-His)]·H2O; [Pd(l-Asn)-(l-His)]·2H2O; [Pd(l-Gln) (l-His)]·2H2O. The ternary systems involving palladium (II), l-His, and an amino acid l-AA with an OH or a CONH2 group (AA=Asn, Gln, serinate, threoninate, or homoserinate) exhibit absorption maxima at 300–311 nm with ε 310–440 and negative and positive circular dichroism (CD) peaks at 307–328 nm and ≈300 nm, respectively. 13C and 1H NMR spectra gave the amounts of the cis and trans isomers of Pd(His)-(AA) in solution and confirmed that Pd(His)(AA) has an N3O donor set in the coordination plane. Whereas the CD magnitude additivity holds for the ternary systems Pd (l-His)(B) with B=glycinate, l-alaninate, or l-valinate and the systems with l-histidine methyl ester (l-HisOMe) and l-AA, Pd(l-HisOMe)(l-AA), the magnitudes for Pd (l-His) (l-AA) deviate significantly from those estimated on the basis of the additivity. The CD magnitude anomaly as well as the geometrical and rotational isomer populations calculated from the 1H NMR spectra establishes the existence of the ligand-ligand interaction between the carboxylato group of His and the hydroxyl or amido group of AA in solution.