Cobalt(II), nickel(II) and zinc(II) complexes of peptide group containing amino acids Bis(acetyl-DL-valinate)metal(II) complexes

First row transition metal(II) complexes of N- acetyl-DL-valine of the type M(AcVal)2·xH2O (M = Co(II), Ni(II) and x = 2; M = Zn(II) and X = O) and their amine adducts of the type M(AcVal)2B2 xH2O (M = Co(II), Ni(II) and Zn(II); B = pyridine, 3- and 4-methylpyridine, 1,10-phenthroline) were prepared and investigated by means of magnetic measurements, electronic, infrared and 1H n.m.r. spectroscopy. Magnetic moments and electronic spectra of the Co(II) and Ni(II) compounds, consistent with hexacoordinated metal(II) with some distortion from the Oh symmetry, suggest the presence of MO6 and MO4N2 chromophores for the hydrate and base adducts, respectively. By comparing the Dq values of the present complexes and those of the other N-substituted arnino acids previously studied, a spectrochemical series of the amino acid ligands is constructed. Also the infrared spectra agree with the coordination of the amino acid toward the carboxyl group. The trans-effect of the amines, with respect to that of the water, results in a weakening of the N-acetyl-DL-valinate coordination strength in the order py > 4pic > 3pic, as steric effects prevail over the inductive effects. The solution electronic and infrared spectra, which are very similar to those of the solid compounds, and the 1H n.m.r. spectra of the diamagnetic zinc(II) complexes indicate that the same complexes also exist in solution. The coupling of the CHNH group in the 1H n.m.r. spectra and the shift of the ν(NH) and δ(NH) vibrations in the infrared spec- tra of the solution, with respect to the solid com- plexes, indicate that the hydrogen bonding of the NH group present in the solid complexes it removed in chloroform solution.