Synthesis and characterization of transition metal encapsulated derivatives of 16- and 18- membered [N₄] macrocycles modified with four carbonyl functions bearing cyclic tetrapeptide features

Abstract

Condensation reactions of 1,2-diaminoethane or 1 ,3-diaminopropane with cyclic dicarboxylic acid anhydrides (succinic anhydride or phthalic anhydride) as well as their corresponding acids in 1:1 mole ratio in dioxane have afforded 16- or 18- membered macrocycles (L¹- L⁴). Physico-chemical and spectroscopic data reveal the presence of cyclic tetraamide/tetrapeptide functions. The macrocycles behave as strong chelating agents forming stable solid complexes with the stoichiometries [MLX ₂] (L = L¹- L⁴; M = Fe, Co or Cu; X = Cl or ClO₄) and [MLX₂]X (M = Fe or Cr). Spectral data indicate coordination through amide/peptide nitrogen without its deprotonation and suggest that metal ions lie in the basal

plane of the macrocyclic cavity. The coordination geometry has been ascertained from magnetic moment measurements, ligand field spectra and EPR studies. The anisotropic EPR signals observed for Cu (II) complexes with <i>g</i>_{<span dir="RTL" style="font-size:12.0pt;font-family:" times="" new="" roman";mso-bidi-language:he"="" lang="HE">║</span>}<i> </i>˃<i>g_┴ </i>> 2.0 confirm tetragonality in the geometry. However, the corresponding orbital reduction parameter <i>k_┴>k_║</i>and the magnitude of <i>G</i><4.0 suggest a significant contribution of the exchange coupling in the distortion of the <i>O</i>_h geometry of Cu (II) complex.